Comparative canine Pharmacokinetics–Pharmacodynamics of Fospropofol Disodium Injection, Propofol Eemulsion, and Cyclodextrin-Enabled Propofol Solution Following Bolus Parenteral Administration

NOTE Comparative Canine Pharmacokinetics–Pharmacodynamics of Fospropofol Disodium Injection, Propofol Emulsion, and Cyclodextrin-Enabled Propofol Solution Following Bolus Parenteral Administration MICHELLE P. MCINTOSH,1 ROGER A. RAJEWSKI2 1

Drug Delivery, Disposition and Dynamics, Monash Institute of Pharmaceutical Sciences, Parkville, Victoria 3052, Australia

2

Department of Pharmaceutical Chemistry, University of Kansas, Lawrence, Kansas 66047

Received 3 March 2012; revised 22 April 2012; accepted 26 April 2012 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/jps.23195 ABSTRACT: The pharmacokinetics and pharmacodynamics of fospropofol (FP) disodium injection, propofol emulsion (PE), and cyclodextrin-enabled propofol (CDP) solution following bolus parenteral administration in dogs was evaluated. Three healthy male beagle dogs were treated in a three-way cross-over study (14 day washout period) with 6 mg/kg propofol equivalents. Blood samples were collected predose and at 16 points postdose through 1440 min and analyzed for propofol and FP, when appropriate. From 5 min predose to 30 min postdose, brain electrical activity [electroencephalography (EEG)] was recorded and analyzed by power spectrum analysis techniques. Each formulation appeared to be well tolerated with transient discomfort observed in the PE and CDP animals and minor excitability in the FP animals prior to loss of consciousness. Blood propofol followed three-compartment pharmacokinetic behavior and derived parameters were not statistically different except for elimination half-life from the CDP formulation and onset, and duration of anesthesia from the FP formulation. The effect site concentrations at 50% the maximum EEG effect for the FP and CDP formulations were approximately one-half that of the PE formulation. Onset and duration of anesthesia are correlated with modeled effect site propofol concentrations. The implications of formulation on pain on injection and propofol activity are discussed. © 2012 Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci Keywords: propofol; pain on injection; cyclodextrin; fospropofol; pharmacodynamics; pharmacokinetics; prodrugs; mathematical model

INTRODUCTION Propofol is the drug of choice for induction and maintenance of anesthesia because it results in a smooth and rapid onset of action and a short duration makes dose titration possible.1,2 The only significant drawback of propofol is that it causes a sensation of pain on injection in 60% of patients, and one-third of those patients describe the pain as severe or excruciating.3–6 Many interventions have been investigated in efforts to alleviate the pain associated with propofol administration such as pretreatment and/or coadministration

Correspondence to: Roger A. Rajewski (Telephone: +785-8645158; Fax: + 785-864-5736; E-mail: [email protected]) Journal of Pharmaceutical Sciences © 2012 Wiley Periodicals, Inc. and the American Pharmacists Association

with lidocaine, opioids, ketamine, or non-steroidal anti-inflammatory drugs. Numerous formulation approaches have been investigated in an attempt to overcome the challenge of pain on injection with propofol, including cyclodextrin-enabled formulations,7,8 micelle formulations,9 and prodrug approaches.10–12 The most successful propofol prodrug investigated to date is fospropofol (FP), a water-soluble prodrug that is formulated as an aqueous solution.13–15 Following administration, FP undergoes hydrolysis by alkaline phophatases to produce propofol, phosphate, and formaldehyde (Fig. 1). FP reportedly causes less pain at the injection site than emulsion formulations of propofol16 ; however, there is a dearth of information regarding the pharmacokinetics (PK)–pharmacodynamics (PD) relationship of the JOURNAL OF PHARMACEUTICAL SCIENCES

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O O P ONa ONa O

OH

+

+

phosphatase

Propofol

O- Na O P O-Na+ OH Phosphate

O

+ H

H

Fomaldehyde

Fospropofol Disodium

Figure 1. Chemical structure of fospropofol disodium and its metabolites.

prodrug since most publications on the subject have been retracted due to potential analytical errors.17–19 Cyclodextrin-enabled formulations have been shown to rapidly and quantitatively deliver propofol to the systemic circulation, resulting in rapid onset of anesthesia.13,14 However, these formulations have been associated with an increase in pain at the injection site.20 The objectives of this study were to evaluate the PK and PD equivalence of three propofol (2, 6diisopropylphenol) formulations and to develop a PK/ PD model that provides insight into the rate of systemic exposure of propofol following administration of each formulation. The formulations evaluated were an aqueous solution of FP disodium in normal saline (phophono-O-methyl-2, 6-diisoporpylphenol, disodium salt), the commercial propofol emulsion (PE), and an aqueous solution of propofol solubilized by hydroxypropyl-$-cyclodextrin [cyclodextrinenabled propofol (CDP); 0.125 M].

