Inter-individual differences in the glycine conjugation of salicylic acid [proceedings]

In vivo effects of naproxen, salicylic acid, and valproic acid on the pharmacokinetics of trichloroethylene and metabolites in rats

It was recently demonstrated that some drugs modulate in vitro metabolism of trichloroethylene (TCE) in humans and rats. The objective was to assess in vivo interactions between TCE and three drugs: naproxen (NA), valproic acid (VA), and salicylic acid (SA). Animals were exposed to TCE by inhalation (50 ppm for 6 h) and administered a bolus dose of drug by gavage, equivalent to 10-fold greater than the recommended daily dose. Samples of blood, urine, and collected tissues were analyzed by headspace gas chromatography coupled to an electron capture detector for TCE and metabolites (trichloroethanol [TCOH] and trichloroacetate [TCA]) levels. Coexposure to NA and TCE significantly increased (up to 50%) total and free TCOH (TCOHtotal and TCOHfree, respectively) in blood. This modulation may be explained by an inhibition of glucuronidation. VA significantly elevated TCE levels in blood (up to 50%) with a marked effect on TCOHtotal excretion in urine but not in blood. In contrast, SA produced an increase in TCOHtotal levels in blood at 30, 60, and 90 min and urine after coexposure. Data confirm in vitro observations that NA, VA, and SA affect in vivo TCE kinetics. Future efforts need to be directed to evaluate whether populations chronically medicated with the considered drugs display greater health risks related to TCE exposure.

Toxicological implications of polymorphic drug metabolism

The occurrence of genetic polymorphisms of drug metabolism means that populations contain subgroups (phenotypes) that differ sharply in their abilities to effect a number of metabolic reactions. Because of this, major interphenotype differences occur in responsiveness to drugs and toxic substances. The well established genetic polymorphisms of acetylation and hydrolysis illustrate the important association that exists between phenotype and propensity to develop toxic and exaggerated responses to some substances. Recently, for metabolic oxidation, a new genetic polymorphism of drug metabolism has been described and it promises to provide a better understanding of inter-individual variability in the metabolic handling of, and responsiveness to, drugs and toxic substances. The following effects of the polymorphism are described here: (a) its influence in determining variable presystemic metabolism and hence systemic drug availability; (b) its role in determining alternative toxic pathways of metabolism in individuals who have a genetically determined impairment of oxidative capacity and (c) its influence on the development of agranulocytosis associated with metiamide administration.

UGT1A6 polymorphism and salicylic acid glucuronidation following aspirin

OBJECTIVES

In vivo, aspirin (acetylsalicylic acid) is rapidly deacetylated to form salicylic acid, which then undergoes primary or secondary glucuronidation catalyzed by UDP-glucuronosyltransferases (UGTs). The variant UGT1A6*2 (T181A, R184S) is associated with altered enzyme function. Our objective was to compare salicylic acid glucuronidation in individuals with different UGT1A6 genotypes.

METHODS

Following orally dosing with 650 mg aspirin, saliva and urine samples were collected over a period of 24 h from healthy individuals with homozygous wild-type UGT1A6 *1/*1 (n=19) and homozygous variant UGT1A6 *2/*2 (T181A, R184S) (n=9) genotypes.

RESULTS

No statistically significant differences were observed in salivary pharmacokinetic parameters. Urinary excretion of the sum of aspirin and its metabolites (salicyluric acid, salicyluric acid phenolic glucuronide, salicyl phenolic glucuronide, salicyl acyl glucuronide, salicylic acid) during the early period of 2-4 h of collection was significantly lower in UGT1A6 *1/*1 than in UGT1A6 *2/*2 individuals. Further, UGT1A6 *1/*1 individuals excreted a lower percentage of aspirin and its metabolites in the first 12 h and a greater percentage after 12 h than UGT1A6 *2/*2 individuals.

CONCLUSIONS

The variant UGT1A6*2 or polymorphisms in other UGTs that are in linkage disequilibrium with UGT1A6*2 may confer more rapid glucuronidation of salicylic acid than the wild-type UGT1A6 *1/*1.

