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).

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In order to investigate structure‐activity relationships in the antiproliferative darpone series, the derivatives 8a and 8b lacking the aryl substituents in either 2‐ or 4‐position were synthesized and evaluated for cancer cell growth inhibition. Since both 8a and 8b were devoid of noteworthy antiproliferative activity, both aryl substituents can be considered as part of the darpone pharmacophore.

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Aryl Rings Are Part of the Darpone Pharmacophore.-In order to investigate structure-activity relationships in the antiproliferative darpone series, the derivatives (VI) and (VIII) are synthesized and evaluated for cancer cell growth inhibition. Since both (VI) and (VIII) are devoid of noteworthy antiproliferative activity, both aryl substituents can be considered as part of the darpone pharmacophore.

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