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Effects of three antagonists on selected pharmacodynamic effects of sublingually administered detomidine in the horse
Veterinary Anaesthesia and Analgesia, 2014, 41, 36–47
doi:10.1111/vaa.12081
RESEARCH PAPER
Effects of three antagonists on selected pharmacodynamic effects of sublingually administered detomidine in the horse Heather K Knych*† & Scott D Stanley*† *K. L. Maddy Equine Analytical Chemistry Laboratory, School of Veterinary Medicine, University of California, Davis, CA, USA †Department of Veterinary Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, CA, USA
Correspondence: Heather K Knych, K.L. Maddy Equine Analytical Chemistry Laboratory, University of California, Davis, School of Veterinary Medicine, West Health Science Drive, Davis, CA 95616, USA. E-mail: [email protected]
Abstract Objective To describe the effects of alpha2-adrenergic receptor antagonists on the pharmacodynamics of sublingual (SL) detomidine in the horse. Study design Randomized crossover design. Animals Nine healthy adult horses with an average age of 7.6 6.5 years. Methods Four treatment groups were studied: 1) 0.04 mg kg1 detomidine SL; 2) 0.04 mg kg1 detomidine SL followed 1 hour later by 0.075 mg kg1 yohimbine intravenously (IV); 3) 0.04 mg kg1 detomidine SL followed 1 hour later by 4 mg kg1 tolazoline IV; and 4) 0.04 mg kg1 detomidine SL followed 1 hour later by 0.12 mg kg1 atipamezole IV. Each horse received all treatments with a minimum of 1 week between treatments. Blood samples were obtained and plasma analyzed for yohimbine, atipamezole and tolazoline concentrations by liquid chromatographymass spectrometry. Behavioral effects, heart rate and rhythm, glucose, packed cell volume (PCV) and plasma proteins were monitored. Results Chin-to-ground distance increased following administration of the antagonists, however, this effect was transient, with a return to pre-reversal 36
values as early as 1 hour. Detomidine induced bradycardia and increased incidence of atrioventricular blocks were either transiently or incompletely antagonized by all antagonists. PCV and glucose concentrations increased with tolazoline administration, and atipamezole subjectively increased urination frequency but not volume. Conclusions and clinical relevance At the doses administered in this study, the alpha2-adrenergic antagonistic effects of tolazoline, yohimbine and atipamezole on cardiac and behavioral effects elicited by SL administration of detomidine are transient and incomplete. Keywords alpha2-adrenergic agonist, antagonist, atipamezole, detomidine, equine, tolazoline, yohimbine.
Introduction Alpha2-adrenergic agonists, such as detomidine, are commonly used in equine medicine for procedures necessitating sedation and analgesia. Detomidine is available in injectable formulations for both intravenous (IV) and intramuscular (IM) administration and the pharmacokinetics and pharmacodynamics following administration via either of these two administration routes have been well characterized (Grimsrud et al. 2009; Hubbell et al. 2009; Mama et al. 2009). Most recently a sublingual (SL) gel
Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley formulation received approval by the Food and Drug Administration for use in the horse. Studies assessing the pharmacodynamics of SL administration of detomidine have described many effects comparable to those observed with parenteral administration (Kaukinen et al. 2010; Knych & Stanley 2011). For example, Kaukinen et al. (2010) and Knych & Stanley (2011) reported a statistically significant decrease in chin-to-ground distance following SL administration with maximal sedation occurring 40 minutes post administration and lasting for approximately 2 hours. Similar to that observed following IV and IM administration, SL administration of detomidine decreased heart rate (HR) and increased the incidence of atrioventricular (AV) blocks (Kaukinen et al. 2010; Knych & Stanley 2011). Alpha2-adrenergic antagonists are often used to reverse the cardiovascular and central nervous system (CNS) depressant effects of alpha2-adrenergic agonists following IV or IM administration. The three most commonly used alpha2-adrenergic antagonists in veterinary medicine are atipamezole, yohimbine and tolazoline. These drugs differ in their relative effectiveness as reversal agents which, in sheep, has been attributed to differences in their alpha2:alpha1-adrenergic receptor selectivity ratio (Schwartz & Clark 1998). Furthermore, the effectiveness of the antagonism appears to be dependent on the dose administered as well as the time administered relative to the alpha2-adrenergic agonist (Kollias-Baker et al. 1993; Ramseyer et al. 1998; Hubbell & Muir 2006). The alpha2D-adrenergic receptor is believed to be responsible for the sedative and cardiac depressant effects observed following administration of alpha2-adrenergic agonists. Of the three agents, atipamezole is the most selective for the alpha2D-adrenergic receptor (Schwartz & Clark 1998), at least in sheep. However, at the doses studied thus far (100–160 lg kg1), atipamezole has been shown to only partially reverse detomidine induced sedation, bradycardia and incidence of AV conduction disturbances following administration to the horse (Raekallio et al. 1990; Ramseyer et al. 1998; Hubbell & Muir 2006). Yohimbine is less selective for the alpha2-adrenergic receptor than atipamezole. At all doses studied, yohimbine effectively and quickly reverses detomidine induced sedation following both epidural and IV administration of detomidine (Skarda & Muir 1999; Knych et al. 2012b). Knych et al. (2012b) reported that the initial alertness, observed following yohim-
bine administration subsequent to detomidine, was followed by a return to a state of sedation, although to a lesser extent than that observed immediately following detomidine administration. Detomidine induced bradycardia and AV conduction disturbances are also rapidly, albeit transiently, reversed following administration of yohimbine (Skarda & Muir 1999; Knych et al. 2012b). Of the three antagonists used in veterinary medicine, tolazoline has the lowest affinity for the alpha2-adrenergic receptor (Schwartz & Clark 1998). Similar to the effects noted for atipamezole and yohimbine, tolazoline transiently and incompletely antagonizes sedation and cardiac effects following either an IV bolus or infusion of detomidine (Kollias-Baker et al. 1993; Carroll et al. 1997; Hubbell & Muir 2006). Although the effects of alpha2-adrenergic antagonists appear to be transient and incomplete, administration may decrease the time to recovery following IV and IM detomidine administration (Hubbell & Muir 2006). The purpose of the study described here was twofold. First we sought to describe the pharmacokinetics of the three selected reversal agents when administered subsequent to detomidine. The second goal was to evaluate the effects of the three adrenergic receptor antagonists in reversing the sedative and cardiovascular effects of SL administered detomidine and to assess the effects on parameters known to be affected by alpha2adrenergic agonists such as packed cell volume (PCV), plasma proteins (PP), and plasma glucose concentrations. Materials and methods Animals Nine healthy horses consisting of eight Thoroughbreds and one Quarter Horse (five geldings and four mares) with a mean SD weight of 528 58 kg and an age of 7.6 6.5 years were studied. Food was withheld for 12 hours prior to and for approximately 8 hours following drug administration. Water was available ad libitum throughout the study. Before beginning this study, horses were determined healthy and free of cardiovascular diseases by physical examination, cardiac auscultation, complete blood count (CBC) and a serum biochemistry panel that included aspartate aminotransferase (AST), creatine phosphokinase (CK), alkaline phosphatase (ALP), total
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Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley
bilirubin, sorbitol dehydrogenase (SDH), blood urea nitrogen (BUN) and creatinine. Blood analyses were performed by the William R. Pritchard Clinical Pathology Laboratory of the University of California, Davis, USA using their standard protocols. Horses did not receive any medications for at least 2 weeks prior to commencement of this study. This study was approved by the Institutional Animal Care and Use Committee of the University of California, Davis. Drug administration Four treatments were administered: 1) detomidine (0.04 mg kg1; Dormosedan Gel; Pfizer Animal Health, NY, USA) SL; 2) detomidine (0.04 mg kg1) SL followed 1 hour later by yohimbine (0.075 mg kg1; Yobine; Lloyd Laboratories, IA, USA) IV; 3) detomidine (0.04 mg kg1) SL followed 1 hour later by tolazoline (4 mg kg1; Tolazine; Lloyd Laboratories) IV; and 4) detomidine (0.04 mg kg1) SL followed 1 hour later by atipamezole (0.12 mg kg1; Antisedan; Pfizer Animal Health) IV. Doses of the antagonists were chosen based on approved doses (tolazoline) or selected from the literature if there was not an approved dose for horses (atipamezole and yohimbine). Each horse received all treatments with a minimum of 1 week elapsing between each treatment. Assignment to a particular group for a particular week was made by random selection of horse, treatment and week using a random number generator. Prior to drug administration, a 14-gauge catheter was aseptically placed in both external jugular veins, with the exception of horses receiving only detomidine, when only one venous catheter was placed. The right jugular vein catheter was used for administration of the antagonist and the other catheter was used for blood collection. Each horse was weighed immediately prior to drug administration. Sample collection and analysis Blood samples for analysis of plasma concentrations of the antagonists were collected at time 0 (immediately prior to detomidine administration) and at 1 hour post detomidine administration (immediately prior to antagonist administration) and at 5, 10, 15, 30, 45 minutes and 1, 1.5, 2, 2.5, 3, 4, 5, 6, 8, 12, 18, 24, 36, 48 and 72 hours post administration of the antagonist. Prior to drawing each sample of blood for analysis of drug 38
concentrations, 10 mL of blood was aspirated and discarded from the catheter and T-Port extension set (combined internal volume <2 mL). The catheter was flushed with 10 mL of a dilute heparinized saline solution (10 IU mL1) following each sample collection. Catheters were removed either following dosing (right side) or following collection of the 8-hour blood sample (left side) and the remaining samples were collected by direct venepuncture. Blood samples were collected into EDTA blood tubes (Kendall/Tyco Healthcare, MA, USA) and were centrifuged at 3000 g for 10 minutes. Plasma was immediately transferred into storage cryovials (Phenix Research Products, NC, USA) and stored at 20 °C until analysis, approximately 1 month following collection of the final time point. Additional blood samples were collected for analysis of plasma glucose concentrations, PCV and PP concentrations. Samples for plasma glucose analysis were collected at time 0 (immediately prior to detomidine administration), 15, 30 and 45 minutes and 1 hour post detomidine administration (prior to antagonist administration) and 15, 30 and 45 minutes and 1, 1.5, 2, 3, 4 and 6 hours post antagonist administration. Samples were collected from the jugular vein catheter into heparin blood tubes (Kendall/Tyco Healthcare, MA, USA) and were immediately placed on ice until centrifugation at 3000 g for 10 minutes at 4 °C. Analyses were performed by the William R Pritchard Clinical Pathology Laboratory of the University of California, Davis using their standard protocol for glucose analysis. Samples for PCV and PP determination were collected into heparinized syringes at time 0 (immediately prior to detomidine administration), 5, 10, 15, 30 and 45 minutes and 1 hour post detomidine administration and at 5, 10, 15, 30 and 45 minutes and 1, 1.5, 2, 3 and 4 hours post antagonist administration. PCV was measured by using microhematocrit and PP by using a refractometer. Determination of plasma drug concentrations Plasma yohimbine and tolazoline concentrations were measured using Liquid Chromatography-Mass spectrometry as reported previously (DiMaio Knych et al. 2011; Knych & Stanley 2011; Casbeer & Knych 2013). Analytical reference standards for atipamezole, and internal standards (IS) medetomidine, were obtained from Sigma Aldrich (MO, USA)
