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Bioavailability and Bioequivalence of Two Formulations of Etodolac (Tablets and Suppositories)
Bioavailability and Bioequivalence of Two Formulations of Etodolac (Tablets and Suppositories) I. T. MOLINA-MARTINEZ*, R. HERRERO*, J. A. A. MARTINEZ-TOBED', AND R. CADORNIGA*'
GUTIERREZS,
J. M. IGLESlAS*, J. L. FABREGAS',
Received November 5, 1990, from the *Department of Pharmacy and Pharmaceutical Technology, Faculty of Pharmacy, Universidad Complutense, 28040 Madrid, the *Department of Internal Medicine, Hospital Clinic0 de San Carlos, 28040 Madrid, and the sLaboratoriosAlmirall, Cardoner 68, 08024 Barcelona, Spain. Accepted for publication July 15, 1992.
Abstract 0 We studied the influence of administration route on the biopharmaceutical behavior of etodolac. The levels obtained in plasma when the same dose of etodolac is administered orally (tablets, dosage form A) and rectally (suppositories, dosage form 6) were compared. The study was done in a crossover design with healthy volunteers of both sexes, of average build, and younger than 35 years of age. From the concentration in plasma-time data, the maximum concentration in plasma (C,d, time to C,,, and area under the curve up to the last measurableconcentration (AUC;) or infinity (AUC,") were calculated and no compared by analysis of variance. With the exception of C,,, significant differences between treatments were found in the rest of the parameters. Finally, with formulation A (tablets) as a reference, the relative bioavailability was established, on the basis of the ratio @:A) of AUCL and AUC,", within the range 100 f 20%. The results indicate that the two routes of administration are bioequivalent and that the rectal route is an alternative administration route for etodolac.
There are many bioequivalence studies in the literature in which a drug at the same dose and with the same dosage form is developed with distinct formulation or technological processes. These studies provide insight into the influence that formulation and process have on the biopharmaceutical parameters that define the bioavailability. However, bioavailability depends not only on the formulation but also on the administration route. Hence, we compared the levels in plasma reached when the same drug (etodolac) is administered at the same dose level orally (tablets) and rectally (suppositories). Etodolacl-3 is a nonsteroidal agent with anti-inflammatory and analgesic activity. The fact that etodolac is not notably susceptible to first-pass metabolism eliminates a possible cause for variation in the biopharmaceutical behavior between the two administration routes tested. Therefore, the bioavailability will be influenced by the availability or release of the drug from the dosage form used and by the characteristics of the absorption sites in each case. The interest in this study rests with the possible utilization of an alternative to the oral administration route for instances when functional or anatomical abnormality makes oral administration unadvisable.
Experimental Section SubjectsSubjects were selected from healthy volunteers of both sexes, aged ltT-40, with a weight within 10%of the ideal weight for their stature and physique. The volunteers had no clinical history of cardiovascular, renal, hepatic, or gastrointestinal diseases. Furthermore, they had no dependence on drugs or alcohol. Individuals with allergies to nonsteroidal anti-inflammatory drugs were excluded. Volunteers were clinically examined and the appropriate blood and biochemical tests were performed before and after the trials to check their state of health. Before the start of treatment, volunteers gave consent in writing after having been informed of the nature of the
*
0022-3549/93/0200-0211$02.50/0 0 1993, American Pharmaceutical Association
trial. Ten volunteers (six male and four female) who fulfilled all the requirements were selected. They were between 22 and 34 years of age (27.7 f 4.1) and weighed between 47 and 83 kg (67.8 4 14.6). Study Design and Protocol-Two different formulations of 200 mg of etodolac were examined: formulation A (reference), tablets (batch D-78043, Lab. Almirall, S.A., Barcelona, Spain), and formulation B (test), suppositories (batch D-85008, Lab. Almirall, S.A.).The formulations were administered according to a two-way crossover design with a 7-day drug-free interval between the administrations. After an overnight fast, each subject ingested a tablet with 100 mL of water. In the case of the suppositories, the rectum was emptied the night before the trial with a glycerin suppository. Blood samples were collected in plastic heparinized tubes at the following times: 0 (predose),0.5, 1,2, 3, 4, 6, 8, 10, 24, and 25 h postdose. The plasma was immediately separated by centrifugation (900 x g)and stored at -20 "C until analysis. The samples were kept away from light to prevent degradation. Analysis of Plasma Samples-Concentrations of etodolac in plasma were quantitated by the high-performance liquid chromatography method described by Cosyns et a l . 