Efficacy and Safety of Abemaciclib, an Inhibitor of CDK4 and CDK6, for Patients with Breast Cancer, Non-Small Cell Lung Cancer, and Other Solid Tumors

Predictive biomarkers for molecularly targeted therapies and immunotherapies in breast cancer

Globally, breast cancer is the most common malignancy in women. Substantial efforts have been made to develop novel therapies, including targeted therapies and immunotherapies, for patients with breast cancer who do not respond to standard therapies. Consequently, new targeted therapies, such as cyclin-dependent kinase 4 and 6 inhibitors, poly (ADP-ribose) polymerase inhibitors, phosphoinositide 3-kinase inhibitor, and antibody-drug conjugates targeting human epidermal growth factor receptor 2 or trophoblast cell surface antigen-2, and immune checkpoint inhibitor targeting programmed cell death-1, have been developed and are now in clinical use. However, only some patients have benefited from these novel therapies; therefore, the identification and validation of reliable or more accurate biomarkers for predicting responses to these agents remain a major challenge. This review summarizes the currently available predictive biomarkers for breast cancer and describes recent efforts undertaken to identify potential predictive markers for molecularly targeted therapies and immune checkpoint inhibitors.

Precision neuro-oncology: a pilot analysis of personalized treatment in recurrent glioma

PURPOSE

When brain cancer relapses, treatment options are scarce. The use of molecularly matched targeted therapies may provide a feasible and efficacious way to treat individual patients based on the molecular tumor profile. Since little information is available on this strategy in neuro-oncology, we retrospectively analyzed the clinical course of 41 patients who underwent advanced molecular testing at disease relapse.

METHODS

We performed Sanger sequencing, targeted next generation sequencing, and immunohistochemistry for analysis of potential targets, including programmed death ligand 1, cyclin D1, phosphorylated mechanistic target of rapamycin, telomerase reverse transcriptase promoter mutation, cyclin-dependent kinase inhibitor 2A/B deletion, or BRAF-V600E mutation. In selected patients, whole exome sequencing was conducted.

RESULTS

The investigation included 41 patients, of whom 32 had isocitrate dehydrogenase (IDH) wildtype glioblastoma. Molecular analysis revealed actionable targets in 31 of 41 tested patients and 18 patients were treated accordingly (matched therapy group). Twenty-three patients received molecularly unmatched empiric treatment (unmatched therapy group). In both groups, 16 patients were diagnosed with recurrent IDH wildtype glioblastoma. The number of severe adverse events was comparable between the therapy groups. Regarding the IDH wildtype glioblastoma patients, median progression-free survival (mPFS) and median overall survival (mOS) were longer in the matched therapy group (mPFS: 3.8 versus 2.0 months, p = 0.0057; mOS: 13.0 versus 4.3 months, p = 0.0357).

CONCLUSION

These encouraging data provide a rationale for molecularly matched targeted therapy in glioma patients. For further validation, future study designs need to additionally consider the prevalence and persistence of actionable molecular alterations in patient tissue.

Investigational cyclin-dependent kinase 4/6 inhibitor GLR2007 demonstrates activity against isocitrate dehydrogenase wild-type glioblastoma and other solid tumors in mice xenograft models

Cyclin-dependent kinases, CDK4 and CDK6, are essential in regulating the cell cycle, which is disrupted in cancers like isocitrate dehydrogenase wild-type glioblastoma (GBM). Currently marketed CDK4/6 inhibitors, including abemaciclib, have shown preclinical efficacy in solid tumors, but factors such as poor blood–brain barrier (BBB) penetration limit their efficacy in GBM. GLR2007 is an investigational CDK4/6 inhibitor with the potential for improved BBB penetration. In vitro assays were used to assess the potency and inhibition of CDK4/6 enzymatic activity of GLR2007. Using in vivo assays, the distribution of radiolabeled GLR2007 in rats was determined through quantitative whole-body autoradiography. The antitumor efficacy of GLR2007 was evaluated in human GBM and breast cancer orthotopic mice xenograft models, and human lung, colorectal, and liver cancer in a subcutaneous xenograft model. In tumor cell line proliferation assays, GLR2007 inhibited proliferation at lower concentration values than abemaciclib in 19 of 20 GBM, five of seven breast, 20 of 21 lung, and 24 of 24 liver cancer cell lines. Total levels of radiolabeled GLR2007 in the brains of rats exceeded those in plasma by 2.3–4.5-fold from 2–6 hours after dosing. A xenograft model showed that, compared with vehicle control, 50 mg/kg GLR2007 induced 95.9% tumor growth inhibition (TGI) (P<0.001) in GBM orthotopic xenografts, 81.4% TGI (P=0.037) in breast cancer orthotopic xenografts, and 91.5% TGI (P<0.001) in colorectal cancer subcutaneous xenografts. These studies show possible BBB penetration of GLR2007 and demonstrate its potential as a CDK4/6 inhibitor for the treatment of solid tumors, including GBM.

Discovery of 2H-benzo[b][1,4]oxazin-3(4H)-one derivatives as potent and selective CDK9 inhibitors that enable transient target engagement for the treatment of hematologic malignancies

Cyclin-dependent kinase 9 (CDK9) is a transcriptional regulator and a potential therapeutic target in hematologic malignancies. Selective and transient CDK9 inhibition reduces Mcl-1 expression and induces apoptosis in Mcl-1-dependent tumor cells for survival. Here, we describe our efforts to discover a novel series of 2H-benzo[b][1,4]oxazin-3(4H)-one as CDK9 inhibitors. Compound 32k was identified as a selective CDK9 inhibitor with short pharmacokinetic and physicochemical properties suitable for intravenous administration. Short-term treatment with 32k resulted in a rapid dose-dependent decrease in cellular p-Ser2-RNAPII, Mcl-1 and c-Myc, leading to apoptosis in the MV4-11 cell line. Correspondingly, significant in vivo antitumor efficacy was observed in xenograft models derived from multiple hematological tumors with intermittent 32k dosing. These results provide evidence that selective transient CDK9 inhibitors could be used for the treatment of hematologic malignancies.

Targeting CDK4 and 6 in Cancer Therapy: Emerging Preclinical Insights Related to Abemaciclib

Pharmacologic inhibitors of cyclin-dependent kinases 4 and 6 (CDK4 and 6) are approved for the treatment of subsets of patients with hormone receptor positive (HR+) breast cancer (BC). In metastatic disease, strategies involving endocrine therapy combined with CDK4 and 6 inhibitors (CDK4 and 6i) improve clinical outcomes in HR+ BCs. CDK4 and 6i prevent retinoblastoma tumor suppressor protein phosphorylation, thereby blocking the transcription of E2F target genes, which in turn inhibits both mitogen and estrogen-mediated cell proliferation. In this review, we summarize preclinical data pertaining to the use of CDK4 and 6i in BC, with a particular focus on several of the unique chemical, pharmacologic, and mechanistic properties of abemaciclib. As research efforts elucidate the novel mechanisms underlying abemaciclib activity, potential new applications are being identified. For example, preclinical studies have demonstrated abemaciclib can exert antitumor activity against multiple tumor types and can cross the blood-brain barrier. Abemaciclib has also demonstrated distinct activity as a monotherapeutic in the treatment of BC. Accordingly, we also discuss how a greater understanding of mechanisms related to CDK4 and 6 blockade highlight abemaciclib's unique in-class properties, and could pave new avenues for enhancing its therapeutic efficacy.

Drug Repurposing, a Fast-Track Approach to Develop Effective Treatments for Glioblastoma

Glioblastoma (GBM) remains one of the most difficult tumors to treat. The mean overall survival rate of 15 months and the 5-year survival rate of 5% have not significantly changed for almost 2 decades. Despite progress in understanding the pathophysiology of the disease, no new effective treatments to combine with radiation therapy after surgical tumor debulking have become available since the introduction of temozolomide in 1999. One of the main reasons for this is the scarcity of compounds that cross the blood-brain barrier (BBB) and reach the brain tumor tissue in therapeutically effective concentrations. In this review, we focus on the role of the BBB and its importance in developing brain tumor treatments. Moreover, we discuss drug repurposing, a drug discovery approach to identify potential effective candidates with optimal pharmacokinetic profiles for central nervous system (CNS) penetration and that allows rapid implementation in clinical trials. Additionally, we provide an overview of repurposed candidate drug currently being investigated in GBM at the preclinical and clinical levels. Finally, we highlight the importance of phase 0 trials to confirm tumor drug exposure and we discuss emerging drug delivery technologies as an alternative route to maximize therapeutic efficacy of repurposed candidate drug.

