Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UMLS:C0036572 (seizures)
 80,221 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Since its introduction in 1987, zidovudine monotherapy has been the treatment of choice for patients with HIV infection. Unfortunately it has been established that the beneficial effects of zidovudine are not sustained due to the development of resistant viral strains. This has led to the strategy of combination therapy, and in 1995 treatment with zidovudine plus didanosine, or zidovudine plus zalcitabine, was demonstrated to be more effective than zidovudine monotherapy in preventing disease progression and reducing mortality in patients with HIV disease. Recent work demonstrates an even greater antiviral effect from triple therapy with 2 nucleosides, zidovudine plus zalcitabine with the addition of saquinavir, a new protease inhibitor drug. The HIV protease enzyme is responsible for the post-translational processing of gag and gag-pol polyprotein precursors, and its inhibition by drugs such as saquinavir, ritonavir, indinavir and VX-478 results in the production of non-infectious virions. As resistance may also develop to the protease inhibitors they may be used in combination, and future strategies may well include quadruple therapy with 2 nucleoside analogues plus 2 protease inhibitors. Administration of protease inhibitors alone or in combination with other drugs does raise a number of important pharmacokinetic issues for patients with HIV disease. Some protease inhibitors (e.g. saquinavir) have kinetic profiles characterised by reduced absorption and a high first pass effect, resulting in poor bioavailability which may be improved by administrating with food. Physiological factors including achlorhydria, malabsorption and hepatic dysfunction may influence the bioavailability of protease inhibitors in HIV disease. Protease inhibitors are very highly bound to plasma proteins (> 98%), predominantly to alpha 1-acid glycoprotein. This may influence their antiviral activity in vitro and may also predispose to plasma protein displacement interactions. Such interactions are usually only of clinical relevance if the metabolism of the displaced drug is also inhibited. This is precisely the situation likely to pertain to the protease inhibitors, as ritonavir may displace other protease inhibitor drugs, such as saquinavir, from plasma proteins and inhibit their metabolism. Protease inhibitors are extensively metabolised by the cytochrome P450 (CYP) enzymes present in the liver and small intestine. In vitro studies suggest that the most influential CYP isoenzyme involved in the metabolism of the protease inhibitors is CYP3A, with the isoforms CYP2C9 and CYP2D6 also contributing. Ritonavir has an elimination half-life (t1/2 beta) of 3 hours, indinavir 2 hours and saquinavir between 7 and 12 hours. Renal elimination is not significant, with less than 5% of ritonavir and saquinavir excreted in the unchanged form. As patients with HIV disease are likely to be taking multiple prolonged drug regimens this may lead to drug interactions as a result of enzyme induction or inhibition. Recognised enzyme inducers of CYP3A, which are likely to be prescribed for patients with HIV disease, include rifampicin (rifampin) [treatment of pulmonary tuberculosis], rifabutin (treatment and prophylaxis of Mycobacterium avium complex), phenobarbital (phenobarbitone), phenytoin and carbamazepine (treatment of seizures secondary to cerebral toxoplasmosis or cerebral lymphoma). These drugs may reduce the plasma concentrations of the protease inhibitors and reduce their antiviral efficacy. If coadministered drugs are substrates for a common CYP enzyme, the elimination of one or both drugs may be impaired. Drugs which are metabolised by CYP3A and are likely to be used in the treatment of patients with HIV disease include the azole antifungals, macrolide antibiotics and dapsone; therefore, protease inhibitors may interact with these drugs. (ABSTRACT TRUNCATED)
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PMID:Protease inhibitors in patients with HIV disease. Clinically important pharmacokinetic considerations. 908 59

