Pharmacodynamic and pharmacokinetic interactions of hydroalcoholic leaf extract of Centella asiatica with valproate and phenytoin in experimental models of epilepsy in rats

ETHNOPHARMACOLOGICAL RELEVANCE

Centella asiatica (CA) is commonly used herbal medicine for treatment of epilepsy. CA has CYP2C9, CYP2D6 and CYP3A4 enzymes inhibition property and used as an adjuvant therapy with conventional antiepileptic drugs (AEDs). That may be responsible for herb-drug interaction.

AIM OF THE STUDY

The present study was planned to evaluate interactions profile of hydroalcoholic extract Centella asiatica (HECA) with antiepileptic drugs in experimental models of epilepsy in rats.

MATERIALS AND METHODS

Wistar rats (175-200 g) were used. In the pharmacodynamic interaction study, seizures were induced using pentylenetetrazole (PTZ) (60 mg/kg, i.p.) and maximal electroshock seizure (MES) (70 mA for 0.2 s). The therapeutic and sub-therapeutic doses of valproate (VPA) and phenytoin (PHT) were co-administrated with HECA in PTZ and MES model of seizures respectively. Behavioural parameters were assessed using elevated plus maze test and passive avoidance paradigm. Rat brain oxidative stress parameters were also assessed. In the pharmacokinetic interaction study, the serum levels of the VPA and PHT were estimated at different time intervals by HPLC and pharmacokinetic parameters were analyzed by WinNonlin software.

RESULTS

The VPA and PHT produced complete protection against seizures in their therapeutic doses but not with sub-therapeutic doses. However, co-administration of HECA with a sub-therapeutic dose of VPA and PHT enhanced the protection of seizures and significantly (p < 0.001) attenuated the seizure induced oxidative stress and cognitive impairment. It also significantly increased (p < 0.001) serum levels of VPA and PHT. The alterations in pharmacokinetic parameters (maximum serum concentration, area under the curve, clearance) of AEDs were also found with co-administration of HECA.

CONCLUSION

The results suggested that co-administration of HECA could improve the therapeutic efficacy of VPA and PHT. But, alteration in pharmacokinetic parameters revel that needs critical medical supervision to avoid any toxic reactions.

Efficacy, Tolerability and Serum Phenytoin Levels after Intravenous Fosphenytoin Loading Dose in Children with Status Epilepticus

OBJECTIVE

To evaluate the efficacy and tolerability of intravenous fosphenytoin in children with status epilepticus, and resulting serum total phenytoin levels.

METHODS

In this prospective study, 51 children aged less than 18 years received intravenous loading dose of fosphenytoin (18-20 mg/kg). Serum total phenytoin levels were estimated at 90 -100 minutes. Outcomes studied were (i) seizure control and local and/or systemic adverse effects in next 24 hours and (ii) phenytoin levels and its correlation with dose received, seizure control and adverse effects.

RESULTS

The actual dose of fosphenytoin received varied from 15.1 to 25 mg/kg. Seizures were controlled in 45 (88%) children and, two required additional dose of 10 mg/kg. None of the children showed any local or systemic adverse effects. Serum total phenytoin levels were in the therapeutic range (10-20 µg/mL) in 12 (23.5%), sub-therapeutic in 16 (31.3%) and supra-therapeutic in 25 (49%) children. There was weak correlation of the phenytoin levels with dose of fosphenytoin received, seizure control, or adverse effects.

CONCLUSIONS

Intravenous fosphenytoin loading dose of 20 mg/kg is effective in controlling seizures in 88% of children with status epilepticus, with a good safety profile. Seizure control and adverse effects appear to be independent of serum total phenytoin levels achieved.

Analysis of phenytoin drug concentration for evaluation of clinical response, uncontrolled seizures and toxicity

The narrow therapeutic index, non-linear pharmacokinetics and unpredictable absorption require regular therapeutic monitoring of phenytoin. The influence of genetic differences, sex, age and race on the phenytoin plasma levels and its metabolites is well recognized. This study is aimed at evaluating phenytoin plasma drug concentration and its relationship with clinical response, persistent seizures and toxicity in different gender and various age groups of Chinese epileptic patients. This knowledge will help the clinicians in adjusting the drug dosages of phenytoin in various sub-groups of epileptic patients for enhancing the safety, efficacy and minimizing the toxicity of phenytoin. A total of 48 plasma samples of epileptic patients for measuring the plasma phenytoin concentration were received. Only patients displaying persistent seizures or suspected of adverse effects were requested for drug monitoring. All these samples were analyzed for therapeutic drug monitoring with Enzyme-multiplied immunoassay technique. Surprisingly, it was found that majorities (85.5%) of samples were out of the reference range, of which 69% of samples were in sub-therapeutic levels and 16.5% of samples were above therapeutic levels. Only 14.5% of all samples had phenytoin levels in the therapeutic range. The difference in plasma concentration of phenytoin was notably altered in gender and various age groups. Careful attention must be applied to specific gender and particular age group on an individual basis in the interpretation of plasma concentration results, in order to facilitate the modification of doses and develop the best approach in treatment and to obtain the desired clinical response because multiple factors can affect the phenytoin plasma concentration. Through these results, it can be concluded that a good correlation exists between phenytoin plasma concentration and clinical response. Therefore, regular therapeutic monitoring of phenytoin and screening of HLA-A, B, C and DRB1 genotypes before prescribing phenytoin in epileptic patients is essentially required to achieve maximum clinical response and prevent the serious toxicity.

Evaluation of phenytoin serum levels following a loading dose in the acute hospital setting

PURPOSE

Due to the complex pharmacokinetic profiles of phenytoin (PHT) and fosphenytoin (FOS), achieving sustained, targeted serum PHT levels in the first day of use is challenging.

METHODS

A population based approach was used to analyze total serum PHT (tPHT) level within 2-24h of PHT/FOS loading with or without supplementary maintenance or additional loading doses among PHT-naïve patients in the acute hospital setting. Adequate tPHT serum level was defined as ≥20μg/mL.

RESULTS

Among 494 patients with 545 tPHT serum levels obtained in the first 2-24h after the loading dose (LD), tPHT serum levels of either

CONCLUSION

Close laboratory surveillance and PHT/FOS dose adjustments are recommended to ensure adequate and sustained tPHT serum levels early in treatment. Free serum PHT level is the preferred method of drug monitoring.

