Blood brain barrier (BBB) presents a significant challenge to the delivery of therapeutic agents to the brain. Tight junctions prevent free diffusion of small polar molecules across this barrier. However, entry of large lipid soluble molecules is also limited across BBB by the presence of several ABC (ATP binding cassette) efflux transporters i.e., P- glycoprotein (P-gp) and multi-drug resistance associated proteins (MRPs). Inclusion of HIV protease inhibitors (PIs) in `highly active antiretroviral therapy'(HAART) has substantially improved clinical outcomes in AIDS patients. However, a major challenge with current anti-HIV drug regimen is the inability of PIs to attain sufficient concentrations in CNS resulting in persistent viral replication and a HIV sanctuary site in the brain parenchyma. Sub-therapeutic concentration of PIs in CNS also leads to drug resistance and AIDS related dementia. One of the major factors that limit CNS permeation of PIs is the presence of efflux proteins i.e, P-gp and MRPs on brain capillary endothelial cells. Therefore the objective of this application is to develop novel prodrug strategies to circumvent ABC transporter mediated efflux and to improve brain permeation of HIV protease inhibitors. Prodrug modification of PIs is one such promising strategy where the ligand linked PIs can bypass efflux processes by becoming substrates for influx transporters such as vitamin, and amino acid transporters present on the blood brain barrier. We hypothesize that the ligand coupled PIs will permeate CNS efficiently by binding and translocating by membrane influx transporters/receptors and bypass membrane efflux pumps like P-gp and MRP. Our preliminary results clearly indicate that prodrugs designed for influx transporters can efficiently bypass efflux pumps. We also propose a novel pro-prodrug strategy where a dipeptide prodrug moiety will be designed so that it can be identified by the peptide transporter present on the intestinal epithelium. Once the dipeptide prodrug reaches systemic circulation it will be converted to amino acid prodrug which can then be identified by the amino acid transporters on the BBB. This pro-prodrug strategy will not only result in increased oral absorption of poorly absorbed PIs but also can result in higher CNS concentrations. Rat brain endothelial cells co-cultured with astrocytes will be employed as a model mimicking BBB to study permeation of the synthesized prodrugs. Finally, in-vivo rat brain microdialysis will be employed to delineate CNS pharmacokinetics of prodrugs following oral and IV administrations. Thus, overall hypothesis of this grant application is to design prodrug and pro-prodrug derivatives of PIs that may result in (a) circumvention of P-gp and MRP mediated efflux (b) higher permeability due to enhanced uptake by influx transporters, (c) higher oral bioavailability (d) lower plasma protein binding (e) higher metabolic stability against CYP 450 enzymes (f) higher aqueous solubility and (g) higher brain uptake.
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