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Metabolism and Disposition of the Thienopyridine Antiplatelet Drugs Ticlopidine, Clopidogrel, and Prasugrel in Humans Nagy A. Farid, Atsushi Kurihara and Steven A. Wrighton J. Clin. Pharmacol. 2010; 50; 126 originally published online Nov 30, 2009; DOI: 10.1177/0091270009343005

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

Nagy A. Farid, PhD, Atsushi Kurihara, PhD, and Steven A. Wrighton, PhD

Ticlopidine, clopidogrel, and prasugrel are thienopyridine and clopidogrel thiolactones helps explain some of the prodrugs that inhibit adenosine-5′-diphosphate (ADP)– observed drug-drug interactions with these molecules and, mediated platelet aggregation in vivo. These compounds more important, the role of CYP2C19 genetic polymor- are converted to thiol-containing active metabolites through phism on the pharmacokinetics of and pharmacodynamic a corresponding thiolactone. The 3 compounds differ in response to clopidogrel. The lack of drug interaction poten- their metabolic pathways to their active metabolites in tial and the absence of CYP2C19 genetic effect result in a humans. Whereas ticlopidine and clopidogrel are metabo- predictable response to thienopyridine antiplatelet therapy lized to their thiolactones in the liver by cytochromes P450, with prasugrel. Current literature shows that greater ADP- prasugrel proceeds to its thiolactone following hydrolysis mediated IPA is associated with significantly better clinical by carboxylesterase 2 during absorption, and a portion of outcomes for patients with acute coronary syndrome. prasugrel’s active metabolite is also formed by intestinal CYP3A. Both ticlopidine and clopidogrel are subject to Keywords: Clopidogrel; drug interaction; genetic poly- major competing metabolic pathways to inactive metabo- morphism; metabolism; pharmacokinetics; lites. Thus, varying efficiencies in the formation of active platelet aggregation; prasugrel; thienopyri- metabolites affect observed effects on the onset of action dine; ticlopidine and extent of inhibition of platelet aggregation (IPA). Journal of Clinical Pharmacology, 2010;50:126-142 Knowledge of the CYP-dependent formation of ticlopidine © 2010 the American College of Clinical Pharmacology

latelet activation and aggregation play important periprocedural pharmacologic intervention to reduce Proles in occlusive vascular events. Release of the risk of postprocedural thrombosis. On the basis adenosine-5′-diphosphate (ADP) from activated of conclusive data showing the benefit of optimal platelets is one of the primary mediators of platelet platelet inhibition on cardiovascular risk, the aggregation, leading to a sustained response via acti- American Heart Association/American College of vation of P2Y12 receptors. Inhibition of platelet Cardiology guidelines recommend that all patients aggregation with a combination of aspirin and a undergoing PCI receive antiplatelet therapy before thienopyridine antiplatelet drug is an important strat- and after the procedure.2 Because most PCI proce- egy for preventing ischemic events in patients with dures employ stenting, these guidelines recommend acute coronary syndrome (ACS), including those the dual (aspirin and clopidogrel) therapy for at least undergoing percutaneous coronary intervention (PCI).1 1 month after placement of a bare metal stent and for PCI is a common procedure to treat ACS and requires 12 months after placement of a drug-eluting stent (DES). Current European guidelines recommend that patients with non-ST segment myocardial infarction From Eli Lilly and Company, Indianapolis, Indiana (Dr Farid, Dr Wrighton) (NSTEMI) or STEMI be treated with a clopidogrel and Daiichi Sankyo Company, Limited, Tokyo, Japan (Dr Kurihara). 300-mg or 600-mg loading dose (LD), followed by a Submitted for publication April 1, 2009; revised version accepted June 18, daily maintenance dose (MD) of 75 mg for 12 months 2009. Address for correspondence: Nagy A. Farid, PhD, c/o Christopher 3 S. Konkoy, PhD, Eli Lilly and Company, Lilly Corporate Center, Indianapolis, unless contraindicated by excessive bleeding risk. IN 46285; e-mail: [email protected]; [email protected]. The thienopyridines—ticlopidine, clopidogrel, DOI: 10.1177/0091270009343005 and prasugrel—are oral prodrugs that require in vivo

126 • J Clin Pharmacol 2010;50:126-142 METABOLISM AND DISPOSITION OF THIENOPYRIDINES conversion to a thiol-containing pharmacologically recent studies demonstrated that prasugrel achieves active metabolite. The thiol moiety of the active a faster, higher, and a more consistent level of inhi- metabolite binds specifically and irreversibly to bition of platelet aggregation, both in healthy par- cysteine residues of the platelet P2Y12 purinergic ticipants and patients with coronary artery receptor, thus inhibiting ADP-mediated platelet acti- disease.19,20 These observed differences between pra- vation and aggregation.4 sugrel and clopidogrel were evaluated to determine Ticlopidine (Ticlid) was the first compound of if they would translate into clinical differences and this class, introduced to the market in 1979 for pre- benefits. The TRITON-TIMI 38 study showed that vention of thrombotic stroke. The recommended prasugrel 60 mg LD/10 mg MD was superior to ticlopidine dose is 250 mg twice daily. The most clopidogrel 300 mg LD/75 mg MD in preventing the common side effects observed with ticlopidine composite endpoint of cardiovascular death, nonfa- treatment include gastrointestinal disturbances, tal MI, or nonfatal stroke in patients undergoing primarily vomiting, diarrhea, nausea, dyspepsia, antiplatelet therapy for a median duration of 14.5 and purpura. In addition, skin rash appears to be a months.21 The benefit of prasugrel over clopidogrel common problem.5 More serious, potentially life- was achieved with a cost of increased noncoronary threatening side effects include hyponatremia, artery bypass graft-related major bleeding (2.4% vs nephrotic syndrome,6 hepatotoxicity,7,8 neutropenia, 1.8% for prasugrel and clopidogrel, respectively). and thrombocytopenia.6,9 The higher bleeding risk was primarily in patients Clopidogrel (Plavix/Iscover) was first launched in less than 60 kg, patients 75 years old and older, and 1998 in the United States and in European countries patients with a history of transient ischemic attack in 1999 for the reduction of atherosclerotic events (TIA) or stroke. Net clinical benefit (rate of prevent- in patients with stroke, myocardial infraction (MI), ing composite endpoint vs rate of noncoronary artery or peripheral arterial disease. The recommended bypass graft bleeding) favored prasugrel in landmark daily dose was 75 mg, a dose chosen because it pro- analyses of both early (up to 3 days) and late (from 3 duced comparable inhibition of platelet aggregation days through the trial’s duration) events,22 consistent to that produced by 250 mg ticlopidine twice daily.10 with the superiority of prasugrel over a standard Adverse events noted with clopidogrel include gas- regimen of clopidogrel for both early and ongoing trointestinal bleeding, pruritus, and rash. Rarely, pharmacologic management of patients undergoing thrombotic thrombocytopenic purpura has been PCI. reported.11 Ticlopidine and clopidogrel were both The PRINCIPLE-TIMI-44 study showed that prasu- shown to have significant improvements over aspi- grel 60 mg LD produced more rapid onset, higher, rin alone in the treatment of patients with stroke or and more consistent levels of platelet inhibition than myocardial infarctions.12 In addition, patients treated clopidogrel 600 mg LD given at least 30 minutes with clopidogrel, as a single 300-mg LD followed by prior to catheterization. Similarly, prasugrel 10 mg/d 75-mg daily MDs, and aspirin demonstrated a 20% produced greater and more consistent levels of plate- relative risk reduction in cardiovascular events com- let inhibition than did clopidogrel 150 mg/d.23 pared to the aspirin-alone group.13 However, sub- Although the 3 thienopyridines share the same stantial interindividual variability in the inhibition mechanism of action, significant differences in their of platelet aggregation (IPA) by clopidogrel in patients metabolism, pharmacokinetics (PK), platelet response, has been observed, and this variability in some cases and drug-drug interactions have been observed in correlated with the risk of recurrent cardiovascular studies conducted in animals and humans. Progress events. In fact, a large number of investigators in understanding the reasons for these differences observed that 15% to more than 40% of the patients has evolved through elucidation of the compounds’ responded poorly to clopidogrel. (The percentages respective metabolic and activation pathways, poten- cited depend on the methodology used to evaluate tial for inhibition of cytochrome P450 (CYP) enzymes, platelet aggregation and the criteria used by the and also the development of sensitive and specific investigators to determine the threshold of response.) analytical methods to determine the plasma concen- In addition, several investigators reported a correla- trations of the active metabolites. tion between the platelet response to clopidogrel This review describes the metabolism, disposi- and patient outcome.14-18 tion, and PK of the 3 thienopyridines and how these Prasugrel (Effient/Efient) is the newest member characteristics affect platelet aggregation and clinical of this class of drugs. In comparison to clopidogrel, outcomes.

