Phenytoin Binding Research Articles

Interaction of Nasogastric Phenytoin and Enteral Feeding Solution

To the Editor: — Drug interactions are of great concern to geriatricians. Phenytoin, the most commonly prescribed antiepileptic, interacts with a wide variety of medications. We wish to draw attention to the interaction between phenytoin and enteral feeding preparations such as Ensure and Osmolite, since the simultaneous use of phenytoin and enteral formulae is not uncommon. We report a case of an elderly woman whose serum phenytoin level was markedly depressed during concomitant administration of Osmolite. An 80-year-old woman was hospitalized with a right hemispheric cerebral infarct. Examination demonstrated stupor and a left hemiparesis. An EEG revealed status epilepticus and she received intravenous loading of phenytoin followed by a maintenance dose of 400 mg daily through a nasogastric tube, resulting in a serum level of 18.7 mcg/mL (therapeutic: 10–20 mcg/mL). Her mental status improved. Due to poor oral intake, Osmolite feeding via nasogastric rube at 50 cc/hr was begun on the sixth hospital day. Over the next four days, this was increased to 100 cc/hr for 16 hours a day. On the tenth day, the serum phenytoin level was 9.3 mcg/mL. A new loading dose of 800 mg phenytoin was given via the tube in two portions and the daily dose was increased to 500 mg. On the 12th day, the phenytoin level was 5 mcg/mL. The patient received 1000 mg orally in two portions but only had a level of 9.8 mcg/mL the next morning. On the 13th and 14th hospital days, she received 500 mg of phenytoin daily. Simultaneously, her Osmolite was discontinued and a pureed diet begun. On the 25th day, the serum phenytoin level was 13.3 mcg/mL and remained in therapeutic range thereafter without further dosage alterations. The patient was receiving no medications known to depress phenytoin levels. Phenytoin has a narrow therapeutic index. One reason for the difficulty in maintaining therapeutic levels may be an interaction between orally administered phenytoin and enteral feeding preparations. This was first noted by Baur1 in young head trauma patients whose phenytoin dose requirement plummeted when enteral feedings were discontinued. A postulated mechanism for this interaction is binding of phenytoin to calcium caseinate or magnesium present in complete diet formulae, rendering the drug unavailable for absorbtion.2 Hypoalbuminemia, frequently seen in patients who receive enteral formulae, can lead to a decline in total serum phenytoin levels but does not explain this apparent drug-nutrient interaction, as a similar decline in serum phenytoin levels has been noted in patients with normal albumin.3 No data exist as to the incidence of this drug-nutrient interaction. A review of common reference sources failed to reveal mention of the interaction. There are a few case reports.3–5 The interaction is clearly important, since subtherapeutic levels may result during enteral feeding and toxic levels may appear after withdrawal. Monitoring of serum levels and alert adjustment in dosage are necessary.

Interaction between vigabatrin and phenytoin.

1. The study was designed to determine the mechanism by which vigabatrin causes a fall in plasma phenytoin concentrations when added to the drug therapy of eight epileptic patients. 2. Total plasma phenytoin concentration was measured before and at intervals during 5 weeks' treatment with vigabatrin. 3. Plasma protein binding of phenytoin, the urinary ratio of phenytoin to 5-(p-hydroxyphenyl)-5-phenylhydantoin, and antipyrine clearance were measured before and at the end of treatment period. 4. Mean plasma phenytoin concentration fell significantly by 23% during the fifth week. 5. No change was found in any of the other measures. 6. Although an interaction between phenytoin and vigabatrin has been confirmed, the mechanism has not been elucidated.

Modulation of phenytoin teratogenicity and embryonic covalent binding by acetylsalicylic acid, caffeic acid, and α-phenyl- N-t-butylnitrone: Implications for bioactivation by prostaglandin synthetase

