Multiple roles of cyclin-dependent kinase 4/6 inhibitors in cancer therapy

Strategy of combining CDK4/6 inhibitors with other therapies and mechanisms of resistance

Cell cycle-dependent protein kinase 4/6 (CDK4/6) is a crucial kinase that regulates the cell cycle, essential for cell division and proliferation. Hence, combining CDK4/6 inhibitors with other anti-tumor drugs is a pivotal clinical strategy. This strategy can efficiently inhibit the growth and division of tumor cells, reduce the side effects, and improve the quality of life of patients by reducing the dosage of combined anticancer drugs. Furthermore, the combination therapy strategy of CDK4/6 inhibitors could ameliorate the drug resistance of combined drugs and overcome the CDK4/6 resistance caused by CDK4/6 inhibitors. Various tumor treatment strategies combined with CDK4/6 inhibitors have entered the clinical trial stage, demonstrating their substantial clinical potential. This study reviews the research progress of CDK4/6 inhibitors from 2018 to 2022, the related resistance mechanism of CDK4/6 inhibitors, and the strategy of combination medication.

Post-marketing safety concerns with palbociclib: a disproportionality analysis of the FDA adverse event reporting system

OBJECTIVES

To explore the association between palbociclib and related adverse events (AEs) in the real world through U.S. Food and Drug Administration Adverse Event Reporting System (FAERS) database.

METHODS

The signal strength of palbociclib-related AEs was done by disproportionality analysis. Clinical priority of palbociclib-related AEs was scored and ranked by assessing five different features. Outcome analysis, time to onset analysis, dose-report /AEs number analysis, and stratification analysis were all performed.

RESULTS

There were 61,821 'primary suspected (PS)' reports of palbociclib and 195,616 AEs associated with palbociclib. The four algorithms simultaneously detected 18 positive signals at the SOC level, and 65 positive signals at the PT level. Bone marrow failure, neuropathy, peripheral, pleural effusion, myelosuppression, pulmonary edema, and pulmonary thrombosis were also found to have positive signals. Gender (female vs male, χ2 = 5.287, p = 0.022) and age showed significant differences in serious and non-serious reports. Palbociclib-related AEs had a median onset time of 79 days (interquartile range [IQR] 20-264 days).

CONCLUSIONS

The study identified potential Palbociclib-related AEs and offered warnings for special AEs, providing further data for palbociclib safety studies in breast cancer patients. Nonetheless, prospective clinical trials are needed to validate these results and explain their relationship.

Progression from ductal carcinoma in situ to invasive breast cancer: molecular features and clinical significance

Ductal carcinoma in situ (DCIS) represents pre-invasive breast carcinoma. In untreated cases, 25-60% DCIS progress to invasive ductal carcinoma (IDC). The challenge lies in distinguishing between non-progressive and progressive DCIS, often resulting in over- or under-treatment in many cases. With increasing screen-detected DCIS in these years, the nature of DCIS has aroused worldwide attention. A deeper understanding of the biological nature of DCIS and the molecular journey of the DCIS-IDC transition is crucial for more effective clinical management. Here, we reviewed the key signaling pathways in breast cancer that may contribute to DCIS initiation and progression. We also explored the molecular features of DCIS and IDC, shedding light on the progression of DCIS through both inherent changes within tumor cells and alterations in the tumor microenvironment. In addition, valuable research tools utilized in studying DCIS including preclinical models and newer advanced technologies such as single-cell sequencing, spatial transcriptomics and artificial intelligence, have been systematically summarized. Further, we thoroughly discussed the clinical advancements in DCIS and IDC, including prognostic biomarkers and clinical managements, with the aim of facilitating more personalized treatment strategies in the future. Research on DCIS has already yielded significant insights into breast carcinogenesis and will continue to pave the way for practical clinical applications.

The Evolving Pathways of the Efficacy of and Resistance to CDK4/6 Inhibitors in Breast Cancer

The approval of cyclin-dependent kinase 4 and 6 inhibitors (CDK4/6i) in combination with endocrine therapy (ET) has remarkably improved the survival outcomes of patients with advanced hormone receptor-positive (HR+) breast cancer (BC), becoming the new standard of care treatment in these patients. Despite the efficacy of this therapeutic combination, intrinsic and acquired resistance inevitably occurs and represents a major clinical challenge. Several mechanisms associated with resistance to CDK4/6i have been identified, including both cell cycle-related and cell cycle-nonspecific mechanisms. This review discusses new insights underlying the mechanisms of action of CDK4/6i, which are more far-reaching than initially thought, and the currently available evidence of the mechanisms of resistance to CDK4/6i in BC. Finally, it highlights possible treatment strategies to improve CDK4/6i efficacy, summarizing the most relevant clinical data on novel combination therapies involving CDK4/6i.

Hormone Receptors and Epithelial Ovarian Cancer: Recent Advances in Biology and Treatment Options

Epithelial ovarian cancer (EOC) is a significant cause of cancer-related mortality in women. Despite advances in diagnosis and treatment, EOC remains a challenging disease to manage, and the 5-year survival rate is still poor. The role of hormone receptors (HRs) in EOC carcinogenesis and prognosis has been actively explored; however, the role of hormone therapy (HT) in the treatment of these tumors is not well established. Most available data on HT mainly come from retrospective series and small early clinical trials. Several of these studies suggest that HT may have a role in adjuvant, maintenance therapy, or in the case of recurrent disease, especially for some subtypes of EOC (e.g., low-grade serous EOC). Furthermore, HT has recently been combined with targeted therapies, but most studies evaluating these combinations are still ongoing. The main aim of this review is to provide an overview of the progress made in the last decade to characterize the biological and prognostic role of HRs for EOC and the developments in their therapeutic targeting through HT.

Emerging Therapies for Breast Cancer

The steady, incremental improvements in outcomes for both early-stage and advanced breast cancer patients are, in large part, attributable to the success of novel systemic therapies. In this review, we discuss key conceptual paradigms that have underpinned this success including (1) targeting the driver: the identification and targeting of major oncoproteins in breast cancers; (2) targeting the lineage pathway: inhibition of those pathways that drive normal mammary epithelial cell proliferation that retain importance in cancer; (3) targeting precisely: the application of molecular classifiers to refine therapy selection for specific cancers, and of antibody-drug conjugates to pinpoint tumor and tumor promoting cells for eradication; and (4) exploiting synthetic lethality: leveraging unique vulnerabilities that cancer-specific molecular alterations induce. We describe promising examples of novel therapies that have been discovered within each of these paradigms and suggest how future drug development efforts might benefit from the continued application of these principles.

Exploring the Molecular Complexity of Medulloblastoma: Implications for Diagnosis and Treatment

Medulloblastoma is the most common malignant brain tumor in children. Over the last few decades, significant progress has been made in revealing the key molecular underpinnings of this disease, leading to the identification of distinct molecular subgroups with different clinical outcomes. In this review, we provide an update on the molecular landscape of medulloblastoma and treatment strategies. We discuss the four main molecular subgroups (WNT-activated, SHH-activated, and non-WNT/non-SHH groups 3 and 4), highlighting the key genetic alterations and signaling pathways associated with each entity. Furthermore, we explore the emerging role of epigenetic regulation in medulloblastoma and the mechanism of resistance to therapy. We also delve into the latest developments in targeted therapies and immunotherapies. Continuing collaborative efforts are needed to further unravel the complex molecular mechanisms and profile optimal treatment for this devastating disease.

