CDK4/6 inhibition in luminal breast cancer

Dual-target inhibitors based on ERα: Novel therapeutic approaches for endocrine resistant breast cancer

Estrogen receptor alpha (ERα), a nuclear transcription factor, is a well-validated therapeutic target for more than 70% of all breast cancers (BCs). Antagonizing ERα either by selective estrogen receptor modulators (SERMs) or selective estrogen receptor degraders (SERDs) forms the foundation of endocrine therapy and has achieved great success in the treatment of ERα positive (ERα+) BCs. Unfortunately, despite initial effectiveness, endocrine resistance eventually emerges in up to 30% of ERα+ BC patients and remains a significant medical challenge. Several mechanisms implicated in endocrine resistance have been extensively studied, including aberrantly activated growth factor receptors and downstream signaling pathways. Hence, the crosstalk between ERα and another oncogenic signaling has led to surge of interest to develop combination therapies and dual-target single agents. This review briefly introduces the synergisms between ERα and another anticancer target and summarizes the recent advances of ERα-based dual-targeting inhibitors from a medicinal chemistry perspective. Accordingly, their rational design strategies, structure-activity relationships (SARs) and biological activities are also dissected to provide some perspectives on future directions for ERα-based dual target drug discovery in BC therapy.

CDK4 Amplification in Esophageal Squamous Cell Carcinoma Associated With Better Patient Outcome

In the present study, we aimed to investigate the clinical and prognostic values of CDK4 amplification and improve the risk stratification in patients with esophageal squamous cell carcinoma. CDK4 amplification was analyzed by fluorescence in situ hybridization using tissue microarray consisting of representative tissues of 520 patients with esophageal squamous cell carcinoma, and its correlation with clinicopathological features and clinical outcomes were evaluated. CDK4 amplification was found in 8.5% (44/520) of patients with esophageal squamous cell carcinoma. CDK4 amplification was negatively correlated with disease progression (P = 0.003) and death (P = 0.006). Patients with CDK4 amplification showed a significantly better disease-free survival (P = 0.016) and overall survival (P = 0.023) compared with those patients without CDK4 amplification. When patients were further stratified into I–II stage groups and III–IV stage groups, CDK4 amplification was significantly associated with both better disease-free survival (P = 0.023) and overall survival (P = 0.025) in the I–II stage group rather than the III–IV stage group. On univariate and multivariate analysis, invasive depth and CDK4 amplification were associated with disease-free survival and overall survival. Taken together, CDK4 amplification was identified as an independent prognostic factor for survival, which could be incorporated into the tumor–node–metastasis staging system to refine risk stratification of patients with esophageal squamous cell carcinoma.

Preclinical development of G1T38: A novel, potent and selective inhibitor of cyclin dependent kinases 4/6 for use as an oral antineoplastic in patients with CDK4/6 sensitive tumors

Inhibition of the p16^INK4a/cyclin D/CDK4/6/RB pathway is an effective therapeutic strategy for the treatment of estrogen receptor positive (ER⁺) breast cancer. Although efficacious, current treatment regimens require a dosing holiday due to severe neutropenia potentially leading to an increased risk of infections, as well as tumor regrowth and emergence of drug resistance. Therefore, a next generation CDK4/6 inhibitor that can inhibit proliferation of CDK4/6-dependent tumors while minimizing neutropenia could reduce both the need for treatment holidays and the risk of inducing drug resistance.

Here, we describe the preclinical characterization and development of G1T38; a novel, potent, selective, and orally bioavailable CDK4/6 inhibitor. In vitro, G1T38 decreased RB1 (RB) phosphorylation, caused a precise G1 arrest, and inhibited cell proliferation in a variety of CDK4/6-dependent tumorigenic cell lines including breast, melanoma, leukemia, and lymphoma cells. In vivo, G1T38 treatment led to equivalent or improved tumor efficacy compared to the first-in-class CDK4/6 inhibitor, palbociclib, in an ER⁺ breast cancer xenograft model. Furthermore, G1T38 accumulated in mouse xenograft tumors but not plasma, resulting in less inhibition of mouse myeloid progenitors than after palbociclib treatment. In larger mammals, this difference in pharmacokinetics allowed for 28 day continuous dosing of G1T38 in beagle dogs without producing severe neutropenia. These data demonstrate G1T38 has unique pharmacokinetic and pharmacodynamic properties, which result in high efficacy against CDK4/6 dependent tumors while minimizing the undesirable on-target bone marrow activity, thus potentially allowing G1T38 to be used as a continuous, daily oral antineoplastic agent.