Teratogenicity of the anticonvulsant drug phenytoin is thought to involve its bioactivation by cytochromes P-450 to a reactive arene oxide intermediate. We hypothesized that phenytoin also may be bioactivated to a teratogenic free radical intermediate by another enzymatic system, prostaglandin synthetase. To evaluate the teratogenic contribution of this latter pathway, an irreversible inhibitor of prostaglandin synthetase, acetylsalicylic acid (ASA), 10 mg/kg intraperitoneally (ip), was administered to pregnant CD-1 mice at 9:00 am on Gestational Days 12 and 13, 2 hr before phenytoin, 65 mg/kg ip. Other groups were pretreated 2 hr prior to phenytoin administration with either the antioxidant caffeic acid or the free radical spin trapping agent α-phenyl- N-t-butylnitrone (PBN). Caffeic acid and PBN were given ip in doses that respectively were up to 1.0 to 0.05 molar equivalents to the dose of phenytoin. Dams were killed on Day 19 and the fetuses were assessed for teratologic anomalies. A similar study evaluated the effect of ASA on the in vivo covalent binding of radiolabeled phenytoin administered on Day 12, in which case dams were killed 24 hr later on Day 13. ASA pretreatment produced a 50% reduction in the incidence of fetal cleft palates induced by phenytoin ( p < 0.05), without significantly altering the incidence of resorptions or mean fetal body weight. Pretreatment with either caffeic acid or PBN resulted in dose-related decreases in the incidence of fetal cleft palates produced by phenytoin, with maximal respective reductions of 71 and 82% at the highest doses of caffeic acid and PBN ( p < 0.05). Caffeic acid and PBN also significantly reduced the incidence of fetal resorptions produced by phenytoin, but not the fetal weight loss. In viable embryos, ASA pretreatment reduced the covalent binding of phenytoin to embryonic protein by 43% ( p < 0.05). Binding of phenytoin to embryonic resorptions was equally high with and without ASA pretreatment, and within each treatment group was 3- to 10-fold higher than that in the respective placentas and associated viable embryos ( p < 0.05). These results suggest that prostaglandin synthetase may contribute to the enzymatic bioactivation of phenytoin to a teratogenic free radical intermediate.

Enhancement of phenytoin binding to tissues in rats by heat treatment

Phenytoin binding to heat-treated tissue homogenates has been examined to characterize the phenytoin binding to tissues. The binding to the heat-treated tissue homogenates was enhanced in all tissues studied compared with controls. The heating might produce the changes in conformation of proteins in tissues and then enhance phenytoin binding to tissue homogenates.

Human brain phenytoin: correlation with unbound and total serum concentrations

Simultaneous brain (gray matter) and serum specimens from 18 patients treated with oral phenytoin, who underwent cortical resections for intractable seizures, were analyzed by HPLC. The correlation between brain phenytoin concentration and unbound (free) phenytoin ( r = 0.98–0.99) was significantly better than the correlation between brain phenytoin and total serum phenytoin ( r = 0.90–0.93). Phenytoin concentrations in tissue from epileptic foci were slightly lower than brain phenytoin concentrations in non-epileptic regions in the same patient (means 15.0 vs. 15.5 μg/g, P < 0.05). The results support the value of monitoring unbound phenytoin in clinical situations where phenytoin binding is highly variable.

The effect of renal transplantation on a major endogenous ligand retained in uremic serum.

The effects of renal transplantation on serum concentrations of 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF) and indole-3-acetic acid (IAA), which are endogenous ligands retained in uremic serum, and on phenytoin binding to serum protein were investigated. IAA, a weakly bound ligand, was rapidly excreted by the transplanted kidney during the first one to three days after renal transplantation, but CMPF, a strongly bound ligand, was slowly excreted. The binding defect of phenytoin was partially corrected by transplantation during the period of study. The results suggested that the prolonged drug binding defect observed despite successful renal transplantation is caused by a slower decrease of strongly bound ligands such as CMPF retained in uremic serum; hypoalbuminemia, usually observed after transplantation, may also contribute to this phenomenon.

Phenceptin: a biomimetic model of the phenytoin receptor

5,5-Diphenylhydanytoin (phenytoin) is the most widely used anticonvulsant drug, but has many side effects. Although its chemical mode of action is unknown, phenytoin is believed to function primarily by interference with the transport of sodium ions across the neuronal membrane. Structure–activity and lipophilicity–activity studies suggest that the drug interacts with its receptor through hydrogen bonding to the N3—C4 amide bond, and an aromatic–aromatic interaction with the C5 substituent. Since sodium channels are cysteine-rich peptides, whose function depends upon the cysteine[Formula: see text]cystine redox process, it has been hypothesized that the action of the phenytoin receptor may be mimicked by a properly designed cyclodepsipeptide containing a cystinyl moiety, a cavity lined with five oxygen atoms oriented in the trigonal-bipyramidal manner appropriate for selective transport of sodium ions, and a site for the binding of phenytoin. A computer programme and strategy were developed to permit the three-dimensional structures of potential target molecules to be viewed, prior to synthesis. Use of this programme led to the discovery of Boc-L-cystinyl-glycyl-L-prolyl-glycyl-L-prolyl-L-cystine-OCHPh2. This compound, termed phenceptin, was synthesized from a linear precursor containing tert-butoxycarbonyl protection at the N-terminus, benzhydryl ester protection at the C-terminus, and trityl protection at sulfur. Detritylation and cyclization to phenceptin were accomplished with iodine in methanol–pyridine. Using an n-octanol membrane to study the kinetics of ion transport, phenceptin was found to transport sodium ions selectively, but only in its oxidized, cyclic form. This transport was inhibited significantly by one mol-equiv. of phenytoin, and not at all by biologically inactive analogs of the drug. The nature of the binding of phenytoin to phenceptin was examined by nuclear magnetic resonance, in n-C8D17-OH solvent, and found to involve hydrogen bonding of the drug to a glycine residue whose oxygen atom is involved in complexation to sodium ions.

