Phenytoin concentrations in mixed, parotid and submandibular saliva and serum measured by radioimmunoassay.

1 Concentrations of phenytoin in mixed, parotid and submandibular saliva and serum were determined in normal subjects after an oral dose, using a specific double antibody radioimmunoassay which requires only 20 micronl fluid. 2 Semi-log concentration-time plots of phenytoin concentration in mixed saliva and serum gave good parallelism after the initial 14 h post-administration period. 3 The mean ratio of the mixed saliva: serum phenytoin concentration was 10.3% +/- 1.5 (s.d.) in seven normal subjects. 4 Phenytoin concentrations found in separate parotid and submandibular salivary fractions did not differ but were significantly greater (P less than 0.001) than those found in mixed saliva. 5 Phenytoin concentrations in all salivary fractions were independent of the volume of fluid produced and the degree of stimulation. 6 The rate of phenytoin secretion in the parotid and submandibular fluid was proportional to the salivary flow rate. 7 These data suggest that mixed saliva may be a suitable medium for the monitoring of phenytoin concentrations and may provide a non-invasive alternative to the direct determination of phenytoin in serum.     

大鼠静注中毒剂量苯妥英钠后唾液与血浆及组织中药物浓度相关性研究

目的：探讨中毒剂量苯妥英钠（DPH—Na）给药后唾液与血浆、组织间浓度的相关性。方法：大鼠静注中毒剂量DPH-Na后。采集血液、组织及唾液样品,HPLC法测定其中DPH—Na浓度,线性回归法计算唾液与血浆及组织间浓度的相关关系。结果：大鼠DPH—Na中毒后在唾液中有少量排泄,唾液与血液及脑组织中DPH—Na浓度间存在良好相关关系,与唾液浓度／其它组织浓度比（S／T比）之间均存在显著相关性。结论：DPH—Na中毒后,可以通过测定唾液中浓度了解其在血液及组织中分布情况。     

Compliance with anticonvulsant therapy in a hospital clinic and in the community

1 Seizure control, saliva anticonvulsant concentration, prescribing habits and compliance with anticonvulsant medication have been compared in 86 epileptic subjects attending either a specialist hospital clinic or general practice surgeries. 2 Of all subjects experiencing recurrent seizures 70% had saliva concentrations of phenytoin below the range equivalent to the 'therapeutic range' of plasma concentration. Mean saliva phenytoin concentrations did not differ significantly between the two treatment settings and were low largely because of the low mean dosage prescribed. 3 Eleven subjects in all had no detectable phenytoin in their and could clearly be identified as noncompliant. Freedom from seizures appeared to predispose to poor compliance in these subjects as well as among those admitting repeated omission of doses.     

Improved dissolution and bioavailability of phenytoin by sulfobutylether-β-cyclodextrin ((SBE)7m-β-CD) and hydroxypropyl-β-cyclodextrin (HP-β-CD) complexation

The inclusion complexation of phenytoin with charged and neutral water-soluble cyclodextrins (CDs), (SBE)_7m-β-CD and HP-β-CD, was studied in order to improve the low aqueous solubility and incomplete oral bioavailability of phenytoin. Effects of CDs on the aqueous solubility of phenytoin were determined by phase-solubility method at pH 7.4 and 11.0. Solubility of phenytoin increased as a function of CD concentration, showing A_L type diagrams for both (SBE)_7m-β-CD and HP-β-CD which indicate a formation of 1:1-complexes. Solid inclusion complexes of phenytoin with (SBE)_7m-β-CD and HP-β-CD were prepared by freeze-drying. Dissolution rate of phenytoin was increased with inclusion complexes as well as with phenytoin/HP-β-CD physical mixture in vitro. Also the freeze-drying of phenytoin tended to enhance the dissolution of phenytoin in vitro. However, plain phenytoin (300.0 mg) pharmacokinetics after oral administration as a crystal form and as a freeze-dried form were comparable in dogs. CD-based formulations of phenytoin increased peak plasma concentration of phenytoin about 1.6-fold and bioavailability (AUC_{0–24 h}) of phenytoin about 2-fold compared to plain phenytoin. Oral pharmacokinetics were not statistically different among various CD formulations. This study indicates that increased bioavailability of phenytoin in the presence of CDs was due to an increased extent of drug dissolution.     

