Evaluation of a limited sampling method used to determine the bioequivalence of highly variable drugs with long half-lives.

The usefulness of a limited sampling method (LSM) to determine the bioequivalence of highly variable drugs with long half-lives was investigated. The LSM uses multiple linear regression of observed drug plasma concentrations versus area under the curve (AUC) or C(max) (peak plasma concentration) to obtain a best set of coefficients, concentration times and intercept based upon the regression coefficient, R(2), to predict the selected pharmacokinetic parameter (i.e. AUC or C(max)). The LSM, used successfully in clinical settings, has also been suggested for data analysis of in vivo bioequivalence studies. Because the method has not yet been thoroughly tested under many conditions likely to be encountered in bioequivalence studies, a further investigation of the method's applicability to bioequivalence determination was undertaken. In the present study, training and test data sets incorporating various levels of intrasubject variability in clearance (CL) with different ratios for fraction absorbed (Fa) of test and reference drug formulations were used to further evaluate the applicability of the limited sampling method (LSM) to the evaluation of bioequivalence for drugs with long half-lives of elimination. Both simulated (a one-compartment pharmacokinetic (PK) model with first-order elimination) and experimental data were used in the study. The results indicated that the determination of bioequivalence using the LSM was significantly influenced by the ratio of Fa(test)/Fa(reference) and by the level of intrasubject error (variability) in CL. Therefore, use of the LSM to determine bioequivalence of drugs with long half-lives and highly variable in CL seems suitable only for formulations that have point estimates of Fa(test)/Fa(reference) within the range of 0.9-1.10. Because the Fa ratio range would have to be verified through use of observed data obtained from a pilot study, the practical utility of the LSM in the determination of bioequivalence would be severely limited.     

The bioequivalence of highly variable drugs and drug products

'Highly variable drugs' have been defined as those drugs for which the within-subject variability (WSV) equals or exceeds 30% of the maximum concentration (Cmax) and/or the area under the concentration versus time curve (AUC). Despite the fact that highly variable drugs are generally safe with flat dose response curves, the bioequivalence of their formulations is a problem because the high variability means that large numbers of subjects are required to give adequate statistical power. Highly variable drug products are poor quality formulations where high within-formulation variability (e.g. tablet to tablet variability) poses a problem rather than high innate WSV of the drug itself. A further problem caused by high variability is that a subset of the population may respond differently to the two formulations producing a significant subject x formulation interaction. Practical examples are shown using replicate designs. The methods proposed to deal with the problems posed by highly variable drugs include: (i) Drug regulatory jurisdictions states that the 90% confidence interval (90% CI) around the test to reference geometric mean ratio (GMR) is required to fit with bioequivalence acceptance limits of 0.8 - 1.25 for both Cmax and AUC. The WSV for single point estimation of Cmax is often greater than that for AUC. One strategy therefore is not to require a 90% CI for Cmax of drugs that do not exhibit a toxicity associated with Cmax and merely require the GMR to fall within the acceptance limits. (ii) To arbitrarily broaden the bioequivalence acceptance limits. For example, to permit a sponsor to justify the use of wider limits e.g the 90% CI around the GMR of Cmax values might be required to fit within acceptance limits of 0.75 - 1.33 or even 0.70 - 1.42. (iii) A more systematic approach would be to broaden the acceptance limits by scaling to either the residual variance from a 2-period design or to the WSV of the reference product in a replicate design. Subsequent evaluations of scaling procedures have demonstrated that smaller numbers of subjects are required for bioequivalence studies on formulations of highly variable drugs. A disadvantage of scaling is that the method is less sensitive to differences between the means compared with unscaled treatment, such that the GMR may prove to be unacceptably low or high. This possibility has let to a suggestion that the GMR must fall within acceptance limits of 0.8 - 1.25 in scaled treatments. (iv) A similar method is to scale the metric rather than the acceptance limits. This method was proposed by the United States' Food and Drug Administration in the context of Individual bioequivalence, but may also be applied (v) to average bioequivalence. (vi) To carry out bioequivalence studies at steady state whenever a multiple dose regimen is ethically acceptable for healthy volunteers. This solution is based on the observation that high variability in a single dose study tends to be dampened at steady state, thus increasing statistical power. Drug regulators have not favored this approach on the grounds that bioequivalence testing should be based on the most discriminating test possible. (vii) Finally the use of metabolite data has been proposed since in many (but by no means all) cases, metabolite is less highly variable than that of the parent drug. This subject remains controversial except when the administered substance is a prodrug which converted by metabolism into the active drug.     