Effect of castration on acetyl salicylic acid metabolism in rabbits

OBJECTIVES

The biotransformation of acetyl salicylic acid (ASA) differs within species, and gender differences have been documented and attributed to the effect of sex hormones. Castration remains a standard therapy for men with advanced prostate cancer. We studied the effect of castration on the metabolism of ASA in rabbits to find out whether the metabolism of ASA is adversely affected after castration.

METHODS

ASA in doses of 12.5, 25, and 50 mg/kg body weight was given intravenously to male and female prepubertal and adult rabbits, castrated adult male rabbits, and castrated male rabbits given testosterone (3 animals per group). Blood samples were collected at 0, 10, 30, 60, 120, and 180 minutes. The high-performance liquid chromatography method was used for the quantitation of salicylic acid (SA) in serum. The percentage of SA not metabolized was determined by comparing the serum level at 10 and 180 minutes for each group.

RESULTS

At a dose of 50 mg/kg in the adult rabbits, the mean +/- SD of SA in serum at 10 and 180 minutes was 146.54 +/- 29.54 microg/mL and 19.12 +/- 5.93 microg/mL for males, 158.25 +/- 6.70 microg/mL and 33.24 +/- 2.78 microg/mL for females, 229.72 +/- 47.85 microg/mL and 44.33 +/- 5.64 microg/mL for castrated male rabbits, and 170.88 +/- 12.03 microg/mL and 68.1 +/- 37.54 microg/mL for castrated male rabbits given testosterone, respectively. Also, at 180 minutes, the percentage of SA not metabolized in adult male rabbits was 12.82% +/- 1.65% compared with 21.04% +/- 2.14% (P <0.01) in adult females, 19.53% +/- 1.73% (P <0.01) in castrated adult male rabbits, and 38.95% +/- 19.48% (P <0.001) in castrated male rabbits given testosterone. At all doses of ASA, the serum SA concentration in male and female prepubertal rabbits was not significantly different for each time point.

CONCLUSIONS

These results indicate that male rabbits are able to metabolize ASA faster than are females. After castration, this ability is significantly decreased. If these experimental results are confirmed in humans, men who are undergoing hormonal manipulation for advanced prostate cancer and who require high-dose ASA, such as in the treatment of stroke or rheumatoid arthritis or as an antioxidant, may need lower doses to reduce the possible toxic effects of ASA.

Depletion of plasma glycine and effect of glycine by mouth on salicylate metabolism during aspirin overdose

1. The metabolism of aspirin was investigated in 45 patients who had taken self-administered overdose of aspirin and were treated with fluids only, glycine, N-glycylglycine by mouth, or by sodium bicarbonate i.v. 2. The major metabolite recovered in the urine of patients treated with oral fluids, glycine or N-glycylglycine was salicyluric acid, which accounted for means of 51%, 47% and 38% of the total, respectively; salicylic acid comprised 19%, 29% and 29%. In contrast, salicylic acid (42%) was the major urinary metabolite recovered from patients treated with sodium bicarbonate. 3. Plasma glycine concentrations in healthy volunteers who had taken no aspirin remained constant through the day and were not affected by a therapeutic dose (500 mg) of aspirin. Plasma glycine was consistently lower in patients with aspirin overdose than in these healthy volunteers, suggesting depletion of available glycine. 4. Orally administered glycine and N-glycylglycine increased plasma glycine. While the fraction of total salicylate recovered as salicyluric acid was not altered, the maximum rate of excretion of salicyluric acid was higher in patients who received glycine than in the control group; there was no significant difference in the maximum rate of excretion of salicyluric acid between the group that received glycine and the group that received N-glycylglycine. 5. The data suggest that exogenous glycine increases the rate of formation of salicyluric acid in salicylate overdose.

