Reinvestigation of clopidogrel bioactivation unveils new cytochrome P450-catalyzed thioester cleavage mechanism

The serendipitous prodrug clopidogrel (CPG, M0) is the mainstay antiplatelet drug in clinical use. The thiophene moiety of CPG undergoes ring opening to form the active metabolite (M13) through two steps of cytochrome P450 (CYP)-catalyzed oxidation. The stable intermediate resulting from the first oxidation, 2-oxo-CPG (M2), is proposed to be oxidized to form an S-oxide intermediate (M11), which proceeds with a hydrolytic pathway to yield a sulfenic acid (M12) and subsequently the bioreduced active metabolite (M13). To test the long-standing pathway of M2 to M13 via M11, we have chemically synthesized M11 but found it does not undergo the proposed hydrolytic activation in various conditions including in liver microsomal incubations. To seek an alternative mechanism, ¹⁸O tracing studies were performed with both H₂¹⁸O and ¹⁸O₂, and LC-MS studies show that the carboxylate product moiety acquires its O-atom from oxygen instead of water, which rules out M11 as the bioactivation intermediate. To explain the ¹⁸O tracing results, a one-step Baeyer-Villiger-like mechanism is proposed for the CYP-dependent thioester cleavage, which features the incorporation of the two O-atoms of O₂ into the two product moieties of carboxylate and sulfenic acid. The research presented herein provides a biochemical basis for delineating the clinical pharmacology of a mainstay treatment and expands our understanding of CYP catalysis.