Antimyeloma activity of I -BET151 against stroma and growth factor–treated myeloma cell lines and primary myeloma cells in vitro and in vivo. (A-B) Representative examples and cumulative data of (A) cell cycle and (B) apoptosis analysis of primary CD138⁺ myeloma cells cultured on MS5 stroma cells and in the presence of IL-6 5 ng/mL and treated with DMSO or I-BET151 for 72 hours (n = 4). Data shown as mean ± SEM. (C) KMS11 cells (5 × 10⁶) were injected subcutaneously into the flank of NSG mice. Treatment with I-BET151 (n = 13; 30 mg/kg intraperitoneal injection daily for 21 consecutive days) or vehicle (n = 11; 0.9% NaCl + 10% kleptose + 5% DMSO) was initiated when the maximum dimension of the tumor was ≥5 mm. Relative increase of tumor volume compared with day 1 of treatment is shown. (D) Kaplan-Meier survival analysis of time to progression, defined as time to doubling of tumor size compared with day 1 of treatment. Log-rank test P < .001.