Absence of an interaction between the synthetic pentasaccharide fondaparinux and oral warfarin

Abstract

Aims

To investigate the pharmacokinetic and pharmacodynamic interaction of the antithrombotic pentasaccharide fondaparinux (Org31540/SR90107A), given subcutaneously, and oral warfarin in healthy subjects.

Methods

This study was performed according to a randomised, three-way cross-over, placebo-controlled, double-blind design in 12 healthy male subjects. The treatment consisted of five subcutaneous (s.c.) injections of fondaparinux (4 mg) or placebo at 24 h intervals. Oral dosing of warfarin or placebo was added to the fourth (15 mg) and fifth (10 mg) s.c. injection. Blood samples for pentasaccharide assay, PT and APTT were drawn before the first s.c. dose of the pentasaccharide and over a 6 day period thereafter.

Results

Fondaparinux administered to healthy male volunteers alone or in combination with oral warfarin was well tolerated and no serious adverse events were observed. No differences were found in the AUC (43 vs 44 mg l⁻¹ h), C_max (645 vs 678 ng ml⁻¹) or elimination half-life (13.8 vs 14.1 h) of fondaparinux between the pentasaccharide-only and the combination treatment. The effect of warfarin on PT (mean maximal increase: 8.2 s.) was not influenced by the presence of the pentasaccharide (mean maximal increase in PT: 9.1 s.). After all treatments a small rise in APTT was seen. No further differences could be detected in the pharmacodynamic parameters following the three treatments.

Conclusions

The coadministration of warfarin did not influence the pharmacokinetics of fondaparinux in healthy subjects. PT can still be used to monitor the effect of oral anticoagulants during the switch from antithrombotic treatment with pentasaccharide to full oral anticoagulant therapy.

Introduction

The pentasaccharide fondaparinux (formerly known as Org 31540/SR90107A) is a fully chemically synthesized antithrombotic agent. The chemical structure of the compound is identical to that of the antithrombin (AT) binding domain of heparin. The drug selectively inhibits factor Xa, is devoid of antithrombin activity and does not affect platelet function. As a consequence the drug exhibits a low bleeding tendency, as demonstrated in vitro and in vivo [1–3].

Assessment of the safety and efficacy of fondaparinux in the prophylaxis and treatment of deep venous thrombosis is an integral part of the clinical development of the compound [4]. Simultaneous administration of subcutaneously (s.c.) administered pentasaccharide and oral anticoagulants can be anticipated in clinical practice. The change from pentasaccharide to oral anticoagulants would normally occur using a loading dose of oral anticoagulants followed by regular prothrombin time (PT) checks after 48–72 h. Combination treatment could potentially effect this measurement and lead to erroneous dose adjustment of the warfarin.

The objectives of this study were to evaluate the possible pharmacokinetic and pharmacodynamic interaction between s.c. administered fondaparinux and orally administered warfarin.

Methods

Subjects and design

The Ethics Committee of Leiden University Medical Center approved the protocol. The study was conducted according to a randomized, three-way cross-over, placebo-controlled double-blind study design. Wash-out between the study periods was 2 weeks. Twelve healthy males with a normal coagulation screen (age: 19–27 years, body weight: 61–87 kg) participated in this study after written informed consent was obtained.

Treatments

Subjects received the following treatments during the study: pentasaccharide (Penta-only), oral warfarin (Warf-only) or the combination (Penta + Warf). The Penta-only treatment consisted of five 4 mg sc injections of fondaparinux (Arixta^R) at 24 h intervals and oral placebo tablets concomitant with the 4th and 5th injection. The Warf-only treatment consisted of five s.c. placebo (0.9% saline) injections at 24 h. To the 4th dose 15 mg of oral warfarin sodium (Coumadin^R) was added and together with the 5th injection another 10 mg of warfarin was given. During the combination treatment the subjects received five s.c. injections of the pentasaccharide and oral warfarin as indicated for the mono-treatment. All subjects received a single oral dose of 10 mg of vitamin K (Konakion^R) at 132 h after the first s.c. injection, to reverse any residual warfarin effect (see Figure 1 for a schematic overview of the study design).

Figure 1.

Schematic overview of the study design; closed symbols indicate active treatment pentasaccharide (arrows) or warfarin (circles) while open symbols indicate corresponding placebo treatment. K indicates vit. K administration.

Subjects were studied after an overnight fast. They were admitted to the research unit on the evening before first administration of the pentasaccharide and remained there for 12 h after each s.c. injection. The subjects stayed in the unit from the fourth s.c. injection and first oral administration until at least 36 h after the fifth s.c. injection.

Blood sampling

Blood was collected without the use of a tourniquet from iv cannula which was kept patent by intermittent flushing with of 0.9% saline. Blood samples for baseline values were taken predose. Blood samples for drug-assay, PT and APTT were drawn at regular time-intervals until 144 h after the first s.c. dose of the pentasaccharide. Blood samples for routine haematology and serum biochemistry were taken predose and at the end of each study period.

Laboratory tests

Anti-factor Xa activity in plasma (reflecting the pentasaccharide concentration) was assessed using a validated amidolytic photometric assay at the Department of Drug Metabolism and Kinetics of NV Organon. The APTT/PT assays were performed, using standard procedures with a STA coagulation analyser (Boehringer Mannheim, Diagnostica Stago) with the reagents provided by the manufacturer.