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Key Words

• Fosphenytoin
• Serum phenytoin level
• Status epilepticus

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MESH Headings

• Adolescent
• Adult
• Aged
• Aged, 80 and over
• Anticonvulsants/blood
• Anticonvulsants/therapeutic use
• Child
• Community Health Planning
• Epilepsy/blood
• Epilepsy/drug therapy
• Female
• Humans
• Male
• Middle Aged
• Phenytoin/blood
• Phenytoin/therapeutic use
• Retrospective Studies
• Statistics as Topic
• Young Adult

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Grants

• T32 ES007271 NIEHS NIH HHS
• TL1 TR002000 NCATS NIH HHS
• UL1 TR000042 NCATS NIH HHS
• UL1 TR002001 NCATS NIH HHS

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Affiliation(s)

Olga Selioutski Department of Neurology, Strong Epilepsy Center, University of Rochester School of Medicine and Dentistry, Rochester, NY, USA.
Katherine Grzesik Department of Biostatistics, University of Rochester School of Medicine and Dentistry, Rochester, NY, USA
Olga N Vasilyeva Department of Pharmacy, University of Rochester School of Medicine and Dentistry, Rochester, NY, USA
Ágúst Hilmarsson Landspitali University Hospital, Slettuvegi, Reykjavik, Iceland
A James Fessler Department of Neurology, Strong Epilepsy Center, University of Rochester School of Medicine and Dentistry, Rochester, NY, USA
Lynn Liu Department of Neurology, Strong Epilepsy Center, University of Rochester School of Medicine and Dentistry, Rochester, NY, USA
Robert A Gross Department of Neurology, Strong Epilepsy Center, University of Rochester School of Medicine and Dentistry, Rochester, NY, USA

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Effects of sodium valproate on the serum protein binding of phenytoin, and on liver enzyme activity

The effect of a single dose of sodium valproate on the serum protein binding of phenytoin was studied in 6 epileptic patients receiving phenytoin maintenance therapy. Phenytoin was significantly displaced by valproic acid, but the free concentration of phenytoin was unchanged because of redistribution to tissues. As a result of this effect, total serum phenytoin concentration was significantly lowered. Induction of metabolism probably does not play a part in this interaction because antipyrine half-lives and urinary D-glucaric acid excretion were not altered by chronic administration of sodium valproate to 8 patients receiving this drug alone.

[Effect of capecitabine therapy on the blood levels of antiepileptic drugs - report of two cases]

We report the cases of 2 breast cancer patients who received capecitabine(CAP)and concomitant anticonvulsant therapy with either phenytoin(PHT)or valproate(VPA)for brain metastasis. The effect of CAP on the blood levels of the 2 anticonvulsants was different and it depended on the variation in metabolism of each drug. Case 1 involved a 59-year-old woman with recurrent breast cancer. After radiation therapy for brain metastases, the patient received PHT(400mg/day)to prevent convulsions. After 5 days of PHT administration, CAP therapy was initiated, and her blood PHT levels increased to 33.8 mg/mL. Although the PHT dose was reduced to 300mg/day, the blood PHT levels markedly increased to 45.5 mg/mL 7 days after the withdrawal of CAP. Case 2 involved a 60-year-old woman with breast cancer who underwent surgery for brain metastases and subsequently received controlled-release VPA tablets(400mg/day). No remarkable change was observed in her blood VPA levels during CAP treatment or after CAP withdrawal(the blood VAP level after 7 days of treatment was, 78.4 mg/mL; after 14 days of treatment, 73.2 mg/mL; and 7 days after withdrawal, 79.7 mg/mL). CAP has been reported to inhibit nucleic acid synthesis and/or folic acid activity rather than cytochrome P450(CYP)directly. CAP had a significant effect on the blood levels of PHT, which is metabolized via the CYP pathway. However, VPA levels remained unchanged because VPA metabolism involves other pathways, such as the beta-oxidation and conjugation pathways.

Therapeutic hypothermia decreases phenytoin elimination in children with traumatic brain injury

OBJECTIVE

Preclinical and clinical studies have suggested that therapeutic hypothermia, while decreasing neurologic injury, may also lead to drug toxicity that may limit its benefit. Cooling decreases cytochrome P450 (CYP)-mediated drug metabolism, and limited clinical data suggest that drug levels are elevated. Fosphenytoin is metabolized by cytochrome P450 2C, has a narrow therapeutic range, and is a commonly used antiepileptic medication. The objective of this study was to evaluate the impact of therapeutic hypothermia on phenytoin levels and pharmacokinetics in children with severe traumatic brain injury.

DESIGN

Pharmacokinetic analysis of subjects participating in a multicenter randomized phase III study of therapeutic hypothermia for severe traumatic brain injury.

SETTING

ICU at the Children's Hospital of Pittsburgh.

PATIENTS

Nineteen children with severe traumatic brain injury.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

A sum of 121 total and 114 free phenytoin levels were evaluated retrospectively in 10 hypothermia-treated and nine normothermia-treated children who were randomized to 48 hours of cooling to 32-33°C followed by slow rewarming or controlled normothermia. Drug dosing, body temperatures, and demographics were collected during cooling, rewarming, and posttreatment periods (8 d). A trend toward elevated free phenytoin levels in the hypothermia group (p=0.051) to a median of 2.2 mg/L during rewarming was observed and was not explained by dosing differences. Nonlinear mixed-effects modeling incorporating both free and total levels demonstrated that therapeutic hypothermia specifically decreased the time-variant component of the maximum velocity of phenytoin metabolism (Vmax) 4.6-fold (11.6-2.53 mg/hr) and reduced the overall Vmax by ~50%. Simulations showed that the increased risk for drug toxicity extends many days beyond the end of the cooling period.

CONCLUSIONS

Therapeutic hypothermia significantly reduces phenytoin elimination in children with severe traumatic brain injury leading to increased drug levels for an extended period of time after cooling. Pharmacokinetic interactions between hypothermia and medications should be considered when caring for children receiving this therapy.

The association between CYP 2C9 polymorphism and bone health

PURPOSE

There is a strong scientific rationale to support the view that cytochrome P450 (CYP P450) enzyme-inducing AEDs induce bone loss in patients with epilepsy. However, no study has investigated the association between CYP 2C9 polymorphism and bone mineral density (BMD), 25-hydroxyvitamin D or parathyroid hormone levels in patients with epilepsy. This study sought to determine the association between BMD and CYP 2C9 polymorphism.

