Abstract 4997: Responsiveness to immune checkpoint inhibitors in RET dependent cancers

Background: The RET receptor tyrosine kinase can be oncogenically activated by gene fusions or point mutations. Multikinase inhibitors such as cabozantinib, lenvatinib, and vandetanib have demonstrated activity in RET dependent (RET+) malignancies. Recent development of selective RET inhibitors is poised to alter their treatment paradigm. However, the responsiveness of RET+ malignancies to immune checkpoint inhibition (ICI) is unknown. We compared the time to progression (TTP) on ICI with non-ICI therapy in patients with malignancies harboring activating RET mutations/fusions. Methods: A retrospective chart review of all RET+ patients who were referred to the phase 1 clinical trials program at MD Anderson Cancer Center between September 2014 and October 2018 was conducted. Patients who had not discontinued treatment or had discontinued treatment for reasons besides disease progression were censored. Time to progression was estimated using Kaplan-Meier analysis. Results: Out of 72 patients who had received systemic therapy for RET+ malignancies, 39 (54.2%) harbored RET mutations (RETm) and 33 (45.8%) harbored RET fusions (RETf). Nineteen patients (26.4%) received ICI and 53 (73.6%) received non-ICI therapy. Thirty four patients (47.2%) had medullary thyroid cancer, 29 (40.3%) had NSCLC and 9 (12.5%) had other types of cancer. PD-L1 expression was only available for 7 patients of which 5 (71.4%) were positive (≥ 1%). Patients had received a median of 1 (1-7) prior lines of therapy. Overall median TTP (mTTP) was 10.5 months (95% CI 6.7-30.1). Hazard ratio for ICI vs. non-ICI therapy was 1.73 (0.70, 4.26) p=0.25 in patients with RETf and 8.0 (2.27, 28.2) p=0.0025 in patients with RETm. Conclusions: In patients with RET+ malignancies, mTTP was shorter with ICI compared to non-ICI therapy. The effect of type of therapy was dependent on the type of RET pathway aberration, with a statistically significant higher risk of progression in RETm malignancies treated with ICI compared to non-ICI therapy. Time to ProgressionRETTherapyNmTTP (months)Freedom from Progression at 6 months (%)Freedom from Progression at 12 months (%)FusionNon-ICI218.36335FusionICI123.03624MutationNon-ICI3231.98273MutationICI75.6430 Citation Format: Aparna Hegde, Le Huang, Shuang Liu, Kenneth Hess, Maria Cabanillas, Mimi Hu, Naifa Busaidy, Steven Sherman, George Simon, George Blumenschein, Vassiliki A. Papadimitrakopoulou, David S. Hong, Funda Meric-Bernstam, John Heymach, Vivek Subbiah. Responsiveness to immune checkpoint inhibitors in RET dependent cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4997.