Antiepileptic Drugs, 5th Edition

Phenytoin and Other Hydantoins

58

Chemistry and Biotransformation

Thomas R. Browne MD*

Barbara Leduc PhD**

* Professor of Neurology, Department of Neurology, Boston University School of Medicine, Boston, Massachusetts

** Massachusetts College of Pharmacy and Allied Sciences, Boston, Massachusetts

CHEMISTRY

Phenytoin is the generic name for 5,5-diphenylhydantoin (acid form). The Chemical Abstracts name is 5,5-diphenyl-2,4-imidazolidinedione. It has the chemical structure shown inFigure 58.1. The free acid has a molecular weight of 252.26; the sodium salt has a molecular weight of 274.25, equivalent to acid content of 91.98%. The acid form is used in formulations of aqueous suspensions (Pediatric Dilantin-30 Suspension and Dilantin-125 Suspension; Pfizer, New York, NY) containing 30 mg or 125 mg of phenytoin acid per 5 mL. The free acid also is used in formulating chewable tablets (Dilantin Infatabs; Pfizer) containing 50 mg phenytoin acid per tablet. However, other products are formulated with the sodium salt of phenytoin (phenytoin sodium; acid equivalents = 91.98%). With these preparations, the drug content is expressed in terms of the sodium salt rather than the free acid. Thus, the gelatin capsules (Dilantin Sodium Kapseals; Pfizer) are formulated to contain either 30 mg or 100 mg of phenytoin sodium (= 27.6 mg or 92.0 mg of phenytoin acid equivalents) per capsule. The Mylan extended-release 100-mg capsules also are formulated with sodium phenytoin. This 8% difference in drug content should be taken into account when changing from one product to another. The sodium salt also is used in parenteral formulations [Parenteral Dilantin (Pfizer); phenytoin sodium injection (generic)]. The drug content is given in terms of the sodium salt.

Phenytoin is a weak organic acid that is poorly soluble in water. The apparent dissociation constant (pKNa), representing the pH at which half the drug is ionized, is in the range of 8.1 to 9.2. The acid essentially is nonionized at pH 5.4 (solubility approximately 19.4 µg/g at 25.4°C), whereas at pH 7.4 (approximately 80% nonionized), the acid has a water solubility of 20.5 µg/g (25.2°C). Higher concentrations of phenytoin required strongly alkaline solutions, with solubility measurements of 165 µg/mL at pH 9.1 and 1,520 µg/mL at pH 10. Parenteral phenytoin sodium is made up in an aqueous vehicle containing propylene glycol, ethanol, and sodium hydroxide. It contains 50 mg phenytoin sodium per mL (= 46 mg phenytoin acid per mL). The solubility of phenytoin in blood plasma is approximately 75 µg/mL (37°C), at least in part because of binding of the drug on the plasma proteins.

Phenytoin sodium is not recommended as an analytical standard because of its variable water content (hydrate formation) and partial conversion to the free acid on exposure to carbon dioxide.

This section was based on Glazko (61), which should be consulted for details and references.

BIOTRANSFORMATION

Phenytoin (5,5-diphenylhydantoin; Dilantin) is eliminated almost entirely by metabolic transformation before excretion in the form of metabolites. Less than 5% of an administered dose is excreted unchanged in the urine (45,62, 114,121). The principal metabolic pathway of phenytoin in humans is the 5-(4-hydroxyphenyl)-5-phenylhydantoin (p-HPPH) and dihydrodiol pathway, accounting for 70% to 90% of administered phenytoin (Figure 58.2). The first

P.566


step of this pathway [involving the cytochrome P450 (CYP) enzymes CYP2C9 and CYP2C19] exhibits nonlinear enzyme kinetics, which has significant effects on phenytoin's clinical pharmacokinetics. A number of minor metabolic pathways for phenytoin also have been described, some of which involve the CYP enzymes CYP2C9, CYP2C19, and CYP3A4 (Figure 58.2). Species differences in the biotransformation of phenytoin have been reported.

FIGURE 58.1. Structure of phenytoin.

FIGURE 58.2. Pathways of phenytoin metabolism.

The Para-HPPH and Dihydrodiol Pathways

Para-HPPH accounts for 67% to 88%, and dihydrodiol accounts for 7% to 11% of human urinary metabolites of phenytoin (23,24,32,43,75,90). The first step in this pathway is the formation of an arene oxide intermediate through the enzymes CYP2C9 and CYP2C19 (Figure 58.2). The arene oxide is converted spontaneously to p-HPPH and is converted by the enzyme epoxide hydrolase to dihydrodiol (Figure 58.2).

Cytochrome P450 Enzymes Active in Phenytoin Metabolism and Their Genetic Variants

Enzymes belonging to the CYP2C subfamily and the CYP3A family have been implicated in the biotransformation of phenytoin and its metabolites (40,95). A substantial number of human allelic variants of these enzymes have been identified.

Four members of the CYP2C subfamily, CYP2C8, CYP2C9, CYP2C18, and CYP2C19, have been identified in humans (64,110). The CYP2C enzymes, which constitute approximately 20% of total hepatic CYP content, are encoded by a cluster of four genes at chromosomal location 10q24 (85,138). Both the CYP2C9 and CYP2C19 enzymes are active in phenytoin hydroxylation, with CYP2C9 being the more vigorous catalyst (10,51,82,154, 167). Although earlier in vitro studies suggested a role for CYP2C8 and CYP2C18, their contribution to phenytoin

P.567


metabolism in vivo has been questioned, and studies have indicated that only negligible amounts of CYP2C18 are expressed in human liver (130). CYP2C9 catalyzes the formation of both (R)- and (S)-HPPH [5-(4′-hydroxyphenyl)-,5-phenylhydantoin] but is highly stereoselective for (S)-HPPH, whereas CYP2C19 exhibits little stereoselectivity and thus contributes more to the formation of the (R)-enantiomer (82,110,167). In addition, studies have identified CYP2C19, CYP2C9, and several CYP3A forms as the catalysts for the further hydroxylation of 3′- and 4′-HPPH to the catechol 3′,4′-diHPPH [5-(3′,4′-dihydroxyphenyl)-,5-phenylhydantoin] (40,95).

Four alleles of CYP2C9 have been identified. The wild-type enzyme, CYP2C9*1, contains arginine at position 144 and isoleucine at position 359 (132). The variant CYP2C9*2 contains a single amino acid substitution at position 144 (Arg144Cys), whereas in CYP2C9*3, leucine is substituted at position 359 (Ile359Leu) (39,69,122, 128,141). Another mutation generating an amino acid substitution at position 359 has been identified in CYP2C9*4 (Ile359Thr) (84). (For a current listing of human CYP450 alleles, see www.imm.ki.sec/CYPalleles). Although the activity of CYP2C9*2 for HPPH formation is reported to be only moderately diminished compared with that of CYP2C9*1, CYP2C9*3 shows markedly decreased catalytic activity both in vitro and in vivo (9,40,52,72,110,122, 128,143,154). Interestingly, a white patient with epilepsy displaying a very low clearance of phenytoin was found to be homozygous for CYP2C9*3 (91). The CYP2C9*4 mutation (Ile359Thr) is extremely rare, but was associated also with a diminished rate of phenytoin hydroxylation in vivo (83). It has been suggested that amino acid 359 is located in the enzyme's substrate recognition site; this would explain the large detrimental effect of substitutions at this position (67). To date, poor metabolizers of phenytoin have been shown to have mutations of either CYP2C9 or CYP2C19. Although CYP2C9*3 is the primary determinant of slow phenytoin metabolism, defective CYP2C19 alleles also contribute, especially at high doses (120).

The incidence of allelic variants of CYP2C9 varies significantly between different racial groups. The gene frequency of CYP2C9*2 has been estimated as 0.08 to 0.125 in whites, 0.01 in Africans, and zero in East Asians (9,122,140,141). The predicted frequency of CYP2C9*3 in whites is 0.06 to 0.10, in East Asians, 0.017 to 0.026, and in Africans, 0.005 (9,92,122,140,141,157).

In contrast to CYP2C9, most variant alleles of CYP2C19 identified thus far appear to result in nonfunctional or absent enzymes. The CYP2C19 polymorphism is responsible for the variation in human 4′-hydroxylation of (S)-mephenytoin (41,42). Poor metabolizers of (S)-mephenytoin constitute approximately 2% to 6% of white or African, and 18% to 23% of East Asian populations (92,119,164, 165, 166). Two wild-type and seven defective alleles of CYP2C19 have been identified thus far (87). As with CYP2C9, the incidence of specific alleles varies significantly among different ethnic groups. CYP2C19*1A and CYP2C19*1B are the wild-type forms (129,132). The allelic variants CYP2C19*2 (CYP2C19m1), CYP2C19*3 (CYP2C19 m2), and CYP2C19*5 account for approximately 99% of East Asian, but only 75% to 85% of white poor metabolizers of (S)-mephenytoin (54,87). CYP2C19*2B, CYP2C19*4, CYP2C19*5B, and CYP2C19*6 account for the remainder of slow metabolism in whites (54). The mutations found in CYP2C19*2A and CYP2C19*2B are due to G6A point mutations in exon 5, leading to aberrant splice sites (41,42,80). CYP2C19*3 is due to a G6A point mutation in exon 4, causing premature termination (41,42). In CYP2C19*4, an A→G mutation disrupts the initiation codon (50). CYP2C19*5A contains an amino acid substitution (Arg433Trp), and CYP2C19*5B contains this, plus an additional substitution (Arg433Trp; Ile331Val); both enzymes appear to lack activity (79,80,163). CYP2C19*6 (Argl32Gln; Ile331Val), CYP2C19*7 (splicing defect), and CYP2C19*8 (Trp120Arg) apparently are absent or inactive as well (80,81).

