Orotidine accumulation in human erythrocytes during allopurinol therapy: association with high urinary oxypurinol-7-riboside concentrations in renal failure and in the Lesch-Nyhan syndrome

1. A compound identified as orotidine has been found in the erythrocytes of all subjects on allopurinol. 2. The erythrocyte orotidine concentrations were much higher in patients with renal failure or with the Lesch-Nyhan syndrome. 3. In addition, increased amounts of oxypurinol-7-riboside were excreted in the urine by both of these groups compared with control subjects or with patients with normal renal function on allopurinol. 4. A good correlation was found between urinary oxypurinol-7-riboside excretion and erythrocyte orotidine concentrations. 5. Increased erythrocyte levels of the pyrimidine-sugar UDP-glucose were also found in patients with the highest orotidine levels. 6. The combined results suggest a derangement of pyrimidine nucleotide metabolism during allopurinol therapy. We propose that erythrocyte orotidine formation results primarily from inhibition of orotidine-5'-monophosphate decarboxylase by oxypurinol-7-ribotide.

Inhibition of xanthine oxidase by 4-hydroxy-6-mercaptopyrazolo[3,4-d]pyrimidine

Compound B103U, 4-hydroxy-6-mercaptopyrazolo[3,4-d]pyrimidine, was investigated as an inhibitor of human xanthine oxidase. Studies in vitro demonstrated that it was significantly more potent than oxypurinol, 4,6-dihydroxypyrazolo[3,4-d]pyrimidine. It formed an initial complex with electron-rich (reduced) human xanthine oxidase that was tighter than the corresponding complex formed by oxypurinol. The initial complexes with each inhibitor and reduced enzyme were internally rearranged into more stable complexes with first-order rate constants of 2.5 to 3 per min. However, the half-life of the isomerized (stable) complex with B103U was three to four times longer than the half-life of the analogous complex with oxypurinol. This stability was previously noted by Massey et al. (J. Biol Chem 254: 2837-2844, 1970) with B103U and bovine xanthine oxidase. The overall Ki values accounting for the initial and isomerized complexes were 5 nM for B103U and 100 nM for oxypurinol. B103U was also more potent as an inhibitor of bovine xanthine oxidase-catalyzed generation of superoxide radicals. Studies in mice revealed that the relative in vitro potency of B103U was not sustained in vivo. Compared to the inhibition of xanthine oxidase by oxypurinol, inhibition by B103U was neither more potent nor longer lasting. This shortcoming was not caused by weaker inhibition of mouse xanthine oxidase. Instead, it was the result of poor bioavailability. Plasma levels of available B103U rapidly decreased from samples of mouse and human blood because of reversible binding to serum proteins. B103U was also susceptible to oxidation. Two equivalents of H2O2 stoichiometrically oxidized the 6-thiol substituent to a sulfinic acid. This oxidized product was three orders of magnitude weaker as an inhibitor of xanthine oxidase than was B103U.

Pharmacokinetic parameters of 14C oxithiopurinol in rat

The study with labelled compound has shown that in rat, as in man, thiopurinol is primarily and rapidly converted into oxithiopurinol. More than 80% of the radioactivity of plasma was identified as oxithiopurinol less than 10 minutes after intravenous or oral administration. The parameters of oxithiopurinol were estimated from the decline of radioactivity in blood from 5 minutes to 8 hours after intravenous administration of 4 microCi/100 g of thiopurinol (8 animals) or oxithiopurinol (5 animals). Half lives of oxithiopurinol t 1/2 (beta) were 2.52 +/- 0.78 hr and 2.90 +/- 1.00 hr and clearance 2.00 +/- 0.36 ml/min. and 3.38 +/- 0.69 respectively. Estimate of renal clearance for oxithiopurinol in rat was 4,17 ml/min. The rapidity of hydroxylation and of elimination of oxithiopurinol might be assumed to be the cause of the apparent ineffectiveness of thiopurinol in vivo on xanthine oxidase activity.

[Inactivation by 4,6 disubstituted pyrazolo (3,4-d) pyrimidines of the arsenolysis of guanosine by erythrocyte purine nucleoside phosphorylase]

New drugs are proposed in an attempt to reduce the overformation of uric acid in man. The procedure is based on the inhibition of human purine nucleoside phosphorylase selectively investigated in its phosphorolytic sequence. But for technical reasons, analogous inactivation of the enzyme in its comparative guanosine arsenolysis is subject to experimentation.