© 2013 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia, 41, 36–47
Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley and Orion (Farmos, Finland), respectively. Quantitative analyses were performed on a TSQ Vantage triple quadrupole mass spectrometer (Thermo Scientific, CA, USA) coupled with a Turbulent flow chromatography system (TLX4; ThermoFisher Scientific, MA, USA). Chromatography separation employed an ACE C18, 10 cm 9 2.1 mm, 3 lm, column (Mac Mod, PA, USA) and a linear gradient of acetonitrile (ACN) in water with a constant 0.2% formic acid at a flow rate of 0.35 mL minute1. The initial ACN concentration was held at 2.0% for 0.33 minutes, ramped up to 15% over 0.5 minute and then to 95% over 4.2 minutes and finally up to 99% at 2.97 minutes. The ACN concentration was held at 99% for 0.5 minutes and then the column was re-equilibrated at initial conditions for 3.0 minutes. The assay was validated using the procedure defined in the FDA Guidelines for Industry Bioanalytical Method Validation. Duplicate accurate weighings of 10 mg atipamezole and IS were made and then diluted in methanol to give a 1 mg mL1 solution. These separate stock solutions were further diluted using methanol to create concentrations of 0.01, 1 and 10 ng mL1. The standard solutions were used to prepare calibrations in plasma over the concentration range of 0.1–200 ng mL1 and three quality control (QC) samples. The IS were prepared using the same dilution protocol then combined forming a single IS mixture with a 50 ng lL1 final concentration. A series of three validation runs were carried out on three separate days using three calibration lines and six replicates of each of the three QC samples. The matrix influence was calculated using the injection of extracted equine plasma blank, plasma blank spiked with authentic reference standards and authentic reference standards spiked in solvent. Prior to analysis, plasma samples, standards and quality control samples were allowed to thaw at room temperature. A plasma aliquot of 500 lL was added to the IS medetomidine, (20 ng mL1) solution, and 2 mL 0.6 mol L1 phosphate buffer (pH 6.5). The eluent was collected in 2 mL autosampler vials dried under a stream of nitrogen gas and redissolved using 160 lL of 5% ACN in water with 0.2% formic acid. The injection volumes were 40.0 lL and the TLX4 liquid chromatography system was multiplexed in LX mode for maximum sample throughput. Detection and quantification employed highly selective reaction monitoring (H-SRM) of initial
precursor ion for atipamezole (mass to charge ratio (m/z) 213.1). The response for the major product ions, for atipamezole (m/z 145.1, 117.1 and 91.0) were plotted and peaks at the proper retention time integrated using Quanbrowser software (Thermo Scientific, CA, USA). The software was used to generate calibration curve and quantitate these analytes in all samples. The concentration of atipamezole for each sample (e.g., calibrators, quality control and unknowns) was determined by an IS, specific for each analyte, method using the peak area ratio and linear regression analysis. The response for atipamezole was linear and gave correlation coefficients (R2) of 0.999 or better. The limit of quantitation (LOQ) was defined at the lowest concentration of each analyte quantified with precision of up to 20% and inaccuracy of 20%. The technique was optimized to provide LOQ of 0.1 ng mL1 for atipamezole with a signal to noise 10 times greater than a blank. For atipamezole the calculated concentrations for intra-day and inter-day precision (% relative standard deviation) and accuracy (% of nominal concentration) of six replicate analysis of the 2.0, 9.0, and 20 ng mL1 QC samples. The evaluation of precision and accuracy revealed coefficients of variation and percent inaccuracy of <15%. Pharmacokinetic calculations Nonlinear least square regression was performed on plasma drug (tolazoline, atipamezole and yohimbine) concentration versus time data using commercially available software (WinNonlin Version 5.2, Pharsight, NC, USA). Only data points (plasma concentrations) equal to or above the LOQ for the assays were included in the analysis. Compartmental analysis was used for determination of pharmacokinetic parameters for all antagonists. Based on Coefficient of Variation, Akaike Information Criterion and visual inspection of the residual plots, a two-compartment model (Cp = Aeat + Bebt) gave the best fit to concentration data points from individual animals for all antagonists. The area under the curve (AUC) and area under the moment curve (AUMC) were extrapolated to infinity using the last measured plasma concentration divided by the terminal slope kz. Values for the maximum observed measured plasma concentrations (Cmax (obs)) were determined from the individual plasma concentration versus time curves. Plasma concentration data from all nine horses are reported as
© 2013 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia, 41, 36–47
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Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley
mean ( SD) and pharmacokinetic parameters as mean ( SD), median and range.
Results Pharmacokinetic analysis
Behavioral and physiological responses Monitoring of behavior and assessments of sedation, excitation and agitation were recorded throughout the course of the study by two investigators. Chin-toground distance was measured and recorded at 0 (baseline, BL) immediately prior to detomidine administration and at 15, 30, and 45 minutes, and 1 hour post detomidine administration (preantagonist administration) and at 0.5, 1, 1.5, 2, 2.5 and 4 hours post antagonist administration by measuring the distance from the animal’s chin to the ground. Each horse was equipped with a Holter monitor (Forrest Medical, NY, USA) for continuous long-term recording of heart rate (HR) and rhythm for a minimum of 30 minutes pre-detomidine administration and 5 hours post antagonist administration. HR was calculated by manually counting P-QRS-T complexes over a 1 minute period at 0 (BL), 2, 5, 8, 10, 12, 15, 20, 30 and 45 minutes and 1 hour post detomidine administration (pre-antagonist administration) and at 2, 5, 8, 10, 12, 15, 20, 30, and 45 minutes, and 1, 1.25 1.5, 2, 2.5, 3, 4, 5 hours post antagonist administration. The percentage of atrial signals blocked at the AV node before and after drug administration was calculated using the formula [(atrial rateventricular rate)/ atrial rate] 9 100 and recorded at the same time points used for HR determinations. Urination frequency was also continuously recorded by an observer throughout the monitoring period. Statistical analysis Statistical analyses using commercially available software (SAS, NC, USA) were performed to assess significant differences in physiologic variables, PCV, TP and glucose values both pre and post detomidine and post antagonist administration for individual horses. Raw data for all physiologic variables as well as glucose, PCV and TP were checked for normality using the Wilk-Shapiro test and then log transformed or Winsorized as necessary to bring the residual distribution in close agreement with a normal distribution. Data for all variables were subsequently analyzed using a mixed-model ANOVA with repeated measures. Significance was set a p < 0.05. Data were summarized as mean SD. 40
Semi-log plots of the mean ( SD) plasma concentrations of atipamezole, yohimbine and tolazoline following SL administration of detomidine are depicted in Figures 1a,b and c, respectively. The mean ( SD), median value and range for a number
(a)
(b)
(c)
Figure 1 Semilog plot of mean plasma concentrations versus time for (a) atipamezole (0.12 mg kg1 IV) (b) yohimbine (0.075 mg kg1 IV) and (c) tolazoline (4 mg kg1) following IV administration to horses (n = 9 per group) with previous SL administration of detomidine (0.04 mg kg1).
© 2013 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia, 41, 36–47
Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley Table 1 Pharmacokinetic parameters of tolazoline (4 mg kg1 IV), atipamezole (0.12 mg kg1 IV) or yohimbine (0.075 mg kg1 IV) (n = 9) following administration of SL detomidine (0.04 mg kg1). All values in this table were generated using compartmental analysis
Tolazoline
Atipamezole
Yohimbine
Parameter
Mean (SD)
Median (Range)
Mean (SD)
Median (Range)
Mean (SD)
Median (Range)
A (ng mL1) B (ng mL1) Alpha (hour1) Beta (hour1) t 1/2el (hours)* Vdss (L kg1) V1 (L kg1) V2 (L kg1) CL (mL minute1 kg1) AUC (hour ng mL1) AUMC (hour ng mL1) MRT (hour)
5369 (1254) 713 (325) 2.17 (0.948)
5419 (3116–7274) 777 (318–1092) 2.23 (0.560–3.68)
135 (69.8) 46.5 (12.5) 6.07 (2.61)
130 (15.4–167) 45.0 (27.5–65.5) 5.21 (3.95–12.4)
54.4 (0.118) 9.20 (4.94) 2.47 (0.903)
54.4 (23.4–77.0) 7.60 (4.20–17.4) 2.60 (1.20–3.68)
0.205 (0.0308)
0.205 (0.196–0.249)
0.788 (0.118)
0.841 (0.580–0.911)
0.347 (0.123)
0.372 (0.173–0.527)
3.51 (0.861)
3.37 (2.78–3.61)
0.879 (0.151)
0.824 (0.761–1.19)
2.00 (1.03)
1.87 (1.31–4.0)
1.90 (0.364)
1.72 (1.53–2.50)
1.43 (0.279)
1.44 (1.09–1.99)
2.90 (1.07)
2.44 (1.97–4.14)
0.707 (0.229) 1.19 (0.440) 11.2 (3.11)
646 (478–1231) 1109 (466–1852) 10.7 (7.55–18.4)
0.770 (0.375) 0.663 (0.167) 25.0 (5.30)
0.690 (0.363–1.68) 1.41 (0.746–2.08) 25.4 (16.1–31.5)
1.29 (0.475) 1.61 (0.806) 25.9 (7.52)
1.18 (0.795–2.34) 1.23 (0.969–3.12) 22.9 (18.6–41.2)
6307 (1487)
6231 (3624–8835)
83.9 (20.4)
78.8 (63.6–125)
51.2 (12.2)
54.6 (30.3–67.2)
18,940 (6620–29,905)
85.2 (37.8)
74.6 (42.4–155)
101 (51.7)
90.3 (48.4–184)
2.90 (1.83–3.46)
0.984 (0.231)
0.929 (0.666–1.46)
1.92 (0.668)
1.68 (1.25–3.19)
18,794 (6255) 2.92 (0.515)
*Harmonic mean.
of pharmacokinetic parameters following non-compartmental analysis are listed for atipamezole, yohimbine and tolazoline in Table 1. Behavioral and physiologic responses Maximal sedation, as evidenced by a decrease in chin-to-ground distance relative to BL was observed immediately prior to administration of the antagonist (Fig. 2). An initial increase in chin-to-ground distance was observed in all groups within 5 minutes of administration of the antagonist. While the average chin-to-ground distance remained above pre-antagonist values for the remainder of the observation period, values did not reach BL levels until the end of the observation period at 4 hours. A significant difference in chin-to-ground distance between horses receiving detomidine only and those receiving yohimbine or tolazoline was observed at 1 hour post antagonist administration (Fig. 2). There was no difference in chin-to-ground distance between the detomidine only and detomidine + atipamezole group.
