Paullones constitute a new family of benzazepinones with promising antitumoral properties. They were recently described as potent, ATP-competitive, inhibitors of the cell cycle regulating cyclin-dependent kinases (CDKs). We here report that paullones also act as very potent inhibitors of glycogen synthase kinase-3b (GSK-3b) (IC 50 : 4±80 nm) and the neuronal CDK5/p25 (IC 50 : 20±200 nm). These two enzymes are responsible for most of the hyperphosphorylation of the microtubule-binding protein tau, a feature observed in the brains of patients with Alzheimer's disease and other neurodegenerative`taupathies'. Alsterpaullone, the most active paullone, was demonstrated to act by competing with ATP for binding to GSK-3b. Alsterpaullone inhibits the phosphorylation of tau in vivo at sites which are typically phosphorylated by GSK-3b in Alzheimer's disease. Alsterpaullone also inhibits the CDK5/p25-dependent phosphorylation of DARPP-32 in mouse striatum slices in vitro. This dual specificity of paullones may turn these compounds into very useful tools for the study and possibly treatment of neurodegenerative and proliferative disorders.Keywords: Alzheimer's disease; cyclin-dependent kinase 5; glycogen synthase kinase-3; paullones, kinase inhibitors.Phosphorylation on serine, threonine and tyrosine residues by protein kinases constitute one of most common mechanisms of post-translational modifications of proteins. Among the estimated 2000 human protein kinases, we are presently investigating the functions of two related families of kinases, the cyclin-dependent kinases (CDKs) and glycogen synthase kinases 3 (GSK-3) [1,2] (see also http://www.sdsc.edu/kinases). CDKs play a key role in cell cycle regulation, centrosome duplication, transcription, muscular and neural functions and apoptosis (reviewed in [3±8]). GSK-3 is involved in the WNT signaling pathway [9], cell cycle regulation [10,11]

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 (GI(50) values between 1 and 10 microM) 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 paullone15, 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 (IC(50) = 0.035 microM) and exceeded the in vitro antitumor potency of the other paullones by 1 order of magnitude (log GI(50) mean graph midpoint = -6.4 M).

With a view to the rational design of selective GSK-3beta inhibitors, 3D-QSAR CoMSIA models were developed for the inhibition of the three serine/threonine kinases CDK1/cyclin B, CDK5/p25, and GSK-3beta by compounds from the paullone inhibitor family. The models are based on the kinase inhibition data of 52 paullone entities, which were aligned by a docking routine into the ATP-binding cleft of a CDK1/cyclin B homology model. Variation of grid spacing and column filtering were used during the optimization of the models. The predictive ability of the models was shown by a leave-one-out cross-validation and the prediction of an independent set of test compounds, which were synthesized especially for this purpose. Besides paullones with the basic indolo[3,2-d][1]benzazepine core, the test set comprised novel thieno[3',2':2,3]azepino[4,5-b]indoles, pyrido[2',3':2,3]azepino[4,5-b]indoles, and a pyrido[3',2':4,5]pyrrolo[3,2-d][1]benzazepine. The best statistical values for the CoMSIA were obtained for the CDK1-models (r(2)() = 0.929 and q(2)() = 0.699), which were clearly superior to the models for CDK5 (r(2)() = 0.874 and q(2)() = 0.652) and GSK-3 (r(2)() = 0.871 and q(2)() = 0.554).