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A combined approach to control valproic acid release via novel delivery system of valpromide: A kinetic and pharmacokinetic study
33
A COMBINED APPROACH TO CONTROL VAtPROfC ACID REtEASE VIA NOVEL DELIVERY SYSTEM OF VALPROMIDE: A KINETIC AND PHARMACOKINETIC STUDY
INYRQIXJCTIQN The primaryamide of vafproicr acid, valpromide, is widely used in sume ~~opea~ countries as an antie~~lept~~ and antjp~y~hoti~ drug [l--5] and di~t~bu~~d as an entericcoated tablet under the trade name Depamide@ (Labaz, France f67). Recent reports demonstrated that valpramide is biotransformed to valproic acid (VPA), with species differences among humans, dogs and rats in the extent of this biotransfarmation [3, 5, 811 J . In humans, complete biotransformation of VPD to VPA is observed after oral as well
as after intravenous administration [ ZZ’J, Although their chemical structure is similar, the pharrn~~~k~neti~~ of valpromide is entirely different from that of valproic acid, Valprom~d~ possesses a very high rnet~bol~~ clearance, a very short el~rn~nat~~~ h&f-life and a high vrslume of distribution of about 1 L/kg [,X2]. As a solid, non-hygroscopic, neutral prodrug of valproic acid, valpromide may be used as an alternative to valproic acid and sodium valproate, In humans, the pharmacokinetic profile of valproic acid obtained after oral administration of ~~p~~rnide resembled that of a sustained release furmulation of valproic acid [11, liZI. Based on this kno~I~d~e~ two noveI
34
controlled release dosage forms of valpromide were developed using a biodegradable hydrophylic polymer [ 141. These formulations will be designated hereafter as VPDCR, and VPDCR,, The advantages of controlled release dosage forms of valproic acid were summarized in several recent publications [13, 15, 161. These include minimizing fluctuations in valproic acid plasma concentrations at steady state and decreasing the dosage regimen of the drug during chronic therapy. Valpromide was found to be a prodrug of valproic acid [ 11, 121 and yielding a sustained release profile for valproic acid. A combination of VPD in a sustained release carrier, can result in a very uniform VPA serum level, that may lead to a once-a-day product of valproic acid. The objective of this study was to perform a kinetic analysis of the in vitro dissolution profiles of VPDCR, and VPDCRl and a comparative pharmacokinetic analysis of the serum concentrations of valproic acid obtained after oral administration of these formulations to six healthy subjects.
MATERIAL
AND METHODS
liters of the organic phase were injected into a gas chromatograph (Packard, model 437, Holland) equipped with a flame ionization detector and a recorder (linear model 1210, U.S.A.). The glass column, 180 cm X 2 mm i.d., was packed with 5% free fatty acid phase on 80-100 mesh chromosorb Q (Applied Science Lab., PA, U.S.A.). Flow rates were: carrier gas (nitrogen): 30 mL/min, hydrogen: 20 mL/min, air: 200 mL/min; and system temperatures: injector: 195”C, column lBO”C, detector: 220°C. Samples were chromatographed using an eight-point calibration curve, containing dissolution medium spiked with known amounts of valpromide. The mixture was formulated in two ways: (a) the mixture was directly, with no additional adjuvants, compressed into 13 mm diameter, 4.1 mm height tablets under 3000 kg force, using a manual hydraulic press (Perkin Elmer, U.S.A.}, to give 600 mg tablets. Each tablet contained 300 mg valpromide (VPDCR,); (b) the mixture was moistened by purified water, granulated, dried for 60 minutes at 60°C and sieved. The granules obtained were then pressed into 13 mm diameter 4.0 mm height tablets in the same manner with no additional adjuvants, to give 600 mg tablets. Each tablet contained 300 mg valpromide ( VPDCRz).