Figure 2 Change in chin-to-ground distance (expressed as change from baseline (inches)) with respect to time following SL administration of detomidine (0.04 mg kg1) alone or with subsequent IV administration of atipamezole (0.12 mg kg1), yohimbine (0.075 mg kg1) or tolazoline (4 mg kg1) to horses (n = 9). Data are expressed as mean SD. *Indicates significant differences (p < 0.05) from the value immediately before administration of the antagonist and † from the detomidine only group.
© 2013 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia, 41, 36–47
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Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley
HR decreased significantly (p < 0.05) for all horses following detomidine administration with the decreased HR persisting for approximately 1.5– 2 hours when administration was not followed by an antagonist (Fig. 3). All antagonists elicited an initial increase in HR, but the degree and the duration varied with each drug (Fig. 3). Following yohimbine administration, HR remained near or at BL for 15–20 minutes. HR subsequently decreased and remained below baseline for the remainder of the sampling period (Fig. 3). HR increased to well above baseline at 5 minutes post tolazoline administration, but decreased to well below baseline by 15 minutes and remained lower than baseline for the remainder of the sampling period (Fig. 3). HR increased only slightly following atipamezole administration and never reached BL during the recording period (Fig. 3). Detomidine administration significantly increased the prevalence of AV conduction disturbances. All antagonists effectively decreased the prevalence of the AV blocks initially, although the number of AV blocks increased again by 1– 2 hours for all drugs (Fig. 4). A significant increase in PCV was noted immediately after administration of tolazoline (Fig. 5). PCV remained increased throughout the entire sample collection period. Significant changes in PCV did not occur following administration of either atipamezole or yohimbine. PP did not change significantly from baseline at any of the time points. Detomidine significantly increased glucose concentrations by 45 minutes post administration with
Figure 3 Change in HR (expressed as change from baseline in beats minute1) with respect to time following SL administration of detomidine (0.04 mg kg1) either with or without subsequent IV administration of atipamezole (0.12 mg kg1), yohimbine (0.075 mg kg1) or tolazoline (4 mg kg1) to horses (n = 9). Data are expressed as mean SD. 42
concentrations remaining elevated for 3 hours (Fig. 6 & Table 2). Administration of yohimbine significantly attenuated the detomidine induced hyperglycemia, atipamezole had no effect and tolazoline initially augmented the hyperglycemic effect.
Figure 4 Change in the percentage of AV blocks (expressed as absolute change from baseline) following SL administration of detomidine (0.04 mg kg1) either with or without subsequent IV administration of atipamezole (0.12 mg kg1), yohimbine (0.075 mg kg1) or tolazoline (4 mg kg1) to horses (n = 9). Data are expressed as mean SD.
Figure 5 Change in PCV% (expressed as absolute change from baseline) with respect to time following SL administration of either detomidine (0.04 mg kg1) alone or followed by IV administration of atipamezole (0.12 mg kg1), yohimbine (0.075 mg kg1) or tolazoline (4 mg kg1) to horses (n = 9). Data are expressed as mean SD. *Indicates significant differences (p < 0.05) from the value immediately before administration of the antagonist and † from the detomidine only group.
© 2013 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia, 41, 36–47
Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley Subjectively, urine volume appeared unchanged, however, increases in urination frequency were prominent following administration of atipamezole relative to the other groups.
Discussion A distinct advantage of alpha2-adrenergic agonists, such as detomidine, is the availability of antagonists that can diminish the agonistic effects. Nonetheless, while alpha2-adrenergic antagonists are beneficial in cases of overdoses of alpha2-adrenergic agonists, there are both anecdotal and documented reports of tachypnea, tachycardia, anxiety and death associated with their use (Scofield et al. 2010). In equine medicine, the only FDA approved alpha2-adrenergic antagonist is tolazoline, although yohimbine and atipamezole have been used to reverse the alpha2-adrenergic agonists. The primary goal of the current study was to compare the pharmacokinetics and selected pharmacodynamic effects of these alpha2-adrenergic antagonists subsequent to detomidine administration. It is important to note that only one dose of each antagonist was studied and that these doses may not be equipotent. Doses selected for each alpha2-adrenergic antagonist were chosen based on either the FDA approved dose (4 mg kg1 tolazoline), the dose most commonly reported in the literature (0.075 mg kg1 yohimbine) or based on previous dose-response studies
Figure 6 Plasma glucose concentrations (expressed as mg dL1 change from baseline) following SL administration of either 0.04 mg kg1 detomidine or followed by IV administration of 0.12 mg kg1 atipamezole, 0.075 mg kg1 yohimbine or 4 mg kg1 tolazoline to nine horses. Data are expressed as mean SD.
Table 2 Plasma glucose concentrations (mean SD) for nine horses before and after SL administration of detomidine (0.04 mg kg1) alone or followed by IV administration of atipamezole (0.12 mg kg1), yohimbine (0.075 mg kg1), or tolazoline (4 mg kg1). Antagonists were administered after measurements and blood samples were collected 1 hour after administration of detomidine Plasma [Glucose] (mg dL1) Time 0 minutes (baseline) 15 minutes 30 minutes 45 minutes 1 hour (pre-reversal) 1.25 hours 1.5 hours 1.75 hours 2 hours 2.5 hours 3 hours 3.5 hours 4 hours 5 hours 6 hours
Detomidine 93 9 91 99 121 138
12 8 11a 14a
—
146 32a —
144 139 127 — 110 — 87
48a 39a 43a 26 19
Atipamezole 89 8 89 95 107 121 138 134 138 138 137 127 120 110 90 80
10 17 22a 23a 23b 43 43 44 38 36 32 34 23b 18b
Yohimbine
Tolazoline
88 14 88 93 111 113 111 109 107 106 102 93 93 90 81 82
12 15 17a 27 29 20c 23 20c 15c 14bc 18 16b 20b 23b
91 5 92 92 117 133 191 172 159 148 137 130 117 108 94 87
8 16 18a 18a 22b 19bc 17b 20 21 24 24b 20b 14b 17b
— sample not collected. aSignificantly different from Time 0 (baseline) (p < 0.05); bSignificantly different from Time 1 hour (immediately prior to antagonist administration) (p < 0.05); cSignificantly different from the detomidine only group (p < 0.05). © 2013 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia, 41, 36–47
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Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley
describing the lowest dose producing maximal effect (0.12 mg kg1 atipamezole). In the current study, pharmacokinetic analysis was performed on three alpha2-adrenergic antagonists when administered subsequent to SL detomidine. The clearance and t1/2el of the alpha2adrenergic antagonist, yohimbine, following SL detomidine administration (22.9 mL minute1 kg1 (Cl); 1.87 hours (t1/2el)) differs from previous reports of yohimbine disposition following IV detomidine administration (6.8 mL minute1 kg1 (Cl); 4.4 hours (t1/2el)) (Knych et al. 2012a). In the current study, total yohimbine plasma clearance ranged from 18.6 to 41.2 mL minute1 kg1, indicating that it is a high hepatic extraction ratio drug with extra hepatic metabolism. This is in agreement with that reported in horses (Knych & Stanley 2011; Knych et al. 2012a) and humans (Hedner et al. 1992) when given as a sole agent, but differs from that reported previously for yohimbine given subsequent to IV detomidine in horses (Knych et al. 2012a). Knych et al. (2012a) reported a 50% decrease in yohimbine clearance following IV detomidine administration relative to yohimbine when administered as a sole agent. In that study, investigators hypothesized that the decreased yohimbine clearance was due to detomidine induced changes in hepatic blood flow. The discrepancy between the two studies may be an effect of lower detomidine plasma concentrations immediately prior to antagonist administration in this study relative to the previous one. This can be attributed to either different routes of detomidine administration or the timing of yohimbine administration relative to detomidine. Lower detomidine concentrations would diminish the drug-induced decreases in hepatic blood flow observed in the previous study (Knych et al. 2012a). Differences in yohimbine clearance and therefore the t1/2el may also be due to subject variability (age and metabolic capabilities) between the two studies. Atipamezole is currently labeled to reverse the effects of medetomidine. To the authors’ knowledge, there are no reports describing disposition of atipamezole following administration of detomidine in any species. There are, however, reports describing the pharmacokinetics of atipamezole when administered subsequent to medetomidine in ruminants. Ranheim et al. (1999, 2000) reported clearance values of 48.2 13.1 and 56.3 mL minute1 kg1 and an elimination half-life of 35.2 17.9 and 34.2 11.9 minutes for dairy cows and sheep 44
respectively, following administration of atipamezole subsequent to medetomidine. In the current study, the average clearance and t1/2el for atipamezole when administered subsequent to detomidine was 25.0 5.30 mL minute1 kg1 and 52.7 9 minutes, respectively. Although the clearance is slower and the half-life of elimination more prolonged relative to cows and sheep, it is unclear as to whether this difference is due to species, drug, or dose differences. In the current study, the median clearance, volume of distribution and t1/2el for tolazoline when administered subsequent to detomidine were 10.7 mL minute1 kg1, 1.72 L kg1 and 3.37 hours, respectively. A group receiving only tolazoline was not included in the current study so it is not possible to assess whether detomidine affects the disposition of tolazoline, however, t1/2el, Vdss and Cl are in close agreement with that reported previously for tolazoline administered as a sole agent to the horse (Casbeer & Knych 2013). It should be noted, though that the t1/2el (1 hour) differs significantly from that reported by Plumb (2011) when tolazoline is administered alone. The reason for this large discrepancy is not clear as details of the study used to determine the t1/2el in Plumb (2011) are not available. Although the magnitude and duration of the responses varied among the different antagonists, the effects of all three alpha2-adrenergic antagonists on sedation and HR and rhythm were incomplete and in some cases transient. Only yohimbine completely antagonized the sedative effect of detomidine (chin-to-ground distance at or near predetomidine value). The effect was short lived, lasting an average of 5 minutes before returning to near pre-antagonist levels. These findings were similar to that previously reported for IV administered detomidine (Knych et al. 2012b), whereby horses treated with yohimbine showed signs of alertness within 5 minutes followed by a return to sedation. The effects of tolazoline and atipamezole on detomidine induced changes in chin-to-ground distance were minimal and agree with that reported previously (Ramseyer et al. 1998; Hubbell & Muir 2006). Although the effects persisted slightly longer (15–20 minutes) than changes in chin-to-ground distance, reversal of HR and rhythm changes were similarly relatively short lived. Antagonism of the detomidine induced cardiac effects was most pronounced with yohimbine and tolazoline and least with atipamezole. As atipamezole is not currently
