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Advances in Therapy for Hormone Receptor (HR)-Positive, Human Epidermal Growth Factor Receptor 2 (HER2)-Negative Advanced Breast Cancer Patients Who Have Experienced Progression After Treatment with CDK4/6 Inhibitors

Authors Li C, Li X

Received 23 December 2020

Accepted for publication 13 April 2021

Published 3 May 2021 Volume 2021:14 Pages 2929—2939


Checked for plagiarism Yes

Review by Single anonymous peer review

Peer reviewer comments 3

Editor who approved publication: Dr Alberto Bongiovanni

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Chao Li,1,2 Xujun Li1,2

1Department of Breast Surgery, Hwa Mei Hospital, University of Chinese Academy of Sciences, Ningbo, Zhejiang, 315000, People’s Republic of China; 2Ningbo Institute of Life and Health Industry, University of Chinese Academy of Sciences, Ningbo, Zhejiang, 315000, People’s Republic of China

Correspondence: Chao Li
Department of Breast Surgery, Hwa Mei Hospital, University of Chinese Academy of Sciences, 41 Northwest Street, Ningbo, Zhejiang, 315000, People’s Republic of China
Tel +86 13732213328
Email [email protected]

Abstract: Approximately 70% of breast cancer (BC) cases are hormone receptor-positive (HR+) and human epidermal growth factor receptor 2-negative (HER2-) BC. Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors have acted as star drugs for reversing endocrine therapy (ET) resistance and improving the prognosis of patients with HR+ advanced breast cancer (ABC) since they were initially approved. However, progression eventually occurs. In this review, we summarize the recent treatment strategies post CDK4/6 inhibitors: 1) CDK4/6 inhibitors plus exemestane and everolimus; 2) phosphoinositide-3-kinase (PI3K) inhibitor alpelisib plus fulvestrant for patients with PIK3CA mutation; 3) poly (ADP-ribose) polymerase (PARP) inhibitor for patients with germline PALB2 mutations, somatic BRCA1/2 mutations, or germline BRCA1/2 mutations; 4) exemestane and everolimus; and (5) chemotherapy. These strategies are all supported by evidence from clinical trials and retrospective studies. We also describe potential future treatment strategies post CDK4/6 inhibitors, such as the trophoblast cell surface antigen 2 (Trop-2) directed antibody–drug conjugate, cyclin-dependent kinase 7 (CDK7) inhibitors, and B-cell lymphoma-2 (BCL-2) inhibitors.

Keywords: breast cancer, CDK4/6 inhibitors, endocrine therapy resistance, subsequent therapy


Breast cancer (BC) is the most common malignancy among women worldwide and seriously endangers the lives of patients, especially those with advanced breast cancer (ABC), in which the tumor metastasizes to other organs, such as the lungs, liver, brain, and bones.1–3 Surgery is less effective for ABC, and these patients have a poor prognosis.4–7 Approximately 70% of BC cases are hormone receptor-positive (HR+) and human epidermal growth factor receptor 2-negative (HER2-), which are sensitive to endocrine therapy (ET).8,9 Since tamoxifen was approved in the 1970s, more and more patients have benefited from ET.10 The prognosis of patients with HR+ ABC has been significantly improved.11 With the approval of aromatase inhibitors (AIs; such as exemestane) and selective estrogen receptor downregulators (SERDs; such as fulvestrant and elacestrant), there are now more ET options for HR+ BC.12–15 However, patients eventually develop ET resistance.16–19

Since they were initially approved, cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors (such as palbociclib, ribociclib and abemaciclib) have acted as star drugs for reversing ET resistance and improving the prognosis of patients with HR+ ABC.20–23 With good results from clinical trials, CDK4/6 inhibitors plus AIs or fulvestrant represent the standard first- or second-line therapy for HR+ ABC.24–35 Despite the efficacy of CDK4/6 inhibitors for HR+ ABC, progression eventually occurs.36–38

In this review, we summarize the recent treatment strategies for patients who have experienced progression post CDK4/6 inhibitors, based on evidence from clinical trials and retrospective studies, and describe the potential choices post CDK4/6 inhibitors.