EXPERIMENTAL Fospropofol was synthesized in laboratories at the Center for Drug Delivery Research at the University of Kansas, following the standard procedures.21 All other chemicals were purchased from commercial sources. The concentration of propofol in the blood samples was determined using high-performance liquid chromatography (HPLC) with fluorescence detection based on the work of Plummer,22 with minor modifications. For quantitation of FP, a second HPLC method was employed. A Vydac C4 column (4.6 × 250 mm2 , ˚ pore size, and 5 :m particle size) was used 300 A with fluorescence detection at 260 nm excitation and 290 nm emission. The mobile phase was 70:30 acetonitrile–0.01 M sodium phosphate buffer (pH 3.0) at a flow rate of 1.0 mL/min. FP was extracted from 250 :L plasma aliquots following the addition of 100 :L of a sodium salicylate solution (400 mg/mL). The samples were vortexed for 5 s prior to centrifugation for 90 min in microfiltration tubes with 10,000 NMWL filters at 4000g. Calculation of propofol blood levels was achieved using external standards. JOURNAL OF PHARMACEUTICAL SCIENCES

In Vivo Dog Studies The University of Kansas Institutional Animal Care and Use Committee approved this study, and appropriate guidelines for the use of animals were observed during all aspects of the study. Three healthy male beagle dogs (11.9–12.7 kg) received 6 mg/kg propofol equivalents as an intravenous (i.v.) dose formulated as PE, FP, and CDP in a three-way randomized cross-over study (14 day washout period). Each formulation contained 10 mg/mL propofol equivalents. Dogs were fasted for at least 16 h prior to drug administration, and water was available ad libitum. Intravenous bolus administration was via a 21-gauge butterfly catheter in the left cephalic vein over a 60 s period. Blood samples (2 mL) were obtained via an indwelling catheter in the right cephalic vein or by individual venipuncture and collected into heparinized R  Vacutainers . Blood samples were collected predose (−5 min) and 1, 3, 5, 10, 15, 20, 30, 60, 90, 120, 180, 240, 300, 360, 480, and 1440 min postdose. Blood samples were processed immediately for analyte quantitation. Animal Instrumentation Electroencephalographic (EEG) data was acquired and analyzed on a Biopac Systems MP100WS data acquisition system. Prior to placement of electrodes, the scalp was shaved and wiped clean with aqueous isopropyl alcohol. A ground electrode was located in the middle of the head and two EEG electrodes were placed on the left and right occipital lobe, respectively. Data were acquired at 500 Hz from 5 min predose until 30 min postdose. Data analysis was performed using Acknowledge software (Version 3.2.7; Biopac Systems, Inc., California). Electroencephalographic data was analyzed by power spectrum analysis techniques.23 The raw data was filtered with a 50 Hz low pass filter and a 2.5 Hz high pass filter to eliminate interference from power source noise and muscle movement. A fast Fourier transform was then performed on each 5-s epoch to convert the time domain data to the frequency domain. The amplitudes of the individual frequency components were then squared and the total electrical DOI 10.1002/jps

PHARMACOKINETICS–PHARMACODYNAMICS OF PROPOFOL FORMULATIONS IN DOGS

output (power spectrum) for the epoch was calculated. On the basis of this total, the frequencies were calculated under which 95% (spectral edge) of the total electrical activity occurred. The time of onset of anesthesia was reported as the time period between the beginning of the injection and the time to one-half the maximal observed spectral edge frequency decrease. The duration of anesthesia was calculated as the interval between the time for one-half the largest frequency decrease in the spectral edge following injection and the time for the frequency to return to 50% of the baseline value.

a

3

Sample propofol

Shallow

Central

Deep

kelim Propofol dose

b

PK/PD Modeling Traditional compartmental analysis of the propofol blood concentration versus time profile for each dog was employed utilizing SAAM II software (Version 1.2.1; University of Washington, Washington). Determination of the appropriate compartmental model was performed using the Akaike information criterion (AIC) calculated by the SAAM II software.24 The AIC is useful when performing model comparisons and discrimination of the appropriate number of compartments in compartmental modeling. The AIC is a function of the goodness of fit, the number of adjustable model and variance parameters, and the total number of data points. In deciding between two or more potential models, the model with the lowest AIC value (the model that most simply explains the data with the least number of parameters) is the best one.25 For PE and CDP formulations, the propofol data through 480 min was best fit to a three-compartment mamillary model using a series of differential equations, and the PK parameters were derived following the standard methods of Gibaldi and Perrier,26 with the general assumption that elimination was from the central compartment only (Fig. 2a). For FP administration, the FP data were best fit to a onecompartment model, with enzymatic conversion resulting in propofol input to the central compartment of the three-compartment model with a simultaneous fit of the propfol data (Fig. 2b). A simultaneous fit of the spectral edge data to an inhibitory Emax model described by Eq. 1 was also performed.

