Cyclin-dependent kinase inhibitor Roscovitine induces apoptosis in chronic lymphocytic leukemia cells

Prognostic insights and immune microenvironment delineation in acute myeloid leukemia by ferroptosis-derived signature

Acute myeloid leukemia (AML) represents as a prevalent and formidable hematological malignancy, characterized by notably low 5-year survival rates. Ferroptosis has been found to be correlated with cancer initiation, therapeutic response, and clinical outcome. Nevertheless, the involvement of Ferroptosis-related genes (FRGs) in AML remains ambiguous. Five independent AML cohorts totaling 1,470 (GSE37642, GSE12417, GSE10358, Beat-AML, and TCGA-AML) patients with clinical information were used to systematically investigated the influence of these FRGs expression on outcome and tumor microenvironment. The integration of these datasets led to the subdivision into training and validation sets. Nineteen FRGs were identified as correlated with the overall survival (OS) of AML patients, primarily enriched in ferroptosis, fatty acid metabolism, and leukemia-related signaling pathways. The prognostic signature, consisting of 11 FRGs, was formulated using LASSO-Cox stepwise regression analysis. Patients with high-risk scores exhibited reduced survival compared to those in the low-risk group. The receiver operating characteristic (ROC) analysis underscored the signature's robust predictive accuracy. The high predictive efficacy was confirmed by both internal and external validation datasets. Leukemia and signaling related to immune regulation were mainly enriched pathways of the differentially expressed genes by comparing high- and low-risk groups. The immune composition deconvolution might indicate an immunosuppressive niche in the high-risk patients. The pRRophetic algorithm exploration unveiled chemical drugs with potentially sensitivity among patients in both groups. Collectively, our study developed a ferroptosis-derived prognostic signature that provides the OS prediction and identifies the immune microenvironment for AML patients on large-scale AML cohorts.

Molecular Pharmacology of Multitarget Cyclin-Dependent Kinase Inhibitors in Human Colorectal Carcinoma Cells

BACKGROUND

Colorectal cancer (CRC) is a leading cause of cancer death. Certain signaling pathways are implicated in colorectal carcinogenesis. Cyclin-dependent kinases (CDKs) are commonly hyperactivated in CRC and hence multitarget CDK inhibitors serve as promising therapeutic drugs against CRC.

OBJECTIVE

Off-target effects of multitarget CDK inhibitors with differential CDK inhibitory spectrum viz. P276-00 (also known as riviciclib), roscovitine and UCN-01 on CRC cell lines of varied genetic background were delineated.

METHOD

Protein expression was analyzed for key signaling proteins by western blotting. β-catenin localization was assessed using immunofluorescence. HIF-1 transcriptional activity and target gene expression were studied by reporter gene assay and RT-PCR respectively. Anti-migratory and anti-angiogenic potential was evaluated by wound healing assay and endothelial tube formation assay.

RESULTS

CDK inhibitors modulated various signaling pathways in CRC and for certain proteins showed a highly cell line-dependent response. Riviciclib and roscovitine inhibited HIF-1 transcriptional activity and HIF-1α accumulation in hypoxic HCT116 cells. Both of these drugs also abrogated migration of HCT116 and in vitro angiogenesis in HUVECs.

CONCLUSION

Anticancer activity of multitarget CDK inhibitors can be certainly attributed to their off-target effects and should be analyzed while assessing their therapeutic utility against CRC.

Cellular and Molecular Mechanisms of R/S-Roscovitine and CDKs Related Inhibition under Both Focal and Global Cerebral Ischemia: A Focus on Neurovascular Unit and Immune Cells

Ischemic stroke is the second leading cause of death worldwide. Following ischemic stroke, Neurovascular Unit (NVU) inflammation and peripheral leucocytes infiltration are major contributors to the extension of brain lesions. For a long time restricted to neurons, the 10 past years have shown the emergence of an increasing number of studies focusing on the role of Cyclin-Dependent Kinases (CDKs) on the other cells of NVU, as well as on the leucocytes. The most widely used CDKs inhibitor, (R)-roscovitine, and its (S) isomer both decreased brain lesions in models of global and focal cerebral ischemia. We previously showed that (S)-roscovitine acted, at least, by modulating NVU response to ischemia. Interestingly, roscovitine was shown to decrease leucocytes-mediated inflammation in several inflammatory models. Specific inhibition of roscovitine majors target CDK 1, 2, 5, 7, and 9 showed that these CDKs played key roles in inflammatory processes of NVU cells and leucocytes after brain lesions, including ischemic stroke. The data summarized here support the investigation of roscovitine as a potential therapeutic agent for the treatment of ischemic stroke, and provide an overview of CDK 1, 2, 5, 7, and 9 functions in brain cells and leucocytes during cerebral ischemia.

Pyrimidine: A promising scaffold for optimization to develop the inhibitors of ABC transporters

The multidrug resistance (MDR) phenomenon in cancer cells is the major obstacle leading to failure of chemotherapy accompanied by the feature of intractable and recurrence of cancers. As significant contributors that cause MDR, ABC superfamily proteins can transport the chemotherapeutic drugs out of the tumor cells by the energy of adenosine triphosphate (ATP) hydrolysis, thereby reducing their intracellular accumulation. The ABC transports like ABCB1, ABCC1 and ABCG2 have been extensively studied to develop modulators for overcoming MDR. To date, no reversal agents have been successfully marketed for clinical application, and little information about the ABC proteins bound to specific inhibitors is known, which make the design of MDR inhibitors with potency, selectivity and low toxicity a major challenge. In recent years, it has been increasingly recognized that pyrimidine-based derivatives have the potential for reversing ABC-mediated MDR. In this review, we summarized the pyrimidine-based inhibitors of ABC transporters, and mainly focused on their structure optimizations, development strategies and structure-activity relationship studies in hope of providing a reference for medicinal chemists to develop new modulators of MDR with highly potency and fewer side effects.

Roscovitine and Trichostatin A promote DNA damage repair during porcine oocyte maturation

Faithful repair of DNA double-strand breaks in mammalian oocytes is essential for meiotic maturation and embryonic development. In the present study we investigated the roles of Roscovitine and Trichostatin A (TSA) in DNA damage recovery during invitro maturation of porcine oocytes. Etoposide was used to trigger DNA damage in oocytes. When these DNA-damaged oocytes were treated with 2μM Roscovitine, 50nM TSA or both for 22h, first polar body extrusion and blastocyst formation in all treated groups were significantly improved compared with the etoposide-only group. The most significant improvement was observed when Roscovitine was present. Further immunofluorescent analysis of γH2A.X, an indicator of DNA damage, indicated that DNA damage was significantly decreased in all treated groups. This observation was further supported by analysing the relative mRNA abundance of DNA repair-related genes, including meiotic recombination 11 homolog A (MRE11A), breast cancer type 1 susceptibility protein (BRCA1), Recombinant DNA Repair Protein 51 (RAD51), DNA-dependent protein kinase catalytic subunit (PRKDC) and X-ray cross complementing gene 4 (XRCC4). Compared with the etoposide-only group, the experimental group with combined treatment of Roscovitine and TSA showed a significant decrease of all genes at germinal vesicle and MII stages. The Roscovitine-only treatment group revealed a similar tendency. Together, these results suggest that Roscovitine and TSA treatments could increase the capacity of oocytes to recover from DNA damage by enlisting DNA repair processes.

Selective antitumor activity of roscovitine in head and neck cancer

Radiation and chemotherapy that are commonly used to treat human cancers damage cellular DNA. DNA damage appears to be more toxic to cancer cells than normal cells, most likely due to deregulated checkpoint activation and/or deficiency in DNA repair pathways that are characteristics of many tumors. However, unwanted side effects arise as a result of DNA damage to normal cells during the treatment.

Here, we show that roscovitine, a cyclin-dependent kinase (CDK) inhibitor that inhibits CDK-1, CDK-2, CDK-5, CDK-7, and CDK-9 due to competitive binding to the ATP site on the kinases, causes significant DNA damage followed by p53-dependent cell death in human papilloma virus (HPV)-positive, but not in HPV-negative, head and neck cancer cells. Since HPV positivity was a molecular marker for increased sensitivity of cells to roscovitine, we reasoned that systemic roscovitine administration would not be toxic to healthy HPV-negative tissue. Indeed, low roscovitine doses significantly inhibited the growth of HPV-associated xenografted tumors in mice without causing any detectable side effects.

