Introduction
Cardiovascular diseases (CVDs) remain the leading cause of mortality worldwide. Estimated 20.5 million deaths were reported globally due to cardiac conditions in 2021 which showed a marked increase from 12.1 million CVD deaths recorded in 1990.1 Of these deaths, more than 50% are secondary to coronary artery disease (CAD), with acute coronary syndrome (ACS) accounting for approximately 10% of all admissions presenting to emergency care physicians.2 Currently, available anti-platelet agents have several limitations including inadequate efficacy, risk of bleeding and variability in individual response.3 Dual anti-platelet therapy with aspirin and P2Y12 inhibitors, drugs from thienopyridine class, is therefore the cornerstone in managing patients with ACS, undergoing percutaneous coronary intervention (PCI) and preventing stent thrombosis in both the acute and long-term therapy.4 5 Many patients continue to have recurrent atherothrombotic events despite receiving dual anti-platelet therapy.6 While the use of clopidogrel has undoubtedly been a beneficial advancement in treating ACS in both short and long-term morbidity and mortality,7 it remains a treatment with several shortcomings.
Clopidogrel is a prodrug and its clinical efficacy appears to be a function of the amount of enzymatically derived active thiol metabolite formed. Clopidogrel is first metabolised to the intermediate metabolite, 2-oxo-clopidogrel, followed by metabolism to an active thiol metabolite in vivo. Most of the drug is inactivated by carboxylesterase-1, and only 15% is converted to the active metabolite H4 via a two-step process in the liver, influenced by CYP2C19 polymorphisms. CYP2C19 is crucial in both the first (45%) and second (21%) metabolic steps, while CYP3A4 enzyme plays a significant role in the second step (40%). Therefore, any changes in CYP2C19 activity impacts thiol metabolite formation significantly and hence also the response to treatment.8 9 The metabolism of clopidogrel to its active metabolite can be impaired by genetic variations in CYP2C19 or by drugs that inhibit CYP2C19, such as omeprazole or esomeprazole.
It has been established that all patients receiving clopidogrel do not benefit equally and poor metabolisers (PM) of CYP2C19 are at increased risk of ischaemic events after PCI,4 9 suggesting that CYP2C19 polymorphism is a clinically relevant determinant of response to clopidogrel. In 2010, the US FDA had imposed a box warning on clopidogrel bisulfate tablets (Plavix) stating that ‘effectiveness of clopidogrel bisulfate depends on conversion to an active metabolite by the cytochrome P450 (CYP) system, principally CYP2C19 enzyme. Consider use of another platelet P2Y12 inhibitor in patients identified as CYP2C19 poor metabolisers’.10
Patients with carriers of reduced-function CYP2C19 alleles (PMs and intermediate metabolisers) produce lower levels of clopidogrel’s active metabolite. Such patients show diminished platelet aggregation inhibition and are at higher risk of major adverse cardiovascular events, including a threefold greater risk of stent thrombosis;11 12 hence, newer anti-platelet agents are preferred in such patients. Although the newer agents like Prasugrel and Ticagrelor are devoid of such problems (genetic polymorphism), the CV benefit comes with the cost of increased bleeding risk that includes fatal intracranial hemorrhage.13
There is a considerable heterogeneity observed in the activity of CYP450 enzymes in humans, and it has become apparent that individuals have polymorphisms in CYP2C19 resulting in low or non-responsiveness to clopidogrel known as ‘clopidogrel resistance’.11 14 The frequency of CYP2C19*2 allele associated with PM type has been reported to be 47.23% in CAD patients in India.15 Studies by Adithan et al16 have reported incidence of CYP2C19*2 of around 37.9% in south Indian general population and 35.5% by Shalia et al17 in western Indian general population.
In the recently published real-world data, comprising of East Asians who had undergone drug-eluting stent implantation and received clopidogrel-based anti-platelet therapy indicated that intermediate metabolisers (IMs) or PMs (~62.1% of the total population evaluated) had a higher risk of cardiac death, myocardial infarction and stent thrombosis at 5-year follow-up compared with normal metabolisers.18 The most common CYP2C19 LOF allele is *2 with allele frequencies of ~15% in Caucasians and Africans and 29–35% in Asians, indicating the prevalence of IM and PM is more in Asian population than western population.12 18 This high prevalence of CYP2C19 polymorphism renders clopidogrel ineffectiveness or adds variability in effectiveness in PMs and intermediate metabolisers, respectively.
Hence, to overcome the shortcomings of clopidogrel associated mainly with CYP2C19 metabolism, we have developed a novel anti-platelet agent AT-10 (2-oxo-clopidogrel bisulfate), which is a clopidogrel derivative that is metabolically converted in one CYP-dependent step, to produce the active metabolite similar to the approved listed drug Plavix. The metabolic pathway of clopidogrel and AT-10 is depicted in figure 1.
Metabolic pathway of Clopidogrel.
AT-10 has the same chemical structure as that of 2-oxo-clopidogrel. It can exist in four chiral isomers: SS, SR, RR and RS. In clopidogrel, one chiral carbon has S-configuration fixed; thus, 2-oxo-clopidogrel metabolite of clopidogrel can exist in two chiral isomers (SS and SR). AT-10 is a single diastereomer of 2-oxo-clopidogrel having 7aS, 2S-configuration that generates the active metabolite H4 similar to clopidogrel. AT-10 is an isomerically purified form (7aS,2’S-configuration) of 2-oxo-clopidogrel, which is a desired intermediate metabolite formed during the in vivo bioactivation of the dosed drug clopidogrel. More recently, similar approach in this therapeutic area has been adopted by some researchers for their novel ester prodrug, Vicagrel as an antiplatelet drug.19
The response to AT-10 may not be influenced by genetic CYP2C19 polymorphisms to the extent of its influence in clopidogrel, thus eliminating the need for characterisation of clopidogrel responsiveness and thereby maintaining effectiveness in all patients, including those patients who are identified as CYP2C19 PMs and intermediate metabolisers. Prior to this Indian phase 1 study, one proof of concept clinical study was performed with an objective of evaluating and establishing the dose-response relationship of single and multiple doses of AT-10 ranging from 7.5 mg to 40 mg in healthy human subjects where 40 mg dose of AT-10 was found to be safe, well-tolerated and comparable with the clopidogrel 300 mg dose with regard to % inhibition of platelet aggregation (IPA) at 6 hours following the single loading dose (LD). Hence, AT-10 40 mg was selected as the LD. Since the maintenance dose (MD) of clopidogrel (75 mg) is a quarter of the LD of clopidogrel (300 mg), 10 mg dose of AT-10 (a quarter of the LD, 40 mg) was selected as the MD in this study. Thus, in light of promising data obtained from earlier studies, we first time evaluated safety, tolerability, pharmacodynamics and pharmacokinetics of AT-10 in healthy Indian subjects in comparison with standard dosage regimen of clopidogrel. It is important to note that AT-10 is an intermediate metabolite in the metabolic pathway of clopidogrel to its active metabolite. Therefore, this study does not represent the first exposure of AT-10 in humans. It does, however, represent the direct administration of AT-10 to humans.