Effect of phenytoin on serum disopyramide concentrations

The effects of phenytoin on serum disopyramide concentrations in 10 volunteers were studied. Leading to the study was the case of a 59-year-old man who received phenytoin and disopyramide and who required unusually high doses of disopyramide and an unusually high serum concentration of this drug to control his ventricular tachycardia. Ten healthy men 23-36 years of age each received a single oral dose of disopyramide phosphate 300 mg. Periodic blood samples were obtained for 24 hours after the dose. On day 2, the subjects began a 13-day course of oral phenytoin sodium 300 mg/day, and on day 14 each again received a single oral dose of disopyramide, after which blood samples were obtained. Serum disopyramide concentrations were determined by enzyme-mediated immunoassay and gas chromatography and serum phenytoin concentrations by fluorescence polarization immunoassay. Pharmacokinetic values before and after phenytoin administration were calculated. The mean area under the serum concentration-time curve for disopyramide and the disopyramide half-life and elimination rate constant were significantly different before and after phenytoin treatment. The maximum serum disopyramide concentrations were not significantly different. After phenytoin therapy, subjective complaints of anticholinergic effects increased in number and severity. An interaction between disopyramide and phenytoin appears to exist and to be caused by an increase in the hepatic metabolism of disopyramide.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pharmacokinetics of adimolol after single and multiple dose administration in healthy volunteers

The pharmacokinetic properties of adimolol (MEN 935), a new antihypertensive agents with predominantly beta-receptor blocking and additional alpha-adrenolytic activity were investigated in healthy volunteers. Study A subjects (n = 6) received single intravenous doses of 5 mg adimolol and single oral doses of 200 mg capsules, 200 mg tablets and 100 mg tablets on four occasions separated by at least two weeks. Study B subjects (n = 6) were given single intravenous doses of 5 mg and single oral doses of 100 mg of the 14C-labelled drug on two different occasions. Study C subjects (n = 6) were administered multiple oral doses of 100 mg adimolol daily for five days, and three weeks later 50 mg daily for five days. Adimolol plasma concentrations were assayed over seven days following each single dose using a specific and sensitive high-pressure liquid chromatographic method. The plasma concentration data obtained from the single i.v. dose studies were individually fitted to an open four-compartment model. To describe mathematically the single oral dose plasma level data, two compartments were added to the model to take care of the absorption. Irrespective of the route of administration, the doses and formulations given, all plasma concentration curves could be described with similar pharmacokinetic parameters. Plasma concentration curves predicted by the open four-compartment model were fully confirmed by the actual data obtained after chronic oral administration. The terminal half-life averaged 12 h following intravenous and 15 h after oral administration. The peak plasma concentration was reached on average 4 h following oral administration.(ABSTRACT TRUNCATED AT 250 WORDS)     

苯妥英钠,卡马西平在血清,血清超滤液,唾液中浓度的监测和相关性探讨

本文应用超滤法测定血清中游离苯妥英钠、卡马西平浓度。并采用荧光偏振免疫法对患者血清、血清超滤液进行２种抗癫痫药物的测定及唾液中苯妥英钠液度的测定，研究它们的相关性。实验结果表明：唾液、超滤液中苯妥英钠浓度分别为血清药物浓度的１０．６３％和９．５１％，超滤液中卡马西平浓度为血清药物浓度的２２．５８％，且均具有良好的相关性，通过回收率及重现性试验证明本方法可用于常规监测。     

Monitoring phenytoin therapy using citric acid-stimulated saliva in infants and children

Two factors have limited the use of saliva in monitoring phenytoin therapy: availability of adequate volume of clear saliva and lack of a sensitive phenytoin assay. The applicability of citric acid-stimulated saliva and of a sensitive analytical assay (fluorescence polarization immunoassay, "TDx" Abbott) was evaluated in this study. Phenytoin was measured in paired plasma-saliva specimens from epileptic children during the long-term or the initial phase of phenytoin therapy. Analysis was carried out in plasma and in the clear supernatant of saliva (following centrifugation). Pooled-estimate SD of the analytical assay variability was 0.175 micrograms/ml for plasma total phenytoin, 0.063 for plasma free phenytoin, and 0.009 for saliva phenytoin. Recovery measurements of phenytoin spiked into saliva samples gave a coefficient of variation of less than 5%. Correlations between saliva and total plasma phenytoin levels and between saliva and free plasma phenytoin levels were strong and highly significant (r = 0.99, p less than 0.01). The percentage of temporal fluctuation (as determined by saliva phenytoin profiles) during 10-24 h ranged between 25.5-177 (mean, 58.3; SD, 47.3). Ratios of plasma total phenytoin/saliva phenytoin and of plasma free phenytoin/saliva phenytoin levels were 9.54 +/- 1.05 and 0.71 +/- 0.09, respectively. Dialysis experiments showed no binding of phenytoin to saliva supernatant. The greater saliva phenytoin concentrations as compared to plasma free phenytoin concentrations could be due to active transport of phenytoin from plasma to saliva. Measurement of phenytoin in citric acid-stimulated saliva by fluorescent polarization immunoassay is a reliable, noninvasive, and convenient method for monitoring phenytoin therapy in children.     