Given that inhibition of CDKs has been shown to inhibit replication of several viruses, we suggest that roscovitine treatment may represent a selective and safe targeted therapeutic option against HPV-positive head and neck cancer.

Mitochondrial dysfunction RAD51, and Ku80 proteolysis promote apoptotic effects of Dinaciclib in Bcl-xL silenced cells

In the present study, we investigated the effect of CDK inhibitors (ribociclib, palbociclib, seliciclib, AZD5438, and dinaciclib) on malignant human glioma cells for cell viability, apoptosis, oxidative stress, and mitochondrial function using various assays. None of the CDK inhibitors induced cell death at a clinically relevant concentration. However, low nanomolar concentrations of dinaciclib showed higher cytotoxic activity against Bcl-xL silenced cells in a time- and concentration-dependent manner. This effect was not seen with other CDK inhibitors. The apoptosis-inducing capability of dinaciclib in Bcl-xL silenced cells was evidenced by cell shrinkage, mitochondrial dysfunction, DNA damage, and increased phosphatidylserine externalization. Dinaciclib was found to disrupt mitochondrial membrane potential, resulting in the release of cytochrome c, AIF, and smac/DIABLO into the cytoplasm. This was accompanied by the downregulation of cyclin-D1, D3, and total Rb. Dinaciclib caused cell cycle arrest in a time- and concentration-dependent manner and with accumulation of cells in the sub-G1 phase. Our results also revealed that dinaciclib, but not ribociclib or palbociclib or seliciclib or AZD5438 induced intrinsic apoptosis via upregulation of the levels of pro-apoptotic proteins (Bax and Bak), resulting in the activation of caspases and cleavage of PARP. We also found an additional mechanism for the dinaciclib-induced augmentation of apoptosis due to abrogation RAD51-cyclin D1 interaction, specifically proteolysis of the DNA repair proteins RAD51 and Ku80. Our results suggest that successfully interfering with Bcl-xL function may restore sensitivity to dinaciclib and could hold the promise for an effective combination therapeutic strategy.

Targeting MCL-1 sensitizes human esophageal squamous cell carcinoma cells to cisplatin-induced apoptosis

Background

Esophageal squamous cell carcinoma (ESCC) is one of the most lethal malignancies in China and is an exceptionally drug-resistant tumor with a 5-year survival rate less than 15%. Cisplatin is the most commonly used conventional chemotherapeutic drug for the treatment of ESCC, but some patients have a poor response to cisplatin-based chemotherapy. New strategies that could enhance chemosensitivity to cisplatin are needed.

Methods

We used reverse transcription-RCR (RT-PCR), immunoblot, immunohistochemical (IHC) staining, anchorage-dependent and -independent growth assays, co-immunoprecipitation (Co-IP) assay, RNA interference and in vivo tumor growth assay to study the expression of MCL-1 in ESCCs and the response of ESCC cells to cisplatin.

Results

The present study showed that MCL-1 expression was significantly increased in ESCC tissues compared to normal adjacent tissues and was associated with depth of invasion and lymph node metastasis. Knockdown of MCL-1 produced significant chemosensitization to cisplatin in association with caspase-3 activation and PARP cleavage in KYSE150 and KYSE510 cells. The selective MCL-1 inhibitor UMI-77 caused dissociation of MCL-1 from the proapoptotic protein BAX and BAK, and enhanced KYSE150 and KYSE510 cells to cisplatin-induced apoptosis accompanied by caspase-3 activation and PARP cleavage.

Conclusions

The current study suggests that MCL-1 contributes to the development of ESCC and is a promising therapeutic target for chemosensitization of ESCC cells to cisplatin. This might provide a scientific basis for developing effective approaches to treat the subset of ESCCs patients with MCL-1 overexpression.

Recent progress of cyclin-dependent kinase inhibitors as potential anticancer agents

Deregulation of the cell cycle is a common feature in human cancer. The inhibition of cyclin-dependent kinases (CDKs), which play a crucial role in control of the cell cycle, has always been one of the most promising areas in cancer chemotherapy. This review first summarizes the biology of CDKs and then focuses on the recent advances in both broad-range and selective CDK inhibitors during the last 5 years. The design rationale, structural optimization and structure–activity relationships analysis of these small molecules have been discussed in detail and the key interactions with the amino-acid residues of the most important compounds are highlighted. Future perspectives for CDKs inhibitors will be defined in the development of highly selective CDK inhibitors, an accurate knowledge of gene control mechanism and further predictive biomarker research.

Cyclin-Dependent Kinase Inhibitor P1446A Induces Apoptosis in a JNK/p38 MAPK-Dependent Manner in Chronic Lymphocytic Leukemia B-Cells

CDK (cyclin-dependent kinase) inhibitors have shown remarkable activity in CLL, where its efficacy has been linked to inhibition of the transcriptional CDKs (7 and 9) and deregulation of RNA polymerase and short-lived pro-survival proteins such as MCL1. Furthermore, ER (endoplasmic reticulum) stress has been implicated in CDK inhibition in CLL. Here we conducted a pre-clinical study of a novel orally active kinase inhibitor P1446A in CLL B-cells. P1446A inhibited CDKs at nanomolar concentrations and induced rapid apoptosis of CLL cells in vitro, irrespective of chromosomal abnormalities or IGHV mutational status. Apoptosis preceded inactivation of RNA polymerase, and was accompanied by phosphorylation of stress kinases JNK (c-Jun N-terminal kinase) and p38 MAPK (mitogen-activated protein kinase). Pharmacologic inhibitors of JNK/p38 MAPK conferred protection from P1446A-mediated apoptosis. Treatment with P1446A led to a dramatic induction of NOXA in a JNK-dependent manner, and sensitized CLL cells to ABT-737, a BH3-mimetic. We observed concurrent activation of apoptosis stress-inducing kinase 1 (ASK1) and its interaction with inositol-requiring enzyme 1 (IRE1) and tumor necrosis factor receptor-associated factor 2 (TRAF2) in CLL cells treated with P1446A, providing insights into upstream regulation of JNK in this setting. Consistent with previous reports on limited functionality of ER stress mechanism in CLL cells, treatment with P1446A failed to induce an extensive unfolded protein response. This study provides rationale for additional investigations of P1446A in CLL.

Polyamines modulate the roscovitine-induced cell death switch decision autophagy vs. apoptosis in MCF-7 and MDA-MB-231 breast cancer cells

Current clinical strategies against breast cancer mainly involve the use of anti‑hormonal agents to decrease estrogen production; however, development of resistance is a major problem. The resistance phenotype depends on the modulation of cell‑cycle regulatory proteins, cyclins and cyclin‑dependent kinases. Roscovitine, a selective inhibitor of cyclin‑dependent kinases, shows high therapeutic potential by causing cell‑cycle arrest in various cancer types. Autophagy is a type of cell death characterized by the enzymatic degradation of macromolecules and organelles in double‑ or multi‑membrane autophagic vesicles. This process has important physiological functions, including the degradation of misfolded proteins and organelle turnover. Recently, the switch between autophagy and apoptosis has been proposed to constitute an important regulator of cell death in response to chemotherapeutic drugs. The process is regulated by several proteins, such as the proteins of the Atg family, essential for the initial formation of the autophagosome, and PI3K, important at the early stages of autophagic vesicle formation. Polyamines (PAs) are small aliphatic amines that play major roles in a number of eukaryotic processes, including cell proliferation. The PA levels are regulated by ornithine decarboxylase (ODC), the rate‑limiting enzyme in PA biosynthesis. In this study, we aimed to investigate the role of PAs in roscovitine‑induced autophagic/apoptotic cell death in estrogen receptor‑positive MCF‑7 and estrogen receptor‑negative MDA‑MB‑231 breast cancer cells. We show that MDA‑MB‑231 cells are more resistant to roscovitine than MCF‑7 cells. This difference was related to the regulation of autophagic key molecules in MDA‑MB‑231 cells. In addition, we found that exogenous PAs have a role in the cell death decision between roscovitine‑induced apoptosis or autophagy in MCF‑7 and MDA‑MB‑231 breast cancer cells.