Population and individual bioequivalence: lessons from real data and simulation studies

The Food and Drug Administration (FDA) has proposed replacing the 1992 average bioequivalence (ABE) with population and individual bioequivalence (PBE & IBE), as outlined in the preliminary draft guidance of December 1997, which was subsequently replaced by the draft guidances of August 1999 and resolved in the final guidance of October 2000. This has led to considerable public debate among regulatory, academic, and industry experts at numerous conferences (e.g., FDA/AAPS March 1998, FDA/AAPS August-September 1999, FDA Pharmaceutical Sciences Advisory Committee September 1999) and in the literature. The final guidance calls for ABE to remain as the primary criterion by which new formulations may be judged ready for access to the marketplace. In addition, the FDA recommends the use of replicate study designs for the specific drug classes of controlled-release formulations and highly variable drugs. The final guidance also alludes to the possibility of a sponsor requesting alternative criteria such as PBE and IBE following consultation with the FDA. This procedure amounts to a data collection period during which data suitable to evaluate the operating characteristics of PBE and IBE would be generated, analyzed, and discussed among interested parties. A comprehensive review of currently available databases is useful in determining the ultimate value of this data collection period. This report provides an update to the previous publication by the authors. In all, 28 data sets from 20 replicate cross-over bioequivalence studies have been analyzed (n = 12-96) using the statistical methodology in the most recent FDA draft guidance. The results are presented below. ABE Pass: ABE Fail: Total: AUC/Cmax AUC/Cmax AUC/Cmax AUC/Cmax Pass PBE & IBE 20/14 1/3 21/17 Pass IBE only 1/0 0/0 1/0 Fail PBE and IBE 0/2 0/1 0/3 Fail IBE only 2/3 4/5 6/8 Total 23/19 5/9 28/28 Review of the database reveals many interesting features, most notably the lack of consistent results within a given data set across all three criteria. The sensitivity of subject-by-formulation interaction to sample size and inherent variability of the compounds is further explored through simulation studies. It is concluded that additional simulation assessments must be considered when evaluating the value of a data collection period for PBE and IBE assessment. It will be shown that definitive conclusions regarding some of the operating characteristics of PBE and IBE can be achieved through a combination of data-driven hypotheses followed by simulation studies to further evaluate the hypotheses. Some recommendations for further data collection will be made.     

Impact of truncated area under the curve on failed bioequivalence studies: a computer simulation analysis

The common measures used in a bioequivalence study are area under the curve (AUC) and the maximum plasma concentration. Estimation of AUC requires frequent blood samples. For long half-life drugs, sampling for long periods of time may become cumbersome. To resolve this issue some investigators have suggested the use of truncated AUC in bioequivalence studies for long half-life drugs. The suggested length of time for the truncated AUC is 72 hours. Many studies have been conducted to show that truncated AUC till 72 hours is a suitable approach. However, the suitability of truncated AUC for failed bioequivalence study has not been demonstrated. This report of simulated plasma concentration versus time data evaluates the suitability of truncated AUC for failed bioequivalence study of two hypothetical drugs. The results of the study indicate that the truncated approach for the estimation of the AUC for long half-life drugs in bioequivalence studies may be useful but it also increases the probability of accepting drugs as being bioequivalent when they are not.     

