Phenytoin-folic acid: a review

Nutritional neuropathies

Neuropathies associated with nutritional deficiencies are routinely encountered by the practicing neurologist. Although these neuropathies assume different patterns, most are length-dependent, sensory axonopathies. Cobalamin deficiency neuropathy is the exception, often presenting with a non-length-dependent sensory neuropathy. Patients with cobalamin and copper deficiency neuropathy characteristically have concomitant myelopathy, whereas vitamin E deficiency is uniquely associated with a spinocerebellar syndrome. In contrast to those nutrients for which deficiencies produce neuropathies, pyridoxine toxicity results in a non-length-dependent sensory neuronopathy. Deficiencies occur in the context of malnutrition, malabsorption, increased nutrient loss (such as with dialysis), autoimmune conditions such as pernicious anemia, and with certain drugs that inhibit nutrient absorption. When promptly identified, therapeutic nutrient supplementation may result in stabilization or improvement of these neuropathies.

Natural Health Product Interactions with Medication

Natural health products (or dietary supplements) refer to those products found in oral dosage forms, containing 1 or more active ingredients considered to be a nutrient, an herbal product, or any other nonnutrient/nonherbal substance. Their use continues to increase in the general population and in patients seen by nutrition support clinicians. Aside from an appraisal of product safety and effectiveness, attention should be paid to the potential for these product ingredients to interact with medication. Estimates are that at least 15 million adults in the United States are at risk for supplement-drug interactions. These can occur through both pharmacokinetic and pharmacodynamic mechanisms. This review describes the influence of dietary supplements on both the disposition and the effect of medication and provides numerous examples. Patients at greatest risk for interactions are those with chronic disease, who use multiple medications-particularly those with a narrow therapeutic range-have genetic variants in drug metabolism, impaired organ function, and are at either end of the age spectrum. Knowledge of the specific effects on drug absorption, metabolism, and effect is still incomplete. Relative to the large number of possible interactions between supplements and medication, only a small number of combinations have been examined or reported. The greatest limiting factor remains the quality or reliability of the existing evidence, as many widely accepted interactions are only theoretical based either on in vitro data or known pharmacology. A distinction needs to be clearly drawn between "documented" interactions and "potential" interactions. Although drug-drug interactions have been widely recognized, supplement-drug interactions may be as important to recognize, report, and manage.

Effects of phenytoin sodium on doubling time, deoxyuridine suppression, 3H-methotrexate uptake and 57Co-cyanocobalamin uptake in HL60 cells

Phenytoin sodium (5-50 micrograms/ml) caused a dose-dependent prolongation of the doubling time of the human promyelocytic leukaemia cell line, HL60. This effect was unassociated with any alteration in cell viability. HL60 cells which were pre-incubated with 15 micrograms/ml phenytoin sodium for 1 or 48 h and then incubated with the same concentration of the drug plus either 3H-methotrexate (3H-MTX) or 57Co-cyanocobalamin for 90 min, showed an altered accumulation of both radioactive compounds when compared with control cells. Control cells were not pre-incubated with the drug and were subsequently studied in the absence of the drug. Pre-incubation with the drug for 1 h resulted in a 34% increase, and pre-incubation for 48 h in a 19% reduction in the accumulation of 3H-MTX. Pre-incubation for 1 or 48 h caused a 29% reduction in the accumulation of 57Co-cyanocobalamin. Cells cultured in the presence of 15 micrograms/ml phenytoin sodium for 48 h also gave a slightly increased deoxyuridine-suppressed value; this abnormality was partially corrected by the addition of 50 micrograms/ml folinic acid to the test system but was unaffected by the addition of 1 microgram/ml cyanocobalamin. The data indicate that the effects of phenytoin sodium on the proliferation of HL60 cells may have been slightly mediated via a reduced uptake of folate and possibly also of vitamin B12. They also suggest that one of the mechanisms underlying some of the undesirable effects of long-term therapy with phenytoin may be a drug-related impairment of both folate and vitamin B12 uptake by certain cells, including haemopoietic and neural cells.

Vitamin B12, folic acid, and the nervous system

There are many reasons for reviewing the neurology of vitamin-B12 and folic-acid deficiencies together, including the intimate relation between the metabolism of the two vitamins, their morphologically indistinguishable megaloblastic anaemias, and their overlapping neuropsychiatric syndromes and neuropathology, including their related inborn errors of metabolism. Folates and vitamin B12 have fundamental roles in CNS function at all ages, especially the methionine-synthase mediated conversion of homocysteine to methionine, which is essential for nucleotide synthesis and genomic and non-genomic methylation. Folic acid and vitamin B12 may have roles in the prevention of disorders of CNS development, mood disorders, and dementias, including Alzheimer's disease and vascular dementia in elderly people.

