Paullones, a series of cyclin-dependent kinase inhibitors: Synthesis, evaluation of CDK1/cyclin B inhibition, and in vitro antitumor activity

The paullones represent a novel class of small molecule cyclin-dependent ki nase (CDK) inhibitors. To investigate structure-activity relationships and to develop paullones with antitumor activity, derivatives of the lead struc ture kenpaullone (9-bromo-7,12-dihydroindolo-[3,2-d][1]benzazepin-6(5H)-one , 4a) were synthesized. Paullones with different substituents in the 2-, 3- , 4-, 9-, and 11-positions were prepared by a Fischer indole reaction start ing from 1H-[1]benzazepine-2,5(3H,4H)-diones 5. Selective substitutions at either the lactam or the indole nitrogen atom were accomplished by treating kenpaullone with alkyl halides in the presence of sodium hydride/THF or po tassium hydroxide/acetone, respectively. S-Methylation of the kenpaullone-d erived thiolactam 18 yielded the methylthioimidate 19, which gave the hydro xyamidine 20 upon reaction with hydroxylamine. The new paullones were teste d both in a CDK1/ cyclin B inhibition assay and in the in vitro antitumor c ell line-screening program of the National Cancer Institute (NCI). With res pect to the CDK1/cyclin B inhibition, electron-withdrawing substituents in the 9-position as well as a 2,3-dimethoxy substitution on the paullone basi c scaffold turned out to be favorable. A 9-trifluoromethyl substituent was found to be equivalent to the 9-bromo substituent of kenpaullone. Replaceme nt of the 9-bromo substituent of kenpaullone by a 9-cyano or 9-nitro group produced a substantial increase in enzyme-inhibiting potency. Substitutions in other positions or the replacement of the lactam moiety led to decrease d CDK1 inhibition. Noteworthy in vitro antitumor activities (GI(50) values between 1 and 10 mu M) were found with the 9-bromo-2,3-dimethoxy-7,12-dihyd roindolo[3,2-d]-[1]benzazepin-6(5H)-one (4t), its 9-trifluoromethyl analogu e 4u, the 12-Boc-substituted paullone 15, and the methylthioimidate 19, res pectively. The 9-nitro-7,12-dihydroindolo[3,2-d][1]benzazepin-6(5H)-one (4j , named alsterpaullone) showed a high CDK1/cyclin B inhibitory activity (IC 50 = 0.035 mu M) and exceeded the in vitro antitumor potency of the other p aullones by 1 order of magnitude (log GI(50) mean graph midpoint = -6.4 M).