Wee1 is a tyrosine kinase that phosphorylates and inactivates CDC2 and is involved in G 2 checkpoint signaling. Because p53 is a key regulator in the G 1 checkpoint, p53-deficient tumors rely only on the G 2 checkpoint after DNA damage. Hence, such tumors are selectively sensitized to DNA-damaging agents by Wee1 inhibition. Here, we report the discovery of a potent and selective smallmolecule inhibitor of Wee1 kinase, MK-1775. This compound inhibits phosphorylation of CDC2 at Tyr15 (CDC2Y15), a direct substrate of Wee1 kinase in cells. MK-1775 abrogates G 2 DNA damage checkpoint, leading to apoptosis in combination with DNA-damaging chemotherapeutic agents such as gemcitabine, carboplatin, and cisplatin selectively in p53-deficient cells. In vivo, MK-1775 potentiates tumor growth inhibition by these agents, and cotreatment does not significantly increase toxicity. The enhancement of antitumor effect by MK-1775 was well correlated with inhibition of CDC2Y15 phosphorylation in tumor tissue and skin hair follicles. Our data indicate that Wee1 inhibition provides a new approach for treatment of multiple human malignancies.

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A new topoisomerase II inhibitor, designated BE-22179, was isolated from the culture broth of Streptomyces sp. A22179, which resembles "Streptomyces gangtokensis". The inhibitor was extracted from the mycelial cake of the culture broth with organic solvent and successively purified by silica gel chromatography. BE-22179 inhibited topoisomerase II strongly but not topoisomerase I and showedpotent antitumor activity against various tumor cell lines both in vitro and in vivo.

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The ratio of LD 10 /GID 75 indicates the therapeutic window of an anti-tumor agent. Although the ratios of doxorubicin, paclitaxel and cisplatin against PC-3 were < < < <0.3, < < < <0.5 and < < < <0.2, J-107088 showed the widest therapeutic window among the anti-tumor drugs tested. J-107088 was also effective on cells that had acquired resistance related to P-glycoprotein. Furthermore, J-107088 was found to be highly effective in inhibiting proliferation of micro-metastases of tumors to the liver in mice. Therefore, J-107088 is considered to be a promising candidate as an anti-tumor drug for treatment of solid tumors in humans.