In the last decades targeted drugs have improved cancer treatment, but revealed to be ineffective mainly in the treatment of solid tumors, largely because of tumor heterogeneity, activation of redundant pathways, and drug resistance. A common and central signal transduction mechanism in many oncogenic pathways is the phosphorylation of proteins at serine or threonine residues followed by proline (S/T-P). Importantly, the phospho-S/T-P motifs of these proteins are recognized by the peptidyl-prolyl cis/trans isomerase (PPIase) PIN1, which catalyzes the cis-trans or trans-cis conformational change around the S-P or T-P bond. Among PPIases, PIN1 is the only enzyme able to efficiently bind proteins containing phosphorylated S/T-P motifs. As a consequence, the phosphorylation dependent prolyl-isomerase PIN1 acts as a critical modifier of multiple signaling pathways. It is overexpressed in the majority of cancers and its activity strongly contributes to tumor initiation and progression. Conversely, inactivation of PIN1 function curbs tumor growth and cancer stem cell expansion, restores chemosensitivity and blocks metastatic spread, thus providing the rationale for a therapeutic strategy based on PIN1 inhibition. Notwithstanding, potent PIN1 inhibitors are still missing from the arsenal of anti-cancer drugs. By a mechanism-based screening we have identified a novel covalent PIN1 inhibitor, KPT-6566, able to selectively inhibit PIN1 among other prolyl-isomerases, and target it for degradation. We demonstrate that KPT-6566 covalently binds to the catalytic site of PIN1. This interaction results in the release of a quinine-mimicking drug that generates reactive oxygen species and DNA damage inducing cell death specifically in cancer cells. Accordingly, KPT-6566 treatment impairs PIN1-dependent cancer phenotypes in vitro and growth of lung metastasis in vivo.
