NeoPalAna: Neoadjuvant Palbociclib, a Cyclin-Dependent Kinase 4/6 Inhibitor, and Anastrozole for Clinical Stage 2 or 3 Estrogen Receptor-Positive Breast Cancer

Cancer cell cycle dystopia: heterogeneity, plasticity, and therapy

The mammalian cell cycle has been extensively studied regarding cancer etiology, progression, and therapeutic intervention. The canonical cell cycle framework is supported by a plethora of data pointing to a relatively simple linear pathway in which mitogenic signals are integrated in a stepwise fashion to allow progression through G1/S with coordinate actions of cyclin-dependent kinases (CDK)4/6 and CDK2 on the RB tumor suppressor. Recent work on adaptive mechanisms and intrinsic heterogeneous dependencies indicates that G1/S control of the cell cycle is a variable signaling pathway rather than an invariant engine that drives cell division. These alterations can limit the effectiveness of pharmaceutical agents but provide new avenues for therapeutic interventions. These findings support a dystopian view of the cell cycle in cancer where the canonical utopian cell cycle is often not observed. However, recognizing the extent of cell cycle heterogeneity likely creates new opportunities for precision therapeutic approaches specifically targeting these states.

Functional genomic analysis of CDK4 and CDK6 gene dependency across human cancer cell lines

Cyclin-dependent kinase 4 (CDK4) and cyclin-dependent kinase 6 (CDK6) are key cell cycle regulators that are frequently dysregulated in human malignancies. CDK4/6 inhibitors are clinically approved for the treatment of hormone receptor-positive, HER2-negative (HR+/HER2-) breast cancer, but improved specificity and reduced toxicity might expand their use to other indications. Through analysis of publicly available genome-wide loss-of-function data combined with single and dual-targeting CRISPR assays, we found differential cell proliferation vulnerability of cell lines to either CDK4 deletion alone, CDK6 deletion alone, combined CDK4/CDK6 deletion, or neither. CDK6 expression was the best single predictor of CDK4 (negatively correlated) and CDK6 (positively correlated) dependencies in the cancer cell lines, with adenocarcinoma cell lines being more sensitive to CDK4 deletion and hematologic and squamous cancer cell lines being more sensitive to CDK6 deletion. RB-E2F signaling was confirmed as a main downstream node of CDK4/6 in these experiments as shown by the survival effects of RB1 deletion. Finally, we show in a subset of cancer cell lines not dependent on CDK4/6 that CDK2-CCNE1 is an important alternative dependency for cell proliferation. Together, our comprehensive data exploration and functional experiments delineate the landscape of pan-cancer CDK4/6 gene dependencies and define unique cancer cell populations that might be sensitive to CDK4-selective or CDK6-selective inhibitors.

Cyclin-Dependent Kinase 4 and 6 Inhibitors: A Quantum Leap in the Treatment of Advanced Breast Cancers

Breast cancer (BC) is defined as an uncontrolled growth of breast cells that affected 2.3 million women in 2020 alone. Until a few years earlier, radiotherapy and chemotherapy were the most commonly used treatments in treating BC; however, many trials and studies were conducted to test the competence of cyclin-dependent kinases 4/6 (CDK4/6) in arresting the cell cycle, and it was found that they were highly influential in halting the disease from progressing. Palbociclib, ribociclib, and abemaciclib are the three drugs that have been approved by the US Food and Drug Administration (FDA) and are even more efficient when used in combination with aromatase inhibitors and fulvestrant. This article aimed to explain the effect of CDK4/6 inhibitors on tumor cells and their efficacy in combination with other drugs. We further explored the development of resistance to these treatments and future possibilities.

Cyclin-dependent kinase 4 and 6 inhibitors in combination with neoadjuvant endocrine therapy in estrogen receptor-positive early breast cancer: a systematic review and meta-analysis

The combination of cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors and endocrine treatment has benefited patients with estrogen receptor-positive/human epidermal growth factor receptor 2-negative (ER + /HER2-) metastatic breast cancer; however, its effects in the neoadjuvant setting for ER + /HER2- early breast cancer (EBC) are unclear. Systematic searches were performed in PubMed, Embase, Cochrane Library, and major oncological meetings for trials of CDK4/6 inhibitors plus neoadjuvant endocrine treatment (NET) vs. NET/neoadjuvant chemotherapy (NACT) alone up to January 30, 2021. We assessed the efficacy of CDK4/6 inhibitors plus NET vs. NET/NACT alone in ER + /HER2- EBC. Six studies that included 803 patients treated with CDK4/6 inhibitors plus NET vs. NET/NACT alone were used. Compared with NET/NACT alone, CDK4/6 inhibitors plus NET increased the complete cell cycle arrest (CCCA) rate (OR, 9.00; 95% CI, 5.42-14.96; P < 0.001). Nonsignificant differences between CDK4/6 inhibitors and NET/NACT alone occurred in the preoperative endocrine prognostic index (PEPI)-0 rate (OR, 1.13; 95% CI, 0.59-2.18; P = 0.71), pathological complete response (pCR) rate (OR, 0.75; 95% CI, 0.13-4.29; P = 0.74), objective response rate (ORR) (OR, 0.70; 95% CI, 0.21-2.29; P = 0.55), and disease control rate (DCR) (OR, 1.16; 95% CI, 0.47-2.89; P = 0.74). CDK4/6 inhibitors plus NET indicated a high risk of neutropenia (OR, 56.43; 95% CI, 15.76-202.11; P < 0.001) as an adverse effect (AE) and elevated alanine aminotransferase (ALT) level (OR, 15.30; 95% CI, 2.02-115.98; P = 0.008) as grade 3/4 AEs. Compared with NET/NACT alone, CDK4/6 inhibitors plus NET increased CCCA rate in ER + /HER2- EBC patients. CDK4/6 inhibitors plus NET did not substantially improve the PEPI-0 rate, pCR rate, ORR, or DCR. The combination increased the risk of neutropenia and elevated ALT levels. In the neoadjuvant setting, addition of CDK4/6 inhibitors to NET may be an option for treating ER + /HER2- EBC.

How to Treat HR+/HER2- Metastatic Breast Cancer Patients after CDK4/6 Inhibitors: An Unfinished Story

CDK4/6 inhibitors in association with endocrine therapy represent the best therapeutic choice for either endocrine-sensitive or resistant hormone-receptor-positive advanced breast cancer patients. On the contrary, the optimal therapeutic strategy after the failure of CDK4/6 inhibitors-based treatment still remains an open question worldwide. In this review, we analyze the most studied mechanisms of resistance to CDK4/6 inhibitors treatment, as well as the most significant results of retrospective and prospective trials in the setting of progression after CDK4/6 inhibitors, to provide the reader a comprehensive overview from both a preclinical and especially a clinical perspective. In our opinion, an approach based on a deeper knowledge of resistance mechanisms to CDK4/6 inhibitors, but also on a careful analysis of what is done in clinical practice, can lead to a better definition of prospective randomized trials, to implement a personalized sequence approach, based on molecular analyses.

Serum thymidine kinase activity in patients with hormone receptor-positive and HER2-negative metastatic breast cancer treated with palbociclib and fulvestrant

BACKGROUND

Biomarkers for cyclin-dependent kinase 4/6 inhibitors, such as palbociclib, for patients with hormone receptor-positive/HER2-negative metastatic breast cancer are lacking. Thymidine kinase is a proliferation marker downstream of the cyclin-dependent kinase 4/6 pathway. We prospectively investigated the prognostic role of serum thymidine kinase activity (sTKa), in patients treated with Palbociclib + fulvestrant.

PATIENTS AND METHODS

PYTHIA was a phase II, single-arm, multicentre, trial that enrolled 124 post-menopausal women with endocrine-resistant hormone receptor-positive/HER2-negative metastatic breast cancer. Serum samples were collected pre-treatment (pre-trt; n = 122), at day 15 of cycle 1 (D15; n = 108), during the one week-off palbociclib before initiating cycle 2 (D28; n = 108) and at end of treatment (n = 76). sTKa was determined centrally using Divitum®, a refined ELISA-based assay with a limit of detection of 20 Divitum Units (Du)/L. The primary study endpoint was progression-free survival, assessed for its association with pre- and on-treatment sTKa.

RESULTS

Data from 122 women were analysed. Pre-treatment sTKa was not associated with clinical characteristics and moderately correlated with tissue Ki-67. Palbociclib + fulvestrant markedly suppressed sTKa levels at D15, with 83% of patients recording levels below limit of detection. At D28, sTKa showed a rebound in 60% of patients. At each timepoint, higher sTKa was associated with shorter progression-free survival (each p < 0.001), with the strongest effect at D15.

