Flavopiridol can be safely administered using a pharmacologically derived schedule and demonstrates activity in relapsed and refractory non-Hodgkin's lymphoma

A preclinical platform for assessing antitumor effects and systemic toxicities of cancer drug targets

Anticancer drug development campaigns often fail due to an incomplete understanding of the therapeutic index differentiating the efficacy of the agent against the cancer and its on-target toxicities to the host. To address this issue, we established a versatile preclinical platform in which genetically defined cancers are produced using somatic tissue engineering in transgenic mice harboring a doxycycline-inducible short hairpin RNA against the target of interest. In this system, target inhibition is achieved by the addition of doxycycline, enabling simultaneous assessment of efficacy and toxicity in the same animal. As proof of concept, we focused on CDK9—a cancer target whose clinical development has been hampered by compounds with poorly understood target specificity and unacceptable toxicities. We systematically compared phenotypes produced by genetic Cdk9 inhibition to those achieved using a recently developed highly specific small molecule CDK9 inhibitor and found that both perturbations led to robust antitumor responses. Remarkably, nontoxic levels of CDK9 inhibition could achieve significant treatment efficacy, and dose-dependent toxicities produced by prolonged CDK9 suppression were largely reversible upon Cdk9 restoration or drug withdrawal. Overall, these results establish a versatile in vivo target validation platform that can be employed for rapid triaging of therapeutic targets and lend support to efforts aimed at advancing CDK9 inhibitors for cancer therapy.

Flavonoids and Related Members of the Aromatic Polyketide Group in Human Health and Disease: Do They Really Work?

Some aromatic polyketides such as dietary flavonoids have gained reputation as miraculous molecules with preeminent beneficial effects on human health, for example, as antioxidants. However, there is little conclusive evidence that dietary flavonoids provide significant leads for developing more effective drugs, as the majority appears to be of negligible medicinal importance. Some aromatic polyketides of limited distribution have shown more interesting medicinal properties and additional research should be focused on them. Combretastatins, analogues of phenoxodiol, hepatoactive kavalactones, and silymarin are showing a considerable promise in the advanced phases of clinical trials for the treatment of various pathologies. If their limitations such as adverse side effects, poor water solubility, and oral inactivity are successfully eliminated, they might be prime candidates for the development of more effective and in some case safer drugs. This review highlights some of the newer compounds, where they are in the new drug pipeline and how researchers are searching for additional likely candidates.

A novel CDK9 inhibitor increases the efficacy of venetoclax (ABT-199) in multiple models of hematologic malignancies

MCL-1 is one of the most frequently amplified genes in cancer, facilitating tumor initiation and maintenance and enabling resistance to anti-tumorigenic agents including the BCL-2 selective inhibitor venetoclax. The expression of MCL-1 is maintained via P-TEFb-mediated transcription, where the kinase CDK9 is a critical component. Consequently, we developed a series of potent small-molecule inhibitors of CDK9, exemplified by the orally active A-1592668, with CDK selectivity profiles that are distinct from related molecules that have been extensively studied clinically. Short-term treatment with A-1592668 rapidly downregulates RNA pol-II (Ser 2) phosphorylation resulting in the loss of MCL-1 protein and apoptosis in MCL-1-dependent hematologic tumor cell lines. This cell death could be attenuated by either inhibiting caspases or overexpressing BCL-2 protein. Synergistic cell killing was also observed between A-1592668 or the related analog A-1467729, and venetoclax in a number of hematologic cell lines and primary NHL patient samples. Importantly, the CDK9 inhibitor plus venetoclax combination was well tolerated in vivo and demonstrated efficacy superior to either agent alone in mouse models of lymphoma and AML. These data indicate that CDK9 inhibitors could be highly efficacious in tumors that depend on MCL-1 for survival or when used in combination with venetoclax in malignancies dependent on MCL-1 and BCL-2.

