The present invention relates to a pharmaceutical composition comprising an organic arsenic compound having the structure wherein R 1 and R 2 are independently alkyls with 1-10 carbon atoms; X is S or Se; R 3 is -COOH, -CH 2 -COOH, -CH 2 -CH 2 -COOH, -CH(CH 3 )-COOH, -CH(CH 2 -CH 3 )-COOH, or -CH 2 -CH 2 -CH 2 -COOH; n is 0 or 1; R 4 is -OH, -H, -CH 3 , or a glutamine substituent; or R 3 and R 4 , together with C 1 and C 2 , form an unsubstituted or substituted aromatic ring; and R 5 is -OH or a glycine substituent; or a pharmaceutically acceptable salt thereof.