Apoptosis inducers in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is characterized by a typical defect in apoptosis and is still an incurable disease. Numerous apoptosis inducers have been described. These synthetic compounds and natural products (mainly derived from plants) display antileukemic properties in vitro and in vivo and some have even been tested in the clinic in CLL. They act through several different mechanisms. Most of them involve proteins of the Bcl-2 family, which are the key regulators in triggering the mitochondrial pathway of caspase-dependent apoptosis. Thus, the Mcl-1/Noxa axis appeared as a target. Here I overview natural and synthetic apoptosis inducers and their mechanisms of action in CLL cells. Opportunities for developing novel, apoptosis-based therapeutics are presented.

CDK9-mediated transcription elongation is required for MYC addiction in hepatocellular carcinoma

One-year survival rates for newly diagnosed hepatocellular carcinoma (HCC) are <50%, and unresectable HCC carries a dismal prognosis owing to its aggressiveness and the undruggable nature of its main genetic drivers. By screening a custom library of shRNAs directed toward known drug targets in a genetically defined Myc-driven HCC model, we identified cyclin-dependent kinase 9 (Cdk9) as required for disease maintenance. Pharmacological or shRNA-mediated CDK9 inhibition led to robust anti-tumor effects that correlated with MYC expression levels and depended on the role that both CDK9 and MYC exert in transcription elongation. Our results establish CDK9 inhibition as a therapeutic strategy for MYC-overexpressing liver tumors and highlight the relevance of transcription elongation in the addiction of cancer cells to MYC.

Reduction of phosphorylated Thr-161 Cdk1 level participates in roscovitine-induced Fas ligand-mediated apoptosis in rat eggs cultured in vitro

The present study was aimed to find out whether roscovitine reduces phosphorylated Thr-161 of cyclin-dependent kinase 1 (Cdk1) level and induces egg apoptosis through Fas ligand (FasL)-mediated pathway. For this purpose, ovulated eggs were cultured in media 199 with or without various concentrations of roscovitine (0, 25, 50, 100, 200 μM) for 3 h in vitro. The morphological apoptotic changes, phosphorylation status of Cdk1, FasL concentration, caspase-8 and caspase-3 activities, and DNA fragmentation were analyzed. Data of the present study suggest that roscovitine significantly reduced Thr-161 phosphorylated Cdk1 level without altering the total level of Cdk1 and induced cytoplasmic fragmentation, a morphological apoptotic feature in a concentration-dependent manner. The roscovitine-induced cytoplasmic fragmentation was associated with increased FasL concentration. The increased FasL concentration induced caspase-8 followed by caspase-3 activities. The increased caspases activity finally induced DNA fragmentation in eggs that showed cytoplasmic fragmentation. Taken together, these results suggest that roscovitine reduced phosphorylated Thr-161 of Cdk1 level and induces apoptosis through FasL-mediated pathway in rat eggs cultured in vitro.

Dinaciclib for the treatment of breast cancer

INTRODUCTION

Cyclin-dependent kinases (CDK) represent attractive targets in oncology due to their key role in controlling gene transcription and cell cycle progression. Dinaciclib (MK-7965, formerly SCH727965) is a relatively novel CDK 1/2/5/9 inhibitor that has shown promising results in preclinical studies and an acceptable safety profile in Phase I clinical trials. It is currently under clinical evaluation for the treatment of hematological and solid malignancies, including breast cancer.

AREAS COVERED

This review summarizes the current understanding of CDK's role in physiology and cancer, and the therapeutic value of blocking their pathways in breast cancer. Particularly, the article reviews the preclinical and clinical data for dinaciclib in its use for the treatment of breast cancer.

EXPERT OPINION

A better understanding of the molecular mechanisms underlying cell cycle dysregulation in cancer is needed in order to develop novel CDK inhibitors. Additionally, further efforts are needed to identify potential biomarkers of dinaciclib efficacy, which could allow a better selection of patients enrolled in clinical trials. Moreover, combination therapies with dinaciclib or other CDK and chemotherapy, endocrine therapy or targeted therapies might be further evaluated in breast cancer patients.

A novel Cdk9 inhibitor preferentially targets tumor cells and synergizes with fludarabine

Cdk9 is a key elongation factor for RNA transcription and functions by phosphorylating the C-terminal domain of RNA polymerase II. Here we present direct evidence that cdk9 is important for cancer cell survival and describe the characterization of the potent cdk9 inhibitor CDKI-73 in primary human leukemia cells. CDKI-73 induced caspase-dependent apoptosis that was preceded by dephosphorylation of cdk9 and serine 2 of RNA polymerase II. CDKI-73 was more potent than the pan-cdk inhibitor flavopiridol and showed >200-fold selectivity against primary leukemia cells when compared with normal CD34+ cells. Furthermore, CDKI-73 was equipotent in poor prognostic sub-groups of leukemia patients and showed cytotoxic synergy with the nucleoside analog fludarabine. The Mechanism of synergy was associated with CDKI-73-mediated transcriptional inhibition of MCL1 and XIAP that was maintained when used in combination with fludarabine. Our data present a strong rationale for the development of cdk9 inhibitors such as CDKI-73 as anticancer therapeutics.

Inhibition of polyamine oxidase prevented cyclin-dependent kinase inhibitor-induced apoptosis in HCT 116 colon carcinoma cells

Roscovitine and purvalanol are novel cyclin-dependent kinase (CDK) inhibitors that prevent cell proliferation and induce apoptotic cell death in various cancer cell lines. Although a number of studies have demonstrated the potential apoptotic role of roscovitine, there is limited data about the therapeutic efficiency of purvalanol on cancer cells. The natural polyamines (PAs) putrescine, spermidine, and spermine have essential roles in the regulation of cell differentiation, growth, and proliferation, and increased levels of these compounds have been associated with cancer progression. Recently, depletion of intracellular PA levels because of modulation of PA catabolic enzymes was shown to be an indicator of the efficacy of chemotherapeutic agents. In this study, our aim was to investigate the potential role of PA catabolic enzymes in CDK inhibitor-induced apoptosis in HCT 116 colon carcinoma cells. Exposure of cells to roscovitine or purvalanol decreased cell viability in a dose- and time-dependent manner. The selected concentrations of roscovitine and purvalanol inhibited cell viability by 50 % compared with control cells and induced apoptosis by activating the mitochondria-mediated pathway in a caspase-dependent manner. However, the apoptotic effect of purvalanol was stronger than that of roscovitine in HCT 116 cells. In addition, we found that CDK inhibitors decreased PA levels and significantly upregulated expression of key PA catabolic enzymes such as polyamine oxidase (PAO) and spermine oxidase (SMO). MDL-72,527, a specific inhibitor of PAO and SMO, decreased apoptotic potential of CDK inhibitors on HCT 116 cells. Moreover, transient silencing of PAO was also reduced prevented CDK inhibitor-induced apoptosis in HCT 116 cells. We conclude that the PA catabolic pathway, especially PAO, is a critical target for understanding the molecular mechanism of CDK inhibitor-induced apoptosis.

Randomized phase II trial of the cyclin-dependent kinase inhibitor dinaciclib (MK-7965) versus capecitabine in patients with advanced breast cancer

INTRODUCTION

Effective therapies after failure of treatment with anthracyclines and taxanes are needed for patients with metastatic breast cancer. Dinaciclib (MK-7965, formerly SCH727965), a small-molecule cyclin-dependent kinase inhibitor, has demonstrated antitumor activity in phase I studies with solid-tumor patients. This phase II trial was designed to assess the efficacy and safety of dinaciclib compared with that of capecitabine in women with previously treated advanced breast cancer.

PATIENTS AND METHODS

Patients were randomized to receive either dinaciclib at 50 mg/m(2), administered as a 2-hour infusion every 21 days, or 1250 mg/m(2) capecitabine, administered orally twice daily in 21-day cycles.