CONCLUSIONS

STKa is an independent prognostic biomarker in patients treated with palbociclib. High pre-treatment sTKa and its incomplete suppression during treatment may identify patients with poorer prognosis and primary resistance. This warrants validation in prospective comparative trials. CLINICALTRIALS.

GOV IDENTIFIER

NCT02536742; EudraCT 2014-005387-15.

The Role of the CDK4/6 Inhibitor Ribociclib in Locally Advanced and Oligometastatic Hormone Receptor Positive, Her2 Negative, Advanced Breast Cancer: Case Series and Review of the Literature

The recent addition of cyclin-dependent kinase 4 (CDK4) and CDK6 inhibitors to endocrine therapy has remarkably improved the outcome of patients affected with hormone receptor positive (HR+), human epidermal grow factor receptor 2 negative (HER2 -) advanced breast cancer (ABC). Ribociclib showed to be effective across most subgroups, regardless of the number and the site of metastasis. Up to 10% of patients with ABC, reported an oligometastatic condition, recently defined as a slow-volume metastatic disease with limited number and size of metastatic lesions (up to 5 and not necessarily in the same organ), potentially amenable for local treatment, aimed at achieving a complete remission status. Despite the wide use of CDK4/6 inhibitors in HR+, HER2-, ABC treatment, data regarding both locally advanced, inoperable disease and oligometastatic conditions are still poor. We reported a review and case series of HR+, HER2-, ABC patients treated with ribociclib as first-line therapy, for a locally advanced and oligometastatic conditions, reporting an impressive response and good safety profile.

Breast Cancer; Discovery of Novel Diagnostic Biomarkers, Drug Resistance, and Therapeutic Implications

Breast cancer is the second most reported cancer in women with high mortality causing millions of cancer-related deaths annually. Early detection of breast cancer intensifies the struggle towards discovering, developing, and optimizing diagnostic biomarkers that can improve its prognosis and therapeutic outcomes. Breast cancer-associated biomarkers comprise macromolecules, such as nucleic acid (DNA/RNA), proteins, and intact cells. Advancements in molecular technologies have identified all types of biomarkers that are exclusively studied for diagnostic, prognostic, drug resistance, and therapeutic implications. Identifying biomarkers may solve the problem of drug resistance which is a challenging obstacle in breast cancer treatment. Dysregulation of non-coding RNAs including circular RNAs (circRNAs) and microRNAs (miRNAs) initiates and progresses breast cancer. The circulating multiple miRNA profiles promise better diagnostic and prognostic performance and sensitivity than individual miRNAs. The high stability and existence of circRNAs in body fluids make them a promising new diagnostic biomarker. Many therapeutic-based novels targeting agents have been identified, including ESR1 mutation (DNA mutations), Oligonucleotide analogs and antagonists (miRNA), poly (ADP-ribose) polymerase (PARP) in BRCA mutations, CDK4/6 (cell cycle regulating factor initiates tumor progression), Androgen receptor (a steroid hormone receptor), that have entered clinical validation procedure. In this review, we summarize the role of novel breast cancer diagnostic biomarkers, drug resistance, and therapeutic implications for breast cancer.

CDK4/6 inhibitors: mechanisms of resistance and potential biomarkers of responsiveness in breast cancer

Hormone receptor (HR)-positive, HER2-negative tumors represent the most common form of metastatic breast cancer (MBC), and endocrine therapy has been the mainstay treatment for several decades. Recently, a novel drug class called CDK4/6 inhibitors in combination with endocrine therapy have remarkably improved the outcome of patients with HR-positive, HER2-negative MBC by targeting the cell cycle machinery and overcoming aspects of endocrine resistance. Several potential cell-cycle-specific and nonspecific mechanisms of resistance to CDK4/6 inhibitors have been reported in recent studies. This review discusses potential resistance mechanisms to CDK4/6 inhibitors, the use of biomarkers to guide treatment for HR-positive, HER2-negative MBC and possible approaches to overcome resistance to CDK4/6 inhibitors.

Adjuvant Palbociclib for Early Breast Cancer: The PALLAS Trial Results (ABCSG-42/AFT-05/BIG-14-03)

PURPOSE

Palbociclib is a cyclin-dependent kinase 4 and 6 inhibitor approved for advanced breast cancer. In the adjuvant setting, the potential value of adding palbociclib to endocrine therapy for hormone receptor-positive breast cancer has not been confirmed.

PATIENTS AND METHODS

In the prospective, randomized, phase III PALLAS trial, patients with hormone receptor-positive, human epidermal growth factor receptor 2-negative early breast cancer were randomly assigned to receive 2 years of palbociclib (125 mg orally once daily, days 1-21 of a 28-day cycle) with adjuvant endocrine therapy or adjuvant endocrine therapy alone (for at least 5 years). The primary end point of the study was invasive disease-free survival (iDFS); secondary end points were invasive breast cancer-free survival, distant recurrence-free survival, locoregional cancer-free survival, and overall survival.

RESULTS

Among 5,796 patients enrolled at 406 centers in 21 countries worldwide over 3 years, 5,761 were included in the intention-to-treat population. At the final protocol-defined analysis, at a median follow-up of 31 months, iDFS events occurred in 253 of 2,884 (8.8%) patients who received palbociclib plus endocrine therapy and in 263 of 2,877 (9.1%) patients who received endocrine therapy alone, with similar results between the two treatment groups (iDFS at 4 years: 84.2% v 84.5%; hazard ratio, 0.96; CI, 0.81 to 1.14; P = .65). No significant differences were observed for secondary time-to-event end points, and subgroup analyses did not show any differences by subgroup. There were no new safety signals for palbociclib in this trial.

CONCLUSION

At this final analysis of the PALLAS trial, the addition of adjuvant palbociclib to standard endocrine therapy did not improve outcomes over endocrine therapy alone in patients with early hormone receptor-positive breast cancer.

Phosphorylation-Induced Ubiquitination and Degradation of PXR through CDK2-TRIM21 Axis

Pregnane X receptor (PXR) is a member of the nuclear receptor superfamily that is activated by a variety of endogenous metabolites or xenobiotics. Its downstream target genes are involved in metabolism, inflammation and processes closely related to cancer. However, the stability regulation of PXR protein resulting from post-translational modification is still largely undefined. In the present study, primary mouse hepatocytes, hepatoma HepG2 cells and HEK 293T cells were used to investigate gene expression and protein interactions. The role of kinases was evaluated by RNA interference and overexpression constructs with or without PXR phosphorylation site mutations. The activity of CYP3A4 and P-gp was determined by enzymatic and substrate accumulation assays. It was found that E3 ubiquitin ligase TRIM21 mediates the ubiquitination and degradation of PXR and plays an important role in regulating the activity of PXR. On this basis, PXR phosphorylation-associated kinases were evaluated regarding regulation of the stability of PXR. We found cyclin dependent kinase 2 (CDK2) exclusively phosphorylates PXR at Ser350, promotes its disassociation with Hsp90/DNAJC7, and leads to subsequent TRIM21-mediated PXR ubiquitination and degradation. As well-known CDK inhibitors, dinaciclib and kenpaullone stabilize PXR and result in elevated expression and activity of PXR-targeted DMETs, including carboxylesterases, CYP3A4 and P-gp. The suppressed degradation of PXR by CDK2 inhibitors denotes dinaciclib-induced promotion of PXR-targeted genes. The findings of CDK2-mediated PXR degradation indicate a wide range of potential drug–drug interactions during clinical cancer therapy using CDK inhibitors and imply an alternative direction for the development of novel PXR antagonists.

Effects of neoadjuvant trastuzumab, pertuzumab and palbociclib on Ki67 in HER2 and ER-positive breast cancer

The crosstalk between estrogen and HER2 receptors and cell-cycle regulation sustains resistance to endocrine therapy of HER2- and hormone receptor-positive breast cancer. We earlier reported that women with HER2 and ER-positive breast cancer receiving neoadjuvant dual HER2-block and palbociclib in the NA-PHER2 trial had Ki67 decrease and 27% pathological complete responses (pCR). We extended NA-PHER2 to Cohort B using dual HER2-block and palbociclib without fulvestrant and report here Ki67 drops at week-2 (mean change −25.7), at surgery (after 16 weeks, mean change −9.5), high objective response (88.5%) and pCR (19.2%). In Cohort C [Ki67 > 20% and HER2_low (IHC 1+/2+ without gene amplification)], women also received fulvestrant, had dramatic Ki67 drop at week 2 (−29.5) persisting at surgery (−19.3), and objective responses in 78.3%. In view of the favorable tolerability and of the efficacy-predictive value of Ki67 drop at week-2, the chemotherapy-free approach of NA-PHER2 deserves further investigation in HER2 and ER-positive breast cancer. The trial is registered with ClinicalTrials.gov, number NCT02530424.

