Anti-HIV Drug Human Biotransformation and Structure-Activity Relationship of Pregnane X Receptor Activation
Lade, Julie M
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With the advent of antiretroviral drug therapy in the late 1980s, human immunodeficiency virus (HIV) is no longer a death sentence, but remains to be a chronic disease. Of concern, however, an estimated 2.1 million individuals were newly infected with HIV globally as of 2015. Further, HIV remains to be a pandemic that requires attention from scientific researchers to push forward the development of novel drug therapies that mitigate viral resistance and improve overall safety profiles. Rilpivirine (RPV) a second generation non-nucleoside reverse transcriptase inhibitor (NNRTI) was FDA approved in 2011 for administration to treatment-naïve HIV-infected individuals. Prior to this work, the hepatic metabolism of RPV was unknown. By in vitro assay, we observed that cytochrome P450s CYP3A4 and CYP3A5 primarily contributed to the oxidative metabolism of RPV and that uridine diphosphate glucuronosyl transferases UGT1A1 and UGT1A4 could contribute to the conjugative metabolism of RPV and its metabolites. In these studies, it was observed RPV could modulate CYP3A4 mRNA expression through activation of the nuclear receptor and xenobiotic sensor pregnane X receptor (PXR). In line with this, efavirenz (EFV) a first generation NNRTI and the most-widely prescribed antiretroviral despite exhibiting a poor safety profile, has been previously observed to activate PXR. Using an array of structural analogs of EFV, we explored the structure-activity relationship of EFV-mediated activation of PXR. In doing so, we identified a lower size limit ranging of > 223 Da to < 289 Da for ligand-dependent activation of PXR. In addition, we observed that the addition of a hydroxyl group to the Cl-phenyl ring of EFV did not impede ligand binding but attenuated PXR activation potentially through destabilizing a key hydrophobic area within the ligand-binding pocket. Building upon this work, we investigated other hepatic pathways that may be stimulated by EFV and/or impacted by PXR activation. To this end, we observed EFV, but not RPV, could disrupt hepatic cholesterol homeostasis by stimulating lipid droplet formation as evidenced in primary mouse hepatocytes. EFV was also observed to increase the expression of the nuclear receptor small heterodimer partner (SHP), which has been implicated in facilitating the developments of dyslipidemia. However, further studies need to be performed in order to determine whether an EFV-mediated increase of SHP has a causal role in lipid droplet formation. In addition to studying antiretrovirals within the context of chronic HIV maintenance therapy, we also investigated RPV and the nucleoside reverse transcriptase inhibitor tenofovir (TFV) as pharmacological agents for HIV pre-exposure prophylaxis (PrEP). Moreover, in retrospective analyses, we identified genetic variants in the genes encoding enzymes responsible for RPV and TFV metabolism and activation, respectively. These findings lay the groundwork for future pharmacogenetic studies investigating inter-individual variability and toxicity associated with HIV PrEP.