Sustained reductions in oxipurinol renal clearance during a restricted diet

Allopurinol transport in human erythrocytes

The mechanism of allopurinol [4-hydroxypyrazolo(3,4-d)pyrimidine] transport into human erythrocytes was investigated with an inhibitor stop assay. Allopurinol transport could be resolved into two components: (1) a saturable system and (2) a non-saturable process, which most likely represents non-facilitated diffusion. Allopurinol transport had a Km of 268 mumol/L and a Vmax of 28 pmol/microL intracellular volume/sec; the non-saturable component was 0.0195/sec. Mutual inhibition studies showed that the competitive Ki values of hypoxanthine and adenine on allopurinol transport were 120 and 3 mumol/L, respectively. These Ki values as well as the IC50 values of 100-150 mumol/L for hypoxanthine and 3-10 mumol/L for adenine were similar to the corresponding transport Km values of these bases, which are 128 and 8 mumol/L, respectively. The Ki of allopurinol on hypoxanthine transport was 274 mumol/L and thus nearly identical to its Km. Thus in erythrocytes the uricostatic agent allopurinol is an alternative substrate for the purine transport system, but lacks the exceptional high affinity it has for xanthine oxidase. This could explain the paradoxical clinical side effect of allopurinol, namely that it can provoke an attack of gout. Theophylline, a methylated purine, inhibited allopurinol transport with an IC50 of 200-400 mumol/L. Oxypurinol [4,6-dihydroxypyrazolo(3,4-d)pyrimidine], the main metabolite of allopurinol, also inhibited allopurinol transport with an IC50 of 20-40 mumol/L. This is noteworthy, since allopurinol and oxypurinol do not share the same transport system in the kidney.

Absorption and disposition kinetics of chlorothiazide in protein-calorie malnutrition

The influence of dietary protein deficiency on the absorption and disposition kinetics of chlorothiazide was investigated in male Sprague-Dawley rats fed for 4 weeks on a 23 per cent (control) or a 5 per cent (low) protein diet ad libitum. Chlorothiazide in plasma and urine was determined by a sensitive and specific HPLC assay. Following an intravenous dose of 10 mg kg-1 chlorothiazide, there was a significant decrease in the total plasma clearance (Cl) per kg of body weight from 1.80 +/- 0.15 to 1.29 +/- 0.15 l h-1 kg-1 and apparent steady-state volume of distribution from 0.65 +/- 0.13 to 0.38 +/- 0.07 l kg-1 in the protein-deficient rats. However, no significant difference was found in the two groups of animals with respect to mean residence time (MRT) and free fraction of drug in plasma. The mean harmonic half-life was increased from 72 to 91 min in the protein-deficient rats. The urinary recovery of unchanged chlorothiazide in 48 h was essentially complete in both groups of animals. The absorption of chlorothiazide, as assessed by the mean urinary recovery of unchanged drug after oral administration, was 66 per cent and 68 per cent in normal and protein-deficient rats, respectively.

Effect of dietary protein on renal tubular clearance of drugs in humans

Diet is one of many factors that influence the pharmacokinetics of drugs. The level of protein intake has been found to significantly influence drug metabolism and glomerular filtration, both of which play an important role in the clearance of drugs. Recently, a marked change, resulting from restricted dietary protein intake, has been reported in the handling of several drugs which are reabsorbed and/or secreted by the renal tubules. In studies of healthy volunteers on protein-restricted diets the renal clearance and fractional excretion of model compounds have been altered, falling to 30% of values obtained on normal diets in the case of the weak acids oxipurinol and uric acid; the fractional excretion of the weak base cimetidine has been increased by 30%. These studies have also found that the change in the renal clearance of both acids is sustained with prolonged dietary protein-calorie restriction, and that, for oxipurinol, the magnitude of the change is directly related to the quantity of protein in the diet, the change is related specifically to the protein content in the diet (and not the total calories), the onset of change is rapid, and on a low-protein diet the renal clearance undergoes marked diurnal variation. The mechanism for the alteration in tubular function is not clear, but may be related to renal haemodynamic changes or competition for transport associated with protein intake. Regardless of the mechanism, these results have important implications for pharmacokinetic research and clinical practice.

Reduced renal clearance of oxypurinol during a 400 calorie protein-free diet

A decrease in dietary protein intake lowers the clearance of a number of substances excreted principally by the kidney including uric acid and oxypurinol, the major metabolite of allopurinol. We studied the kinetics of uric acid and oxypurinol in seven healthy volunteers on a normal protein diet (2600 calories; 100 g protein) followed by a 400 calorie, protein-free diet. A 600 mg dose of allopurinol was given orally after 6 days of the normal protein diet and again after 2 days of the 400 calorie, protein-free diet. Two major findings emerged: first, the renal clearance of oxypurinol was reduced from 21.2 +/- 1.9 ml/min during the normal protein diet to 12.3 +/- 1.2 ml/min (P less than .05) during the 400 calorie, protein-free diet, and second, there was a striking diurnal difference in oxypurinol renal clearance with a 41% decrease in the oxypurinol clearance at night (8 PM to 8 AM) versus day (8 AM to 8 PM) on the 400 calorie, protein-free diet.

The influence of nutrition on the systemic availability of drugs. Part II: Drug metabolism and renal excretion

Plasma concentrations of an active compound vary between individuals and within a subject, even if the same drug dosage is used. These differences are related to variations of drug absorption from the gastrointestinal tract, of presystemic drug metabolism in the intestine or the liver, and of drug elimination from the systemic circulation, for instance by hepatic metabolism or renal excretion. All of these processes can be modified by nutrition. However it is necessary to emphasize the significance of the pharmaceutical formulation for variations of both drug absorption and elimination. The role of nutrition should receive adequate attention during therapeutic use of drugs as well as in drug disposition studies. Recent guidelines for the assessment of the systemic availability of drugs therefore do recommend nutrition to be controlled.