Contribution of human esterases to the metabolism of selected drugs of abuse

Human esterases such as the human carboxylesterases (hCES) are important for the catalytic ester hydrolysis of xenobiotics and they play an important role in the detoxification of drugs (e.g., cocaine) but also in the activation of prodrugs (e.g., ramipril). Therefore, the aim of the presented study was to characterize the enzyme-catalyzed ester hydrolysis of ten drugs (cocaine, dimethocaine, ethylphenidate, 4-fluoro-3α-tropacocaine, 4-fluoro-3β-tropacocaine, heroin, methylphenidate, mitragynine, ramipril, and thebacon) by different esterase-containing systems (recombinant hCES1b, hCES1c, and hCES2, pooled human liver microsomes, pooled human liver S9 fraction, and pooled human plasma). Michaelis-Menten kinetic studies were done using in vitro incubations with the aforementioned enzyme-containing systems and LC coupled to ion trap MS for analysis. Ramipril and heroin were used as known model substrates to ensure reliable incubation conditions. The hydrolysis reactions followed classic Michaelis-Menten kinetics with exception of cocaine and 4-fluoro-3α-tropacocaine, for which hydrolysis rate was too low for reliable modeling. The substrates were mainly metabolized by the following enzymes systems: cocaine, hCES1c; dimethocaine, human plasma esterases; ethylphenidate, hCES1c; 4-fluoro-3β-tropacocaine, human plasma esterases; heroin, hCES2; methylphenidate, hCES1c; mitragynine, hCES1c; ramipril, hCES1b; thebacon, hCES2. Compounds bearing a small alcohol part and a larger acyl part showed higher affinity to hCES1 while those with a large alcohol part showed higher affinity to hCES2. The collected data are important for prediction of drug-drug or drug-food interactions as well as for individual variations in metabolism of drugs of abuse due to enzyme polymorphisms.