© 2013 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia, 41, 36–47
Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley approved for use in the horse, the dose administered in the current study was selected based upon a review of the literature. The most effective dose may not yet be established and may vary depending on the desired effect. A significant increase in PCV was observed following tolazoline administration, but not following atipamezole or yohimbine. The most likely explanation for this finding is either direct stimulation or attenuation of sympathetic inhibition, which could lead to splenic contraction, most likely mediated by alpha adrenergic receptors. While this hypothesis is further supported by the increase in HR above BL recorded immediately following tolazoline administration, the specific mechanism responsible for the sympathetic stimulation cannot be elucidated from the current study. In addition to its antagonistic effects at the alpha2-adrenergic receptor, tolazoline has antagonistic effects at alpha1-adrenergic, histamine and cholinergic receptors as well as direct effects on the vascular endothelium. Of the three antagonists under current discussion, tolazoline is considered to be least selective for the alpha2-adrenergic receptor. However, equipotent doses of each antagonist were not established, and the possibility that the effect is mediated by the alpha2-adrenergic receptor cannot be ruled out. Consistent with previous reports describing the effects of detomidine on plasma glucose concentrations (Gasthuys et al. 1987; Knych et al. 2012b), in the current study detomidine significantly increased plasma glucose concentrations. This effect induced by xylazine, another alpha2-adrenergic agonist, has been attributed to inhibition of insulin release from pancreatic beta cells (Eichner et al. 1979; Goldfine & Arieff 1979; Feldberg & Symonds 1980; Brockman 1981; Hsu & Hummer 1981; Thurmon et al. 1982, 1984). Furthermore, Angel and coworkers (Angel & Langer 1988; Angel et al. 1990) demonstrated that the inhibition of insulin release was mediated through postsynaptic adrenoreceptors located on the pancreatic cells, specifically the alpha2A subtype and that alpha2-adrenergic receptor antagonists, such as yohimbine, block the hyperglycemic effect of alpha2-adrenergic agonists (Angel et al. 1990; Oda et al. 1991). Schwartz & Clark (1998) have previously demonstrated that in sheep, the affinities of yohimbine and atipamezole are similar at the alpha2A-adrenergic receptors. In the current study, while yohimbine decreased detomidine induced hyperglycemia, atipamezole
did not have a significant effect on the hyperglycemic effect until 4 hours post administration, suggesting that the decrease in glucose was more likely due to cessation of detomidine induced effects as opposed to being due to atipamezole. There are two possible explanations for this deviation from the expected outcome. The anti-hyperglycemic effect may be dose-dependent and a higher dose of atipamezole may be necessary to reverse the alpha2-agonistic effect and in turn result in decreased plasma glucose concentrations. Secondarily, the similarity in affinity of atipamezole for the alpha2A-adrenergic receptor subtype in sheep (Schwartz & Clark 1998) may be species specific and may not hold true for horses. In contrast to the effects observed with atipamezole and yohimbine, tolazoline initially increased the detomidine induced hyperglycemia. While the effect was temporary, lasting for only 1 hour, the magnitude of the change was such that the level of statistical significance was high. To the authors’ knowledge, there are no reports describing the molecular mechanisms of tolazoline with respect to its effects on glucose, however, similar to changes in PCV, the most likely explanation for the increase in plasma glucose concentrations observed in the current study is increased sympathetic stimulation. Antagonism of insulin by stress hormones, such as cortisol can also lead to hyperglycemia. Neither insulin nor cortisol concentrations were measured in the current study, but Carroll et al. (1997) have reported increases in cortisol concentrations in horses following tolazoline administration subsequent to detomidine. In the current study, we sought to characterize the pharmacokinetics of three alpha2-adrenergic receptor antagonists as well as pharmacodynamic effects following SL administration of detomidine in the horse, utilizing doses for each that appear in the literature. The antagonistic effects of tolazoline, yohimbine and atipamezole on HR and rhythm changes and behavioral effects elicited by SL administered detomidine appear to be incomplete. Acknowledgements Financial support for this study was provided by the California Department of Food and Agriculture’s Equine Medication Monitoring Program and by the California Horse Racing Board. The authors would like to thank Dr. Eugene Steffey for editorial assis-
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tance and Carley Corado, Vanessa Covarrubias and Dan McKemie for technical assistance. References Angel I, Langer SZ (1988) Adrenergic-induced hyperglycemia in anaesthetized rats: involvement of peripheral alpha 2-adrenoreceptors. Eur J Pharmacol 154, 191–196. Angel I, Niddam R, Langer SZ (1990) Involvement of alpha-2 adrenergic receptor subtypes in hyperglycemia. J Vet Pharmacol Ther 254, 877–882. Brockman RP (1981) Effect of xylazine on plasma glucose, glucagon and insulin concentration in sheep. Res Vet Sci 30, 383–384. Carroll GL, Matthews NS, Hartsfield SM et al. (1997) The effect of detomidine and its antagonism with tolazoline on stress-related hormones, metabolites and physiologic responses and behavior in awake ponies. Vet Surg 26, 69–77. Casbeer HC, Knych HK (2013) Pharmacokinetics and pharmacodynamics effects of tolazoline following intravenous administration to horses. Vet J 196, 504– 509. DiMaio Knych HK, Steffey EP, Deuel JL et al. (2011) Pharmacokinetics of yohimbine following intravenous administration to horses. J Vet Pharmacol Ther 34, 58–63. Eichner RD, Prior RL, Kvasnicka WG (1979) Xylazineinduced hyperglycemia in beef cattle. Am J Vet Res 40, 127–129. Feldberg W, Symonds HW (1980) Hyperglycemic effect of xylazine. J Vet Pharmacol Ther 3, 197–202. Gasthuys F, Terpstra P, van den Hende C et al. (1987) Hyperglycaemia and diuresis during sedation with detomidine in the horse. Zentralb Veterinarmed A 34, 641–648. Goldfine ID, Arieff AI (1979) Rapid inhibition of basal and glucose-stimulated insulin release by xylazine. Endocrinology 105, 920–922. Grimsrud KN, Mama KR, Thomasy SM et al. (2009) Pharmacokinetics of detomidine and its metabolites following intravenous and intramuscular administration in horses. Equine Vet J 41, 361–365. Hedner T, Edgar B, Edvinsson L (1992) Yohimbine pharmacokinetics and interaction with the sympathetic nervous system in normal volunteers. Eur J Clin Pharmacol 43, 651–656. Hsu WH, Hummer SK (1981) Xylazine induced hyperglycemia in cattle: a possible involvement of alpha 2 adrenergic receptors regulating insulin release. Endocrinology 109, 825–829. Hubbell JAE, Muir WW (2006) Antagonism of detomidine sedation in the horse using intravenous tolazoline or atipamezole. Equine Vet J 38, 238–241. Hubbell JAE, Sams RA, Schmall LM et al. (2009) Pharmacokinetics of detomidine administered to horses
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at rest and after maximal exercise. Equine Vet J 41, 419– 422. Kaukinen H, Aspegren J, Hyppa S et al. (2010) Bioavailability of detomidine administered sublingually to horses as an oromucosal gel. J Vet Pharmacol Ther 34, 76–81. Knych HK, Stanley SD (2011) Pharmacokinetics and pharmacodynamics of detomidine following sublingual administration to the horse. Am J Vet Res 72, 1378– 1385. Knych HK, Steffey EP, Stanley SD (2012a) The effects of yohimbine on the pharmacokinetic parameters of detomidine in the horse. Vet Anaesth Analg 39, 221– 229. Knych HK, Covarrubias V, Steffey EP (2012b) Effect of yohimbine induced changes in behavior, cardiac and selected blood parameters in the horse. Vet Anaesth Analg 39, 583–584. Kollias-Baker CA, Court MH, Williams LL (1993) Influence of yohimbine and tolazoline on the cardiovascular, respiratory, and sedative effects of xylazine in the horse. J Vet Pharmacol Ther 16, 350–358. Mama KR, Grimsrud K, Snell T et al. (2009) Plasma concentrations, behavioral and physiologic effects following intravenous and intramuscular detomidine in horses. Equine Vet J 41, 772–777. Oda S, Fujimura H, Sasaki Y et al. (1991) Alpha-2 adrenergic modulation of glucagon and insulin secretions in sheep. Tohoku J Exp Med 163, 101–110. Plumb DC (2011) Plumb’s Veterinary Drug Handbook (7th edn). Wiley Blackwell, Ames, USA. Raekallio M, Vainio O, Karjalainen J (1990) The influence of atipamezole on the cardiovascular effects of detomidine in horses. J Assoc Vet Anaesth 17, 50–53. Ramseyer B, Schmucker N, Schatzmann U et al. (1998) Antagonism of detomidine sedation with atipamezole in horses. J Vet Anaesth 25, 47–51. Ranheim B, Arnemo JM, Ryeng KA et al. (1999) A pharmacokinetic study including some relevant clinical effects of medetomidine and atipamezole in lactating dairy cows. J Vet Pharmacol Ther 22, 368–373. Ranheim B, Arnemo JM, Stuen S et al. (2000) Medetomidine and atipamezole in sheep: disposition and clinical effects. J Vet Pharmacol Ther 23, 401–404. Schwartz DD, Clark TP (1998) Selectivity of atipamezole, yohimbine and tolazoline for alpha-2 adrenergic receptor subtypes: implications for clinical reversal of alpha-2 adrenergic receptor mediated sedation in sheep. J Vet Pharmacol Ther 21, 342–347. Scofield DB, Alexander DL, Franklin RP et al. (2010) Review of fatalities and adverse reactions after administration of a-2 adrenergic agonist reversal agents in the horse. Proceedings of the 56th American Association of Equine Practitioners, Baltimore, USA. p. 44–49. Skarda RT, Muir WW (1999) Effects of intravenously administered yohimbine on antinociceptive,
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Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley cardiorespiratory and postural changes induced by epidural administration of detomidine hydrochloride solution to healthy mares. Am J Vet Res 60, 1262–1270. Thurmon JC, Neff-Davis C, Davis LE et al. (1982) Xylazine hydrochloride-induced hyperglycemia and hypoinsulinemia in thoroughbred race horses. J Vet Pharmacol Ther 5, 241–245.
Thurmon JC, Steffey EP, Zinkl JG et al. (1984) Xylazine causes transient dose-related hyperglycemia and increased urine volumes in mares. Am J Vet Res 45, 224–227. Received 6 July 2012; accepted 10 December 2012.
Corrigendum In the publication ‘Effects of positive end-expiratory pressure titration on gas exchange, respiratory mechanics and hemodynamics in anesthetized horses’, errors were made in two equations in the section ‘Anesthesia and monitoring’: 1 The alveolar oxygen tension should be calculated using the following alveolar gas equation: PAO2 ¼ ½FiO2 ðPb 47 mmHgÞ ½PaCO2 ð1=0:8Þ 2 The physiologic right-to-left shunt equation should be calculated using the following modified shunt equation for patients breathing 100% oxygen: _ Qt _ ¼ f½PðA aÞO2 0:003=4 þ ½PðA aÞO2 0:003gg 100 Qs= These errors do not affect the data or results of the paper. Reference Ambrosio AM, Ida KK, Souto MT, Oshiro AH, Fantoni DT. (2013) Effects of positive end-expiratory pressure titration on gas exchange, respiratory mechanics and hemodynamics in anesthetized horses. Vet Anaesth Analg 40, 564–572.
