Effect of CDK4/6 Inhibitors on Tumor Immune Microenvironment.

Cancer is an abnormal proliferation of cells that is stimulated by cyclin-dependent kinases (CDKs) and defective cell cycle regulation. The essential agent that drive the cell cycle, CDK4/6, would be activated by proliferative signals. Activated CDK4/6 results in the phosphorylation of the neuroblastoma protein (RB) and the release of the transcription factor E2F, which promotes the cell cycle progression. CDK4/6 inhibitor (CDK4/6i) has been currently a research focus, which inhibits the CDK4/6-RB-E2F axis, thereby reducing the cell cycle transition from G1 to S phase and mediating the cell cycle arrest. This action helps achieve an anti-tumor effect. Recent research has demonstrated that CDK4/6i, in addition to contributing to cell cycle arrest, is also essential for the interaction between the tumor cells and the host immune system, i.e., activating the immune system, strengthening the tumor antigen presentation, and reducing the number of regulatory T cells (Treg). Additionally, CDK4/6i would elevate the level of PD-L1, an immunosuppressive factor, in tumor cells, and CDK4/6i in combination with anti-PD-L1 therapy would more effectively reduce the tumor growth. Our results showed that CDK4/6i caused autophagy and senescence in tumor cells. Herein, the impact of CDK4/6i on the immune microenvironment of malignant tumors was mainly focused, as well as their interaction with immune checkpoint inhibitors in affecting anti-tumor immunity.

Flow cytometry-assisted quantification of cell cycle arrest in cancer cells treated with CDK4/6 inhibitors.

Cyclin-dependent kinase 4 (CDK4) and CDK6 inhibitors (i.e., palbociclib, abemaciclib, and ribociclib) are well known for their capacity to mediate cytostatic effects by promoting cell cycle arrest in the G1 phase, thus inhibiting cancer cell proliferation. Cytostatic effects induced by CDK4/6 inhibitors can be transient or lead to a permanent state of cell cycle arrest, commonly defined as cellular senescence. Induction of senescence is often associated to metabolic modifications and to the acquisition of a senescence-associated secretory phenotype (SASP) by cancer cells, which in turn can promote or limit antitumor immunity (and thus the efficacy of CDK4/6 inhibitors) depending on SASP components. Thus, although accumulating evidence suggests that anti-cancer effects of CDK4/6 inhibitors also depend on the promotion of antitumor immune responses, assessing cell cycle arrest and progression in cells treated with palbociclib remains a key approach for investigating the efficacy of CDK4/6 inhibitors. Here, we describe a method to assess cell cycle distribution simultaneously with active DNA replication by flow cytometry in cultured hormone receptor-positive breast cancer MCF7 cells.

The CDK4/6 inhibitor revolution - a game-changing era for breast cancer treatment.

Cyclin-dependent kinase (CDK) 4/6 inhibition in combination with endocrine therapy is the standard-of-care treatment for patients with advanced-stage hormone receptor-positive, HER2 non-amplified (HR+HER2-) breast cancer. These agents can also be administered as adjuvant therapy to patients with higher-risk early stage disease. Nonetheless, the clinical success of these agents has created several challenges, such as how to address acquired resistance, identifying which patients are most likely to benefit from therapy prior to treatment, and understanding the optimal timing of administration and sequencing of these agents. In this Review, we describe the rationale for targeting CDK4/6 in patients with breast cancer, including a summary of updated clinical evidence and how this should inform clinical practice. We also discuss ongoing research efforts that are attempting to address the various challenges created by the widespread implementation of these agents.

Design and Synthesis of New bis-oxindole and Spiro(triazole-oxindole) as CDK4 Inhibitors with Potent Anti-breast Cancer Activity.

