Cinnamaldehydes inhibit cyclin dependent kinase 4/cyclin D1

Cinnamaldehyde as a Potential Cathepsin-B Inhibitor: A Comparative Investigation with some Commercial Anticancer Drugs

Cancer is a leading cause of death worldwide, surpassed only by heart disease. Despite improved diagnosis and treatment, cancer cells still evade normal physiological processes such as apoptosis, metabolism, angiogenesis, cell cycle, and epigenetics. To mitigate the numerous side effects linked to chemotherapy, leveraging natural products emerged as a promising alternative, either alone or in tandem with traditional agents. Cinnamaldehyde, an active ingredient of Cinnamomum cassia's stem bark has emerged as a molecule of research with diverse pharmacological properties. In the present study, we report an in silico potential of cinnamaldehyde (CM) potential as an anticancer agent across thirteen anti-cancer targets in comparison with chlorambucil (CB), docetaxel (DOC), melphalan (MP). Computational tools such as DFT, CHEM3D, molinspiration, vNNADMET, SWISS ADME, admetSAR, galaxyrefine, iGEMDOCK, and DS-Visualizer were employed. Additionally, anti-cathepsin B activity was assessed for cinnamaldehyde and the commercial drugs CB, DOC, MP and the results showed 52.76, 62.41, 72.48 and 65.52 % inhibition respectively which is comparable. The results supported molecular docking using iGEMDOCK. Both in silico and experimental findings substantiate cinnamaldehyde as a promising drug for cancer treatment including metastasis and invasion where cathepsin B involvement is indicated.

Recent Progress in CDK4/6 Inhibitors and PROTACs

Cell division in eukaryotes is a highly regulated process that is critical to the life of a cell. Dysregulated cell proliferation, often driven by anomalies in cell Cyclin-dependent kinase (CDK) activation, is a key pathological mechanism in cancer. Recently, selective CDK4/6 inhibitors have shown clinical success, particularly in treating advanced-stage estrogen receptor (ER)-positive and human epidermal growth factor receptor 2 (HER2)-negative breast cancer. This review provides an in-depth analysis of the action mechanism and recent advancements in CDK4/6 inhibitors, categorizing them based on their structural characteristics and origins. Furthermore, it explores proteolysis targeting chimers (PROTACs) targeting CDK4/6. We hope that this review could be of benefit for further research on CDK4/6 inhibitors and PROTACs.

A review of cinnamaldehyde and its derivatives as antibacterial agents

There is a continuing rise in the occurrence of multidrug-resistant bacterial infections. Antibiotic resistance to currently available antibiotics has become a global health issue leading to an urgent need for alternative antibacterial strategies. There has been a renewed interest in the development of antibacterial agents from natural sources, and trans-cinnamaldehyde is an example of a naturally occurring compound that has received significant attention in recent years. Trans-Cinnamaldehyde has been shown to possess substantial antimicrobial activity, as well as an array of other medicinal properties, and represents an intriguing hit compound from which a number of derivatives have been developed. In some cases, these derivatives have been shown to possess improved activity, not only compared to trans-cinnamaldehyde but also to commonly used antibiotics. Therefore, understanding the antibacterial mechanisms of action that these compounds elicit is imperative in order to facilitate their development and the development of new antibacterial agents that could exploit similar mechanistic approaches. The purpose of this review is to provide an overview of current knowledge on the antibacterial activity and mechanisms of action of cinnamaldehyde and its derivatives, and to highlight significant contributions made in this research area. It is hoped that the findings presented in this work will aid the future development of new antibacterial agents.

Medicinal attributes of major phenylpropanoids present in cinnamon

Background

Excessive production of free radicals has been implicated in many diseases including cancer. They are highly reactive and bring about oxidation of biomolecules i.e., proteins, lipids and nucleic acids which are associated with many degenerative diseases. Natural products acting as antioxidants have ability to neutralize free radicals and their actions and hence they mitigate their harmful effects. The present study was designed to investigate pharmacological properties viz., antioxidant, antibacterial and antiproliferative activities of cinnamaldehyde and eugenol, the two naturally occurring phenylpropanoids present in Cinnamomum spp. and other plants.