Tablet preparation Dissolution rate test
Controlled release tablets were prepared by mixing valpromide (Labaz, Paris, France) with a hydrophylic biodegradable polymer in a ratio of 1:l 1141. The mixture was sampled and analysed for valpromide content and homogeneity by the following procedure: each 100 mg sample was dispersed into 50 mL of purified water. After dilution with an additional amount of purified water, 1 mL of the aqueous solution was extracted with 500 PL of chloroform (B.D.H., Poole, England) containing benzyl alcohol as an internal standard (B.D.H., Poole, England). The mixture was vortexed for 15 s and centrifuged at 4000 rpm for 15 min. The centrifugation was done in order to get rid of turbidity due to presence of the enzyme pancreatin. Three micro-
The in vitro dissolution rate tests of the two types of tablets, VPDCR, and VPDCR* were studied in a rotating basket system that provided for the possibility of examining two tablets at a time [ 17, IS], at a constant speed of 100 rpm (Fisher “Steady Speed”, U.S.A.), using 500 mL simulated gastric fluid TS followed by 500 mL simulated intestinal fluid TS U.S.P. XX [19] for 1 hour and 23 hours, respectively, so that sink conditions were maintained during the study [20]. The dissolution media were sampled at 0.5, 1, 1.5, 2,3,4, 5,6,7, 8 and 24 hours. Equal volumes of dissolution medium were immediately returned to the dissolution flask after each sampling. The samples were centrifuged im-
35
mediately at 4000 rpm for 5 minutes and assayed for valpromide content according to the method previously described [ 211. Human studies
Depamide@ (Labaz, Paris, France), VPDCR, and VPDCRz were administered orally to six male volunteers (one subject dropped from the study before the administration of VPDCR2). The volunteers, aged 25-33 years, weighed between 65-82 kg and were selected on the basis of a negative medical history and physical examination, and normal routine blood chemical analysis and morphology. Written informed consent was obtained from each volunteer and the entire clinical experiment had been approved by the Helsinki Committee of the Hadassah Medical Center and the Israeli Ministry of Health. Each volunteer received, at separate times, a single dose of three tablets (900 mg) of VPDCR,, VPDCR*, and Depamide@, at 7 a.m. after an overnight fast. Food was withheld from 12 hours before to 6 hours after dosing. Venous blood samples (10 mL) were taken via an indwelling catheter from the forearm vein at 0, 0.5, 1, 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 24, 30, 39 and 48 hours after dosing. Between two consecutive studies, there was a washout period of 3 weeks. Sera were immediately separated by centrifugation at 7000 rpm for 15 minutes and stored at -20°C. Before assay, the serum was allowed to reach room temperature, vortexed, centrifuged and the residual clot removed. Valproic acid was assayed by a gas chromatographic method [21]. Each sample was extracted into chloroform and chromatographed in comparison with an eight-point calibration curve containing serum from each volunteer before each treatment; time = 0, spiked with known amounts of valproic acid and valpromide. The AUC (area under the serum concentration vs. time curve) was calculated using the trapezoidal rule with extrapolation to infinity [22]. The relative and absolute bioavailability of VPDCR, and VPDCRz was
calculated from the ratio of the AUCs of each one of the two formulations relative to Depamide@ and an i.v. preparation of VPD which had been administered to the same six subjects in a previous study [ 121.
RESULTS AND DISCUSSION Dissolution-kinetic tablets
model of VPDCRl
and VPDC&
A degradation phenomenon, symmetrical in nature, was observed during the dissolution study of VPDCRi and VPDCRz tablets. At the end of the attrition of each tablet, a spherical cylinder-like residue could be identified in each compartment of the dissolution rate apparatus. As the dissolution process was performed under well-stirred conditions and sink conditions were maintained, the system could be analysed according to the second special case of Hixson-Crowell’s cube root law [23] represented by the equation: k W0 113_ W’” = 3 _ t = K’t
(1)
a
where W. is the initial amount of valpromide in the tablet, W is the amount of valpromide retained at time t, calculated from W. less the amount found in the dissolution medium at time t, K’ is the slope of the straight line of Wo1’3 - W”j versus time, and a is the reciprocal value of the two-thirds-root density of valpromide tablet, multiplied by an appropriate geometric shape factor, assuming a three-dimensional symmetrical attrition of a cylinder [ 241. Hixson-Crowell graphical treatment for VPDCR, and VPDCRz are presented in Fig. 1 and the appropriate values for densities, shape factors and cube root law rate constants are summarized in Table 1. The dissolution data of VPDCR, and VPDCRz were computerized, using a non-linear least squares kinetic program [25] and the values of cube root law constants obtained matched well with the values obtained using the graphic method.