Biomarkers of Response and Resistance to Palbociclib Plus Letrozole in Patients With ER+/HER2- Breast Cancer

PURPOSE

To determine (i) the relationship between candidate biomarkers of the antiproliferative (Ki67) response to letrozole and palbociclib alone and combined in ER⁺/HER2^- breast cancer; and (ii) the pharmacodynamic effect of the agents on the biomarkers.

EXPERIMENTAL DESIGN

307 postmenopausal women with ER⁺/HER2^- primary breast cancer were randomly assigned to neoadjuvant treatment with letrozole for 14 weeks; letrozole for 2 weeks, then letrozole+palbociclib to 14 weeks; palbociclib for 2 weeks, then letrozole+palbociclib to 14 weeks; or letrozole+palbociclib for 14 weeks. Biopsies were taken at baseline, 2 and 14 weeks and surgery at varying times after stopping palbociclib. Immunohistochemical analyses were conducted for Ki67, c-PARP, ER, PgR, RB1, CCNE1, and CCND1.

RESULTS

Higher baselines ER and PgR were significantly associated with a greater chance of complete cell-cycle arrest (CCCA: Ki67 <2.7%) at 14 weeks and higher baseline Ki67, c-PARP, and CCNE1 with a lower chance. The interaction with treatment was significant only for c-PARP. CCND1 levels were decreased c.20% by letrozole at 2 and 14 weeks but showed a tendency to increase with palbociclib. CCNE1 levels fell 82% (median) in tumors showing CCCA but were unchanged in those with no CCCA. Only 2/9 tumors showed CCCA 3-9 days after stopping palbociclib. ESR1 mutations were found in 2/4 tumors for which surgery took place ≥6 months after starting treatment.

CONCLUSIONS

High CCNE1 levels were confirmed as a biomarker of resistance to letrozole+palbociclib. Ki67 recovery within 3-9 days of discontinuing palbociclib indicates incomplete suppression of proliferation during the "off" week of its schedule.

Integrating CDK4/6 inhibitors in the treatment of patients with early breast cancer

CDK4/6 inhibitors have an established role in the treatment of hormone receptor positive HER2-negative advanced breast cancer. All studies conducted in metastatic breast cancer showed a benefit in delaying progression when added to standard endocrine therapy, regardless of therapy line, pretreatment, menopausal status, site of metastasis, CDK4/6 inhibitor used and associated endocrine therapy. A benefit in overall survival has also been demonstrated. In early breast cancer, only the MonarchE study has shown an improved invasive disease-free survival with abemaciclib taken for 2 years, whereas the Penelope-B did not meet the primary endpoint and the PALLAS study was terminated early for futility. Studies conducted in the neoadjuvant setting might help to explain the discordant results.

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CDK4/6 inhibitors increase PFS in advanced breast cancer in all subgroups.

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2-years abemaciclib added to endocrine therapy improves invasive disease-free survival in high-risk breast cancer.

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Palbociclib did not improve invasive disease-free survival in early breast cancer.

Surrogate endpoints for early-stage breast cancer: a review of the state of the art, controversies, and future prospects

Drug approval for early-stage breast cancer (EBC) has been historically granted in the context of registration trials based on adequate outcomes such as disease-free survival and overall survival. Improvements in long-term outcomes have made it more difficult to demonstrate the clinical benefit of a new cancer drug in large, randomized, comparative clinical trials. Therefore, the use of surrogate endpoints rather than traditional measures allows for cancer drug trials to proceed with smaller sample sizes and shorter follow-up periods, which reduces drug development time. Among surrogate endpoints for breast cancer, the increase in pathological complete response (pCR) rates was considered appropriate for accelerated drug approval. The association between pCR and long-term outcomes was strongest in patients with aggressive tumor subtypes, such as triple-negative and human epidermal growth factor receptor 2 (HER2)-positive/hormone receptor-negative breast cancers. Whereas in hormone receptor-positive/HER2-negative EBC, the most accepted surrogate markers for endocrine therapy-based trials include changes in Ki67 and the preoperative endocrine prognostic index. Beyond the classic endpoints, further prognostic tools are required to provide EBC patients with individualized and effective therapies, and the neoadjuvant setting provides an excellent platform for drug development and biomarker discovery. Nowadays, the availability of multigene signatures is offering a standardized quantitative and reproducible tool to potentiate the efficacy of standard treatment for high-risk patients and develop de-escalated treatments for patients at lower risk of relapse. In this article, we first evaluate the surrogacies used for long-term outcomes and the underlying evidence supporting the use of each surrogate endpoint for the accelerated or regular drug approval process in EBC. Next, we provide an overview of the most recent studies and innovative strategies in a (neo)adjuvant setting as a platform to accelerate new drug approval. Finally, we highlight some clinical trials aimed at tailoring systemic treatment of EBC using prognosis-related factors or early biomarkers of drug sensitivity or resistance.

Resistance to CDK4/6 Inhibitors in Estrogen Receptor-Positive Breast Cancer

Estrogen receptor-positive (ER+) breast cancer is the most common form of breast cancer. Antiestrogens were the first therapy aimed at treating this subtype, but resistance to these warranted the development of a new treatment option. CDK4/6 inhibitors address this problem by halting cell cycle progression in ER+ cells, and have proven to be successful in the clinic. Unfortunately, both intrinsic and acquired resistance to CDK4/6 inhibitors are common. Numerous mechanisms of how resistance occurs have been identified to date, including the activation of prominent growth signaling pathways, the loss of tumor-suppressive genes, and noncanonical cell cycle function. Many of these have been successfully targeted and demonstrate the ability to overcome resistance to CDK4/6 inhibitors in preclinical and clinical trials. Future studies should focus on the development of biomarkers so that patients likely to be resistant to CDK4/6 inhibition can initially be given alternative methods of treatment.

Endocrine Treatment for Breast Cancer Patients Revisited-History, Standard of Care, and Possibilities of Improvement

PURPOSE OF REVIEW

Due to the findings of current studies and the approval of novel substances for the therapy of hormone-receptor-positive breast cancer patients, the established standards of endocrine treatment are changing. The purpose of this review is to give an overview of the history of endocrine treatment, to clarify its role in the present standard of care, and to discuss the possibilities of improvement.

RECENT FINDINGS

Tamoxifen, aromatase inhibitors, and fulvestrant are the main drugs that have been used for decades in the therapy of hormone-receptor-positive breast cancer patients. However, since a relevant number of women suffer at some point from disease recurrence or progression, several novel substances are being investigated to overcome resistance mechanisms by interfering with certain signaling pathways, such as the PI3K/AKT/mTOR or the CDK4/6 pathways. mTOR and CDK4/6 inhibitors were the first drugs approved for this purpose and many more are in development.

SUMMARY

Endocrine treatment is one of the best tolerable cancer therapies available. Continuous investigation serves to improve patients' outcomes and modernize the current standard of care. Considering the resistance mechanisms and substances analyzed against these, endocrine treatment of hormone-receptor-positive breast cancer is on the brink of a new era.

Resistance to endocrine therapy in HR + and/or HER2 + breast cancer: the most promising predictive biomarkers

Breast cancer is the most common cancer in women. It is a heterogeneous disease, encompassing different biological subtypes that differ in histological features, outcomes, clinical behaviour and different molecular subtypes. Therapy has progressed substantially over the past years with a reduction both for locoregional and systemic therapy. Endocrine therapies have considerably reduced cancer recurrence and mortality. Despite the major diagnostic and therapeutic innovations, resistance to therapy has become a main challenge, especially in metastatic breast cancer, and became a major factor limiting the use of endocrine therapeutic agents in ER positive breast cancers. Approximately 50% of patients with ER positive metastatic disease achieve a complete or partial response with endocrine therapy. However, in the remaining patients, the benefit is limited due to resistance, intrinsic or acquired, resulting in disease progression and poor outcome.Tumour heterogeneity as well as acquired genetic changes and therapeutics pressure have been involved in the endocrine therapy resistance. Nowadays, targeted sequencing of genes involved in cancer has provided insights about genomic tumour evolution throughout treatment and resistance driver mutations. Several studies have described multiple alterations in receptor tyrosine kinases, signalling pathways such as Phosphoinositide-3-kinase-protein kinase B/Akt/mTOR (PI3K/Akt/mTOR) and Mitogen-activated protein kinase (MAPK), cell cycle machinery and their implications in endocrine treatment failure.One of the current concern in cancer is personalized therapy. The focus has been the discovery of new potentially predictive biomarkers capable to identify reliably the most appropriate therapy regimen and which patients will experience disease relapse. The major concern is also to avoid overtreatment/undertreatment and development of resistance.This review focuses on the most promising predictive biomarkers of resistance in estrogen receptor-positive breast cancer and the emerging role of circulating free-DNA as a powerful tool for longitudinal monitoring of tumour molecular profile throughout treatment.

