Abstract 1184: Combination pembrolizumab and radiotherapy induces anti-tumor immune responses in immunologically-cold non-small cell lung cancer

Abstract Introduction: Primary resistance to immune checkpoint inhibition (ICI) is in part driven by a “cold” tumor microenvironment in the context of low PD-L1 expression, low tumor mutation burden or oncogenic mutations that drive tumor immune exclusion. Radiation has the potential to sensitize tumors to ICI through extra-tumoral T cell activation in response to tumor antigens released from irradiated cancer cells (abscopal effect). Here, we investigate tumor microenvironment and T cell repertoire reshaping leveraging serial biospecimens from a phase 2 randomized clinical trial of stereotactic body radiation therapy followed by pembrolizumab (SBRT arm) vs. pembrolizumab alone (control arm) in non-small cell lung cancer (NSCLC; NCT02492568). Methods: We employed whole exome sequencing, RNA sequencing and T cell receptor (TCR) sequencing to analyze 293 serial tumor and peripheral blood samples collected at baseline and after 8 weeks of treatment from 72 patients in the control (n=35) and SBRT (n=37) arms. Immunologically-cold tumors were defined as low TMB (<300 mutations per exome, n=43), PD-L1 null (0% expression on immunohistochemistry, n=41), or WNT-activated (presence of mutations affecting genes in the WNT pathway, n=10). TCR clonotypic differential abundance analyses and RNA sequencing deconvolution and gene set enrichment analyses were performed on baseline and on-therapy blood and tumor samples from the control and SBRT arms. Results: Patients with immunologically-cold tumors in the SBRT arm had significantly longer progression free survival compared to the pembrolizumab monotherapy arm (log rank p=0.029 for TMB-low, p=0.022 for PD-L1-null, p=0.037 for the WNT-activated subsets). Induction of interferon-gamma, interferon-alpha, antigen processing and presentation, and inflammatory response gene sets was significantly enriched on-therapy in the SBRT arm (FDR adjusted p<0.01 for TMB-low, PD-L1-null, and WNT-activated tumors). We observed greater enrichment in newly expanded TCR clones on-therapy intratumorally and in peripheral blood in the SBRT arm compared to the control arm (mean 16.17 vs 7.54 clonotypes, p=0.022 in tumor; mean 4.56 vs 1.00, p=0.039 in blood; Mann-Whitney U test). Similarly, we noted greater expansion of pre-existing TCR clones in the SBRT arm than within the control arm (mean 34.83 vs 18.69, p=0.021 in tumor; mean 21.61 vs 8.15, p=0.009 in blood; Mann-Whitney U test). Within the SBRT arm, there was no significant difference in T cell expansion between TMB-stratified (<300 mutations vs >=300 mutations) or PD-L1-stratified (0% vs >= 1% expression) groups. Conclusions: SBRT followed by pembrolizumab was associated with priming and augmenting anti-tumor immune responses in immunologically-cold NSCLC, which opens a therapeutic window of opportunity for radiotherapy to overcome primary resistance to immune checkpoint inhibition. Citation Format: Justin Huang, Willemijn S. Theelen, Zineb Belcaid, Mimi Najjar, Christopher Cherry, Archana Balan, James R. White, Noushin Niknafs, Jaime Wehr, M.M. van den Heuvel, Rachel Karchin, Paul Baas, Victor E. Velculescu, Valsamo Anagnostou. Combination pembrolizumab and radiotherapy induces anti-tumor immune responses in immunologically-cold non-small cell lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1184.