Anti-cathepsin D antibody-drug conjugates for the targeted delivery of monomethyl auristatin F and E elicit potent anticancer activity in breast cancer

Breast cancer (BC) is the leading cause of cancer deaths in women. Patients with estrogen receptor-positive (ER⁺) and human epidermal growth factor receptor 2-negative or low (HER2^-; HER2^low) BC exhibit a late recurrence pattern and resistance to endocrine therapy. Antibody-drug conjugates (ADCs) are new anticancer agents that currently show a remarkable acceleration in preclinical and clinical development. The aspartic protease cathepsin D (cath-D), a poor prognosis marker in BC, is a tumor cell-associated protein with pro-tumor activity that is highly and specifically secreted by BC cells. We recently showed using the human anti-cath-D antibody F1M1 that cath-D is a potent target for antibody-based therapy in BC. Here, we asked whether cath-D is a suitable target for ADC-based therapy in non-amplified ER⁺/HER2^low BC. To this aim, we bioconjugated F1M1 to the cytotoxic payload monomethyl auristatin F (MMAF) via a non-cleavable linker (for the release of a non-cell permeable MMAF metabolite) and to its cell permeable derivative monomethyl auristatin E (MMAE) via a protease-sensitive cleavable linker, to form the ADCs F1M1-MMAF and F1M1-MMAE respectively, with a drug-to-antibody ratio close to four. These F1M1-based ADCs were specifically internalized and showed cytotoxic activity in the nanomolar range in the ER⁺/HER2^low MCF-7 and endocrine therapy-resistant ER⁺/HER2^low MCF-7/LCC2 BC cell lines. In vivo, three injections of F1M1-based ADCs led to long-lasting tumor growth inhibition in mice harboring MCF-7/LCC2 cell xenografts. In conclusion, we showed for the first time that cath-D is a potent target for ADC in BC.