MUC1 induces M2 type macrophage influx during postpartum mammary gland involution and triggers breast cancer

The microenvironment of postpartum mammary gland involution (PMI) has been linked to the increased risk of breast cancer and poor outcome of patients. Nevertheless the mechanism underlying regulates the microenvironment remains largely unknown. MUC1, which is abnormally overexpressed in most breast cancer, is physiologically expressed in PMI. Using MUC1 cytoplasm domain (MUC1-CD) transgenic mice, we reveal that the overexpression of MUC1-CD in mammary epithelial cells increases M2 type macrophage infiltration in PMI. By sustain activating p50, MUC1 upregulates M2 macrophage chemo-attractants and the anti-apoptotic protein Bcl-xL. Because of the tumor promotional microenvironments and reduced apoptosis, MUC1-CD delays PMI process and results in atypical phenotype in multiparous mice mammary. This finding is further supported by the positive association between the expression of MUC1 and p50 in Luminal A and Luminal B subtypes through analyzing breast cancer databases. Taken together, our study demonstrates that MUC1-CD plays an important role in regulating microenvironment of PMI and promoting postpartum mammary tumorigenicity, providing novel prevention and treatment strategies against postpartum breast cancer.

Recent advances of cyclin-dependent kinases as potential therapeutic targets in HR+/HER2- metastatic breast cancer: a focus on ribociclib

In normal cell cycle progression, transition of G0/G1 phase to synthesis (S) phase for breast and other cells is regulated by association of cyclin D and cyclin-dependent kinases 4 and 6 (CDK4/6) that leads to phosphorylation of retinoblastoma (Rb) protein. Imbalance of this cyclin D-CDK4/6-inhibitors of CDK4/6-Rb phosphorylation pathway is associated with tumorigenesis of hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) breast cancers. Despite effective first-line endocrine therapy, HR+/HER2- metastatic breast cancers remain still incurable. Currently, advances in understanding of cell cycle checkpoints are evolving as promising strategy to target in treatment of various types of cancers including breast cancer. Therapies that target this cell cycle machinery in HR+/HER2- breast cancers are getting approval by the US Food and Drug administration (FDA) including ribociclib (LEE011). Ribociclib got the first FDA approval in March 13, 2017, as an initial therapy for HR+/HER2- advanced or metastatic breast cancer in combination with an aromatase inhibitor. This review, therefore, addresses the role of selective CDK4/6 inhibitors in advanced or metastatic breast cancer with a specific focus on ribociclib. Some findings of clinical trials involving ribociclib found pivotal benefits of ribociclib in HR+/HER2- metastatic breast cancer in terms of prolonging progression-free survival and objective response rates. Daily dosage range of the drug for such benefits is 50-900 mg with common daily doses of 400 or 600 mg and 600 mg in early and advanced breast cancer therapies, respectively. Along with its therapeutic benefits, however, more incident but manageable dose-limiting grade 3 or 4 toxicities, primarily hematologic adverse events, are common in patients treated with ribociclib. Generally, there are several active clinical trials undergoing to investigate the clinical efficacy and toxicity profile of the drug in various cancerous conditions other than breast cancer and will likely benefit patients with other cancer types.

A current and comprehensive review of cyclin-dependent kinase inhibitors for the treatment of metastatic breast cancer

BACKGROUND

Resistance to endocrine treatment generally occurs over time, especially in the metastatic stage. In this paper, we aimed to review the mechanisms of cyclin-dependent kinase (CDK) 4/6 inhibition and clinical usage of new agents in the light of recent literature updates.

SCOPE

A literature search was carried out using PubMed, Medline and ASCO and ESMO annual-meeting abstracts by using the following search keywords; "palbociclib", "abemaciclib", "ribociclib", "cyclin-dependent kinase inhibitors" and "CDK 4/6" in metastatic breast cancer (MBC). The last search was on 10 June 2017.