Neoadjuvant Endocrine Therapy in Clinical Practice: A Review

Importance

In clinical practice, neoadjuvant endocrine therapy (NET) is rarely used despite being an effective treatment modality able to downstage tumors and facilitate breast-conserving surgery.

Observations

Using data from studies conducted since 2000, we provide readers with a critical in-depth review on clinical aspects related to the application of NET in the treatment of hormone receptor (HR)-positive/ERBB2 (formerly HER2)-negative breast cancer. This includes an overview of patient-selection criteria, regimen choice, treatment duration, evaluation of response by imaging, interpretation of pathology after treatment, and surgical considerations. Areas of controversy include the use of gene-expression tests for patient selection, treatment of premenopausal women, surgical management of the axilla after NET, and adjuvant systemic therapy decision-making, including the use of chemotherapy.

Conclusions and Relevance

NET is an optimal treatment modality for a considerable proportion of postmenopausal women diagnosed with HR-positive tumors. The treatment landscape for HR-positive breast cancer is evolving, with novel agents and the growing use of gene expression profiling to define treatment selection. As such, it is likely that NET use will increase and the practical considerations outlined here will become more important.

Restoring order at the cell cycle border: Co-targeting CDK4/6 and CDK2

Overcoming resistance to CDK4/6 inhibitors is a major clinical challenge. In this issue of Cancer Cell, Freeman-Cook et al. study mechanisms of resistance to CDK4/6 inhibitors by employing a CRISPRa screen. They identify the cyclin E-CDK2 axis and Myc signaling as key pathways of resistance and develop PF-06873600, a selective CDK2/4/6 inhibitor.

Neoadjuvant Therapy of Cyclin-Dependent Kinase 4/6 Inhibitors Combined with Endocrine Therapy in HR+/HER2- Breast Cancer: A Systematic Review and Meta-Analysis

BACKGROUND

Cyclin-dependent kinase (CDK) 4/6 inhibitors have been advocated for adjuvant therapy of metastatic hormone receptor (HR)+/human epidermal growth factor receptor 2 (HER2)- breast cancer (BC). However, the efficiency of adding CDK 4/6 inhibitors to neoadjuvant therapy was not unequivocal.

OBJECTIVE

The aim of the study was to evaluate the efficiency and toxicity of neoadjuvant CDK 4/6 inhibitors + endocrine therapy (ET) versus neoadjuvant endocrine monotherapy or standard neoadjuvant chemotherapy in HR+/HER2- BC.

METHOD

We searched PubMed, the Cochrane Library, Web of Science, and Embase online databases for randomized controlled trials and single-arm studies written in English until April 2021.

RESULTS

Five studies comparing CDK 4/6 inhibitors + ET as neoadjuvant treatments to ET alone and 2 studies comparing neoadjuvant CDK 4/6 inhibitors + ET to neoadjuvant chemotherapy were analysed. Neoadjuvant CDK 4/6 inhibitors + ET improved the rate of complete cell cycle arrest (CCCA: central Ki67 < 2.7%, odds ratio [OR] = 7.91, 95% confidence interval [CI] = 4.81-13.03, p < 0.001), increased the risk of adverse events (AEs; especially ≥3 AEs; AEs of all grades: OR = 9.10, 95% CI = 2.39-34.58, p = 0.001; AEs ≥3: OR = 12.24, 95% CI = 4.17-35.88, p < 0.001), led to no significant differences in pathological complete response (pCR) in patients with BC (OR = 0.34, 95% CI = 0.04-2.85, p = 0.318) compared to endocrine monotherapy. Moreover, subgroup analysis showed that the 3 types of CDK 4/6 inhibitors all improved the rate of CCCA (ribociclib: OR = 10.31, 95% CI = 3.59-29.61, p < 0.001; palbociclib: OR = 7.39, 95% CI = 1.26-43.40, p = 0.027, and abemaciclib: OR = 8.28, 95% CI = 3.41-20.11, p < 0.001). Compared to neoadjuvant chemotherapy, neoadjuvant CDK 4/6 inhibitors plus ET decreased the risk of AEs ≥3 (OR = 0.50, 95% CI = 0.29-0.87, p = 0.015) and showed similar ability to reach pCR (OR = 0.50, 95% CI = 0.12-2.07, p = 0.342) and reduce the residual cancer burden (RCB, RCB 0-1: OR = 0.47, 95% CI = 0.18-1.22, p = 0.121; RCB 2-3: OR = 2.30, 95% CI = 0.89-5.91, p = 0.084).

CONCLUSIONS

The results suggested that combination therapy had increased efficacy and toxicity compared to endocrine monotherapy and showed similar efficacy to and better safety than neoadjuvant chemotherapy.

Combined endocrine and targeted therapy in luminal breast cancer

Introduction: For decades, endocrine therapy has been the cornerstone of management for luminal breast cancer. Despite the substantial benefit derived by patients from endocrine therapy, primary and secondary resistances to endocrine therapy are serious clinical issues.Areas covered: Today, in the advanced setting, three distinct classes of targeted agents mTOR, CDK 4/6, and PI3K inhibitors, are approved for use. CDK 4/6 inhibitors have improved outcomes substantially, changing the natural history of advanced luminal breast cancer. Current studies seek to bring CDK 4/6 inhibitors to the early setting. This review will cover all available data on target therapy combinations with endocrine therapy for both the early and advanced settings, including approved drugs and agents in development.Expert opinion: Combined endocrine and target therapy has changed the landscape in advanced disease. In early disease, it is possible to have a large impact, particularly in patients with higher risk of relapse. Trials like ADAPTCYCLE seek to leverage neoadjuvant data to de-escalate treatment, substituting chemotherapy for CDK 4/6 inhibitors. In advanced diseases, studies such as PADA-1 point toward a future in which ctDNA will be used to define management before clinical progression occurs.

Activation of the IFN Signaling Pathway is Associated with Resistance to CDK4/6 Inhibitors and Immune Checkpoint Activation in ER-Positive Breast Cancer

PURPOSE

Cyclin-dependent kinase 4 (CDK4) and CDK6 inhibitors (CDK4/6i) are highly effective against estrogen receptor-positive (ER⁺)/HER2^- breast cancer; however, intrinsic and acquired resistance is common. Elucidating the molecular features of sensitivity and resistance to CDK4/6i may lead to identification of predictive biomarkers and novel therapeutic targets, paving the way toward improving patient outcomes.

EXPERIMENTAL DESIGN

Parental breast cancer cells and their endocrine-resistant derivatives (EndoR) were used. Derivatives with acquired resistance to palbociclib (PalboR) were generated from parental and estrogen deprivation-resistant MCF7 and T47D cells. Transcriptomic and proteomic analyses were performed in palbociclib-sensitive and PalboR lines. Gene expression data from CDK4/6i neoadjuvant trials and publicly available datasets were interrogated for correlations of gene signatures and patient outcomes.

RESULTS

Parental and EndoR breast cancer lines showed varying degrees of sensitivity to palbociclib. Transcriptomic analysis of these cell lines identified an association between high IFN signaling and reduced CDK4/6i sensitivity; thus an "IFN-related palbociclib-resistance Signature" (IRPS) was derived. In two neoadjuvant trials of CDK4/6i plus endocrine therapy, IRPS and other IFN-related signatures were highly enriched in patients with tumors exhibiting intrinsic resistance to CDK4/6i. PalboR derivatives displayed dramatic activation of IFN/STAT1 signaling compared with their short-term treated or untreated counterparts. In primary ER⁺/HER2^- tumors, the IRPS score was significantly higher in lumB than lumA subtype and correlated with increased gene expression of immune checkpoints, endocrine resistance, and poor prognosis.

CONCLUSIONS

Aberrant IFN signaling is associated with intrinsic resistance to CDK4/6i. Experimentally, acquired resistance to palbociclib is associated with activation of the IFN pathway, warranting additional studies to clarify its involvement in resistance to CDK4/6i.

