Discovery of 4 0 -(1,4-dihydro-indeno[1,2-c]pyrazol-3-yl)- benzonitriles and 4 0 -(1,4-dihydro-indeno[1,2-c]pyrazol-3-yl)- pyridine-2 0 -carbonitriles as potent checkpoint kinase 1 (Chk1) inhibitors Zhi-Fu Tao, * Gaoquan Li, Yunsong Tong, Kent D. Stewart, Zehan Chen, Mai-Ha Bui, Philip Merta, Chang Park, Peter Kovar, Haiying Zhang, Hing L. Sham, Saul H. Rosenberg, Thomas J. Sowin and Nan-Horng Lin Cancer Research, Global Pharmaceutical Research and Development, Abbott Laboratories, Abbott Park, IL 60064, USA Received 19 June 2007; revised 20 July 2007; accepted 23 July 2007 Available online 25 August 2007 Abstract—An extensive structure–activity relationship study of the 3-position of a series of tricyclic pyrazole-based Chk1 inhibitors is described. As a result, 4 0 -(1,4-dihydro-indeno[1,2-c]pyrazol-3-yl)-benzonitriles (4) and 4 0 -(1,4-dihydro-indeno[1,2-c]pyrazol-3-yl)- pyridine-2 0 -carbonitriles (29) emerged as new lead series. Compared with the original lead compound 2, these new leads fully retain the biological activity in both enzymatic inhibition and cell-based assays. More importantly, the new leads 4 and 29 exhibit favorable physicochemical properties such as lower molecular weight, lower Clog P, and the absence of a hydroxyl group. Furthermore, struc- ture–activity relationship studies were performed at the 6- and 7-positions of 4, which led to the identification of ideal Chk1 inhib- itors 49, 50, 51, and 55. These compounds not only potently inhibit Chk1 in an enzymatic assay but also significantly potentiate the cytotoxicity of DNA-damaging agents in cell-based assays while they show little single agent activity. A cell cycle analysis by FACS confirmed that these Chk1 inhibitors efficiently abrogate the G2/M and S checkpoints induced by DNA-damaging agent. The current work paved the way to the identification of several potent Chk1 inhibitors with good pharmacokinetics that are suitable for in vivo study with oral dosing. Ó 2007 Elsevier Ltd. All rights reserved. Cancer is a top killer of human beings. There is great ur- gency to develop highly efficacious and minimally toxic treatments for cancer. Although tremendous progress has been achieved in the development of novel cancer treatments, most of the current cancer drugs usually exhibit high toxicity and are severely resisted by tumor cells in the clinic. This dilemma is particularly true for DNA-damaging agents, the mainstay of cancer treat- ment. 1 To improve the efficacy and lower the toxicity of DNA-damaging anticancer drugs, the development of adjuvant therapeutics has been aggressively pursued in recent years. 2,3 Such treatments may either sensitize tumor tissue or protect normal tissue from DNA damage. Checkpoint kinase 1 (Chk1), a serine/threonine protein kinase which plays a key role in DNA damage-induced checkpoints, 4,5 has emerged as an attractive chemosensi- tization anticancer target. 6,7 Chk1 inhibitors have been demonstrated to abrogate the DNA damage-induced S and G2 checkpoints and disrupt the DNA repair pro- cess, resulting in premature chromosome condensation and leading to cell death; this preferentially sensitizes tumor cells, especially p53-null cells, to various DNA- damaging agents. 5–17 An optimal therapeutic window may be achieved for Chk1 inhibitors in the clinic be- cause normal cells can be arrested in the G1 phase and are less affected by S and G2 checkpoint abrogation in response to DNA damage. 10 Several classes of Chk1 inhibitors have been recently reported. 10–27 UCN-01 is the most extensively studied Chk1 inhibitor and is now in phase I/II clinical tri- als. 14,15 It abrogates both the S and G2 checkpoints and sensitizes tumor cells to a wide spectrum of 0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2007.07.102 Keywords: Antitumor agent; Chk1 inhibitors; Checkpoint kinase 1; Pyrazole; Structure–activity relationship; DNA damage; Combination therapy. * Corresponding author. Tel: +1 847 938 6772; fax: +1 847 935 5165; e-mail: Zhi-Fu.Tao@abbott.com Bioorganic & Medicinal Chemistry Letters 17 (2007) 5944–5951