Thiomersal

Thiomersal (INN), or thimerosal (USAN, JAN), also sold under the name merthiolate, is an organomercury compound. It is a well-established antiseptic and antifungal agent.

It has been used as a preservative in vaccines, immunoglobulin preparations, skin test antigens, antivenins, ophthalmic and nasal products, and tattoo inks. Despite the scientific consensus that fears about its safety are unsubstantiated, its use as a vaccine preservative has been exploited by anti-vaccination groups.

Morris Kharasch, a chemist then at the University of Maryland, filed a patent application for thiomersal in 1927; Eli Lilly later marketed the compound under the trade name Merthiolate. In vitro tests conducted by Lilly investigators H. M. Powell and W. A. Jamieson found that it was forty to fifty times as effective as phenol against Staphylococcus aureus. It was used to kill bacteria and prevent contamination in antiseptic ointments, creams, jellies, and sprays used by consumers and in hospitals, including nasal sprays, eye drops, contact lens solutions, immunoglobulins, and vaccines. Thiomersal was used as a preservative (bactericide) so that multidose vials of vaccines could be used instead of single-dose vials, which are more expensive. By 1938, Lilly's assistant director of research listed thiomersal as one of the five most important drugs ever developed by the company.

One of the earliest synthetic methods was a reaction between ethylmercury chloride and thiosalicylic acid in alkaline solution, with subsequent acidification. Various minor modifications of this are possible, but the modern synthesis is essentially the same.

Thiomersal features mercury(II) with a coordination number 2, i.e. two ligands are attached to Hg, the thiolate and the ethyl group. The carboxylate group confers solubility in water. Like other two-coordinate Hg(II) compounds, the coordination geometry of Hg is linear, with a 180° S-Hg-C angle. Typically, organomercury thiolate compounds are prepared from organomercury chlorides.

Thiomersal's main use is as an antiseptic and antifungal agent, due to its oligodynamic effect. In multidose injectable drug delivery systems, it prevents serious adverse effects such as the Staphylococcus infection that, in one 1928 incident, killed 12 of 21 children vaccinated with a diphtheria vaccine that lacked a preservative. Unlike other preservatives at the time, such as phenol and cresol, thiomersal does not reduce the potency of the vaccines that it protects. Bacteriostatics such as thiomersal are not needed in single-dose injectables.

In the United States, the European Union, and a few other affluent countries, thiomersal is no longer used as a preservative in routine childhood vaccination schedules. In the U.S., all vaccines routinely recommended for children 6 years of age and younger are available in formulations that do not contain thiomersal. Two vaccines (a TD and the single-dose version of the trivalent influenza vaccine Fluvirin) may contain a trace of thiomersal from steps in manufacture, at less than 1 microgram of mercury per dose. The multi-dose versions of some trivalent and quadrivalent influenza vaccines can contain up to 25 micrograms of mercury per dose from thiomersal. Also, four rarely used treatments for pit viper, coral snake, and black widow venom contain thiomersal.

Outside North America and Europe, many vaccines contain thiomersal; the World Health Organization reported no evidence of toxicity from thiomersal in vaccines and no reason on safety grounds to change to more expensive single-dose administration. The United Nations Environment Program backed away from an earlier proposal of putting thiomersal on the list of banned vaccine compounds as part of its campaign to reduce mercury exposure. It stated that eliminating it in multi-dose vaccines, primarily used in developing countries, would lead to high cost and a refrigeration requirement that developing countries could ill afford. At the Minamata Convention on Mercury in 2013, thiomersal was excluded from the treaty.