Retrospective review of metastatic hormone receptor-positive inflammatory breast cancer patients reveals poor responses to cyclin dependent kinase 4/6 inhibition

Abstract Background Patients with inflammatory breast cancer (IBC) have aggressive biology and relatively inferior responses to standard-of-care (SOC) therapies. Understanding the efficacy of SOC therapies in IBC is critical to optimize outcomes. Our objective was to assess the progression-free survival (PFS) of metastatic hormone receptor-positive HER2-negative/low (HR+HER2−) IBC patients treated with CDK4/6 inhibitors (CDKIs) and hormonal therapy (HT). Methods Data from 58 IBC patients with metastatic HR + /HER2- IBC from a single institution were reviewed. The medians (95% confidence intervals) of overall survival (OS), PFS, and time on treatment (ToT) from the time of CDKI initiation were reported via the Kaplan‒Meier method. Differences were tested by the log-rank test. Results We identified 58 patients (including 16 with de novo stage IV disease). The median OS, PFS, and ToT in the total cohort were 26 (16, 37), 7 (5, 10), and 7 (5, 10) months (mos), respectively. No differences were observed between pre-menopausal patients and postmenopausal patients. The OS, PFS, and ToT rates at the initial diagnosis of Stage III later developing metastatic breast cancer (MBC, N = 42) and de novo IV (N = 16) patients were 19 (15, 34) vs 34 (21, NR), 7 (5, 14) vs 9 (6, NR), and 6 (5, 10) vs 9 (4, NR) mos, respectively (ns). OS, PFS, and ToT in patients receiving CDKI in the first-line vs second-line metastatic setting were 27 (19, 44) vs 17 (12, 39), 7 (5, 15) vs 6 (3, NR), and 7 (5, 15) vs 6 (3, 20) mos, respectively (ns). Among the patients initially diagnosed with stage III disease later progressing to MBC, brain metastases were observed in 12/42 patients. Thirty-eight patients underwent genomic testing either before CDKI treatment (N = 21) or at progression (N = 17). Among the 38 patients who underwent genomic testing, 34 had mutations, most commonly in TP53, PIK3CA, FGFR1, CCND1, and ARID1A. ESR1 mutations were present in 0% of the samples tested prior to CDKI treatment, and 29% of the samples tested at progression. Conclusions Patients with metastatic HR+HER2− IBC demonstrated a shorter time on treatment suggesting shorter duration of response on CDKI + HT, which is markedly inferior to reported data for non-IBC patients from phase III trials.