LAUSR

5510 Background: Single agent aromatase inhibitor (AI) therapy is associated with limited clinical activity in ovarian cancer (OC) and endometrial cancers (EC). AI therapy was associated with a progression free survival (PFS) at 12 weeks of only 20% in relapsed OC (Bowman et al, 2002) and a median PFS of 1 month in relapsed EC (Rose et al, 2000). In Estrogen Receptor (ER) positive metastatic breast cancer, clinical studies had shown a significant prolongation of PFS with the addition of the cyclin kinase 4/6 inhibitor ribociclib to AI (Hortobagyi et al, 2016). Here, we report the results of a phase 2 clinical trial of the combination of ribociclib and letrozole in patients with relapsed ER positive OC or EC. Objectives: Primary endpoint was the proportion of patients with relapsed ER positive OC or EC alive and progression-free after 12 weeks of therapy (PFS12) with the combination of ribociclib given at a dose of 400 mg orally daily and letrozole 2.5 mg orally daily. A PFS of 45% was considered a favorable result based on the data referenced above from Bowman et al. Methods: Eligibility criteria included patients with relapsed ER positive OC or EC, with measurable disease, not previously treated with ribociclib or AIs. Xenografts were created from CT guided tumor biopsies at baseline to assess feasibility. Results: A total of 40 patients were enrolled (20 with OC and 20 with EC) ) with a median age of 61 years (range: 30-82) and 64.5 (range: 52-75) in the OC and EC groups respectively. Among the OC patients, 17 had high grade serous carcinomas and 3 had low grade serous carcinomas. 11 EC patients had endometrioid cancers (3 with grade 1 tumors) and 9 had high grade serous tumors. Ten out of 20 OC patients and 11/20 EC patients were alive and progression-free at 12 weeks (PFS12 of 50 and 55%, respectively). The most common grade 3 or higher adverse events (occurring in at least 5 pts) were leukopenia (18%), lymphopenia (18%), neutropenia (13%), and fatigue (13%). 34 tumor biopsies were suitable for injection into mice and 44% engrafted. ER expression persisted through multiple passages in mice. Two of three EC PDX models exhibited improved PFS with letrozole/ribociclib compared to letrozole alone. Conclusions: The combination of ribociclib and letrozole is associated with a promising 50% and 55% PFS12 in patients with ER positive relapsed OC or EC respectively. Creation of xenograft tumor models from CT guided biopsies of OC and EC tumors was feasible. Clinical trial information: NCT02657928.