Cellular Inhibitor of Apoptosis Protein 1 (cIAP1), Cellular Inhibitor of Apoptosis Protein 2 (cIAP2), and X-linked Inhibitor of Apoptosis Protein (XIAP) (cIAP1/2 and XIAP)

Cellular Inhibitor of Apoptosis Protein 1 (cIAP1), Cellular Inhibitor of Apoptosis Protein 2 (cIAP2), and X-linked Inhibitor of Apoptosis Protein (XIAP) are key members of the Inhibitor of Apoptosis Protein (IAP) family that regulate cell death and survival [1, 11]. XIAP is the most potent endogenous inhibitor of caspases, directly binding to and neutralizing caspases-3, -7, and -9 to prevent the execution of apoptosis [1, 18]. In contrast, cIAP1 and cIAP2 primarily function as E3 ubiquitin ligases that regulate NF-kappaB signaling and the assembly of pro-survival signaling complexes, such as those associated with TNF receptors [13, 14]. These proteins are frequently overexpressed in various cancers, including head and neck, breast, and prostate cancers, where they contribute to tumor cell survival and resistance to therapy [6, 15]. Therapeutic strategies targeting these proteins utilize small-molecule Smac mimetics, which mimic the endogenous IAP antagonist Smac/DIABLO [1, 7]. These drugs induce the rapid auto-ubiquitination and degradation of cIAPs and antagonize XIAP-caspase interactions, thereby restoring apoptotic sensitivity and promoting tumor cell death, often in a TNF-alpha-dependent manner [3, 8]. Clinical development of these inhibitors, such as xevinapant and tolinapant, has focused on sensitizing tumors to chemotherapy and radiotherapy [7, 11]. Notable safety concerns associated with this class of drugs include cytokine release syndrome and hepatotoxicity, which are linked to the systemic induction of pro-inflammatory cytokines [7].

Other names

BIRC2BIRC3BIRC4API1API2API3HIAP1HIAP2HIAP3MIHAMIHBMIHCRNF48RNF49XLP2

Smac mimetics bind to the BIR2 and BIR3 domains of cIAP1, cIAP2, and XIAP, mimicking the N-terminal AVPI motif of the endogenous antagonist Smac/DIABLO [1, 5]. Binding to cIAP1 and cIAP2 triggers their rapid auto-ubiquitination and proteasomal degradation, which leads to the stabilization of NF-kappaB-inducing kinase (NIK) and activation of the non-canonical NF-kappaB pathway, while also sensitizing cells to TNF-alpha-mediated apoptosis [3, 8]. Simultaneously, these agents bind to the BIR domains of XIAP to directly relieve its inhibition of caspases-3, -7, and -9, thereby lowering the threshold for apoptotic cell death [1, 11].