Selective protection of normal cells from chemotherapy, while killing drug-resistant cancer cells

Cancer therapy is limited by toxicity in normal cells and drug-resistance in cancer cells. Paradoxically, cancer resistance to certain therapies can be exploited for protection of normal cells, simultaneously enabling the selective killing of resistant cancer cells by using antagonistic drug combinations, which include cytotoxic and protective drugs. Depending on the mechanisms of drug-resistance in cancer cells, the protection of normal cells can be achieved with inhibitors of CDK4/6, caspases, Mdm2, mTOR, and mitogenic kinases. When normal cells are protected, the selectivity and potency of multi-drug combinations can be further enhanced by adding synergistic drugs, in theory, eliminating the deadliest cancer clones with minimal side effects. I also discuss how the recent success of Trilaciclib may foster similar approaches into clinical practice, how to mitigate systemic side effects of chemotherapy in patients with brain tumors and how to ensure that protective drugs would only protect normal cells (not cancer cells) in a particular patient.

Leveraging the replication stress response to optimize cancer therapy

High-fidelity DNA replication is critical for the faithful transmission of genetic information to daughter cells. Following genotoxic stress, specialized DNA damage tolerance pathways are activated to ensure replication fork progression. These pathways include translesion DNA synthesis, template switching and repriming. In this Review, we describe how DNA damage tolerance pathways impact genome stability, their connection with tumorigenesis and their effects on cancer therapy response. We discuss recent findings that single-strand DNA gap accumulation impacts chemoresponse and explore a growing body of evidence that suggests that different DNA damage tolerance factors, including translesion synthesis polymerases, template switching proteins and enzymes affecting single-stranded DNA gaps, represent useful cancer targets. We further outline how the consequences of DNA damage tolerance mechanisms could inform the discovery of new biomarkers to refine cancer therapies.

Applications and mechanisms of the cyclin-dependent kinase 4/6 inhibitor, PD-0332991, in solid tumors

Abnormal CDK4/6-Rb-E2F signal transduction is a common finding in tumors and is a driving factor for the excessive proliferation of various tumor cells. PD-0332991, a highly specific, small molecule inhibitor for CDK4 and 6, has been shown to inhibit tumor growth by abrogating the phosphorylating capacity of CDK4/6 and suppressing Rb phosphorylation. It has been promoted for the treatment of breast cancer and potentially for other tumor types such as liver cancers, lung cancers and sarcomas. Due to the risk of monotherapy resistance, PD-0332991 is commonly used in combination with other drugs. Such combination treatments have proved able to inhibit tumor proliferation more effectively, induce stronger senescence and apoptosis, and enhance the efficiency of immunotherapy. Therefore, tumor cells with senescence induced by PD-0332991 are now used as ideal screening tools of cytolytic drugs with more efficient and thorough anti-tumor properties. With more extensive understandings about the branching points between senescence and apoptosis, it is possible to refine the dosage of PD-0332991. Better characterization of resistant cells, of inhibitors and of adverse effects such as leukopenia are needed to overcome obstacles in the use of PD-0332991. In this review of PD-0332991 research, we hope to provide guidance of transitions from laboratory findings to clinical applications of PD-0332991 and to facilitate PD-0332991-based multi-inhibitor combination therapies for various tumors.

Molecular Mechanisms of Anti-Estrogen Therapy Resistance and Novel Targeted Therapies

Breast cancer (BC) is the most commonly diagnosed cancer in women, constituting one-third of all cancers in women, and it is the second leading cause of cancer-related deaths in the United States. Anti-estrogen therapies, such as selective estrogen receptor modulators, significantly improve survival in estrogen receptor-positive (ER+) BC patients, which represents about 70% of cases. However, about 60% of patients inevitably experience intrinsic or acquired resistance to anti-estrogen therapies, representing a major clinical problem that leads to relapse, metastasis, and patient deaths. The resistance mechanisms involve mutations of the direct targets of anti-estrogen therapies, compensatory survival pathways, as well as alterations in the expression of non-coding RNAs (e.g., microRNA) that regulate the activity of survival and signaling pathways. Although cyclin-dependent kinase 4/6 and phosphatidylinositol 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) inhibitors have significantly improved survival, the efficacy of these therapies alone and in combination with anti-estrogen therapy for advanced ER+ BC, are not curative in advanced and metastatic disease. Therefore, understanding the molecular mechanisms causing treatment resistance is critical for developing highly effective therapies and improving patient survival. This review focuses on the key mechanisms that contribute to anti-estrogen therapy resistance and potential new treatment strategies alone and in combination with anti-estrogen drugs to improve the survival of BC patients.

Structure-guided design and development of cyclin-dependent kinase 4/6 inhibitors: A review on therapeutic implications

Cyclin-dependent kinase 6 (EC 2.7.11.22) play significant roles in numerous biological processes and triggers cell cycle events. CDK6 controlled the transcriptional regulation. A dysregulated function of CDK6 is linked with the development of progression of multiple tumor types. Thus, it is considered as an effective drug target for cancer therapy. Based on the direct roles of CDK4/6 in tumor development, numerous inhibitors developed as promising anti-cancer agents. CDK4/6 inhibitors regulate the G1 to S transition by preventing Rb phosphorylation and E2F liberation, showing potent anti-cancer activity in several tumors, including HR+/HER2- breast cancer. CDK4/6 inhibitors such as abemaciclib, palbociclib, and ribociclib, control cell cycle, provoke cell senescence, and induces tumor cell disturbance in pre-clinical studies. Here, we discuss the roles of CDK6 in cancer along with the present status of CDK4/6 inhibitors in cancer therapy. We further discussed, how structural features of CDK4/6 could be implicated in the design and development of potential anti-cancer agents. In addition, the therapeutic potential and limitations of available CDK4/6 inhibitors are described in detail. Recent pre-clinical and clinical information for CDK4/6 inhibitors are highlighted. In addition, combination of CDK4/6 inhibitors with other drugs for the therapeutic management of cancer are discussed.

