Abstract

Drug-drug interactions (DDIs) with the HIV protease inhibitors (PIs) are complex, paradoxical (e.g. ritonavir/alprazolam), and involve multiple mechanisms. As part of a larger study to better understand these DDIs and to devise a framework for in vitro to in vivo prediction of these DDIs, we determined the inductive effect of ~ 2 weeks of administration of two prototypic PIs, nelfinavir (NFV), ritonavir (RTV), and rifampin (RIF, induction positive control), on cytochrome P450 enzymes, CYP1A2, 2B6, 2C9 and 2D6 and the inductive or inductive plus inhibitory effect of NFV, RTV or RIF on CYP3A and P-glycoprotein in healthy human volunteers. Statistically significant induction of CYP1A2 (2.1, 2.9 and 2.2-fold), 2B6 (1.8, 2.4 and 4.0-fold) and 2C9 (1.3, 1.8 and 2.6-fold) was observed after NFV, RTV or RIF treatment respectively (as expected, CYP2D6 was not induced). Moreover, we accurately predicted the in vivo induction of these enzymes by quantifying their induction by the PIs in human hepatocytes and by using RIF as an in vitro to in vivo scalar. Based on the modest in vivo induction of CYP1A2, 2B6 or 2C9, the in vivo paradoxical DDIs with the PIs are likely explained by mechanisms other than induction of these enzymes such as induction of other metabolic enzymes, transporters, or both.