An Updated Review of Smac Mimetics, LCL161, Birinapant, and GDC-0152 in Cancer Treatment

Inhibitor of apoptosis proteins (IAPs) are suggested as therapeutic targets for cancer treatment. Smac/DIABLO is a natural IAP antagonist in cells; therefore, Smac mimetics have been developed for cancer treatment in the past decade. In this article, we review the anti-cancer potency and novel molecular targets of LCL161, birinapant, and GDC-0152. Preclinical studies demonstrated that Smac mimetics not only induce apoptosis but also arrest cell cycle, induce necroptosis, and induce immune storm in vitro and in vivo. The safety and tolerance of Smac mimetics are evaluated in phase 1 and phase 2 clinical trials. In addition, the combination of Smac mimetics and chemotherapeutic compounds was reported to improve anti-cancer effects. Interestingly, the novel anti-cancer molecular mechanism of action of Smac mimetics was reported in recent studies, suggesting that many unknown functions of Smac mimetics still need to be revealed. Exploring these currently unknown signaling pathways is important to provide hints for the modification and combination therapy of further compounds.

Clinical positioning of the IAP antagonist tolinapant (ASTX660) in colorectal cancer

Cancer cells frequently express elevated levels of Inhibitor of Apoptosis Proteins (IAPs): cIAPI, cIAP2 and XIAP. Elevated expression of cIAP1 and cIAP2 (but not XIAP) significantly correlated with poor prognosis in microsatellite stable (MSS) stage-III colorectal cancer (CRC) patients treated with adjuvant chemotherapy, suggesting their involvement in promoting resistance. Preclinical analysis of the IAP inhibitor tolinapant in CRC cell lines demonstrated robust on-target effects and caspase-8-dependent apoptosis that was inhibited by the caspase-8 paralogs FLIP and, unexpectedly, caspase-10. Importantly, tolinipant-induced apoptosis was augmented by standard-of-care chemotherapy (FOLFOX) in CRC disease models, due (at least in part) to FOLFOX-induced downregulation of Class-I histone deacetylases, leading to acetylation of the FLIP-binding partner Ku70 and downregulation of FLIP. Moreover, this effect could be phenocopied using a Class-I HDAC inhibitor. Further analyses revealed that caspase-8-knockout RIPK3-positive CRC models were sensitive to tolinostat-induced necroptosis, an effect that could be exploited with the FDA-approved caspase inhibitor emricasan. Our study provides evidence for immediate clinical exploration of tolinapant in combination with FOLFOX chemotherapy in poor prognosis MSS CRC with elevated cIAP1/2 expression.

Molecular subtype-specific responses of colon cancer cells to the SMAC mimetic Birinapant

Colorectal cancer is a molecularly heterogeneous disease. Responses to genotoxic chemotherapy in the adjuvant or palliative setting vary greatly between patients, and colorectal cancer cells often resist chemotherapy by evading apoptosis. Antagonists of an inhibitor of apoptosis proteins (IAPs) can restore defective apoptosis signaling by degrading cIAP1 and cIAP2 proteins and by inhibition of XIAP. Due to the multiple molecular mechanisms-of-action of these targets, responses to IAP antagonist may differ between molecularly distinct colon cancer cells. In this study, responses to the IAP antagonist Birinapant and oxaliplatin/5-fluorouracil (5-FU) were investigated in 14 colon cancer cell lines, representing the consensus molecular subtypes (CMS). Treatment with Birinapant alone did not result in a substantial increase in apoptotic cells in this cell line panel. Annexin-V/PI assays quantified by flow cytometry and high-content screening showed that Birinapant increased responses of CMS1 and partially CMS3 cell lines to oxaliplatin/5-FU, whereas CMS2 cells were not effectively sensitized. FRET-based imaging of caspase-8 and -3 activation validated these differences at the single-cell level, with CMS1 cells displaying sustained activation of caspase-8-like activity during Birinapant and oxaliplatin/5-FU co-treatment, ultimately activating the intrinsic mitochondrial apoptosis pathway. In CMS2 cell lines, Birinapant exhibited synergistic effects in combination with TNFα, suggesting that Birinapant can restore extrinsic apoptosis signaling in the context of inflammatory signals in this subtype. To explore this further, we co-cultured CMS2 and CMS1 colon cancer cells with peripheral blood mononuclear cells. We observed increased cell death during Birinapant single treatment in these co-cultures, which was abrogated by anti-TNFα-neutralizing antibodies. Collectively, our study demonstrates that IAP inhibition is a promising modulator of response to oxaliplatin/5-FU in colorectal cancers of the CMS1 subtype, and may show promise as in the CMS2 subtype, suggesting that molecular subtyping may aid as a patient stratification tool for IAP antagonists in this disease.