Utilizing the HiBiT System to Identify CARM1 Degraders for Targeted Cancer Therapy.

Preclinical studies validated coactivator-associated arginine methyltransferase 1 (CARM1) as a targetable therapeutic vulnerability, leading to the development of Proteolysis-Targeting Chimeras that specifically degrade CARM1. These compounds face significant translational challenges, including poor oral bioavailability and limited metabolic stability, which require extensive optimization. To identify more drug-like CARM1 degraders, we developed a high-throughput screening platform. We enabled antibody-free monitoring of CARM1 levels by fusing a HiBiT tag to CARM1 in MCF7 breast cancer cells. Complementation with LgBiT produces luciferase activity. Using this platform, we screened 1408 plant-derived natural product fractions to identify compounds that reduce CARM1 protein levels. This screen revealed two promising natural compounds, kusunokinin and exostemin, that specifically target CARM1 for degradation with selectivity over other protein arginine methyltransferases. Both compounds demonstrated functional anticancer activity, significantly inhibiting breast cancer cell colony formation and migration. Kusunokinin and exostemin represent lead compounds for developing next-generation CARM1-targeted therapeutics with enhanced translational potential.