Background: In the randomized, open-label, phase 3 KEYNOTE-119 study (NCT02555657), OS was not significantly different with pembro monotherapy versus chemo in second- or third-line settings for mTNBC; however, the pembro treatment effect increased with increasing PD-L1 enrichment. We evaluated the association of TMB with efficacy of pembro monotherapy versus chemo in patients with previously treated mTNBC. Methods: Patients with centrally confirmed TNBC and 1 or 2 prior systemic treatments for metastatic disease were enrolled. Patients were randomly assigned 1:1 to pembro 200 mg Q3W or single-agent chemo per investigator’s choice of capecitabine, eribulin, gemcitabine, or vinorelbine. Association of TMB, as measured by FoundationOne CDx (Foundation Medicine), with response was an exploratory objective evaluated using receiver operator characteristic (ROC) analysis, logistic regression (ORR), and Cox regression (OS; PFS) within treatment arms; estimates of efficacy based on TMB cutpoint used a prespecified cutpoint of 10 mut/Mb. Results: TMB data were available for 253/601 (42.1%) treated patients (pembro, n = 132; chemo, n = 121); baseline characteristics were similar to that of the overall study population. One-sided P values for the association of TMB and clinical outcomes in pembro-treated patients were 0.154 for ORR, 0.014 for PFS, and 0.018 for OS; the area under the ROC curve ([AUROC] 95% CI) for predicting ORR was 0.58 (0.43-0.73). Two-sided P values for the association of TMB and clinical outcomes in chemo-treated patients were 0.114 for ORR, 0.478 for PFS, and 0.906 for OS; AUROC (95% CI) was 0.43 (0.27-0.59). Twenty-six patients had TMB ≥10 mut/Mb. Thus, the prevalence of TMB ≥10 mut/Mb was ~10%. Outcomes based on TMB cutpoint are reported in the Table. Conclusions: Data from this exploratory analysis from KEYNOTE-119 suggest a potential positive association between TMB and clinical benefit with pembro but not chemo in patients with mTNBC. Although precision is limited by sample size and the number of patients with TMB ≥10 mut/Mb, ORR and HRs for OS suggested a trend towards increased benefit with pembro versus chemo in patients with TMB ≥10 mut/Mb. Clinical trial information: NCT02555657.