Review 127 FARID ET AL

PHARMACODYNAMIC RESPONSE available, an increased number of metabolites could be quantified. LC/MS/MS methods were developed Determination of the effect of thienopyridines on ex and validated for the measurement of prasugrel’s vivo platelet aggregation was the method chosen by active metabolite, R-138727, and major inactive various investigators to help evaluate their response.24 metabolites in human plasma.35 The assay limit of Studies in animals showed that the inhibition of quantitation is 0.5 ng/mL for R-138727 and 1 ng/mL platelet aggregation after oral dosing with the 3 for the inactive metabolites. To assist in understand- thienopyridines was time and dose dependent. In ing the PK and pharmacodynamics (PD) of clopi- these studies, it was also shown that in rats dosed dogrel through its active metabolite, an assay for the with 1 of these 3 thienopyridines, prasugrel was quantification of clopidogrel’s active metabolite in approximately 100- and 10-fold more potent at inhib- human plasma was also developed and validated.36 iting ex vivo platelet aggregation than ticlopidine To accurately determine plasma concentrations of and clopidogrel, respectively.25,26 However, in vitro, prasugrel and clopidogrel active metabolites, these the active metabolites of clopidogrel and prasugrel metabolites must be stabilized by derivatization were equipotent with respect to their ability to with 2-bromo-3′-methoxyacetophenone immediately inhibit platelet aggregation,27 and the results pro- upon collection (in blood and prior to separation vided by Yoneda et al28 suggested that the active of the plasma). The omission of the derivatization metabolite of ticlopidine has approximately one step typically results in low and unreliable concentra- tenth the potency of the other 2 thienopyridines. tions of the active metabolite that cannot be used in deriving meaningful conclusions with respect to PK BIOANALYTICAL METHODS TO DETERMINE and PK/PD relationships. PLASMA CONCENTRATIONS OF THE THIENOPYRIDINES AND THEIR METABOLITES THE METABOLISM AND DISPOSITION OF TICLOPIDINE The plasma concentrations of a drug and/or its active metabolite are typically of major interest in In humans, the onset of activity after initiation of oral pharmacokinetic studies. Moreover, in this class of treatment with ticlopidine (250 mg twice daily) compounds, it was eventually demonstrated that the occurred between 24 and 48 hours, with maximal concentrations of their respective active metabolites inhibition of ADP-induced platelet aggregation occur- in human plasma were an extension of the metabolic ring 3 to 5 days after initial dosing.37 Approximately pathways and correlated with the observed effects on 85% of a ticlopidine oral dose was absorbed, with platelet inhibition. For ticlopidine, methods were peak plasma concentrations (Cmax) of ticlopidine developed for the determination of its concentra- occurring 2 hours after the dose. The median ticlopi- 29 tions in human plasma. However, no methods were dine Cmax after the first 250-mg dose was 310 ng/mL, reported for the determination of the active metabolite which increased to 990 ng/mL after 21 days of dosing of ticlopidine in plasma. every 12 hours.29 Ticlopidine exhibited nonlinear Plasma concentrations of clopidogrel were gener- PK, and its clearance decreased significantly after ally too low (<2 ng/mL) to be quantified after a 75-mg repeated dosing. The median elimination half-life 30 dose, although Nirogi et al have recently reported a (t1/2) of ticlopidine after multiple dosing was 29 hours sensitive liquid chromatography/tandem mass spec- but increased to 4 to 5 days in elderly participants.6,29 trometry (LC/MS/MS) method for the determination Steady-state concentrations of ticlopidine in plasma of clopidogrel concentrations in plasma. Therefore, were achieved in 5 days but require 2 to 3 weeks of early bioanalytical methods quantified the pharma- dosing in the elderly.37 These data indicate that ticlo- cologically inactive clopidogrel acid metabolite in pidine inhibits its own metabolism in humans, most human plasma.31,32 likely by inhibiting CYP2B6 and CYP2C19, as will be Prasugrel has not been detected in human plasma, discussed later. so in the initial pharmacokinetic and absorption Ticlopidine represented 5% of the plasma radio- studies with prasugrel, some of prasugrel’s inactive activity after a single dose of [14C]ticlopidine, which metabolites were measured and their derived phar- increased to 15% at steady state. Sixty percent of the macokinetic parameters used as indicators of the administered radioactivity was recovered in the urine absorption and metabolism of prasugrel.33,34 As refer- and 23% in the feces. Almost 8% of the dose was ence standards for additional metabolites became eliminated in the feces as ticlopidine, either through

128 • J Clin Pharmacol 2010;50:126-142 METABOLISM AND DISPOSITION OF THIENOPYRIDINES

