Abstract: The human C-C chemokine receptor type 5 (CCR5) is a validated therapeutic target in HIV, and an important emerging target in a host of additional indications including Chagas disease, cancer, and liver and kidney fibrosis. While the FDA-approved drug Marviroc has proven effective as an allosteric inhibitor of CCR5-mediated HIV entry, resistance to Maraviroc is a known complication. An attractive alternative strategy to allosteric inhibition is to ablate production of CCR5. It has recently been discovered that translation of CCR5 is subject to a process known as a -1 programmed ribosomal frameshift, or -1 PRF. In this process, interaction of the ribosome with a structured portion of the CCR5 mRNA causes it to shift backwards 1 nucleotide. This results in production of a non-functional protein, and triggers degradation of CCR5 mRNA via nonsense-mediated decay. We hypothesize that compounds able to specifically bind to and stabilize this structured portion of the CCR5 mRNA, termed a frameshift- stimulatory sequence, or FSS, will act as frameshift enhancers and constitute a new approach to inhibiting CCR5 production. Our hypothesis is grounded on our success with using an innovative library screening strategy, termed Resin-Bound Dynamic Combinatorial Chemistry (RBDCC) to identify compounds able to selectively bind an FSS RNA responsible for frameshifting in HIV. Medicinal chemistry efforts have resulted in the production of compounds with nanomolar affinity, high selectivity, and antiviral activity in human cells. We will apply an analogous approach to the identification of compounds binding the CCR5 FSS RNA, and will develop a new target- competitive method for RBDCC screening. Libraries will incorporate substituted diketopiperazines (a privileged substructure in medicinal chemistry), thus addressing an urgent need for new RNA-binding chemotypes. After prioritizing hit compounds based on biophysical measurements, including assessment of binding to off-target RNA sequences, a panel of cellular assays will be employed to assess the ability of CCR5 FSS-targeting compounds to reduce production of the receptor, and inhibit HIV infectivity. Completion of this research will provide the first compounds able to influence a human frameshifting event, and proof-of-concept for a novel CCR5 RNA-targeted therapeutic strategy.

Project Narrative: CCR5 is a human cell-­surface receptor protein used by HIV for cellular  entry. Inhibition of CCR5 has also been suggested as an important therapeutic goal in many  other human diseases, including cancer, fibrosis, and Chagas disease. We propose to identify  compounds able to interfere with production of CCR5 via a novel RNA-­mediated mechanism,  termed frameshifting.