Journal of
J.Neurol. 216, 155--162 (1977)
Neurology t~ by Springer-Verlag 1977
The Bioavailability of Phenytoin* R. Gugler l**, W. Fr6scher 2, M. Eichelbaum l, and G. Hildenbrand 3 t Medizinische Universit/itsklinik (Direktor: Prof. Dr. H. J. Dengler), 2Universit/itsnervenklinik (Komm. Direktor: Prof. Dr. H. Penin), 3Institut fiir Medizinische Statistik, Dokumentation und Datenverarbeitung (Direktor: Prof. Dr. G. Oberhoffer), all: D-5300Bonn-Venusberg, Federal Republic of Germany Summary. The bioavailability of three commercial products of phenytoin (Epanutin, sodium salt; Phenhydan, calcium salt; Zentropil, free acid) was studied relative to a standard solution of sodium phenytoin. Each preparation was given for 14 days in a daily dose of 300 mg according to a cross over design. The three brands tested were equivalent as to their bioavailability with respect to plasma concentrations during one dosing interval, the area under the plasma level time curve, the time of peak plasma concentration, and the urinary excretion of the primary metabolite. Therefore, these phenytoin brands are bioequivalent and exchangeable for antiepileptic treatment. The standard solution of sodium phenytoin had a higher bioavailability as compared to the commercial products. Key words: Phenytoin - Bioavailability - Antiepileptic therapy - Epilepsy.
Zusammenfassung. Die biologische Verffigbarkeit der 3 Handelspr~iparate von Phenytoin, Epanutin® (Natriumsalz), Phenhydan® (Calciumsalz) und Zentropil® (freie S~iure), wurde im Vergleich zu einer w~issrigen Standardform von Phenytoin-Natrium untersucht. Nach einem cross-over-Plan wurde jede Pr~iparation fiber 14 Tage in einer Dosis von 300mg verabreicht. Die 3 Handelspr~iparate hatten gleiche biologische Verffigbarkeit im Hinblick auf alle Plasmakonzentrationen wahrend eines Dosisintervalles im steady state, die Fl~che unter der Plasmaspiegelkurve, den Zeitpunkt des maximalen Plasmaspiegels und die Urinausscheidung des Hauptmetaboliten. Diese Pr~parate von Phenytoin sind damit bio~iquivalent und in der Therapie austauschbar. Die Standardform yon Phenytoin-Natrium hatte eine bessere biologische Verfiigbarkeit als die Handelspr~iparate. Phenytoin (DPH) plays a major role in the treatment of several types of epilepsy, particularly of major and psychomotor seizures. Although a correlation between the plasma concentration of phenytoin and the therapeutic effect has been * Supported by Bundesministerium ffir Jugend, Familie und Gesundheit (342-4801-8/3) ** Corresponding author
156
R. Gugler et al.
e s t a b l i s h e d ( K u t t a n d M c D o w e l l , 1968; L u n d et al., 1964), a satisfactory corr e l a t i o n does n o t exist between the dose a n d the p l a s m a level ( W o o d b u r y et al., 1972). This lack o f c o r r e l a t i o n is due to genetic differences in the m e t a b o l i s m o f p h e n y t o i n ( K u t t et al., 1964) o r o f dose d e p e n d e n t p h a r m a c o k i n e t i c s in the m a j o r i t y o f cases ( A r n o l d a n d G e r b e r , 1970; G e r b e r a n d W a g n e r , 1972). A g o o d b i o a v a i l a b i l i t y o f a n y p r e p a r a t i o n used s h o u l d eliminate a n o t h e r variable in the d r u g t r e a t m e n t o f epilepsy, i.e. i n c o m p l e t e a n d consequently i n c o n s t a n t a b s o r p t i o n o f p h e n y t o i n . P h e n y t o i n is one o f those drugs, o f which differences in b i o a v a i l a b i l i t y are c o n s i d e r e d to be o f p a r t i c u l a r i m p o r t a n c e ( K o c h - W e s e r , 1974): p h e n y t o i n is a highly p o t e n t d r u g which is a d m i n i s t e r e d to seriously ill patients, has a n a r r o w t h e r a p e u t i c range a n d is k n o w n to be slowly a b s o r b e d . T h e p r e s e n t s t u d y was u n d e r t a k e n to investigate the b i o a v a i l a b i l i t y o f three p r e p a r a t i o n s consisting o f the s o d i u m salt, the calcium salt, a n d the acid f o r m o f p h e n y t o i n . T h e different p r e p a r a t i o n s were c o m p a r e d with a s t a n d a r d s o l u t i o n p r e p a r e d as a s u s p e n s i o n o f p h e n y t o i n sodium. In o r d e r to m i m i c the real p a t i e n t s i t u a t i o n the s t u d y was p e r f o r m e d using a chronic t r e a t m e n t design, since d a t a o b t a i n e d f r o m a single dose o f drugs with n o n linear p h a r m a c o k i n e t i c s c a n n o t be a p p l i e d to the c o n d i t i o n s o f long t e r m t h e r a p y .