METHODS

Ninety-three patients taking phenytoin as monotherapy were examined for CYP 2C9 polymorphism, vitamin D level and parathyroid hormone level and underwent basic chemistry testing. The bone mineral density of the lumbar spine and left femur were measured using dual-energy X-ray absorptiometry.

RESULTS

The results indicated that about 18.3% of the patients with epilepsy were positive for CYP2C9*3. Furthermore, bone mineral density was associated with CYP 2C9 polymorphism epileptic patients. Specifically, patients with 2C9 polymorphism had higher T-scores and Z-scores of the femoral neck (p=0.02 and 0.04, respectively), but not of the lumbar spine (p=0.27 and 0.06, respectively). There was also a trend of having higher serum PTH levels and statistically significantly lower 25-hydroxyvitamin D levels in patients with wild type than in those compared with CYP 2C9 polymorphism (p=0.05 and 0.03, respectively). Additionally, the patients with CYP 2C9 polymorphism had higher plasma levels of phenytoin, particularly when compared with those with wild type (p=0.01). However, there was no association between serum levels of phenytoin and low BMD at femoral neck or lumbar spine.

CONCLUSION

CYP 2C9 polymorphism is associated with higher BMD, independent of plasma levels of phenytoin.

Phenotypic interaction of simultaneously administered isoniazid and phenytoin in patients with tuberculous meningitis or tuberculoma having seizures

Treatment of tuberculous meningitis or tuberculoma has become complicated because of adverse drug interactions found amongst antitubercular and anticonvulsant drugs. The aim of the study is to evaluate the effect of simultaneously administered isoniazid (300 mg/day) and phenytoin (300 mg/day) on 60 patients with tuberculous meningitis or tuberculoma having seizures. Plasma samples were analyzed for isoniazid, acetylated-isoniazid (AcINH) and phenytoin levels by high performance liquid chromatography at 3h of drugs administration and patients were classified as rapid or slow acetylator on the basis of metabolic ratio of isoniazid (Rm) and percentage of acetylated-isoniazid (%AcINH). Out of 60 patients studied, 23 were slow acetylators and 37 were rapid acetylators. Slow acetylators revealed higher plasma isoniazid levels and lower plasma AcINH levels, metabolic ratio and %AcINH as compared to rapid acetylators. Plasma phenytoin levels were found to be significantly higher (above therapeutic range) in slow acetylators as compared to rapid acetylators. Plasma phenytoin concentration was moderately strong, negatively correlated with metabolic ratio (r=-0.439, P<0.001) and %AcINH (r=-0.729, P<0.001). Eight comatose patients (34.8%) also showed significantly higher plasma phenytoin levels. Our results suggest that assessment of acetylator status and plasma phenytoin level is critical for dose optimization of isoniazid and phenytoin and to predict the patients at risk of intoxication.

Variability of carbamazepine and valproate concentrations in elderly nursing home residents

PURPOSE

Measuring antiepileptic drug (AED) concentrations is common practice in nursing homes. Phenytoin (PHT) concentrations fluctuate substantially in many nursing home residents under constant dose conditions; however, the stability of other AED concentrations has not been studied. We investigated the variability of carbamazepine (CBZ) and valproate (VPA) concentrations under constant dose conditions in US nursing home residents.

METHODS

A database of elderly persons (≥65 years) in 119 nursing homes throughout the US was reviewed for residents with at least one measurement of total PHT, CBZ or VPA. Inclusion criteria for this study were three or more serum concentration measurements while on the same dose of CBZ or VPA, a two-month minimum stay, and no interfering co-medications (inducers or inhibitors). Enrollment occurred over a 2-year period. Data were collected on residents for a minimum of 6 months.

KEY FINDINGS

Of the 593 residents identified, 245 had CBZ or VPA concentrations measured and 44 (18%) met inclusion criteria (22 on CBZ and 22 VPA). Some subjects had little variability in AED concentrations, others had large fluctuations. Total CBZ concentrations within individuals varied as little as 0mg/L to as much as 6.3mg/L and total VPA concentrations as little as 10.0mg/L to as much as 77.6mg/L.

SIGNIFICANCE

The variability of PHT, CBZ, and VPA concentrations in many but not all nursing home residents implies that a re-evaluation of the role of AED concentration measurements in the management of patients is needed. Strategies for use and interpretation of AED concentration measurements need to be reevaluated.

Charcoal hemoperfusion for phenytoin intoxication

Accidental ingestion of phenytoin can lead to severe neurological sequelae. Charcoal hemoperfusion decreases phenytoin levels but has never been reported before in children. We present a child with accidental ingestion of phenytoin who responded to charcoal hemoperfusion.

[Effect of high altitude hypoxia on the activity and protein expression of CYP2C9 and CYP2C19]

This study is to investigate the effect of high altitude hypoxia on the activity and protein expression of CYP2C9 and CYP2C19. Rats from plain (P) and rats with acute middle altitude hypoxia (AMH), chronic middle altitude hypoxia (CMH), acute high altitude hypoxia (AHH) and chronic high altitude hypoxia (CHH) were administered orally phenytoin sodium (PHT) and omeprazole (OMZ) to evaluate the activity of CYP2C9 and CYP2C19, separately. The serum concentrations of PHT and metabolite 4'-hydroxyphenytoin (HPPH) at 12 h after treatment and the serum concentrations of OMZ and metabolite 5-hydroxy omeprazole (5-OHOMZ) at 3 h after treatment were determined by RP-HPLC. The activity of CYP2C9 and CYP2C19 was evaluated by the ratio of HPPH to PHT and the ratio of 5-OHOMZ to OMZ, respectively. The protein expressions of CYP2C9 and CYP2C19 were determined by ELISA method. The activities of CYP2C9 (HPPH/PHT) in P, AMH, CMH, AHH and CHH were 0.67 +/- 0.31, 0.75 +/- 0.29, 0.76 +/- 0.23, 0.79 +/- 0.31 and 0.75 +/- 0.18, respectively, and the activities of CYP2C19 (5-OHOMZ/OMZ) in P, AMH, CMH, AHH and CHH were 0.17 +/- 0.06, 0.20 +/- 0.10, 0.11 +/- 0.05, 0.37 +/- 0.13 and 0.19 +/- 0.05, respectively. The protein expressions of CYP2C9 in P, AMH, CMH, AHH and CHH were 4.20 +/- 1.27, 3.95 +/- 0.81, 3.93 +/- 1.11, 4.32 +/- 1.03 and 4.12 +/- 0.86 ng x g(-1), respectively, and the protein expressions of CYP2C19 in P, AMH, CMH, AHH and CHH were 3.91 +/- 1.82, 3.63 +/- 2.07, 2.55 +/- 0.85, 4.78 +/- 2.37 and 3.51 +/- 1.03 ng x g(-1), respectively. The activities and protein expressions of CYP2C9 in AMH, CMH, AHH and CHH were not significantly different with those of P. The protein expressions of CYP2C19 in AMH, CMH, AHH and CHH were not significantly different with those of P, but the activity of CYP2C19 in AHH was significantly higher than that of P. This study found significant changes in the activity of CYP2C19 under the special environment of acute high altitude hypoxia.