Carbamazepine: a bioequivalence study and limited sampling modeling

OBJECTIVES: To assess the bioequivalence of 2 formulations of carbamazepine and to develop and validate limited sampling strategy (LSS) models for estimating the area under the plasma concentration-time curve (AUC0-infinity) and the peak plasma concentration (Cmax) of carbamazepine. METHODS: Twenty-four (12 men, 12 women) healthy volunteers received single oral doses (400 mg) of carbamazepine, as reference and test conventional-release formulations, in a standard 2-sequence, 2-period crossover design. Bioequivalence assessment was based on the individual ratios of log-transformed values of AUC0-infinity and Cmax LSS modeling was developed in a training set of 12 randomly assigned volunteers and was validated on the other 12 subjects (validation set). RESULTS: Carbamazepine AUC0-infinity and Cmax can be accurately predicted (R2 = 0.89 - 0.95, precision = 2.6 - 7.2%) by single-point (72 h) and 2-point LSS models (6, 32 h), respectively. Bioequivalence assessments based on LSS-derived AUC0-infinity and Cmax provided results similar to those obtained using all the concentration-in-plasma data points, and indicated that the 2 formulations are bioequivalent. CONCLUSION: One-and 2-point LSS models provided accurate estimates of carbamazepine's AUC0-infinity and Cmax, and allowed correct assessment of bioequivalence between the formulations studied.     

Estimated coefficient of variation values for sample size planning in bioequivalence studies

OBJECTIVE: The aim of the present communication is to provide information regarding the intrasubject coefficent of variation obtained from 30 bioequivalence studies covering 16 drugs which can be used for estimation of sample size. Additionally, an attempt was also made to estimate the test power of each of the studies conducted. METHODS: The intrasubject coefficient of variation was estimated from the residual mean square error obtained from analysis of variance of the parameters AUC0-infinity, Cmax and Cmax/AUC0-infinity after logarithmic transformation. The test power in the analyses of the above parameters was subsequently estimated using nomograms provided by Diletti et al. [1991]. RESULTS AND CONCLUSION: Thirty products covering 16 drugs were studied in which 22 were immediate-release (including one dispersible tablet) and 8 were sustained-release formulations. The intrasubject coefficient of variation for the parameter AUC0-infinity was smaller than Cmax, and hence considerably more studies were able to attain a power of greater than 80% using 12 volunteers for the AUC0-infinity, compared to the Cmax. However, the variability in the Cmax could be reduced by using the parameter Cmax/ AUC0-infinity, and thus, provide a more realistic estimation of sample size, since the latter reflects only the rate of absorption and not both the rate and extent as in the case of Cmax [Endrenyi et al. 1991].     

Limited sampling model for the estimation of pharmacokinetic parameters in children

A limited sampling model (LSM) is proposed for the first-time assessment of pharmacokinetic parameters (area under the concentration-time curve (AUC), Cmax, and T1/2) in children after a single oral dose of drug. Three drugs were evaluated in this study. The LSM was developed for each drug from the data of 10 healthy adult volunteers. The relationship at selected time points between plasma concentration and the AUC or Cmax was evaluated by multiple linear regression. The multiple linear regression that gave the best correlation coefficient (r) for 3 sampling times versus AUC or Cmax was chosen as the LSM. Pharmacokinetic parameters generated using sparse sampling (3 blood samples) were compared with pharmacokinetic parameters generated using extensive sampling (>7 blood samples). The results indicated that a limited sampling model can be developed from adult data to estimate pharmacokinetic parameters in children with fair degree of accuracy.     

Comparative bioavailability of two immediate-release tablets of lisinopril/hydrochlorothiazide in healthy volunteers