Absorption characteristics of three phenytoin sodium products after administration of oral loading doses

The absorption characteristics of three phenytoin sodium products given orally as loading doses in five healthy men were studied. Extended phenytoin sodium capsules, prompt phenytoin sodium capsules, and phenytoin sodium injection were administered in a randomized, crossover trial as single 18-mg/kg doses and as divided doses of 6 mg/kg every three hours for three doses. Each dose was given with 200 ml of water, and a two-week washout period followed each treatment. The maximum plasma concentration (Cmax), time to reach maximum plasma concentration, time to reach the lower end (10 mg/liter) of the therapeutic range, time to reach a plasma concentration greater than 15 mg/liter, and time within the therapeutic range were determined for each loading-dose regimen. Prompt phenytoin sodium capsules (prompt PHT) given in divided doses produced a mean Cmax of 22.0 mg/liter, which was significantly higher than that observed with any of the other loading-dose regimens. In addition, all subjects receiving prompt PHT in divided doses had plasma phenytoin concentrations of 10 mg/liter within six hours; only this treatment produced plasma concentrations greater than 15 mg/liter at nine hours in all subjects. Plasma concentrations remained within the therapeutic range (10-20 mg/liter) for 81 and 78% of the first 24-hour period for prompt PHT in divided and single doses, respectively. Adverse effects were minimal in all regimens. The prompt-release phenytoin sodium capsules used in this study may provide an alternative means for rapidly achieving therapeutic phenytoin concentrations in situations where i.v. administration is not indicated or practical.(ABSTRACT TRUNCATED AT 250 WORDS)     

The disposition and bioavailability of intravenous and oral nalbuphine in healthy volunteers

The pharmacokinetics of intravenous and oral nalbuphine were studied in 24 healthy male volunteers ranging in age from 21 to 30 years. On separate test days over a five-week period, subjects received single doses of each of four different formulations of nalbuphine, with a one-week washout period between treatments: 10 mg intravenously administered over two minutes, 45 mg orally given as a solution, and 45 mg orally administered in two tablet formulations (formulation A and formulation B). Blood samples were collected over 48 hours postadministration, and plasma nalbuphine concentrations were determined by reversed-phase high-performance liquid chromatography (HPLC) with electrochemical detection. The mean nalbuphine plasma concentration five minutes after 10 mg intravenously was 53 ng/mL, and the half-life of nalbuphine with this route of administration was 2.3 hours. In contrast, mean maximum nalbuphine concentrations (Cmax) after the three orally administered preparations ranged from 14.4 to 15.5 ng/mL, and occurred 0.9 to 1.2 hours after dose administration. Mean elimination half-lives after administration of the three nalbuphine oral formulations were essentially identical, ranging from 6.9 to 7.7 hours. Nalbuphine plasma concentration curves decayed biexponentially regardless of route of administration or type of formulation. Absolute bioavailability of the orally administered forms of nalbuphine ranged from 16.4 to 17.4% and Cmax and AUC data further established the bioequivalence of the three oral formulations. The low absolute bioavailability and prolonged elimination half-life of nalbuphine associated with oral administration are likely due to extensive first-pass metabolism and enterohepatic circulation, respectively.     

