Bioequivalence of methylphenidate immediate-release tablets using a replicated study design to characterize intrasubject variability

Within-subject pharmacokinetic variability has a strong influence on individual exposure ratios in bioequivalence studies, hence on drug formulation interchangeability

INTRODUCTION

Bioequivalence between a reference and a generic drug is based on the hypothesis that a ± 20% change in blood exposure (or ± 10% for drugs with narrow therapeutic index, NTI) following the generic/reference switch will not have any therapeutic consequences. However, the individual exposure ratio between generic and reference can be higher than 1.20 (or 1.10). This study aims to analyse the different parameters influencing the individual exposure ratio, hence the conditions for reference/generic interchangeability.

METHODS

Bioequivalence studies with a double cross-over design for a virtual drug were simulated using 100 random sets of 12, 24, 48 or 100 pairs of areas under the curve (AUC), varying the generic/reference AUC geometric mean ratios between 0.80 and 1.25 and the within-subject exposure variance of the reference and the generic formulations.

RESULTS

The proportion of subjects with an exposure generic/reference ratio outside the ± 10% or ± 20% acceptance intervals increases when (1) the reference within-subject variance increases; (2) the ratio of the generic within-subject variance on the reference within-subject variance increases; and (3) the generic/reference mean AUC ratio diverges from 1.0. When only considering replicated administrations of the reference, the individual exposure ratio increases with the within-subject variance, yielding values outside the usually accepted individual exposure ratio range of 0.5 to 2 for drugs with narrow therapeutic index as soon as the within-subject variance standard deviation is ≥ 0.25 (equivalent to within-patient CV% > 25%).

CONCLUSIONS

Interchangeability between reference and generic formulations, especially for drugs with narrow therapeutic index can only be assumed if, the within-subject variance of generic is less or equal to the within-subject variance of reference or, if this is not the case, if the distribution of the generic/generic individual exposure ratios is included within the therapeutic margins of the reference drug.

Interchangeability between Generic and Reference Products: Limits of Average Bioequivalence Methodology

Marketing authorisation of generic drugs is based on a demonstration of the "average" bioequivalence (ABE), with acceptance limits of 0.8-1.25 for the 90% confidence interval (CI) of the ratio (generic versus reference) of geometric means of exposure (whole blood, serum or plasma drug concentration). However, when interchangeability of reference by one generic is considered during treatment of a given patient, such methodology cannot guarantee the lack of therapeutic impact especially for drugs with narrow therapeutic index. This review article describes the basis and limits of ABE methodology, and the adaptations that have been proposed by regulatory agencies. For highly variable drugs, given their large therapeutic margin, regulatory agencies even allow widening of the bioequivalence acceptance limits. For drugs with a narrow therapeutic index, the average bioequivalence methodology has been amended differently by regulatory agencies. The European Medicine Agency only requires the narrowing of the ABE acceptance limits to the 0.9-1.10 range. The US Food and Drug Administration (FDA) has proposed to narrow the ABE acceptance limits according to the reference within-subject variance. The FDA requires a fully replicate cross-over study (with four periods) which allows one to compare the within-subject variance between generic and reference drug, and to detect any subject-by-formulation interaction. Indeed, any within-subject variance difference or subject by formulation interaction is an obstacle to interchangeability at the individual level. These amendments for the ABE do not fundamentally change the fact that individual ratios of exposure (generic/reference) will vary to a larger extent than the ratio of their means. For these reasons, since true individual bioequivalence studies cannot be performed for practical reasons and statistical issues, one can suggest that, in addition to the usual average bioequivalence criteria, the limits of the 95% confidence interval of the individual generic/reference exposure ratios could be used to allow interchangeability during treatment (at least for narrow therapeutic index drugs). Limit values of such CI for interchangeability acceptance should be scaled to the therapeutic margin of the reference drug. Regulatory agencies could conduct calculations based on real datasets of bioequivalence studies to determine if such criteria could be acceptable to allow interchangeability.

