Nagy A. Farid

Common polymorphisms of CYP2C19 and CYP2C9 affect the pharmacokinetic and pharmacodynamic response to clopidogrel but not prasugrel

Summary.  Background: Thienopyridines are metabolized to active metabolites that irreversibly inhibit the platelet P2Y12 adenosine diphosphate receptor. The pharmacodynamic response to clopidogrel is more variable than the response to prasugrel, but the reasons for variation in response to clopidogrel are not well characterized.

Identification of the Human Cytochrome P450 Enzymes Involved in the Two Oxidative Steps in the Bioactivation of Clopidogrel to Its Pharmacologically Active Metabolite

The aim of the current study is to identify the human cytochrome P450 (P450) isoforms involved in the two oxidative steps in the bioactivation of clopidogrel to its pharmacologically active metabolite. In the in vitro experiments using cDNA-expressed human P450 isoforms, clopidogrel was metabolized to 2-oxo-clopidogrel, the immediate precursor of its pharmacologically active metabolite. CYP1A2, CYP2B6, and CYP2C19 catalyzed this reaction. In the same system using 2-oxo-clopidogrel as the substrate, detection of the active metabolite of clopidogrel required the addition of glutathione to the system. CYP2B6, CYP2C9, CYP2C19, and CYP3A4 contributed to the production of the active metabolite. Secondly, the contribution of each P450 involved in both oxidative steps was estimated by using enzyme kinetic parameters. The contribution of CYP1A2, CYP2B6, and CYP2C19 to the formation of 2-oxo-clopidogrel was 35.8, 19.4, and 44.9%, respectively. The contribution of CYP2B6, CYP2C9, CYP2C19, and CYP3A4 to the formation of the active metabolite was 32.9, 6.76, 20.6, and 39.8%, respectively. In the inhibition studies with antibodies and selective chemical inhibitors to P450s, the outcomes obtained by inhibition studies were consistent with the results of P450 contributions in each oxidative step. These studies showed that CYP2C19 contributed substantially to both oxidative steps required in the formation of clopidogrel active metabolite and that CYP3A4 contributed substantially to the second oxidative step. These results help explain the role of genetic polymorphism of CYP2C19 and also the effect of potent CYP3A inhibitors on the pharmacokinetics and pharmacodynamics of clopidogrel in humans and on clinical outcomes.

Effects of the Proton Pump Inhibitor Lansoprazole on the Pharmacokinetics and Pharmacodynamics of Prasugrel and Clopidogrel

Prasugrel and clopidogrel, thienopyridine prodrugs, are each metabolized to an active metabolite that inhibits the platelet P2Y12 ADP receptor. In this open‐label, 4‐period crossover study, the effects of the proton pump inhibitor lansoprazole on the pharmacokinetics and pharmacodynamics of prasugrel and clopidogrel were assessed in healthy subjects given single doses of prasugrel 60 mg and clopidogrel 300 mg with and without concurrent lansoprazole 30 mg qd. Cmax and AUC0‐tlast of prasugrels active metabolite, R‐138727, and clopidogrels inactive carboxylic acid metabolite, SR26334, were assessed. Inhibition of platelet aggregation (IPA) was measured by turbidimetric aggregometry 4 to 24 hours after each treatment. Lansoprazole (1) decreased R‐138727 AUC0‐tlast and Cmax by 13% and 29%, respectively, but did not affect IPA after the prasugrel dose, and (2) did not affect SR62334 exposure but tended to lower IPA after a clopidogrel dose. A retrospective tertile analysis showed in subjects with high IPA after a clopidogrel dose alone that lansoprazole decreased IPA, whereas IPA was unaffected in these same subjects after a prasugrel dose. The overall data suggest that a prasugrel dose adjustment is not likely warranted in an individual taking prasugrel with a proton pump inhibitor such as lansoprazole.