AIM: Two formulations of lisinopril/hydrochlorothiazide (20 mg/12.5 mg) were evaluated for bioequivalence after single dosing in healthy volunteers. METHODS: The study was conducted according to an open, randomized, 2-period crossover design with a 2-week washout interval between doses. Twenty-four volunteers participated and all completed the study successfully. Lisinopril and hydrochlorothiazide were determined in plasma by HPLC. The pharmacokinetic parameters AUC(0-t), AUC(0-infinity), Cmax and Cmax/AUC(0-infinity) were tested for bioequivalence after logarithmic transformation of data and ratios of tmax were evaluated non-parametrically. RESULTS: For lisinopril, the parametric analysis revealed the following test/reference ratios and their confidence intervals (90% CI): 1.01 (0.84-1.22) for AUC(0-t), 0.98 (0.81-1.19) for AUC(0-infinity), 1.02 (0.83-1.25) for Cmax and 1.03 (0.99-1.08) for Cmax/AUC(0-infinity). The 90% CI for tmax was 0.94-1.07. All parameters showed bioequivalence between both formulations. As for hydrochlorothiazide, test/reference ratios and their confidence intervals (90% CI) were: 1.05 (0.95-1.17), 1.02 (0.93-1.12) for AUC(0-infinity), 0.99 (0.89-1.07) for Cmax and 0.97 (0.90-1.04) for Cmax/AUC(0-infinity). The 90% CI for tmax was 1.00-1.41. All parameters showed bioequivalence between both formulations except for tmax. A discrete fall in both systolic (SBP) and diastolic (DBP) blood pressure was observed after drug administration. The time course of both parameters was similar for the 2 formulations. Heart rates also followed a similar time profile. CONCLUSIONS: The bioequivalence of the 2 formulations of lisinopril/hydrochlorothiazide was demonstrated.     

Relative bioavailability study of two nifedipine tablet formulations in healthy male volunteers

OBJECTIVE: To assess the bioequivalence of two oral formulations containing 10 mg of nifedipine. The test preparation were Macorel tablets, the reference preparation were Adalat tablets. SUBJECTS, MATERIAL AND METHODS: The study was designed as a single-dose, three-period crossover randomized design to 18 non-smoker, healthy male volunteers under fasting conditions. Seventeen volunteers completed the study. Plasma samples were analyzed for nifedipine by HPLC after solid-phase extraction. The pharmacokinetic parameters used to assess the bioequivalence of the two formulations were AUC(0-infinite) and AUC(0-t) for the extent of absorption and Cmax and Tmax for the rate of absorption. Statistical comparisons of AUC(0-infinite) AUC(0-t), and Cmax data were evaluated after logarithmic transformation by two-way analysis of variance (ANOVA), and differences of Tmax were tested non-parametricaly. RESULTS: Point estimates (90% confidence intervals) of the test/reference ratios were 97.4% (87.6%-108.3%) for AUC(0-infinite) 97.0% (85.6%-110.1%) for AUC0-t, and 107.7% (89.1%-130.7%) for Cmax. No statistically significant difference was found for Tmax and elimination half-life values. CONCLUSION: Therefore, in accordance with the European Union bioequivalence requirements, the test and reference nifedipine preparations are bioequivalent for both the extent and the rate of absorption.     

Comparison of the Bayesian approach and a limited sampling model for the estimation of AUC and Cmax: a computer simulation analysis.

OBJECTIVES: To compare two limited sampling methods (Bayesian and the limited sampling model) for the estimation of AUC and Cmax following a single oral dose of a hypothetical drug. METHODS: The plasma concentration vs time data sets for 50 subjects using a linear one- or two-compartment pharmacokinetic model were generated by simulation. The limited sampling model (LSM) was developed using samples from 10 subjects using one or two time points. The simulated plasma concentrations were also used for Bayesian evaluation. Bayesian analysis was performed on Non-Mem and mean pharmacokinetic parameters used for simulation were assumed as population pharmacokinetic parameters. In addition a test drug was also used to compare the predicted AUC and Cmax for the two approaches. RESULTS: Both methods were validated in 40 subjects for the hypothetical drug and in 12 subjects for the test drug. Both methods provided good estimates of AUC and Cmax. CONCLUSION: The results indicate that the LSM is similar to the Bayesian method and may be used in lieu of the Bayesian approach in estimating AUC and Cmax using one or two samples in clinical settings without detailed pharmacokinetic studies.     