Targeting CDK4/6 in mantle cell lymphoma

Targeting the cell cycle represents a rational approach to mantle cell lymphoma (MCL) therapy, as aberrant expression of cyclin D1 and dysregulation of CDK4 underlie cell cycle progression and proliferation of MCL cells. Although cell cycle cancer therapy was historically ineffective due to a lack of selective and effective drugs, this landscape changed with the advent of selective and potent small-molecule oral CDK4/6 inhibitors. Here, we review the anti-tumor activities and clinical data of selective CDK4/6 inhibitors in MCL. We summarize the known mechanism of action of palbociclib, the most specific CDK4/6 inhibitor to date, and the strategy to leverage this specificity to reprogram MCL for a deeper and more durable clinical response to partner drugs. We also discuss integrative longitudinal functional genomics as a strategy to discover tumor-intrinsic genomic biomarkers and tumor-immune interactions that potentially contribute to the clinical response to palbociclib in combination therapy for MCL. Understanding the genomic basis for targeting CDK4/6 and the mechanisms of action and resistance in MCL may advance personalized therapy for MCL and shed light on drug resistance in other cancers.

Differential expression of cyclin-dependent kinases in the adult human retina in relation to CDK inhibitor retinotoxicity

Cyclin-dependent kinases (CDKs) are a family of kinases associated predominantly with cell cycle control, making CDK inhibitors interesting candidates for anti-cancer therapeutics. However, retinal toxicity (loss of photoreceptors) has been associated with CDK inhibitors, including the pan-CDK inhibitor AG-012896. The purpose of this research was to use a novel planar sectioning technique to determine CDK expression profiles in the ex vivo human retina with the aim of identifying isoforms responsible for CDK retinotoxicity. Four CDK isoforms (CDK11, 16, 17 and 18) were selected as a result of IC₅₀ data comparing neurotoxic (AG-012986 and NVP-1) and non-neurotoxic (dinaciclib and NVP-2) CDK inhibitors, with IC₅₀s at CDK11 showing a clear difference between the neurotoxic and non-neurotoxic drugs. CDK11 was maximally expressed in the photoreceptor layer, whereas CDK16, 17 and 18 showed maximal expression in the inner nuclear layer. CDK5 (an isoform associated with retinal homeostasis) was maximally expressed in the retinal ganglion cell layer. Apart from CDK18, each isoform showed expression in the photoreceptor layer. The human Müller cell line MIO-M1 expressed CDK5, 11, 16 and 17 and AG-01298 (0.02-60 µM) caused a dose-dependent increase in MIO-M1 cell death. In conclusion, CDK11 appears the most likely candidate for mediation of photoreceptor toxicity. RNA profiling can be used to determine the distribution of genes of interest in relation to retinal toxicity in the human retina.

Gene mutations and actionable genetic lesions in mantle cell lymphoma

Mutations and epigenetic alterations are key events in transforming normal cells to cancer cells. Mantle cell lymphoma (MCL), a non-Hodgkin's lymphoma of the B-cell, is an aggressive malignancy with poor prognosis especially for those patients who are resistant to the frontline drugs. There is a great need to describe the molecular basis and mechanism of drug resistance in MCL to develop new strategies for treatment. We reviewed frequent somatic mutations and mutations involving the B-cell pathways in MCL and discussed clinical trials that attempted to disrupt these gene pathways and/or epigenetic events. Recurrent gene mutations were discussed in the light of prognostic and therapeutic opportunity and also the challenges of targeting these lesions. Mutations in the ATM, CCND1, TP53, MLL2, TRAF2 and NOTCH1 were most frequently encountered in mantle cell lymphoma. Translational models should be built that would assess mutations longitudinally to identify important compensatory, pro-survival and anti-apoptic pathways and actionable genetic targets.

G2/M inhibitors as pharmacotherapeutic opportunities for glioblastoma: the old, the new, and the future

Glioblastoma (GBM) is one of the deadliest tumors and has a median survival of 3 months if left untreated. Despite advances in rationally targeted pharmacological approaches, the clinical care of GBM remains palliative in intent. Since the majority of altered signaling cascades involved in cancer establishment and progression eventually affect cell cycle progression, an alternative approach for cancer therapy is to develop innovative compounds that block the activity of crucial molecules needed by tumor cells to complete cell division. In this context, we review promising ongoing and future strategies for GBM therapeutics aimed towards G2/M inhibition such as anti-microtubule agents and targeted therapy against G2/M regulators like cyclin-dependent kinases, Aurora inhibitors, PLK1, BUB, 1, and BUBR1, and survivin. Moreover, we also include investigational agents in the preclinical and early clinical settings. Although several drugs were shown to be gliotoxic, most of them have not yet entered therapeutic trials. The use of either single exposure or a combination with novel compounds may lead to treatment alternatives for GBM patients in the near future.

