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Pharmacokinetics and pharmacodynamics of intra-graft iloprost in femorodistal bypass surgery
Prostaglandins, Leukotrienes and Essential Fatty Acids (1997) 56(5), 389-393 © Pearson ProfessionalLtd 1997
Pharmacokinetics and pharmacodynamics of intra.graft iloprost in femorodistal bypass surgery H. R. W a t s o n , ~ F. C. T. Smith, 2 C. P. S h e a r m a n , 2 M. H i l d e b r a n d 3 ~Cardiovascular Clinical Development, Schering AG, D-13342 Berlin, Germany 2Department of Surgery, Queen Elizabeth Medical Centre, Birmingham, UK 3Department of Pharmacokinetics, Schering AG, D-t3342 Berlin, Germany
Summary Intra-graft injection of the prostacyclin analogue, iloprost, was performed at the end of femorodistal bypass procedures in 12 patients with severe peripheral arterial occlusive disease. Iloprost plasma levels were measured and compared with changes in haemodynamics. There was a high initial iloprost plasma level (mean 625 pg/ml) which dropped to a mean of 50 pg/ml after 15 min. This correlated with an immediate reduction in systolic blood pressure which had returned to pretreatment levels after 15 min. In contrast, the vascular resistance distal to the graft showed a reduction after 5 min which was maintained for at least 20 rain after iloprost injection and the mean blood flow through the graft increased steadily throughout the same period of measurement. The study showed an effect of iloprost on blood pressure which correlated with plasma levels, but the time course of the changes in distal vascular resistance and graft blood flow demonstrated an effect more prolonged than the half-life of iloprost.
INTRODUCTION
Iloprost is a chemically stable analogue of prostacyclin which has been proposed as a means of improving graft blood flow after difficult femorodistal bypass surgery. The optimal dose, given directly into the arterial circulation by a slow intra-graft injection, was found to be 3000 ng. 1 This dose has been shown to produce a sustained improvement in graft blood flow.2 The pharmacokinetics of sustained intravenous infusions of iloprost have been studied in healthy volunteers, 3 an elderly population 4 and in patients with severe peripheral arterial disease, s These studies have demonstrated that iloprost has a biphasic plasma half-life with mean a-phases of 3-5 min and mean ~-phases of 15-30 min. Although no human pharmacokinetic studies of intra-arterial iloprost have been performed, these Received 15 October 1996 Accepted 22 October 1996 Correspondence to: H. R. Watson, Tel. 00 49 30 468 4958; Fax. 00 49 30 468 1269
results suggest that haemodynamic effects of a single intra-graft injection of iloprost after bypass surgery should be short-lived. The aim of the present study was to establish the pharmacokinetic parameters of a single injection of 3000 ng iloprost into the graft at the end of femorodistal bypass surgery and to compare the plasma concentration profile with the effects of iloprost on graft blood flow and systemic haemodynamics. METHODS Patients
All patients underwent femorodistal bypass surgery- for severe ischaemia of the lower limb at the Queen Elizabeth Hospital in Birmingham. Patients with abnormal haemostasis or severe liver disease were excluded. Ethical approval for the study was obtained from the Research Ethics Committee of the South Birmingham Health Authority and all patients gave written informed consent prior to entry in the study. 389
390
Watson et al
Surgical procedures and treatment
Patients received a femorodistal bypass graft under general anaesthesia. Grafts consisted of either autogenous vein or a combination of vein and prosthetic material in the form of sequential bypasses via a blind popliteal segment. The bypass grafts all had the proximal anastomosis at the femoral artery bifurcation. The distal anastomoses were divided equally between the tibioperoneal trunk (3 cases), the anterior and posterior tibial arteries (3 cases each) and the peroneal artery (3 cases). Nine patients received vein grafts and three composite grafts of vein plus polytetrafluoroethylene. The prosthetic graft was used to bypass to a blind popliteal segment and a vein graft taken sequentially from that to the distal run off vessel. This technique was used when autologous vein was of poor quality or unavailable. The mean minimum graft diameter was 3.9 mm + 0.3 (range 2.5-6.0 mm). After completion of the distal anastomosis, the arterial clamps were removed and blood flow through the graft was established. This was measured until a stable reading over 10 min was obtained. Fifteen mfllilitres of saline solution containing 3000 ng of iloprost were then infused over a period of 2 min into the proximal end of the graft through a side branch of the vein. All patients were operated upon under general anaesthesia and received the following agents as part of the anaesthetic regimen: an intravenous induction agent (thiopentone, methohexitone or etomidate), an analgesic (fentanyl, alfentanil or papaveretum), a muscle relaxant (atracurium or vecuronium) and an antibiotic (cefuroxime, ciprofloxacin, flucloxacillin, gentamicin, metronidazole or vancomycin).