The importance of sample size,log-mean ratios,and intrasubject variability in the acceptance criteria of 108 bioequivalence studies

PURPOSE: Fulfilling bioequivalence criteria with highly variable drugs is difficult. The aim of this study was to compare the importance of sample size, intrasubject variability, and the point estimate of test and reference formulations with regard to meeting bioequivalence (BE) criteria [maximum observed plasma concentration (C(max)) and area under the concentration-time curve (AUC)]. METHODS: We compared 137 pairs of data from BE studies with a conventional number of subjects, approximately 31-32 volunteers, developed in the last 10 years. RESULTS: The third part of the studies failed to demonstrate BE, in part due to an unacceptable difference between the mean ratios (T/R) (18) but also due to high variability with small differences between formulations (17). Increasing the number of subjects is hard to justify, and expanding the confidence interval (CI) was insufficient for the most highly variable drugs. CONCLUSIONS: Therefore, for low-variable drugs, the difference between formulations was the cornerstone of the fulfillment of BE criteria, but for highly variable drugs, the intrasubject coefficient of variability (ICV) was decisive. Our proposal is that for highly variable drugs that fall outside BE 90% CI limits could result in BE in the absence of formulation effect and maximal differences between formulations below 20%.     

Non-traditional study designs to demonstrate average bioequivalence for highly variable drug products

OBJECTIVE: To demonstrate average bioequivalence, the ninety-percent confidence intervals (CI) on the ratio of geometric means for area under the concentration-time curve (AUC) and maximum observed plasma concentration (Cmax) must lie within 0.80-1.25. Demonstration of average bioequivalence (ABE) for highly variable drug products requires large numbers of subjects in a standard, adequately powered, two-period crossover. METHODS: Application of non-traditional study designs can help to meet this hurdle. Study design and analysis for replicate and group sequential-replicate study designs are presented and illustrated using examples. It is demonstrated how to use such approaches to meet the difficult regulatory hurdle of average bioequivalence for a highly variable drug product. RESULTS: To illustrate, data are provided from three separate ABE studies for a highly variable drug product at three dosage strengths. In all three studies, a replicate study design was used to compensate for high intrasubject variation. Additionally, for the last study, a group sequential study design was imposed to provide early evidence of conclusive results. CONCLUSION: Replicate designs and group-sequential designs in bioequivalence should be used to demonstrate average bioequivalence for highly variable drug products or when uncertain of true intrasubject variability in order to ensure conclusive study results.     

Bioequivalence of two oral ciprofloxacin tablets formulations

The relative bioavailability of a new 750 mg tablet formulation of ciprofloxacin (test formulation supplied by Dr. August Wolff GmbH and Co., Germany) was compared with that of Ciprobay tablets 750 mg (reference formulation from Bayer Vital GmbH and Co., Germany). Twenty-four healthy volunteers (12 male and 12 female) were included in this single-dose, 2-sequence, crossover randomized study. Blood samples were obtained prior to dosing and at 0.5, 0.75, 1, 1.5, 2, 3, 4, 6, 8, 10, 12, 18, 24 and 30 hours after drug administration. Plasma concentrations of ciprofloxacin were determined by HPLC. No differences were found when the in vitro dissolution profiles of both formulations were compared. The pharmacokinetic parameters AUC(0-t), AUC(0-infinity), Cmax and Cmax/AUC(0-infinity) were tested for bioequivalence after log-transformation of data, and ratios of tmax were evaluated nonparametrically. The parametric analysis revealed the following mean values for the test/reference ratios (90% standard confidence intervals in parenthesis (ln-transformed data): 1.01 (0.95-1.07) for AUC(0-t), 0.99 (0.93-1.05) for AUC(0-infinity), 1.05 (0.97-1.14) for Cmax and 1.06 (0.97-1.15) for Cmax/AUC(0-infinity). The nonparametric confidence interval for tmax was 0.77-1.15. All parameters showed bioequivalence between both formulations as their confidence intervals were within the bioequivalence acceptable range of 0.80-1.25 limits; the 90% confidence interval for tmax slightly exceeded limits of bioequivalence. We conclude that both formulations show bioequivalence for both the rate and the extent of absorption.     

The role of metabolites in a bioequivalence study II: amoxapine, 7-hydroxyamoxapine, and 8-hydroxyamoxapine.