RESULTS

An unplanned interim analysis showed that the time to disease progression was inferior with dinaciclib treatment compared with capecitabine treatment; therefore, the trial was stopped after 30 patients were randomized. Dinaciclib treatment demonstrated antitumor activity in 2 of 7 patients with estrogen receptor-positive and human epidermal growth factor receptor 2-negative metastatic breast cancer (1 confirmed and 1 unconfirmed partial response), as well as acceptable safety and tolerability. Grade 3 or 4 treatment-related adverse events were common and included neutropenia, leukopenia, increase in aspartate aminotransferase, and febrile neutropenia. Population pharmacokinetic model-predicted mean dinaciclib exposure (area under the concentration-time curve extrapolated to infinity [AUC[I]]) at 50 mg/m(2) was similar to that observed in a previous phase I trial, and no drug accumulation was observed after multiple-dose administration.

CONCLUSION

Although dinaciclib monotherapy demonstrated some antitumor activity and was generally tolerated, efficacy was not superior to capecitabine. Future studies may be considered to evaluate dinaciclib in select patient populations with metastatic breast cancer and in combination with other agents.

A first-in-human, phase 1, dose-escalation study of dinaciclib, a novel cyclin-dependent kinase inhibitor, administered weekly in subjects with advanced malignancies

Background

Dinaciclib, a small-molecule, cyclin-dependent kinase inhibitor, inhibits cell cycle progression and proliferation in various tumor cell lines in vitro. We conducted an open-label, dose-escalation study to determine the safety, tolerability, and bioactivity of dinaciclib in adults with advanced malignancies.

Methods

Dinaciclib was administered starting at a dose of 0.33 mg/m², as a 2-hour intravenous infusion once weekly for 3 weeks (on days 1, 8, and 15 of a 28-day cycle), to determine the maximum administered dose (MAD), dose-limiting toxicities (DLTs), recommended phase 2 dose (RP2D), and safety and tolerability. Pharmacodynamics of dinaciclib were assessed using an ex vivo phytohemagglutinin lymphocyte stimulation assay and immunohistochemistry staining for retinoblastoma protein phosphorylation in skin biopsies. Evidence of antitumor activity was assessed by sequential computed tomography imaging after every 2 treatment cycles.

Results

Forty-eight subjects with solid tumors were treated. The MAD was found to be 14 mg/m² and the RP2D was determined to be 12 mg/m²; DLTs at the MAD included orthostatic hypotension and elevated uric acid. Forty-seven (98%) subjects reported adverse events (AEs) across all dose levels; the most common AEs were nausea, anemia, decreased appetite, and fatigue. Dinaciclib administered at the RP2D significantly inhibited lymphocyte proliferation, demonstrating a pharmacodynamic effect. Ten subjects treated at a variety of doses achieved prolonged stable disease for at least 4 treatment cycles.

Conclusions

Dinaciclib administered every week for 3 weeks (on days 1, 8, and 15 of a 28-day cycle) was generally safe and well tolerated. Initial bioactivity and observed disease stabilization support further evaluation of dinaciclib as a treatment option for patients with advanced solid malignancies.

Trial registration

ClinicalTrials.gov # NCT00871663

Identification and structural-functional analysis of cyclin-dependent kinases of the cattle tick Rhipicephalus (Boophilus) microplus

Cyclin-dependent kinases (CDKs) are a family of serine/threonine kinases essential for cell cycle progression. Herein, we describe the participation of CDKs in the physiology of Rhipicephalus microplus, the southern cattle tick and an important disease vector. Firstly, amino acid sequences homologous with CDKs of other organisms were identified from a R. microplus transcriptome database in silico. The analysis of the deduced amino acid sequences of CDK1 and CDK10 from R. microplus showed that both have caspase-3/7 cleavage motifs despite their differences in motif position and length of encoded proteins. CDK1 has two motifs (DKRGD and SAKDA) located opposite to the ATP binding site while CDK10 has only one motif (SLLDN) for caspase 3–7 near the ATP binding site. Roscovitine (Rosco), a purine derivative that inhibits CDK/cyclin complexes by binding to the catalytic domain of the CDK molecule at the ATP binding site, which prevents the transfer of ATP's γphosphoryl group to the substrate. To determine the effect of Rosco on tick CDKs, BME26 cells derived from R. microplus embryo cells were utilized in vitro inhibition assays. Cell viability decreased in the Rosco-treated groups after 24 hours of incubation in a concentration-dependent manner and this was observed up to 48 hours following incubation. To our knowledge, this is the first report on characterization of a cell cycle protein in arachnids, and the sensitivity of BME26 tick cell line to Rosco treatment suggests that CDKs are potential targets for novel drug design to control tick infestation.

Targeting cyclin-dependent kinases in anti-neoplastic therapy

Cell cycle progression is controlled by sequential activation of cyclin-dependent kinases (CDKs), which are often deregulated in cancer. Consequently numerous pharmacological inhibitors of CDKs have been developed with the aim of treating cancers. The article briefly reviews CDK inhibitors and their use to treat cancers, with specific focus on the use of biomarkers and drugs combination to improve their therapeutic efficacy.

Emerging drug profile: cyclin-dependent kinase inhibitors

Abstract As the rational application of targeted therapies in cancer supplants traditional cytotoxic chemotherapy, there is an ever-greater need for a thorough understanding of the complex machinery of the cell and an application of this knowledge to the development of novel therapeutics and combinations of agents. Here, we review the current state of knowledge of the class of targeted agents known as cyclin-dependent kinase (CDK) inhibitors, with a focus on chronic lymphocytic leukemia (CLL). Flavopiridol (alvocidib) is the best studied of the CDK inhibitors, producing a dramatic cytotoxic effect in vitro and in vivo, with the principal limiting factor of acute tumor lysis. Unfortunately, flavopiridol has a narrow therapeutic window and is relatively non-selective with several off-target (i.e. non-CDK) effects, which prompted development of the second-generation CDK inhibitor dinaciclib. Dinaciclib appears to be both more potent and selective than flavopiridol, with at least an order of magnitude greater therapeutic index, and is currently in phase III clinical trials. In additional to flavopiridol and dinaciclib, we also review the current status of other members of this class, and provide commentary as to the future direction of combination therapy including CDK inhibitors.

Inhibition of NF-κB-mediated signaling by the cyclin-dependent kinase inhibitor CR8 overcomes prosurvival stimuli to induce apoptosis in chronic lymphocytic leukemia cells

PURPOSE

Chronic lymphocytic leukemia (CLL) is currently incurable with standard chemotherapeutic agents, highlighting the need for novel therapies. Overcoming proliferative and cytoprotective signals generated within the microenvironment of lymphoid organs is essential for limiting CLL progression and ultimately developing a cure.

EXPERIMENTAL DESIGN

We assessed the potency of cyclin-dependent kinase (CDK) inhibitor CR8, a roscovitine analog, to induce apoptosis in primary CLL from distinct prognostic subsets using flow cytometry-based assays. CLL cells were cultured in in vitro prosurvival and proproliferative conditions to mimic microenvironmental signals in the lymphoid organs, to elucidate the mechanism of action of CR8 in quiescent and proliferating CLL cells using flow cytometry, Western blotting, and quantitative real-time PCR.

RESULTS

CR8 was 100-fold more potent at inducing apoptosis in primary CLL cells than roscovitine, both in isolated culture and stromal-coculture conditions. Importantly, CR8 induced apoptosis in CD40-ligated CLL cells and preferentially targeted actively proliferating cells within these cultures. CR8 treatment induced downregulation of the antiapoptotic proteins Mcl-1 and XIAP, through inhibition of RNA polymerase II, and inhibition of NF-κB signaling at the transcriptional level and through inhibition of the inhibitor of IκB kinase (IKK) complex, resulting in stabilization of IκBα expression.

CONCLUSIONS

CR8 is a potent CDK inhibitor that subverts pivotal prosurvival and proproliferative signals present in the tumor microenvironment of CLL patient lymphoid organs. Our data support the clinical development of selective CDK inhibitors as novel therapies for CLL.

