Abstract

Although many of the clinically significant drug interactions of the anti-human immunodeficiency virus (HIV) protease inhibitors (PIs) can be explained by their propensity to inactivate CYP3A enzymes, paradoxically these drugs cause (or lack) interactions with CYP3A substrates that cannot be explained by this mechanism (e.g., alprazolam). To better understand these paradoxical interactions (or lack thereof), we determined the cytochromes P450 and transporters induced by various concentrations (0-25 μM) of two PIs, ritonavir and nelfinavir, and rifampin (positive control) in primary human hepatocytes. At 10 μM, ritonavir and nelfinavir suppressed CYP3A4 activity but induced its transcripts and protein expression (19- and 12- and 12- and 6-fold, respectively; a >2-fold change over control was interpreted as induction). At 10 μM, rifampin induced CYP3A4 transcripts, CYP3A protein, and activity by 23-, 12-, and 13-fold, respectively. The induction by rifampin of CYP3A activity was significantly correlated with its induction of CYP3A4 transcripts (r = 0.96, p < 0.05) and CYP3A protein (r = 0.89, p < 0.05). All three drugs (10 μM) induced CYP2B6 activity by 2- to 4-fold, CYP2C8 and 2C9 activity by 2- to 4-fold and the transcripts of CYP2B6, 2C8, and 2C9 by >3-, 5-, and 3-fold, respectively. CYP2C19 and 1A2 activity and transcripts were modestly induced (2-fold), whereas, as expected, CYP2D6 was not induced by any of the drugs. Of the transporters studied, protease inhibitors moderately induced multidrug resistance 1 (ABCB1) and multidrug resistance-associated protein (ABCC1) transcripts but had no or minimal effect on the transcripts of breast cancer resistance protein (ABCG2), organic anion-transporting peptide (OATP) 1B1 (SLCO1B1), or OATP1B3 (SLCO1B3). On the basis of these data, we concluded that many of the paradoxical drug interactions (or lack thereof) with the PIs are metabolismrather than transporter-based and are due to induction of CYP2B6 and 2C enzymes.