Targeted Therapy of HPV Positive and Negative Tonsillar Squamous Cell Carcinoma Cell Lines Reveals Synergy between CDK4/6, PI3K and Sometimes FGFR Inhibitors, but Rarely between PARP and WEE1 Inhibitors

Human papillomavirus positive (HPV+) tonsillar and base of tongue squamous cell carcinoma (TSCC/BOTSCC) have a favorable outcome, but upon relapse, survival is poor and new therapeutical options are needed. Recently, we found synergistic effects by combining the food and drug administration approved (FDA) phosphoinositide 3-kinase (PI3K) and fibroblast-growth-factor-receptor (FGFR) inhibitors BYL719 and JNJ-42756493 on TSCC cell lines. Here this approach was extended and Cyclin-Dependent-Kinase-4/6 (CDK4/6) and Poly-ADP-ribose-polymerase (PARP) and WEE1 inhibitors PD-0332991, and MK-1775 respectively were also examined. HPV+ CU-OP-2, -3, -20, and HPV- CU-OP-17 TSCC cell lines were treated with either BYL719 and JNJ-42756493, PD-0332991 BMN-673 and MK-1775 alone or in different combinations. Viability, proliferation, and cytotoxicity were followed by WST-1 assays and the IncuCyte S3 Live® Cell Analysis System. All inhibitors presented dose-dependent inhibitory effects on tested TSCC lines. Synergy was frequently obtained when combining CDK4/6 with PI3K inhibitors, but only sometimes or rarely when combining CDK4/6 with FGFR inhibitors or PARP with WEE1 inhibitors. To conclude, using CDK4/6 with PI3K or FGFR inhibitors, especially PD-0332991 with BYL719 presented synergy and enhanced the decrease of viability considerably, while although dose dependent responses were obtained with PARP and WEE1 inhibitors (BMN-673 and MK-1775 resp.), synergy was rarely disclosed.

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.

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.

CDK4/6 Inhibitors in Combination Therapies: Better in Company Than Alone: A Mini Review

The cyclin D-CDK4/6 complexes play a pivotal role in controlling the cell cycle. Deregulation in cyclin D-CDK4/6 pathway has been described in many types of cancer and it invariably leads to uncontrolled cell proliferation. Many efforts have been made to develop a target therapy able to inhibit CDK4/6 activity. To date, three selective CDK4/6 small inhibitors have been introduced in the clinic for the treatment of hormone positive advanced breast cancer patients, following the impressive results obtained in phase III clinical trials. However, since their approval, clinical evidences have demonstrated that about 30% of breast cancer is intrinsically resistant to CDK4/6 inhibitors and that prolonged treatment eventually leads to acquired resistance in many patients. So, on one hand, clinical and preclinical studies fully support to go beyond breast cancer and expand the use of CDK4/6 inhibitors in other tumor types; on the other hand, the question of primary and secondary resistance has to be taken into account, since it is now very clear that neoplastic cells rapidly develop adaptive strategies under treatment, eventually resulting in disease progression. Resistance mechanisms so far discovered involve both cell-cycle and non-cell-cycle related escape strategies. Full understanding is yet to be achieved but many different pathways that, if targeted, may lead to reversion of the resistant phenotype, have been already elucidated. Here, we aim to summarize the knowledge in this field, focusing on predictive biomarkers, to recognize intrinsically resistant tumors, and therapeutic strategies, to overcome acquired resistance.

Cyclin-dependent kinase 4-related tubular epithelial cell proliferation is regulated by Paired box gene 2 in kidney ischemia-reperfusion injury

Paired box 2 (Pax2) is a transcription factor essential for kidney development and is reactivated in proximal tubular epithelial cells (PTECs) during recovery from kidney injury. However, the role of Pax2 in this process is still unknown. Here the role of Pax2 reactivation during injury was examined in the proliferation of PTECs using an ischemia-reperfusion injury (IRI) mouse model. Kidney proximal tubule-specific Pax2 conditional knockout mice were generated by mating kidney androgen-regulated protein-Cre and Pax2 flox mice. The degree of cell proliferation and fibrosis was assessed and a Pax2 inhibitor (EG1) was used to evaluate the role of Pax2 in the hypoxic condition of cultured PTECs (O₂ 5%, 24 hours). The number of Pax2-positive cells and Pax2 mRNA increased after IRI. Sirius red staining indicated that the area of interstitial fibrosis was significantly larger in knockout mice 14 days after IRI. The number of Ki-67-positive cells (an index of proliferation) was significantly lower in knockout than in wild-type mice after IRI, whereas the number of TUNEL-positive cells (an index of apoptotic cells) was significantly higher in knockout mice four days after IRI. Expression analyses of cell cycle-related genes showed that cyclin-dependent kinase 4 (CDK4) was significantly less expressed in the Pax2 knockout mice. In vitro data showed that the increase in CDK4 mRNA and protein expression induced by hypoxia was attenuated by EG1. Thus, Pax2 reactivation may be involved in PTEC proliferation by activating CDK4, thereby limiting kidney fibrosis.

Cyclin-Dependent Kinase 4 and 6 Inhibitors: A Quantum Leap in the Treatment of Advanced Breast Cancers

Breast cancer (BC) is defined as an uncontrolled growth of breast cells that affected 2.3 million women in 2020 alone. Until a few years earlier, radiotherapy and chemotherapy were the most commonly used treatments in treating BC; however, many trials and studies were conducted to test the competence of cyclin-dependent kinases 4/6 (CDK4/6) in arresting the cell cycle, and it was found that they were highly influential in halting the disease from progressing. Palbociclib, ribociclib, and abemaciclib are the three drugs that have been approved by the US Food and Drug Administration (FDA) and are even more efficient when used in combination with aromatase inhibitors and fulvestrant. This article aimed to explain the effect of CDK4/6 inhibitors on tumor cells and their efficacy in combination with other drugs. We further explored the development of resistance to these treatments and future possibilities.

CDK4/6 inhibitors: mechanisms of resistance and potential biomarkers of responsiveness in breast cancer

Hormone receptor (HR)-positive, HER2-negative tumors represent the most common form of metastatic breast cancer (MBC), and endocrine therapy has been the mainstay treatment for several decades. Recently, a novel drug class called CDK4/6 inhibitors in combination with endocrine therapy have remarkably improved the outcome of patients with HR-positive, HER2-negative MBC by targeting the cell cycle machinery and overcoming aspects of endocrine resistance. Several potential cell-cycle-specific and nonspecific mechanisms of resistance to CDK4/6 inhibitors have been reported in recent studies. This review discusses potential resistance mechanisms to CDK4/6 inhibitors, the use of biomarkers to guide treatment for HR-positive, HER2-negative MBC and possible approaches to overcome resistance to CDK4/6 inhibitors.

The Renaissance of Cyclin Dependent Kinase Inhibitors

Cyclin-dependent kinases (CDK) regulate cell cycle progression. During tumor development, altered expression and availability of CDKs strongly contribute to impaired cell proliferation, a hallmark of cancer. In recent years, targeted inhibition of CDKs has shown considerable therapeutic benefit in a variety of tumor entities. Their success is reflected in clinical approvals of specific CDK4/6 inhibitors for breast cancer. This review provides a detailed insight into the molecular mechanisms of CDKs as well as a general overview of CDK inhibition. It also summarizes the latest research approaches and current advances in the treatment of head and neck cancer with CDK inhibitors. Instead of monotherapies, combination therapies with CDK inhibitors may especially provide promising results in tumor therapy. Indeed, recent studies have shown a synergistic effect of CDK inhibition together with chemo- and radio- and immunotherapy in cancer treatment to overcome tumor evasion, which may lead to a renaissance of CDK inhibitors.

