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.     

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。     

CDK4/6抑制剂联合mTOR抑制剂在乳腺癌中的作用机制

CDK-RB-E2F通路和PI3K-AKT-mTOR通路对乳腺癌耐药机制起到了关键作用。通过对两个通路的研究发现,在治疗激素受体阳性的乳腺癌时,CDK4/6抑制剂与内分泌药物联合使用可以提高患者的生存率,mTOR抑制剂也显示出抗肿瘤的实力。最近的研究表明,mTOR抑制剂和CDK4/6抑制剂联合使用可以进一步抑制CDK-RB-E2F通路激活,协同控制肿瘤细胞增殖。同时发现CDK4/6抑制剂耐药患者的CDK-RB-E2F通路重新激活,仍对mTOR抑制剂敏感。还有研究表明CDK4/6抑制剂和mTOR抑制剂可以通过自噬作用协同控制肿瘤的发生。本文针对两药联合在乳腺癌中的作用机制进行综述。     

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抑制剂的联合治疗模式

 细胞周期素依赖激酶（CDKs）抑制剂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，CDK4/6抑制剂还可以联合表皮生长因子受体（EGFR）抑制剂、磷脂酰肌醇-3激酶（PI3K）/哺乳动物雷帕霉素靶蛋白（mTOR）抑制剂、化学治疗、免疫治疗、分子靶向治疗和其他治疗。联合治疗模式克服了CDK4/6抑制剂的耐药并提高了临床疗效，开启了肿瘤精准治疗的一扇新窗口。     

Ribociclib for the treatment of hormone receptor-positive,human epidermal growth factor receptor 2-negative advanced breast cancer

Introduction: The emergence of cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitors marked a significant advancement in the treatment of advanced breast cancer. Ribociclib is an orally bioavailable, highly selective inhibitor of CDK4/6. In combination with various endocrine therapies, ribociclib has demonstrated clinical activity as a first-line therapy for patients with HR+, HER2? advanced breast cancer, without compromising the favorable toxicity profile associated with endocrine therapy. Thus, ribociclib is now considered a new standard of care for HR+, HER2? advanced breast cancer.

Areas covered: This review provides a concise overview of the preclinical and clinical development of ribociclib, including evidence of its clinical activity and safety profile when combined with endocrine therapy in HR+, HER2? advanced breast cancer.

Expert commentary: CDK4/6 inhibition represents a promising treatment option for patients with HR+ metastatic breast cancer. Ribociclib significantly improved progression-free survival in patients receiving first-line endocrine therapy for HR+, HER2? advanced breast cancer. Planned and ongoing trials investigating ribociclib in combination with other endocrine therapies and in various clinical settings will help to determine the optimal treatment sequence for different patient populations.     

Activation of PI3K/Akt/mTOR signaling in the tumor stroma drives endocrine therapy-dependent breast tumor regression

Improved efficacy of neoadjuvant endocrine-targeting therapies in luminal breast carcinomas could be achieved with optimal use of pathway targeting agents. In a mouse model of ductal breast carcinoma we identify a tumor regressive stromal reaction that is induced by neoadjuvant endocrine therapy. This reparative reaction is characterized by tumor neovascularization accompanied by infiltration of immune cells and carcinoma-associated fibroblasts that stain for phosphorylated ribosomal protein S6 (pS6), downstream the PI3K/Akt/mTOR pathway. While tumor variants with higher PI3K/Akt/mTOR activity respond well to a combination of endocrine and PI3K/Akt/mTOR inhibitors, tumor variants with lower PI3K/Akt/mTOR activity respond more poorly to the combination therapy than to the endocrine therapy alone, associated with inhibition of stromal pS6 and the reparative reaction. In human breast cancer xenografts we confirm that such differential sensitivity to therapy is primarily determined by the level of PI3K/Akt/mTOR in tumor cells. We further show that the clinical response of breast cancer patients undergoing neoadjuvant endocrine therapy is associated with the reparative stromal reaction. We conclude that tumor level and localization of pS6 are associated with therapeutic response in breast cancer and represent biomarkers to distinguish which tumors will benefit from the incorporation of PI3K/Akt/mTOR inhibitors with neoadjuvant endocrine therapy.     

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。     

Endocrine resistance associated with activated ErbB system in breast cancer cells is reversed by inhibiting MAPK or PI3K/Akt signaling pathways

