Arsphenamine

Arsphenamine, also known under its trade name Salvarsan, is a drug that was used to treat syphilis. It was the first modern chemotheraputic agent. Paul Ehrlich, in his search for a magic bullet that would attack a disease causing agent, and leave human tissue untouched, found this drug (the 606th he searched) in 1909. The structure is:

It was the first important antisyphillitic, though was phased out in the 1930s by better arsenical compounds, and eventually altogether by penicillin.