Phase 1b clinical trial of ado-trastuzumab emtansine and ribociclib for HER2-positive metastatic breast cancer

Patients with HER2+ metastatic breast cancer are often treated with a multitude of therapies in the metastatic setting, and additional strategies to prolong responses to anti-HER2 therapies are needed. Preclinical evidence suggests synergy between cyclin-dependent kinase 4 and 6 (CDK 4/6) inhibitors and anti-HER2 therapies. We conducted a phase 1b study of ribociclib and ado-trastuzumab emtansine (T-DM1) in patients with advanced/metastatic HER2-positive breast cancer previously treated with trastuzumab and a taxane in any setting, with four or fewer prior lines of therapy in the metastatic setting. A standard 3 + 3 dose-escalation design was used to evaluate various doses of ribociclib in combination with T-DM1, starting at 300 mg. The primary objective was to determine the maximum tolerated dose and/or recommended phase 2 dose (RP2D) of ribociclib in combination with T-DM1. A total of 12 patients were enrolled. During dose-escalation, patients received doses of ribociclib of 300 mg (n = 3), 400 mg (n = 3), 500 mg (n = 3), and 600 mg (n = 3). No dose-limiting toxicities were observed. The majority of toxicities were Grade 1 and 2, and the most common Grade 3 toxicities were neutropenia (33%), leukopenia (33%), and anemia (25%). After a median follow-up of 12.4 months, the median PFS was 10.4 months (95% confidence interval, 2.7-19.3). Based on the pharmacokinetic analysis, adverse events, and dose reductions, 400 mg was determined to be the RP2D for ribociclib given on days 8-21 of a 21-day cycle with T-DM1.

Inhibition of CDK4/6 as Therapeutic Approach for Ovarian Cancer Patients: Current Evidences and Future Perspectives

Simple Summary

Altered regulation of the cell cycle is a hallmark of cancer. The recent clinical success of the inhibitors of CDK4 and CDK6 has convincingly demonstrated that targeting cell cycle components may represent an effective anti-cancer strategy, at least in some cancer types. However, possible applications of CDK4/6 inhibitors in patients with ovarian cancer is still under evaluation. Here, we describe the possible biological role of CDK4 and CDK6 complexes in ovarian cancer and provide the rationale for the use of CDK4/6 inhibitors in this pathology, alone or in combination with other drugs. This review, coupling basic, preclinical and clinical research studies, could be of great translational value for investigators attempting to design new clinical trials for the better management of ovarian cancer patients.

Abstract

Alterations in components of the cell-cycle machinery are present in essentially all tumor types. In particular, molecular alterations resulting in dysregulation of the G1 to S phase transition have been observed in almost all human tumors, including ovarian cancer. These alterations have been identified as potential therapeutic targets in several cancer types, thereby stimulating the development of small molecule inhibitors of the cyclin dependent kinases. Among these, CDK4 and CDK6 inhibitors confirmed in clinical trials that CDKs might indeed represent valid therapeutic targets in, at least some, types of cancer. CDK4 and CDK6 inhibitors are now used in clinic for the treatment of patients with estrogen receptor positive metastatic breast cancer and their clinical use is being tested in many other cancer types, alone or in combination with other agents. Here, we review the role of CDK4 and CDK6 complexes in ovarian cancer and propose the possible use of their inhibitors in the treatment of ovarian cancer patients with different types and stages of disease.

Selective cyclin-dependent kinase 4/6 inhibitors as anticancer drugs: Moving beyond hormone receptor-positive breast cancer

The cyclin D-cyclin-dependent kinase (CDK) 4/6 pathway controls the cell cycle machinery by regulating the G1-to-S-phase transition. Dysregulation of this pathway, resulting in increased cellular proliferation, is frequently observed in a variety of human cancers. Activation of cyclin D-CDK 4/6 pathway can occur through different mechanisms, including gene amplification/rearrangement, loss of negative regulatory factors, epigenetic modifications, and point mutations of different components of this pathway. Quite conspicuously, CDK 4/6 inhibitors have emerged as promising anticancer agents in various tumors in which CDK 4/6 has a pivotal role in the G1-to-S-phase cell cycle transition. The clinical use of first-generation, nonselective pan-CDK inhibitors was not progressed beyond early phase trials, due to unacceptable toxicity and lack of efficacy noted with these agents. The emergence of selective CDK 4/6 inhibitors, including ribociclib, abemaciclib, and palbociclib, has enabled us to effectively target cyclin D-CDK 4/6 pathway, at the cost of acceptable toxicity. The results of landmark phase III trials investigating palbociclib and ribociclib in advanced hormone receptor (HR)-positive breast cancer have demonstrated a substantial clinical benefit with a well-tolerated toxicity profile. Mechanisms of acquired resistance to selective CDK 4/6 inhibitors are beginning to emerge. Clearly, a detailed understanding of these resistance mechanisms is very much essential for the rational development of post-CDK 4/6 inhibitor therapeutic strategies. Extending the use of selective CDK 4/6 inhibitors beyond HR-positive breast cancer is a challenging task and will likely require identification of clinically meaningful biomarkers to predict response and the use of combination approaches to optimize CDK 4/6 targeting.

Epithelial proliferation and cell cycle dysregulation in kidney injury and disease

Various cellular insults and injury to renal epithelial cells stimulate repair mechanisms to adapt and restore the organ homeostasis. Renal tubular epithelial cells are endowed with regenerative capacity, which allows for a restoration of nephron function after acute kidney injury. However, recent evidence indicates that the repair is often incomplete, leading to maladaptive responses that promote the progression to chronic kidney disease. The dysregulated cell cycle and proliferation is also a key feature of renal tubular epithelial cells in polycystic kidney disease and HIV-associated nephropathy. Therefore, in this review, we provide an overview of cell cycle regulation and the consequences of dysregulated cell proliferation in acute kidney injury, polycystic kidney disease, and HIV-associated nephropathy. An increased understanding of these processes may help define better targets for kidney repair and combat chronic kidney disease progression.

Advances in Rodent Models for Breast Cancer Formation, Progression, and Therapeutic Testing

Breast cancer is a highly complicated disease. Advancement in the treatment and prevention of breast cancer lies in elucidation of the mechanism of carcinogenesis and progression. Rodent models of breast cancer have developed into premier tools for investigating the mechanisms and genetic pathways in breast cancer progression and metastasis and for developing and evaluating clinical therapeutics. Every rodent model has advantages and disadvantages, and the selection of appropriate rodent models with which to investigate breast cancer is a key decision in research. Design of a suitable rodent model for a specific research purpose is based on the integration of the advantages and disadvantages of different models. Our purpose in writing this review is to elaborate on various rodent models for breast cancer formation, progression, and therapeutic testing.

In vitro to Clinical Translation of Combinatorial Effects of Doxorubicin and Abemaciclib in Rb-Positive Triple Negative Breast Cancer: A Systems-Based Pharmacokinetic/Pharmacodynamic Modeling Approach

Background

Doxorubicin (DOX) and its pegylated liposomal formulation (L_DOX) are the standard of care for triple-negative breast cancer (TNBC). However, resistance to DOX often occurs, motivating the search for alternative treatment approaches. The retinoblastoma protein (Rb) is a potential pharmacological target for TNBC treatment since its expression has been associated with resistance to DOX-based therapy.

