Potential clinical activity of pembrolizumab monotherapy in ovarian sex cords, rare epithelial carcinoma, and other rare ovarian tumor histotypes: The French AcSe pembrolizumab study from Unicancer.

5572 Background: AcSé Pembrolizumab is a Phase 2, non-randomized parallel arms, multicentric basket trial investigating the efficacy and safety of pembrolizumab monotherapy in different cohorts of patients with rare cancers (NCT03012620). Here we report the results in the rare ovarian tumors cohort. Methods: Selected histotypes were all rare ovarian cancers (incidence < 6/100,000/year). Main inclusion criteria were age > 18, ECOG PS≤1, resistant disease to platinum based chemotherapy, and systematic histological central review by expert pathologist from TMRG network. Patients (pts) received pembrolizumab 200 mg IV on Day 1 of every 21-day cycle for a maximum of 2 years. The primary endpoint was the confirmed objective response rate according to RECIST v1.1 at 12 weeks. Secondary endpoints included best response rate, duration of response, progression-free survival (PFS), overall survival (OS), and safety. The 7 subgroups of pts analyzed were carcinosarcoma (CS), clear cell carcinoma (CCC), low grade serous carcinoma (LGSC), mucinous carcinoma (MEOC), sex cord tumors (SCT), germ cell tumor (GCT), and smarcA4 deficient hypercalcemic ovarian tumor (SCHOCCT). Results: 62 pts from 22 centers, were included from 08/2017 to 12/2020. Median Age was 53.5 years old [36-64]. Median number of previous lines of chemotherapy was 2 (range 1-4). The median number of cycles was 8 (range, 1-35) with 44 pts (70.9%) who discontinued the trial after a mean number of 6.8 cycles. There were 2 pts (3.2%) with partial response (PR) at 12 weeks. The best response in ITT was complete response (CR) in 1 patient (1%), PR in 3 (14.3%), and stable disease (SD) in 21 (33.8%). The occurrence of best response depended on the histotype with 1 CR (33%) in GCT (cancerized teratoma), 2 PR (20%) in CCC, and 1 PR (4%) in LGSC. 4/4 pts (100%) reported PD as best response in SCOOHT (Table 1). Median duration of response or stabilized disease was 7.8 months [IQR, 4.1 to 9.0]. At the data cut off, 6-month PFS was 29% [19.7-42.8] and 6-month OS was 77.8% [67.7-89.3] on the overall population. Outcomes differed according to subgroups and will be presented. There were a total of 62 adverse events (AEs) reported in 28 pts. For 5 pts (8%) AEs lead to drug discontinuation. AEs were of grade 1 (n = 9), grade 2 (n = 8), or grade ≥ 3 (n = 45: 42 grade 3, 2 grade 4, and 1 grade 5). Conclusions: Pembrolizumab is safe and well tolerate in this population of rare ovarian cancer pts. AcSé study reports prolonged responses in very selected subtypes of rare ovarian tumor (CCC, cancerized teratoma, and LGCS). Acknowledgements: TMRG (national cancer network dedicated to rare gynecological tumors), GINECO group for partnership, La Ligue Nationale contre le Cancer, INCa and MSD. Clinical trial information: NCT03012620. [Table: see text]