Influence of NAR and TAL. Naringin, one of the major constituents of grapefruit juice, interacts with intestinal transporters as Oatp/OATP or P-gp [40,41] and also with metabolism enzymes (CYP3A4) [19,42,43]. Based on the literature reports, the variety of drugs affected by grapefruit juice includes SQV, TAL, cyclosporine, simvastatin, midazolam, colchicine or montelukast [31,40,44]. On these bases, we suspected that the concomitant administration of NAR, SQV and TAL [32,45,46] could restrict the absorption of any of them to various extents because of interferences on absorption or secretion transporters. To understand the potential interaction among SQV, TAL and NAR, a competitive inhibition model was checked.
Concentrations of NAR were chosen according to the following criteria: a commercial grapefruit juice concentration, 1400 μM, and a NAR concentration 50 times lower than the commercial concentration assayed, NAR 28 μM [29,41]. According to our findings, the flavonoid acts as an inhibitor of SQV influx and has a slight effect on the intestinal efflux of the drug (model 9, table 3), leading to a statistically significant decrease in SQV ka (p < 0.05) when the drug is administered at the 5 μM concentration with NAR (fig. 1).
Results reported indicate that the inhibitory effect of NAR on influx and efflux carriers involved on SQV absorption is concentration dependent of the drug and concentration independent of NAR. The net result when SQV is perfused at 5 μM in the presence of NAR at 28 or 1400 μM is a similar reduction of SQV absorption, which seems to be independent of the concentration of NAR used in this study. This SQV ka reduction was not observed at 100 μM concentration of SQV co-administered with NAR 1400 μM (p > 0.05). On these bases, NAR seems to affect predominantly the influx carrier of SQV at lower doses of SQV.
Only when TAL, a potent inhibitor of mdr1, is co-administered with NAR and SQV, the ka of the protease inhibitor significantly increases 2.07 times (p < 0.01) (fig. 1), reflecting that TAL has a profound impact on efflux of SQV.
Taking into account that NAR preferentially inhibits OATP over P-gp [30,32] and TAL is considered a candidate for the determination of intestinal P-glycoprotein function , data reported in this study suggest that OATP/Oatp and MDR1/Mdr1 play roles in the intestinal absorption of SQV as influx and efflux transporters, respectively. In this way, the reported km value of SQV for OATP1A2-mediated transport is 36.4 μM, and OATP1A2-mediated NAR transport had an IC50 value of 3.6 μM .
Pharmacokinetic model tools used in this in situ study do not provide direct evidence of the involvement of Oatp or gp-P on the SQV absorption process, but the results obtained are consistent with the role of these carriers in SQV uptake described by other authors [14,17,18,21]. This is why these tools can be considered to be of great utility in this type of studies, because they allow us to avoid more complex techniques and reduce the number of animals used. In the light of this result, we can conclude that SQV absorption in the rat small intestine is mediated by carriers shared by NAR and TAL.