Pharmacokinetics
Blood samples for the determination of iloprost concentration in the plasma were taken from an antecubital vein. Sampling times were 1, 3, 5, 10, 15, 30, 60 and 120 min after administration of iloprost. For each sample 3-4 ml blood was taken into a heparinised tube and centrifuged at 2200 x g for 10-15 min. Each sample was centrifuged within an hour of sampling. 1.5-2.0 ml of plasma from each sample were stored at -20°C. Iloprost was measured by a sensitive and specific scintillation proximity assay (SPA)? An antiserum raised against iloprost-9-pentynyl BSA. Iloprost-3H-methylester with a specific activity of 50.6 ci/mmol was used as a tracer. SPA analyses were carried out with 0.15 or 0.2 ml plasma adjusted to pH 2 with 1 N HCL and extracted with 2.5 ml diethylether. The organic layer was separated after ethanol/dry ice solidification. The residue was redissolved in 0.25 ml of BSA buffer, 0.21 ml was pipetted and tracer (1600 or 2600cpm) plus anti-rabbit-protein SPA
reagent (Amersham 1:15 000 dilution) suspended in BSA were added. The samples were shaken overnight and measured in a Pharmacia Beta-Plate counter. Calibration samples were obtained with human plasma containing 0-1000pg/ml. Quality control samples with 0, 50, 100 and 200 pg/ml were processed as described above. Pharmacodynamics
Systolic and diastolic blood pressure measurements were recorded via a radial artery cannula immediately before iloprost treatment and monitored thereafter for any effect of the treatment on systemic haemodynamics. Blood flow and mean graft pressure were measured using a Doppler Flowmeter (OpDop 130, SciMed, Bristol, UK) and pressure transducer (Baxter Healthcare Corp., Amsterdam, The Netherlands) connected to a silastic cannula introduced into the graft through an unligated side branch. Peripheral resistance, calculated by the OpDop 130 software from mean graft pressure and blood flow, was expressed in peripheral resistance units (PRU) where 1 PRU = 1 mmHg/ml-1/min-~. All haemodynamic measurements were recorded before and at 5, 10, 15 and 20min after iloprost treatment. Graft patency was confirmed 24 h and 7 days postoperatively using Duplex ultrasound scanning. All adverse medical events occurring during this period were recorded. Statistics and calculations
Pharmacokinetic and pharmacodynamic data are described by means, standard errors and ranges. Pharmacodynamic data were analyzed using paired Student's t-test for within patient changes. Pharmacokinetic data were calculated by TOPFIT7 using an open two compartment model and applying a weighting function of y = 1/x2 to obtain best fit results. The area under the plasma concentration curve (AUC) is conventionally obtained by extrapolating to infinity using the terminal half-life (AUC0_~). Calculations were additionally made of the area under the curve for the period studied (AUCo_t). Total clearance was derived based on AUC0_t. Volumes of distribution were calculated for the central compartment and at steady state. RESULTS
Twelve patients undergoing femorodistal bypass surgery were entered into the study ranging in age from 62 to 79 years (mean 70 years + 2). These included six males and six females. Five were smokers. The mean weight of the patients was 72 kg + 3 kg (range 4 7 - 9 2 kg). All patients were suffering from critical leg ischaemia with either rest pain (11 cases) and/or ulcers (5 cases). They had a mean
Prostaglandins, Leukotrienes and Essential Fatty Acids (1997) 56(5), 389-393
© Pearson Professional Ltd 1997
Pharmacokinetics and pharmacodynamics of intra-graft iloprost in femorodistal bypass surgery
ankle-brachial pressure index of 0.28 _+ 0.06 (range 0-0.59). Two of the patients were diabetic. Pharmacokinetics
Iloprost assays were performed on the plasma samples obtained from eight patients. Technical problems in the storage of plasma samples from the other four cases resulted in thawing and the loss of these data. The pharmacokinetic results from the eight patients satisfactorily completed are summarized in Table 1. The mean floprost plasma concentrations over time are shown in Figure 1. The mean peak plasma concentration was 625 pg/ml, with a considerable variation between the individual values. The mean had fallen to 50 pg/ml 15 min after iloprost administration. The initial distribution phase half-lives ranged from 1.3 to 4.1 rain. The terminal disposition phase half-lives fell into two groups. In four patients values of 6.6 to 11.7 rain were obtained and in the other four patients the values obtained from model-fitting were in the range 79-99 min. The long terminal-phase half-lives calculated in this second group are due to low iloprost levels (< 50 pg/ml) still being detectable in the period between 15 and 120 min. The shorter terminal-phase of 6.6 to 11.7 min is considered more reliable and these are date which are summarized in Table 1. The differences
between AUC values obtained from available data and from model-fitting (Table 1) reflect the long terminal half-lives calculated for some patients. Total clearance was estimated from AUCo_t as 15.8 + 2.3 ml/min/kg. Haemodynamics
Mean systolic and diastolic blood pressures before iloprost were 140 mmHg + 8 and 70 mmHg + 3 respectively. Distinct and statistically significant reductions in systolic and diastolic blood pressure were seen at 5 and 10 min after iloprost administration (Table 2). The pretreatment heart rate of 67 beats/min + 2 showed a small but statistically significant increase at the same time points (Table 2). All of these changes in systemic haemodynamics were transient and no significant effects of iloprost could be seen after 15 min. Initial graft blood flow before iloprost administration was 104 ml/min + 24 with a calculated peripheral resis-
Table 2 Changes in systemic haemodynamics Time after iloprost infusion (min) (n = 12) 5 10 15 30 60 Systolic blood pressure (% change)
mean S.E. minimum maximum P
-18 3 -35 1 0.0001
-13 4 -38 3 0.004
-4 3 -21 22 n.s.
-3 5 -38 25 n.s.
1 4 -19 22 n.s.
