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

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.

Synergistic Anticancer Effect of a Combination of Berbamine and Arcyriaflavin A against Glioblastoma Stem-like Cells

Glioblastoma multiforme (GBM) is the most aggressive form of brain tumor. Relapse is frequent and rapid due to glioblastoma stem-like cells (GSCs) that induce tumor initiation, drug resistance, high cancer invasion, immune evasion, and recurrence. Therefore, suppression of GSCs is a powerful therapeutic approach for GBM treatment. Natural compounds berbamine and arcyriaflavin A (ArcA) are known to possess anticancer activity by targeting calcium/calmodulin-dependent protein kinase II gamma (CaMKIIγ) and cyclin-dependent kinase 4 (CDK4), respectively. In this study, we evaluated the effects of concurrent treatment with both compounds on GSCs. Combined treatment with berbamine and ArcA synergistically inhibited cell viability and tumorsphere formation in U87MG- and C6-drived GSCs. Furthermore, simultaneous administration of both compounds potently inhibited tumor growth in a U87MG GSC-grafted chick embryo chorioallantoic membrane (CAM) model. Notably, the synergistic anticancer effect of berbamine and ArcA on GSC growth is associated with the promotion of reactive oxygen species (ROS)- and calcium-dependent apoptosis via strong activation of the p53-mediated caspase cascade. Moreover, co-treatment with both compounds significantly reduced the expression levels of key GSC markers, including CD133, integrin α6, aldehyde dehydrogenase 1A1 (ALDH1A1), Nanog, Sox2, and Oct4. The combined effect of berbamine and ArcA on GSC growth also resulted in downregulation of cell cycle regulatory proteins, such as cyclins and CDKs, by potent inactivation of the CaMKIIγ-mediated STAT3/AKT/ERK1/2 signaling pathway. In addition, a genetic knockdown study using small interfering RNAs (siRNAs) targeting either CaMKIIγ or CDK4 demonstrated that the synergistic anticancer effect of the two compounds on GSCs resulted from dual inhibition of CaMKIIγ and CDK4. Collectively, our findings suggest that a novel combination therapy involving berbamine and ArcA could effectively eradicate GSCs.

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.

How selective are clinical CDK4/6 inhibitors?

Pharmacological inhibition of cyclin-dependent kinase 4/6 (CDK4/6) has emerged as an efficient approach for treating breast cancer, and its clinical potential is expanding to other cancers. CDK4/6 inhibitors were originally believed to act by arresting proliferation in the G1 phase, but it is gradually becoming clear that the cellular response to these compounds is far more complex than this. Multiple context-dependent mechanisms of action are emerging, involving modulation of quiescence, senescence, autophagy, cellular metabolism, and enhanced tumor cell immunogenicity. These mechanisms may be driven by interactions with unexpected targets. We review cellular responses to the Food and Drug Administration-approved CDK4/6 inhibitors palbociclib, ribociclib, and abemaciclib, and summarize available knowledge of other drugs undergoing clinical trials, including data on their off-target landscapes. We emphasize the importance of comprehensively characterizing drugs' selectivity profiles to maximize their clinical efficacy and safety and to facilitate their repurposing to treat additional diseases based on their target spectrum.

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.

Halogenation-Guided Chemical Screening Provides Insight into Tjipanazole Biosynthesis by the Cyanobacterium Fischerella ambigua

Halogenated natural products (HNPs) show a wide range of interesting biological activities. Chemistry-guided screening with a software tool dedicated to identifying halogenated compounds in HPLC-MS data indicated the presence of several uncharacterised HNPs in an extract of the cyanobacterium Fischerella ambigua (Näg.) Gomont 108b. Three new natural products, tjipanazoles K, L, and M, were isolated from this strain together with the known tjipanazoles D and I. Taking into account the structures of all tjipanazole derivatives detected in this strain, reanalysis of the tjipanazole biosynthetic gene cluster allowed us to propose a biosynthetic pathway for the tjipanazoles. As the isolated tjipanazoles show structural similarity to arcyriaflavin A, an inhibitor of the clinically relevant multidrug-transporter ABCG2 overexpressed by different cancer cell lines, the isolated compounds were tested for ABCG2 inhibitory activity. Only tjipanazole K showed appreciable transporter inhibition, whereas the compounds lacking the pyrrolo[3,4-c] ring or featuring additional chloro substituents were found to be much less active.

