Genetically determined adverse drug reactions involving metabolism

Association of CYP2D6 100 C > T and 2850 C > T polymorphisms with generalized tonic clonic seizures among Indians

Multiple avenues of research have provided evidence for the role of genetic and environmental factors in epilepsy. Previous studies indicated an association of debrisoquine hydroxylase (CYP2D6) with susceptibility to epilepsy. In this study, association of CYP2D6 100C > T and 2850C > T polymorphisms with generalized tonic clonic seizures (GTCS) among Indians has been analysed using case-control approach. A significant association of 2850C > T (P = 0.015) has been observed. Comparison between phenytoin toxic and others among patients showed no association of these polymorphisms with phenytoin toxicity.

Dextromethorphan metabolic phenotyping in an Iranian population

OBJECTIVE

CYP2D6 polymorphism of drug metabolism represents an important source of interindividual and interethnic variation in drug response. Since this polymorphism has not been studied in an Iranian population, the present study was undertaken.

METHODS

Two hundred healthy unrelated Iranian subjects participated in this study. Phenotyping was based on high-performance liquid chromatography determination of the dextromethorphan/total dextrorphan molar ratios as metabolic ratios (MRs) in plasma samples collected at 3 h after oral administration of 30 mg dextromethorphan hydrobromide. Since the dextromethorphan detection limit of 5 ng/ml achieved in the simultaneous assay for dextromethorphan and its metabolites was not adequate to identify intermediate metabolizers (IMs), 80 of 200 samples selected randomly were also assayed using a modified, more sensitive procedure with a dextromethorphan detection limit of 1 ng/ml.

RESULTS

Poor and extensive metabolizers (EMs) could be identified distinctly. A 520-fold interindividual variation in dextromethorphan MRs was observed in this study. In contrast to undetectable dextrorphan and hydroxymorphinan concentrations, clearly determinable dextromethorphan concentrations higher than 10 ng/ml were observed in plasma samples of poor metabolizers (PMs). Considering the antimode of 0.3, five (2.5%, 95% confidence interval of 0.34-4.66) volunteers were identified as PMs. Using the more sensitive method, dextromethorphan was quantified in 4 (one PM) of 80 samples. Excluding the PM, a Shapiro-Wilk test indicated a non-normal distribution of MRs (P < 0.01) in the latter population.

CONCLUSIONS

From this study it can be concluded that the frequency of PMs in an Iranian population is 2.5% (95% confidence interval of 0.34-4.66). IMs could be identified using dextromethorphan plasma assays with detection limits of at least 1 ng/ml. However, the phenotype-genotype relationships in this respect remain to be established.

Pharmacologic issues in the critically ill

The pharmacotherapy of critically ill patients poses numerous challenges to the ICU team. Polypharmacy and alterations in drug disposition are common in the ICU; critically ill patients have limited physiologic reserve to deal with adverse drug events. Careful prescribing, based upon sound pharmacologic principles, decreases the potential for preventable adverse events and maximizes the opportunity for successful therapy. A systematic approach to reporting, analysis, and prevention of errors is a further step in our ultimate goal to provide optimal care for the vulnerable patients whom we support in our ICUs.

