Characterization of the mechanism of action of new HIV-1 reverse transcriptase-associated ribonuclease H inhibitors

HIV-1 the Reverse Transcriptase (RT), the most renowned retroviral specific enzyme, was the first anti-HIV target to be exploited, as. HIV-1 RT combines two functions essential for viral replication: DNA polymerase, synthesis DNA either in a RNA dependent (RDDP) or DNA dependent (DDDP) manner, and Ribonuclease H (RNase H) . The RNase H activity catalyzes highly specific hydrolytic events on the RNA strand of the RNA/DNA replication intermediate, critical to the synthesis of integration-competent double-stranded proviral DNA. Because of its essential role, RNase H is a promising target for drug development. However, despite years of efforts, no RNase H inhibitor (RHI) has yet reached clinical approval. In this work we pursued the identification and characterization of new promising RHIs targeting either the RNase H active site itself (RNase H active site chelating agents) or both RNase H and RDDP activities (allosteric dual inhibitors). The first approach faced the challenging nature of the RNase H active site region, the morphology of wich is, more open than that of the relatively similar,HIV-1 integrase (IN). This hampers the identification of a druggable pocket. We initially used Foamy Virus RT as a tool, to perform NMR and docking analyses on the interaction between FV RT RNase H domain and a previously identified diketo acid (DKA) derivative, inhibitor RDS1643. The amino acid residues of the FV RNase H active site region (T641, I647, Y672 and W703) were established to be important for the interaction with the inhibitor and analogous residues were successfully identified in the HIV-1 RNase H domain using structural overlays. Further docking and site directed mutagenesis studies were performed using six couples of ester/acid DKA, derived from RDS1643, showing for the first time, a broad interaction between RHIs and conserved residues in the HIV-1 RNase H active site region (R448, N474, Q475, Y501 and R557). Moreover, ester and acid derivatives exhibited a different binding orientation, that reflected a different specificity for RNase H versus IN. Among the synthesised derivatives one, RDS1759, showed to be an RNase H selective active site inhibitor characterized also, for the first time,in cell-based assays. The second approach focused on the determination of the mechanism of action of a new isatine-derived RNase H/RDDP dual inhibitor, RMNC6. Docking analysis and site directed mutagenesis results suported the hypothesis of a two-sites mode of action, with an independent role for two pockets,to be further characterized for a rational optimization of the scaffold.