CDK 4/6 inhibitors for the treatment of meningioma

Meningiomas are the most common non-metastatic brain tumors, and although the majority are relatively slow-growing and histologically benign, a subset of meningiomas are aggressive and remain challenging to treat. Despite a standard of care that includes surgical resection and radiotherapy, and recent advances in meningioma molecular grouping, there are no systemic medical options for patients with meningiomas that are resistant to standard interventions. Misactivation of the cell cycle at the level of CDK4/6 is common in high-grade or molecularly aggressive meningiomas, and CDK4/6 has emerged as a potential target for systemic meningioma treatments. In this review, we describe the preclinical evidence for CDK4/6 inhibitors as a treatment for high-grade meningiomas and summarize evolving clinical experience with these agents. Further, we highlight upcoming clinical trials for patients meningiomas, and discuss future directions aimed at optimizing the efficacy of these therapies and selecting patients most likely to benefit from their use.

Comprehensive Genomic Profiling Reveals Clinical Associations in Response to Immune Therapy in Head and Neck Cancer

Comprehensive genomic profiling (CGP) provides information regarding cancer-related genetic aberrations. However, its clinical utility in recurrent/metastatic head and neck cancer (R/M HNC) remains unknown. Additionally, predictive biomarkers for immune checkpoint inhibitors (ICIs) should be fully elucidated because of their low response rate. Here, we analyzed the clinical utility of CGP and identified predictive biomarkers that respond to ICIs in R/M HNC. We evaluated over 1100 cases of HNC using the nationwide genetic clinical database established by the Center for Cancer Genomics and Advanced Therapeutics (C-CAT) and 54 cases in an institution-based study. The C-CAT database revealed that 23% of the cases were candidates for clinical trials, and 5% received biomarker-matched therapy, including NTRK fusion. Our institution-based study showed that 9% of SCC cases and 25% of salivary gland cancer cases received targeted agents. In SCC cases, the tumor mutational burden (TMB) high (≥10 Mut/Mb) group showed long-term survival (>2 years) in response to ICI therapy, whereas the PD-L1 combined positive score showed no significant difference in progression-free survival. In multivariate analysis, CCND1 amplification was associated with a lower response to ICIs. Our results indicate that CGP may be useful in identifying prognostic biomarkers for immunotherapy in patients with HNC.

Genomic Features of Organ-Specific Metastases in Lung Adenocarcinoma

Background

The genomic features of cancer cells may confer the metastatic ability of lung adenocarcinoma (LUAD) to metastasize to specific organs. We aimed to identify the differences in genomic alterations between patients with primary LUAD with and without metastases and to elucidate the metastatic biology that may help developing biomarker-directed therapies for advanced or metastatic disease.

Methods

A retrospective cohort of 497 patients with LUAD including 388 primary tumors (PR), 53 bone metastases (MT-bone), 30 liver metastases (MT-liver), and 26 brain metastases (MT-brain) was tested for genomic alterations by a next-generation sequencing assay.

Results

The EGFR, TP53, TERT, LRP1B, CDKN2A, ERBB2, ALK, and KMT2C genes had a high frequency of mutations, and the mutations were shared by PR and metastases groups. TP53 and EGFR were the most common mutated genes. In comparison with PR, KRAS, STK11, ATM, NPM1, and ROS1 were significantly mutated in MT-brain, and TP53, MYC, RSPO2, CDKN2a, and CDKN2B were significantly mutated in MT-liver. The frequencies of TP53, CDKN2A, MTAP, PRKCI, and APC mutations were higher in MT-bone than that in PR. The ERBB, phosphoinositide-3-kinase/protein kinase B (PI3K-AKT), cell cycle, Fibroblast growth factor (FGF), and homologous recombination deficiency signaling pathways were affected in both PR and metastases, and there is higher frequency of mutations in metastases. Moreover, the co-mutations in patients with PR and metastasis were respectively analyzed. In addition, the programmed death ligand 1 (PD-L1) level was obviously related to tumor stage and tumor metastases, and the tumor mutational burden was correlated to clinicopathological features including age, gender, pathological stages, and tumor metastases. FGFR1, KAT6A, MYC, RAD21, TP53, and DAXX were also dramatically correlated to the tumor mutational burden.

Conclusion

Metastases are the most devastating stage of tumors and the main cause of cancer-related deaths. Our results provided a clinically relevant view of the tumor-intrinsic mutational landscape of patients with metastatic LUAD.

CDK4/6 inhibition enhances oncolytic virus efficacy by potentiating tumor-selective cell killing and T cell activation in refractory glioblastoma

Glioblastoma multiforme (GBM) is among the most aggressive human cancers. Although oncolytic virus (OV) therapy has been proposed as a potential approach to treat GBM, it frequently fails because GBM cells are usually nonpermissive to OV. Here, we describe a dual-step drug screen for identifying chemical enhancers of oncolytic virus in GBM. From a high-throughput screen of 1416 FDA-approved drugs, an inhibitor of CDK4/6 was identified as the top enhancer, selectively increasing potency of two OV strains, VSVΔ51 and Zika virus. Mechanistically, CDK4/6 inhibition promoted autophagic degradation of MAVS, resulting in impaired antiviral responses and enhanced tumor-selective replication of VSVΔ51 in vitro and in vivo. CDK4/6 inhibition cooperated with VSVΔ51 to induce severe DNA damage stress and amplify oncolysis. In GBM xenograft models, combined treatment with CDK4/6 inhibitor and VSVΔ51 significantly inhibited tumor growth and prolonged the survival of tumor-bearing mice. Further investigation revealed that CDK4/6 inhibitor and VSVΔ51 synergistically induced immunogenic cell death and boosted anti-tumor immunity. Together, this study features a promising approach of treating aggressive GBM through the combination of CDK4/6 inhibitor with OV.

Continuous treatment with abemaciclib leads to sustained and efficient inhibition of breast cancer cell proliferation

Abemaciclib is an oral, selective cyclin-dependent kinase 4 & 6 inhibitor (CDK4 & 6i), approved for hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2–) advanced breast cancer (ABC) as monotherapy for endocrine refractory disease, and with endocrine therapy (ET) for initial treatment and after progression on ET. Abemaciclib has also shown clinical activity in combination with ET in patients with high risk early BC (EBC). Here, we examined the preclinical attributes of abemaciclib and other CDK4 & 6i using biochemical and cell-based assays. In vitro, abemaciclib preferentially inhibited CDK4 kinase activity versus CDK6, resulting in inhibition of cell proliferation in a panel of BC cell lines with higher average potency than palbociclib or ribociclib. Abemaciclib showed activity regardless of HER2 amplification and phosphatidylinositol 3-kinase (PI3KCA) gene mutation status. In human bone marrow progenitor cells, abemaciclib showed lower impact on myeloid maturation than other CDK4 & 6i when tested at unbound concentrations similar to those observed in clinical trials. Continuous abemaciclib treatment provided profound inhibition of cell proliferation, and triggered senescence and apoptosis. These preclinical results support the unique efficacy and safety profile of abemaciclib observed in clinical trials.

Cyclin-dependent kinases as potential targets for colorectal cancer: past, present and future

Colorectal cancer (CRC) is a common cancer in the world and its prevalence is increasing in developing countries. Deregulated cell cycle traverse is a hallmark of malignant transformation and is often observed in CRC as a result of imprecise activity of cell cycle regulatory components, viz. cyclins and cyclin-dependent kinases (CDKs). Apart from cell cycle regulation, some CDKs also regulate processes such as transcription and have also been shown to be involved in colorectal carcinogenesis. This article aims to review cyclin-dependent kinases as potential targets for CRC. Furthermore, therapeutic candidates to target CDKs are also discussed.