Methods

The antioxidant potential of test compounds was evaluated by measuring DPPH free radical scavenging, reducing power and metal ion chelating activities. Protection against membrane damage was assayed by inhibition of lipid peroxidation in rat liver homogenate. Antibacterial activity was measured by Kirby-Bauer disc diffusion method while antiproliferative activity of test compounds was measured by sulforhodamine-B (SRB) assay.

Results

Eugenol exhibited noticeable antioxidant potential in DPPH radical scavenging (81 %) and reducing power (1.12) assays at 1.0 μM/ml and 0.1 μM/ml concentrations, respectively. IC₅₀ value of eugenol for radical scavenging activity was found to be 0.495 μM/ml. Cinnamaldehyde demonstrated considerable metal ion chelating ability (75 %) at 50 μM/ml and moderate lipo-protective activity in lipid peroxidation assay at 3 μM/ml. In addition cinnamaldehyde also showed appreciable antibacterial activity (zone of inhibition 32–42 mm) against Bacillus cereus (MTCC 6840), Streptococcus mutans (MTCC 497), Proteus vulgaris (MTCC 7299), Salmonella typhi (MTCC 3917) and Bordetella bronchiseptica (MTCC 6838) while eugenol produced moderate activity at 80 μM/disc. Cinnamaldehyde exhibited comparatively better antiproliferative potential against breast (T47D) and lung (NCI-H322) cancer cell lines than eugenol in SRB assay at 50 μM concentration.

Conclusion

Cinnamaldehyde possessed metal ion chelating, lipo-protective, antibacterial and antiproliferative activities while eugenol showed potent H-atom donating potential indicating radical quenching and reducing power abilities. Medicinal attributes shown by both the compounds indicated their usefulness in food and pharmaceutical sector.

Cinnamaldehydes in Cancer Chemotherapy

Cinnamaldehyde and cinnamaldehyde-derived compounds are candidates for the development of anticancer drugs that have received extensive research attention. In this review, we summarize recent findings detailing the positive and negative aspects of cinnamaldehyde and its derivatives as potential anticancer drug candidates. Furthermore, we describe the in vivo pharmacokinetics and metabolism of cinnamaldehydes. The oxidative and antioxidative properties of cinnamaldehydes, which contribute to their potential in chemotherapy, have also been discussed. Moreover, the mechanism(s) by which cinnamaldehydes induce apoptosis in cancer cells have been explored. In addition, evidence of the regulatory effects of cinnamaldehydes on cancer cell invasion and metastasis has been described. Finally, the application of cinnamaldehydes in treating various types of cancer, including breast, prostate, and colon cancers, has been discussed in detail. The effects of cinnamaldehydes on leukemia, hepatocellular carcinoma, and oral cancer have been summarized briefly. Copyright © 2016 John Wiley & Sons, Ltd.

In silico and in vitro studies of cinnamaldehyde and their derivatives against LuxS in Streptococcus pyogenes: effects on biofilm and virulence genes

The LuxS-based signalling pathway has an important role in physiological and pathogenic functions that are capable of causing different infections. In the present study, cinnamaldehyde (CN) and their derivatives were evaluated for their inhibitory efficiency against LuxS by molecular modelling, docking, dynamics and free-energy calculations. Sequence and structure-similarity analysis of LuxS protein, five different amino acids were found to be highly conserved, of which GLY128 was identified as the key residue involved in the effective binding of the ligands. Quantum-polarized ligand docking protocol showed that 2nitro and 4nitro CN has a higher binding efficiency than CN, which very well corroborates with the in vitro studies. COMSTAT analysis for the microscopic images of the S. pyogenes biofilm showed that the ligands have antibiofilm potential. In addition, the results of quantitative polymerase chain reaction (qPCR) analysis revealed that the transcripts treated with the compounds showed decrease in luxS expression, which directly reflects with the reduction in expression of speB. No substantial effect was observed on the virulence regulator (srv) transcript. These results confirm that speB is controlled by the regulation of luxS. The decreased rate of S. pyogenes survival in the presence of these ligands envisaged the fact that the compounds could readily enhance opsonophagocytosis with the reduction of virulence factor secretion. Thus, the overall data supports the use of CN derivatives against quorum sensing-mediated infections caused by S. pyogenes.

