Inhibitor of Apoptosis Protein (IAP) Antagonists in Anticancer Agent Discovery: Current Status and Perspectives

Apoptosis, an important form of programmed cell death (PCD), is a tightly regulated cellular process to eliminate unwanted or damaged cells. Resistance of apoptosis is a hallmark of cancer cells. Inhibitor of apoptosis proteins (IAPs) is a class of key apoptosis regulators that promote cancer cell resistant to apoptosis, particularly in cancer treatment. Disrupting the binding of IAPs with their functional partners therefore is a promising strategy to restore the apoptotic response to proapoptotic stimuli, particularly those introduced by standard cancer therapies. The most successful example is the use of small molecules to mimic the IAP-binding motif of an endogenous IAP antagonist, second mitochondria-derived activator of caspase (SMAC). Here we will review the functions of IAPs, the structural interactions of IAPs with SMAC, four generations of SMAC-mimetic IAP antagonists, and representative antagonists in clinical evaluations, focusing on research articles over the past 15 years. Outlooks and perspectives on the associated challenges are provided as well.