Targeting of CDK9 with indirubin 3'-monoxime safely and durably reduces HIV viremia in chronically infected humanized mice

Successful propagation of HIV in the human host requires entry into a permissive cell, reverse transcription of viral RNA, integration into the human genome, transcription of the integrated provirus, and assembly/release of new virus particles. Currently, there are antiretrovirals against each of these viral steps, except for provirus transcription. An inhibitor of HIV transcription could both increase potency of treatment and suppress drug-resistant strains. Cellular cyclin-dependent kinase 9 (CDK9) serves as a cofactor for the HIV Tat protein and is required for effective transcription of the provirus. Previous studies have shown that the CDK9 inhibitor Indirubin 3’-monoxime (IM) inhibits HIV transcription in vitro and in short-term in vivo studies of HIV acute infection in humanized mice (PBMC-NSG model), suggesting a therapeutic potential. The objective of this study is to evaluate the toxicity, pharmacokinetics and long-term antiviral activity of IM during chronic HIV infection in humanized mice (HSC-NSG model). We show that IM concentrations above EC₅₀ values are rapidly achieved and sustained for > 3 h in plasma, and that non-toxic concentrations durably reduce HIV RNA levels. In addition, IM enhanced the antiviral activity of antiretrovirals from the reverse transcriptase, protease and integrase inhibitor classes in in vitro infectivity assays. In summary, IM may enhance current antiretroviral treatments and could help achieve a “functional cure” in HIV patients by preventing expression of proviruses.

Overview of CDK9 as a target in cancer research

CDK9 is a protein in constant development in cancer therapy. Herein we present an overview of the enzyme as a target for cancer therapy. We provide data on its characteristics and mechanism of action. In recent years, CDK9 inhibitors that have been designed with molecular modeling have demonstrated good antitumoral activity in vitro. Clinical studies of the drugs flavopiridol, dinaciclib, seliciclib, SNS-032 and RGB-286638 used as CDK9 inhibitors are also reviewed, with their additional targets and their relative IC50 values. Unfortunately, treatment with these drugs remains unsuccessful and involves many adverse effects. We could conclude that there are many small molecules that bind to CDK9, but their lack of selectivity against other CDKs do not allow them to get to the clinical use. However, drug designers currently have the tools needed to improve the selectivity of CDK9 inhibitors and to make successful treatment available to patients.

A new molecular paradigm in mycosis fungoides and Sézary syndrome

Mycosis Fungoides (MF) and Sézary Syndrome (SS) are clonal proliferations of mature T-cells manifesting as lymphoproliferative disorders in which the neoplastic cells show a strong propensity for skin-homing. While the predominant site of presentation in MF is the skin, the peripheral blood carries a significant tumor burden in Sézary Syndrome such that it resembles a "leukemic" disease. While the genetic basis of these diseases has been studied using different approaches in the previous years, recent genome-wide studies employing massively parallel sequencing techniques now offer new insights into the molecular pathogenesis of these diseases. In this chapter, we discuss the recent findings elucidating the genomic landscape of MF and SS. The pathways targeted by mutational alterations are discussed and a model for understanding the pathogenesis of these diseases is proposed. It is anticipated that prognostic stratification and therapeutic targeting based on mutational signatures will be achieved in the near future based on the improved understanding of the molecular pathogenesis of these diseases.