Although CYP2C9 is the primary determinant, a number of studies have confirmed that the activity of CYP2C19 is clinically significant to overall phenytoin metabolism. Diminished renal elimination of (R)-HPPH and total HPPH, a substantially decreased urinary (R)-HPPH/phenytoin ratio, elevated serum phenytoin concentrations, and an elevated Michaelis constant (Km) for phenytoin have been demonstrated in Japanese subjects who were either heterozygous or homozygous for mutant CYP2C19 alleles (82,110,116,120,122,157,158). Furthermore, ticlopidine, a potent inhibitor of CYP2C19, has been implicated in drug interactions resulting in phenytoin toxicity (47).

The gene frequency of CYP2C19*2 has been estimated as 0.11 to 0.13 in whites and in Africans, but 0.27 to 0.37 in East Asians (16,30,92,112,133,142,163). In contrast, the CYP2C19*3 allele is rare or absent in whites, whereas its frequency is approximately 0.5 to 0.11 in East Asians (92,133,163). Interestingly, in Japanese but not whites, mutations of CYP2C18 and CYP2C19 appear to be completely linked, that is, the occurrence of CYP2C18m2 coincides with that of CYP2C19*32, and CYP2C18ml occurs with CYP2C19*3 (96,109,116). The CYP2C9 mutations were found to be independent of either CYP2C18 and CYP2C19 alleles (86).

Collectively, the CYP3A enzymes constitute most of the total hepatic CYP. Four genes have been described: CYP3A4, CYP3A5, CYP3A7, and a newly identified form, CYP3A43 (56,147). cDNA analysis of CYP3A4, CYP3A5, and CYP3A7 sequences confirms at least 90% homology between them (71,160). CYP3A43 is predicted to be at least 71% homologous with other CYP3A forms (56). The CYP3A forms display broad and overlapping substrate specificities,

P.568


and are thought to participate in the metabolism of at least 50% of all drugs (49,103). Although CYP3A4 exhibited the greatest activity, recombinant CYP3A4, CYP3A5, and CYP3A7 isoforms were shown to be capable of catalyzing the conversion of 3′- and 4′-HPPH to the catechol diHPPH in vitro (40,93). In humans, total CYP3A activity displays large interindividual and interethnic variations that might be due to differences in induction, inhibition, or regulation of expression (97,138). To facilitate study of the control of CYP3A gene expression, the human CYP3A gene locus at 7q21.1 has been sequenced (56).

CYP3A4 is the most common form of CYP450 in liver and intestine. CYP3A4*1A is the wild-type form (65). Although CYP3A4*1B contains an A→G point mutation in the 5′-flanking region, there appears to be no difference in the level of hepatic expression of the enzyme compared with the wild-type form (89,161). The frequency of this mutation is 4.2% to 9% in whites, 53% to 66% in African Americans, and 0% in Taiwanese subjects (134,156). CYP3A4*2 contains a Ser222Pro substitution, resulting in lower in vitro nifedipine metabolism but, curiously, no difference in testosterone β-hydroxylation (134). This variant was found in 2.7% of white subjects, but was absent in both East Asians and African Americans (134). A CYP3A4*3 variant, containing a Met445Thr substitution, was found in only one Chinese subject (132). Recently, Hsieh et al. identified three rare variants in Chinese subjects, CYP3A4*4 (Ilel18Val), CYP3A4*4 (Prol02Arg), and CYP3A4*6 (frameshift), which were associated with decreased β-hydroxycortisol formation (77).

CYP3A5 appears to be expressed considerably more frequently than previously thought (13,97). The isoform constitutes up to 50% of total CYP3A in subjects who express the gene, and may be an important contributor to interindividual and interracial differences in CYP3A-dependent drug disposition (97). Only subjects having at least one copy of CYP3A5*1 express large amounts of the enzyme (95). The CYP3A5*1B and CYP3A5*1C variants differ from the wild-type gene (CYP3A5*1A) by having nucleotide substitutions in the promoter region, although these mutations are not associated with differences in enzyme activity in vivo (1,97). The CYP3A5*2, CYP3A5*3, and CYP3A5*6 variants appear to result in absence of the hepatic enzyme (89,97). Hepatic CYP3A5 is expressed much more frequently in African Americans (66%) than in whites (33%) (97).

CYP3A7 is the fetal form of the enzyme, although expression has been known to persist into adulthood in some people (70,135). A partial explanation for this continued expression may be the discovery of the CYP3A7*1C variant, in which a segment of the CYP3A4 promoter is substituted into the equivalent CYP3A7 promoter region. Interestingly, this CYP3A7*1C allele is three times more common in African Americans than in whites (97).

Arene Oxide

The arene oxide of phenytoin has never been isolated from plasma or urine, presumably because it is rapidly converted to further metabolic products. The existence of an arene oxide intermediate in the formation of p-HPPH was suggested by early kinetic experiments (148) and was established using the “NIH shift” technique (38,117).

The NIH shift technique depends on the observation that arene oxides spontaneously break down to form a hydroxylated metabolite, and that when this happens the hydrogen molecule at the site of the hydroxyl group is lost 50% of the time and is shifted (NIH shift) to the adjacent carbon, where the oxygen molecule had been attached 50% of the time. The NIH shift experiment of Claesen et al. (38) is shown in Figure 58.3. Racemic 5-(4-deuteriophenyl)-5-phenylhydantoin (p-[2H]-DPH) was administered to volunteers, and urine was collected. After enzymatic hydrolysis of the urine, deuterium retention by p-HPPH metabolites was determined. The expected percentage of deuterium retention by p-HPPH in the presence of an arene oxide/NIH shift pathway was 75%, and the measured values were 65% to 75% in four volunteers.

Phenytoin is known to exhibit nonlinear pharmacokinetics in humans (see later), implying that substrate saturation must be occurring in one or more of the enzymes metabolizing phenytoin. The CYP2C9 and CYP2C19 enzymes appear to be the site of substrate saturation resulting in nonlinear pharmacokinetics in humans based on the following observations: (a) Browne et al. (18) demonstrated that the rate of urinary excretion of p-HPPH varies inversely with phenytoin plasma concentration in humans (r = -0.640, p < 0.005) (9); (b) Tsuru et al. (149) demonstrated that the rate of formation of p-HPPH from rat hepatocytes and rat hepatic microsomes varies inversely with phenytoin concentration in the medium. Phenytoin is the probable substance causing competitive inhibition of the arene oxidase enzyme (25). There is some evidence that high concentrations of p-HPPH competitively inhibit hydroxylation of phenytoin in animals (76), but this phenomenon has not been demonstrated in vivo in humans (25,32). Arene oxides have attracted considerable attention because of their reactivity and possible role in toxicity and teratogenicity mechanisms.

Para-HPPH and Meta-HPPH

The principal urinary metabolite of phenytoin is p-HPPH in humans (27,28,146). Most p-HPPH is excreted as a glucuronide, and only small amounts of free p-HPPH are found in human urine (31,114). Very small amounts of m-HPPH also are found in human urine (5,29,63). Species differences in the patterns of p-HPPH and m-HPPH exist (5,28,73,121). The details of these differences have been reviewed previously (17).

P.569

FIGURE 58.3. Arene oxide-NIH shift pathway for para-hydroxylation of phenytoin. (From Claesen M, Moustafa MAA, Adline J, et al. Evidence for an arene oxide-NIH shift pathway in the metabolic conversion of phenytoin to 5-(4-hydroxyphenyl)-5-phenylhydantoin in the rat and in man. Drug Metab Dispos 1982;10:667-671, with permission.)

Mechanism of Formation

NIH shift experiments indicate that most p-HPPH in humans is produced through an arene oxide intermediate (see earlier). CYP2C9 and CYP2C19 are the enzymes responsible for the formation of arene oxide (see earlier).

A minority of patients are “slow metabolizers” of phenytoin (32,63,98,99,152,153). In these individuals, toxic phenytoin plasma concentrations develop at average dosing rates of phenytoin. Mutations of CYP2C9 or CYP2C19 are the causes of slowed metabolism (see earlier).

Dihydrodiol

We (17) have reviewed the key papers on phenytoin dihydrodiol.

Mechanism of Formation

Phenytoin dihydrodiol is believed to be formed from phenytoin arene oxide through the enzyme epoxide hydrolase (Figures 58.2 and 58.4). This belief is supported by several lines of evidence. First, the usual metabolic pathway leading to formation of dihydrodiols is conversion of an epoxide intermediate to a dihydrodiol by the enzyme epoxide hydrolase (32,68,88,123). Second, administration of an epoxide hydrolase inhibitor (1,2-epoxy-3,3,3-trichlorophropane) to pregnant Swiss mice reduced the incidence of phenytoin-related teratogenesis and the covalent binding of 14C radioactivity in

P.570


the fetus after administration of 14C-labeled phenytoin, while maternal phenytoin serum concentrations remained unchanged (111). This implies that the initial step of phenytoin metabolism (presumably the arene oxide formation) is unaffected by epoxide hydrolase inhibition and that a metabolic intermediate with teratogenic and covalent binding properties (presumably arene oxide) accumulates in greaterthan-usual amounts when epoxide hydrolase is inhibited. Third, incubation of phenytoin with human liver fractions known to contain epoxide hydrolase results in the formation of phenytoin dihydrodiol (127). There is evidence that chronic administration of phenytoin may induce epoxide hydrolase activity in humans (18,127).