Diastolic blood pressure (% change)
mean S.E. minimum maximum P
-19 4 -33 7 0.0002
-14 4 -35 11 0.002
-5 4 -26 15 n.s.
-2 3 -13 16 n.s.
4 5 -19 36 n.s.
Heart rate (% change)
mean S.E. minimum maximum P
4 2 -4 17 0.018
5 10 1 6 0 -5 11 77 0.0006 n.s.
3 3 -10 17 n.s.
20 5 -2 51 0.003
Table 1 Pharmacokinetic parameters Parameter
n
mean
S,E.
01 min (pg/ml) AUCo_t (pg.min/ml) AUC0_= (pg.min/m0 t , ~ (rain) t ~ (min)
8 8 8 8 4 8 8 8
625 3333 7225 1.9 8.4 15.8 0.04 0.25
95 673 963 0.3 1.2 2.3 0.01 0.08
Cldata (ml/min/kg) Vdo (I/kg) Vdss (I/kg)
391
900 j_ Table 3 Changes in peripheral resistance and graft blood flow
800 t E ,~
?_o
700
Time after iloprost infusion (min) 5 10 15 20
600
soo
E °= = o
400
~-
200
300
100
0 ~ 0
~I
i
30
60
90
120
Time (min)
Fig, 1 Plasma concentration of iloprost after 3000 ng intra-graft injection.
© Pearson Professional Ltd 1997
Peripheral resistance (% change)
n mean S.E. minimum maximum P
12 -27 7 -64 7 0.002
12 -30 13 -78 90 0.049
12 -27 12 -80 62 0.040
6 -30 11 -7t 0 0.048
Graft blood flow (% change)
n mean S.E. minimum maximum P
12 32 14 -22 126 0.045
12 58 24 -57 256 0.037
12 63 25 -36 278 0.029
6 78 45 -7 294 0.14
Prostaglandins, Leukotrienes and Essential Fatty Acids (1997) 56(5), 389-393
392
Watson et al
8OO
tance of 1.32 pru + 0.26. The mean peripheral resistance was significantly reduced by a minimum of 27% throughout the period of measurement and the volume blood flow through the graft was significantly increased compared to values immediately prior to iloprost injection (Table 3). The fall in resistance in the distal vascular bed occurred within 5 min and was maintained at a constant level for at least 20 min. The effect on volume blood flow on the other hand, although statistically significant after 5 min, increased steadily over the 20 min for which it was measured. A comparison of changes in floprost plasma concentration with systolic blood pressure, peripheral vascular resistance and graft blood flow (Fig. 2) shows an effect on blood pressure coinciding with iloprost plasma levels while the vascular resistance distal to the graft remains decreased and the volume flow increases even as the plasma concentration falls.
7O0 600.
soo -: 400 • (J
E a-
Seven days postoperatively rest pain had resolved in 10 of 11 patients and healing of an ischaemic ulcer had occurred in one out of five patients. Graft occlusion occurred in one case after 24 h and another after 7 days giving a 7-day primary patency rate of 83%. No surgical complications or other adverse medical events were reported in the first 24 h after surgery. Subsequently one patient developed a tibial wound infection over the graft and another developed a mild hyperglycaemia which was controlled with an oral hypoglycaemic agent. Neither of these two adverse events were attributed to the study drug.
200 100 0
0
o ~
Clinical outcome
300
0 o
.............................................................
-20
09 •Q .
-30
0 o
-10
~
-30
v
"~
-40
D -50
80-
DISCUSSION
70-
As a result of improved techniques in peripheral bypass surgery, procedures in the lower limb are being performed ever more distally. Bypass grafts to the calf vessels and beyond are now regularly performed, but they continue to have a high failure rate, up to 45% at 12 months in some series. 8 The majority of distal bypasses which fail, do so from 6 weeks onwards due to the development of graft stenoses or, in the longer term, progression of atherosclerosis. However, over 20% in a recent large study failed in the first 6 weeks? Technical error is undoubtedly responsible for some immediate graft occlusions, but graft thrombogenicity, resistance in the distal vascular bed and blood flow through the graft also play a role in early graft failures. It has been observed that graft blood flow immediately after completion of the bypass remains very poor in some patients despite patent vessels distal to the graft. This has been termed the 'low-reflow' phenomenon 1° and
60~-
/
50
"a o
~g (.9
30 2O 10 0
............................................................. I
I
I
I
I
0
5
10
15
20
Time (rain) Fig, 2 Comparison of iloprost plasma concentration and haemodynamics after 3000 ng intra-graft injection.
