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Birinapant not only binds to the isolated BIR3 domains of cIAP1, cIAP2, XIAP but the single BIR domain of ML-IAP with high affinity and degrades TRAF2-bound cIAP1 and cIAP2 rapidly accordingly inhibiting the activation of TNF-mediated NF- kB. Additionally, birinapantcan promote the formation of caspase-8: RIPK1 complex in response to TNF stimulation, which result in downstream caspasesactivation [4].

In the inorganic SUM149- and SUM190-derived cells, which with differential XIAP expression (SUM149 wtXIAP, SUM190 shXIAP) and other high cIAP1/2 but low XIAP binding affinity bivalent Smac mimetic GT13402, XIAP inhibition are needed for increasing TRAIL potency. Opposite, single agent efficacy of Birinapant is owing to pan-IAP antagonism. Rapid cIAP1 degradation was caused by birinapant, as well as NF-κB activation, PARP cleavage andcaspase activation. While combined withTNF-α, showing strong combination activity, the combination was more effective than individual. The response in spheroid models was conserved, whereas in vivo birinapant inhibited tumor growth without adding TNF-α in vitro to resistant cell lines. In a parental cell line, TNF-αcombined withbirinapantinhibited the growth of a melanoma cell line with acquired resistance to the same extent of BRAF inhibition [1, 2].

Drug treatment increased the mean [18F]ICMT-11 tumor uptake with a peak at 24 hours for CPA (40 mg/kg; AUC40-60: 8.04 ± 1.33 and 16.05 ± 3.35 %ID/mL × min at baseline and 24 hours, respectively) and 6 hours for birinapant (15 mg/kg; AUC40-60: 20.29 ± 0.82 and 31.07 ± 5.66 %ID/mL × min, at baseline and 6 hours, respectively). Voxel-based spatiotemporal analysis of tumor-intrinsic heterogeneity showed that [18F] ICMT-11 could detect the discrete pockets of caspase-3 activation. Caspase-3 activation that measured ex vivo associated with the increased tumor [18F] ICMT-11, and early radiotracer uptake predicted apoptosis, distinct from the glucose metabolism with [18F] fluorodeoxyglucose-PET, which depicted the continuous loss of cell viability [3].

References:

1.Allensworth JL, Sauer S, Lyerly HK, et al. Smac mimetic Birinapant induces apoptosis and enhances TRAIL potency in inflammatory breast cancer cells in an IAP-dependent and TNF-a-independent mechanism. Breast Cancer Research, 2013, 137:359-371.

2.Krepler C, Chunduru SK, Halloran MB, et al. The novel SMAC mimetic birinapant exhibits potent activity against human melanoma cells. Clinical Cancer Research, 2013, 19 (7): 1784-1794.

3.Nguyen QD, Lavdas I, Gubbins J, et al. Temporal and Spatial Evolution of Therapy-Induced Tumor Apoptosis Detected by Caspase-3–Selective Molecular Imaging. Clinical Cancer Research, 2013, 19 (14): 3914-3924.

4.Benetatos CA, Mitsuuchi Y, Burns JM, et al. Birinapant (TL32711), a Bivalent SMAC Mimetic, Targets TRAF2-Associated cIAPs, Abrogates TNF-Induced NF-kB Activation, and Is Active in Patient-Derived Xenograft Models. 2014, 13(4):867-879.

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