Delivery\nof Monomethyl Auristatin F to the Tumor Microenvironment\nwith Noninternalizing Fibroblast Activation Protein-Cleavable Small\nMolecule–Drug Conjugates Elicits Potent <i>In Vivo</i> Anticancer Activity

OncoFAP is an ultrahigh affinity ligand of fibroblast\nactivation\nprotein (FAP), a tumor-associated antigen overexpressed in the stroma\nof the majority of solid tumors. OncoFAP has been previously implemented\nas a tumor-homing moiety for the development of small molecule drug\nconjugates (SMDCs). In the same context, the glycine–-proline\ndipeptide was included with the aim to selectively undergo cleavage\nonly in the presence of the target FAP, triggering the consequent\nrelease of the cytotoxic payload in the tumor microenvironment. In\nthis work, we evaluate the use of monomethyl auristatin F (MMAF) as\na payload, a close derivative of MMAE bearing a charged carboxylic\nacid that hampers its cellular permeability, typically employed in\nthe development of internalizing antibody–drug conjugates.\nThe novel OncoFAP-GlyPro-MMAF and the previously described OncoFAP-GlyPro-MMAE\nwere compared in a head-to-head therapeutic experiment in mice bearing\nFAP-positive tumors. Surprisingly, the MMAF conjugate mediated potent\nantitumor activity, despite its poor cellular permeability.