The High Atherosclerotic Risk Among Epileptics: the Atheroprotective Role of Multivitamins

Neurologists have little concern about the high atherosclerotic risk among epileptics. Recent evidences mount that chronic epilepsy and prolonged use of antiepileptic drugs (AEDs) are associated with multiple risk factors that are critically implicated in pathobiology and dysfunction of the vessel wall through complex molecular mechanisms that promote atherogenesis. This review is concerned with three metabolic alterations, which are attributed as major risk factors for atherosclerosis among epileptics: altered metabolism of a) homocysteine (Hcy), b) lipids and lipoproteins, and c) uric acid. Most conventional AEDs reduce folic acid levels, thereby raising Hcy levels. Hyperhomosysteinemia is recently believed to induce endothelial dysfunction and promote atherosclerosis through complex oxidative and excitatory neurotoxic molecular mechanisms. However, Hcy itself is a convulsing substance with increased seizure recurrence and intractability to antiepileptic medications. AEDs can disturb lipid metabolism with resultant hypercholestrolemia and dyslipidemia, common recognized risks for atherosclerosis. Altered uric acid metabolism is common among epileptics. Uric acid has been implicated in endothelial cell damage and decreased endothelial nitric oxide bioavailability. In the presence of atherosclerotic milieu, uric acid interacts with other substrate toxicities and increased reactive oxygen species, accelerating atherosclerosis. The above information forms the rationale for future routine screening and correction of such metabolic alterations in epileptics. A convincing argument now develops that routine polyvitamin supplementation (folic acid, vitamin B12, vitamin B6, vitamin C, vitamin E, and beta-carotene) becomes increasingly important for women and men receiving AEDs at all ages. The atheroprotective effect of multivitamins is through their antioxidant and anti-inflammatory effects together with their lipid and Hcy lowering effects.

Multiple congenital anomalies associated with in utero exposure of phenytoin: Possible hypoxic ischemic mechanism?

BACKGROUND

The characteristics of the phenotype of the malformed phenytoin-exposed infant can help to clarify the mechanism of the drug's teratogenesis. One postulated mechanism is vascular disruption.

CASE

An infant who was exposed to phenytoin as monotherapy throughout pregnancy was born with the following abnormalities: midface hypoplasia, digit hypoplasia with syndactyly in the hands and feet, meningomyelocele, talipes equinovarus, and a long skin pedicle on the back. The mother was also exposed to cigarette smoking and alcohol during the pregnancy.

CONCLUSIONS

The malformations of the hands and feet, and the talipes deformity are potential effects of vascular disruption, a postulated fetal effect of both phenytoin and cigarette smoking. The mechanism of the teratogenicity of phenytoin may have included episodes of bradyarrhythmia in the fetus; however, no such episodes were documented.

Drug-nutrient interactions: a review

Concurrent administration of medications and nutrients can lead to interactions that change the absorption or metabolism of the medication or nutrient. Some of these interactions have little or no impact on the patient while others may be fatal. The objective of this article is to review the mechanisms of various drug-nutrient interactions. Topics to be discussed include specific populations at risk of interactions, nutrients that have a positive and negative effect on drug absorption, nutrients that result in alterations of drug metabolism, and a variety of pharmacologic interactions of medications with nutrients. It is vital that healthcare providers are familiar with drug-nutrient interactions and continue to educate themselves and their patients to optimize the effectiveness and minimize the toxicities of medications.