Design and synthesis of pyrimidine molecules endowed with thiazolidin-4-one as new anticancer agents

Design and synthesis of new pyrimidine derivatives clubbed with thiazolidin-4-one from 4-(2-chlorophenyl)-6-(2,4-dichlorophenyl)pyrimidin-2-amine and their in vitro anticancer activities were screened at National Cancer Institute (NCI), USA against full NCI 60 cell lines. Compound 2 (NSC: 765735) exhibited remarkable growth inhibition at single dose (10 μM) and encourage chosen for broadcast at 10-fold dilutions of five different concentrations (0.01, 0.1, 1, 10 and 100 μM). The compound 2 was found better quality for Lung cancer cell line (HOP-92) by viewing growth inhibition (GI50 0.52) and no cytotoxicity seen (LC50 > 100). Molecular docking study was performed using Maestro 9.0 (Schrodinger Inc. USA) to provide binding mode into binding sites of CDK2. Compound 2 could be used as a lead compound for developing new potential anticancer agents.

Effects of an Indolocarbazole-Derived CDK4 Inhibitor on Breast Cancer Cells

Introduction: Cyclin D1 (D1) binds to cyclin-dependent kinases (CDK) 4 or 6 to form a holoenzyme that phosphorylates the Rb protein to promote cell cycle progression from G1 to S phase. Therefore, targeting CDK4/6 may be a good strategy for chemotherapy of cancer. We performed a proof-of-principle study to determine the effect of Naphtho [2, 1-α] pyrrolo [3, 4-c] carbazole-5, 7 (6H, 12H)-dione (NPCD), a novel CDK4 inhibitor, on breast cancer cell lines.

Methods: NPCD was synthesized and purified to over 99% purity verified by HPLC. MCF7, MB231, MCF15, T47D and GI101Ap human breast cancer cells were analyzed for the efficacy of NPCD with MTT and clonogenic assays, with FACS and staining for ethidium bromide and acridine orange for cell death and cell cycle profile. Western blot, reverse transcription and PCR were used for studies of gene expression, and co-immunoprecipitation for protein-complex formation.

Results: MTT assay showed that NPCD caused growth arrest and apoptosis of MCF7, MDA-MB231, T47D, MCF15 and GI101Ap cells with an IC50 ranging between 3 to 8 µM given as a single dose. The growth arrest persisted for many days after cessation of the treatment, as shown in a clonogenic assay. NPCD could induce or reduce the D1 and CDK4 protein levels, depending on the cell line, but this effect was not correlated with its efficacy. Phosphorylation of D1 at Thr286 was decreased but it unexpectedly did not correlate with the change in D1 level in the cell lines studied. Phosphorylation of the Rb protein was decreased as expected whereas the p27kip1 protein level was decreased unexpectedly. Protein levels of p21cip1, CDK2 and cyclin E were also decreased in some, but not all, of the cell lines, whereas the mRNA levels of D1, CDK4, cyclin E, CDK2, p27kip1 and p21cip1 were increased in different cell lines.

Conclusions: NPCD can cause long-lasting growth arrest and cell death of breast cancer cell lines at an IC50 of 3-8 µM. Decreased phosphorylation of Rb by D1-CDK4/6 and decreased p27kip1 protein level may be part of the underlying mechanism.