BACKGROUND: Since CDKs have been demonstrated to be overexpressed in a wide spectrum of human malignancies, their inhibition has been cited as an effective technique for anticancer drug development. METHODS: In this context, new bis-oxindole/spiro-triazole-oxindole anti-breast cancer drugs with potential CDK4 inhibitory effects were produced in this work. The novel series of bis-oxindole/spirotriazole- oxindole were synthesized from the reaction of bis-oxindole with the aniline derivatives then followed by 1,3-dipolar cycloaddition of hydrazonoyl chloride. RESULTS: The structure of these bis-oxindole/spiro-triazole-oxindole series was proven based on their spectral analyses. Most bis-oxindole and bis-spiro-triazole-oxindole compounds effectively inhibited the growth of MCF-7 (IC50 = 2.81-17.61 µM) and MDA-MB-231 (IC50 = 3.23-7.98 µM) breast cancer cell lines with low inhibitory activity against normal WI-38 cells. While the reference doxorubicin showed IC50 values of 7.43 µM against MCF-7 and 5.71 µM against the MDA-MB-231 cell line. Additionally, compounds 3b, 3c, 6b, and 6d revealed significant anti-CDK4 activity (IC50 = 0.157- 0.618 µM) compared to palbociclib (IC50 = 0.071 µM). Subsequent mechanistic investigations demonstrated that 3c was able to trigger tumor cell death through the induction of apoptosis. Moreover, it stimulated cancer cell cycle arrest in the G1 phase. Furthermore, western blotting disclosed that the 3c-induced cell cycle arrest may be mediated through p21 upregulation. CONCLUSION: According to all of the findings, bis-oxindole 3c shows promise as a cancer treatment targeting CDK4.

CDK4/6 inhibition in hormone receptor-positive/HER2-negative breast cancer: Biological and clinical aspects.

A dysregulated cell division, one of the key hallmarks of cancer, results in uncontrolled cellular proliferation. This aberrant process, mediated by a dysregulated cell-cycle machinery and overactivation of cyclin-dependent kinase (CDK) 4 and 6, can potentially promote tumorigenesis. The clinical application of CDK 4/6 inhibitors, developed to inhibit cell-cycle progression, in the treatment regimens of breast cancer (BC) patients is expanding. Currently, three agents, ribociclib, palbociclib, and abemaciclib, are approved for treating patients with hormone receptor-positive and human epidermal growth factor receptor 2 (HER2)-negative metastatic BC. In addition, abemaciclib is FDA and EMA-approved for patients with hormone receptor-positive HER2-negative, node-positive, early BC at high risk of recurrence. Emerging data suggest potential anti-tumor effects beyond cell cycle arrest, providing novel insights into the agent's mechanisms of action. As a result, a broader application of the CDK4/6 inhibitors in patients with cancer is achieved, contributing to enhanced optimized treatment in the adjuvant and neoadjuvant settings. Herein, the immunomodulatory activities of CDK4/6 inhibitors, their impact on the cell's metabolic state, and the effect on the decision of the cell to undergo quiescence or senescence are discussed. Moreover, this review provides an update on clinical trial outcomes and the differences in the underlying mechanisms between the distinct CDK4/6 inhibitors.

Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors: existing and emerging differences.

The cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors palbociclib, ribociclib, and abemaciclib are standard-of-care therapy for hormone receptor-positive advanced or metastatic breast cancer, based on randomized trials showing improved progression-free survival for all 3 drugs and overall survival for ribociclib and abemaciclib. Results in early breast cancer are discordant, with sustained improvement in invasive disease-free survival demonstrated for abemaciclib but not other CDK4/6 inhibitors to date. We review nonclinical studies exploring mechanistic differences between the drugs, the impact of continuous dosing on treatment effect, and translational research into potential resistance mechanisms and prognostic and predictive markers. We focus particularly on how emerging findings may help us understand similarities and differences between the available CDK4/6 inhibitors. Even at late-stage clinical development, there remains much to learn about how agents in this class exert their varying effects.

CDK4/6 and autophagy inhibitors synergize to suppress the growth of human head and neck squamous cell carcinomas.

Head and neck squamous cell carcinoma (HNSCC) accounts for over 10,000 deaths in the United States annually. Approximately 80% of HNSCC are human papillomavirus (HPV)-negative which have an overall poorer prognosis compared to the HPV-positive disease. Treatment options are mainly nontargeted chemotherapy, radiation, and surgery. The cyclin-d-CDK4/6-RB pathway, which regulates cell cycle progression, is often deregulated in HNSCC, making it an attractive therapeutic target. In the current study, we investigated the therapeutic effects of cyclin-dependent kinase 4/6 (CDK4/6) inhibitors in preclinical models of HNSCCs. Our results show that the specific CDK4/6 inhibitor, abemaciclib, inhibited cell growth, and induced apoptosis in HNSCC cell lines. We also demonstrated that both the pro-survival autophagy pathway and the ERK pathway in HNSCC cells were activated with abemaciclib treatment through the generation of reactive oxygen species (ROS). Coinhibition of CDK4/6 and autophagy synergistically decreased cell viability, induced apoptosis, and inhibited tumor growth in both in vitro and in vivo preclinical HNSCC models. These results reveal a potential therapeutic strategy that supports the rationale for further clinical development of a combination of CDK4/6 and autophagy inhibitors in HNSCC.