The elderly have a relatively high risk of developing adverse drug reactions. Phenytoin continues to be a preferred drug for treating generalised tonic-clonic seizures in the elderly and simple partial seizures that generalise. Phenytoin is eliminated almost entirely by hepatic oxidation. The principle enzymes responsible are cytochrome P450 (CYP)2C9 and CYP2C19. CYP2C9 is saturated by therapeutic doses of phenytoin, and at steady state both enzymes are probably operant in most people. The nonlinear pharmacokinetics of phenytoin make it a difficult drug for which to establish safe and effective administration regimens. An important area of inquiry is whether the differential disposition kinetics of phenytoin in the elderly render its administration an even more difficult challenge. Moreover, since the elderly are generally subject to more polypharmacy than younger adults, are they, as a result, subject to either more frequent or more severe drug interactions with phenytoin than younger adults? In order to examine these issues we were interested in learning the extent to which old age might affect the plasma protein binding of phenytoin, its hepatic metabolism and, ultimately, its pharmacokinetic profile. With regard to the latter we looked carefully at the methods that have been used to characterise the disposition kinetics of phenytoin in general, and in the elderly, in particular. There are many conflicting findings with regard to the effect of age on the disposition kinetics of phenytoin. However, the strategies used for estimating kinetic parameters for phenytoin [viz the maximum rate of metabolism/elimination (Vmax) and the Michaelis-Menton constant (Km)] exhibit deficiencies that could account for some of the disparate findings. Certainly, more careful prospective studies focusing on the effects of age on phenytoin disposition kinetics are warranted. However, in light of the information currently available, no special attention need be paid to the initiation of phenytoin administration in elderly patients who are taking multiple anticonvulsants. On the other hand, for the elderly receiving phenytoin monotherapy, the initiation of phenytoin administration should occur at lower doses than would be customary for younger adults, and phenytoin blood concentrations should be appropriately monitored in order to evaluate individual Vmax and Km values for informed dosage adjustments.
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PMID:Differential kinetics of phenytoin in elderly patients. 1050 15

The authors report on a Japanese adult male patient with a long history of partial seizures that were poorly controlled by conventional doses of phenytoin and other drugs. His treatment was complicated by toxic symptoms and an excessive serum phenytoin concentration, 32.6 microg/mL at a dose of 187.5 mg/day. Polymerase chain reaction-restriction fragment length polymorphism analysis disclosed heterozygosity involving cytochrome P450 subfamilies 2C9 (*1/*3) and 2C19 (*1/*3). Currently, it is generally accepted that the former mutation is responsible for the CYP2C9 poor metabolizer phenotype. Pharmacokinetic parameters were estimated by a kinetic analysis, MULTI, using 17 observed dose-concentration data sets: a lower Vmax (5.6 mg/kg/day) and a higher Km (11.5 microg/mL) were observed. Although phenytoin is metabolized predominantly by CYP2C9 with a minor contribution of CYP2C19, patients with the Leu359 variant should be monitored closely when treated with a moderate to high daily dose of phenytoin.
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PMID:Genetic polymorphism of the CYP2C subfamily and excessive serum phenytoin concentration with central nervous system intoxication. 1077 39

A 31-year-old woman who had a severe head injury was treated with oral phenytoin (100 mg 3 times a day) to prevent posttraumatic seizures. On day 10 of phenytoin treatment, 3 hours after the morning dose, the patient manifested neurologic signs compatible with phenytoin intoxication. Thus drug serum concentrations were monitored daily for 12 days. The elimination half-life was 103 hours, namely, about 5 times longer than the mean value generally quoted (22 hours). In the absence of any acquired predisposing factor for phenytoin toxicity, genetic mutations in the cytochrome P450 (CYP) enzymes responsible for phenytoin metabolism (CYP2C9 and CYP2C19) were suspected. Genotyping revealed that the patient was homozygous for the CYP2C9*3 allele (CYP2C9*3/*3) and heterozygous for the CYP2C19*2 allele (CYP2C19*1/*2). In view of the markedly reduced metabolic activity of CYP2C*3 in comparison with the wild-type enzyme (about one fifth) and of the minor role of CYP2C19 in phenytoin metabolism, it is likely that CYP2C9*3 mutation was largely responsible for drug overdose.
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PMID:Severe phenytoin intoxication in a subject homozygous for CYP2C9*3. 1167 55