Apoptosis deregulation in CLL

The description of apoptosis and the identification of the genes that regulate it have proved pivotal to our understanding of how cancer cells accumulate and ultimately cause morbidity and mortality. It has become increasingly clear that in CLL the balance between the pro- and anti-apoptotic members of the BCL2 family of apoptotic regulatory proteins is critical in the development and clinical progression of CLL. Furthermore, the apoptotic potential of the CLL cell determines chemotherapy sensitivity and ultimately progression-free and overall survival. The unravelling of the BCL2 story in CLL has led to the development of a whole new class of therapeutic agents-the BH3 mimetics-which are significantly more targeted than conventional chemo-immunotherapy and therefore promise potent clinical activity coupled with reduced toxicity.

Inhibition of T-type calcium channels disrupts Akt signaling and promotes apoptosis in glioblastoma cells

Glioblastoma multiforme (GBM) are brain tumors that are exceptionally resistant to both radio- and chemotherapy regimens and novel approaches to treatment are needed. T-type calcium channels are one type of low voltage-gated channel (LVCC) involved in embryonic cell proliferation and differentiation; however they are often over-expressed in tumors, including GBM. In this study, we found that inhibition of T-type Ca(2+) channels in GBM cells significantly reduced their survival and resistance to therapy. Moreover, either T-type selective antagonists, such as mibefradil, or siRNA-mediated knockdown of the T-type channel alpha subunits not only reduced cell viability and clonogenic potential, but also induced apoptosis. In response to channel blockade or ablation, we observed reduced phosphorylation of Akt and Rictor, suggesting inhibition of the mTORC2/Akt pathway. This was followed by reduction in phosphorylation of anti-apoptotic Bad and caspases activation. The apoptotic response was specific for T-type Ca(2+) channels, as inhibition of L-type Ca(2+) channels did not induce similar effects. Our results implicate T-type Ca(2+) channels as distinct entities for survival signaling in GBM cells and suggest that they are a novel molecular target for tumor therapy.

Roscovitine sensitizes leukemia and lymphoma cells to tumor necrosis factor-related apoptosis-inducing ligand-induced apoptosis

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a death ligand with selective antitumor activity. However, many primary tumors are TRAIL resistant. Previous studies reported that roscovitine, a cyclin-dependent kinase inhibitor, sensitized various solid cancer cells to TRAIL. We show that roscovitine and TRAIL demonstrate synergistic cytotoxicity in hematologic malignant cell lines and primary cells. Pretreatment of TRAIL-resistant leukemia cells with roscovitine induced enhanced cleavage of death-inducing signaling complex-bound proximal caspases after exposure to TRAIL. We observed increased levels of both pro- and antiapoptotic BCL-2 proteins at the mitochondria following exposure to roscovitine. These results suggest that roscovitine induces priming of cancer cells for death by binding antiapoptotic BCL-2 proteins to proapoptotic BH3-only proteins at the mitochondria, thereby decreasing the threshold for diverse proapoptotic stimuli. We propose that the mitochondrial priming and enhanced processing of apical caspases represent major molecular mechanisms of roscovitine-induced sensitization to TRAIL in leukemia/lymphoma cells.

Characterization of cyclin E expression in multiple myeloma and its functional role in seliciclib-induced apoptotic cell death

Multiple Myeloma (MM) is a lymphatic neoplasm characterized by clonal proliferation of malignant plasma cell that eventually develops resistance to chemotherapy. Drug resistance, differentiation block and increased survival of the MM tumor cells result from high genomic instability. Chromosomal translocations, the most common genomic alterations in MM, lead to dysregulation of cyclin D, a regulatory protein that governs the activation of key cell cycle regulator – cyclin dependent kinase (CDK). Genomic instability was reported to be affected by over expression of another CDK regulator - cyclin E (CCNE). This occurs early in tumorigenesis in various lymphatic malignancies including CLL, NHL and HL. We therefore sought to investigate the role of cyclin E in MM. CCNE1 expression was found to be heterogeneous in various MM cell lines (hMMCLs). Incubation of hMMCLs with seliciclib, a selective CDK-inhibitor, results in apoptosis which is accompanied by down regulation of MCL1 and p27. Ectopic over expression of CCNE1 resulted in reduced sensitivity of the MM tumor cells in comparison to the paternal cell line, whereas CCNE1 silencing with siRNA increased the cell sensitivity to seliciclib. Adhesion to FN of hMMCLs was prevented by seliciclib, eliminating adhesion–mediated drug resistance of MM cells. Combination of seliciclib with flavopiridol effectively reduced CCNE1 and CCND1 protein levels, increased subG1 apoptotic fraction and promoted MM cell death in BMSCs co-culture conditions, therefore over-coming stroma-mediated protection. We suggest that seliciclib may be considered as essential component of modern anti MM drug combination therapy.

The impact of CDK inhibition in human malignancies associated with pronounced defects in apoptosis: advantages of multi-targeting small molecules

Malignant cells in chronic lymphocytic leukemia (CLL) and related diseases are heterogeneous and consist primarily of long-lived resting cells in the periphery and a minor subset of dividing cells in proliferating centers. Both cell populations have different molecular signatures that play a major role in determining their sensitivity to therapy. Contemporary approaches to treating CLL are heavily reliant on cytotoxic chemotherapeutics. However, none of the current treatment regimens can be considered curative. Pharmacological CDK inhibitors have extended the repertoire of potential drugs for CLL. Multi-targeted CDK inhibitors affect CDKs involved in regulating both cell cycle progression and transcription. Their interference with transcriptional elongation represses anti-apoptotic proteins and, thus, promotes the induction of apoptosis. Importantly, there is evidence that treatment with CDK inhibitors can overcome resistance to therapy. The pharmacological CDK inhibitors have great potential for use in combination with other therapeutics and represent promising tools for the development of new curative treatments for CLL.

The Aurora A and B kinases are up-regulated in bone marrow-derived chronic lymphocytic leukemia cells and represent potential therapeutic targets

BACKGROUND

The malignant B cells in chronic lymphocytic leukemia receive signals from the bone marrow and lymph node microenvironments which regulate their survival and proliferation. Characterization of these signals and the pathways that propagate them to the interior of the cell is important for the identification of novel potential targets for therapeutic intervention.

DESIGN AND METHODS

We compared the gene expression profiles of chronic lymphocytic leukemia B cells purified from bone marrow and peripheral blood to identify genes that are induced by the bone marrow microenvironment. Two of the differentially expressed genes were further studied in cell culture experiments and in an animal model to determine whether they could represent appropriate therapeutic targets in chronic lymphocytic leukemia.

RESULTS

Functional classification analysis revealed that the majority of differentially expressed genes belong to gene ontology categories related to cell cycle and mitosis. Significantly up-regulated genes in bone marrow-derived tumor cells included important cell cycle regulators, such as Aurora A and B, survivin and CDK6. Down-regulation of Aurora A and B by RNA interference inhibited proliferation of chronic lymphocytic leukemia-derived cell lines and induced low levels of apoptosis. A similar effect was observed with the Aurora kinase inhibitor VX-680 in primary chronic lymphocytic leukemia cells that were induced to proliferate by CpG-oligonucleotides and interleukin-2. Moreover, VX-680 significantly blocked leukemia growth in a mouse model of chronic lymphocytic leukemia.

CONCLUSIONS

Aurora A and B are up-regulated in proliferating chronic lymphocytic leukemia cells and represent potential therapeutic targets in this disease.

Roscovitine inhibits CaV3.1 (T-type) channels by preferentially affecting closed-state inactivation

T-type calcium channels (Ca(V)3) play an important role in many physiological and pathological processes, including cancerogenesis. Ca(V)3 channel blockers have been proposed as potential cancer treatments. Roscovitine, a trisubstituted purine, is a cyclin-dependent kinase (CDK) inhibitor that is currently undergoing phase II clinical trials as an anticancer drug and has been shown to affect calcium and potassium channel activity. Here, we investigate the effect of roscovitine on Ca(V)3.1 channels. Ca(V)3.1 channels were transiently expressed in human embryonic kidney 293 cells, and currents were recorded by using the whole-cell patch-clamp technique. Roscovitine blocks Ca(V)3.1 channels with higher affinity for depolarized cells (EC₅₀ of 10 μM), which is associated with a negative shift in the voltage dependence of closed-state inactivation. Enhanced inactivation is mediated by roscovitine-induced acceleration of closed-state inactivation and slowed recovery from inactivation. Small effects of roscovitine were also observed on T-channel deactivation and open-state inactivation, but neither could explain the inhibitory effect. Roscovitine inhibits Ca(V)3.1 channels within the therapeutic range (10-50 μM) in part by stabilizing the closed-inactivated state. The ability of roscovitine to block multiple mediators of proliferation, including CDKs and Ca(V)3.1 channels, may facilitate its anticancer properties.

