Biol. Pharm. Bull. 30(8) 1379—1383 (2007)

not only cancer cells but also normal cells. Because these drugs are not specifically selective for cancer cells, patients suffer from adverse side effects due to the loss of healthy cells. Therefore, it is important to reduce such side effects caused by anti-cancer drugs. DNA damage induces signal transduction pathways called checkpoints, which delay cell cycle progression and permit repair of DNA. Checkpoints arrest cells in G1 phase to prevent replication of damaged DNA and in G2 phase to prevent segregation of damaged chromosomes during mitosis. Normal human cells can repair DNA in the G1 and G2 phases, whereas most cancer cells have mutations in genes involved in the G1 checkpoint, such as p53, p16 or Rb, and can repair DNA only in the G2 phase. Accordingly, disruption of the G2 checkpoint in cancer cells is expected to increase sensitivity to DNA-damaging reagents like bleomycin, cisplatin and camptothecin, causing selective cell death of cancer cells due to their accumulation of mutation in G1 checkpoint genes. Therefore, the G2 checkpoint is a potential target for development of a novel therapy against intractable cancers. At the G2 checkpoint, DNA damage activates the ATM (ataxia telangiectasia-mutated) and ATR (A-T and rad3-related) members of the phosphoinositide kinase family. A signal is then transmitted through the downstream protein kinases Chk1 and Chk2, which are able to phosphorylate Cdc25C at Ser 216. This phosphorylation is thought to directly prevent Cdc25C from activating the Cdc2 kinase. Chk1 and Chk2 can also phosphorylate and activate Wee1, a kinase that catalyzes Cdc2 inhibitory phosphorylation. Several G2 checkpoint inhibitors have been reported to data. For example, UCN-01, a derivative of staurosporine, abrogates DNA damage-induced S and G2 arrest due to inhibition of Chk1 and perhaps Chk2. Although Phase I clinical trials of UCN-01 have been completed, UCN-01 was found to bind readily to human plasma proteins and to inhibit kinases other than Chk1 and Chk2, which may result in unwanted side effects in patients. SB-218078 and Go6979 were reported recently to specifically inhibit Chk1, and their binding activity to human plasma proteins was stronger than that of UCN-01. Caffeine and PD0166285 inhibit the checkpoint kinases ATM and ATR and Wee1 kinase, respectively. Suganuma et al. previouly reported that the short peptide corresponding to amino acids 211— 221 of human Cdc25C, which is an important phosphatase in the G2 checkpoint, fused with a part of HIV-TAT can efficiently disrupt the cell cycle G2 checkpoint, which is activated by DNA damage, and can thereby sensitize cancer cells but not normal cells to anticancer reagents. On the basis of these findings, more potent analogs of this peptide have been developed as G2 checkpoint inhibitors. Jurkat, a human T-cell leukemia-derived cell line, depends on the G2 checkpoint to repair DNA damage since it lacks functional p53. We established a cell-based assay using Jurkat cells to screen culture broths of microorganisms for G2 checkpoint inhibitors. During the course of the continuous screening program, two malformins, previously reported to cause malformation of the stems and petioles of bean plants and to be potentiators of fibrinolytic activity, were isolated from the culture broth of strain Aspergillus niger FKI-2342 (Fig. 1). In this study, we describe the activity of malformins A1 and C as G2 checkpoint inhibitors.