Shifting the Antibody–Drug Conjugate Paradigm: A Trastuzumab-Gold-Based Conjugate Demonstrates High Efficacy against Human Epidermal Growth Factor Receptor 2-Positive Breast Cancer Mouse Model

Antibody–drug conjugates (ADCs) combine the selectivity of monoclonal antibodies (mAbs) with the efficacy of chemotherapeutics to target cancers without toxicity to normal tissue. Clinically, most chemotherapeutic ADCs are based on complex organic molecules, while the conjugation of metallodrugs to mAbs has been overlooked, despite the resurgent interest in metal-based drugs as cancer chemotherapeutics. In 2019, we described the first gold ADCs containing gold–triphenylphosphane fragments as a proof of concept. The ADCs (based on the antibody trastuzumab) were selective and highly active against HER2-positive breast cancer cells. In this study, we developed site-specific ADCs (Thio-1b and Thio-2b) using the cysteine-engineered trastuzumab derivative THIOMAB antibody technology with gold(I)-containing phosphanes and a maleimide-based linker amenable to bioconjugation (1b and 2b). In addition, we developed lysine-directed ADCs with gold payloads based on phosphanes and N-heterocyclic carbenes featuring an activated ester moiety (2c and 5c) with trastuzumab (Tras-2c and Tras-5c) and another anti-HER2 antibody, pertuzumab (Per-2c and Per-5c). Both sets of ADCs demonstrated significant anticancer potency in vitro assays. Based on these results, one ADC (Tras-2c), containing the [Au(PEt3)] fragment present in FDA-approved auranofin, was selected for an in vivo antitumor efficacy study. Immunocompromised mice xenografted with the HER2-positive human cancer cell line SKBR-3 exhibited almost complete tumor reduction and low toxicity with intravenous administration of Tras-2c. With this highly selective targeting system, we demonstrated that a subnanomolar cytotoxicity profile in cells is not required for an impressive antitumor effect in a mouse xenograft model.