Clinical trials and transethnic pharmacology

Optimization of the use of Calcineurin inhibitors in liver transplantation

Calcineurin inhibitors (CNIs), such as cyclosporin A and tacrolimus, are the cornerstone of maintenance immunosuppressive regimens in liver transplantation. CNIs prevent rejection by inhibition of calcineurin, via which lymphocyte proliferation and interleukin (IL)-2 production is prevented. Tacrolimus is now the first-choice immunosuppressant after liver transplantation, since it is associated with fewer episodes of rejection than cyclosporin A. In this review we will discuss interindividual differences, which influence tacrolimus metabolism. Because of these factors and the narrow therapeutic index of tacrolimus, monitoring of drug trough levels is necessary. Furthermore, we will discuss studies concerning conversion from the tacrolimus twice daily to tacrolimus once daily formulation in stable LT patients. Due to adverse effects of CNIs, such as chronic renal failure, hypertension, de novo malignancy and new-onset diabetes mellitus, CNI minimization strategies have been developed, which will be discussed too.

Ethnic or Racial Differences Revisited

Ethnic or racial differences in pharmacokinetics and pharmacodynamics have been attributed to the distinctions in the genetic, physiological and pathological factors between ethnic/racial groups. These pharmacokinetic/pharmacodynamic differences are also known to be influenced by several extrinsic factors such as socioeconomic background, culture, diet and environment. However, it is noted that other factors related to dosage regimen and dosage form have largely been ignored or overlooked when conducting or analysing pharmacokinetic/pharmacodynamic studies in relation to ethnicity/race. Potential interactions can arise between the characteristics of ethnicity/race and a unique feature of dosage regimen or dosage form used in the study, which may partly account for the observed pharmacokinetic/pharmacodynamic differences between ethnic/racial groups. Ethnic/racial differences in pharmacokinetics/pharmacodynamics can occur from drug administration through a specific route that imparts distinct pattern of absorption, distribution, transport, metabolism or excretion. For example, racial differences in the first-pass metabolism of a drug following oral administration may not be relevant when the drug is applied to the skin. On the other hand, ethnic/racial difference in pharmacokinetics/pharmacodynamics can also happen via two different routes of drug delivery, with varying levels of dissimilarity between routes. For example, greater ethnic/racial differences were observed in oral clearance than in systemic clearance of some drugs, which might be explained by the pre-systemic factors involved in the oral administration as opposed to the intravenous administration. Similarly, changes in the dose frequency and/or duration may have profound impact on the ethnic/racial differences in pharmacokinetic/pharmacodynamic outcome. Saturation of enzymes, transporters or receptors at high drug concentrations is a possible reason for many observed ethnic/racial discrepancies between single- and multiple-dose regimens, or between low- and high-dose administrations. The presence of genetic polymorphism of enzymes and/or transporters can further complicate the analysis of pharmacokinetic/pharmacodynamic data in ethnic/racial populations. Even within the same dosage regimen, the use of different dosage forms may trigger significantly different pharmacokinetic/pharmacodynamic responses in various ethnic/racial groups, given that different dosage forms may exhibit different rates of drug release, may release the drug at different sites, and/or have different retention times at specific sites of the body. It is thus cautioned that the pharmacokinetic/pharmacodynamic data obtained from different ethnic/racial groups cannot be indiscriminately compared or combined for analysis if there is a lack of homogeneity in the apparent 'extrinsic' factors, including dosage regimen and dosage form.

Mechanisms of clinically relevant drug interactions associated with tacrolimus

The clinical management of tacrolimus, a macrolide used as immunosuppressant after transplantation, is complicated by its narrow therapeutic index in combination with inter- and intraindividually variable pharmacokinetics. As a substrate of cytochrome P450 (CYP) 3A enzymes and P-glycoprotein, tacrolimus interacts with several other drugs used in transplantation medicine, which also are known CYP3A and/or P-glycoprotein inhibitors and/or inducers. In clinical studies, CYP3A/P-glycoprotein inhibitors and inducers primarily affect oral bioavailability of tacrolimus rather than its clearance, indicating a key role of intestinal P-glycoprotein and CYP3A. There is an almost complete overlap between the reported clinical drug interactions of tacrolimus and those of cyclosporin. However, in comparison with cyclosporin, only few controlled drug interaction studies have been carried out, but tacrolimus drug interactions have been extensively studied in vitro. These results are inconsistent and are of poor predictive value for clinical drug interactions because of false negative results. P-glycoprotein regulates distribution of tacrolimus through the blood-brain barrier into the brain as well as distribution into lymphocytes. Interaction of other drugs with P-glycoprotein may change tacrolimus tissue distribution and modify its toxicity and immunosuppressive activity. There is evidence that ethnic and gender differences exist for tacrolimus drug interactions. Therapeutic drug monitoring to guide dosage adjustments of tacrolimus is an efficient tool to manage drug interactions. In the near future, progress can be expected from studies evaluating potential pharmacokinetic interactions caused by herbal preparations and food components, the exact biochemical mechanism underlying tacrolimus toxicity, and the potential of inhibition of CYP3A and P-glycoprotein to improve oral bioavailability and to decrease intraindividual variability of tacrolimus pharmacokinetics.

