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

High-throughput virtual screening followed by in vitro investigation to identify new lead inhibitors of Cyclin Dependent Kinase 4

In the family of serine/threonine kinases, Cyclin Dependent Kinase 4 (CDK4), is an important regulator in numerous signal transduction pathways. The cell cycle is dysregulated in human breast adenocarcinoma (MCF-7). A set of various categorical QSAR models were generated and validated in the current examination. A recursive partition model, with predictive ability shown by an accuracy of greater than 0.90, was used for virtual screening of 500,000 molecules. Following a consecutive series of molecular docking procedures, followed by pharmacokinetic analysis of 49759 molecules predicted to have pIC50 greater than 7.39, 25 molecules displayed properties that could be described as drug-like. We selected the lead molecules in the MCF-7 cell line based on its ability to promote cell cycle progression.

Selective inhibition of CDK4/6: A safe and effective strategy for developing anticancer drugs

The sustained cell proliferation resulting from dysregulation of the cell cycle and activation of cyclin-dependent kinases (CDKs) is a hallmark of cancer. The inhibition of CDKs is a highly promising and attractive strategy for the development of anticancer drugs. In particular, third-generation CDK inhibitors can selectively inhibit CDK4/6 and regulate the cell cycle by suppressing the G1 to S phase transition, exhibiting a perfect balance between anticancer efficacy and general toxicity. To date, three selective CDK4/6 inhibitors have received approval from the U.S. Food and Drug Administration (FDA), and 15 CDK4/6 inhibitors are in clinical trials for the treatment of cancers. In this perspective, we discuss the crucial roles of CDK4/6 in regulating the cell cycle and cancer cells, analyze the rationale for selectively inhibiting CDK4/6 for cancer treatment, review the latest advances in highly selective CDK4/6 inhibitors with different chemical scaffolds, explain the mechanisms associated with CDK4/6 inhibitor resistance and describe solutions to overcome this issue, and briefly introduce proteolysis targeting chimera (PROTAC), a new and revolutionary technique used to degrade CDK4/6.

Synthesis of pyrrolo[3,4-c]carbazole-1,3(2H,6H)-diones via addition/cyclization reactions between 3-chloro-4-indolylmaleimides and alkynes

3-Chloro-4-indolylmaleimides and two different alkynes are used as the starting materials in a novel and highly effective Pd-catalyzed addition/C–H activation/cyclization sequence for the synthesis of pyrrolo[3,4- c]carbazole-1,3(2 H,6 H)-diones. The desired products are obtained in moderate to excellent yields. Such compounds show a wide range of biological activities.

Carbazole Derivatives as Antiviral Agents: An Overview

Keywords: carbazole; tetrahydrocarbazole; antiviral agents.

Synthesis and Initial in Vivo Studies with [(11)C]SB-216763: The First Radiolabeled Brain Penetrative Inhibitor of GSK-3

Quantifying glycogen synthase kinase-3 (GSK-3) activity in vivo using positron emission tomography (PET) imaging is of interest because dysregulation of GSK-3 is implicated in numerous diseases and neurological disorders for which GSK-3 inhibitors are being considered as therapeutic strategies. Previous PET radiotracers for GSK-3 have been reported, but none of the published examples cross the blood-brain barrier. Therefore, we have an ongoing interest in developing a brain penetrating radiotracer for GSK-3. To this end, we were interested in synthesis and preclinical evaluation of [(11)C]SB-216763, a high-affinity inhibitor of GSK-3 (K i = 9 nM; IC50 = 34 nM). Initial radiosyntheses of [(11)C]SB-216763 proved ineffective in our hands because of competing [3 + 3] sigmatropic shifts. Therefore, we have developed a novel one-pot two-step synthesis of [(11)C]SB-216763 from a 2,4-dimethoxybenzyl-protected maleimide precursor, which provided high specific activity [(11)C]SB-216763 in 1% noncorrected radiochemical yield (based upon [(11)C]CH3I) and 97-100% radiochemical purity (n = 7). Initial preclinical evaluation in rodent and nonhuman primate PET imaging studies revealed high initial brain uptake (peak rodent SUV = 2.5 @ 3 min postinjection; peak nonhuman primate SUV = 1.9 @ 5 min postinjection) followed by washout. Brain uptake was highest in thalamus, striatum, cortex, and cerebellum, areas known to be rich in GSK-3. These results make the arylindolemaleimide skeleton our lead scaffold for developing a PET radiotracer for quantification of GSK-3 density in vivo and ultimately translating it into clinical use.

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.

Marine pyrrolocarbazoles and analogues: synthesis and kinase inhibition

Granulatimide and isogranulatimide are alkaloids obtained from marine sources which have been shown to inhibit cell-cycle G2-checkpoint, targeting more particularly checkpoint 1 kinase (Chk1). At a structural level, they possess a characteristic pyrrolocarbazole framework also shared by the well-known rebeccamycin and staurosporine microbial metabolites which have been described to inhibit topoisomerase I and diverse kinases, respectively. This review reports precisely on the synthesis and kinase inhibitory activities of pyrrolocarbazole-based analogues of granulatimide.