Continue CDK4/6 Inhibitors and Add a Subsequent Line of Therapy

CDK4/6 Inhibitors Plus Mammalian Target of Rapamycin (mTOR) Inhibitor (Everolimus) and Steroidal AI (Exemestane)

At present, CDK4/6 inhibitors are combined with ET drugs. When progression occurs, CDK4/6 inhibitors can be continued with other ET drugs. The Phase II Triniti-1 trial was presented at the 2019 American Society of Clinical Oncology (ASCO) annual meeting and assessed the ongoing use of the CDK4/6 inhibitor ribociclib plus everolimus and exemestane post progression on CDK4/6 inhibitors.39 This regimen demonstrated a clinical benefit rate (CBR) at week 24 of 41%, and the median progression-free survival (mPFS) was 5.7 months in patients who had experienced progression post CDK4/6 inhibitors. The median overall survival (mOS) was not estimable at the data cutoff point. Subgroup analysis showed that the regimen had relatively poor efficacy in patients with estrogen receptor 1 (ESR1) mutation (6.9 vs 3.5 months; hazard ratio: 1.76, 95% confidence interval [CI]: 1.01–3.05).

CDK4/6 Inhibitors Plus Immunotherapy

In vitro research has shown that CDK4/6 inhibitors plus an anti-programmed cell death 1 ligand 1 (PD-L1) drug is a more effective regimen than either drug alone.40 Therefore, the phase II PACE trial is a randomized, open-label, multicenter trial assessing the utility of ongoing CDK4/6 inhibitors plus fulvestrant following progression on CDK4/6 inhibitors plus AIs.41 The patients in this trial were randomized into three groups: group A: fulvestrant monotherapy; group B: ongoing CDK4/6 inhibitor palbociclib plus fulvestrant; and Group C: anti-PD-L1 drug (avelumab) plus ongoing CDK4/6 inhibitor palbociclib and fulvestrant. We are looking forward to the trial results. The trial will indicate whether it is effective to continue the CDK4/6 inhibitor plus fulvestrant or plus an anti-PD-L1 drug and fulvestrant, among patients who developed resistance on CDK4/6 inhibitors plus AIs.

Change to a Regimen without CDK4/6 Inhibitors (Including ET-Based Regimens)

Phosphoinositide-3-Kinase (PI3K) Inhibitor (Alpelisib) Plus Fulvestrant

The PI3K pathway is frequently mutated in HR+ BC, which can lead to ET resistance.42,43 About 40% of PIK3CA mutations in HR+ BC lead to excessive PI3K pathway activation.44–47 PI3K inhibitors can inhibit the growth of estrogen-independent ER+ BC cells that exhibit PI3K pathway activation.48,49 Alpelisib (byl719) is a highly selective inhibitor of the PI3Kα subtype.50 The Phase III SOLAR-1 trial presented at the 2018 San Antonio Breast Cancer Symposium (SABCS)51,52 showed that the mPFS for patients with PI3KCA mutation was prolonged by alpelisib plus fulvestrant compared to placebo plus fulvestrant (11 vs 5.3 months, hazard ratio: 0.50–0.85, p=0.00065). For patients pretreated with CDK4/6 inhibitors, the mPFS of alpelisib plus fulvestrant was also prolonged compared to the control group (5.5 vs 1.8 months, hazard ratio: 0.48, 95% CI: 0.17–1.36), indicating that the regimen was effective among patients with PI3KCA mutation who were pretreated with CDK4/6 inhibitors. However, in this trial, only 20 patients had had PI3KCA mutation and had previously used CDK4/6 inhibitors, so the small sample size might have influenced the results. Next, the phase II BYLieve trial was presented at the 2020 ASCO annual meeting,53 and it assessed the efficacy of alpelisib plus fulvestrant or letrozole in patients with PIK3CA-mutated HR+, HER2- ABC post CDK4/6 inhibitors. Patients in Cohort A had developed resistance during treatment with CDK4/6 inhibitors plus AI and were treated with alpelisib plus fulvestrant. Their mPFS was 7.3 months, and 50.4% were alive without disease progression at 6 months. With a well-characterized safety profile and a sample size of 121 patients, the BYLieve trial supported the use of alpelisib plus fulvestrant for patients with PIK3CA-mutated HR+, HER2-, ABC post CDK4/6 inhibitors, confirming the SOLAR-1 results.

mTOR Inhibitor (Everolimus) Plus Steroidal AI (Exemestane)

In the phase III BOLERO-2 trial, exemestane plus everolimus significantly improved mPFS compared to exemestane plus placebo (11 vs 4.1 months).54,55 Subgroup analysis found that the more lines of treatment patients had received, the more benefits patients obtained from everolimus.56 However, none of the patients in this trial were previously treated with CDK4/6 inhibitors and there were no trials that directly assessed the efficacy of exemestane plus everolimus in patients post CDK4/6 inhibitors.

