Development of PRC1 Inhibitors Employing Fragment-Based Approach and NMR-Guided Optimization

Polycomb Repressive Complex 1 (PRC1) is associated with transcriptional silencing, and its dysregulation plays an important role in various cancers. Well-characterized PRC1 inhibitors can facilitate the exploration of PRC1 inhibition as therapeutic agents. By employing an NMR-based fragment screening approach, we have previously identified a very weak millimolar ligand RB-1, which directly binds to RING1B-BMI1. Then, we reported a low-micromolar PRC1 inhibitor, RB-3, which is active in leukemic cells, showing inhibition of H2A ubiquitylation and modulation of target genes. Here, we describe details of the optimization campaign of RB-1 into potent PRC1 inhibitors by guiding the SAR employing two NMR approaches and a probe-based biochemical assay. These efforts, combined with medicinal chemistry optimization, resulted in the development of RB-3 and slightly improved RB-4. We have demonstrated that RB-4 binds to both RING1A and RING1B proteins and inhibits the activity of RING1B-BMI1 and RING1B-PCGF1, representing both canonical and noncanonical PRC1 complexes.