Figure 1. Metabolic pathway of ticlopidine. excretion in the bile and/or due to the lack of absorp- CYP(s) contributing to the N-dealkylation step of tion. Ticlopidine is 98% bound to plasma proteins.38 ticlopidine have not been identified. Ticlopidine is rapidly metabolized by the liver. Initially, 4 ticlopidine metabolites were identified in THE METABOLISM AND humans, but the pharmacologically active metabolite DISPOSITION OF CLOPIDOGREL was not detected or identified. The main metabolite, 2-chlorohippuric acid (Figure 1), appeared rapidly in Clopidogrel appears to be rapidly absorbed, as deter- the urine, whereas ticlopidine was present in the mined by the appearance of its pharmacologically urine in only trace amounts.29,37 The N-dealkylation inactive carboxylic acid metabolite in human plasma pathway that eventually results in 2-chlorohippuric (Figure 2). The Cmax of clopidogrel’s acid metabolite acid formation also produced tetrahydrothienopyri- was 2780 ng/mL and was generally achieved 1 hour dine. Ticlopidine N-oxide and a hydroxy metabolite (a after a 75-mg clopidogrel oral dose.43 The exposure non-sulfur-containing compound) were also identified to the acid metabolite appeared proportional to as metabolites (Figure 1). clopidogrel doses between 50 and 150 mg. Clopido- Although ticlopidine was known to be a prodrug, grel acid metabolite accounted for 71% of the 14C the structure of its pharmacologically active metab- concentration in human plasma at 1 hour after a olite was not known. However, the structure of 75-mg dose of [14C]clopidogrel.44 The exposure to ticlopidine active metabolite was deduced after the clopidogrel active metabolite in humans was later structures of prasugrel and clopidogrel active metab- found to be less than dose proportional, likely due to olites were independently disclosed in 2000.26,39 In saturable absorption, metabolism, or both.45-48 In gen- 2004, Yoneda et al28 isolated and characterized the eral, quadrupling a clopidogrel dose from 75 mg to active metabolite of ticlopidine after in vitro incuba- 300 mg resulted in approximately 3 times greater AUC tion of 2-oxo-ticlopidine with homogenates prepared of the active metabolite. The Cmax and AUC0-t of clopi- from phenobarbital-induced rat livers (Figure 1). dogrel active metabolite after a 300-mg LD were CYP2C19 and CYP2B6 were shown to contribute approximately 70 ng/mL and 90 ng⋅h/mL, respec- to the metabolic transformation of ticlopidine to tively, and were approximately 30 ng/mL and 30 ng⋅h/ 2-oxo-ticlopidine.40,41 Dalvie and O’Connell42 dem- mL after a 75-mg MD. In addition, doubling the clopi- onstrated that, in vitro, CYP3A also contributes to dogrel dose from 300 mg to 600 mg resulted in only a the metabolism of ticlopidine, resulting in the for- 44% increase in the AUC of its active metabolite. Time mation of a pyridinium metabolite and oxidation to Cmax of clopidogrel active metabolite was typically of the carbon next to the nitrogen atom but not achieved between 0.5 and 1 hour after an oral dose. 2-oxo-ticlopidine. However, the CYP(s) involved in Clopidogrel is 98% bound to plasma proteins.49 the formation of ticlopidine active metabolite from The effect of daily 75-mg doses of clopidogrel on its thiolactone intermediate are unknown; likewise, platelet aggregation typically reaches steady state

Review 129 FARID ET AL

Figure 2. Metabolic pathway of clopidogrel.

3 to 7 days after initiation of treatment.11 Cadroy et al50 carboxylesterase.55 In initial studies investigating showed that during the first 24 hours of a 75-mg the in vivo formation of clopidogrel active metabo- clopidogrel dose given with aspirin, IPA was not lite, Savi et al demonstrated the importance of hepatic significantly different from predose values. However, metabolism in eliciting clopidogrel’s effect on plate- a single 300-mg clopidogrel LD resulted in platelet let aggregation56 and that the active metabolite was inhibition values 6 hours after dosing that were com- formed through an intermediate, 2-oxo-clopidogrel parable to those achieved after a few days of dosing (Figure 2).39 Kazui et al57 demonstrated that clopi- with 75 mg clopidogrel. This highlighted the impor- dogrel remained essentially unchanged when incu- tance of administering an LD to patients with ACS, bated with a human small intestine S9 fraction but with or without coronary stent implantation, in was hydrolyzed to its carboxylic acid metabolite which effective antithrombotic responses are needed when incubated with a human liver S9 fraction in as early as possible to prevent thrombosis. Thebault the absence of NADPH and to the carboxylic acid et al51 reported that single loading doses of clopi- and 2-oxo-clopidogrel in the presence of NADPH. dogrel 100, 200, 400, and 600 mg dose dependently Incubation of clopidogrel or 2-oxo-clopidogrel with increased ex vivo IPA induced by 5 µM ADP up to human liver microsomes and NADPH, in the presence the 400-mg dose; no further increase in IPA was of a reducing agent (such as glutathione), produced observed for the 600-mg dose. Several other investi- the clopidogrel active metabolite (Figure 2).39 gators evaluated the IPA of clopidogrel 600 mg and The absolute S-configuration at the benzylic carbon 900 mg against the 300-mg LD. In most cases, the of clopidogrel, which was found to be stable in vivo, 600-mg LD increased mean IPA between 10 and 20 is required for biological effect on platelets, and so is percentage points, 4 to 6 hours after dosing, com- the Z configuration of the ethylenic bond at carbon pared with the 300-mg LD. However, the 900-mg LD 3.58 However, the effect of the configuration of the did not provide any significant additional increase thiol group of the active metabolite on the expression in IPA.48,52-54 These observations support the concept of activity on platelets was not determined. that increases in clopidogrel dosing produce less The CYP forms involved in producing the phar- than proportional increases in clopidogrel active macologically active metabolite from clopidogrel metabolite concentrations. An important finding in were identified as CYP1A2 in rats59 and CYP3A in all of these reports was that intersubject variability humans.60 In addition, several recent clinical studies in the response to the clopidogrel 300-mg LD did not have shown that loss of catalytic activity of CYP3A4, decrease by increasing the dose to 600 mg. CYP3A5, or CYP2C19 due to enzyme inhibition or The first clopidogrel metabolite identified was genetic polymorphism has a negative effect on the PK the carboxylic acid derivative (Figure 2), which was of clopidogrel active metabolite and its effect on later shown to be produced through hydrolysis by platelets, indicating that these CYPs play a significant human carboxylesterase 1 (hCE1), primarily a hepatic role in the metabolism of clopidogrel to its active

130 • J Clin Pharmacol 2010;50:126-142 METABOLISM AND DISPOSITION OF THIENOPYRIDINES

