MK-8776, a novel Chk1 inhibitor, exhibits an improved radiosensitizing effect compared to UCN-01 by exacerbating radiation-induced aberrant mitosis

Motofumi Suzuki; Tohru Yamamori; Tomoki Bo; Yuri Sakai; Osamu Inanami

doi:10.1016/j.tranon.2017.04.002

MK-8776, a novel Chk1 inhibitor, exhibits an improved radiosensitizing effect compared to UCN-01 by exacerbating radiation-induced aberrant mitosis

Transl Oncol. 2017 Aug;10(4):491-500. doi: 10.1016/j.tranon.2017.04.002. Epub 2017 May 24.

Authors

Motofumi Suzuki¹, Tohru Yamamori², Tomoki Bo¹, Yuri Sakai¹, Osamu Inanami³

Affiliations

¹ Laboratory of Radiation Biology, Department of Applied Veterinary Sciences, Division of Veterinary Medicine, Faculty of Veterinary Medicine, Hokkaido University, Sapporo, Japan.
² Laboratory of Radiation Biology, Department of Applied Veterinary Sciences, Division of Veterinary Medicine, Faculty of Veterinary Medicine, Hokkaido University, Sapporo, Japan. Electronic address: yamamorit@vetmed.hokudai.ac.jp.
³ Laboratory of Radiation Biology, Department of Applied Veterinary Sciences, Division of Veterinary Medicine, Faculty of Veterinary Medicine, Hokkaido University, Sapporo, Japan. Electronic address: inanami@vetmed.hokudai.ac.jp.

Abstract

Checkpoint kinase 1 (Chk1) is an evolutionarily conserved serine/threonine kinase that plays an important role in G₂/M checkpoint signaling. Here, we evaluate the radiosensitizing effects of a novel selective Chk1 inhibitor MK-8776, comparing its efficacy with a first-generation Chk1 inhibitor UCN-01, and attempt to elucidate the mechanism of radiosensitization. In a clonogenic survival assay, MK-8776 demonstrated a more pronounced radiosensitizing effect than UCN-01, with lower cytotoxicity. Importantly, radiosensitization by MK-8776 can be achieved at doses as low as 2.5 Gy, which is a clinically applicable irradiation dose. MK-8776, but not UCN-01, exacerbated mitotic catastrophe (MC) and centrosome abnormalities, without affecting repair kinetics of DNA double strand breaks. Furthermore, live-cell imaging revealed that MK-8776 significantly abrogated the radiation-induced G₂/M checkpoint, prolonged the mitotic phase, and enhanced aberrant mitosis. This suggests that Chk1 inhibition by MK-8776 activates a spindle assembly checkpoint and increases mitotic defects in irradiated EMT6 cells. In conclusion, we have shown that, at minimally toxic concentrations, MK-8776 enhances radiation-induced cell death through the enhancement of aberrant mitosis and MC, without affecting DNA damage repair.