Methods
Subjects. Eight healthy volunteers (4 male, 4 female), age range between 23 and 35 years, participated in the study. Subjects were judged as being healthy from the history, physical examination, and routine laboratory tests. In particular, no evidence of hepatic or renal disease or of congestive heart failure existed. The subjects had to be without any medication for at least 2 weeks prior to the study, and no drugs in addition to the test drugs were allowed throughout the study. Administration of Drugs. The study was carried out using a cross over design. The duration of the study was 8 weeks. Each subject received every phenytoin preparation chronicly over a period of 2 weeks. The daily dose was 300 mg of phenytoin to be taken once a day in the morning between 0700 and 0800 h. The subjects were allowed a light continental breakfast immediately after drug intake. The following preparation were used: A = Epanutin (sodium salt; Parke-Davis; Lot No. DbKE 703). B =Phenhydan (calcium salt; Desitin; Lot No. 186054). C =Zentropil (acid; Nordmark; Lot No. 1013). D = Standard solution ,(sodium salt). According to the study design the sequence of drug intake was A-B-C-D, B-C-D-A, C-D-AB and D-A-B-C for 2 subjects each. Samples. Blood (6 ml) for determination of phenytoin concentration in plasma was obtained from the subjects on days 12, 13 and 14 of each treatment period immediately prior to the next dose. On day 14 additional blood samples were drawn 1, 2, 4, 6, 8, 10 and 24h after the last dose. Urine was collected over 24h for 3 days at the end of each treatment period for measurement of p-hydroxyphenyl-phenylhydantoin (p-HPPH) during one dosing interval in steady state. Drug Assays.Phenytoin was determined according to a gas chromatographic method following methylation of the compound (Berlin et al., 1972); p-HPPH in urine was measured by the method of Atkinson et al. (1970).
The Bioavailability of Phenytoin
157
Bioavailability Parameters. The parameters used to determine the bioavailability were the
following: 1. Plasma concentration in steady state under chronic treatment. C s ~ - m i n = plasma concentration prior to the next dose (mean of 4 determinations). Css.... = peak plasma concentration following administration of the dose. 2. Area under the plasma level time curve (AUC) during one dosing interval calculated by the trapezoidal rule. 3. Time of maximum plasma concentration (Tm~x). 4. Excretion of the metabolite (p-HPPH) over 24 h in steady state (mean of 3 determinations). Statistical Methods. Rank analysis of variance (Friedman, 1937) was used for statistical analysis. In case of significance (~2 _>X20.05:3= 7.81) multiple comparisons (Wilcoxon and Wilcox, 1964) were applied.
Results M e a n plasma concentrations of all subjects during one dosing interval for the four preparations tested are shown in Figure 1. At time 0 (prior to the daily dose) no significant difference in plasma concentrations was f o u n d between the preparations. A t 1, 4, 6, a n d 8 h following the dose, the concentrations o f the standard solution (D) were significantly higher than the concentrations o f E p a n u t i n (A) a n d P h e n h y d a n (B), and were also higher than the concentrations o f Zentropil (C) at 2, 3, and 10 h post dose. Epanutin, P h e n h y d a n a n d Zentropil were at no time significantly different f r o m each other, although in the graph the plasma level curve o f Zentropil appears to be located between the other two preparations a n d the s t a n d a r d form. The plasma concentrations prior to the dose (C .... in) are listed in Table 1 (mean o f determinations on four different days). The differences were n o t 20 E 'e o.
0_ 5
"rime [h3
Fig. 1. Mean values of the plasma level curves of 4 phenytoin preparations (A = Epanutin®, B= Phenhydan®, C= Zentropil®, D= standard solution) during one dosing interval under chronic therapy
158
R. Gugler et al.
Subject
1
2 3 4 5 6 7 8 Mean SD a
Aa
Ba
Ca
D
4.1 4.0 2.8 15.4 3.8 7.2 5.1 3.3
6.1 6.8 3.5 16.4 4.3 7.3 3.3 2.7
8.7 5.5 2.4 20.6 5.0 5.5 8.8 3.0
11.7 6.8 2.7 22.1 4.7 6.4 9.6 3.5
5.7 1.5
6.3 1.6
7.4 2.0
8.4 2.2
Table 1. Plasma concentrations (ttg/ml) prior to the dose (mean of 4 determinations) of 4 phenytoin preparations during chronic therapy. Mean and standard deviation (SD)
NO di~rence from D (P>0.05)
Su~ect
1
2 3 4 5 6 7 8 Mean SD a
Preparation
Preparation W
~
D
4.4 7.3 4.9 21.3 7.5 12.4 10.7 6.8
10.4 I0.0 7.1 20.3 6.0 12.5 5.0 4.2
12.8 6.7 7.0 25.6 9.9 9.4 13.6 6.4
19.8 16.1 7.7 25.1 10.2 14.1 14.5 7.2
9.4 1.9
9.4 1.8
11.4
14.3 2.2
6.3
Table2. Peak plasma concentrations (C . . . . ) during one dosing interval of 4 phenytoin preparations tested
Different from D (P0.05)
