| dc.description.abstract | Clopidogrel is a thienopyridine antiplatelet prodrug that was approved by the US FDA in 1997 and quickly supplanted ticlopidine as the primary drug therapy for reducing atherothrombotic events. It is converted to its pharmacologically active metabolite H4, which irreversibly inactivates the P2Y12 receptor on platelets, through two sequential reactions that are catalyzed mainly by CYP2C19. Common clinical practice involved the coadministration of a proton pump inhibitor (PPI, including omeprazole, esomeprazole, lansoprazole, pantoprazole, and rabeprazole) with clopidogrel to decrease the risk of upper gastrointestinal bleeding. This practice was formalized for high risk patients by the American Heart Association (and others) in 2008. By 2009, numerous publications described an unexpected decrease in clopidogrel efficacy when coadministered with PPIs, prompting both the US Food & Drug Administration (FDA) and European Medicines Agency (EMA) to issue recommendations discouraging the concomitant use of PPIs and clopidogrel. Proton pump inhibitors are also metabolized by CYP2C19. It seemed reasonable to conclude that, despite their relatively short plasma half-lives, PPIs might competitively inhibit CYP2C19, thereby reducing the efficacy of clopidogrel. In 2010, as numerous publications emerged, both regulatory agencies restricted subsequent warnings to only omeprazole and esomeprazole. The interaction between clopidogrel and PPIs, and the potential mechanisms responsible for it, continues to be a subject of much debate in 2015. This dissertation describes research that contributes to the progress made in understanding the basis for the interaction between clopidogrel and PPIs since the time of the initial regulatory statements, and in particular, why only omeprazole and esomeprazole are implicated in this drug interaction. The initial studies in this dissertation identified omeprazole (a racemic mixture of R- and S-enantiomers) and esomeprazole (the S-enantiomer) as not only competitive inhibitors, but more importantly, metabolism-dependent inhibitors (MDIs) of CYP2C19 in human liver microsomes (HLM), human hepatocytes and recombinant CYP2C19. In contrast, lansoprazole and pantoprazole did not cause metabolism-dependent inhibition (MDI) of CYP2C19. In addition to its clinical relevance, these observations are important because they underscore the importance of using a low concentration of enzyme and a short incubation time with the CYP marker substrate in order to detect MDI of CYP enzymes in vitro. In many previous studies of CYP2C19 inhibition by omeprazole or esomeprazole, the concentration of HLM was too high and/or the substrate incubation time was too long to detect MDI. The kinetic parameters for CYP2C19 inactivation by omeprazole, namely kinact and KI, were determined and used in a physiologically based pharmacokinetic (PBPK) model to predict the degree of CYP2C19 inactivation under clinical conditions. Omeprazole and esomeprazole were subsequently shown to be irreversible MDIs of CYP2C19, which explained why the decrease in clopidogrel efficacy could not be prevented in clinical studies by simply separating the doses of clopidogrel from omeprazole or esomeprazole. Subsequent studies demonstrated that, like the parent drug, two of the three major metabolites of omeprazole are also irreversible MDIs of CYP2C19. The kinetic parameters for CYP2C19 inactivation by these metabolites were determined and, along with those for omeprazole and esomeprazole, used in a mechanistic static model to predict the reduction of H4 formation from clopidogrel under clinical conditions. The model slightly overpredicted (by a factor of 2) the ability of omeprazole to block the conversion of clopidogrel to H4, its pharmacologically active metabolites, but otherwise established that inactivation of CYP2C19 is the likely mechanism for the clinical interaction between omeprazole/esomeprazole and clopidogrel. Esomeprazole and its two inhibitory metabolites, namely omeprazole sulfone and 5 O desmethylomeprazole, were subsequently determined to meet several criteria for mechanism-based inhibition (a special case of irreversible MDI). In addition, studies were initiated to test the hypothesis that the mechanism of CYP2C19 inactivation by esomeprazole and its metabolites involves the formation of a benzylic radical (on the 5,,S-methyl group) that binds covalently to the heme moiety. This hypothesis was based on the observation that the 5,,S methyl group is present on the pyridine ring of those compounds that irreversibly inactivate CYP2C19, namely omeprazole, esomeprazole, omeprazole sulfone and 5 O desmethylomeprazole, but absent from those compounds that did not inactivate CYP2C19, namely lansoprazole, pantoprazole and 5,,S-hydroxyomeprazole. Based on this hypothesis, the investigational PPI, tenatoprazole, which contains a 5,,S-methyl group, was correctly predicted to cause MDI of CYP2C19 whereas ilaprazole and rabeprazole, which lack a 5,,S-methyl group, did not cause MDI of CYP2C19. These results suggest that the investigational PPI, tenatoprazole, but not the clinically used PPIs ilaprazole or rabeprazole, may compromise the therapeutic effectiveness of clopidogrel. Finally, studies were performed in an attempt to provide direct evidence for the proposed mechanism of inactivation of CYP2C19 by esomeprazole, namely the formation of a heme adduct. The potential for the formation of a heme adduct in incubations of esomeprazole in HLM was evaluated by UHPLC analysis with UV/VIS detection and high resolution mass spectrometry (HRMS) with post-acquisition mass-defect filtering to identify heme and heme-containing adducts. Incubation of esomeprazole with NADPH-fortified HLM resulted in a substantial decrease in the amount of heme detectable by UHPLC with either UV absorbance or HRMS and appeared to show the formation of a heme adduct based on mass-defect filtering and isotopic distribution. However, the putative heme adduct was subsequently identified as a dimer of esomeprazole sulfone (a metabolite of esomeprazole formed by CYP3A4/5). Although an adduct between heme and a metabolite of esomeprazole was not ultimately identified, the potential for an unusual analytical artifact was revealed; namely, that sulfur-containing drugs can be converted to metabolites that closely resemble a heme adduct based on mass-defect filtering and isotopic distribution. In summary, this dissertation supports the hypothesis that irreversible inactivation of CYP2C19 is the mechanism by which omeprazole and esomeprazole reduce the efficacy of clopidogrel. This property is not shared by lansoprazole, pantoprazole, rabeprazole or ilaprazole. These findings support regulatory agencies¡¦ recommendations that, in order to reduce the risk of gastrointestinal bleeding, clopidogrel should not be coadministered with omeprazole or esomeprazole but should be coadministered with other PPIs that do not inactivate CYP2C19. | |