4 With 0.5 mL of plasma, the limit for quantitation was 0.2 &mL, with a coefficientof variation (CV)of 5.6% (six replicates in the same study). Interday reproducibility was confirmed by comparing the slopes of standard curves (n = 5);a CV of 4.3% was determined. Data Processing-The pharmacokinetic parameters were determined from the profiles of concentration in plasma versus time (plasma concentration-time), and both formulations were compared , C and the time at statistically. The maximum concentration ) which this concentration was reached (tmm)were used as measures of absorption and rate. The areas under the plasma concentration-time curves up to the last measurable concentration (AUC;) or infinity (AUC;) express the magnitudes of absorption. The AUCS was calculated by the trapezoidal rule up to the last sampling time ( t )and the total area was determined by adding AUC; and the amount resulting from dividing the concentration at "t" (C,) by the apparent elimination rate constant for the drug in plasma KeJ:
TheKe, was calculated by least-squares regression from the data for the last points of the plasma concentration-time curves. Finally, the apparent plasma half-life (tllZ)was calculated as t,,2 = In 2/Kep Statistical Analysis-The parameters were compared by analysis of variance (ANOVA) for crossover design in which sources of variation are subjects, time periods, treatments, and residuals. The power of each ANOVA was calculated by the Winer method6 with 20% of the mean value of a parameter resulting from test~treatment. The B:A ratios of pharmacokinetic parameters were examined by the 20/80 rule.6.7 The confidence limits were computed by determining conventional 90%(1-2a) confidence limits of the difference between the mean values obtained.
Results and Discussion Table I gives the mean concentrations of etodolacin plasma, standardized to a body weight of 70 kg, and the CVs obtained at the different times after administration of the formulaJournal of Pharmaceutical Sciences 1 211 Vol. 82, No. 2, February 1993
Table I-Mean Concentrations of Etodolac In Plasma for Each Formulatlon
Mean Concentration (CV, %), mg/L Formulation A Formulation B
Time, h
4.54 (71.8) 17.30(40.0) 14.91 (23.5) 10.43(20.9) 7.72(25.9) 4.44(24.9) 3.24 (32.2) 2.77(30.0) 0.82 (64.5) 0.77(78.6)
0.5 1 .o 2.0 3.0 4.0 6.0 8.0 10.0 24.0 25.0
8.56(23.8) 12.69 (22.4) 12.29(18.3) 9.56(30.7) 6.66 (21.8) 4.85(23.4) 3.47(28.3) 2.89 (32.5) 0.80(58.5) 0.78(64.9)
tions. A plot of these results is shown in Figure 1. The parameters used to measure bioavailability were the AUC; and AUC,"for the amount absorbed and t,, and C, for the absorption rate and they were calculated in a modelindependent manner. Table I1 shows the mean values for the above parameters along with Kel and t,. Analysis (by ANOVA) of the bioavailability data and their logarithmic transformations (Table 111)shows there were no significant differences between formulations in AUC;, AUC,", and t,,. However, the C, values for the two formulations were significantly different. The period has almost no influence as a source of variation (Table 111).The statistical powers to detect a 20% difference between treatments, based on the AUC;, AUC,",and ,t values, were >95%, thus establishing the validity of the analyses by ANOVA. Although the interindividual differences in the AUCs are not excessively high (CV = 20-30%, Table 111, they were significant because the error due to chance or random variability was low. These results indicate that the amount of drug absorbed in both cases is the same. The differences found in C,,, a parameter influenced by the extent as well as the absorption rate, seem to originate mostly from the rate of the absorption process. Because etodolac is not noticeably affected by first-pass metabolism, differences between the amount of drug absorbed by the oral and rectal routes would be mainly due to the release of the drug from the dosage form used and, to a lesser extent, to the administration route. It is assumed that the absorption of etodolac is by a simple passive diffusion process. We believe that differences in absorption rate exist between both routes of administration, although the ,t values do not indicate this. The possible influences on ,t are hidden by the high interindividual dispersion found because ,t is a variable that can only have discrete values that coincide with the 2o