FIGURE 58.4. Scheme depicting stereoselective metabolic pathways involved in the production of 5-(4-hydroxyphenyl)-5-phenylhydantoin (p-HPPH) and dihydrodiol (DHD). Possible stereoselective direct hydroxylation pathways are indicated with an “X.” [From Maguire JH, Butler TC, Dudley KH. Absolute configuration of (+)-5-(3-hydroxyphenyl)-5-phenylhydantoin, the major metabolite of 5,5-diphenylhydantoin in the dog. J Med Chem 1978;21:1194-1297, with permission.]

Stereoselective Formation of HPPH and Dihydrodiol

Phenytoin is a prochiral compound. Introduction of a hydroxyl group in one of the phenyl rings leads to the creation of a chiral center and results in the formation of enantiomeric phenolic metabolites (Figure 58.4). CYP2C19 catalyzes the formation of both (R)-HPPH and (S)-HPPH, whereas CYP2C9 is more stereoselective in the formation of (S)-HPPH (82,110,167). The p-HPPH from human urine consists of a 10:1 mixture of levorotatory and dextrorotatory isomers (29,51,108). The amount of m-HPPH in human urine is too low to permit isolation and measurement of optical rotation. Most dihydrodiol in human urine is in the (S) configuration [3:1 ratio of (S):(R)] (105,107,108,124). Although an arene oxide of phenytoin has not been directly isolated and characterized, a large body of evidence has accumulated in support of the existence of (S) and (R) arene oxide intermediates. The proposed pathway is depicted in Figure 58.4. There are species differences in the chirality of p-HPPH, m-HPPH, and dihydrodiol excreted in the urine (29,51,105,107,108).

Effects of Head Trauma and Pregnancy

After head trauma, there is decrease in plasma albumin concentration that begins immediately, reaches a minimum at 5 to 7 days, and does not return to normal for 3 to 4 weeks (2). After head trauma, there also is an increase in phenytoin metabolism (3,15). These changes are accompanied by a decrease in free and total phenytoin plasma concentration and an increase in free fraction of phenytoin (2,4,15). Frequent monitoring of free phenytoin plasma concentration and frequent adjustment of phenytoin dosage often are necessary after head injury.

Phenytoin does not appear to be metabolized to p-HPPH or derivatives by the human placenta (94).

Minor Metabolic Pathways

Catechol and 3-O-Methylcatechol Metabolites

Relatively small amounts of a 3,4-catechol metabolite, -(3,4-dihydroxyphenyl)-5-phenylhydantoin, and a 3-O-methylcatechol metabolite, 5-(4-hydroxy-3-methoxyphenyl)-5- phenylhydantoin, have been identified in the urine of humans and other species (12,14,33,34,115). The same metabolites also have been identified as glucuronides in the bile from isolated, perfused rat liver (60).

The catechol could be formed through dehydrogenation of the dihydrodiol metabolite by a mechanism similar to those described by Ayengar et al. (8). In a preliminary experiment,14C-labeled dihydrodiol metabolite isolated from rat urine was administered perorally to rats. It did not result in the appearance of the catechol metabolite in urine (33). The catechol metabolite could be formed by further hydroxylation of m-HPPH or p-HPPH. Gerber and Thompson (57) identified small amounts of the catechol and 3-O-methylcatechol in rat bile by gas chromatography/mass spectrometry (GC/MS) techniques after administration of m-HPPH or p-HPPH to rats or by addition of these compounds to an isolated, perfused rat liver preparation. More recently, Billings and Fischer (12) and Chow and Fischer (36) provided evidence that the catechol metabolite of phenytoin was formed from both dihydrodiol and p-HPPH in rats and mice. However, the predominant route was through p-HPPH (36).

In rat urine, the concentration of the 3-O-methylcatechol metabolite was approximately fivefold greater than that of the catechol, indicating extensive O-methylation in this species (33). Evidence obtained by Chang et al. (34) indicates that the 3-O-methylcatechol metabolite of phenytoin is formed from the catechol. Administration of the synthetic 3,4-dihydroxycatechol to rats resulted in the prompt appearance of 3-O-methylcatechol in the urine (33). It is reasonable to assume that the enzyme catechol-O-methyltransferase is involved in the formation of this metabolite because it is known to methylate other catechols, including catecholamines (7).

4,4N-Dihydroxy Metabolite and N-Glucuronide of Phenytoin

Two minor metabolites of phenytoin—namely, 5,5-bis(4-hydroxyphenyl) hydantoin (145) and an N-glucuronide of phenytoin (139)—were identified in rats and in humans (Figure 58.2). The 4,4N-dihydroxy metabolite was excreted as a glucuronide and accounted for approximately 1% of the total hydroxylated metabolites. The pathway that leads to the formation of this metabolite is not known. When p-HPPH was added to the perfusate of an isolated rat liver preparation, there was no evidence of the formation of the 4,4N-dihydroxy metabolite (145). However, addition of the synthetic 4,4N-dihydroxy compound to the same in vitro preparation (145) resulted in the formation of a monoglucuronide, a trihydroxyphenytoin glucuronide, and a dihydroxymethoxyphenytoin glucuronide, indicating further hydroxylation of the dihydroxy metabolite. Whether the trihydroxylated product represents normal phenytoin metabolites in intact animals is not known.

P.571

The glucuronide of phenytoin was isolated from a patient receiving phenytoin and was characterized as the N-3 glucuronide by GC/MS. The same metabolite also was present in the bile of an isolated, perfused rat liver preparation. The structure assignment was based on the mass spectra of various permethylated derivatives and a comparison of the reaction of the metabolite and 5,5-diphenyl-3-methylhydantoin with diazomethane (139). Subsequently, Hassell et al. (73) reported that phenytoin N-glucuronide was the major metabolite in cat urine.

A number of other minor metabolites of phenytoin, some not yet identified, are known to exist (31,32).

The metabolite distribution patterns in human urine were reported by Chang and coworkers (32,35): 68% to 81% p-HPPH, 7% to 11% dihydrodiol, 2.5% 3-O-methylcatechol, 1% unchanged phenytoin, and approximately 1% catechol. Metabolites in human feces (1% of fecal radioactivity) were identified as p-HPPH (50% to 80%), dihydrodiol (15% to 35%), and catechol (5% to 10%). Later studies have confirmed these values (43).

SPECIES DIFFERENCES IN METABOLISM

With the exception of the dog and cat, the major metabolic product of phenytoin in all species, including humans, is a glucuronide conjugate of p-HPPH (17,66,106). Significant amounts of free p-HPPH were observed in the urine of mice and rats (32). The dihydrodiol metabolite was found in higher concentrations in urine of rats and monkeys than in that of mice, dogs, or cats. The catechol and 3-O-methylcatechol metabolites were detected in rat urine after repeated administration of phenytoin in the diet and accounted for 2% and 20%, respectively, of the total metabolite present (34). In dog urine, the major metabolite was the glucuronide conjugate of m-HPPH, whereas phenytoin N-glucuronide was the major urinary product in cat.

Species differences in the formation of glucuronide conjugates of m-HPPH or p-HPPH in rat or dog liver supernatant were reported by Gabler (53). Conjugation of m-HPPH was greater than that of p-HPPH in the dog liver, whereas the opposite effect was found with rat liver supernatant.

NONLINEAR PHARMACOKINETIC PROPERTIES OF PHENYTOIN

The rate of change of plasma concentration (C) of a drug by an enzyme system can be expressed by the Michaelis-Menten equation:

where t is time, Vmax is the maximum velocity of the enzyme system, and Km is the Michaelis constant of the enzyme system (plasma concentration at which half of the maximum velocity of the enzyme system is attained). Mean steady-state plasma drug concentration (Css) can be expressed as

where R is dosing rate (102). When C is similar to or greater than Km, dC/dt varies in a nonlinear fashion with C; when R is equal to or greater than 0.1 Vmax, Css will vary in a nonlinear fashion with R. These observations are the basis of nonlinear pharmacokinetics.

The mean apparent value for phenytoin Km in adults is 6.2 µg/mL, with a range of 1.5 to 30.7 µg/mL based on 55 reported determinations; the mean apparent value for phenytoin Vmax in adults is 0.45 µg/mL/hr, with a range of 0.14 to 1.36 µg/mL/hr based on 54 reported determinations (6,19,48,55,59,74,113,126). These values appear to be determined principally by CYP2C9 and CYP2C19 Km and Vmax values. The other pathways shown in Figure 58.2 potentially could have modifying effects on the apparent values of Km and Vmax for phenytoin in humans (104). However, attempts to demonstrate an effect of these other pathways on phenytoin pharmacokinetic parameters in humans have been negative (23).

The apparent Km values computed for humans are based on total (protein-bound and non-protein-bound) phenytoin plasma concentration. Because only non-protein-bound phenytoin can be acted on by the metabolizing enzyme system and the non-protein-bound fraction for phenytoin is approximately 10% in humans (162), the Km of the enzyme responsible for parahydroxylation of phenytoin actually should be approximately 0.6 µg/mL. This prediction has been verified in rat liver microsomes (149).