BACKGROUND: Amoxapine is a dibenzoxazepine type tricyclic antidepressant. The mechanism of clinical action in patients is not well understood. In animals, amoxapine blocks the reuptake of norepinephrine and, to a lesser extent serotonin, into their respective neurons and blocks the response of dopaminergic receptors to dopamine. The major metabolite, 8-hydroxyamoxapine, has similar norepinephrine uptake inhibitory action to the parent drug, but has a more pronounced inhibitory action on serotonin uptake. Another major metabolite, 7-hydroxyamoxapine blocks post-synaptic dopamine receptors. SUBJECTS AND METHODS: The present study was a traditional two-treatment, two-period, two-sequence crossover design with the primary objective to investigate the average bioequivalence of two formulations of amoxapine. Secondary objectives were to explore the potential roles of metabolites and truncated (partial) areas in bioequivalence studies. Serial plasma samples were harvested from immediately pre dose to 96 hours post dose. The parent drug and the two hydroxy metabolites were monitored by validated HPLC procedures. Geometric mean ratios and 90% confidence intervals (90% CIs) were calculated for Cmax, AUCinfinity, the truncated areas of AUC, AUCinfinity/Cmax, and the truncated areas of AUC/Cmax. RESULTS: The results indicated that the two formulations were bioequivalent in terms of the conventional parameters Cmax and AUC for all three analytes in the sense that the 90% CIs fitted entirely within preset bioequivalence limits of 80-125%. Moreover, the 90% CIs for the truncated areas AUC2.0hr through AUClast and Cmax/AUC1.0hr through Cmax/AUClast of all three analytes also fell entirely within bioequivalence limits of 80-125%. It was concluded that it was unnecessary to have harvested plasma samples beyond 12 hours, in which case additional plasma samples could have been devoted to the more precise estimation of tmax and Cmax. CONCLUSION: Of the three analytes, test and reference individual plasma concentration versus time curves of 8-hydroxyamoxapine were more closely superimposable than those of the other two analytes. Any decision to use metabolite data in bioequivalence studies, however, must be made a priori to avoid introduction of bias arising from selective post hoc manipulation of the raw data; and to facilitate the design of blood sampling schedules based on prior information about the tmax of the selected analyte.     

In vitro and in vivo equivalence of two oral atenolol tablet formulations

A randomised, cross-over, open study of bioequivalence between two different atenolol (CAS 29122-68-7) tablet formulations is presented. An in vitro comparative study between the two formulations was also performed. Both products meet the USP 23 (United States Pharmacopea) specification. The values of similarity factor (f2) and difference factor (f1) obtained ensure sameness or equivalence of the two dissolution curves. Twenty-four healthy volunteers (male/female) participated in the bioequivalence study. Each treatment was given as a single 100-mg tablet following an overnight fast. Atenolol concentrations in plasma were determined up to 30 h after treatment by HPLC. The pharmacokinetic parameters AUC0-infinity, Cmax and Cmax/AUC0-infinity were tested for bioequivalence after logarithmic transformation of data and ratios of tmax were evaluated nonparametrically. The parametric analysis revealed the following test/reference ratios and their 90% confidence intervals (90% CI): 1.06 (0.99-1.13) for AUC, 1.07 (0.97-1.18) for Cmax, and 0.99 (0.94-1.07) for Cmax/AUC0-infinity. The 90% CI for tmax was 0.91-1.23. All parameters showed bioequivalence between both formulations. A discrete fall in both systolic (SBP) and diastolic (DBP) blood pressure was observed after the drug administration. The fall extent (approximately 11 mmHg in supine position) and the time course of both parameters after the drug administration was similar for both formulations. Minimal values for SBP and DBP were achieved at 6 h after the drug administration for both formulations. Heart rates were also reduced after the administration of both formulations of atenolol in a similar extent (12 b.p.m.) and following a similar time profile (i.e. maximal reductions were observed between 1 and 3 h after the drug administration). It can be concluded that both formulations are equivalent in vitro and in vivo.     