Prostaglandins, Leukotrienes and Essential Fatty Acids (1997) 56(5), 389-393
© Pearson Professional Ltd 1997
Pharmacokinetics and pharmacodynamics of intra-graft iloprost in femorodistal bypass surgery
these patients are t h o u g h t to be at particular risk of graft t h r o m b o s i s in the immediate postoperative period. The origins of this p h e n o m e n o n are not certain, b u t a c c u m u l a t i o n of white cells u and vascular endothelial swelling 12 have b o t h b e e n detected in the distal vascular b e d after partial ischaemia Femorodistal bypass surgery entails a period of partial ischaemia in an already ischaemic circulation, while the m a i n arteries are clamped followed b y reperfusion. It is possible that a severely impaired microcirculation results in some cases leading to difficulty in establishing an adequate blood flow t h r o u g h the graft immediately after surgery. Iloprost has b e e n extensively investigated in peripheral vascular diseases and the results presented here show that iloprost injected intra-arterially at the end of distal bypass surgery has a plasma half-life comparable with that found during intravenous infusions in non-surgical patients. 5 The time-course of platelet aggregation inhibition and vasodilation with iloprost have previously b e e n s h o w n to correlate with the plasma concentration, a concentration of 50 pg/ml being the threshold for detection of these effects. 3 The duration of the effects of iloprost on systemic blood pressure in this study correlates well with the duration of the plasma concentration over 50 pg/ml. In a g r e e m e n t with previous studies, ~3more sustained effects were seen on distal vascular resistance and graft blood flow. The full duration of the beneficial effect on blood flow could not be investigated here due to the invasive nature of the techniques and the need to complete the operation. However, the effects seen were in a g r e e m e n t with previous studies incorporating a placebo control group and in which less invasive, b u t more indirect assessments of blood flow showed an effect in excess of 24 h. 2 The difference b e t w e e n the plasma half-life and the transient reduction in systemic blood pressure on the one h a n d and the sustained effects on distal blood flow on the other suggests that the m e c h a n i s m of this effect is not a simple vasodilation of the distal vasculature. An alternative possibility is that it is microcirculatory in origin, lloprost is k n o w n to reduce capillary endothelial swelling and white cell a c c u m u l a t i o n after ischaemia. 11,12 It m a y be possible to attenuate the effects of ischaemia on the microcirculation in femorodistal bypass surgery even with a short intra-graft administration of iloprost. Studying drug administration during a surgical procedure has inherent problems. The administration of other medications concomitantly, such as anaesthetic agents, is unavoidable. The use of prosthetic material in three of the operations could in t h e o r y also have influenced the results, b u t no consistent differences were evident in these patients. Notwithstanding these limitations of the study, the data show that the time course of the effect
© Pearson Professional Ltd 1997
393
of iloprost on blood pressure correlated well with the plasma concentration, b u t that the effects on graft blood flow and distal resistance in the operated leg outlasted the presence of iloprost at a concentration necessary to give rise to platelet inhibition and vasodilation. Longer infusions of iloprost might be expected to have a more prolonged antithrombotic effect, but the intra-graft injection of 3000 ng produces only transient systemic h a e m o dynamic effects and appears to be sufficient to improve graft blood flow after distal bypass surgery. ACKNOWLEDGEMENT
This study was supported by a financial grant from Schering AG, Berlin.
REFERENCES
1. Shearman C. P., Hickey N. C., Simms M. H. Femorodistal graft flow augmentation with the prostacyclin analogue iloprost. Eur J Vasc Surg 1990; 4: 455-457. 2. Hickey N. C., Shearman C. P., Crowson M. C., Simms M. H., Watson H. R. Iloprost improves femorodistal graft flow after a single bolus injection. EurJ Vasc Surg 1991, 3: 19-22. 3. Kranse W., Krais T. Pharmacokinetics and pharmacodynamics of the prostacyclin analogue iloprost in man. EurJ Clin Pharmacol 1986; 30: 61-68. 4. Krause W., Krais T. Pharmacokinetics and pharmacodynamics of radio-labeled iloprost in elderly volunteers. EurJ Clin Pharmaco11987; 32: 597-605. 5. Hildebrand M., Kranse W., Oberender H. A., Zurdel-Dfllinger S., Junger M., Bodenburg H. Pharmacokinetics of iloprost in patients with severe peripheral arterial occlusive disease. Eicosanoids 1990; 3: 145-148. 6. Hildebrand M., Louton T., Schutt A. Comparison of bioanalytical determinations of iloprost, a chemically stable PGI2 mimetic, by conventional radioimmunoassay (R/A) and scintillation proximity assay (SPA). Eicosanoids 1992; 3: 5-8. 7. Heinzel G., Woloszczak R., Thomann P. TOPFIT 2.0, Stuttgart, Jena, New York: Gustav Fischer Verlag, 1993. 8. Harling H., Lorentzen J. E., Nielsen O. M., Buchardt Hansen H. J. Femorodistal bypass - a worthwhile procedure? EurJ Vasc Surg 1987; 1: 133-135. 9. Horrocks M. Prospective study of 12 month outcome after distal bypass surgery. Abstract: Vascular Surgical Society of Great Britain and Northern Ireland, Edinburgh, UK, November 1994. 10. Cronenstrand R., Ekestrom S. Blood flow after peripheral arterial reconstruction. Scand J Thorac Cardiovasc Surg 1970; 4: 159-171. 11. Thomson I. A., Egginton S., Hudlick~ O., Simms M. H. Iloprost reduces leukocyte adhesion in skeletal muscle venules following ischaemia in a rat model of femorodistal bypass. EurJ Vasc Surg 1994; 8: 335-341. 12. Thomson I. A., Egginton S., Hudlickdt O., Simms M. H. Iloprost reduces calf muscle capillary endothelial swelling during femorodistal bypass grafting. Abstract: Royal Australasian College of Surgeons, Dunedin, NZ, August 1993. 13. Smith F. C. T., Tsang G. M. K., Watson H. R., Shearman C. P. floprost reduces peripheral resistance during femorodistal reconstruction. Eur J Vasc Surg 1992; 6: 194-198.
