High clinical activity of pembrolizumab in chordoma, alveolar soft part sarcoma (ASPS) and other rare sarcoma histotypes: The French AcSé pembrolizumab study from Unicancer.

11520 Background: AcSé Pembrolizumab is a Phase 2, non-randomized parallel arms, open-label, multicentric study from Unicancer investigating the efficacy and safety of pembrolizumab monotherapy in different cohorts of patients with rare cancers (NCT03012620). Here we report the results of pembrolizumab in the rare sarcoma cohort. Methods: Selected histotypes were all rare sarcomas patients (pts) (incidence < 0.2/100,000/year). Main inclusion criteria were age > 18, ECOG PS≤1 and advanced or metastatic disease resistant to standard treatment. Patients received pembrolizumab 200 mg IV as a 30-minute infusion 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 clinical benefit rate, duration of response, progression-free survival (PFS), overall survival (OS), and safety. Five groups of pts were distinguished, namely chordoma, alveolar soft-part sarcoma (ASPS), desmoplastic small round cell tumor (DSRCT), smarca4 deficient malignant rhabdoid tumor (SMRT), and other histotypes. Results: 98 patients including 34 with chordoma, 14 ASPS, 11 SMRT, 8 DSCRT and 31 with other histotypes, were included from July 2017 to December 2020. The median number of cycles was 5 (range, 1 to 35) with 78 (79.6%) patients who discontinued the trial after a median of 4 cycles. There were 6 (7.3%) partial response (PR) at 12 weeks. The best response was CR in 1 patient (1%), PR in 14 patients (14.3%), and stable disease (SD) in 33 (33.7%). Median duration of response was 8.2 months [IQR, 4.1 to 9.0]. The occurrence of best response depended on the histotype, with 3 (8.8%) responses in chordoma, 7 (50%) in ASPS, 3 (27%) in SMRT, 1 (12.5%) in DSCRT and 1 (3.2%) in other histotypes (p = 0.0011). At the data cut off, median PFS was 2.75 months, and median OS was 19.7 months on the overall population. Outcomes differed according to the histotype group, with the 12 months PFS rates at 31.2% (chordoma), 35.7% (ASPS), 18.2% (SMRT), 0% (DSCRT) and 3.3% (other), respectively (p < 0.0001), and median PFS at 6.6 (chordoma), 7.5 (ASPS), 1.1 (SMRT), 2.1 (DSCRT) and 2.1 months (other), while 1-year OS rates were 76.6% (chordoma), 85.7% (ASPS), 36.4% (SMRT), 17.5% (DSCRT) and 42.9% (other) with median OS only reached for SMRT (2.4 months), DSRCT (10 months), and the other histotype group (7.1 months) (p = 0.004). The side effect profile of pembrolizumab was similar to other tumor type. Conclusions: Pembrolizumab is safe and well tolerate in this pop od STS pts, AcSé study reports high levels response rate and prolonged activity in selected subtypes of rare sarcomas. Clinical trial information: NCT03012620.