arsenic trioxide (AR-seh-nik try-OK-side) is a drug used to treat acute promyelocytic leukemia (APL) that has a certain chromosome abnormality or a fusion gene called PML/RAR-alpha. It is used with tretinoin in some newly diagnosed patients. It is used alone in patients whose cancer has not gotten better or has come back after treatment with other anticancer drugs. Arsenic trioxide is also being studied in the treatment of other types of cancer. Arsenic trioxide damages a fusion protein made by the PML/RAR-alpha fusion gene. This may help keep cancer cells from growing and may kill them. Also called Trisenox.