Clinical predictors of long-term response to capecitabine in metastatic breast cancer (MBC).

1088 Background: Select patients (pts) with metastatic breast cancer (MBC) treated with capecitabine (cape) experience long-term disease control. We aimed to evaluate the clinical and biological predictors of long-term response. Methods: Pts receiving cape monotherapy for breast cancer from 01/2006 to 01/2016 at Princess Margaret Cancer Centre in Toronto, Canada (N = 352) or Alberta, Canada (N = 798) were identified through central pharmacy records. A median time-to-progression (MTTP) 3-fold higher (19.3 months) than those seen in published studies for patients treated with cape monotherapy at 1000mg/m2 was applied to select for pts with long-term response. MBC pts meeting these criteria were identified through chart review, with collection of clinical, pathological, and survival outcomes in addition to oncologist assessed (o) best response (eg: oCR, oPR, oSD) by radiology report review. Descriptive statistics, Kaplan-Meier and Cox-regressions were applied. Results: Overall, 41 (4%) pts met long-term response criteria. Median age of the study cohort was 62 (range 40-80), with 39% metastatic at diagnosis and 76% post-menopausal. At initiation of cape, a majority of pts were HR positive (85%), with an LDH < ULN (56%), and had bone (83%) or visceral (63%) metastases. Only 1 (2%) patient was HER2+. In the metastatic setting, most patients were chemotherapy-naive (83%) and had received 0-1 lines of hormonal therapy (61%). Median treatment duration with cape was 2.2 years (range 1.7-5.7) with 37% of pts having ≥40 cycles (range 22-94). Visceral (49%), bone (34%), and lymph nodes (24%) were the most common sites of progression, with 41% requiring 1 dose reduction and 27% requiring 2 dose reductions of cape. Overall response rate was 56% (oCR = 2%, oPR = 54%) with 44% having oSD as best response. Median follow-up was 6.8 years (95%CI: 5.5-8.2). Median PFS was 2.3 years (2.0-2.6), with a median OS of 3.8 years (2.9-4.6). 5 patients (12%) remained alive at data-cutoff, and 1 (2%) remained on treatment after 94 cycles of cape. Conclusions: Clinical features associated with long-term response on cape include HR positive, HER2 negative postmenopausal patients with bone predominant metastasis who have received 0-2 lines of prior hormone therapy and 0-1 lines of chemotherapy in the metastatic setting. This is the largest reported analysis of MBC patients with prolonged responses to cape. In the absence of randomized controlled trials; real-world evidence could aid clinicians in the optimal patient selection for treatment with cape.