LY2874455 and Abemaciclib Reverse FGF3/4/19/CCND1 Amplification Mediated Gefitinib Resistance in NSCLC

Non-small cell lung carcinoma (NSCLC) patients who initially received tyrosine kinase inhibitor (TKI) therapy often acquired resistance via multiple complex mechanisms. The amplification of FGF3/4/19/CCND1 on chromosome 11q13 was found in many cancers with TKI resistance. However, the role of these amplifications in TKI-resistant NSCLC remains uncovered. Here, we generated the FGF3/4/19/CCND1 amplification model in the NSCLC cell lines PC-9 and HCC827. Upregulation of FGF3/4/19/CCND1 strongly promoted cell proliferation and gefitinib resistance in NSCLC cells. To find out the potential therapeutic strategies, we screened the combination of inhibitors against the FGF/FGFR signaling pathway and the CCND1/CDK4 complex and revealed that gefitinib combined with LY2874455 and abemaciclib exhibited the most effective inhibition of resistance in vitro and in vivo. Mechanistically, FGFs/CCND1 activated the MAPK pathway, which was abolished by the combination drugs. Our study provides a rationale for clinical testing of dual targeting FGFR and CCND1 with LY2874455 and abemaciclib in NSCLC patients who harbored FGF3/4/19/CCND1 amplification.

Gilteritinib Enhances Anti-Tumor Efficacy of CDK4/6 Inhibitor, Abemaciclib in Lung Cancer Cells

Abemaciclib is a cyclin-dependent kinases 4/6 (CDK4/6) inhibitor approved for the treatment of metastatic breast cancer. Preclinical studies suggest that abemaciclib has the potential for lung cancer treatment. However, several clinical trials demonstrate that monotherapy with abemaciclib has no obvious superiority than erlotinib to treat lung cancer patients, limiting its therapeutic options for lung cancer treatment. Here, we show that the US Food and Drug Administration (FDA)-approved drug, gilteritinib, enhances the cytotoxicity of abemaciclib through inducing apoptosis and senescence in lung cancer cells. Interestingly, abemaciclib in combination with gilteritinib leads to excessive accumulation of vacuoles in lung cancer cells. Mechanistically, combined abemaciclib and gilteritinib induces complete inactivation of AKT and retinoblastoma (Rb) pathways in lung cancer cells. In addition, RNA-sequencing data demonstrate that combination of abemaciclib and gilteritinib treatment induces G2 phase cell-cycle arrest, inhibits DNA replication, and leads to reduction in homologous recombination associated gene expressions. Of note, abemaciclib-resistant lung cancer cells are more sensitive to gilteritinib treatment. In a mouse xenograft model, combined abemaciclib and gilteritinib is more effective than either drug alone in suppressing tumor growth and appears to be well tolerated. Together, our findings support the combination of abemaciclib with gilteritinib as an effective strategy for the treatment of lung cancer, suggesting further evaluation of their efficacy is needed in a clinical trial.

Efficacy, safety and toxicity management of adjuvant abemaciclib in early stage HR+/HER2- high-risk breast cancer

INTRODUCTION

The majority of the over 250,000 new cases of invasive breast cancer diagnosed in the United States is driven by hormone receptor signaling (HR+). Since 2015, cyclin-dependent kinase 4 and 6 inhibitors (CDK4/6i) have become the standard in combination with endocrine therapy (ET) for patients facing metastatic disease.

AREAS COVERED

There are now three approved agents in the metastatic setting; abemaciclib, ribociclib, and palbociclib. Due to the almost doubling of progression free survival (PFS) and improvement in overall survival (OS) in the metastatic setting, studies were conducted to examine the benefit of adding CDK4/6i in the adjuvant setting for those patients at high risk for recurrence. Despite negative results of PALLAS (palbociclib) in this setting, monarchE (abemaciclib) showed an improvement in invasive disease-free survival (IDFS) and distant recurrence free survival (DRFS) at the 3 year time point for patients with high risk tumor characteristics leading to its approval. Herein, we discuss the data, the population studied and the population in which abemaciclib is approved as well as safety, tolerability, and dose reductions for practical management of these patients.

EXPERT OPINION

Abemaciclib is appropriate and beneficial for those patients with high-risk, node-positive, hormonally-driven breast cancer.

Implementation of a combined CDK inhibition and arginine-deprivation approach to target arginine-auxotrophic glioblastoma multiforme cells

Constitutive activation of cyclin-dependent kinases (CDKs) or arginine auxotrophy are hallmarks of Glioblastoma multiforme (GBM). The latter metabolic defect renders tumor cells vulnerable to arginine-depleting substances, such as arginine deiminase from Streptococcus pyogenes (SpyADI). Previously, we confirmed the susceptibility of patient-derived GBM cells towards SpyADI as well as CDK inhibitors (CDKis). To improve therapeutic effects, we here applied a combined approach based on SpyADI and CDKis (dinaciclib, abemaciclib). Three arginine-auxotrophic patient-derived GBM lines with different molecular characteristics were cultured in 2D and 3D and effects of this combined SpyADI/CDKi approach were analyzed in-depth. All CDKi/SpyADI combinations yielded synergistic antitumoral effects, especially when given sequentially (SEQ), i.e., CDKi in first-line and most pronounced in the 3D models. SEQ application demonstrated impaired cell proliferation, invasiveness, and viability. Mitochondrial impairment was demonstrated by increasing mitochondrial membrane potential and decreasing oxygen consumption rate and extracellular acidification rate after SpyADI/abemaciclib monotherapy or its combination regimens. The combined treatment even induced autophagy in target cells (abemaciclib/SpyADI > dinaciclib/SpyADI). By contrast, the unfolded protein response and p53/p21 induced senescence played a minor role. Transmission electron microscopy confirmed damaged mitochondria and endoplasmic reticulum together with increased vacuolization under CDKi mono- and combination therapy. SEQ-abemaciclib/SpyADI treatment suppressed the DSB repair system via NHEJ and HR, whereas SEQ-dinaciclib/SpyADI treatment increased γ-H2AX accumulation and induced Rad51/Ku80. The latter combination also activated the stress sensor GADD45 and β-catenin antagonist AXIN2 and induced expression changes of genes involved in cellular/cytoskeletal integrity. This study highlights the strong antitumoral potential of a combined arginine deprivation and CDK inhibition approach via complex effects on mitochondrial dysfunction, invasiveness as well as DNA-damage response. This provides a good starting point for further in vitro and in vivo proof-of-concept studies to move forward with this strategy.

Targeting Cell Cycle Progression in HER2+ Breast Cancer: An Emerging Treatment Opportunity

The development of HER2-targeted therapies has dramatically improved patient survival and patient management and increased the quality of life in the HER2+ breast cancer patient population. Due to the activation of compensatory pathways, patients eventually suffer from resistance to HER2-directed therapies and develop a more aggressive disease phenotype. One of these mechanisms is the crosstalk between ER and HER2 signaling, especially the CDK4/6-Cyclin D-Rb signaling axis that is commonly active and has received attention for its potential role in regulating tumor progression. CDK 4/6 inhibitors interfere with the binding of cell-cycle-dependent kinases (CDKs) with their cognate partner cyclins, and forestall the progression of the cell cycle by preventing Rb phosphorylation and E2F release that consequentially leads to cancer cell senescence. CDK 4/6 inhibitors, namely, palbociclib, ribociclib, and abemaciclib, in combination with anti-estrogen therapies, have shown impressive outcomes in hormonal receptor-positive (HR+) disease and have received approval for this disease context. As an extension of this concept, preclinical/clinical studies incorporating CDK 4/6 inhibitors with HER2-targeted drugs have been evaluated and have shown potency in limiting tumor progression, restoring therapeutic sensitivity, and may improving the management of the disease. Currently, several clinical trials are examining the synergistic effects of CDK 4/6 inhibitors with optimized HER2-directed therapies for the (ER+/-) HER2+ population in the metastatic setting. In this review, we aim to interrogate the burden of HER2+ disease in light of recent treatment progress in the field and examine the clinical benefit of CDK 4/6 inhibitors as a replacement for traditional chemotherapy to improve outcomes in HER2+ breast cancer.