Eadie et al. (48) performed the largest comprehensive comparison of phenytoin Km and Vmax values in children and adults. The Km values from 21 adults (mean = 5.8 µg/mL) and 15 children (mean = 5.3 µg/mL) were not significantly different. The Vmax values from 21 adults (mean = 0.48 µg/mL/hr, assuming phenytoin volume of distribution = 0.7 L/kg) were significantly (p < .025) less than the Vmax values from 15 children (mean = 0.74 µg/mL/hr, assuming phenytoin volume of distribution = 0.7 L/kg). These observations predict that the clearance (Vmax/[Km + C]) of phenytoin should be greater in children than in adults. This prediction is confirmed by the observations that the elimination half-life of phenytoin is shorter in children than in adults and that the average dosing rate of phenytoin (in milligrams per kilogram per day) required to achieve a given plasma concentration is greater in children than in adults (46,98).

The Vmax for phenytoin increases significantly during pregnancy (44). This explains part or all of the observed decreases in phenytoin steady-state plasma concentration during pregnancy (equation [2]).

P.572

Phenytoin exhibits nonlinear pharmacokinetic properties in most patients because the usual therapeutic plasma concentration values (10 to 20 µg/mL; Chapter 60) exceed the usual Km (6.2 µg/mL), and the usual dosing rate (0.15 to 0.45 µg/mL/hr) is greater than 0.1 times the usual value of Vmax (0.45 µg/mL/hr). Note that the metabolism of phenytoin is linear at low plasma concentrations and nonlinear at therapeutic and higher plasma concentrations (151). The consequences of nonlinear pharmacokinetics are discussed later.

Steady-State Plasma Concentration Varies in a Nonlinear Fashion with Dosing Rate

For drugs with nonlinear pharmacokinetics, steady-state plasma concentration increases faster than dosing rate when dosing rate is increased, and plasma concentration decreases faster than dosing rate when dosing rate is decreased (equation 2) (27,44,131) (Figure 58.5). Thus, the steady-state plasma concentration of a drug with nonlinear pharmacokinetic properties at one dosing rate does not directly predict the steady-state plasma concentration of the drug at another dosing rate.

FIGURE 58.5. Relationship between serum phenytoin concentration and daily dose in five patients. Each point represents the mean (± standard deviation) of three to eight measurements of serum phenytoin concentration at steady state. The curves were fitted by computer using the Michaelis-Menten equation. (From Richens A, Dunlop A. Serum phenytoin levels in the management of epilepsy. Lancet 1975;2:247-248, with permission.)

If the clinician attempts to increase or decrease the phenytoin steady-state plasma concentration by simple linear extrapolation from a known plasma concentration versus dosing rate point, the result often is an unexpectedly high or low plasma concentration when the new steady-state value is attained (Figure 58.5). Numerous mathematical and tabular methods have been published that claim to be able to predict the phenytoin dosing rate necessary to produce a given steady-state plasma concentration from a single steady-state plasma concentration versus dose point, and these methods have been critically reviewed elsewhere (7,19,27,37,78,118,126,136,137). A useful rule of thumb in titrating phenytoin dosage upward in adults is to increase dosing rate in increments of 100 mg/day at monthly intervals (see later) until a steady-state phenytoin plasma concentration of 5 to 10 µg/mL (a value approximately equal to Km) is attained; later increases should not exceed 50 mg/day at monthly intervals.

P.573

Generic Equivalence

Nonlinear pharmacokinetics complicate the issue of generic equivalence of phenytoin products. The weighted mean value for absolute bioavailability of brand-name (Dilantin Kapseals, 100 mg) phenytoin was 86% in three studies (18). Less-than-complete absorption of brand-name phenytoin is, at least in part, a consequence of the use of a sustained-release preparation (Chapter 13). Different generic preparations of phenytoin thus have the potential to differ in absolute bioavailability from the brand-name preparation by +14% and -14% or more. Because of phenytoin's nonlinear pharmacokinetics, a 14% difference in bioavailability would result in a >14% increase or decrease in steady-state plasma concentration. A national epidemic of phenytoin intoxication occurred in Australia when a more bioavailable formulation was substituted for an older formulation (125,150). Ludden et al. (104) and Browne et al. (26) have reviewed the effect of nonlinear pharmacokinetics on bioavailability studies in more detail.

Clearance Varies Inversely, and Elimination Half-Life Directly, with Plasma Concentration

Drug clearance is equal to Vmax/(Km + C). Drug elimination half-life is equal to 0.693 × volume of distribution/clearance. Thus, phenytoin clearance varies inversely with plasma concentration, and phenytoin elimination half-life varies directly with plasma concentration (18,20,27) (Figure 58.6). Browne et al. (20,27) described and validated a method for calculating phenytoin elimination half-life at any given phenytoin plasma concentration if the patient's Km and Vmax values for phenytoin are known. The results were as follows for a group of six adult men on phenytoin monotherapy (plasma concentration - mean calculated elimination half-life): 1 µg/mL—12.8 hours; 10 µg/mL—25.8 hours; 20 µg/mL—40.2 hours; 40 µg/mL—69.1 hours. Note that the often-quoted elimination half-life of 24 hours for phenytoin applies principally to plasma concentration values in the low therapeutic range (10 µg/mL) and that the elimination half-life often is longer at higher plasma concentrations (Figure 58.6). The range of elimination half-life values at phenytoin plasma concentration 40 µg/mL was 37.1 to 96.8 hours. Because of phenytoin's long and variable elimination half-life values at toxic plasma concentration values, the time required for phenytoin plasma concentration to fall from a toxic value to a therapeutic value cannot be predicted in a given individual. In such circumstances, the clinician must withhold phenytoin and obtain daily plasma concentration determination values until the plasma concentration has fallen back into the therapeutic range.

FIGURE 58.6. Phenytoin clearance and elimination half-life values determined by stable isotope tracer techniques at 30 different serum concentration values in 18 adult men on phenytoin monotherapy. (Based in part on data from references 18, 21, and 23, with permission.)

P.574

Time to Reach Steady-State Plasma Concentration Varies Nonlinearly with Dosing Rate and Linearly with Plasma Concentration

As phenytoin plasma concentration rises, phenytoin clearance decreases (see earlier). This results in a further rise in phenytoin plasma concentration and a further decrease in phenytoin clearance. This self-propagating cycle can require a long period to go to completion. The time (t) required to attain a given plasma concentration can be computed by the equation

FIGURE 58.7. Plot of minimum serum concentration after the Nth dose, Cmin, versus dose number n. Symbols and dosing rates (g/day) are: ♦, 0.50; ●, 0.40; ◊, 0.30; ▪, 0.25; and▲, 0.20. (From Wagner JG. Time to reach steady state and prediction of steady state concentration for drugs obeying Michaelis-Menten elimination kinetics. J Pharmacokinet Biopharm 1978;6:209-225, with permission.)

where Vd is the volume of distribution, Cs,t is the plasma concentration at time t, and Cs,0 is the plasma concentration at time 0 (89). Assuming average values for Km and Vmax, it is possible to compute an accumulation 1/half-life (t½A) for phenytoin as follows:

where t½,A has units of days, and Css has units of µg/mL (100). Equations [3] and [4] predict, and empirical data confirm, the following: (a) the time to reach steady-state plasma concentration varies nonlinearly with dosing rate; (b)

P.575


the time to reach steady-state plasma concentration varies linearly with plasma concentration; and (c) the time required to attain new steady-state plasma concentration values after starting phenytoin therapy or increasing or decreasing phenytoin dosing rate may be as long as 28 days (19,27, 101,144,155) (Figures 58.7 and 58.8). Therefore, a plasma phenytoin concentration value measured less than 28 days after a change in phenytoin dosing rate may not be an accurate indication of the ultimate new steady-state plasma concentration that will result from the change in dosing rate.

FIGURE 58.8. Changes in serum phenytoin concentration after reduction in phenytoin dosing rate from 250 to 200 mg/day. Serum phenytoin concentration stabilized after day 23. (From Theodore WH, Qu P, Tsay JY, et al. Phenytoin: the pseudosteady-state phenomenon. Clin Pharmacol Ther 1984;25:822-825, with permission.)

TABLE 58.1. PHENYTOIN BIOTRANSFORMATION AND PHARMACOKINETIC VALUES FOR SIX PATIENTS AT THREE TIMES DURING MONOTHERAPY DETERMINED WITH 150-MG TRACER DOSES OF STABLE ISOTOPE-LABELED PHENYTOINa

Week 0b

Week 4c

Week 12d

Significancee

Rate of formation of p-HPPH (mg labeled p-HPPH excreted per 48 hr):

98.5 ± 31.45f

82.2 ± 20.1

69.2 ± 31.9

p < .01

Clearance (mL/min/kg)g:

0.587 ± 0.149

0.456 ± 0.147

0.387 ± 0.187

p < .05

Elimination half-life (hr)g:

13.2 ± 3.6

18.4 ± 5.0

25.9 ± 9.7

p < .01

p-HPPH, 5-(p-hydroxyphenyl)-5-phenylhydantoin
aFrom Browne TR, Evans JE, Szabo GK, et al. Studies with stable isotopes: I. changes in phenytoin pharmacokinetics and biotransformation during monotherapy. J Clin Pharmacol 1985;25:43-50, with permission.)

b Week 0 = value from single-dose (150 mg) study performed before monotherapy.

c Week 4 = value after 4 weeks on monotherapy (300 mg/day).

d Week 12 = value after 12 weeks on monotherapy (300-500 mg/day).

e Difference among values for weeks 0, 4, and 12 by analysis of variance for one group with repeated measures.

f Mean ± standard deviation, here and throughout table.

g Value for tracer dose of phenytoin.