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doi:10.1111/vaa.12081
RESEARCH PAPER
Effects of three antagonists on selected pharmacodynamic effects of sublingually administered detomidine in the horse Heather K Knych*† & Scott D Stanley*† *K. L. Maddy Equine Analytical Chemistry Laboratory, School of Veterinary Medicine, University of California, Davis, CA, USA †Department of Veterinary Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, CA, USA
Correspondence: Heather K Knych, K.L. Maddy Equine Analytical Chemistry Laboratory, University of California, Davis, School of Veterinary Medicine, West Health Science Drive, Davis, CA 95616, USA. E-mail: [email protected]
Abstract Objective To describe the effects of alpha2-adrenergic receptor antagonists on the pharmacodynamics of sublingual (SL) detomidine in the horse. Study design Randomized crossover design. Animals Nine healthy adult horses with an average age of 7.6 6.5 years. Methods Four treatment groups were studied: 1) 0.04 mg kg1 detomidine SL; 2) 0.04 mg kg1 detomidine SL followed 1 hour later by 0.075 mg kg1 yohimbine intravenously (IV); 3) 0.04 mg kg1 detomidine SL followed 1 hour later by 4 mg kg1 tolazoline IV; and 4) 0.04 mg kg1 detomidine SL followed 1 hour later by 0.12 mg kg1 atipamezole IV. Each horse received all treatments with a minimum of 1 week between treatments. Blood samples were obtained and plasma analyzed for yohimbine, atipamezole and tolazoline concentrations by liquid chromatographymass spectrometry. Behavioral effects, heart rate and rhythm, glucose, packed cell volume (PCV) and plasma proteins were monitored. Results Chin-to-ground distance increased following administration of the antagonists, however, this effect was transient, with a return to pre-reversal 36
values as early as 1 hour. Detomidine induced bradycardia and increased incidence of atrioventricular blocks were either transiently or incompletely antagonized by all antagonists. PCV and glucose concentrations increased with tolazoline administration, and atipamezole subjectively increased urination frequency but not volume. Conclusions and clinical relevance At the doses administered in this study, the alpha2-adrenergic antagonistic effects of tolazoline, yohimbine and atipamezole on cardiac and behavioral effects elicited by SL administration of detomidine are transient and incomplete. Keywords alpha2-adrenergic agonist, antagonist, atipamezole, detomidine, equine, tolazoline, yohimbine.
Introduction Alpha2-adrenergic agonists, such as detomidine, are commonly used in equine medicine for procedures necessitating sedation and analgesia. Detomidine is available in injectable formulations for both intravenous (IV) and intramuscular (IM) administration and the pharmacokinetics and pharmacodynamics following administration via either of these two administration routes have been well characterized (Grimsrud et al. 2009; Hubbell et al. 2009; Mama et al. 2009). Most recently a sublingual (SL) gel
Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley formulation received approval by the Food and Drug Administration for use in the horse. Studies assessing the pharmacodynamics of SL administration of detomidine have described many effects comparable to those observed with parenteral administration (Kaukinen et al. 2010; Knych & Stanley 2011). For example, Kaukinen et al. (2010) and Knych & Stanley (2011) reported a statistically significant decrease in chin-to-ground distance following SL administration with maximal sedation occurring 40 minutes post administration and lasting for approximately 2 hours. Similar to that observed following IV and IM administration, SL administration of detomidine decreased heart rate (HR) and increased the incidence of atrioventricular (AV) blocks (Kaukinen et al. 2010; Knych & Stanley 2011). Alpha2-adrenergic antagonists are often used to reverse the cardiovascular and central nervous system (CNS) depressant effects of alpha2-adrenergic agonists following IV or IM administration. The three most commonly used alpha2-adrenergic antagonists in veterinary medicine are atipamezole, yohimbine and tolazoline. These drugs differ in their relative effectiveness as reversal agents which, in sheep, has been attributed to differences in their alpha2:alpha1-adrenergic receptor selectivity ratio (Schwartz & Clark 1998). Furthermore, the effectiveness of the antagonism appears to be dependent on the dose administered as well as the time administered relative to the alpha2-adrenergic agonist (Kollias-Baker et al. 1993; Ramseyer et al. 1998; Hubbell & Muir 2006). The alpha2D-adrenergic receptor is believed to be responsible for the sedative and cardiac depressant effects observed following administration of alpha2-adrenergic agonists. Of the three agents, atipamezole is the most selective for the alpha2D-adrenergic receptor (Schwartz & Clark 1998), at least in sheep. However, at the doses studied thus far (100–160 lg kg1), atipamezole has been shown to only partially reverse detomidine induced sedation, bradycardia and incidence of AV conduction disturbances following administration to the horse (Raekallio et al. 1990; Ramseyer et al. 1998; Hubbell & Muir 2006). Yohimbine is less selective for the alpha2-adrenergic receptor than atipamezole. At all doses studied, yohimbine effectively and quickly reverses detomidine induced sedation following both epidural and IV administration of detomidine (Skarda & Muir 1999; Knych et al. 2012b). Knych et al. (2012b) reported that the initial alertness, observed following yohim-
bine administration subsequent to detomidine, was followed by a return to a state of sedation, although to a lesser extent than that observed immediately following detomidine administration. Detomidine induced bradycardia and AV conduction disturbances are also rapidly, albeit transiently, reversed following administration of yohimbine (Skarda & Muir 1999; Knych et al. 2012b). Of the three antagonists used in veterinary medicine, tolazoline has the lowest affinity for the alpha2-adrenergic receptor (Schwartz & Clark 1998). Similar to the effects noted for atipamezole and yohimbine, tolazoline transiently and incompletely antagonizes sedation and cardiac effects following either an IV bolus or infusion of detomidine (Kollias-Baker et al. 1993; Carroll et al. 1997; Hubbell & Muir 2006). Although the effects of alpha2-adrenergic antagonists appear to be transient and incomplete, administration may decrease the time to recovery following IV and IM detomidine administration (Hubbell & Muir 2006). The purpose of the study described here was twofold. First we sought to describe the pharmacokinetics of the three selected reversal agents when administered subsequent to detomidine. The second goal was to evaluate the effects of the three adrenergic receptor antagonists in reversing the sedative and cardiovascular effects of SL administered detomidine and to assess the effects on parameters known to be affected by alpha2adrenergic agonists such as packed cell volume (PCV), plasma proteins (PP), and plasma glucose concentrations. Materials and methods Animals Nine healthy horses consisting of eight Thoroughbreds and one Quarter Horse (five geldings and four mares) with a mean SD weight of 528 58 kg and an age of 7.6 6.5 years were studied. Food was withheld for 12 hours prior to and for approximately 8 hours following drug administration. Water was available ad libitum throughout the study. Before beginning this study, horses were determined healthy and free of cardiovascular diseases by physical examination, cardiac auscultation, complete blood count (CBC) and a serum biochemistry panel that included aspartate aminotransferase (AST), creatine phosphokinase (CK), alkaline phosphatase (ALP), total
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bilirubin, sorbitol dehydrogenase (SDH), blood urea nitrogen (BUN) and creatinine. Blood analyses were performed by the William R. Pritchard Clinical Pathology Laboratory of the University of California, Davis, USA using their standard protocols. Horses did not receive any medications for at least 2 weeks prior to commencement of this study. This study was approved by the Institutional Animal Care and Use Committee of the University of California, Davis. Drug administration Four treatments were administered: 1) detomidine (0.04 mg kg1; Dormosedan Gel; Pfizer Animal Health, NY, USA) SL; 2) detomidine (0.04 mg kg1) SL followed 1 hour later by yohimbine (0.075 mg kg1; Yobine; Lloyd Laboratories, IA, USA) IV; 3) detomidine (0.04 mg kg1) SL followed 1 hour later by tolazoline (4 mg kg1; Tolazine; Lloyd Laboratories) IV; and 4) detomidine (0.04 mg kg1) SL followed 1 hour later by atipamezole (0.12 mg kg1; Antisedan; Pfizer Animal Health) IV. Doses of the antagonists were chosen based on approved doses (tolazoline) or selected from the literature if there was not an approved dose for horses (atipamezole and yohimbine). Each horse received all treatments with a minimum of 1 week elapsing between each treatment. Assignment to a particular group for a particular week was made by random selection of horse, treatment and week using a random number generator. Prior to drug administration, a 14-gauge catheter was aseptically placed in both external jugular veins, with the exception of horses receiving only detomidine, when only one venous catheter was placed. The right jugular vein catheter was used for administration of the antagonist and the other catheter was used for blood collection. Each horse was weighed immediately prior to drug administration. Sample collection and analysis Blood samples for analysis of plasma concentrations of the antagonists were collected at time 0 (immediately prior to detomidine administration) and at 1 hour post detomidine administration (immediately prior to antagonist administration) and at 5, 10, 15, 30, 45 minutes and 1, 1.5, 2, 2.5, 3, 4, 5, 6, 8, 12, 18, 24, 36, 48 and 72 hours post administration of the antagonist. Prior to drawing each sample of blood for analysis of drug 38
concentrations, 10 mL of blood was aspirated and discarded from the catheter and T-Port extension set (combined internal volume <2 mL). The catheter was flushed with 10 mL of a dilute heparinized saline solution (10 IU mL1) following each sample collection. Catheters were removed either following dosing (right side) or following collection of the 8-hour blood sample (left side) and the remaining samples were collected by direct venepuncture. Blood samples were collected into EDTA blood tubes (Kendall/Tyco Healthcare, MA, USA) and were centrifuged at 3000 g for 10 minutes. Plasma was immediately transferred into storage cryovials (Phenix Research Products, NC, USA) and stored at 20 °C until analysis, approximately 1 month following collection of the final time point. Additional blood samples were collected for analysis of plasma glucose concentrations, PCV and PP concentrations. Samples for plasma glucose analysis were collected at time 0 (immediately prior to detomidine administration), 15, 30 and 45 minutes and 1 hour post detomidine administration (prior to antagonist administration) and 15, 30 and 45 minutes and 1, 1.5, 2, 3, 4 and 6 hours post antagonist administration. Samples were collected from the jugular vein catheter into heparin blood tubes (Kendall/Tyco Healthcare, MA, USA) and were immediately placed on ice until centrifugation at 3000 g for 10 minutes at 4 °C. Analyses were performed by the William R Pritchard Clinical Pathology Laboratory of the University of California, Davis using their standard protocol for glucose analysis. Samples for PCV and PP determination were collected into heparinized syringes at time 0 (immediately prior to detomidine administration), 5, 10, 15, 30 and 45 minutes and 1 hour post detomidine administration and at 5, 10, 15, 30 and 45 minutes and 1, 1.5, 2, 3 and 4 hours post antagonist administration. PCV was measured by using microhematocrit and PP by using a refractometer. Determination of plasma drug concentrations Plasma yohimbine and tolazoline concentrations were measured using Liquid Chromatography-Mass spectrometry as reported previously (DiMaio Knych et al. 2011; Knych & Stanley 2011; Casbeer & Knych 2013). Analytical reference standards for atipamezole, and internal standards (IS) medetomidine, were obtained from Sigma Aldrich (MO, USA)