A retrospective study conducted in Portland showed that this regimen has the same effects on patients with HR+ ABC regardless of prior CDK4/6 inhibitor use (mPFS: 3.6 vs 4.2 months for prior CDK4/6 inhibitor use and no prior use, respectively, hazard ratio: 1.22, 95% CI: 0.65–2.28, p=0.538; mOS: 15.6 vs 11.3 months, respectively, hazard ratio: 0.70, 95% CI: 0.35–1.40, p=0.308).57 The study involved 43 patients, 17 who had received prior CDK4/6 inhibitors and 26 who had not. Patient characteristics, including other prior therapies and metastasis sites, were not significantly different. Thus, everolimus plus exemestane may be effective for HR+ ABC regardless of prior CDK4/6 inhibitor use.

Poly (ADP-Ribose) Polymerase (PARP) Inhibitors

PARP inhibitors such as olaparib and talazoparib had good therapeutic effects on HER2- patients with germline BRCA mutation.58,59 However, the patients had not received prior CDK4/6 inhibitors. The phase II TBCRC 048 trial of olaparib monotherapy in metastatic BC patients with germline or somatic mutations in homologous recombination pathway genes was presented at the 2020 ASCO annual meeting.60 It showed that olaparib was effective for some patients post CDK4/6 inhibitors. In this trial, 93% of the HR+ and HER2- patients were previously treated with CDK4/6 inhibitors. Olaparib had significant effects on both patients with germline PALB2 mutations (objective response rate [ORR]: 82%, CBR at 18 weeks: 100%, mPFS: 13.3 months, 90% CI: 12 months to not reached) and patients with somatic BRCA1/2 mutations (ORR: 50%, CBR at 18 weeks: 67%, mPFS: 6.3 months, 90% CI: 4.4 months to not reached). This provides a new choice for patients with germline PALB2 mutations or somatic BRCA1/2 mutations post CDK4/6 inhibitor resistance.

The phase III EMBRACA trial compared the safety and efficacy of talazoparib monotherapy vs protocol-specific physician’s choice in patients with locally advanced BC with germline BRCA mutations.61 Prespecified subgroup analysis showed prolonged mPFS with talazoparib (9.4 vs 6.7 months, hazard ratio: 0.47, 95% CI: 0.32–0.71) for HR+/HER2- patients. However, there was no prespecified subgroup analysis of patients pretreated with CDK4/6 inhibitors. Talazoparib may be useful for patients pretreated with CDK4/6 inhibitors. Thus, PARP inhibitors may be tried for patients with germline BRCA1/2 mutations post CDK4/6 inhibitor resistance.60–62 All the above-mentioned clinical trials and retrospective studies are shown in Table 1.

Table 1 Clinical Trials and Retrospective Studies Post CDK4/6 Inhibitors Mentioned

Other Targets

Trophoblast cell-surface antigen-2 (Trop-2) is expressed in epithelial cancers, including HR+ ABC, and it is associated with worse survival.63,64 A Trop-2 directed antibody–drug conjugate sacituzumab govitecan (IMMU-132) has shown benefit in HR+ ABC.65 This therapeutic agent is a potentially valuable for patients pretreated with CDK4/6 inhibitors. The phase III TROPICS-02 trial is currently investigating this agent.66

Cyclin-dependent kinase 7 (CDK7) inhibitors are emerging as promising BC drugs, being effective for HR+ BC in vitro and in vivo.67,68 A Phase I trial (NCT03363893) of patients pretreated with CDK4/6 inhibitors is ongoing.

B-cell lymphoma-2 (BCL2) is an estrogen-responsive gene that is overexpressed in approximately 80% of primary HR+ BC cases.69–71 Preclinical data (based on patient-derived xenograft models) indicate that BCL-2 inhibitors may be effective in HR+ BC.72 A phase I trial (NCT03584009) of patients pretreated with CDK4/6 inhibitors is ongoing.