Figure 3. Prasugrel metabolic pathway of interest. metabolite.45,61-63 The contribution of human CYP1A2 of clopidogrel in the incubation mixture, and that to the formation of the thiolactone, 2-oxo-clopidogrel, approximately 50% of the produced thiolactone was has been confirmed by Kurihara et al.64 These authors hydrolyzed to the corresponding inactive acid. In also demonstrated that CYP2C19 is a major contribu- total, these data suggest that only a small percentage tor to 2-oxo-clopidogrel formation from clopidogrel, (ie, 10% or less) of a clopidogrel dose is ultimately followed by CYP2B6. Furthermore, by following the converted to its active metabolite. formation of the product of each oxidative step by LC/MS/MS, these authors clearly demonstrated THE METABOLISM AND that CYP3A does not contribute to the formation of DISPOSITION OF PRASUGREL the thiolactone metabolite from clopidogrel; rather, CYP3A contributes to the formation of the active Studies have shown that prasugrel is rapidly metabolite from the thiolactone. In total, 4 CYP absorbed, is extensively metabolized (Figure 3), and forms contribute to the formation of clopidogrel is not detected in human or animal plasma.66,67 active metabolite from 2-oxo-clopidogrel: CYP3A, The Cmax of its pharmacologically active metabolite, CYP2B6, CYP2C19, and CYP2C9.64 R-138727, was achieved about 30 minutes after an The above indicates that 2 competing metabolic oral dose. The exposure to prasugrel metabolites is pathways occur in the liver: the hydrolysis of clopi- essentially proportional to the administered dose dogrel by hCE1 to form its inactive acid metabolite in humans.33,34,46 Following a 60-mg LD and a 10-mg and the CYP-catalyzed oxidation of clopidogrel to MD, the Cmax values for R-138727 were typically 2-oxo-clopidogrel. Similarly, there are 2 competing about 453 ng/mL and 56 ng/mL, respectively, and the pathways for 2-oxo-clopidogrel: its hydrolysis by corresponding AUC0-t values were 460 and 54 ng⋅h/ hCE1 to its carboxylic acid analog65 and its oxidation mL.46 R-138727 was detectable in human plasma for by CYP3A, CYP2B6, CYP2C19, and CYP2C9 to the about 24 hours after a 60-mg LD and for 6 to 8 hours active metabolite. Indeed, Hagihara et al65 recently after a 10-mg MD (data on file, Daiichi Sankyo, Inc. reported that following incubation of clopidogrel in and Eli Lilly and Company). The concentrations of human liver microsomes, about 90% of clopidogrel R-95913 and R-119251 peaked at 0.5 hours, the same was converted to its carboxylic acid metabolite, that time as R-138727, and declined in parallel with each 2-oxo-clopidogrel accounted for 224 fmol/min/µM other and with the active metabolite. Concentrations

Review 131 FARID ET AL of R-106583 were higher than those of R-138727, Prasugrel, like ticlopidine and clopidogrel, is peaked at 1 hour, and declined slower than those of extensively metabolized and is now the best charac- R-138727 and the 2 other metabolites. These metabo- terized of the thienopyridines with respect to metabo- lites did not accumulate during multiple dosing. lism. The hydrolysis of prasugrel, which is mediated Approximately 21% of a 15-mg [14C]prasugrel dose by hCE2, primarily an intestinal enzyme, forms the was eliminated in the feces within the first 48 hours thiolactone R-95913 (Figure 3) through keto-enol tau- after oral administration to healthy participants, tomerism. R-95913 is metabolized to prasugrel’s active indicating that at least 79% of the prasugrel dose was metabolite, R-138727, which is then metabolized to 2 absorbed.66 Overall, about 68% of the [14C]prasugrel inactive compounds: R-106583 (by S-methylation) dose was excreted in the urine and 27% in the feces and R-119251 (by conjugation with cysteine) (Figure 3). in the form of inactive metabolites over a period of R-119251 contains a disulfide bond, which can be 10 days, indicating that urinary excretion is the reduced in vivo back to R-138727. major pathway for the elimination of prasugrel After a single 15-mg [14C]prasugrel dose to human metabolites. The terminal t1/2 of R-138727 is 7.4 participants, R-106583 was the major metabolite cir- hours and that of plasma radioactivity was approxi- culating in plasma, representing 26% of the AUC0-12 h mately 8 days. Prasugrel and R-138727 are not of the radioactivity.66 The 4 key metabolites—R-95913, detected in human urine or feces. The binding of R-138727, R-119251, and R-106583—combined com- R-138727 to plasma proteins could not be deter- prised approximately 70% of the plasma radioactiv- mined but was 98% in a 4% human serum albumin ity at 15 minutes and 30 minutes after dosing, solution in phosphate buffer, pH 7.4. suggesting rapid metabolism of prasugrel via the A combination of loading and maintenances doses pathway leading to the active metabolite (data on of prasugrel was evaluated for their effects on IPA in file, Study H7T-LC-TAAB, Daiichi Sankyo, Inc. and stable aspirin-treated patients with coronary artery Eli Lilly and Company). The AUC0-0.25 h of R-138727 disease.20 In this population, a 40-mg or 60-mg LD of and its 2 downstream metabolites, R-106583 and prasugrel produced a faster onset of IPA than 300 mg R-119251, accounted for about 55% of the compara- clopidogrel and also a higher degree of inhibition: ble area for plasma radioactivity (Table I). These data 70% for the 60-mg prasugrel dose compared with suggest strongly that at least half of the prasugrel 31% for the 300-mg clopidogrel dose, 6 hours after dose was rapidly converted to its active metabolite. the dose. A dose-dependent effect of the daily MD of This is further supported by the elimination results prasugrel on the IPA was observed, with the 5-mg of prasugrel metabolites in human urine and feces in prasugrel dose resulting in essentially similar IPA to which metabolites not derived from the active the 75-mg clopidogrel dose, 34.5% versus 31.2%, metabolite pathway were estimated to account for respectively, whereas the IPA after 10-mg daily doses roughly 53% of the administered dose (data on file, of prasugrel was 57.5%.20 This and additional clinical Study H7T-LC-TAAB, Daiichi Sankyo, Inc. and Eli studies led to the selection of prasugrel loading and Lilly and Company). maintenance doses of 60 mg and 10 mg, respectively, The single major metabolite of prasugrel found in for subsequent clinical evaluations.19,20,68 human urine was M1 (Figure 3), which accounted The antiplatelet effects of prasugrel 60 mg LD and for 21.3% ± 5.1% (mean ± SD; n = 5) of the dose. The 10 mg MD were compared in several studies to the major metabolites found in feces were R-106583 and responses of clopidogrel 300- and/or 600-mg LDs M1. The formation of M1 from prasugrel was pro- and 75- or 150-mg MDs. Typically, prasugrel 60 mg posed to be through a pathway that involves an LD achieved IPA values (20 µM ADP) of about 50% isomer of R-95913, which was metabolized to a thi- thirty minutes after the dose, which increased to one. Formation of the corresponding ketone from approximately 75% to 80% at 1 hour after the dose. the thione, followed by reduction, yielded M1.66 It is In these studies, maximum IPA with clopidogrel likely that formation of the hydroxy metabolite of 300 mg and 600 mg was 50% and 69%, respectively, ticlopidine (Figure 1) follows a similar pathway. and occurred 6 hours after the dose. In addition, a Both hCE1 and hCE2 are capable of catalyzing the 10-mg MD of prasugrel produced mean IPA values conversion of prasugrel to R-95913, but the hydroly- that were consistently higher than those obtained by sis rate with hCE2 was 25 times that with hCE1.71 It either 75 mg or 150 mg of clopidogrel. Another find- was also found that incubation of prasugrel in ing of these studies was that prasugrel produced human small intestine S9 fraction resulted in a rapid more consistent IPA between participants compared decrease in prasugrel concentration and a corres­ with clopidogrel.19,20,45,48,68-70 ponding increase in R-95913 formation.57 In addition,

