New Xanthine oxidase inhibitors are in development and these need to be evaluated reliably before being introduced into clinical investigations. The presence of valid, easy and reliable in vivo bioassy to evaluate the Xanthine oxidase inhibition is of utmost importance to achieve steady paces in drug discovery attempts to treat gout disease. The objective of the current study was to develop and validate a bioassay procedure that can be employed for in vivo evaluation of drugs investigated as xanthine oxidase inhibitors. Exploiting rats as an animal model to carry out the bioassay is proposed to establish a novel approach to screen and evaluate the xanthine oxidase inhibitory activity of potential agents and/or plant extracts. Allopurinol was used as a reference treatment to inhibit Xanthine oxidase while the degree of enzyme inhibition was indirectly measured by determining the blood levels of 6-mercaptopurine (6-MP) as a surrogate chemical marker. In this model, serum obtained from: untreated rats, rats treated with 6-MP alone and rats treated with 6-MP and different doses of allopurinol were analyzed, for levels of 6-mercaptopurine. An elevated plasma 6-mercaptopurine level in the allopurinol treated rats as compared to untreated rats was an indication of an in vivo inhibition of the enzyme Xanthine oxidase.

Key words: Xanthine oxidase inhibitors, 6-Mercaptopurine, allopurinol, gout, bioassay, animal model.