Mechanisms of Resistance to CDK4/6 Inhibitors in Breast Cancer and Potential Biomarkers of Response

Identifying prognostic biomarkers for palbociclib add-on therapy in fulvestrant-resistant breast cancer using cell-free DNA sequencing

BACKGROUND

The FUTURE trial (UMIN000029294) demonstrated the safety and efficacy of adding palbociclib after fulvestrant resistance in patients with hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2-) advanced and metastatic breast cancer (ABC/MBC). In this planned sub-study, cancer panel sequencing of cell-free DNA (cfDNA) was utilized to explore prognostic and predictive biomarkers for further palbociclib treatment following fulvestrant resistance.

MATERIALS AND METHODS

Herein, 149 cfDNA samples from 65 patients with fulvestrant-resistant disease were analysed at the time of palbociclib addition after fulvestrant resistance (baseline), on day 15 of cycle 1, and at the end of treatment using the assay for identifying diverse mutations in 34 cancer-related genes.

RESULTS

During the course of treatment, mutations in ESR1, PIK3CA, FOXA1, RUNX1, TBX3, and TP53 were the most common genomic alterations observed. Analysis of genomic mutations revealed that before fulvestrant introduction, baseline PIK3CA mutations were marginally lower in metastatic aromatase inhibitor (AI)-treated patients compared to adjuvant AI-treated patients (P = 0.063). Baseline PIK3CA mutations were associated with poorer progression-free survival [hazard ratio: 1.62, P = 0.04]. Comparative analysis between baseline and early-changing gene mutations identified poor prognostic factors including early-changing MAP3K1 mutations (hazard ratio: 4.66, P = 0.04), baseline AR mutations (hazard ratio: 3.53, P = 0.04), and baseline PIK3CA mutations (hazard ratio: 3.41, P = 0.02). Notably, the relationship between ESR1 mutations and mutations in PIK3CA, MAP3K1, and TP53 weakened as treatment progressed. Instead, PIK3CA mutations became correlated with TP53 and FOXA1 mutations.

CONCLUSIONS

Cancer panel testing for cfDNA identified prognostic and predictive biomarkers for palbociclib add-on therapy after acquiring fulvestrant resistance in patients with HR+/HER2- ABC/MBC.

The Role of CD4/6 Inhibitors in Breast Cancer Treatment

Over the last decade, treatment paradigms for breast cancer have undergone a renaissance, particularly in hormone-receptor-positive/HER2-negative breast cancer. These revolutionary therapies are based on the selective targeting of aberrancies within the cell cycle. This shift towards targeted therapies has also changed the landscape of disease monitoring. In this article, we will review the fundamentals of cell cycle progression in the context of the new cyclin-dependent kinase inhibitors. In addition to discussing the currently approved cyclin-dependent kinase inhibitors for breast cancer, we will explore the ongoing development and search for predictive biomarkers and modalities to monitor treatment.

Pooled ctDNA analysis of MONALEESA phase III advanced breast cancer trials

BACKGROUND

The phase III MONALEESA trials tested the efficacy and safety of the cyclin-dependent kinase (CDK)4/6 inhibitor ribociclib with different endocrine therapy partners as first- or second-line treatment of hormone receptor-positive/human epidermal growth factor receptor 2-negative advanced breast cancer (ABC). Using the largest pooled biomarker dataset of the CDK4/6 inhibitor ribociclib in ABC to date, we identified potential biomarkers of response to ribociclib.

PATIENTS AND METHODS

Baseline circulating tumour DNA from patients in the MONALEESA trials was assessed using next-generation sequencing. An analysis of correlation between gene alteration status and progression-free survival (PFS) was carried out to identify potential biomarkers of response to ribociclib.

RESULTS

Multiple frequently altered genes were identified. Alterations in ERBB2, FAT3, FRS2, MDM2, SFRP1, and ZNF217 were associated with a greater PFS benefit with ribociclib versus placebo. Patients with high tumour mutational burden (TMB) and with ANO1, CDKN2A/2B/2C, and RB1 alterations exhibited decreased sensitivity to ribociclib versus placebo.

CONCLUSIONS

Although exploratory, these results provide insight into alterations associated with the improved response to ribociclib treatment and may inform treatment sequencing in patients with actionable alterations following progression on CDK4/6 inhibitors. Validation of potential biomarkers identified here and development of prospective trials testing their clinical utility are warranted.

CLINICALTRIALS

GOV IDENTIFIERS

NCT01958021, NCT02422615, NCT02278120.

Clinical and Genomic Characteristics of Patients with Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-Negative Metastatic Breast Cancer Following Progression on Cyclin-Dependent Kinase 4 and 6 Inhibitors

PURPOSE

We explored the clinical and genomic characteristics of hormone receptor-positive (HR+), HER2-negative (HER2-) metastatic breast cancer (MBC) after progression on cyclin-dependent kinase 4 and 6 inhibitors (CDK4 and 6i) ± endocrine therapy (ET) to understand potential resistance mechanisms that may aid in identifying treatment options.

EXPERIMENTAL DESIGN

Patients in the United States with HR+, HER2- MBC had tumor biopsies collected from a metastatic site during routine care following progression on a CDK4 and 6i ± ET (CohortPost) or prior to initiating CDK4 and 6i treatment (CohortPre) and analyzed using a targeted mutation panel and RNA-sequencing. Clinical and genomic characteristics were described.

RESULTS

The mean age at MBC diagnosis was 59 years in CohortPre (n = 133) and 56 years in CohortPost (n = 223); 14% and 45% of patients had prior chemotherapy/ET, and 35% and 26% had de novo stage IV MBC, respectively. The most common biopsy site was liver (CohortPre, 23%; CohortPost, 56%). CohortPost had significantly higher tumor mutational burden (TMB; median 3.16 vs. 1.67 Mut/Mb, P < 0.0001), ESR1 alteration frequency (mutations: 37% vs. 10%, FDR < 0.0001; fusions: 9% vs. 2%, P = 0.0176), and higher copy-number amplification of genes on chr12q15, including MDM2, FRS2, and YEATS4 versus patients in the CohortPre group. In addition, CDK4 copy-number gain on chr12q13 was significantly higher in CohortPost versus CohortPre (27% vs. 11%, P = 0.0005).

CONCLUSIONS

Distinct mechanisms potentially associated with resistance to CDK4 and 6i ± ET, including alterations in ESR1 and amplification of chr12q15 and CDK4 copy-number gain, were identified.

Is Palbociclib a cost-effective strategy in the second-line treatment of metastatic breast cancer in Iran?

BACKGROUND & AIMS

This study evaluates the cost-effectiveness of Palbociclib in the second-line treatment of hormone receptor-positive (HR+) human epidermal growth factor receptor 2-negative (HER2-) metastatic breast cancer (MBC) in Iran.

METHODS

The present economic evaluation used a partitioned survival model (PSM). This model compares lifetime costs and disease outcomes among groups receiving different medication combinations containing Palbociclib, Fulvestrant, Everolimus, Ribociclib, and Abemaciclib as the second-line therapy for HR+/HER2- MBC. The model was conducted from Iran's healthcare perspective, structured with 1-month cycles, and the evaluation time horizon in the base analysis was set to 180 cycles (15 years). Transition probabilities were extracted using the survival curves. The cost information was extracted based on the year 2020. The Quality Adjusted Life Years (QALY) was considered the final outcome unit, and the cost-effectiveness of different combinations is calculated as cost per QALY. The annual discount rate of 5% was considered for costs and QALYs. Two times Iran's GDP per capita (800,000,000 IRR = US$5934) was used as the threshold. Finally, due to the uncertainty of some parameters, deterministic and probabilistic sensitivity analyses were carried out.

RESULTS

The base case results showed that the highest cost was for the 'Ribociclib+ Fulvestrant' combination (US$89,629.56), and the lowest price was for the 'Iranian Everolimus + Fulvestrant' combination (US$10,740.09). 'Palbociclib + Fulvestrant' brings about the highest value of 1.456 incremental QALYs compared to other strategies. Finally, the 'Iranian Palbociclib + Fulvestrant' was the cost-effective combination, with an incremental cost-effectiveness ratio (ICER) of US$4,201 compared to other strategies. The base case results were supported by the probabilistic sensitivity analysis. Deterministic sensitivity analysis showed that the cost of Iranian Palbociclib has a threshold of US$582.99.