Identification of bisindolylmaleimides and indolocarbazoles as inhibitors of HCV replication by tube-capture-RT-PCR

We devised a screening method for hepatitis C virus (HCV) inhibitors by exploiting the JFH1 viral culture system. The viral RNA released in the medium was adsorbed onto PCR plates, and real-time RT-PCR was performed by directly adding the one-step RT-PCR reaction mixture to the wells. The "tube-capture-RT-PCR" method obviates the need for labor-intensive RNA isolation and should allow high-throughput screening of HCV inhibitors. To substantiate the validity of the assay for drug screening, a pilot screen of an inhibitor library composed of 95 compounds was performed. In addition to the known inhibitors of HCV replication included in the library, the assay identified the PKC inhibitor bisindolylmaleimide I (BIM I) as an HCV replication inhibitor. BIM I was also effective in reducing the viral protein level in genotype 1b and 2a subgenomic replicon cells, indicating inhibition of HCV replication. Further assays revealed that a broad range of bisindolylmaleimides and indolocarbazoles inhibit HCV, but no correlation was found between the PKC inhibition pattern and anti-HCV activity. These series of compounds represent new classes of inhibitors that may warrant further development.

Recent progress in the discovery and development of cyclin-dependent kinase inhibitors

Cyclin-dependent kinases (CDKs) have long been known to be the main facilitators of the cell proliferation cycle. However, they also play important roles in the regulation of the RNA polymerase II transcription cycle. Cancer cells display aberrant cell cycle regulation to gain proliferative advantages and they also appear to have an exaggerated dependence on RNA polymerase II transcriptional activity to sustain pro-survival and antiapoptotic signalling. A picture is now starting to emerge that both the cell-cycle and transcriptional functions of CDKs can be exploited pharmacologically with CDK inhibitors that possess appropriate selectivity profiles. In this article, recent advances into these mechanistic insights and how they can guide clinical development in terms of choice of indication are reviewed, as well as combinations with existing chemotherapies. An overview is also given of recent clinical trial results with the lead CDK inhibitor drug candidates seliciclib (CYC202, (R)-roscovitine; Cyclacel) and alvocidib (flavopiridol; Aventis-NCI), as well as the development of other clinical entries and advanced preclinical compounds. The discussion focuses on oncology, but we point out recent results with CDK inhibitors in virology and nephrology.

1,7-annulated indolocarbazoles as cyclin-dependent kinase inhibitors

The synthesis and kinase inhibitory activity of a series of novel 1,7-annulated indolocarbazoles 6 and 16 is described. These compounds exhibited potent inhibitory activity against cyclin-dependent kinase 4 and good antiproliferative activity in a human colon carcinoma cell line.

Specific inhibition of cyclin-dependent kinase 4/6 by PD 0332991 and associated antitumor activity in human tumor xenografts

PD 0332991 is a highly specific inhibitor of cyclin-dependent kinase 4 (Cdk4) (IC50, 0.011 μmol/L) and Cdk6 (IC50, 0.016 μmol/L), having no activity against a panel of 36 additional protein kinases. It is a potent antiproliferative agent against retinoblastoma (Rb)-positive tumor cells in vitro, inducing an exclusive G1 arrest, with a concomitant reduction of phospho-Ser780/Ser795 on the Rb protein. Oral administration of PD 0332991 to mice bearing the Colo-205 human colon carcinoma produces marked tumor regression. Therapeutic doses of PD 0332991 cause elimination of phospho-Rb and the proliferative marker Ki-67 in tumor tissue and down-regulation of genes under the transcriptional control of E2F. The results indicate that inhibition of Cdk4/6 alone is sufficient to cause tumor regression and a net reduction in tumor burden in some tumors.

Synthesis of 1,7-annulated indoles and their applications in the studies of cyclin dependent kinase inhibitors

The synthesis of a novel series of 1,7-annulated indolocarbazoles 2 and 16 is described. These compounds were found to be potent cyclin dependent kinase inhibitors with good antiproliferative activity against two human carcinoma cell lines. These inhibitors also arrested tumor cells at the G1 phase and inhibited pRb phosphorylation.

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.

Novel, potent and selective cyclin D1/CDK4 inhibitors: indolo[6,7-a]pyrrolo[3,4-c]carbazoles

The synthesis and CDK inhibitory properties of a series of indolo[6,7-a]pyrrolo[3,4-c]carbazoles is reported. In addition to their potent CDK activity, the compounds display antiproliferative activity against two human cancer cell lines. These inhibitors also effect strong G1 arrest in these cell lines and inhibit Rb phosphorylation at Ser780 consistent with inhibition of cyclin D1/CDK4.