Cinnamaldehydes inhibit thioredoxin reductase and induce Nrf2: potential candidates for cancer therapy and chemoprevention

Trans-cinnamaldehyde (CA) and its analogs 2-hydroxycinnamaldehyde and 2-benzoyloxycinnamaldehyde have been reported to possess antitumor activity. CA is also a known Nrf2 activator. In this study, a series of ortho-substituted cinnamaldehyde analogs was synthesized and screened for antiproliferative and thioredoxin reductase (TrxR)-inhibitory activities. Whereas CA was weakly cytotoxic and TrxR inhibiting, hydroxy and benzoyloxy substitutions resulted in analogs with enhanced antiproliferative activity paralleling increased potency in TrxR inactivation. A novel analog, 5-fluoro-2-hydroxycinnamaldehyde, was identified as exhibiting the strongest antitumor effect (GI(50) 1.6 microM in HCT 116 cells) and TrxR inhibition (IC(50) 7 microM, 1 h incubation with recombinant TrxR). CA and its 2-hydroxy- and 2-benzoyloxy-substituted analogs possessed dual TrxR-inhibitory and Nrf2-inducing effects, both attributed to an active Michael acceptor pharmacophore. At lethal concentrations, TrxR-inhibitory potencies correlated with the compounds' antiproliferative activities. The penultimate C-terminal selenocysteine residue was shown to be a possible target. Conversely, at sublethal concentrations, these agents induced an adaptive antioxidant response through Nrf2-mediated upregulation of phase II enzymes, including TrxR induction. We conclude from the results obtained that TrxR inactivation contributes at least partly to cinnamaldehyde cytotoxicity. These Michael acceptor molecules can potentially be exploited for use in different concentrations in chemotherapeutic and chemopreventive strategies.

Synthesis and biological evaluation of cinnamyl compounds as potent antitumor agents

A series of cinnamyl compounds related to 2'-hydroxycinnamaldehyde were synthesized and their antitumor effects against human cancer cells evaluated. Hydroxylamine derivative 6 inhibited the growth of human cancer cells and human colon tumor xenograft in nude mice. Its antitumor effects belong to the induction of apoptosis and arresting cell cycle at G(2)/M phase, which is confirmed by detection of apoptosis markers and cell cycle analysis.

Inhibition of chitin synthases and antifungal activities by 2'-benzoyloxycinnamaldehyde from Pleuropterus ciliinervis and its derivatives

In the course of search for potent chitin synthase inhibitors from natural resources, a novel chitin synthases inhibitor, 2'-benzoyloxycinnamaldehyde (2'-BCA) (I), was isolated from the aerial parts of Pleuropterus ciliinervis NAKAI. 2'-BCA inhibited chitin synthase 1 and 2 of Saccharomyces cerevisiae with the IC50s of 54.9 and 70.8 microg/ml, respectively, whereas it exhibited no inhibitory activity for chitin synthase 3 up to 280 microg/ml. Its derivatives, 2'-chloro- (V) and 2(-bromo-cinnamaldehyde (VI), each showed 1.9 and 2.7-fold stronger inhibitory activities than 2'-BCA, with the IC50s of 37.2 and 26.6 microg/ml, respectively. Especially, the IC50 of compound VI against chitin synthase 2 represented 1.7-fold more potent inhibitory activity than polyoxin D, a well-known chitin synthase inhibitor. Furthermore, compounds V and VI showed potent antifungal activities against various fungi including human pathogenic fungi, with a particularly strong inhibitory activity against Cryptococcus neoformans (MIC = 16 microg/ml). Although the chemical synthesis of this compound has been reported, the present study is the first report to describe the isolation of 2'-BCA from natural resources and chitin synthases inhibitory activities of its derivatives. These results suggested that 2'-BCA and its derivatives can potentially serve as useful lead compounds for development of antifungal agents.

Synthesis and biological evaluation of dimeric cinnamaldehydes as potent antitumor agents

It has been reported that 2-hydroxycinnamaldehyde and 2-benzoyl-oxycinnamaldehyde inhibited the activity of farnesyl protein transferase, angiogenesis, cell-cell adhesion, and tumor growth in vivo model. In order to improve its anti-tumor activity, dimeric cinnamaldehydes have been synthesized based on 2-hydroxycinnamaldehyde. The synthesized compounds strongly inhibited the growth of human colon tumor cells with GI50 values of 0.6-10 microM. Especially, 2-piperazine derivative blocked in vivo growth of human colon tumor xenograft in nude mice at 10 mg/kg. It was found that their anti-tumor effects induce apoptosis and cell cycle arrest at G2/M phase by the compounds. It was confirmed by detection of apoptosis markers such as activated caspase-3 and cleaved PARP, and cell cycle analysis. The dimeric compounds also inhibited Cdc25B phosphatase which is essential for preinitiating G2/M transition and S phase progression.

