Background: Studies have suggested that pts with TMB-high tumors could derive clinical benefit from atezolizumab, a PD-L1 inhibitor; however, these studies used inconsistent TMB cutoffs. We report efficacy and safety data of atezolizumab in adult and pediatric pts with TMB-high advanced/metastatic solid tumors from Cohort D of the TAPISTRY trial (NCT04589845), using two TMB cutoffs: ≥13 mutations [mut]/Mb and ≥16 mut/Mb. Methods: TAPISTRY is a phase II, global, open-label, multicohort basket trial evaluating the efficacy and safety of multiple therapies in pretreated pts with advanced/metastatic solid tumors. Pts in Cohort D had advanced unresectable/metastatic, PD-L1 inhibitor-naïve, TMB-high (≥13 mut/Mb) solid tumors. Atezolizumab was given every 21 days, at 1200 mg in pts ≥18 years old, and at ≥15 mg/kg (up to 1200 mg) in pts <18 years old. Tumor responses were assessed per RECIST v1.1. Primary endpoint: objective response rate (ORR) by independent review committee (IRC) in pts with TMB ≥16 mut/Mb. Secondary endpoints included ORR by IRC in pts with TMB ≥13 mut/Mb, duration of response (DoR), progression-free survival (PFS), overall survival (OS) and safety. Results: At data cut-off (Nov 9, 2023), 150 pts with TMB ≥13 mut/Mb were enrolled. In the safety-evaluable population (n = 148), median age was 63 years (range 11–86); 56% of pts were male, and 56% had received ≥2 prior lines of therapy (median 2; range 0–14). The efficacy-evaluable population included 129 pts with TMB ≥13 mut/Mb (TMB ≥16 mut/Mb; n = 111); the most common tumor types were colorectal (n = 40; 31%), breast, and gastroesophageal cancer (n = 11 each; 9%). Key outcomes are presented (Table). After a median follow-up of 9.8 months, ORR by IRC was comparable between pts with TMB ≥16 mut/Mb (22.5%) and pts with TMB ≥13 mut/Mb (20.2%). Responses were seen across a variety of tumor types. DoR 6- and 12-month event-free rates were 79% and 72%, respectively. Median PFS was short, suggesting fast disease progression in non-responders. Fatigue (22%) and anemia (20%) were the most common adverse events. Safety of atezolizumab was consistent with its known profile. Conclusions: Atezolizumab was well tolerated and led to antitumor activity in pts with TMB-high solid tumors. Responses were seen across a variety of tumor types. Clinical trial information: NCT04589845.

(TR)AE, (treatment-related) adverse event.