160
R. Gugler et al.
between A, B and C (Table 3). If the standard solution (D) is regarded as 100%, the mean relative bioavailability is 67% for Epanutin, 66% for Phenhydan and 83% for Zentropil. The time of peak plasma concentration shows no consistant pattern for the individual subjects as well as for any particular phenytoin preparation (Table 4). The mean value of Tmax is greater for preparation B (5.8h) and C (5.6h) as compared to A (3.8 h) and D (3.0 h); however, this difference is not statistically significant. The renal excretion of the main metabolity of phenytoin (p-HPPH) during one dosing interval is not significantly different between preparations (Table 5).
Discussion
The bioavailability of phenytoin has not been studied extensively before (Dill et al., 1956; Glazko, 1972; Arnold et al., 1970; Lund et al., 1974; Lund, 1974; Albert et al., 1974; Pentika'inen et al., 1975; Gugler et al., 1976). The majority of results on the bioavailability of this drug were obtained using a single dose design; one study has been performed in steady state (Lund, 1974). It has been demonstrated with digoxin, however, that testing the bioavailability under a multiple dose regimen appears to give more reliable data (Huffman et al., 1974), since the results are not influenced by the particular experimental conditions of one study day; drug concentrations in plasma or urine are well above the sensitivity limits of any assay; the area under the plasma level curve and the renal excretion of the drug or its metabolite can be estimated with more accuracy during one dosing interval than by extrapolation to infinity after a single dose; multiple dosing is for most drugs the real mode of application. One additional reason for using a chronic treatment design is the non-linear pharmacokinetics of phenytoin. The higher the plasma concentrations of phenytoin, the more likely they are in the range of saturation of elimination, i.e. small differences in low concentrations seen after a single dose can be magnified and can thus become clinically more relevant in the high concentration range obtained during chronic therapy. This effect of non-linear pharmacokinetics is indeed reflected in the results of the present study. During one dosing interval plasma concentrations at time 0 and 24 h, which are in the lower range, are not significantly different between preparations, whereas significant differences are recorded from 1 to 10h after dosing. When the results of all bioavailability parameters tested are judged in context, the commercial preparations of phenytoin, Epanutin, Phenhydan and Zentropil were found not to be different from each other. Differences in the area under the plasma level curve and in the concentrations at several time points during one dosing interval were smaller than the day to day variation during chronic therapy. Thus, the conclusion appears to be justified that these phenytoin preparations are bioequivalent and are exchangeable with each other without any significant effect on drug efficacy. These results are of importance in the light of considerable differences in bioavailability of phenytoin reported from Australia and Scandinavia, where dramatic signs of toxicity or ineffectiveness were redorded when a
The Bioavailability of Phenytoin
161
different brand of phenytoin was prescribed, and even when the formulation of one brand was changed (Eadie et al., 1968; Rail, 1968; Tyrer et al., 1970; Lund, 1974; Pentikainen et al., 1975). O f all bioavailability parameters tested the area under the curve seemed to be the best measure for evaluation of bioavailability. The time of peak plasma concentration was not particularly consistent in the present study. With the elimination of the metabolite p - H P P H only 60% of the dose was recovered. Another variable using this test is the risk of incomplete urine collection. Plasma concentrations at various time points after dosing reflect only a small section of one dosing interval. The standard solution of phenytoin sodium showed at most time points during one dosing interval and in the area under the curve a better bioavailability when compared to the three commercial products of this study. I f the standard solution is regarded as the oral form with the best obtainable bioavailability, the possibility of improvement in the formulation exists for all other preparations tested here. One of the manufactures (Desitin) has meanwhile changed the galenic formulation with the result of an obvious increase in bioavailability (Gugler et al., 1977). Dill et al. (1956), Glazko and Chang (1972) and Lund (1974) draw the conclusion from the results of their studies that the sodium salt of phenytoin is better absorbed than the free acid. The data reported in the present study are in contradiction to this hypothesis. The bioavailability of the acid form (Zentropil) was identical to the sodium salt (Epanutin) and to the calcium salt (Phenhydan) of phenytoin. Our results indicate that it is not primarily the chemistry of phenytoin, but rather the details of the formulation, i.e. excipients, particle shape or particle size, that affect the bioavailability of phenytoin. In this respect the bioavailability problems of phenytoin are of the same nature as demonstrated for digoxin (Jounela et al., 1975; Rietbrock, 1976).