Correlation between serum and salivary phenytoin concentrations in Thai epileptic children

OBJECTIVE

To study the correlation between serum and salivary phenytoin concentration in Thai epileptic children.

MATERIAL AND METHOD

Children aged 5 to 12 years with diagnosed epilepsy who received phenytoin monotherapy seen in the neurological clinic at Queen Sirikit National Institute of Child Health were studied. The recruited patients were required to have good compliance, normal albumin level, and no evidence of cancer, HIV infection, hepatic, renal and salivary glands disease. Blood and saliva samples were collected and measured phenytoin level by fluorescence-polarization immunoassay technique.

RESULTS

Thirty patients, 19 males and 11 females, were studied. The average (mean +/- SD) age and weight were 8.24 +/- 2.09 years and 27.76 +/- 9.86 Kilograms. Both serum and salivary phenytoin levels correlated with phenytoin doses as exponential type (R2 = 0.4188, 0.3682, respectively). Equations for describing serum and salivary phenytoin levels by phenytoin dose were y = 0.7403e(0.3952x) and y = 0.1431e(0.3072x) respectively. Serum and salivary phenytoin levels were closely correlated as linear type (R = 0.880, R2 = 0.967). The obtained equation of this relationship was y = 10.165x, where y = serum phenytoin level and x = salivary phenytoin level. Adverse drug reactions were found in 5 patients (6.6%), horizontal nystagmus 2 cases, hirsutism 2 cases and gingival hyperplasia 1 case.

CONCLUSION

High correlation between serum and salivary phenytoin levels supported the use of saliva instead of blood for phenytoin monitoring in Thai children which were difficult in blood collection and had psychological trauma. The obtained equations in the present study could be applied for adjusting the dosage regimen and monitoring by using salivary phenytoin level in clinical practice.

[A case of toxicity caused by drug interaction between capecitabine and phenytoin in patient with colorectal cancer]

We present a case of toxicity caused by a drug interaction between capecitabine and phenytoin (PHT). The drug combination elevated the plasma level of PHT in a patient on chemotherapy with capecitabine for colorectal cancer. Our patient was a 44-year-old woman diagnosed with epilepsy in her 20's, being treated with valproic acid (VPA) and PHT. Adjuvant chemotherapy with capecitabine began following surgery for colorectal cancer. Seven weeks later, she developed numbness, dizziness, dysarthria and difficulty walking, and was hospitalized for investigation. Her serum PHT level was elevated at 35. 1 μg/ mL. This case suggests that when capecitabine and PHT are coadministered, PHT levels should be monitored frequently, and that PHT dosage should be adjusted accordingly if it cannot be replaced by an alternative anticonvulsant.

Oral dosing requirements for phenytoin in the first three months of life

BACKGROUND

Historically, physicians have been reluctant to maintain infants on phenytoin (PHT) following initial stabilization with intravenous loading doses, as therapeutic blood levels are difficult to achieve with conventional oral doses, and there is concern that high doses will result in toxicity.

OBJECTIVES

To determine the oral dose of PHT required to achieve therapeutic blood concentrations, without clinical toxicity, in the first weeks of life.

METHODS

Eight infants with seizures were treated with phenytoin from 2 weeks to 3 months of age. Total and free phenytoin concentrations, and urine phenytoin metabolite (p-hydroxyphenytoin) were measured every 2 weeks. Parents were asked to note seizure frequency and complete a questionnaire about possible side effects every 2 weeks.

RESULTS

No infants had seizures and no clinical side effects were noted. Doses required to achieve therapeutic serum concentrations ranged from 10-20mg/kg/day, considerably higher than doses required in adults. Free phenytoin levels were 8-13% of total serum concentrations, similar to ratios reported in adults.

CONCLUSION

To achieve therapeutic serum phenytoin levels in infants, doses of 10-20 mg/kg/day are required. These higher doses can be safely administered without clinical toxicity.

A common polymorphism in the SCN1A gene associates with phenytoin serum levels at maintenance dose

OBJECTIVES

A broad range of phenytoin doses is used in clinical practice, with the final 'maintenance' dose normally determined by trial and error. A common functional polymorphism in the SCN1A gene (one of the genes encoding the drug target) has been previously associated with maximum dose of phenytoin used clinically, and also maximum dose of carbamazepine, another antiepileptic drug with the same drug target.

METHODS

We have related variation at the SCN1A IVS5-91 G>A polymorphism to maximum dose and to maintenance dose of phenytoin in 168 patients with epilepsy treated with phenytoin. We also related genotype to phenytoin serum levels at maximum dose and at maintenance dose of phenytoin. We genotyped the polymorphism using an Applied Biosystems Taqman assay.

RESULTS

The polymorphism is associated with phenytoin serum concentration at maintenance dose (P=0.03). In a reduced cohort of 71 patients receiving phenytoin monotherapy this association is also significant (P=0.03). Neither association remains significant after Bonferroni correction for multiple testing.

CONCLUSIONS

These results are not a replication of the original study. They do, however, support the hypothesis that this polymorphism influences the clinical use of phenytoin. They also demonstrate the utility of using multiple phenotypes in pharmacogenetics studies, particularly when attempting to separate pharmacokinetic and pharmacodynamic effects. As the SCN1A polymorphism affects phenytoin pharmacodynamics, it is particularly useful to obtain data on serum levels in addition to dose because association of a pharmacodynamic variant may be stronger with serum levels than dose as the serum level may eliminate or reduce pharmacokinetic variability.