Effect of saliva flow rate on saliva phenytoin concentrations: implications for therapeutic monitoring

The effect of atropine-induced reductions in saliva flow rate on saliva phenytoin concentrations were evaluated in a randomised placebo-controlled crossover study in a group of epileptic patients stabilised on the drug.Pretreatment with atropine caused significant reductions in saliva flow rates during the first 4 h, compared to saline. The AUC_{0–4 h} for saliva flow rate was significantly reduced by atropine (245 g vs 327 g) and the saliva phenytoin AUC_{0–4 h} was significantly increased (5.6 g · ml^–1 · h vs 4.5 g · ml^–1 · h) without affecting plasma phenytoin concentrations. The saliva/plasma phenytoin AUC_{0–4 h} ratio was therefore significantly increased by atropine (0.15 vs 0.12). However, there was a poor correlation between saliva/plasma phenytoin concentration ratios and saliva flow rates for the two treatments in the individual patients (correlation coefficient ranged from 0.25 to 0.65).These findings demonstrate that saliva phenytoin concentrations are increased by reductions in saliva flow rate. Caution is therefore required when saliva phenytoin concentrations are used for therapeutic monitoring in the presence of factors which may affect saliva flow rate.     

Rapid and slow release phenytoin in epileptic patients at steady state: Assessment of relative bioavailability utilizing Michaelis-Menten parameters

The bioavailability of phenytoin from rapid release capsule and oral solution formulations relative to that of a slow release capsule formulation was assessed in five patients who had participated in a three-way crossover study performed at steady state. The subjects then underwent dosage adjustment utilizing the slow release formulation, and estimates of their Michaelis-Menten parameters thus obtained were utilized in calculating the relative bioavailabilities. In addition, expected changes in steady-state plasma phenytoin concentrations were calculated assuming initial levels of 15 mg/liter, with increases and decreases in bioavailability of 10%. The consequences of such alterations in the extent of phenytoin absorption or average content of the dosage form may be clinically significant, particularly where the initial phenytoin level is equal to or greater than the patient's operative K_m.The authors gratefully acknowledge the support of FDA Contract #223-76-3019 and NINCDS Contract #N01-NS-5-2327 in the performance of this research.     

Saliva-resistant coating of tablets prevents oral release of penicillin Plasma but not saliva equivalence

Objective: To investigate saliva and plasma concentrations of penicillin after the intake of a conventional phenoxymethylpenicillin (PcV) tablet and a tablet with saliva-resistant coating (PcVsr), both containing 1?g penicillin. Methods: The study had an open randomized crossover design and involved 24 healthy subjects. Saliva and blood were sampled intermittently for 6?h after tablet intake. Results: Within the first 10?min after tablet intake penicillin was detected in saliva in ten subjects taking PcV and in none taking PcVsr (P?<?0.001). These initial saliva concentrations were short-lasting, but in some subjects 50 to 100 times higher than those following the peak concentration in plasma, i.e. at 40?min or more after swallowing. From 40?min and onwards the saliva concentrations of penicillin were very similar for the two formulations. The elimination of high initial saliva concentrations may diminish ecological disturbances of the mouth flora as well as removing the unpleasant taste of penicillin. The plasma concentrations of penicillin were similar for the two formulations throughout the 6-h sampling period and the mean ratio of the area under the plasma concentration-time curve was 99% for PcVsr in relation to PcV, the 90% confidence interval being 86–115%. The corresponding values for the maximum plasma concentration were 108% and 93–127%. The time to maximum concentration was 45?min for PcVsr and 41?min for PcV. Thus, with regard to standard criteria which are based on systemic (plasma) concentrations, the formulations were bioequivalent despite the substantial difference in initial local (saliva) concentrations. Conclusion: Saliva-resistant coating of tablets can prevent oral release of penicillin without affecting the plasma concentrations. From a clinical point of view both local and systemic equivalence should be established before bioequivalence is assumed.     

Comparison of plasma and saliva concentrations of levetiracetam following administration orally as a tablet and as a solution in healthy adult volunteers

OBJECTIVE: Concentrations in saliva, as an alternative to concentrations in blood, can be advantageous for the monitoring of antiepileptic agents. This study assesses the relationship between saliva and plasma concentrations of levetiracetam after administration orally as a solution and as a tablet. The possibility that saliva concentrations of the drug are altered by contamination in the buccal cavity was also examined. METHODS: 4 healthy male subjects received a single 750 mg oral dose of levetiracetam as a 10% solution and 4 subjects received three 250 mg tablets (750 mg). Levetiracetam concentrations in plasma and saliva were monitored for 24 hours post dose. RESULTS: In subjects receiving the levetiracetam solution, maximum saliva concentrations were observed at the first collection point (15 min) after administration and these were 19-74 times higher than corresponding plasma levels. The mean saliva/plasma ratio rapidly decreased thereafter, becoming stable after 4 hours. In subjects receiving tablets, levetiracetam concentration profiles for saliva paralleled the plasma concentration profiles with a fairly constant saliva/plasma concentration ratio throughout the 24-hour sampling period. A significant linear correlation between levetiracetam saliva and plasma concentrations was demonstrated (Pearson r = 0.88; p < 0.001 for tablet (n = 35) and r = 0.87; p < 0.001 for solution at times > or = 4 hours post-dose (n = 20)). The saliva to plasma concentration ratio was 1.11 (95% confidence interval: 0.99 - 1.22) following tablet intake, and 1.55 (95% CI: 1.34 1.77) following oral solution (> or = 4 hours post dose). CONCLUSIONS: Using saliva to monitor therapeutic exposure to levetiracetam is feasible beginning 15 minutes after tablet intake but beginning 4 hours after intake of an oral solution.     

