Among the 27 patients with known LDH level at baseline, greater than two-fold elevations in baseline LDH levels were associated with rapid disease progression: five of five (100%) patients with LDH ≥ 800 IU/L experienced PD within 8 weeks of study entry compared with 11 of 22 (50.0%) patients with LDH < 800 IU/L. This suggests that patients with mTNBC and high LDH may not have the opportunity to derive benefit from pembrolizumab, which in TNBC is associated with a longer median time to response (18 weeks; range, 7 to 32 weeks) than cytotoxic chemotherapy.