T
Table Il-Mean of Etodolac
Pharmacoklnetlc Parameters of Each Formulatlon
Parameter
-
AUC;, mg h/L AUC,", mg h/L Cma, mg/L Lax9
h
Y,, h-' h. h
Formulation A
Formulation B
(CV, %)
(CV, 96)
96.14(21.6) 105.91 (30.9) 19.68(18.2) 1.3 (37.2) 0.097(19.8) 7.45(27.1)
92.40(23.5) 101.26 (29.1) 13.63(18.4) 1.6 (43.7) 0.100 (16.6) 7.10(20.4)
Table Ill-Probablllty of 'F' Obtained from ANOVA'
Source of Variation Formulation Period Subject
(4 Values for Sources of Variation AUC,"
AUCL >0.25 (>0.25) >0.75 (>0.50) <0.005' (<0.005)*
Cm,
Lax
(>0.25) >0.25
(
(r0.25) >0.25
>0.75 (>0.75) <0.001 (<0.001)*
20.50 (>0.25) 20.75 (20.75)
>0.50 p0.75) >0.25 (r0.25)
aValues in the parentheses are the values for corresponding In-transformed data. Significantly differentat a = 0.05. (Y
sampling time. Also, when comparing two administration routes that normally present different lag times (tl,different distance from the absorption zone), the most appropriate parameter for representing the distinct absorption rate constant would be (t,, - tl) and not tm,. This value can not be determined without assuming a specific disposition model. If we compare the mean values of these parameters, using the (B:A) ratio value given in percentages and their 90% confidence limits (Table IV),we see that although ANOVA did not show a significant difference in ,t there is a difference in behavior between treatments when the general rule allowing 20% as a variation limit is applied as a criterion in bioequivalence tests.6,' These results confirm that absorption rates are different for both formulations. The mean residence time was not used as a rate parameter because its variation in drugs that have a slow elimination constant is more representative of the possible interindividual variation of disposition than variation produced by the formulation in absorption of the drug.8 The confidence limits for the testxeference ratios for the mean AUC; and AUC,"indicate that these values are entirely within the tolerance intervals of bioequivalence (220%of the reference in the comparison). The statistical powers of the t test for difference between the two means5 revealed that >80% of the subjects showed relative bioavailabilities of 100 ? 20% for AUC; and AUC,"ratios. On the basis of a strict interpretation of the superimposition principle for the profiles of drug level in blood after a single dose, the treatments examined would be judged as bioinequivalent. However, when considering the specific therapeutic effect of the drug belonging to the pharmaceutical form under Table IV-Ratlo In Percentage of the Mean Parameters of Bloavallablllty between the Two Formulatlons and Thelr Confldence Llmlts (a= 0.1)
Parameter AUC; AUC," Flgure I-Mean
Time (h)
concentration of etodolac in plasma of 10 healthy volunteers after administration of single doses of 200 mg of etodolac. Key: (0)oral route; (0)rectal route. 212 1 Journal of Pharmaceutical Sciences Vol. 82, No. 2, February 7993
Cmax tmax a
0:A
Confidence Limits
96.11 95.61 69.26' 123.08a
87.97-104.24 86.45-104.77 52.54-85.97' 84.1616799'
Does not fulfill the 20% rule.
study and its dosage regimen,l-3 a report of bioequivalence should be given. When the absorption rate decreases, the interval between the C,, and Cminat steady state is reduced and the adivity and/or toxicity arising from these two concentrations remain within the margins for the standard treatment.
References and Notes 1. Lynch, S.; Brogden, R. N. Drugs 1986,31, 28S300. 2. Kraml, M.; Cosyns, L.; Hicks, D. R.; Simon, J.; Mullane, J. F.;
Dvornik, D. Biopharm. Drug Dispos. 1984,5, 63-74. 3. Dey, M.; Enever, R.; Marino, M.; Michelucci, J.; Smith, D.; Warner, R.; Weierstall, R. Int. J. Pharm. 1989,49, 121-128. 4. COSynS, L.; Spain, M.; ~ ~L. J. Pharm. l SCi. , 1983,72,275-277. 5. Winer, B. J. Statistical Principles in Experimental Design, 2nd ed.; Maraw-Hil1 N~~ Yo&, 1971; pp 19-29 and 222-225. 6. Ritschel, W. A. Meth. End. Exptl. Qin. Pharmacol. 1987, 9, 453-459. 7. Glasser, A. C. Meth. Find. Exptl. Clin. Pharmacol. 1987, 9, 449452. 8. Concheiro, A.; Llabres, M.; Martinez, R.; Vila, J. L. Biopharm. Drug Dispos. 1986, 7, 253-264.
