Treatment of cancer cells lacking p53 function with G2 checkpoint inhibitors sensitizes them to the toxic effects of DNA damage and has been proposed as a strategy for cancer therapy. However, few inhibitors are known, and they have been found serendipitously. We report the development of a G2 checkpoint inhibition assay that is suitable for high-throughput screening and its application to a screen of 1300 natural extracts. We present the isolation of a new G2 checkpoint inhibitor, the structurally novel compound isogranulatimide. In combination with γ-irradiation, isogranulatimide selectively kills MCF-7 cells lacking p53 function.
Supported by the Canadian Breast Cancer Research Initiative (to M. R.), National Cancer Institute of Canada (to R. J. A.), Natural Sciences and Engineering Research Council of Canada (R. J. A.), and Fundação de Amparo à Pesquisa do Estado de São Paulo, Brazil (to R. G. S. B.).