CONCLUSIONS

The 'Iranian Palbociclib + Fulvestrant' combination was cost-effective in second-line therapy for HR+ HER2- MBC in Iran.

Role of identified proteins in the proteome profiles of CDK4/6 inhibitor-resistant breast cancer cell lines

Abemaciclib (Ab) and palbociclib (Pb) are CDK4/6 inhibitors used to cure advanced breast cancer (BC). However, acquired resistance is a major challenge. The molecular mechanisms and signature proteins of therapy resistance for Ab and Pb drugs need to be explored. Here we developed resistant cells for Ab and Pb drugs in MCF-7 cell lines and explored the mechanisms and signature proteins of therapy resistance in BC. Proteome profiling was performed using the label-free proteome-orbitrap-fusion-MS-MS technique. Gene ontology (GO)-terms, KEGG pathways and network analysis were performed for the proteome data. Drug-resistant cells showed increased drug tolerance, enhanced colony formation potential and an increased gap-healing tendency for the respective drug. Up-regulation of survival genes (BCL-2 and MCL-1) and down-regulation of apoptosis inducers were observed. Drug-resistance markers (MDR-1 and ABCG2 (BCRP)) along with ESR-1, CDK4, CDK6, and cyclin-D1 genes were up-regulated in resistant cells. A total of 237 and 239 proteins were found to be differentially expressed in the Ab and Pb-resistant cells, respectively. Down-regulated proteins induce apoptosis signalling and nucleotide metabolisms and restrict EGFR signalling; however, up-regulated proteins induce Erk, wnt-β-catenin, VEGFR-PI3K-AKT, glucose transportation, and hypoxia signalling pathways and regulate hydrogen peroxide signalling pathways. The panel of identified proteins associated with these pathways might have characteristics of molecular signature and new drug targets for overcoming drug resistance in breast cancer.

AKT/mTOR signaling modulates resistance to endocrine therapy and CDK4/6 inhibition in metastatic breast cancers

Endocrine therapy (ET) in combination with CDK4/6 inhibition is routinely used as first-line treatment for HR+/HER2- metastatic breast cancer (MBC) patients. However, 30-40% of patients quickly develop disease progression. In this open-label multicenter clinical trial, we utilized a hypothesis-driven protein/phosphoprotein-based approach to identify predictive markers of response to ET plus CDK4/6 inhibition in pre-treatment tissue biopsies. Pathway-centered signaling profiles were generated from microdissected tumor epithelia and surrounding stroma/immune cells using the reverse phase protein microarray. Phosphorylation levels of the CDK4/6 downstream substrates Rb (S780) and FoxM1 (T600) were higher in patients with progressive disease (PD) compared to responders (p = 0.02). Systemic PI3K/AKT/mTOR activation in tumor epithelia and stroma/immune cells was detected in patients with PD. This activation was not explained by underpinning genomic alterations alone. As the number of FDA-approved targeted compounds increases, functional protein-based signaling analyses may become a critical component of response prediction and treatment selection for MBC patients.

Combined inhibition of ACLY and CDK4/6 reduces cancer cell growth and invasion

The use of small molecule kinase inhibitors, which target specific enzymes that are overactive in cancer cells, has revolutionized cancer patient treatment. To treat some types of breast cancer, CDK4/6 inhibitors, such as palbociclib, have been developed that target the phosphorylation of the retinoblastoma tumor suppressor gene. Acquired resistance to CDK4/6 inhibitors may be due to activation of the AKT pro‑survival signaling pathway that stimulates several processes, such as growth, metastasis and changes in metabolism that support rapid cell proliferation. The aim of the present study was to investigate whether targeting ATP citrate lyase (ACLY), a downstream target of AKT, may combine with CDK4/6 inhibition to inhibit tumorigenesis. The present study determined that ACLY is activated in breast and pancreatic cancer cells in response to palbociclib treatment and AKT mediates this effect. Inhibition of ACLY using bempedoic acid used in combination with palbociclib reduced cell viability in a panel of breast and pancreatic cancer cell lines. This effect was also observed using breast cancer cells grown in 3D cell culture. Mechanistically, palbociclib inhibited cell proliferation, whereas bempedoic acid stimulated apoptosis. Finally, using Transwell invasion assays and immunoblotting, the present study demonstrated that ACLY inhibition blocked cell invasion, when used alone or in combination with palbociclib. These data may yield useful information that could guide the development of future therapies aimed at the reduction of acquired resistance observed clinically.

CDK4/6 inhibitor resistance: A bibliometric analysis

BACKGROUND

Cyclin-dependent kinases (CDKs) 4/6 inhibitors are a type of cell cycle regulation that prevents cell proliferation by blocking retinoblastoma protein (Rb) phosphorylation in the G1 to S phase transition. CDK 4/6 inhibitors are currently used mainly in patients with hormone receptor-positive/human epidermal growth factor receptor 2 (HER2) negative breast cancer in combination with endocrine therapy. However, primary or acquired resistance to drugs severely affect drug efficacy. Our study aims at summarizing and visualizing the current research direction and development trend of CDK4/6 inhibitor resistance to provide clinicians and research power with a summary of the past and ideas for the future.

METHODS

The Web of Science Core Collection and PubMed was searched for all included articles on CDK4/6 inhibitor resistance for bibliometric statistics and graph plotting. The metrological software and graphing tools used were R language version 4.2.0, Bibliometrix 4.0.0, Vosviewer 1.6.18, GraphPad Prism 9, and Microsoft Excel 2019.

RESULTS

A total of 1278 English-language articles related to CDK4/6 inhibitor resistance were included in the Web of Science core dataset from 1996-2022, with an annual growth rate of14.56%. In PubMed, a total of 1123 articles were counted in the statistics, with an annual growth rate of 17.41% Cancer Research is the most included journal (102/1278, 7.98%) with an impact factor of 13.312 and is the Q1 of the Oncology category of the Journal Citation Reports. Professor Malorni Luca from Italy is probably the most contributing author in the current field (Publications 21/1278, 1.64%), while Prof. Turner Nicholas C from the USA is perhaps the most authoritative new author in the field of CDK4/6 inhibitor resistance (Total Citations2584, M-index 1.429). The main research efforts in this field are currently focused on Palbociclib and Abemaciclib. Studies on drug resistance mechanisms or post-drug resistance therapies focus on MEK inhibitors and related pathways, PI3K-AKT-MTOR pathways or inhibitors, EGFR-related pathways, EGFR inhibitors, TKI inhibitors, MAPK pathways and inhibitors, and so on.

CONCLUSION

This study provides researchers with a reliable basis and guidance for finding authoritative references, understanding research trends, and mining research neglect directions.

Resistance to CDK4/6 inhibition: Mechanisms and strategies to overcome a therapeutic problem in the treatment of hormone receptor-positive metastatic breast cancer

Selective CDK4/6 inhibitors, such as palbociclib, ribociclib, and abemaciclib, have been approved in combination with hormone therapy for the treatment of patients with HR+, HER2-negative advanced or metastatic breast cancer (mBC). Despite their promising activity, approximately 10 % of patients have de novo resistance, while the rest of them will develop acquired resistance after 24-28 months when used as first-line therapy and after a shorter period when used as second-line therapy. Various mechanisms of resistance to CDK4/6 inhibitors have been described, including cell cycle-related mechanisms, such as RB loss, p16 amplification, CDK6 or CDK4 amplification, and cyclin E-CDK2 amplification. Other bypass mechanisms involve the activation of FGFR or PI3K/AKT/mTOR pathways. Identifying the different mechanisms by which resistance to CDK4/6 inhibitors occurs may help to design new treatment strategies to improve patient outcomes. This review presents the currently available knowledge on the mechanisms of resistance to CDK4/6 inhibitors, explores possible treatment strategies that could overcome this therapeutic problem, and summarizes relevant recent clinical trials.