Mathematically Modeling the Effect of Endocrine and Cdk4/6 Inhibitor Therapies on Breast Cancer Cells.

Mathematical modeling of cancer systems is beginning to be used to design better treatment regimens, especially in chemotherapy and radiotherapy. The effectiveness of mathematical modeling to inform treatment decisions and identify therapy protocols, some of which are highly nonintuitive, is because it enables the exploration of a huge number of therapeutic possibilities. Considering the immense cost of laboratory research and clinical trials, these nonintuitive therapy protocols would likely never be found by experimental approaches. While much of the work to date in this area has involved high-level models, which look simply at overall tumor growth or the interaction of resistant and sensitive cell types, mechanistic models that integrate molecular biology and pharmacology can contribute greatly to the discovery of better cancer treatment regimens. These mechanistic models are better able to account for the effect of drug interactions and the dynamics of therapy. The aim of this chapter is to demonstrate the use of ordinary differential equation-based mechanistic models to describe the dynamic interactions between the molecular signaling of breast cancer cells and two key clinical drugs. In particular, we illustrate the procedure for building a model of the response of MCF-7 cells to standard therapies used in the clinic. Such mathematical models can be used to explore the vast number of potential protocols to suggest better treatment approaches.

Identification of the KIF and MCM protein families as novel targets for combination therapy with CDK4/6 inhibitors in bladder cancer.

CDK4/6 inhibitors have proven their potency for the treatment of cancer but only in combination with hormone or targeted therapies. The aim of this study was the identification of molecules that are involved in response mechanisms to CDK4/6 inhibitors and the development of novel combination therapies with corresponding inhibitors in bladder cancer. Genes of response to therapy and genes that confer resistance to the CDK4/6 inhibitor palbociclib were identified by performing an analysis of published literature and own published data using a CRISPR-dCas9 genome wide gain of function screen. Genes that were down-regulated upon treatment were compared with genes that confer resistance when up-regulated. Two of the top 5 genes were validated by quantitative PCR and western blotting upon treatment with palbociclib in the bladder cancer cell lines T24, RT112 and UMUC3. As inhibitors for combination therapy, we used ciprofloxacin, paprotrain, ispinesib and SR31527. Analysis of synergy was done using the "zero interaction potency" model. Cell growth was examined using sulforhodamine B staining. A list of genes that met the requirements for inclusion in the study was generated from 7 publications. Of the 5 most relevant genes, MCM6 and KIFC1 were chosen and their down-regulation upon treatment with palbociclib was confirmed by qPCR and immunoblotting. The combination of inhibitors against both, KIFC1 and MCM6 with PD resulted in a synergistic inhibition of cell growth. We have identified 2 molecular targets whose inhibition has promising potential for effective combination therapies with the CDK4/6 inhibitor palbociclib.

PFKFB3 mediates tubular cell death in cisplatin nephrotoxicity by activating CDK4.

Nephrotoxicity is a major side effect of cisplatin, a widely used cancer therapy drug. However, the mechanism of cisplatin nephrotoxicity remains unclear and no effective kidney protective strategies are available. Here, we report the induction of 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 (PFKFB3) in both in vitro cell culture and in vivo mouse models of cisplatin nephrotoxicity. Notably, PFKFB3 was mainly induced in the nucleus of kidney tubular cells, suggesting a novel function other than its canonical role in glycolysis. Both pharmacological inhibition and genetic silencing of PFKFB3 led to the suppression of cisplatin-induced apoptosis in cultured renal proximal tubular cells (RPTCs). Moreover, cisplatin-induced kidney injury or nephrotoxicity was ameliorated in renal proximal tubule-specific PFKFB3 knockout mice. Mechanistically, we demonstrated the interaction of PFKFB3 with cyclin-dependent kinase 4 (CDK4) during cisplatin treatment, resulting in CDK4 activation and consequent phosphorylation and inactivation of retinoblastoma tumor suppressor (Rb). Inhibition of CDK4 reduced cisplatin-induced apoptosis in RPTCs and kidney injury in mice. Collectively, this study unveils a novel pathological role of PFKFB3 in cisplatin nephrotoxicity through the activation of the CDK4/Rb pathway, suggesting a new kidney protective strategy for cancer patients by blocking PFKFB3.