IPP51, a chalcone acting as a microtubule inhibitor with in vivo antitumor activity against bladder carcinoma

We previously identified 1-(2,4-dimethoxyphenyl)-3-(1-methylindolyl) propenone (IPP51), a new chalcone derivative that is capable of inducing prometaphase arrest and subsequent apoptosis of bladder cancer cells. Here, we demonstrate that IPP51 selectively inhibits proliferation of tumor-derived cells versus normal non-tumor cells. IPP51 interfered with spindle formation and mitotic chromosome alignment. Accumulation of cyclin B1 and mitotic checkpoint proteins Bub1 and BubR1 on chromosomes in IPP51 treated cells indicated the activation of spindle-assembly checkpoint, which is consistent with the mitotic arrest. The antimitotic actions of other chalcones are often associated with microtubule disruption. Indeed, IPP51 inhibited tubulin polymerization in an in vitro assay with purified tubulin. In cells, IPP51 induced an increase in soluble tubulin. Furthermore, IPP51 inhibited in vitro capillary-like tube formation by endothelial cells, indicating that it has anti-angiogenic activity. Molecular docking showed that the indol group of IPP51 can be accommodated in the colchicine binding site of tubulin. This characteristic was confirmed by an in vitro competition assay demonstrating that IPP51 can compete for colchicine binding to soluble tubulin. Finally, in a human bladder xenograft mouse model, IPP51 inhibited tumor growth without signs of toxicity. Altogether, these findings suggest that IPP51 is an attractive new microtubule-targeting agent with potential chemotherapeutic value.

A phase 1 clinical trial of flavopiridol consolidation in chronic lymphocytic leukemia patients following chemoimmunotherapy

Patients with chronic lymphocytic leukemia (CLL) who receive chemoimmunotherapy and do not achieve complete remission experience significantly shortened progression-free interval (PFS). Additionally, the majority of patients treated for relapsed disease demonstrate evidence of measurable disease. Eradication of minimal residual disease (MRD) results in improved PFS and overall survival. Maintenance therapy might result in eradication of MRD and improve response duration but might be associated with an increase in incidence of infectious complications. Flavopiridol is a broad cyclin-dependent kinase (CDK) inhibitor with established safety and efficacy in patients with relapsed CLL, particularly patients with high-risk cytogenetic features. A pharmacologically derived schedule was utilized as consolidation therapy in this phase I study to assess the safety and feasibility of outpatient therapy with flavopiridol in patients with low tumor burden. Flavopiridol was administered as a 30-min loading dose of 30 mg/m(2) followed by a 4-h infusion of 30 mg/m(2) once weekly for 3 weeks every 5 weeks (1 cycle) for planned 2 cycles in ten patients. Therapy was extremely well tolerated and no patient developed acute tumor lysis syndrome. The most common toxicities were gastrointestinal. Of the patients, 22 % improved their response from a PR to CR. Eighty-eight percent experienced a reduction in tumor burden as measured by extent of bone marrow involvement including patients with del17p and complex karyotype. The study establishes the safety and efficacy of flavopiridol as consolidation therapy after chemoimmunotherapy for patients with CLL. Further evaluation is required in larger trials for the utility of CDK inhibitors as consolidation or maintenance strategies.Registration number at ClinicalTrials.gov: NCT00377104.

Novel synthesised flavone derivatives provide significant insight into the structural features required for enhanced anti-proliferative activity

Synthesis and antiproliferative evaluation of a library of 76 methoxy and hydroxy flavones, and their 4-thio analogues showed that the novel thioflavones 15f and 16f exhibit 7–46 fold greater anti-proliferative potency than the natural flavone chrysin (2d).

Up-regulation of CDK9 kinase activity and Mcl-1 stability contributes to the acquired resistance to cyclin-dependent kinase inhibitors in leukemia

Flavopiridol is a small molecule inhibitor of cyclin-dependent kinases (CDK) known to impair global transcription via inactivation of positive transcription elongation factor b. It has been demonstrated to have significant activity predominantly in chronic lymphocytic leukemia and acute myeloid leukemia in phase I/II clinical trials while other similar CDK inhibitors are vigorously being pursued in pre-clinical and clinical studies. Although flavopiridol is a potent therapeutic agent against blood diseases, some patients still have primary or acquired resistance throughout their clinical course. Considering the limited knowledge of resistance mechanisms of flavopiridol, we investigated the potential mechanisms of resistance to flavopiridol in a cell line system, which gradually acquired resistance to flavopiridol in vitro, and then confirmed the mechanism in patient samples. Herein, we present that this resistant cell line developed resistance through up-regulation of phosphorylation of RNA polymerase II C-terminal domain, activation of CDK9 kinase activity, and prolonged Mcl-1 stability to counter flavopiridol's drug actions. Further analyses suggest MAPK/ERK activation-mediated Mcl-1 stabilization contributes to the resistance and knockdown of Mcl-1 in part restores sensitivity to flavopiridol-induced cytotoxicity. Altogether, these findings demonstrate that CDK9 is the most relevant target of flavopiridol and provide avenues to improve the therapeutic strategies in blood malignancies.