36
TABLE
1
Calculated
values of density,
VPDCR, VPDCR, %alculated bCalculated ‘Correlation
shape
factor
and cube root
law rate constants
for VPDCR,
and VPDCR,
Shape factor
g/mL
Rate g h“
constanta (K’), cmmZ X 10’
Rate constantb (K’), g h-l cm-’ x 10’
1.27 7.27
1.10 1.30
2.54 8.96
(0.994)c (0.996)’
2.55 8.82
Density,
by the graphical method. by non-linear kinetic computer coefficient.
program.
Cb(mgIL)
Fig. 1. Cube root law plots of valpromide released into U.S.P. gastrointestinal fluids from VPDCR, (c,) and VPDCR, (a).
For the sake of comparison it should be mentioned that the dissolution of the Depamide@ tablet was completed within half an hour under the same experimental conditions. The threefold differences in dissolution rate constants between VPDCRl and VPDCRz can be explained by the different nature of the two formulations. The granulation process leads to a more porous product, hence VPDCRz degrades faster due to more itensive leaching of the gastrointestinal medium into it. Pharmacokinetic
studies
No valpromide concentrations in serum could be determined in any of the subjects after oral administration of VPDCRI, VPDCR, and Depamide@ [ 111 . Mean valproic acid serum concentrations obtained after VPDCR 1, VPDCRz administration in comparison to Depamide@ are presented in Fig. 2. Table 2 presents a summary of some pharmacokinetic parameters calculated from the serum concen-
Fig. 2. Mean serum concentrations of valproic acid obtained after oral administration (900 mg) to six healthy subjects of: Depamide@ (a), VPDCR, (o), VPDCR, (e). (VPDCR, was administered only to five subjects.)
tration data of valproic acid after administration of the three VPD formulations. From Table 2 and Fig. 2 it can be seen that the bioavailability of VPDCRz was not significantly different from Depamide@, while the bioavailability of VPDCR, was significantly lower than Depamide@. After the administration of VPDCR1, a very uniform serum concentration of about 25 pg/mL was observed between 6 to 30 hours after dosing. This prolonged absorption caused a decrease in the extent of the absorption. After administration of the identical dose of VPDCR* a uniform serum concentration of 38 pg/mL was obtained from 6 to 16 hours after dosing. VPDCRz was found to be bioequivalent to Depamide@ as far as the extent of absorption. However, the rate of absorption of valproic acid after the administration of VPDCR, was more sustained and controlled than after the administration of Depamide’. This slow absorption of valproic acid implies less fluctuations in VPA at
37
TABLE
2
Summary of mean (+ SD.) pharmacokinetie parameters of valproic 3 X 300 mg of Depamide@, VPDCR, and VPDCR, to six volunteers Pharmacokinetic parameter
Cb max (mg/L~ tmax (hl F (%) F’ (%I Cb rnh 224 h @w/L) Cb(,tmax (mglL) Cb(ss)min
@wiGI
% fluctuation
acid obtained
after
the administration
of
Formulation Depamidea
VPDCR,
VPDCR,
65.5 ?: 14.2 9.2 f 3.0 67.5 f 23.0 25.3 t 9.2 120.0 rt 60.1 45.5 i 19.9 158.7 + 41.6
27.9 10.0 49.6 73.5 20.0 43.7 31.3 46.8
44.8 11.8 71.6 106.0 29.7 68.1 48.8 73.6
i 7.7 t 3.2 i 7.0 t 10.3 + 4.8 1. 18.0 + 12.3 + 19.2
t f i t i: + i i
12.2 4.1 18.9 24.4 13.3 21.8 22.5 21.3
= peak serum concentration; t,,, = time to reach Cb,,; F = absolute bioavailability; F’ = bioavailability C&n, relative to Depamide@ ; Cb,, , 24 h = serum concentration of valproic acid 24 hours after valpromide administra= theoretical steady-state peak serum level when VPD formulations were administered chronically tion; Cb(s)rnax every 24 hours; Cbcsjmin = theoretical steady-state lower serum level when VPD formulations were administered chronically every 24 hours.