Modulation of antiepileptic effect of phenytoin and carbamazepine by melatonin in mice

The pharmacokinetic and pharmacodynamic interaction of phenytoin and carbamazepine with melatonin was studied in a maximal electroshock seizure (MES) model in mice. The anticonvulsant ED(20), ED(33), ED(50) and ED(100) of phenytoin and carbamazepine, and ED(50) of melatonin were determined. Thereafter, the subanticonvulsant doses of phenytoin and carbamazepine were combined with ED(50) dose of melatonin. In combination with melatonin, 100% protection against seizures was achieved with phenytoin and carbamazepine in doses as low as ED(50) and ED(33), respectively. Serum levels of phenytoin and carbamazepine in animals that received ED(50) dose of phenytoin and carbamazepine per se, were not significantly different to those of the groups that received melatonin also. The study suggests that the synergistic antiepileptic effect is most likely a pharmacodynamic interaction, and not due to pharmacokinetic changes. Melatonin, thus, can be a potential adjunct to antiepileptic drugs, achieving a therapeutic effect at lower concentrations, hence limiting their dose-related toxicities.

Differences between the measured and calculated free serum phenytoin concentrations in epileptic patients

PURPOSE

The pharmacokinetics of phenytoin is complicated by genetic and environmental differences. It is, therefore, important to monitor the serum concentrations in patients who receive phenytoin. Because most of the phenytoin in serum is bound to proteins, the level of serum albumin influences the amount of free phenytoin.

MATERIALS AND METHODS

We compared the measured and calculated free phenytoin levels in epileptic patients who were taking phenytoin monotherapy, using the Sheiner-Tozer equation. A total of 49 patients (30 men and 19 women; age range, 15 - 87 years) were included in the study and their trough serum phenytoin and albumin concentrations were analyzed.

RESULTS

The linear correlation between free and total phenytoin concentrations was moderate (r = 0.822, p < 0.001). The mean difference between measured and calculated free phenytoin was large (0.65 +/- 0.88 microg/mL; 95% confidence interval (CI), -1.11 to 2.41). After dividing the patients into groups by albumin concentration, hypoalbuminemic patients (< 3.5 g/dL) more often had a greater percent difference (> or = 20%) than observed in the normoalbuminemic (> or = 3.5 g/dL) group.

CONCLUSION

In hypoalbuminemic patients, the measurement of free phenytoin level is necessary to properly evaluate the phenytoin level than that calculated from total phenytoin level.

Free phenytoin concentration measurement in brain extracellular fluid: a pilot study

This article investigates the relationship between brain extracellular fluid free phenytoin concentration and plasma free phenytoin concentration in adults with acute brain injury. Daily cerebral microdialysate free phenytoin concentration was measured in eight adults with acute brain injury and compared with simultaneous measurement of plasma free phenytoin concentration. The group data revealed no significant correlation between microdialysate and plasma free phenytoin concentration (r = 0.34, p = 0.41). However, in two patients, with a sufficient number of samples for intra-individual analysis, there was a significant correlation between microdialysate and plasma free phenytoin concentration (r = 0.92, p < 0.001 and r = 0.88, p < 0.01). In vitro microdialysis relative recovery for phenytoin was 2.1%. In the context of acute brain injury, measurement of free plasma phenytoin concentration may not provide an accurate reflection of regional brain extracellular fluid free phenytoin concentration and may have limitations with respect to achieving reproducible brain extracellular fluid free phenytoin concentrations. This has implications for dosing regimens relying on plasma phenytoin levels.

Changes in plasma phenytoin level following craniotomy

Phenytoin is often used to prevent postcraniotomy seizures, but is not always effective. We investigate changes in plasma phenytoin level ([phenytoin]) following craniotomy. The [phenytoin] in 28 patients who were receiving phenytoin (oral/ intravenous) and undergoing a craniotomy were prospectively measured 24 h preoperatively, immediately pre- and postcraniotomy, 24 and 48 h postoperatively. Factors examined included patients' age, sex, pathology, preoperative [phenytoin], operative duration and blood loss. Fifteen patients had [phenytoin] concentrations outside the therapeutic range. Twenty-five patients experienced a decrease in [phenytoin] immediately postcraniotomy: pre-, post- and 24 h postcraniotomy mean [phenytoin] were 13.4, 10.0 and 12.9 mg/l, respectively. Preoperative [phenytoin], operative duration and blood loss had significant correlation with the decrease in [phenytoin] (p < 0.05). In conclusion, < 50% of the patients had therapeutic preoperative [phenytoin]. In most patients, [phenytoin] decreases by 26% after craniotomy and returns to preoperative level within 24 h. These may contribute to early postoperative seizure development.

The effect of different oral anticoagulants on diphenylhydantoin (DPH) and tolbutamide metabolism

The effect of bishydroxycoumarin, phenprocoumon, warfarin and phenindione on the metabolism of diphenylhydantoin (DPH) and tolbutamide has been studied in 54 patients. The half-lives of DPH and tolbutamide in blood following i.v. injections were studied in 33 patients before and after one week of anticoagulant treatment. Bishydroxycoumarin increased the mean half-life values of DPH from 8.8 to 37.4 hours and of tolbutamide from 4.9 to 17.5. Phenprocoumon prolonged DPH half-life from a mean value of 9.9 to 14.0 hours but did not change the tolbutamide half-life. Warfarin and phenindione did not affect DPH or tolbutamide half-lives. Steady state concentration studies in 21 patients showed a rise in serum DPH during bishydroxycoumarin and phenprocoumon treatment but not during treatment with warfarin and phenindione. A rise in serum tolbutamide was noted during treatment with bishydroxycoumarin. These findings suggest that bishydroxycoumarin inhibits the betabolism of DPH and tolbutamide and that phenprocoumon inhibits DPH metabolism. No effect on DPH and tolbutamide metabolism could be demonstrated following administration of warfarin and phenindione.

The effect of different sulfonamides on phenytoin metabolism in man

The influence on the metabolism of phenytoin of some sulfonamides given in common clinical doses has been studied. In single dose experiments sulfaphenazole increased phenytoin half-life (T/2) by 237% and decreased phenytoin metabolic clearance rate (MCR) by 67%. Sulfadiazine, sulfamethiazole, sulfamethoxazole + trimethoprim and trimethoprim increased phenytoin T/2 by 80, 66, 39 and 51% respectively, and decreased phenytoin MCR by 45, 36, 27 and 30% respectively. Sulfamethoxazole gave a small but significant increase in phenytoin T/2 but not a corresponding fall in phenytoin MCR. No changes were found in phenytoin T/2 and MCR after treatment with sulfamethoxypyridazine, sulfadimethoxine and sulfamethoxydiazine. Steady state experiments confirmed the findings of the single dose experiments. It is suggested that sulfaphenazole, sulfadiazine, sulfamethizole, sulfamethoxazole + trimethoprim and trimethoprim inhibit hepatic metabolism of phenytoin.