Cytochrome P450 3A Inhibition by Ketoconazole Affects Prasugrel and Clopidogrel Pharmacokinetics and Pharmacodynamics Differently

Prasugrel and clopidogrel inhibit platelet aggregation through active metabolite formation. Prasugrels active metabolite (R‐138727) is formed primarily by cytochrome P450 (CYP) 3A and CYP2B6, with roles for CYP2C9 and CYP2C19. Clopidogrels activation involves two sequential steps by CYP3A, CYP1A2, CYP2C9, CYP2C19, and/or CYP2B6. In a randomized crossover study, healthy subjects received a loading dose (LD) of prasugrel (60 mg) or clopidogrel (300 mg), followed by five daily maintenance doses (MDs) (15 and 75 mg, respectively) with or without the potent CYP3A inhibitor ketoconazole (400 mg/day). Subjects had a 2‐week washout between periods. Ketoconazole decreased R‐138727 and clopidogrel active metabolite Cmax (maximum plasma concentration) 34–61% after prasugrel and clopidogrel dosing. Ketoconazole did not affect R‐138727 exposure or prasugrels inhibition of platelet aggregation (IPA). Ketoconazole decreased clopidogrels active metabolite AUC0–24 (area under the concentration–time curve to 24 h postdose) 22% (LD) to 29% (MD) and reduced IPA 28% (LD) to 33% (MD). We conclude that CYP3A4 and CYP3A5 inhibition by ketoconazole affects formation of clopidogrels but not prasugrels active metabolite. The decreased formation of clopidogrels active metabolite is associated with reduced IPA.

Metabolism and Disposition of the Thienopyridine Antiplatelet Drugs Ticlopidine, Clopidogrel, and Prasugrel in Humans

Ticlopidine, clopidogrel, and prasugrel are thienopyridine prodrugs that inhibit adenosine‐5′‐diphosphate (ADP)‐mediated platelet aggregation in vivo. These compounds are converted to thiol‐containing active metabolites through a corresponding thiolactone. The 3 compounds differ in their metabolic pathways to their active metabolites in humans. Whereas ticlopidine and clopidogrel are metabolized to their thiolactones in the liver by cytochromes P450, prasugrel proceeds to its thiolactone following hydrolysis by carboxylesterase 2 during absorption, and a portion of prasugrels active metabolite is also formed by intestinal CYP3A. Both ticlopidine and clopidogrel are subject to major competing metabolic pathways to inactive metabolites. Thus, varying efficiencies in the formation of active metabolites affect observed effects on the onset of action and extent of inhibition of platelet aggregation (IPA). Knowledge of the CYP‐dependent formation of ticlopidine and clopidogrel thiolactones helps explain some of the observed drug‐drug interactions with these molecules and, more important, the role of CYP2C19 genetic polymorphism on the pharmacokinetics of and pharmacodynamic response to clopidogrel. The lack of drug interaction potential and the absence of CYP2C19 genetic effect result in a predictable response to thienopyridine antiplatelet therapy with prasugrel. Current literature shows that greater ADP‐mediated IPA is associated with significantly better clinical outcomes for patients with acute coronary syndrome.

Interactions of two major metabolites of prasugrel, a thienopyridine antiplatelet agent, with the cytochromes P450.