Influence of continuous ambulatory peritoneal dialysis on serum alpha 1-acid glycoprotein concentration and drug binding.

The influence of continuous ambulatory peritoneal dialysis (CAPD) on the concentrations of alpha 1-acid glycoprotein in serum and dialysate and on the serum binding of oxprenolol, propranolol and phenytoin has been studied. Before starting CAPD treatment, the serum binding of oxprenolol and propranolol was higher and that of phenytoin lower than in healthy volunteers, and the serum alpha 1-AGP concentration was higher. During the first days to weeks after starting CAPD, the serum alpha 1-AGP concentration rose with a concomitant increase in the binding of oxprenolol and propranolol. Subsequently, the alpha 1-AGP level and the binding of oxprenolol and propranolol decreased to the values found before starting CAPD. The binding of phenytoin showed little change. The concentration of alpha 1-AGP in dialysate was 2 to 5% of that in serum.

Drug protein binding in chronic renal failure: Evaluation of nine drugs

In this study, changes of protein binding of nine drugs were evaluated. In addition, theophylline and phenytoin, the two drugs with the most substantial and progressive decrease in protein binding, were further studied by high performance liquid chromatography (HPLC)-fractions of ultrafiltrate of normal and uremic serum, in an attempt to identify substances causing drug protein binding inhibition. There was a marked decline of the protein binding of theophylline, phenytoin and methotrexate (dialyzed patients vs. normals: -20.1, -16.0 and -15.1%, respectively). There was a rise in the protein binding of propranolol, cimetidine and clonidine. The changes observed for diazepam, prazosin and imipramine were less marked. For phenytoin, theophylline, methotrexate and diazepam, protein binding was inversely correlated to the serum creatinine (r = 0.87, 0.80, 0.79 and 0.67, P less than 0.001), and a less pronounced but still significant positive correlation was found for clonidine (r = 0.46, P less than 0.01). Ultrafiltrate, obtained during a hemofiltration session, inhibited protein binding of theophylline and phenytoin in a dose dependent way. After separation of this ultrafiltrate by HPLC, it appeared that for both theophylline and phenytoin at least a part of this inhibitory activity corresponded to the elution zone of hippuric acid. For theophylline two other inhibitory zones were further recognized: one corresponding to the elution zone of NaCl and one in which the responsible substance remained unidentified. Hippuric acid in solution inhibited protein binding of theophylline and phenytoin in a dose dependent way. In conclusion, protein binding of several drugs currently used in renal failure is affected in parallel with renal function, which might affect the therapeutic effectiveness of the drugs.(ABSTRACT TRUNCATED AT 250 WORDS)

Is Free Fraction Measurement of Phenytoin Always Necessary in Pediatric Epileptic Patients?

Sixty-two serum samples from 28 pediatric epileptic patients under treatment with phenytoin [diphenylhydantoin (DPH)] were obtained to study the correlation between total and free serum DPH concentrations. The effect of coadministered antiepileptic drugs on DPH protein binding was also studied. Binding of DPH to serum protein was assessed by ultrafiltration, and total and free DPH concentrations were determined by fluorescence polarization immunoassay. A strong correlation existed between the total and free concentrations [r = 0.958, p less than 0.001, standard error of estimate (+/- 1 SY) = 0.214]. The mean value for the DPH free fraction was 8.9% (range 5.7-16.3%). The samples obtained from patients on combination therapy with valproic acid (VPA) showed a larger DPH free fraction and greater variation. Exclusion of the 16 samples from patients concomitantly taking DPH and VPA yielded a better correlation (n = 46, r = 0.983, p less than 0.001, +/- 1 SY = 0.145). The mean free fraction in the patients not taking VPA was 7.7% (range 5.7-9.0%). The effect of VPA on the protein binding of DPH was also studied by addition of the same antiepileptic drugs to normal human plasma; the results were similar to those obtained for epileptic patients. These findings suggest that the free DPH fraction can be predicted from the total concentration, even when the drug is coadministered with other antiepileptic drugs, the sole exception being VPA.