Topiramate and zonisamide prevent paradoxical intoxication induced by carbamazepine and phenytoin

The mechanisms of paradoxical aggravation of epileptic seizures induced by selected antiepileptic drugs (AEDs) remain unclear. The present study addressed this issue by determining the seizure-threshold doses of carbamazepine (CBZ) and phenytoin (PHT), as well the dose-dependent effects of CBZ, PHT, and carbonic anhydrase-inhibiting AEDs, acetazolamide (AZM), topiramate (TPM), and zonisamide (ZNS), on neurotransmitter release in rat hippocampus. The dose-dependent effects of AEDs on hippocampal extracellular levels of glutamate (Glu), GABA, norepinephrine (NE), dopamine (DA), and serotonin (5-HT) were determined by microdialysis with high-speed and high-sensitive extreme liquid chromatography. Proconvulsive effects of AEDs were determined by telemetric-electrocorticography. Therapeutically relevant doses of AZM, CBZ, TPM, and ZNS increased hippocampal extracellular levels of GABA, NE, DA, and 5-HT, while PHT had no effect. Supratherapeutic doses of AZM, CBZ, PHT, TPM, and ZNS decreased extracellular levels of GABA, NE, DA, and 5-HT, without affecting Glu levels. Toxic doses of CBZ and PHT produced seizures (paradoxical intoxication), markedly increasing all transmitter levels, but TPM and ZNS even at toxic doses did not produce seizure. Co-administration experiments showed that therapeutically relevant doses of CBZ or PHT reduced the seizure-threshold doses of PHT or CBZ, respectively. In contrast, therapeutically relevant doses of AZM, TPM, and ZNS elevated the seizure-threshold doses of CBZ and PHT. These results suggested that blockade of high percentage of the population of voltage-dependent sodium channels by CBZ and PHT might be important in inducing paradoxical intoxication/reaction, and that inhibition of carbonic anhydrase inhibits this effect. TPM and ZNS are candidate first-choice agents in treatment of epilepsy when first-line AEDs are ineffective.

Enhanced clearance of highly protein-bound drugs by albumin-supplemented dialysate during modeled continuous hemodialysis

BACKGROUND

In 2006, there were 16 796 toxic exposures attributed to valproic acid (VPA), carbamazepine (CBZ) and phenytoin (PHT) reported to the US Toxic Exposure Surveillance System. Of these, 30% (5046) were treated in a health care facility with 12 cases resulting in death. These drugs are highly protein bound and poorly dialyzable; however, it has been suggested that albumin-supplemented dialysate may enhance dialytic clearance. We investigated whether the addition of albumin to dialysate affects dialytic clearance of VPA, CBZ and PHT.

METHODS

VPA, CBZ and PHT were added to a bovine blood-based in vitro continuous hemodialysis circuit, which included a polysulfone or an AN69 hemodialyzer. VPA, CBZ and PHT clearances were calculated from spent dialysate and pre-dialyzer plasma concentrations. VPA, CBZ and PHT clearances with control (albumin-free) dialysate were compared to clearances achieved with 2.5% or 5% human albumin-containing dialysate. The influences of blood flow (180 and 270 mL/min) and dialysate flow (1, 2 and 4 L/h) on dialysis clearance were also assessed.

RESULTS

The addition of 2.5% albumin to dialysate significantly enhanced dialytic clearance of VPA and CBZ, but not PHT. Use of 5% albumin dialysate further increased VPA and CBZ clearance. Overall, drug clearance was related directly to dialysate flow but independent of blood flow.

CONCLUSION

Continuous hemodialysis with albumin-supplemented dialysate significantly enhanced VPA and CBZ, but not PHT, clearance compared to control dialysate. Continuous hemodialysis with albumin-supplemented dialysate may be a promising therapy to enhance dialytic clearance of selected highly protein-bound drugs.

Elevated thyroid-stimulating hormone in a 65-year-old patient with phenytoin toxicity

Phenytoin has a narrow therapeutic range and exhibits concentration-dependent kinetics within this therapeutic window. Therefore, small changes in dose or metabolism can have a large effect on serum concentrations. Therapeutic drug monitoring may be helpful in identifying drug toxicity. A 65-year-old male with a history of hydrocephalus was admitted for generalized weakness and altered mental status. Laboratory testing revealed an elevated phenytoin level, and the patient's phenytoin was discontinued. Further laboratory tests led to the diagnosis of hypothyroidism, and treatment was initiated. The complicated interaction between phenytoin and thyroid hormones is described. Pharmacists can play a role in therapeutic drug monitoring and the identification of drug-disease interactions. Pharmacist drug counseling can help patients and family members identify adverse effects and toxicity and know when to seek medical assistance.

Therapeutic drug monitoring of antiepileptic drugs

Commonly used conventional antiepileptic drugs for pharmacotherapy in epilepsy are phenytoin, carbamazepine and valproic acid. These drugs have complex pharmacokinetic properties leading to fluctuation in their plasma level at given same therapeutic dose. The present study was done to monitor their plasma levels. A prospective observational study was conducted at National Public Health Laboratory. After taking detail history, blood samples were taken from epileptic patients of all age groups and both gender who were on usual therapeutic dose of one or two combined antiepileptic drugs. Plasma level of these drugs were analyzed by using Fluorescence Polarization Immuno Assay (FPIA) technique. Out of total 417 testing, 81 were tested for phenytoin , 241 for carbamazepine and 95 for valproic acid. Their levels were further analyzed to find therapeutic, subtherapeutic and toxic levels. Out of total 81 blood samples tested for phenytoin, 38.8% had plasma drug at therapeutic level, 38.8% at subtherapeutic level and 28.4% had toxic level. Carbamazepine was tested in 241 samples and 79.3% cases had at therapeutic drug level, 15.8% had subtherapeutic drug level and 4.9% had toxic level. Out of 95 samples tested for valproic acid, 62% had therapeutic level and 20% had subtherapeutic and 18% had toxic level of drug. Therapeutic drug monitoring of phenytoin showed wide fluctuation in its plasma level. Its toxic and subtherapeutic levels were quite high. It is suggested that the dose of phenytoin should be adjusted after regular plasma level monitoring only. Monitoring of carbamazepine and valproic acid were also helpful when their toxicity and efficacy are doubtful.

Effects of etoricoxib on the pharmacokinetics of phenytoin

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.