References Albert, K. S., Sakmar, E., Hallmark, M. R., Weidler, D., Wagner, J. G.: Bioavailability of diphenylhydantoin. Clin. Pharmacol. Ther. 16, 727--735 (1974) Arnold, K., Gerber, N.: The rate of decline of diphenylhydantoin in human plasma. Clin. Pharmacol. Ther. 11, 121--134 (1970) Arnold, K., Gerber, N., Levy, G.: Absorption and dissolution studies in sodium diphenylhydantoin Capsules. Canad. J. Pharm. Sci. 5, 89--92 (1970) Atkinson, A. J., MacGee, J., Strong, J., Garteiz, D., Gaffney, T. E.: Identification of 5-metahydroxyphenyl-5-phenylhydantoin as a metabolite of diphenylhydantoin. Biochem. Pharmacol. 19, 2483--2491 (1970) Berlin, A., Agurell, S., Borg§, O., Lund, L., Sj6qvist, F.: Micromethod for the determination of diphenylhydantoin in plasma and cerebrospinal fluid. A comparison between a gas chromatographic and a spectrophotometric method. Scand. J. clin. Lab. Invest. 29, 281--287 (1972) Dill, W. A., Kazenko, A., Wolf, L. M., Glazko, A. J.: Studies on 5,5-diphenylhydantoin (dilantin) in animals and man. J. Pharmacol. Exptl. Ther. 118, 270--279 (1956) Eadie, M. J., Sutherland, J. M., Tyrer, J. H.: Dilantin overdosage. Med. J. Aust. 2, 515 (1968) Friedman, M.: J. Amer.. Statist. Assoc. 32, 675--701 (1937)
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Gerber, N., Wagner, J. G.: Explanation of dose-dependent decline of diphenylhydantoin plasma levels by fitting to the integrated form of the Michaelis-Menten equation. Res. Comm. Chem. Path. Pharmacol. 3, 455----466 (1972) Glazko, A. J.: Diphenylhydantoin. Pharmacol. 8, 163--177 (1972) Glazko, A. J., Chang, T.: In: Antiepileptic Drugs, (D. M. Woodbury, J. Penry, Schmidt, R.P.). New York: Raven Press 1972 Gugler, R., Manion, C. V., Azarnoff, D. L.: Phenytoin: Pharmac'okinetics and bioavailability. Clin. Pharmacol. Ther. 19, 135--142 (1976) Gugler, R., Eichelbaum, M., Fr6scher, W.: In preparation Huffman, D. H., Manion, C. V., Azarnoff, D. L.: Absorption of digoxin from different oral preparations in normal subjects during steady state. Clin. Pharmacol. Ther. 16, 310--317 (1974) Jounela, A. J., Pentikainen, P. J., Sothman, A.: Effect of particle size on the bioavailability of digoxin. Europ. J. clin. Pharmacol. 8, 365--370 (1975) Kutt, H., Wolk, B. A., Scherman, R., McDowell, F.: Insufficient parahydroxylation as a cause of diphenylhydantoin toxicity. Neurology 14, 542--548 (1964) Kutt, M., McDowell, F.: Management of epilepsy with diphenylhydantoin sodium. J. Amer. med. Ass. 203, 167--170 (1968) Koch-Weser, J.: Bioavailability of drugs. New Engl. J. Med. 291, 503--506 (1974) Lund, M., J6rgensen, R. S., Kiihl, V.: Serum diphenylhydantoin (phenytoin) in ambulant patients with epilepsy. Epilepsia (Boston) 5, 51--58 (1964) Lund, L., Alvan, G., Berlin, A., Alexanderson, B.: Pharmacokinetics of single and multiple doses of phenytoin in man. Europ. J. clin. Pharmacol. 7, 81--86 (1974) Lund, L.: Generic inequivalence of two different pharmaceutical formulations of phenytoin. Europ. J. olin. Pharmacol. 7, 119--124 (1974) Pentik~iinen, P. J., Neuvonen, P. J., Elfving, S. M.: Bioavailabilityof four brands of phenytoin tablets. Europ. J. clin. Pharmacol. 9, 213--218 (1975) Rail, L.: Dilantin overdosage. Med. J. Aust. 2, 339 (1968) Rietbrock, N.: Therapeutische Gleichwertigkeit und Ungleichwertigkeit chemisch identischer Stoffe. Arzneimittelforsch. 26, 135 (1976) Tyrer, J. H., Eadie, M. J., Sutherland, J. M., Hooper, W. D.: Outbreak of anticonvulsant intoxication in an Australian city. Brit. IVied. J. IV, 271--273 (1970) Wilcoxon, F., Wilcox, R. A.: Lederle Laboratories. New York: Pearl River 1964 Woodbury, D. M., Penry, J. K., Schmidt, R. P. (eds.): Antiepileptic drugs, Kap. 18 u. 19. New York: 1972 Received March 12, 1977
J.Neurol. 216, 155--162 (1977)
Neurology t~ by Springer-Verlag 1977
The Bioavailability of Phenytoin* R. Gugler l**, W. Fr6scher 2, M. Eichelbaum l, and G. Hildenbrand 3 t Medizinische Universit/itsklinik (Direktor: Prof. Dr. H. J. Dengler), 2Universit/itsnervenklinik (Komm. Direktor: Prof. Dr. H. Penin), 3Institut fiir Medizinische Statistik, Dokumentation und Datenverarbeitung (Direktor: Prof. Dr. G. Oberhoffer), all: D-5300Bonn-Venusberg, Federal Republic of Germany Summary. The bioavailability of three commercial products of phenytoin (Epanutin, sodium salt; Phenhydan, calcium salt; Zentropil, free acid) was studied relative to a standard solution of sodium phenytoin. Each preparation was given for 14 days in a daily dose of 300 mg according to a cross over design. The three brands tested were equivalent as to their bioavailability with respect to plasma concentrations during one dosing interval, the area under the plasma level time curve, the time of peak plasma concentration, and the urinary excretion of the primary metabolite. Therefore, these phenytoin brands are bioequivalent and exchangeable for antiepileptic treatment. The standard solution of sodium phenytoin had a higher bioavailability as compared to the commercial products. Key words: Phenytoin - Bioavailability - Antiepileptic therapy - Epilepsy.
