Arthralgia induced by endocrine therapy with or without cyclin-dependent kinase 4/6 inhibitors in breast cancer: A systematic review and meta-analysis

Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) in combination with endocrine therapy (ET) have been approved for breast cancer (BC) treatment. Several trials suggested that arthralgia was reduced in patients treated with ET plus CDK4/6i compared with that in those with ET-alone. We aimed to compare arthralgia rates in BC patients treated with/without CDK4/6i. We reviewed randomized controlled phase II/III trials investigating CDK4/6i with ET in hormone receptor-positive and epidermal growth factor 2-negative BC. Publications were retrieved from PubMed from January 2014 to April 2021. We compared arthralgia rates between patients who were administered ET plus CDK4/6i (CDK4/6i group) and those treated with ET-alone (control group). We reviewed 12 trials that reported data on adverse effects for arthralgia. These trials included 17,440 patients (9255 in the CDK4/6i group and 8185 in the control group). The arthralgia rate in the CDK4/6i group was significantly lower than that in the control group (27.6% vs. 34.8%, p < .001), especially in early BC (28.8% vs. 37.3%, p < .001). These suggested that the arthralgia rate in patients treated with ET plus CDK4/6i was lower than that in patients treated with ET-alone and that CDK4/6i may decrease the arthralgia rate in BC patients treated with ET, especially in early BC.     

Prognostic importance of CDK4/6-specific activity as a predictive marker for recurrence in patients with endometrial cancer,with or without adjuvant chemotherapy

Background:

Pathologically low-risk endometrial cancer patients do not receive postoperative treatment; however, 10–15% of these patients show recurrence with poor prognosis. We evaluated the clinical importance of cyclin-dependent kinase 4/6 (CDK4/6) activity, and its significance as a novel biomarker for the prognosis and chemo-sensitivity of endometrioid endometrial carcinoma (EEC).

Methods:

Cyclin-dependent kinase 4/6 expression and enzyme activity in 109 tumour samples from patients with EEC were examined with a cell-cycle profiling (C2P) assay. CDK4/6-specific activity (CDK4/6SA) was determined, and its relationship with clinicopathological factors and expression of Ki-67 was analysed.

Results:

CDK4/6-specific activity was significantly correlated with Ki-67 (P=0.035), but not with any other clinicopathological characteristics. CDK4/6SA was significantly higher (P=0.002) in pathologically low-risk patients (not receiving adjuvant chemotherapy, n=74) than in intermediate- or high-risk patients (receiving adjuvant chemotherapy, n=35). In addition, patients with high CDK4/6SA (>3.0) showed significantly (P=0.024) shorter progression-free survival (PFS) than those with low CDK4/6SA (<3.0). Although Ki-67 expression itself was not a marker for prognosis, the combination of high CDK4/6SA and high Ki-67 expression (>15%) was robustly associated with shorter PFS (P=0.015), and this combination was an independent poor prognostic factor in the low-risk group. Inversely, in the intermediate-/high-risk group, patients with high CDK4/6SA had a tendency of a more favourable prognosis compared with patients with low CDK4/6SA (P=0.063).

Conclusions:

CDK4/6-specific activity can be used as a biomarker to predict prognosis and, possibly, chemo-sensitivity. The combination of Ki-67 expression might strengthen the clinical usefulness of CDK4/6SA as a biomarker.     

CDK4/6 inhibitors have potent activity in combination with pathway selective therapeutic agents in models of pancreatic cancer

Pancreatic ductal adenocarcinoma (PDA) has a poor prognosis, in part, due to the therapy-recalcitrant nature of the disease. Loss of the CDK4/6 inhibitor CDKN2A is a signature genetic event in PDA. Therefore, PDA may be amenable to treatment with pharmaceutical CDK4/6 inhibitors. Surprisingly, response to CDK4/6 inhibition was highly variable in PDA models, and associated with differential suppression of gene expression. Mitotic genes were repressed and FOXM1 was uniformly attenuated; however, genes involved in DNA replication were uniquely suppressed in sensitive models. Aberrant induction of Cyclin E1 was associated with resistance, and knockdown demonstrated synergistic suppression of the cell cycle with CDK4/6 inhibition. Combination therapies are likely required for the effective treatment of disease, and drug screening demonstrated additive/antagonistic interactions with CDK4/6 inhibitors. Agents dependent on mitotic progression (taxanes/PLK1 inhibitors) were antagonized by CDK4/6 inhibition, while the response to 5-FU and gemcitabine exhibited drug specific interactions. PI3K/MTOR and MEK inhibitors potently cooperated with CDK4/6 inhibition. These agents were synergistic with CDK4/6 inhibition, blocked the aberrant upregulation of Cyclin E1, and yielded potent inhibition of tumor cell growth. Together, these data identify novel mechanisms of resistance to CDK4/6 inhibitions and provide a roadmap for combination therapies in the treatment of PDA.     

