Background: Glucocorticoid receptor (GR) activity inhibits chemotherapy-induced apoptosis, and GR antagonism with m enhances chemotherapy sensitivity in GR+ breast (B) and ovarian cancer (OC) cells. C+G is a commonly used regimen for B and OC. We report the results of a phase I trial of the GR antagonist m plus C+G in patients with advanced B and OC. Methods: A standard “3+3” dose escalation phase I study was performed. Objectives were to assess the safety and tolerability of the regimen, and to determine the recommended phase 2 (RP2D) dose of M+C+G. C+G was administered on days 1 and 8 of a 21 day cycle, and m was administered the day prior to and the day of chemotherapy. The starting dose level (DL) was 1, with additional DLs as follows in the table. Results: 31 patients (pts) with a median age of 54 years (range 32-76) were enrolled. 18 pts had BC (3 ER+, 15 triple-negative), and 13 had high grade serous OC (11 platinum-sensitive, 2 platinum-resistant). The median number of prior therapies for advanced BC was 1 (range 0-5) and for OC was 2 (range 1-3). Dose de-escalation was necessary due to the DLT of neutropenia. After DL -3, prophylactic G-CSF (PGF) was instituted. The RP2D was C AUC 2, G 600 mg/m2, m 300 mg with PGF administered on day 9. Of the BC pts, 2 had a complete response (CR), 2 had a partial response (PR), 8 had stable disease (SD), 4 had progressive disease (PD). Of the OC pts, there was 1 CR (CR2 lasted > 27 mos; CR1 lasted only 8 mos), 1 PR, 6 SD, and 3 PD. 4 pts were inevaluable for response. Conclusions: These data suggest that M+C+G is safe and tolerable, and the most common DLT is neutropenia. This was easily managed with the institution of PGF. Studies correlating tumor GR expression with response are ongoing, and may help identify patients who are most likely to benefit from this combination. Clinical trial information: NCT02046421

*Prophylactic G-CSF administered on day 9