In vivo pharmacokinetic-pharmacodynamic relationship and in vitro equivalence of two oral furosemide tablet formulations

A randomized, cross-over, open study of bioequivalence between two different furosemide (CAS 54-31-9) formulations was performed; simultaneously, diuretic effects (urine output, sodium, potassium and chloride excretion) were also compared. Both products meet the British Pharmacopoeia specification and the results of a previous in vitro comparative study ensure equivalence of the two dissolution curves. Twenty-four healthy volunteers (male/female) participated in the bioequivalence study. Each treatment was given as a single 40-mg tablet following an overnight fast. Furosemide concentrations in plasma (measured by HPLC) and electrolyte amounts in urine were determined up to 12 h after treatment. The pharmacokinetic parameters AUC0-infinity, Cmax and Cmax/AUC0-infinity were tested for bioequivalence after ln-transformation of data and ratios of tmax were evaluated nonparametrically. The parametric analysis revealed the following test/reference ratios and their 90% confidence intervals (90% CI): 1.06 (0.94-1.19) for AUC0-infinity, 1.12 (0.96-1.31) for Cmax, and 1.06 (0.97-1.16) for Cmax/AUC0-infinity. The 90% CI for tmax was 0.55-1.00. Bioequivalence between both formulations was concluded for all parameters except for tmax. No significant diuretic differences between both formulations (test and reference) were observed after drug administration in relation to the baseline period. Systolic and diastolic blood pressure and heart rate showed a similar time-course after the drug administration and there were no differences between both formulations. Both products were well tolerated. It can be concluded that both formulations are equivalent in vitro and in vivo.     

Evaluation of Bioequivalence of Highly Variable Drugs Using Clinical Trial Simulations. II: Comparison of Single and Multiple-Dose Trials Using AUC and Cmax

Purpose. Evaluating of the effects of high intrasubject variability in clearance (CL) and volume of distribution (V), on 90% confidence intervals (CIs) for AUC (Area Under the concentration Curve) in single and multiple-dose bioequivalence studies. The main methodology was Monte Carlo simulation, and we also used deterministic simulation, and examination of clinical trials. The results are compared with those previously observed for Cmax (maximum concentration.) Methods. The time course of drug concentration in plasma was simulated using a one-compartment model with log-normal statistical distributions of intersubject and intrasubject variabilities in the pharmacokinetic parameters. Both immediate-release and prolonged-release products were simulated using several levels of intrasubject variability in single-dose and multiple-dose studies. Simulations of 2000 clinical bioequivalence trials per condition (138 conditions) with 30 subjects in each crossover trial were carried out. Simulated data were compared with data from actual bioequivalence trials. Results. The current simulations for AUC show similar probabilities of failure for single-dose and multiple-dose bioequivalence studies, even with differences in the rate of absorption or fraction absorbed. AUC values from prolonged-release scenario studies are more sensitive to changes in the first order absorption rate constant ka, and to variability in CL and V than AUC from studies of immediate-release studies. Conclusions. We showed that multiple-dose designs for highly variable drugs do not always reduce intrasubject variability in either AUC or Cmax, although the behavior of AUC differs from Cmax. Single dose AUC to the last quantifiable concentration was more reliable than either single dose AUC extrapolated to infinity, or multiple dose AUC during a steady-state interval. Multiple-dose designs may not be the best solution for assessing bioequivalence of highly variable drugs.     

Evaluation of bioequivalence of isoniazid and pyrazinamide in three and four drugs fixed dose combinations using WHO simplified protocol.

The reliable supply of quality drugs in the form of fixed dose combination (FDC) is an essential part of tuberculosis treatment. The objective of this investigation was to evaluate whether the World Health Organization (WHO) simplified screening protocol for the bioequivalence assessment of rifampicin can be used for the evaluation of other components of FDC so as to ensure the bioavailability of all drugs at tissue site. These bioequivalence studies were conducted on 20 and 22 healthy male volunteers for evaluation of three and four drugs FDC formulations, respectively. Both studies were conducted as randomized, open, crossover trials and sampling schedule was upto 8h according to WHO recommended protocol for evaluation of rifampicin bioequivalence. The bioequivalence of isoniazid and pyrazinamide were estimated using AUC(0-8), AUC(0-alpha), and C(max). FDC formulation was considered bioequivalent to separate formulations for isoniazid and pyrazinamide if bioequivalence limit fall in between 0.80 and 1.25. Bioequivalence estimates of AUC(0-8) and AUC(0-alpha) for isoniazid and all the three pharmacokinetic measures of pyrazinamide were within the acceptable limits, whereas C(max) of isoniazid from four drugs FDC was outside the limit when evaluated by two-way ANOVA. After evaluation of isoniazid and pyrazinamide based on their pharmacokinetics, it was found that C(max) is being affected by limited sampling time points of WHO protocol. Further, AUC was a robust parameter unaffected by sampling schedule adopted. The WHO simplified protocol for assessment of rifampicin is also suitable for evaluating bioequivalence of isoniazid and pyrazinamide from FDC formulations. However, for comparison of rate of absorption by means of C(max), careful evaluation of concentration-time profile along with pharmacokinetics is necessary before final judgment.     

