ARTICLE ABSTRACTPurpose: Tumor mutational burden detected by tissue next-generation sequencing (NGS) correlates with checkpoint inhibitor response. However, tissue biopsy may be costly and invasive. We sought to investigate the association between hypermutated blood-derived circulating tumor DNA (ctDNA) and checkpoint inhibitor response.Experimental Design: We assessed 69 patients with diverse malignancies who received checkpoint inhibitor–based immunotherapy and blood-derived ctDNA NGS testing (54–70 genes). Rates of stable disease (SD) ≥6 months, partial and complete response (PR, CR), progression-free survival (PFS), and overall survival (OS) were assessed based on total and VUS alterations.Results: Statistically significant improvement in PFS was associated with high versus low alteration number in variants of unknown significance (VUS, >3 alterations versus VUS ≤3 alterations), SD ≥6 months/PR/CR 45% versus 15%, respectively; P = 0.014. Similar results were seen with high versus low total alteration number (characterized plus VUS, ≥6 vs. <6). Statistically significant OS improvement was also associated with high VUS alteration status. Two-month landmark analysis showed that responders versus nonresponders with VUS >3 had a median PFS of 23 versus 2.3 months (P = 0.0004).Conclusions: Given the association of alteration number on liquid biopsy and checkpoint inhibitor–based immunotherapy outcomes, further investigation of hypermutated ctDNA as a predictive biomarker is warranted. Clin Cancer Res; 23(19); 5729–36. ©2017 AACR.