Ginkgolide A Participates in LPS-Induced PMVEC Injury by Regulating miR-224 and Inhibiting p21 in a Targeted Manner.

Most studies have focused on the protective effects of ginkgolide A against ischemia/reperfusion-induced cardiomyopathy and injury of the brain, liver, and other organs, but there are few reports about the protection of lung tissues. This study was designed to clarify the protection of ginkgolide A against lipopolysaccharide (LPS)-induced pulmonary microvascular endothelial cell (PMVEC) injury. PMVECs were extracted and fell into control, LPS, and ginkgolide A groups. Next, we delved into the growth activity and apoptosis rate of cells via the CCK-8 assay and Hoechst staining, independently. Beyond that, western blotting (WB) was implemented for measurement of the expressions of cyclin D1, cyclin-dependent kinase 4 (CDK4), and CDK inhibitor (p21) that pertained to the cell cycle. The target sites of ginkgolide A were confirmed by miRNA array and real-time quantitative PCR. The relationship between miR-224 and p21 was analyzed using dual-luciferase reporter gene assay. Compared with the control group, the LPS group and ginkgolide A group had significantly decreased cell growth activity and relative expressions of cyclin D1 and CDK4 and elevated apoptosis rate and p21 expression. Pronounced elevations were observable in the cell growth activity and expressions of cyclin D1, CDK4, and p21, while the ginkgolide A group presented with a reduced apoptosis rate in comparison with the LPS group (P < 0.05). MiR-224 was the target of ginkgolide A, which had targeted regulatory effects on p21. Ginkgolide A can modulate miR-224 expression and regulate p21 expression in a targeted manner to enhance the resistance of PMVECs to LPS-induced cell apoptosis.

Protective Effect of Ginsenosides Rg1 on Ischemic Injury of Cardiomyocytes After Acute Myocardial Infarction.

Acute myocardial infarction (AMI) leads to anoxia and ischemia of cardiomyocytes, followed by apoptosis. This study investigated the protective effect of ginsenoside Rg1 (Rg1) on myocardial ischemia injury in rats with AMI. Rats were randomly divided into five groups: group A (blank control group), group B (hypoxia/reoxygenation group), group C (hypoxia/reoxygenation + 10 mg/L Rg1), group D (hypoxia/reoxygenation + 20 mg/L Rg1) and group E (hypoxia/reoxygenation + 40 mg/L Rg1). The survival rate, apoptosis rate, expression of cyclin-dependent kinase 4 (CDK4), fibroblast growth factor 9 (FGF9), superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px), microvessel density and myocardial infarction area of rats in each group were compared. The expressions of CDK4 and FGF9, the contents of SOD and GSH-Px in groups C, D and E injected with Rg1 were significantly promoted compared to group B without Rg1 injection (P < 0.05). The survival rate of myocardial cells was significantly increased while the apoptosis rate was significantly decreased in group C, D, E compared to group B (P < 0.05). On the 3rd, 7th and 10th day following Rg1 treatment, the infarct area of E group was significantly decreased in three groups C, D, E, and the microvessel density of infarct area was significantly increased compared with group B (P < 0.05). So, Rg1 can improve the survival rate of myocardial cells, reduce the apoptosis rate and the area of myocardial infarction, and increase the microvessel density of infarct area, thus playing a protective role in ischemic myocardial cells of AMI rats.

Palbociclib Induces the Apoptosis of Lung Squamous Cell Carcinoma Cells via RB-Independent STAT3 Phosphorylation.

Lung squamous cell carcinoma (LUSC) treatment response is poor and treatment alternatives are limited. Palbociclib, a cyclin-dependent kinase (CDK) 4/6 inhibitor, has recently been approved for hormone receptor-positive breast cancer patients and applied in multiple preclinical models, but its use for LUSC therapy remains elusive. Here, we investigated whether palbociclib induced cell apoptosis and dissected the underlying mechanism in LUSC. We found that palbociclib induced LUSC cell apoptosis through inhibition of Src tyrosine kinase/signal transducers and activators of transcription 3 (STAT3). Interestingly, palbociclib reduced STAT3 signaling in LUSC cells interfered by retinoblastoma tumor-suppressor gene (RB), suggesting that pro-apoptosis effect of palbociclib was independent of classic CDK4/6-RB signaling. Furthermore, palbociclib could suppress IL-1ß and IL-6 expression, and therefore blocked Src/STAT3 signaling, which were rescued by either recombinant human IL-1ß or IL-6. Moreover, Myc mediated the sensitivity of LUSC cells to palbociclib. Our discoveries demonstrated that palbociclib induces apoptosis of LUSC cells through the Src/STAT3 axis in an RB-independent manner, and provided a reliable experimental basis of clinical studies in LUSC patients.

