Paracetamol metabolism in African villagers

Inter-ethnic differences in pharmacokinetics-is there more that unites than divides?

Inter-ethnic variability in pharmacokinetics (PK) has been attributed to several factors ranging from genetic to environmental. It is not clear how current teaching in higher education (HE) reflects what published literature suggests on this subject. This study aims to gain insights into current knowledge about inter-ethnic differences in PK based on reports from published literature and current teaching practices in HE. A systematic literature search was conducted on PubMed and Scopus to identify suitable literature to be reviewed. Insights into inter-ethnic differences in PK teaching among educators in HE and industry were determined using a questionnaire. Thirty-one percent of the studies reviewed reported inter-ethnic differences in PK, of these, 37% of authors suggested genetic polymorphism as possible explanation for the inter-ethnic differences observed. Other factors authors proposed included diet and weight differences between ethnicities. Most respondents (80%) who taught inter-ethnic difference in PK attributed inter-ethnic differences to genetic polymorphism. While genetic polymorphism is one source of variability in PK, the teaching of genetic polymorphism is better associated with interindividual variabilities rather than inter-ethnic differences in PK as there are no genes with PK implications specific to any one ethnic group. Nongenetic factors such as diet, weight, and environmental factors, should be highlighted as potential sources of interindividual variation in the PK of drugs.

Paracetamol metabolism and related genetic differences

Paracetamol (acetaminophen) is a worldwide used analgesic and antipyretic drug. It is metabolised via several metabolic pathways, including glucuronidation, sulfation, oxidation, hydroxylation, and deacetylation: Hepatic and other organ damage may occur, especially in overdose, because of the accumulation of a toxic metabolite. Intersubject and ethnic differences have been reported in paracetamol metabolism activation, suggesting possible differences in susceptibility to toxicity and in pain alleviation, linked to different pharmacogenetic profiles. This article aims at reviewing, in the literature, the links between paracetamol metabolism and enzyme genotypes in the context of toxic side effects and efficacy of paracetamol in therapeutics.

Effects of Combined UDP-Glucuronosyltransferase (UGT) 1A1*28 and 1A6*2 on Paracetamol Pharmacokinetics in β-Thalassemia/HbE

In addition to pathophysiological changes, genetic variations can alter drug pharmacokinetics in patients with thalassemia. Numerous drugs are metabolized by UDP-glucuronosyltransferases (UGT) including paracetamol (PCM), a widely used analgesic. Co-occurrence of the UGT1A1 polymorphism (UGT1A1*28) and the UGT1A6 polymorphism (UGT1A6*2) may affect PCM glucuronidation. To elucidate the effect of these combined polymorphisms on the PCM metabolism in thalassemic patients, 15 beta-thalassemia/hemoglobin E subjects with three different UGT1A genotypes received a single oral dose of 1,000 mg PCM. Drug disposition was determined by HPLC. Patients who have UGT1A6*2 without UGT1A1*28 showed a significant, lower area under concentration-time curve (AUC(0)-->infinity) of PCM, PCM-glucuronide and PCM-sulfate than those of the patients with wild-type UGT1A1 and UGT1A6 (p < 0.05). In addition, a high elimination rate constant and clearance of PCM and its metabolites were also found in these patients (p < 0.05). Ourstudy suggests that a subtherapeutic level of PCM may occur in patients who have UGT1A6*2 without UGT1A1*28.

Differences in the single-oral-dose pharmacokinetics and urinary excretion of paracetamol and its conjugates between Hong Kong Chinese and Caucasian subjects

BACKGROUND AND OBJECTIVES

The present study was conducted to determine if ethnic differences exist for single oral dose pharmacokinetics of paracetamol and its conjugates between Hong Kong Chinese and Caucasian subjects.

METHODS

Twenty healthy Chinese (n = 11) and Caucasian (n = 9) subjects, aged 21-44 years, 11 male and nine female, were given oral paracetamol syrup 20 mg/kg, following an overnight fast. Paracetamol and its metabolites (glucuronide, sulphate, cysteine and mercapturic acid conjugates) were measured in serial plasma samples (0.25, 0.5, 0.75, 1.0, 1.5, 2, 3,...,12, 24 h) and urine collections (0-24 h) by high-performance liquid chromatography.