With the development of treatment and detection technologies, many other potential therapeutic targets have been found among patients with CDK4/6 inhibitor resistance (Figure 1).36,73 A better understanding of the mechanism of CDK4/6 inhibitor resistance may improve the rational selection of next-line therapy.37,38 Loss of retinoblastoma protein (RB), p16 amplification, CCNE1 overexpression, fibroblast growth factor receptor 1 (FGFR1) amplification, mitotic Aurora kinase (AURKA) amplification, E2F amplification, and cyclin-dependent kinase 2 (CDK2) overexpression have all been reported to be associated with CDK4/6 inhibitor resistance.73–83 We can use tissue or liquid biopsies to identify potential therapeutic targets in patients with CDK4/6 inhibitor resistance and provide individualized therapy based on the results.73 For example, if ESR1 mutation is found, we can use SERD drugs such as fulvestrant or elacestrant to deal with the ESR1 mutation.84,85 For patients with FGFR1 amplification, AURKA amplification, or CDK2 overexpression, FGFR1 inhibitors, AURKA inhibitors, and CDK2 inhibitors, respectively, could be used to treat patients who developed resistance during CDK4/6 inhibitor use.78–80 However, FGFR1 inhibitors, AURKA inhibitors, CDK2 inhibitors, and other new targeted drugs treating cancer associated with CDK4/6 inhibitor resistance are still in development for clinical trials (Table 2).86–88

Figure 1 Mechanisms underlying CDK4/6 inhibitor resistance. Multiple factors involved in cell cycle regulation are associated with CDK4/6 inhibitor resistance, such as loss of RB, p16 amplification, CCNE1 overexpression, FGFR1 amplification, AURKA amplification, and E2F amplification.

Abbreviations: CDK, cyclin-dependent kinase; RB, retinoblastoma protein; AURKA, mitotic Aurora kinase; MEK, mitogen-activated ERK-activating kinase; mTOR, mammalian target of rapamycin; PIP2, phosphatidylinositol-4, 5-bisphosphate; PIP3, phosphatidylinositol-3,4,4-trisphosphate; PI3K, phosphoinositide-3-kinase; PTEN, phosphatase and tensin homolog; RAS, rat sarcoma; ERK, extracellular signal-regulated kinases; FGFR1, fibroblast growth factor receptor 1.

Table 2 Other Clinical Trials Post CDK4/6 Inhibitors Globally


Chemotherapy is also a good choice for patients who develop resistance during CDK4/6 inhibitor use.89 With the approval of new and classic chemotherapeutics such as anthracyclines, taxanes, nanoparticle albumin-bound (nab)-paclitaxel, vinorelbine, capecitabine, platinum, and eribulin, which have all been shown to be effective for ABC in clinical trials, we now have more choices for first-line and later chemotherapy.90–97 There are three ongoing clinical trials (TATEN, TROPICS-02, NCT04134884) to assess the effect of chemotherapy post CDK4/6 inhibitors progression (Table 2). Due to its different mechanisms of action against BC, chemotherapy, a cytotoxic treatment, is effective for patients post CDK4/6 inhibitor resistance.


Since the approval of CDK4/6 inhibitors for patients with HR+ ABC, they have been accepted by global experts as they can reverse ET resistance and significantly improve the prognosis of these patients. As star drugs, CDK4/6 inhibitors have achieved amazing success by prolonging the PFS and OS of patients with HR+ ABC. Therefore, according to the National Comprehensive Cancer Network (NCCN) guidelines, CDK4/6 inhibitors have gradually been promoted from late- to first- and second-line treatment, indicating that they have good therapeutic effects.

However, in ABC, no matter how good CDK4/6 inhibitors are, resistance eventually occurs. How to deal with CDK4/6 inhibitor resistance will be a major BC research topic going forward. In this study, we have discussed Phase I, II and III clinical trials and retrospective studies post CDK4/6 inhibitors to try to answer this question. The current evidence supports the following conclusions regarding therapeutic strategies post CDK4/6 inhibitor use: (1) CDK4/6 inhibitors plus exemestane and everolimus have clinical benefits (mPFS: 5.7 months). However, for patients with ESR1 mutation, the effect is much lower than for patients with the wildtype gene. CDK4/6 inhibitors plus immunotherapeutic PD-L1 inhibitors were effective for HR+ BC in vitro, but there is not yet any obvious evidence from clinical trials. We are looking forward to the results of the PACE trial. (2) CDK4/6 inhibitor regimens can be replaced with a different regimen (including an ET-based regimen). For example, for patients with PIK3CA mutation, the PIK3 inhibitor alpelisib plus fulvestrant improves clinical outcomes (mPFS: 5.5–7.3 months according to the SOLAR-1 and BYLieve trials). Limited evidence suggests that everolimus plus exemestane is an effective post CDK4/6 inhibitors (mPFS: 3.6 months; mOS: 15.6 months). The phase II TBCRC 048 trial showed that for patients with germline PALB2 mutations or somatic BRCA1/2 mutations, olaparib can be used post CDK4/6 inhibitors. Olaparib or talazoparib can also be attempted in patients with germline BRCA1/2 mutations post CDK4/6 inhibitors. (3) When patients develop CDK4/6 inhibitor resistance, ET does not necessarily need to be continued, as chemotherapy can be started. As the earliest systemic treatment for BC, chemotherapy significantly improves prognosis. And with the approval of new and classic chemotherapeutics, we now have more choice for first-line or later chemotherapy strategies that are effective against CDK4/6 inhibitor resistance.