Fospropofol dose

Sample fospropofol

Central

Sample propofol

Enzymatic conversion to propofol Shallow

Central

Deep

kelim Figure 2. A schematic representation of threecompartment pharmacokinetic model used in the evaluation of (a) propofol emulsion and cyclodextrinenabled formulations, and (b) the fospropofol disodium solution formulation.

spectral edge, and γ is the Hill factor, a measure of the responsiveness of the model to the effect. Pharmacokinetic and PD data were statistically evaluated using one-way repeated measures analysis of variance, and where necessary followed by Tukey’s post-hoc test (SigmaStat, SPSS Inc.). The results were considered statistically significant at 5% significance level (p < 0.05).

RESULTS AND DISCUSSION E = E0 −

( Emax × Ce ( ( EC50 + Ce

(1)

In Eq. 1, E is the observed spectral edge frequency, E0 is the baseline frequency prior to drug administration, Emax is the maximum suppression in the spectral edge, Ce is the concentration of propofol in the effect compartment, EC50 is the drug concentration in the effect compartment providing a 50% reduction in the DOI 10.1002/jps

Figure 3a depicts a representative blood concentration versus time profile after i.v. bolus administration of 6 mg/kg of propofol equivalents as FP, PE, or CDP to a single dog, on three separate occasions. As shown in the figure, FP exhibits one-compartment behavior with a half-life of 7.2 ± 0.82 min. The disappearance of FP is associated with an initial increase in propofol concentration through the first 5 min following injection (see inset to Fig. 3a showing data through first 35 min). Injection of equivalent amounts of propofol JOURNAL OF PHARMACEUTICAL SCIENCES

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Figure 3. (a) Blood propofol and plasma fospropofol concentrations following administration of the three title propofol formulations. Lines represent best-fit values according to the respective models in Figures 2a and 2b.  Fospropofol,  propofol from fospropofol, • propofol emulsion, and ◦ cyclodextrin-enabled solution. (b) Spectral edge as a function of time. (c) Modeled effect site concentration of propofol. Solid line from fospropofol, dotted line from cyclodextrin-enabled formulation, and dashed line from propofol emulsion.

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from PE and CDP resulted in slightly higher blood levels from PE, relative to CDP. The PK data are consistent with the observations of Dutta and Ebling27,28 that the emulsion formulation decreases the first-pass loss of propofol in the lung relative to non-lipid formulations. The mean PK parameters determined for the three formulations are shown in Table 1. The PK parameters describing FP were not significantly different to those determined for PE. The parameters describing the CDP were similar to those obtained for PE and FP, with the exception of the elimination half-life value (10.51 ± 0.80 min), which was significantly longer than the propofol half-life determined following PE and FP administration at 6.57 ± 0.55 min (p = 0.017) and 7.63 ± 1.60 min (p = 0.025), respectively. The etiology of the longer half-life from the CDP formulation is unknown and may be a result of equal weighting of all time-dependent data in the fitting process. Differences in half-life were not observed in porcine studies evaluating a sulfobutyl ether cyclodextrin-solubilized propofol formulation relative to PE using controlled rate infusions.8 Each formulation was well tolerated, with transient discomfort evident during the start of the PE and CDP injections. All animals receiving these formulations were unconscious before the end of the injection. The FP formulations did not elicit signs of discomfort on injection; however, the animals did demonstrate signs of excitability immediately prior to loosing consciousness. Pain on injection of propofol arises from the amount and rate of propofol available to the aqueous phase.29–31 As the inclusion kinetics between propofol and the cyclodextrin approaches aqueous diffusion rate limitations (association constant approximately 3000 M−1 ),7 high free concentrations of propofol are available immediately on injection of CDP, resulting in a high incidence of pain.20 The free concentration of propofol from PE should be lower than that from CDP as the availability of propofol is limited by propofol diffusivity out of the oil droplets. The diffusivity is sufficiently high to produce pain on injection.20 FP has limited pain on injection because the injection site is exposed to FP and there is no propofol present until enzymatic conversion of the prodrug to propofol has begun.16 The corresponding EEG 95% power spectrum profiles for the blood versus time profiles in Figure 3a are presented in Figure 3b. All three formulations induced a similar depth of anesthesia. The maximum effect on EEG suppression (see Table 1) was 16.15 ± 0.96, 16.16 ± 1.51, and 16.78 ± 1.92 Hz for PE, FP, and CDP, respectively. The Emax values were not statistically different (p > 0.05), which suggests that

