Mechanism for effective lymphoid cell and tissue loading following oral administration of nucleotide prodrug GS-7340.

GS-7340 is a prodrug of tenofovir (TFV) that more efficiently delivers TFV into lymphoid cells and tissues than the clinically used prodrug TFV disoproxil fumarate, resulting in higher antiviral potency at greatly reduced doses and lower systemic TFV exposure. First-pass extraction by the intestine and liver represents substantial barriers to the oral delivery of prodrugs designed for rapid intracellular hydrolysis. In order to understand how GS-7340 reduces first-pass clearance to be an effective oral prodrug, its permeability and stability were characterized in vitro and detailed pharmacokinetic studies were completed in dogs. GS-7340 showed concentration-dependent permeability through monolayers of caco-2 cells and dose-dependent oral bioavailability in dogs, increasing from 1.7% at 2 mg/kg to 24.7% at 20 mg/kg, suggesting saturable intestinal efflux transport. Taking into account a 65% hepatic extraction measured in portal vein cannulated dogs, high dose GS-7340 is nearly completely absorbed. Consistent with the proposed role of intestinal efflux transport, coadministration of low dose GS-7340 with a transport inhibitor substantially increased GS-7340 exposure. The result of effective oral absorption and efficient lymphoid cell loading was reflected in the high and persistent levels of the pharmacologically active metabolite, TFV diphosphate, in peripheral blood mononuclear cells following oral administration to dogs. In conclusion, GS-7340 reaches the systemic circulation to effectively load target cells by saturating intestinal efflux transporters, facilitated by its high solubility, and by maintaining sufficient stability in intestinal and hepatic tissue.

Cobicistat boosts the intestinal absorption of transport substrates, including HIV protease inhibitors and GS-7340, in vitro.

The experimental pharmacoenhancer cobicistat (COBI), a potent mechanism-based inhibitor of cytochrome P450 3A enzymes, was found to inhibit the intestinal efflux transporters P-glycoprotein and breast cancer resistance protein. Consistent with its transporter inhibition, COBI significantly increased the absorptive flux of potential candidates for clinical coadministration, including the HIV protease inhibitors atazanavir and darunavir and the lymphoid cell- and tissue-targeted prodrug of the nucleotide analog tenofovir, GS-7340, through monolayers of Caco-2 cells in vitro.

Effects of human immunodeficiency virus protease inhibitors on the intestinal absorption of tenofovir disoproxil fumarate in vitro.

Human immunodeficiency virus protease inhibitors (PIs) modestly affect the plasma pharmacokinetics of tenofovir (TFV; -15% to +37% change in exposure) following coadministration with the oral prodrug TFV disoproxil fumarate (TDF) by a previously undefined mechanism. TDF permeation was found to be reduced by the combined action of ester cleavage and efflux transport in vitro. Saturable TDF efflux observed in Caco-2 cells suggests that at pharmacologically relevant intestinal concentrations, transport has only a limited effect on TDF absorption, thus minimizing the magnitude of potential intestinal drug interactions. Most tested PIs increased apical-to-basolateral TDF permeation and decreased secretory transport in MDCKII cells overexpressing P-glycoprotein (Pgp; MDCKII-MDR1 cells) and Caco-2 cells. PIs were found to cause a multifactorial effect on the barriers to TDF absorption. All PIs showed similar levels of inhibition of esterase-dependent degradation of TDF in an intestinal subcellular fraction, except for amprenavir, which was found to be a weaker inhibitor. All PIs caused a dose-dependent increase in the accumulation of a model Pgp substrate in MDCKII-MDR1 cells. Pgp inhibition constants ranged from 10.3 microM (lopinavir) to >100 microM (amprenavir, indinavir, and darunavir). Analogous to hepatic cytochrome P450-mediated drug interactions, we propose that the relative differences in perturbations in TFV plasma levels when TDF is coadministered with PIs are based in part on the net effect of inhibition and induction of intestinal Pgp by PIs. Combined with prior studies, these findings indicate that intestinal absorption is the mechanism for changes in TFV plasma levels when TDF is coadministered with PIs.