Single institution retrospective review of patients with metastatic hormone receptor positive breast cancer receiving first line therapy of CDK4/6 inhibitors.

Abstract

e13090 Background: Breast cancer continues to be the most commonly diagnosed cancer in women. The majority of these cancers are estrogen positive (ER+) and about 6% of new cases are stage 4 at time of diagnosis. The current standard of care for postmenopausal women with stage 4 ER+ breast cancer is a combination of an aromatase inhibitor (AI) with a cyclin-dependent kinase (CDK) 4/6 inhibitor. Based on Medicare data claim analysis from IQVIA, CDK 4/6 inhibitors are being used nationally in the first line setting in 56% of cases where 24% of cases were treated with AI monotherapy. The state of Kentucky currently lies below this national average by using this treatment regimen in 38% of cases where 47% of cases are treated with AI monotherapy. The reason for this discrepancy between the national average and Kentucky is currently unknown. To further understanding, we have conducted an internal review of adherence to the guidelines. Methods: This is a single center retrospective study done at University of Louisville, Brown Cancer Center. Data was collected on patients who had biopsy proven metastatic hormone positive breast cancer from July 2021 until May 2023 (n=41). Collected characteristics include age on diagnosis, tumor grade, sites of metastatic disease, first line treatment received, choice of CDK4/6 inhibitor if received, and prior exposure to chemotherapy or anti-estrogen therapy. Results: In our cohort of 41 patients, we found that 63.4% (n=26) of patients received an AI and CDK4/6 inhibitor in the first line. There were 4.88% (n=2) of patients who received AI monotherapy, 21.9% (n=9) received Fulvestrant and CDK4/6 inhibitor, 4.88 % (n=2) received AI and Fulvestrant and 4.88% (n=2) received Fulvestrant alone. In total, patients who received estrogen therapy plus CDK4/6 inhibitor totaled 85% of new patients (n=35). Conclusions: This internal review showed our institution to be compliant with endocrine therapy and CDK 4/6 inhibitor in front-line treatment for the majority of metastatic ER+ patients. Given the supportive data for use of CDK 4/6 inhibitors in front-line treatment, the compliance is expected to be much higher. We recommend continued discussion amongst oncology providers to help identify barriers to treatment and determine the causes for poor compliance in the state of Kentucky.