RNF2 mediates pulmonary fibroblasts activation and proliferation by regulating mTOR and p16-CDK4-Rb1 signaling pathway.

BACKGROUND: Pulmonary fibrosis (PF) is a chronic, progressive interstitial lung disease with unknown etiology, associated with increasing morbidity and pessimistic prognosis. Pulmonary fibroblasts (PFbs) are the key effector cells of PF, in which abnormal activation and proliferation is an important pathogenesis of PF. Ring finger protein 2 (RNF2), is identified as the catalytic subunit of poly-comb repressive complex 1, which is closely related to occurrence and development of lung cancer, but its function in PF has not been revealed. In this paper, we sought to identify the regulatory role of RNF2 in lung fibrogenesis and its underlying mechanisms. METHODS: The expression of RNF2 in lung fibrosis tissue (human and Bleomycin-induced mouse) and cell model (TGF-ß1-induced HFL1 cells) was examined by immunoblotting analysis and immunofluorescence. Western blot, qRT-PCR were performed to evaluate the expression of pro-fibrogenic cytokines (including α-SMA, ECM and MMPs/ TIMPs) induced by TGF-ß1 in HFL1 cells. Cell proliferation, cycle progression and apoptosis were examined by fow cytometric. Molecular interactions were tested by Co-IP assays. RESULTS: RNF2 expression was elevated in PF tissues compared to normal adjacent tissues and in PFbs (HFL1) induced by TGF-ß1. Furthermore, knockdown of RNF2 could evidently inhibit the abnormal expression of pro-fibrogenic cytokines (including α-SMA, ECM and MMPs/TIMPs) induced by TGF-ß1 in HFL1 cells. Functionally, RNF2 silencing could significantly suppress TGF-ß1-induced anomalous proliferation, cell cycle progression, apoptosis and autophagy in HFL1 cells. Mechanistically, RNF2 deficiency could effectively inhibit the abnormal activation of mTOR signaling pathway in TGF-ß1-induced HFL1 cells, and mTOR pathway had feedback regulation on the expression of RNF2. Further studies RNF2 could regulate the phosphorylation level of RB1 through interacting with p16 to destroy the binding of p16 and CDK4 competitively. Simultaneously, overexpression of RNF2 could show the opposite results. CONCLUSIONS: These results indicated that RNF2 is a potent pro-fibrogenic molecule for PFbs activation and proliferation through mTOR and p16-CDK4-Rb signaling pathways, and RNF2 inhibition will be a potential therapeutic avenue for treating PF.

Artificial Assembled Macrophage Co-Deliver Black Phosphorus Quantum Dot and CDK4/6 Inhibitor for Colorectal Cancer Triple-Therapy.

In recent years, therapeutic strategies based on macrophages have been inspiringly developed, but due to the high intricacy and immunosuppression of the tumor microenvironment, the widespread use of these strategies still faces significant challenges. Herein, an artificial assembled macrophage concept (AB@LM) was presented to imitate the main antitumor abilities of macrophages of tumor targeting, promoting the antitumor immunity, and direct tumor-killing effects. The artificial assembled macrophage (AB@LM) was prepared through an extrusion method, which is to fuse the macrophage membrane with abemaciclib and black phosphorus quantum dot (BPQD)-loaded liposomes. AB@LM showed good stability and tumor targeting ability with the help of macrophage membrane. Furthermore, AB@LM reversed the immunosuppressive tumor microenvironment by inhibiting regulatory T cells (Tregs) and stimulating the maturation of antigen-presenting cells to activate the antitumor immune response through triggering an immunogenic cell death effect. More importantly, in the colorectal tumor model in vivo, a strong cooperative therapeutic effect of photo/chemo/immunotherapy was observed with high tumor inhibition rate (95.3 ± 2.05%). In conclusion, AB@LM exhibits excellent antitumor efficacy by intelligently mimicking the abilities of macrophages. A promising therapeutic strategy for tumor treatment based on imitating macrophages was provided in this study.

Targeting cyclin-dependent kinase 4/6 as a therapeutic approach for mucosal melanoma.