Fractionated stereotactic radiotherapy boost and weekly paclitaxel in malignant gliomas clinical and pharmacokinetics results

Management of Malignant Gliomas continues to be a challenge. We prospectively studied the role of adding weekly Paclitaxel to Fractionated Stereotactic Radiation Therapy (FSRT) in the treatment of Malignant Gliomas. Twenty-three Glioblastoma Multiforme and two Anaplastic Astrocytoma were studied. Patients received 46 Gy at 2 Gy/fraction followed by a boost utilizing FSRT at a fraction of 2.5 Gy for 8 fractions. Paclitaxel is delivered concomitantly at 150 mg/m(2) weekly for six cycles. Eighteen patients had pharmacokinetic assays of Paclitaxel levels. All patients were followed until death or for a maximum of 36 months. The overall survival of the whole group was 14 months. The median survival for RPA prognostic classes III, IV, V, and VI were 20, 14, 12, and 11 months. Higher survival (14 months) was noted in the subtherapeutic phenytoin level group compared to 10 months in the therapeutic group (P=0.271). No grade 4 CTCAE (version 3.0) toxicities were observed. Enhanced survival was demonstrated with gross tumor resection (20.8 months), KPS > or =80 (18.7 months) and age < or =60 years (27 months) as compared to subtotal resection or biopsy (12.1 months, P< 0.005), KPS < or =70 (10.8 months, P=0. 005) and older age > 60 (10.46 months, P=0.006), respectively. Our study suggests that: i) the use of weekly Paclitaxel and FSRT in Gliomas is well tolerated with a survival of 14 months; ii) the regimen resulted in improvement of survival of RPA classes IV, V, VI; and iii) the use of FSRT boost may be studied with other chemotherapeutic agents to see if superior results can be attained.

Contributions of CYP2C9/CYP2C19 genotypes and drug interaction to the phenytoin treatment in the Korean epileptic patients in the clinical setting

We examined the contribution of CYP2C9 and CYP2C19 genotypes and drug interactions to the phenytoin metabolism among 97 Korean epileptic patients to determine if pharmacogenetic testing could be utilized in routine clinical practice. The CYP2C9 polymorphism is a wellknown major genetic factor responsible for phenytoin metabolism. The CYP219 polymorphism, with a high incidence of variant alleles, has a minor influence on phenytoin treated Koran patients. Using a multiple regression model for evaluation of the CYP2C9 and CYP2C19 genotypes, together with other non-genetic variables, we explained 39.6% of the variance in serum phenytoin levels. Incorporation of genotyping for CYP2C9 and CYP2C19 into a clinical practice may be of some help in the determination of phenytoin dosage. However, because concurrent drug treatment is common in patients taking phenytoin and many environmental factors are likely to play a role in drug metabolism, these factors may overwhelm the relevance of CYP polymorphisms in the clinical setting. Further investigations with an approach to dose assessment that includes comprehensive interpretation of both pharmacogenetic and pharmacokinetic data along with understanding of the mechanism of drug interactions in dosage adjustment is warranted.

Robust isocratic high performance liquid chromatographic method for simultaneous determination of seven antiepileptic drugs including lamotrigine, oxcarbazepine and zonisamide in serum after solid-phase extraction

A rapid, simple and robust method is presented for the simultaneous determination of seven antiepileptic drugs (AEDs), including primidone, phenobarbital, phenytoin, carbamazepine with its two major metabolites carbamazepine-10,11-epoxide and carbamazepine-10,11-(trans)-dihydrodiol and the new AEDs lamotrigine, hydroxycarbazepine (active metabolite of oxcarbazepine) and zonisamide in serum by high performance liquid chromatography (HPLC)-diode array detector (DAD). After solid-phase extraction, separation is achieved on an Alltima 3C18 analytical column using isocratic elution with a mixture of acetonitrile, methanol and phosphate buffer at 45 degrees C. The method is exhaustively validated, including experimental design in combination with statistical evaluation (ANOVA) to study the robustness of chromatography and sample preparation. Commonly co-administered antiepileptic drugs do not interfere with the method. Intra-day precision (RSD<1.9%), linearity, lower limit of quantitation (LOQ<0.065 mg/l) and robustness make the method suitable for daily therapeutic drug monitoring and pharmacokinetic studies.

Possible Mechanisms for the Pharmacokinetic Interaction Between Phenytoin and Folinate in Rats

The plasma concentration of phenytoin (PHT) is decreased by coadministration of folinate (leucovorin; LV), a folate (FA) analogue. The aim of this study was to examine the effect of LV on the pharmacokinetics of PHT in rats in vivo and to investigate the mechanism of the interaction. LV (50 mg/kg) was administered orally to rats concomitantly given intravenous PHT (50 mg/kg) to evaluate the effect of LV on the pharmacokinetics of PHT. The effect of LV on the plasma protein binding of PHT was investigated by using plasma from rats that had received oral LV. We also examined the effects of LV on the uptake of PHT into isolated rat hepatocytes and on the metabolism of PHT in isolated rat hepatocytes and rat hepatic microsomes. LV significantly increased the systemic clearance (2-fold) and liver-to-blood partition coefficient (1.24-fold) of PHT. However, it did not affect the plasma protein binding or hepatic uptake of PHT. LV increased the metabolism of PHT in isolated rat hepatocytes, with a significant 1.41-fold increase in the maximum rate of metabolism and a decrease in the Michaelis-Menten constant. On the other hand, 5-methyltetrahydrofolate (5-MTHF), a primary metabolite of LV and FA, significantly increased p-hydroxylation of PHT in rat hepatic microsomes, whereas LV and FA themselves had no effect. In conclusion, these results suggest that, in rats, LV, an FA analogue, decreases the plasma concentration of PHT by increasing the hepatic metabolism of PHT, and the increase in the PHT metabolism is, at least in part, attributable to 5-MTHF.