Antiarrhythmic agents: drug interactions of clinical significance

The management of cardiac arrhythmias has grown more complex in recent years. Despite the recent focus on nonpharmacological therapy, most clinical arrhythmias are treated with existing antiarrhythmics. Because of the narrow therapeutic index of antiarrhythmic agents, potential drug interactions with other medications are of major clinical importance. As most antiarrhythmics are metabolised via the cytochrome P450 enzyme system, pharmacokinetic interactions constitute the majority of clinically significant interactions seen with these agents. Antiarrhythmics may be substrates, inducers or inhibitors of cytochrome P450 enzymes, and many of these metabolic interactions have been characterised. However, many potential interactions have not, and knowledge of how antiarrhythmic agents are metabolised by the cytochrome P450 enzyme system may allow clinicians to predict potential interactions. Drug interactions with Vaughn-Williams Class II (beta-blockers) and Class IV (calcium antagonists) agents have previously been reviewed and are not discussed here. Class I agents, which primarily block fast sodium channels and slow conduction velocity, include quinidine, procainamide, disopyramide, lidocaine (lignocaine), mexiletine, flecainide and propafenone. All of these agents except procainamide are metabolised via the cytochrome P450 system and are involved in a number of drug-drug interactions, including over 20 different interactions with quinidine. Quinidine has been observed to inhibit the metabolism of digoxin, tricyclic antidepressants and codeine. Furthermore, cimetidine, azole antifungals and calcium antagonists can significantly inhibit the metabolism of quinidine. Procainamide is excreted via active tubular secretion, which may be inhibited by cimetidine and trimethoprim. Other Class I agents may affect the disposition of warfarin, theophylline and tricyclic antidepressants. Many of these interactions can significantly affect efficacy and/or toxicity. Of the Class III antiarrhythmics, amiodarone is involved in a significant number of interactions since it is a potent inhibitor of several cytochrome P450 enzymes. It can significantly impair the metabolism of digoxin, theophylline and warfarin. Dosages of digoxin and warfarin should empirically be decreased by one-half when amiodarone therapy is added. In addition to pharmacokinetic interactions, many reports describe the use of antiarrhythmic drug combinations for the treatment of arrhythmias. By combining antiarrhythmic drugs and utilising additive electrophysiological/pharmacodynamic effects, antiarrhythmic efficacy may be improved and toxicity reduced. As medication regimens grow more complex with the aging population, knowledge of existing and potential drug-drug interactions becomes vital for clinicians to optimise drug therapy for every patient.

Seizures associated with therapeutic doses of venlafaxine and trimipramine

OBJECTIVE

To report a patient having a first-time seizure after receiving venlafaxine and trimipramine for depression.

CASE SUMMARY

A 25-year-old white woman with chronic depression was treated with venlafaxine 150 mg/d and trimipramine 50 mg/d. Eleven days after increase of the trimipramine dosage to 100 mg/d, she was hospitalized because of seizures suggesting a secondary generalized grand-mal episode. The electroencephalogram showed a pathologic pattern with several generalized epileptiform discharges. Because of suspected drug-induced seizures, both antidepressants were stopped. After antidepressant drug cessation, the patient was symptom free and had no further seizure episodes within the following 12 months of follow-up. No other potential cause for the seizure episode could be identified.

DISCUSSION

Both venlafaxine and trimipramine have been associated with seizures, mainly after overdose. Venlafaxine-associated seizures at therapeutic doses have not been reported in the literature. We hypothesize that a pharmacodynamic or pharmacokinetic drug interaction between venlafaxine and trimipramine involving the CYP2D6 isoenzyme may have played a role in inducing the seizures.

CONCLUSIONS

Clinicians should be aware of the proepileptogenic effect of venlafaxine and trimipramine at therapeutic doses and that this combination may eventually increase the risk of seizures.

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.

CYP2D6 mutations and therapeutic outcome in schizophrenic patients

STUDY OBJECTIVE

To investigate whether a relationship exists between the most common known cytochrome P450 (CYP) isozyme 2D6 mutations and schizophrenia. Because most antipsychotic and antidepressant agents interact with CYP2D6, we also investigated clinical outcomes in schizophrenic poor metabolizers (PMs) and extensive metabolizers (EMs).

DESIGN

Prospective, observational study.

SETTING

Two psychiatric hospitals and a university-affiliated nonpsychiatric hospital.

SUBJECTS

Thirty-nine consecutive schizophrenic patients (POP 1), 89 schizophrenics of French Canadian origin (POP 2), and 384 healthy French Canadians (POP 3).

INTERVENTION

All study subjects were genotyped for CYP2D6 mutant alleles. POP 1 patients were evaluated before and after 21 or more days of treatment with antipsychotic drugs metabolized at least in part by CYP2D6.