Journal of PharmaceuticalSciences I 213 Vol. 82, No. 2, February 1993
GUTIERREZS,
J. M. IGLESlAS*, J. L. FABREGAS',
Received November 5, 1990, from the *Department of Pharmacy and Pharmaceutical Technology, Faculty of Pharmacy, Universidad Complutense, 28040 Madrid, the *Department of Internal Medicine, Hospital Clinic0 de San Carlos, 28040 Madrid, and the sLaboratoriosAlmirall, Cardoner 68, 08024 Barcelona, Spain. Accepted for publication July 15, 1992.
Abstract 0 We studied the influence of administration route on the biopharmaceutical behavior of etodolac. The levels obtained in plasma when the same dose of etodolac is administered orally (tablets, dosage form A) and rectally (suppositories, dosage form 6) were compared. The study was done in a crossover design with healthy volunteers of both sexes, of average build, and younger than 35 years of age. From the concentration in plasma-time data, the maximum concentration in plasma (C,d, time to C,,, and area under the curve up to the last measurableconcentration (AUC;) or infinity (AUC,") were calculated and no compared by analysis of variance. With the exception of C,,, significant differences between treatments were found in the rest of the parameters. Finally, with formulation A (tablets) as a reference, the relative bioavailability was established, on the basis of the ratio @:A) of AUCL and AUC,", within the range 100 f 20%. The results indicate that the two routes of administration are bioequivalent and that the rectal route is an alternative administration route for etodolac.
There are many bioequivalence studies in the literature in which a drug at the same dose and with the same dosage form is developed with distinct formulation or technological processes. These studies provide insight into the influence that formulation and process have on the biopharmaceutical parameters that define the bioavailability. However, bioavailability depends not only on the formulation but also on the administration route. Hence, we compared the levels in plasma reached when the same drug (etodolac) is administered at the same dose level orally (tablets) and rectally (suppositories). Etodolacl-3 is a nonsteroidal agent with anti-inflammatory and analgesic activity. The fact that etodolac is not notably susceptible to first-pass metabolism eliminates a possible cause for variation in the biopharmaceutical behavior between the two administration routes tested. Therefore, the bioavailability will be influenced by the availability or release of the drug from the dosage form used and by the characteristics of the absorption sites in each case. The interest in this study rests with the possible utilization of an alternative to the oral administration route for instances when functional or anatomical abnormality makes oral administration unadvisable.
Experimental Section SubjectsSubjects were selected from healthy volunteers of both sexes, aged ltT-40, with a weight within 10%of the ideal weight for their stature and physique. The volunteers had no clinical history of cardiovascular, renal, hepatic, or gastrointestinal diseases. Furthermore, they had no dependence on drugs or alcohol. Individuals with allergies to nonsteroidal anti-inflammatory drugs were excluded. Volunteers were clinically examined and the appropriate blood and biochemical tests were performed before and after the trials to check their state of health. Before the start of treatment, volunteers gave consent in writing after having been informed of the nature of the
*
0022-3549/93/0200-0211$02.50/0 0 1993, American Pharmaceutical Association
trial. Ten volunteers (six male and four female) who fulfilled all the requirements were selected. They were between 22 and 34 years of age (27.7 f 4.1) and weighed between 47 and 83 kg (67.8 4 14.6). Study Design and Protocol-Two different formulations of 200 mg of etodolac were examined: formulation A (reference), tablets (batch D-78043, Lab. Almirall, S.A., Barcelona, Spain), and formulation B (test), suppositories (batch D-85008, Lab. Almirall, S.A.).The formulations were administered according to a two-way crossover design with a 7-day drug-free interval between the administrations. After an overnight fast, each subject ingested a tablet with 100 mL of water. In the case of the suppositories, the rectum was emptied the night before the trial with a glycerin suppository. Blood samples were collected in plastic heparinized tubes at the following times: 0 (predose),0.5, 1,2, 3, 4, 6, 8, 10, 24, and 25 h postdose. The plasma was immediately separated by centrifugation (900 x g)and stored at -20 "C until analysis. The samples were kept away from light to prevent degradation. Analysis of Plasma Samples-Concentrations of etodolac in plasma were quantitated by the high-performance liquid chromatography method described by Cosyns et a l . 