Active renal secretion of TFV across proximal tubules occurs via uptake from the circulation into the basolateral side of tubules by human organic anion transporters 1 and 3 (hOAT1 and

hOAT3) coupled with efflux out of the apical side of tubules into urine by multidrug resistance protein-4 (MRP4) [34] and MRP2 [35] (although the role of the latter transporter at the renal tubule remains controversial [34]). In vitro cell-based transport models have shown that APV has minimal effects on hOAT1 and hOAT3 (20% inhibition when given at APV therapeutic Cmax) [34]. Its effects on MRP4 and MRP2 have not been evaluated to date. As the minor hOAT1 and hOAT3 effects do not explain the small decrease in TFV Cmin and AUC we saw during FPV or FPV/RTV coadministration with TDF, it is probable that the interaction responsible for this overall pharmacokinetic change occurs at the gut level. TDF, but not Y-27632 mouse TFV, is a substrate for the intestinal efflux transmitter P-glycoprotein (P-gp)

LBH589 molecular weight [9], which APV may induce [36], thereby reducing TFV absorption. TFV Cmax was the pharmacokinetic parameter most reduced during coadministration, yet the maximum decrease was by only 25%, as noted following concurrent use of the unboosted FPV regimen with TDF. The reduction in TFV Cmin and AUC was less during the TDF+FPV/RTV period relative to the TDF+ unboosted FPV period, possibly because the P-gp-inhibitory effect of RTV may have partially counteracted the P-gp-induction effect of APV. TPV and NFV also induce intestinal P-gp [36,37], while ATV and LPV markedly inhibit P-gp [38], contributing to their TFV exposure-elevating effects.

It is unclear why TDF coadministration would increase APV concentrations, as TDF does not affect cytochrome P450 3A4 (CYP3A4) metabolism [9], the primary metabolic pathway for APV, nor does it affect P-gp [39,40], for which APV is a substrate. The increase in APV plasma concentrations during TDF coadministration is in contrast to the reduction in ATV and LPV concentrations seen when unboosted ATV, ATV/RTV or LPV/RTV is given with TDF [13,26,28], which is postulated to occur because of a physicochemical reaction these PIs have with TDF at 4-Aminobutyrate aminotransferase the time of their absorption in the intestine [11]. The combination of TDF with either FPV or FPV/RTV was well tolerated, with no unexpected adverse events observed. In the study as a whole, we noted a high incidence of maculopapular rash (38%) in various dosing cohorts: FPV alone (n=6), TDF/FPV (n=4), TDF/FPV+RTV (n=4) and FPV/RTV (n=1). The high frequency of rash in our study is in stark contrast to the low rates reported in the ALERT trial which evaluated TDF–FPV/RTV among HIV-infected patients [4], but it is consistent with reports of other pharmacokinetic trials of FPV in healthy volunteers [19,41].