Loss in MCL-1 function sensitizes non-Hodgkin's lymphoma cell lines to the BCL-2-selective inhibitor venetoclax (ABT-199)

As a population, non-Hodgkin's lymphoma (NHL) cell lines positive for the t(14;18) translocation and/or possessing elevated BCL2 copy number (CN; BCL2(High)) are exquisitely sensitive to navitoclax or the B-cell lymphoma protein-2 (BCL-2)-selective inhibitor venetoclax. Despite this, some BCL2(High) cell lines remain resistant to either agent. Here we show that the MCL-1-specific inhibitor A-1210477 sensitizes these cell lines to navitoclax. Chemical segregation of this synergy with the BCL-2-selective inhibitor venetoclax or BCL-XL-selective inhibitor A-1155463 indicated that MCL-1 and BCL-2 are the two key anti-apoptotic targets for sensitization. Similarly, the CDK inhibitor flavopiridol downregulated MCL-1 expression and synergized with venetoclax in BCL2(High) NHL cell lines to a similar extent as A-1210477. A-1210477 also synergized with navitoclax in the majority of BCL2(Low) NHL cell lines. However, chemical segregation with venetoclax or A-1155463 revealed that synergy was driven by BCL-XL inhibition in this population. Collectively these data emphasize that BCL2 status is predictive of venetoclax potency in NHL not only as a single agent, but also in the adjuvant setting with anti-tumorigenic agents that inhibit MCL-1 function. These studies also potentially identify a patient population (BCL2(Low)) that could benefit from BCL-XL (navitoclax)-driven combination therapy.

Do mantle cell lymphomas have an 'Achilles heel'?

PURPOSE OF REVIEW

Mantle cell lymphoma (MCL) is a mature B-cell malignancy that continues to have a high mortality rate. In this article, we discuss key pathogenic pathways in MCL biology and their possible therapeutic targeting.

RECENT FINDINGS

In addition to cyclin-D1, the transcription factor SOX-11 emerged as a common characteristic of MCL. Genomic studies have identified a number of recurrently mutated genes; in order of descending frequency these include ATM, CCND1, UBR5, TP53, BIRC3, NOTCH1/2 and TRAF2. However, no clear oncogenic driver has been identified. In contrast, several observations indicate that MCL cells are antigen-experienced cells and that the tumor microenvironment and B-cell receptor engagement are important. This is underscored by the impressive clinical responses achieved with the Bruton's tyrosine kinase inhibitor ibrutinib. Recently identified activating mutations in the noncanonical nuclear factor-kappa B pathway could give rise to ibrutinib resistance. Poly-ADP ribose polymerase and aurora kinase inhibitors may be synthetic lethal with the common aberrations in DNA damage pathways found in MCL. Also, ABT-199, a potent and selective inhibitor of B-cell lymphoma 2, has promising activity in early studies.

SUMMARY

MCL is a heterogeneous disease, and no single Achilles heel has been identified. Nevertheless, genomic, molecular and clinical studies have revealed vulnerabilities that can be exploited for effective therapy.