steady-state serum concentrations during multiple dosing with VPDCR*. This was substantiated by a series of theoretical steady-state calculations based on the experimental results obtained in this study. Equations (2) and (3) were used for these repetitive dosing steady-state concentrations
[261. Cb(ss)max
Cbmax = 1
where Cb(,),, and Cb(s)min are the peak and trough drug serum concentrations, respectively, at steady state: 7 is a dosing interval of 24 hours. Cb,,, is peak serum level at single dose administration, Cb,i, is serum level at 7 hours after dosing and K is the elimination rate constant of the drug. The results of these steady-state calculations are summarized in Table 2. Under these conditions, Depamide@ and VPDCRz are both candidates for once-a-day of valpromide (T = 24 h) chronic treatment as they both possess a steady-state peak serum
level within the therapeutic window of valproic acid (SO-100 pg/mL) [27], although their lower serum level at steady state does not enter the desired range (Table 2). However, the percentage of serum fluctuations of valproic acid as calculated by eqn. (4) [28] was higher after Depamide@ administration than after the administration of VPDCR2. Cb(ss)max
- Cb(,)min
X 100 = % fluctuation
(4)
Cb(ss)min
The % fluctuation values obtained, 48.7 rt: 27.2 for VPDCR2 and 114.9 i 11.3 for Depamide@ (mean i SD.), demonstrate quite clearly the advantages of VPDCRz as a once-a-day controlled release drug delivery system of valproic acid. A neutral, solid non-hygrosopic controlled release formulation of valpromide such as VPDCR, can be a good pharmaceutical alternative for standard and sustained release formulations of valproic acid.
ACKNOWLEDGEMENTS
This work was supported by Grant No. 2127 of the Israel National Council for Research and Development and is included in
38
the dissertation project of Abraham Rubinstein as partial fulfillment of the Doctor of Philosophy degree requirements of the Hebrew University of Jerusalem. The authors wish to thank Mr. Zahy Mattar for his skillful technical assistance. The valpromide sample obtained from Labaz (Sanofi) France, is gratefully acknowledged.
13
14
15
REFERENCES 1
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P. Favel, J. Cartier, J.P. Gratadow and G. Gratadow, Depamide Q in the treatment of epilepsy: A clinical trial, Epilepsia, 14 (1973) 329-334. R. Musolino, G. Gallitto, L. Morgant, F. Pisani and R. Di Perri, The anti-epileptic properties of n-dipropylacetamide (Depamide@), Acta. Neurol. 2 (1980) 107-114. F. Pisani and R. Di Perri, Some clinical pharmacological aspects of n-dipropylacetamide, Ital. Neurol. Sci., 4 (1980) 245-249. F. Pisani, A. Fazio, G. Oteri and R. Di Perri, Dipropylacetic acid plasma levels: Diurnal fluctuations during chronic treatment with dipropylacetamide, Ther. Drug Monitor, 3 (1981) 297301. F. Pisani, A.A. D’Agastano, A. Fazio, G. Oteri, G. Pimerano and R. Di Perri, Increased dipropylacetic acid bioavailability from dipropylacetamide by food, Epilepsia, 23 (1982) 115-121. J.E.F. Reynolds, Martindale - The Extra Pharmacopoeia, 28th edn., Pharmaceutical Press, London, 1982, p. 1250. F. Schobben, Valproic acid : Pharmacokinetic aspects, Br. J. Clin. Pratt., Suppl. 27 (1983) 48-51. F. Pisani, A. Fazio, G. Oteri and R. Di Perri, A study on the metabolism of dipropylacetamide to dipropylacetic acid in rats, J. Pharm. Pharmacol., 34 (1982) 45-46. M. Bialer, A. Rubinstein, A comparative study on the pharmacokinetics of valpromide after intravenous administration in dogs, J. Pharm. Pharmacol., 35 (1983) 607-609. M. Bialer and A. Rubinstein, Pharmacokinetics of valpromide in dogs after various modes of administration, Biopharm. Drug Dispos., 5 (1984) 177-183. M. Bialer, A. Rubinstein, I. Raz and 0. Abramsky, Pharmacokinetics of valpromide after oral administration of a solution and a tablet to healthy volunteers, Eur. J. Clin. Pharmacoi., 27 (1984) 501-503. M. Bialer, A. Rubinstein, J. Dubrovsky, I. Raz and 0. Abramsky, A comparative pharmacokinetic study of valpromide and valproic acid after intravenous administration in humans, Int. J. Pharm., 23 (1985) 25-33.