2′-Benzoyloxycinnamaldehyde Induces Apoptosis in Human Carcinoma via Reactive Oxygen Species

2'-hydroxycinnamaldehyde (HCA) has been shown to have inhibitory effects on farnesyl protein transferase in vitro, angiogenesis, and tumor cell growth. However, mechanism for these inhibitions remains unknown. As a derivative of HCA, BCA (2'-benzoyl-oxycinnamaldehyde) was synthesized by replacing hydroxyl group with benzoyl-oxyl group. When p53-mutated cancer cell lines (MDA-MB-231 breast cancer cell and SW620 colon cancer cell) were treated with 10 microM HCA or BCA, it induced growth arrest and apoptosis of tumor cells. Markers of apoptosis such as degradations of chromosomal DNA and poly(ADP-ribose) polymerase and activation of caspase-3 were detected after HCA or BCA treatment. BCA-induced apoptosis was blocked by pretreatment of cells with anti-oxidants, glutathione, or N-acetyl-cysteine. In addition, BCA-induced activation of caspase-3 and degradation of poly(ADP-ribose) polymerase were abolished by pretreatment of cells with the anti-oxidants. These results suggest that reactive oxygen species are major regulator of BCA-induced apoptosis. HCA or BCA-induced accumulation of reactive oxygen species was detected by using DCF-DA, an intracellular probe of oxidative stress. Furthermore, when BCA (100 mg/kg) was administrated intraperitoneally or orally into a nude mouse, it inhibited >88 or 41% of tumor growth, respectively, without any detectable weight change. These results suggest that BCA is a good drug candidate for cancer therapy.

Aryl[ a ]pyrrolo[3,4- c ]carbazoles as selective cyclin D1-CDK4 inhibitors

The synthesis of new analogues of Arcyriaflavin A in which one indole ring is replaced by an aryl or heteroaryl ring is described. These new series of aryl[a]pyrrolo[3,4-c]carbazoles were evaluated as inhibitors of Cyclin D1-CDK4. A potent and selective D1-CDK4 inhibitor, 7a (D1-CDK4 IC(50)=45 nM), has been identified. The potency, selectivity profile against other kinases, and structure-activity relationship (SAR) trends of this class of compounds are discussed.