Antiamoebic Activities of Indolocarbazole Metabolites Isolated from Streptomyces sanyensis Cultures

Indolocarbazoles are a family of natural alkaloids characterized by their potent protein kinase and topoisomerase I inhibitory activity. Among them, staurosporine (1) has exhibited promising inhibitory activity against parasites. Based on new insights on the activity and mechanism of action of STS in Acanthamoeba parasites, this work reports the isolation, identification, and the anti-Acanthamoeba activity of the minor metabolites 7-oxostaurosporine (2), 4′-demethylamino-4′-oxostaurosporine (3), and streptocarbazole B (4), isolated from cultures of the mangrove strain Streptomyces sanyensis. A clear correlation between the antiparasitic activities and the structural elements and conformations of the indolocarbazoles 1–4 was observed. Also, the study reveals that 7-oxostaurosporine (2) affects membrane permeability and causes mitochondrial damages on trophozoites of A. castellanii Neff.

How Selective Are Pharmacological Inhibitors of Cell-Cycle-Regulating Cyclin-Dependent Kinases?

Cyclin-dependent kinases (CDKs) are an important and emerging class of drug targets for which many small-molecule inhibitors have been developed. However, there is often insufficient data available on the selectivity of CDK inhibitors (CDKi) to attribute the effects on the presumed target CDK to these inhibitors. Here, we highlight discrepancies between the kinase selectivity of CDKi and the phenotype exhibited; we evaluated 31 CDKi (claimed to target CDK1-4) for activity toward CDKs 1, 2, 4, 5, 7, 9 and for effects on the cell cycle. Our results suggest that most CDKi should be reclassified as pan-selective and should not be used as a tool. In addition, some compounds did not even inhibit CDKs as their primary cellular targets; for example, NU6140 showed potent inhibition of Aurora kinases. We also established an online database of commercially available CDKi for critical evaluation of their utility as molecular probes. Our results should help researchers select the most relevant chemical tools for their specific applications.

Arcyriaflavin a, a cyclin D1-cyclin-dependent kinase4 inhibitor, induces apoptosis and inhibits proliferation of human endometriotic stromal cells: a potential therapeutic agent in endometriosis

BACKGROUND

We previously showed that microRNA-503 (miR-503) transfection into endometriotic cyst stromal cells (ECSCs) induced cell cycle arrest at the G0/G1 phase by suppressing cyclin D1. This finding prompted us to evaluate the potential therapeutic effects of cyclin D1 inhibitors in endometriotic cells. This study aimed to determine whether arcyriaflavin A, a representative inhibitor of cyclin D1-cyclin-dependent kinase 4 (CDK4), is beneficial in the treatment of endometriosis.

METHODS

ECSCs were isolated from the ovarian endometriotic tissues of 32 women. The effects of arcyriaflavin A on cell viability and proliferation, vascular endothelial growth factor A expression, apoptosis, and cell cycle progression were evaluated using a modified methylthiazoletetrazolium assay, enzyme-linked immunosorbent assay (ELISA), Caspase-Glo® 3/7 assay, and flow cytometry.

RESULTS

Arcyriaflavin A significantly inhibited cell viability, proliferation, and angiogenesis of ECSCs as assessed using the 5-bromo-2-deoxyuridine (BrdU) and methylthiazoletetrazolium bromide (MTT) assays, and vascular endothelial growth factor (VEGF) ELISA. Arcyriaflavin A induced apoptosis as shown in the Caspase-Glo® 3/7 assay and cell death detection ELISA whilethe cell cycle was arrested at the G0/G1 phase.

CONCLUSION

The findings indicate that cyclin D1-CDK4 inhibitors may be promising candidates for the treatment of endometriosis. This is the first study to demonstrate the potential usefulness of arcyriaflavin A as a therapeutic agent for endometriosis. Further studies of the effects of cyclin D1-CDK4 inhibitors on endometriosis may provide useful information on pathogenesis and treatment.

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.

Kinase Inhibitor Screening Identifies Cyclin-Dependent Kinases and Glycogen Synthase Kinase 3 as Potential Modulators of TDP-43 Cytosolic Accumulation during Cell Stress