Zusammenfassung. Die biologische Verffigbarkeit der 3 Handelspr~iparate von Phenytoin, Epanutin® (Natriumsalz), Phenhydan® (Calciumsalz) und Zentropil® (freie S~iure), wurde im Vergleich zu einer w~issrigen Standardform von Phenytoin-Natrium untersucht. Nach einem cross-over-Plan wurde jede Pr~iparation fiber 14 Tage in einer Dosis von 300mg verabreicht. Die 3 Handelspr~iparate hatten gleiche biologische Verffigbarkeit im Hinblick auf alle Plasmakonzentrationen wahrend eines Dosisintervalles im steady state, die Fl~che unter der Plasmaspiegelkurve, den Zeitpunkt des maximalen Plasmaspiegels und die Urinausscheidung des Hauptmetaboliten. Diese Pr~parate von Phenytoin sind damit bio~iquivalent und in der Therapie austauschbar. Die Standardform yon Phenytoin-Natrium hatte eine bessere biologische Verfiigbarkeit als die Handelspr~iparate. Phenytoin (DPH) plays a major role in the treatment of several types of epilepsy, particularly of major and psychomotor seizures. Although a correlation between the plasma concentration of phenytoin and the therapeutic effect has been * Supported by Bundesministerium ffir Jugend, Familie und Gesundheit (342-4801-8/3) ** Corresponding author
156
R. Gugler et al.
e s t a b l i s h e d ( K u t t a n d M c D o w e l l , 1968; L u n d et al., 1964), a satisfactory corr e l a t i o n does n o t exist between the dose a n d the p l a s m a level ( W o o d b u r y et al., 1972). This lack o f c o r r e l a t i o n is due to genetic differences in the m e t a b o l i s m o f p h e n y t o i n ( K u t t et al., 1964) o r o f dose d e p e n d e n t p h a r m a c o k i n e t i c s in the m a j o r i t y o f cases ( A r n o l d a n d G e r b e r , 1970; G e r b e r a n d W a g n e r , 1972). A g o o d b i o a v a i l a b i l i t y o f a n y p r e p a r a t i o n used s h o u l d eliminate a n o t h e r variable in the d r u g t r e a t m e n t o f epilepsy, i.e. i n c o m p l e t e a n d consequently i n c o n s t a n t a b s o r p t i o n o f p h e n y t o i n . P h e n y t o i n is one o f those drugs, o f which differences in b i o a v a i l a b i l i t y are c o n s i d e r e d to be o f p a r t i c u l a r i m p o r t a n c e ( K o c h - W e s e r , 1974): p h e n y t o i n is a highly p o t e n t d r u g which is a d m i n i s t e r e d to seriously ill patients, has a n a r r o w t h e r a p e u t i c range a n d is k n o w n to be slowly a b s o r b e d . T h e p r e s e n t s t u d y was u n d e r t a k e n to investigate the b i o a v a i l a b i l i t y o f three p r e p a r a t i o n s consisting o f the s o d i u m salt, the calcium salt, a n d the acid f o r m o f p h e n y t o i n . T h e different p r e p a r a t i o n s were c o m p a r e d with a s t a n d a r d s o l u t i o n p r e p a r e d as a s u s p e n s i o n o f p h e n y t o i n sodium. In o r d e r to m i m i c the real p a t i e n t s i t u a t i o n the s t u d y was p e r f o r m e d using a chronic t r e a t m e n t design, since d a t a o b t a i n e d f r o m a single dose o f drugs with n o n linear p h a r m a c o k i n e t i c s c a n n o t be a p p l i e d to the c o n d i t i o n s o f long t e r m t h e r a p y .