Methods

DOX (0.01–20 μM) combination with abemaciclib (ABE, 1–6 μM) was evaluated over 72 hours on Rb-positive (MDA-MB-231) and Rb-negative (MDA-MB-468) TNBC cells. Combination indices (CI) for DOX+ABE were calculated using Compusyn software. The TNBC cell viability time-course and fold-change from the control of phosphorylated-Rb (pRb) protein expression were measured with CCK8-kit and enzyme-linked immunosorbent assay. A cell-based pharmacodynamic (PD) model was developed, where pRb protein dynamics drove cell viability response. Clinical pharmacokinetic (PK) models for DOX, L_DOX, and ABE were developed using data extracted from the literature. After scaling cancer cell growth to clinical TNBC tumor growth, the time-to-tumor progression (TTP) was predicted for human dosing regimens of DOX, ABE, and DOX+ABE.

Results

DOX and ABE combinations were synergistic (CI<1) in MDA-MB-231 and antagonistic (CI>1) in MDA-MB-468. The maximum inhibitory effects (Imax) for both drugs were set to one. The drug concentrations producing 50% of Imax for DOX and ABE were 0.565 and 2.31 μM (MDA-MB-231) and 0.121 and 1.61 μM (MDA-MB-468). The first-orders rate constants of abemaciclib absorption (k_a) and doxorubicin release from L_DOX (k_Rel) were estimated at 0.31 and 0.013 h⁻¹. Their linear clearances were 21.7 (ABE) and 32.1 L/h (DOX). The estimated TTP for intravenous DOX (75 mg/m2 every 21 days), intravenous L_DOX (50 mg/m² every 28 days), and oral ABE (200 mg twice a day) were 125, 31.2, and 8.6 days shorter than drug-free control. The TTP for DOX+ABE and L_DOX+ABE were 312 days and 47.5 days shorter than control, both larger than single-agent DOX, suggesting improved activity with the DOX+ABE combination.

Conclusion

The developed translational systems-based PK/PD model provides an in vitro-to-clinic modeling platform for DOX+ABE in TNBC. Although model-based simulations suggest improved outcomes with combination over monotherapy, tumor relapse was not prevented with the combination. Hence, DOX+ABE may not be an effective treatment combination for TNBC.

Inhibiting CDK4/6 in Breast Cancer with Palbociclib, Ribociclib, and Abemaciclib: Similarities and Differences

The cyclin-dependent kinase (CDK) 4/6 inhibitors belong to a new class of drugs that interrupt proliferation of malignant cells by inhibiting progression through the cell cycle. Three such inhibitors, palbociclib, ribociclib, and abemaciclib were recently approved for breast cancer treatment in various settings and combination regimens. On the basis of their impressive efficacy, all three CDK4/6 inhibitors now play an important role in the treatment of patients with HR+, HER2- breast cancer; however, their optimal use still needs to be established. The three drugs have many similarities in both pharmacokinetics and pharmacodynamics. However, there are some differences on the basis of which the choice for a particular CDK4/6 inhibitor for an individual patient can be important. In this article, the clinical pharmacokinetic and pharmacodynamic profiles of the three CDK4/6 inhibitors are reviewed and important future directions of the clinical applicability of CDK4/6 inhibitors will be discussed.

Preclinical discovery and development of abemaciclib used to treat breast cancer

Introduction: Cyclin-dependent kinase (CDK) 4/6 inhibitors have altered the standard-of-care treatment for patients with ER-positive, HER2-negative metastatic breast cancer. One such inhibitor, abemaciclib, a reversible ATP-competitive CDK4/6 inhibitor developed by Eli Lilly and Company, was approved by the FDA for ER-positive, HER2-negative metastatic breast cancer.Areas covered: Preclinical studies revealed abemaciclib's distinct structure, efficacy as monotherapy, and ability to penetrate the Central Nervous System. In this review, the authors have examined the literature regarding the development of CDK 4/6 inhibitors before providing a focused review on the preclinical discovery and development of abemaciclib. The authors then conclude their manuscript by providing their expert opinion and future perspectives.Expert opinion: Understanding the genesis and evolution from concept to approval and beyond will allow one to analyze the impact of abemaciclib. With its unique characteristics, abemaciclib has provided a meaningful addition to the therapeutic arsenal for metastatic breast cancer. There is, however, a need for predictive biomarkers to identify patients who may not benefit from or may develop resistance to CDK4/6 inhibition.

Mechanisms of resistance to cyclin-dependent kinase 4/6 inhibitors

Cyclin-dependent kinase (CDK) 4/6 inhibitors have emerged in the treatment of metastatic hormone receptor (HR)-positive and human epidermal growth factor receptor 2 (HER2)-negative breast cancer. However, most patients will eventually present disease progression, highlighting the inevitable resistance of cancer cells to CDK4/6 inhibition. Several studies have suggested that resistance mechanisms involve aberrations of the molecules that regulate the cell cycle, and the re-wiring of the cell to escape CDK4/6 dependence and turn to alternative pathways. Loss of retinoblastoma function, overexpression of CDK 6, upregulation of cyclin E, overexpression of CDK 7, and dysregulation of several signaling pathways, notably the PI3/AKT/mTOR pathway, have been implicated in the development of resistance to CDK4/6 inhibitors. Overlap with endocrine resistance mechanisms might be possible. Combinational therapeutic strategies should be explored in order to prevent resistance and optimize the management of patients after progression under CDK 4/6 inhibition.

Selective CDK4/6 Inhibitors: Biologic Outcomes, Determinants of Sensitivity, Mechanisms of Resistance, Combinatorial Approaches, and Pharmacodynamic Biomarkers

CDK4/6 inhibitors are now part of the standard armamentarium for hormone receptor-positive breast cancer. In this article, we review the biologic outcomes imposed by these drugs on cancer cells, determinants of response, mechanisms of intrinsic and acquired resistance, as well as combinatorial approaches emanating from mechanistic studies that may allow use of these agents to extend beyond breast cancer. In addition, we will address tumor-, imaging-, and blood-based pharmacodynamic biomarkers that can inform rationally designed trials as clinical development continues.

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

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

Synthesis and Clinical Development of Palbociclib: An overview

Breast cancer is the second most commonly identified cancer in women in the United States after skin cancer. The past few years have seen a substantial increase in breast cancer awareness campaigns and active research in fields of diagnosis and targeted therapy. These factors have led to a better mechanistic understanding of the disease, detection at earlier stages and more personalized approach to treatment, ultimately causing a crucial increase in the survival rates after detection. However, with the advances in treatment, cases of patients developing primary resistance and acquired resistance are increasing. Most of the breast cancers which develop resistance to therapy are ER+ and are typically treated with tamoxifen and fulvestrant. These drugs either lower the levels of estrogen or inhibit the receptors for estrogen and prevent the tumor from spreading. Around one third of women treated with these drugs develop resistance to them, lowering their chances of survival. This has directed to the search of newer drug therapies to target advanced breast cancer and resistance. One of these efforts has resulted in the development of Palbociclib, a first in class inhibitor of cyclin dependent kinases 4 and 6 (CDK4 and CDK6), which was granted accelerated approval from FDA for combination therapy in postmenopausal women with ER+, HER2- metastatic breast cancer. This review is focused on the various aspects of "Palbociclib" including its synthesis, molecular modeling studies and efficacy and safety profile with clinical trials data.