Endocrine therapy resistance is one of the main challenges in the treatment of estrogen receptor positive (ER+) breast cancer patients. This study showed that two ER+ human breast carcinoma cell lines derived from MCF‐7 (MVLN cells) that have acquired under OH‐Tamoxifen selection two distinct phenotypes of endocrine resistance both displayed constitutive activation of the PI3K/Akt and MAPK pathways. Aberrant expression and activation of the ErbB system (phospho‐EGFR, phospho‐ErbB2, phospho‐ErbB3, over‐expression of ErbB4 and over‐expression of several ErbB ligands) were also observed in the two resistant cell lines, suggesting the existence of an autocrine loop leading to constitutive activation of MAPK and PI3K/Akt survival pathways. The recent clinical use of specific signal transduction inhibitors is one of the most promising therapeutic approaches in breast cancers. The MEK inhibitor PD98059 and the PI3K inhibitor LY294002 were both able to enhance the cytostatic effect of OH‐Tamoxifen or fulvestrant on MVLN sensitive cells. In the two resistant cell lines, inhibition of the MAPK or the PI3K/Akt pathways associated with endocrine therapy was sufficient to reverse OH‐Tamoxifen or fulvestrant resistance. Investigating the effect of a combination of both inhibitors on the reversion of OH‐Tamoxifen and fulvestrant resistance in the two resistant cell lines suggested that, in clinical practice, a strategy combining the two inhibitors would be the best approach to target the different endocrine resistance phenotypes possibly present in a tumor. In conclusion, the combination of MAPK and PI3K inhibitors represents a promising strategy to overcome endocrine therapy resistance in ER+ breast cancer patients.     

激素受体阳性乳腺癌靶向治疗进展

内分泌治疗是激素受体阳性乳腺癌的主要治疗手段。内分泌耐药是这部分患者肿瘤复发或进展的主要原因。近期研究发现一系列导致激素受体阳性乳腺癌不依赖雌激素的抵抗机制,开发出相应的靶向治疗药物,其中包括细胞周期蛋白依赖性激酶4/6抑制剂、mTOR抑制剂、表皮生长因子受体抑制剂、抗血管生成药物、组蛋白去乙酰化酶抑制剂、成纤维细胞生长因子受体抑制剂、胰岛素样生长因子受体抑制剂,以及免疫检查点抑制剂等。这些药物被用于阻断耐药通路并提高内分泌治疗疗效,其中已经被批准上市的靶向药物有依维莫司和palbociclib。本文将对内分泌联合靶向治疗的药物研究进展进行综述。      

CDK4/6 inhibitor plus endocrine therapy for hormone receptor‐positive,HER2‐negative metastatic breast cancer: The new standard of care

Patients presenting with hormone receptor‐positive (HR⁺), human epidermal growth factor receptor 2‐negative (HER2^–) metastatic breast cancer (MBC) are usually treated with endocrine therapy (ET), except if there is a concern about endocrine resistance or a need to achieve rapid disease control due to visceral crisis. The combination of CDK4/6 inhibitor + ET has now replaced single‐agent ET as the standard first‐line treatment; and it can also be considered a standard option in the second‐line setting. This review briefly summarizes recently reported efficacy findings from the key phase III clinical trials of CDK4/6 inhibitor + ET in patients with HR⁺/HER2^– MBC, including evidence that adding a CDK4/6 inhibitor to ET improves overall survival and does so without reducing patients’ quality of life. There is still much to learn regarding the use of CDK4/6 inhibitors and how they may be optimally integrated into clinical practice. In particular, there is a need for specific biomarkers that help predict the likelihood of response or resistance to CDK4/6 inhibitor therapy; and for data to guide treatment decisions when a patient's disease progresses on a CDK4/6 inhibitor.     

Everolimus Plus Exemestane for the Treatment of Advanced Breast Cancer: A Review of Subanalyses from BOLERO-2

Hormone receptor–positive breast cancer is typically managed with endocrine therapies. However, resistance to endocrine therapy results in disease progression in a large proportion of breast cancers. Through the understanding of the mechanisms of endocrine resistance, identification of implicated pathways and targets has led to the development of novel agents targeting these pathways. Phosphoinositide 3-kinase/protein kinase B/mammalian target of rapamycin (PI3K/AKT/mTOR) pathway aberrations are common in breast cancer, with increased PI3K/AKT/mTOR signaling associated with resistance to endocrine and human epidermal growth factor receptor 2 (HER2)–targeted therapies. The mTOR inhibitor everolimus, in combination with exemestane, has been approved for patients with advanced hormone receptor–positive/HER2-negative breast cancer who progress on prior nonsteroidal aromatase inhibitor therapy based on results reported in the Breast Cancer Trials of Oral Everolimus-2 (BOLERO-2) study. This review will summarize the overall findings from BOLERO-2 and will consider available subanalyses by age, Asian origin, visceral or bone metastases, and prior therapy, with the aim of identifying populations most likely to benefit from everolimus therapy. The review will also summarize safety findings and their management and the effects of everolimus on quality of life.Abbreviations: AE, adverse event; BSAP, bone-specific alkaline phosphatase; CBR, clinical benefit rate; CR, complete response; CTX, C-terminal cross-linking telopeptide of type 1 collagen; HER2, human epidermal growth factor receptor 2; HR, hazard ratio; NIP, noninfectious pneumonitis; ORR, objective response rate; PI3K/AKT/mTOR, phosphoinositide 3-kinase/protein kinase B/mammalian target of rapamycin; PFS, progression-free survival; P1NP, amino-terminal propeptide of type 1 collagen; PR, partial response; QoL, quality of life; TDD, time to definitive deterioration     

