Assessment of cryopreserved human hepatocytes as a model system to investigate sulfation and glucuronidation and to evaluate inhibitors of drug conjugation

Cultured cryopreserved human hepatocytes are extensively used as a model system for studying drug metabolism, although they remain poorly characterized in respect of the major conjugation reactions glucuronidation and sulfation. Using paracetamol (acetaminophen), we assessed eleven samples of cryopreserved human hepatocytes for their suitability to investigate the simultaneous glucuronidation and sulfation of xenobiotics and evaluated inhibitors of conjugation. Kinetic characterization showed broadly similar values for paracetamol conjugation by hepatocytes (as reported in the literature for in vitro systems), with Km values of approximately 6 mM and 0.3 mM for glucuronidation and sulfation, respectively. Substantial interindividual differences were observed. The hepatocytes demonstrated a strong dose-dependent switch from a preponderance of sulfation at low concentrations of paracetamol to glucuronidation at higher doses, consistent with routes of clearance in vivo. A number of drugs, some of which such as probenecid and sulfinpyrazone are known to interact with paracetamol in vivo, were demonstrated to inhibit the sulfation and/or glucuronidation of paracetamol in hepatocytes, demonstrating the potential application of this model system for studying drug-drug interactions involving conjugation.