One of the major differences between the older antiepileptic drugs (AEDs) and the newer AEDs is the potential of the older AEDs for significant interactions with other medications. Many of the drug-drug interactions involving the older AEDs are reciprocal, i.e., both drugs affect each other. In contrast, the newer AEDs have either no or limited drug interaction potential. Despite our extensive understanding of and our ability to predict drug-drug interactions, serious drug interactions still occur. More than 30% of all new seizures occur in the elderly, and because this population may be taking a variety of other medications the addition of an AED can have profound impact on these other therapies. In women, the use of enzyme-inducing AEDs can cause significant alterations of sex hormones and can decrease the efficacy of oral contraceptives. In children and adults, the use of enzyme inducers may result in long-term endocrine effects, including bone loss and lipid, thyroid, and sex hormone abnormalities. Phenytoin and phenobarbital are metabolized by cytochrome P450 isozymes, with activity dependent on genetic polymorphism (CYP2C9, CYP2C19). The dosing of the newer AEDs is not affected by genetic polymorphism. The decreased induction and inhibition effects and the lack of significant genetic polymorphism of the newer AEDs allow increased ease of use and perhaps greater safety, especially for patients taking multiple medications.
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PMID:Pharmacogenetics and enzyme induction/inhibition properties of antiepileptic drugs. 1555 48

Alternative therapy including herbal drugs and complementary medicine is becoming increasingly popular. However, the rise in the incidence of herb-drug interactions is causing concern, especially in the absence of warning labels addressing potential adverse effects. We present the case of a 55-year-old male who suffered a fatal breakthrough seizure, with no evidence of non-compliance with his anticonvulsant medications. The autopsy report revealed subtherapeutic serum levels for both anticonvulsants Depakote and Dilantin. Concomitant with his prescribed medications, the decedent was also self-medicating with a cornucopia of herbal supplements and nutraceuticals, prominent among which was Ginkgo biloba. Ginkgo, an herbal extract from the leaves of the Ginkgo biloba tree, has been used medicinally for centuries and has been touted as a cure for a variety of medical conditions. The induction of Cytochrome P450 enzymes by components of herbal drugs has been known to affect the metabolism of various drugs. Dilantin is primarily metabolized by CYP2C9, and secondarily metabolized by CYP2C19. Valproate metabolism is also modulated in part by CYP2C9 and CYP2C19. A recent study revealed significant inductive effect of ginkgo on CYP2C19 activity. CYP2C19 induction by ginkgo could be a plausible explanation for the subtherapeutic levels of Dilantin and Depakote. Additionally, ginkgo nuts contain a potent neurotoxin, which is known to induce seizure activity. Evidence of other herbal drugs diminishing the efficacy of anticonvulsant medication does exist; however, there has been only one other documented instance of ginkgo potentiating seizure activity in the presence of anticonvulsant therapy. Highlighting the potential adverse effects and drug interactions of ginkgo on the packaging of the drug may help prevent inadvertent use in vulnerable individuals.
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PMID:Fatal seizures due to potential herb-drug interactions with Ginkgo biloba. 1641 14

Phenytoin is a first-line drug for the treatment of status epilepticus. We report a case of phenytoin intoxication after intravenous phenytoin loading in a patient with clozapine-related seizures. To our knowledge, this is the first description of phenytoin intoxication due to CYP2C9 inhibition by clozapine. This case report is important because it supports the use of a lower intravenous loading dose of phenytoin in patients with clozapine-related status epilepticus.
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PMID:Phenytoin intoxication in a clozapine-related prolonged seizure. 1796 58