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Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley and Orion (Farmos, Finland), respectively. Quantitative analyses were performed on a TSQ Vantage triple quadrupole mass spectrometer (Thermo Scientific, CA, USA) coupled with a Turbulent flow chromatography system (TLX4; ThermoFisher Scientific, MA, USA). Chromatography separation employed an ACE C18, 10 cm 9 2.1 mm, 3 lm, column (Mac Mod, PA, USA) and a linear gradient of acetonitrile (ACN) in water with a constant 0.2% formic acid at a flow rate of 0.35 mL minute1. The initial ACN concentration was held at 2.0% for 0.33 minutes, ramped up to 15% over 0.5 minute and then to 95% over 4.2 minutes and finally up to 99% at 2.97 minutes. The ACN concentration was held at 99% for 0.5 minutes and then the column was re-equilibrated at initial conditions for 3.0 minutes. The assay was validated using the procedure defined in the FDA Guidelines for Industry Bioanalytical Method Validation. Duplicate accurate weighings of 10 mg atipamezole and IS were made and then diluted in methanol to give a 1 mg mL1 solution. These separate stock solutions were further diluted using methanol to create concentrations of 0.01, 1 and 10 ng mL1. The standard solutions were used to prepare calibrations in plasma over the concentration range of 0.1–200 ng mL1 and three quality control (QC) samples. The IS were prepared using the same dilution protocol then combined forming a single IS mixture with a 50 ng lL1 final concentration. A series of three validation runs were carried out on three separate days using three calibration lines and six replicates of each of the three QC samples. The matrix influence was calculated using the injection of extracted equine plasma blank, plasma blank spiked with authentic reference standards and authentic reference standards spiked in solvent. Prior to analysis, plasma samples, standards and quality control samples were allowed to thaw at room temperature. A plasma aliquot of 500 lL was added to the IS medetomidine, (20 ng mL1) solution, and 2 mL 0.6 mol L1 phosphate buffer (pH 6.5). The eluent was collected in 2 mL autosampler vials dried under a stream of nitrogen gas and redissolved using 160 lL of 5% ACN in water with 0.2% formic acid. The injection volumes were 40.0 lL and the TLX4 liquid chromatography system was multiplexed in LX mode for maximum sample throughput. Detection and quantification employed highly selective reaction monitoring (H-SRM) of initial
precursor ion for atipamezole (mass to charge ratio (m/z) 213.1). The response for the major product ions, for atipamezole (m/z 145.1, 117.1 and 91.0) were plotted and peaks at the proper retention time integrated using Quanbrowser software (Thermo Scientific, CA, USA). The software was used to generate calibration curve and quantitate these analytes in all samples. The concentration of atipamezole for each sample (e.g., calibrators, quality control and unknowns) was determined by an IS, specific for each analyte, method using the peak area ratio and linear regression analysis. The response for atipamezole was linear and gave correlation coefficients (R2) of 0.999 or better. The limit of quantitation (LOQ) was defined at the lowest concentration of each analyte quantified with precision of up to 20% and inaccuracy of 20%. The technique was optimized to provide LOQ of 0.1 ng mL1 for atipamezole with a signal to noise 10 times greater than a blank. For atipamezole the calculated concentrations for intra-day and inter-day precision (% relative standard deviation) and accuracy (% of nominal concentration) of six replicate analysis of the 2.0, 9.0, and 20 ng mL1 QC samples. The evaluation of precision and accuracy revealed coefficients of variation and percent inaccuracy of <15%. Pharmacokinetic calculations Nonlinear least square regression was performed on plasma drug (tolazoline, atipamezole and yohimbine) concentration versus time data using commercially available software (WinNonlin Version 5.2, Pharsight, NC, USA). Only data points (plasma concentrations) equal to or above the LOQ for the assays were included in the analysis. Compartmental analysis was used for determination of pharmacokinetic parameters for all antagonists. Based on Coefficient of Variation, Akaike Information Criterion and visual inspection of the residual plots, a two-compartment model (Cp = Aeat + Bebt) gave the best fit to concentration data points from individual animals for all antagonists. The area under the curve (AUC) and area under the moment curve (AUMC) were extrapolated to infinity using the last measured plasma concentration divided by the terminal slope kz. Values for the maximum observed measured plasma concentrations (Cmax (obs)) were determined from the individual plasma concentration versus time curves. Plasma concentration data from all nine horses are reported as
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Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley
mean ( SD) and pharmacokinetic parameters as mean ( SD), median and range.
Results Pharmacokinetic analysis
Behavioral and physiological responses Monitoring of behavior and assessments of sedation, excitation and agitation were recorded throughout the course of the study by two investigators. Chin-toground distance was measured and recorded at 0 (baseline, BL) immediately prior to detomidine administration and at 15, 30, and 45 minutes, and 1 hour post detomidine administration (preantagonist administration) and at 0.5, 1, 1.5, 2, 2.5 and 4 hours post antagonist administration by measuring the distance from the animal’s chin to the ground. Each horse was equipped with a Holter monitor (Forrest Medical, NY, USA) for continuous long-term recording of heart rate (HR) and rhythm for a minimum of 30 minutes pre-detomidine administration and 5 hours post antagonist administration. HR was calculated by manually counting P-QRS-T complexes over a 1 minute period at 0 (BL), 2, 5, 8, 10, 12, 15, 20, 30 and 45 minutes and 1 hour post detomidine administration (pre-antagonist administration) and at 2, 5, 8, 10, 12, 15, 20, 30, and 45 minutes, and 1, 1.25 1.5, 2, 2.5, 3, 4, 5 hours post antagonist administration. The percentage of atrial signals blocked at the AV node before and after drug administration was calculated using the formula [(atrial rateventricular rate)/ atrial rate] 9 100 and recorded at the same time points used for HR determinations. Urination frequency was also continuously recorded by an observer throughout the monitoring period. Statistical analysis Statistical analyses using commercially available software (SAS, NC, USA) were performed to assess significant differences in physiologic variables, PCV, TP and glucose values both pre and post detomidine and post antagonist administration for individual horses. Raw data for all physiologic variables as well as glucose, PCV and TP were checked for normality using the Wilk-Shapiro test and then log transformed or Winsorized as necessary to bring the residual distribution in close agreement with a normal distribution. Data for all variables were subsequently analyzed using a mixed-model ANOVA with repeated measures. Significance was set a p < 0.05. Data were summarized as mean SD. 40
Semi-log plots of the mean ( SD) plasma concentrations of atipamezole, yohimbine and tolazoline following SL administration of detomidine are depicted in Figures 1a,b and c, respectively. The mean ( SD), median value and range for a number
(a)
(b)
(c)
Figure 1 Semilog plot of mean plasma concentrations versus time for (a) atipamezole (0.12 mg kg1 IV) (b) yohimbine (0.075 mg kg1 IV) and (c) tolazoline (4 mg kg1) following IV administration to horses (n = 9 per group) with previous SL administration of detomidine (0.04 mg kg1).
© 2013 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia, 41, 36–47
Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley Table 1 Pharmacokinetic parameters of tolazoline (4 mg kg1 IV), atipamezole (0.12 mg kg1 IV) or yohimbine (0.075 mg kg1 IV) (n = 9) following administration of SL detomidine (0.04 mg kg1). All values in this table were generated using compartmental analysis
Tolazoline
Atipamezole
Yohimbine
Parameter
Mean (SD)
Median (Range)
Mean (SD)
Median (Range)
Mean (SD)
Median (Range)
A (ng mL1) B (ng mL1) Alpha (hour1) Beta (hour1) t 1/2el (hours)* Vdss (L kg1) V1 (L kg1) V2 (L kg1) CL (mL minute1 kg1) AUC (hour ng mL1) AUMC (hour ng mL1) MRT (hour)
5369 (1254) 713 (325) 2.17 (0.948)
5419 (3116–7274) 777 (318–1092) 2.23 (0.560–3.68)
135 (69.8) 46.5 (12.5) 6.07 (2.61)
130 (15.4–167) 45.0 (27.5–65.5) 5.21 (3.95–12.4)
54.4 (0.118) 9.20 (4.94) 2.47 (0.903)
54.4 (23.4–77.0) 7.60 (4.20–17.4) 2.60 (1.20–3.68)
0.205 (0.0308)
0.205 (0.196–0.249)
0.788 (0.118)
0.841 (0.580–0.911)
0.347 (0.123)
0.372 (0.173–0.527)
3.51 (0.861)
3.37 (2.78–3.61)
0.879 (0.151)
0.824 (0.761–1.19)
2.00 (1.03)
1.87 (1.31–4.0)
1.90 (0.364)
1.72 (1.53–2.50)
1.43 (0.279)
1.44 (1.09–1.99)
2.90 (1.07)
2.44 (1.97–4.14)
0.707 (0.229) 1.19 (0.440) 11.2 (3.11)
646 (478–1231) 1109 (466–1852) 10.7 (7.55–18.4)
0.770 (0.375) 0.663 (0.167) 25.0 (5.30)
0.690 (0.363–1.68) 1.41 (0.746–2.08) 25.4 (16.1–31.5)
1.29 (0.475) 1.61 (0.806) 25.9 (7.52)
1.18 (0.795–2.34) 1.23 (0.969–3.12) 22.9 (18.6–41.2)
6307 (1487)
6231 (3624–8835)
83.9 (20.4)
78.8 (63.6–125)
51.2 (12.2)
54.6 (30.3–67.2)
18,940 (6620–29,905)
85.2 (37.8)
74.6 (42.4–155)
101 (51.7)
90.3 (48.4–184)
2.90 (1.83–3.46)
0.984 (0.231)
0.929 (0.666–1.46)
1.92 (0.668)
1.68 (1.25–3.19)
18,794 (6255) 2.92 (0.515)
*Harmonic mean.
of pharmacokinetic parameters following non-compartmental analysis are listed for atipamezole, yohimbine and tolazoline in Table 1. Behavioral and physiologic responses Maximal sedation, as evidenced by a decrease in chin-to-ground distance relative to BL was observed immediately prior to administration of the antagonist (Fig. 2). An initial increase in chin-to-ground distance was observed in all groups within 5 minutes of administration of the antagonist. While the average chin-to-ground distance remained above pre-antagonist values for the remainder of the observation period, values did not reach BL levels until the end of the observation period at 4 hours. A significant difference in chin-to-ground distance between horses receiving detomidine only and those receiving yohimbine or tolazoline was observed at 1 hour post antagonist administration (Fig. 2). There was no difference in chin-to-ground distance between the detomidine only and detomidine + atipamezole group.
