Marvin C. Meyer

Pharmacokinetics of chlorpheniramine, phenytoin, glipizide and nifedipine in an individual homozygous for the CYP2C9*3 allele.

Genetic polymorphisms in the cytochrome P450 (CYP) family are widely known to contribute to interindividual differences in the pharmacokinetics of many drugs. Several alleles for the CYP2C9 gene have been reported. Individuals homozygous for the Leu359 variant (CYP2C9*3) have been shown to have significantly lower drug clearances compared with Ile359 (CYP2C9*1) homozygous individuals. A male Caucasian who participated in six bioavailability studies in our laboratory over a period of several years showed extremely low clearance of two drugs: phenytoin and glipizide (both substrates of CYP2C9), but not for nifedipine (a CYP3A4 substrate) and chlorpheniramine (a CYP2D6 substrate). His oral clearance of phenytoin was 21% of the mean of the other 11 individuals participating in the study, and his oral clearance of glipizide, a second generation sulfonylurea structurally similar to tolbutamide, was only 188% of the mean of the other 10 individuals. However, his oral clearance of nifedipine and chlorpheniramine did not differ from individuals in other studies performed at our laboratories. An additional blood sample was obtained from this individual to determine if he possessed any of the known CYP2C9 or CYP2C19 allelic variants that would account for his poor clearance of the CYP2C9 substrates (phenytoin and glipizide) compared with the CYP3A4 (nifedipine) and CYP2D6 (chlorpheniramine) substrates. The results of the genotype testing showed that this individual was homozygous for the CYP2C9*3 allele and did not possess any of the known defective CYP2C19 alleles. This study establishes that the Leu359 mutation is responsible for the phenytoin and glipizide/tolbutamide poor metabolizer phenotype.

The Bioinequivalence of Carbamazepine Tablets with a History of Clinical Failures

The bioavailability of three lots of a generic 200-mg carbamazepine tablet, which had been withdrawn from the market, was compared to the bioavailability of one lot of the innovator product in 24 healthy volunteers. Fifty-three lots of the generic product had been recalled by the manufacturer because of concerns over reports of clinical failures for several of the lots. The three generic lots tested in this study exhibited a wide range of bioavailability, as well as large differences in the in vitro dissolution rates. The mean maximum carbamazepine plasma concentrations for two of the generic lots were only 61-74% that of the innovator product, while the third lot was 142% of the innovator. The mean areas under the plasma concentration-time curve for the three generic lots ranged from 60 to 113% that of the innovator product. The results clearly indicate a significant difference in the rate and extent of absorption of the generic products compared to the innovator, as well as among the generic lots. A good relationship was found between the in vivo parameters and the in vitro dissolution results for the four dosage forms.

The application of an artificial neural network and pharmacokinetic simulations in the design of controlled-release dosage forms

The objective of this work is to use an artificial neural network (ANN) and pharmacokinetic simulations in the design of controlled-release formulations with predictable in vitro and in vivo behavior. Seven formulation variables and three other tablet variables (moisture, particle size and hardness) for 22 tablet formulations of a model sympathomimetic drug were used as the ANN model input, and in vitro dissolution-time profiles at ten different sampling times were used as output. An ANN model was constructed by selecting the optimal number of iterations and model structure (the number of hidden layers and number of hidden layer nodes). The optimized ANN model was used for prediction of formulations based on two desired target in vitro dissolution-time profiles and two desired bioavailability profiles. For three of the four predicted formulations there was very good agreement between the ANN predicted and the observed in vitro and simulated in vivo properties. This work illustrates the potential for an artificial neural network, along with pharmacokinetic simulations, to assist in the development of complex dosage forms.

A Modern View of Excipient Effects on Bioequivalence: Case Study of Sorbitol

PurposeTo examine the effect of common excipients such as sugars (sorbitol versus sucrose) on bioequivalence between pharmaceutical formulations, using ranitidine and metoprolol as model drugs.MethodsTwo single-dose, replicated, crossover studies were first conducted in healthy volunteers (Nu2009=u200920 each) to compare the effect of 5xa0Gm of sorbitol and sucrose on bioequivalence of 150xa0mg ranitidine or 50xa0mg metoprolol in aqueous solution, followed by a single-dose, nonreplicated, crossover study (Nu2009=u200924) to determine the threshold of sorbitol effect on bioequivalence of 150xa0mg ranitidine in solution.ResultsRanitidine Cmax and AUC(0–∞) were decreased by ∼50% and 45%, respectively, in the presence of sorbitol versus sucrose. Similarly, sorbitol reduced metoprolol Cmax by 23% but had no significant effect on AUC(0–∞). An appreciable subject-by-formulation interaction was found for ranitidine Cmax and AUC(0–∞), as well as metoprolol Cmax. Sorbitol decreased the systemic exposure of ranitidine in a dose-dependent manner and affected bioequivalence at a level of 1.25xa0Gm or greater.ConclusionsAs exemplified by sorbitol, some common excipients have unexpected effect on bioavailability/bioequivalence, depending on the pharmacokinetic characteristics of the drug, as well as the type and amount of the excipient present in the formulation. More research is warranted to examine other ‘common’ excipients that may have unintended influence on bioavailability/bioequivalence.

