694

samples, we should have been able to identify and measure these metabolites. In contrast to Waring and Kupfer we did not observe significant biotransformation to sulphoxides of carbocisteine, methylcisteine, or corresponding N-acetylated metabolites. Only minor amounts of epimeric carbocisteine sulphoxides, never exceeding 1% of doses could be detected during the three 8-h collection periods. Occasionally we also observed the renal excretion of minor amounts of methylcisteine sulphoxide. However, the studies with the 13C-isotopomer of carbocisteine proved that this metabolite did not originate from administered carbocisteine but rather from dietary sources such as vegetables.6 In the 23 individuals studied so far we have no indication of polymorphic sulphoxidation of carbocisteine. In agreement with the data reported by Kupfer, Idle, and their colleagues we observed substantial excretion of carbocisteine within the first 8 h only. The major metabolites formed after the administration of carbocisteine were thiodiglycolic acid, which is also an endogenous compound, and its sulphoxide. In addition, we observed the excretion of a pyruvate-derived metabolite (3-carboxyrnethylthio)lactic acid. By monitoring the excretion of unchanged drug and nitrogen-free metabolites, up to 70% of dose was recovered in urine over 24 h. Other groups6--8 have not been able to confirm the original findings of Waring et al and it is unlikely that carbocisteine sulphoxidation is polymorphic, at least in respect of the metabolites claimed to be formed after the administration of carbocisteine. The fact that we could not confmn the formation of cysteinyl sulphoxide metabolites does not, however, exclude the possibility of differences in the metabolic handling of carbocisteine because this has been reported in certain disease conditions such as primary biliary cirrhosis.9 Since in our studies 30-60% of the dose administered could not be accounted for by carbocisteine, thiodiglycolic acid, the corresponding sulphoxide, or the lactate metabolite, other metabolites with yet unknown structures are being formed, and these need to be identified. Dr Margarete Fischer-Bosch-Institut fur Klinische Pharmakologie, D-7000 Stuttgart 50, West Germany

CLAUS O. MEESE UTE HOFMANN MICHEL EICHELBAUM

Specht D, Ratge D, Kohse KP, Meese CO, Eichelbaum M, Wisser H. Metabolismus und Ausscheidung von-S-carboxymethyl-L-cystein. J Clin Chem Clin Biochem 1988; 26: 769-70 2. Specht D, Meese CO, Ratge D, Eichelbaum M, Wisser H. The metabolic pattern of S-carboxymethyl-L-cysteine. a new study with HPLC and a novel ex vivo carbon-13-NMR-method. Fresenius J Anal Chem 1990; 337: 63-64. 3. Meese CO, Specht D, Ratge D, et al. Reinvestigation of S-carboxymethyl-L-cysteine metabolism m humans by novel stable isotope tracer studies. NaunynSchmiedeberg’s Arch Pharmacol Suppl 1990; 341: R9. 4. Meese CO, Specht D, Fischer P. Revision of S-carboxymethyl-L-cysteine metabolism by 13C-NMR spectroscopy. Fresenius J Anal Chem 1990; 337: 130-31. 5. Meese CO, Specht D, Hofmann U. Synthesis of metabolites of S-carboxymethyl-Lcysteine and S-methyl-L-cysteine and some isotopically labelled (2H, 13C) analogues. Arch Pharm (Weinheim) (in press). 6. Karim EFIA, Millership JS, Temple DJ, Woolfson AD. The influence of diet on drug metabolism studies of S-carboxymethyl-L-cysteine. Int J Pharmaceutics 1989, 52: 1.

155-58. 7. Brockmoller J, Simane ZJ, Roots I. HPLC-analysis of S-carboxymethylcysteine and its sulphoxide metabolites. Drug Metab Drug Interact 1988; 6: 447-56. 8. Staffeldt B, Brockmoller J, Roots I. Evaluation of possible polymorphism in sulfoxidation of carboxystein analysed by HPLC-methods. Eur J Pharmacol 1990; 183: 627-28. 9. Olomu AB, Bickers CR, Waring RH, et al. High incidence of poor sulphoxidation in patients with primary biliary cirrhosis. N Engl J Med 1988; 318: 1089-92.

Debrisoquine hydroxylation phenotyping: do we

expect too much?

SIR,-Further to earlier Lancet correspondence’ on the apparently elusive nature and arbitrary positioning of metabolic ratio (MR) anti-mode in population studies of debrisoquine metabolism, we would like to add our own observations. Data for 1616 healthy British whites, investigated for debrisoquine hydroxylation polymorphism, were found to cover almost the entire range of possible oxidation capacities, from 0-4% to 98-0% urinary recovery as 4-hydroxydebrisoquine. The distribution was significantly non-gaussian (skewness —0’56;

Percentage score).

recovery

as

4-hydroxydebrisoquine (normalised

kurtosis 2-42). Exploration of the distribution of normalised data by maximum likelihood analysis2 showed that the model best fitting the data was one in which there were three independent modes, all with different variances (figure). The largest mode (76-6% of subjects) contained "extensive metabolisers", with mean MR of 06 and 61-9% recovery as 4-hydroxydebrisoquine; the second (166%) consisted of "intermediate metabolisers" (mean MR 3-5, 22-3% recovery); and the third (6-8%) contained the "poor metabolisers" (mean MR 32’1, 3.0% recovery). We hesitate to suggest that these three modes directly represent the three possible genotypes as predicted by a simple mendelian model since the volume of the modes only approximated to Hardy-Weinberg proportions. Two anti-modes were apparent within the idealised distribution, corresponding to MR values of 19 and 14-0. As Idle has highlighted, closer inspection of published population data has revealed alternative possible positions for anti-modes between MR values of 1-8 and 6-0 (engulfing the current 1-9), besides the generally accepted anti-mode at 12-6. Where a variable discerned in vivo, such debrisoquine 4-hydroxylation has a pattern of overlapping distributions rather than of distinctly separated parts and the position of an anti-mode will be susceptible to many factors. Strict adherence to such an anti-mode value becomes ineffectual. Although different methods of mathematical dissection can equally justify different anti-modes such values should not become sacrosanct. They are a useful internal guide, permitting division of a continuous spread of oxidising capacities within a sample but they may not be applicable to other racial groups and may not be even within different populations of the same race. In such studies the sample sizes will never be so large that no mathematical assumptions need be made, and not every influencing factor (most of which are not known) will be completely controlled. It is not surprising, therefore, that definitions of predicted oxidation capacity made upon information obtained at the molecular level may not directly relate to observations made at the "whole body" level.3 It is a fallacy to expect absolute concordance for all planes of expression, from the level looked at by the molecular biologist to that of the clinical explorer. Department of Pharmacology and Toxicology, St Mary’s Hospital Medical School, London W2 1PG, UK

S. C. MITCHELL

AFRC Institute of Animal Physiology and Genetics Research, Edinburgh Research Station, Roslin, Midlothian

C.S.HALEY

1. Idle JR. Poor metabolisers of debrisoquine reveal their true colours. Lancet 1989, ii. 1097. 2. Maclean CJ, Morton NE, Elston RC, Yee, S. Skewness in conmingled distributions. Biometrics 1976; 32: 695-99. 3. Yue QY, Bertilsson L, Dahl-Puustinen ML, et al. Disassociation between debrisoquine hydroxylation phenotype and genotype among Chinese Lancet 1989, 334: 870.

Debrisoquine hydroxylation phenotyping: do we expect too much?

694 samples, we should have been able to identify and measure these metabolites. In contrast to Waring and Kupfer we did not observe significant biot...
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