Can ex vivo evaluation (testing) predict the sensitivity of CLL cells to therapy with purine analogs in conjunction with an alkylating agent? A comparison of in vivo and ex vivo responses to treatment

Malfunctions in the regulation of apoptosis cause the accumulation of malignant, long-lived B CD19+/CD5+ cells in chronic lymphocytic leukemia (CLL). The primary goal in CLL therapy is to overcome resistance to apoptosis and efficiently trigger programmed cell death in leukemic cells. This study demonstrated that the in vivo responses of malignant cells from CLL patients after administration of purine analogs (cladribine/fludarabine) with cyclophosphamide vary significantly. For comparative purposes, the sensitivity of leukemic cells obtained from the same CLL patients to conventional purine analogs and the selective CDK inhibitor R-roscovitine (ROSC) was determined, with and without the addition of an alkylating agent, prior to the onset of in vivo therapy. The kinetics and rate of spontaneous and drug-induced apoptosis of CLL cells under ex vivo conditions differed significantly between patients, mirroring the variability observed during in vivo treatment. Interestingly, individual patients' leukemic cells were comparably sensitive to the drugs under both conditions. Of the drugs examined, ROSC exerted the highest therapeutic efficacy under ex vivo conditions. Our results indicate that ex vivo testing might be useful for identifying the most potent first-line therapeutic regimen for specific CLL patients and possibly for the design of therapies tailored for individual CLL patients.

Enhancement of carboplatin- and quercetin-induced cell death by roscovitine is Akt dependent and p53 independent in hepatoma cells

PURPOSE

Hepatocellular carcinoma (HCC) is a common malignancy worldwide and has an annual occurrence of one million new cases. Novel therapeutic strategies of increased efficacy in the treatment of HCC-bearing patients would certainly be helpful. Hence, the authors explored the effect of combination treatment of roscovitine with chemotherapeutic drugs or quercetin (Qctn) in hepatoma cells, HepG2 and Hep3B.

METHODS

Cell viability was assessed by MTT assay, cell growth assay, and nuclear morphological changes by DAPI staining. The altered expression of signaling proteins and apoptotic molecules was established by Western blotting.

RESULTS

Roscovitine pretreatment considerably enhanced the drugs and Qctn-induced cell death in HepG2 and Hep3B cells. The exploratory studies revealed that augmented cell killing in HepG2 and Hep3B was mediated via Akt pathway and was independent of p53. pAkt was found to be significantly downregulated in combination treatment of roscovitine with carboplatin or Qctn. Corresponding to reduced expression of pAkt, the downstream molecules Bcl-2 and proactive forms of caspase 9 and caspase 3 were also downregulated indicating apoptosis.

CONCLUSIONS

The present study reports for the first time, in hepatoma cells, the potentiation of carboplatin- and Qctn-induced cell death by the cell cycle inhibitor roscovitine. Roscovitine can thus be considered as a potential therapeutic target in combination with chemotherapeutic drugs or Qctn for treatment of HCC.

Polyamine depletion enhances the roscovitine-induced apoptosis through the activation of mitochondria in HCT116 colon carcinoma cells

Small molecule inhibitors of cyclin-dependent kinases (CDKs) show high therapeutic potential in various cancer types which are characterized by the accumulation of transformed cells due to impaired apoptotic machinery. Roscovitine, a CDK inhibitor showed to be a potent apoptotic inducer in several cancer cells. Polyamines, putrescine, spermidine and spermine, are biogenic amines involved in many cellular processes, including apoptosis. In this study, we explored the potential role of polyamines in roscovitine-induced apoptosis in HCT116 colon cancer cells. Roscovitine induced apoptosis by activating mitochondrial pathway caspases and modulating the expression of Bcl-2 family members. Depletion of polyamines by treatment with difluoromethylornithine (DFMO) increased roscovitine-induced apoptosis. Transient silencing of ornithine decarboxylase, polyamine biosynthesis enzyme and special target of DFMO also increased roscovitine-induced apoptosis in HCT116 cells. Interestingly, additional putrescine treatment was found pro-apoptotic due to the presence of non-functional ornithine decarboxylase (ODC). Finally, roscovitine altered polyamine catabolic pathway and led to decrease in putrescine and spermidine levels. Therefore, the metabolic regulation of polyamines may dictate the power of roscovitine induced apoptotic responses in HCT116 colon cancer cells.

Interference with ER-α enhances the therapeutic efficacy of the selective CDK inhibitor roscovitine towards ER-positive breast cancer cells

In recent years many risk factors for the development of breast cancer that are linked to estrogens have been identified, and roscovitine (ROSC), a selective cyclin-dependent kinase (CDK) inhibitor, has been shown to be an efficient inhibitor of the proliferation of human breast cancer cells. Therefore, we have examined the possibility that interference with estrogen signaling pathways, using tamoxifen (TAM), a selective estrogen receptor modulator (SERM), could modulate the efficacy of treatment with ROSC. In conjunction with TAM, ROSC exhibited enhanced anti-proliferative activity and CDK inhibition, particularly in estrogen-dependent MCF-7 cells. The interaction between both drugs was synergistic. However, in ER-α-negative cells the interaction was antagonistic. Exposure of MCF-7 cells to ROSC abolished the activating phosphorylation of CDK2 and CDK7 at Ser(164/170). This in turn prevented the phosphorylation of the carboxyl-terminal repeat domain of RNA Polymerase II and ER-α at Ser(118), resulting in the down-regulation of the latter. Concomitantly, wt p53 was strongly activated by phosphorylation at Ser(46). Our results demonstrate that ROSC negatively affects the functional status of ER-α, making it potentially useful in the treatment of estrogen-dependent breast cancer cells.

Ibulocydine is a novel prodrug Cdk inhibitor that effectively induces apoptosis in hepatocellular carcinoma cells

Hepatocellular carcinoma (HCC) is frequently associated with abnormalities in cell cycle regulation, leading to increased activity of cyclin-dependent kinases (Cdks) due to the loss, or low expression of, Cdk inhibitors. In this study, we showed that ibulocydine (an isobutyrate prodrug of the specific Cdk inhibitor, BMK-Y101) is a candidate anti-cancer drug for HCC. Ibulocydine has high activity against Cdk7/cyclin H/Mat1 and Cdk9/cyclin T. Ibulocydine inhibited the growth of HCC cells more effectively than other Cdk inhibitors, including olomoucine and roscovitine, whereas ibulocydine as well as the other Cdk inhibitors and BMK-Y101 minimally influenced the growth of normal hepatocyte cells. Ibulocydine induced apoptosis in HCC cells, most likely by inhibiting Cdk7 and Cdk9. In vitro treatment of HCC cells with ibulocydine rapidly blocked phosphorylation of the carboxyl-terminal domain (CTD) of the large subunit of RNA polymerase II, a process mediated by Cdk7/9. Anti-apoptotic gene products such as Mcl-1, survivin, and X-linked IAP (XIAP) are crucial for the survival of many cell types, including HCC. Following the inhibition of RNA polymerase II phosphorylation, ibulocydine caused rapid down-regulation of Mcl-1, survivin, and XIAP, thus inducing apoptosis. Furthermore, ibulocydine effectively induced apoptosis in HCC xenografts with no toxic side effects. These results suggest that ibulocydine is a strong candidate anti-cancer drug for the treatment of HCC.

