Allopurinol and its active metabolite, oxypurinol, inhibits the enzyme xanthine oxidase, blocking the conversion of the oxypurines hypoxanthine and xanthine to uric acid. Elevated concentrations of oxypurine and oxypurine inhibition of xanthine oxidase through negative feedback results in a decrease in the concentrations of uric acid in the serum and urine. Allopurinol also facilitates the incorporation of hypoxanthine and xanthine into DNA and RNA, leading to a feedback inhibition of de novo purin synthesis and a decrease in serum uric acid concentrations as a result of an increase in nucleotide concentration.
Allopurinol, a structural analog of the natural purine base hypoxanthine, is used to prevent gout and renal calculi due to either uric acid or calcium oxalate and to treat uric acid nephropathy, hyperuricemia, and some solid tumors.
LD50=214 mg/kg (in mice)