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M. Bialer, M. Friedman, J. Dubrovsky, I. Raz and 0. Abramsky, Pharmacokinetie evaluation of novel sustained release dosage form of valproic acid in humans, Biopharm. Drug Dispos., 6 (1985) 401- 411. M. Friedman, M. Bialer, A. Rubinstein and J. Dubrovsky, Novel controlled release dosage forms of valproic acid, Israel Patent Application No. 74,468, February 28, 1985. M. Bialer, M. Friedman and J. Dubrovsky, Comparative pharmaeokinetic analysis of a novel sustained release dosage form of valproic acid in dogs, Biopharm. Drug Dispos., 5 (1984) l-10. M. Bialer, M. Friedman and J. Dubrovsky, Effect of sustained release on the ph~macokinetics of valproic acid in dogs, Int. J. Pharm., 20 (1985) 53-63. D.L. Simmons, M. Frechette, R.J. Ranz, N.S. Chen and N.K. Patel, A rotating compartmentalized disk for dissolution rate determinations, Can. J. Pharm. Sci., 7 (1972) 62-65. M. Donbrow and E. Touitou, The dissolution mechanism in system undergoing complexation: salicylamide in caffeine solution, J. Pharm. Pharmacol., 29 (1977) 524-528. The United States Pharmacopoeia, Twentieth Revision, United States Pharmacopoeia1 Convention Inc., Rockville, 1980. M. Gibaldi and S. Feldman, Establishment of sink conditions in dissolution rate determination: Theoretical considerations and application to non-disintegrating dosage forms, J. Pharm. Sci., 56 (1967) 1238-1242. M. Bialer, M. Friedman and A. Rubinstein, A rapid gas chromatographic assay for monitoring of valpromide and valproic acid in plasma, J. Pharm. Sci., 73 (1984) 991-993. M. Gibaldi and D. Perrier, Pharmacokinetics, 2nd edn., Marcel Dekker, New York, NY, 1982, pp. 445-449. A.W. Hixson and J.H. Crowell, Dependence of reaction velocity upon surface and agitation. I. Theoretical considerations, Ind. Eng. Chem., 23 (1931) 923-931. E.L. Parrot, D.E. Wurster and T. Higuchi, Drugs release from solids, I. Some factors influencing the solution rate. J. Amer. Pharm. Assoc. Sci., 44 (1955) 269-273. S. Benita, J. Shani, M. Abdulrazik and A. Samuni, Controlled release of radioprotective agents from matrix tablets - Effects of preparative conditions on release rates, J. Pharm. Pharmacol., 36 (1984) 222-228. M. Gibaldi and D. Perrier, Pharmacokinetics, 2nd end., Marcel Dekker, New York, NY, 1982, pp. 113-144. R. Gugler and E. Von Unruh, Clinical pharmacokinetics of valproic acid, Clin. Pharmacokinet., 5 (1980) 67-83. M. Gibaldi, Prolonged-release medication II, Perspec. Clin. Pharmacol., 2 (1984) 25---27.
A COMBINED APPROACH TO CONTROL VAtPROfC ACID REtEASE VIA NOVEL DELIVERY SYSTEM OF VALPROMIDE: A KINETIC AND PHARMACOKINETIC STUDY
INYRQIXJCTIQN The primaryamide of vafproicr acid, valpromide, is widely used in sume ~~opea~ countries as an antie~~lept~~ and antjp~y~hoti~ drug [l--5] and di~t~bu~~d as an entericcoated tablet under the trade name Depamide@ (Labaz, France f67). Recent reports demonstrated that valpramide is biotransformed to valproic acid (VPA), with species differences among humans, dogs and rats in the extent of this biotransfarmation [3, 5, 811 J . In humans, complete biotransformation of VPD to VPA is observed after oral as well
as after intravenous administration [ ZZ’J, Although their chemical structure is similar, the pharrn~~~k~neti~~ of valpromide is entirely different from that of valproic acid, Valprom~d~ possesses a very high rnet~bol~~ clearance, a very short el~rn~nat~~~ h&f-life and a high vrslume of distribution of about 1 L/kg [,X2]. As a solid, non-hygroscopic, neutral prodrug of valproic acid, valpromide may be used as an alternative to valproic acid and sodium valproate, In humans, the pharmacokinetic profile of valproic acid obtained after oral administration of ~~p~~rnide resembled that of a sustained release furmulation of valproic acid [11, liZI. Based on this kno~I~d~e~ two noveI
34
controlled release dosage forms of valpromide were developed using a biodegradable hydrophylic polymer [ 141. These formulations will be designated hereafter as VPDCR, and VPDCR,, The advantages of controlled release dosage forms of valproic acid were summarized in several recent publications [13, 15, 161. These include minimizing fluctuations in valproic acid plasma concentrations at steady state and decreasing the dosage regimen of the drug during chronic therapy. Valpromide was found to be a prodrug of valproic acid [ 11, 121 and yielding a sustained release profile for valproic acid. A combination of VPD in a sustained release carrier, can result in a very uniform VPA serum level, that may lead to a once-a-day product of valproic acid. The objective of this study was to perform a kinetic analysis of the in vitro dissolution profiles of VPDCR, and VPDCRl and a comparative pharmacokinetic analysis of the serum concentrations of valproic acid obtained after oral administration of these formulations to six healthy subjects.