Cyclin-dependent kinase 4/6 activity is a critical determinant of pre-replication complex assembly

Cyclin-dependent kinases (CDKs) are important in regulating cell cycle transitions, particularly in coordinating DNA replication. Although the role of CDK2 activity on the replication apparatus has been extensively studied, the role of CDK4/6 in DNA replication control is less understood. Through targeted inhibition of CDK4/6 activity, we demonstrate that CDK4/6 kinase activity promotes cdc6 and cdt1 expression, and pre-replication complex (pre-RC) assembly in cycling cells. Conversely, CDK2 inhibition had no effect on the pre-RC assembly. The inhibition of pre-RC assembly is dependent on a functional retinoblastoma (RB) protein, which mediates downstream effects. As such, CDK4/6 inhibition has minimal effect on the replication apparatus in the absence of RB. The requirement of CDK4/6 was further interrogated using cells lacking D-type cyclins, in which replication complexes form normally, and correspondingly CDK4/6 inhibition had no effect on cell cycle or replication control. However, in the absence of D-type cyclins, CDK2 inhibition resulted in the attenuation of cdc6 and cdt1 levels, suggesting overlapping roles for CDK4/6 and CDK2 in regulating replication protein activity. Finally, CDK4/6 inhibition prevented the accumulation of cdc6 and cdt1 as cells progressed from mitosis through the subsequent G(1). Combined, these studies indicate that CDK4/6 activity is important in regulating the expression of these critical mediators of DNA replication.     

CDK4/6抑制剂耐药机制的研究进展

细胞周期依赖性激酶4/6（cyclin-dependent kinase 4/6,CDK4/6)抑制剂能作用于过度活化的CDK4/6,恢复正常细胞周期,并通过触发免疫,改变肿瘤微环境等发挥抗肿瘤作用。目前,CDK4/6抑制剂的问世极大地改善了激素受体阳性（hormone receptor-positive,HR+）、人表皮生长因子受体2阴性（human epidermal growth factor receptor 2-negative,HER2-)晚期乳腺癌患者的预后,使无进展生存期(progression-free-survival,PFS)增加近一倍,且不良反应可控。尽管如此,这类患者最终仍会因CDK4/6抑制剂耐药而出现疾病进展。CDK4/6抑制剂的耐药机制十分复杂,尚未完全明确。预测其早期耐药或治疗敏感的生物标记物也有待确定。本文对CDK4/6抑制剂治疗的作用机制及耐药机制进行梳理和总结,并对疾病进展后的治疗策略作简要讨论。     

Outcomes and toxicity of concurrent CDK4/6 inhibitor and locoregional radiotherapy for patients with de novo metastatic breast cancer

The objective of the present study was to assess the outcomes and toxicity of patients treated with concurrent administration of CDK4/6 inhibitors (CDK4/6i) and locoregional radiation therapy (RT), including the breast with a boost or the thoracic wall after mastectomy and the regional lymph node areas. We retrospectively analyzed data from 27 patients with hormone receptor-positive, HER2-negative de novo metastatic breast cancer treated with CDK4/6i and concomitant locoregional RT in 2017/2022. Survival rates were calculated by Kaplan-Meier method. Prognostic factors were tested with log-rank test. CDK4/6i was used as the first systemic metastatic treatment for all the patients, and the median overall treatment time was 26 months. The median time from initiation of CDK4/6i to the start of RT was 10 months (IQR: 7-14 months). The median duration of concomitant CDK4/6i and RT administration was 21 days (IQR: 14.5-23 days). After a median follow-up of 19 months (IQR: 14-36 months), 1 patient died, 11/27 had distant metastases and 1 patient had local recurrence, respectively. The 1- and 3-years progression-free survival (PFS) were 61.4% (95% CI: 45.1%-83.7%) and 53.7% (35.8%-80.5%), respectively. The acute toxicities most observed during RT were neutropenia (44%) and dermatitis (37%). Dermatitis was significantly more frequent in patients with large target volumes (CTV > 911 cc and PTV > 1285 cc). CDK4/6i had to be discontinued in five patients during RT (due to toxicity in three cases and disease progression in two cases). One patient has developed grade 2 late pulmonary fibrosis. Finally, our study demonstrated that concurrent administration of locoregional RT and CDK4/6i did not induce severe late toxicity for most patients.     