Novel sequential treatment with palbociclib enhances the effect of cisplatin in RB-proficient triple-negative breast cancer

Background

Triple-negative breast cancer (TNBC) is a highly aggressive malignancy that lacks sensitivity to chemotherapy, endocrine therapy or targeted therapy. CDK4/6 inhibitors, combined with endocrine therapy, have been shown to be effective in postmenopausal women with HR-positive, HER2-negative advanced or metastatic breast cancer. Therefore, we investigated whether the CDK4/6 inhibitor palbociclib (PD) could enhance the effects of cisplatin (CDDP) on TNBC.

Methods

The effects of different drug regimens consisting of PD and CDDP on MDA-MB-231 and RB-knockdown MDA-MB-231 (sh-MDA-MB-231) cells were assessed in vitro and in vivo. MDA-MB-468 and RB-overexpressing MDA-MB-468 cells were used to assess the effect of the PD-CDDP regimens in vitro. Immunoblotting illustrated the role of the cyclin D1/RB/E2F axis signalling pathway.

Results

PD induced G1 phase cell cycle arrest in the MDA-MB-231 cell line. However, synchronous treatment with PD and CDDP for 24 h, treatment with PD for 24 h followed by CDDP and treatment with CDDP for 24 h followed by PD had no influence on MDA-MB-231 cell apoptosis. We further investigated the effect of PD or CDDP withdrawal on the effects of sequential treatment and found that PD treatment for 48 h followed by withdrawal for 48 h and subsequent CDDP treatment (PD-CDDP) significantly increased apoptosis and inhibited the cell viability and colony formation of MDA-MB-231 cells, while with other regimens, PD and CDDP had an additive or antagonistic response. The preferential use of PD increased DNA damage induced by CDDP, as measured through γH2AX immunofluorescence. These findings were not observed in sh-MDA-MB-231 cells, and experiments to assess cell function in MDA-MB-468 and RB-overexpressing MDA-MB-468 cells yielded similar results, which indicated that PD enhanced the sensitivity of TNBC cells to CDDP in an RB-dependent manner. In vivo, compared with single drug treatment, combination treatment inhibited tumour growth and Ki-67 expression in MDA-MB-231 xenograft models. Western blot analysis revealed that PD enhanced sensitivity to CDDP through the CDK4/6-cyclin D1-RB-E2F pathway.

Conclusions

Pre-treatment with PD synchronized the tumour cell cycle through the CDK4/6-cyclin D1-RB-E2F pathway, which increased the antitumour effect of CDDP. Thus, PD-CDDP might be an effective treatment for RB-proficient TNBC patients.

Synchronization of human retinal pigment epithelial-1 cells in mitosis

Human retinal pigment epithelial-1 (RPE-1) cells are increasingly being used as a model to study mitosis because they represent a non-transformed alternative to cancer cell lines, such as HeLa cervical adenocarcinoma cells. However, the lack of an efficient method to synchronize RPE-1 cells in mitosis precludes their application for large-scale biochemical and proteomics assays. Here, we report a protocol to synchronize RPE-1 cells based on sequential treatments with the Cdk4 and Cdk6 inhibitor PD 0332991 (palbociclib) and the microtubule-depolymerizing drug nocodazole. With this method, the vast majority (80-90%) of RPE-1 cells arrested at prometaphase and exited mitosis synchronously after release from nocodazole. Moreover, the cells fully recovered and re-entered the cell cycle after the palbociclib-nocodazole block. Finally, we show that this protocol could be successfully employed for the characterization of the protein-protein interaction network of the kinetochore protein Ndc80 by immunoprecipitation coupled with mass spectrometry. This synchronization method significantly expands the versatility and applicability of RPE-1 cells to the study of cell division and might be applied to other cell lines that do not respond to treatments with DNA synthesis inhibitors.

Targeting Cell Cycle in Breast Cancer: CDK4/6 Inhibitors

Deregulation of cell cycle, via cyclin D/CDK/pRb pathway, is frequently observed in breast cancer lending support to the development of drugs targeting the cell cycle control machinery, like the inhibitors of the cycline-dependent kinases (CDK) 4 and 6. Up to now, three CDK4/6 inhibitors have been approved by FDA for the treatment of hormone receptor-positive (HR+), HER2-negative metastatic breast cancer. These agents have been effective in improving the clinical outcomes, but the development of intrinsic or acquired resistance can limit the efficacy of these treatments. Clinical and translational research is now focused on investigation of the mechanism of sensitivity/resistance to CDK4/6 inhibition and novel therapeutic strategies aimed to improve clinical outcomes. This review summarizes the available knowledge regarding CDK4/6 inhibitor, the discovery of new biomarkers of response, and the biological rationale for new combination strategies of treatment.

CDK4/6 Inhibitors in Breast Cancer Treatment: Potential Interactions with Drug, Gene, and Pathophysiological Conditions

Palbociclib, ribociclib, and abemaciclib belong to the third generation of cyclin-dependent kinases inhibitors (CDKis), an established therapeutic class for advanced and metastatic breast cancer. Interindividual variability in the therapeutic response of CDKis has been reported and some individuals may experience increased and unexpected toxicity. This narrative review aims at identifying the factors potentially concurring at this variability for driving the most appropriate and tailored use of CDKis in the clinic. Specifically, concomitant medications, pharmacogenetic profile, and pathophysiological conditions could influence absorption, distribution, metabolism, and elimination pharmacokinetics. A personalized therapeutic approach taking into consideration all factors potentially contributing to an altered pharmacokinetic/pharmacodynamic profile could better drive safe and effective clinical use.

MAPRE1 promotes cell cycle progression of hepatocellular carcinoma cells by interacting with CDK2

Targeting cyclin-dependent kinases (CDKs) is a promising method of therapy for cancer. Unfortunately, the efficacy of CDK inhibitors in hepatocellular carcinoma (HCC) is limited, due in part to incomplete understanding of cell cycle progression and a lack of specific biomarkers to adequately identify which patients may be responsive to CDK inhibitors. In the present study, we report that microtubule-associated protein RP/EB family member 1 (MAPRE1), a gene involved in cell cycle and microtubule regulation, is significantly increased in HCC tissue, promotes HCC cell proliferation, enhances in vitro tumorigenesis, and associates with poor prognosis of HCC. We demonstrate that MAPRE1 binds with CDK2, resulting in the hyperphosphorylation of the CDK2 Thr161 residue in HCC cells. Our findings reveal that targeting MAPRE1 might be an effective therapeutic strategy in HCC, and suggest that MAPRE1 expression might provide a promising biomarker to stratify patients with HCC who may benefit from treatment with CDK inhibitors.