Landmark trials in the medical oncology management of metastatic breast cancer

Significant advances in the management of metastatic breast cancer (MBC) have guided more personalized treatment according to disease biology and led to improved survival outcomes and quality of life for patients. In this review, we discuss landmark clinical trials in medical oncology that have shaped the current standard of care for MBC. Combinations of endocrine therapy with cyclin-dependent kinase 4/6 inhibitors have led to substantial improvements in overall survival, thus becoming standard first-line treatment for patients with HR-positive MBC. Inhibition of the PI3K and mTOR pathway is another promising strategy to overcome resistance to endocrine therapy. HER2-targeted therapies have also evolved with the addition of pertuzumab to trastuzumab plus a taxane demonstrating remarkable overall survival advantage in patient with HER2-positive MBC. In second or later line therapies, novel anti-HER2 antibody-drug conjugates and TKIs have durable antitumor activity, survival benefit, and encouraging efficacy in the subgroup of patients with brain metastases. Triple negative breast cancer remains the most challenging subtype due to lack of druggable targets. Immunotherapy for patients with PDL-1 expression on tumor infiltrating immune cells and poly (ADP-ribose) polymerase inhibitors for those with germline BRCA1/2 mutations are the latest approved targeted strategies in this population. Numerous obstacles still exist in treating MBC, especially for patients whose disease develops resistance to available agents. Future research is eagerly awaited to address the optimal sequence or combination of therapies and to identify better biomarkers to guide precision medicine.     

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。     

PI3K/AKT/mTOR pathway activation in primary and corresponding metastatic breast tumors after adjuvant endocrine therapy

Both preclinical and clinical data suggest that activation of the PI3K/AKT/mTOR pathway in response to hormonal therapy results in acquired endocrine therapy resistance. We evaluated differences in activation of the PI3K/AKT/mTOR pathway in estrogen receptor α (ERα) positive primary and corresponding metastatic breast cancer tissues using immunohistochemistry for downstream activated proteins, like phosphorylated mTOR (p‐mTOR), phosphorylated 4E Binding Protein 1 (p‐4EBP1) and phosphorylated p70S6K (p‐p70S6K). For p‐mTOR and p‐4EBP1, the proportion of immunostained tumor cells (0–100%) was scored. Cytoplasmic intensity (0–3) was assessed for p‐p70S6K. The difference between expression of these activated PI3K/AKT/mTOR proteins‐ in primary and metastatic tumor was calculated and tested for an association with adjuvant endocrine therapy. In patients who had received endocrine therapy (N = 34), p‐mTOR expression increased in metastatic tumor lesions compared to the primary tumor (median difference 45%), while in patients who had not received adjuvant endocrine therapy (N = 37), no difference was found. Similar results were observed for p‐4EBP1 and p‐p70S6K expression. In multivariate analyses, adjuvant endocrine therapy was significantly associated with an increase in p‐mTOR (p = 0.01), p‐4EBP1 (p = 0.03) and p‐p70S6K (p = 0.001), indicating that compensatory activation of the PI3K/AKT/mTOR pathway might indeed be a clinically relevant resistance mechanism resulting in acquired endocrine therapy resistance.     

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.     

乳腺癌内分泌治疗加时代

激素受体（HR）阳性乳腺癌对内分泌治疗有效但也会产生耐药,以内分泌治疗为基础的联合治疗克服了耐药并提高了内分泌治疗的效果。乳腺癌的治疗已进入内分泌治疗加时代,这包括乳腺癌内分泌治疗联合分子靶向药物、血管生成抑制剂、磷脂酰肌醇-3激酶（PI3K）抑制剂、哺乳动物雷帕霉素靶蛋白(mTOR)抑制剂、细胞周期素依赖激酶（CDK)4/6抑制剂、抗HER2剂和表观遗传调节剂。本文系统总结乳腺癌内分泌联合治疗的临床研究结果和潜在药物。     

CDK4/6 inhibition in early and metastatic breast cancer: A review

Breast cancer (BC) is responsible for 14% of cancer-related deaths in women [1]. Cyclin-dependent kinase (CDK) 4/6 inhibitors are a new class of selective drugs, offering an effective and tolerable treatment. CDK4/6 inhibitors induce cell cycle arrest in the G1 phase, and may thereby prevent tumour progression. Three CDK4/6 inhibitors have been tested in clinical BC trials: palbociclib, ribociclib, and abemaciclib. The Food and Drug Administration (FDA) and European Commission (EMA) have approved palbociclib for the treatment of patients HR+ HER2- locally advanced or metastatic BC (aBC) in combination with an aromatase inhibitor as initial therapy in postmenopausal women or in combination with fulvestrant in women who have received prior endocrine therapy. Ribociclib has been approved by the FDA in combination with an aromatase inhibitor as initial therapy for postmenopausal women with HR+ HER2- aBC. Moreover, CDK4/6 inhibitors have shown promising results in the (neo)adjuvant setting. In this review, the principal completed and ongoing clinical trials in aBC are reviewed for both the metastatic as (neo)adjuvant setting. Tables will provide a complete overview of the ongoing clinical trials. At last, the future perspectives of these CDK4/6 inhibitors are discussed.