Statistical analysis

The drug-concentration time profiles were analysed using noncompartmental pharmacokinetic techniques with the WinNonlin program (Version 1.1, Scientific Consulting, Inc.). Points included for calculation of terminal half-life were automatically determined by the program and visually checked for adequacy. The following parameters were derived: the AUC up to the last measurable concentration using the linear trapezoidal rule (AUC(0,last)), the AUC extrapolated to infinity (AUC(0,∞)), terminal half-life (t_1/2), the clearance/F (calculated using AUC(0,∞) and assuming a total dose of 20 mg fondaparinux), the AUC from 72 h (from the first administration of warfarin) up to the last measurable concentration (AUC(72 h,last)), the peak concentration (C_max) and the time to reach peak concentration (t_max). The last two parameters were calculated relative to the final s.c. administration.

APTT and PT were analysed using both the uncorrected area under the effect curve (AUEC) and on basis of baseline-corrected data (AUEC above average prevalue) divided by the corresponding time span. This prevalue correction results in a weighted average increase above baseline for which 95% confidence intervals were calculated to test whether a significant increase from baseline occurred. Maximal effect (E_max) and the time to reach this maximal effect (t_max) on the basis of observed data were calculated for APTT and PT (only for treatments with Warf-only and Penta + Warf).

All contrasts were calculated using paired t-tests on untransformed measures except for AUC and C_max parameters that were log-transformed. Log-transformed contrasts were back-transformed, resulting in parameters that can be interpreted as percentage increase due to the addition of warfarin along with 95% confidence intervals. Statistical analysis and calculations were performed using SPSS for Windows V6.1 (SPSS, Inc., Chicago, IL).

Results

All subjects completed the study without serious adverse events. Two subjects reported mild gastrointestinal complaints after intake of placebo-tablets. Mild headaches were reported by four subjects (a total of six episodes) and three subjects reported an episode of malaise. Minor haematomas around venipuncture sites were reported for four subjects. One subject reported a bleeding episode of approximately 10 min, after he cut himself while shaving and one subject had a spontaneous nose-bleed in between two occasions.

Pharmacokinetics

The mean plasma concentration-time curves for the treatments are shown in Figure 2. No statistically significant difference between Penta-only and the combination treatment was found. Co-administration of warfarin led to a nonsignificant increase in C_max of 5.1% (95% CI: −1.1, + 11.7%), AUC(0,∞) of 2.5% (−1.1, + 6.2%) and AUC(72,last) of 2.8% (−2.8, + 8.7%).

Figure 2.

Mean pentasaccharide plasma concentration-time curves (a), mean PT values (b) and mean APTT values (c) per treatment (▪ pentasaccharide; ▴ pentasaccharide plus warfarin; • warfarin). The s.c. pentasaccharide injections and oral warfarin adminstrations are indicated.The arrow indicates the timepoint at which vitamin K was administered.

Pharmacodynamics

The mean PT-time curves for the different treatments are shown in Figure 2. The treatment with Warf-only as well as Penta + Warf resulted in a statistically significant increase in AUEC and E_max compared with the Penta-only treatment. The difference was 2.6 s (95% CI: + 1.9, + 3.3 s) and 2.9 s (+ 1.8, + 3.9 s), respectively, for the AUECs and 8.2 s (+ 3.6, + 12.7 s) and 9.1 s (+ 4.0, + 14.2 s) for E_max. No statistically significant difference was found between the Warf-only treatment and the combination treatment for AUEC (both with and without prevalue correction), E_max and t_max.

The APTT increased above baseline after Penta-only (mean increase: 3.1 s; + 0.7, + 5.5 s) and after Warf-only (3.2 s; + 0.3, + 6.2 s). The combination treatment resulted in a greater increase (5.0 s; 95% CI: 4.0, 5.9), which however, was not significantly higher than the summed increase after single drug treatment (Figure 2). This suggests a possible additive effect of both drugs on APTT.

Discussion

The objective of this study was to investigate the possible pharmacokinetic and pharmacodynamic interaction of s.c. administered pentasaccharide and oral warfarin in healthy male volunteers. For warfarin a loading dose regimen was chosen to avoid the cumbersome procedure of attaining stable oral anticoagulant levels in healthy volunteers. It is recognized that with this dose regimen a full suppression of all vitamin K-dependent clotting factors is not reached. However, it is reflective of the clinical state during which pentasaccharides and oral anticoagulants will be administered concomitantly. This is of particular importance for the effects of the combination of the two drugs on the PT at the time the switch from pentasaccharide treatment to oral anticoagulants occurs clinically. In addition, previous studies using a single loading dose of 25 mg of warfarin have been proven to be provide useful information on this relevant issue [5–8].

The pharmacokinetic profile and the estimated parameters of pentasaccharide given in combination with warfarin were identical to that of pentasaccharide alone. Hence, no pharmacokinetic interaction was observed.

PT is the most commonly used test to monitor orally administered anticoagulants [9]. In this study the PT did not change from baseline with the pentasaccharide-only treatment. Treatment with warfarin alone or in combination with the pentasaccharide prolonged PT as expected. However, the presence of the pentasaccharide did not influence the effect of warfarin on the PT. It can thus be concluded that PT can remain to be used to monitor the effect of oral anticoagulants during the switch from anticoagulant treatment with pentasaccharide to oral anticoagulant therapy.

The administration of pentasaccharide alone or warfarin alone was followed by a small (approximately 3 s) rise in APTT. The increase in APTT after the combination treatment was only slightly more than the APTT-increase after the monotherapy.

In conclusion, concomitant oral administration of warfarin does not affect the pharmacokinetics of the novel pentasaccharide fondaparinux in healthy volunteers. In addition, warfarin-induced increase in PT (INR) was not influenced by concomitant subcutaneous pentasaccharide treatment. Accordingly, if this finding can be extrapolated to patients, PT can safely be used to monitor the effect of warfarin during the switch from pentasaccharide treatment to warfarin.

This study was sponsored by NV Organon, Oss, The Netherlands.