The Renaissance of CDK Inhibitors in Breast Cancer Therapy: An Update on Clinical Trials and Therapy Resistance

Simple Summary

Cyclin-dependent kinase inhibitors (palbociclib (Ibrance), ribociclib (Kisqali), and abemaciclib (Verzenio)), targeting aberrant cell-cycle activity have been evaluated extensively in clinical trials. Significant delays in progression free survival and overall survival are now documented with each agent in estrogen receptor positive and human epidermal growth factor receptor two negative advanced breast cancer including luminal B breast cancer. Therapy resistance, driven by chromosomal instability, results in genomic rearrangements, activation of cell-cycle components (cyclin E/cdk2 in Rb⁻ tumors, cyclin D1 in growth factor activated pathways), and the immune response. Molecular analysis of therapy resistant tumors may provide the rational basis for new therapies (brivanib, CYC065, WEE1 kinase and other inhibitors). Luminal B breast cancer is enriched for cyclin D1 overexpression and the chromosomal instability gene signature. The molecular mechanisms governing chromosomal instability in luminal B breast cancer remain poorly understood. Co-targeting of chromosomal instability may potentially reduce the prevalent escape mechanisms that reduce the effectiveness of cyclin-dependent kinase inhibitors.

Abstract

Cyclin-dependent kinases (CDKs) govern cell-cycle checkpoint transitions necessary for cancer cell proliferation. Recent developments have illustrated nuanced important differences between mono CDK inhibitor (CDKI) treatment and the combination therapies of breast cancers. The CDKIs that are currently FDA-approved for breast cancer therapy are oral agents that selectively inhibit CDK4 and CDK6, include palbociclib (Ibrance), ribociclib (Kisqali), and abemaciclib (Verzenio). CDKI therapy is effective in hormone receptor positive (HR⁺), and human epidermal growth factor receptor two negative (HER2⁻) advanced breast cancers (ABC) malignancies, but remains susceptible due to estrogen and progesterone receptor overexpression. Adding a CDK4/6I to endocrine therapy increases efficacy and delays disease progression. Given the side effects of CDKI, identifying potential new treatments to enhance CDKI effectiveness is essential. Recent long-term studies with Palbociclib, including the PALLAS and PENELOPE B, which failed to meet their primary endpoints of influencing progression-free survival, suggest a deeper mechanistic understanding of cyclin/CDK functions is required. The impact of CDKI on the anti-tumor immune response represents an area of great promise. CDKI therapy resistance that arises provides the opportunity for specific types of new therapies currently in clinical trials.

CDK4/6 inhibitor resistance mechanisms and treatment strategies (Review)

In recent years, the incidence rate of breast cancer has increased year by year, and it has become a major threat to the health of women globally. Among all breast cancer subtypes, the hormone receptor (HR)⁺/human epidermal growth factor receptor 2 (HER2)⁻ luminal subtype breast cancer is the most common form of breast cancer. Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors, the hotspots in the field of targeted therapy for breast cancer, have proved to exhibit a good effect on patients with HR⁺/HER2⁻ breast cancer in a number of clinical trials, but the problem of drug resistance is inevitable. At present, three specific CDK4/6 inhibitors (palbociclib, ribociclib and abemaciclib) have been approved by the USA Food and Drug Administration for the first-line treatment of HR⁺/HER2⁻ breast cancer. The drug resistance mechanisms of CDK4/6 inhibitors can be divided into cell cycle-specific resistance and cell cycle non-specific resistance. With the discovery of the drug resistance mechanism of CDK4/6 inhibitors, various targeted strategies have been proposed. The present review mainly discusses the mechanism of CDK4/6 inhibitors, drug resistance mechanisms and treatment strategies after resistance.

CDK4: a master regulator of the cell cycle and its role in cancer

The cell cycle is regulated in part by cyclins and their associated serine/threonine cyclin-dependent kinases, or CDKs. CDK4, in conjunction with the D-type cyclins, mediates progression through the G₁ phase when the cell prepares to initiate DNA synthesis. Although Cdk4-null mutant mice are viable and cell proliferation is not significantly affected in vitro due to compensatory roles played by other CDKs, this gene plays a key role in mammalian development and cancer. This review discusses the role that CDK4 plays in cell cycle control, normal development and tumorigenesis as well as the current status and utility of approved small molecule CDK4/6 inhibitors that are currently being used as cancer therapeutics.

Evaluating the efficacy of an innovative herbal formulation (HF6) on different human cancer cell lines

Cancer is reported to be the leading cause of death and illness worldwide. This research aims to evaluate the phytochemicals, antioxidant, cytotoxic, and apoptotic activities of the polyherbal formulation HF6. HF6 was prepared by blending equal quantities of plants powder, namely, Curcuma longa, Salvia officinalis, Cinnamomum zeylanicum, Capsicum annuum, Zingiber officinale, and Syzygium aromaticum, and later extracted using hexane (HF6H), chloroform (HF6C), ethyl acetate (HF6E), and methanol (HF6M) in Soxhlet apparatus. Among the four different extracts, only the hexane extract (HF6H) was significantly effective. The HF6H extract showed antioxidant and anticancer potentials against different cancer cell lines, and moderate cytotoxicity against non-cancer cells, rendering it a promising remedy. In addition, it exerted tremendous cytotoxic effects on MCF-7, Huh-7, HCT116, MDA-MB-231, LoVo, and HepG2 cells with IC50 values of 2.02, 4.5, 6.9, 11.4, 23.5, and 34.7 µg/mL, respectively. The morphological hallmarks of apoptosis such as the rounding of cells, loss of contact with neighboring cells, formation of cell membrane blebbing, and microspike protrusion were detected using several different techniques. DAPI staining revealed apoptotic nuclear morphology such as condensation and DNA fragmentation. The morphological changes of MCF7 cells were also analyzed by AO/EB fluorescence staining. MCF7-stained green cells were viable cells, whereas the treated cells showed fragmented green nuclei representing early apoptosis. The phytochemical screening of HF6H showed positive results regarding the presence of alkaloids, polyphenols, flavonoids, and sterols. The GC-MS (gas chromatography-mass spectrometry) analysis of the HF6H extract indicated the presence of 12 compounds, mainly trans-caryophyllene (21.55%), cis-isoeugenol (18.42%), acetyleugenol (17.53%), alpha farnesene (10.0%), and zingiberene (8.55%). However, further investigation could be carried out to examine the toxicity of the extract on animal models.

Small Molecule Inhibitors in Adult High-Grade Glioma: From the Past to the Future

Glioblastoma is the most common primary malignant tumor in the brain and has a dismal prognosis despite patients accepting standard therapies. Alternation of genes and deregulation of proteins, such as receptor tyrosine kinase, PI3K/Akt, PKC, Ras/Raf/MEK, histone deacetylases, poly (ADP-ribose) polymerase (PARP), CDK4/6, branched-chain amino acid transaminase 1 (BCAT1), and Isocitrate dehydrogenase (IDH), play pivotal roles in the pathogenesis and progression of glioma. Simultaneously, the abnormalities change the cellular biological behavior and microenvironment of tumor cells. The differences between tumor cells and normal tissue become the vulnerability of tumor, which can be taken advantage of using targeted therapies. Small molecule inhibitors, as an important part of modern treatment for cancers, have shown significant efficacy in hematologic cancers and some solid tumors. To date, in glioblastoma, there have been more than 200 clinical trials completed or ongoing in which trial designers used small molecules as monotherapy or combination regimens to correct the abnormalities. In this review, we summarize the dysfunctional molecular mechanisms and highlight the outcomes of relevant clinical trials associated with small-molecule targeted therapies. Based on the outcomes, the main findings were that small-molecule inhibitors did not bring more benefit to newly diagnosed glioblastoma, but the clinical studies involving progressive glioblastoma usually claimed “noninferiority” compared with historical results. However, as to the clinical inferiority trial, similar dosing regimens should be avoided in future clinical trials.