A randomized,crossover, assessor-blind study of the bioequivalence of a single oral dose of 200 mg of four formulations of phenytoin sodium in healthy,normal Indian volunteers

The objective of the study was to compare the bioavailability of a single oral 200-mg dose of four brands of phenytoin sodium available in the Indian market. Dilantin, Epsolin, and M-toin were compared with Eptoin, which was taken as the reference standard. A randomized, assessor-blind, four-way crossover study was done in 12 healthy Indian volunteers. The study was conducted at a clinical pharmacology ward at King Edward VII Memorial Hospital, a tertiary referral center in Mumbai (Bombay). All 12 subjects received a single oral 200-mg dose of all the formulations with a 2-week washout period between the formulations. Blood samples for plasma phenytoin levels were collected at 0, 0.5, 1, 2, 3, 4, 5, 6, 8, 10, 12, 24, 48, and 72 hours. Safety was measured by pretreatment and posttreatment biochemical investigations, physical examination, and ECG. The pharmacokinetics of the four brands of phenytoin were calculated by maximum plasma concentration (C(max)), time to reach C(max) (t(max)), area under the concentration versus time curve for time 0 to 72 hours (AUC(0-72)), and from time 0 to infinity (AUC(0- infinity)). For all brands, 90% CI of all untransformed and log transformed pharmacokinetic parameters failed to remain within prescribed limits of 80% to 120% for untransformed data and 80% to 125% for log transformed data. Since phenytoin obeys Micheles Mentens kinetics, the AUC methodology used for comparison would give only an approximate indication of relative bioavailability. M-toin was shown to be bioinequivalent to Eptoin. The other comparisons indicate but do not prove bioinequivalence of the other brands. The results of the study show that in India switching phenytoin brands could have significant implications and is not advisable once a patient is carefully titrated on one formulation.     

Developmental toxicity and pharmacokinetics of oral and intravenous phenytoin in the rat

Correlations between oral and intravenous (i.v.) doses of phenytoin, maternal plasma levels, and subsequent developmental toxicity were examined in the Sprague-Dawley rat. Oral administration of 150 to 1500 mg/kg and i.v. administration of 25 to 100 mg/kg phenytoin from gestational days (GD) 8 to 17 resulted in a dose-dependent increase in maternal death and toxicity [impaired motor function, decreased maternal weight gain (oral dose only)], embryolethality, and intrauterine growth retardation, in addition to significant increases in craniofacial (1125 mg/kg oral; 75 mg/kg i.v.) and urogenital (1125 mg/kg oral) malformations. Pharmacokinetic sampling in oral and i.v. groups on GD 8-9 and 16-17 revealed significant increases in maternal drug exposure over the treatment period, as evidenced by 2- to 3-fold increases in total plasma phenytoin (bound + free) half-life, area under the concentration curve, peak concentration (oral dose only), and decreases in clearance. These findings emphasize the importance of pharmacokinetics in the evaluation of phenytoin-induced developmental toxicity.     

