Effect of Intrinsic and Extrinsic Factors on the Clinical Pharmacokinetics and Pharmacodynamics of Prasugrel

Thienopyridines are inactive prodrugs that are converted in vivo to active metabolites, which irreversibly bind to and inactivate platelet P2Y₁₂ receptors, and inhibit platelet activation and aggregation. Prasugrel is a third-generation thienopyridine, recently approved for prevention of thrombotic cardiovascular complications in patients with an acute coronary syndrome undergoing percutaneous coronary intervention. Prasugrel is converted to its active metabolite (Pras-AM; compound R-138727) in two sequential steps: (i) rapid and complete hydrolysis by intestinal human carboxylesterase-2 to form a thiolactone intermediate; and (ii) oxidation of the thiolactone by cytochrome P450 (CYP) enzymes in the gut and/or the liver. CYP3A and CYP2B6 are the primary CYPs contributing to Pras-AM formation, with smaller contributions from CYP2C9 and CYP2C19. Prasugrel is rapidly absorbed and metabolized, with Pras-AM plasma concentrations peaking at about 0.5 hours after oral administration; this helps to account for the rapid onset of inhibition of platelet aggregation (IPA) achieved by prasugrel.

In the clinical pharmacology programme for prasugrel, bodyweight had the greatest effect of all covariates that were tested. In the phase III TRITON-TIMI 38 trial, the mean exposure to Pras-AM was 42% greater in patients weighing <60kg than in patients with the study population median bodyweight of 85 kg. In a pharmacodynamic meta-analysis of data from healthy subjects a decrease of 1 kg in bodyweight was associated with an increase in IPA of approximately 0.26 percentage points (p< 0.0001). Pras-AM exposure was greater in subjects aged ≥75 years, but exposure differences were not as large as those for bodyweight. Pras-AM exposure was greater in Asians than in Caucasians, but this appeared to result from a disproportionately greater exposure difference in Asian subjects with low bodyweight. Sex and allelic variation in CYPs 1A2, 2B6, 2C19, 2C9, 3A4 and 3A5 appeared to have no clinically relevant effect on Pras-AM exposure or IPA. Consistent with the lack of association between genetic status and these pharmacokinetic and pharmacodynamic results in healthy subjects, no significant association was detected between these allelic variants and the composite primary endpoint (cardiovascular death, non-fatal myocardial infarction or non-fatal stroke) in the TRITON-TIMI 38 trial. Studies in renally impaired subjects and subjects with mild or moderate hepatic impairment have indicated that dose adjustment is not required in these patient populations.

Prasugrel has few clinically significant drug-drug interactions. Potent CYP3A inhibitors, gastric acid suppressants and food have been shown to reduce the rate of formation of Pras-AM but not its overall exposure. This pharmacokinetic effect reduced the rate of onset of IPA after a loading dose but did not affect the peak IPA after a loading dose or the IPA during maintenance dosing. Potent induction of CYP3A, as well as smoking - which induces CYP1A2 — did not affect Pras-AM exposure or IPA. Prior treatment with clopidogrel did not influence tolerability to prasugrel and did not appear to alter IPA during prasugrel treatment. Prasugrel did not affect the activities of CYP2C9, CYP2C19 or P-glycoprotein, but it weakly inhibited CYP2B6. The inhibition of CYP2B6 is potentially clinically significant only for drugs that have a narrow therapeutic window and have CYP2B6 as the primary elimination pathway. No interaction was detected between prasugrel and heparin. Although prasugrel did not alter warfarin pharmacokinetics, prasugrel and warfarin should not be used together, because of an increased bleeding risk associated with their concomitant use.