Exploring a "Nature-Inspired" Strategy for Enhancing the Pharmacology of Small Molecules: Towards the Development of Next Generation Anti-Infectives.

Abstract

HIV is now a pandemic of staggering proportions. Unfortunately, the current generation of antiretroviral therapeutics is only partially and temporarily effective in suppressing viral replication due to rapid metabolism leading to liver toxicity, resistance, and poor pharmacokinetics. Thus, there is a burgeoning need to develop innovative approaches for prolonging the lifetimes of these drugs.

Here, we propose a novel and potentially general methodology for enhancing the stability of HIV-1 protease inhibitors and other small molecules. This strategy emerged from our interest in understanding the unusual pharmacology behind the natural product FK506, a compound that is an excellent substrate for metabolic P450 enzymes in vitro, yet has an unexpectedly long half-life in humans (t½ ~40 hours). This apparent contradiction may partially be explained by the observation that FK506 is predominantly sequestered into the cytosol of peripheral blood cells. Both erythrocytes and leukocytes are a rich source of FKBP, yet little metabolism occurs here as these cells do not express significant levels of P450 enzymes. Therefore, we hypothesized that FKBP binding compounds, such as FK506, might be protected from exposure to metabolic enzymes by partitioning into this protected cellular niche.

During my thesis research, we tested this model by designing and synthesizing a series of bifunctional, FKBP-binding small molecules and evaluating their pharmacokinetic properties in vitro and in vivo. These efforts led to the discovery of SLFavir, an FKBP-binding antiviral with nanomolar potency against HIV-1 protease. We determined that SLFavir is sequestered into the cytosol of erythrocytes and that its lifetime in mice is improved by >20-fold. Furthermore, we observed that binding to FKBP partially blocks its interactions with the CYP3A4 P450 isozyme, making it less susceptible to degradation. Finally, to enable modular synthesis of additional bifunctional compounds, we developed a chemical platform using microwave-assisted olefin cross metathesis to rapidly append FK506 to other drugs, including the antibiotics ampicillin and ciprofloxacin. The key discovery made during my thesis research is that covalently tethering FKBP-binding groups to existing drugs dramatically improves their persistence via cellular partitioning. We expect this “nature-inspired” strategy to yield antivirals and other small molecules with exciting new pharmacokinetic profiles.