Typical Changes in Phenytoin Pharmacokinetics during Chronic Administration

When a patient starts phenytoin monotherapy, his or her plasma phenytoin concentration progressively rises. This has the following effects on phenytoin biotransformation and pharmacokinetic values (18): Rate of formation of p-HPPH decreases, clearance decreases, and elimination half-life increases (Figure 58.6). Table 58.1 illustrates typical values for these changes in a group of six patients started on phenytoin monotherapy.

ACKNOWLEDGMENT

This work was supported in part by the Department of Veterans Affairs.

CONVERSION

Conversion Factor:

Conversion:

(µg/m) × 3.96 = µmol/L ÷ 3.96 = µg/mL

(µmol/L) ÷ 3.96 = µg/mL

P.576

REFERENCES

  1. Aoyama T, Yamano S, Waxman DJ, et al. Cytochrome P450-hPCN3, a novel cytochrome P-450IIIA gene product that is differentially expressed in adult human liver: cDNA and deduced amino acid sequence in distinct specificities of cDNA-expressed hpCN1 and hpCN3 for the metabolism of steroid hormones and cyclosporine. J Biol Chem1989;264:100388-100395.
  2. Anderson GD, Gidal BE, Hendryx RJ, et al. Decreased plasma protein binding of valproate in patients with acute head trauma. Br J Clin Pharmacol1994;37:559-562.
  3. Anderson GD, Kaines KB, Kobayashi KA, et al. Phenytoin pharmacokinetics in post-traumatic head injury patients. Epilepsia1991;32[Suppl 3]:9(abstr).
  4. Anderson GD, Pak C, Doane KW, et al. Revised Winter-Tozer equation for normalized phenytoin concentrations in trauma and elderly patients with hypoalbuminemia. Ann Pharmacother1997;31:279-284.
  5. Atkinson AJ Jr, MacGee J, Strong J, et al. Identification of 5-methohydroxyphenyl-5-phenyl hydantoin as a metabolite of diphenylhydantoin. Biochem Pharmacol1970;19:2483-2491.
  6. Atkinson AJ, Shaw JM. Pharmacokinetic study of a patient with diphenylhydantoin toxicity. Clin Pharmacol Ther1973;14: 521-528.
  7. Axelrod J, Tomachick R. Enzymatic O-methylation of epinephrine and other catechols. J Biol Chem1950;233:702-705.
  8. Ayengar PK, Hayaisnio M, Nakajima M, et al. Enzymatic aromatization of 3,5-cyclohexadiene-1,2-diol. Biochim Biophys Acta1959;33:111-119.
  9. Aynacioglu AS, Brockmoller J, Bauer S, et al. Frequency of cytochrome P450 CYP2C9 variants in a Turkish population and functional relevance for phenytoin. Br J Clin Pharmacol1999;48:409-15.
  10. Bajpai M, Roskos LK, Shen DD, et al. Roles of cytochrome P4502C9 and cytochrome P4502C19 in the stereoselective metabolism of phenytoin to its major metabolite. Drug Metab Dispos1996;24:1401-1403.
  11. Billings RE. Sex differences in rats in the metabolism of phenytoin to 5-(3,4-dihydroxyphenyl)-5-phenyl hydantoin. J Pharmacol Exp Ther1983;225:630-636.
  12. Billings RE, Fischer LJ. Oxygen-18 incorporation studies of the metabolism of phenytoin to the catechol. Drug Metab Dispos1985;13:312-317.
  13. Boobis AR, Edwards RJ, Adams DA, et al. Dissecting the function of cytochrome P450. Br J Clin Pharmacol1996;42:81-89.
  14. Borga O, Garle M, Gutova M. Identification of 5-(3,4-dihydroxyphenyl)-5-phenylhydantoin as a metabolite of 5,5-diphenylhydantoin (phenytoin) in rats and man. Pharmacology1972;7:129-137.
  15. Boucher BA, Rodman JH, Jaresko GS, et al. Phenytoin pharmacokinetics in critically ill trauma patients. Clin Pharmacol Ther1988;44:675-683.
  16. Brockmoller J, Rost KL, Gross D, et al. Phenotyping of CYP2C19 with enantiospecific HPLC-quantification of R- and S-mephenytoin and comparison with the intron4/exon5 G—A-splice site mutation. Pharmacogenetics1995;5:80-88.
  17. Browne TR, Chang T. Phenytoin: biotransformation. In: Levy RH, Dreifuss FE, Mattson RH, et al., eds. Antiepileptic drugs,3rd ed. New York: Raven Press, 1989:197-213.
  18. Browne TR, Evans JE, Szabo GK, et al. Studies with stable isotopes: I. changes in phenytoin pharmacokinetics and biotransformation during monotherapy. J Clin Pharmacol1985;25: 43-50.
  19. Browne TR, Greenblatt DJ, Evans JE, et al. Determination of the in vivo Kmand Vmax of a drug with tracer studies. J Clin Pharmacol 1987;27:321-324.
  20. Browne TR, Greenblatt DJ, Evans JE, et al. Estimation of a drug's elimination half-life at any serum concentration when the drug's Kmand Vmax are known: calculation and validation with phenytoin. J Clin Pharmacol 1987;27:318-320.
  21. Browne TR, Szabo GK, Evans JE, et al. Phenobarbital does not alter phenytoin steady-state serum concentration or pharmacokinetics. Neurology1988;38:639-642.
  22. Browne TR, Szabo GK, Evans JE, et al. Carbamazepine increases phenytoin serum concentration and reduces phenytoin clearance. Neurology1988;38:1146-1150.
  23. Browne TR, Davondi H, Donn KH, et al. Bioavailability of ACC-9653 (phenytoin prodrug). Epilepsia1989;30[Suppl 2]: 527-532.
  24. Browne TR, Szabo GK, Schumacher GE, et al. Effects of epoxide hydrolase and other minor metabolic pathways on phenytoin's non-linear pharmacokinetics. J Clin Pharmacol1989;29: 857.
  25. Browne TR, Szabo GK, Walsh CT, et al. New pharmacokinetic methods: II. determination of presence or absence of product inhibition in drugs with non-linear pharmacokinetics. J Clin Pharmacol1990;30:578-584.
  26. Browne TR, Szabo GK, Schumacher GE, et al. Bioavailability studies of drugs with non-linear pharmacokinetics: I. tracer dose A.U.C. varies directly with serum concentration.J Clin Pharmacol1992;32:1141-1145.
  27. Browne TR, Szabo GK, Evans JE, et al. Studies of nonlinear pharmacokinetics with stable isotope labeled phenytoin. In: Baillie TA, Jones JP, eds. Synthesis and applications of isotopically labeled compounds.Amsterdam: Elsevier, 1994:157-162.
  28. Butler TC. The metabolic conversion of 5,5-diphenylhydantoin to 5-(p-hydroxyphenyl)-5-phenylhydantoin. J Pharmacol Exp Ther1957;199:1-11.
  29. Butler TC, Dudley KH, Johnson D, et al. Studies of the metabolism of 5,5-diphenylhydantoin relating principally to the stereoselectivity of the hydroxylation reactions in man and the dog. J Pharmacol Exp Ther1976;199:82-92.
  30. Chang M, Dahl ML, Tybring G, et al. Use of omeprazole as a probe for CYP2C19 phenotype in Swedish Caucasians: comparison with S-mephenytoin hydroxylation phenotype and CYP2C19 genotype. Pharmacogenetics1995;5:358-363.
  31. Chang T, Glazko AJ. Diphenylhydantoin: biotransformation. In: Woodbury DM, Penry JK, Schmidt RP, eds. Antiepileptic drugs.New York: Raven Press, 1972:149-162.
  32. Chang T, Glazko AJ. Phenytoin: biotransformation. In: Woodbury DM, Penry JK, Pippenger CE, eds. Antiepileptic drugs,2nd ed. New York: Raven Press, 1982:204-226.
  33. Chang T, Okerholm RA, Glazko AJ. Identification of 5-(3,4-dihydroxyphenyl)-5-phenylhydantoin: a metabolite of 5,5-diphenylhydantoin (Dilantin) in rat urine. Anal Lett1972;5: 195-202.
  34. Chang T, Okerholm RA, Glazko AJ. A 3-O-methylated catechol metabolite of diphenylhydantoin (Dilantin) in rat urine. Res Commun Chem Pathol Pharmacol1972;4:13-23.
  35. Chang T, Young R, Maschewske E, et al. Metabolite studies with 13C-14C doubly labeled phenytoin in human subjects. Epilepsia1977;18:191.
  36. Chow SA, Fischer LJ. Phenytoin metabolism in mice. Drug Metab Dispos1982; 10:156-160.
  37. Choy M, Winter ME. Comparing a mass balance algorithm with a Baysian regression analysis computer program for predicting serum phenytoin concentrations. Am J Health Syst Pharm1998; 55:2392-2396.