MEASUREMENTS AND MAIN RESULTS

Whole blood was collected to determine CYP2D6 alleles *1, *3, *4, *5, *6, and *7 using standard restriction fragment length polymorphisms and polymerase chain reaction techniques. In comparison, CYP2D6 genotypes were determined in POP 2 and POP 3. Twenty-three (59.0%) of 39 patients in POP 1 were genotypically EM homozygotes, 15 (38.4%) were EM heterozygotes, and 1 (2.6%) was a PM. Similar genotype distributions were determined in POP 2 and in POP 3. Genotype distributions for all three populations were in Hardy-Weinberg equilibrium (p>0.05), and there was no significant difference among them (p=0.857). In POP 1, no differences were seen among genotypes in disease symptom severity, number and severity of adverse drug effects, or attitudes toward drug treatment at baseline and at the end of the study. In fact, all patients improved significantly during their hospital stay (all p<0.05), although independent of the CYP2D6 genotype.

CONCLUSION

Common CYP2D6 mutant alleles were not associated with schizophrenia or with disease symptoms, antipsychotic-related adverse effects, or attitudes toward treatment.

Differential gene expression in drug metabolism and toxicology: practicalities, problems and potential

1. An important feature of the work of many molecular biologists is identifying which genes are switched on and off in a cell under different environmental conditions or subsequent to xenobiotic challenge. Such information has many uses, including the deciphering of molecular pathways and facilitating the development of new experimental and diagnostic procedures. However, the student of gene hunting should be forgiven for perhaps becoming confused by the mountain of information available as there appears to be almost as many methods of discovering differentially expressed genes as there are research groups using the technique. 2. The aim of this review was to clarify the main methods of differential gene expression analysis and the mechanistic principles underlying them. Also included is a discussion on some of the practical aspects of using this technique. Emphasis is placed on the so-called 'open' systems, which require no prior knowledge of the genes contained within the study model. Whilst these will eventually be replaced by 'closed' systems in the study of human, mouse and other commonly studied laboratory animals, they will remain a powerful tool for those examining less fashionable models. 3. The use of suppression-PCR subtractive hybridization is exemplified in the identification of up- and down-regulated genes in rat liver following exposure to phenobarbital, a well-known inducer of the drug metabolizing enzymes. 4. Differential gene display provides a coherent platform for building libraries and microchip arrays of 'gene fingerprints' characteristic of known enzyme inducers and xenobiotic toxicants, which may be interrogated subsequently for the identification and characterization of xenobiotics of unknown biological properties.

Cardiovascular pharmacology of aging

Despite the relative paucity of drug trials in the old and especially the very old (> 85 years), some general principles of pharmacology in the aging patient can be taken from available data and clinical experience. The pharmacokinetic changes most consistently seen with aging occur in the volume of distribution, clearance, and half-life of a drug. Renal drug clearance is consistently diminished with aging. Hepatic metabolism is more variably affected, and in contrast to renal clearance, no reliable formula exists to estimate hepatic drug clearance. Pharmacodynamic changes, although present, are less well studied or described in the elderly. Drug interactions and adverse drug reactions increase with increasing numbers of medications prescribed and represent a complex interplay of age, underlying disease, and number and types of medications. The clinical caveats that apply to drug prescription in the very old include reduced starting doses with slow incremental increases; elimination of unnecessary medications; and anticipating and monitoring for drug interactions and ADRs, especially when prescribing warfarin, digoxin, and amiodarone. Future studies that look at the aging patient in the presence of effects of age, physiology, gender, comorbid illness, and multiple drug therapies may help evolve a new set of paradigms for geriatric drug prescribing.

Genetic polymorphisms of human N-acetyltransferase, cytochrome P450, glutathione-S-transferase, and epoxide hydrolase enzymes: relevance to xenobiotic metabolism and toxicity

In this review, an overview is presented of the current knowledge of genetic polymorphisms of four of the most important enzyme families involved in the metabolism of xenobiotics, that is, the N-acetyltransferase (NAT), cytochrome P450 (P450), glutathione-S-transferase (GST), and microsomal epoxide hydrolase (mEH) enzymes. The emphasis is on two main topics, the molecular genetics of the polymorphisms and the consequences for xenobiotic metabolism and toxicity. Studies are described in which wild-type and mutant alleles of biotransformation enzymes have been expressed in heterologous systems to study the molecular genetics and the metabolism and pharmacological or toxicological effects of xenobiotics. Furthermore, studies are described that have investigated the effects of genetic polymorphisms of biotransformation enzymes on the metabolism of drugs in humans and on the metabolism of genotoxic compounds in vivo as well. The effects of the polymorphisms are highly dependent on the enzyme systems involved and the compounds being metabolized. Several polymorphisms are described that also clearly influence the metabolism and effects of drugs and toxic compounds, in vivo in humans. Future perspectives in studies on genetic polymorphisms of biotransformation enzymes are also discussed. It is concluded that genetic polymorphisms of biotransformation enzymes are in a number of cases a major factor involved in the interindividual variability in xenobiotic metabolism and toxicity. This may lead to interindividual variability in efficacy of drugs and disease susceptibility.