The Relative Bioavailability and In Vivo-In Vitro Correlations for Four Marketed Carbamazepine Tablets

AbstractPurpose. To determine if three marketed generic carbamazepine tablets were bioequivalent to the innovator formulation, as well as to each other. In addition, to examine in vivo-in vitro relationships among the four formulations.nMethods. Each formulation was given as a single dose to 18 healthy male and female subjects using a crossover design. Blood samples were collected for 169 hr. Carbamazepine was assayed by HPLC with UV detection.nResults.In vivo fraction absorbed plots indicated that the three generic formulations were absorbed more rapidly than the innovator product, and the mean time of maximum plasma concentration was 6−7 hr sooner for the generic formulations. The mean maximum plasma concentration ranged from 17−19 percent higher for the generic products compared to the innovator, and the 90% confidence limits for Cmax data ranged from 111% to 126%. The mean AUC(0−∞) for the generic products ranged from 101−104% compared to the innovator, and the confidence limits for AUC ranged from 97−108%.nConclusions. The generic products were all more rapidly absorbed than the innovator, but simulations of steady-state concentrations indicated that it would be unlikely that these differences would have any significant clinical effect. An excellent association was seen between the Cmax and the percent of drug dissolved in vitro. The correlation was used to accurately predict the Cmax of four other 200 mg tablets evaluated in an earlier study.

The Effect of Gelatin Cross-Linking on the Bioequivalence of Hard and Soft Gelatin Acetaminophen Capsules

AbstractPurpose. To determine if changes in the in vitrodissolution of hard and soft gelatin acetaminophen capsules, which resultfrom gelatin crosslinking, are predictive of changes in the bioavailabilityof the capsules in humans.nMethods. Both hard and soft gelatin capsules were“stressed” by a controlled exposure to formaldehyde, resultingin unstressed, moderately stressed and highly stressed capsules. invitro dissolution studies were conducted using water or SGF with andwithout pepsin as the media. Separate 24-subject, 3-way crossover humanbioequivalence studies were performed on the unstressed and stressedacetaminophen capsules. Plasma acetaminophen was determined by highperformance liquid chromatography (HPLC) for 12 hr after each dose.nResults. The in vitro rate of dissolution of hardand soft gelatin capsules was decreased by crosslinking. The bioequivalencestudies showed that both hard and soft gelatin capsules, which failed tomeet the USP dissolution specification in water, but complied when tested inSGF containing pepsin, were bioequivalent to the unstressed controlcapsules. The capsules that were cross-linked to the greatest extent werenot bioequivalent to the unstressed control capsules, based on Cmax. Atrend toward an increase in Cmax with increased level of cross-linking wasobserved, but this was only significant for the severely stressedcapsules.nConclusions. On the basis of this study a two-tier invitro dissolution test was developed using enzymes to distinguishbetween bioequivalent and bioinequivalent gelatin capsules.

Carbamazepine level-A in vivo-in vitro correlation (IVIVC) : A scaled convolution based predictive approach

A method is presented for prediction of the systemic drug concentration profile from in vitro release/dissolution data for a drug formulation. The method is demonstrated using four different tablet formulations containing 200 mg carbamazepine (CZM), each administered in a four way cross‐over manner to 20 human subjects, with 15 blood samples drawn to determine the resulting concentration profile. Amount versus time dissolution data were obtained by a 75 rpm paddle method for each formulation. Differentiation, with respect to time, of a monotonic quadratic spline fitted to the dissolution data provided the dissolution rate curve. The dissolution curve was through time and magnitude scaling mapped into a drug concentration curve via a convolution by a single exponential, and the estimated unit impulse response function. The method was tested by cross‐validation, where the in vivo concentration profiles for each formulation were predicted based on correlation parameters determined from in vivo–in vitro data from the remaining three formulations. The mean prediction error (MPE), defined as the mean value of 100% x(observed−predicted)/observed was calculated for all 240 cross‐validation predictions. The mean values of MPE were in the range of 10–36% (average 22%) with standard deviations (S.D.s) in the range of 9–33% (average 13%), indicating a good prediction performance of the proposed in vivo–in vitro correlation (IVIVC) method. Copyright