P.577

  1. Claesen M, Moustafa MAA, Adline J, et al. Evidence for an arene oxide-NIH shift pathway in the metabolic conversion of phenytoin to 5-(4-hydroxyphenyl)-5-phenylhydantoin in the rat and in man. Drug Metab Dispos1982;10:667-671.
  2. Crespi CL, Miller VP. The R144C change in the CYP2C9*2 allele alters interaction of the cytochrome P450 with NADPH: cytochrome P450 oxidoreductase. Pharmacogenetics1997;7: 203-210.
  3. Cuttle L, Munns A, Hogg NA, et al. Phenytoin metabolism by human cytochrome P450: involvement of P450 3A and 2C forms in secondary metabolism and drug-protein adduct formation. Drug Metab Dispos2000;28:945-950.
  4. deMorais SMF, Wilkinson GR, Blaisdell J, et al. The major defect responsible for the polymorphism of S-mephenytoin metabolism in humans. J Biol Chem1994;269:15419-15422.
  5. deMorais SM, Wilkinson GR, Blaisdell J, et al. Identification of a new genetic defect responsible for the polymorphism of (S)-mephenytoin metabolism in Japanese. Mol Pharmacol1994;46: 594-598.
  6. Dickinson RG, Hooper WD, Patterson M, et al. Extent of urinary excretion of p-hydroxyphenytoin in healthy subjects given phenytoin. Ther Drug Monit1985;7:283-289.
  7. Dickinson RG, Hooper WD, Wood B, et al. The effect of pregnancy in humans on the pharmacokinetics of stable isotope labeled phenytoin. Br J Clin Pharmacol1989;28:17-27.
  8. Dill WA, Kazenko A, Wold LM, et al. Studies on 5,5-diphenylhydantoin (Dilantin) in animals and man. J Pharmacol Exp Ther1956;118:270-279.
  9. Dodson WE. Phenytoin elimination in childhood: effect of concentration-dependent kinetics. Neurology1980;30:196-199.
  10. Donahue SR, Flockhart DA, Abernethy DR, et al. Ticlopidine inhibition of phenytoin metabolism mediated by potent inhibition of 2C19. Clin Pharmacol Ther1997;62:572-577.
  11. Eadie MJ, Tyrer JH, Bochner F, et al. The elimination of phenytoin in man. Clin Exp Pharmacol Physiol1976;3:217-224.
  12. Evans WE, Relling MV. Pharmacogenetics: translating functional genomics into rational therapeutics. Science1999;286: 487-491.
  13. Ferguson RJ, deMorais SM, Benhamou S, et al. A new genetic defect in human CYP2C19: mutation of the initiation codon is responsible for poor metabolism of S-mephenytoin.J Pharmacol Exp Ther1998;284:356-361.
  14. Fritz S, Linder W, Roots I, et al. Stereochemistry of aromatic phenytoin hydroxylation in humans. J Pharmacol Exp Ther1987;241:615-622.
  15. Furuya H, Fernandez-Salguero P, Gregory W, et al. Genetic polymorphism of CYP2C9 and its effect on warfarin maintenance dose requirement in patients undergoing anticoagulation therapy. Pharmacogenetics1995;5:389-392.
  16. Gabler WL. A method for assaying conjugation of diphenylhydantoin metabolites. Fed Proc1974;33:525.
  17. Garcia-Barcelo M, Chow LY, Kum Chiu HF, et al. Frequencies of defective CYP2C19 alleles in a Hong Kong Chinese population: detection of the rare allele CYP2C19*4. Clin Chem1999; 45:2273-2274.
  18. Garrettson LK, Jusko WJ. Diphenylhydantoin elimination kinetics in overdosed children. Clin Pharmacol Ther1975;17: 481-491.
  19. Gellner K, Eiselt R, Hustert E, et al. Genomic organization of the human CYP3A locus: identification of a new, inducible CYP3A gene. Pharmacogenetics2001; 11:111-121.
  20. Gerber N, Thompson RM. Identification of catechol glucuronide metabolites of hydroxydiphenylhydantoins (m-HPPH, p-HPPH) in rat bile. Fed Proc1974;33:525.25.
  21. Gerber N, Wagner JG. Explanation of dose-dependent decline of diphenylhydantoin levels by fitting to the integrated form of the Michaelis-Menten equation. Res Commun Chem Pathol Pharmacol1972;3:455-466.
  22. Gerber N, Lynn R, Oates J. Acute intoxication with 5,5-diphenylhydantoin (Dilantin) associated with impairment of biotransformation: plasma levels urinary metabolites; and studies in healthy volunteers. Ann Intern Med1972;77:756-771.
  23. Gerber N, Seibert RA, Thompson RM. Identification of a catechol glucuronide metabolite of 5,5-diphenylhydantoin (DPH) in rat bile by gas chromatography (GC) and mass spectrometry (MS). Res Commun Chem Path Pharmacol1973; 6:499-511.
  24. Glazko AJ. Phenytoin: chemistry and methods of determination. In: Levy RH, Mattson RH, Meldrum B, et al., eds. Antiepileptic drugs,3rd ed. New York: Raven Press, 1989:159-160.
  25. Glazko AJ, Chang T, Baukema J, et al. Metabolic disposition of diphenylhydantoin in normal human subjects following intravenous administration. Clin Pharmacol Ther1969;10:498-504.
  26. Glazko AJ, Peterson FE, Smith TC, et al. Phenytoin metabolism in subjects with long and short plasma half-lives. Ther Drug Monit1982;4:281-292.
  27. Goldstein JA, deMorais SMF. Biochemistry and molecular biology of the human CYP2C subfamily. Pharmacogenetics1994;4: 285-289.
  28. Gonzalez FJ, Schmid BJ, Umeno M, et al. Human P450PCN1: sequence, chromosome localization, and direct evidence through cDNA expression that P450PCN1 is nifedipine oxidase. DNA1988;7:79-86.
  29. Gorvin JH, Brownlee G. Metabolism of 5,5-diphenylhydantoin in the rabbit. Nature1957;179:1248.
  30. Gotoh O. Substrate recognition sites in cytochrome P450 family 2 (CYP2) proteins inferred from comparative analyses of amino acid and coding nucleotide sequences. J Biol Chem1992; 267:83-90.
  31. Grover PL, Hewer A, Sims P. Epoxides as microsomal metabolites of polycyclic hydrocarbons. FEBS Lett1971;18:76-80.
  32. Haining RL, Hunter AP, Veronese ME, et al. Allelic variants of human cytochrome P450 2C9: baculovirus-mediated expression, purification, structural characterization, substrate stereospecificity, and prochiral selectivity of the wild-type and I359L mutant forms. Arch Biochem Biophys1996;333:447-458.
  33. Hakkola J, Tanaka E, Pelkonen O. Developmental expression of cytochrome P450 enzymes in human liver. Pharmacol Toxicol1998;82:209-217.
  34. Hashimoto H, Toide K, Katamura R, et al. Gene structure of CYP3A4, an adult-specific form of cytochrome P450 in human livers, and its transcriptional control. Eur J Biochem1993;218: 585-595.
  35. Hashimoto Y, Otsuki Y, Odani A, et al. Effect of CYP2C polymorphisms on the pharmacokinetics of phenytoin in Japanese patients with epilepsy. Biol Pharm Bull1996;19:1103-1105.
  36. Hassell TM, Maguire JH, Cooper CG, et al. Phenytoin administered in the cat after long-term administration. Epilepsia1984; 25:556-563.
  37. Holcomb R, Lynn R, Harvey B, et al. Intoxication with 5,5-diphenylhydantoin (Dilantin): clinical features, blood levels, urinary metabolites and metabolic changes in a child. J Pediatr1972;80:627-632.
  38. Horning MG, Lertratanangkoon K. Effect of chronic administration on urinary profiles of phenytoin metabolites. Fed Proc1977;36:966.
  39. Horning MG, Stratton C, Wilson A, et al. Detection of 5-(3,4-di-p-phdroxy-1,5-cyclohexadien-1-yl)-5-phenylhydantoin as a major metabolite of 5,5-diphenylhydantoin (Dilantin) in newborn human. Anal Lett1971;4:537-545.