RESULTS

In Chinese subjects, the (mean range) peak plasma concentration of paracetamol was 23.8 mug/mL (17.9-32.3) and time to attain this peak 0.66 h (0.5-0.75). This was lower (P < 0.015) at 18.7 microg/mL (14.4-22.9) and achieved later (P < 0.033) at 1.06 h (0.5-2.0) in Caucasians. In Chinese subjects, plasma levels of glucuronide were lower, sulphate higher and cysteine conjugates significantly lower than in Caucasians (P < 0.05). Chinese subjects excreted 6% more sulphate and 5% less glucuronide. They also excreted significantly less mercapturic acid conjugates (P < 0.001).

DISCUSSION AND CONCLUSION

Chinese subjects show more rapid absorption of paracetamol, a tendency to produce less glucuronide but more sulphate conjugates and reduced production of cysteine and mercapturic acid conjugates. The latter may help to protect against hepatotoxicity following paracetamol overdose.

Paracetamol elimination in Chinese and Indians in Singapore

The 24 h urinary excretion of paracetamol and its conjugates after oral administration of about 15 mg/kg was studied in 24 Chinese and 24 Indian healthy young adult male volunteers living in Singapore. The Indians excreted a significantly lower fraction of sulphate conjugate (28.9% compared to 35.9% in the Chinese), and a correspondingly higher fraction as glucuronide conjugate (62.2% compared to 54.5% in the Chinese). There was no difference between the ethnic groups in terms of the urinary recovery of unchanged paracetamol (Indians 3.4%, Chinese 3.6%), glutathione-derived cysteine (Indians 2.3% and Chinese 2.2%) and mercapturic acid (Indians 3.6%, Chinese 3.8%) conjugates. The total fraction of administered paracetamol recovered in the urine was 84.7% and 85.7% in the Indian and the Chinese groups, respectively. Although the total glutathione derived conjugates recovered in the two ethnic groups were similar (Indians 5.9%, Chinese 6%), the values were lower than those reported previously for Caucasians in Scotland (9.3%).

Paracetamol conjugation: an interethnic and dietary study

To test whether dietary or hereditary factors affect paracetamol metabolism, two groups of Venda and a group of Caucasian medical students were investigated. The Venda groups were selected as traditionally living villagers and those who followed a Western life-style. Salivary concentrations of paracetamol and urinary amounts of the glucuronide and sulphate metabolites eliminated over 22 h were determined by HPLC. The metabolite formation rate constants and the percentage of the dose eliminated as each metabolite were calculated. No significant differences were found between the data for total Venda, rural Venda, westernized Venda and Caucasian students for the calculated metabolite parameters. Thus environmental effects showed no apparent influence on the sulphide and glucuronide conjugation of paracetamol, and no hereditary effect was evident between the Venda and Caucasians.

Inter-subject and ethnic differences in paracetamol metabolism

The 24 h urinary excretion of paracetamol and its metabolites following a single oral dose of 1.5 g was compared in 111 Caucasians (Scotland), 67 West Africans (Ghana) and 20 East Africans (Kenya). The fractional recovery of the mercapturic acid and cysteine conjugates of paracetamol was 9.3% in the Caucasians compared with only 5.2% and 4.4% in the Ghanaians and Kenyans respectively (P = less than 0.0005). This probably indicates markedly reduced metabolic activation of paracetamol in the Africans. There were no ethnic differences in the sulphate conjugation of paracetamol, but the mean fractional recovery of the glucuronide conjugate in Caucasians (54%) was significantly less than in the Africans (58%). The sulphate conjugation of paracetamol was increased and glucuronide conjugation reduced in Caucasian females compared with males. A similar trend was seen in the Ghanaians but there were no other significant sex differences. The range of intersubject variation in the metabolic activation of paracetamol was sixty fold compared with only a three fold variation in glucuronide and sulphate conjugation. This has important implications for susceptibility to paracetamol hepatotoxicity following overdosage especially in a small subgroup showing extensive metabolic activation. These ethnic differences in paracetamol metabolism may be related to genetic or environmental factors including differences in diet and protein intake.