Drug-drug interactions in breast cancer patients treated with CDK4/6 inhibitors

CDK4/6 inhibitors are a new class of anticancer drugs used for the treatment of women with hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced or metastatic breast cancer with disease progression following endocrine therapy. Polypharmacy is a well-known problem in advanced cancer causing potential drug-drug interactions (DDIs), which, in turn, may limit the therapeutic value of CDK4/6 inhibitors. Therefore, understanding the mechanisms underlying potential DDIs in patients taking CDK4/6 inhibitors may be useful in decision-making processes and represent an important step towards treatment personalization. The present review is aimed at describing the potential DDIs that might occur in breast cancer patients receiving CDK4/6 inhibitors based on direct evidence from the literature and mechanistic considerations tailored on specific class of drugs used in combination.     

CDK4/6抑制剂在激素受体阳性进展期乳腺癌治疗中的研究进展

内分泌治疗因其兼具良好的疗效及安全性,是激素受体阳性进展期乳腺癌患者的重要治疗手段。近年来内分泌领域进展迅速,很多新型药物相继出现,其中包括多种可以逆转或延迟内分泌耐药的周期蛋白依赖性激酶(cyclin-dependent kinase,CDK)4/6抑制剂。CDK4/6抑制剂联合内分泌治疗可为激素受体阳性进展期乳腺癌患者带来生存获益、延迟至化疗时间,正逐渐改变国内外激素受体阳性进展期乳腺癌的治疗模式。本文将就CDK4/6抑制剂在激素受体阳性进展期乳腺癌中的治疗进展进行综述。     

Comparative efficacy and safety of CDK4/6 and PI3K/AKT/mTOR inhibitors in women with hormone receptor-positive,HER2-negative metastatic breast cancer: a systematic review and network meta-analysis

BackgroundCDK4/6 inhibitors and PI3K/AKT/mTOR inhibitors are both emerging agents for hormonal receptor (HR) positive and human epidermal growth factor receptor 2 (HER2) negative metastatic breast cancer. Evidence for the comparisons from head-to-head comparative trials is currently insufficient. This meta-analysis assessed the comparative efficacy and safety of these two groups of agents for HR+/HER2- metastatic breast cancer.MethodsSystematic searches of PubMed, Embase, CENTRAL, SciSearch between January 2010 to December 2019 were conducted. Randomized controlled trials (RCTs) which evaluated clinical benefits and toxicities of CDK4/6 inhibitors or PI3K/AKT/mTOR inhibitors plus endocrine therapy were adopted. Primary endpoints were progression-free survival (PFS) and overall survival (OS). Secondary endpoint was treatment-related adverse event (TRAE). Pooled hazard ratio (HR) and risk rate (RR) were used to assess the differences between CDK4/6 and PI3K/AKT/mTOR inhibitors.ResultsA total of twenty RCTs including 9771 participants were identified in this study. Pooled results showed that PFS was considerably prolonged by targeted therapy plus endocrine therapy. PFS was relatively better in CDK4/6 inhibitors than that of PI3K inhibitor group (HR, 1.43; 95%CrI, 1.12-1.61). Similar results were demonstrated in results after balancing lines of therapy or metastatic sites, both in viscera and bone-only. Coalesced outcomes revealed that CDK4/6 inhibitors plus endocrine therapy could significantly improve OS (HR, 0.78; 95%CrI, 0.65-0.94) than PI3K/mTOR inhibitors. Safety profiles of diarrhea and rash were consistent between CDK4/6 inhibitors and PI3K/AKT/mTOR inhibitors with no difference of estimated RR. Several TRAEs signified specificity, for instance, myelosuppression in CDK4/6 inhibitors or hyperglycemia in PI3K/mTOR inhibitors.ConclusionsClinical efficacy is in favor of CDK4/6 inhibitors, and safety profiles are comparable between CDK4/6 inhibitors or PI3K/AKT/mTOR inhibitors plus endocrine therapy.     

