Abstract

The purpose of this study was to characterize the concentration-response effects of cyclophosphamide (CPA) with and without dexamethasone (DEX; 10 μM) on the expression of CYP3A4 and CYP2B6 in cultured human hepatocytes at concentrations representative of standard- and high-dose CPA therapy (25 to 750 μM). CPA produced concentration-dependent increases in CYP3A4 and CYP2B6 activity and immunoreactive protein that peaked at 250 and 125 μM, respectively, and declined thereafter. The inductive effect of CPA alone and in combination with DEX was greater in magnitude for CYP2B6 compared with CYP3A4. To further examine the inductive effect of CPA on CYP3A4, CPA (250 μM) and DEX (10 μM) alone and in combination were examined in 10 hepatocyte preparations. The combination of CPA and DEX yielded higher rates of 6β-hydroxytestosterone formation than either agent alone. However, the effect was less than additive in human hepatocyte cultures with relatively high baseline CYP3A4 activity and additive or synergistic in human hepatocyte cultures with relatively low baseline CYP3A4 activity. Induction index was highly correlated with CYP3A4 baseline activity for both CPA (r² = 0.75) and CPA plus DEX (r² = 0.85). To investigate the potential mechanism for CPA-induced increases in CYP3A4 activity, the ability of CPA alone and in combination with DEX to activate pregnane X receptor (PXR) was explored using transient transfection assays. CPA produced a dose-dependent increase in PXR activation that was maximal at the highest CPA concentration studied (500 μM). The addition of DEX to CPA resulted in a minor increase in PXR activation compared with CPA alone. These results indicate that CPA alone and in combination with DEX differentially induces the expression of CYP3A4 and 2B in a concentration-dependent manner, which may be mediated partially through activation of PXR. The impact of these effects on the efficacy and toxicity of CPA therapy warrants further investigation.