1325 Preclinical activity and safety profile of JANX007, a novel PSMA-targeting tumor-activated T Cell engager for treatment of metastatic castration-resistant prostate cancer

Background Metastatic castration-resistant prostate cancer (mCRPC) remains an incurable disease. Bispecific T cell engag-ers (TCEs) targeting prostate-specific membrane antigen (PSMA) and CD3 on T cells showed great clinical potential for the treatment of mCRPC. However, cytokine release syn-drome (CRS) and poor pharmacokinetic (PK) profile hinder their further development. To overcome these challenges, Janux has developed JANX007, a tumor-activated T cell engager (TRACTr) with enhanced safety and PK properties. JANX007 is a humanized trispecific protein that contains PSMA- and CD3-binding domains, an albumin binding domain to extend circulating half-life, and a CD3 inhibitory peptide mask fused to the molecule through tumor protease cleavable linker. Only when tumor-resident proteases cleave the TRACTr and enable mask separation can the resulting active molecule bind CD3. This cleavage-dependent CD3 agonism can poten-tially limit systemic toxicity associated with broad T cell activation. Methods Peptide masks against the CD3 binding domain were identified via phage display. Mask efficiency was evaluated using human CD3 ELISAs. Masking and cleavable linker stability was characterized in human (healthy and mCRPC donor) and cynomolgus monkey serum. JANX007-induced cleavage-dependent activation of T cells was evaluated in human PBMC/prostate tumor cell in vitro co-culture assays.