Cyclin-dependent kinase inhibitor fadraciclib (CYC065) depletes anti-apoptotic protein and synergizes with venetoclax in primary chronic lymphocytic leukemia cells

Fadraciclib (CYC065) is a second-generation aminopurine CDK2/9 inhibitor with increased potency and selectivity toward CDK2 and CDK9 compared to seliciclib (R-roscovitine). In chronic lymphocytic leukemia (CLL), a disease that depends on the over-expression of anti-apoptotic proteins for its survival, inhibition of CDK9 by fadraciclib reduced phosphorylation of the C-terminal domain of RNA polymerase II and blocked transcription in vitro; these actions depleted the intrinsically short-lived anti-apoptotic protein Mcl-1 and induced apoptosis. While the simulated bone marrow and lymph node microenvironments induced Mcl-1 expression and protected CLL cells from apoptosis, these conditions did not prolong the turnover rate of Mcl-1, and fadraciclib efficiently abrogated the protective effect. Further, fadraciclib was synergistic with the Bcl-2 antagonist venetoclax, inducing more profound CLL cell death, especially in samples with 17p deletion. While fadraciclib, venetoclax, and the combination each had distinct kinetics of cell death induction, their activities were reversible, as no additional cell death was induced upon removal of the drugs. The best combination effects were achieved when both drugs were maintained together. Altogether, this study provides a rationale for the clinical development of fadraciclib in CLL, either alone or in combination with a Bcl-2 antagonist.

Targeting CDK4 and CDK6 in cancer

Cyclin-dependent kinase 4 (CDK4) and CDK6 are critical mediators of cellular transition into S phase and are important for the initiation, growth and survival of many cancer types. Pharmacological inhibitors of CDK4/6 have rapidly become a new standard of care for patients with advanced hormone receptor-positive breast cancer. As expected, CDK4/6 inhibitors arrest sensitive tumour cells in the G1 phase of the cell cycle. However, the effects of CDK4/6 inhibition are far more wide-reaching. New insights into their mechanisms of action have triggered identification of new therapeutic opportunities, including the development of novel combination regimens, expanded application to a broader range of cancers and use as supportive care to ameliorate the toxic effects of other therapies. Exploring these new opportunities in the clinic is an urgent priority, which in many cases has not been adequately addressed. Here, we provide a framework for conceptualizing the activity of CDK4/6 inhibitors in cancer and explain how this framework might shape the future clinical development of these agents. We also discuss the biological underpinnings of CDK4/6 inhibitor resistance, an increasingly common challenge in clinical oncology.

The Emerging Role of Cyclin-Dependent Kinase Inhibitors in Treating Diet-Induced Obesity: New Opportunities for Breast and Ovarian Cancers?

Overweight and obesity constitute the most impactful lifestyle-dependent risk factors for cancer and have been tightly linked to a higher number of tumor-related deaths nowadays. The excessive accumulation of energy can lead to an imbalance in the level of essential cellular biomolecules that may result in inflammation and cell-cycle dysregulation. Nutritional strategies and phytochemicals are gaining interest in the management of obesity-related cancers, with several ongoing and completed clinical studies that support their effectiveness. At the same time, cyclin-dependent kinases (CDKs) are becoming an important target in breast and ovarian cancer treatment, with various FDA-approved CDK4/6 inhibitors that have recently received more attention for their potential role in diet-induced obesity (DIO). Here we provide an overview of the most recent studies involving nutraceuticals and other dietary strategies affecting cell-cycle pathways, which might impact the management of breast and ovarian cancers, as well as the repurposing of already commercialized chemotherapeutic options to treat DIO.

Cancer cell cycle dystopia: heterogeneity, plasticity, and therapy

The mammalian cell cycle has been extensively studied regarding cancer etiology, progression, and therapeutic intervention. The canonical cell cycle framework is supported by a plethora of data pointing to a relatively simple linear pathway in which mitogenic signals are integrated in a stepwise fashion to allow progression through G1/S with coordinate actions of cyclin-dependent kinases (CDK)4/6 and CDK2 on the RB tumor suppressor. Recent work on adaptive mechanisms and intrinsic heterogeneous dependencies indicates that G1/S control of the cell cycle is a variable signaling pathway rather than an invariant engine that drives cell division. These alterations can limit the effectiveness of pharmaceutical agents but provide new avenues for therapeutic interventions. These findings support a dystopian view of the cell cycle in cancer where the canonical utopian cell cycle is often not observed. However, recognizing the extent of cell cycle heterogeneity likely creates new opportunities for precision therapeutic approaches specifically targeting these states.

Expression of CDK6 in Stomach Cancer and the Effect of CDK4/6 Inhibitor PD-0332991 on the Function of Stomach Cancer Cells

Objective

To study the expression and prognostic value of CDK6 in stomach cancer and the function of CDK4/6 inhibitor PD-0332991 on the proliferation of stomach cancer cells.

Methods

Immunohistochemistry was used to detect the expression of CDK6 in stomach cancer tissues and adjacent normal tissues and to analyze the effect of CDK6 on clinicopathological parameters of stomach cancer patients. Kaplan-Meier plotter was employed to study the relationship between CDK6 and overall survival in stomach cancer. Western blot and RT-PCR were used to detect protein and gene expression of CDK6 in different cells. The effects of CDK4/6 inhibitor PD-0332991 on apoptosis and aging of stomach cancer cells were detected by flow cytometry and β-galactosidase aging staining assay. The effects of CDK4/6 inhibitor PD-0332991 on the invasion and migration of stomach cancer cells were explored by the wound healing experiment and the Transwell experiment. The supernatant of stomach cancer cells was collected, and the effect of CDK4/6 inhibitor PD-0332991 on tumor markers of stomach cancer cells was detected by biochemical immunoassay.

Results

(1) CDK6 was highly expressed in stomach cancer tissues and cells. (2) Abnormally elevated CDK6 expression results in shorter survival in stomach cancer patients. (3) CDK4/6 inhibitor PD-0332991 could block the proliferation of stomach cancer cells, but not stomach epithelial proliferation. PD-0332991 could inhibit the secretion of pro-GRP by MGC 823. (4) PD-0332991 could advance the development of the apoptosis and senescence of stomach cancer cells and suppressed the invasion and migration of stomach cancer cells.

Conclusion

CDK6 expression is elevated in gastric cancer, and the CDK4/6 inhibitor PD-0332991 can remarkably promote apoptosis and senescence of stomach cancer cells and effectively inhibit the migration and invasion of stomach cancer cells.

Endocrine resistant breast cancer: brain metastasis

Endocrine resistant breast cancer metastasis continues to serve as a significant clinical challenge with high morbidity and mortality for patients. As the number of breast cancer cases continues to rise, the rate of brain metastasis has also increased. For single lesions or a large symptomatic lesion with other smaller lesions, surgical resection is a viable option in non-eloquent regions. Stereotactic radiosurgery is a great option for post-operative therapy or for 10 or fewer small lesions (< 3 cm in size). Whole-brain radiation can be used sparingly for large tumor burdens but should encompass hippocampus sparing techniques. Chemotherapy options have remained relatively limited due to decreased permeability of the blood-brain barrier. Emerging monoclonal antibody treatments have offered initial promise, especially for endocrine resistant breast cancer metastasis.

CDK4/6 Inhibition Suppresses p73 Phosphorylation and Activates DR5 to Potentiate Chemotherapy and Immune Checkpoint Blockade

Targeting cyclin-dependent kinases 4 and 6 (CDK4/6) is a successful therapeutic approach against breast and other solid tumors. Inhibition of CDK4/6 halts cell cycle progression and promotes antitumor immunity. However, the mechanisms underlying the antitumor activity of CDK4/6 inhibitors are not fully understood. We found that CDK4/6 bind and phosphorylate the p53 family member p73 at threonine 86, which sequesters p73 in the cytoplasm. Inhibition of CDK4/6 led to dephosphorylation and nuclear translocation of p73, which transcriptionally activated death receptor 5 (DR5), a cytokine receptor and key component of the extrinsic apoptotic pathway. p73-mediated induction of DR5 by CDK4/6 inhibitors promoted immunogenic cell death of cancer cells. Deletion of DR5 in cancer cells in vitro and in vivo abrogated the potentiating effects of CDK4/6 inhibitors on immune cytokine TRAIL, 5-fluorouracil chemotherapy, and anti-PD-1 immunotherapy. Together, these results reveal a previously unrecognized consequence of CDK4/6 inhibition, which may be critical for potentiating the killing and immunogenic effects on cancer cells.

