Abstract P5-21-30: Retrospective review of palbociclib (Pal) efficacy and benefit from subsequent treatments following Pal progression in patients (pts) with hormone receptor positive (HR+) and HER2 negative (HER2-) metastatic breast cancer (MBC)

Background The cyclin-dependent kinase (CDK) 4/6 inhibitor Pal is approved for HR+ HER2- MBC. However, the optimal therapy following Pal progression is unknown. Therefore we conducted this retrospective study to review Pal efficacy and summarize the practice pattern and responses to subsequent treatments post Pal progression. Methods We performed a chart review of pts with HR+ HER2- MBC who began Pal treatment at Washington University Siteman Cancer Center between Feb 16, 2015 and July 13, 2016 and collected information on pts demographics, diagnosis, and treatment history. Duration of therapy was used to calculate the progression free survival (PFS) for each regimen. Treatment was considered first-line if administered without any prior systemic therapy or at least 1 year from completion of adjuvant hormonal therapy (HT). Treatments received after progression on 1st line therapy or upon relapse during or within 1 year from the completion of adjuvant HT were considered second-line regimens. Statistical analyses were performed on SAS software, version 9.4. The Kaplan-Meier method was used to generate time-to-event curves, from which median PFS was calculated. A stratified log-rank test was used for all comparisons, and the P value derived from the comparison was reported. Results We completed a chart review for 81 pts (78 female and 3 male; 63 Caucasian, 14 African American, and 4 other races) with HR+ HER2- MBC (68 were ER+PR+, 13 were ER+PR-) who received Pal plus letrozole (n=65) or fulvestrant (n=15) or anastrozole (n=1), with a median age of 62.0 years (range 28.1 - 85.6) at the start of Pal. The median follow up was 20.0 months (mos) (range 10.8 – 27.9). 25 pts were still on Pal treatment. The median PFS on Pal was 19.9 mos in the first-line setting (n=20), compared to 12.1 mos and 4.4 mos in the second-line (n=14) and subsequent lines (n=47), respectively (p=0.0287). Among the 54 pts who progressed on Pal, 38 moved on to the next treatment. 20 pts received chemotherapy and 16 pts received HT or a HT combination. 2 pts received fulvestrant plus Pal upon progression on letrozole plus Pal, and treatment was still ongoing at 4 mos and 7 mos of follow up, respectively. The most common treatments post Pal were single-agent capecitabine (Cape) (n=9) and the combination of exemestane (Exe) and everolimus (Eve) (n=8). The median PFS was 4.7 mos with Cape compared to 8.4 mos with Exe and Eve (p=0.60). The median PFS was 4.7 mos for the 20 pts who received chemo, whereas the median PFS was 4.9 mos with subsequent HT (n=16) (p=0.75). Conclusion Pal plus letrozole or fulvestrant is effective for the treatment of HR+ HER2- MBC, with activity observed beyond the 1st and 2nd line treatment settings. The PFS of Pal observed in this single center retrospective study is consistent with that of published data. Single-agent cape or the Exe and Eve combination were common treatment choices following progression on Pal. Although the study is limited by its small sample size, the median PFS of 8.4 mos with Exe and Eve indicates its potential efficacy in the setting of Pal progression. Additional pts and followup data will be presented. Citation Format: Xi J, Oza A, Thomas S, Naughton M, Ademuyiwa F, Weilbaecher KN, Suresh R, Bose R, Cherian MA, Hernandez-Aya L, Frith A, Peterson LL, Krishnamurthy J, Ma CX. Retrospective review of palbociclib (Pal) efficacy and benefit from subsequent treatments following Pal progression in patients (pts) with hormone receptor positive (HR+) and HER2 negative (HER2-) metastatic breast cancer (MBC) [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P5-21-30.