Phenytoin-induced alterations in craniofacial gene expression

In utero exposure to the anticonvulsant drug phenytoin has been shown to alter normal embryonic development, leading to a pattern of dysmorphogenesis known as the Fetal Hydantoin Syndrome. This embryopathy is characterized by growth retardation, microcephaly, mental deficiency, and craniofacial malformations, although the precise mechanism(s) by which phenytoin alters normal developmental pathways remains unknown. To better understand the molecular events involved in the pathogenesis of phenytoin-induced congenital defects, alterations in gene expression were examined during critical periods of craniofacial development. Pregnant SWV mice were administered phenytoin (60 mg/kg/day) from gestational day 6.5 until they were sacrificed at selected developmental time points. Tissue from the craniofacial region of control and exposed embryos was isolated, and samples were subjected to in situ transcription, antisense RNA amplification, and hybridization on reverse Northern blots to quantitatively assess expression of 36 candidate genes. Chronic phenytoin exposure significantly altered expression of several genes at distinct times during morphogenesis. Results of these studies show that expression of the retinoic acid receptors (RAR) alpha, beta, and gamma were significantly increased by phenytoin exposure. Elevations in gene expression of laminin beta 1, and the growth factors IGF-2, TGF alpha, and TGF beta 1, were also demonstrated in the craniofacial region of phenytoin-exposed embryos. As several of these genes are transcriptionally regulated by retinoic-acid-responsive elements in their promoter regions, phenytoin-induced alterations in expression of the RAR isoforms may have severe downstream consequences in the regulation of events necessary for normal craniofacial development. Such alterations occurring coordinately at critical times during craniofacial development may account for the dysmorphogenesis often associated with phenytoin exposure.

Drug and environmental factors associated with adverse pregnancy outcomes. Part I: Antiepileptic drugs, contraceptives, smoking, and folate

OBJECTIVE

Part I of this review examines the relationship between antiepileptic drugs (AEDs) and pregnancy outcomes. Drug-induced folate deficiency and the role of AED metabolism are emphasized. Part II will discuss periconceptional folate supplementation for prevention of birth defects. Part III will discuss the mechanism of folate's protective effect, therapeutic recommendations, compliance, and cost.

DATA SOURCES

A MEDLINE search was conducted for journal articles published through December 1997. Additional sources were obtained from Current Contents and citations from the references obtained. Search terms included phenytoin, carbamazepine, phenobarbital, primidone, valproic acid, oral contraceptives, clomiphene, drug-induced abnormalities, spina bifida, anencephaly, neural tube defect, folate, folic acid, and folic acid deficiency.

STUDY SELECTION

Relevant animal and human studies examining the effects of AEDs, smoking, and oral contraceptives on folate status and pregnancy outcome are reviewed.

DATA EXTRACTION

Studies and case reports were interpreted. Data extracted included dosing, serum and red blood cell folate concentrations, teratogenicity of anticonvulsant medications, metabolism of AEDs and folate, and genetic susceptibility to AED-induced teratogenicity.

DATA SYNTHESIS

Low serum and red blood cell folate concentrations are associated with adverse pregnancy outcomes. Decreases in serum folate are seen with AEDs, oral contraceptives, and smoking. Since similar birth defects are observed with multiple AEDs, metabolism of aromatic AEDs to epoxide metabolites and genetic factors may play a role in teratogenesis.

CONCLUSIONS

Adequate prepregnancy planning is essential for women who have epilepsy. Women receiving folate-lowering drugs may be at increased risk of adverse pregnancy outcomes. Therefore, epileptic women contemplating pregnancy should be treated with the minimum number of folate-lowering drugs possible and receive folic acid supplementation.

Phenytoin-folic acid interaction

OBJECTIVE

To review information regarding the dual and interdependent drug-nutrient interaction between phenytoin and folic acid and other literature involving phenytoin and folic acid.

DATA SOURCES

Information was retrieved from a MEDLINE search of English-language literature conducted from 1983 (time of the last review) to March 1995. Search terms included folic acid, phenytoin, and folic acid deficiency. Additional references were obtained from Current Contents and from the bibliographies of the retrieved references.

STUDY SELECTION

All human studies examining the effects of phenytoin on serum folate concentrations and folic acid supplementation on serum phenytoin concentrations were selected. These included studies of patients with epilepsy and healthy volunteers as well as case reports. Case reports were included because of the extensive length of time needed to study this drug interaction.

DATA EXTRACTION

Data extracted included gender, dosing, serum folate concentrations if available, pharmacokinetics, and adverse events.

DATA SYNTHESIS

Serum folate decreases when phenytoin therapy is initiated alone with no folate supplementation. Folic acid supplementation in folate-deficient patients with epilepsy changes the pharmacokinetics of phenytoin, usually leading to lower serum phenytoin concentrations and possible seizure breakthrough. Folate is hypothesized to be a cofactor in phenytoin metabolism and may be responsible for the "pseudo-steady-state," which is a concentration where phenytoin appears to be at steady-state, but in reality, is not. Phenytoin and folic acid therapy initiated concomitantly prevents decreased folate and phenytoin obtains steady-state concentrations sooner.