Prediction of Carboxylesterase 1-mediated In Vivo Drug Interaction between Methylphenidate and Cannabinoids using Static and Physiologically Based Pharmacokinetic Models

The use of cannabis products has increased substantially. Cannabis products have been perceived and investigated as potential treatments for attention-deficit/hyperactivity disorder (ADHD). Accordingly, co-administration of cannabis products and methylphenidate (MPH), a first-line medication for ADHD, is possible. Oral MPH undergoes extensive presystemic metabolism by carboxylesterase 1 (CES1), a hepatic enzyme which can be inhibited by two prominent cannabinoids, Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD). This prompts further investigation into the likelihood of clinical interactions between MPH and these two cannabinoids through CES1 inhibition. In the present study, inhibition parameters were obtained from a human liver S9 system and then incorporated into static and physiologically-based pharmacokinetic (PBPK) models for prediction of potential clinical significance. The inhibition of MPH hydrolysis by THC and CBD was reversible, with estimated unbound inhibition constants (Ki,u) of 0.031 and 0.091 µM, respectively. The static model predicted a mild increase in MPH exposure by concurrent THC (34%) and CBD (94%) from smoking a cannabis cigarette and ingestion of prescriptive CBD, respectively. PBPK models suggested no significant interactions between single doses of MPH and CBD (2.5 - 10 mg/kg) when administered simultaneously, while a mild interaction (area under drug concentration-time curve increased by up to 55% and maximum concentration by up to 45%) is likely if multiple doses of CBD (10 mg/kg twice daily) are administered. In conclusion, the pharmacokinetic disposition of MPH can be potentially influenced by THC and CBD under certain clinical scenarios. Whether the magnitude of predicted interactions translates into clinically relevant outcomes requires verification in an appropriately designed clinical study. SIGNIFICANCE STATEMENT: This work demonstrated a potential mechanism of drug-drug interactions between methylphenidate (MPH) and two major cannabinoids (Δ9-tetrahydrocannabinol [THC] and cannabidiol [CBD]) not previously reported. We predicted a mild interaction between MPH and THC when the cannabinoid exposure occurred via cannabis smoking. Mild interactions between MPH and CBD were predicted with multiple oral administrations of CBD.

Methylphenidate for Treating ADHD: A Naturalistic Clinical Study of Methylphenidate Blood Concentrations in Children and Adults With Optimized Dosage

Background

Methylphenidate (MPH), along with behavioral and psychosocial interventions, is the first-line medication to treat attention-deficit hyperactivity disorder (ADHD) in Sweden. The dose of MPH for good symptom control differs between patients. However, studies of MPH concentration measurement in ADHD treatment are limited.

Objective

To describe blood and oral fluid (OF) concentrations of MPH after administration of medication in patients with well-adjusted MPH treatment for ADHD, and to identify the most suitable matrix for accurate MPH concentration during treatment.

Methods

Patients were recruited from Child and Adolescent Psychiatry (CAP), General Psychiatry (GP), and the Department of Dependency (DD). Blood and OF samples were collected in the morning before MPH administration as well as 1 and 6 h after administration of the prescribed morning dose of MPH.

Results

Fifty-nine patients aged between 9 and 69 years, 76 % males. The daily dose of MPH varied from 18 to 180 mg, but the median daily dose per body weight was similar, approximately 1.0 mg/kg body weight. The median MPH concentration in blood 1 and 6 h after the morning dose was 5.4 and 9.3 ng/mL, respectively. Highly variable OF-to-blood ratios for MPH were found at all time points for all three groups.

Conclusions

Weight is a reliable clinical parameter for optimal dose titration. Otherwise, MPH blood concentration might be used for individual dose optimization and for monitoring of the prescribed dose. Relying only on the outcome in OF cannot be recommended for evaluation of accurate MPH concentrations for treatment monitoring.