SIGNIFICANCE

This work demonstrates how inhibition of CDK4/6 sensitizes cancer cells to chemotherapy and immune checkpoint blockade and may provide a new molecular marker for improving CDK4/6-targeted cancer therapies. See related commentary by Frank, p. 1170.

An UHPLC-MS/MS method for quantification of the CDK4/6 inhibitor abemaciclib in human serum

BACKGROUND

Abemaciclib is a new oral targeted treatment option for patients with advanced breast cancer. The emerging field of oral antitumor therapeutics presents challenges for both patients and healthcare teams; non-adherence and high inter-individual pharmacokinetic variability can influence response rates.

METHODS

For monitoring abemaciclib in human sera, a rapid novel ultra-high-performance liquid chromatography-tandem mass spectrometry method was developed and fully validated. Sample preparation was based on a protein precipitation step followed by on-line solid phase extraction. Chromatographic separation was achieved using a biphenyl column and the isotope labeled standard abemaciclib-d8 was used for quantification.

RESULTS

The method showed linearity over a wide calibration range from 20.0 to 2500 ng/mL. With accuracies and precisions of ≤13.9% and ≤4.42%, respectively, the validation results were within the criteria of acceptance. The fitness of the method was tested by monitoring abemaciclib levels under compassionate use for a single individual.

CONCLUSIONS

The novelty of the presented two dimensional isotope dilution UHPLC-MS/MS method is in the semi-automated sample preparation, which results in negligible matrix effects, thereby allowing the introduction of abemaciclib into robust routine therapeutic drug monitoring (TDM). This method provides an efficient tool to verify the usefulness of personalized anticancer therapy in clinical practice.

FDA-Approved Small Molecule Compounds as Drugs for Solid Cancers from Early 2011 to the End of 2021

Solid cancers are the most common types of cancers diagnosed globally and comprise a large number of deaths each year. The main challenge currently in drug development for tumors raised from solid organs is to find more selective compounds, which exploit specific molecular targets. In this work, the small molecule drugs registered by the Food and Drug Administration (FDA) for solid cancers treatment between 2011 and 2022 were identified and analyzed by investigating a type of therapy they are used for, as well as their structures and mechanisms of action. On average, 4 new small molecule agents were introduced each year, with a few exceptions, for a total of 62 new drug approvals. A total of 50 of all FDA-approved drugs have also been authorized for use in the European Union by the European Medicines Agency (EMA). Our analysis indicates that many more anticancer molecules show a selective mode of action, i.e., 49 targeted agents, 5 hormone therapies and 3 radiopharmaceuticals, compared to less specific cytostatic action, i.e., 5 chemotherapeutic agents. It should be emphasized that new medications are indicated for use mainly for monotherapy and less for a combination or adjuvant therapies. The comprehensive data presented in this review can serve for further design and development of more specific targeted agents in clinical usage for solid tumors.

Targeting CDK4/6 for Anticancer Therapy

Cyclin-dependent kinase 4/6 (CDK4/6) are key regulators of the cell cycle and are deemed as critical therapeutic targets of multiple cancers. Various approaches have been applied to silence CDK4/6 at different levels, i.e., CRISPR to knock out at the DNA level, siRNA to inhibit translation, and drugs that target the protein of interest. Here we summarize the current status in this field, highlighting the mechanisms of small molecular inhibitors treatment and drug resistance. We describe approaches to combat drug resistance, including combination therapy and PROTACs drugs that degrade the kinases. Finally, critical issues and perspectives in the field are outlined.

CDK4/6 inhibitors induce replication stress to cause long-term cell cycle withdrawal

CDK4/6 inhibitors arrest the cell cycle in G1-phase. They are approved to treat breast cancer and are also undergoing clinical trials against a range of other tumour types. To facilitate these efforts, it is important to understand why a cytostatic arrest in G1 causes long-lasting effects on tumour growth. Here, we demonstrate that a prolonged G1 arrest following CDK4/6 inhibition downregulates replisome components and impairs origin licencing. Upon release from that arrest, many cells fail to complete DNA replication and exit the cell cycle in a p53-dependent manner. If cells fail to withdraw from the cell cycle following DNA replication problems, they enter mitosis and missegregate chromosomes causing excessive DNA damage, which further limits their proliferative potential. These effects are observed in a range of tumour types, including breast cancer, implying that genotoxic stress is a common outcome of CDK4/6 inhibition. This unanticipated ability of CDK4/6 inhibitors to induce DNA damage now provides a rationale to better predict responsive tumour types and effective combination therapies, as demonstrated by the fact that CDK4/6 inhibition induces sensitivity to chemotherapeutics that also cause replication stress.

Circulating Cell-Free DNA Reflects the Clonal Evolution of Breast Cancer Tumors

Simple Summary

Liquid biopsy of cell-free DNA (cfDNA) is proposed as potential method for the early detection of breast cancer (BC) metastases and following the clonal evolution of BC. Though the use of liquid biopsy is widely discussed, only few studies have demonstrated such usage so far. The aim of this study was to evaluate how accurately cfDNA resembles the genetic profile of tumor DNA and how liquid biopsy reflects the clonal evolution of 18 Eastern-Finnish BC cases with locoregional or distant metastases. Although notable discordance between the sequenced cfDNA and tumor DNA was observed, our results show that liquid biopsy reflects the heterogeneity and clonal evolution of BC and may help to identify potential driver variants and therapeutic targets that are not detected with the sequencing of tumor DNA. This information may be used to detect metastatic BC earlier and to support decision-making in clinical practice.

Abstract

Liquid biopsy of cell-free DNA (cfDNA) is proposed as a potential method for the early detection of breast cancer (BC) metastases and following the clonal evolution of BC. Though the use of liquid biopsy is a widely discussed topic in the field, only a few studies have demonstrated such usage so far. We sequenced the DNA of matched primary tumor and metastatic sites together with the matched cfDNA samples from 18 Eastern Finnish BC patients and investigated how well cfDNA reflected the clonal evolution of BC interpreted from tumor DNA. On average, liquid biopsy detected 56.2 ± 7.2% of the somatic variants that were present either in the matched primary tumor or metastatic sites. Despite the high discordance observed between matched samples, liquid biopsy was found to reflect the clonal evolution of BC and identify novel driver variants and therapeutic targets absent from the tumor DNA. Tumor-specific somatic variants were detected in cfDNA at the time of diagnosis and 8.4 ± 2.4 months prior to detection of locoregional recurrence or distant metastases. Our results demonstrate that the sequencing of cfDNA may be used for the early detection of locoregional and distant BC metastases. Observed discordance between tumor DNA sequencing and liquid biopsy supports the parallel sequencing of cfDNA and tumor DNA to yield the most comprehensive overview for the genetic landscape of BC.

Abemaciclib in Combination With Endocrine Therapy for Patients With Hormone Receptor-Positive, HER2-Negative Metastatic Breast Cancer: A Phase 1b Study

Background

Cyclin-dependent kinases (CDK) 4 and 6 regulate G1 to S cell cycle progression and are often altered in cancers. Abemaciclib is a selective inhibitor of CDK4 and CDK6 approved for administration on a continuous dosing schedule as monotherapy or as combination therapy with an aromatase inhibitor or fulvestrant in patients with advanced or metastatic breast cancer. This Phase 1b study evaluated the safety and tolerability, pharmacokinetics, and antitumor activity of abemaciclib in combination with endocrine therapy for metastatic breast cancer (MBC), including aromatase inhibitors (letrozole, anastrozole, or exemestane) or tamoxifen.

Patients and Methods

Women ≥18 years old with hormone receptor positive (HR+), human epidermal growth factor receptor 2 negative (HER2-) MBC were eligible for enrollment. Eligibility included measurable disease or non-measurable but evaluable bone disease by Response Evaluation Criteria in Solid Tumours (RECIST) v1.1, Eastern Cooperative Oncology Group performance status 0–1, and no prior chemotherapy for metastatic disease. Adverse events were graded by the National Cancer Institute Common Terminology Criteria for Adverse Events v4.0 and tumor response were assessed by RECIST v1.1.

