Atovaquone inhibits constitutive STAT3 tyrosine phosphorylation, STAT3 target gene expression, and viability of STAT3-dependent cancer cells through apoptosis. (A) Atovaquone at the indicated concentrations (μM) was used to treat U266 (2.5 hours), HEL (6 hours), or INA-6 cells (4 hours). Treatment with JAK inhibitor 1 (a direct pharmacological JAK inhibitor) at 1 μM for the same duration served as a control and comparison. Data are representative of 2 independent experiments. (B) Expression of endogenous STAT3 target genes was assayed by quantitative reverse transcription polymerase chain reaction in U266 cells treated with atovaquone for 6 hours. Data are representative means ± standard error from 1 of 2 independent experiments. (C) Viable cell number response curve to atovaquone (72-hour treatment) of hematological cancer cells that harbor STAT3 activation, compared with viability of nonmalignant PBMCs (average of 4 donors), which lack STAT3 activation. Data are means from 2 independent experiments. (D) Atovaquone (15 μM) treatment of INA-6 cells for 24 hours, followed by staining of cells with annexin V/PI to assay apoptosis. Data are representative of 2 independent experiments.