Abnormal processing of TAR DNA binding protein 43 (TDP-43) has been identified as a major factor in neuronal degeneration during amyotrophic lateral sclerosis (ALS) or frontotemporal lobar degeneration (FTLD). It is unclear how changes to TDP-43, including nuclear to cytosolic translocation and subsequent accumulation, are controlled in these diseases. TDP-43 is a member of the heterogeneous ribonucleoprotein (hnRNP) RNA binding protein family and is known to associate with cytosolic RNA stress granule proteins in ALS and FTLD. hnRNP trafficking and accumulation is controlled by the action of specific kinases including members of the mitogen-activated protein kinase (MAPK) pathway. However, little is known about how kinase pathways control TDP-43 movement and accumulation. In this study, we used an in vitro model of TDP-43-positve stress granule formation to screen for the effect of kinase inhibitors on TDP-43 accumulation. We found that while a number of kinase inhibitors, particularly of the MAPK pathways modulated both TDP-43 and the global stress granule marker, human antigen R (HuR), multiple inhibitors were more specific to TDP-43 accumulation, including inhibitors of cyclin-dependent kinases (CDKs) and glycogen synthase kinase 3 (GSK3). Close correlation was observed between effects of these inhibitors on TDP-43, hnRNP K and TIAR, but often with different effects on HuR accumulation. This may indicate a potential interaction between TDP-43, hnRNP K and TIAR. CDK inhibitors were also found to reverse pre-formed TDP-43-positive stress granules and both CDK and GSK3 inhibitors abrogated the accumulation of C-terminal TDP-43 (219–414) in transfected cells. Further studies are required to confirm the specific kinases involved and whether their action is through phosphorylation of the TDP-43 binding partner hnRNP K. This knowledge provides a valuable insight into the mechanisms controlling abnormal cytoplasmic TDP-43 accumulation and may herald new opportunities for kinase modulation-based therapeutic intervention in ALS and FTLD.

Testing the promiscuity of commercial kinase inhibitors against the AGC kinase group using a split-luciferase screen

Using a newly developed competitive binding assay dependent upon the reassembly of a split reporter protein, we have tested the promiscuity of a panel of reported kinase inhibitors against the AGC group. Many non-AGC targeted kinase inhibitors target multiple members of the AGC group. In general, structurally similar inhibitors consistently exhibited activity toward the same target as well as toward closely related kinases. The inhibition data was analyzed to test the predictive value of either using identity scores derived from residues within 6 Å of the active site or identity scores derived from the entire kinase domain. The results suggest that the active site identity in certain cases may be a stronger predictor of inhibitor promiscuity. The overall results provide general guidelines for establishing inhibitor selectivity as well as for the future design of inhibitors that either target or avoid AGC kinases.

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.

Ca(2+) permeable AMPA receptor induced long-term potentiation requires PI3/MAP kinases but not Ca/CaM-dependent kinase II

Ca(2+) influx via GluR2-lacking Ca(2+)-permeable AMPA glutamate receptors (CP-AMPARs) can trigger changes in synaptic efficacy in both interneurons and principle neurons, but the underlying mechanisms remain unknown. We took advantage of genetically altered mice with no or reduced GluR2, thus allowing the expression of synaptic CP-AMPARs, to investigate the molecular signaling process during CP-AMPAR-induced synaptic plasticity at CA1 synapses in the hippocampus. Utilizing electrophysiological techniques, we demonstrated that these receptors were capable of inducing numerous forms of long-term potentiation (referred to as CP-AMPAR dependent LTP) through a number of different induction protocols, including high-frequency stimulation (HFS) and theta-burst stimulation (TBS). This included a previously undemonstrated form of protein-synthesis dependent late-LTP (L-LTP) at CA1 synapses that is NMDA-receptor independent. This form of plasticity was completely blocked by the selective CP-AMPAR inhibitor IEM-1460, and found to be dependent on postsynaptic Ca(2+) ions through calcium chelator (BAPTA) studies. Surprisingly, Ca/CaM-dependent kinase II (CaMKII), the key protein kinase that is indispensable for NMDA-receptor dependent LTP at CA1 synapses appeared to be not required for the induction of CP-AMPAR dependent LTP due to the lack of effect of two separate pharmacological inhibitors (KN-62 and staurosporine) on this form of potentiation. Both KN-62 and staurosporine strongly inhibited NMDA-receptor dependent LTP in control studies. In contrast, inhibitors for PI3-kinase (LY294002 and wortmannin) or the MAPK cascade (PD98059 and U0126) significantly attenuated this CP-AMPAR-dependent LTP. Similarly, postsynaptic infusion of tetanus toxin (TeTx) light chain, an inhibitor of exocytosis, also had a significant inhibitory effect on this form of LTP. These results suggest that distinct synaptic signaling underlies GluR2-lacking CP-AMPAR-dependent LTP, and reinforces the recent notions that CP-AMPARs are important facilitators of synaptic plasticity in the brain.

Efficient palladium-catalyzed synthesis of 3-aryl-4-indolylmaleimides

Improved palladium catalysts for the Suzuki coupling of 3-bromo-1-methyl-4-(2-methyl-3-indolyl)maleimide have been developed. The coupling of both aryl- and heteroarylboronic acids proceeds smoothly in good to excellent yields at low catalyst loading.