P.578

  1. Hsieh KP, Lin YY, Cheng CL, et al. Novel mutations of CYP3A4 in Chinese. Drug Metab Dispos2001;29:268-273.
  2. Hudson SA, Farqyhar DL, Thompson P, et al. Phenytoin dosage individualization: five methods compared in the elderly. J Clin Pharmacol Ther1990;15:25-34.
  3. Ibeanu GC, Blaisdell J, Ghanayem BI, et al. An additional defective allele, CYP2C19*5, contributes to the S-mephenytoin poor metabolizer phenotype in Caucasians.Pharmacogenetics1998;8:129-135.
  4. Ibeanu GC, Goldstein JA, Meyer U, et al. Identification of new human CYP2C19 alleles (CYP2C19*6 and CYP2C19*2B) in a Caucasian poor metabolizer of mephenytoin. J Pharmacol Exp Ther1998;286:1490-1495.
  5. Ibeanu GC, Blaisdell J, Ferguson RJ, et al. A novel transversion in the intron 5 donor splice junction of CYP2C19 and a sequence polymorphism in exon 3 contribute to the poor metabolizer phenotype for the anticonvulsant drug S-mephenytoin. J Pharmacol Exp Ther1999;290:635-640.
  6. Ieiri I, Mamiya K, Urae A, et al. Stereoselective 4′-hydroxylation of phenytoin: relationship to (S)-mephenytoin polymorphism in Japanese. Br J Clin Pharmacol1997;43:441-445.
  7. Ieiri I, Tainaka H, Morita T, et al. Polymorphism of the cytochrome P450 (CYP) 2C9 gene in Japanese epileptic patients: genetic analysis of the CYP2C9 locus.Pharmacogenetics2000;10:85-89.
  8. Imai J, Ieiri I, Mamiya K, et al. Polymorphism of cytochrome P450 (CYP) 2C9 gene in Japanese epileptic patients: genetic analysis of the CYP2C9 locus. Pharmacogenetics2000;10: 85-89.
  9. Inoue K, Yamazaki H, Imiya K, et al. Relationship between CYP2C9 and CYP2C19 genotypes and tolbutamide methyl hydroxylation and S-mephenytoin 4′-hydroxylation activities in livers of Japanese and Caucasian populations. Pharmacogenetics1997;7:103-113.
  10. Inoue K, Yamazaki H, Shimada T. Linkage between the distribution of mutations in the CYP2C18 and CYP2C19 genes in the Japanese and Caucasian. Xenobiotica1998; 28:403-411.
  11. Itoh K, Inoue K, Nakao H, et al. Polymerase chain reaction-single-strand conformation polymorphism based determination of two major genetic defects responsible for a phenotypic polymorphism of cytochrome P450 (CYP) 2C19 in the Japanese population. Anal Biochem2000;284:160-162.
  12. Jerina DM, Daly JW, Witkop B, et al. 1,2-Naphthalene oxide as an intermediate in the microsomal hydroxylation of naphthalene. Biochemistry1970;9:147-155.
  13. Jounadi Y, Hyrailles V, Gervot L, et al. Detection of CYP3A5 allelic variant: a candidate for the polymorphic expression of the protein? Biochem Biophys Res Commun1996;221:466-470.
  14. Jusko WJ. Bioavailability and disposition kinetics of phenytoin in man. In: Kellaway P, Petersen I, eds. Quantitative analytic studies in epilepsy.New York: Raven Press, 1976:115-136.
  15. Kidd RS, Straughn AB, Meyer MC, et al. Pharmacokinetics of chlorpheniramine, phenytoin, glipizide, and nifedipine in an individual homozygous for the CYP2C9*3 allele.Pharmacogenetics1999;9:71-80.
  16. Kimura M, Ieire I, Mamiya K, et al. Genetic polymorphisms of the cytochrome P450s, CYP2C19 and CCP2C9 in Japanese population. Ther Drug Monit1998;20:243-247.
  17. Kimura SJ, Pastewka J, Gelboin HV, et al. cDNA and amino acid sequences of two members of the human P450 IIC gene subfamily. Nucleic Acids Res1987;15:10053-10054.
  18. Kluck RM, Cannell GR, Hooper WD, et al. Disposition of phenytoin and phenybarbitone in isolated perfused human placenta. Clin Pharmacol Physiol1988;15:827-836.
  19. Komatsu T, Yamazaki H, Asahi S, et al. Formation of a dihydroxy metabolite of phenytoin in human liver microsomes /cytosol: roles of cytochromes P450 2C9, 2C19, and 3A4.Drug Metab Dispos2000;28:1361-1368.
  20. Kubota T, Hibi N, Chiba K. Linkage of mutant alleles of CYP2C18 and CYP2C19 in a Japanese population. Biochem Pharmacol1998;55:2039-2042.
  21. Kuehl P, Zhang J, Lin Y, et al. Sequence diversity in CYP3A promoters and characterization of the genetic basis of polymorphic CYP3A5 expression. Nat Genet2001;27:383-391.
  22. Kutt H. Phenytoin: relation of plasma concentration to seizure control. In: Woodbury DM, Penry JK, Pippenger CE, eds. Antiepileptic drugs,2nd ed. New York: Raven Press, 1982: 241-246.
  23. Kutt H, Wolk M, Scherman R, et al. Insufficient parahydroxylation as a cause of diphenylhydantoin toxicity. Neurology1964; 14:542-548.
  24. Lai C, Moore P, Matier WL, et al. Bioequivalence of ACC-9653, a phosphate ester prodrug of phenytoin, to phenytoin sodium after i.v. administration in dogs. Pharmacologist1987; 29:143.
  25. Lam G, Chiou WL. Integrated equation to evaluate accumulation profiles of drugs eliminated by Michaelis-Menten kinetics. J Pharmacokinet Biopharm1979;7:227-232.
  26. Levy RH. General principles: drug absorption, distribution and elimination. In: Woodbury DM, Penry JK, Pippenger CE, eds. Antiepileptic drugs,2nd ed. New York: Raven Press, 1982:11-24.
  27. Li AP, Kaminski DL, Rasmussen A. Substrates of human hepatic cytochrome P450 3A4. Toxicology1995;104:1-8.
  28. Ludden TM, Allerheiligen SR, Browne TR, et al. Sensitivity analysis of the effect of bioavailability or dosage form content on mean steady state phenytoin concentration.Ther Drug Monit1991;13:120-125.
  29. Maguire JH, Wilson DC. Urinary dihydrodiol metabolites of phenytoin: high performance liquid chromatography assay of diastereometric composition. J Chromatogr1985;342:323-332.
  30. Maguire JH, Butler TC, Dudley KH. Absolute configuration of (+)-5-(3-hydroxyphenyl)-5-phenylhydantoin, the major metabolite of 5,5-diphenylhydantoin in the dog. J Med Chem1978; 21:1194-1297.
  31. Maguire JH, Butler TC, Dudley KH. Absolute configurations of the dihydrodiol metabolites of 5,5-diphenylhydantoin (phenytoin) from rat, dog, and human urine. Drug Metab Dispos1980;8:325-331.
  32. Maguire JH, McClanahan JS. Evidence for stereoselective production of phenytoin (5,5-diphenylhydantoin) arene oxides in man. Life Sci1983;897-902.
  33. Mamiya K, Ieiri I, Miyahara S, et al. Association of polymorphisms in the cytochrome P450 (CYP) 2C19 and 2C18 genes in Japanese epileptic patients. Pharmacogenetics1998;8:87-90.
  34. Mamiya K, Ieieri I, Shimamoto J, et al. The effects of genetic polymorphisms of CYP2C9 and CYP2C19 on phenytoin metabolism in Japanese adult patients with epilepsy: studies with stereoselective hydroxylation and population pharmacokinetics. Epilepsia1998; 39:1317-1323.
  35. Martz F, Failinger C III, Blake D. Phenytoin teratogenesis: correlation between embryopathic effects and covalent binding of putative arene oxide metabolite in gestational tissue. J Pharmacol Exp Ther1977;203:231-239.
  36. Masimirembwa C, Bertilsson L, Johansson I, et al. Phenotyping and genotyping of S-mephenytoin hydroxylase (cytochrome P450 2C19) in a Shona population of Zimbabwe.Clin Pharmacol Ther1995;57:656-661.
  37. Mawer GE, Mullen PW, Rodgers M, et al. Phenytoin dose adjustment in epileptic patients. Br J Clin Pharmacol1974;1: 163-168.
  38. Maynert EW. The metabolic fate of diphenylhydantoin in the dog, rat, and man. J Pharmacol Exp Ther1960;130:275-284.