132 • J Clin Pharmacol 2010;50:126-142 METABOLISM AND DISPOSITION OF THIENOPYRIDINES

Table I Exposure to Prasugrel Active and Inactive Metabolites as a Percentage of the Plasma Radioactivity in Humans After a Single 15-mg Oral Dose of [14C]Prasugrel

Percentage of 14C Percentage of 14C Concentration AUC

Metabolite 0.25 h 0.5 h AUC0-0.25 h AUC0-0.5 h R-95913 17.6 14.6 17.6 16.2 R-138727 30.8 19.8 30.8 24.6 R-119251 14.1 10.9 14.1 13.3 R-106583 12.9 16.3 12.9 14.2 All 4 metabolites 71.0 67.1 71.0 70.4 R-138727+R-119251+ R-106583 55.4 48.2 55.4 49.9

Data on file, Study H7T-LC-TAAB, Daiichi Sankyo, Inc and Eli Lilly and Company. results of a Caco-2 monolayer metabolism and trans- whether the blood was sampled after the first dose of port study demonstrated the complete conversion of the drug or after a few weeks of pharmacotherapy prasugrel to R-95913 during absorption. Regardless with prasugrel.76 This stereoselectivity in the dispo- of apical or basolateral administration of prasugrel sition of R-138727 could be due to its stereoselective to the Caco-2 cells, prasugrel was found only in the formation, metabolic clearance, or a combination of donor buffer and not the receiver buffer, strongly both. The observed stereoselectivity in the active suggesting that prasugrel does not pass through the metabolite formation in humans from R-95913 sug- intestine intact.71 Thus, it was concluded that all of gests that either the opening of the thiolactone ring R-95913 was formed during the absorption of prasu- in R-95913, a CYP-mediated process,72 is stereose- grel through the intestine. The extent of R-95913 lective with chirality preserved up to the step lead- formation from prasugrel was later determined in ing to R-138727 formation, or that the ultimate step vitro following incubation of prasugrel in human leading to R-138727 formation is stereoselective, as liver microsomes (which contain hCE1). R-95913 described previously.76 Two in vitro studies may was produced at a rate of 5520 fmol/min/µM of pra- support this stereoselectivity in the disposition of sugrel in the incubation mixture, even without the R-138727. In the first study, the ratio of the R-138727 contribution of hCE2.65 isomer pairs (calculated as [(R,S) + (R,R)]/[(S,R) + Four CYPs are capable of forming prasugrel active (S,S)]) after incubating the expressed CYP3A4 or metabolite R-138727 from its thiolactone, R-95913. CYP3A5 with 20 µM R-95913 was 1.6.73 In the sec- The main contributors are CYP3A4 and CYP2B6, ond study, the S-methylation of R-138727 by thiol with smaller contributions by CYP2C9 and CYP2C19.72 S-methyl transferase appeared to be stereoselective, Additional in vitro data showed that CYP3A4 and favoring the formation of the S-methyl analogs of the CYP3A5 are similarly efficient in converting R-95913 least potent isomer pair (S,R and S,S) of R-138727.77 to R-138727.73 Prasugrel was developed as a race- Together with the stereoselective formation of the mate because the administration of either of its metabolites by the CYP3A forms, this finding may enantiomers to dogs resulted in the equal formation partly explain the in vivo profile of R-138727 in of the 4 stereoisomers of R-95913. Thus, R- and plasma where the RS- and RR-forms are the major S-configurations at the benzylic carbon of prasugrel enantiomers. interconvert in vivo.74 Like R-95913, R-138727 con- CYP3A represents about 80% of the intestinal tains 2 chiral centers, and its 4 stereoisomers have CYP forms.78 Considering the role of hCE2 in the varying affinities for the P2Y12 receptor, with the (R,S) formation of R-95913 and that of CYP3A in R-138727 and (R,R) isomers being the most potent (the first formation from R-95913,71,72 the role of intestinal letter refers to the configuration of the carbon carry- metabolism in the formation of prasugrel active ing the thiol group) (Figure 4).75 In humans, the (R,S) metabolite becomes apparent, suggesting that a large and (R,R) isomers (R-125690 and R-125689, respec- proportion of active metabolite is formed during tively) comprised about 84% of the R-138727 in first-pass metabolism by intestinal CYP3A. This is plasma, with the (S,R) and (S,S) pair (R-125688 and supported by the results of a study in humans given R-125687, respectively) accounting for about 16%. prasugrel and ketoconazole, a potent CYP3A4/5 The ratio was consistent among participants, inhibitor, in which ketoconazole coadministration regardless of the dose, time of sample collection, or resulted in a 46% reduction in the Cmax of R-138727

Review 133 FARID ET AL

Figure 4. Effect of the isomers of prasugrel’s active metabolite, R-138727, on platelet aggregation induced by 10 µM adenosine-5′- 75 diphosphate (ADP). IC50 values for binding to the P2Y12 receptor: R-125690: 0.19 µM (66.4 ng/mL); R-125689: 3.1 µM (1083 ng/mL); R-125687: 28 µM (9784 ng/mL); and R-125688: 36 µM (12579 ng/mL). after a prasugrel 60-mg LD, but the overall exposure A comparison of the key PK and PD data for ticlo- (AUC) was bioequivalent with and without ketocon- pidine, clopidogrel, and prasugrel is presented in 45 azole. This change in R-138727 Cmax was consistent Table III. with the effect of inhibition of intestinal CYP3A activity by ketoconazole on the initial formation of EFFECT OF THE THIENOPYRIDINES R-138727 during the absorption process. In the same ON CYP CATALYTIC ACTIVITY study, the AUC of R-95913 doubled, whereas its Cmax increased by 71% to 93%, with no change to tmax or In vitro, ticlopidine was found to be a potent t1/2 (Table II). Insofar as AUC and Cmax reflect bio- mechanism-based inhibitor of CYP2B6 and CYP­ 38,41,81 availability whereas t1/2 depends directly on hepatic 2C19 and is also an inhibitor of CYP1A2 and clearance, these differential pharmacokinetic effects CYP2D6.81-83 In addition, 2-oxo-ticlopidine inhib- 84 on R‑95913 AUC and Cmax are indicative of a sub- ited CYP2B6 with similar potency to ticlopidine itself. strate for which metabolism by intestinal CYP3A is Clopidogrel was found to be a mechanism-based important, but hepatic CYP3A-mediated metabolism inhibitor of CYP2B641 and an inhibitor of CYP2C19 83,85 is less important. These results also suggest that (IC50 = 0.524 µM). However, 2-oxo-clopidogrel was R-95913 has secondary metabolic pathway(s) that may a much weaker inhibitor of CYP2B6 and CYP2C19, be CYP3A dependent, in addition to that resulting in such that its ability to inhibit these 2 isoforms would R-138727 formation. This is supported by the obser- not be of clinical importance.83,84 The thiol-containing vation that coadministration of the CYP inducer active metabolites of ticlopidine and clopidogrel, as rifampin with prasugrel resulted in reduction in the well as the major circulating metabolite of clopido-