CONCLUSIONS

Folic acid supplementation should be initiated each time phenytoin therapy commences because of the hypothesized cofactor mechanism, decreased adverse effects associated with folate deficiency, and better seizure control with no perturbation of phenytoin pharmacokinetics.

Folic acid improves phenytoin pharmacokinetics

Phenytoin (PHT) therapy to control seizures decreases serum folate levels in half of epileptic patients, thus increasing the risk of folate depletion. Supplementation with folic acid prevents deficiency but also changes PHT pharmacokinetics. Kinetic monitoring of PHT when folic acid is provided as a supplement has not been reported in women of child-bearing age. This study of six fertile women examined the interdependence of PHT and folic acid in a randomized crossover study of two treatments: treatment 1 consisted of 300 mg sodium PHT per day and treatment 2 consisted of 300 mg sodium PHT plus 1 mg folic acid per day. Dietary folic acid intake was calculated daily. During treatment 1, serum folate level decreased 38.0 +/- 18.6% (mean +/- standard deviation) and serum PHT concentration was in the low therapeutic range (43.92 +/- 14.52 mumol/L). During treatment 2, serum folate level increased 26.0 +/- 33.4%, and serum PHT level (39.04 +/- 14.16 mumol/L) was similar to that in treatment 1. Only one subject attained PHT steady state during treatment 1, but four subjects achieved steady state during treatment 2. Dietary folate intakes during treatments 1 and 2 were not significantly different. This study suggests an interdependence between PHT and folic acid and supports the observation that fertile women treated with PHT require folic acid supplementation to maintain a normal serum folate level.

Phenytoin pharmacokinetics: before and after folic acid administration

Phenytoin (PHT) exhibits linear and Michaelis-Menten pharmacokinetics. PHT decreases serum folate; the vitamin folic acid (FA) is hypothesized to be a cofactor in the metabolism of PHT. The depletion of serum folate may explain the unpredictability of measured total serum PHT concentrations and time to steady state as compared with the Michaelis-Menten predictive calculations. We examined PHT pharmacokinetics before and after FA supplementation in 13 healthy male volunteers. The study was divided into two phases. Phase I determined V(max) (mg/day) and Km (micrograms/ml) of PHT to calculate PHT doses needed for the second phase. Phase II was a four-way cross-over study to examine the effect of 1 and 5 mg FA on total serum PHT concentrations 1 microgram/ml less and 5 micrograms/ml greater than the subject's Km, Km - 1 and Km + 5, respectively. Predicted versus measured total serum PHT concentrations, t90% (days to steady state), and the effect of FA were calculated for Km - 1 and Km + 5 before and after 1 or 5 mg FA. The measured total serum PHT concentration was always greater than the calculated concentration (p less than 0.05), and t90% was always longer than the calculated t90% (p less than 0.05) for Km - 1 before FA (all subjects decreased serum FA); the same was observed for Km + 5. If folate is assumed to be a cofactor in PHT metabolism, these results are expected, because depletion of the vitamin would indicate less folate to drive the metabolism of PHT, resulting in higher total serum PHT concentrations and longer time to reach steady state.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in phenytoin biotransformation and susceptibility to congenital malformations: a review

The clinical variability of teratogenic response to fetal drug exposure has been well documented. Metabolic differences in biotransformation have been shown to extend to multiple drugs and may involve many steps in drug metabolism with alterations of key intermediates. Although metabolic differences have been reported to be associated with complications of medication use, it has only recently been appreciated that such differences also may be associated in the unborn with the potential for the disruption of normal embryologic development and the production of congenital malformations. It has long been suspected that the teratogenicity of phenytoin may be mediated not only by the parent compound, but also by toxic intermediary metabolites that are produced during the biotransformation of the parent compound. Recent work elucidating differences in isoenzyme forms of cytochrome P-450 enzyme systems, glutathione, and microsomal epoxide hydrolase has provided increased interest in the multiple individual pharmacogenetic differences that may be significant factors affecting increased susceptibility to birth defects in individuals and families with fetal exposure to phenytoin.