Absorption Differences between Immediate-Release Dexmethylphenidate and dl-Methylphenidate

The postulate that twice the milligram/kilogram dose of dl-methylphenidate (dl-MPH) would result in equal exposure to d-MPH compared with half that milligram/kilogram dose of the chiral switch product dexmethylphenidate (d-MPH) was tested. Using a randomized, crossover study design, 12 men and 12 women received either immediate-release (IR) dl-MPH (0.3 mg/kg) or IR d-MPH (0.15 mg/kg). Relative bioavailability comparisons included partial area under the plasma concentration-time curves (pAUC0-3 h) for d-MPH. The pAUC0-3 h is a new regulatory metric presently only required for bioequivalence testing of a specific dl-MPH modified-release product. The geometric mean ratios for both the Cmax and area under the plasma concentration-time curve (AUC0-∞) were within the 90% confidence interval (CI) regulatory range of 0.8-1.25, indicating that these two drugs were bioequivalent in terms of d-MPH. However, the pAUC0-3 h geometric mean ratio for d-MPH after IR dl-MPH versus IR d-MPH was 0.76 (P < 0.001; 90% CI, 0.67-0.87), showing significantly less early exposure to the d-isomer than IR d-MPH. The 1-hour d-MPH concentration after dl-MPH was 56% of that after the enantiopure drug. The maximum d-MPH plasma concentration (Cmax) for dl-MPH was also significantly lower for dl-MPH (P < 0.05; CI, 1.02-1.19), whereas the AUC0-∞ ratio of 0.89 was not significantly different (P = 0.21; CI, 0.98-1.13). The AUC0-3 h difference reported here points to the potential limitations of using bioequivalence for sound predictions of dose-response relationships. Knowledge of the greater early exposure to d-MPH after the pure d-isomer drug compared with the racemate may contribute to drug individualization/optimization in the treatment of attention deficit hyperactivity disorder.

Ethylphenidate as a selective dopaminergic agonist and methylphenidate-ethanol transesterification biomarker

We review the pharmaceutical science of ethylphenidate (EPH) in the contexts of drug discovery, drug interactions, biomarker for dl-methylphenidate (MPH)-ethanol exposure, potentiation of dl-MPH abuse liability, contemporary "designer drug," pertinence to the newer transdermal and chiral switch MPH formulations, as well as problematic internal standard. d-EPH selectively targets the dopamine transporter, whereas d-MPH exhibits equipotent actions at dopamine and norepinephrine transporters. This selectivity carries implications for the advancement of tailored attention-deficit/hyperactivity disorder (ADHD) pharmacotherapy in the era of genome-based diagnostics. Abuse of dl-MPH often involves ethanol coabuse. Carboxylesterase 1 enantioselectively transesterifies l-MPH with ethanol to yield l-EPH accompanied by significantly increased early exposure to d-MPH and rapid potentiation of euphoria. The pharmacokinetic component of this drug interaction can largely be avoided using dexmethylphenidate (dexMPH). This notwithstanding, maximal potentiated euphoria occurs following dexMPH-ethanol. C57BL/6 mice model dl-MPH-ethanol interactions: an otherwise depressive dose of ethanol synergistically increases dl-MPH stimulation; a substimulatory dose of dl-MPH potentiates a low, stimulatory dose of ethanol; ethanol elevates blood, brain, and urinary d-MPH concentrations while forming l-EPH. Integration of EPH preclinical neuropharmacology with clinical studies of MPH-ethanol interactions provides a translational approach toward advancement of ADHD personalized medicine and management of comorbid alcohol use disorder.

