Growth inhibitory effect of THZ1 on ATL cells. (A) Four ATL/HTLV-1–infected T-cell lines (TL-Om1 and ATL-55T[–], MT-2 and ATL-43b[–]) and Jurkat cells were treated with THZ1 for 3 days. The cell viability was measured using a CellTiter Glo assay, and the results are shown as a percent of the DMSO-treated control. The data are presented as the mean ± standard deviation of duplicate experiments. *P < .05, **P < .01, ***P < .001 by 2-sample, 2-tailed Student t test compared with the DMSO-treated control. (B) Primary leukemia cells from 5 ATL patients (ATL5, 6, 7, 8, and 11) were treated with THZ1 at the indicated concentrations for 2 days (ATL6, 7, and 8) or 3 days (ATL5 and 11). The cell viability was measured, and the results are shown as a percentage of the DMSO-treated control. The data are presented as the mean ± standard deviation of duplicate experiments. (C) TL-Om1 and MT-2 cells were treated with THZ1 for 5 days. The cell viability was measured, and the results are shown as a percentage of the DMSO-treated control at day 1. (D) Two THZ1-sensitive (TL-Om1 and ATL-55T[–]) and 2 THZ1-resistant (MT-2 and ATL-43b[−]) cell lines were treated with THZ1 (0, 100, or 300 nM) for 24 hours. Whole-cell extracts were subjected to western blot analysis with antibodies specific for total RNA polymerase II (Pol II), Pol II phosphorylated at serine 5 (Ser 5) or serine 2 (Ser 2), or PARP (apoptosis marker). α-tubulin was used as a loading control.