Mucosal melanoma is a rare but devastating subtype of melanoma which typically has a worse prognosis than other melanoma subtypes. Large-scale next-generation sequencing studies, including our recent research, have also proved that the molecular landscape and potential oncogenic drivers of mucosal melanoma remain distinct from that of cutaneous melanoma. Recently, a number of selective cyclin-dependent kinase 4 (CDK4)/6 inhibitors have been approved for clinical application in breast cancer or entered phase III clinical trial in other solid tumors. Additionally, we have revealed that the dysregulation of cell cycle progression, caused by CDK4 amplification, is a key genetic feature in half of mucosal melanoma and targeting of CDK4 in selected mucosal melanoma patients is a potentially promising direction for precision cancer treatment by using molecular-characterized mucosal melanoma patient-derived-xenograft models. This review summarizes the current literature regarding CDK4/6 dysregulation in mucosal melanoma, preclinical and clinical studies of CDK4/6 inhibitors and potential combinational strategies in treating mucosal melanoma.

Pan-cancer analysis reveals synergistic effects of CDK4/6i and PARPi combination treatment in RB-proficient and RB-deficient breast cancer cells.

DNA damage results in mutations and plays critical roles in cancer development, progression, and treatment. Targeting DNA damage response in cancers by inhibiting poly-(ADP-ribose) polymerases (PARPs) offers an important therapeutic strategy. However, the failure of PARP inhibitors to markedly benefit patients suggests the necessity for developing new strategies to improve their efficacy. Here, we show that the expression of cyclin-dependent kinase 4/6 (CDK4/6) complex members significantly correlates with mutations (as proxies of DNA damages), and that the combination of CDK4/6 and PARP inhibitors shows synergy in both RB-proficient and RB-deficient breast cancer cells. As PARPs constitute sensors of DNA damage and are broadly involved in multiple DNA repair pathways, we hypothesized that the combined inhibition of PARPs and DNA repair (or repair-related) pathways critical for cancer (DRPCC) should show synergy. To identify druggable candidate DRPCC(s), we analyzed the correlation between the genome-wide expression of individual genes and the mutations for 27 different cancer types, assessing 7146 exomes and over 1,500,000 somatic mutations. Pathway enrichment analyses of the top-ranked genes correlated with mutations indicated "cell cycle pathway" as the top candidate DRPCC. Additionally, among functional cell-cycle complexes, the CDK4/6 complex showed the most significant negative correlation with mutations, also suggesting that combined CDK4/6 and PARP inhibition might exhibit synergy. Furthermore, combination treatment showed synergy in not only RB-proficient but also RB-deficient breast cancer cells in a reactive oxygen species-dependent manner. These findings suggest a potential therapeutic strategy to improve the efficacy of PARP and CDK4/6 inhibitors in cancer treatment.

Emerging data and future directions for CDK4/6 inhibitor treatment of patients with hormone receptor positive HER2-non-amplified metastatic breast cancer.

Cyclin-dependent kinase (CDK4/6) inhibitors in combination with endocrine therapy are currently the optimal first line treatment for hormone receptor (HR) positive, human epidermal growth factor receptor 2 (HER2) non-amplified metastatic breast cancer (MBC). However, not all patients benefit from this treatment and all patients will inevitably progress. Identifying therapeutic strategies in this setting is therefore of immediate clinical importance. We present an overview of the mechanisms of resistance to CDK4/6 inhibitors and review potential biomarkers that may guide therapy selection. We also discuss the use of CDK4/6 inhibitors in the context of non-HR-positive/HER2-non-amplified breast cancer and in combination with therapies other than endocrine therapy.

Hormone receptor positive, HER2 negative metastatic breast cancer: Impact of CDK4/6 inhibitors on the current treatment paradigm.

Resistance to endocrine therapy is a significant therapeutic challenge in the treatment of women with hormone receptor positive (HR+), human epidermal growth factor receptor 2 negative (HER2-) advanced breast cancer. Cyclin-dependent kinase (CDK)4/6 inhibitors in combination with endocrine therapy have been shown to improve progression free survival, overall response rate and clinical benefit rate in women with HR+ HER2- metastatic breast cancer compared with endocrine therapy alone. This review examines the clinical evidence to support the use of CDK4/6 inhibitors in first and second line settings. Practical guidance is provided for the use of CDK4/6 inhibitors, including tolerability data, monitoring requirements and management of key toxicities for each of the available agents.