Statistical analysis of polymorphic drug metabolism data using the Rosin Rammler Sperling Weibull distribution

Thiopurine S-methyltransferase activity in Nigerians: phenotypes and activity reference values

Objectives

This study assessed the activity of thiopurine S-methyltransferase (TPMT) in Nigerians with a view to providing data on susceptibility to thiopurine toxicity, and as well generate reference activity values for clinical use.

Results

TPMT activity, expressed as the amount of 6MMP in ng/mL after 1 h incubation at 37 °C per haemoglobin (U/g Hb), varied between 2.34 and 63.50 U/g Hb in the study population. Poor metabolic phenotypes, characterised by an activity values below 8.41 U/g Hb, were observed in 20% of the study subjects. Intermediate metabolizers had activity values between 8.41 and 16.13 U/g Hb. Fast and very fast metabolizers were characterised by activity values of 16.20–56.22 and > 56.22 U/g Hb, respectively. These findings suggest that a potentially huge discordance between TPMT phenotype and genotype exist in Nigerians, and emphasizes the superiority of a prior determination of TPMT metabolic phenotype in ensuring the safety of thiopurine drug administration in the population.

Relationship between metabolic phenotypes and genotypes of CYP1A2 and CYP2A6 in the Nigerian population

BACKGROUND

CYP1A2 and CYP2A6 are polymorphic drug-metabolising enzymes that are also implicated in the activation of procarcinogens in humans. Some of their alleles and haplotypes, often varied in prevalence across populations, are thought to influence activity despite the known contribution of environmental factors. This study assessed the potential influence of some genetic variants of CYP1A2 and CYP2A6 on metabolic phenotypes in Nigerians.

METHODS

Genomic DNA was extracted from blood samples of 100 healthy, unrelated subjects for whom CYP1A2 and CYP2A6 phenotypes had previously been determined, alongside an additional 80 other individuals for whom phenotype data were unavailable. The samples were screened for CYP1A2 (*1C,*1D,*1E,*1F, *3,*4,*6,*7) and CYP2A6 (*9,*11,*17) alleles using the Sequenom MassARRAY platform for some alleles and direct Sanger sequencing for others. The genetic data acquired were subsequently analysed for haplotypes and assessed for concordance with phenotypes.

RESULTS

All five CYP1A2 haplotypes (CYP1A2*1F, 1J, 1N, 1L, 1W) identified in the Nigerian population were not significantly predictive of metabolic phenotypes. Heterozygous CYP1A2*1J carriers and homozygous CYP1A2*1W carriers showed statistically insignificant decrease in CYP1A2 activity. The CYP2A6*9/*17 genotype was, however, significantly associated with the CYP2A6-poor metabolic phenotype, whereas CYP2A6*9 or CYP2A6*17 alone did not show any such association. CYP2A6*11 was not detected in the population.

CONCLUSIONS

Our findings suggest that CYP1A2 alleles or haplotypes were not predictive of metabolic phenotypes in the Nigerian population. Carriers of CYP2A6*9/*17 genotype are likely to be poor metabolisers of CYP2A6 substrates and may experience adverse reactions or poor efficacy while using drugs metabolised mainly by CYP2A6.

Investigation of a correlation between monoamine oxidase B and catechol-O-methyltransferase activity in human blood cells

Monoamine oxidase B (MAO-B) and catechol-O-methyltransferase (COMT) are pivotal enzymes in the catabolism of several neurotransmitters. MAO-B and COMT activity can be reliably measured in human platelets and erythrocytes, respectively. This study investigated whether a correlation exists between the activity of the two enzymes in a panel of 47 elderly subjects (age range 55-80 years). No correlation was apparent between the two activities (r(2)<0.01), which suggests that, genetically, they are determined independently. COMT activity in a panel of 163 subjects showed a bimodal distribution with a nadir at approximately 38 pmol/h/mg Hb.