Zonisamide in pediatric epilepsy: review of the Japanese experience

Zonisamide is a novel anticonvulsant that is structurally and mechanistically unique, compared with other antiepilepsy drugs. Available in Japan and South Korea since 1989, it was approved in the United States in the year 2000 as adjunctive therapy for partial seizures in adults. There has been extensive clinical trial and clinical practice experience with zonisamide therapy in Japanese children. Open-label data from pediatric clinical trials conducted in Japan suggest that zonisamide is well tolerated and effective against partial- and generalized-onset seizures in children. Despite this wealth of open-label data, no formal pharmacokinetic studies and only one well-controlled trial of zonisamide's efficacy and safety in Japanese children have been completed to date. No controlled clinical trials of zonisamide in children have been completed in the United States or Europe. Additional controlled trials in children with partial- or generalized-onset seizures, infantile spasms, and Lennox-Gastaut syndrome are warranted to further delineate zonisamide's broad spectrum of efficacy and tolerability in children.

The influence of ethnicity and antidepressant pharmacogenetics in the treatment of depression

Antidepressant disposition can be influenced by a variety of CYP isozymes and their effects in the treatment of depression are reviewed. The CYP isozymes 2D6, 3A4, 1A2 and 2C are discussed in regard to antidepressant drug pharmacokinetics, clinical relevance and variability in activity for each isozyme. Polymorphism has been identified with CYP 2D6 and 2C19. Disposition of antidepressants which are substrates of these two isozymes can also be influenced and contributes towards the wide interpatient and interethnic variability found with these drugs. Antidepressants (especially SSRIs) can be CYP isozyme inhibitors and produce significant drug-drug interactions.

Pharmacokinetics and pharmacodynamics of gliclazide in Caucasians and Australian Aborigines with type 2 diabetes

AIMS

Gliclazide pharmacokinetics and pharmacodynamics were assessed in 9 Caucasians and 10 Australian Aborigines with uncomplicated type 2 diabetes.

METHODS

Subjects were on a stable dose of 80 mg gliclazide twice daily, took 160 mg on the morning of study and had a standard breakfast. No further gliclazide was given over the next 48 h. Regular blood samples were drawn for serum glucose, insulin and gliclazide assay. Gliclazide was measured using h.p.l.c. Noncompartmental analysis was used to describe primary data. A multicompartment model incorporating entero-hepatic recirculation was fitted to group mean serum gliclazide profiles.

RESULTS

The Caucasians were older than the Aborigines (mean +/- s.d. age 53.4 +/- 12.2 vs 40.3 +/- 6.9 years, P < 0.05) but had similar diabetes duration, body mass index and glycated haemoglobin. Noncompartmental analysis revealed no between-group differences in gliclazide kinetics. Post-breakfast serum glucose and insulin responses were also similar apart from a longer time to maximum concentration (tmax) for glucose amongst the Aborigines (2.6 +/- 0.4 vs 2.2 +/- 0. 3 h in Caucasians; P = 0.024). Gliclazide tmax exhibited a skewed unimodal distribution and was not associated with gliclazide maximum concentration, or glucose or insulin responses. Most patients had a serum gliclazide profile suggestive of enterohepatic recirculation and/or biphasic absorption. Model-derived estimates of the extent of putative enterohepatic recirculation were 30% and 20% of dose in Caucasians and Aborigines, respectively.

CONCLUSIONS

Gliclazide is equally effective in Caucasian and Aboriginal diabetic patients. The pharmacokinetics of oral gliclazide appear more complex than previously thought. Gliclazide pharmacodynamics are unrelated to rate and extent of absorption, consistent with a threshold concentration for hypoglycaemic effect.