CDK4/6抑制剂抗肿瘤作用研究进展

细胞周期素依赖性蛋白激酶4/6（cyclin-dependent kinases 4/6,CDK4/6）与细胞周期蛋白D（cyclin D）结合,通过调节细胞G1-S期转换,成为细胞周期调控机制的核心部分。CDK4/6-cyclin D-INK4-Rb通路的异常活化将会导致癌细胞失控性生长,该通路的异常存在于多种恶性肿瘤中,因此CDK4/6成为潜在的治疗靶点。本文对CDK4/6的作用机制,及主要几种CDK4/6抑制剂的研究进展进行综述,旨在探讨CDK4/6抑制剂在恶性肿瘤治疗中的应用前景及优化手段。     

CDK4, CDK6, cyclin D1, p16(INK4a) and EGFR expression in glioblastoma with a primitive neuronal component

Glioblastoma with primitive neuroectodermal tumor-like component (GBM-PNET) is a rare variant of glioblastoma, which was renamed as glioblastoma with a primitive neuronal component (GBM-PN) in new WHO classification of tumours of the central nervous system in 2016. There are few publications on the investigation of GBM-PN. In this study, PCR mRNA arrays on 6 cases of conventional GBM and 10 cases of GBM-PN showed high mRNA level of CDK4 in GBM-PN and low mRNA level of EGFR in GBM-PN. Immunohistochemical stains on tissue microarrays with 28 cases of conventional GBM and 13 cases of GBM-PN demonstrated that CDK4 was selectively expressed in the primitive neuronal component of all GBM-PN cases while EGFR was positive in conventional GBM and glial component of GBM-PN, but was negative in the primitive neuronal component of all GBM-PN cases. Immunohistochemical stains with antibodies against proteins that interact with CDK4 in cell cycle regulation, such as CDK6, cyclin D1 and p16(INK4a), were performed on these GBM-PN and GBM cases. CDK6 was patchily positive in rare cases of GBM-PN and cyclin D1 was negative in GBM-PN cases. p16(INK4a) is traditionally known as an inhibitor of CDK4 and CDK6. p16(INK4a) might not be the inhibitor of CDK4 in GBM-PN cases because seven GBM-PN cases were positive for both CDK4 and p16(INK4a). It indicates that CDK4 and p16(INK4a) might play a crucial role in GBM-PN pathogenesis. Since CDK4 and EGFR are highly expressed in the primitive neuronal component and in the glial component of GBM-PN respectively, the combination of CDK4/6 inhibitor and targeted therapy against EGFR might be potential effective therapeutic regimen for GBM-PN. CDK4 and EGFR immuohistochemical stain patterns make the diagnosis of GBM-PN much easier.     

SPH3643: A novel cyclin‐dependent kinase 4/6 inhibitor with good anticancer efficacy and strong blood‐brain barrier permeability

The cyclin‐dependent kinase (CDK)4/6‐cyclin D1‐Rb‐p16/ink4a pathway is responsible for regulating cell progression past the G₁ restriction point during the cell cycle. The development of a majority of human tumors is associated with dysregulation of this pathway, resulting in increased cancer cell proliferation. Both CDK4 and CDK6, well‐validated cancer drug targets, function primarily as catalytic enzymes that mediate the phosphorylation of retinoblastoma protein (Rb). Here, we determined that SPH3643 is a novel potent antiproliferative agent that inhibits CDK4/6 kinase activity. In biochemical assays, SPH3643 showed more potent inhibition of both CDK4 and CDK6 than did 2 published CDK4/6 inhibitors, LY2835219 and palbociclib, and had better selectivity than LY2835219. Further in vitro study revealed that SPH3643 blocked Cdk/Rb signaling by inhibiting the phosphorylation of Rb^Ser780 and arrested the MCF‐7 cancer cells at G₀/G₁ phase, resulting in marked inhibition of the proliferation of Rb‐positive cancer cell lines. In vivo SPH3643 treatment in mice bearing xenograft tumor models of breast cancer, colon cancer, acute myelocytic leukemia, and glioblastoma resulted in significant decreases in tumor growth. SPH3643 was able to particularly strongly inhibit glioblastoma (U87‐MG) cell growth in the brains of orthotopic carcinoma xenograft mice due to its high degree of intracerebral penetration and significant persistence in this setting. Together these results revealed that SPH3643 is a potent, orally active small‐molecule inhibitor of CDK4/6 with robust anticancer efficacy and a high degree of blood‐brain barrier permeability.     

