Patients with Metastatic Breast Cancer Enrolled in Phase I Clinical Trials: Clinical Outcomes and Cohort Trends.

1099 Background: Phase I clinical trials have traditionally enrolled patients with advanced solid tumors and many providers perceive the likelihood of clinical benefit as low. The purpose of this study was to evaluate clinical outcomes for patients with metastatic breast cancer enrolled on Phase I clinical trials and explore differences in outcomes for patients enrolled in all-comer versus breast cancer-specific cohorts. Methods: We performed a retrospective chart review of patients with metastatic breast cancer enrolled in Phase I clinical trials at the University of Colorado Cancer Center from 2012-2018. We included trials with Phase I and/or Phase Ib in the title. Studies or cohorts enrolling patients with ≥ 3 tumor types were considered all-comer and those with enrollment restricted to breast cancer or a breast cancer subtype were considered breast cancer-specific. Results: A total of 208 patients were enrolled in Phase I clinical trials, 168 in breast cancer-specific cohorts and 40 in all-comer trials. Patients on average were 56.9 years old (range 31-79), 98.6% (205/208) female, 1.4% (3/208) male, 57.2% ER+/Her2-, 30.1% ER-/Her2- and 11.1% Her2+. Patients received on average 2.1(range 0-10) prior lines of chemotherapy in the metastatic setting. Patients enrolled on Phase I clinical trials remained on study without progression on average for 138 days (CI 95%, 112.64 to 163.91). Patients enrolled on breast cancer-specific studies remained on study for 152 days (CI 95%, 120.66 to 182.56) compared to 82 days (CI 95%, 59.43 to 105.13) for those enrolled on all-comer trials, p< 0.05. Patients went off study for disease progression (83.17%), adverse events (7.69%), and other (9.14%), including withdrawal of consent. Conclusions: Patients with metastatic breast cancer previously treated with multiple lines of chemotherapy in the metastatic setting enrolled in Phase I clinical trials received clinical benefit from treatment that is favorable compared to historical controls of late-line chemotherapy. The majority of patients were treated on breast cancer-specific cohorts consistent with trends in Phase I trial design including more tumor specific cohorts.