Figure 2 Change in chin-to-ground distance (expressed as change from baseline (inches)) with respect to time following SL administration of detomidine (0.04 mg kg1) alone or with subsequent IV administration of atipamezole (0.12 mg kg1), yohimbine (0.075 mg kg1) or tolazoline (4 mg kg1) to horses (n = 9). Data are expressed as mean SD. *Indicates significant differences (p < 0.05) from the value immediately before administration of the antagonist and † from the detomidine only group.
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Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley
HR decreased significantly (p < 0.05) for all horses following detomidine administration with the decreased HR persisting for approximately 1.5– 2 hours when administration was not followed by an antagonist (Fig. 3). All antagonists elicited an initial increase in HR, but the degree and the duration varied with each drug (Fig. 3). Following yohimbine administration, HR remained near or at BL for 15–20 minutes. HR subsequently decreased and remained below baseline for the remainder of the sampling period (Fig. 3). HR increased to well above baseline at 5 minutes post tolazoline administration, but decreased to well below baseline by 15 minutes and remained lower than baseline for the remainder of the sampling period (Fig. 3). HR increased only slightly following atipamezole administration and never reached BL during the recording period (Fig. 3). Detomidine administration significantly increased the prevalence of AV conduction disturbances. All antagonists effectively decreased the prevalence of the AV blocks initially, although the number of AV blocks increased again by 1– 2 hours for all drugs (Fig. 4). A significant increase in PCV was noted immediately after administration of tolazoline (Fig. 5). PCV remained increased throughout the entire sample collection period. Significant changes in PCV did not occur following administration of either atipamezole or yohimbine. PP did not change significantly from baseline at any of the time points. Detomidine significantly increased glucose concentrations by 45 minutes post administration with
Figure 3 Change in HR (expressed as change from baseline in beats minute1) with respect to time following SL administration of detomidine (0.04 mg kg1) either with or without subsequent IV administration of atipamezole (0.12 mg kg1), yohimbine (0.075 mg kg1) or tolazoline (4 mg kg1) to horses (n = 9). Data are expressed as mean SD. 42
concentrations remaining elevated for 3 hours (Fig. 6 & Table 2). Administration of yohimbine significantly attenuated the detomidine induced hyperglycemia, atipamezole had no effect and tolazoline initially augmented the hyperglycemic effect.
Figure 4 Change in the percentage of AV blocks (expressed as absolute change from baseline) following SL administration of detomidine (0.04 mg kg1) either with or without subsequent IV administration of atipamezole (0.12 mg kg1), yohimbine (0.075 mg kg1) or tolazoline (4 mg kg1) to horses (n = 9). Data are expressed as mean SD.
Figure 5 Change in PCV% (expressed as absolute change from baseline) with respect to time following SL administration of either detomidine (0.04 mg kg1) alone or followed by IV administration of atipamezole (0.12 mg kg1), yohimbine (0.075 mg kg1) or tolazoline (4 mg kg1) to horses (n = 9). Data are expressed as mean SD. *Indicates significant differences (p < 0.05) from the value immediately before administration of the antagonist and † from the detomidine only group.
© 2013 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia, 41, 36–47
Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley Subjectively, urine volume appeared unchanged, however, increases in urination frequency were prominent following administration of atipamezole relative to the other groups.
Discussion A distinct advantage of alpha2-adrenergic agonists, such as detomidine, is the availability of antagonists that can diminish the agonistic effects. Nonetheless, while alpha2-adrenergic antagonists are beneficial in cases of overdoses of alpha2-adrenergic agonists, there are both anecdotal and documented reports of tachypnea, tachycardia, anxiety and death associated with their use (Scofield et al. 2010). In equine medicine, the only FDA approved alpha2-adrenergic antagonist is tolazoline, although yohimbine and atipamezole have been used to reverse the alpha2-adrenergic agonists. The primary goal of the current study was to compare the pharmacokinetics and selected pharmacodynamic effects of these alpha2-adrenergic antagonists subsequent to detomidine administration. It is important to note that only one dose of each antagonist was studied and that these doses may not be equipotent. Doses selected for each alpha2-adrenergic antagonist were chosen based on either the FDA approved dose (4 mg kg1 tolazoline), the dose most commonly reported in the literature (0.075 mg kg1 yohimbine) or based on previous dose-response studies
Figure 6 Plasma glucose concentrations (expressed as mg dL1 change from baseline) following SL administration of either 0.04 mg kg1 detomidine or followed by IV administration of 0.12 mg kg1 atipamezole, 0.075 mg kg1 yohimbine or 4 mg kg1 tolazoline to nine horses. Data are expressed as mean SD.
Table 2 Plasma glucose concentrations (mean SD) for nine horses before and after SL administration of detomidine (0.04 mg kg1) alone or followed by IV administration of atipamezole (0.12 mg kg1), yohimbine (0.075 mg kg1), or tolazoline (4 mg kg1). Antagonists were administered after measurements and blood samples were collected 1 hour after administration of detomidine Plasma [Glucose] (mg dL1) Time 0 minutes (baseline) 15 minutes 30 minutes 45 minutes 1 hour (pre-reversal) 1.25 hours 1.5 hours 1.75 hours 2 hours 2.5 hours 3 hours 3.5 hours 4 hours 5 hours 6 hours
Detomidine 93 9 91 99 121 138
12 8 11a 14a
—
146 32a —
144 139 127 — 110 — 87
48a 39a 43a 26 19
Atipamezole 89 8 89 95 107 121 138 134 138 138 137 127 120 110 90 80
10 17 22a 23a 23b 43 43 44 38 36 32 34 23b 18b
Yohimbine
Tolazoline
88 14 88 93 111 113 111 109 107 106 102 93 93 90 81 82
12 15 17a 27 29 20c 23 20c 15c 14bc 18 16b 20b 23b
91 5 92 92 117 133 191 172 159 148 137 130 117 108 94 87
8 16 18a 18a 22b 19bc 17b 20 21 24 24b 20b 14b 17b
— sample not collected. aSignificantly different from Time 0 (baseline) (p < 0.05); bSignificantly different from Time 1 hour (immediately prior to antagonist administration) (p < 0.05); cSignificantly different from the detomidine only group (p < 0.05). © 2013 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia, 41, 36–47
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Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley
describing the lowest dose producing maximal effect (0.12 mg kg1 atipamezole). In the current study, pharmacokinetic analysis was performed on three alpha2-adrenergic antagonists when administered subsequent to SL detomidine. The clearance and t1/2el of the alpha2adrenergic antagonist, yohimbine, following SL detomidine administration (22.9 mL minute1 kg1 (Cl); 1.87 hours (t1/2el)) differs from previous reports of yohimbine disposition following IV detomidine administration (6.8 mL minute1 kg1 (Cl); 4.4 hours (t1/2el)) (Knych et al. 2012a). In the current study, total yohimbine plasma clearance ranged from 18.6 to 41.2 mL minute1 kg1, indicating that it is a high hepatic extraction ratio drug with extra hepatic metabolism. This is in agreement with that reported in horses (Knych & Stanley 2011; Knych et al. 2012a) and humans (Hedner et al. 1992) when given as a sole agent, but differs from that reported previously for yohimbine given subsequent to IV detomidine in horses (Knych et al. 2012a). Knych et al. (2012a) reported a 50% decrease in yohimbine clearance following IV detomidine administration relative to yohimbine when administered as a sole agent. In that study, investigators hypothesized that the decreased yohimbine clearance was due to detomidine induced changes in hepatic blood flow. The discrepancy between the two studies may be an effect of lower detomidine plasma concentrations immediately prior to antagonist administration in this study relative to the previous one. This can be attributed to either different routes of detomidine administration or the timing of yohimbine administration relative to detomidine. Lower detomidine concentrations would diminish the drug-induced decreases in hepatic blood flow observed in the previous study (Knych et al. 2012a). Differences in yohimbine clearance and therefore the t1/2el may also be due to subject variability (age and metabolic capabilities) between the two studies. Atipamezole is currently labeled to reverse the effects of medetomidine. To the authors’ knowledge, there are no reports describing disposition of atipamezole following administration of detomidine in any species. There are, however, reports describing the pharmacokinetics of atipamezole when administered subsequent to medetomidine in ruminants. Ranheim et al. (1999, 2000) reported clearance values of 48.2 13.1 and 56.3 mL minute1 kg1 and an elimination half-life of 35.2 17.9 and 34.2 11.9 minutes for dairy cows and sheep 44
respectively, following administration of atipamezole subsequent to medetomidine. In the current study, the average clearance and t1/2el for atipamezole when administered subsequent to detomidine was 25.0 5.30 mL minute1 kg1 and 52.7 9 minutes, respectively. Although the clearance is slower and the half-life of elimination more prolonged relative to cows and sheep, it is unclear as to whether this difference is due to species, drug, or dose differences. In the current study, the median clearance, volume of distribution and t1/2el for tolazoline when administered subsequent to detomidine were 10.7 mL minute1 kg1, 1.72 L kg1 and 3.37 hours, respectively. A group receiving only tolazoline was not included in the current study so it is not possible to assess whether detomidine affects the disposition of tolazoline, however, t1/2el, Vdss and Cl are in close agreement with that reported previously for tolazoline administered as a sole agent to the horse (Casbeer & Knych 2013). It should be noted, though that the t1/2el (1 hour) differs significantly from that reported by Plumb (2011) when tolazoline is administered alone. The reason for this large discrepancy is not clear as details of the study used to determine the t1/2el in Plumb (2011) are not available. Although the magnitude and duration of the responses varied among the different antagonists, the effects of all three alpha2-adrenergic antagonists on sedation and HR and rhythm were incomplete and in some cases transient. Only yohimbine completely antagonized the sedative effect of detomidine (chin-to-ground distance at or near predetomidine value). The effect was short lived, lasting an average of 5 minutes before returning to near pre-antagonist levels. These findings were similar to that previously reported for IV administered detomidine (Knych et al. 2012b), whereby horses treated with yohimbine showed signs of alertness within 5 minutes followed by a return to sedation. The effects of tolazoline and atipamezole on detomidine induced changes in chin-to-ground distance were minimal and agree with that reported previously (Ramseyer et al. 1998; Hubbell & Muir 2006). Although the effects persisted slightly longer (15–20 minutes) than changes in chin-to-ground distance, reversal of HR and rhythm changes were similarly relatively short lived. Antagonism of the detomidine induced cardiac effects was most pronounced with yohimbine and tolazoline and least with atipamezole. As atipamezole is not currently
