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.     

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.     

The next era of treatment for hormone receptor-positive,HER2-negative advanced breast cancer: Triplet combination-based endocrine therapies

Until recently, the standard of care for hormone receptor-positive (HR+) breast cancer was single-agent endocrine therapy, which aims to prevent estrogen receptor signaling. This therapeutic strategy has extended survival without the toxicity associated with chemotherapy, but primary endocrine therapy resistance is common, and secondary resistance develops over time. Adjunct downstream inhibition of the cyclin-dependent kinase (CDK)4/6 pathway, intended to delay and prevent endocrine therapy resistance, has further extended progression-free survival in patients receiving endocrine therapy; however, resistance still eventually develops in these patients. Addition of phosphatidylinositol-3 kinase (PI3K) or mammalian target of rapamycin (mTOR) inhibitors to combined CDK4/6 and endocrine inhibitor regimens may help prolong CDK4/6 inhibitor sensitivity. Early trials combining CDK4/6 inhibitors, PI3K or mTOR inhibitors, and endocrine therapy have shown encouraging signs of clinical activity. However, further research is needed to help understand the extent of treatment benefit from triplet therapy and where this strategy will fit in the treatment sequence for patients with HR+ breast cancer.     

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.     

Novel Strategies in Hormone Receptor-Positive Advanced Breast Cancer: Overcoming Endocrine Resistance

Resistance to standard endocrine therapies in hormone receptor-positive advanced breast cancer represents a significant clinical challenge. Different intracellular signaling pathways mediate independent activation of the estrogen receptor (ER), promoting tumor cell proliferation despite anti-hormonal treatment. Recently, the inhibition of cell cycle regulators, CDK4 and CDK6, has demonstrated to significantly enhance the effectiveness of endocrine therapy by overcoming or delaying resistance to estrogen blockade. Strategies such as inhibition of the PI3K/mTOR pathway or epigenetic modulation of ER-related gene expression are closely following the trail of CDK inhibitors. Here, we seek to review the most recent efforts to improve outcomes in these patients, in an attempt to extend endocrine treatment and defer the need for cytotoxic regimens. We also discuss future directions to be considered in the treatment of ER-positive disease, as mechanisms of resistance to these new agents arise.     

CDK4/6抑制剂在乳腺癌治疗中的研究进展

细胞增殖、生长和分裂等过程受到细胞周期的严格调控,其调控机制在肿瘤的发生发展中发挥重要作用。近年来,多项研究显示,细胞周期蛋白依赖性激酶4和6（CDK4/6）抑制剂对于雌激素受体阳性或人类表皮生长因子受体阳性的乳腺癌患者具有较好的疗效,然而其在三阴性乳腺癌患者中的作用仍存在争议,故探究相关分子标志物以进一步筛选最能从该治疗中获益的乳腺癌人群对临床具有重要意义。此外视网膜母细胞瘤基因（Rb）也在乳腺癌中扮演关键角色,而cyclin D-CDK4/6-Rb信号通路因具有调控细胞周期限制点的作用,被认为是乳腺癌潜在的治疗靶点。本文将对CDK4/6抑制剂在乳腺癌治疗中的研究进展作一综述。     

激素受体阳性晚期乳腺癌内分泌治疗选择

激素受体阳性乳腺癌占所有乳腺癌的70%。内分泌治疗是这个亚型乳腺癌的主要治疗手段,最常见药物有他莫昔芬和芳香酶抑制剂如阿拉曲唑、来曲唑和依西美坦。全文重点总结新型内分泌治疗药物,如雌激素受体降解剂(Fulvestrant),以及新的靶向药物如mTOR抑制剂(Everolimus)、CDK4/6抑制剂(Palbociclib、Ribociclib和Abemaciclib)和PI3K抑制剂(Alpelisib、Buparlisib和Pictilisib)等。新的靶向药物联合内分泌治疗已经改变了临床实践,延长激素受体阳性晚期乳腺癌患者的生存期。     

CDK4/6抑制剂联合内分泌治疗晚期乳腺癌

内分泌治疗因疗效显著并具有安全性,是激素受体阳性(HR+)晚期乳腺癌患者的主要治疗方法。近年来内分泌领域发展迅速,如何延迟或逆转内分泌耐药及内分泌治疗新药物成为临床研究关注的焦点。研究发现,内分泌治疗耐药可能与CDK-RB-E2F通路有关,针对该通路的细胞周期蛋白依赖性激酶(CDK)4/6抑制剂可显著延缓HR+晚期乳腺癌患者内分泌耐药。CDK4/6抑制剂与内分泌药物联合使用,可提高HR+晚期乳腺癌患者的治疗客观缓解率,并可显著改善无进展生存期(PFS)。现就CDK4/6抑制剂的作用机制、药物有效性和安全性及相关临床试验做一综述。     

