“This is consistent with the … that the intracellular concentrations of TPP and TPPR-MP are sufficient … inhibit the conversion of allopurinol to allopurinol ribonucleotide (HPPR-MP) by the hypoxanthine-guanine phosphoribosyltransferase by 30 per cent and the amination of HPPR-MP by adenylosuccinate synthetase by 50 per cent respectively.”