P-glycoprotein (P-gp) is an efflux pump responsible for limiting oral bioavailability, tissue penetration and increasing metabolism of the HIV protease inhibitor saquinavir (SQV). The objective of this study is to investigate whether prodrug derivatization of SQV to novel dipeptide prodrugs Val-Val-saquinavir (Val-Val-SQV) and Gly-Val-saquinavir (Gly-Val-SQV) targeting peptide transporters can enhance cellular permeability of saquinavir and modulate P-gp mediated efflux. Uptake and transport studies were conducted employing MDCKII-MDRI cell line at 37 degrees C for 10 min and 3 h, respectively. Uptake of [3H]ritonavir and [3H]erythromycin, utilized as model P-gp substrates, was carried out in the presence of inhibitory concentration of SQV and its peptide prodrugs. Bidirectional transport studies were conducted on MDCKII-MDR1 cells grown over membrane inserts. Uptake of [3H]erythromycin by MDCKII-MDR1 cells exhibited a four-fold increase in the presence of 75 microM SQV. However, equimolar concentrations of Val-Val-SQV and Gly-Val-SQV showed only 2.5-fold increase in [3H]erythromycin uptake. Concentration dependent inhibition of [3H]glycylsarcosine (Gly-Sar), a model peptide transporter substrate, was observed in the presence of SQV prodrugs. Transepithelial transport studies of Val-Val-SQV and Gly-Val-SQV exhibited an enhanced absorptive flux and reduced secretory flux relative to studies employing SQV. These results are very likely due to decreased efflux of SQV dipeptide prodrugs by P-gp. Peptide prodrug derivatization constitutes an exciting strategy to improve intestinal absorption and oral bioavailability of SQV.