Abstract: Novel cytotoxic agent useful against malignant tumors. It provides a stable composition of matter based on the cytotoxic activity of two synergistically acting toxins which sequence are herein described. The basic amphipathic peptide binds to the cell membrane causing perturbation of the lipid bilayer. The non-covalent heterodimer complex dissociates and the phospholiphase A.sub.2 subunit (B) binds to the cell membrane. Subunit A acts as a chaperon preventing non-specific binding of the phospholipase A.sub.2, it has no enzymatic activity. The basic amphipathic peptide increases the effect of the phospholipase A.sub.2 subunit. Cell death is caused by the enzymatic hydrolysis of cell membrance phospholipids. Both the non-covalent heterodimer complex and the basic amphipathic peptide used were purified from the venoms of Crotalus durissus terrificus and Naja naja atra, respectively.

Abstract: The present invention provides a stable composition of matter based on the cytotoxic activity of a basic phospholipase A.sub.2 of molecular weight 14,500 and isoelectric point 9.6-9.7 (crotoxin B) isolated from the venom of Crotalus durissus terrificus which in complex with a specific, non-enzymatic, peptide of molecular weight 9,500 and isoelectric point 3.5-3.7 (crotoxin A) displays a preferential cytotoxic activity against various types of tumor cells. When administered parenterally in an acceptable vehicle and in pharmacologically efficient amounts to animals and humans the complex is useful in the treatment of malignant tumors in advanced stages. The method for purification of the active components, the preparation in a pharmacologically acceptable form, and the method of therapeutic use of the present composition of matter are also disclosed.