Palbociclib in breast cancer neoadjuvant setting

Cyclin-dependent kinase 4/6 inhibitors represent a major advance in breast cancer treatment, emerging as the standard of care of the initial treatment of hormone receptor-positive and HER2-negative metastatic breast cancer. Their activity in this subset of patients leads to interest in their use in the adjuvant and neoadjuvant settings. This case report presents a real-life case of cyclin-dependent kinase 4/6 inhibitors use in a patient initially considered to have stage IV luminal HER2-negative breast cancer with liver metastasis. The discrepancy of treatment response between the breast tumor and liver node led to a repetition of the liver biopsy, which revealed metastasis of a neuroendocrine tumor of unknown primary. The breast tumor showed a partial response, and the initial therapeutic strategy was then redefined for curative intent. While cyclin-dependent kinase 4/6 inhibitors are not yet approved for clinical practice in the neo / adjuvant treatment of hormone receptor-positive breast cancer, this case report portrays a successful example of its application in a neoadjuvant setting.

The Effects of HER2 on CDK4/6 Activity in Breast Cancer

BACKGROUND

CDK4/6 inhibitors have been used to treat hormone receptor-positive HER2-negative advanced breast cancer. Their benefit in HER2-positive breast cancer has not been determined yet. In this study, we investigated the effects of HER2 on CDK4/6 activity by assessing the level of downstream phosphorylated retinoblastoma protein (pRb) in HER2-positive breast cancer (HER2 positivity is defined by immunohistochemical study or FISH, regardless of ER status) to determine if these cases may be responsive to CDK4/6 inhibitors.

MATERIALS AND METHODS

One hundred and thirty cases of breast biopsies with invasive carcinoma were collected, including 77 cases of HER2+ (39 cases of ER +PR±HER2+ and 38 cases of ER-PR-HER2+) and 53 cases of HER2- (ER-PR-HER2-) breast cancer. Immunohistochemical study of pRb was performed and the pRb level was assessed by H-score (intensity x percentage of positive cells).

RESULTS

The pRb H-score ranges from 3 to 270. The average H-scores for the ER-PR-HER2+, ER+PR±HER2+ and ER-PR-HER2- groups are 115.8 ± 75.8, 93.1 ± 68.6 and 63.1 ± 65.6, respectively. By comparison, HER2+ cases have significantly higher pRb levels than HER2- cases (P = .001). Among HER2+ cases, there was a trend of positive correlation between the HER2 gene copy number, and the pRb level although not statistically significant (r = 0.192, 95% CI, [-0.033, 0.399], P = .09).

CONCLUSION

In breast cancer, HER2 positivity leads to significantly higher levels of CDK4/6 activity as reflexed by pRb. Breast cancer that is positive for HER2 may respond to CDK4/6 inhibitors and pRb may potentially be used as a biomarker to predict the responsiveness.

Cyclin-dependent kinase (CDK) inhibitors in solid tumors: a review of clinical trials

Cyclin-dependent kinases (CDKs) play a key regulating role in the cell cycle, which is almost universally altered in cancer, leading to sustained proliferation. Early pan-CDK inhibitors showed poor results in clinical trials for solid malignancies, as the lack of selectivity produced significant toxicity. The production of more selective inhibitors led to significant developments in cancer therapy, as CDK4/6 inhibitors in combination with endocrine therapy changed the landscape of the treatment of hormone-receptor positive (HR +) metastatic breast cancer. Recently, Trilaciclib demonstrated benefits regarding hematological toxicity compared to placebo when administered in combination with chemotherapy in small cell lung cancer. Newer agents, such as SY-5609, a selective CDK7 inhibitor, have also shown promising results in early clinical trials. In this paper, we review the data from clinical trials of CDK inhibitors in solid tumors, either as a monotherapy or in combination with other agents, with an emphasis on novel agents and potential new indications for this drug class.

Radiological and pathological assessment of response to neoadjuvant CDK4/6 inhibitor and endocrine treatments in a real-life setting-initial results

BACKGROUND

Neoadjuvant endocrine therapy is an alternative to neoadjuvant chemotherapy in women with inoperable luminal-like breast cancers. Neoadjuvant cyclin-dependent kinase 4/6 inhibitor treatment combined with endocrine treatment (CDK4/6I + E) is interesting given the combination's utility in the treatment of metastatic breast cancer. Currently, the literature on the radiological response evaluation of patients treated with neoadjuvant CDK4/6I + E in a real-life setting is scarce.

PURPOSE

To conduct a radiological response evaluation of patients treated with neoadjuvant CDK4/6I + E in a real-life setting.

MATERIAL AND METHODS

We retrospectively reviewed clinical, pathological, and radiological findings of six patients with luminal-like breast cancers treated with neoadjuvant CDK4/6I + E treatment. The radiological neoadjuvant CDK4/6I + E response was evaluated with the RECIST 1.1 criteria and the pathological residual disease was assessed using the Residual Cancer Burden (RBC) criteria.

RESULTS

None of the patients achieved a complete radiological magnetic resonance imaging (MRI)-determined response or a complete pathological response; three (50%) patients had a partial radiological response; in the three others, the disease remained stable radiologically. All of the tumors were rendered susceptible to surgical treatment. Two out of six (33.3%) patients had a moderate response (RBC-II); four (66.7%) had an extensive residual disease (RBC-III) in the final surgical sample.

CONCLUSION

Although none of the patients achieved a pathologically complete response, neoadjuvant CDK4/6I + E treatment rendered all tumors operable. MRI appears to be reliable in the assessment of the neoadjuvant CDK4/6I + E treatment response in a real-life setting. Larger studies are warranted to confirm these results.

The role of CDK4/6 inhibitors in early breast cancer

The use of cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) has proven to be a successful strategy in the treatment of advanced hormone receptor-positive (HR+) and human epidermal growth factor receptor 2-negative (HER2-) breast cancer (BC), leading to a strong interest in their possible role in the treatment of early luminal BC. In this review we collect the most relevant and recent information on the use of CDK4/6i for the treatment of early BC in the neoadjuvant and adjuvant settings. Specifically, we evaluate the results of the large phase 3 adjuvant trials recently released, which have yielded apparently divergent results. We also examine the relevance of biomarkers as response predictive factors for CDI4/6i, the combination between radiotherapy and CDK4/6i, and provide a critical discussion on the evidence that we have so far and future directions of the role of these drugs in the treatment of early BC.

Surgical Management of Axilla Following Neoadjuvant Endocrine Therapy

BACKGROUND

Randomized clinical trials support deescalation of axillary surgery in breast cancer patients with low-volume axillary disease treated with a surgery-first approach. However, few data exist to guide axillary surgery following neoadjuvant endocrine therapy (NET). Therefore, we evaluated the extent and outcomes of axillary surgery in a contemporary cohort of NET patients, a treatment approach that has become particularly relevant during the coronavirus disease-19 (COVID-19) pandemic.

PATIENTS AND METHODS

We identified invasive breast cancer patients treated with NET between October 2008 and November 2019. Patients presenting with stage IV disease or recurrent disease were excluded. Statistical analyses were performed using chi-square, Fisher's exact, and Wilcoxon rank-sum tests.

RESULTS

194 invasive breast cancers in 186 patients (median age 66 years) were evaluated; 81 patients had breast-conserving surgery (BCS), while 113 underwent mastectomy. Eighty-four patients (43.3%) were biopsy-proven cN+ with 4/84 (4.8%) ypN0 following NET. Among cN+ patients, 14 (16.7%) had sentinel lymph node biopsy (SLNB) only, 27 (32.1%) had SLNB + axillary lymph node dissection (ALND), and 43 (51.2%) had ALND. Among 110 cN0 patients, 99 had axillary surgery with 28/99 (28.3%) ypN+: SLNB in 83 (75.5%), SLNB+ALND in 14 (12.7%), and ALND in 2 (1.8%). Among all ypN+ patients, 23/108 (21.3%) had SLNB alone: 18/43 (41.9%) of BCS and 5/65 (7.7%) mastectomy patients (p < 0.001). After median follow-up of 35 months, no regional recurrences were observed.

CONCLUSIONS

Among biopsy-proven cN+ NET patients, we observed deescalation of axillary surgery in selected patients, despite a low nodal pathologic complete response (pCR) rate, without nodal recurrences. These data suggest that patients with low-volume axillary disease treated with NET may be managed similarly to patients treated with a surgery-first approach.