Development of a physiologically based model to describe the pharmacokinetics of methylphenidate in juvenile and adult humans and nonhuman primates

The widespread usage of methylphenidate (MPH) in the pediatric population has received considerable attention due to its potential effect on child development. For the first time a physiologically based pharmacokinetic (PBPK) model has been developed in juvenile and adult humans and nonhuman primates to quantitatively evaluate species- and age-dependent enantiomer specific pharmacokinetics of MPH and its primary metabolite ritalinic acid. The PBPK model was first calibrated in adult humans using in vitro enzyme kinetic data of MPH enantiomers, together with plasma and urine pharmacokinetic data with MPH in adult humans. Metabolism of MPH in the small intestine was assumed to account for the low oral bioavailability of MPH. Due to lack of information, model development for children and juvenile and adult nonhuman primates primarily relied on intra- and interspecies extrapolation using allometric scaling. The juvenile monkeys appear to metabolize MPH more rapidly than adult monkeys and humans, both adults and children. Model prediction performance is comparable between juvenile monkeys and children, with average root mean squared error values of 4.1 and 2.1, providing scientific basis for interspecies extrapolation of toxicity findings. Model estimated human equivalent doses in children that achieve similar internal dose metrics to those associated with pubertal delays in juvenile monkeys were found to be close to the therapeutic doses of MPH used in pediatric patients. This computational analysis suggests that continued pharmacovigilance assessment is prudent for the safe use of MPH.

Pharmacokinetics of coadministration of guanfacine extended release and methylphenidate extended release

Background

α₂-Adrenoceptor agonists are used adjunctively to psychostimulants in treating attention-deficit/hyperactivity disorder (ADHD) when psychostimulants alone do not sufficiently reduce symptoms. However, data on the pharmacokinetic profiles and safety of combination treatments in ADHD are needed.

Objective

The primary objective of this study was to evaluate the pharmacokinetic profiles of guanfacine extended release (GXR) and methylphenidate hydrochloride (MPH) extended release, alone and in combination.

Study Design

This was an open-label, randomized, three-period crossover, drug–drug interaction study.

Setting

The study was conducted at a single clinical research center.

Participants

Thirty-eight healthy adults were randomized in this study.

Interventions

Subjects were administered single oral doses of GXR (Intuniv^®; Shire Development LLC, Wayne, PA, USA) 4 mg, MPH (Concerta^®; McNeil Pediatrics, Titusville, NJ, USA) 36 mg, or GXR and MPH combined.

Main Outcome Measures

Guanfacine, dexmethylphenidate (d-MPH), and l-methylphenidate (l-MPH) levels were measured with blood samples collected predose and up to 72 h postdose. Safety evaluations included treatment-emergent adverse events (TEAEs), vital signs, and electrocardiograms (ECGs).

Results

Thirty-five subjects completed the study. Analyses of the 90 % confidence intervals (CIs) for the geometric mean ratios of the maximum plasma concentration (C_max) and area under the concentration–time curve extrapolated to infinity (AUC_∞) values for guanfacine and d-MPH following administration of GXR or MPH alone or combined met strict bioequivalence criteria (90 % CIs within the interval of 0.80–1.25). Overall, combining GXR and MPH did not alter the pharmacokinetic parameters of either medication. Sixteen subjects (42.1 %) had at least one TEAE. The most commonly reported TEAEs included headache and dizziness following GXR, MPH, and GXR and MPH combined. Two subjects had clinically significant abnormalities in ECG results following coadministration: both events were mild and resolved the same day.

Conclusions

In this short-term, open-label study of healthy adults, coadministration of GXR and MPH did not result in significant pharmacokinetic drug–drug interactions. No unique TEAEs were observed with coadministration of GXR and MPH compared with either treatment alone.

[Methylphenidate in the treatment of children with attention-deficit hyperactivity disorder: monitoring in biological matrices]

Attention-deficit hyperactivity disorder (ADHD) has emerged in the last few years as the most commonly diagnosed and treated psychiatric disorder in the paediatric population. In 1980's, methylphenidate (MFD) a psychomotor stimulant drug, was approved in Spain for the symptomatic therapy of ADHD. Since then, MFD has become one of the most extensively prescribed and studied treatment for ADHD both in children and adults. In this paper, the main pharmacological issues of MFD are reviewed, focusing on its pharmacokinetics in conventional (blood and urine) and non-conventional (hair, oral fluid and sweat) biological matrices, its pharmaceutical preparations, therapeutic levels and side effects.