Cmax and AUC of R-95913 by 68% to 84%, compared grel, the acid derivative, were not found to be inhibi- 83 to prasugrel alone, while not affecting the exposure tors of the various CYP forms (IC50 > 50 µM). to the active metabolite or the 2 downstream metab- Because prasugrel is rapidly hydrolyzed in the olites (Table II).79 intestine and is not detected in plasma, in vitro CYP Significant involvement of intestinal enzymes inhibition studies were conducted with the hydrolysis also helps explain the rapid appearance of prasugrel’s product, the thiolactone R-95913. In vitro, R-95913 active metabolite in plasma and the lack of effect of inhibited CYP2C8, CYP2C9, CYP2C19, CYP2D6, and moderate hepatic impairment on the PK of prasugrel’s CYP3A4, with Ki values ranging from 7.2 µM to 82 80 72 active metabolite. Based on both the in vitro and µM, but did not inhibit CYP1A2. These Ki values in vivo data, it was concluded that a significant por- significantly exceeded the highest observed Cmax of tion of R-95913 is oxidized to the active metabolite of R-95913 (248 nM) in any participant receiving a prasugrel during intestinal absorption. 10-mg prasugrel daily dose (data on file, Study

134 • J Clin Pharmacol 2010;50:126-142 METABOLISM AND DISPOSITION OF THIENOPYRIDINES

Table II Effect of Enzyme Inhibition and Induction on the Exposure Parameters for the Thiolactone of Prasugrel, R-95913, in Healthy Participants When Prasugrel Is Administered Alone and With Ketoconazole or Rifampicin

Prasugrel Prasugrel + Alone (n = 18) Ketoconazole (n = 18)

Geometric Geometric Ratio of LS Geometric Parameter Mean (CV%) Mean (CV%) Means (90% CI)

Prasugrel 60-mg LD

AUC0-t, ng⋅h/mL 332 (45.8) 660 (35.6) 1.96 (1.60, 2.40) Cmax, ng/mL 193 (56.3) 331 (37.3) 1.71 (1.33, 2.21) Prasugrel 15-mg MD

AUC0-t, ng⋅h/mL 90.9 (50.9) 191 (33.2) 2.02 (1.64, 2.49) Cmax, ng/mL 52.6 (41.4) 102 (38.4) 1.93 (1.54, 2.41)

Prasugrel Prasugrel + Alone (n = 30) Rifampin (n = 29)

Geometric Geometric Ratio of LS Geometric Parameter Mean (CV%) Mean (CV%) Means (90% CI)

Prasugrel 60-mg LD

AUC0-t, ng⋅h/mL 371 (32.7) 98.6 (31.6) 0.265 (0.241, 0.291) Cmax, ng/mL 172 (40.1) 54.7 (43.0) 0.318 (0.282, 0.358) Prasugrel 10-mg MD

AUC0-t, ng⋅h/mL 58.6 (41.6) 9.31 (48.8) 0.159 (0.138, 0.182) Cmax, ng/mL 33.1 (38.3) 6.98 (51.2) 0.209 (0.181, 0.242)

Data on file, Study H7T-EW-TAAK, Daiichi Sankyo, Inc. and Eli Lilly and Company and Farid et al.79 CI, confidence interval; CV, coefficient of variation; LD, loading dose; LS, least squares; MD, maintenance dose.

H7T-LC-TAAV, Daiichi Sankyo, Inc. and Eli Lilly of ticlopidine at steady state (during 250-mg twice- and Company). In addition, prasugrel active metabo- daily dosing) are in the 3-µM to 8-µM range, whereas lite, R-138727, and its main circulating metabolite, those for clopidogrel are typically <14 nM. R-106583, did not inhibit any of the CYPs indi- Symptomatic phenytoin toxicity has been reported in cated above.72 Thus, prasugrel’s metabolites R-95913, patients administered both ticlopidine and phenytoin, R-138727, and R-106583 are unlikely to inhibit the suggesting that ticlopidine inhibits CYP2C19 in CYP-mediated metabolism of coadministered drugs. humans.88,89 Subsequently, ticlopidine was found Clinical studies confirmed several of the in vitro to decrease the activity of CYP2C19 in humans, findings regarding the abilities of the thienopyridines resulting in a significant decrease in apparent to inhibit various CYP forms. Coadministration of clearance of omeprazole from 25.7 L/h to 10.8 L/h.90 ticlopidine with theophylline to healthy participants Ieiri et al91 reported that although the PK of omepra- decreased theophylline clearance from 0.682 mL/kg/ zole were different between participants who are min to 0.431 mL/kg/min and increased its elimina- CYP2C19 homozygous extensive metabolizers, CYP 86 tion t1/2 from 8.6 to 12.2 hours. Results of this 2C19 heterozygous extensive metabolizers, and study suggested that ticlopidine inhibits CYP1A2 in CYP2C19 ho­mozygous poor metabolizers, these dif- humans. Clopidogrel did not affect the PK of theo- ferences disappeared after coadministration of phylline.87 The ability of ticlopidine, but not clopi- ticlopidine (200 mg/d), with all participants dem- dogrel, to inhibit the metabolism of compounds onstrating the pharmacokinetic profile of the homozy- metabolized by CYP1A2 may be explained by the gous poor metabolizers and indicating the inhibition differences in their plasma concentration. Although of CYP2C19 by ticlopidine. the IC50 values for ticlopidine and clopidogrel for the The observed in vitro inhibitory effect of ticlo- inhibition of CYP1A2 (12.4 µM and 24.3 µM, respec- pidine and clopidogrel on CYP2B6 was confirmed 83 tively) are of comparable magnitude, the Cmax values in a clinical study using bupropion as a substrate

Review 135 FARID ET AL

Table III Summary of Thienopyridine Pharmacokinetics and Pharmacodynamics

Ticlopidine, Clopidogrel, Prasugrel, 250 mg bida 300/75 mg (LD/MD)b 60/10 mg (LD/MD)b

Pharmacokinetic estimates 28 47 47 tmax, h 2.0 0.5-1.0 0.5 28 46 46 Cmax, ng/mL 990 70/28 453/56 28 46 46 AUC0-t, ng⋅h/mL 4060 90/29 460/54 28 c t1/2, h 29 NA 7.4 Pharmacodynamic response (20 µM ADP) IPA (time from LD), % (h) NA 43 (6)46 79 (1)46 IPA (MD), % 20-30109,110 34-5246 60-7146

