Dually targeting the epigenetic proteins lysine specific demethylase 1 (LSD1) and histone deacetylases (HDACs) that play a key role in cancer cells by modulating gene repressor complexes including CoREST will have a profound effect in inhibiting tumour growth. Here, we evaluated JBI-097 a dual LSD1/HDAC6 inhibitor, for its in vitro and in vivo activities in various tumor models. In vitro, JBI-097 showed a strong potency in inhibiting LSD1 and HDAC6 enzymatic activities with the isoform selectivity over other HDACs. Cell-based experiments demonstrated a superior anti-proliferative profile against haematological and solid tumor cell lines. JBI-097 also showed strong modulation of HDAC6 and LSD1 specific biomarkers, alpha-tubulin, CD86, CD11b, and GFi1b. In vivo, JBI-097 showed a stronger effect in erythroleukemia, multiple myeloma xenograft models, and in CT-26 syngeneic model. JBI-097 also showed efficacy as monotherapy and additive or synergistic efficacy in combination with the standard of care or with immune checkpoint inhibitors. These and other findings suggest that JBI-097 could be a promising molecule for targeting the LSD1 and HDAC6. Further studies are warranted to elucidate the mechanism of action.

Introduction: Protein arginine methyltransferases (PRMTs) transfer a methyl group from the cofactor S-adenosyl-L-methionine (SAM) to the arginine of a substrate protein (1,2). PRMT5 is a type II protein arginine methyltransferase that catalyzes the symmetrical dimethylation. PRMT5 drives transcriptional suppression of cell cycle control and tumor suppressor genes by hypermethylating promoter histones H3R8 and H4R3. Recently, dysregulation of the splicing machinery has been identified to be one of the therapeutic vulnerabilities for PRMT5 inhibition in multiple cancers. Therefore, inhibitors selectively targeting PRMT5 could be of high clinical value in cancers with defects in spliceosome machinery. In this regard, we are developing a series of novel substrate competitive PRMT5 inhibitors to target brain cancer and solid tumors with brain metastasis. Methods: Structure based drug design was used to identify novel PRMT5 inhibitors. To assess in vitro potency, flash plate assay was used. Cell based activity was assessed by measuring the symmetrical dimethylation of SmD3, intron retention and long-term cell proliferation assays. Tumor growth inhibition was measured in orthotopic glioblastoma models in mice. Results: Our lead PRMT5 inhibitor JBI-778 had a strong in vitro potency against PRMT5 where the EC50 was ~5 nM and a similar cellular potency (< 10 nM) in inhibiting symmetric dimethylation of arginine. JBI-778 showed a strong anti-proliferative activity in select Neuroblastoma, Medulloblastoma, NSCLC, and glioma cell lines where the IC50s ranged from 27 to 700 nM. Of note, NSCLC cell lines with mutation in the spliceosome protein RBM10 appeared to be more sensitive to JBI-778 treatment, which is consistent with enhanced retention of introns that are reported to be PRMT5 substrates including ATM, POLD and PNISR. JBI-778 showed an excellent oral bioavailability with a very high and sustained brain exposure in rodents. In U87-MG human GBM orthotopic model, oral administration of the JBI-778 resulted in strong and dose dependent efficacy with complete (~95 %) tumor growth inhibition and regression in few mice and translated into significant survival advantage. In 005 mouse GBM orthotopic model that mimics human GBM, JBI-778 showed a strong tumor growth inhibition and significantly extended survival (40 in control vs 58 days in 778 treated, p<0.05). IND-enabling GLP toxicology studies conducted in rodent and non-rodent species did not detect anemia or thrombocytopenia even at higher than efficacious concentrations. Conclusion: PRMT5 inhibitor JBI-778 is able to penetrate the brain in all animal species tested and it is highly efficacious in orthotopic glioma models. Its differentiated mechanism makes it a potential option for the treatment of gliomas and brain metastasis with a focus on patients with spliceosome mutations. IND application for JBI-778 has been approved by FDA and a FIH trial is being planned. Citation Format: Dhanalakshmi Sivanandhan, Chandru Gajendran, Naveen Sadhu M, Zainuddin Mohammed, Ramachandraiah Gosu, Divsha Sher, Shahar Mansur, Dinorah Friedmann-Morvinski, Sridharan Rajagopal, Luca Rastelli. JBI-778, a novel brain-penetrant small molecule PRMT5 inhibitor for treatment of cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6269.