The biotransformation of prasugrel to R-138727 (2-[1-2-cyclopropyl-1-(2-fluorophenyl)-2-oxoethyl]-4-mercapto-3-piperidinylidene]acetic acid) involves rapid deesterification to R-95913 (2-[2-oxo-6,7-dihydrothieno[3,2-c]pyridin-5(4H)-yl]-1-cyclopropyl-2-(2-fluorophenyl)ethanone) followed by cytochrome P450 (P450)-mediated formation of R-138727, the metabolite responsible for platelet aggregation. For identification of the P450s responsible for the formation of the active metabolite, the current studies were conducted with R-95913 as the substrate. Incubations required supplementation with reduced glutathione. Hyperbolic kinetics (Km 21–30 μM), consistent with a single enzyme predominating, were observed after incubations with human liver microsomes. Correlation analyses revealed a strong relationship between R-138727 formation and CYP3A-mediated midazolam 1′-hydroxylation (r2 = 0.98; p < 0.001) in a bank of characterized human liver microsomal samples. The human lymphoblast-expressed enzymes capable of forming R-138727, in rank order of rates, were CYP3A4>CYP2B6>CYP2C19≈CYP2C9>CYP2D6. A monoclonal antibody to CYP2B6 and the CYP3A inhibitor ketoconazole substantially inhibited R-138727 formation, whereas inhibitors of CYP2C9 (sulfaphenazole) and CYP2C19 (omeprazole) did not. Scaling of in vitro intrinsic clearance values from expressed enzymes to the whole liver using a relative abundance approach indicated that either CYP3A4 alone or CYP3A4 and CYP2B6 are the major contributors to R-138727 formation. R-95913 and R-138727 were also examined for their ability to inhibit metabolism mediated by five P450s. R-138727 did not inhibit the P450s tested. In vitro, R-95913 inhibited CYP2C9, CYP2C19, CYP2D6, and CYP3A, with Ki values ranging from 7.2 μM to 82 μM, but did not inhibit CYP1A2. These Ki values exceed circulating concentrations in humans by 3.8- to 43-fold. Therefore, neither R-95913 nor R-138727 is expected to substantially inhibit the P450-mediated metabolism of coadministered drugs.

The Disposition of Prasugrel, a Novel Thienopyridine, in Humans

Prasugrel, a prodrug, is a novel and potent inhibitor of platelet aggregation in vivo. The metabolism of prasugrel and the elimination and pharmacokinetics of its active metabolite, 2-[1-[2-cyclopropyl-1-(2-fluorophenyl)-2-oxoethyl]-4-mercapto-3-piperidinylidene]acetic acid (R-138727), three inactive metabolites, and radioactivity were determined in five healthy male subjects after a single 15-mg (100 μCi) p.o. dose of [14C]prasugrel. Prasugrel was rapidly absorbed, and maximum plasma concentrations of radioactivity and R-138727 were achieved in 30 min, indicating rapid formation of R-138727. Prasugrel was extensively metabolized in humans, first by hydrolysis to a thiolactone, followed by ring opening to form R-138727, which was further metabolized by S-methylation and conjugation with cysteine. Total radioactivity was higher in plasma than in blood, suggesting limited penetration of prasugrel metabolites into red blood cells. Approximately 70% of the dose was excreted in the urine and 25% in the feces.

Increased active metabolite formation explains the greater platelet inhibition with prasugrel compared to high-dose clopidogrel.

Prasugrel pharmacodynamics and pharmacokinetics after a 60-mg loading dose (LD) and daily 10-mg maintenance doses (MD) were compared in a 3-way crossover study to clopidogrel 600-mg/75-mg and 300-mg/75-mg LD/MD in 41 healthy, aspirin-free subjects. Each LD was followed by 7 days of daily MD and a 14-day washout period. Inhibition of platelet aggregation (IPA) was assessed by turbidometric aggregometry (20 and 5 μM ADP). Prasugrel 60-mg achieved higher mean IPA (54%) 30 minutes post-LD than clopidogrel 300-mg (3%) or 600-mg (6%) (P < 0.001) and greater IPA by 1 hour (82%) and 2 hours (91%) than the 6-hour IPA for clopidogrel 300-mg (51%) or 600-mg (69%) (P < 0.01). During MD, IPA for prasugrel 10-mg (78%) exceeded that of clopidogrel (300-mg/75-mg, 56%; 600-mg/75-mg, 52%; P < 0.001). Active metabolite area under the concentration-time curve (AUC0-tlast) after prasugrel 60-mg (594 ng·hr/mL) was 2.2 times that after clopidogrel 600-mg. Prasugrel active metabolite AUC0-tlast was consistent with dose-proportionality from 10-mg to 60-mg, while clopidogrel active metabolite AUC0-tlast exhibited saturable absorption and/or metabolism. In conclusion, greater exposure to prasugrels active metabolite results in faster onset, higher levels, and less variability of platelet inhibition compared with high-dose clopidogrel in healthy subjects.