Results

Sixty-seven patients were enrolled and received abemaciclib 200 mg every 12 hours in combination with letrozole (Part A, n=20), anastrozole (Part B, n=16), tamoxifen (Part C, n=16), or exemestane (Part D, n=15). The most common treatment-emergent adverse events (TEAE) were diarrhea, fatigue, nausea, and abdominal pain. Grade 4 TEAEs were reported in five patients (one each with hyperglycemia, hypertension, neutropenia, procedural hemorrhage, and sepsis). There was no effect of abemaciclib or endocrine therapy on the pharmacokinetics of any combination study drug. Across all treated patients, the median progression-free survival was 25.4 months (95% confidence interval: 18.0, 35.8). The objective response rate was 38.9% in 36 patients with measurable disease.

Conclusions

Abemaciclib in combination with multiple endocrine therapy options exhibited manageable safety and promising antitumor activity in patients with HR+, HER2- MBC.

Clinical Trial Registration

https://clinicaltrials.gov/, identifier NCT02057133

Mechanisms of Cell Cycle Arrest and Apoptosis in Glioblastoma

Cells of glioblastoma, the most frequent primary malignant brain tumor, are characterized by their rapid growth and infiltration of adjacent healthy brain parenchyma, which reflects their aggressive biological behavior. In order to maintain their excessive proliferation and invasion, glioblastomas exploit the innate biological capacities of the patients suffering from this tumor. The pathways involved in cell cycle regulation and apoptosis are the mechanisms most commonly affected. The following work reviews the regulatory pathways of cell growth in general as well as the dysregulated cell cycle and apoptosis relevant mechanisms observed in glioblastomas. We then describe the molecular targeting of the current established adjuvant therapy and present ongoing trials or completed studies on specific promising therapeutic agents that induce cell cycle arrest and apoptosis of glioblastoma cells.

Understanding Retinoblastoma Post-Translational Regulation for the Design of Targeted Cancer Therapies

Simple Summary

Rb1 is a regulator of cell cycle progression and genomic stability. This review focuses on post-translational modifications, their effect on Rb1 interactors, and their role in intracellular signaling in the context of cancer development. Finally, we highlight potential approaches to harness these post-translational modifications to design novel effective anticancer therapies.

Abstract

The retinoblastoma protein (Rb1) is a prototypical tumor suppressor protein whose role was described more than 40 years ago. Together with p107 (also known as RBL1) and p130 (also known as RBL2), the Rb1 belongs to a family of structurally and functionally similar proteins that inhibits cell cycle progression. Given the central role of Rb1 in regulating proliferation, its expression or function is altered in most types of cancer. One of the mechanisms underlying Rb-mediated cell cycle inhibition is the binding and repression of E2F transcription factors, and these processes are dependent on Rb1 phosphorylation status. However, recent work shows that Rb1 is a convergent point of many pathways and thus the regulation of its function through post-translational modifications is more complex than initially expected. Moreover, depending on the context, downstream signaling can be both E2F-dependent and -independent. This review seeks to summarize the most recent research on Rb1 function and regulation and discuss potential avenues for the design of novel cancer therapies.

Biological Effects of Cyclin-Dependent Kinase Inhibitors Ribociclib, Palbociclib and Abemaciclib on Breast Cancer Bone Microenvironment

The CDK4/6 inhibitors (CDKi) palbociclib, ribociclib, and abemaciclib are currently approved in combination with anti-estrogen therapy for the treatment of advanced and/or metastatic hormone receptor-positive/HER2-neu-negative breast cancer patients. Given the high incidence of bone metastases in this population, we investigated and compared the potential effects of palbociclib, ribociclib, and abemaciclib on the breast cancer bone microenvironment. Primary osteoclasts (OCs) and osteoblasts (OBs) were obtained from human monocyte and mesenchymal stem cells, respectively. OC function was evaluated by tartrate-resistant acid phosphatase assay and real-time PCR; OB activity was assessed by an alizarin red assay. OB/breast cancer co-culture models were generated via the seeding of MCF-7 cells on a layer of OBs, and tumor cell proliferation was analyzed using flow cytometry. Here, we showed that ribociclib, palbociclib, and abemaciclib exerted similar inhibitory effects on the OC differentiation and expression of bone resorption markers without affecting OC viability. On the other hand, the three CDKi did not affect the ability of OB to produce bone matrix, even if the higher doses of palbociclib and abemaciclib reduced the OB viability. In OB/MCF-7 co-culture models, palbociclib demonstrated a lower anti-tumor effect than ribociclib and abemaciclib. Overall, our results revealed the direct effects of CDKi on the tumor bone microenvironment, highlighting differences potentially relevant for clinical practice.

Abemaciclib: The First FDA-Approved CDK4/6 Inhibitor for the Adjuvant Treatment of HR+ HER2- Early Breast Cancer

OBJECTIVE

To review the new indication of cyclin-dependent kinase (CDK4/6) inhibitor abemaciclib for the adjuvant treatment of hormone receptor positive (HR+), human epidermal growth factor receptor 2 negative (HER2-), axillary lymph node (LN) positive early breast cancer (EBC) at high risk of recurrence and a Ki-67 ≥20%.

DATA SOURCES

A literature search was performed through PubMed, ClinicalTrials.gov, and Food and Drug Administration (FDA) website (February 1, 2018, to December 23, 2021) to identify relevant information.

STUDY SELECTION AND DATA EXTRACTION

Human and animal studies related to pharmacology, pharmacokinetics, efficacy, and safety of abemaciclib were identified.

DATA SYNTHESIS

Addition of abemaciclib to standard of care endocrine therapy (ET) for patients with high-risk clinicopathologic features and Ki-67 ≥20% demonstrated 30% reduction in the risk of developing invasive disease and distant recurrence. At 15.5 months, abemaciclib + ET demonstrated a significant improvement in invasive disease-free survival (IDFS) vs ET alone (hazard ratio [HR], 0.75; 95% confidence interval [CI], 0.60-0.93, P = 0.01). At 27 months, IDFS benefit was maintained (HR, 0.70; 95% CI, 0.59-0.82, P < 0.0001). Diarrhea occurred in more than 80% of patients in the abemaciclib arm.

RELEVANCE TO PATIENT CARE AND CLINICAL PRACTICE

This review describes the clinical applicability of adjuvant abemaciclib for patients with HR+, HER2- EBC at high risk for recurrence.

CONCLUSION

Adjuvant abemaciclib significantly reduces the risk for early development of invasive disease and distant recurrence in patients with HR+, HER2- node positive EBC. Longer follow-up is needed to determine the impact of adjuvant abemaciclib on late disease recurrence and survival outcomes.

A Functional Precision Oncology Approach to Identify Treatment Strategies for Myxofibrosarcoma Patients

In this era of precision medicine, numerous workflows for the targeting of high-recurrent mutations in common tumor types have been developed, leaving patients with rare diseases with few options. Here, we implement a functional precision oncology approach utilizing comprehensive genomic profiling in combination with high-throughput drug screening, to identify tumor-specific drug sensitivities for patients with rare tumor types such as myxofibrosarcoma. From a patient with a high-grade myxofibrosarcoma, who was enrolled in the Englander Institute for Precision Medicine (EIPM) program, we established patient-derived 3D sarco-spheres and xenograft models for functional testing. In the absence of a large cohort of clinically similar cases, high-throughput drug screening was performed on the patient-derived cells, and compared with two other myxofibrosarcoma lines and a benign fibroblast line to functionally identify tumor-specific drug sensitivities. The addition of functional drug sensitivity testing to complement genomic profiling identified multiple therapeutic options that were further validated in patient derived xenograft models. Genomic analyses detected the frequently known codeletion of the tumor suppressors CDKN2A/B together with the methylthioadenosine phosphorylase (MTAP) and a TP53 E286fs*50 mutation. High-throughput drug screening demonstrated tumor-specific sensitivity to compounds targeting the cell cycle. Based on genomic analysis and high-throughput drug screening, we show that targeting the cell cycle in these tumors is a powerful approach. IMPLICATIONS: This study demonstrates the potential of functional testing to aid clinical decision making for patients with rare or molecularly complex malignancies when combined with comprehensive genomic profiling.

