Among the ten pharmacological inhibitors of cyclindependent kinases (CDKs) currently in clinical trials, the purine roscovitine (CYC202, Seliciclib) is undergoing phase 2 trials against non-small-cell lung and nasopharyngeal cancers. An extensive medicinal chemistry study, designed to generate more potent analogues of roscovitine, led to the identification of an optimal substitution at the N6 position (compound CR8). An extensive selectivity study (108 kinases) highlights the exquisite selectivity of CR8 for CDK1/2/3/5/7/9. CR8 was 2-to 4-fold more potent than (R)-roscovitine at inhibiting these kinases. Cocrystal structures of (R)-CR8 and (R)-roscovitine with pCDK2/cyclin A showed that both inhibitors adopt essentially identical positions. The cellular effects of CR8 and (R)-roscovitine were investigated in human neuroblastoma SH-SY5Y cells. CR8 inhibited the phosphorylation of CDK1 and 9 substrates, with a 25-50 times higher potency compared to (R)-roscovitine. CR8 was consistently more potent than (R)-roscovitine at inducing apoptotic cell death parameters: 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium reduction (40-fold), lactate dehydrogenase release (35-fold), caspases activation (68-fold) and poly-(ADP-ribose)polymerase cleavage (50-fold). This improved cell death-inducing activity of CR8 over (R)-roscovitine was observed in 25 different cell lines. Altogether these results show that second-generation analogues of (R)-roscovitine can be designed with improved antitumor potential.

Protein kinases represent promising anticancer drug targets. We describe here the meriolins, a new family of inhibitors of cyclin-dependent kinases (CDK). Meriolins represent a chemical structural hybrid between meridianins and variolins, two families of kinase inhibitors extracted from various marine invertebrates. Variolin B is currently in preclinical evaluation as an antitumor agent. A selectivity study done on 32 kinases showed that, compared with variolin B, meriolins display enhanced specificity toward CDKs, with marked potency on CDK2 and CDK9. The structures of pCDK2/cyclin A/variolin B and pCDK2/cyclin A/meriolin 3 complexes reveal that the two inhibitors bind within the ATP binding site of the kinase, but in different orientations. Meriolins display better antiproliferative and proapoptotic properties in human tumor cell cultures than their parent molecules, meridianins and variolins. Phosphorylation at CDK1, CDK4, and CDK9 sites on, respectively, protein phosphatase 1A, retinoblastoma protein, and RNA polymerase II is inhibited in neuroblastoma SH-SY5Y cells exposed to meriolins. Apoptosis triggered by meriolins is accompanied by rapid Mcl-1 down-regulation, cytochrome c release, and activation of caspases. Meriolin 3 potently inhibits tumor growth in two mouse xenograft cancer models, namely, Ewing's sarcoma and LS174T colorectal carcinoma. Meriolins thus constitute a new CDK inhibitory scaffold, with promising antitumor activity, derived from molecules initially isolated from marine organisms. [Cancer Res 2007;67(17):8325-34]

We report the synthesis and biological characterization of 3-(pyrimidin-4-yl)-7-azaindoles (meriolins), a chemical hybrid between the natural products meridianins and variolins, derived from marine organisms. Meriolins display potent inhibitory activities toward cyclin-dependent kinases (CDKs) and, to a lesser extent, other kinases (GSK-3, DYRK1A). The crystal structures of 1e (meriolin 5) and variolin B (Bettayeb, K.; Tirado, O. M.; Marionneau-Lambert, S.; Ferandin, Y.; Lozach, O.; Morris, J.; Mateo-Lozano, S.; Drückes, P.; Schächtele, C.; Kubbutat, M.; Liger, F.; Marquet, B.; Joseph, B.; Echalier, A.; Endicott, J.; Notario, V.; Meijer, L. Cancer Res. 2007, 67, 8325-8334) in complex with CDK2/cyclin A reveal that the two inhibitors are orientated in very different ways inside the ATP-binding pocket of the kinase. A structure-activity relationship provides further insight into the molecular mechanism of action of this family of kinase inhibitors. Meriolins are also potent antiproliferative and proapoptotic agents in cells cultured either as monolayers or in spheroids. Proapoptotic efficacy of meriolins correlates best with their CDK2 and CDK9 inhibitory activity. Meriolins thus constitute a promising class of pharmacological agents to be further evaluated against the numerous human diseases that imply abnormal regulation of CDKs including cancers, neurodegenerative disorders, and polycystic kidney disease.