Transdermal and oral dl-methylphenidate-ethanol interactions in C57BL/6J mice: transesterification to ethylphenidate and elevation of d-methylphenidate concentrations

We tested the hypothesis that C57BL/6J mice will model human metabolic interactions between dl-methylphenidate (MPH) and ethanol, placing an emphasis on the MPH transdermal system (MTS). Specifically, we asked: (1) will ethanol increase d-MPH biological concentrations, (2) will MTS facilitate the systemic bioavailability of l-MPH, and (3) will l-MPH enantioselectively interact with ethanol to yield l-ethylphenidate (l-EPH)? Mice were dosed with MTS (¼ of a 12.5 cm(2) patch on shaved skin) or a comparable oral dl-MPH dose (7.5 mg/kg), with or without ethanol (3.0 g/kg), and then placed in metabolic cages for 3 h. MPH and EPH isomer concentrations in blood, brain, and urine were analyzed by gas chromatographic-mass spectrometry monitoring of N-(S)-prolylpiperidyl fragments. As in humans, MTS greatly facilitated the absorption of l-MPH in this mouse strain. Similarly, ethanol led to the enantioselective formation of l-EPH and to an elevation in d-MPH concentrations with both MTS and oral MPH. Although only guarded comparisons between MTS and oral MPH can be made due to route-dependent drug absorption rate differences, MTS was associated with significant MPH-ethanol interactions. Ethanol-mediated increases in circulating concentrations of d-MPH carry toxicological and abuse liability implications should this animal model hold for ethanol-consuming attention-deficit hyperactivity disorder patients or coabusers.

Pharmacokinetics of methylphenidate following two oral formulations (immediate and sustained release) in the dog

Methylphenidate (MPH) is an immediate-release (IR) or sustained-release (SR) drug used to treat attention-deficit hyperactivity disorder. Eight dogs were randomly assigned to two treatment groups, using an open, single-dose, two-treatment, two-period, randomized, crossover design. Each subject received a single dose of 20 mg d,l-MPH IR or SR tablet. After blood collections at specific times, the concentrations of d,l-MPH in plasma were evaluated by high-performance liquid chromatography. Following both IR and SR oral administration of d,l-MPH, the animals did not show any side effects, except that mild hyperkinesia was observed in a few subjects belonging to the IR treatment group. After both administrations, the concentration data for d,l-MPH in plasma displayed a characteristic, one-compartment drug model. The relative bioavailability of the SR formulation was 30.58 +/- 13.73%. Significant differences between the two administrations were found in T(max), C(max), AUC, and Cl. Despite low drug concentrations in the blood, the SR formulation ensured uniformity of d,l-MPH plasma concentrations and, thus, a simpler and easier titration. In conclusion, the tested dosage appears to be too low for clinical application in canines, and an increase in dosing is suggested. Further pharmacodynamics studies are necessary to support this speculation.