Chemotherapy and CDK4/6 Inhibitors: Unexpected Bedfellows

Cyclin-dependent kinases 4 and 6 (CDK4/6) have emerged as important therapeutic targets. Pharmacologic inhibitors of these kinases function to inhibit cell-cycle progression and exert other important effects on the tumor and host environment. Because of their impact on the cell cycle, CDK4/6 inhibitors (CDK4/6i) have been hypothesized to antagonize the antitumor effects of cytotoxic chemotherapy in tumors that are CDK4/6 dependent. However, there are multiple preclinical studies that illustrate potent cooperation between CDK4/6i and chemotherapy. Furthermore, the combination of CDK4/6i and chemotherapy is being tested in clinical trials to both enhance antitumor efficacy and limit toxicity. Exploitation of the noncanonical effects of CDK4/6i could also provide an impetus for future studies in combination with chemotherapy. Thus, while seemingly mutually exclusive mechanisms are at play, the combination of CDK4/6 inhibition and chemotherapy could exemplify rational medicine.

Sensitivity to differential NRF1 gene signatures contributes to breast cancer disparities

PURPOSE

Nuclear respiratory factor 1 (NRF1) drives estrogen-dependent breast tumorigenesis. Herein we examined the impact of NRF1 activity on the aggressiveness and disparate molecular signature of breast cancer in Black, White, Asian, and Hispanic women.

METHODS

NRF1 activity by transcription factor target enrichment analysis and causal NRF1-target gene signatures by Bayesian Network Inference with Java Objects (BANJO) and Markov Chain Monte Carlo (MCMC)-based gene order were examined in The Cancer Genome Atlas (TCGA) breast cancer cohorts.

RESULTS

We are the first to report increased NRF1 activity based on its differential effects on genome-wide transcription associated with luminal A and B, HER2+ and triple-negative (TN) molecular subtypes of breast cancer in women of different race/ethnicity. We observed disparate NRF1 motif-containing causal gene signatures unique to Black, White, Asian, and Hispanic women for luminal A breast cancer. Further gene order searches showed molecular heterogeneity of each subtype of breast cancer. Six different gene order sequences involving CDK1, HMMR, CCNB2, CCNB1, E2F1, CREB3L4, GTSE1, and LMNB1 with almost equal weight predicted the probability of luminal A breast cancer in whites. Three different gene order sequences consisting of CCNB1 and GTSE1, and CCNB1, LMNB1, CDK1 or CASP3 predicted almost 100% probability of luminal B breast cancer in whites; CCNB1 and LMNB1 or GTSE predicted 100% HER2+ breast cancer in whites. GTSE1 and TUBA1C combined together predicted 100% probability of developing TNBC in whites; NRF1, TUBA1B and BAX with EFNA4, and NRF1 and BTRC predicated 100% TNBC in blacks. High expressor NRF1 TN breast tumors showed unfavorable prognosis with a high risk of breast cancer death in white women.

CONCLUSION

Our findings showed how sensitivity to high NRF1 transcriptional activity coupled with its target gene signatures contribute to racial differences in luminal A and TN breast cancer subtypes. This knowledge may be useful in personalized intervention to prevent and treat this clinically challenging problem.

Senescence as a therapeutically relevant response to CDK4/6 inhibitors

Cyclin-dependent kinases 4 and 6 (CDK4/6) phosphorylate and inhibit retinoblastoma (RB) family proteins. Hyperphosphorylated RB releases E2F transcription factors, activating a transcriptional program that initiates S phase. Due to the critical role that this pathway has in regulating cell cycle progression, inhibiting CDK4/6 is an attractive therapeutic strategy. Indeed, CDK4/6 inhibitors in combination with antiestrogens produce a significant benefit in patients with ER+/HER2- breast cancer. Clinical trials are currently investigating if the use of CDK4/6 inhibitors alone or in combination can be extended to other cancer types. Inhibition of CDK4/6 can result in different cell fates such as quiescence, senescence, or apoptosis. Senescence is a stress response that can be induced by stimuli that include oncogenic activation, chemotherapy, irradiation, and targeted therapies such as CDK4/6 inhibitors. Senescent cells undergo a stable cell cycle arrest and produce a bioactive secretome that remodels their microenvironment and engages the immune system. In this review, we analyze the therapeutic relevance of senescence induction by CDK4/6 inhibitors. We also discuss how different therapies, including checkpoint inhibitors and drugs targeting MEK or PI3K, can be used in combination with CDK4/6 inhibitors to reinforce or exploit senescence. Recently, a lot of effort has been put into identifying compounds that selectively kill senescent cells (termed senolytics). Thus, sequential treatment with senolytics might be an additional strategy to potentiate the antitumor effects of CDK4/6 inhibitors.

Combination of cyclin-dependent kinase and immune checkpoint inhibitors for the treatment of bladder cancer

BACKGROUND

Perturbation of the CDK4/6 pathway is frequently observed in advanced bladder cancer. We investigated the potential of targeting this pathway alone or in combination with chemotherapy or immunotherapy as a therapeutic approach for the treatment of bladder cancer METHODS: The genetic alterations of the CDK4/6 pathway in bladder cancer were first analyzed with The Cancer Genome Atlas database and validated in our bladder cancer patient-derived tumor xenografts (PDXs). Bladder cancer cell lines and mice carrying PDXs with the CDK4/6 pathway perturbations were treated with a CDK4/6 inhibitor palbociclib to determine its anticancer activity and the underlying mechanisms. The combination index method was performed to assess palbociclib and gemcitabine drug-drug interactions. Syngeneic mouse bladder cancer model BBN963 was used to assess whether palbociclib could potentiate anti-PD1 immunotherapy.

RESULTS

Of the 413 bladder cancer specimens, 79.2% harbored pertubations along the CDK4/6 pathway. Palbociclib induced G0/G1 cell cycle arrest but with minimal apoptosis in vitro. In mice carrying PDXs, palbociclib treatment reduced tumor growth and prolonged survival from 14 to 32 days compared to vehicle only controls (p = 0.0001). Palbociclib treatment was associated with a decrease in Rb phosphorylation in both cell lines and PDXs. Palbociclib and gemcitabine exhibited antagonistic cytotoxicity in vitro (CI > 3) and in vivo, but palbociclib significantly enhanced the treatment efficacy of anti-PD1 immunotherapy and induced CD8+ T lymphocyte infiltration in syngeneic mouse models.

CONCLUSIONS

The CDK4/6 pathway is feasible as a potential target for the treatment of bladder cancer, especially in combination with immunotherapy. A CDK4/6 inhibitor should not be combined with gemcitabine.

MicroRNA-Mediated Suppression of the TGF-β Pathway Confers Transmissible and Reversible CDK4/6 Inhibitor Resistance

CDK4/6 inhibition is now part of the standard armamentarium for patients with estrogen receptorpositive (ER⁺) breast cancer, so that defining mechanisms of resistance is a pressing issue. Here, we identify increased CDK6 expression as a key determinant of acquired resistance after palbociclib treatment in ER⁺ breast cancer cells. CDK6 expression is critical for cellular survival during palbociclib exposure. The increased CDK6 expression observed in resistant cells is dependent on TGF-b pathway suppression via miR-432-5p expression. Exosomal miR-432-5p expression mediates the transfer of the resistance phenotype between neighboring cell populations. Levels of miR-432-5p are higher in primary breast cancers demonstrating CDK4/6 resistance compared to those that are sensitive. These data are Furthermore confirmed in pre-treatment and post-progression biopsies from a parotid cancer patient who had responded to ribociclib, demonstrating the clinical relevance of this mechanism. Finally, the CDK4/6 inhibitor resistance phenotype is reversible in vitro and in vivo by a prolonged drug holiday.