4 With 0.5 mL of plasma, the limit for quantitation was 0.2 &mL, with a coefficientof variation (CV)of 5.6% (six replicates in the same study). Interday reproducibility was confirmed by comparing the slopes of standard curves (n = 5);a CV of 4.3% was determined. Data Processing-The pharmacokinetic parameters were determined from the profiles of concentration in plasma versus time (plasma concentration-time), and both formulations were compared , C and the time at statistically. The maximum concentration ) which this concentration was reached (tmm)were used as measures of absorption and rate. The areas under the plasma concentration-time curves up to the last measurable concentration (AUC;) or infinity (AUC;) express the magnitudes of absorption. The AUCS was calculated by the trapezoidal rule up to the last sampling time ( t )and the total area was determined by adding AUC; and the amount resulting from dividing the concentration at "t" (C,) by the apparent elimination rate constant for the drug in plasma KeJ:
TheKe, was calculated by least-squares regression from the data for the last points of the plasma concentration-time curves. Finally, the apparent plasma half-life (tllZ)was calculated as t,,2 = In 2/Kep Statistical Analysis-The parameters were compared by analysis of variance (ANOVA) for crossover design in which sources of variation are subjects, time periods, treatments, and residuals. The power of each ANOVA was calculated by the Winer method6 with 20% of the mean value of a parameter resulting from test~treatment. The B:A ratios of pharmacokinetic parameters were examined by the 20/80 rule.6.7 The confidence limits were computed by determining conventional 90%(1-2a) confidence limits of the difference between the mean values obtained.
Results and Discussion Table I gives the mean concentrations of etodolacin plasma, standardized to a body weight of 70 kg, and the CVs obtained at the different times after administration of the formulaJournal of Pharmaceutical Sciences 1 211 Vol. 82, No. 2, February 1993
Table I-Mean Concentrations of Etodolac In Plasma for Each Formulatlon
Mean Concentration (CV, %), mg/L Formulation A Formulation B
Time, h
4.54 (71.8) 17.30(40.0) 14.91 (23.5) 10.43(20.9) 7.72(25.9) 4.44(24.9) 3.24 (32.2) 2.77(30.0) 0.82 (64.5) 0.77(78.6)
0.5 1 .o 2.0 3.0 4.0 6.0 8.0 10.0 24.0 25.0
8.56(23.8) 12.69 (22.4) 12.29(18.3) 9.56(30.7) 6.66 (21.8) 4.85(23.4) 3.47(28.3) 2.89 (32.5) 0.80(58.5) 0.78(64.9)
tions. A plot of these results is shown in Figure 1. The parameters used to measure bioavailability were the AUC; and AUC,"for the amount absorbed and t,, and C, for the absorption rate and they were calculated in a modelindependent manner. Table I1 shows the mean values for the above parameters along with Kel and t,. Analysis (by ANOVA) of the bioavailability data and their logarithmic transformations (Table 111)shows there were no significant differences between formulations in AUC;, AUC,", and t,,. However, the C, values for the two formulations were significantly different. The period has almost no influence as a source of variation (Table 111).The statistical powers to detect a 20% difference between treatments, based on the AUC;, AUC,",and ,t values, were >95%, thus establishing the validity of the analyses by ANOVA. Although the interindividual differences in the AUCs are not excessively high (CV = 20-30%, Table 111, they were significant because the error due to chance or random variability was low. These results indicate that the amount of drug absorbed in both cases is the same. The differences found in C,,, a parameter influenced by the extent as well as the absorption rate, seem to originate mostly from the rate of the absorption process. Because etodolac is not noticeably affected by first-pass metabolism, differences between the amount of drug absorbed by the oral and rectal routes would be mainly due to the release of the drug from the dosage form used and, to a lesser extent, to the administration route. It is assumed that the absorption of etodolac is by a simple passive diffusion process. We believe that differences in absorption rate exist between both routes of administration, although the ,t values do not indicate this. The possible influences on ,t are hidden by the high interindividual dispersion found because ,t is a variable that can only have discrete values that coincide with the 2o
