Background: In concert with oxidative metabolism, intestinal absorption and renal elimination determine the pharmacokinetics of anti-viral drugs, including both HIV protease inhibitors and nucleoside phosphonates. At both sites, specific transporters, e.g., the classical organic anion and ATP-driven excretory ATPases, govern transepithelial drug flux. These same processes are also found at the blood/brain and blood/cerebrospinal fluid barriers, where they limit penetration of drugs into the central nervous system. In addition, carrier-mediated drug accumulation within excretory organs, liver and kidney, often result in dose-limiting cellular toxicity. These studies are designed to use the cloned transporters in combination with cell and tissue flux studies to define the critical steps in AIDS anti-viral drug pharmacokinetics and to evaluate the effectiveness of transport modulation in enhancing intestinal drug entry, increasing CNS penetration, and reducing nephrotoxicity. Active transport of anionic and cationic drugs plays a critical role in their intestinal absorption, their penetration across the blood-CSF and blood-brain barriers, and their accumulation within excretory organs. Using the cloned organic anion transporters, rOAT1 and hOAT1, transiently expressed in Xenopus oocytes and stably expressed in a renal epithelial cell line, MDCK cells, we have demonstrated that the nucleoside phosphonate antiviral drugs are excellent substrates for both transporters, with particularly high affinity for hOAT1, indicating that the basis for their known dose-limiting renal toxicity in man is their extensive accumulation within the proximal tubular epithelium via this transporter (Mol. Pharm., 1999). In addition, we have demonstrated the presence of the rat homolog of this carrier, rOAT1, in the blood-CSF barrier (choroid plexus), suggesting that CSF to blood transport contributes to the limited penetrance of these drugs into the brain and thus, their limited effectiveness there (JBC, 1999). Finally, in collaboration with Dr. Miller, we have shown that the nucleoside phosphonates are also substrates for ATP-driven MRP2-mediated transport across the blood-brain barrier, i.e., the brain capillary endothelium (unpublished).