Development of pteridin-7(8H)-one analogues as highly potent cyclin-dependent kinase 4/6 inhibitors: Synthesis, structure-activity relationship, and biological activity

CDK4/6 have been validated as the cancer therapeutic targets. Here, we describe a series of pteridin-7(8H)-one analogues as potent CDK4/6 inhibitors. Among them, the most promising compound 7s demonstrated remarkable and broad-spectrum antiproliferative activities toward HCT116, HeLa, MDA-MB-231, and HT-29 cells with IC₅₀ values of 0.65, 0.70, 0.39, and 2.53 μM, respectively, which were more potent than that of the anticancer drug Palbociclib. Interestingly, 7s also manifested the greatest inhibitory activities toward both CDK4/cyclin D3 and CDK6/cyclin D3 (IC₅₀ = 34.0 and 65.1 nM, respectively), which was comparable with Palbociclib. Additionally, molecular simulation indicated that 7s bound efficiently at the ATPbindingsitesofCDK4 and CDK6. Further mechanistic studies revealed that compound 7s could concentration-dependently induce cell cycle arrest and apoptosis in HeLa cells. Takentogether, 7s represents a promising novel CDK4/6 inhibitor for the potential treatment of cancer.

Molecular mechanisms of chemo- and radiotherapy resistance and the potential implications for cancer treatment

Cancer is a leading cause of death worldwide. Surgery is the primary treatment approach for cancer, but the survival rate is very low due to the rapid progression of the disease and presence of local and distant metastasis at diagnosis. Adjuvant chemotherapy and radiotherapy are important components of the multidisciplinary approaches for cancer treatment. However, resistance to radiotherapy and chemotherapy may result in treatment failure or even cancer recurrence. Radioresistance in cancer is often caused by the repair response to radiation-induced DNA damage, cell cycle dysregulation, cancer stem cells (CSCs) resilience, and epithelial-mesenchymal transition (EMT). Understanding the molecular alterations that lead to radioresistance may provide new diagnostic markers and therapeutic targets to improve radiotherapy efficacy. Patients who develop resistance to chemotherapy drugs cannot benefit from the cytotoxicity induced by the prescribed drug and will likely have a poor outcome with these treatments. Chemotherapy often shows a low response rate due to various drug resistance mechanisms. This review focuses on the molecular mechanisms of radioresistance and chemoresistance in cancer and discusses recent developments in therapeutic strategies targeting chemoradiotherapy resistance to improve treatment outcomes.

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.

NP-ALT, a Liposomal:Peptide Drug, Blocks p27Kip1 Phosphorylation to Induce Oxidative Stress, Necroptosis, and Regression in Therapy-Resistant Breast Cancer Cells

Resistance to cyclin D-CDK4/6 inhibitors (CDK4/6i) represents an unmet clinical need and is frequently caused by compensatory CDK2 activity. Here we describe a novel strategy to prevent CDK4i resistance by using a therapeutic liposomal:peptide formulation, NP-ALT, to inhibit the tyrosine phosphorylation of p27Kip1(CDKN1B), which in turn inhibits both CDK4/6 and CDK2. We find that NP-ALT blocks proliferation in HR+ breast cancer cells, as well as CDK4i-resistant cell types, including triple negative breast cancer (TNBC). The peptide ALT is not as stable in primary mammary epithelium, suggesting that NP-ALT has little effect in nontumor tissues. In HR+ breast cancer cells specifically, NP-ALT treatment induces ROS and RIPK1-dependent necroptosis. Estrogen signaling and ERα appear required. Significantly, NP-ALT induces necroptosis in MCF7 ESR^Y537S cells, which contain an ER gain of function mutation frequently detected in metastatic patients, which renders them resistant to endocrine therapy. Here we show that NP-ALT causes necroptosis and tumor regression in treatment naïve, palbociclib-resistant, and endocrine-resistant BC cells and xenograft models, demonstrating that p27 is a viable therapeutic target to combat drug resistance. IMPLICATIONS: This study reveals that blocking p27 tyrosine phosphorylation inhibits CDK4 and CDK2 activity and induces ROS-dependent necroptosis, suggesting a novel therapeutic option for endocrine and CDK4 inhibitor-resistant HR+ tumors.

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.

Advances in endocrine and targeted therapy for hormone-receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer

Nearly 70% of breast cancer (BC) is hormone-receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative, and endocrine therapy is the mainstay of treatment for this subtype. However, intrinsic or acquired endocrine resistance can occur during the endocrine treatment. Based on insights of endocrine resistance mechanisms, a number of targeted therapies have been and continue to be developed. With regard to HR-positive, HER2-negative advanced BC, aromatase inhibitor (AI) is superior to tamoxifen, and fulvestrant is a better option for patients previously exposed to endocrine therapy. Targeted drugs, such as cyclin-dependent kinases (CDK) 4/6 inhibitors, mammalian target of rapamycin (mTOR) inhibitors, phosphoinositide-3-kinase (PI3K) inhibitors, and histone deacetylase (HDAC) inhibitors, play a significant role in the present and show a promising future. With the application of CDK4/6 inhibitors becoming common, mechanisms of acquired resistance to them should also be taken into consideration.

Chaperonin-Containing TCP1 Complex (CCT) Promotes Breast Cancer Growth Through Correlations With Key Cell Cycle Regulators

Uncontrolled proliferation as a result of dysregulated cell cycling is one of the hallmarks of cancer. Therapeutically targeting pathways that control the cell cycle would improve patient outcomes. However, the development of drug resistance and a limited number of inhibitors that target multiple cell cycle modulators are challenges that impede stopping the deregulated growth that leads to malignancy. To advance the discovery of new druggable targets for cell cycle inhibition, we investigated the role of Chaperonin-Containing TCP1 (CCT or TRiC) in breast cancer cells. CCT, a type II chaperonin, is a multi-subunit protein-folding complex that interacts with many oncoproteins and mutant tumor suppressors. CCT subunits are highly expressed in a number of cancers, including breast cancer. We found that expression of one of the CCT subunits, CCT2, inversely correlates with breast cancer patient survival and is subject to copy number alterations through genomic amplification. To investigate a role for CCT2 in the regulation of the cell cycle, we expressed an exogenous CCT2-FLAG construct in T47D and MCF7 luminal A breast cancer cells and examined cell proliferation under conditions of two-dimensional (2D) monolayer and three-dimensional (3D) spheroid cultures. Exogenous CCT2 increased the proliferation of cancer cells, resulting in larger and multiple spheroids as compared to control cells. CCT2-expressing cells were also able to undergo spheroid growth reversal, re-attaching, and resuming growth in 2D cultures. Such cells gained anchorage-independent growth. CCT2 expression in cells correlated with increased expression of MYC, especially in spheroid cultures, and other cell cycle regulators like CCND1 and CDK2, indicative of a novel activity that could contribute to the increase in cell growth. Statistically significant correlations between CCT2, MYC, and CCND1 were shown. Since CCT2 is located on chromosome 12q15, an amplicon frequently found in soft tissue cancers as well as breast cancer, CCT2 may have the basic characteristics of an oncogene. Our findings suggest that CCT2 could be an essential driver of cell division that may be a node through which pathways involving MYC, cyclin D1 and other proliferative factors could converge. Hence the therapeutic inhibition of CCT2 may have the potential to achieve multi-target inhibition, overcoming the limitations associated with single agent inhibitors.

A cyclin D-CDK6 dimer helps to reshuffle cyclin-dependent kinase inhibitors (CKI) to overcome TGF-beta-mediated arrest and maintain CDK2 activity

The cyclin D-CDK4/6 complex has two distinct functions. Its kinase-dependent role involves its ability to act as serine/threonine kinase, responsible for phosphorylation of substrates required for cell cycle transitions, while its kinase-independent function involves its ability to act as a reservoir for p27^Kip1. This association sequesters p27 from cyclin E-CDK2 complexes, allowing them to remain active. The aim of this current study is two-fold: to understand the contribution of the kinase-dependent and kinase-independent functions of CDK4 and CDK6 in epithelial cells and to directly compare CDK4 and CDK6 in a simple model system, TGF-β treatment, where arrest is initiated by the expression of p15^Ink4b. Cells that overexpressed a catalytically inactive, p15-insensitive CDK6 variant (p27 sequestration only mutant) were able to overcome TGF-β-mediated arrest by maintaining CDK2 activity, while cells expressing the identical mutations in CDK4 were not. This result can be partially explained by the presence of a previously unidentified cyclin D-CDK6 dimer, which serves as a sink for free p27 during TGF-β treatment, enabling CDK2 to remain inhibitor free. The use of the TGF-β model system and the characterization of CDK pool dynamics and p27 switching is relevant to the CDK4/6 specific inhibitors, such as palbociclib, whose mechanism of action may resemble that of p15.

