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The structure of arsphenamine was believed to be A until 2005, when new research suggested the true structure was in fact a mixture of the trimer B and the pentamer C

Arsphenamine is a drug that was used to treat syphilis and trypanosomiasis. It was the first modern chemotherapeutic agent. Sahachiro Hata discovered the anti-syphilitic activity of this compound in 1908 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 anti-microbial activity. 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 marketed under the trade name Salvarsan in 1910. It was also called 606, because it was the 606th compound synthesized for testing. Salvarsan was the first organic anti-syphillitic, and a great improvement over the inorganic mercury compounds that had been used previously. A more soluble (but slightly less effective) arsenical compound, Neosalvarsan, (neoarsphenamine), 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.

The bacterium that causes syphilis is a spirochete, Treponema pallidum. Arsphenamine is not toxic to spirochetes until it has been converted to an active form by the body, so the discovery of this drug could not have been made without Hata's animal testing. After leaving Erlich's laboratory, Hata continued parallel investigation of the new medicine in Japan.^[1]

The structure was believed to be:

However, in 2005, researchers determined Arsphenamine's structure to be a compound of a cyclic trimer and a pentamer[1].

1. ^ Izumi, Yoshio; and Isozumi, Kazuo. (2001). Modern Japanese medical history and the European influence. Keio Journal of Medicine 50 (2), 91-99. PMID 11450598.

1. Nicholas C. Lloyd, Hugh W. Morgan, Prof., Brian K. Nicholson, Prof. *, Ron S. Ronimus, Dr. (2005). The Composition of Ehrlich's Salvarsan: Resolution of a Century-Old Debate Angewandte Chemie 117 (6), 963-966. PMID 15624113.
2. Yarnell, A. (2005) Salvarsan 'Chemical and Engineering News' 83(25).

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