New targeted chemotherapeutics are urgently needed to minimize off-target toxicity and reduce the high-mortality rate associated with metastatic prostate cancer. Herein, we report on the modular synthesis, pharmacokinetics, and efficacy of two small-molecule-drug conjugates (SMDC) targeted to prostate-specific membrane antigen (PSMA) incorporating either: (i) a cathepsin-B-cleavable valine-citrulline (Val-Cit), or (ii) an acid-cleavable phosphoramidate linker. Crucial components used in the design of the conjugates include: (i) CTT1298, a nanomolar affinity ligand that binds irreversibly to PSMA and has proven in past studies to rapidly internalize and shuttle payloads into PSMA-expressing prostate cancer cells, (ii) MMAE, a known potent cytotoxic payload, and (iii) an albumin-binder, proven to improve residence time of drug conjugates. At dose of 0.8 mg/kg (∼250 nmol/kg), the two SMDCs showed significant efficacy in a PSMA(+) PC3-PIP mouse model of human prostate cancer compared with controls, without inducing systemic toxicity. Though localization of the SMDCs was observed in tissues apart from the tumor, release of MMAE was observed predominantly in tumor tissue, at levels that were 2-3 orders of magnitude higher than non-target tissues. Furthermore, SMDC2, which incorporated a novel pH-responsive phosporamidate linker, demonstrated significantly improved efficacy over SMDC1 that has a Val-Cit linker, with a 100% survival over 90 days and 4 out of 8 mice showing complete tumor growth inhibition after 6 weekly doses of 0.8 mg/kg (244 nmol/kg). Our findings demonstrate the potential of irreversible PSMA inhibitors combined with pH-responsive linkers as a way to specifically deliver chemotherapeutic drugs to prostate cancer tumors with minimal toxicity.
©2022 American Association for Cancer Research.