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Arsphenamine, also known as Salvarsan and compound 606, is a drug that was introduced at the beginning of the 1910s as the first effective treatment for syphilis, and was also used to treat trypanosomiasis.  It is an organoarsenic molecule, and was the first modern chemotherapeutic agent.
Sahachiro Hata and Paul Ehrlich discovered the antisyphilitic activity of this compound in 1909 in the laboratory of Paul Ehrlich, during a survey of hundreds of newly synthesized organic arsenical compounds. Ehrlich had theorized that by screening many compounds, a drug could be discovered with antimicrobial activity without killing the human. Ehrlich's team began their search for such a "magic bullet" among chemical derivatives of the dangerously toxic drug atoxyl. This was the first organized team effort to optimize the biological activity of a lead compound through systematic chemical modifications, the basis for nearly all modern pharmaceutical research.
Arsphenamine was originally called "606" because it was the sixth in the sixth group of compounds synthesized for testing; it was marketed by Hoechst AG under the trade name Salvarsan in 1910. Salvarsan was the first organic antisyphilitic, and a great improvement over the inorganic mercury compounds that had been used previously. It was distributed as a yellow, crystalline, hygroscopic powder that was highly unstable in air. This significantly complicated administration, as the drug had to be dissolved in several hundred milliliters of distilled, sterile water with minimal exposure to air to produce a solution suitable for injection. Some of the side effects[which?] attributed to Salvarsan were thought to be caused by improper handling and administration, causing Ehrlich, who worked assiduously to standardize practices, to observe, "the step from the laboratory to the patient's bedside ... is extraordinarily arduous and fraught with danger." 
Ehrlich's laboratory developed a more soluble (but slightly less effective) arsenical compound, Neosalvarsan (neoarsphenamine), which was easier to prepare, and it became available in 1912. These arsenical compounds came with considerable risk of side effects, and they were supplanted as treatments for syphilis in the 1940s by penicillin.
From Salvarsan's discovery until recently, it was believed that the structure featured an As=As double bond. However, in 2005, an extensive mass spectral analysis showed the actual structure is most likely to be a mixture of the cyclic trimer and a pentamer. The revised structure features As-As single bonds, not double bonds.