Structure-activity relationships of diamine inhibitors of cytochrome P450 (CYP) 3A as novel pharmacoenhancers. Part II: P2/P3 region and discovery of cobicistat (GS-9350).

The HIV protease inhibitor (PI) ritonavir (RTV) has been widely used as a pharmacoenhancer for other PIs, which are substrates of cytochrome P450 3A (CYP3A). However the potent anti-HIV activity of ritonavir may limit its use as a pharmacoenhancer with other classes of anti-HIV agents. Ritonavir is also associated with limitations such as poor physicochemical properties. To address these issues a series of compounds with replacements at the P2 and/or P3 region was designed and evaluated as novel CYP3A inhibitors. Through these efforts, a potent and selective inhibitor of CYP3A, GS-9350 (cobicistat) with improved physiochemical properties was discovered.

Structure-activity relationships of diamine inhibitors of cytochrome P450 (CYP) 3A as novel pharmacoenhancers, part I: core region.

Ritonavir (RTV), an HIV-1 protease inhibitor (PI), is also a potent mechanism-based inhibitor of human cytochrome P450 3A (CYP3A) and has been widely prescribed as a pharmacoenhancer. As a boosting agent for marketed PIs, it reduces pill burden, and improves compliance. Removal of the hydroxyl group from RTV reduces, but does not eliminate HIV PI activity and does not affect CYP3A inhibition. Herein we report the discovery of a novel series of CYP3A inhibitors that are devoid of antiviral activity. The synthesis and evaluation of analogs with extensive modifications of the 1,4-diamine core along with the structure activity relationships with respect to anti-HIV activity, CYP3A inhibitory activity, selectivity against other CYP enzymes and the human pregnane X receptor (PXR) will be discussed.

Cobicistat (GS-9350): A Potent and Selective Inhibitor of Human CYP3A as a Novel Pharmacoenhancer.

Cobicistat (3, GS-9350) is a newly discovered, potent, and selective inhibitor of human cytochrome P450 3A (CYP3A) enzymes. In contrast to ritonavir, 3 is devoid of anti-HIV activity and is thus more suitable for use in boosting anti-HIV drugs without risking selection of potential drug-resistant HIV variants. Compound 3 shows reduced liability for drug interactions and may have potential improvements in tolerability over ritonavir. In addition, 3 has high aqueous solubility and can be readily coformulated with other agents.

Nucleotide analogue prodrug tenofovir disoproxil enhances lymphoid cell loading following oral administration in monkeys.

The antiviral drug tenofovir (TFV) is orally administered as the fumarate salt of its disoproxil prodrug (TFV disoproxil fumarate (TDF)). TFV is a dianion at physiological pH and, as a result, has poor lipid membrane permeability. Administration of the lipophilic and cell-permeable prodrug, TFV disoproxil, enhances the oral absorption of TFV. In order to determine whether oral administration of TDF also increases distribution to sites of viral infection, the plasma and circulating lymphoid cell pharmacokinetics of TFV and its phosphorylated metabolites were assessed following a single oral TDF or subcutaneous TFV administration at doses yielding equivalent plasma exposures to TFV in macaques. Despite TFV disoproxil's lack of plasma stability and undetectable levels in the first plasma samples taken, oral administration of TDF resulted in 7.9-fold higher peripheral blood mononuclear cell exposures to the active metabolite, TFV-diphosphate. The apparent plasma terminal half-life (t(1/2)) of TFV was also longer following oral TDF relative to subcutaneous TFV administration (median t(1/2) of 15.3 and 3.9 h, respectively), suggesting broader distribution to cells and tissues outside of the central plasma compartment. In conclusion, the disoproxil pro-moiety enhances not only the oral absorption of TFV but also tissue and lymphoid cell loading. These results illustrate that administration of even a fleeting prodrug can increase target tissue loading and give valuable insight for future prodrug development.

Mechanism of active renal tubular efflux of tenofovir.

Tenofovir (TFV) undergoes renal elimination by a combination of glomerular filtration and active tubular secretion. While transporter-mediated uptake of TFV from the blood into proximal-tubule cells has been well characterized, comparatively little is known about the efflux system responsible for transporting TFV into the lumen during active tubular secretion. Therefore, members of the ATP-binding cassette family of efflux pumps expressed at the apical side of proximal-tubule cells were studied for the ability to transport TFV. Studies in multiple independent in vitro systems show TFV not to be a substrate for P glycoprotein (Pgp) or multidrug resistance protein type 2 (MRP2). In contrast to Pgp and MRP2, TFV was observed to be a substrate for MRP4. TFV accumulated to fivefold lower levels in MRP4-overexpressing cells, and its accumulation could be increased by an MRP inhibitor. Furthermore, MRP4-overexpressing cells were found to be 2.0- to 2.5-fold less susceptible to cytotoxicity caused by TFV. ATP-dependent uptake of TFV was observed in membrane vesicles containing MRP4 but not in vesicles lacking the transporter. On the basis of these and previous results, the molecular transport pathway for the active tubular secretion of TFV through renal proximal-tubule cells involves uptake from the blood mediated by human organic anion transporters 1 and 3 and efflux into urine by MRP4. A detailed understanding of the molecular mechanism of TFV active tubular secretion will facilitate the assessment of potential renal drug-drug interactions with coadministered agents.