Bioequivalence study of two limaprost alfadex 5 microg tablets in healthy subjects: moisture-resistant tablet (dextran formulation) versus standard tablet (lactose formulation)

OBJECTIVE: A study was conducted to assess the bioequivalence of two limaprost alfadex 5 microg tablets, a moisture-resistant tablet (dextran formulation) and a standard tablet (lactose formulation). MATERIALS AND METHODS: The clinical investigation was designed as a randomized, open-labeled, two-part, two-treatment, two-period crossover study, in 120 healthy male volunteers. One tablet of either formulation was administered with 200 ml of water after 10-hour overnight fast. After dosing, serial blood samples were collected for a period of 6 hours. Plasma harvested from blood was analyzed for limaprost by a validated LC/MS/MS method. The peak plasma concentration (Cmax) values and time associated with the maximal concentration (tmax) were obtained from the observed data. The elimination rate constant (lambda z) was obtained as the slope of the linear regression of the log-transformed concentration values vs. time data in the terminal phase, and the elimination half-life (t1/2) was calculated as 0.693/lambda z. The area under the curve to the last measurable point (AUC0-t) was estimated by the linear trapezoidal rule. The analysis of variance (ANOVA) was carried out using log-transformed AUC0-t, AUC0-A yen and Cmax and untransformed tmax, and 90% confidence intervals for AUC0-t and Cmax were calculated. If the 90% confidence intervals (CI) for both AUC0-t and Cmax fell fully within the interval 80 - 125%, the bioequivalence of the two formulations was established. RESULTS: The means of AUC0-t were 0.779 vs. 0.754 pg x h/ml (test vs. reference), and the means of the Cmax were 1.26 vs. 1.12 pg/ml (test vs. reference). The geometric mean ratios of the test formulation to reference formulation for AUC0-t and Cmax were 104.0 and 112.4%, respectively, and the 90% CI for AUC0-t and Cmax were 100.7 - 107.4% and 105.6 - 119.6%, respectively. Both 90% CI for AUC0-t and Cmax fell within the Ministry of Health, Labour and Welfare of Japan accepted bioequivalence range of 80 - 125%. CONCLUSIONS: Based on the results, the moisture-resistant tablet was determined to be bioequivalent to the standard tablet.     

A perspective for biowaivers of human bioequivalence studies on the basis of the combination of the ratio of AUC to the dose and the biopharmaceutics classification system

The ratio of AUC to the dose (AUC/dose) was previously found as a parameter that predicts a risk of bioinequivalence of oral drug products. On the basis of the combination of this parameter and the biopharmaceutics classification system (BCS), a perspective for biowaivers of human bioequivalence studies is discussed. Databases of bioequivalence studies using immediate-release solid oral dosage forms were disclosed by 6 Japanese generic pharmaceutical companies, and the number of subjects required for demonstrating bioequivalence between generic and reference products was plotted as a function of AUC/dose for each BCS category. A small variation in the number of subjects was constantly observed in bioequivalence studies using dosage forms containing an identical BCS class 1 or class 3 drug, even though formulations of the generic product differ between companies. The variation was extremely enlarged when the drugs were substituted with BCS class 2 drugs. Rate-determining steps in oral absorption of highly water-soluble BCS class 1 and class 3 drugs are independent of formulations when there is no significant difference in the in vitro dissolution profiles between formulations. The small variation observed for both BCS categories indicates that the number of subjects converges into one value for each drug. Our analysis indicates the appropriateness of biowaiver of bioequivalence studies for immediate-release solid oral dosage forms containing not only BCS class 1 drugs but also class 3 drugs.