Aberrantly activated anti-apoptotic signalling mechanisms in chronic lymphocytic leukaemia cells: clues to the identification of novel therapeutic targets

Chronic lymphocytic leukaemia (CLL) is the commonest haematological malignancy in the western world and is incurable by cytotoxic therapy. Considerable research effort has identified the signal transduction pathways in CLL cells that contribute to anti-apoptotic signalling. Some pathways are constitutively activated in CLL cells but upregulated in normal cells only when protein tyrosine kinases (PTKs) are activated by ligands. This review describes which PTKs are aberrantly activated in CLL cells and are potential targets for inhibition. Additional potential targets within pathways downstream of these PTKs include Mek/Erk, mTorc1, protein kinase C, PI-3 kinase/Akt, nuclear factor-κB and cyclin-dependent protein kinase. Numerous studies have identified chemical agents and antibodies that selectively kill CLL cells, irrespective of their genetic resistance to conventional chemotherapeutic agents, and which can overcome cytoprotective microenvironmental signalling. These studies have resulted in identification of novel therapies, some of which are currently undergoing clinical trials. In vitro and animal model studies and clinical trials could determine which inhibitors of which targets are the likely to be most effective and least toxic either singly or in combination.

The cyclin-dependent kinase inhibitor SCH 727965 (dinacliclib) induces the apoptosis of osteosarcoma cells

Although rare, osteosarcoma is an aggressive cancer that often metastasizes to the lungs. Toward the goal of developing new treatment options for osteosarcoma, we show that the cyclin-dependent kinase (CDK) inhibitor SCH 727965 (SCH) induces the apoptosis of several osteosarcoma cell lines including those resistant to doxorubicin and dasatinib. Cell lines prepared in our laboratory from patients who had received adjuvant chemotherapy and explants derived from a human osteosarcoma xenograft in mice were also responsive to SCH. Apoptosis occurred at low nanomolar concentrations of SCH, as did CDK inhibition, and was p53-independent. SCH activated the mitochondrial pathway of apoptosis as evidenced by caspase-9 cleavage and accumulation of cytoplasmic cytochrome c. Amounts of the apoptotic proteins Bax and Bim increased in mitochondria, whereas amounts of the antiapoptotic proteins Mcl-1 and Bcl-x(L) declined. Osteosarcoma cells apoptosed when codepleted of CDK1 and CDK2 but not when depleted of other CDK combinations. We suggest that SCH triggers the apoptosis of osteosarcoma cells by inactivating CDK1 and CDK2 and that SCH may be useful for treatment of drug-resistant osteosarcomas. SCH also induced the apoptosis of other sarcoma types but not of normal quiescent osteoblasts or fibroblasts.

Roscovitine inhibits EBNA1 serine 393 phosphorylation, nuclear localization, transcription, and episome maintenance

Latent Epstein-Barr virus (EBV) infection causes human lymphomas and carcinomas. EBV usually persists as an episome in malignant cells. EBV episome persistence, replication, and gene expression are dependent on EBNA1 binding to multiple cognate sites in oriP. To search for inhibitors of EBNA1- and oriP-dependent episome maintenance or transcription, a library of 40,550 small molecules was screened for compounds that inhibit EBNA1- and oriP-dependent transcription and do not inhibit EBNA1- and oriP-independent transcription. This screening identified roscovitine, a selective inhibitor of cyclin-dependent kinase 1 (CDK1), CDK2, CDK5, and CDK7. Based on motif predictions of EBNA1 serine 393 as a CDK phosphorylation site and (486)RALL(489) and (580)KDLVM(584) as potential cyclin binding domains, we hypothesized that cyclin binding to EBNA1 may enable CDK1, -2, -5, or -7 to phosphorylate serine 393. We found that Escherichia coli-expressed EBNA1 amino acids 387 to 641 were phosphorylated in vitro by CDK1-, -2-, -5-, and -7/cyclin complexes and serine 393 phosphorylation was roscovitine inhibited. Further, S393A mutation abrogated phosphorylation. S393A mutant EBNA1 was deficient in supporting EBNA1- and oriP-dependent transcription and episome persistence, and roscovitine had little further effect on the diminished S393A mutant EBNA1-mediated transcription or episome persistence. Immunoprecipitated FLAG-EBNA1 was phosphorylated in vitro, and roscovitine inhibited this phosphorylation. Moreover, roscovitine decreased nuclear EBNA1 and often increased cytoplasmic EBNA1, whereas S393A mutant EBNA1 was localized equally in the nucleus and cytoplasm and was unaffected by roscovitine treatment. These data indicate that roscovitine effects are serine 393 specific and that serine 393 is important in EBNA1- and oriPCp-dependent transcription and episome persistence.

Homoharringtonine reduced Mcl-1 expression and induced apoptosis in chronic lymphocytic leukemia

Homoharringtonine (HHT) is a plant alkaloid that inhibits the elongation phase of translation that is currently in clinical trials. Because the intrinsically short-lived antiapoptotic protein myeloid cell leukemia-1 (Mcl-1) has been reported to support the survival of chronic lymphocytic leukemia (CLL) cells, we hypothesized that inhibition of protein synthesis by HHT would decrease Mcl-1 expression and induce apoptosis in CLL. In primary CLL cells, HHT induced significant apoptosis independent of the prognostic characteristics of the patients. This was associated with inhibition of translation and decreased Mcl-1 levels in CLL cells. Mcl-1 reduction was evident as early as 2 hours and continued to decrease in the next 6-8 hours, whereas cell death started in 2 hours and continued to increase for 24 hours. Reduction of the Mcl-1 level was due to translation inhibition and proteasome degradation rather than to transcription inhibition or caspase cleavage. HHT and the transcription inhibitor SNS-032 induced synergistic cell killing. Although stromal cells induced Mcl-1 expression and protected CLL cells from the toxicity of fludarabine, this induction was reversed by HHT, which overcame stromal cell-mediated protection. Thus, these results provide a rationale for clinical development of HHT in CLL as single agent or in combinations.

Cell death induction in resting lymphocytes by pan-Cdk inhibitor, but not by Cdk4/6 selective inhibitor

Immunosuppression is one of the common side effects of many anti-tumor agents targeting proliferating cells. We previously reported the development of a new class of pan-cyclin-dependent kinase (Cdk) inhibitor compounds that induce immunosuppression in rodents. Here, we demonstrated that a pan-Cdk inhibitor, Compound 1 very rapidly reduced white blood cells in mice, only 8 h after administration. Compound 1 induced death of peripheral blood cells or purified resting (non-stimulated) lymphocytes ex vivo. Cell death was induced very rapidly, after 4 h of incubation, suggesting that acute immunosuppression observed in rodents might be, at least in part, due to direct cytotoxic effects of Compound 1 on resting lymphocytes. While cell cycle-related Cdks were not activated, the carboxyl terminal domain (CTD) of the largest subunit of RNA polymerase II was phosphorylated, indicating activation of Cdk7 or Cdk9, which phosphorylates this domain, in resting lymphocytes. Indeed, the pan-Cdk inhibitor suppressed CTD phosphorylation in resting cells at the dose required for cell death induction. Inhibition of Cdk7 or Cdk9 by Compound 1 was also confirmed by suppression of nuclear factor-kappa B (NF-κB)-dependent transcription activity in the human cancer cell line U2OS. Interestingly, a Cdk4/6 inhibitor with selectivity against Cdk7 and Cdk9 did not induce cell death in resting lymphocytes. These results suggest that CTD phosphorylation possibly by Cdk7 or Cdk9 might be important for survival of resting lymphocytes and that Cdk inhibitors without inhibitory activity on these kinases might be an attractive agent for cancer chemotherapy.

Phase I and pharmacologic study of SNS-032, a potent and selective Cdk2, 7, and 9 inhibitor, in patients with advanced chronic lymphocytic leukemia and multiple myeloma

PURPOSE

SNS-032 is a highly selective and potent inhibitor of cyclin-dependent kinases (Cdks) 2, 7, and 9, with in vitro growth inhibitory effects and ability to induce apoptosis in malignant B cells. A phase I dose-escalation study of SNS-032 was conducted to evaluate safety, pharmacokinetics, biomarkers of mechanism-based pharmacodynamic (PD) activity, and clinical efficacy.

PATIENTS AND METHODS

Parallel cohorts of previously treated patients with chronic lymphocytic leukemia (CLL) and multiple myeloma (MM) received SNS-032 as a loading dose followed by 6-hour infusion weekly for 3 weeks of each 4-week course.

