Abstract 4596: Targeting arginine methylome in 9p21/MTAP-deleted malignant cancers with a next generation PRMT5-specific inhibitor CTS3497

Abstract It remains largely challenging to treat cancer patients with chromosome 9p21-deletion which occurs in approximately 15% of human cancers. Recently, specific targeting of protein arginine methyltransferase 5 (PRMT5), a key member of the type-II PRMT family and a master epigenetic modulator of arginine methylome essential for cancer progression, has emerged as a promising therapy in various hematological and solid tumors with MTAP deletion serving as a precision biomarker. To optimize the potential of targeting PRMT5, we developed CTS3497, a potent, brain-penetrable, orally bioavailable MTA-cooperative PRMT5 inhibitor, using our EpigenPLUSTM platform. The overall excellent drug-like properties of CTS3497 support fast track IND application, and it is anticipated to enter Phase 1 trials in 2024. CTS3497 demonstrated potent PRMT5 inhibitory activity in cellular pharmacodynamics (PD) assays, evidenced by the suppression of symmetric dimethylarginine (SDMA). Additionally, it exhibited profound cell growth inhibition with a low single-digit nM IC50 and 171-fold high selectivity for MTAPnull over isogenic MTAPwt cell lines. Moreover, CTS3497 displayed strong inhibitory effects on cell growth across diverse panels of MTAPnull cancer cell lines and patient-derived xenograft organoids (PDXOs) spanning various lineages. In vivo, oral administration of CTS3497 resulted in significant tumor regression in multiple MTAP-deleted xenograft models, notably MTAPnull brain tumors. The observed anti-tumor activity correlated with PRMT5 modulation, as indicated by reduced SDMA levels. Impressively, sustained tumor growth inhibition led to the complete tumor regression, with no regrowth even after discontinuation of CTS3497 treatment. The brain penetration property of CTS3497 underscores its therapeutic potential for both primary brain tumors, and brain metastases arising from specific cancers like lung cancer. At the molecular level, PRMT5 inhibition by CTS3497 induced substantial and distinctive RNA splicing defects and differentially expressed gene signatures, impacting key cellular activities downstream of arginine and epigenetic reprogramming in MTAPnull cancer cells. Furthermore, a synergistic anti-tumor effect was observed in multiple MTAP-deleted tumors when combining PRMT5 inhibition with CTS2190, an investigational type-I PRMT inhibitor currently in phase I/II clinical trials. In summary, CTS3497, a PRMT5 inhibitor selectively targeting MTAPnull tumor cells, demonstrated robust and durable anti-tumor responses both in vitro and in vivo across diverse lineages of MTAP-deleted tumors. Its superior ADME properties and favorable safety profiles observed in preclinical studies mark a breakthrough in the next generation epigenetic therapy. CTS3497 emerges as a promising therapeutic option for patients with MTAP-deleted malignancies. Citation Format: Yilin Liu, Long Wang, Hui Shi, Yuanyuan Liu, Qiugeng Ouyang, Xingnian Fu, Meng Wang, Jiannan Guo, Yiqin Wang, Youzhen Wang, Guoliang Xu, Yuan Mi, Haiping Wu. Targeting arginine methylome in 9p21/MTAP-deleted malignant cancers with a next generation PRMT5-specific inhibitor CTS3497 abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 4596.