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Table 1. Pharmacokinetic and Pharmacodynamic Parameters Determined for Propofol Following Three-Compartmental Analysis Coupled with an Effect Site Compartment (n = 3)

Cmax (:M) Vc (L/kg) Half-life (min) CL [mL/(min kg)] AUC [:mol/(min mL)] EC50 (:M) Emax (Hz) Onset time (s) Duration (s) γ (Hill factor)

PE

FP

CDP

55.20 ± 7.97 0.72 ± 0.11 6.57 ± 0.55 77 ± 14 0.454 ± 0.099 0.64 ± 0.25 16.15 ± 0.96 24.0 ± 1.73 870 ± 207 5.2 ± 3.5

41.67 ± 5.67 0.95 ± 0.14 7.63 ± 1.6 90 ± 28 0.382 ± 0.121 0.39 ± 0.11 16.16 ± 1.51 124 ± 15∗ 1448 ± 155∗ 35 ± 21∗

40.06 ± 12.02 1.04 ± 0.30 10.51 ± 0.80∗ 69 ± 20 0.457 ± 0.087 0.36 ± 0.12 16.78 ± 1.92 24.7 ± 0.58 1103 ± 169 7.1 ± 4.2

∗ p < 0.05. PE, propofol emulsion; FP, fospropofol disodium solution; CDP, cyclodextrin-enabled propofol solution.

there is no difference in the ability of propofol to reach the effect site once released from the formulations. The EC50 propofol concentrations calculated from the direct effect PD model were 0.64 ± 0.25, 0.39 ± 0.11, and 0.36 ± 0.12 :M for PE, FP, and CDP, respectively. Although the EC50 values are not statistically different, the data are consistent with studies showing that clinical parameters are attained at lower modeled effect site propofol concentrations from FP relative to PE.32 This effect is evident in the modeled effect site propofol concentrations depicted in Figure 3c, which show that effect site levels of propofol required to attain a 50% reduction in the spectral edge are considerably higher for PE relative to FP and CDP. The Hill factor was also significantly higher for FP relative to PE and CDP, demonstrating a steeper concentration– effect curve. This behavior can be clearly observed in the concentration–effect hysteresis curves in Figure 4. Although blood propofol from PE reaches higher concentrations than that from CDP and FP, the slope

of the line (sensitivity) on approach to similar effect levels is lower. The sensitivity of the model is highest for propofol from FP, followed by CDP. The attainment of clinical parameters at lower effect site propofol concentrations from FP and the steep concentration–effect may be a result of competitive protein binding of FP with propofol, leading to a higher free fraction of propofol from FP relative to other propofol formulations at the same blood propofol concentrations. Propofol is highly protein bound in dogs, ranging from 98% to 99%.33 This behavior has been studied in-depth for another phosphonooxymethyl prodrug, fosphenytoin, which competitively displaces its active metabolite, phenytoin, from plasma protein binding sites.34,35 Following i.v. administration of fosphenytoin at clinically relevant levels, this displacement causes up to twice the amount of unbound plasma phenytoin for 30–60 min.35 Analogous behavior may be causing the attainment of clinical parameters at earlier and lower propofol concentrations from FP relative to PE formulations.32 The onset of observable sedation following PE and CDP were not significantly different to one another; however, the onset following FP was significantly delayed, which is an expected consequence of the enzymatic conversion of FP to propofol. The duration of sedation following FP was significantly greater than the duration after PE, although there was no significant difference between CDP and FP (see rise in spectral edge frequency in Fig. 3b). Again, this effect is evident in the modeled effect site propofol concentrations in Figure 3c where the propofol concentrations from FP decrease at a much slower rate than those from PE and CDP.

ACKNOWLEDGMENTS Figure 4. Blood propofol concentration–effect hysteresis plot for propofol emulsion (gray line), cyclodextrin-enabled formulation (gray dotted line), and the fospropofol disodium solution formulation (black line). Arrows indicate the temporal aspect of the effect. DOI 10.1002/jps

This work was supported in part by the Kansas Technology Enterprise Corporation through the Centers of Excellence program. The authors gratefully acknowledge the assistance of James Bresnahan JOURNAL OF PHARMACEUTICAL SCIENCES

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DVM, David Kosednar and Daniel Hurt in the conduct of these studies.

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