Radiosensitization of Head and Neck Squamous Cell Carcinoma by a SMAC-Mimetic Compound, SM-164, Requires Activation of Caspases

Chemoradiation is the treatment of choice for locally advanced head and neck squamous cell carcinoma (HNSCC). However, radioresistance, which contributes to local recurrence, remains a significant therapeutic problem. In this study, we characterized SM-164, a small second mitochondria-derived activator of caspase -mimetic compound that promotes degradation of cellular inhibitor of apoptosis-1(cIAP-1; also known as baculoviral IAP repeat-containing protein 2, BIRC2) and releases active caspases from the X-linked inhibitor of apoptosis inhibitory binding as a radiosensitizing agent in HNSCC cells. We found that SM-164 at nanomolar concentrations induced radiosensitization in some HNSCC cell lines in a manner dependent on intrinsic sensitivity to caspase activation and apoptosis induction. Blockage of caspase activation via short interfering RNA knockdown or a pan-caspase inhibitor, z-VAD-fmk, largely abrogated SM-164 radiosensitization. On the other hand, the resistant lines with a high level of Bcl-2 that blocks caspase activation and apoptosis induction became sensitive to radiation on Bcl-2 knockdown. Mechanistic studies revealed that SM-164 radiosensitization in sensitive cells was associated with NF-κB activation and TNFα secretion, followed by activation of caspase-8 and -9, leading to enhanced apoptosis. Finally, SM-164 also radiosensitized human tumor xenograft while causing minimal toxicity. Thus, SM-164 is a potent radiosensitizer via a mechanism involving caspase activation and holds promise for future clinical development as a novel class of radiosensitizer for the treatment of a subset of head and neck cancer patients.