Folate treatment of diphenylhydantoin-induced gingival hyperplasia

It has recently been reported that folic acid supplementation reduced DPH-induced gingival hyperplasia in cat and in a pilot study also in man. The present study was performed to further evaluate this therapy in man. Twenty-three children with DPH-treatment for more than 1 yr, and eight children with short-time DPH-treatment were randomly assigned to groups with and without daily supplementation of folic acid (5 mg Folacin) for 1 yr. Although the DPH-levels were in many cases below the lower reference value, the seizure control of the children was good before and during the year of study. The plasma and red cell folate levels were within or above the given reference values in all but one child. There were no significant changes in the size of the gingival hyperplasias after 1 yr of folate supplementation. Nine severely mentally retarded DPH-treated adults were also given supplementation with folic acid. Their serum DPH-levels were above the higher reference values both at the start and during the study. Their plasma and red cell folic acid levels were below the reference value at the start of the study, but as a consequence of the Folacin supplementation these values rose. The size of the gingival hyperplasias was significantly reduced. Seizure control was unchanged. Folate levels should be checked and supplementation with folic acid considered in patients on long-term anticonvulsive multipharmacy therapy.

Decrease of serum folates in healthy male volunteers taking phenytoin

The effect of phenytoin (PHT) on serum folate and the effect of additional oral folic acid (FA) on serum folate during continued treatment with PHT were studied in 13 healthy male subjects 20-35 years of age. The study was divided into two phases: Phase I determined Vmax (mg/kg/day) and Km (microgram/ml) of PHT in order to calculate the PHT doses needed for the second phase. Phase II was a four-way cross-over study to examine the effect of 1 and 5 mg FA on total serum PHT concentrations 1 microgram/ml less and 5 micrograms/ml greater than the subject's Km, Km-1 and Km+5, respectively. Both phases examined the effect of PHT on serum folate. In Phase I, serum folate decreased by a mean and standard deviation of 42.15 +/- 21.44% after an average of 24.15 +/- 5.63 days of PHT administration, with a mean steady-state total serum PHT concentration of 8.45 +/- 2.70 micrograms/ml. Mean percentage decreases in serum folate before the addition of 1 and 5 mg FA in Phase II were 12.80 +/- 31.45% and 23.24 +/- 21.24% for Km-1 and Km+5, respectively. The average numbers of days of PHT administration and total serum PHT concentrations before FA administration were 9.52 +/- 3.34 and 15.84 +/- 7.02 days, and 2.60 +/- 2.18 and 8.64 +/- 3.44 micrograms/ml, for Km-1 and Km+5, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

The clinical importance of subnormal folate levels in epileptic patients on anticonvulsant therapy

A neurological theory was obtained and examination performed on 62 outpatient epileptics on anticonvulsant therapy. Blood counts, folate and B12 assays were performed on all patients and on a control group of 59 adult non-epileptic neurological outpatients. None of the anticonvulsant treated group had clinical peripheral neuropathy; there was one patient with microcytic anaemia and one with normochromic, normocytic anaemia. In 5 of this group the mean corpuscular volume (MCV) was slightly raised but there was no significant overall difference from the control group. In 17 patients serum folate was subnormal and in 7 the red cell folate was subnormal and this was significantly different to the control group (P less than 0.001). Vitamin B12 levels were normal in all subjects. It is concluded that despite subnormal measured folate levels, there is no increased incidence of clinical peripheral neuropathy or of significant macrocytosis. In view of this, we recommend that folate replacement should not be given to non anaemic asymptomatic patients, with subnormal folate levels, on anticonvulsant therapy.

A comparative study of laboratory parameters in head-injured patients receiving either phenytoin or placebo for 24 months

The effects of chronic phenytoin therapy on serum calcium, phosphorus, folate, and various hematological indices were assessed. One hundred and fifty-one patients, ages 18 months to 81 years, received phenytoin in a previously-conducted, double-blind, placebo-controlled study. Of the patients receiving phenytoin, initially 127 were evaluable while for control patients receiving placebo, 116 were evaluable. All patients had various laboratory parameters monitored at one day post-loading dose, one week, 1,3,6,9,12,15,18,21, and 24 months. Laboratory values examined were serum calcium, phosphorus, folate, white blood cell count with differential, hemoglobin, hematocrit, and red blood cell and platelet counts. A statistical analysis using the t-test method was employed to evaluate data. Data are reported as mean values +/- standard deviation. Patients suffering early hypersensitivity, manifested by a morbilliform skin rash, were removed from the drug by day 30 and were not included in the chronic therapy review. Results indicate that the various laboratory values examined were not significantly affected by phenytoin administration in the patient population. Therefore, chronic phenytoin therapy following the initial hypersensitivity period does not cause abnormal laboratory values as followed in this study.