Methods
Subjects. Eight healthy volunteers (4 male, 4 female), age range between 23 and 35 years, participated in the study. Subjects were judged as being healthy from the history, physical examination, and routine laboratory tests. In particular, no evidence of hepatic or renal disease or of congestive heart failure existed. The subjects had to be without any medication for at least 2 weeks prior to the study, and no drugs in addition to the test drugs were allowed throughout the study. Administration of Drugs. The study was carried out using a cross over design. The duration of the study was 8 weeks. Each subject received every phenytoin preparation chronicly over a period of 2 weeks. The daily dose was 300 mg of phenytoin to be taken once a day in the morning between 0700 and 0800 h. The subjects were allowed a light continental breakfast immediately after drug intake. The following preparation were used: A = Epanutin (sodium salt; Parke-Davis; Lot No. DbKE 703). B =Phenhydan (calcium salt; Desitin; Lot No. 186054). C =Zentropil (acid; Nordmark; Lot No. 1013). D = Standard solution ,(sodium salt). According to the study design the sequence of drug intake was A-B-C-D, B-C-D-A, C-D-AB and D-A-B-C for 2 subjects each. Samples. Blood (6 ml) for determination of phenytoin concentration in plasma was obtained from the subjects on days 12, 13 and 14 of each treatment period immediately prior to the next dose. On day 14 additional blood samples were drawn 1, 2, 4, 6, 8, 10 and 24h after the last dose. Urine was collected over 24h for 3 days at the end of each treatment period for measurement of p-hydroxyphenyl-phenylhydantoin (p-HPPH) during one dosing interval in steady state. Drug Assays.Phenytoin was determined according to a gas chromatographic method following methylation of the compound (Berlin et al., 1972); p-HPPH in urine was measured by the method of Atkinson et al. (1970).
The Bioavailability of Phenytoin
157
Bioavailability Parameters. The parameters used to determine the bioavailability were the
following: 1. Plasma concentration in steady state under chronic treatment. C s ~ - m i n = plasma concentration prior to the next dose (mean of 4 determinations). Css.... = peak plasma concentration following administration of the dose. 2. Area under the plasma level time curve (AUC) during one dosing interval calculated by the trapezoidal rule. 3. Time of maximum plasma concentration (Tm~x). 4. Excretion of the metabolite (p-HPPH) over 24 h in steady state (mean of 3 determinations). Statistical Methods. Rank analysis of variance (Friedman, 1937) was used for statistical analysis. In case of significance (~2 _>X20.05:3= 7.81) multiple comparisons (Wilcoxon and Wilcox, 1964) were applied.
Results M e a n plasma concentrations of all subjects during one dosing interval for the four preparations tested are shown in Figure 1. At time 0 (prior to the daily dose) no significant difference in plasma concentrations was f o u n d between the preparations. A t 1, 4, 6, a n d 8 h following the dose, the concentrations o f the standard solution (D) were significantly higher than the concentrations o f E p a n u t i n (A) a n d P h e n h y d a n (B), and were also higher than the concentrations o f Zentropil (C) at 2, 3, and 10 h post dose. Epanutin, P h e n h y d a n a n d Zentropil were at no time significantly different f r o m each other, although in the graph the plasma level curve o f Zentropil appears to be located between the other two preparations a n d the s t a n d a r d form. The plasma concentrations prior to the dose (C .... in) are listed in Table 1 (mean o f determinations on four different days). The differences were n o t 20 E 'e o.
0_ 5
"rime [h3
Fig. 1. Mean values of the plasma level curves of 4 phenytoin preparations (A = Epanutin®, B= Phenhydan®, C= Zentropil®, D= standard solution) during one dosing interval under chronic therapy
