Ultrasensitive ctDNA monitoring reveals early predictors of immunotherapy response in advanced cancer

Circulating tumor DNA (ctDNA)-based response assessment is appealing but limited by conventional analytical thresholds. We utilized a whole genome sequencing based, tumor-informed ultrasensitive ctDNA assay which tracked ~1800 somatic mutations to analyze 227 longitudinal plasma samples from 39 patients with advanced/metastatic cancers receiving immune checkpoint inhibitors (ICIs). ctDNA was detected from 2.0-239,315 PPM (median limit of detection: 1.77 PPM), with 33% of positive detections below 100 PPM. Early molecular response, defined as >50% ctDNA reduction or sustained ctDNA negativity from baseline to first follow-up, strongly predicted improved progression-free survival (PFS) (hazard ratio (HR) = 0.09, 95% CI: 0.02-0.39, p = 0.001) and was independently prognostic of PFS. Molecular complete response (mCR), defined as any ctDNA clearance, predicted overall survival and PFS, with 1-year PFS of 87% in mCR patients versus 16% in non-mCR patients (HR = 0.14, 95% CI: 0.04-0.50, p = 0.003). The high-sensitivity ctDNA monitoring may enable precise, real-time evaluation of ICI response to guide clinical decision-making.