Bioequivalence of Methylphenidate Immediate-Release Tablets Using a Replicated Study Design to Characterize Intrasubject Variability

AbstractPurpose. To determine the relative bioavailability of two marketed,immediate-release methylphenidate tablets. The study used a replicatedstudy design to characterize intrasubject variability, and determinebioequivalence using both average and individual bioequivalencecriteria.nMethods. A replicated crossover design was employed using 20subjects. Each subject received a single 20 mg dose of the reference tableton two occasions and two doses of the test tablet on two occasions.Blood samples were obtained for 10 hr after dosing, and plasma wasassayed for methylphenidate by GC/MS.nResults. The test product was more rapidly dissolved in vitro and morerapidly absorbed in vivo than the reference product. The mean Cmaxand AUC(0 − ∞) differed by 11% and 9%, respectively. Using anaverage bioequivalence criterion, the 90% confidence limits for theLn-transformed Cmax and AUC(0 − ∞), comparing the two replicatesof the test to the reference product, fell within the acceptable range of80–125%. Using an individual bioequivalence criterion the test productfailed to demonstrate equivalence in Cmax to the reference product.nConclusions. The test and reference tablets were bioequivalent usingan average bioequivalence criterion. The intrasubject variability of thegeneric product was greater and the subject-by-formulation interactionvariance was borderline high. For these reasons, the test tablets werenot individually bioequivalent to the reference tablets.

Bioavailability of morphine after administration of a new bioadhesive buccal tablet

A new bioadhesive buccal morphine tablet was developed for controlled release delivery of drug and improved bioavailability compared with oral controlled release tablet. In order to characterize the pharmacokinetic properties of this bioadhesive buccal formulation, a bioavailability study was performed in 12 healthy volunteers who received: a 30 mg oral controlled release tablet (A); a 20 mg aqueous solution retained in the mouth for 10 min (B); and the 60 mg bioadhesive buccal tablet placed between the lower gum and lip for 6 h (C). The mean amount of morphine absorbed from the solution was very low, only 2 mg of the 20 mg dose. After administration of forms A and C, plasma levels exhibit typical sustained release concentration–time curves. The mean amount of drug recovered from the residual bioadhesive buccal tablet after 6 h indicated that approximately 50% of the dose was released from the bioadhesive buccal tablet. The relative bioavailability of the buccal tablet (corrected for residual unabsorbed dose) compared with the controlled‐release tablet was 98% based on the morphine AUC values. Good correlations between the AUC and the Cmax of the bioadhesive tablet for the drug and metabolite plotted versus the amount of morphine absorbed were found.

Variability in the Bioavailability of Phenytoin Capsules in Males and Females

AbstractPurpose. To determine inter-lot and intra-subject variability in the bioavailability of the 100 mg extended phenytoin sodium capsules. In addition, to determine the effect of gender and menstrual cycle on phenytoin bioavailability.nMethods. Three different lots of extended phenytoin sodium capsules were given to 12 healthy male and 12 healthy female subjects in a crossover fashion. One of the lots was also given a second time to each subject. Plasma phenytoin was determined, using an HPLC assay, in samples collected over a 73-hr period after each dose.nResults. The mean Cmax for the four administrations ranged from 1.71-1.79 μg/ml and mean AUC(0-∞) values from ranged 53.0-54.1 μg*hr/ml. The elimination half-life was 3 hr shorter, and the AUC(0-∞) adjusted for the mg/kg dose was 30% lower for females. Average bioequivalence was demonstrated between the three lots for both Cmax and AUC(0-∞) based on the BE limit of 80-125%. Further, all confidence intervals of AUC(0-∞) fell within the limit of 90-111%. There were no differences in the confidence limits for Cmax and AUC(0-∞) determined separately for males and females. Also, there was no difference in the mean Cmax or AUC(0-∞) for females when analyzed as a function of the week of their menstrual cycle. Individual bioequivalence was demonstrated between three lots of phenytoin using the constant-scaled method, but not the reference-scaled method.nConclusions. There was very little difference in the bioavailability of the three lots of phenytoin. Females exhibited a lower AUC(0-∞) than males after adjustment of dose for body weight, but their inclusion in the study did not affect the assessment of bioequivalence. When dose was not adjusted for body weight, no difference in AUC(0-∞) was seen between males and females.