Chemoresistance and Metastasis in Breast Cancer Molecular Mechanisms and Novel Clinical Strategies

Breast cancer is the most common malignant tumor in females worldwide. Chemotherapy is the standard breast cancer treatment; however, chemoresistance is often seen in patients with metastatic breast cancer. Owing to high heterogeneity, the mechanisms of breast cancer chemoresistance and metastasis have not been fully investigated. The possible molecular mechanisms of chemoresistance in breast cancer include efflux transporters, signaling pathways, non-coding RNAs, and cancer stem cells. However, to overcome this hurdle, the use of novel clinical strategies such as drug carriers, immunotherapy, and autophagy regulation, are being investigated. The goal of this review is to summarize the current data about the molecular mechanisms of breast cancer chemoresistance and the novel clinical strategies; thus, providing a useful clinical tool to explore optimal treatment for breast cancer.

The evolution of cyclin dependent kinase inhibitors in the treatment of cancer

INTRODUCTION

The cell cycle cyclin dependent kinases (CDKs) play a critical role in controlling the transition between cell cycle phases, as well as cellular transcription. Aberrant CDK activation is common in cancer, and deregulation of the cell cycle a key hallmark of cancer. Although CDK4/6 inhibitors are now a standard-of-care option for first- and second-line HR+HER2- metastatic breast cancer, resistance inevitably limits their clinical benefit.

AREAS COVERED

Early pan-CDK inhibitors targeted the cell cycle and RNA polymerase II phosphorylation, but were complicated by toxicity, providing a rationale and need for the development of selective CDK inhibitors. In this review, we highlight selected recent literature to provide a narrative review summarizing the current CDK inhibitor therapeutic landscape. We detail the challenges associated with targeting CDKs for the treatment of breast and other cancers and review emerging biomarkers that may aid response prediction. We also discuss the risk-benefit ratio for CDK therapy and explore promising combination approaches.

EXPERT OPINION

Although CDK inhibitors may stem the proliferation of cancer cells, resistance remains an issue, and currently there are limited biomarkers to predict response to therapy. Ongoing research investigating CDK inhibitors in cancer is of paramount importance to define appropriate and effective treatment regimens.

Patient-derived scaffolds as a drug-testing platform for endocrine therapies in breast cancer

Three-dimensional cell culture platforms based on decellularised patient-based microenvironments provide in vivo-like growth conditions allowing cancer cells to interact with intact structures and components of the surrounding tissue. A patient-derived scaffold (PDS) model was therefore evaluated as a testing platform for the endocrine therapies (Z)-4-Hydroxytamoxifen (4OHT) and fulvestrant as well as the CDK4/6-inhibitor palbociclib, monitoring the treatment responses in breast cancer cell lines MCF7 and T47D adapted to the patient-based microenvironments. MCF7 cells growing in PDSs showed increased resistance to 4OHT and fulvestrant treatment (100- and 20-fold) compared to 2D cultures. Quantitative PCR analyses of endocrine treated cancer cells in PDSs revealed upregulation of pluripotency markers further supported by increased self-renewal capacity in sphere formation assays. When comparing different 3D growth platforms including PDS, matrigel, gelatin sponges and 3D-printed hydrogels, 3D based cultures showed slightly varying responses to fulvestrant and palbociclib whereas PDS and matrigel cultures showed more similar gene expression profiles for 4OHT treatment compared to the other platforms. The results support that the PDS technique maximized to provide a multitude of smaller functional PDS replicates from each primary breast cancer, is an up-scalable patient-derived drug-testing platform available for gene expression profiling and downstream functional assays.

[Targeting the cyclin-dependent kinases 4/6 in advanced breast cancers]

The historical median survival of advanced luminal breast cancer does not exceed four years. The deciphering of the mechanisms of resistance to hormone therapy has led to the development of inhibitors of cyclin D dependent kinases (CDK4 and 6). Three drugs, palbociclib, ribociclib and abemaciclib, very similar pharmacologically, have been evaluated in the context of pivotal, randomized phase III trials. Strikingly and regardless of the endocrine therapy backbone, and in both hormone-sensitive and hormone-resistant patients, the addition of a CDK4 / 6 inhibitor doubles progression-free survival with a hazard ratio always around 0.55. The benefit in overall survival begins to be demonstrated. This review presents all published results, as well as the main safety data.

Circulating Biomarkers of CDK4/6 Inhibitors Response in Hormone Receptor Positive and HER2 Negative Breast Cancer

CDK4/6 inhibitors (CDK4/6i) and endocrine therapy are the standard treatment for patients with hormone receptor-positive and HER2 negative (HR+/HER2-) metastatic breast cancer. Patients might show intrinsic and acquired resistance, which leads to treatment failure and progression. Circulating biomarkers have the potential advantages of recognizing patients who might not respond to treatment, monitoring treatment effects and identifying markers of acquired resistance during tumor progression with a simple withdrawal of peripheral blood. Genomic alterations on circulating tumor DNA and serum thymidine kinase activity, but also circulating tumor cells, epigenetic or exosome markers are currently being tested as markers of CDK4/6i treatment response, even though none of these have been integrated into clinical practice. In this review, we discuss the recent advancements in the development of circulating biomarkers of CDK4/6i response in patients with HR+/HER2-breast cancer.

Breast Cancer Resistance to Cyclin-Dependent Kinases 4/6 Inhibitors: Intricacy of the Molecular Mechanisms

Breast cancer is a common malignant tumor in women, with a highest incidence and mortality among all of the female malignant tumors. Notably, targeted therapy has achieved impressive success in the treatment of breast cancer. As one class of the anti-tumor targeted therapeutics, Cyclin-Dependent Kinases 4/6CDK4/6inhibitors have shown good clinical activity in treating breast cancer. Nevertheless, despite the promising clinical outcomes, intrinsic or acquired resistance to CDK4/6 inhibitors has limited the benefits of this novel target therapy. In the present review, we provide an overview of the currently known molecular mechanisms of resistance to CDK4/6 inhibitors, and discuss the potential strategies to overcoming drug resistance improving the outcomes for breast cancer patients treated with CDK4/6 inhibitors.

The Present and Future of Neoadjuvant Endocrine Therapy for Breast Cancer Treatment

Simple Summary

The treatment of breast cancer has evolved considerably over the last two decades, leading toward individualized disease management. Hormone-sensitive breast cancers constitute the vast majority of cases and endocrine therapy is the mainstay of their treatment. On the other hand, neoadjuvant or pre-surgical treatments provide a number of advantages for tumor management. In this review we will discuss the existing evidence on neoadjuvant endocrine therapy, as well as its possible future indications.

Abstract

Endocrine therapy (ET) has established itself as an efficacious treatment for estrogen receptor-positive (ER+) breast cancers, with a reduction in recurrence rates and increased survival rates. The pre-surgical approach with chemotherapy (NCT) has become a common form of management for large, locally advanced, or high-risk tumors. However, a good response to NCT is not usually expected in ER+ tumors. Good results with primary ET, mainly in elderly women, have encouraged studies in other stages of life, and nowadays neoadjuvant endocrine treatment (NET) has become a useful approach to many ER+ breast cancers. The aim of this review is to provide an update on the current state of art regarding the present and the future role of NET.

Palbociclib for Residual High-Risk Invasive HR-Positive and HER2-Negative Early Breast Cancer-The Penelope-B Trial

PURPOSE

About one third of patients with hormone receptor-positive, human epidermal growth factor receptor 2-negative breast cancer who have residual invasive disease after neoadjuvant chemotherapy (NACT) will relapse. Thus, additional therapy is needed. Palbociclib is a cyclin-dependent kinase 4 and 6 inhibitor demonstrating efficacy in the metastatic setting.

PATIENTS AND METHODS

PENELOPE-B (NCT01864746) is a double-blind, placebo-controlled, phase III study in women with hormone receptor-positive, human epidermal growth factor receptor 2-negative primary breast cancer without a pathological complete response after taxane-containing NACT and at high risk of relapse (clinical pathological staging-estrogen receptor grading score ≥ 3 or 2 and ypN+). Patients were randomly assigned (1:1) to receive 13 cycles of palbociclib 125 mg once daily or placebo on days 1-21 in a 28-day cycle in addition to endocrine therapy (ET). Primary end point is invasive disease-free survival (iDFS). Final analysis was planned after 290 iDFS events with a two-sided efficacy boundary P < .0463 because of two interim analyses.