158
R. Gugler et al.
Subject
1
2 3 4 5 6 7 8 Mean SD a
Aa
Ba
Ca
D
4.1 4.0 2.8 15.4 3.8 7.2 5.1 3.3
6.1 6.8 3.5 16.4 4.3 7.3 3.3 2.7
8.7 5.5 2.4 20.6 5.0 5.5 8.8 3.0
11.7 6.8 2.7 22.1 4.7 6.4 9.6 3.5
5.7 1.5
6.3 1.6
7.4 2.0
8.4 2.2
Table 1. Plasma concentrations (ttg/ml) prior to the dose (mean of 4 determinations) of 4 phenytoin preparations during chronic therapy. Mean and standard deviation (SD)
NO di~rence from D (P>0.05)
Su~ect
1
2 3 4 5 6 7 8 Mean SD a
Preparation
Preparation W
~
D
4.4 7.3 4.9 21.3 7.5 12.4 10.7 6.8
10.4 I0.0 7.1 20.3 6.0 12.5 5.0 4.2
12.8 6.7 7.0 25.6 9.9 9.4 13.6 6.4
19.8 16.1 7.7 25.1 10.2 14.1 14.5 7.2
9.4 1.9
9.4 1.8
11.4
14.3 2.2
6.3
Table2. Peak plasma concentrations (C . . . . ) during one dosing interval of 4 phenytoin preparations tested
Different from D (P0.05)
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between A, B and C (Table 3). If the standard solution (D) is regarded as 100%, the mean relative bioavailability is 67% for Epanutin, 66% for Phenhydan and 83% for Zentropil. The time of peak plasma concentration shows no consistant pattern for the individual subjects as well as for any particular phenytoin preparation (Table 4). The mean value of Tmax is greater for preparation B (5.8h) and C (5.6h) as compared to A (3.8 h) and D (3.0 h); however, this difference is not statistically significant. The renal excretion of the main metabolity of phenytoin (p-HPPH) during one dosing interval is not significantly different between preparations (Table 5).
Discussion
The bioavailability of phenytoin has not been studied extensively before (Dill et al., 1956; Glazko, 1972; Arnold et al., 1970; Lund et al., 1974; Lund, 1974; Albert et al., 1974; Pentika'inen et al., 1975; Gugler et al., 1976). The majority of results on the bioavailability of this drug were obtained using a single dose design; one study has been performed in steady state (Lund, 1974). It has been demonstrated with digoxin, however, that testing the bioavailability under a multiple dose regimen appears to give more reliable data (Huffman et al., 1974), since the results are not influenced by the particular experimental conditions of one study day; drug concentrations in plasma or urine are well above the sensitivity limits of any assay; the area under the plasma level curve and the renal excretion of the drug or its metabolite can be estimated with more accuracy during one dosing interval than by extrapolation to infinity after a single dose; multiple dosing is for most drugs the real mode of application. One additional reason for using a chronic treatment design is the non-linear pharmacokinetics of phenytoin. The higher the plasma concentrations of phenytoin, the more likely they are in the range of saturation of elimination, i.e. small differences in low concentrations seen after a single dose can be magnified and can thus become clinically more relevant in the high concentration range obtained during chronic therapy. This effect of non-linear pharmacokinetics is indeed reflected in the results of the present study. During one dosing interval plasma concentrations at time 0 and 24 h, which are in the lower range, are not significantly different between preparations, whereas significant differences are recorded from 1 to 10h after dosing. When the results of all bioavailability parameters tested are judged in context, the commercial preparations of phenytoin, Epanutin, Phenhydan and Zentropil were found not to be different from each other. Differences in the area under the plasma level curve and in the concentrations at several time points during one dosing interval were smaller than the day to day variation during chronic therapy. Thus, the conclusion appears to be justified that these phenytoin preparations are bioequivalent and are exchangeable with each other without any significant effect on drug efficacy. These results are of importance in the light of considerable differences in bioavailability of phenytoin reported from Australia and Scandinavia, where dramatic signs of toxicity or ineffectiveness were redorded when a
The Bioavailability of Phenytoin
161
different brand of phenytoin was prescribed, and even when the formulation of one brand was changed (Eadie et al., 1968; Rail, 1968; Tyrer et al., 1970; Lund, 1974; Pentikainen et al., 1975). O f all bioavailability parameters tested the area under the curve seemed to be the best measure for evaluation of bioavailability. The time of peak plasma concentration was not particularly consistent in the present study. With the elimination of the metabolite p - H P P H only 60% of the dose was recovered. Another variable using this test is the risk of incomplete urine collection. Plasma concentrations at various time points after dosing reflect only a small section of one dosing interval. The standard solution of phenytoin sodium showed at most time points during one dosing interval and in the area under the curve a better bioavailability when compared to the three commercial products of this study. I f the standard solution is regarded as the oral form with the best obtainable bioavailability, the possibility of improvement in the formulation exists for all other preparations tested here. One of the manufactures (Desitin) has meanwhile changed the galenic formulation with the result of an obvious increase in bioavailability (Gugler et al., 1977). Dill et al. (1956), Glazko and Chang (1972) and Lund (1974) draw the conclusion from the results of their studies that the sodium salt of phenytoin is better absorbed than the free acid. The data reported in the present study are in contradiction to this hypothesis. The bioavailability of the acid form (Zentropil) was identical to the sodium salt (Epanutin) and to the calcium salt (Phenhydan) of phenytoin. Our results indicate that it is not primarily the chemistry of phenytoin, but rather the details of the formulation, i.e. excipients, particle shape or particle size, that affect the bioavailability of phenytoin. In this respect the bioavailability problems of phenytoin are of the same nature as demonstrated for digoxin (Jounela et al., 1975; Rietbrock, 1976).