FINDINGS

CDKs and cyclins are two molecules that have a key role in cell cycle progression. Today, there are three highly selective CDK4/6 inhibitors in clinical development - palbociclib, ribociclib and abemaciclib. Palbociclib and ribociclib were recently approved by the US FDA in combination with letrozole for the treatment of MBC in a first-line setting, as well as palbociclib in combination with fulvestrant for hormone-receptor (HR)-positive MBC that had progressed while on previous endocrine therapy according to the PALOMA-1, MONALEESA-2 and PALOMA-3 trials, respectively. In the recently published randomized phase III MONARCH 2 trial, abemaciclib plus letrozole had longer progression-free survival and higher objective response rates with less serious adverse events in advanced HR-positive breast cancer previously treated with hormonal treatment.

CONCLUSION

CDK4/6 inhibition is a new and promising target for patients with hormone-receptor-positive MBC. Both palbociclib and ribociclib showed significant additive benefit for patients receiving first-line treatment for HR-positive, epidermal growth factor receptor-2-negative advanced breast cancer. Palbociclib and abemaciclib also had significant activity in combination with fulvestrant for patients with MBC that progressed on previous endocrine therapy.

CDK4/6 inhibitor-SHR6390 exerts potent antitumor activity in esophageal squamous cell carcinoma by inhibiting phosphorylated Rb and inducing G1 cell cycle arrest

Background

Cell cycle dysregulation is common in human malignancies, and CDK4/6 inhibitors targeting cell cycle have potential antitumor activity. SHR6390 is a novel small molecule inhibitor specifically targeting the CDK4/6 pathway. However, the role of SHR6390 in esophageal squamous cell carcinoma (ESCC) remains unknown, which will be investigated in our study.

Methods

Eca 109, Eca 9706, and KYSE-510 ESCC cell lines were chosen for further analysis. The effect of SHR6390 on cell viability, cell cycle and cell apoptosis, the status of kinases in Cyclin D1-CDK4/6-Rb pathway were determined by MTS assay, flow cytometry, and western blotting, respectively. Cell-derived and patient-derived xenografts were established to investigate the effects of drugs in vivo.

Results

SHR6390 could suppress cell proliferation in vitro cell lines and inhibit tumor growth in vivo PDX models with different drug susceptibility. The effective treatment of SHR6390 induced the inhibition of phosphorylated Rb and cell cycle arrest at G1 phase both in cell lines and in xenografts. SHR6390 combined with paclitaxel or cisplatin offered synergistic inhibitory effects in cell-derived xenografts especially in Eca 9706 xenografts which showed relative lower sensitivity of SHR6390 single. Moreover, low expression of CDK6 and/or high expression of Cyclin D1 might be associated with high sensitivity of SHR6390, which would be validated in the future.

Conclusions

CDK4/6 inhibitor-SHR6390 exerted potential antitumor activity against ESCC cell lines and xenografts, and evaluation of CDK6 and Cyclin D1 expressions might be helpful to select patients beneficial from SHR6390, which provided evidences for future clinical trials.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-017-1231-7) contains supplementary material, which is available to authorized users.

HR+, HER2- Advanced Breast Cancer and CDK4/6 Inhibitors: Mode of Action, Clinical Activity, and Safety Profiles

BACKGROUND

Cyclin-dependent kinase (CDK) 4/6 inhibitor-based therapies have shown great promise in improving clinical outcomes for patients with hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) advanced breast cancer.

OBJECTIVES

1. Discuss the mode of action of the three CDK4/6 inhibitors in late clinical development: palbociclib (PD-0332991; Pfizer), ribociclib (LEE011; Novartis), and abemaciclib (LY2835219; Lilly). 2. Describe the efficacy and safety data relating to their use in HR+, HER2- advanced breast cancer. 3. Discuss the key side effects associated with CDK4/6 inhibitors along with considerations for adverse event management and patient monitoring.

METHOD

Relevant information and data were assimilated from manuscripts, congress publications, and online sources.

RESULTS

CDK4/6 inhibitors have demonstrated improved progression-free survival in combination with endocrine therapy compared with endocrine therapy alone. The side-effect profile of each agent is described, along with implications for patient monitoring, and considerations for patient care providers and pharmacists.

CONCLUSION

Addition of a CDK4/6 inhibitor to endocrine therapy increases efficacy and delays disease progression. Insight into the unique side-effect profiles of this class of agents and effective patient monitoring will facilitate the successful use of CDK4/6 inhibitor-based therapies in the clinic.