Phase II study of atezolizumab in advanced alveolar soft part sarcoma (ASPS).

11519 Background: ASPS constitutes < 1% of soft tissue sarcomas and frequently presents in adolescents and young adults. There are no approved therapies for ASPS. We are currently evaluating the clinical activity of atezolizumab (atezo), an anti-PD-L1 antibody, in patients (pts) with advanced ASPS. Methods: This is a multicenter, open-label, single-arm phase II study where atezo is administered at a fixed dose of 1200 mg in adults or 15 mg/kg (1200 mg max) in pediatric pts age ≥2 once Q21 days. The primary objective is to determine the objective response rate (ORR) of atezo using RECIST 1.1. Secondary objectives include duration of response and correlating response with the immune effects of atezo in blood and paired tumor biopsies (pre- and post-treatment). Tumor specimens were analyzed with multiplex immunofluorescence immuno-oncology panels to quantify CD8+, PD-1+, and PD-L1+ cells/mm 2 in the tumor microenvironment. CD8+ density was calculated as the total number of CD8+ cells divided by the entire area (mm 2 ) of the tumor and invasive margins of the biopsy. Results: As of February 4, 2021, 44 pts have been enrolled. The median age in the study was 31 years (range, 12–70) with equal male: female distribution. 54.5% of pts were Caucasian. Baseline ECOG ≤1 was present in 97.7%. The median time on study was 11.5 months (range, 0.8–40.3 months). At data cutoff, response evaluation was available for 43 pts with an ORR of 37.2% (16/43). One pt experienced a complete response and 15 pts experienced a partial response (PR), of which 14 were confirmed. The median time to confirmed response was 3.5 months (range, 2.1–14.9 months). The median duration of confirmed response was 16.5 months (range, 4.9–38.1 months). Stable disease (SD) was present in 58.1% (25/43). One or more grade 3 adverse events potentially related to atezo were identified in 16.3% (7/43) pts. These include diarrhea, hypothyroidism, transaminitis, anemia, vertigo, extremity pain, myalgia, pneumonitis, rash, and stroke (n = 1 each). No grade 4 or 5 events have been reported. Among 8 cases with evaluable biopsy pairs, both baseline and C3D1 specimens in all cases demonstrated CD8+ T cell infiltration and PD-L1 expression. PD-1 expression was detected at baseline in 5 cases and at C3D1 in 7 cases. In 6 cases (3 SDs and 3 PRs), treatment did not change CD8+ cell density. In the other 2 cases (both PRs), CD8+ density increased > 3x above baseline by C3D1. Analysis of T cell activation using pharmacodynamic response biomarkers, along with whole exome and RNA-seq to evaluate the genomic and transcriptomic landscape of ASPS, are ongoing. Conclusions: Atezo is well tolerated and demonstrates promising single agent activity with durable responses in advanced ASPS. Preliminary tumor biomarker analysis confirms the presence of multiple PD-1/PD-L1 immune checkpoint (IC) components, indicating that advanced ASPS is an ideal candidate for therapeutic IC inhibition. Funded by NCI Contract No HHSN261200800001E. Clinical trial information: NCT03141684.