Ultrasensitive ctDNA monitoring to reveal early predictors of immunotherapy success in advanced cancer.

2561 Background: The potential of immune checkpoint inhibition (ICI) therapy is constrained by the inability to predict patient response. Circulating tumor DNA (ctDNA) has emerged as a promising tool for real-time response tracking and early prediction of therapeutic outcomes. However, the clinical utility of ctDNA-based liquid biopsy faces a critical challenge: reliable detection in low-shedding tumors and during dramatic therapeutic responses when ctDNA levels approach the analytical threshold. We overcome this technical limitation, achieving the precise longitudinal monitoring needed to optimize ICI therapy. Methods: We analyzed longitudinal plasma samples from 43 patients with treatment-refractory metastatic cancers spanning 8 distinct groups, composed primarily of GI (n = 17) and gynecological cancers (n = 6). Patients underwent a median of 1 previous line of therapy (range 0-8). Using NeXT Personal, an ultra-sensitive personalized liquid biopsy approach, we tracked up to 1,800 patient-specific somatic variants per case across 250 plasma samples. This methodology achieves exceptional analytical sensitivity, detecting circulating tumor DNA at levels as low as 1-3 parts per million (PPM). Results: ctDNA was detected across five orders of magnitude (2.0-239,315 PPM, median LOD: 1.76 PPM), with 31% of positive signals falling in the ultrasensitive range below 100 PPM. Early molecular response, measured by > 50% ctDNA reduction or sustained ctDNA negativity from baseline to first follow-up (median 23 days), strongly predicted improved progression-free survival (PFS) (HR = 0.22, 95% CI 0.07-0.70, p = 0.006), representing a 3-fold increase in 1 year PFS rates. Achievement of durable molecular complete response (dmCR), defined as sustained ctDNA clearance > 120 days, emerged as a powerful predictor of PFS, with dmCR patients maintaining 100% progression-free status at 12 months compared to 63% in non-dmCR patients (HR = 0.10, 95% CI 0.01-0.92, p = 0.017). This survival advantage persisted at 18 months with 80% PFS in dmCR patients versus 21% in non-dmCR patients. Conclusions: Early ctDNA kinetics predict long-term ICI outcomes across multiple advanced cancer types. The ability to detect ultra-low ctDNA levels proved critical for accurate minimal residual disease assessment, even in this heavily pretreated cohort. These results establish high-sensitivity ctDNA monitoring as an essential tool for precise, real-time evaluation of immunotherapy response to guide clinical decision-making. Clinical trial information: NCT02478931 .