Amanitin-Based Fc-Small Molecule Drug Conjugates with Non-Cleavable Linker: A Novel Therapeutic Strategy for Prostate Cancer Targeted Therapy

Prostate cancer remains a major global health burden, with limited options and poor prognosis in advanced stages. To extend survival and improve quality of life for patients, targeted therapies have become an emerging treatment modality. Antibody-drug conjugates (ADCs) have shown huge clinical success but only limited therapeutic effects in prostate cancer since their large size limits tumor penetration. Small-molecule drug conjugates (SMDCs), consisting of a small molecule as binding moiety and a cytotoxic drug, offer advantages due to their smaller size, but their short plasma half-life and poor efficacy hampered their clinical breakthrough so far. Fc-grafted SMDCs (Fc-SMDCs) combine a small-molecule drug conjugate with the half-life extending Fc fragment of an antibody. So far, the Fc fragments have been attached directly to the Small-molecule drug complex making an enzymatic cleavable linker for payload release an indispensable prerequisite. We report a first-in-class Fc-SMDC targeting PSMA and carrying α-amanitin with a non-cleavable linker. The payload is conjugated directly to the Fc region via an engineered cysteine, to separate it from the targeting moiety. This approach enables in contrast to conventional Fc-SMDCs the use of non-cleavable linkers for minimal premature drug release, increased plasma stability and low systemic toxicity. Due to the non-cleavable linker, our conjugate showed a high tolerability and an extended half-life resulting in prolonged tumor exposure and an excellent anti-tumor efficacy in xenograft models. Together with the favorable tolerability in non-human primates, these findings highlight the potential as a next-generation treatment for prostate cancer.