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G₂ checkpoint abrogators as anticancer drugs

CanBas Co. Ltd., Numazu, Japan

Requests for Reprints: Takumi Kawabe, CanBas Co. Ltd., MAKIYA Building 5F, 9 Tooriyoko-cho, Numazu, 410-0891, Japan. Phone: 81-55-954-3666; Fax: 81-55-954-3668. E-mail: takumi{at}canbas.co.jp

Many conventional anticancer treatments kill cells irrespective of whether they are normal or cancerous, so patients suffer from adverse side effects due to the loss of healthy cells. Anticancer insights derived from cell cycle research has given birth to the idea of cell cycle G₂ checkpoint abrogation as a cancer cell specific therapy, based on the discovery that many cancer cells have a defective G₁ checkpoint resulting in a dependence on the G₂ checkpoint during cell replication.

Damaged DNA in humans is detected by sensor proteins (such as hHUS1, hRAD1, hRAD9, hRAD17, and hRAD26) that transmit a signal via ATR to CHK1, or by another sensor complex (that may include H2AX, 53BP1, BRCA1, NBS1, hMRE11, and hRAD50), the signal of which is relayed by ATM to CHK2. Most of the damage signals originated by the sensor complexes for the G₂ checkpoint are conducted to CDC25C, the activity of which is modulated by 14-3-3. There are also less extensively explored pathways involving p53, p38, PCNA, HDAC, PP2A, PLK1, WEE1, CDC25B, and CDC25A.

This review will examine the available inhibitors of CHK1 (Staurosporin, UCN-01, Go6976, SB-218078, ICP-1, and CEP-3891), both CHK1 and CHK2 (TAT-S216A and debromohymenialdisine), CHK2 (CEP-6367), WEE1 (PD0166285), and PP2A (okadaic acid and fostriecin), as well as the unknown checkpoint inhibitors 13-hydroxy-15-ozoapathin and the isogranulatimides. Among these targets, CHK1 seems to be the most suitable target for therapeutic G₂ abrogation to date, although an unexplored target such as 14-3-3 or the strategy of targeting multiple proteins at once may be of interest in the future.