Membrane permeability of HIV-1 protease inhibitors.
According to the 2012 UNAIDS global report, sub-Saharan Africa hosts 69% of the world’s total population living with HIV, South Africa being the most affected with a reported 24% incidence rate. To date, extensive research is being conducted globally, particularly involving anti-HIV treatment that targets the retroviral enzymes: reverse transcriptase, integrase and protease. The discovery of inhibitors to HIV protease which disrupts virion protein assembly has made this enzyme a prime target of anti-retroviral therapies, thus there exists a concerted research initiative to identify compounds with HIV protease inactivation potential. This study employs HIV protease that is isolated and purified from a genetically modified HIV protease overexpressing Escherichia coli strain to monitor the inhibitory capacity of new lead compounds. Optimized growth conditions for HIV protease production displayed that the use of chemically defined media resulted in higher yields of the enzyme. Recent research studies have shown that peptide-based cage and glycosylated compounds displayed HIV protease inhibitor activity in cell free enzymatic reactions that are comparable to commercially available HIV protease inhibitors. However, in contrast it has also been reported that these inhibitors are inactive in whole T-cell assays, when employing HIV infected CD4 cells. It is a well-known fact that potential new chemical entities that do not possess oral bioavailability, in terms of their absorption properties, are not successful candidates within the drug discovery industry. Following this, the current study was designed to determine if inefficient membrane permeability of these promising anti-HIV protease lead compounds could result in their inactivity in whole T-cell assays. Two different methods were considered, a cell-based method using the Madin Darby Canine Kidney strain I (MDCKI) cell line and a non-cell based method, the parallel artificial membrane permeability assay (PAMPA). MDCKI cells have been extensively used to form monolayers that mimic human intestinal membranes whilst the PAMPA utilizes an artificial lipid membrane composition on a filter support. Data from permeability assays using the novel chemically synthesized inhibitors have been compared to commercially available drugs, antipyrine, metoprolol and caffeine, which displayed efficient membrane permeability characteristics, thereby validating the assay. The results indicated that novel cage-derived and glycosylated peptide inhibitors do not possess sufficient passive diffusion properties which may explain their inactivity in whole T-cell assays.