Cornell et al. demonstrate a mechanism of acquired CDK4/6 inhibitor resistance that is independent of inherent genetic mutations, is conferred through extracellular signaling, and is reversible in vitro and in vivo. Resistance was mediated by exosomal miRNA, causing increased expression of CDK6 to overcome G1 arrest and promote cell survival.

Efficacy and Safety of Cyclin-Dependent Kinases 4 and 6 Inhibitors in HR+/HER2- Advanced Breast Cancer

Purpose

To assess the efficacy and safety of cyclin-dependent kinases 4 and 6 inhibitors (CDKi) combined with endocrine therapy (ET) in women with hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer (ABC) and compare the efficacy of different CDKi (palbociclib, ribociclib, or abemaciclib).

Materials and Methods

This study based on randomized Phase 2 or 3 trials of CDKi plus ET compared with placebo plus ET for women with HR+/HER2−ABC and identify relevant randomized clinical trials (RCTs) published prior to February 2020. The primary endpoint was progression-free survival (PFS), the secondary endpoints included overall survival (OS), objective response rate (ORR), clinical benefit response (CBR) and safety. The PROSPERO registry number is 42018081105.

Results

The results from eight trials including 4580 participants were pooled. Evidence indicated that the PFS of CDKi group was significantly prolonged (hazard ratio [HR] 0.55, 95% confidence interval [CI] 0.50–0.60, P < 0.01) compared with placebo group. The ORR and CBR were better (risk ratio [RR] 1.47, 95% CI 1.30–1.67, P < 0.01; 1.24, 95% CI 1.15–1.35, P < 0.01) in the CDKi group. The OS of CDKi group (HR 0.75, 95% CI 0.67–0.85, P < 0.01) was significantly longer than ET alone. Subgroup analyses confirmed that the benefit was consistent across most subgroups. Subgroup analyses showed no statistically significant difference of PFS among three CDKi: palbociclib vs ribociclib (HR 0.55, 95% CI 0.49–0.60, P = 0.34), palbociclib vs abemaciclib (HR 0.53, 95% CI, 0.47–0.59, P = 0.61), and ribociclib vs abemaciclib (HR 0.56, 95% CI, 0.51–0.62, P = 0.72). Treatment-related grade 3 or 4 hematologic adverse events (AEs) were more frequently in CDKi group.

Conclusion

CDKi combined with ET can significantly prolong PFS and improve the ORR, CBR and OS in patients with HR+/HER2− ABC. However, the advantage of different CDKi has not been established.

The application and prospect of CDK4/6 inhibitors in malignant solid tumors

Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors, which block the transition from the G1 to S phase of the cell cycle by interfering with Rb phosphorylation and E2F release, have shown potent antitumor activity and manageable toxicity in HR+/HER2− breast cancer patients. Some clinical trials involving CDK4/6 inhibitors in other tumors have achieved preliminary impressive efficacy. Whether CDK4/6 inhibitors possess great potential as broad-spectrum antitumor drugs and how to maximize their clinical benefits remain uncertain. TCGA database analysis showed that CDK4/6 genes and related genes are widely expressed among various tumors, and high or moderate expression of CDK4/6 genes commonly indicates poor survival. CDK4/6 gene expression is significantly higher in COAD, ESCA, STAD, LIHC, and HNSC, suggesting that CDK4/6 inhibitors could be more efficacious in those tumors. Moreover, network analysis with the STRING database demonstrated that CDK4/6-related proteins were co-expressed or co-occurred with the classical tumor signaling pathways, such as the cell cycle pathway, RAS pathway, PI3K pathway, Myc pathway, and p53 pathway. The extensive antitumor effects of CDK4/6 inhibitors may be achieved by synergizing or antagonizing with other signaling molecule inhibitors, and combination therapy might be the most effective treatment strategy. This article analyzed the feasibility of expanding the application of CDK4/6 inhibitors at the genetic level and further summarized the associated clinical/preclinical studies to collect supportive evidence. This is the first study that presents a theoretical foundation for CDK4/6 inhibitor precision therapy via combined analysis of comprehensive gene information and clinical research results.

The Pharmacological Profile of Cyclin-dependent Kinase (CDK) 4/6 Inhibitors: Clinical Management of Toxicity and Drug Interactions Related to CDK 4/6 Inhibitor-based Treatment in Advanced/Metastatic Breast Cancer

The emergence of cyclin-dependent kinase (CDK) 4 and 6 inhibitors has brought a new approach in the treatment of advanced hormone receptor (HR) positive breast cancer and human epidermal growth factor (HER) 2 negative breast cancer. To date, three CDK 4/6 inhibitors, palbociclib, ribociclib, and abemaciclib, are approved by the Food and Drug Administration (FDA); the first two agents are approved by the European Medicines Agency (EMA) as well. The family of CDKs consists of key regulatory enzymes that play a significant role in cell cycle progression. The aim of this review is to give an overview of the mechanism of action and the efficacy of CDK4/6 inhibitors and to highlight the most serious adverse events and the drug interactions related to these agents.

Cyclin D-CDK4/6 functions in cancer

The mammalian cell cycle is driven by a complex of cyclins and their associated cyclin-dependent kinases (CDKs). Abnormal dysregulation of cyclin-CDK is a hallmark of cancer. D-type cyclins and their associated CDKs (CDK4 and CDK6) are key components of cell cycle machinery in driving G1 to S phase transition via phosphorylating and inactivating the retinoblastoma protein (RB). A body of evidence shows that the cyclin Ds-CDKs axis plays a critical role in cancer through various aspects, such as control of proliferation, senescence, migration, apoptosis, and angiogenesis. CDK4/6 dual-inhibitors show significant efficacy in pre-clinical or clinical cancer therapies either as single agents or in combination with hormone, chemotherapy, irradiation or immune treatments. Of note, as the associated partner of D-type cyclins, CDK6 shows multiple distinct functions from CDK4 in cancer. Depletion of the individual CDK may provide a therapeutic strategy for patients with cancer.

CDK4/6 Inhibitors Impair Recovery from Cytotoxic Chemotherapy in Pancreatic Adenocarcinoma

Inhibition of the cell-cycle kinases CDK4 and CDK6 is now part of the standard treatment in advanced breast cancer. CDK4/6 inhibitors, however, are not expected to cooperate with DNA-damaging or antimitotic chemotherapies as the former prevent cell-cycle entry, thus interfering with S-phase- or mitosis-targeting agents. Here, we report that sequential administration of CDK4/6 inhibitors after taxanes cooperates to prevent cellular proliferation in pancreatic ductal adenocarcinoma (PDAC) cells, patient-derived xenografts, and genetically engineered mice with Kras G12V and Cdkn2a-null mutations frequently observed in PDAC. This effect correlates with the repressive activity of CDK4/6 inhibitors on homologous recombination proteins required for the recovery from chromosomal damage. CDK4/6 inhibitors also prevent recovery from multiple DNA-damaging agents, suggesting broad applicability for their sequential administration after available chemotherapeutic agents.