The present invention relates to treating malignancies such as tumors or cancers by orally administering lyophilized compositions comprising arsenic to a subject in such need. Malignancies include various hematological malignancies, such as acute myeloid leukemia (AML) including acute promyelocytic leukemia (APL), myelodysplastic syndrome (MDS), multiple myeloma (MM) and lymphomas and solid tumors including glioblastoma multiforme and breast cancer. Arsenic treatment has shown great promise in the treatment of several cancers but requires daily intravenous (IV) administration. This invention relates to a novel formulation comprising a lyophilized compositions comprising arsenic. As a result, the formulation facilitates a systemic bioavailability comparable to that of intravenous (IV) administration of arsenic trioxide currently practiced. The present invention also relates to a method for lyophilizing the arsenic trioxide, preparing the oral formulation comprising lyophilized compositions comprising arsenic, and a method for treating a subject with malignancies using the oral formulation.