Ki67 and progesterone receptor status predicts sensitivity to palbociclib: a real-world study

Background

Palbociclib combined with endocrine therapy has been approved as a front-line treatment for hormone receptor positive (HR+), human epidermal growth factor receptor 2 negative (HER2-) advanced breast cancer (ABC). A key challenge remains to uncover biomarkers to identify those patients who may benefit from palbociclib treatment.

Methods

We retrospectively analyzed the values of Ki67 and progesterone receptor (PR) as detected by immunohistochemistry in 81 ABC patients with palbociclib and hormone therapy treatment, and evaluated the impact on progression-free survival (PFS).

Results

In the total population, women with Ki67 ≥14% had marginally significantly shorter PFS than those with Ki67 <14% (P=0.062). Patients with Ki67 ≥30% had significantly shorter PFS than those with Ki67 <30% (P=0.048). Meanwhile, PR ≥20% was associated with longer PFS. Moreover, the change of Ki67 or PR from primary tissue to metastatic lesions was related to PFS. As for the hormone therapy subgroup, there were significant associations between Ki67 and PR levels and PFS in the aromatase inhibitors (AIs) subgroup. Patients with Ki67 ≥14% or Ki67 ≥30% had shorter PFS than those with Ki67 <14% or Ki67 <30%, respectively (P=0.024, P<0.001). Additionally, the change of Ki67 or PR from primary tissue to metastatic lesions was related to PFS. When both Ki67 and PR were considered, there were significant differences between the different cohorts. Compared with patients with Ki67 ≥14% and PR <20%, those with Ki67 <14% and PR ≥20% had significantly longer PFS. In addition, patients with Ki67 <30% and PR ≥20% had significantly longer PFS than those with Ki67 ≥30% and PR <20%. Furthermore, in the AI cohort, patients with Ki67 <14% and PR ≥20% had significantly longer PFS than those with Ki67 ≥14% and PR <20%. Women with Ki67 <30% and PR ≥20% had significantly longer PFS than those with Ki67 ≥30% and PR <20%.

Conclusions

The present study indicates that both Ki67 and PR have great impacts on palbociclib and hormone therapy and may contribute to selecting more effective partners for palbociclib.

Circulating tumor cells and palbociclib treatment in patients with ER-positive, HER2-negative advanced breast cancer: results from a translational sub-study of the TREnd trial

Background

Circulating tumor cells (CTCs) are prognostic in patients with advanced breast cancer (ABC). However, no data exist about their use in patients treated with palbociclib. We analyzed the prognostic role of CTC counts in patients enrolled in the cTREnd study, a pre-planned translational sub-study of TREnd (NCT02549430), that randomized patients with ABC to palbociclib alone or palbociclib plus the endocrine therapy received in the prior line of treatment. Moreover, we evaluated RB1 gene expression on CTCs and explored its prognostic role within the cTREnd subpopulation.

Methods

Forty-six patients with ER-positive, HER2-negative ABC were analyzed. Blood samples were collected before starting palbociclib treatment (timepoint T0), after the first cycle of treatment (timepoint T1), and at disease progression (timepoint T2). CTCs were isolated and counted by CellSearch® System using the CellSearch™Epithelial Cell kit. Progression-free survival (PFS), clinical benefit (CB) during study treatment, and time to treatment failure (TTF) after study treatment were correlated with CTC counts. Samples with ≥ 5 CTCs were sorted by DEPArray system® (DA). RB1 and GAPDH gene expression levels were measured by ddPCR.

Results

All 46 patients were suitable for CTCs analysis. CTC count at T0 did not show significant prognostic value in terms of PFS and CB. Patients with at least one detectable CTC at T1 (n = 26) had a worse PFS than those with 0 CTCs (n = 16) (p = 0.02). At T1, patients with an increase of at least three CTCs showed reduced PFS compared to those with no increase (mPFS = 3 versus 9 months, (p = 0.004). Finally, patients with ≥ 5 CTCs at T2 (n = 6/23) who received chemotherapy as post-study treatment had a shorter TTF (p = 0.02). Gene expression data for RB1 were obtained from 19 patients. CTCs showed heterogeneous RB1 expression. Patients with detectable expression of RB1 at any timepoint showed better, but not statistically significant, outcomes than those with undetectable levels.

Conclusions

CTC count seems to be a promising modality in monitoring palbociclib response. Moreover, CTC count at the time of progression could predict clinical outcome post-palbociclib. RB1 expression analysis on CTCs is feasible and may provide additional prognostic information. Results should be interpreted with caution given the small studied sample size.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13058-021-01415-w.

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.

Selective inhibition of CDK4/6: A safe and effective strategy for developing anticancer drugs

The sustained cell proliferation resulting from dysregulation of the cell cycle and activation of cyclin-dependent kinases (CDKs) is a hallmark of cancer. The inhibition of CDKs is a highly promising and attractive strategy for the development of anticancer drugs. In particular, third-generation CDK inhibitors can selectively inhibit CDK4/6 and regulate the cell cycle by suppressing the G1 to S phase transition, exhibiting a perfect balance between anticancer efficacy and general toxicity. To date, three selective CDK4/6 inhibitors have received approval from the U.S. Food and Drug Administration (FDA), and 15 CDK4/6 inhibitors are in clinical trials for the treatment of cancers. In this perspective, we discuss the crucial roles of CDK4/6 in regulating the cell cycle and cancer cells, analyze the rationale for selectively inhibiting CDK4/6 for cancer treatment, review the latest advances in highly selective CDK4/6 inhibitors with different chemical scaffolds, explain the mechanisms associated with CDK4/6 inhibitor resistance and describe solutions to overcome this issue, and briefly introduce proteolysis targeting chimera (PROTAC), a new and revolutionary technique used to degrade CDK4/6.

CDK4/6 inhibitors: A focus on biomarkers of response and post-treatment therapeutic strategies in hormone receptor-positive HER2-negative breast cancer

CDK4/6 inhibitors (CDK4/6i) in combination with endocrine therapy are the mainstay of treatment for patients with hormone receptor-positive, HER2 negative (HR+/HER2neg) metastatic breast cancer. However, resistance - either de novo or acquired - invariably occurs, leading to treatment failure and cancer progression. Genomic alterations, gene expression data and circulating biomarkers have been correlated to response to treatment, but to date no biomarker has been approved to stratify patients. Treatment strategies after progression on CDK4/6i are yet to be standardized. Current approaches include endocrine therapy alone or in combination with target therapy, or chemotherapy. New agents are in clinical development based on potential mechanisms of acquired resistance. Here we will review recent advancements in biomarkers of response to CDK4/6i, and in post- treatment therapeutic strategies.

Exploring treatment with Ribociclib alone or in sequence/combination with Everolimus in ER+HER2-Rb wild-type and knock-down in breast cancer cell lines

Background

Breast cancer (BC) is the second most common type of cancer worldwide. Among targeted therapies for Hormone Receptor-positive (HR⁺) and Human Epidermal growth factor Receptor 2-negative (HER2⁻) BC, the Cyclin-Dependent Kinases (CDK4/6) are targeted by inhibitors such as Ribociclib (Rib); however, resistance to CDK4/6 inhibitors frequently develops. The aim of this work is to assess in vitro activity of Rib and Everolimus (Eve) in HR⁺HER2⁻ MCF-7 and HR⁻HER2⁻BT-549 BC cell lines.

Methods

HR⁺HER2⁻ MCF-7 and HR⁻HER2⁻ BT-549 BC cell lines were treated with increasing concentration of Rib and Eve (up to 80 μg/mL) for 48–72 h. Subsequently, HR⁺HER2⁻ MCF-7 cells were silenced for Retinoblastoma (Rb) gene, and thus, the effect of Rib in sequential or concurrent schedule with Eve for the treatment of both Rb wild type or Rb knock-down MCF-7 in vitro was evaluated. Cell viability of HR⁺HER2⁻ MCF-7cells treated with sequential and concurrent dosing schedule was analyzed by MTT assay. Moreover, cell cycle phases, cell death and senescence were evaluated by cytofluorimetric analysis after treatment with Rib or Eve alone or in combination.

