biostudies-literatureUnknownJaishankar PNIAID NIH HHSNIGMS NIH HHSThe COVID-19 pandemic made plain the need for effective antivirals acting on novel antiviral targets, among which viral macrodomains have attracted considerable attention. We recently described AVI-4206 (<b>1</b>), a potent and selective inhibitor of the SARS-CoV-2 ADP-ribosylhydrolase Mac1 based on a 9<i>H</i>-pyrimido[4,5-<i>b</i>]indole core, the first Mac1 inhibitor to demonstrate antiviral efficacy in mouse models of SARS-CoV-2 infection, but requiring IP administration and frequent dosing. Herein we describe an extensive, structurally enabled medicinal chemistry effort to identify orally bioavailable Mac1 inhibitors by addressing permeability and efflux liabilities of <b>1</b> and many of its analogs. Multiple strategies were pursued to overcome these issues, including replacing a urbioRxiv : the preprint server for biology2025.10.11.681833https://www.ebi.ac.uk/biostudies/studies/S-EPMC12632963biostudies-literatureDiscovery of AVI-6451, a Potent and Selective Inhibitor of the SARS-CoV-2 ADP-Ribosylhydrolase Mac1 with Oral Efficacy in vivo.PMC12632963P30 GM133894U19 AI171110P30 GM124169Rachman MMCorrey GJKrogan NJJaishankar PMontano MFraser JSRenslo ARTaha TYShoichet BKZheng JPampel JStevens MGVHantz ERAshworth AOtt MMatsui YTogo TRosecrans JDiolaiti MEfalseDiscovery of AVI-6451, a Potent and Selective Inhibitor of the SARS-CoV-2 ADP-Ribosylhydrolase Mac1 with Oral Efficacy in vivo.The COVID-19 pandemic made plain the need for effective antivirals acting on novel antiviral targets, among which viral macrodomains have attracted considerable attention. We recently described AVI-4206 (<b>1</b>), a potent and selective inhibitor of the SARS-CoV-2 ADP-ribosylhydrolase Mac1 based on a 9<i>H</i>-pyrimido[4,5-<i>b</i>]indole core, the first Mac1 inhibitor to demonstrate antiviral efficacy in mouse models of SARS-CoV-2 infection, but requiring IP administration and frequent dosing. Herein we describe an extensive, structurally enabled medicinal chemistry effort to identify orally bioavailable Mac1 inhibitors by addressing permeability and efflux liabilities of <b>1</b> and many of its analogs. Multiple strategies were pursued to overcome these issues, including replacing a ur2025-01-01T00:00:00Z2025 Oct2026-06-04T03:16:02.101Z2026-06-04T03:11:17.733ZS-EPMC126329634127916010.1101/2025.10.11.681833