ADP, adenosine-5′-diphosphate; IPA, inhibition of platelet aggregation; LD, loading dose; MD, maintenance dose. aSteady-state pharmacokinetic parameters reported for ticlopidine. bRespective active metabolite pharmacokinetic parameters reported for clopidogrel and prasugrel. cClopidogrel active metabolite concentrations are typically below the assay limit of quantitation (0.5 ng/mL) by 2 to 4 hours after the dose (eg, see Farid 46 et al ), making reliable estimation of its terminal t1/2 unattainable. because its hydroxylation is a pathway almost which is metabolized by conjugation, reduced the exclusively catalyzed by CYP2B6.92-94 Ticlopidine inhibitory effect of clopidogrel on platelet aggrega- and clopidogrel increased the mean AUC for bupro- tion in a dose-dependent manner. However, the pion by 85% and 60%, respectively, and decreased number of participants per atorvastatin dose group the mean AUC of hydroxybupropion by 84% and was very small. Since the initial study, a few reports 52%.94 Prasugrel was found to have a weaker inhib- have agreed with that finding, whereas several others itory effect on CYP2B6 in humans because the AUC have not.97 A prospectively designed, statistically of hydroxybupropion was 9.64 µg⋅h/mL when bupro- powered crossover study was conducted to assess pion was given with prasugrel versus 12.3 µg⋅h/mL the effect of atorvastatin, given at the highest approved when given alone, a decrease of 23%.95 There have dose, 80 mg, on the platelet response to clopidogrel been no other reports to indicate inhibitory effects (300 mg LD/75 mg MD) or prasugrel (60 mg LD/10 of prasugrel or clopidogrel on other CYP forms in mg MD). The pharmacokinetic parameters for each humans. active metabolite were determined as well as the Taken together, these data suggest that the extent of IPA when each thienopyridine was given mechanism-based inhibition of the catalytic activ- alone and with atorvastatin. The results showed that ity of CYP2B6 and CYP2C19 by ticlopidine and statin administration did not negatively affect the PK clopidogrel occurs during the first oxidative step or PD response to either prasugrel or clopidogrel.46 that leads to the formation of their respective thio- Because CYP3A forms contribute to the formation lactones. Because the formation of prasugrel’s thio- of the active metabolites of clopidogrel and prasugrel lactone, R-95913, is not CYP dependent, such an from their respective thiolactones, the effect of inhibitory effect would not occur during R-95913 CYP3A4 and CYP3A5 inhibition with ketoconazole formation. on the PK of both active metabolites and their effects on platelets were determined in healthy participants in a EFFECT OF CYP INHIBITION OR GENETIC randomized crossover study.45 The participants POLYMORPHISMS ON THE THIENOPYRIDINES received an LD/MD of prasugrel 60/15 mg or clopi- dogrel 300/75 mg (5 daily MDs) without or with This area of investigation has gained significant ketoconazole at 400 mg/d. Ketoconazole decreased momentum over the past 6 years because of observa- the Cmax values of the active metabolites 34% to 61% tions regarding the effect of CYP inhibition or genetic after prasugrel and clopidogrel dosing. Although polymorphism on the platelet response to clopidog- ketoconazole did not affect R-138727 AUC or prasu- rel therapy. One of the main contributing factors to grel’s inhibition of platelet aggregation, it decreased this area of interest was the identification of patients clopidogrel’s active metabolite AUC0-24 22% (LD) to who respond poorly to clopidogrel, as previously 29% (MD) and reduced inhibition of platelet aggrega- mentioned.13-18,70 In 2003, Lau et al96 reported that tion 28 percentage points (LD) to 33 percentage atorvastatin, a CYP3A substrate, but not pravastatin, points (MD). This study showed that CYP3A4 and

136 • J Clin Pharmacol 2010;50:126-142 METABOLISM AND DISPOSITION OF THIENOPYRIDINES

CYP3A5 inhibition by ketoconazole affects the extent CYP2C19*2 allele was associated with decreased of active metabolite formation of clopidogrel, but not clopidogrel-inhibited platelet aggregation and may prasugrel, and that the decreased formation of clopi- be associated with poorer clinical outcome compared dogrel active metabolite was associated with its with patients who do not carry this loss-of-function reduced ability to inhibit platelet aggregation. allele.99,100 Similarly, Close et al101 reported that of Another study by Suh et al61 showed that administra- 349 healthy participants, those who were carriers of tion of itraconazole, a selective CYP3A4 inhibitor, the reduced-function CYP2C19 allele had a 53% with clopidogrel to participants who did not express (P = .0003) reduction in plasma AUC of the clopi- CYP3A5 lacked the ability to inhibit platelet aggrega- dogrel active metabolite after a 300-mg LD, resulting tion. They also reported that atherothrombotic events in less reduction (by 19 percentage points) in MPA, tended to occur more frequently in patients treated compared with those classified as EMs, 4 hours after with clopidogrel who did not express CYP3A5. clopidogrel 300 mg LD (P = .04). The CYP2C19 Another enzyme that contributes to both oxida- genetic effect on response to clopidogrel was consis- tive steps of clopidogrel metabolism is CYP2C19, a tently observed after both the LD (600 and 300 mg) polymorphic enzyme. Two of the earliest publica- and MD (75 mg). These observations have been sub- tions to describe the effect of CYP2C19 polymor- stantiated in larger studies that concluded that among phism on clopidogrel were by Hulot et al62 and Brandt ACS patients treated with clopidogrel, carriers of a et al.63 Hulot et al showed that participants who were CYP2C19 reduced-function allele had diminished CYP2C19 extensive metabolizer (EM) homozygotes platelet inhibition, resulting in higher rates of stent (*1/*1) had the ability to inhibit platelet aggregation thrombosis and major adverse cardiovascular after daily 75 mg clopidogrel over a 7-day period. events.102,103 For prasugrel, none of the genetic vari- Maximum platelet aggregation (MPA) in response to ants tested had a clinically or statistically significant 10 µM ADP decreased from 76.2% ± 6.8% predose effect on active metabolite exposure or reduction in 101 to 48.9% ± 14.9% on day 7 of treatment (P < .001). P2Y12 inhibition. However, there was no effect of clopidogrel on Small et al104 assessed the effect of the proton platelet aggregation in the CYP2C19 heterozygous pump inhibitor (PPI) lansoprazole on the PK and (EM/poor metabolizer [PM]) participants (*1/*2; PD of loading doses of prasugrel (60 mg) and clopi- baseline was 77.2% ± 4.3% vs 71.8% ± 14.6% on dogrel (300 mg) in healthy participants. Although day 7, P = .22), suggesting that the formation of the lansoprazole coadministration resulted in a 19% clopidogrel active metabolite was significantly decrease in prasugrel active metabolite AUC, there reduced in the heterozygous population. In the sec- was no effect on the IPA after the prasugrel dose. ond study, the PK of and the PD response to a clopi- Lansoprazole did not affect the exposure to clopi- dogrel 300-mg LD and a prasugrel 60-mg LD were dogrel inactive acid metabolite but resulted in lower evaluated in a retrospective analysis of 2 clinical tri- IPA after the clopidogrel dose, suggesting an effect als that included 71 participants. The authors found on the extent of clopidogrel active metabolite for- that genetic variations in CYP2C19 resulting in the mation. (The active metabolite was not measured loss of catalytic function were associated with because its assay had not been developed at the decreased exposure to the active metabolite of clopi- time of study.) Retrospective tertile analysis showed dogrel and thus decreased its ability to inhibit plate- that the negative effect of lansoprazole on IPA after let aggregation. There was no effect of CYP2C19 dosing with clopidogrel was most evident in the polymorphisms on the PK of prasugrel active metab- participants who responded best to clopidogrel olite or, accordingly, its effect on platelets.63 Recently, alone. The analysis also showed that although lanso- Kim et al98 demonstrated that exposure to clopi- prazole lowered IPA in participants treated with dogrel prodrug correlated with CYP2C19 genotype clopidogrel, it had no effect on the IPA response to after treating participants with a 300-mg clopidogrel prasugrel in these same participants. LD, followed by a 75-mg dose for 2 days. Exposure The clearance of both PPIs omeprazole and esome- to clopidogrel was lowest in the homozygous EMs prazole (the S-enantiomer of omeprazole, a CYP2C19 (CYP2C19*1/*1), intermediate in the heterozygous substrate and inhibitor) decreases with repeated (EM/PM) group (*1/*2 and *1/*3), and highest in dosing, suggesting that these compounds impair the the PMs (*2/*2 and *2/*3). Accordingly, the ability enzymatic activity of CYP2C19.105 Gilard et al106 of clopidogrel to inhibit platelet aggregation decreased reported that omeprazole significantly reduced the in that same order. Studies in patients undergoing ability of clopidogrel to inhibit platelet aggregation PCI also demonstrated that the presence of a in patients undergoing coronary stent implantation.

