Arsthinol is an antiprotozoal agent which may have anti-cancer activity. It was found that arsthinol, a trivalent organoarsenic compound (dithiarsolane), has been active in vitro on leukemia cell lines and offers a better therapeutic index than arsenic trioxide, as estimated by the ratio LD50/IC50. Arsthinol induced growth inhibition of NB4 cells at lower concentration (IC50 (concentration inhibiting growth by 50%) = 0.78 +/- 0.08 micromol/l after 24 h) than As(2)O(3) (IC50 = 1.60 +/- 0.23 micromol/l after 24 h) or melarsoprol (IC50 = 1.44 +/- 0.08 micromol/l after 24 h). Arsthinol-cyclodextrin complex demonstrated to have was more effective than arsenic trioxide (As2O3) and melarsoprol on the U87 MG cell line. Importantly, in the in vivo study, significant antitumor activity against heterotopic xenografts was observed after i.p. administration.