Interaction of CYP3A4 with Rationally Designed Ritonavir Analogues: Impact of Steric Constraints Imposed on the Heme-Ligating Group and the End-Pyridine Attachment

A four-drug combination oral tablet of darunavir, cobicistat, emtricitabine, and tenofovir alafenamide for the treatment of HIV-1 infection in adults

INTRODUCTION

Darunavir (DRV)/cobicistat (COBI)/emtricitabine (FTC)/tenofovir alafenamide (TAF) is the only protease inhibitor-based single-tablet regimen (STR) approved for the treatment of HIV infection of adults and pediatric patients weighing at least 40 kg. DRV/COBI/FTC/TAF has demonstrated to be an effective regimen, to have a high genetic barrier to resistance, and to be well tolerated.

AREAS COVERED

The authors summarize the chemistry and pharmacology of DRV, COBI, FTC, and TAF and discuss trials conducted on antiretroviral therapy (ART)-naïve and -experienced people living with HIV designed to evaluate safety, tolerability, and efficacy of the STR. This work also reports studies comparing DRV/COBI/FTC/TAF with competitive agents in real-world settings.

EXPERT OPINION

Despite the availability of newer antiretroviral drugs and strategies in the management of HIV infection, including long-acting therapies, DRV/COBI/FTC/TAF is still considered an alternative regimen for the treatment of ART-naïve adults. DRV/COBI/FTC/TAF is an effective, well-tolerated, and safe antiretroviral regimen and represents a valid option for people who need to switch therapy due to tolerability issues, such as the onset of neuropsychiatric effects related to integrase strand transfer inhibitors, or virological failure.

The binding of nitrogen-donor ligands to the ferric and ferrous forms of cytochrome P450 enzymes

The cytochrome P450 superfamily of heme-thiolate monooxygenase enzymes can catalyse various oxidation reactions. The addition of a substrate or an inhibitor ligand induces changes in the absorption spectrum of these enzymes and UV-visible (UV-vis) absorbance spectroscopy is the most common and readily available technique used to interrogate their heme and active site environment. Nitrogen-containing ligands can inhibit the catalytic cycle of heme enzymes by interacting with the heme. Here we evaluate the binding of imidazole and pyridine-based ligands to the ferric and ferrous forms of a selection of bacterial cytochrome P450 enzymes using UV-visible absorbance spectroscopy. The majority of these ligands interact with the heme as one would expect for type II nitrogen directly coordinated to a ferric heme-thiolate species. However, the spectroscopic changes observed in the ligand-bound ferrous forms indicated differences in the heme environment across these P450 enzyme/ligand combinations. Multiple species were observed in the UV-vis spectra of the ferrous ligand-bound P450s. None of the enzymes gave rise to the isolation of a single species with a Soret band at ∼442-447 nm, indicative of a 6-coordinate ferrous thiolate species with a nitrogen-donor ligand. A ferrous species with Soret band at ∼427 nm coupled with an α-band of increased intensity was observed with the imidazole ligands. With some enzyme-ligand combinations reduction resulted in breaking of the iron‑nitrogen bond yielding a 5-coordinate high-spin ferrous species. In other instances, the ferrous form was readily oxidised back to the ferric form on addition of the ligand.

Crystal Structure of CYP3A4 Complexed with Fluorol Identifies the Substrate Access Channel as a High-Affinity Ligand Binding Site

Cytochrome P450 3A4 (CYP3A4) is a major human drug-metabolizing enzyme, notoriously known for its extreme substrate promiscuity, allosteric behavior, and implications in drug–drug interactions. Despite extensive investigations, the mechanism of ligand binding to CYP3A4 is not fully understood. We determined the crystal structure of CYP3A4 complexed with fluorol, a small fluorescent dye that can undergo hydroxylation. In the structure, fluorol associates to the substrate channel, well suited for the binding of planar polyaromatic molecules bearing polar groups, through which stabilizing H-bonds with the polar channel residues, such as Thr224 and Arg372, can be established. Mutagenesis, spectral, kinetic, and functional data confirmed the involvement but not strict requirement of Thr224 for the association of fluorol. Collectively, our data identify the substrate channel as a high-affinity ligand binding site and support the notion that hydrophobic ligands first dock to the nearby peripheral surface, before migrating to the channel and, subsequently, into the active site.