Metabolism of aspirin after therapeutic and toxic doses

1 The urinary recovery of metabolites of aspirin (ASA) was studied in 45 volunteers who took a therapeutic dose (600 mg) of ASA by mouth and in 37 patients who took ASA in overdose. 2 The main metabolite recovered from the volunteers was the glycine conjugate, salicyluric acid (SUA), which accounted for 75.01 +/- 1.19% of total urinary metabolites, whereas salicylic acid (SA) accounted for 8.82 +/- 0.56%. Recovery of SUA was negatively correlated with that of SA (r = -0.8625, P less than 0.001). 3. In 24 patients with admission plasma salicylate concentrations of 240-360 mg l-1, SUA accounted for 46.66 +/- 3.22% and SA for 31.88 +/- 4.02%. 4. In 13 patients with admission plasma salicylate concentrations of 715-870 mg l-1, SUA accounted for 21.57 +/- 3.65% and SA for 64.72 +/- 4.82%. 5. Reduced excretion of salicylate as SUA was also accompanied by increased elimination as gentisic acid and salicylic acid phenolic glucuronide indicating that the unsaturated processes that lead to the formation of these metabolites contribute significantly (22-23%) to the inactivation of large doses of salicylate. 6. While the Michalis-Menten kinetics of ASA have been well demonstrated at lower doses, our findings illustrate the progressive saturation of SUA formation under conditions of increasing ASA load to toxic amounts and raise issues about the in-vivo glycine pool when ASA is taken in overdose.

The metabolism of aspirin in man: a population study

The metabolism of a 900 mg oral dose of aspirin has been investigated in 129 healthy volunteers. For this purpose, the 0-12 h urine was collected and analysed for the following excretion products: salicylic acid, its acyl and phenolic glucuronides, salicyluric acid, its phenolic glucuronide and gentisic acid. The total excretion of salicylate and metabolites was normally distributed within the population group studied, showing a 2.5-fold variation: a mean of 68.1% of the dose was recovered in 12 h. The excretion of salicylic acid was found to be highly variable within the study panel (1.3-31% of dose in 12 h), and was related to both urine volume and pH. Salicyluric acid was the major metabolite in the majority of the volunteers and its excretion was normally distributed amongst the study panel. The elimination of this metabolite ranged from 19.8 to 65% of the dose and was related to the total recovery of salicylate. The excretion of the two salicyl glucuronides was highly variable, ranging from 0.8 to 42% of the dose. The elimination of the glucuronides was inversely related to that of salicyluric acid. Gentisic acid and salicyluric acid phenolic glucuronide were minor metabolites of salicylate, accounting for 1 and 3% of the dose, respectively. The recovery of gentisic acid was statistically significantly greater in female subjects than in males, whilst the opposite was found for salicyluric acid and total salicylate. However, these differences were small in magnitude.

Sex differences in salicylic acid metabolism in Nigerian subjects

The urinary metabolites of a single dose (1 g) of salicylic acid were investigated in black Nigerian subjects (78 females 44 males). Qualitatively, the major metabolites were the glycine and glucuronic acid conjugates. Quantitatively, there was a statistically significant difference in the level of these metabolites between female and male subjects (P less than 0.001) (using Student's t-test). The results of the present study compared with earlier published data show a statistically significant quantitative difference between black Nigerians and Caucasians (P less than 0.001). The results suggest possible racial and sex differences in the metabolism of salicylic acid.

The effects of age and sex on the disposition of acetylsalicylic acid and its metabolites

The disposition of a low dose (600 mg) of acetylsalicylic acid (ASA) and its metabolites (salicylate, salicyluric acid and salicyl glucuronides) was studied in 25 male and female patients of different ages. Plasma levels of ASA and salicylate were found to be significantly higher in the females (young and elderly), whereas plasma levels of salicyluric acid were found to be significantly higher in the elderly (male and female) groups. The higher plasma levels of ASA and salicylate in the females appear to be due to an intrinsically lower metabolic activity in that sex, while the lower clearance of salicyluric acid leads to the accumulation of that compound in the aged. No age and sex effects were found to influence the volumes of distribution of ASA, salicylate and salicyluric acid.

The metabolism of [carboxyl-14C]aspirin in man

1. [carboxyl-14C]Aspirin has been orally administered to four male volunteers and the urinary metabolites examined by paper chromatography, t.l.c., h.p.l.c. and mass spectrometry. 2. 14C Radioactivity was eliminated rapidly in the urine, 94 to 98% of the dose in the first 24 h and approx. 1% in 24-48 h. 3. The major urinary metabolite was salicyluric acid (56-68% dose). In addition, free salicylic and gentisic acids were also detected as were both the acyl and phenolic glucuronides of salicylate. 4. A phenolic glucuronide of salicyluric acid has also been identified. The importance of this metabolite in relation to analytical methods for salicylphenolic glucuronide determination is discussed. 5. The presence of other di- and tri-hydroxybenzoic acids or gentisuric acid could not be demonstrated.