Next, we sought to characterize potential toxic side effects of compounds 1, 2, and 3. Preclinical models such as the colony forming unit megakaryocyte assay (CFU-Mk) have been successfully used to predict drug-induced thrombocytopenia in the clinic. Using such an assay, Faivre et al. showed the clinical candidate, pan-D2 inhibitor ABBV-744, was much better tolerated than pan-BET inhibitor ABBV-075 (EC50 = 645 nM vs 3 nM respectively). The latter was shown to induce BET-protein-dependent reversible thrombocytopenia in clinical trials. Using the CFU-Mk assay, we tested the effects of I-BET151, iBET-BD2, 1, 2, and 3. Similar to ABBV-744, iBET-BD2 had a nanomolar EC50 of 380 nM, supporting a better-tolerated inhibitor (Figure ). Alternatively, pan-D1-biased inhibitor, 3, induced a toxicity profile similar to pan-BET inhibitors with an EC50 = 90 nM. This result was consistent with pan-D1 inhibitors phenocopying pan-BET inhibitors. However, a recent study in rats using a pan-D1 inhibitor did not show adverse effects on isolated platelets over a 10-day treatment period, despite significant platelet reduction from ABBV-075 administration. However, the in vivo stability of this pan-D1 inhibitor was called into question. Surprisingly, I-BET151, 1, and 2 were the most tolerated (EC50 = 1.28 μM, 0.67 μM and 1.83 μM respectively). The tolerance of 1 and 2 may be partially explained as BRD2 and BRD3 were shown to be necessary for erythroid maturation, while the tolerance of I-BET151 remains unclear. Given that I-