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Targeting ML-IAP for the Design of Cancer Therapeutics


Metastatic malignant melanoma is the leading cause of skin cancer-related death with a 5-year survival rate smaller than 19%. Most malignant melanomas overexpress an oncogene called melanoma inhibitor of apoptosis (ML-IAP), a member of inhibitor of apoptosis (IAP) proteins, that also include oncogenic proteins, XIAP, cIAP1, and cIAP2. These proteins render cancer cells resistant to apoptosis induced by anti-cancer therapies. ML-IAP is overexpressed in melanoma and several other solid tumors, but it is not present in normal adult tissues, making it potentially an ideal target for novel apoptosis-based therapies. However, studies to validate its potential as a therapeutic target have been hampered by the lack of potent and selective pharmacological inhibitors.

To this end, we first characterized IAP antagonists that have been recently designed to mimic the interactions between an endogenous IAP antagonist, namely the second mitochondria-derived activator of caspases protein (SMAC), and IAPs. SMAC pro-apoptotic activity is based on a conserved IAP-binding motif of sequence Alanine-Valine-Proline-Isoleucine/Phenylalanine (AVPI/F). This tetra-peptide releases caspases that were sequestered by IAPs, thus restoring apoptosis. Several AVPF mimetics have been reported that target all IAPs indiscriminately, making it difficult to dissect the role of one oncogenic IAP versus another using these available pharmacological inhibitors.

Hence, first we employed a highly innovative structure-driven approach to target covalently a nucleophilic Lysine (Lys) residue present in the AVPI/F binding site of both ML-IAP and XIAP, but not in cIAP1 or cIAP2. Pioneering the use of certain Lys-targeting electrophiles including aryl-sulfonyl fluorides and aryl-fluorosulfates, and exploiting subtle structural differences in the sub-pockets that accommodate the third and the fourth residue of AVPF between XIAP and ML-IAP, we were able to derive agent 142I5 as a first-in-class potent and selective Lys-covalent ML-IAP inhibitor. We demonstrate that the ML-IAP targeting agent 142I5 is as effective as pan-IAP inhibitors in restoring apoptosis in apoptosis resistant melanoma cell lines. In summary, we derived an innovative and unprecedented pharmacological tool targeting ML-IAP covalently that can be used to further characterize the role of this oncogene in cancer resistance and could provide a valuable steppingstone for the development of novel apoptosis-based therapeutics.

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