Sulphadimidine acetylation status in Gujarati and Marathi population.
Genetic polymorphism in the ability of humans to acetylate isoniazide and sulphadimidine and to oxidize debrisoquine is of interest to physicians, as a number of drugs are metabolized by the same pathways.
Thus, procainamide, hydrallazine, dapsone, sulphadimidine, nitrazepam (after enzymatic reduction of nitro group) are metabolized by the same pathway as isoniazid., Efficacy and toxicity of these drugs differ between acetylator phenotypes. The relevance of acetylation status to efficacy and toxicity depends on other routes of elimination available for the drug, concentration effect relation etc. Thus, fast acetylators require higher doses of procainamide while slow acetylators are at greater risk of developing SLE early in treatment with procainamide or hydrallazine. On the other hand with dapsone, there is no relationship between acetylation phenotype and response, as there are other pathways of elimination. Determination of acetylation status will be helpful in identifying individuals at risk of developing side effects and modifying doses appropriately before commencing treatment.,
We investigated slow and fast acetylator phenotypes in Gujarati and Marathi population in Bombay using sulphadimidine as the test drug,
Genetic polymorphism is also observed in debrisoquine oxidation. Several drugs are oxidized by the same pathways as debrisoquine e.g. phenformin, nortriptyline, perhexiline, metoprolol, bufuralol, oxprenolol and sparteine. Some of the individuals who were phenotyped for acetylation status were also phenotyped for debrisoquine oxidation status to assess whether the two polymorphisms are linked.
The study was carried out in 54 healthy male and female volunteers from Gujarati and Marathi communities in Bombay. These two communities are known to be genetically homogenous.
Volunteers with any abnormal finding on clinical biochemical examination and those with history of drug allergy, alcohol intake, history of jaundice or renal disease in past 3 months, history of drug intake in past 15 days and of steroid intake in past 1 month were excluded. For phenotyping for acetylation, the method described by Rao et al and Price Evans was followed.
Volunteers were given sulphadimidine base 44 mg/kg as powder after overnight fasting, and were not allowed anything orally for further 2 hours to ensure uniformity of absorption. After this time, normal fluid and food intake was permitted. Normal activity was permitted throughout the study. At the end of 5 hours, volunteers emptied their bladder and were given 200 ml of water. Urine passed between 5th and 6th hour and blood at 6th hour was collected. For the determination of sulphadimidine and its metabolite, Bratton and Marshall's method was used. The sensitivity of the method in our laboratory was 1 mg%, recovery 99% and day to day variation 6%. Per cent acetylated sulphadimidine was calculated as:
----------- X 100.
For 23 volunteers who participated in this study debrisoquine oxidation study data was available. Correlation between debrisoquine metabolic ratio and per cent acetylated sulphadimidine in 6th hour plasma was calculated.
(Debrisoquine oxidation study was carried out in collaboration with St. Mary's Hospital, London, in 127 volunteers. Method described by Mahgoub was followed. Debrisoquine and 4-hydroxydebri-soquine were estimated in 0-8 hr urine sample after administering 10 mg debrisoquine. Metabolic ratio was calculated by dividing debrisoquine by 4-hydroxydebrisoquine value in 0-8 hour urine (unpublished).
Fifty-four healthy volunteers (44 males and 10 females with age ranging from 20 to 49 years, height 152-176 cm and weight 38- 61.5 kg) completed the study. Per cent acetylated sulphadimidine excreted in urine ranged from 35 to 96 while that in plasma ranged from 10 to 79. The histogram of per cent acetylated sulpha in 6th hr plasma and in 5-6 hr urine is given in [Fig. 1]. Out of the total 54 volunteers who participated, 47 gave plasma and urine samples while 7 gave only urine samples. Among 47 subjects who gave both plasma and urine samples, the plasma acetylated sulphadimidine was less than 30% in 22, while it was more than 40% in 25 subjects. Acetylated sulphadimidine in urine sample was less than 70% in 28 while it was more than 70% in 26 volunteers.
Using criteria of Price Evans (% acetylated sulpha less than 37% in 6 hr plasma and less than 70% in 5-6 hr urine in slow acetylators) two volunteers were phenotyped as slow acetylators by urinary data but rapid acetylators by plasma data. By criteria of Rao et al (% acetylated sulphadimidine less than 25% in 6 hour plasma and less than 70% in 5-6 hour urine in slow acetylators), such discripancy occurred in 7 volunteers.