Inhibition of ABCG2-mediated transport by protein kinase inhibitors with a bisindolylmaleimide or indolocarbazole structure

ABCG2 is a transporter with potential importance in cancer drug resistance, drug oral absorption, and stem cell biology. In an effort to identify novel inhibitors of ABCG2, we examined the ability of commercially available bisindolylmaleimides (BIM) and indolocarbazole protein kinase inhibitors (PKI) to inhibit ABCG2, given the previous demonstration that the indolocarbazole PKI UCN-01 interacted with the transporter. At a concentration of 10 micromol/L, all of the compounds tested increased intracellular fluorescence of the ABCG2-specific substrate pheophorbide a in ABCG2-transfected HEK-293 cells by 1.3- to 6-fold as measured by flow cytometry; the ABCG2-specific inhibitor fumitremorgin C increased intracellular fluorescence by 6.6-fold. In 4-day cytotoxicity assays, wild-type ABCG2-transfected cells were not more than 2-fold resistant to any of the compounds, suggesting that the PKIs are not significantly transported by ABCG2. BIMs I, II, III, IV, and V, K252c, and arcyriaflavin A were also able to inhibit [(125)I]iodoarylazidoprazosin labeling of ABCG2 by 65% to 80% at 20 micromol/L, compared with a 50% to 70% reduction by 20 micromol/L fumitremorgin C. K252c and arcyriaflavin A were the most potent compounds, with IC(50) values for inhibition of [(125)I]iodoarylazidoprazosin labeling of 0.37 and 0.23 micromol/L, respectively. K252c and arcyriaflavin A did not have any effect on the ATPase activity of ABCG2. Four minimally toxic compounds--BIM IV, BIM V, arcyriaflavin A, and K252c-reduced the relative resistance of ABCG2-transfected cells to SN-38 in cytotoxicity assays. We find that indolocarbazole and BIM PKIs directly interact with the ABCG2 protein and may thus increase oral bioavailability of ABCG2 substrates.

In vitro antitumor properties of a novel cyclin-dependent kinase inhibitor, P276-00

Cyclin-dependent kinases (Cdk) and their associated pathways represent some of the most attractive targets for the development of anticancer therapeutics. Based on antitumor activity in animal models, a variety of Cdk inhibitors are undergoing clinical evaluation either as a single agent or in combination with other approved drugs. In our anticancer drug discovery program, a novel series of flavones have been synthesized for evaluation against the activity of Cdk4-D1. This enzyme catalyzes the phosphorylation of retinoblastoma protein, thus inhibiting its function. We have identified a series of potent Cdk4-D1 inhibitors with IC(50) below 250 nmol/L. In this report, we have described the properties of one of the best compound, P276-00 of the flavone's series. P276-00 shows 40-fold selectivity toward Cdk4-D1, compared with Cdk2-E. The specificity toward 14 other related and unrelated kinases was also determined. P276-00 was found to be more selective with IC(50)s <100 nmol/L for Cdk4-D1, Cdk1-B, and Cdk9-T1, as compared with other Cdks, and less selective for non-Cdk kinases. It showed potent antiproliferative effects against various human cancer cell lines, with an IC(50) ranging from 300 to 800 nmol/L and was further compared for its antiproliferative activity against cancer and normal fibroblast cell lines. P276-00 was found to be highly selective for cancer cells as compared with normal fibroblast cells. To delineate its mechanism of action, the effect of P276-00 on cell cycle proteins was studied in human breast cancer cell line (MCF-7) and human non-small cell lung carcinoma (H-460). A significant down-regulation of cyclin D1 and Cdk4 and a decrease in Cdk4-specific pRb Ser(780) phosphorylation was observed. P276-00 produced potent inhibition of Cdk4-D1 activity that was found to be competitive with ATP and not with retinoblastoma protein. The compound also induced apoptosis in human promyelocytic leukemia (HL-60) cells, as evidenced by the induction of caspase-3 and DNA ladder studies. These data suggest that P276-00 has the potential to be developed as an anti-Cdk chemotherapeutic agent.

Synthesis and anticancer activity of new pyrrolocarbazoles and pyrrolo-β-carbolines

'Bended' 1, 3 or 'linear' 2 pyrrolidino-fused (aza)carbazoles were prepared and screened towards a few cancer-related targets. Whereas 'bended' derivatives 1 and 3 proved to be weakly toxic, several members of the 'linear' family strongly interact with DNA, especially derivative 28a.

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.