Review 137 FARID ET AL

In contrast, Siller-Matula et al107 found that coad- ­compared with the carboxymethyl group at this ministration of either esomeprazole or pantoprazole position in clopidogrel, thus imparting stability with clopidogrel did not affect the platelet response toward enzymatic hydrolysis at this site. Another to clopidogrel, implying the absence of a class effect key difference is the presence of the acetoxy group among PPIs. Lack of information regarding identifica- at the second position of the thiophene ring in pra- tion of patients as responders or nonresponders to sugrel, thus eliminating the need of CYP-dependent clopidogrel before initiation of treatment may under- oxidation to produce the thiolactone. The net effect lie the apparent lack of agreement between the results is that upon complete hydrolysis by carboxyle- of these studies. Considering the significant role sterases, the formed thiolactone is readily available CYP2C19 plays in the bioactivation of clopidogrel, for the CYP-dependent step that produces the active ideally a prospectively designed, statistically pow- metabolite. Replacement of the chlorine atom of ered crossover design study in clopidogrel respond- clopidogrel by fluorine in prasugrel also appears to ers is needed before a conclusion can be made improve the antiplatelet effect in the rat (US Patent regarding possible interaction between clopidogrel 5,288,726 [1994]). and omeprazole or esomeprazole. Moreover, based The net effect of differences between the meta- on the known differences in the hepatic metabolism bolic pathways of prasugrel and clopidogrel is that of the PPIs, and because other PPIs have not been significantly more of the prasugrel thiolactone inter- shown to be CYP2C19 inhibitors in vivo, these mediate is formed relative to that for clopidogrel. observations will most likely be limited to omepra- This results in the formation of more active metabo- zole and esomeprazole. lite after a prasugrel dose compared with a clopido- grel dose. The data presented here show that the DISCUSSION thiolactone and the active metabolite of prasugrel are formed faster during the absorption/first-pass Antiplatelet therapy has proven to be crucial for man- process compared with clopidogrel. As stated above, aging patients with ACS and coronary artery disease a 10-mg dose of prasugrel produces twice as much of and in patients undergoing PCI. Clopidogrel differs its active metabolite compared to the 75-mg clopi- from ticlopidine by the addition of the carboxymethyl dogrel dose, and a 60-mg dose of prasugrel produces group at the benzylic carbon. The addition of this side 5 times the active metabolite produced from a 300-mg chain apparently decreases or possibly eliminates the LD of clopidogrel (calculated from AUC data). metabolic pathway that leads to the cleavage of ticlo- Clinically, this translates into having a faster onset pidine into the thienopyridine and o-chlorobenzyl of action, greater extent of IPA, and a more consis- moieties. It also appears that the increased lipophilic tent response to treatment for participants on prasu- character in the benzylic region may result in grel relative to those on clopidogrel. In addition, improved potency toward the P2Y12 receptor, thus because the formation of prasugrel’s thiolactone is not increasing IPA, because the active metabolite of CYP dependent, the potential for clinically significant clopidogrel is approximately 10 times more potent drug interactions between other CYP-metabolized than that of ticlopidine, as mentioned above. The drugs and prasugrel is essentially eliminated. In combination of these 2 factors would explain why addition, there is less chance for other enzyme the use of a lower daily clopidogrel dose, 75 mg, pro- inhibitors or genetic polymorphisms to alter the duces the same degree of platelet inhibition in pharmacodynamic effect of prasugrel on platelets. humans as that produced by 500 mg of ticlopidine Knowing that the pharmacologically active metab- daily. Both ticlopidine and clopidogrel are metabo- olites of prasugrel and clopidogrel are equipotent, lized in the liver. One could easily predict the involve- the ability to quantify these metabolites in human ment of CYP forms in the formation of ticlopidine and plasma enabled the development of a correlation clopidogrel thiolactones in the first of 2 steps in their (Figure 5) between exposure to the active metabolite biotransformation to their respective active metabo- and the extent of IPA.108 The correlation showed that lites because 1 oxygen atom is introduced into each the active metabolites of prasugrel and clopidogrel compound at the second position of the thiophene have the same exposure-IPA relationship and aided ring. However, in vitro data showed that CYP3A does the interpretation of clinical observations and find- not contribute to the formation of the thiolactones of ings. Thus, the platelet-inhibitory effects of the active ticlopidine and clopidogrel.42,64 metabolites of prasugrel and clopidogrel depend on Structurally, clopidogrel and prasugrel differ in their respective AUCs. Whereas a 60-mg prasugrel several key elements. Prasugrel contains a cyclo­ LD results in a pharmacodynamic response at or near propylcarbonyl group at the benzylic carbon atom the Emax, that from a 300-mg clopidogrel LD will fall

138 • J Clin Pharmacol 2010;50:126-142 METABOLISM AND DISPOSITION OF THIENOPYRIDINES

relevant effects by P2Y12 antagonists are unclear, the present clinical data indicate that greater degrees of inhibition of ADP-mediated platelet aggregation are associated with significant reduction in stent throm- bosis and more effective prevention of ischemic events.21

The authors thank Christopher S. Konkoy, PhD, and Julie A. Sherman, AAS, both of Eli Lilly and Company, for their expert assistance in preparing this manuscript. Financial support: This study was funded by Daiichi Sankyo Company, Limited and Eli Lilly and Company.

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