Evasion of efflux and enhancing bioavailability of Amprenavir by prodrug derivatization

Millions of people worldwide have been infected with human immunodeficiency (HIV). Highly active antiretroviral therapy (HAART) has substantially improved the clinical management of HIV-1 infection. Inclusion of HIV protease inhibitors (PIs) in HAART has substantially improved clinical outcomes of HIV infected patients. However complete eradication of virus poses an unmet goal. Major challenge with current anti-HIV drug regimen is the inability of PIs to attain sufficient concentrations in sanctuary sites such as brain resulting in persistent viral replication and may lead to viral resistance. Sub-therapeutic concentration of PIs in brain leads to drug resistance and HIV associated dementia. One of the major factors which limit central nervous system (CNS) permeation of PIs is the presence of efflux proteins on brain capillary endothelial cells. There are several reports demonstrating that PIs are substrates of efflux proteins which limit their oral bioavailability and brain permeation. Various approaches have been employed to evade the efflux and improve the bioavailability, however improving brain absorption still remains as an elusive goal. The overall objective of this dissertation project was to improve oral and brain bioavailability of Amprenavir (APV) by prodrug strategies targeting nutrient transporters to bypass P-glycoprotein (P-gp) mediated efflux. APV has low bioavailability and negligible brain absorption which is attributed by low aqueous solubility, efflux mediated by multidrug resistance proteins, extensive metabolism and protein binding. In order to improve oral bioavailability and absorption across blood brain interfaces, di-peptide and amino acid prodrugs of APV (Val-Val-APV, Gly-Val-APV, Val-APV) were synthesized and evaluated for interaction with efflux and influx transporters, enzymatic hydrolysis, and bioavailability. These prodrugs targeting peptide transporters were demonstrated to evade P-gp mediated efflux and exhibit better permeability characteristics compared to APV. All these prodrugs have higher aqueous solubility. Enzymatic stability and metabolism studies demonstrated that these prodrugs undergo enzymatic hydrolysis and regenerate parent drug which exerts pharmacological action. Oral and brain absorption of prodrugs was much higher as compared to APV in Sprague-Dawley rats. Thus transporter targeted prodrug modification of P-gp substrates could lead to shielding of these drug molecules from efflux pumps and improve bioavailability of PIs and other P-gp substrates.