Results

The sequential treatment didn’t produce a significant increase of cytotoxicity, compared to Rib alone. Instead, the cotreatment synergized to increase the cytotoxicity compared to Rib alone. The cotreatment reduced the percentage of cells in S and G2/M phases and induced apoptosis. Rib triggered senescence and Eve completely reversed this effect in Rb wild type BC cells. Rib also showed Rb-independent effects as shown by results in Rb knock-down MCF-7.

Conclusion

Overall, the Rib/Eve concurrent therapy augmented the in vitro cytotoxic effect, compared to Rib/Eve sequential therapy or single treatments.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-020-07619-1.

FZR1 as a novel biomarker for breast cancer neoadjuvant chemotherapy prediction

The concept of breast-conserving surgery is a remarkable achievement of breast cancer therapy. Neoadjuvant chemotherapy is being used increasingly to shrink the tumor prior to surgery. Neoadjuvant chemotherapy is reducing the tumor size to make the surgery with less damaging to surrounding tissue and downstage locally inoperable disease to operable. However, non-effective neoadjuvant chemotherapy could increase the risks of delaying surgery, develop unresectable disease and metastatic tumor spread. The biomarkers for predicting the neoadjuvant chemotherapy effect are scarce in breast cancer treatment. In this study, we identified that FZR1 can be a novel biomarker for breast cancer neoadjuvant chemotherapy according to clinical patient cohort evaluation and molecular mechanism investigation. Transcriptomic data analysis indicated that the expression of FZR1 is correlated with the effect of neoadjuvant chemotherapy. Mechanistically, we demonstrate that FZR1 is pivotal to the chemotherapy drugs induced apoptosis and cell cycle arrest. FZR1 is involved in the stability of p53 by impairing the phosphorylation at ser15 site. We demonstrate that the expression of FZR1 detected by quantification of IHC can be an effective predictor of neoadjuvant chemotherapy in animal experiment and clinical patient cohort. To obtain more benefit for breast cancer patient, we propose that the FZR1 IHC score using at the clinical to predict the effect of neoadjuvant chemotherapy.

Therapy options after CDK4/6 inhibitors for HR+, HER2- postmenopausal metastatic/recurrent breast cancer in Japan: a role for mammalian target of rapamycin inhibitors?

Despite advances in the treatment of hormone receptor-positive, HER2- metastatic breast cancer, the disease is rarely curable. In this review, we focus on the use of CDK4/6 inhibitors, examining clinical experience and the mechanisms underlying the development of resistance, and evaluating treatment options after failure to respond to CDK4/6 inhibitors. Current basic research supports the use of mammalian target of rapamycin inhibitors after CDK4/6 inhibitor failure; however, more data are needed, particularly regarding treatment sequencing. Real-world data studies may help to fill the current knowledge gap, particularly where large-scale randomized controlled studies are not feasible.

Abemaciclib: safety and effectiveness of a unique cyclin-dependent kinase inhibitor

INTRODUCTION

The discovery and the clinical availability of novel cyclin-dependent kinases 4 and 6 inhibitors have profoundly changed the therapeutic scenario of metastatic hormone receptor-positive breast carcinoma. Among these inhibitors, abemaciclib can induce potent and sustained cell cycle arrest and immune system stimulation.

AREAS COVERED

This review summarizes the safety profile and clinical efficacy data on abemaciclib alone or in combination with aromatase inhibitors or fulvestrant in metastatic hormone receptor-positive breast carcinoma. The management of patients treated with abemaciclib is the object of this paper.

EXPERT OPINION

As shown in phase 2 and 3 clinical trials on efficacy and tolerability, abemaciclib is a potentially convenient, safe, and effective agent for the treatment of patients with advanced hormone receptor-positive patients. Orally administered abemaciclib in combination with aromatase inhibitors or fulvestrant has the potential to allow significant improvement in survival outcomes, quality of life, response rate, and duration of response even in poor prognosis subgroups. Adequate patients' information, clinical selection, and prompt, proactive management of side effects are mandatory.

Therapeutic opportunities for PLK1 inhibitors: Spotlight on BRCA1-deficiency and triple negative breast cancers

Polo-Like Kinases (PLKs) are central players of mitotic progression in Eukaryotes. Given the intimate relationship between cell cycle progression and cancer development, PLKs in general and PLK1 in particular have been thoroughly studied as biomarkers and potential therapeutic targets in oncology. The oncogenic properties of PLK1 overexpression across different types of human cancers are attributed to its roles in promoting mitotic entry, centrosome maturation, spindle assembly and cytokinesis. While several academic labs and pharmaceutical companies were able to develop potent and selective inhibitors of PLK1 (PLK1i) for preclinical research, such compounds have reached only limited success in clinical trials despite their great pharmacokinetics. Even though this could be attributed to multiple causes, the housekeeping roles of PLK1 in both normal and cancer cells are most likely the main reason for clinical trials failure and withdraw due to toxicities issues. Therefore, great efforts are being invested to position PLK1i in the treatment of specific types of cancers with revised dosages schemes. In this mini review we focus on two potential niches for PLK1i that are supported by recent evidence: triple negative breast cancers (TNBCs) and BRCA1-deficient cancers. On the one hand, we recollect several lines of strong evidence indicating that TNBCs are among the cancers with highest PLK1 expression and sensitivity to PLK1i. These findings are encouraging because of the limited therapeutics options available for TNBC patients, which rely mainly on classic chemotherapy. On the other hand, we discuss recent evidence that unveils synthetic lethality induction by PLK1 inhibition in BRCA1-deficient cancers cells. This previously unforeseen therapeutic link between PLK1 and BRCA1 is promising because it defines novel therapeutic opportunities for PLK1i not only for breast cancer (i.e. TNBCs with BRCA1 deficiencies), but also for other types of cancers with BRCA1-deficiencies, such as pancreatic and prostate cancers.

Glucose Metabolic Reprogramming of ER Breast Cancer in Acquired Resistance to the CDK4/6 Inhibitor Palbociclib

The majority of breast cancers express the estrogen receptor (ER) and are dependent on estrogen for their growth and survival. Endocrine therapy (ET) is the standard of care for these tumors. However, a superior outcome is achieved in a subset of ER positive (ER⁺)/human epidermal growth factor receptor 2 negative (HER2⁻) metastatic breast cancer patients when ET is administrated in combination with a cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitor, such as palbociclib. Moreover, CDK4/6 inhibitors are currently being tested in ER⁺/HER2⁺ breast cancer and reported encouraging results. Despite the clinical advances of a combinatorial therapy using ET plus CDK4/6 inhibitors, potential limitations (i.e., resistance) could emerge and the metabolic adaptations underlying such resistance warrant further elucidation. Here we investigate the glucose-dependent catabolism in a series of isogenic ER⁺ breast cancer cell lines sensitive to palbociclib and in their derivatives with acquired resistance to the drug. Importantly, ER⁺/HER2⁻ and ER⁺/HER2⁺ cell lines show a different degree of glucose dependency. While ER⁺/HER2⁻ breast cancer cells are characterized by enhanced aerobic glycolysis at the time of palbociclib sensitivity, ER⁺/HER2⁺ cells enhance their glycolytic catabolism at resistance. This metabolic phenotype was shown to have prognostic value and was targeted with multiple approaches offering a series of potential scenarios that could be of clinical relevance.

Potent Cell-Cycle Inhibition and Upregulation of Immune Response with Abemaciclib and Anastrozole in neoMONARCH, Phase II Neoadjuvant Study in HR+/HER2- Breast Cancer

PURPOSE

neoMONARCH assessed the biological effects of abemaciclib in combination with anastrozole in the neoadjuvant setting.

PATIENTS AND METHODS

Postmenopausal women with stage I-IIIB HR⁺/HER2^- breast cancer were randomized to a 2-week lead-in of abemaciclib, anastrozole, or abemaciclib plus anastrozole followed by 14 weeks of the combination. The primary objective evaluated change in Ki67 from baseline to 2 weeks of treatment. Additional objectives included clinical, radiologic, and pathologic responses, safety, as well as gene expression changes related to cell proliferation and immune response.