mTOR inhibitors and PIK3 inhibitors act as upstream signaling pathway of CDK4/6 inhibitors and are related to the causes of CDK4/6 inhibitor resistance. Thus, these drugs may be able to overcome CDK4/6 inhibitor resistance. PAPP inhibitors, chemotherapy, and other targeted drugs work in other pathways, causing cancer cell apoptosis in order to overcome CDK4/6 inhibitor resistance.

With a better understanding of BC, we can further our understanding of the mechanisms of CDK4/6 inhibitor resistance. Using tissue and liquid biopsies, we will be able to identify the mutations leading to the resistance and then use the mutations as targets and develop drugs to treat these mutations, which could provide individualized treatment options. For example, if the ESR1 mutation is found, we can use SERD drugs such as fulvestrant or elacestrant. Regarding other mutations, such as AURKA or FGFR2 mutations, the corresponding drugs are still in clinical trials. We believe that, in the near future, based on the mechanism of drug resistance, targeted treatment based on tissue or liquid biopsies could benefit patients. Future treatments will be more precise. With the development of effective targeted drugs, such as Trop-2 directed antibody–drug conjugate, CDK-7 inhibitor, BCL-2 inhibitor, we can delay or even eliminate CDK4/6 inhibitor resistance. Additionally, we can continue to use chemotherapy to overcome CDK4/6 inhibitor resistance.


BC, breast cancer; HR+, hormone receptor positive; Her2-, human epidermal growth factor receptor 2 negative; ABC, advanced breast cancer; ET, endocrine therapy; AIs, aromatase inhibitors; CDK, cyclin-dependent kinase; PARP, poly (ADP-ribose) polymerase; Trop-2, trophoblast cell surface antigen 2; CDK7, cyclin-dependent kinase 7; BCL-2, B-cell lymphoma-2; SERDs, selective estrogen receptor downregulators; CI, confidence interval; PD-L1, programmed cell death 1 ligand 1; PI3K, Phosphoinositide-3-kinase; mTOR, mammalian target of rapamycin; CBR, clinical benefit rate; PFS, progression-free survival; OS, overall survival; ESR1, estrogen receptor 1; SABCS, San Antonio Breast Cancer Symposium; ASCO, American Society of Clinical Oncology; NCCN: National Comprehensive Cancer Network; RB, retinoblastoma protein; AURKA, mitotic Aurora kinase; MEK, mitogen-activated ERK-activating kinase; PIP2, phosphatidylinositol-4, 5-bisphosphate; PIP3, phosphatidylinositol-3,4,4-trisphosphate; PI3K, phosphoinositide-3-kinase; PTEN, phosphatase and tensin homolog; RAS, rat sarcoma; ERK, extracellular signal-regulated kinase; FGFR1, fibroblast growth factor receptor 1; HR, hormone receptor; RIB, ribociclib; EVE, everolimus; EXE, exemestane; mPFS, median progression-free survival; mOS, median overall survival; ORR, objective response rate; Ful, fulvestrant; Pal, palbociclib; Ave, avelumab; NA, not available. TAM, tamoxifen; LHRHa, luteinizing Hormone Releasing Hormone analogues; ER, estrogen receptor.

Data Sharing Statement

Not applicable.

Ethics Approval and Consent to Participate

Not applicable.

Consent for Publication

Not applicable.


The authors thank the members of their department for their research work.

Author Contributions

Both authors made substantial contributions to review conception and design, acquisition of data, and interpretation of data. Chao Li drafted the manuscript and Xujun Li revised it critically for important intellectual content. Both authors agreed to submit to the journal. Both authors gave final approval for the version to be published and agreed to be accountable for all aspects of the work.


No funding support.


The authors report no conflicts of interest in this work.


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