CDK4/6 inhibitors in advanced breast cancer, what is beyond?

Facing the CDK4/6i resistance dilemma in patients with breast cancer, exploration of the resistance mechanism and possible reverse strategy: A narrative review

Breast cancer is one of the highest rates of malignancy of women, approximate 70% metastatic breast cancer are hormone receptor positive (HR+) and human epidermal growth factor receptor 2 negative (HER2-). Hormone therapy is the primary strategy of HR+/HER2- metastatic breast cancer. With the permission of cyclin-dependent kinase 4 and 6 inhibitors (CDK4/6i), progress free survival and overall survival were significantly licensed. However, inevitable outcome of CDK4/6i resistance has become the main reason that restricts the clinical benefit of patients. In recent years, the research on dealing with drug resistance has become a hot topic, a large number of molecular mechanisms have been focused, and a lot of experiments have been carried out at the preclinical level. This review summarizes the current knowledge of CDK4/6i resistance mechanism, systematically expounds the signaling pathways and targets leading to CDK4/6i resistance, analyzes different ways and mechanisms, and provides theoretical guidance for the clinical reversal of endocrine therapy resistance.

Thirteen-year analyses of medical oncology outpatient day clinic data: a changing field

Background

Novel treatment modalities like targeted therapy and immunotherapy are currently changing treatment strategies and protocols in the field of medical oncology.

Methods

Numbers of patients and patient contacts admitted to medical oncology day clinics of a large European academic cancer centre in the period from 2006 to 2018 were analysed using our patient administration system.

Results

A patient cohort of 9.870 consecutive individual patients with 125.679 patient contacts was descriptively and retrospectively characterised. Mean age was 59.9 years. A substantial increase in both individual patients treated per year (+45.4%; 2006: 1.100; 2018: 1.599) and annual patient contacts (+63.3%; 2006: 8.857; 2018: 14.467) between 2006 and 2018 was detected. Hence and most interestingly, the ratio of visits per patient increased by approximately one visit per patient per year over the last 12 years (+12.4%; 2006: 8.0; 2018: 9.0). Further, a decrease of patient contacts in more prevalent entities like breast cancer was found, while contacts for orphan diseases like myeloma and sarcoma increased substantially. Interestingly, female patients showed more per patient contacts as compared with men (13.5 vs 11.9). Lastly, short-term safety data of outpatient day clinic admissions are reported.

Conclusions

We present a representative and large set of patient contacts over time that indicates an increasing load in routine clinical work of outpatient cancer care. Increases observed were highest for orphan diseases, likely attributed to centralisation effects and increased treatment complexity.

Short-term CDK4/6 Inhibition Radiosensitizes Estrogen Receptor-Positive Breast Cancers

PURPOSE

Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors have improved progression-free survival for metastatic, estrogen receptor-positive (ER⁺) breast cancers, but their role in the nonmetastatic setting remains unclear. We sought to understand the effects of CDK4/6 inhibition (CDK4/6i) and radiotherapy in multiple preclinical breast cancer models.

EXPERIMENTAL DESIGN

Transcriptomic and proteomic analyses were used to identify significantly altered pathways after CDK4/6i. Clonogenic assays were used to quantify the radiotherapy enhancement ratio (rER). DNA damage was quantified using γH2AX staining and the neutral comet assay. DNA repair was assessed using RAD51 foci formation and nonhomologous end joining (NHEJ) reporter assays. Orthotopic xenografts were used to assess the efficacy of combination therapy.

RESULTS

Palbociclib significantly radiosensitized multiple ER⁺ cell lines at low nanomolar, sub IC₅₀ concentrations (rER: 1.21-1.52) and led to a decrease in the surviving fraction of cells at 2 Gy (P < 0.001). Similar results were observed in ribociclib-treated (rER: 1.08-1.68) and abemaciclib-treated (rER: 1.19-2.05) cells. Combination treatment decreased RAD51 foci formation (P < 0.001), leading to a suppression of homologous recombination activity, but did not affect NHEJ efficiency (P > 0.05). Immortalized breast epithelial cells and cells with acquired resistance to CDK4/6i did not demonstrate radiosensitization (rER: 0.94-1.11) or changes in RAD51 foci. In xenograft models, concurrent palbociclib and radiotherapy led to a significant decrease in tumor growth.

CONCLUSIONS

These studies provide preclinical rationale to test CDK4/6i and radiotherapy in women with locally advanced ER⁺ breast cancer at high risk for locoregional recurrence.