MATERIAL
AND METHODS
liters of the organic phase were injected into a gas chromatograph (Packard, model 437, Holland) equipped with a flame ionization detector and a recorder (linear model 1210, U.S.A.). The glass column, 180 cm X 2 mm i.d., was packed with 5% free fatty acid phase on 80-100 mesh chromosorb Q (Applied Science Lab., PA, U.S.A.). Flow rates were: carrier gas (nitrogen): 30 mL/min, hydrogen: 20 mL/min, air: 200 mL/min; and system temperatures: injector: 195”C, column lBO”C, detector: 220°C. Samples were chromatographed using an eight-point calibration curve, containing dissolution medium spiked with known amounts of valpromide. The mixture was formulated in two ways: (a) the mixture was directly, with no additional adjuvants, compressed into 13 mm diameter, 4.1 mm height tablets under 3000 kg force, using a manual hydraulic press (Perkin Elmer, U.S.A.}, to give 600 mg tablets. Each tablet contained 300 mg valpromide (VPDCR,); (b) the mixture was moistened by purified water, granulated, dried for 60 minutes at 60°C and sieved. The granules obtained were then pressed into 13 mm diameter 4.0 mm height tablets in the same manner with no additional adjuvants, to give 600 mg tablets. Each tablet contained 300 mg valpromide ( VPDCRz).
Tablet preparation Dissolution rate test
Controlled release tablets were prepared by mixing valpromide (Labaz, Paris, France) with a hydrophylic biodegradable polymer in a ratio of 1:l 1141. The mixture was sampled and analysed for valpromide content and homogeneity by the following procedure: each 100 mg sample was dispersed into 50 mL of purified water. After dilution with an additional amount of purified water, 1 mL of the aqueous solution was extracted with 500 PL of chloroform (B.D.H., Poole, England) containing benzyl alcohol as an internal standard (B.D.H., Poole, England). The mixture was vortexed for 15 s and centrifuged at 4000 rpm for 15 min. The centrifugation was done in order to get rid of turbidity due to presence of the enzyme pancreatin. Three micro-
The in vitro dissolution rate tests of the two types of tablets, VPDCR, and VPDCR* were studied in a rotating basket system that provided for the possibility of examining two tablets at a time [ 17, IS], at a constant speed of 100 rpm (Fisher “Steady Speed”, U.S.A.), using 500 mL simulated gastric fluid TS followed by 500 mL simulated intestinal fluid TS U.S.P. XX [19] for 1 hour and 23 hours, respectively, so that sink conditions were maintained during the study [20]. The dissolution media were sampled at 0.5, 1, 1.5, 2,3,4, 5,6,7, 8 and 24 hours. Equal volumes of dissolution medium were immediately returned to the dissolution flask after each sampling. The samples were centrifuged im-
35
mediately at 4000 rpm for 5 minutes and assayed for valpromide content according to the method previously described [ 211. Human studies
Depamide@ (Labaz, Paris, France), VPDCR, and VPDCRz were administered orally to six male volunteers (one subject dropped from the study before the administration of VPDCR2). The volunteers, aged 25-33 years, weighed between 65-82 kg and were selected on the basis of a negative medical history and physical examination, and normal routine blood chemical analysis and morphology. Written informed consent was obtained from each volunteer and the entire clinical experiment had been approved by the Helsinki Committee of the Hadassah Medical Center and the Israeli Ministry of Health. Each volunteer received, at separate times, a single dose of three tablets (900 mg) of VPDCR,, VPDCR*, and Depamide@, at 7 a.m. after an overnight fast. Food was withheld from 12 hours before to 6 hours after dosing. Venous blood samples (10 mL) were taken via an indwelling catheter from the forearm vein at 0, 0.5, 1, 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 24, 30, 39 and 48 hours after dosing. Between two consecutive studies, there was a washout period of 3 weeks. Sera were immediately separated by centrifugation at 7000 rpm for 15 minutes and stored at -20°C. Before assay, the serum was allowed to reach room temperature, vortexed, centrifuged and the residual clot removed. Valproic acid was assayed by a gas chromatographic method [21]. Each sample was extracted into chloroform and chromatographed in comparison with an eight-point calibration curve containing serum from each volunteer before each treatment; time = 0, spiked with known amounts of valproic acid and valpromide. The AUC (area under the serum concentration vs. time curve) was calculated using the trapezoidal rule with extrapolation to infinity [22]. The relative and absolute bioavailability of VPDCR, and VPDCRz was
calculated from the ratio of the AUCs of each one of the two formulations relative to Depamide@ and an i.v. preparation of VPD which had been administered to the same six subjects in a previous study [ 121.