RESULTS

One thousand two hundred fifty patients were randomly assigned. The median age was 49.0 years (range, 19-79), and the majority were ypN+ with Ki-67 ≤ 15%; 59.4% of patients had a clinical pathological staging-estrogen receptor grading score ≥ 3. 50.1% received aromatase inhibitor, and 33% of premenopausal women received a luteinizing hormone releasing hormone analog in addition to either tamoxifen or an aromatase inhibitor. After a median follow-up of 42.8 months (92% complete), 308 events were confirmed. Palbociclib did not improve iDFS versus placebo added to ET-stratified hazard ratio, 0.93 (95% repeated CI, 0.74 to 1.17) and two-sided weighted log-rank test (Cui, Hung, and Wang) P = .525. There was no difference among the subgroups. Most common related serious adverse events were infections and vascular disorders in 113 (9.1%) patients with no difference between the treatment arms. Eight fatal serious adverse events (two palbociclib and six placebo) were reported.

CONCLUSION

Palbociclib for 1 year in addition to ET did not improve iDFS in women with residual invasive disease after NACT.

In Silico-Analysis of the Multi-Omics Data Identified the Ataxia Telangiectasia Mutated Gene as a Potential Biomarker of Breast Invasive Carcinoma

Background: The elevated global burden of the breast invasive carcinoma (BRIC) and lack of appropriate biomarkers for its early detection and treatment requires extensive investigation to enhance understanding regarding BRIC associated molecular alterations. Ataxia telangiectasia mutated (ATM) is a multifunctional tumor suppressor gene, which participates in the DNA damage response pathway and cellular checkpoint activation. Several studies have reported the reduction of ATM expression as a reliable biomarker of BRIC. However, its role as a clinicopathological feature-specific biomarker still needs to be explored. Aim: The present study was designed to investigate the mutational spectrum and expression variations of ATM in BRIC patients exhibiting various clinicopathological features. Furthermore, we also performed a correlational analysis of clinicopathological feature-specific ATM expression with its promoter methylation, status genetic alterations, copy number variation (CNVs), overall survival (OS), and effectiveness of various anticancerous drugs in BRIC patients. Methods: We utilized multiple online platforms, including UALCN, cBioportal, and CCLE GDSC tool kit. Results: The ATM exhibited decreased expression in the majority of the BRIC patients, and its promoter was hypermethylated compared to healthy controls. Hence, the degree of promoter methylation and ATM expression level were inversely correlated in BRIC. In addition, we also investigated if BRIC patients that had higher ATM expression had lower OS. We found that elevated expression of ATM was found to promoted or decreased the effectiveness of various anticancer drugs. Conclusion: This study revealed the overall and clinicopathological feature-specific role of the ATM, gene, however, these findings need to be validated via larger scale studies.

Neoadjuvant Endocrine Therapy in Breast Cancer Management: State of the Art

Endocrine therapy is the mainstay of treatment in HR+/HER2- breast cancers, which represent about 70% of all breast cancers. Neoadjuvant therapy has been developed since the 1990s to address several issues, including breast-conserving surgery (BCS) and improvement of survival rates. For a long time, neoadjuvant endocrine therapy (NET) was confined to frail patients in order to improve surgery outcome. Since the 2000s, NET now plays a central role as a research tool for predictive endocrine sensitivity biomarkers and targeted therapies. One of the major issues in early HR+/HER2- breast cancer is to identify patients in whom chemotherapy can be safely withheld. In vivo assessment of response to NET might be the best treatment strategy to address this issue.

Distinct mechanisms of resistance to fulvestrant treatment dictate level of ER independence and selective response to CDK inhibitors in metastatic breast cancer

Background

Resistance to endocrine treatment in metastatic breast cancer is a major clinical challenge. Clinical tools to predict both drug resistance and possible treatment combination approaches to overcome it are lacking. This unmet need is mainly due to the heterogeneity underlying both the mechanisms involved in resistance development and breast cancer itself.

Methods

To study the complexity of the mechanisms involved in the resistance to the selective estrogen receptor degrader (SERD) fulvestrant, we performed comprehensive biomarker analyses using several in vitro models that recapitulate the heterogeneity of developed resistance. We further corroborated our findings in tissue samples from patients treated with fulvestrant.

Results

We found that different in vitro models of fulvestrant resistance show variable stability in their phenotypes, which corresponded with distinct genomic alterations. Notably, the studied models presented adaptation at different cell cycle nodes to facilitate progression through the cell cycle and responded differently to CDK inhibitors. Cyclin E2 overexpression was identified as a biomarker of a persistent fulvestrant-resistant phenotype. Comparison of pre- and post-treatment paired tumor biopsies from patients treated with fulvestrant revealed an upregulation of cyclin E2 upon development of resistance. Moreover, overexpression of this cyclin was found to be a prognostic factor determining resistance to fulvestrant and shorter progression-free survival.

Conclusions

These data highlight the complexity of estrogen receptor positive breast cancer and suggest that the development of diverse resistance mechanisms dictate levels of ER independence and potentially cross-resistance to CDK inhibitors.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13058-021-01402-1.

Molecular Biomarkers for Contemporary Therapies in Hormone Receptor-Positive Breast Cancer

Systemic treatment of hormone receptor-positive (HR+) breast cancer is undergoing a renaissance, with a number of targeted therapies including CDK4/6, mTOR, and PI3K inhibitors now approved for use in combination with endocrine therapies. The increased use of targeted therapies has changed the natural history of HR+ breast cancers, with the emergence of new escape mechanisms leading to the inevitable progression of disease in patients with advanced cancers. The identification of new predictive and pharmacodynamic biomarkers to current standard-of-care therapies and discovery of new therapies is an evolving and urgent clinical challenge in this setting. While traditional, routinely measured biomarkers such as estrogen receptors (ERs), progesterone receptors (PRs), and human epidermal growth factor receptor 2 (HER2) still represent the best prognostic and predictive biomarkers for HR+ breast cancer, a significant proportion of patients either do not respond to endocrine therapy or develop endocrine resistant disease. Genomic tests have emerged as a useful adjunct prognostication tool and guide the addition of chemotherapy to endocrine therapy. In the treatment-resistant setting, mutational profiling has been used to identify ESR1, PIK3CA, and AKT mutations as predictive molecular biomarkers to newer therapies. Additionally, pharmacodynamic biomarkers are being increasingly used and considered in the metastatic setting. In this review, we summarise the current state-of-the-art therapies; prognostic, predictive, and pharmacodynamic molecular biomarkers; and how these are impacted by emerging therapies for HR+ breast cancer.

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.

Circulating tumor DNA dynamics in advanced breast cancer treated with CDK4/6 inhibition and endocrine therapy

Circulating tumor DNA (ctDNA) levels may predict response to anticancer drugs, including CDK4/6 inhibitors and endocrine therapy combinations (CDK4/6i+ET); however, critical questions remain unanswered such as which assay or statistical method to use. Here, we obtained paired plasma samples at baseline and week 4 in 45 consecutive patients with advanced breast cancer treated with CDK4/6i+ET. ctDNA was detected in 96% of cases using the 74-gene Guardant360 assay. A variant allele fraction ratio (VAFR) was calculated for each of the 79 detected mutations between both timepoints. Mean of all VAFRs (mVAFR) was computed for each patient. In our dataset, mVAFR was significantly associated with progression-free survival (PFS). Baseline VAF, on-treatment VAF or absolute changes in VAF were not associated with PFS, nor were CA-15.3 levels at baseline, week 4 or the CA-15.3 ratio. These findings demonstrate that ctDNA dynamics using a standardized multi-gene panel and a unique methodological approach predicts treatment outcome. Clinical trials in patients with an unfavorable ctDNA response are needed.

Inhibiting CDK4/6 in Breast Cancer with Palbociclib, Ribociclib, and Abemaciclib: Similarities and Differences

The cyclin-dependent kinase (CDK) 4/6 inhibitors belong to a new class of drugs that interrupt proliferation of malignant cells by inhibiting progression through the cell cycle. Three such inhibitors, palbociclib, ribociclib, and abemaciclib were recently approved for breast cancer treatment in various settings and combination regimens. On the basis of their impressive efficacy, all three CDK4/6 inhibitors now play an important role in the treatment of patients with HR+, HER2- breast cancer; however, their optimal use still needs to be established. The three drugs have many similarities in both pharmacokinetics and pharmacodynamics. However, there are some differences on the basis of which the choice for a particular CDK4/6 inhibitor for an individual patient can be important. In this article, the clinical pharmacokinetic and pharmacodynamic profiles of the three CDK4/6 inhibitors are reviewed and important future directions of the clinical applicability of CDK4/6 inhibitors will be discussed.

