222P Poor response to cyclin dependent kinase 4/6 inhibition in metastatic inflammatory breast cancer

Patients with inflammatory breast cancer (IBC) have a poor prognosis owing to the aggressive biology of the disease and relatively inferior response to standard of care therapies. Cyclin dependent kinase 4/6 inhibitors (CDKI) are utilized in the metastatic setting for hormone receptor-positive HER2-negative/low (HR+ HER2-) breast cancer. The outcomes of these patients treated with CDKI combined with hormone directed therapy are presented. Patients with HR+ HER2- IBC for which CDKI was administered in the metastatic setting were identified from the IBC registry at MD Anderson Cancer Center (N= 58). Medians (95% confidence intervals) of overall survival (OS), progression free survival (PFS), and time on treatment (ToT) from the time of CDKI initiation are reported by Kaplan-Meier methods. Differences are tested by log-rank test. All patients (42 stage III, 16 de novo stage IV) received induction chemotherapy at diagnosis. Median OS, PFS, and ToT in the total cohort was 26 (16, 37), 7 (5, 10), and 7 (5, 10) months (mos), respectively. No differences were observed between pre and post-menopausal patients. OS, PFS, and ToT in initial diagnosis of Stage III (N=42) vs IV (N=16) patients was 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). Patients treated with abemaciclib (N=11), Palbociclib (N=36), and ribociclib (N=9) show PFS of 7 (3, NR), 6 (4, 10), 9 (7, NR) mos; and ToT of 6 (3, NR), 6 (4, 9), 9 (7, NR) mos. OS, PFS, and ToT in patients receiving CDKI in the fistr-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). Brain metastases were observed in 12/42 stage III patients. Patients with metastatic HR+ HER2- IBC demonstrate a poor response to CDKI based therapy. Patients receiving CDKI in the first vs second-line setting demonstrated a relatively superior outcome. Overall, however, outcomes are significantly inferior to historical data compared to non-IBC. Small sample size precludes definitive conclusions and data should be interpreted cautiously.