P.579

  1. Midha KK, Hindmarsh KW, McGilvrary IJ, et al. Identification of urinary catechol and methylated catechol metabolites of phenytoin in human, monkeys, and dogs by GLC and GLC-mass spectrometry. J Pharm Sci1977;66:1596-1602.
  2. Mizugaki M, Hiratsuka M, Agatsuma Y, et al. Rapid detection of CYP2C18 genotypes by real-time fluorescence polymerase chain reaction. J Pharm Pharmacol2000;52:199-205.
  3. Moustafa MAA, Claesen M, Adline J, et al. Evidence for an arene-3,4-oxide as metabolic intermediate in the meta- and para-hydroxylation of phenytoin in the dog. Drug Metab Dispos1983;11:574-580.
  4. Mullen PW, Foster RW. Comparative evaluation of six techniques for determining the Michaelis-Menten parameters relating phenytoin dose and steady state serum concentrations. J Pharm Pharmacol1979;31:100-104.
  5. Nakamura K, Goto F, Ray WA, et al. Interethnic differences in genetic polymorphism of debrisoquine and mephenytoin hydroxylation between Japanese and Caucasian populations. Clin Pharmacol Ther1985;38:402-408.
  6. Ninomiya H, Mamiya K, Matsuo S, et al. Genetic polymorphism of the CYP2C subfamily and excessive serum phenytoin concentration with central nervous system intoxication. Ther Drug Monit2000;22:230-232.
  7. Noach EL, Woodbury DM, Goodman LS. Studies on the absorption, distribution, fate and excretion of 4-14C-labeled diphenylhydantoin. J Pharmacol Exp Ther1958;122:301-314.
  8. Odani A, Hashimoto Y, Otsuki Y, et al. Genetic polymorphism of the CYP2C subfamily and its effect on the pharmacokinetics of phenytoin in Japanese patients with epilepsy.Clin Pharmacol Ther1997;62:287-292.
  9. Oesch F, Kaubisch N, Jerina DM, et al. Hepatic epoxide hydrase: structure-activity relationships for substrates and inhibitors. Biochemistry1971;10:4858-4866.
  10. Poupaert JH, Cavalier R, Claesen MH, et al. Absolute configuration of the major metabolite of 5,5-diphenylhydantoin, 5-(4-hydroxyphenyl)-5-phenylhydantoin. J Med Chem1975;18: 1268-1271.
  11. Rail L. Dilantin overdosage. Med J Aust1968;2:339.
  12. Rambeck B, Boenigk HE, Dunlop A, et al. Predicting phenytoin dose: A revised monogram. Ther Drug Monit1979;1:325-333.
  13. Rane A, Peug D. Phenytoin enhances epoxide metabolism in human fetal liver cultures. Drug Metab Dispos1985;13: 382-385.
  14. Rettie AE, Haining RL, Bajpai M, et al. A common genetic basis for idiosyncratic toxicity of warfarin and phenytoin. Epilepsy Res1999;35:253-255.
  15. Richardson TH, Jung F, Griffin KJ, et al. A universal approach to the expression of human and rabbit cytochrome P450s of the 2C subfamily in Escherichia coli. Arch Biochem Biophys1995; 323:87-96.
  16. Richardson TH, Griffin KJ, Jung F, et al. Targeted antipeptide antibodies to cytochrome P450 2C18 based on epitope mapping of an inhibitory monoclonal antibody to P450 2C51. Arch Biochem Biophys1997;338:157-162.
  17. Richens A, Dunlop A. Serum phenytoin levels in the management of epilepsy. Lancet1975;2:247-248.
  18. Romkes M, Faletto MB, Blaisdell JA, et al. Cloning and expression of complementary DNAs for multiple members of the human cytochrome P450IIC subfamily. Biochemistry1991;30: 3247-3255.
  19. Ruas JL, Lechner MC. Allele frequency of CYP2C19 in a Portuguese population. Pharmacogenetics1997;7:333-335.
  20. Sata F, Sapone A, Elizondo G, et al. CYP3A4 allelic variants with amino acid substitutions in exons 7 and 12: evidence for an allelic variant with altered catalytic activity.Clin Pharmacol Ther2000;67:48-56.
  21. Schuetz J, Beach P, Guzelian PS. Selective expression of cytochrome P450 CYP3A mRNAs in embryonic and adult human liver. Pharmacogenetics1994;4:11-20.
  22. Schumacher GE. Using pharmacokinetics in drug therapy: VI. comparing methods for dealing with nonlinear drugs like phenytoin. Am J Hosp Pharm1980;37:128-132.
  23. Sheiner LB, Beal SL. Evaluation of population methods for estimating pharmacokinetic parameters: I. Michaelis-Menten model: routine pharmacokinetic data. J Pharmacokinet Biopharm1980;8:553-571.
  24. Shimada T, Yamazaki H, Mimura M, et al. Interindividual variations in human liver cytochrome P-450 enzymes involved in the oxidation of drugs, carcinogens, and toxic chemicals: studies with liver microsomes of 30 Japanese and 30 Caucasians. J Pharmacol Exp Ther1994;270:414-423.
  25. Smith RG, Daves GD Jr, Lynn RK, et al. Hydantoin ring glucuronidation: characterization of a new metabolite of 5,5-diphenylhydantoin in man and the rat. Biomed Mass Spectrom1977;4:275-279.
  26. Stubbins MJ, Haries LW, Smith G, et al. Genetic analysis of the human cytochrome P450 CYP2C9 locus. Pharmacogenetics1996;6:429-439.
  27. Sullivan-Klose TH, Ghanayem BI, Bell DA, et al. The role of the CYP2C9-Leu359 allelic variant in the tolbutamide polymorphism. Pharmacogenetics1996;6:341-349.
  28. Takakubo F, Kuwano A, Kondo I. Evidence that poor metabolizers of (S)-mephenytoin could be identified by haplotypes of CYP2C19 in Japanese. Pharmacogenetics1996;6:265-267.
  29. Takanashi K, Tainaka H, Kobayashi K, et al. CYP2C9Ile359 and Leu359 variants: enzyme kinetic study with seven substrates. Pharmacogenetics2000;10:95-104.
  30. Theodore WH, Qu P, Tsay JY, et al. Phenytoin: the pseudosteady-state phenomenon. Clin Pharmacol Ther1984;25:822-825.
  31. Thompson RM, Beghin J, Fife WK, et al. 5,5-Bis(4-hydroxyphenyl)hydantoin, a minor metabolite of diphenylhydantoin (Dilantin) in the rat and human. Drug Metab Dispos1976; 4:349-356.
  32. Thompson RM, Gerber N, Seibert RA, et al. A rapid method for the mass spectrometric identification of glucuronides and other polar drug metabolites in permethylated rat bile. Drug Metab Dispos1973;1:489-505.
  33. Thummel KE, Wilkinson GR. In vitro and in vivo drug interactions involving human CYP3A. Annu Rev Pharmacol Toxicol1998;38:389-430.
  34. Tomaszewski JE, Jerina DM, Daly JW. Deuterium isotope effect during formation of phenols by hepatic monoxygenases: evidence for an alternative to the arene oxide pathway. Biochemistry1975;14:2024-2031.
  35. Tsuru M, Erickerson RR, Holtzman JL. The metabolism of phenytoin by isolated hepatocytes and hepatic microsomes from male rats. J Pharmacol Exp Ther1982;222:658-661.
  36. Tyrer JH, Eadie MJ, Sutherland JM. Outbreak of anticonvulsant intoxication in an Australian city. BMJ1970;4:271-273.
  37. Valodia PN, Seymour SA, McFayden ML, et al. Validation of population pharmacokinetic parameters of phenytoin using a parallel Michaelis-Menten and first-order elimination model. Ther Drug Monit2000;22:313-319.
  38. Vasko MR, Bell RD, Daly DD, et al. Inheritance of phenytoin hypometabolism: a kinetic study in one family. Clin Pharmacol Ther1979;27:96-103.
  39. Vermeij P, Ferrari MD, Buruma PJ, et al. Inheritance of poor phenytoin parahydroxylation capacity in a Dutch family. Clin Pharmacol Ther1988;44:588-593.
  40. Veronese ME, Doecke CJ, Mackenzie PI, et al. Site-directed mutation studies of human liver cytochrome P-450 isoenzymes in the 2C subfamily. Biochem J1993;289:533-538.

P.580

  1. Wagner JG. Time to reach steady state and prediction of steady state concentration for drugs obeying Michaelis-Menten elimination kinetics. J Pharmacokinet Biopharm1978;6:209-225
  2. Walker AH, Jaffe JM, Gunasegaram S, et al. Characterization of an allelic variant in the nifedipine-specific element of CYP3A4: ethnic distribution and implications for prostate cancer risk. Hum Mutat1998;12:289-293.
  3. Wang SL, Huang JD, Lai MD, et al. Detection of CYP2C9 polymorphism based on the polymerase chain reaction in Chinese. Pharmacogenetics1995;5:37-42.
  4. Watanabe M, Iwahashi K, Kugoh T, et al. The relationship between phenytoin pharmacokinetics and the CYP2C19 genotype in Japanese epileptic patients. Clin Neuropharmacol1998; 21:122-126.
  5. Weber BL, Rebbeck TR. Characterization of an allelic variant in the nifedipine-specific element of CYP3A4: ethnic distribution and implications for prostate cancer risk. Hum Mutat1998;12:289-293.
  6. Watkins PB. Noninvasive tests of CYP3A enzymes. Pharmacogenetics1994;4:171-184.
  7. Westlind A, Lofberg L, Tindberg N, et al. Interindividual differences in hepatic expression of CYP3A4: relationship to genetic polymorphism in the 5′-upstream regulatory region. Biochem Biophys Res Commun1999;259:201-205.
  8. Woodbury DM. Phenytoin: absorption, distribution, and excretion. In: Woodbury DM, Penry JK, Pippenger CE, eds. Antiepileptic drugs,2nd ed. New York: Raven Press, 1982: 191-207.
  9. Xiao ZS, Goldstein JA, Xie HG, et al. Differences in the incidence of the CYP2C19 polymorphism affecting the S-mephenytoin phenotype in Chinese Han and Bai populations and identification of a new rare CYP2C19 mutant allele. J Pharmacol Exp Ther1997;281:604-609.
  10. Xie HG, Kim RB, Stein CM, et al. Genetic polymorphism of (S)-mephenytoin 4′-hydroxylation in populations of African descent. Br J Clin Pharmacol1999;48:402-408.
  11. Xie HG, Stein CM, Kim RB, et al. Allelic, genotypic and phenotypic distributions of S-mephenytoin 4′-hydroxylase (CYP2C19) in healthy Caucasian populations of European descent throughout the world. Pharmacogenetics1999;9: 539-549.
  12. Xie HG. Genetic variations of S-mephenytoin 4′-hydroxylase (CYP2C19) in the Chinese population. Life Sci2000;66: PL175-PL181.
  13. Yasumori T, Chen LS, Li QH, et al. Human CYP2C-mediated stereoselective phenytoin hydroxylation in Japanese: difference in chiral preference of CYP2C9 and CYP2C19.Biochem Pharmacol1999;57:1297-1303.

Previous
Page
Next
Page

Contents


If you find an error or have any questions, please email us at admin@doctorlib.org. Thank you!