CDK4/6抑制剂治疗三阴性乳腺癌的研究进展

细胞周期的调控机制在肿瘤的发生、发展中发挥着重要作用。多项研究表明，在雌激素受体阳性、人类表皮生长因子受体阴性的乳腺癌患者中，细胞周期蛋白依赖性激酶4/6（cyclin-dependent kinase 4/6，CDK4/6）抑制剂具有较好的疗效，然而其在三阴性乳腺癌患者中的疗效仍需进一步探讨。Cyclin D-CDK4/6-INK4-Rb-E2F信号通路因具有调控细胞周期检查点的作用，被认为是乳腺癌潜在的治疗靶点。寻找三阴性乳腺癌与CDK4/6抑制剂关联的生物标志物，探究合理的药物配伍，筛选能从中获益的靶向人群，对临床工作具有重要意义。对CDK4/6抑制剂在三阴性乳腺癌的研究进展进行综述，探讨其应用前景及优化手段。     

Aspects biologiques,précliniques et cliniques de l’association de radiothérapie et d’inhibiteur de CDK 4/6

Several clinical studies have shown that CDK4/6 inhibitors (CDK4/6i) improve survival in patients with metastatic or locally advanced HR-positive, HER-2-negative breast cancer (BC). The aim of this review was to synthesize the biological, preclinical and clinical aspects of the treatment of BC with CDK4/6i, with a focus on the combination of CDK4/6i and radiotherapy. The DNA damage induced after exposure of cells to ionizing radiation activates control pathways that inhibit cell progression in the G1 and G2 phases and induce a transient delay in progression in the S phase. These checkpoints are in particular mediated by cyclin-dependent kinases (CDK) 4/6 activated by cyclin D1. Several preclinical studies have shown that CDK4/6i could be used as radiosensitizers in non-small cell lung cancer, medulloblastoma, brainstem glioma and breast cancer. CDK4/6 inhibition also protected against radiation-induced intestinal toxicities by inducing redistribution of quiescent intestinal progenitor cells, making them less radiosensitive. Clinical data on the combination of CDK inhibitors and radiotherapy for both locoregional and metastatic irradiation are based on retrospective data. Nevertheless, the most optimal therapeutic sequence would be radiotherapy followed by palbociclib. Pending prospective clinical trials, the concomitant combination of the two treatments should be done under close supervision.     

Therapy after cyclin-dependent kinase inhibition in metastatic hormone receptor-positive breast cancer: Resistance mechanisms and novel treatment strategies

Endocrine therapy has been the standard of care for patients with metastatic hormone receptor (HR)-positive, HER2-negative breast cancer since the 1970s, improving survival while avoiding the toxicities associated with cytotoxic chemotherapy. However, all HR-positive tumors ultimately develop resistance to endocrine therapy. Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors have more recently become an important component of the management of this breast cancer subtype, significantly delaying time to the disease progression and improving survival when combined with endocrine therapy. However, as with endocrine therapy alone, treatment resistance remains a universal phenomenon. As more women receive CDK4/6 inhibitors as part of their treatment, the management of de novo and acquired resistance to combined CDK4/CDK6 inhibitor plus endocrine therapy regimens has emerged as an important clinical challenge. Several resistance mechanisms have been described, including alterations in the CDK4/6/cyclin D complex or its major effector retinoblastoma protein (pRb), bypass signaling through other cyclin/CDK complexes and activation of upstream signaling pathways, in particular the PI3K/mTOR pathway, but robust biomarkers to predict resistance remain elusive, and the role for continuing CDK4/6 inhibitors after progression remains under investigation. Novel strategies being evaluated in clinical trials include the continuation of CDK4/6 inhibitors through progression, as well as triplet therapy combinations with PI3K inhibitors or immune checkpoint inhibitors.     

