Abstract 6557: Longitudinal cell-free tumor load dynamics represent an early endpoint for immunotherapy response in non-small cell lung cancer

Abstract Background: Liquid biopsies can inform treatment strategies by rapidly and accurately assessing tumor burden; with emerging data supporting the clinical utility of circulating cell-free tumor DNA (ctDNA) as an early endpoint of immunotherapy response. Methods: We performed targeted error-correction sequencing of ctDNA (n=330) and matched white blood cell (WBC) DNA (n=109) from 109 patients with advanced non-small cell lung cancer (NSCLC) who received immune checkpoint inhibitor monotherapy (n=79) or immunotherapy-containing combination regimens (n=30). Variant cellular origin was determined using a tumor-independent WBC DNA-informed approach. The maximum mutant allele fraction (maxMAF) of tumor-derived mutations was tracked longitudinally to assess changes in total cell-free tumor load (cfTL). Results: Among 3, 418 variants with resolved cellular origin, 2, 452 (72%) were tumor-derived, 125 (4%) were germline, while 841 (25%) were related to clonal haematopoiesis (CH). Cell-free DNA fragments harbouring tumor-derived mutations were significantly shorter than fragments containing wild-type alleles; however, fragment length alone was insufficient to distinguish CH variants from wild-type. Overall, 97 (89%) patients with 1 detectable tumor mutation were considered for cfTL tracking and molecular response assessment. Clearance of cfTL indicated molecular response (mR) whereas persistence indicated molecular progression (mPD). Time-course analyses, aiming to establish the optimal timing for molecular response assessment, revealed that early cfTL clearance within 3-9 weeks was significantly linked to improved survival outcomes. Notably, blood tumor mutation burden above or below 16 mutations/megabase showed no association with overall survival (OS; log rank p=0. 98) or progression-free survival (PFS; log rank p=0. 95). Of 81 patients evaluated for early on-therapy molecular responses, 28 (35%) were classified as having an mR, while 53 (65%) patients were classified in the mPD category. The median time to mR was 5 weeks in patients receiving immune checkpoint monotherapy and 6 weeks in patients receiving chemo-immunotherapy combination regimens. Patients in the mR group achieved longer OS (log rank p=0. 001) and PFS (log rank p=0. 0003) compared to those with mPD. Assessment of landmark PFS at 6 months demonstrated a significant association between ctDNA molecular response and durable clinical benefit (DCB) in patients receiving immune checkpoint monotherapy (p 0. 001, Fisher’s exact test), and a numerical association with DCB in patients on chemo-immunotherapy (p=0. 07, Fisher’s exact test). Conclusions: Longitudinal tracking of ctDNA provides a real-time and accurate approach for monitoring immunotherapy response. These findings underscore the clinical utility of employing ctDNA molecular response to inform clinical decision-making. Citation Format: Lavanya Sivapalan, Bahar Alipanahi, Archana Balan, Jamie E. Medina, Noushin Niknafs, Erica Peters, Jaime Wehr, Alissa Konicki, Mimi Najjar, Bryan Chesnick, Gavin Pereira, Sarah Rongione, Nisha Rao, Michael Rongione, Brittany Shavatt, Lorenzo Rinaldi, Benjamin Levy, Victor E. Velculescu, Patrick M. Forde, Valsamo Anagnostou. Longitudinal cell-free tumor load dynamics represent an early endpoint for immunotherapy response in 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 (6Suppl): Abstract nr 6557.