Longitudinal genome‐wide aneuploidy measurements in circulating cell‐free <scp>DNA</scp> to predict lack of benefit from pembrolizumab in patients with metastatic urothelial cancer

Accurate prediction of lack of benefit from pembrolizumab in patients with metastatic urothelial cancer (mUC) is an unmet need. We investigated the dynamics of circulating tumor DNA (ctDNA) load, estimated using the modified fast aneuploidy screening test-sequencing system (mFast-SeqS), as a potential biomarker for early on-treatment identification of treatment response. A total of 104 patients with mUC treated with pembrolizumab from two prospective biomarker discovery trials were included and mFast-SeqS was performed on paired blood samples collected at baseline and on-treatment. Patients with a high on-treatment aneuploidy score (≥ 5, n = 26) had a shorter median OS than patients with a low (< 5) score (n = 76) (3 vs 17 months: P-value< 0.001). Patients with an increased (n = 10), stable (n = 66), or decreased (n = 28) on-treatment score relative to their baseline score had a median PFS of 1.5, 4.0, and 8.3 months, respectively. Median OS was 3.0, 11.1, and 18.7 months, respectively. In patients with mUC treated with pembrolizumab, the on-treatment mFast-SeqS-based ctDNA level and its dynamics relative to baseline are independent prognostic markers that can be used to identify patients that are unlikely to benefit from pembrolizumab.