Using plasma data and Price Evans criteria for phenotyping the slow acetylator gene frequency could be estimated as 0.684 with 95% confidence limits 0.674-0.694 (calculated by method described by Emery et al). The population distribution of different phenotypes could be estimated as 46.8% of slow acetylators, 43.2% hetero hygotes and 10% homozygote rapid acetylators.
The debrisoquine metabolic ratio ranged from 0.2-44.7 in 23 volunteers. No correlation was observed between debrisoquine metabolic ratio and % acetylated sulphadimidine in 6 hour plasma.
In the present study, acetylation phenotypes in Gujarati and Marathi populations were investigated using sulphadimidine as the marker drug.
Choice of drug and dose: A number of test drugs and discriminatory parameters have been used by various workers to assess acetylation status [Table I]. We selected sulphadimidine as the test drug as the samples are stable over prolonged period and method of analysis is easy and fast and slow acetylators can be distinguished by measuring % acetylated sulphadimidine in urine or plasma. Different workers have used different doses of sulphadimidine, viz. 10 mg/kg,20 40-44 mg/kg, and 160 mg/kg of metabolic active mass. Sulphadimidine kinetics have been reported to be dose dependent, fraction of free sulphadimidine that gets converted to acetyl sulphadimidine appears to decrease as amount of sulphadimidine increase, hence it is better to avoid doses as high as 160 mg/kg dose. Use of 10 mg/kg is not suitable if spectrophotometric method of analysis is being used as the sensitivity of the method may be inadequate for some patients hence dose of 40-44 mg/kg (44 mg/kg was calculated as 2 mg/100 Ib person) was chosen as the most suitable dose for the present study.
Various workers have used either urine or plasma as test sample. Rao et al observed satisfactory agreement between isoniazid method and sulphadimidine method, however, 3% of their patients were misclassified by blood sulphadimidine test and 1% by urine test. Price Evans noted that serum test appears to be a better phenotypic discriminant than urine, histogram of serum sample showing clear cut bimodal distribution. Talseth and Landmark observed that phenotyping using urine is more susceptible to error than is plasma analysis in patients with chronic renal failure when large doses of sulpha were used. Du souich et al analysed the potential influences of changes in absorption and urinary elimination rate constants on markers of acetylation capacity. Their results showed that 6 hour plasma sample is least sensitive to such changes and suggested that as the most appropriate method for phenotyping.
There is also difference of opinion as to most appropriate cut off point for phenotyping. Rao et al reported that 25% acetylated sulpha in plasma and 70% in urine should be cut off point, values below these representing slow acetylators. Price Evans and Olsen et al, however, observed 37% as the most appropriate cut off point for plasma and 70-71% for urine. We observe that distinct separation between two phenotypes is possible with plasma but not with urine. With our data, the cut off with plasma samples is 35. Using criteria mentioned by Price Evans and Olsen et al discrepancy in phenotyping with plasma data or urine data, occurred in two volunteers, while using cut off point mentioned by Rao et al discrepancy occurred in 7 volunteers. Using 35% acetylated sulphadimidine in 6 hour plasma as the cut off point the slow acetylator gene frequency in the Gujarati and Marathi population was found to be 0.684. In South Indians the slow gene frequency has been shown to be 0.77 which is comparable to our observation.
There was no correlation between debrisoquine metabolic ratio and % acetylated sulpha in 6 hour plasma indicating that the two pathways are not co-inherited. Both defects have autosomal recessive mode of inheritance but the gene loci for the two polymorphism are not linked.
Thus sulphadimidine in the dose 44 mg/kg and discriminating parameter 6 hour plasma is suitable for phenotyping individuals for acetylation status. The slow acetylation gene frequency in Gujarati and Marathi population is 0.684. There is no correlation between sulphadimidine acetylation status and debrisoquine oxidation status.
This work was supported by Department of Science and Technology, Government of India Research Grant No. HCS/DST/730/79 which is gratefully acknowledged. The authors wish to thank the Dean, Seth G. S. Medical College and K.E.M. Hospital, Bombay, for permitting to carryout the study.