Prostaglandins, Leukotrienes and Essential Fatty Acids (1997) 56(5), 389-393
Pharmacokinetics and pharmacodynamics of intra.graft iloprost in femorodistal bypass surgery H. R. W a t s o n , ~ F. C. T. Smith, 2 C. P. S h e a r m a n , 2 M. H i l d e b r a n d 3 ~Cardiovascular Clinical Development, Schering AG, D-13342 Berlin, Germany 2Department of Surgery, Queen Elizabeth Medical Centre, Birmingham, UK 3Department of Pharmacokinetics, Schering AG, D-t3342 Berlin, Germany
Summary Intra-graft injection of the prostacyclin analogue, iloprost, was performed at the end of femorodistal bypass procedures in 12 patients with severe peripheral arterial occlusive disease. Iloprost plasma levels were measured and compared with changes in haemodynamics. There was a high initial iloprost plasma level (mean 625 pg/ml) which dropped to a mean of 50 pg/ml after 15 min. This correlated with an immediate reduction in systolic blood pressure which had returned to pretreatment levels after 15 min. In contrast, the vascular resistance distal to the graft showed a reduction after 5 min which was maintained for at least 20 rain after iloprost injection and the mean blood flow through the graft increased steadily throughout the same period of measurement. The study showed an effect of iloprost on blood pressure which correlated with plasma levels, but the time course of the changes in distal vascular resistance and graft blood flow demonstrated an effect more prolonged than the half-life of iloprost.
INTRODUCTION
Iloprost is a chemically stable analogue of prostacyclin which has been proposed as a means of improving graft blood flow after difficult femorodistal bypass surgery. The optimal dose, given directly into the arterial circulation by a slow intra-graft injection, was found to be 3000 ng. 1 This dose has been shown to produce a sustained improvement in graft blood flow.2 The pharmacokinetics of sustained intravenous infusions of iloprost have been studied in healthy volunteers, 3 an elderly population 4 and in patients with severe peripheral arterial disease, s These studies have demonstrated that iloprost has a biphasic plasma half-life with mean a-phases of 3-5 min and mean ~-phases of 15-30 min. Although no human pharmacokinetic studies of intra-arterial iloprost have been performed, these Received 15 October 1996 Accepted 22 October 1996 Correspondence to: H. R. Watson, Tel. 00 49 30 468 4958; Fax. 00 49 30 468 1269
results suggest that haemodynamic effects of a single intra-graft injection of iloprost after bypass surgery should be short-lived. The aim of the present study was to establish the pharmacokinetic parameters of a single injection of 3000 ng iloprost into the graft at the end of femorodistal bypass surgery and to compare the plasma concentration profile with the effects of iloprost on graft blood flow and systemic haemodynamics. METHODS Patients
All patients underwent femorodistal bypass surgery- for severe ischaemia of the lower limb at the Queen Elizabeth Hospital in Birmingham. Patients with abnormal haemostasis or severe liver disease were excluded. Ethical approval for the study was obtained from the Research Ethics Committee of the South Birmingham Health Authority and all patients gave written informed consent prior to entry in the study. 389
390
Watson et al
Surgical procedures and treatment
Patients received a femorodistal bypass graft under general anaesthesia. Grafts consisted of either autogenous vein or a combination of vein and prosthetic material in the form of sequential bypasses via a blind popliteal segment. The bypass grafts all had the proximal anastomosis at the femoral artery bifurcation. The distal anastomoses were divided equally between the tibioperoneal trunk (3 cases), the anterior and posterior tibial arteries (3 cases each) and the peroneal artery (3 cases). Nine patients received vein grafts and three composite grafts of vein plus polytetrafluoroethylene. The prosthetic graft was used to bypass to a blind popliteal segment and a vein graft taken sequentially from that to the distal run off vessel. This technique was used when autologous vein was of poor quality or unavailable. The mean minimum graft diameter was 3.9 mm + 0.3 (range 2.5-6.0 mm). After completion of the distal anastomosis, the arterial clamps were removed and blood flow through the graft was established. This was measured until a stable reading over 10 min was obtained. Fifteen mfllilitres of saline solution containing 3000 ng of iloprost were then infused over a period of 2 min into the proximal end of the graft through a side branch of the vein. All patients were operated upon under general anaesthesia and received the following agents as part of the anaesthetic regimen: an intravenous induction agent (thiopentone, methohexitone or etomidate), an analgesic (fentanyl, alfentanil or papaveretum), a muscle relaxant (atracurium or vecuronium) and an antibiotic (cefuroxime, ciprofloxacin, flucloxacillin, gentamicin, metronidazole or vancomycin).