Properties of Phenytoin Binding Sites in the Rat Brain: Regional Distribution and Postnatal Development

Specific [3H]phenytoin binding in rat cortical membranes is associated with a micromolar affinity site through which diazepam can exert an enhancing effect. In contrast to diazepam, ( + )bicuculline, isoguvacine and pentobarbital inhibited Phenytoin binding, while GABA and (-)baclofen had no effect. Decreased Phenytoin binding and a loss of the diazepam effect were observed in phospholipase A2-treated membranes. Binding in the absence and presence of diazepam demonstrated differential postnatal development. Furthermore, the degree to which binding was enhanced by diazepam varied from one brain region to another. The regional variations of Phenytoin binding, both in the presence and absence of diazepam, did not correlate positively with the regional distribution of either the central benzodiazepine receptors or the peripheral type binding sites in the brain.

Properties of phenytoin binding sites in the rat brain: regional distribution and postnatal development.

Specific [3H]phenytoin binding in rat cortical membranes is associated with a micromolar affinity site through which diazepam can exert an enhancing effect. In contrast to diazepam, (+)bicuculline, isoguvacine and pentobarbital inhibited phenytoin binding, while GABA and (-)baclofen had no effect. Decreased phenytoin binding and a loss of the diazepam effect were observed in phospholipase A2-treated membranes. Binding in the absence and presence of diazepam demonstrated differential postnatal development. Furthermore, the degree to which binding was enhanced by diazepam varied from one brain region to another. The regional variations of phenytoin binding, both in the presence and absence of diazepam, did not correlate positively with the regional distribution of either the central benzodiazepine receptors or the peripheral type binding sites in the brain.

Protein binding of phenytoin and lidocaine in pediatric patients with type I diabetes mellitus.

We evaluated serum protein binding of phenytoin and lidocaine, the extent of albumin and hemoglobin glycation, and alpha-1-acid glycoprotein concentrations in 47 children and adolescents (9-17 years) with type I diabetes mellitus (DM). Serum was incubated with phenytoin (n = 47) and lidocaine (n = 32) to yield total concentrations of 19.4 +/- 1.34 and 4.6 +/- 0.5 mg/l, respectively. A serum ultrafiltrate was prepared from an aliquot of each sample by membrane centrifugation. Total and free concentrations of phenytoin (free fraction 8.3 +/- 1.0%) and lidocaine (free fraction 25.8 +/- 11.6%) were determined using a fluorescence polarization immunoassay technique. A linear relationship (r = 0.63, p = 0.0001, y = 40.1 + [-0.2x]) was also found between the alpha-1-acid glycoprotein concentration (68.5 +/- 20.9 mg%, range 47.2-134.7 mg%) and lidocaine-free fraction (n = 32). No relationship existed between the extent of glycated albumin and the lidocaine-free fraction, in contrast to the linear correlation (r = 0.49, p = 0.0005, y = 3.7 + 0.4x) found between the extent of glycated albumin and the free fraction of phenytoin in serum (n = 47). A similar correlation (r = 0.49, p = 0.014, y = 3.7 + 0.3x) was found between glycated albumin (%) and the phenytoin-free fraction (3.9 +/- 0.9%) when examined in a separate (n = 17) set of patient samples spiked to contain a total serum phenytoin concentration of 32.6 +/- 1.4 (range 29.4-35.7) mg/l. Our data demonstrate a predictable increase in the free fraction of phenytoin as the extent of albumin glycation increases in pediatric patients with type I DM who are in poor glycemic control. This relationship was not found for lidocaine, a representative basic compound. These findings suggest that glycation of albumin alters the binding of phenytoin in children and adolescents with type I DM.

The protein binding of phenytoin, propranolol, diazepam, and AL01576 (an aldose reductase inhibitor) in human and rat diabetic serum.

The extent of serum protein binding of AL01576, phenytoin (DPH), diazepam (DIAZ), and propranolol (PRO) was evaluated in a group of nondiabetic and a group of insulin-dependent diabetic subjects, as well as in streptozotocin-treated rats. Both serum glucose and glucosylated protein levels were elevated in the diabetic patient population (179 and 150% of control values, respectively). The mean free fractions (fp) of AL01576, DPH, and PRO were not statistically different for the two human groups. The DIAZ fp was slightly elevated (P less than 0.05) in the diabetic patients (mean = 0.016) compared to the control group (mean fp = 0.014). An acute (less than 3 days) and chronic (greater than 20 days) diabetic rodent model was evaluated using Sprague-Dawley rats following streptozotocin administration (60 mg/kg i.p.). Both diabetic rat groups exhibited substantial increases in serum glucose, free fatty acids (FFA), and protein glucosylation compared to controls. The fp of AL01576 was increased in both the acute (mean = 0.248) and the chronic (mean = 0.202) condition compared to controls (mean = 0.163). The fp of DPH was also markedly increased in the acute (mean = 0.348) and the chronic (mean = 0.280) models compared to untreated controls (mean = 0.207). DIAZ and PRO binding was largely unaffected by the streptozotocin treatment. In vitro studies of purified human albumin suggest that a considerable degree of glucosylation would need to be present in diabetic serum before it would effectively alter drug binding. Our data suggest that only minor drug-serum binding changes occur in diabetic patients who are otherwise healthy and whose disease is well controlled.