Variability and impact on design of bioequivalence studies

In 2008, the European Agency for the Evaluation of Medicinal Products released a draft guidance on the investigation of bioequivalence for immediate release dosage forms with systemic action to replace the former guidance of a decade ago. Revisions of the regulatory guidance are based upon many questions over the past years and sometimes continuing scientific discussions on the use of the most suitable statistical analysis methods and study designs, particularly for drugs and drug products with high within-subject variability. Although high within-subject variability is usually associated with a coefficient of variation of 30% or more, new approaches are available in the literature to allow a gradual increase and a levelling off of the bioequivalence limits to some maximum wider values (e.g. 75-133%), dependent on the increase in the within-subject variability. The two-way, cross-over single dose study measuring parent drug is still the design of first choice. A partial replicate design with repeating the reference product and scaling the bioequivalence for the reference variability are proposed for drugs with high within-subject variability. In case of high variability, more regulatory authorities may accept a two-stage or group-sequential bioequivalence design using appropriately adjusted statistical analysis. This review also considers the mechanisms why drugs and drug products may exhibit large variability. The physiological complexity of the gastrointestinal tract and the interaction with the physicochemical properties of drug substances may contribute to the variation in plasma drug concentration-time profiles of drugs and drug products and to variability between and within subjects. A review of submitted bioequivalence studies at the Food and Drug Administration's Office of Generic Drugs over the period 2003-2005 indicated that extensive pre-systemic metabolism of the drug substance was the most important explanation for consistently high variability drugs, rather than a formulation factor. These scientific efforts are expected to further lead to revisions of earlier regulatory guidance in other regions as is the current situation in Europe.

Evolution of stimulants to treat ADHD: transdermal methylphenidate

OBJECTIVE

The following comprehensive review describes the evolution of stimulant drug formulations used in the treatment of attention-deficit/hyperactivity disorder (ADHD). Emphasis is placed on the basic and clinical pharmacology of the dl-methylphenidate (MPH) transdermal system (MTS).

METHODS

The pharmacokinetic and pharmacodynamic literature pertaining to MPH and amphetamine enantiomers was reviewed in the context of ADHD therapy and MTS as a treatment option.

RESULTS

MTS incorporates MPH into an adhesive monolithic matrix, using the free base form of the drug to facilitate transdermal absorption. MTS technology minimizes contact dermatitis by eliminating to need for percutaneous penetration enhancers. After a lag time of approximately 2 h, plasma concentrations of the therapeutic d-MPH isomer become detectable, then continuously rise over the course of the recommended 9 h wear time. Concentrations of l-MPH typically attain 40-50% that of d-MPH (vs. 1-2% following oral MPH). Unauthorized MTS removal poses some misuse liability and over 50% of MTS drug content remains in the discarded system.

CONCLUSIONS

While liquid or chewable MPH formulations overcome potential swallowing difficulties, as do sprinkled once-daily extended-release (ER) MPH products, only MTS addresses swallowing difficulties while also offering a flexible individualized MPH exposure time in a once-daily MPH regimen.

Influence of ethanol and gender on methylphenidate pharmacokinetics and pharmacodynamics

This study explores the hypotheses that: (1) ethanol will interact with dl-Methylphenidate (MPH) to enantioselectively elevate plasma d-MPH, and primarily yield l-ethylphenidate as a transesterification metabolite; (2) women will exhibit lower relative bioavailability of MPH than men; and (3) sex-dependent differences in subjective effects will exist. dl-MPH HCl (0.3 mg/kg) was administered orally 30 min before ethanol, 30 min after ethanol (0.6 gm/kg), or without ethanol, in a randomized, normal subject three-way crossover study of 10 men and 10 women. Pharmacokinetic parameters were compared. Subjective effects were recorded using visual analog scales. One subject was a novel poor MPH metabolizer whose data were analyzed separately. Ethanol after or before MPH significantly (P<0.0001) elevated the geometric mean for the maximum d-MPH plasma concentration (C(max) (+/-SD)) from 15.3 (3.37) ng/ml to 21.5 (6.81) and 21.4 (4.86), respectively, and raised the corresponding geometric mean for the area under the concentration-time curve values from 82.9 (21.7) ng ml/h to 105.2 (23.5) and 102.9 (19.2). l-MPH was present in plasma only at 1-3% of the concentration of d-MPH, except in the poor metabolizer where l-MPH exceeded that of d-MPH. The metabolite l-ethylphenidate frequently exceeded 1 ng/ml in plasma, whereas d-ethylphenidate was detected only in low pg/ml concentrations. Women reported a significantly greater stimulant effect than men when questioned "Do you feel any drug effect?" (P<0.05), in spite of lower mean plasma d-MPH area under the response-time curves in women. Ethanol elevates plasma d-MPH C(max) and area under the concentration-time curve by approximately 40% and 25%, respectively. If the poor metabolizer of MPH proves to be a distinct phenotype, determining the genetic mechanism may be of value for individualizing drug therapy. The more pronounced stimulant effects experienced by women have sex-based abuse liability implications.