Polyclonal RB1 mutations and acquired resistance to CDK 4/6 inhibitors in patients with metastatic breast cancer

BackgroundWhile deregulation of the cyclin D1-CDK4/6-retinoblastoma pathway is common in hormone receptor positive (HR+) breast cancer, Rb is usually intact in HR+ breast cancer, and targeted CDK 4/6 inhibitors that act upstream of Rb, are routinely being utilized in clinical practice. However, factors that can lead to clinical resistance to CDK 4/6 inhibitors are not known.Patients and methodsWe identified patients who had pre- and post-genotyping in tissue and peripheral blood samples after receiving CDK 4/6 inhibitors. Genotyping was carried out in tumor tissue or blood collected before start of CDK 4/6 inhibitor and after disease progression on CDK 4/6 inhibitor, covering more than 90% of the coding region in RB1.ResultsWe identified detectable acquired RB1 mutations in circulating tumor DNA (ctDNA) after exposure to CDK4/6 inhibitor (palbociclib, palbociclib, ribociclib) for 5, 8, and 13 months, respectively, in three patients. The RB1 mutations included substitution in donor splicing site of exon 8 of the RB1 gene in patient #1; substitution in donor splicing site of exon 22 of RB1 gene, exon 19 deletion, exon 3 insertion in patient #2; and RB1 exon 16 H483Y mutation in patient #3. None of these RB1 mutations were present in the pre-CDK 4/6 specimen highlighting these molecular alterations, which lead to functional loss of Rb1, likely emerged under selective pressure from the CDK4/6 inhibitor potentially confering therapeutic resistance.ConclusionThis is the first clinical report to describe the emergence of somatic RB1 mutations after exposure to palbociclib or ribociclib, in patients with metastatic breast cancer. Further research is needed to validate these findings, identify how these mutations temporally emerge under selective pressure of CDK 4/6 inhibitor, and develop rational therapeutic strategies.     

Palbociclib enhances activin‐SMAD‐induced cytostasis in estrogen receptor‐positive breast cancer

Cyclin‐dependent kinase (CDK) 4 and CDK6 inhibitors are effective therapeutic options for hormone receptor (HR)‐positive, human epidermal growth factor receptor 2 (HER2)‐negative advanced breast cancer. Although CDK4/6 inhibitors mainly target the cyclin D‐CDK4/6‐retinoblastoma tumor suppressor protein (RB) axis, little is known about the clinical impact of inhibiting phosphorylation of other CDK4/6 target proteins. Here, we focused on other CDK4/6 targets, SMAD proteins. We showed that a CDK4/6 inhibitor palbociclib and activin‐SMAD2 signaling cooperatively inhibited cell cycle progression of a luminal‐type breast cancer cell line T47D. Palbociclib enhanced SMAD2 binding to the genome by inhibiting CDK4/6‐mediated linker phosphorylation of the SMAD2 protein. We also showed that cyclin G2 plays essential roles in SMAD2‐dependent cytostatic response. Moreover, comparison of the SMAD2 ChIP‐seq data of T47D cells with those of Hs578T (triple‐negative breast cancer cells) indicated that palbociclib augmented different SMAD2‐mediated functions based on cell type, and enhanced SMAD2 binding to the target regions on the genome without affecting its binding pattern. In summary, palbociclib enhances the cytostatic effects of the activin‐SMAD2 signaling pathway, whereas it possibly strengthens the tumor‐promoting aspect in aggressive breast cancer.     

Clinical development of CDK4/6 inhibitor for breast cancer

Endocrine therapy is the mainstay of treatment for patients with estrogen receptor positive (ER+)/HER2-negative (HER2?) metastatic breast cancer (MBC). Many clinicians consider the sequential endocrine therapy is gold standard strategy because of better outcome and the maintenance of a better quality of life (QOL) for MBC patients. However, clinical practice shall be changed according to development of CDK4/6 inhibitor in current. CDK4/6 is key kinase which promote the cell cycle, and especially the expression of cyclin D1 and the activation of CDK4/6 to drive breast cancer proliferation. Currently positive data of several clinical trials using three CDK4/6 inhibitors (palbocilcib, ribociclib, abemaciclib) were published and primary endpoint were met in all phase III studies. Therefore, practice change of endocrine therapy has been achieved in ER positive MBC. This review will present clinical trial data, including both the efficacy and safety of CDK4/6 inhibitors for MBC, and describe the designs of the mainly ongoing clinical trials examining CDK4/6 inhibitors for the treatment of MBC and EBC.