Pharmacokinetics
Blood samples for the determination of iloprost concentration in the plasma were taken from an antecubital vein. Sampling times were 1, 3, 5, 10, 15, 30, 60 and 120 min after administration of iloprost. For each sample 3-4 ml blood was taken into a heparinised tube and centrifuged at 2200 x g for 10-15 min. Each sample was centrifuged within an hour of sampling. 1.5-2.0 ml of plasma from each sample were stored at -20°C. Iloprost was measured by a sensitive and specific scintillation proximity assay (SPA)? An antiserum raised against iloprost-9-pentynyl BSA. Iloprost-3H-methylester with a specific activity of 50.6 ci/mmol was used as a tracer. SPA analyses were carried out with 0.15 or 0.2 ml plasma adjusted to pH 2 with 1 N HCL and extracted with 2.5 ml diethylether. The organic layer was separated after ethanol/dry ice solidification. The residue was redissolved in 0.25 ml of BSA buffer, 0.21 ml was pipetted and tracer (1600 or 2600cpm) plus anti-rabbit-protein SPA
reagent (Amersham 1:15 000 dilution) suspended in BSA were added. The samples were shaken overnight and measured in a Pharmacia Beta-Plate counter. Calibration samples were obtained with human plasma containing 0-1000pg/ml. Quality control samples with 0, 50, 100 and 200 pg/ml were processed as described above. Pharmacodynamics
Systolic and diastolic blood pressure measurements were recorded via a radial artery cannula immediately before iloprost treatment and monitored thereafter for any effect of the treatment on systemic haemodynamics. Blood flow and mean graft pressure were measured using a Doppler Flowmeter (OpDop 130, SciMed, Bristol, UK) and pressure transducer (Baxter Healthcare Corp., Amsterdam, The Netherlands) connected to a silastic cannula introduced into the graft through an unligated side branch. Peripheral resistance, calculated by the OpDop 130 software from mean graft pressure and blood flow, was expressed in peripheral resistance units (PRU) where 1 PRU = 1 mmHg/ml-1/min-~. All haemodynamic measurements were recorded before and at 5, 10, 15 and 20min after iloprost treatment. Graft patency was confirmed 24 h and 7 days postoperatively using Duplex ultrasound scanning. All adverse medical events occurring during this period were recorded. Statistics and calculations
Pharmacokinetic and pharmacodynamic data are described by means, standard errors and ranges. Pharmacodynamic data were analyzed using paired Student's t-test for within patient changes. Pharmacokinetic data were calculated by TOPFIT7 using an open two compartment model and applying a weighting function of y = 1/x2 to obtain best fit results. The area under the plasma concentration curve (AUC) is conventionally obtained by extrapolating to infinity using the terminal half-life (AUC0_~). Calculations were additionally made of the area under the curve for the period studied (AUCo_t). Total clearance was derived based on AUC0_t. Volumes of distribution were calculated for the central compartment and at steady state. RESULTS
Twelve patients undergoing femorodistal bypass surgery were entered into the study ranging in age from 62 to 79 years (mean 70 years + 2). These included six males and six females. Five were smokers. The mean weight of the patients was 72 kg + 3 kg (range 4 7 - 9 2 kg). All patients were suffering from critical leg ischaemia with either rest pain (11 cases) and/or ulcers (5 cases). They had a mean
Prostaglandins, Leukotrienes and Essential Fatty Acids (1997) 56(5), 389-393
© Pearson Professional Ltd 1997
Pharmacokinetics and pharmacodynamics of intra-graft iloprost in femorodistal bypass surgery
ankle-brachial pressure index of 0.28 _+ 0.06 (range 0-0.59). Two of the patients were diabetic. Pharmacokinetics
Iloprost assays were performed on the plasma samples obtained from eight patients. Technical problems in the storage of plasma samples from the other four cases resulted in thawing and the loss of these data. The pharmacokinetic results from the eight patients satisfactorily completed are summarized in Table 1. The mean floprost plasma concentrations over time are shown in Figure 1. The mean peak plasma concentration was 625 pg/ml, with a considerable variation between the individual values. The mean had fallen to 50 pg/ml 15 min after iloprost administration. The initial distribution phase half-lives ranged from 1.3 to 4.1 rain. The terminal disposition phase half-lives fell into two groups. In four patients values of 6.6 to 11.7 rain were obtained and in the other four patients the values obtained from model-fitting were in the range 79-99 min. The long terminal-phase half-lives calculated in this second group are due to low iloprost levels (< 50 pg/ml) still being detectable in the period between 15 and 120 min. The shorter terminal-phase of 6.6 to 11.7 min is considered more reliable and these are date which are summarized in Table 1. The differences
between AUC values obtained from available data and from model-fitting (Table 1) reflect the long terminal half-lives calculated for some patients. Total clearance was estimated from AUCo_t as 15.8 + 2.3 ml/min/kg. Haemodynamics
Mean systolic and diastolic blood pressures before iloprost were 140 mmHg + 8 and 70 mmHg + 3 respectively. Distinct and statistically significant reductions in systolic and diastolic blood pressure were seen at 5 and 10 min after iloprost administration (Table 2). The pretreatment heart rate of 67 beats/min + 2 showed a small but statistically significant increase at the same time points (Table 2). All of these changes in systemic haemodynamics were transient and no significant effects of iloprost could be seen after 15 min. Initial graft blood flow before iloprost administration was 104 ml/min + 24 with a calculated peripheral resis-
Table 2 Changes in systemic haemodynamics Time after iloprost infusion (min) (n = 12) 5 10 15 30 60 Systolic blood pressure (% change)
mean S.E. minimum maximum P
-18 3 -35 1 0.0001
-13 4 -38 3 0.004
-4 3 -21 22 n.s.
-3 5 -38 25 n.s.
1 4 -19 22 n.s.