CDK inhibitors in cancer therapy,an overview of recent development

Dysregulated cell division, which leads to aberrant cell proliferation, is one of the key hallmarks of cancer. Therefore, therapeutic targets that block cell division would be effective for cancer treatment. Cell division is mainly controlled by a complex composed of cyclin and cyclin dependent kinases (CDKs). To date, the CDK inhibitors (CDKIs), specifically the ones that block the enzyme activity of CDK4 and CDK6 (CDK4/6), have been approved by FDA for the treatment of metastatic hormone receptor positive breast cancer. However, due to the non-selectivity and significant toxicity, most of the first generation CDK inhibitors (so called pan-CDK inhibitors that target several CDKs), have not been approved for clinical application. Despite this, great efforts and progress have been made to enable pan-CDK inhibitors application in the clinical setting. Notably, the development of combination therapy strategies in recent years has made it possible to reduce the toxicity and side effects of pan-CDK inhibitors. Thus, as a combination therapy approach, pan-CDK inhibitors regain great potential in clinical application. In this review, we introduced the CDK family members and discussed their major functions in cell cycle controlling. Then, we summarized the research progress regarding CDK inhibitors, especially those other than CDK4/6 inhibitors. We reviewed first-generation pan-CDKIs Flavopiridol and Roscovitine, and second-generation CDKIs Dinaciclib, P276-00, AT7519, TG02, Roniciclib, RGB-286638 by focusing on their developing stages, clinical trials and targeting cancers. The specific CDKIs, which targets to increase specificity and decrease the side effects, were also discussed. These CDKIs include CDK4/6, CDK7, CDK9, and CDK12/13 inhibitors. Finally, the efficacy and discrepancy of combination therapy with CDK inhibitors and PD1/PDL1 antibodies were analyzed, which might give insights into the development of promising strategy for cancer treatment.     

Practical Treatment Strategies and Future Directions After Progression While Receiving CDK4/6 Inhibition and Endocrine Therapy in Advanced HR+/HER2− Breast Cancer

Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) in combination with backbone endocrine therapy have markedly improved progression-free survival and overall survival over endocrine therapy alone in advanced hormone receptor–positive, HER2-negative (HR⁺/HER2⁻) breast cancer and are the standard of care in the first- or second-line setting. There are few data to drive decision making for subsequent treatment strategies after inevitable disease progression after CDK4/6i. Information about the genomic landscape of CDK4/6i-resistant disease is emerging. Resistance mechanisms appear to be varied, but mutations in PIK3CA and ESR1, which can be acquired while receiving treatment, are frequent. Activating PIK3CA mutations are present in up to 35% of patients and are now the most actionable genomic alteration in HR⁺/HER2⁻ advanced breast cancer with the recent approval of alpelisib and fulvestrant. Everolimus-based combinations and chemotherapy appear to have continued efficacy after progression while receiving CDK4/6i, although historical data on benefit include CDK4/6i-naive patients. Use of selective estrogen down-regulators over aromatase inhibitors is best once the patient has an acquired ESR1 mutation. Tumor biopsy with genomic sequencing and repeat biomarker analysis in patients with CDK4/6i- and endocrine-resistant disease will be integral to guide subsequent treatment strategies and to inform clinical trial eligibility. Promising novel therapeutics in CDK4/6i-resistant disease including oral selective estrogen down-regulators, fibroblast growth factor receptor antagonists, and immunotherapy will be discussed.     