T
Table Il-Mean of Etodolac
Pharmacoklnetlc Parameters of Each Formulatlon
Parameter
-
AUC;, mg h/L AUC,", mg h/L Cma, mg/L Lax9
h
Y,, h-' h. h
Formulation A
Formulation B
(CV, %)
(CV, 96)
96.14(21.6) 105.91 (30.9) 19.68(18.2) 1.3 (37.2) 0.097(19.8) 7.45(27.1)
92.40(23.5) 101.26 (29.1) 13.63(18.4) 1.6 (43.7) 0.100 (16.6) 7.10(20.4)
Table Ill-Probablllty of 'F' Obtained from ANOVA'
Source of Variation Formulation Period Subject
(4 Values for Sources of Variation AUC,"
AUCL >0.25 (>0.25) >0.75 (>0.50) <0.005' (<0.005)*
Cm,
Lax
(>0.25) >0.25
(
(r0.25) >0.25
>0.75 (>0.75) <0.001 (<0.001)*
20.50 (>0.25) 20.75 (20.75)
>0.50 p0.75) >0.25 (r0.25)
aValues in the parentheses are the values for corresponding In-transformed data. Significantly differentat a = 0.05. (Y
sampling time. Also, when comparing two administration routes that normally present different lag times (tl,different distance from the absorption zone), the most appropriate parameter for representing the distinct absorption rate constant would be (t,, - tl) and not tm,. This value can not be determined without assuming a specific disposition model. If we compare the mean values of these parameters, using the (B:A) ratio value given in percentages and their 90% confidence limits (Table IV),we see that although ANOVA did not show a significant difference in ,t there is a difference in behavior between treatments when the general rule allowing 20% as a variation limit is applied as a criterion in bioequivalence tests.6,' These results confirm that absorption rates are different for both formulations. The mean residence time was not used as a rate parameter because its variation in drugs that have a slow elimination constant is more representative of the possible interindividual variation of disposition than variation produced by the formulation in absorption of the drug.8 The confidence limits for the testxeference ratios for the mean AUC; and AUC,"indicate that these values are entirely within the tolerance intervals of bioequivalence (220%of the reference in the comparison). The statistical powers of the t test for difference between the two means5 revealed that >80% of the subjects showed relative bioavailabilities of 100 ? 20% for AUC; and AUC,"ratios. On the basis of a strict interpretation of the superimposition principle for the profiles of drug level in blood after a single dose, the treatments examined would be judged as bioinequivalent. However, when considering the specific therapeutic effect of the drug belonging to the pharmaceutical form under Table IV-Ratlo In Percentage of the Mean Parameters of Bloavallablllty between the Two Formulatlons and Thelr Confldence Llmlts (a= 0.1)
Parameter AUC; AUC," Flgure I-Mean
Time (h)
concentration of etodolac in plasma of 10 healthy volunteers after administration of single doses of 200 mg of etodolac. Key: (0)oral route; (0)rectal route. 212 1 Journal of Pharmaceutical Sciences Vol. 82, No. 2, February 7993
Cmax tmax a
0:A
Confidence Limits
96.11 95.61 69.26' 123.08a
87.97-104.24 86.45-104.77 52.54-85.97' 84.1616799'
Does not fulfill the 20% rule.
study and its dosage regimen,l-3 a report of bioequivalence should be given. When the absorption rate decreases, the interval between the C,, and Cminat steady state is reduced and the adivity and/or toxicity arising from these two concentrations remain within the margins for the standard treatment.
References and Notes 1. Lynch, S.; Brogden, R. N. Drugs 1986,31, 28S300. 2. Kraml, M.; Cosyns, L.; Hicks, D. R.; Simon, J.; Mullane, J. F.;
Dvornik, D. Biopharm. Drug Dispos. 1984,5, 63-74. 3. Dey, M.; Enever, R.; Marino, M.; Michelucci, J.; Smith, D.; Warner, R.; Weierstall, R. Int. J. Pharm. 1989,49, 121-128. 4. COSynS, L.; Spain, M.; ~ ~L. J. Pharm. l SCi. , 1983,72,275-277. 5. Winer, B. J. Statistical Principles in Experimental Design, 2nd ed.; Maraw-Hil1 N~~ Yo&, 1971; pp 19-29 and 222-225. 6. Ritschel, W. A. Meth. End. Exptl. Qin. Pharmacol. 1987, 9, 453-459. 7. Glasser, A. C. Meth. Find. Exptl. Clin. Pharmacol. 1987, 9, 449452. 8. Concheiro, A.; Llabres, M.; Martinez, R.; Vila, J. L. Biopharm. Drug Dispos. 1986, 7, 253-264.
Journal of PharmaceuticalSciences I 213 Vol. 82, No. 2, February 1993