cisapride receptor Nowadays more than kinases have

Nowadays, more than 500 kinases have been identified of human genome [2]. Imatinib (Gleevec) was the first tyrosine kinase inhibitor approved by the US Food and Drug Administration (FDA) for the treatment of chronic myeloid leukemia [3], and kinases have become an attractive target for the development of anti-malignant agents. To date, 30 more kinase inhibitors have achieved FDA approval and currently a number of experimental kinase inhibitors are enrolled in clinical trials [4], [5]. Aurora kinases belong to the serine/threonine kinase family, and are critical regulators of mitosis. Three human paralogues, Aurora A, B, and C, have been identified, each with a distinct subcellular location and function [6], [7]. Aurora A is involved in chromosome maturation, bipolar spindle assembly and orientation in the late S phase and early M phase. Aurora B acts primarily during the M phase and is required for proper chromosome segregation and cytokinesis [8]. Overexpression of both Aurora A and Aurora B was found in various human malignancies, e.g. glioma, breast, ovarian and thyroid cancers [9], [10]. Currently, several Aurora inhibitors, such as pan-Aurora inhibitor VX-680 [11], PHA-739358 [12] and AT-9283 [13], Aurora-A-selective MLN-8237 [14] and MK-5108 [15], and Aurora-B-selective AZD-1152 [16] are in clinical development to treat variety of cancers (Fig. 1). Fragment-based drug discovery (FBDD) has been widely adopted by the pharmaceutical industry over the past decades [17]. This method allows the identification of small fragments which bind only weakly to the biological targets and postulates that further high-affinity leads could be generated via the combination of the small fragments. In contrast to high-throughput screening (HTS), which heavily relies on the screening of a great quantity of chemical libraries, FBDD is a more efficient methodology in designing drug-like molecules [18]. Ligand efficiency (LE) is a measurement of ligand activity normalized by the number of heavy cisapride receptor (HA), defined as the ratio of Gibbs free energy (ΔG) to HA of the compound: ΔG/HA (where ΔG = −RTlnKi or −RTlnIC50). LE values can be used to prioritize the lead compounds with favorable ligand efficiency as well as pharmacological properties. However, LE values were noted to be higher on average for smaller molecules than for the larger ones [19], [20]. Accordingly, fit quality (FQ) was therefore proposed to transform LE into metrics that are more reliable within a broader range of molecule sizes. FQ values are obtained by normalizing the LE with scaled values defined as the ratio of LE to LE_Scale of the compound: LE/LE_Scale. FQ score could be a size-independent parameter to assess suitable fragments, where FQ near to 1 indicated an optimal ligand binding [21], [22], [23]. Herein, we report the discovery of a series of indazole derivatives as novel Aurora kinase inhibitors using knowledge-based drug design and in silico FBDD. Screening of our in-house database by sub-structure screening, identified compound 8 which showed weak inhibition against Aurora A (IC50 = 13 μM). Compound 9a was further designed using in silico techniques, and resulting in 10-fold potency improvement. Finally, two additional binding regions were explored, which resulted in compound 17, a potent Aurora A inhibitor (IC50 = 26 nM) (Scheme 1).
Chemistry The preparation of hit compound 8 and its derivatives is illustrated in Scheme 2. Iodination of commercially available indazole 6 with iodine and potassium hydroxide afforded 3-iodo-indazole 7, which underwent Suzuki coupling with 3-aminophenylboronic acid to give aniline 8 in a good yield. Amide bond formation through HOBt-mediated coupling or acylation was carried out with aniline 8 to give the desired analogues 9a-f as well as 9g (from the hydrolysis of 9d under basic condition). The preparation of C-5 substituted indazoles is outlined in Scheme 3. Intermediate 11 was prepared from 5-nitroindazole as described [24]. The amino group of 3-iodo-5-amino indazole 11 was reacted with various electrophiles to provide a series C-5 substituted 3-iodo-indazoles 12a-j. Palladium catalyzed Suzuki coupling was performed between 3-aminophenylboronic acid and 3-iodo-indazoles 12a-j to give the corresponding aniline analogues 13a-j, which were further reacted with maleic anhydride to afford the final compounds 14–23. Analogues 24–30 of compound 17 were prepared either by HOBt-mediated amide-bond formation of 13d with the corresponding carboxylic acids or monoethyl fumarate following by hydrolysis and hydrogenation. In similar manner, synthesis of the 6-substituted analog 33 was achieved as depicted in Scheme 4.