Discovery of HRO761, a novel, first-in-class clinical stage WRN inhibitor with potent anti-tumor activity in microsatellite instability high cancers

The RecQ DNA helicase WRN was identified as a synthetic lethal target in MSI^high tumors by several genetic screens. Despite recent advances in the treatment of MSI^high cancers by immune checkpoint inhibitors, a significant proportion of patients still fails to respond to or relapses after single agent anti-PD1 or combination of anti-PD1 plus anti-CTLA4 treatments. We present the discovery, biochemical, structural and pharmacological characterization of the first potent and selective WRN helicase inhibitor, HRO761. Iterative high throughput screening with a more sensitive assay format informed by a covalent hit resulted in a single validated hit amid thousands of artefacts. Despite a lipophilic efficient(lipE)-driven optimisation, functional activity could only be obtained at the cost of high molecular weight (MW). The simultaneous optimization of permeability and lipE was achieved by chameleonic transformations identified with the help of a computational assay. Despite a MW above 700 Da, HRO761 has high permeability, low lipophilicity, and low clearance, leading to excellent PK profiles across preclinical species. HRO761 is an allosteric WRN inhibitor that binds at the interface of the D1 and D2 helicase domains, thus locking WRN in an inactive conformation. Pharmacological inhibition of WRN by HRO761 recapitulates the phenotype observed by WRN genetic suppression, validating WRN as a therapeutic target in MSI^high cancers. A phase 1 clinical trial with HRO761 is currently ongoing to assess the safety, tolerability, and preliminary anti-tumor activity in patients with MSI^high colorectal cancer and other MSI^high solid tumors.