A phase II trial of atezolizumab and bevacizumab in patients with advanced, progressive neuroendocrine tumors (NETs).

619 Background: Neuroendocrine tumors (NETs) are relatively rare and heterogeneous tumors arising throughout the aerodigestive tract, which are incurable and life-limiting when metastatic. Prior studies of checkpoint inhibitors in NET patients have yielded minimal evidence of efficacy. Historically, effective therapies for advanced, progressive NET yield response rates less than 10% and progression-free survival (PFS) durations of approximately 11 months, as compared to approximately 4.5 months with placebo. Methods: We undertook a phase II basket study of atezolizumab in combination with bevacizumab in patients with rare cancers, and present here the data from the pancreatic NET (pNET) cohort and extrapancreatic NET (epNET) cohort, each of which included 20 patients with grade 1-2 NET that was progressive under any prior therapy. Patients received 1200mg of atezolizumab and 15mg/kg of bevacizumab IV q 21 days. The primary endpoint was confirmed objective response by RECIST 1.1. Results: The confirmed objective response rate with this combination was 20% (95% CI 6-44%) in the pNET cohort and 15% (95% CI 3-38%) in the epNET cohort. The median PFS in the pNET cohort is 19.6 months (95% CI 10.6-NR), while it was 14.9 months (95% CI 6.1-NR) in the epNET cohort, 1-year PFS was 75% and 52%, respectively. The combination was well-tolerated in this patient population, with the most common related treatment-emergent adverse events being hypertension (47.5%), proteinuria (37.5%), and fatigue (35%). The most common related grade 3/4 adverse events were hypertension (20%) and proteinuria (7.5%). Conclusions: The combination of atezolizumab and bevacizumab demonstrated moderate clinical activity in patients with advanced NETs. As pre-treatment and on-treatment biopsies were obtained for all patients, correlations with immune infiltration, mutations, and transcriptome alterations should provide additional insight into the mechanisms of response and resistance. Clinical trial information: NCT03074513.