References Albert, K. S., Sakmar, E., Hallmark, M. R., Weidler, D., Wagner, J. G.: Bioavailability of diphenylhydantoin. Clin. Pharmacol. Ther. 16, 727--735 (1974) Arnold, K., Gerber, N.: The rate of decline of diphenylhydantoin in human plasma. Clin. Pharmacol. Ther. 11, 121--134 (1970) Arnold, K., Gerber, N., Levy, G.: Absorption and dissolution studies in sodium diphenylhydantoin Capsules. Canad. J. Pharm. Sci. 5, 89--92 (1970) Atkinson, A. J., MacGee, J., Strong, J., Garteiz, D., Gaffney, T. E.: Identification of 5-metahydroxyphenyl-5-phenylhydantoin as a metabolite of diphenylhydantoin. Biochem. Pharmacol. 19, 2483--2491 (1970) Berlin, A., Agurell, S., Borg§, O., Lund, L., Sj6qvist, F.: Micromethod for the determination of diphenylhydantoin in plasma and cerebrospinal fluid. A comparison between a gas chromatographic and a spectrophotometric method. Scand. J. clin. Lab. Invest. 29, 281--287 (1972) Dill, W. A., Kazenko, A., Wolf, L. M., Glazko, A. J.: Studies on 5,5-diphenylhydantoin (dilantin) in animals and man. J. Pharmacol. Exptl. Ther. 118, 270--279 (1956) Eadie, M. J., Sutherland, J. M., Tyrer, J. H.: Dilantin overdosage. Med. J. Aust. 2, 515 (1968) Friedman, M.: J. Amer.. Statist. Assoc. 32, 675--701 (1937)
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Gerber, N., Wagner, J. G.: Explanation of dose-dependent decline of diphenylhydantoin plasma levels by fitting to the integrated form of the Michaelis-Menten equation. Res. Comm. Chem. Path. Pharmacol. 3, 455----466 (1972) Glazko, A. J.: Diphenylhydantoin. Pharmacol. 8, 163--177 (1972) Glazko, A. J., Chang, T.: In: Antiepileptic Drugs, (D. M. Woodbury, J. Penry, Schmidt, R.P.). New York: Raven Press 1972 Gugler, R., Manion, C. V., Azarnoff, D. L.: Phenytoin: Pharmac'okinetics and bioavailability. Clin. Pharmacol. Ther. 19, 135--142 (1976) Gugler, R., Eichelbaum, M., Fr6scher, W.: In preparation Huffman, D. H., Manion, C. V., Azarnoff, D. L.: Absorption of digoxin from different oral preparations in normal subjects during steady state. Clin. Pharmacol. Ther. 16, 310--317 (1974) Jounela, A. J., Pentikainen, P. J., Sothman, A.: Effect of particle size on the bioavailability of digoxin. Europ. J. clin. Pharmacol. 8, 365--370 (1975) Kutt, H., Wolk, B. A., Scherman, R., McDowell, F.: Insufficient parahydroxylation as a cause of diphenylhydantoin toxicity. Neurology 14, 542--548 (1964) Kutt, M., McDowell, F.: Management of epilepsy with diphenylhydantoin sodium. J. Amer. med. Ass. 203, 167--170 (1968) Koch-Weser, J.: Bioavailability of drugs. New Engl. J. Med. 291, 503--506 (1974) Lund, M., J6rgensen, R. S., Kiihl, V.: Serum diphenylhydantoin (phenytoin) in ambulant patients with epilepsy. Epilepsia (Boston) 5, 51--58 (1964) Lund, L., Alvan, G., Berlin, A., Alexanderson, B.: Pharmacokinetics of single and multiple doses of phenytoin in man. Europ. J. clin. Pharmacol. 7, 81--86 (1974) Lund, L.: Generic inequivalence of two different pharmaceutical formulations of phenytoin. Europ. J. olin. Pharmacol. 7, 119--124 (1974) Pentik~iinen, P. J., Neuvonen, P. J., Elfving, S. M.: Bioavailabilityof four brands of phenytoin tablets. Europ. J. clin. Pharmacol. 9, 213--218 (1975) Rail, L.: Dilantin overdosage. Med. J. Aust. 2, 339 (1968) Rietbrock, N.: Therapeutische Gleichwertigkeit und Ungleichwertigkeit chemisch identischer Stoffe. Arzneimittelforsch. 26, 135 (1976) Tyrer, J. H., Eadie, M. J., Sutherland, J. M., Hooper, W. D.: Outbreak of anticonvulsant intoxication in an Australian city. Brit. IVied. J. IV, 271--273 (1970) Wilcoxon, F., Wilcox, R. A.: Lederle Laboratories. New York: Pearl River 1964 Woodbury, D. M., Penry, J. K., Schmidt, R. P. (eds.): Antiepileptic drugs, Kap. 18 u. 19. New York: 1972 Received March 12, 1977