Exploring quercetin and luteolin derivatives as antiangiogenic agents

The formation of new blood vessels from the pre-existing vasculature (angiogenesis) is a crucial stage in cancer progression and, indeed, angiogenesis inhibitors are now used as anticancer agents, clinically. Here we have explored the potential of flavonoid derivatives as antiangiogenic agents. Specifically, we have synthesised methoxy and 4-thio derivatives of the natural flavones quercetin and luteolin, two of which (4-thio quercetin and 4-thio luteolin) had never been previously reported. Seven of these compounds showed significant (p < 0.05) antiangiogenic activity in an in vitro scratch assay. Their activity ranged from an 86% inhibition of the vascular endothelium growth factor (VEGF)-stimulated migration (observed for methoxyquercetin at 10 μM and for luteolin at 1 μM) to a 36% inhibition (for thiomethoxy quercetin at 10 μM). Western blotting studies showed that most (4 out of 7) compounds inhibited phosphorylation of the VEGF receptor-2 (VEGFR2), suggesting that the antiangiogenic activity was due to an interference with the VEGF/VEGFR2 pathway. Molecular modelling studies looking at the affinity of our compounds towards VEGFR and/or VEGF confirmed this hypothesis, and indeed the compound with the highest antiangiogenic activity (methoxyquercetin) showed the highest affinity towards VEGFR and VEGF. As reports from others have suggested that structurally similar compounds can elicit biological responses via a non-specific, promiscuous membrane perturbation, potential interactions of the active compounds with a model lipid bilayer were assessed via DSC. Luteolin and its derivatives did not perturb the model membrane even at concentrations 10 times higher than the biologically active concentration and only subtle interactions were observed for quercetin and its derivatives. Finally, cytotoxicity assessment of these flavonoid derivatives against MCF-7 breast cancer cells demonstrated also a direct anticancer activity albeit at generally higher concentrations than those required for an antiangiogenic effect (10 fold higher for the methoxy analogues). Taken together these results show promise for flavonoid derivatives as antiangiogenic agents.

Novel Pharmacotherapies for B-Cell Lymphomas and Leukemias

Novel pharmacotherapeutic agents were recently approved for treatment of low-grade B-cell neoplasms, and many other agents are under investigation. Several agents have demonstrated impressive activity in targeting malignant B-cell processes and specific pathways, all with the potential to expand our ability to effectively treat B-cell malignancies. The inhibitors of several cell regulatory proteins, including Bruton tyrosine kinase (Btk), phosphoinositide 3-kinase (PI3-K), B-cell lymphoma/leukemia-2 (Bcl-2), and histone deacetylases, as well as immunomodulatory agents are a few of the many pharmacotherapeutic agents under study. A comprehensive review and assessment is presented of the current pharmacotherapies under investigation targeting B-cell lymphomas and leukemias.

Synthesis, structure-activity relationship studies, and antibacterial evaluation of 4-chromanones and chalcones, as well as olympicin A and derivatives

On the basis of recently reported abyssinone II and olympicin A, a series of chemically modified flavonoid phytochemicals were synthesized and evaluated against Mycobacterium tuberculosis and a panel of Gram-positive and -negative bacterial pathogens. Some of the synthesized compounds exhibited good antibacterial activities against Gram-positive pathogens including methicillin resistant Staphylococcus aureus with minimum inhibitory concentration as low as 0.39 μg/mL. SAR analysis revealed that the 2-hydrophobic substituent and the 4-hydrogen bond donor/acceptor of the 4-chromanone scaffold together with the hydroxy groups at 5- and 7-positions enhanced antibacterial activities; the 2′,4′-dihydroxylated A ring and the lipophilic substituted B ring of chalcone derivatives were pharmacophoric elements for antibacterial activities. Mode of action studies performed on selected compounds revealed that they dissipated the bacterial membrane potential, resulting in the inhibition of macromolecular biosynthesis; further studies showed that selected compounds inhibited DNA topoisomerase IV, suggesting complex mechanisms of actions for compounds in this series.

Are we a step forward with targeted agents in resolving the enigma of mantle cell lymphoma?

Mantle cell lymphoma has been recognized as a distinct entity from the other non-Hodgkin lymphomas in middle 1990's. It carries a worst prognosis among all mature B-cell malignancies. Cyclin D1 and recently SOX11 are the hallmarks for this disease. Even if it is highly responsive to induction treatment, it remains incurable, since it inevitably relapses. Highly aggressive approaches with stem cell transplantation can shift the survival curve for a bit, but even so the overall survival is not significantly improved in most of the cases. Small portion of patients with this heterogeneous disease have an indolent course with long-term survival. Conventional immunochemotherapy has reached its maximal possibilities, so novel target agents are absolutely warranted. The large number of ongoing early phase trials demonstrated promising results, especially emphasizing agents that target B-cell receptor. They are mostly investigated in relapsed/refractory disease, while front-line approaches with those agents need to be explored in future times.