Factors causing age-dependent changes in phenytoin tissue and serum binding in rats.

The factors that cause age-dependent changes in phenytoin tissue and serum protein binding in rats were studied. It was confirmed that the age-dependent changes in the concentration of tissue constituents to which phenytoin bound mainly governed the change in phenytoin tissue binding. The concentration of the tissue constituents, protein and phospholipid, was changed by the water content in tissues in the growth process of rats. The increase in serum protein binding of phenytoin in the growth process of rats was caused by changes in both serum albumin concentration and binding parameters of phenytoin to serum albumin. The changes in binding parameters of phenytoin to serum albumin were led by those in the molar rations of free fatty acids to albumin.

A major inhibitor of phenytoin binding to serum protein in uremia.

A major endogenous ligand substance, 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF), accumulated in the sera of uremic patients, inhibited phenytoin binding to pooled serum obtained from healthy subjects and to human serum albumin in a concentration usually observed in the sera of patients with uremia. This suggests that CMPF is a major drug-binding inhibitor present in uremic serum and may be one of the so-called 'uremic toxins'.

Phenytoin binding in healthy volunteers.

The pharmacologic effect of phenytoin is directly related to the unbound concentration in the serum, which previously has been reported in the literature to be approximately 10%. The results of 13 out of 14 20-35 year-old healthy male volunteers studied indicate that less than 10% unbound phenytoin is present in the majority of subjects taking two different doses of phenytoin.

Serum Protein Binding of Phenytoin and Valproic Acid in Insulin-Dependent Diabetes Mellitus

The serum protein binding of valproic acid (VPA) and phenytoin (PHT) was determined in spiked serum samples collected from 17 patients with insulin-dependent diabetes mellitus and 16 healthy control subjects. The free fraction of VPA was significantly greater in patients than in controls (7.6 +/- 1.6% vs. 6.2 +/- 1.2%, p less than 0.01); for PHT, free fraction values were similar in the two groups (8.2 +/- 1.1% vs. 8.4 +/- 1.2%). The free fraction of VPA in diabetic patients was positively correlated with free fatty acid (FFA) concentration (r = 0.79, p less than 0.01). No significant relationships could be found between free drug fraction and either serum albumin or glycosylated protein concentration.

Alteration of phenytoin binding by glycosylation of albumin in IDDM.

We measured glycosylated albumin and hemoglobin and serum protein binding of phenytoin in 57 children and adolescents with insulin-dependent diabetes mellitus (IDDM). Serum was incubated with phenytoin to yield concentrations of 15 and 25 mg/L, and a serum ultrafiltrate was prepared from an aliquot of each sample. We observed a linear correlation between glycosylated albumin and the free fraction of phenytoin at serum phenytoin concentrations of 15 mg/L (r = .35, P = .03) and 25 mg/L (r = .40, P = .003). A better correlation existed between the free fraction of phenytoin and total albumin concentrations for both serum concentrations (r = .45, P = .005 for 15 mg/L; r = .56, P = 10-5) for 25 mg/L), whereas the best linear correlation resided between the free fraction of phenytoin and the concentration of nonglycosylated albumin (r = .54, P = .0005 for 15 mg/L; r = .63, P less than 10(-6) for 25 mg/L). There was no correlation between the free fraction of phenytoin and the concentration of glycosylated albumin. Incubation of solutions of glycosylated and nonglycosylated albumin demonstrated significantly lower binding to the glycosylated fraction (P = 8.1 X 10(-6)). These results indicate that glycosylation of albumin diminishes the affinity of the phenytoin binding site on albumin. This alteration may have clinical significance in that it may alter the disposition of phenytoin in patients with IDDM and produce free phenytoin serum concentrations that are not accurately reflected by total serum phenytoin concentrations.

Alteration of phenytoin binding by glycosylation of albumin in IDDM

Alteration of phenytoin binding by glycosylation of albumin in IDDM