Therapeutic drug monitoring and clinical outcomes in epileptic Egyptian patients: a gene polymorphism perspective study

This work was performed to explore the effect of polymorphism in multidrug resistant genes on plasma phenytoin levels and patient outcome to evaluate its involvement in drug resistance and toxicity, which is usually associated with antiepileptic drugs. Therefore, we genotyped the adenosine triphosphate-binding cassette subfamily B member 1 (ABCB1) in 100 patients suffering from partial or generalized tonic-clonic seizures and receiving phenytoin and 50 healthy control subjects. Steady state plasma phenytoin levels were also determined in the epileptic patients. Patients were evaluated after 3 and 6 months and were classified either as drug resistant patients or responsive patients. Results revealed 37 patients with drug responsive epilepsy and 63 patients with drug resistant epilepsy. Genotyping of our patients and control subjects revealed a genotype distribution of CC, CT, TT: 55.50%, 38.00%, 6.50% for drug resistant patients, CC, CT, TT: 13.50%, 46.00%, 40.50% for drug responsive patients, and CC, CT, TT: 24.00%, 48.00%, 28.00% for the control subjects. Patients with drug-resistant epilepsy were more likely to have the CC than the TT genotype compared with either responsive patients (P < 0.0001) or control subjects (P < 0.0001). The C polymorphism was over-represented among patients with drug-resistant epilepsy as compared with either those with drug-responsive epilepsy (P < 0.001) or control subjects (P < 0.001). Of the total 100 epileptic patients, 13 patients had their plasma phenytoin levels exceeding the maximum safe concentration. These 13 patients were more likely to have TT genotype than the CC genotype compared with the remainder of patients who had their plasma phenytoin levels at 20 microg/mL or less. Responsive patients showed no deviation from the control group regarding the genotype (P > 0.05) or allele frequency (P > 0.05). In conclusion, because most of the antiepileptic drugs are multidrug resistant gene substrates, the ABCB1 is thus an important candidate gene for potentially influencing the response to antiepileptic drugs. Our findings suggest that using genotype data may make it possible to safely reduce the time required to reach an effective dose. Therefore, it is a priority to assess the utility of dose adjustment on the basis of genotype for these medicines that are substrates for this gene.

Factors affecting antiepileptic drug pharmacokinetics in community-dwelling elderly

Because aging is associated with changes in physiological processes, it is widely believed that antiepileptic drug pharmacodynamics and pharmacokinetics in elderly patients differ from those in younger adults. In order to better characterize these differences, this chapter reports on preliminary results from an investigation of the effect of age on steady-state phenytoin (PHT) and carbamazepine (CBZ) pharmacokinetics. Parenteral formulations of stable-labeled PHT, fosphenytoin (FOS), and CBZ were administered to elderly (> or =65 years of age) and adult (18-64 years of age) patients on maintenance regimens of PHT or CBZ; a labeled 100-mg dose was infused over 10 min, then the remainder of the patient's AED dose was administered as unlabeled drug. Blood samples were collected just before administration of the labeled drug and for up to 192 h afterward. Samples were then assayed for the concentrations of labeled and unlabeled drug. Preliminary results from 60 patients on PHT therapy (41 elderly, mean age 76 years; 19 younger adults, mean age 41 years) indicate that PHT bioavailability did not differ between the two age groups; however, absorption and elimination half-lives were more variable in the elderly patients. The elimination half-life for the entire patient population was approximately twofold longer than the value reported in the product labeling (40-50 h vs 22 h). Preliminary results from 67 patients on CBZ therapy (14 elderly, mean age 70 years; 53 younger adults, mean age 41 years) showed no apparent difference between elderly and adult patients in any parameter; however, the mean CBZ elimination half-life for the combined groups (21 h) was longer than previous estimates. These results indicate that the effect of age on CBZ and PHT absorption may result in greater variability in plasma concentrations in elderly patients, whereas the effect on half-life is modest.

Drug interaction assessment following concomitant administration of posaconazole and phenytoin in healthy men

OBJECTIVE

Posaconazole is an extended-spectrum triazole antifungal agent for the treatment and prophylaxis of invasive fungal infections. This randomized, open-label, parallel-group, multiple-dose study was conducted in healthy adult volunteers to assess the potential for a drug interaction between phenytoin and the posaconazole tablet formulation.

METHODS

Subjects were randomly assigned for 10 days to one of the following treatments: posaconazole (200 mg once daily), phenytoin (200 mg once daily), or posaconazole (200 mg once daily) and phenytoin (200 mg once daily). Blood samples were collected on days 1 and 10 for pharmacokinetic evaluation of posaconazole and phenytoin concentrations.

RESULTS

A total of 36 healthy men enrolled in the study. On day 1, the maximum plasma concentration (C(max)) and area under the concentration-time curve calculated from time 0-24 h post-dose (AUC(0-24)) were unchanged upon co-administration. At steady state (day 10), co-administration of posaconazole with phenytoin resulted in 44% (p = 0.012) and 52% (p = 0.007) decreases in posaconazole C(max) and AUC(0-24), respectively. These decreases in exposure corresponded with a 90% increase in steady-state clearance of orally administered posaconazole. Phenytoin C(max) and AUC(0-24) were not significantly altered upon co-administration of the two agents, 24% increase in C(max) (p = 0.196) and 25% increase in AUC(0-24) (p = 0.212) values, although inter-subject variability was observed within this group.

CONCLUSION

Because co-administration of phenytoin and posaconazole significantly reduces posaconazole exposure and increases phenytoin levels in some subjects, concomitant use of these agents should be avoided unless the benefit outweighs the risk.

Phenytoin immunoassay measurements in serum samples from patients with renal insufficiency: comparison with high-performance liquid chromatography

The debate continues regarding the possible interference of phenytoin metabolites in phenytoin immunoassays, and its clinical importance for patients with renal failure. The aim of this study was to compare the results obtained using the Abbott fluorescence polarization immunoassay (FPIA), Dade enzyme-multiplied immunoassay technique (EMIT), and high-performance liquid chromatography (HPLC) to establish the significance of the differences in conditions of renal failure. Thirty-six adult patients who had been treated with phenytoin and whose renal function ranged from normal to severely impaired were chosen for this study. In accordance with previously established validation criteria for analytical methods for the determination of drugs, a 15% bias from the HPLC phenytoin values was considered an acceptable limit. The mean (+/-SEM) glomerular filtration rate (GFR) of the patients was 37.5+/-4.6 mL/min (range = 10-102 mL/min). The mean values found using FPIA (10.8+/-1.2 microg/mL) and EMIT (10.8+/-1.3 microg/mL) presented acceptable deviations with respect to HPLC (10.5+/-1.2 microg/mL), and a high correlation was found among the results (N = 36) of the different methods (r > or = 0.987, P < 0.001). An FPIA deviation above the 15% bias limit with respect to HPLC was found only in two cases with very low serum phenytoin concentrations and low GFR values (< 20 mL/min), although it does not appear to be important in terms of adjusting drug dosage. According to our data, FPIA and EMIT gave accurate results for total phenytoin in serum samples from patients with renal failure.