Desipramine clearance in children and adolescents: absence of effect of development and gender

OBJECTIVE

To examine the influence of development and gender on the pharmacokinetics of desipramine (DMI) in the pediatric population.

METHOD

DMI pharmacokinetic parameters were calculated from 407 routinely drawn, dose- and weight-normalized serum concentrations in 173 youths receiving DMI (90 children, 83 adolescents; 29 were female, 144 were male).

RESULTS

Mean pharmacokinetic parameters for the entire population included dose (3.78 +/- 1.51 mg/kg), weight- and dose-normalized serum concentration (45.41 +/- 47.39 [micrograms/L]/[mg/kg]), and DMI clearance (0.68 +/- 1.51 [L/kg]/hr). No between-group differences for children and adolescents were detected in dose (child, adolescent) (3.73 +/- 1.40 mg/kg, 3.83 +/- 1.68 mg/kg), weight- and dose-normalized serum concentrations (44.52 +/- 39.6 [micrograms/L]/[mg/kg], 46.34 +/- 34.89 [micrograms/L]/[mg/kg]; p = .62), and clearance (0.680 +/- 0.890 [L/kg]/hr, 0.695 +/- 1.05 [L/kg]/hr; p = .103). No between-group gender differences were detected in dose (male, female) (3.83 +/- 1.55 mg, 3.39 +/- 1.84 mg), weight- and dose-normalized serum concentrations (45.15 +/- 37.76 [micrograms/L]/[mg/kg], 47.14 +/- 34.96 [micrograms/L]/mg/kg]; p = .720), and clearance (0.699 +/- 0.89 [L/kg]/hr, 0.606 +/- 0.535 [L/kg]/hr; p = .390).

CONCLUSIONS

These results suggest that age and gender do not significantly influence DMI clearance or dose-normalized serum concentrations in the pediatric population.

The serotonin syndrome

The serotonin syndrome is characterized by mental status changes and a variety of autonomic and neuromuscular manifestations. Its duration is usually brief, resolving within hours provided that the inciting agent has been discontinued. In most cases, two or more types of medications known to increase the activity of serotonin at the 5-HT1A receptor are required to produce it, and it frequently begins soon after the initiation of a new treatment regimen. Treatment is largely supportive although limited clinical experience warrants the cautious use of specified agents. Although its overall incidence is unknown, it is probably low, and an appropriate level of suspicion coupled with an adequate knowledge of the patient's drug history remains the mainstay of diagnosis and treatment.

The use of in vitro metabolism techniques in the planning and interpretation of drug safety studies

An important issue in toxicology is the suitability of the data obtained with experimental animals for human risk assessment. Because it is not possible to use humans in long-term toxicological studies, the use of animals will continue. However, the data obtained in animal studies can be better extrapolated to the patient by utilizing bridging studies with in vitro models of human drug metabolism. There are 2 basic categories of in vitro methods for the examination of human liver drug metabolism. The first group of in vitro methods consists of the cellular models, which include primary hepatocytes, liver slices, and cell lines. The second group is the use of preparations of the drug-metabolizing enzymes such as tissue homogenates, subcellular fractions, and isolated enzymes. Studies modeling both the human and experimental animal metabolism of a drug are useful in the design of toxicological studies. In vitro studies can identify metabolites, species-specific metabolic routes, and the experimental animal model that best reflects the potential human exposure to the drug and its metabolites. This information can also be useful in the design of the clinical studies by identifying human metabolites, the enzymes responsible for the metabolic clearance of the drug, the effects of genetics and other host factors on the metabolism of the drug, and potential drug-drug interactions. An example of how such information is generated and interpreted is presented.