Synthesis and Biological Evaluation of RGD and isoDGR–Monomethyl Auristatin Conjugates Targeting Integrin α(V)β(3)

This work reports the synthesis of a series of small‐molecule–drug conjugates containing the α(V)β(3)‐integrin ligand cyclo[DKP‐RGD] or cyclo[DKP‐isoDGR], a lysosomally cleavable Val‐Ala (VA) linker or an “uncleavable” version devoid of this sequence, and monomethyl auristatin E (MMAE) or F (MMAF) as the cytotoxic agent. The conjugates were obtained via a straightforward synthetic scheme taking advantage of a copper‐catalyzed azide–alkyne cycloaddition as the key step. The conjugates were tested for their binding affinity for the isolated α(v)β(3) receptor and were shown to retain nanomolar IC(50) values, in the same range as those of the free ligands. The cytotoxic activity of the conjugates was evaluated in cell viability assays with α(v)β(3) integrin overexpressing human glioblastoma (U87) and human melanoma (M21) cells. The conjugates possess markedly lower cytotoxic activity than the free drugs, which is consistent with inefficient integrin‐mediated internalization. In almost all cases the conjugates featuring isoDGR as integrin ligand exhibited higher potency than their RGD counterparts. In particular, the cyclo[DKP‐isoDGR]‐VA‐MMAE conjugate has low nanomolar IC(50) values in cell viability assays with both cancer cell lines tested (U87: 11.50±0.13 nm; M21: 6.94±0.09 nm) and is therefore a promising candidate for in vivo experiments.