Diastolic blood pressure (% change)
mean S.E. minimum maximum P
-19 4 -33 7 0.0002
-14 4 -35 11 0.002
-5 4 -26 15 n.s.
-2 3 -13 16 n.s.
4 5 -19 36 n.s.
Heart rate (% change)
mean S.E. minimum maximum P
4 2 -4 17 0.018
5 10 1 6 0 -5 11 77 0.0006 n.s.
3 3 -10 17 n.s.
20 5 -2 51 0.003
Table 1 Pharmacokinetic parameters Parameter
n
mean
S,E.
01 min (pg/ml) AUCo_t (pg.min/ml) AUC0_= (pg.min/m0 t , ~ (rain) t ~ (min)
8 8 8 8 4 8 8 8
625 3333 7225 1.9 8.4 15.8 0.04 0.25
95 673 963 0.3 1.2 2.3 0.01 0.08
Cldata (ml/min/kg) Vdo (I/kg) Vdss (I/kg)
391
900 j_ Table 3 Changes in peripheral resistance and graft blood flow
800 t E ,~
?_o
700
Time after iloprost infusion (min) 5 10 15 20
600
soo
E °= = o
400
~-
200
300
100
0 ~ 0
~I
i
30
60
90
120
Time (min)
Fig, 1 Plasma concentration of iloprost after 3000 ng intra-graft injection.
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Peripheral resistance (% change)
n mean S.E. minimum maximum P
12 -27 7 -64 7 0.002
12 -30 13 -78 90 0.049
12 -27 12 -80 62 0.040
6 -30 11 -7t 0 0.048
Graft blood flow (% change)
n mean S.E. minimum maximum P
12 32 14 -22 126 0.045
12 58 24 -57 256 0.037
12 63 25 -36 278 0.029
6 78 45 -7 294 0.14
Prostaglandins, Leukotrienes and Essential Fatty Acids (1997) 56(5), 389-393
392
Watson et al
8OO
tance of 1.32 pru + 0.26. The mean peripheral resistance was significantly reduced by a minimum of 27% throughout the period of measurement and the volume blood flow through the graft was significantly increased compared to values immediately prior to iloprost injection (Table 3). The fall in resistance in the distal vascular bed occurred within 5 min and was maintained at a constant level for at least 20 min. The effect on volume blood flow on the other hand, although statistically significant after 5 min, increased steadily over the 20 min for which it was measured. A comparison of changes in floprost plasma concentration with systolic blood pressure, peripheral vascular resistance and graft blood flow (Fig. 2) shows an effect on blood pressure coinciding with iloprost plasma levels while the vascular resistance distal to the graft remains decreased and the volume flow increases even as the plasma concentration falls.
7O0 600.
soo -: 400 • (J
E a-
Seven days postoperatively rest pain had resolved in 10 of 11 patients and healing of an ischaemic ulcer had occurred in one out of five patients. Graft occlusion occurred in one case after 24 h and another after 7 days giving a 7-day primary patency rate of 83%. No surgical complications or other adverse medical events were reported in the first 24 h after surgery. Subsequently one patient developed a tibial wound infection over the graft and another developed a mild hyperglycaemia which was controlled with an oral hypoglycaemic agent. Neither of these two adverse events were attributed to the study drug.
200 100 0
0
o ~
Clinical outcome
300
0 o
.............................................................
-20
09 •Q .
-30
0 o
-10
~
-30
v
"~
-40
D -50
80-
DISCUSSION
70-
As a result of improved techniques in peripheral bypass surgery, procedures in the lower limb are being performed ever more distally. Bypass grafts to the calf vessels and beyond are now regularly performed, but they continue to have a high failure rate, up to 45% at 12 months in some series. 8 The majority of distal bypasses which fail, do so from 6 weeks onwards due to the development of graft stenoses or, in the longer term, progression of atherosclerosis. However, over 20% in a recent large study failed in the first 6 weeks? Technical error is undoubtedly responsible for some immediate graft occlusions, but graft thrombogenicity, resistance in the distal vascular bed and blood flow through the graft also play a role in early graft failures. It has been observed that graft blood flow immediately after completion of the bypass remains very poor in some patients despite patent vessels distal to the graft. This has been termed the 'low-reflow' phenomenon 1° and
60~-
/
50
"a o
~g (.9
30 2O 10 0
............................................................. I
I
I
I
I
0
5
10
15
20
Time (rain) Fig, 2 Comparison of iloprost plasma concentration and haemodynamics after 3000 ng intra-graft injection.