The Role of CDK4/6 Inhibition in Breast Cancer

Imbalance of the cyclin D and cyclin‐dependent kinase (CDK) pathway in cancer cells may result in diversion away from a pathway to senescence and toward a more proliferative phenotype. Cancer cells may increase cyclin D‐dependent activity through a variety of mechanisms. Therapeutic inhibition of CDKs in tumors to negate their evasion of growth suppressors has been identified as a key anticancer strategy. In this review, we outline the development of CDK inhibitory therapy in breast cancer, including the initial experience with the pan‐CDK inhibitor flavopiridol and the next generation of oral highly selective CDK4 and CDK6 inhibitors PD0332991 (palbociclib), LEE011 (ribociclib), and LY2835219 (abemaciclib). Data from phase I and II studies in estrogen receptor‐positive (ER+) breast cancer demonstrate promising efficacy with manageable toxic effects, chiefly neutropenia. We discuss these studies and the phase III studies that are accruing or nearing completion. We describe the application of such therapy to other breast cancer settings, including HER2‐positive breast cancer and the adjuvant treatment of early breast cancer. We also discuss potential concerns surrounding the combination of CDK inhibitors with chemotherapy and their effects on repair of double‐strand DNA breaks in cancer cells. Oral highly selective CDK inhibitors show great promise in improving the outcomes of patients with ER+ breast cancer, although caution must apply to their combination with other agents and in the early breast cancer setting.     

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.     

Inhibiting CDK in Cancer Therapy: Current Evidence and Future Directions

Cell cycle dysregulation is a hallmark of all cancers, resulting in uncontrolled proliferation. Cyclin dependent kinases (CDKs), a family of proteins that are involved in the regulation of the cell cycle, are frequently overexpressed or mutated in cancer. Hence, CDK-inhibiting drugs have been developed and evaluated as cancer therapeutics. Clinical trials have shown CDK4/6 inhibitors (CDK4/6i) to be relatively safe and effective, and these are now standard of care treatment for advanced hormone receptor positive breast cancer. Some CDK4/6i drugs are also able to cross the blood brain barrier and may, therefore, offer effective therapy for primary and metastatic central nervous system malignancies. Ongoing research is also evaluating CDK4/6i for additional breast cancer subtypes and non-breast malignancies with promising early phase clinical trial results. Finally, pre-clinical research has identified potential biomarkers for CDK4/6i efficacy and is exploring potential resistance mechanisms to this treatment. Further clinical-translational research is needed to advance patient selection and combinatorial treatment strategies with CDK4/6i in breast cancer and other malignancies.     

CDK4/6抑制剂治疗乳腺癌的研究进展

对于HR+/HER-2-乳腺癌患者来说,内分泌治疗扮演着非常重要的角色。长久以来,医学工作者对于此类患者的内分泌耐药机制以及相关治疗方法的讨论也层出不穷。近年来,CDK4/6抑制剂为HR+患者的带来了福音,CDK4/6抑制剂单药治疗与联合内分泌治疗已经用于晚期乳腺癌的治疗。除了内分泌治疗之外,CDK4/6抑制剂还可以与抗HER-2药物、PD-L1、PD-L1抑制剂或其他靶向药物联合,联合治疗在一定程度上克服了CDK4/6抑制剂的耐药情况,并提高了治疗疗效,为乳腺癌患者的精准治疗带来了福音。     

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。     

Mechanism of resistance to endocrine therapy in breast cancer: the important role of PI3K/Akt/mTOR in estrogen receptor-positive,HER2-negative breast cancer

Endocrine therapy is a crucial treatment for estrogen receptor-positive (ER+) breast cancer, with proven clinical benefits. However, adaptive mechanisms emerge in the tumor, causing resistance to endocrine therapy. A better understanding of resistance mechanisms is needed to overcome this problem and to develop new, precise treatment strategies. Accumulating genetic and cancer biological studies demonstrate the importance of understanding the PI3K/Akt/mTOR and CDK4/6/RB pathways in ER+ HER2? breast cancer. PIK3CA (which encodes phosphatidylinositol-4, 5-bisphosphate 3-kinase catalytic subunit α) is frequently mutated in breast cancer, and 30% of advanced ER+ HER2? breast cancers have an activating PIK3CA mutation. AKT1 mutations (E17K) have been found in 1.4–8% of breast cancer patients. ER+ breast cancer patients preferentially demonstrate gain of CCND1 (cyclin D1; 58% in luminal B vs. 29% in luminal A) and CDK4 (25% in luminal B vs. 14% in luminal A) and loss of CDKN2A (p16) and CDKN2C (p18), which are negatively regulated with the cell cycle and are correlated with the CDK4/6/RB pathway. Abnormalities in PI3K/Akt/mTOR and CDK4/6/RB pathways due to genetic alterations result in deregulated kinase activity and malignant transformation. This review focuses on the recent reports of the essential role of PI3K/Akt/mTOR and CDK4/6/RB pathways in ER+ HER2? breast cancer.     

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。