RESULTS AND DISCUSSION Dissolution-kinetic tablets
model of VPDCRl
and VPDC&
A degradation phenomenon, symmetrical in nature, was observed during the dissolution study of VPDCRi and VPDCRz tablets. At the end of the attrition of each tablet, a spherical cylinder-like residue could be identified in each compartment of the dissolution rate apparatus. As the dissolution process was performed under well-stirred conditions and sink conditions were maintained, the system could be analysed according to the second special case of Hixson-Crowell’s cube root law [23] represented by the equation: k W0 113_ W’” = 3 _ t = K’t
(1)
a
where W. is the initial amount of valpromide in the tablet, W is the amount of valpromide retained at time t, calculated from W. less the amount found in the dissolution medium at time t, K’ is the slope of the straight line of Wo1’3 - W”j versus time, and a is the reciprocal value of the two-thirds-root density of valpromide tablet, multiplied by an appropriate geometric shape factor, assuming a three-dimensional symmetrical attrition of a cylinder [ 241. Hixson-Crowell graphical treatment for VPDCR, and VPDCRz are presented in Fig. 1 and the appropriate values for densities, shape factors and cube root law rate constants are summarized in Table 1. The dissolution data of VPDCR, and VPDCRz were computerized, using a non-linear least squares kinetic program [25] and the values of cube root law constants obtained matched well with the values obtained using the graphic method.
36
TABLE
1
Calculated
values of density,
VPDCR, VPDCR, %alculated bCalculated ‘Correlation
shape
factor
and cube root
law rate constants
for VPDCR,
and VPDCR,
Shape factor
g/mL
Rate g h“
constanta (K’), cmmZ X 10’
Rate constantb (K’), g h-l cm-’ x 10’
1.27 7.27
1.10 1.30
2.54 8.96
(0.994)c (0.996)’
2.55 8.82
Density,
by the graphical method. by non-linear kinetic computer coefficient.
program.
Cb(mgIL)
Fig. 1. Cube root law plots of valpromide released into U.S.P. gastrointestinal fluids from VPDCR, (c,) and VPDCR, (a).
For the sake of comparison it should be mentioned that the dissolution of the Depamide@ tablet was completed within half an hour under the same experimental conditions. The threefold differences in dissolution rate constants between VPDCRl and VPDCRz can be explained by the different nature of the two formulations. The granulation process leads to a more porous product, hence VPDCRz degrades faster due to more itensive leaching of the gastrointestinal medium into it. Pharmacokinetic
studies
No valpromide concentrations in serum could be determined in any of the subjects after oral administration of VPDCRI, VPDCR, and Depamide@ [ 111 . Mean valproic acid serum concentrations obtained after VPDCR 1, VPDCRz administration in comparison to Depamide@ are presented in Fig. 2. Table 2 presents a summary of some pharmacokinetic parameters calculated from the serum concen-
Fig. 2. Mean serum concentrations of valproic acid obtained after oral administration (900 mg) to six healthy subjects of: Depamide@ (a), VPDCR, (o), VPDCR, (e). (VPDCR, was administered only to five subjects.)