AKT1 Is Required for a Complete Palbociclib-Induced Senescence Phenotype in BRAF-V600E-Driven Human Melanoma

Cellular senescence is a carefully regulated process of proliferative arrest accompanied by functional and morphologic changes. Senescence allows damaged cells to avoid neoplastic proliferation; however, the induction of the senescence-associated secretory phenotype (SASP) can promote tumor growth. The complexity of senescence may limit the efficacy of anti-neoplastic agents, such as CDK4/6 inhibitors (Cdk4/6i), that induce a senescence-like state in tumor cells. The AKT kinase family, which contains three isoforms that play both unique and redundant roles in cancer progression, is commonly hyperactive in many cancers including melanoma and has been implicated in the regulation of senescence. To interrogate the role of AKT isoforms in Cdk4/6i-induced cellular senescence, we generated isoform-specific AKT knockout human melanoma cell lines. We found that the CDK4/6i Palbociclib induced a form of senescence in these cells that was dependent on AKT1. We then evaluated the activity of the cGAS-STING pathway, recently implicated in cellular senescence, finding that cGAS-STING function was dependent on AKT1, and pharmacologic inhibition of cGAS had little effect on senescence. However, we found SASP factors to require NF-κB function, in part dependent on a stimulatory phosphorylation of IKKα by AKT1. In summary, we provide the first evidence of a novel, isoform-specific role for AKT1 in therapy-induced senescence in human melanoma cells acting through NF-κB but independent of cGAS.

E2F and STAT3 provide transcriptional synergy for histone variant H2AZ activation to sustain glioblastoma chromatin accessibility and tumorigenicity

The histone variant H2AZ is overexpressed in diverse cancer types where it facilitates the accessibility of transcriptional regulators to the promoters of cell cycle genes. However, the molecular basis for its dysregulation in cancer remains unknown. Here, we report that glioblastomas (GBM) and glioma stem cells (GSCs) preferentially overexpress H2AZ for their proliferation, stemness and tumorigenicity. Chromatin accessibility analysis of H2AZ2 depleted GSC revealed that E2F1 occupies the enhancer region within H2AZ2 gene promoter, thereby activating H2AZ2 transcription. Exploration of other H2AZ2 transcriptional activators using a customized "anti-H2AZ2" query signature for connectivity map analysis identified STAT3. Co-targeting E2F and STAT3 synergistically reduced the levels of H2AZ, histone 3 lysine 27 acetylation (H3K27ac) and cell cycle gene transcription, indicating that E2F1 and STAT3 synergize to activate H2AZ gene transcription in GSCs. Remarkably, an E2F/STAT3 inhibitor combination durably suppresses GSC tumorigenicity in an orthotopic GBM xenograft model. In glioma patients, high STAT3 signaling is associated with high E2F1 and H2AZ2 expression. Thus, GBM has uniquely opted the use of E2F1- and STAT3-containing "enhanceosomes" that integrate multiple signaling pathways to achieve H2AZ gene activation, supporting a translational path for the E2F/STAT3 inhibitor combination to be applied in GBM treatment.

Aberrant GIMAP2 expression affects oral squamous cell carcinoma progression by promoting cell cycle and inhibiting apoptosis

GTPases of immunity-associated protein 2 (GIMAP2) is a GTPase family member associated with T cell survival. However, its mechanisms of action in oral squamous cell carcinoma (OSCC) remain largely unknown. Therefore, the present study aimed to elucidate the possible role of GIMAP2 in OSCC development by investigating its expression levels and molecular mechanisms in OSCC. Reverse transcription quantitative PCR, immunoblotting and immunohistochemistry indicated that GIMAP2 expression was significantly upregulated (P<0.05) in OSCC-derived cell lines and primary OSCC specimens compared with that in their normal counterparts. GIMAP2-knockdown OSCC cells exhibited decreased cell growth, which was associated with cyclin-dependent kinase (CDK)4, CDK6 and phosphorylated Rb downregulation and p53 and p21 upregulation. In addition to cell cycle arrest, GIMAP2 affected anti-apoptotic functions in GIMAP2-knockdown cells by upregulating Bcl-2 and downregulating Bax and Bak. These findings indicated that GIMAP2 may significantly influence OSCC development and apoptosis inhibition and thus is a potential biomarker of OSCC.

Pharmacological CDK4/6 inhibition reveals a p53-dependent senescent state with restricted toxicity

Cellular senescence is a state of stable growth arrest and a desired outcome of tumor suppressive interventions. Treatment with many anti‐cancer drugs can cause premature senescence of non‐malignant cells. These therapy‐induced senescent cells can have pro‐tumorigenic and pro‐disease functions via activation of an inflammatory secretory phenotype (SASP). Inhibitors of cyclin‐dependent kinases 4/6 (CDK4/6i) have recently proven to restrain tumor growth by activating a senescence‐like program in cancer cells. However, the physiological consequence of exposing the whole organism to pharmacological CDK4/6i remains poorly characterized. Here, we show that exposure to CDK4/6i induces non‐malignant cells to enter a premature state of senescence dependent on p53. We observe in mice and breast cancer patients that the CDK4/6i‐induced senescent program activates only a partial SASP enriched in p53 targets but lacking pro‐inflammatory and NF‐κB‐driven components. We find that CDK4/6i‐induced senescent cells do not acquire pro‐tumorigenic and detrimental properties but retain the ability to promote paracrine senescence and undergo clearance. Our results demonstrate that SASP composition is exquisitely stress‐dependent and a predictor for the biological functions of different senescence subsets.

The multifaceted mechanisms of malignant glioblastoma progression and clinical implications

With the application of high throughput sequencing technologies at single-cell resolution, studies of the tumor microenvironment in glioblastoma, one of the most aggressive and invasive of all cancers, have revealed immense cellular and tissue heterogeneity. A unique extracellular scaffold system adapts to and supports progressive infiltration and migration of tumor cells, which is characterized by altered composition, effector delivery, and mechanical properties. The spatiotemporal interactions between malignant and immune cells generate an immunosuppressive microenvironment, contributing to the failure of effective anti-tumor immune attack. Among the heterogeneous tumor cell subpopulations of glioblastoma, glioma stem cells (GSCs), which exhibit tumorigenic properties and strong invasive capacity, are critical for tumor growth and are believed to contribute to therapeutic resistance and tumor recurrence. Here we discuss the role of extracellular matrix and immune cell populations, major components of the tumor ecosystem in glioblastoma, as well as signaling pathways that regulate GSC maintenance and invasion. We also highlight emerging advances in therapeutic targeting of these components.

Synergistic combination of cytotoxic chemotherapy and cyclin-dependent kinase 4/6 inhibitors in biliary tract cancers

BACKGROUND AND AIMS

Biliary tract cancers (BTCs) are uncommon, but highly lethal, gastrointestinal malignancies. Gemcitabine/cisplatin is a standard-of-care systemic therapy, but has a modest impact on survival and harbors toxicities, including myelosuppression, nephropathy, neuropathy, and ototoxicity. Whereas BTCs are characterized by aberrations activating the cyclinD1/cyclin-dependent kinase (CDK)4/6/CDK inhibitor 2a/retinoblastoma pathway, clinical use of CDK4/6 inhibitors as monotherapy is limited by lack of validated biomarkers, diffident preclinical efficacy, and development of acquired drug resistance. Emerging studies have explored therapeutic strategies to enhance the antitumor efficacy of CDK4/6 inhibitors by the combination with chemotherapy regimens, but their mechanism of action remains elusive.

APPROACH AND RESULTS

Here, we report in vitro and in vivo synergy in BTC models, showing enhanced efficacy, reduced toxicity, and better survival with a combination comprising gemcitabine/cisplatin and CDK4/6 inhibitors. Furthermore, we demonstrated that abemaciclib monotherapy had only modest efficacy attributable to autophagy-induced resistance. Notably, triplet therapy was able to potentiate efficacy through elimination of the autophagic flux. Correspondingly, abemaciclib potentiated ribonucleotide reductase catalytic subunit M1 reduction, resulting in sensitization to gemcitabine.

CONCLUSIONS

As such, these data provide robust preclinical mechanistic evidence of synergy between gemcitabine/cisplatin and CDK4/6 inhibitors and delineate a path forward for translation of these findings to preliminary clinical studies in advanced BTC patients.

