ST1926 induces growth arrest in malignant T cells. (A) Effects of ST1926 treatment on the growth of HTLV-1–positive (HuT-102, MT-2, and C8166) and HTLV-1–negative (Jurkat, CEM, and MOLT-4) malignant cell lines: cell growth was assayed in quadruplicate wells with the CellTiter 96 nonradioactive cell proliferation kit. Viability results are expressed as percentage of control (0.1% DMSO) and represent the mean of ≥3 independent experiments ± standard error (SE). (B) Primary ATL cells are sensitive to ST1926: primary ATL from 3 patients (patients 1 and 2 newly diagnosed acute ATL and patient 3 relapsed acute ATL) were treated with the indicated concentrations of ST1926, and cell growth was assayed in quadruplicate wells with the CellTiter 96 nonradioactive cell proliferation kit. The results are expressed as percentage of control (0.1% DMSO) ± standard deviation (SD) and are representative of 2 independent experiments. (C) Resting and activated T-lymphocytes are resistant to suprapharmacological concentrations of ST1926. PBMCs were collected from 3 healthy donors. Activated PBMCs were supplemented with 2% PHA. Cells were seeded in 24-well plates and treated with 0.1% DMSO or 5 to 10 μM ST1926 up to 48 hours. Cell growth was assayed in triplicate wells using the Cell Titer 96 nonradioactive cell proliferation kit. Results are expressed as percentage of control (0.1% DMSO) ± SD.