tration data of valproic acid after administration of the three VPD formulations. From Table 2 and Fig. 2 it can be seen that the bioavailability of VPDCRz was not significantly different from Depamide@, while the bioavailability of VPDCR, was significantly lower than Depamide@. After the administration of VPDCR1, a very uniform serum concentration of about 25 pg/mL was observed between 6 to 30 hours after dosing. This prolonged absorption caused a decrease in the extent of the absorption. After administration of the identical dose of VPDCR* a uniform serum concentration of 38 pg/mL was obtained from 6 to 16 hours after dosing. VPDCRz was found to be bioequivalent to Depamide@ as far as the extent of absorption. However, the rate of absorption of valproic acid after the administration of VPDCR, was more sustained and controlled than after the administration of Depamide’. This slow absorption of valproic acid implies less fluctuations in VPA at
37
TABLE
2
Summary of mean (+ SD.) pharmacokinetie parameters of valproic 3 X 300 mg of Depamide@, VPDCR, and VPDCR, to six volunteers Pharmacokinetic parameter
Cb max (mg/L~ tmax (hl F (%) F’ (%I Cb rnh 224 h @w/L) Cb(,tmax (mglL) Cb(ss)min
@wiGI
% fluctuation
acid obtained
after
the administration
of
Formulation Depamidea
VPDCR,
VPDCR,
65.5 ?: 14.2 9.2 f 3.0 67.5 f 23.0 25.3 t 9.2 120.0 rt 60.1 45.5 i 19.9 158.7 + 41.6
27.9 10.0 49.6 73.5 20.0 43.7 31.3 46.8
44.8 11.8 71.6 106.0 29.7 68.1 48.8 73.6
i 7.7 t 3.2 i 7.0 t 10.3 + 4.8 1. 18.0 + 12.3 + 19.2
t f i t i: + i i
12.2 4.1 18.9 24.4 13.3 21.8 22.5 21.3
= peak serum concentration; t,,, = time to reach Cb,,; F = absolute bioavailability; F’ = bioavailability C&n, relative to Depamide@ ; Cb,, , 24 h = serum concentration of valproic acid 24 hours after valpromide administra= theoretical steady-state peak serum level when VPD formulations were administered chronically tion; Cb(s)rnax every 24 hours; Cbcsjmin = theoretical steady-state lower serum level when VPD formulations were administered chronically every 24 hours.
steady-state serum concentrations during multiple dosing with VPDCR*. This was substantiated by a series of theoretical steady-state calculations based on the experimental results obtained in this study. Equations (2) and (3) were used for these repetitive dosing steady-state concentrations
[261. Cb(ss)max
Cbmax = 1
where Cb(,),, and Cb(s)min are the peak and trough drug serum concentrations, respectively, at steady state: 7 is a dosing interval of 24 hours. Cb,,, is peak serum level at single dose administration, Cb,i, is serum level at 7 hours after dosing and K is the elimination rate constant of the drug. The results of these steady-state calculations are summarized in Table 2. Under these conditions, Depamide@ and VPDCRz are both candidates for once-a-day of valpromide (T = 24 h) chronic treatment as they both possess a steady-state peak serum
level within the therapeutic window of valproic acid (SO-100 pg/mL) [27], although their lower serum level at steady state does not enter the desired range (Table 2). However, the percentage of serum fluctuations of valproic acid as calculated by eqn. (4) [28] was higher after Depamide@ administration than after the administration of VPDCR2. Cb(ss)max
- Cb(,)min
X 100 = % fluctuation
(4)
Cb(ss)min
The % fluctuation values obtained, 48.7 rt: 27.2 for VPDCR2 and 114.9 i 11.3 for Depamide@ (mean i SD.), demonstrate quite clearly the advantages of VPDCRz as a once-a-day controlled release drug delivery system of valproic acid. A neutral, solid non-hygrosopic controlled release formulation of valpromide such as VPDCR, can be a good pharmaceutical alternative for standard and sustained release formulations of valproic acid.
ACKNOWLEDGEMENTS
This work was supported by Grant No. 2127 of the Israel National Council for Research and Development and is included in
38
the dissertation project of Abraham Rubinstein as partial fulfillment of the Doctor of Philosophy degree requirements of the Hebrew University of Jerusalem. The authors wish to thank Mr. Zahy Mattar for his skillful technical assistance. The valpromide sample obtained from Labaz (Sanofi) France, is gratefully acknowledged.
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