Allopurinol

Allopurinol is best known as the “gout medicine”. By inhibiting the xanthine oxidase enzyme, it decreases high concentrations of uric acid in the blood. It is used to prevent or treat gout, uric acid–based kidney stones, and (formerly) chemotherapiy-induced uric acid formation in the blood.

Roland K. Robins at New Mexico Highlands University (Las Vegas) and P. Schmidt and J. Druey at CIBA (Basel, Switzerland) independently reported allopurinol syntheses in 1956. At the time it was known that purine derivatives acted as antagonists of some biological processes. Robins believed that derivatives of purine’s isomer pyrazolo [3,4-d]pyrimidine also might be biologically active. He synthesized 15 compounds, including allopurinol. He was looking specifically for new antitumor agents.

Schmidt and Druey also prepared allopurinol and other pyrazolo [3,4-d]pyrimidines in search of new pharmaceuticals for CIBA. In subsequent years, numerous investigators established allopurinol’s medicinal properties. The Food and Drug Administration approved it for use in the United States in 1966.

Allopurinol also has theoretical interest. In 1996, Begoña Hernández, Francisco J. Luque, and Modesto Orozco at the University of Barcelona used computational methods to evaluate the relative stabilities of tautomers of allopurinol and its isomer hypoxanthine.

To avoid having to use a large number of expensive calculations (remember, this was 1996), the authors first used a low-level method to identify the seven most stable forms of both isomers. They then used ab initio quantum mechanical methods to determine the most stable tautomer of each molecule in the gas phase and in aqueous solution.

The tautomer shown here for allopurinol and the equivalent structure of hypoxanthine were the most stable in both media. But the relative stabilities of the other tautomers differed between the two phases. The authors believed that their work would help explain the biological activites of these molecules.