Abstract 3849: Frequent monitoring of NSCLC immunotherapy using an mDETECT liquid biopsy reveals unexpected complexity and opportunities

Abstract The methylation DETEction of Circulating Tumour DNA (mDETECT) assay is a targeted DNA methylation-based Next Generation Sequencing liquid biopsy designed to detect cancer-specific methylation patterns. We have developed a version of our mDETECT assay that sensitively and quantitatively monitors Non-Small Cell Lung Cancer (NSCLC). This targeted approach assesses 23 differentially methylated genes in NSCLC patients covering 229 CpGs. Our assay has a 95% sensitivity at 95% specificity with an AUC of 0.95. We conducted a pilot observational study to frequently measure tumour dynamics in patients undergoing first-line pembrolizumab monotherapy for metastatic NSCLC. 19 participants were recruited prior to the initiation of immunotherapy and followed using the mDETECT NSCLC assay. 40 mL of blood was collected pre-treatment then weekly or biweekly after treatment initiation with some patients being followed for up to 2.3 years. In total we collected 226 samples (median 11.8 timepoints per patient). Radiological assessment was performed approximately every three months as per the standard of care. The primary aim of the study was to determine if overall survival (OS) could be predicted with the mDETECT assay within the first 6 weeks of treatment. Patients were divided by short (< 1 year), medium (1-4 years), and long term (> 4 years) OS. Patients with short OS showed constant or increasing mDETECT levels and never dropped below 80% of their pre-treatment mDETECT level. Patients with long OS showed an immediate decrease within the first 6 weeks of treatment and generally reached undetectable levels. Patients with medium OS showed a slower decline and a higher plateau than the long OS patients. These patterns, while derived from a small pilot cohort, suggest the mDETECT assay can determine within weeks if a patient is responding to immunotherapy. Continued monitoring revealed progression in some initially responding patients, with increasing mDETECT levels detected 4-6 months in advance of radiological progression. Frequent assessment of tumour burden by a liquid biopsy such as mDETECT offers the opportunity to rapidly determine a patient’s response to therapy and potentially modify treatments earlier than with radiology alone and on an ongoing basis. These findings support the feasibility and potential clinical utility of integrating methylation-based ctDNA monitoring into immunotherapy management workflows and justify further prospective validation in a larger study. Citation Format: Mihaela Mates, Andrew Robinson, Harriet Feilotter, Sofia Genta, Keira Frosst, Keira Parr, Garrett Baron, Christopher R. Mueller. Frequent monitoring of NSCLC immunotherapy using an mDETECT liquid biopsy reveals unexpected complexity and opportunities [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 3849.