A new, sensitive graphical method for detecting deviations from the normal distribution of drug responses: the NTV plot

1. A new graphical method was developed for the detection of deviations from the normal distribution. The approach took advantage of the similarity of graphical features of a graded dose-response relationship and a cumulative normal distribution. 2. The behaviour of the new normal test variable (NTV) plot was evaluated, in comparison with that of the probit plot and probability density functions (the generalization of histograms), for various assumed distributions. These included skewed distributions and composites of normal distributions with a variety of separations, ratios of peak sizes and widths. 3. The NTV approach generally detected deviations from the normal distribution more sensitively than the probit plot. 4. The NTV and probit plots may be able to identify biomadality by complementary approaches. 5. The characteristics of the three graphical representations were illustrated by a simulated sample from a composite of normal distributions and by an example of sparteine metabolism in 142 Cuna Amerindians.

Testing for bimodality in frequency distributions of data suggesting polymorphisms of drug metabolism--hypothesis testing

1. The theory of methods of hypothesis testing in relation to the detection of bimodality in density distributions is discussed. 2. Practical problems arising from these methods are outlined. 3. The power of three methods of hypothesis testing was compared using simulated data from bimodal distributions with varying separation between components. None of the methods could determine bimodality until the separation between components was 2 standard deviation units and could only do so reliably (greater than 90%) when the separation was as great as 4-6 standard deviation units. 4. The robustness of a parametric and a non-parametric method of hypothesis testing was compared using simulated unimodal distributions known to deviate markedly from normality. Both methods had a high frequency of falsely indicating bimodality with distributions where the components had markedly differing variances. 5. A further test of robustness using power transformation of data from a normal distribution showed that the algorithms could accurately determine unimodality only when the skew of the distribution was in the range 0-1.45.

Metoprolol. An updated review of its pharmacodynamic and pharmacokinetic properties, and therapeutic efficacy, in hypertension, ischaemic heart disease and related cardiovascular disorders

During the intervening years since metoprolol was first reviewed in the Journal (1977), it has become widely used in the treatment of mild to moderate hypertension and angina pectoris. Although much data have accumulated, its precise mechanisms of action in these diseases remain largely uncertain. Optimum treatment of hypertension and angina pectoris with metoprolol is achieved through dose titration within the therapeutic range. It has been clearly demonstrated that metoprolol is at least as effective as other beta-blockers, diuretics and certain calcium antagonists in the majority of patients. Although a twice daily dosage regimen is normally used, satisfactory control can be maintained in many patients with single daily doses of conventional or, more frequently, slow release formulations. Addition of a diuretic may improve the overall response rate in hypertension. Several controlled trials have studied the effects of metoprolol administered during the acute phase and after myocardial infarction. In early intervention trials a reduction in total mortality was achieved in one moderately large trial of prolonged treatment, but in another, which excluded patients already being treated with beta-blockers or certain calcium antagonists and where treatment was only short term, mortality was significantly reduced only in 'high risk' patients. Overall results with metoprolol have not demonstrated that early intervention treatment in all patients produces clinically important improvement in short term mortality. Thus, the use of metoprolol during the early stages of myocardial infarction is controversial, largely because of the requirement to treat all patients to save a small number at 'high risk'. This blanket coverage approach to treatment may be more justified during the post-infarction follow-up phase since it has been shown that metoprolol slightly, but significantly, reduces the mortality rate for periods of up to 3 years. Metoprolol is generally well tolerated and its beta 1-selectivity may facilitate its administration to certain patients (e.g. asthmatics and diabetics) in whom non-selective beta-blockers are contraindicated. Temporary fatigue, dizziness and headache are among the most frequently reported side effects. After a decade of use, metoprolol is well established as a first choice drug in mild to moderate hypertension and stable angina, and is beneficial in post-infarction patients. Further study is needed in less well established areas of treatment such as cardiac arrhythmias, idiopathic dilated cardiomyopathy and hypertensive cardiomegaly.