Abstract 1145: AGX323 - A SAM-competitive, orally available inhibitor of protein arginine methyltransferase 5 (PRMT5) with potent cellular antiproliferative and in vivo antitumor activity against selected solid cancer types

Background: Protein arginine methyltransferase 5 (PRMT5) is an attractive therapeutic target because of its involvement in key oncogenic mechanisms including epigenetics, DNA response and RNA processing. Early development of PRMT5 inhibitors has focused on hematological malignancies although more recently several compounds have entered the clinic for both liquid and solid malignancies. Early Phase I clinical data indicate dose-limiting hematological toxicity, particularly anemia and thrombocytopenia. We focused our effort in discovering compounds with potent inhibitory activity against cells from solid cancer types known to overexpress PRMT5 and which correlated with poor prognosis (NSCLC, breast, bladder and pancreatic cancers). Methods: Enzyme inhibition was conducted using a PRMT5:MEP50 flash plate assay, with S-adenosylmethionine (SAM) as the methyl donor and histone H4 as substrate. Inhibition of cell proliferation was assessed in various cancer lines using a 7-day cellTitre-Glo (CTG) assay. Binding mode of the inhibitors to PRMT5 was investigated using surface plasmon resonance (SPR). In vivo antitumor activity was evaluated in various human cancer models grown in immunocompromised mice. Results: Structure activity relationship of a series of over 50 close analogs demonstrated that in vitro anticancer activities correlate poorly with enzyme inhibition potency. To understand this discrepancy we used SPR to study the PRMT5 binding mode of cell active and inactive compounds in the presence of various endogenous substrates (SAM, SAH or MTA). We obtained binding affinity (KD), association constant (Ka) and dissociation constant (Kd) of exemplars of active and inactive inhibitors and correlated their relationship to cell activity. The lead compound AGX323 has a KD of 1.8 x 10-10 M to SAM-bound PRMT5 and exhibited potent antiproliferative activity against a panel of over 20 human solid epithelial cancer cell lines from lung, breast, bladder and pancreas, with nM potency. For example, in the H1048 human lung cancer model it has an enzyme inhibition IC50 of 0.8 nM, a cell proliferation IC50 of 1.7 nM and when given to tumor bearing mice it demonstrated robust antitumor activity at well-tolerated dose levels. In nonclinical ADME studies, AGX323 demonstrated favorable drug-like properties: including acceptable oral bioavailability (in mouse, dog and monkey) and systemic exposure following oral administration, excellent metabolic stability, lack of significant drug-drug interaction liabilities (transporters, CYP inhibition, CYP induction). Conclusion: AGX323 demonstrated potentially best-in-class activity in PRMT5 dependent human solid cancer models and exhibited excellent drug-like properties. IND enabling studies are currently planned. Citation Format: Francis Y. F. Lee, Wen-Lian Wu, Zhiqiang Yang, John Tan. AGX323 - A SAM-competitive, orally available inhibitor of protein arginine methyltransferase 5 (PRMT5) with potent cellular antiproliferative and in vivo antitumor activity against selected solid cancer types [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1145.