Therapeutic drug monitoring (TDM) is widely accepted as a method to improve the effectiveness and safety of the first generation antiepileptic drugs (AEDs) and to identify an individual's optimum concentration. Like the older AEDs, the new AEDs also have significant pharmacokinetic variability. A similar relationship between concentration and effect for the new and old AEDs in experimental seizure models suggests that it is reasonable to use TDM for the new AEDs. With the addition of generic formulations of the new AEDs, TDM can play an important role to validate bioequivalence in patients. There is a history of problems with generics of the older AEDs, primarily carbamazepine and phenytoin. The Biopharmaceutics Classification System, which correlates the solubility and permeability of a drug with oral drug absorption, predicts that there should be no significant problems with the majority of the new AEDs. Because of the controversy over the risk-benefit of generic substitution of AEDs, the use of TDM will provide a way to ensure patient safety while establishing that generics of AEDs proven to be bioequivalent in population studies are also bioequivalent in individuals. The goal of personalized medicine is to use genetic testing to target therapy and identify those individuals unlikely to respond to a drug or likely to respond adversely to the same drug. Of all the AEDs, only phenytoin undergoes significant metabolism by cytochrome P450 isozymes with significant genetic polymorphisms (CYP2C9, CYP2C19). Studies are still needed to identify genetic and biomarkers to identify patients at risk for serious idiosyncratic reactions. There have been significant advances in the understanding of the role of genetics in idiopathic as well as acquired epilepsies. Identification of experimental and clinical evidence linking functional changes associated with gene mutations to epilepsy syndromes will help provide new molecular targets for future AEDs.
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PMID:Pharmacokinetic, pharmacodynamic, and pharmacogenetic targeted therapy of antiepileptic drugs. 1836 77

Etoricoxib is presently the most commonly prescribed cyclooxygenase-2 (Cox-2) inhibitor for chronic pain and inflammatory conditions. In clinical practice, phenytoin and etoricoxib are used in chronic conditions of generalized seizure with concomitant chronic pain. Hence, there are chances of drug-drug interaction because modulations of isoenzymes involved in metabolism CYP2C9/10 and CYP2C19 which partially inhibited by etoricoxib. It is important to maintain the therapeutic level of phenytoin in plasma for effective control of seizure. So, the aim of the study was to determine the effect of etoricoxib on the pharmacokinetics of phenytoin in rabbits. In a parallel design study, phenytoin (30 mg/kg/day) was given daily for seven days. On day 7, blood samples were taken at various time intervals between 0-24 h. In etoricoxib group, phenytoin was administered for seven days as above. On day 8, etoricoxib (5.6 mg/kg) along with phenytoin (30 mg/kg/day) was administered and blood samples were drawn as above. Plasma phenytoin levels were assayed by HPLC and pharmacokinetic parameters were calculated. In etoricoxib group, there was a decrease in t(1/2)a phenytoin and t(1/2)el decreased significantly as compared to phenytoin group. Significant changes were observed in the pharmacokinetic parameters in etoricoxib-treated group. These results suggest that etoricoxib alters the pharmacokinetics of phenytoin. Confirmation of these results in human studies will warrant changes in phenytoin dose or frequency when etoricoxib is co-administered with it.
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PMID:Effects of etoricoxib on the pharmacokinetics of phenytoin. 1844 85

Drug treatment of epilepsy is characterized by unpredictability of efficacy, adverse drug reactions, and optimal doses in individual patients, which, at least in part, is a consequence of genetic variation. Since genetic variability in drug metabolism was reported to affect the treatment with phenytoin more than 25 years ago, the ultimate goal of pharmacogenetics is to use the genetic makeup of an individual to predict drug response and efficacy, as well as potential adverse drug events. However, determining the practical relevance of pharmacogenetic variants remains difficult, in part because of problems with study design and replication. This article reviews the published work with particular emphasis on pharmacogenetic alterations that may affect efficacy, tolerability, and safety of antiepileptic drugs (AEDs), including variation in genes encoding drug target (SCN1A), drug transport (ABCB1), drug metabolizing (CYP2C9, CYP2C19), and human leucocyte antigen (HLA) proteins. Although the current studies associating particular genes and their variants with seizure control or adverse events have inherent weaknesses and have not provided unifying conclusions, several results, for example that Asian patients with a particular HLA allele, HLA-B*1502, are at a higher risk for Stevens-Johnson syndrome when using carbamazepine, are helpful to increase our knowledge how genetic variation affects the treatment of epilepsy. Although genetic testing raises ethical and social issues, a better understanding of the genetic influences on epilepsy outcome is key to developing the much needed new therapeutic strategies for individuals with epilepsy.
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PMID:The clinical impact of pharmacogenetics on the treatment of epilepsy. 1912 37


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