Prostaglandins, Leukotrienes and Essential Fatty Acids (1997) 56(5), 389-393
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Pharmacokinetics and pharmacodynamics of intra-graft iloprost in femorodistal bypass surgery
these patients are t h o u g h t to be at particular risk of graft t h r o m b o s i s in the immediate postoperative period. The origins of this p h e n o m e n o n are not certain, b u t a c c u m u l a t i o n of white cells u and vascular endothelial swelling 12 have b o t h b e e n detected in the distal vascular b e d after partial ischaemia Femorodistal bypass surgery entails a period of partial ischaemia in an already ischaemic circulation, while the m a i n arteries are clamped followed b y reperfusion. It is possible that a severely impaired microcirculation results in some cases leading to difficulty in establishing an adequate blood flow t h r o u g h the graft immediately after surgery. Iloprost has b e e n extensively investigated in peripheral vascular diseases and the results presented here show that iloprost injected intra-arterially at the end of distal bypass surgery has a plasma half-life comparable with that found during intravenous infusions in non-surgical patients. 5 The time-course of platelet aggregation inhibition and vasodilation with iloprost have previously b e e n s h o w n to correlate with the plasma concentration, a concentration of 50 pg/ml being the threshold for detection of these effects. 3 The duration of the effects of iloprost on systemic blood pressure in this study correlates well with the duration of the plasma concentration over 50 pg/ml. In a g r e e m e n t with previous studies, ~3more sustained effects were seen on distal vascular resistance and graft blood flow. The full duration of the beneficial effect on blood flow could not be investigated here due to the invasive nature of the techniques and the need to complete the operation. However, the effects seen were in a g r e e m e n t with previous studies incorporating a placebo control group and in which less invasive, b u t more indirect assessments of blood flow showed an effect in excess of 24 h. 2 The difference b e t w e e n the plasma half-life and the transient reduction in systemic blood pressure on the one h a n d and the sustained effects on distal blood flow on the other suggests that the m e c h a n i s m of this effect is not a simple vasodilation of the distal vasculature. An alternative possibility is that it is microcirculatory in origin, lloprost is k n o w n to reduce capillary endothelial swelling and white cell a c c u m u l a t i o n after ischaemia. 11,12 It m a y be possible to attenuate the effects of ischaemia on the microcirculation in femorodistal bypass surgery even with a short intra-graft administration of iloprost. Studying drug administration during a surgical procedure has inherent problems. The administration of other medications concomitantly, such as anaesthetic agents, is unavoidable. The use of prosthetic material in three of the operations could in t h e o r y also have influenced the results, b u t no consistent differences were evident in these patients. Notwithstanding these limitations of the study, the data show that the time course of the effect
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393
of iloprost on blood pressure correlated well with the plasma concentration, b u t that the effects on graft blood flow and distal resistance in the operated leg outlasted the presence of iloprost at a concentration necessary to give rise to platelet inhibition and vasodilation. Longer infusions of iloprost might be expected to have a more prolonged antithrombotic effect, but the intra-graft injection of 3000 ng produces only transient systemic h a e m o dynamic effects and appears to be sufficient to improve graft blood flow after distal bypass surgery. ACKNOWLEDGEMENT
This study was supported by a financial grant from Schering AG, Berlin.
REFERENCES
1. Shearman C. P., Hickey N. C., Simms M. H. Femorodistal graft flow augmentation with the prostacyclin analogue iloprost. Eur J Vasc Surg 1990; 4: 455-457. 2. Hickey N. C., Shearman C. P., Crowson M. C., Simms M. H., Watson H. R. Iloprost improves femorodistal graft flow after a single bolus injection. EurJ Vasc Surg 1991, 3: 19-22. 3. Kranse W., Krais T. Pharmacokinetics and pharmacodynamics of the prostacyclin analogue iloprost in man. EurJ Clin Pharmacol 1986; 30: 61-68. 4. Krause W., Krais T. Pharmacokinetics and pharmacodynamics of radio-labeled iloprost in elderly volunteers. EurJ Clin Pharmaco11987; 32: 597-605. 5. Hildebrand M., Kranse W., Oberender H. A., Zurdel-Dfllinger S., Junger M., Bodenburg H. Pharmacokinetics of iloprost in patients with severe peripheral arterial occlusive disease. Eicosanoids 1990; 3: 145-148. 6. Hildebrand M., Louton T., Schutt A. Comparison of bioanalytical determinations of iloprost, a chemically stable PGI2 mimetic, by conventional radioimmunoassay (R/A) and scintillation proximity assay (SPA). Eicosanoids 1992; 3: 5-8. 7. Heinzel G., Woloszczak R., Thomann P. TOPFIT 2.0, Stuttgart, Jena, New York: Gustav Fischer Verlag, 1993. 8. Harling H., Lorentzen J. E., Nielsen O. M., Buchardt Hansen H. J. Femorodistal bypass - a worthwhile procedure? EurJ Vasc Surg 1987; 1: 133-135. 9. Horrocks M. Prospective study of 12 month outcome after distal bypass surgery. Abstract: Vascular Surgical Society of Great Britain and Northern Ireland, Edinburgh, UK, November 1994. 10. Cronenstrand R., Ekestrom S. Blood flow after peripheral arterial reconstruction. Scand J Thorac Cardiovasc Surg 1970; 4: 159-171. 11. Thomson I. A., Egginton S., Hudlick~ O., Simms M. H. Iloprost reduces leukocyte adhesion in skeletal muscle venules following ischaemia in a rat model of femorodistal bypass. EurJ Vasc Surg 1994; 8: 335-341. 12. Thomson I. A., Egginton S., Hudlickdt O., Simms M. H. Iloprost reduces calf muscle capillary endothelial swelling during femorodistal bypass grafting. Abstract: Royal Australasian College of Surgeons, Dunedin, NZ, August 1993. 13. Smith F. C. T., Tsang G. M. K., Watson H. R., Shearman C. P. floprost reduces peripheral resistance during femorodistal reconstruction. Eur J Vasc Surg 1992; 6: 194-198.
Prostaglandins, Leukotrienes and Essential Fatty Acids (1997) 56(5), 389-393