The present invention is predicated on the realisation that problems with the poor water solubility of arsenic trioxide and the extreme difficulty in dissolving arsenic trioxide in anything other than a very basic solution, could be overcome by forming a much more soluble diarsenic tetraoxide, including the compound NaHAs2O4, prior to its delivery to a patient. Pharmaceutical compositions with such compounds and their use in the treatment of cancers is disclosed.本發明係基於理解三氧化二砷具有不佳的水溶解度以及除了非常鹼性溶液以外之任何條件均極端難以溶解三氧化二砷的問題，在遞送患者之前藉由形成更為可溶的四氧化二砷，較佳地為NaHAs2O4，而予以克服。亦揭示具有該等化合物的醫藥組成物及其治療癌症的用途。
