Abstract 6469: Longitudinal loss of microbiome stability is associated with poor response to immune checkpoint inhibitors in lung cancer and melanoma

Abstract While immune checkpoint inhibitors (ICI)-based regimens and chemo-immunotherapy combinations (chemo-ICI) are now first-line therapy for patients with metastatic non-small cell lung cancer (NSCLC), the number of patients who experience a sustained response to treatment remains limited. The majority of patients will eventually develop progressive disease. Microbiome-based biomarkers offer an opportunity to identify patients who may have a poor response to ICI-based regimens using non-invasive methods, and potentially, the microbiota may be amenable to therapy-enhancing alteration. However, a deep understanding of the longitudinal dynamics of gut microbial features in patients treated with ICIs in NSCLC, a feature of likely importance for microbiome-based therapeutics, remains limited. In this study, we show that patients with NSCLC who experience a poor response to an ICI-based regimen show a loss of intra-individual microbiome stability in the first 4 months during treatment with an ICI-based regimen, independent of antibiotic exposure. Longitudinal loss of microbiome stability was also associated with poor response in patients with melanoma. To identify key microbial species associated with progression, recursive feature elimination with random forest classifiers was used to identify temporally-associated microbial species associated with disease progression. An index of these progression-associated species was able to predict clinical outcomes based on pre-treatment fecal samples, with validation on an independent lung cancer cohort. Together, our data show that microbial instability may be an early indicator of ICI-resistance in patients with NSCLC and melanoma, with the potential to be developed into biomarkers of primary resistance to ICI-based regimens. Citation Format: Yujie Zhao, Jarushka Naidoo, Michael Conroy, Jacqueline T. Ferri, Joell J. Gills, Krista Y. Chen, James R. White, Sara Glass, Willaim O. Assan, Kimberly Peloza, Megan D. Schollenberger, William H. Sharfman, Kristen Marrone, Patrick M. Forde, Julie R. Brahmer, Valsamo (Elsa) Anagnostou, Drew M. Pardoll, Joseph Murray, Evan J. Lipson, Cynthia L. Sears, Fyza Y. Shaikh. Longitudinal loss of microbiome stability is associated with poor response to immune checkpoint inhibitors in lung cancer and melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 6469.