Molecular response cutoffs and ctDNA collection timepoints influence on interpretation of associations between early changes in ctDNA and overall survival in patients treated with anti-PD(L)1 and/or chemotherapy

Background Circulating tumor DNA (ctDNA) is a promising intermediate end point for oncology drug development, potentially accelerating regulatory approvals by providing early insights into treatment response. However, challenges remain in standardizing ctDNA assessment, including optimal blood collection timing and defining molecular response (MR) cutoffs. The ctDNA for Monitoring Treatment Response (ctMoniTR) project, led by Friends of Cancer Research, aggregates patient- level data from clinical trials to evaluate associations between ctDNA changes and overall survival (OS). Methods This analysis included four randomized clinical trials of patients with advanced non- small cell lung cancer (aNSCLC) treated with either anti-programmed death (ligand) 1 (anti-PD(L)1) therapy (with or without chemotherapy) or chemotherapy alone. MR was assessed using three predefined per cent-change thresholds in ctDNA levels (≥50% decrease, ≥90% decrease, and 100% clearance). ctDNA samples were analyzed at two timepoints: an early window (T1, up to 7 weeks post-treatment initiation) and a later window (T2, 7–13 weeks post-treatment initiation). Multivariable Cox proportional hazards models and time-dependent analyses were used to evaluate associations between ctDNA changes and OS. Results A total of 918 patients were included. In the anti-PD(L)1 group, ctDNA reductions at both T1 and T2 were significantly associated with improved OS across all MR thresholds. In the chemotherapy group, associations were weaker at T1 but became more pronounced at T2. Patients with MR at both T1 and T2 had the strongest OS associations. Overall, the results suggest that T2 had marginally stronger association with OS than T1. Conclusions This study supports the potential of ctDNA as an intermediate end point in aNSCLC, with MR at both early (T1) and later (T2) timepoints showing significant associations with OS. Differences in ctDNA dynamics between treatment modalities highlight the importance of considering the timing of blood collection. Further research is needed to determine the optimal time window for assessing ctDNA response. Prospective trials and trial-level meta- analyses will be critical to validating ctDNA as a regulatory- grade intermediate end point for oncology drug development.