The use of the VerifyNow P2Y12 point-of-care device to monitor platelet function across a range of P2Y12 inhibition levels following prasugrel and clopidogrel administration

Variability in response to antiplatelet agents has prompted the development of point-of-care (POC) technology. In this study, we compared the VerifyNow P2Y12 (VN-P2Y12) POC device with light transmission aggregometry (LTA) in subjects switched directly from clopidogrel to prasugrel. Healthy subjects on aspirin were administered a clopidogrel 600 mg loading dose (LD) followed by a 75 mg/d maintenance dose (MD) for 10 days. Subjects were then switched to a prasugrel 60 mg LD and then 10 mg/d MD for 10 days (n = 16), or to a prasugrel 10 mg/d MD for 11 days (n = 19). Platelet function was measured by LTA and VN-P2Y12 at baseline and after dosing. Clopidogrel 600 mg LD/75 mg MD treatment led to a reduction in P2Y(12) reaction units (PRU) from baseline. A switch from clopidogrel MD to prasugrel 60 mg LD/10 mg MD produced an immediate decrease in PRU, while a switch to prasugrel 10 mg MD resulted in a more gradual decline. Consistent with the reduction in PRU, device-reported percent inhibition increased during both clopidogrel and prasugrel regimens. Inhibition of platelet aggregation as measured by LTA showed a very similar pattern to that found with VN-P2Y12 measurement, irrespective of treatment regimens. The dynamic range of VN-P2Y12 appeared to be narrower than that of LTA. With two different thienopyridines, the VN-P2Y12 device, within a somewhat more limited range, reflected the overall magnitude of change in aggregation response determined by LTA. The determination of the clinical utility of such POC devices will require their use in clinical outcome studies.

A Possible Mechanism for the Differences in Efficiency and Variability of Active Metabolite Formation from Thienopyridine Antiplatelet Agents, Prasugrel and Clopidogrel

The efficiency and interindividual variability in bioactivation of prasugrel and clopidogrel were quantitatively compared and the mechanisms involved were elucidated using 20 individual human liver microsomes. Prasugrel and clopidogrel are converted to their thiol-containing active metabolites through corresponding thiolactone metabolites. The formation rate of clopidogrel active metabolite was much lower and more variable [0.164 ± 0.196 μl/min/mg protein, coefficient of variation (CV) = 120%] compared with the formation of prasugrel active metabolite (8.68 ± 6.64 μl/min/mg protein, CV = 76%). This result was most likely attributable to the less efficient and less consistent formation of clopidogrel thiolactone metabolite (2.24 ± 1.00 μl/min/mg protein, CV = 45%) compared with the formation of prasugrel thiolactone metabolite (55.2 ± 15.4 μl/min/mg protein, CV = 28%). These differences may be attributed to the following factors. Clopidogrel was largely hydrolyzed to an inactive acid metabolite (approximately 90% of total metabolites analyzed), and the clopidogrel concentrations consumed were correlated to human carboxylesterase 1 activity in each source of liver microsomes. In addition, 48% of the clopidogrel thiolactone metabolite formed was converted to an inactive thiolactone acid metabolite. The oxidation of clopidogrel to its thiolactone metabolite correlated with variable activities of CYP1A2, CYP2B6, and CYP2C19. In conclusion, the active metabolite of clopidogrel was formed with less efficiency and higher variability than that of prasugrel. This difference in thiolactone formation was attributed to hydrolysis of clopidogrel and its thiolactone metabolite to inactive acid metabolites and to variability in cytochrome P450-mediated oxidation of clopidogrel to its thiolactone metabolite, which may contribute to the poorer and more variable active metabolite formation for clopidogrel than prasugrel.