Targeted Neoadjuvant Therapies in HR+/HER2-Breast Cancers: Challenges for Improving pCR

A strong association of pCR (pathological complete response) with disease-free survival or overall survival is clinically desirable. The association of pCR with disease-free survival or overall survival in ER+/HER2-breast cancers following neoadjuvant systemic therapy (NAT) or neoadjuvant endocrine therapy (NET) is relatively low as compared to the other two subtypes of breast cancers, namely triple-negative and HER2+ amplified. On the bright side, a neoadjuvant model offers a potential opportunity to explore the efficacy of novel therapies and the associated genomic alterations, thus providing a rare personalized insight into the tumor's biology and the tumor cells' response to the drug. Several decades of research have taught us that the disease's biology is a critical factor determining the tumor cells' response to any therapy and hence the final outcome of the disease. Here we propose two scenarios wherein apoptosis can be induced in ER+/HER2- breast cancers expressing wild type TP53 and RB genes following combinations of BCL2 inhibitor, MDM2 inhibitor, and cell-cycle inhibitor. The suggested combinations are contextual and based on the current understanding of the cell signaling in the ER+/HER2- breast cancers. The two combinations of drugs are (1) BCL2 inhibitor plus a cell-cycle inhibitor, which can prime the tumor cells for apoptosis, and (2) BCL2 inhibitor plus an MDM2 inhibitor.

Lobular Breast Cancer: A Review

Invasive lobular carcinoma accounts for 5%-15% of all invasive breast cancers, with a marked increase in incidence rates over the past two decades. Distinctive biological hallmarks of invasive lobular carcinoma include the loss of cell adhesion molecule E-cadherin leading to cells with a discohesive morphology, proliferating into single-file strands and estrogen receptor positivity. These key molecular features can make diagnosis difficult, as invasive lobular carcinoma is challenging to detect both physically and with current standard imaging. Treatment of invasive lobular carcinoma strongly favors endocrine therapy due to low chemosensitivity and lower rates of pathological response as a result. This review will summarize the distinct biological and molecular features of invasive lobular carcinoma, focusing on the diagnostic challenges faced and the subsequent surgical and medical management strategies. Prospective therapeutic options will also be explored, highlighting how furthering our understanding of the unique biology of lobular breast carcinoma is essential in guiding and informing the treatment of patients in the future.

Palbociclib with adjuvant endocrine therapy in early breast cancer (PALLAS): interim analysis of a multicentre, open-label, randomised, phase 3 study

BACKGROUND

Palbociclib added to endocrine therapy improves progression-free survival in hormone-receptor-positive, HER2-negative, metastatic breast cancer. The PALLAS trial aimed to investigate whether the addition of 2 years of palbociclib to adjuvant endocrine therapy improves invasive disease-free survival over endocrine therapy alone in patients with hormone-receptor-positive, HER2-negative, early-stage breast cancer.

METHODS

PALLAS is an ongoing multicentre, open-label, randomised, phase 3 study that enrolled patients at 406 cancer centres in 21 countries worldwide with stage II-III histologically confirmed hormone-receptor-positive, HER2-negative breast cancer, within 12 months of initial diagnosis. Eligible patients were aged 18 years or older with an Eastern Cooperative Oncology Group performance score of 0 or 1. Patients were randomly assigned (1:1) in permuted blocks of random size (4 or 6), stratified by anatomic stage, previous chemotherapy, age, and geographical region, by use of central telephone-based and web-based interactive response technology, to receive either 2 years of palbociclib (125 mg orally once daily on days 1-21 of a 28-day cycle) with ongoing standard provider or patient-choice adjuvant endocrine therapy (tamoxifen or aromatase inhibitor, with or without concurrent luteinising hormone-releasing hormone agonist), or endocrine therapy alone, without masking. The primary endpoint of the study was invasive disease-free survival in the intention-to-treat population. Safety was assessed in all randomly assigned patients who started palbociclib or endocrine therapy. This report presents results from the second pre-planned interim analysis triggered on Jan 9, 2020, when 67% of the total number of expected invasive disease-free survival events had been observed. The trial is registered with ClinicalTrials.gov (NCT02513394) and EudraCT (2014-005181-30).

FINDINGS

Between Sept 1, 2015, and Nov 30, 2018, 5760 patients were randomly assigned to receive palbociclib plus endocrine therapy (n=2883) or endocrine therapy alone (n=2877). At the time of the planned second interim analysis, at a median follow-up of 23·7 months (IQR 16·9-29·2), 170 of 2883 patients assigned to palbociclib plus endocrine therapy and 181 of 2877 assigned to endocrine therapy alone had invasive disease-free survival events. 3-year invasive disease-free survival was 88·2% (95% CI 85·2-90·6) for palbociclib plus endocrine therapy and 88·5% (85·8-90·7) for endocrine therapy alone (hazard ratio 0·93 [95% CI 0·76-1·15]; log-rank p=0·51). As the test statistic comparing invasive disease-free survival between groups crossed the prespecified futility boundary, the independent data monitoring committee recommended discontinuation of palbociclib in patients still receiving palbociclib and endocrine therapy. The most common grade 3-4 adverse events were neutropenia (1742 [61·3%] of 2840 patients on palbociclib and endocrine therapy vs 11 [0·3%] of 2903 on endocrine therapy alone), leucopenia (857 [30·2%] vs three [0·1%]), and fatigue (60 [2·1%] vs ten [0·3%]). Serious adverse events occurred in 351 (12·4%) of 2840 patients on palbociclib plus endocrine therapy versus 220 (7·6%) of 2903 patients on endocrine therapy alone. There were no treatment-related deaths.

INTERPRETATION

At the planned second interim analysis, addition of 2 years of adjuvant palbociclib to adjuvant endocrine therapy did not improve invasive disease-free survival compared with adjuvant endocrine therapy alone. On the basis of these findings, this regimen cannot be recommended in the adjuvant setting. Long-term follow-up of the PALLAS population and correlative studies are ongoing.

FUNDING

Pfizer.

Mechanisms of resistance to cyclin-dependent kinase 4/6 inhibitors

Cyclin-dependent kinase (CDK) 4/6 inhibitors have emerged in the treatment of metastatic hormone receptor (HR)-positive and human epidermal growth factor receptor 2 (HER2)-negative breast cancer. However, most patients will eventually present disease progression, highlighting the inevitable resistance of cancer cells to CDK4/6 inhibition. Several studies have suggested that resistance mechanisms involve aberrations of the molecules that regulate the cell cycle, and the re-wiring of the cell to escape CDK4/6 dependence and turn to alternative pathways. Loss of retinoblastoma function, overexpression of CDK 6, upregulation of cyclin E, overexpression of CDK 7, and dysregulation of several signaling pathways, notably the PI3/AKT/mTOR pathway, have been implicated in the development of resistance to CDK4/6 inhibitors. Overlap with endocrine resistance mechanisms might be possible. Combinational therapeutic strategies should be explored in order to prevent resistance and optimize the management of patients after progression under CDK 4/6 inhibition.

Recent advances in neoadjuvant therapy for breast cancer

Neoadjuvant trials for early breast cancer have accelerated the identification of novel active agents, enabling streamlined conduct of registration trials with fewer subjects. Measurement of neoadjuvant drug effects has also enabled the identification of patients with high risk of distant recurrence and has justified development of additional adjuvant approaches to improve outcomes. Neoadjuvant evaluation of new drugs was significantly improved by the introduction of pathologic complete response (pCR) rate as a quantitative surrogate endpoint for distant disease-free survival (DDFS) and event free survival (EFS). The neoadjuvant phase 2 platform trial I-SPY 2 simultaneously tests multiple drugs across multiple breast cancer subtypes using Bayesian methods of adaptive randomization for assessment of drug efficacy. In addition to the pCR endpoint, the I-SPY 2 trial has demonstrated that the residual cancer burden (RCB) score measures gradations of tumor response that correlate with DDFS and EFS across treatments and subtypes. For HER2-positive and triple-negative breast cancers that have failed to attain pCR with neoadjuvant chemotherapy (NAC), effective modifications of adjuvant treatment have improved outcomes and changed the standard of care for these subtypes. Neoadjuvant therapy is therefore preferred for stage II and III, as well as some stage I, HER2-positive and triple-negative tumors. Neoadjuvant endocrine therapy (NET) strategies have also emerged from innovative trials for stage II and III estrogen receptor (ER)-positive/HER2-negative tumors, as in the ALTERNATE trial. From neoadjuvant trials, opportunities have emerged to de-escalate therapy on the basis of metrics of response to chemotherapy or hormonal therapy. Neoadjuvant therapy for early breast cancer is therefore emerging as a promising approach to accelerate new drug development, optimize treatment strategies, and (where appropriate) de-escalate neoadjuvant therapy.