RESULTS

Abemaciclib, alone or in combination with anastrozole, achieved a significant decrease in Ki67 expression and led to potent cell-cycle arrest after 2 weeks of treatment compared with anastrozole alone. More patients in the abemaciclib-containing arms versus anastrozole alone achieved complete cell-cycle arrest (58%/68% vs. 14%, P < 0.001). At the end of treatment, following 2 weeks lead-in and 14 weeks of combination therapy, 46% of intent-to-treat patients achieved a radiologic response, with pathologic complete response observed in 4%. The most common all-grade adverse events were diarrhea (62%), constipation (44%), and nausea (42%). Abemaciclib, anastrozole, and the combination inhibited cell-cycle processes and estrogen signaling; however, combination therapy resulted in increased cytokine signaling and adaptive immune response indicative of enhanced antigen presentation and activated T-cell phenotypes.

CONCLUSIONS

Abemaciclib plus anastrozole demonstrated biological and clinical activity with generally manageable toxicities in patients with HR⁺/HER2^- early breast cancer. Abemaciclib led to potent cell-cycle arrest, and in combination with anastrozole, enhanced immune activation.

The Ongoing Search for Biomarkers of CDK4/6 Inhibitor Responsiveness in Breast Cancer

CDK4 inhibitors (CDK4/6i), such as palbociclib, ribociclib, and abemaciclib, are approved in combination with hormonal therapy as a front-line treatment for metastatic HR+, HER2- breast cancer. Their targets, CDK4 and CDK6, are cell-cycle regulatory proteins governing the G1-S phase transition across many tissue types. A key challenge remains to uncover biomarkers to identify those patients that may benefit from this class of drugs. Although CDK4/6i addition to estrogen modulation therapy essentially doubles the median progression-free survival, overall survival is not significantly increased. However, in reality only a subset of treated patients respond. Many patients exhibit primary resistance to CDK4/6 inhibition and do not derive any benefit from these agents, often switching to chemotherapy within 6 months. Some patients initially benefit from treatment, but later develop secondary resistance. This highlights the need for complementary or companion diagnostics to pinpoint patients who would respond. In addition, because CDK4 is a bona fide target in other tumor types where CDK4/6i therapy is currently in clinical trials, the lack of target identification may obscure benefit to a subset of patients there as well. This review summarizes the current status of CDK4/6i biomarker test development, both in clinical trials and at the bench, with particular attention paid to those which have a strong biological basis as well as supportive clinical data.

Efficacy of a CDK4/6 Inhibitor in a Patient with Breast Cancer and Liposarcoma: A Case Report and Review of the Literature

BACKGROUND

The cyclin D/cyclin-dependent kinase (CDK)4/6 inhibitor of the CDK4 (INK4)/retinoblastoma (Rb) pathway plays a crucial role in cell cycle progression. Selective CDK4/6 inhibitors specifically target a variety of tumors, with the main focus on hormone receptor(HR)-positive and human epidermal growth factor receptor 2(HER2)-negative breast cancer (BC).

CASE REPORT

We report on the efficacy of neoadjuvant palbociclib and letrozole application in a patient suffering from invasive estrogen receptor (ER)+/HER2- BC and concurrent well-differentiated and dedifferentiated liposarcoma (WD-DDLPS) of the thigh. Clinical and histological workup upon surgery revealed significant regressive changes in both the liposarcoma and the BC. The 24-month follow-up shows no signs of disease.

CONCLUSION

CDK4/6 inhibitors exhibit a high therapeutic potential, although reliable prognostic markers need to be identified.

Molecular mechanisms of resistance to CDK4/6 inhibitors in breast cancer: A review

Deregulation of the cyclin D-CDK4/6-INK4-RB pathway leading to uncontrolled cell proliferation, is frequently observed in breast cancer. Currently, three selective CDK4/6 inhibitors have been FDA approved: palbociclib, ribociclib and abemaciclib. Despite promising clinical outcomes, intrinsic or acquired resistance to CDK4/6 inhibitors has limited the success of these treatments; therefore, the development of various strategies to overcome this resistance is of great importance. We highlight the various mechanisms that are directly or indirectly responsible for resistance to CDK4/6 inhibitors, categorizing them into two broad groups; cell cycle-specific mechanisms and cell cycle-nonspecific mechanisms. Elucidation of the diverse mechanisms through which resistance to CDK4/6 inhibitors occurs, may aid in the design of novel therapeutic strategies to improve patient outcomes. This review summarizes the currently available knowledge regarding mechanisms of resistance to CDK4/6 inhibitors, and possible therapeutic strategies that may overcome this resistance as well.

Cyclin-Dependent Kinases 4/6 Inhibitors in Breast Cancer: Current Status, Resistance, and Combination Strategies

Dysregulated activation of the cyclin-dependent kinases (CDKs) 4/6, leading to uncontrolled cell division, is hallmark of cancers. Further study of the cell cycle will advance the cancer treatment. As powerful and effective drugs, inhibitors of CDK 4/6 have been widely used in clinical practice for several malignancies, particularly against breast cancers driven by the estrogen receptor (ER). Three CDK4/6 inhibitors, including palbociclib (PD0332991), ribociclib (LEE011) and abemaciclib (LY2835219), have been approved by the US Food and Drug Administration (FDA) for the treatment of hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced or metastatic breast cancer. However, CDK4/6 inhibitors act downstream of many mitogenic signaling pathways, and this has implications for resistance. It is worth to note that the mechanisms of resistance are not very clear. Up to now, a small number of preclinical and clinical studies have explored potential mechanisms of CDK4/6 inhibitors resistance in breast cancer. On this basis, rational and effective combination therapy is under development. Here we review the current knowledge about the mechanisms and efficacy of CDK4/6 inhibitors, and summarize data on resistance mechanisms to make future combination therapies more accurate and reasonable.

Mechanisms of Resistance to CDK4/6 Inhibitors: Potential Implications and Biomarkers for Clinical Practice

The recent arrival of CDK4/6 inhibitor agents, with an approximate doubling of progression-free survival (PFS) associated with their use in hormone receptor-positive, HER2-negative advanced breast cancer (BC), has radically changed the approach to managing this disease. However, resistance to CDK4/6 inhibitors is considered a near-inevitability in most patients. Mechanisms of resistance to these agents are multifactorial, and research in this field is still evolving. Biomarkers with the ability to identify early resistance, or to predict the likelihood of successful treatment using CDK4/6 inhibitors are yet to be identified, and represent an area of unmet clinical need. Here we present selected mechanisms of resistance to CDK4/6 inhibitors, largely focussing on roles of Rb, cyclin E1, and the PIK3CA pathway, with discussion of associated biomarkers which have been investigated and applied in recent pre-clinical and clinical studies. These biological drivers may furthermore influence clinical treatment strategies adopted beyond CDK4/6 resistance.

Cyclin-Dependent Kinase 4/6 Inhibitors in Neoadjuvant Endocrine Therapy of Hormone Receptor-Positive Breast Cancer

The landscape of therapeutic options for the treatment of hormone receptor (HR)-positive (HR⁺) HER2^- breast cancer (BC) has been profoundly changed by the introduction of cyclin-dependent kinase 4/6 (CDK4/6) inhibitors into the metastatic setting. Currently all CDK4/6 inhibitors are approved only in the metastatic setting by Food and Drug Administration (FDA) and European Medicine Agency (EMA), whereas their role in the neoadjuvant setting is still at an investigational stage. Exploitation of novel agents such as CDK4/6 inhibitors to improve the efficacy of neoadjuvant endocrine therapy (ET) or to overcome de novo resistance to ET is an area of research under active evaluation. We present a review of the currently available data and ongoing clinical trials that are evaluating the role of CDK4/6 inhibitors in neoadjuvant therapy of HR⁺ HER2^- early BC, and also illustrate translational aspects, such as the potential biomarkers of response to these new therapeutic agents.

