A phase 2 study of anti-PD-L1 antibody (atezolizumab) in grade 2 and 3 chondrosarcoma.

11528 Background: Chondrosarcoma is one of the most common bone malignancies in adults, and the third most common in pediatric patients (pts). The most prevalent subtype, conventional chondrosarcoma, is a slow growing tumor that is historically known to be refractory to chemotherapy. Anecdotal reports indicated a role for anti-PD-(L)1 in the treatment of this disease. This is the first prospective report on the efficacy of the PD-L1-targeting agent, atezolizumab, in this rare disease. Methods: Patients (pts) ages 2 and older with unresectable grade 2 or 3 conventional chondrosarcoma were eligible. No prior anti-PD-(L)1 treatment was allowed, otherwise pts were eligible irrespective of prior therapies as long as protocol-specified washout period requirements were met. Pts received atezolizumab 1200 mg (15 mg/kg with 1200 mg cap in pediatric pts) once every 21 days. Imaging was carried out at end of cycle 3, and then every two cycles. Research biopsies were collected from adult pts prior to C1D1, prior to C3D1, and at progression. Immuno-pharmacodynamic (IO-PD) studies were performed on paired tumor samples and circulating immune cells to help elucidate signaling pathways mediating the immune response, with focus on subsets of effector cells in the tumor microenvironment. Results: A total of 9 pts (7 males, 2 females) were enrolled in 6 centers across the US and Canada. Six pts were Caucasian/White, 1 Asian, 1 Hispanic, and 1 unknown. Median age was 49 years (42-72). No objective responses were seen. Three pts (33%) experienced disease stability (SD) per RECIST 1.1, for a median duration of 21 weeks as of data cutoff (January 2022). A patient with SD remains on active treatment (tx) for 35 weeks. Three patients had no tx-related adverse events (AEs). Six pts (67%) experienced at least one tx-related AE. Two patients experienced > G2 AEs, but only one was considered tx-related (lymphopenia). Immune-related AEs were all G1/2 and included hepatitis (2), hypothyroidism (1), hyperthyroidism (1), and maculopapular rash (1). IO-PD studies are ongoing and will be reported at the conference if available. Conclusions: Atezolizumab was well-tolerated but demonstrated limited activity in this cohort of pts with few treatment options. Ongoing IO-PD studies will provide insight into atezolizumab’s effect upon immune cell content and activation in the tumor microenvironment that will help design future immunotherapy trials in this disease and other sarcoma types. The study was funded by NCI Contract HHSN261201500003I. Clinical trial information: NCT04458922.