Development of Multivalent FAP-Targeted Small Molecule–Drug Conjugates with Tailored MMAE Release Kinetics

Antibody-drug conjugates are one of the most diffused targeted therapeutic modalities for cancer treatment and consist of a tumor-targeted monoclonal antibody connected to a cytotoxic payload, which is released selectively at the tumor site. Small molecule-drug conjugates (SMDC) represent an alternative approach, in which the antibody is replaced by a tumor-homing small organic ligand. Thanks to their small molecular size, SMDCs are characterized by rapid extravasation and enhanced penetration in solid tumors compared with antibody-drug conjugates. We recently developed SMDCs targeting fibroblast activation protein (FAP), a cell surface endopeptidase abundant in the tumor microenvironment, using the highly specific FAP inhibitor OncoFAP as a targeting moiety. In this study, we compared the tumor-targeting properties and in vivo activity of SMDCs based on OncoFAP against products based on a stronger FAP inhibitor (i.e., trivalent OncoFAP), aiming to tune the release kinetic of the cytotoxic payload to the neoplastic site. We compared the kinetic profiles of the monovalent and trivalent derivatives of OncoFAP through in vivo and ex vivo biodistribution and therapy studies. The distinct in vivo monomethyl auristatin E (MMAE) release obtained for OncoFAP-GlyPro-MMAE and TriOncoFAP-GlyPro-MMAE did not lead to substantial differences in therapeutic efficacy in a preclinical FAP-positive cancer model.