Small-molecule cyclin-dependent kinase modulators

Aberrations in cell cycle progression occur in the majority of human malignancies. The main pathway affected is the retinoblastoma (Rb) pathway. The tumor suppressor gene Rb is an important component in the G(1)/S transition and its function is abnormal in most human neoplasms. Loss in Rb function occurs by the hyperactivation of the cyclin-dependent kinases (cdk's). Therefore, modulation of cdk's may have an important use for the therapy and prevention of human neoplasms. Efforts to obtain small-molecule cdk modulators yielded two classes of modulators: direct and indirect modulators. Direct cdk modulators are small molecules that specifically target the ATP binding site of cdk's. Examples for this group include flavopiridol, roscovitine and BMS-387032. In contrast, indirect cdk modulators affect cdk function due to modulation of upstream pathways required for cdk activation. Some examples include perifosine, lovastatin, and UCN-01. The first example of a direct small-molecule cdk modulator tested in the clinic, flavopiridol, is a pan-cdk inhibitor that not only promotes cell cycle arrest but also halts transcriptional elongation, promotes apoptosis, induces differentiation, and has antiangiogenic properties. Clinical trials with this agent were performed with at least three different schedules of administration: 1-, 24- and 72-h infusions. The main toxicities for infusions >/=24-h are secretory diarrhea and proinflammatory syndrome. In addition, patients receiving shorter infusions have nausea/vomiting and neutropenia. A phase II trial of patients with advanced non-small-cell lung carcinoma using the 72-h infusion every 2 weeks was recently completed. The median overall survival for the 20 patients who received treatment was 7.5 months, a survival similar to that obtained in a randomized trial of four chemotherapy regimens containing platinum analogues in combination with taxanes or gemcitabine, or with gefitinib, a recently approved EGFR inhibitor for the treatment of advanced lung cancer. Based on these encouraging results, a phase III trial comparing standard combination chemotherapy versus combination chemotherapy plus flavopiridol is currently under investigation. The second example of direct small-molecule cdk modulator tested in clinical trials is UCN-01 (7-hydroxystaurosporine). UCN-01 has interesting preclinical features: it inhibits Ca(2+)-dependent PKCs, promotes apoptosis, arrests cell cycle progression at G(1)/S, and abrogates checkpoints upon DNA damage. The first phase I trial of UCN-01 demonstrated a very prolonged half-life. Based on this novel feature, UCN-01 is administered as a 72-h continuous infusion every 4 weeks (in second and subsequent cycles UCN-01 is administered as a 36-h infusion). Other shorter schedules (i.e. 3 h) are being tested. Dose-limiting toxicities include nausea/vomiting, hypoxemia, and insulin-resistant hyperglycemia. Combination trials with cisplatin and other DNA-damaging agents are being tested. Recently, phase I trials with two novel small-molecule cdk modulators, BMS 387032 and R-Roscovitine (CYC202), have commenced with good tolerability. In summary, novel small-molecule cdk modulators are being tested in the clinic with interesting results. Although these small molecules are directed towards a very prevalent cause of carcinogenesis, we need to test them in advanced clinical trials to determine the future of this class of agents for the prevention and therapy of human malignancies.

Imidazo[1,2-a]pyridines: a potent and selective class of cyclin-dependent kinase inhibitors identified through structure-based hybridisation

High-throughput screening identified the imidazo[1,2-a]pyridine and bisanilinopyrimidine series as inhibitors of the cyclin-dependent kinase CDK4. Comparison of their experimentally-determined binding modes and emerging structure-activity trends led to the development of potent and selective imidazo[1,2-a]pyridine inhibitors for CDK4 and in particular CDK2.

Cyclin-dependent kinase 4 inhibitors as a treatment for cancer. Part 1: identification and optimisation of substituted 4,6-bis anilino pyrimidines

Using a high-throughput screening campaign, we identified the 4,6-bis anilino pyrimidines as inhibitors of the cyclin-dependent kinase, CDK4. Herein we describe the further chemical modification and use of X-ray crystallography to develop potent and selective in vitro inhibitors of CDK4.

Cyclin-dependent kinase 4 inhibitors as a treatment for cancer. Part 2: identification and optimisation of substituted 2,4-bis anilino pyrimidines

Through chemical modification and X-ray crystallography we identified the 2,4-bis anilino pyrimidines as potent inhibitors of CDK4. Herein, we describe the optimisation of this series.

Antitumor effect of the cinnamaldehyde derivative CB403 through the arrest of cell cycle progression in the G2/M phase

Cinnamaldehydes have been shown to have inhibitory effects on farnesyl protein transferase (FPTase; EC 2.5.1.29) in vitro, angiogenesis, cell-cell adhesion, and tumor cell growth and to be immunomodulators. However, the mechanisms responsible for these effects remain unknown. To elucidate the molecular mechanism of the cinnamaldehyde derivative CB403 for growth inhibition, CB403 was synthesized from 2'-hydroxycinnamaldehyde. CB403-treated cells were weakly adherent to the culture dishes. In addition, CB403 inhibited tumor growth in these cells in a concentration-dependent manner. FACS analysis using human cancer cells treated with this compound showed cell cycle arrest in mitosis, which was correlated with a marked increase in the amount of cyclin B1. Furthermore, CB403 blocked in vivo growth of human colon and breast tumor xenografts without loss of body weight in nude mice. These results support the hypothesis that the cinnamaldehyde derivative CB403 exerts cytostatic properties by inducing mitotic arrest in cancer cells.

