The Influence of Human Plasma on Drug-Metabolizing Enzymes: An Exploratory Study.

1. Endogenous factors can introduce significant interindividual variability in in vitro assessments of drug metabolism and drug-drug interactions.2. The current study evaluated the effects of plasma samples from 12 healthy, non-medicated subjects on the activities of cytochrome P450 (CYP) enzymes.3. The inhibitory effects of pooled plasma across a broader concentration range (3-100%) were assessed on CYP3A activity.4. To further investigate kinetic differences, CYP3A-mediated reactions conducted in plasma were compared with those performed in human serum albumin (HSA) solution.5. Plasma increased the in vitro activities of CYP1A2, CYP2C8, and CYP2C19, while decreasing the activities of CYP3A, and CES1. Interindividual variability was observed for all tested drug metabolic enzymes (DMEs), except CYP3A.6. The inhibitory effect of plasma on CYP3A activity was concentration-dependent, with lower reaction activity observed at higher plasma concentrations.7. Notably, inhibition of CYP3A activity by undiluted plasma and 52 mg/mL HSA buffer was nearly complete. Although both HSA and pooled plasma inhibited in vitro CYP3A4 activity, their effects on kinetic parameters differed.8. These findings identify plasma as a potentially important endogenous modulator of apparent in vitro DME activities and underscore the need for further mechanistic investigation.