Activity and Safety of Palbociclib in Patients with Advanced Gastrointestinal Stromal Tumors Refractory to Imatinib and Sunitinib: A Biomarker-driven Phase II Study

PURPOSE

CDKN2A loss is frequent in gastrointestinal stromal tumors (GISTs) and associated with aggressive outcome. Palbociclib is a CDK4 inhibitor with preclinical antitumor efficacy in tumors with P16/CDKN2A loss.

PATIENTS AND METHODS

This is a multicenter single-arm phase II clinical trial assessing safety and efficacy of palbociclib in patients with advanced GIST bearing CDKN2A gene loss. Adults with unresectable locally advanced or metastatic, refractory to at least imatinib and sunitinib, measurable and documented progressive disease (PD) as per RECIST 1.1, and CDKN2A deletion centrally assessed were eligible. Patients received palbociclib 125 mg orally daily on a 21 days on/7 days off dosing schedule, until PD or unacceptable toxicity. The primary endpoint was 4-month non-PD rate according to RECIST 1.1.

RESULTS

As of May 2017, 71 patients had been included in the study, and 29 patients (40.3%) met the molecular eligibility requirement. Twenty-five patients (86.2%) had grade 1-2 adverse events (AEs) and 12 patients (41.4%) grade 3-4 AEs possibly related to the drug. The planned interim statistical analysis performed after central histologic and radiological review showed that 19 (86.4%) out of the first 22 evaluable patients had PD at 4 months. CDKN2A status had no impact either on overall survival or outcome on previous standard lines of treatment. Translational analysis suggested upregulation of CCNE1 or downregulation of CDKN1A/P21 or LRRC3B as potential mechanisms of resistance.

CONCLUSIONS

Palbociclib has no significant clinical activity as a single agent in P16/CDKN2A -deleted GIST refractory to imatinib and sunitinib.

Tyrosine Phosphorylation of p27Kip1 Correlates with Palbociclib Responsiveness in Breast Cancer Tumor Cells Grown in Explant Culture

Cdk4-targeting drugs, such as palbociclib, are approved for metastatic ER/PR⁺, Her2^- breast cancer. However, other than loss of retinoblastoma, which is very rare in this subset, there are no biomarkers to predict response. Cyclin D or cdk4 levels are not by themselves indicative, because p27Kip1 is required for cyclin D-cdk4 complex activation. Tyrosine phosphorylation of p27, including modification on residue Y88 (pY88), activates DK4-p27, and the pY88 level correlates with palbociclib responsiveness in cell lines. We developed dual IHC staining for p27 and pY88, and found that benign breast epithelium was negative, while breast cancer biopsies (of varied hormonal status) could be stratified for pY88 status. Lack of pY88 suggested that DK4 was inactive, and that these samples would not have the target required for palbociclib response. Tumor resection material was grown in explant culture, treated with palbociclib, and stained with Ki67 as a marker of response. Explants from the no pY88 group were nonresponsive, while explants from the low or high pY88 group responded to drug. IMPLICATIONS: Use of the pY88 biomarker, as a surrogate for cdk4 activity, may identify patients responsive to cdk4-targeting drugs and expand use of this therapy.Visual Overview: http://mcr.aacrjournals.org/content/molcanres/17/3/669/F1.large.jpg.

Cyclin E1 and Rb modulation as common events at time of resistance to palbociclib in hormone receptor-positive breast cancer

CDK4/6 inhibitors represent a new treatment standard for hormone receptor-positive (HR+), HER2-negative advanced breast cancer (BC) patients. Although efficacious, resistance to these agents is universal. Here, we profiled a large panel of HR+ BC cell lines with conditioned resistance to the CDK4/6 inhibitor palbociclib, and analyzed cell cycle-related markers by gene expression profiles (GEP) and western blot (WB). GEP showed high molecular heterogeneity among the models, with E2F targets being significantly enriched both during treatment and at the time of resistance. By both WB and GEP, a common molecular feature at the time of palbociclib resistance was the concomitant overexpression of cyclin E1 and down-regulation of Rb. CCNE1 was the only significantly up-regulated gene among E2F targets at resistance with CCNE1 genomic amplification being observed in two resistant models. Rb was downregulated in all resistant models; a reduction of RB1 copy number was observed in three resistant cell lines. In silico analyses showed that CCNE1/RB1 ratio correlated with palbociclib IC50 in different datasets of both breast and non-breast cancer cell lines, performing better than CCNE1 or RB1 taken separately. Finally, the CCNE1/RB1 ratio was shown to be an adverse prognostic factor in patients with ER+ BC and to be able to discriminate palbociclib-sensitive versus resistant among patients enrolled in the NeoPalAna trial, a neoadjuvant trial testing palbociclib, performing better than CCNE1 or RB1 alone. Our data suggest that the CCNE1/RB1 ratio may be a viable biomarker of palbociclib resistance, warranting further clinical validation.

The expression levels of two genes involved in controlling the cell cycle offers a promising predictive indicator of whether women with hormone receptor–positive breast cancer are likely to develop resistance to the drug palbociclib. A team led by Luca Malorni and Ilenia Migliaccio from the Hospital of Prato, Italy, analyzed gene and protein profiles in a panel of breast cancer cell lines with differential sensitivity to palbociclib. The researchers found that, compared to palbociclib-sensitive cell lines, those that were drug-resistant tended to have elevated expression of one cell-cycle–related gene, called CCNE1, and reduced expression of another, called Rb. The ratio of CCNE1:Rb expression proved to be an accurate prognostic factor among patients enrolled in a clinical trial of palbociclib, performing better than either CCNE1 or Rb levels on their own.

Current Landscape of Targeted Therapies for Hormone-Receptor Positive, HER2 Negative Metastatic Breast Cancer

The majority of deaths from MBC are in patients with hormone receptor (HR) positive, HER2 negative disease. Endocrine therapy (ET) remains the backbone of treatment in these cases, improving survival and quality of life. However, treatment can lose effectiveness due to primary or acquired endocrine resistance. Analysis of mechanisms of ET resistance has led to the development of a new generation of targeted therapies for advanced breast cancer. In addition to anti-estrogen therapy with selective estrogen receptor modulators, aromatase inhibitors, and/or selective estrogen receptor degraders, combinations with cyclin dependent kinase (CDK) 4/6 inhibitors have led to substantial progression free survival (PFS) improvements in the first and second line settings. While the PI3K/AKT/mTOR pathway is known to be an important growth pathway in HR positive breast cancer, PI3K inhibitors have been disappointing due to modest effect sizes and significant toxicity. The mTOR inhibitor everolimus significantly improves progression free survival when added to ET, and recent studies have improved supportive care allowing less toxicity. While these combination targeted therapies improve outcomes and often delay initiation of chemotherapy, long term overall survival data are lacking and data for the ideal strategy for sequencing these agents remains unclear. Ongoing research evaluating potential biomarkers and mechanisms of resistance is anticipated to continue to improve outcomes for patients with HR positive metastatic breast cancer. In this review, we will discuss management and ongoing challenges in the treatment of advanced HR positive, HER2 negative breast cancer, highlighting single agent and combination endocrine therapies, targeted therapies including palbociclib, ribociclib, abemaciclib, and everolimus, and sequencing of therapies in the clinic.

CDK4/6 inhibitors in breast cancer therapy: Current practice and future opportunities

Dysregulation of the cyclin dependent kinase pathway in luminal breast cancer creates a new therapeutic opportunity for estrogen receptor positive breast cancer. Initial pan-CDK inhibitors were associated with extensive toxicities but in recent years, the development of potent specific CDK inhibitors with favorable tolerability has driven renewed interests in this class of targeted therapies. Palbociclib, ribociclib and abemaciclib are specific CDK4/6 inhibitors that have been approved by the U.S. Food and Drug Administration for use in combination with endocrine therapy for women with advanced hormone receptor positive breast cancer. These three anticancer therapeutics were approved based on progression free survival benefit seen on phase III trials with the most common grade 3 treatment-related side effects being neutropenia, fatigue, nausea and diarrhea. Except for estrogen receptor positivity, no biomarkers predictive of response to CDK4/6 inhibitors have been identified to date. Based on mechanistic insights here described, CDK4/6 inhibitors are currently being explored in combination with other agents, including targeted therapies, immunotherapy and chemotherapy.