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 kinase (CDK) inhibitors. To investigate structure−activity relationships and to develop paullones with antitumor activity, derivatives of the lead structure 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 starting 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 potassium hydroxide/acetone, respectively. S-Methylation of the kenpaullone-derived thiolactam 18 yielded the methylthioimidate 19, which gave the hydroxyamidine 20 upon reaction with hydroxylamine. The new paullones were tested both in a CDK1/cyclin B inhibition assay and in the in vitro antitumor cell line-screening program of the National Cancer Institute (NCI). With respect to the CDK1/cyclin B inhibition, electron-withdrawing substituents in the 9-position as well as a 2,3-dimethoxy substitution on the paullone basic scaffold turned out to be favorable. A 9-trifluoromethyl substituent was found to be equivalent to the 9-bromo substituent of kenpaullone. Replacement 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 decreased CDK1 inhibition. Noteworthy in vitro antitumor activities (GI50 values between 1 and 10 μM) were found with the 9-bromo-2,3-dimethoxy-7,12-dihydroindolo[3,2-d][1]benzazepin-6(5H)-one (4t), its 9-trifluoromethyl analogue 4u, the 12-Boc-substituted paullone 15, and the methylthioimidate 19, respectively. 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 (IC50 = 0.035 μM) and exceeded the in vitro antitumor potency of the other paullones by 1 order of magnitude (log GI50 mean graph midpoint = −6.4 M).