Dual effect of valproic acid on the pharmacokinetics of phenytoin

To evaluate the effects of valproic acid on the disposition of phenytoin, a single dose of 600 mg valproic acid and multiple doses of valproic acid (200 mg four times a day for 5 days) were administered together with a single oral dose of 600 mg phenytoin to 12 young male volunteers. Fraction of unbound phenytoin and the area under curve (AUC) of the total and unbound phenytoin in plasma were compared with the control phase in which only 600 mg phenytoin was given. Valproic acid increased the unbound fraction of phenytoin in both single- and multiple-dose studies by 15 per cent and 41 per cent, respectively. Single-dose valproic acid increased the total AUC of phenytoin by 11 per cent. Multiple-dose valproic acid decreased the total AUC by 7 per cent. Single-and multiple-dose valproic acid increased the unbound AUC by 25 per cent and 18 per cent, respectively, probably due to the inhibition on the metabolizing enzymes. We concluded that there are at least two mechanisms involved in valproic acid-phenytoin interaction. Whereas valproic acid displacing phenytoin on the plasma protein decreased the total drug concentration of phenytoin, the enzyme inhibition by valproic acid increased both the total and unbound concentration of phenytoin. The two conflicting mechanisms may result in different effects on the total plasma concentration of phenytoin. Therapeutic drug monitoring based on the total concentration of phenytoin may be misleading when valproic acid is co-administered.     

Plasma levels of lidocaine during combined treatment with phenytoin and procainamide

Summary The effects of phenytoin and procainamide on plasma concentrations of lidocaine have been studied in patients and dogs receiving continuous intravenous infusions of the latter drug. All drugs were given in doses that produced therapeutic plasma concentrations. In the patients, no changes were observed in plasma lidocaine levels after intravenous or intramuscular phenytoin, or after intravenous or oral procainamide. Similarly, in the dogs, intravenous phenytoin had no effect on plasma lidocaine concentrations. However, in both patients and dogs a high incidence of CNS side-effects was recorded during lidocaine — phenytoin combination therapy, which suggests a potential pharmacodynamic interaction between them. The absorption of phenytoin administered intra-muscularly was impaired, probably because of pH-dependent crystallization. This route of administration should be avoided in acute treatment with phenytoin.Dedicated to the late Balzar Alexanderson, M.D.     

Novel mucoadhesive extended release tablets for treatment of oral candidosis: "in vivo" evaluation of the biopharmaceutical performance

Mucoadhesive tablets containing nystatin (10 mg) were evaluated in vivo. The assays were carried out with 12 healthy volunteers and the concentration of nystatin in saliva was determined at different times. Tablets remained attached to the buccal mucosa during 270 min +/- 30 min. No evidence of ulceration or bleeding was observed. Typical appearance of intact human buccal mucosa was seen before and after contact with the tablet. The tablets were well accepted by the volunteers, although most of the volunteers reported a light bitter taste, probably due to nystatin. Concentration of nystatin in saliva was several times higher than MIC over a period of approximately 4.5 h, which was in agreement with the behavior observed in vitro. These results allow us to infer that the administration of these mucoadhesive tablets could be advantageous compared to conventional formulations and mucoadhesive extended-release tablets might produce better therapeutic performance than conventional formulations in the treatment of oral candidosis.     

Probabilistic approach to the establishment of maximal content limits of impurities in drug formulations: the case of parenteral diphenylhydantoic acid

Diphenylhydantoic acid (DPHA) is a degradation product in parenteral formulations of the anticonvulsant phenytoin and the prodrug fosphenytoin. DPHA has also been reported to be a minor metabolite of phenytoin. Levels found in the urine of various species, including humans, after oral or intravenous (iv) phenytoin ranged from undetected to a few percent of administered dose. In the present analysis, the toxicologic profile of DPHA was integrated with exposure data in order to characterize its safety under recommended clinical regimens of fosphenytoin administration. In preclinical safety studies, DPHA was without effect in the Ames assay and at concentrations up to 3000 microg/plate in the presence or absence of metabolic activation, and in the in vitro micronucleus test with acute and 2-week repeated dose studies in Wistar rats at iv doses up to 15 mg/kg. In 4-week studies conducted in rats and dogs receiving fosphenytoin containing DPHA levels up to 1.1%, and in an in vitro structural chromosome aberration test with DPHA levels up to 2.0%, all findings were consistent with known effects of phenytoin (such as CNS signs and increased liver weight), and none were attributed to DPHA. Reports in the literature indicate that in murine in vivo and in vitro models, DPHA has much lower potential for reproductive toxicity than phenytoin. A no-observed-effect level (NOEL) of 15 mg/kg established from the 2-week study in rats was used with probabilistic techniques to estimate tolerable daily doses (TDDs) of DPHA. In this approach, interspecies correction was performed by allometrically scaling the NOEL based on a distributional power of body weight while intraindividual variability was accounted for by selecting the lower percentiles of the population-based distribution of TDDs. The results indicate that a DPHA content limit of 3.0% in an administered dose of fosphenytoin is unlikely to cause adverse effects in patients.