Change in 2,3-DPG, ATP, p50, and t50 from baseline in study subjects. Measures of PD, (A) 2,3-DPG, (B) ATP, (C) p50, and (D) t50, were assessed at steady state (baseline); after 14 ± 3 days of treatment on 5 mg BID, 20 mg BID, 50 mg BID, and 100 mg BID of mitapivat; at the end of taper (1 ± 3 days after the last dose of mitapivat); and at EOS (4 weeks ± 3 days after the last dose of mitapivat). A total of 16 subjects escalated to the 50-mg BID dose, and 9 of 10 subjects completed up to the 100-mg BID dose level. End of taper data were unavailable for 1 subject who self-discontinued treatment without undergoing a taper; EOS data were excluded for 1 subject because of intervening blood transfusion. Missing data reflected in the sample sizes shown (in particular, for p50) were largely the result of disruptions related to the COVID-19 pandemic. Mean change was calculated from baseline, defined as the most recent measurement prior to start of study drug, and reported as percentage change for ease of clinical interpretation. Error bars correspond to SDs. A linear mixed effects model using age and sex as covariates was created for each variable (referred to as the basic model; supplemental Materials), and statistical significance within this model is denoted in the figures by ∗. (A-B) There was a significant mean percentage decrease in 2,3-DPG levels and mean percentage increase in ATP levels compared with baseline at the 20-, 50-, and 100-mg BID dose levels of mitapivat (P < .0001 for all). (C-D) No significant changes from baseline were identified for p50 and t50 values in the basic model, except for a percentage increase p50 at the end-of-taper and end-of-study timepoints (P = .02 and .003, respectively).