RESULTS

There were 19 patients with CLL and 18 with MM treated. Tumor lysis syndrome was the dose-limiting toxicity (DLT) for CLL, the maximum-tolerated dose (MTD) was 75 mg/m(2), and the most frequent grade 3 to 4 toxicity was myelosuppression. One patient with CLL had more than 50% reduction in measurable disease without improvement in hematologic parameters. Another patient with low tumor burden had stable disease for four courses. For patients with MM, no DLT was observed and MTD was not identified at up to 75 mg/m(2), owing to early study closure. Two patients with MM had stable disease and one had normalization of spleen size with treatment. Biomarker analyses demonstrated mechanism-based PD activity with inhibition of Cdk7 and Cdk9, decreases in Mcl-1 and XIAP expression level, and associated CLL cell apoptosis.

CONCLUSION

SNS-032 demonstrated mechanism-based target modulation and limited clinical activity in heavily pretreated patients with CLL and MM. Further single-agent, PD-based, dose and schedule modification is warranted to maximize clinical efficacy.

CDK Inhibitors Roscovitine and CR8 Trigger Mcl-1 Down-Regulation and Apoptotic Cell Death in Neuroblastoma Cells

Neuroblastoma (NB), the most frequent extracranial solid tumor of children accounting for nearly 15% of all childhood cancer mortality, displays overexpression of antiapoptotic Bcl-2 and Mcl-1 in aggressive forms of the disease. The clinical phase 2 drug roscovitine (CYC202, seliciclib), a relatively selective inhibitor of cyclin-dependent kinases (CDKs), and CR8, a recently developed and more potent analog, induce concentration-dependent apoptotic cell death of NB cells (average IC(50) values: 24.2 µM and 0.4 µM for roscovitine and CR8, respectively). Both roscovitine and CR8 trigger rapid down-regulation of the short-lived survival factor Mcl-1 in the 9 investigated human NB cell lines. This effect was further analyzed in the human SH-SY5Y NB cell line. Down-regulation of Mcl-1 appears to depend on inhibition of CDKs rather than on interaction of roscovitine and CR8 with their secondary targets. CR8 is an adenosine triphosphate-competitive inhibitor of CDK9, and the structure of a CDK9/cyclin T/CR8 complex is described. Mcl-1 down-regulation occurs both at the mRNA and protein levels. This effect can be accounted for by a reduction in Mcl-1 protein synthesis, under stable Mcl-1 degradation conditions. Mcl-1 down-regulation is accompanied by a transient increase in free Noxa, a proapoptotic factor. Mcl-1 down-regulation occurs independently of the presence or up-regulation of p53 and of the MYCN status. Taken together, these results suggest that the clinical drug roscovitine and its novel analog CR8 induce apoptotic tumor cell death by down-regulating Mcl-1, a key survival factor expressed in all NB cell lines. CDK inhibition may thus constitute a new approach to treat refractory high-risk NB.

AT7519, a cyclin-dependent kinase inhibitor, exerts its effects by transcriptional inhibition in leukemia cell lines and patient samples

AT7519 is a potent inhibitor of several cyclin-dependent kinases and is currently in early phase clinical development. Recently, cyclin-dependent kinases 7, 8, and 9 have been shown to regulate transcription through phosphorylation of RNA polymerase II. B-cell lymphoproliferative disorders, including chronic lymphocytic leukemia, rely on the expression of transcripts with a short half-life, such as Mcl-1, Bcl-2, and XIAP, for survival. Here, we describe the characterization of AT7519 in leukemia cell lines, and compare and contrast the response in cell lines derived from solid tumors. Finally, we use these mechanistic insights to show activity in peripheral blood mononuclear cells isolated from 16 chronic lymphocytic leukemia patients. AT7519 induced apoptosis at concentrations of 100 to 700 nmol/L and was equally effective regardless of Rai stage or known prognostic markers. Short-term treatments (4-6 hours) resulted in inhibition of phosphorylation of the transcriptional marker RNA polymerase II and downregulation of the antiapoptotic protein Mcl-1, with no effect on either XIAP or Bcl-2 levels. The reduction in Mcl-1 protein level was associated with an increase in cleaved poly(ADP-ribose) polymerase. Together the data suggest AT7519 offers a promising treatment for patients with advanced B-cell leukemia. Mol Cancer Ther; 9(4); 920-8. (c)2010 AACR.

Seliciclib in malignancies

Cyclins and cyclin-dependent kinases (CDK) form a key part of the regulatory proteins that govern the cell cycle. Aberrancy in their function can lead to uncontrolled growth and proliferation of the cells which forms the basis of many human diseases, especially cancers. Seliciclib (CYC202, R-roscovitine) is a second-generation CDK inhibitor that competes for ATP binding sites on these kinases, reducing tumor growth and inducing cell death. It is a direct inhibitor of cyclin E/CDK2 and also has inhibitory effects on cyclin H/CDK7 and cyclin T/CDK9. Seliciclib leads to growth arrest and apoptosis of cell lines through activation of the p53 gene, inhibition of RNA processing and blockage of the RNA polymerase II-dependent transcription, and reduction of anti-apoptotic proteins. Seliciclib has good oral bioavailability, although its absorption is slowed by food. It is distributed rapidly to the body tissues and metabolized rapidly to a carboxylated derivative that is excreted by the kidneys. The major adverse effects of seliciclib are electrolyte disturbances (hypokalemia, hyponatremia), gastrointestinal side effects (nausea, emesis, anorexia), fatigue, transient hyperglycemia, elevation of liver enzymes and reversible elevation of serum creatinine. At present, it is in Phase II trials for non-small cell lung cancer and nasopharyngeal carcinoma.

Flavopiridol in chronic lymphocytic leukemia: a concise review

Patients with chronic lymphocytic leukemia (CLL) with high-risk cytogenetic features such as del(17p13) have limited treatment options and decreased overall survival. Dysfunction of p53 leads to resistance to fludarabine-based therapies. Cyclin-dependent kinase inhibitors (CDKi) are a novel class of agents that induce apoptosis in CLL cells independent of p53 mutational status. The synthetic flavone flavopiridol demonstrated promising in vitro activity in CLL. In initial phase I studies using a continuous infusion dosing schedule in a variety of malignancies, no clinical activity was observed. Detailed pharmacokinetic modeling led to the development of a novel dosing schedule designed to achieve target drug concentrations in vivo. In phase I testing, this dosing schedule resulted in acute tumor lysis syndrome (TLS) as the dose-limiting toxicity. With the implementation of a standardized protocol to prevent severe TLS, flavopiridol was administered safely, and responses were observed in heavily pretreated, fludarabine-refractory patients, cytogenetically high-risk patients, and patients with bulky lymphadenopathy. In a pharmacokinetic analysis, flavopiridol area under the plasma concentration-time curve (AUC) correlated with clinical response and cytokine release syndrome. Phase II studies are under way with encouraging preliminary results. Flavopiridol is currently under active investigation in combination with other agents and as a means to eradicate minimal residual disease in patients following cytoreductive chemotherapy. Several other investigational CDKi in preclinical and early clinical development are briefly discussed in this review.

Novel potent pharmacological cyclin-dependent kinase inhibitors

Progression of the cell cycle is controlled by various activating and inhibiting cellular factors. The subtle balance between these counteracting regulators in normal cells ensures proper cell cycle progression and facilitates cellular responses to a variety of stress stimuli. Key activators include cyclin-dependent kinases (CDKs) and, consequently, loss or inactivation of CDK inhibitors contributes to the escape of cancer cells from cell cycle control and hyperactivation of CDKs occurs in various neurodegenerative disorders. However, these adverse effects may be compensated by pharmacological counterparts. Inhibitors of CDKs representing various classes of compounds with diverse CDK inhibitory patterns have been developed, but inhibitors that have high selectivity and offer highly targeted activity against both cell cycle and transcriptional CDKs are of particular interest. This review focuses on pharmacological CDK inhibitors that have entered clinical trials and some compounds that have been evaluated preclinically. Recent discoveries in cell cycle regulation have provided rationales for clinical applications of CDK inhibitors in both monotherapeutic and combined therapeutic regimens.