Abemaciclib is synergistic with doxorubicin in osteosarcoma pre-clinical models via inhibition of CDK4/6-Cyclin D-Rb pathway

BACKGROUND

Osteosarcoma is a prevalent type of bone tumor in children and adolescents, with limited treatment and poor prognosis. Abemaciclib, an inhibitor of cyclin-dependent kinases 4 and 6 (CDK4/6), is approved for the treatment of advanced breast cancer as single agent therapy and is currently under investigation in clinical trials for the treatment of several solid tumors.

METHODS

The efficacy of abemaciclib was determined using osteosarcoma cellular assays and xenograft mouse model. The combination studies were performed based on the Chou-Talalay method. Immunoblotting analysis was performed to determine the underlying mechanisms of abemaciclib in osteosarcoma cell lines.

RESULTS

Abemaciclib potently inhibits growth, anchorage-independent colony formation and survival of a panel of osteosarcoma cell lines, with IC50 range from 90 nM to >20 μM. In addition, the combination of abemaciclib and doxorubicin is synergistic and antagonistic in abemaciclib-sensitive (IC50 <1 μM) and abemaciclib-resistant (IC50 >1 μM), respectively. Abemaciclib inhibits tumor formation and growth in a dose-dependent manner without causing significant drug toxicity in mice. The combination of abemaciclib and doxorubicin results in much greater efficacy than doxorubicin alone in inhibiting tumor growth throughout the whole treatment duration. Abemaciclib acts on osteosarcoma via suppressing CDK4/6-Cyclin D-Rb pathway.

CONCLUSIONS

Our pre-clinical evidence provides a rationale of initializing clinical trial of investigating the efficacy of abemaciclib in combination with doxorubicin in osteosarcoma patients. Our work also highlights the therapeutic value of CDK4/6 inhibition in osteosarcoma with proper function of Rb.

Leptomeningeal Disease as an Initial Presenting Manifestation in Breast Cancer

Leptomeningeal disease (LMD) occurs in less than 5% of breast cancer patients. Rarely, patients present with LMD at diagnosis. We report a case of a 59-year-old female who presented with lower back pain, radicular neuropathy with lower extremity weakness, and was found to have diffuse LMD. Workup was initiated to evaluate the etiology of LMD and later involved iliac bone biopsy showed metastatic adenocarcinoma consistent with breast carcinoma. Patient received radiation therapy followed by abemaciclib with letrozole and responded well with the treatment.

Cyclin-Dependent Kinase Inhibitors and Their Therapeutic Potential in Colorectal Cancer Treatment

Cyclin-dependent kinases (CDKs) are key players in cell cycle regulation. So far, more than ten CDKs have been described. Their direct interaction with cyclins allow progression through G1 phase, transitions to S and G2 phase and finally through mitosis (M). While CDK activation is important in cell renewal, its aberrant expression can lead to the development of malignant tumor cells. Dysregulations in CDK pathways are often encountered in various types of cancer, including all gastrointestinal (GI) tract tumors. This prompted the development of CDK inhibitors as novel therapies for cancer. Currently, CDK inhibitors such as CDK4/6 inhibitors are used in pre-clinical studies for cancer treatment. In this review, we will focus on the therapeutic role of various CDK inhibitors in colorectal cancer, with a special focus on the CDK4/6 inhibitors.

Therapeutic Drug Monitoring of Protein Kinase Inhibitors in Breast Cancer Patients

Protein kinase inhibitors (PKIs) represent up-to-date therapeutic approach in breast cancer treatment. Although cancer is a rapidly progressive disease, many substances, including PKIs, are usually used at fixed doses without regard to each patient's individuality. Therapeutic drug monitoring (TDM) is a tool that allows individualization of therapy based on drug plasma levels. For TDM conduct, exposure-response relationships of drug substances are required. The pharmacokinetic data and exposure-response evidence supporting the use of TDM for 6 PKIs used in breast cancer treatment, one of the most common female tumour diseases, are discussed in this review.

RB depletion is required for the continuous growth of tumors initiated by loss of RB

The retinoblastoma (RB) tumor suppressor is functionally inactivated in a wide range of human tumors where this inactivation promotes tumorigenesis in part by allowing uncontrolled proliferation. RB has been extensively studied, but its mechanisms of action in normal and cancer cells remain only partly understood. Here, we describe a new mouse model to investigate the consequences of RB depletion and its re-activation in vivo. In these mice, induction of shRNA molecules targeting RB for knock-down results in the development of phenotypes similar to Rb knock-out mice, including the development of pituitary and thyroid tumors. Re-expression of RB leads to cell cycle arrest in cancer cells and repression of transcriptional programs driven by E2F activity. Thus, continuous RB loss is required for the maintenance of tumor phenotypes initiated by loss of RB, and this new mouse model will provide a new platform to investigate RB function in vivo.

Cyclin-dependent kinase 4/6 inhibitors suppress tumor growth in extramammary Paget's disease

Extramammary Paget's disease (EMPD) is a rare adnexal neoplasm commonly seen in the genital areas among the senior population. The prognosis of advanced EMPD is not favorable; thus, the development of potential treatments has long been sought. Cyclin-dependent kinase (CDK) 4/6 inhibitors such as abemaciclib and palbociclib have been proven effective against metastatic breast cancer; however, no studies have addressed CDK4/6 inhibitors as an EMPD treatment. We herein examine the efficacy of CDK4/6 inhibitors against an EMPD patient-derived xenograft (PDX) model. Abemaciclib (50 mg/kg/day) or palbociclib (120 mg/kg/day) was given orally to tumor-bearing NOD/Scid mice over a 3-week period. We also investigated the protein expression levels of CDK4/6 and cyclin D1 through immunohistochemical staining using EMPD clinical samples. Treatment with abemaciclib or palbociclib as a single agent was found to significantly suppress tumor growth in EMPD-PDX. The Ki-67-positive ratio of the treated EMPD-PDX tumors was significantly lower than that of the nontreated tumors. Clinically, the expression levels of CDK4 and cyclin D1 were significantly higher in the EMPD tumor cells than in the normal epidermis. Our results suggest that CDK4/6 inhibitors could be novel and potent therapeutics for the treatment of EMPD.

CDK4/6 inhibitors: A potential therapeutic approach for triple negative breast cancer

Triple negative breast cancer (TNBC) cells lack expression of the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor-2 (HER-2). Thus, TNBC does not respond to hormone-based therapy. TNBC is also an aggressive subtype associated with poorer prognoses compared to other breast cancers. Conventional chemotherapeutics are used to manage TNBC although systemic relapse is common with limited benefits being reported as well as adverse events being documented. Here, we discuss current therapies for TNBC in the neo- and adjuvant settings, as well as recent advancements in the targeting of PD-L1-positive tumors and inclusion of PARP inhibitors for TNBC patients with BRCA mutations. The recent development of cyclin-dependent kinase (CDK) 4/6 inhibitors in ER-positive breast cancers has demonstrated significant improvements in progression free survival in patients. Here, we review preclinical data of CDK 4/6 inhibitors and describe current clinical trials assessing these in TNBC disease.

Therapeutic strtegies of glioblastoma (GBM): The current advances in the molecular targets and bioactive small molecule compounds

Glioblastoma (GBM) is the most common aggressive malignant tumor in brain neuroepithelial tumors and remains incurable. A variety of treatment options are currently being explored to improve patient survival, including small molecule inhibitors, viral therapies, cancer vaccines, and monoclonal antibodies. Among them, the unique advantages of small molecule inhibitors have made them a focus of attention in the drug discovery of glioblastoma. Currently, the most used chemotherapeutic agents are small molecule inhibitors that target key dysregulated signaling pathways in glioblastoma, including receptor tyrosine kinase, PI3K/AKT/mTOR pathway, DNA damage response, TP53 and cell cycle inhibitors. This review analyzes the therapeutic benefit and clinical development of novel small molecule inhibitors discovered as promising anti-glioblastoma agents by the related targets of these major pathways. Meanwhile, the recent advances in temozolomide resistance and drug combination are also reviewed. In the last part, due to the constant clinical failure of targeted therapies, this paper reviewed the research progress of other therapeutic methods for glioblastoma, to provide patients and readers with a more comprehensive understanding of the treatment landscape of glioblastoma.