Effects of gender and race on albuterol pharmacokinetics

We evaluated the effects of race and gender on albuterol pharmacokinetics in 30 patients with moderate asthma (15 blacks, 15 whites, 16 men, 14 women). Subjects received a single dose of albuterol 8 mg oral solution and had blood samples collected at various times for 12 hours after the dose. Albuterol plasma concentrations were determined by HPLC with fluorescence detection, and pharmacokinetics were determined by compartmental analysis. The apparent volume of distribution of albuterol was significantly higher in men than in women (631+/-171 and 510+/-109 L, respectively, p<0.05). Consequently, the maximum concentration was lower in men than women (10.3+/-2.1 and 12.0+/-1.9 ng/ml, respectively, p<0.05). Elimination rates were 0.136+/-0.008 and 0.160+0.012 hour(-1), respectively (p<0.10). When corrected for ideal body weight, apparent volume of distribution was not different by gender. No differences between blacks and whites other than lag time were noted in albuterol kinetics. The greater apparent volume of distribution in men is likely explained by differences in ideal body weight or lean body mass.

Influence of race or ethnicity on pharmacokinetics of drugs

Review of the current literature on racial differences in pharmacokinetics of drugs supports the premise that only pharmacokinetic processes which are biologically or biochemically mediated have the potential to exhibit differences between racial or ethnic groups. Thus, the pharmacokinetic factors which can be expected to potentially exhibit racial differences are (1) bioavailability for drugs which undergo gut or hepatic first-pass metabolism, (2) protein binding, (3) volume of distribution, (4) hepatic metabolism, and (5) renal tubular secretion. Absorption (unless active), filtration at the glomerulus, and passive tubular reabsorption would not be expected to exhibit racial differences. As is evident from this review, there are relatively few drugs for which there is information on ethnic or racial differences in pharmacokinetics. Thus it is often necessary to try to predict whether such differences might exist. Taking into consideration the above factors and evaluation of the pharmacokinetic characteristics of the drug, it should be possible to identify those drugs most likely to exhibit differences in their pharmacokinetics. For example, a drug which is eliminated entirely by the kidneys through filtration and reabsorption and is not highly bound to plasma proteins (or is bound to albumin) is highly unlikely to exhibit racial differences in its kinetics. Conversely, a drug which undergoes significant gut and/or hepatic first-pass metabolism and is highly bound to AGP is much more likely to exhibit kinetic differences between racial groups. A discussion of the impact of racial differences in kinetics on drug response or racial differences in drug efficacy, toxicity, or pharmacodynamics (concentration-response relationship) is beyond the scope of this review. However, a number of the papers described above also evaluated differences in pharmacodynamics or response. Among the comparisons of Chinese and Caucasians, these include the papers on propranolol, morphine, nifedipine, triazolam, diazepam, and omeprazole. For those studies comparing differences in blacks and Caucasians, responses or pharmacodynamics were also determined in the studies of propranolol, trimazosin, and methylprednisolone. Interested readers are also referred to the review by Wood and a more recent review by Kitler for additional discussion of ethnic/racial differences in pharmacodynamics/drug response.

Predominance of slow acetylators of N-acetyltransferase in a Hmong population residing in the United States

Pharmacogenetics can be an important determinant of pharmacologic response. To learn more about interpopulation differences in drug metabolism between ethnically diverse populations of subjects cared for by an International Clinic, a study was conducted to describe the prevalence of fast or slow acetylators of N-acetyltransferase (NAT2) in a population of Hmong residing in Minnesota. Ninety-eight healthy Hmong refugees from Laos volunteered to take caffeine as an oral probe drug to establish acetylator phenotype. Participants were classified as either rapid or slow acetylators based on the urinary molar ratio of select metabolites of caffeine. Assignment of phenotype was based on results from analysis of urine collected subsequent to ingestion of caffeine. The ratio of 5-acetylamino-6-formylamino-3-methyluracil (AFMU) to the combined products of the 7-demethylation pathway of paraxanthine (AFMU, 1-methylxanthine (1X), and 1-methylurate (1U)] formed the basis for this determination. A probit plot of the data collected in our subjects qualified a metabolic ratio of 0.34 as an acceptable cut point for phenotype assignment. Participants with an AFMU/(AFMU + 1X + 1U) ratio of < 0.34 were classified as slow acetylators and all others as rapid acetylators. Analysis of the data suggested a bimodal distribution with an excess (74.5%) of slow acetylators in the population. The predominance of slow acetylators found in the Hmong contrast with the prevalence of slow acetylators seen in other ethnic groups. These findings may have important clinical implications given the large number of Hmong treated each year in our International Clinic and the increasing use of medications metabolized by NAT2 in this population.

Drug-nutrient interactions

Nutrition status plays a significant role in a drug's pharmacodynamics. Some disease states and other special conditions affect nutrient status and a drug's therapeutic efficacy. Many classes of drugs, including antimicrobials, hypoglycemics, and hypocholesterolemic agents, can be affected by the presence of food, with the geriatric patient particularly at risk. While a drug's pharmacokinetic profile can usually be predicted, it can be modified by nutrients and by certain pathophysiologic conditions, including aging, hepatic dysfunction, and micronutrients.