© 2013 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia, 41, 36–47
Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley approved for use in the horse, the dose administered in the current study was selected based upon a review of the literature. The most effective dose may not yet be established and may vary depending on the desired effect. A significant increase in PCV was observed following tolazoline administration, but not following atipamezole or yohimbine. The most likely explanation for this finding is either direct stimulation or attenuation of sympathetic inhibition, which could lead to splenic contraction, most likely mediated by alpha adrenergic receptors. While this hypothesis is further supported by the increase in HR above BL recorded immediately following tolazoline administration, the specific mechanism responsible for the sympathetic stimulation cannot be elucidated from the current study. In addition to its antagonistic effects at the alpha2-adrenergic receptor, tolazoline has antagonistic effects at alpha1-adrenergic, histamine and cholinergic receptors as well as direct effects on the vascular endothelium. Of the three antagonists under current discussion, tolazoline is considered to be least selective for the alpha2-adrenergic receptor. However, equipotent doses of each antagonist were not established, and the possibility that the effect is mediated by the alpha2-adrenergic receptor cannot be ruled out. Consistent with previous reports describing the effects of detomidine on plasma glucose concentrations (Gasthuys et al. 1987; Knych et al. 2012b), in the current study detomidine significantly increased plasma glucose concentrations. This effect induced by xylazine, another alpha2-adrenergic agonist, has been attributed to inhibition of insulin release from pancreatic beta cells (Eichner et al. 1979; Goldfine & Arieff 1979; Feldberg & Symonds 1980; Brockman 1981; Hsu & Hummer 1981; Thurmon et al. 1982, 1984). Furthermore, Angel and coworkers (Angel & Langer 1988; Angel et al. 1990) demonstrated that the inhibition of insulin release was mediated through postsynaptic adrenoreceptors located on the pancreatic cells, specifically the alpha2A subtype and that alpha2-adrenergic receptor antagonists, such as yohimbine, block the hyperglycemic effect of alpha2-adrenergic agonists (Angel et al. 1990; Oda et al. 1991). Schwartz & Clark (1998) have previously demonstrated that in sheep, the affinities of yohimbine and atipamezole are similar at the alpha2A-adrenergic receptors. In the current study, while yohimbine decreased detomidine induced hyperglycemia, atipamezole
did not have a significant effect on the hyperglycemic effect until 4 hours post administration, suggesting that the decrease in glucose was more likely due to cessation of detomidine induced effects as opposed to being due to atipamezole. There are two possible explanations for this deviation from the expected outcome. The anti-hyperglycemic effect may be dose-dependent and a higher dose of atipamezole may be necessary to reverse the alpha2-agonistic effect and in turn result in decreased plasma glucose concentrations. Secondarily, the similarity in affinity of atipamezole for the alpha2A-adrenergic receptor subtype in sheep (Schwartz & Clark 1998) may be species specific and may not hold true for horses. In contrast to the effects observed with atipamezole and yohimbine, tolazoline initially increased the detomidine induced hyperglycemia. While the effect was temporary, lasting for only 1 hour, the magnitude of the change was such that the level of statistical significance was high. To the authors’ knowledge, there are no reports describing the molecular mechanisms of tolazoline with respect to its effects on glucose, however, similar to changes in PCV, the most likely explanation for the increase in plasma glucose concentrations observed in the current study is increased sympathetic stimulation. Antagonism of insulin by stress hormones, such as cortisol can also lead to hyperglycemia. Neither insulin nor cortisol concentrations were measured in the current study, but Carroll et al. (1997) have reported increases in cortisol concentrations in horses following tolazoline administration subsequent to detomidine. In the current study, we sought to characterize the pharmacokinetics of three alpha2-adrenergic receptor antagonists as well as pharmacodynamic effects following SL administration of detomidine in the horse, utilizing doses for each that appear in the literature. The antagonistic effects of tolazoline, yohimbine and atipamezole on HR and rhythm changes and behavioral effects elicited by SL administered detomidine appear to be incomplete. Acknowledgements Financial support for this study was provided by the California Department of Food and Agriculture’s Equine Medication Monitoring Program and by the California Horse Racing Board. The authors would like to thank Dr. Eugene Steffey for editorial assis-
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Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley
tance and Carley Corado, Vanessa Covarrubias and Dan McKemie for technical assistance. References Angel I, Langer SZ (1988) Adrenergic-induced hyperglycemia in anaesthetized rats: involvement of peripheral alpha 2-adrenoreceptors. Eur J Pharmacol 154, 191–196. Angel I, Niddam R, Langer SZ (1990) Involvement of alpha-2 adrenergic receptor subtypes in hyperglycemia. J Vet Pharmacol Ther 254, 877–882. Brockman RP (1981) Effect of xylazine on plasma glucose, glucagon and insulin concentration in sheep. Res Vet Sci 30, 383–384. Carroll GL, Matthews NS, Hartsfield SM et al. (1997) The effect of detomidine and its antagonism with tolazoline on stress-related hormones, metabolites and physiologic responses and behavior in awake ponies. Vet Surg 26, 69–77. Casbeer HC, Knych HK (2013) Pharmacokinetics and pharmacodynamics effects of tolazoline following intravenous administration to horses. Vet J 196, 504– 509. DiMaio Knych HK, Steffey EP, Deuel JL et al. (2011) Pharmacokinetics of yohimbine following intravenous administration to horses. J Vet Pharmacol Ther 34, 58–63. Eichner RD, Prior RL, Kvasnicka WG (1979) Xylazineinduced hyperglycemia in beef cattle. Am J Vet Res 40, 127–129. Feldberg W, Symonds HW (1980) Hyperglycemic effect of xylazine. J Vet Pharmacol Ther 3, 197–202. Gasthuys F, Terpstra P, van den Hende C et al. (1987) Hyperglycaemia and diuresis during sedation with detomidine in the horse. Zentralb Veterinarmed A 34, 641–648. Goldfine ID, Arieff AI (1979) Rapid inhibition of basal and glucose-stimulated insulin release by xylazine. Endocrinology 105, 920–922. Grimsrud KN, Mama KR, Thomasy SM et al. (2009) Pharmacokinetics of detomidine and its metabolites following intravenous and intramuscular administration in horses. Equine Vet J 41, 361–365. Hedner T, Edgar B, Edvinsson L (1992) Yohimbine pharmacokinetics and interaction with the sympathetic nervous system in normal volunteers. Eur J Clin Pharmacol 43, 651–656. Hsu WH, Hummer SK (1981) Xylazine induced hyperglycemia in cattle: a possible involvement of alpha 2 adrenergic receptors regulating insulin release. Endocrinology 109, 825–829. Hubbell JAE, Muir WW (2006) Antagonism of detomidine sedation in the horse using intravenous tolazoline or atipamezole. Equine Vet J 38, 238–241. Hubbell JAE, Sams RA, Schmall LM et al. (2009) Pharmacokinetics of detomidine administered to horses
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at rest and after maximal exercise. Equine Vet J 41, 419– 422. Kaukinen H, Aspegren J, Hyppa S et al. (2010) Bioavailability of detomidine administered sublingually to horses as an oromucosal gel. J Vet Pharmacol Ther 34, 76–81. Knych HK, Stanley SD (2011) Pharmacokinetics and pharmacodynamics of detomidine following sublingual administration to the horse. Am J Vet Res 72, 1378– 1385. Knych HK, Steffey EP, Stanley SD (2012a) The effects of yohimbine on the pharmacokinetic parameters of detomidine in the horse. Vet Anaesth Analg 39, 221– 229. Knych HK, Covarrubias V, Steffey EP (2012b) Effect of yohimbine induced changes in behavior, cardiac and selected blood parameters in the horse. Vet Anaesth Analg 39, 583–584. Kollias-Baker CA, Court MH, Williams LL (1993) Influence of yohimbine and tolazoline on the cardiovascular, respiratory, and sedative effects of xylazine in the horse. J Vet Pharmacol Ther 16, 350–358. Mama KR, Grimsrud K, Snell T et al. (2009) Plasma concentrations, behavioral and physiologic effects following intravenous and intramuscular detomidine in horses. Equine Vet J 41, 772–777. Oda S, Fujimura H, Sasaki Y et al. (1991) Alpha-2 adrenergic modulation of glucagon and insulin secretions in sheep. Tohoku J Exp Med 163, 101–110. Plumb DC (2011) Plumb’s Veterinary Drug Handbook (7th edn). Wiley Blackwell, Ames, USA. Raekallio M, Vainio O, Karjalainen J (1990) The influence of atipamezole on the cardiovascular effects of detomidine in horses. J Assoc Vet Anaesth 17, 50–53. Ramseyer B, Schmucker N, Schatzmann U et al. (1998) Antagonism of detomidine sedation with atipamezole in horses. J Vet Anaesth 25, 47–51. Ranheim B, Arnemo JM, Ryeng KA et al. (1999) A pharmacokinetic study including some relevant clinical effects of medetomidine and atipamezole in lactating dairy cows. J Vet Pharmacol Ther 22, 368–373. Ranheim B, Arnemo JM, Stuen S et al. (2000) Medetomidine and atipamezole in sheep: disposition and clinical effects. J Vet Pharmacol Ther 23, 401–404. Schwartz DD, Clark TP (1998) Selectivity of atipamezole, yohimbine and tolazoline for alpha-2 adrenergic receptor subtypes: implications for clinical reversal of alpha-2 adrenergic receptor mediated sedation in sheep. J Vet Pharmacol Ther 21, 342–347. Scofield DB, Alexander DL, Franklin RP et al. (2010) Review of fatalities and adverse reactions after administration of a-2 adrenergic agonist reversal agents in the horse. Proceedings of the 56th American Association of Equine Practitioners, Baltimore, USA. p. 44–49. Skarda RT, Muir WW (1999) Effects of intravenously administered yohimbine on antinociceptive,
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Effects of antagonists on SL detomidine in the horse HK Knych and SD Stanley cardiorespiratory and postural changes induced by epidural administration of detomidine hydrochloride solution to healthy mares. Am J Vet Res 60, 1262–1270. Thurmon JC, Neff-Davis C, Davis LE et al. (1982) Xylazine hydrochloride-induced hyperglycemia and hypoinsulinemia in thoroughbred race horses. J Vet Pharmacol Ther 5, 241–245.
Thurmon JC, Steffey EP, Zinkl JG et al. (1984) Xylazine causes transient dose-related hyperglycemia and increased urine volumes in mares. Am J Vet Res 45, 224–227. Received 6 July 2012; accepted 10 December 2012.
Corrigendum In the publication ‘Effects of positive end-expiratory pressure titration on gas exchange, respiratory mechanics and hemodynamics in anesthetized horses’, errors were made in two equations in the section ‘Anesthesia and monitoring’: 1 The alveolar oxygen tension should be calculated using the following alveolar gas equation: PAO2 ¼ ½FiO2 ðPb 47 mmHgÞ ½PaCO2 ð1=0:8Þ 2 The physiologic right-to-left shunt equation should be calculated using the following modified shunt equation for patients breathing 100% oxygen: _ Qt _ ¼ f½PðA aÞO2 0:003=4 þ ½PðA aÞO2 0:003gg 100 Qs= These errors do not affect the data or results of the paper. Reference Ambrosio AM, Ida KK, Souto MT, Oshiro AH, Fantoni DT. (2013) Effects of positive end-expiratory pressure titration on gas exchange, respiratory mechanics and hemodynamics in anesthetized horses. Vet Anaesth Analg 40, 564–572.
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