Synthesis of quinolinyl/isoquinolinyl[a]pyrrolo [3,4-c] carbazoles as cyclin D1/CDK4 inhibitors

A novel series of pyrrolo[3,4-c] carbazoles fused with a quinolinyl/isoquinolinyl moiety were synthesized and their D1/CDK4 inhibitory and antiproliferative activity were evaluated. Compound 8H, 14H-isoquinolinyl[6,5-a]-pyrrolo[3,4-c]carbazole-7,9-dione (1d) was found to be a highly potent D1/CDK4 inhibitor with an IC(50) of 69 nM. Compound 1d also inhibited tumor cell growth, arrested tumor cells in G1 phase and inhibited pRb phosphorylation.

Aloisines, a new family of CDK/GSK-3 inhibitors. SAR study, crystal structure in complex with CDK2, enzyme selectivity, and cellular effects

Cyclin-dependent kinases (CDKs) regulate the cell cycle, apoptosis, neuronal functions, transcription, and exocytosis. The observation of CDK deregulations in various pathological situations suggests that CDK inhibitors may have a therapeutic value. In this article, we report on the identification of 6-phenyl[5H]pyrrolo[2,3-b]pyrazines (aloisines) as a novel potent CDK inhibitory scaffold. A selectivity study performed on 26 kinases shows that aloisine A is highly selective for CDK1/cyclin B, CDK2/cyclin A-E, CDK5/p25, and GSK-3 alpha/beta; the two latter enzymes have been implicated in Alzheimer's disease. Kinetic studies, as well as the resolution of a CDK2-aloisine cocrystal structure, demonstrate that aloisines act by competitive inhibition of ATP binding to the catalytic subunit of the kinase. As observed with all inhibitors reported so far, aloisine interacts with the ATP-binding pocket through two hydrogen bonds with backbone nitrogen and oxygen atoms of Leu 83. Aloisine inhibits cell proliferation by arresting cells in both G1 and G2.

p42/p44 MAPKs are intracellular targets of the CDK inhibitor purvalanol

Chemical inhibitors of cyclin-dependent kinases (CDKs) have a great therapeutic potential against various proliferative and neurodegenerative disorders. Intensive screening of a combinatorial chemistry library of 2,6,9-trisubstituted purines has led to the identification of purvalanol, one of the most potent and selective CDK inhibitors to date. In preliminary studies, this compound demonstrates definite anti-mitotic properties, consistent with its nanomolar range efficiency towards purified CDK1 and CDK2. However, the actual intracellular targets of purvalanol remain to be identified, and a method for the determination of its in vivo selectivity was developed. In this technique, cell extracts were screened for purvalanol-interacting proteins by affinity chromatography on immobilized inhibitor. In addition to CDK1, p42/p44 MAPK were found to be two major purvalanol-interacting proteins in five different mammalian cell lines (CCL39, PC12, HBL100, MCF-7 and Jurkat cells), suggesting the generality of the purvalanol/p42/p44 MAPK interaction. The Chinese hamster lung fibroblast cell line CCL39 was used as a model to investigate the anti-proliferative properties of purvalanol. The compound inhibited cell growth with a GI(50) value of 2.5 microM and induced a G2/M block when added to exponentially growing cells. It did not appear to trigger massive activation of caspase. We next tested whether CDKs and p42/p44 MAPK were actually targeted by the compound in vivo. p42/p44 MAPK activity was visualized using an Elk-Gal4 luciferase reporter system and CDK1 activity was detected by the phosphonucleolin level. When cells were treated with purvalanol, p42/p44 MAPK and CDK1 activities were inhibited in a dose-dependent manner. Furthermore, purvalanol inhibited the nuclear accumulation of p42/p44 MAPK, an event dependent on the catalytic activity of these kinases. We conclude that the anti-proliferative properties of purvalanol are mediated by inhibition of both p42/p44 MAPK and CDKs. These observations highlight the potency of moderate selectivity compounds and encourage the search for new therapeutics which simultaneously target distinct but relevant pathways of cell proliferation.