(P 10) ATEZOLIZUMAB IN JAPANESE PATIENTS WITH ADVANCED ALVEOLAR SOFT PART SARCOMA: A MULTI-CENTER, SINGLE-ARM, INVESTIGATOR-INITIATED PHASE 2 TRIAL (NCCH1907)

Objective: Alveolar soft part sarcoma is a rare and clinically indolent soft tissue sarcoma subtype. Here, we report the clinical activity and safety of atezolizumab, an anti-PD-L1 immune checkpoint inhibitor, in Japanese patients with unresectable, recurrent, or metastatic alveolar soft part sarcoma.

Methods: In this study, patients were enrolled from 4 hospitals in Japan. Eligible patients eligible were aged 16 years or older with histologically confirmed, locally advanced or metastatic alveolar soft part sarcoma; an Eastern Cooperative Oncology Group performance status score of 0-1; and adequate bone marrow, hepatic, and renal function. Patients received 1200 mg of atezolizumab every three weeks until disease progression, unacceptable toxicity, or patient refusal. The primary endpoint was the objective response rate (ORR) measured according to the Response Evaluation Criteria in Solid Tumors, version 1.1, evaluated by blinded independent central review. Secondary endpoints were progression-free survival (PFS), overall survival (OS), and safety. This trial is registered with jRCT, number 2031200041.

Results: Between May 2020 and March 2021, 21 patients with alveolar soft part sarcoma were enrolled in the study and deemed eligible for inclusion. The median patient age was 33 years (range, 18-61 years). Fourteen patients (66.7%) received prior systemic treatment; 13 patients received pazopanib. Twenty patients were evaluable for the primary endpoint. The overall ORR was 10.0% (95% confidence interval (CI), 1.2%-31.7%). Complete responses were observed in 2 patients (10.0%), and response continued for more than 1 year. Stable disease was present in 14 patients (70.0%). The disease control rate was 80.0% (95% CI, 56.3%-94.3%). The median PFS was 10.0 months [95% CI, 3.3-16.7] and the median OS was not reached [95% CI, 14.9-not estimable (NE)]. The most common treatment-related adverse events were fever (40.0%) and lymphocyte count decreased (30.0%). The most frequent grade 3 or worse adverse events were lymphocyte count decreased (three [15.0%]), CPK increased, white blood cell count decreased, weight gain, fever, parotitis, anemia, thyroiditis, cancer pain and liver function abnormalities (one [5.0% each]). There were no treatment-related deaths.

Conclusion: Atezolizumab showed modest antitumor activity with manageable toxicities for Japanese patients with unresectable, recurrent, or metastatic alveolar soft part sarcoma.