New methylphenidate formulations for the treatment of attention-deficit/hyperactivity disorder

dl-Methylphenidate (MPH) remains the most widely used pharmacological agent in the treatment of attention-deficit/hyperactivity disorder (ADHD). The predominantly dopaminergic mechanism of the psychostimulant actions has become more clearly defined. Neuroimaging and genetic studies are revealing the underlying neuropathology in ADHD. Novel extended-release (ER) MPH formulations now offer drug delivery options to overcome both the short-term actions of immediate-release (IR) MPH and the acute tolerance associated with the first-generation ER-MPH products. These novel MPH products apply proprietary technologies such as OROS (Alza), Diffucaps (Eurand) and SODAS (Elan) to offer both the convenience of once-a-day administration and absorption profiles resembling, to varying degrees, the standard multiple dose schedules of IR-MPH. The pharmacodynamics of the separate MPH enantiomers is in the process of further neuropharmacological characterisation. It is well established that dl-MPH undergoes marked stereoselective metabolism. Although l-MPH exhibits only minimal oral absorption, it may preferentially penetrate the brain, and interacts with ethanol to form the metabolite ethylphenidate. The newly approved resolved enantiomer product d-MPH is now available in an IR formulation, and when administered at one-half the dose to that of the racemate, is purported to produce a longer duration of clinical effect, despite essentially identical pharmacokinetics. A long-acting formulation of d-MPH, which employs the SODAS technology, is in the advanced stages of clinical development.

Advances in the Pharmacotherapy of Attention-Deficit–Hyperactivity Disorder: Focus on Methylphenidate Formulations

The psychostimulant dl-methylphenidate (MPH) remains the most common drug therapy in child and adolescent psychiatry for the treatment of attention-deficit-hyperactivity disorder (ADHD). Evidence of a dopaminergic basis both for the actions of MPH and for the underlying neuropathology in ADHD continues to mount. Advances in the biopharmaceutics of MPH have been conspicuous. Novel approaches to formulation design have resulted in new MPH delivery options to overcome the short-term actions of both immediate-and sustained-release MPH. New modified-release MPH products offer the convenience of once-daily administration while providing extended absorption profiles that better mimic those of standard schedules of immediate-release MPH (i.e., the absorption phase of MPH better correlates with improved behavioral response than does the elimination phase). The oral bioavailability of MPH in females may be lower than in males. The l-MPH isomer exhibits only negligible oral bioavailability and, further, possesses little intrinsic activity at the dopamine transporter. This notwithstanding, a single-isomer d-MPH immediate-release product is now available for dosing recommended at one-half that of dl-MPH.

Pharmacokinetics of methylphenidate after oral administration of two modified-release formulations in healthy adults

OBJECTIVE

To compare the rate and extent of absorption of DL-threo-methylphenidate (MPH) from two modified-release MPH formulations at their respective recommended starting doses in healthy adult volunteers.

DESIGN

Open-label, randomised, crossover, bioavailability study.

PARTICIPANTS

Twenty healthy adult male and female volunteers.

METHODS

Subjects received single doses of two modified-release formulations of MPH, a 20mg capsule (Ritalin) LA) and an 18 mg tablet (Concerta). A total of 19 plasma samples was collected over 24 hours, and MPH plasma concentrations were determined by liquid chromatography-mass spectrometry (LC-MS/MS). These values were used to calculate standard noncompartmental pharmacokinetic parameters describing the rate (peak concentration and time to peak concentration) and extent (area under the concentration-time curve, AUC) of absorption of the two formulations. The relative bioavailability of the two drugs was assessed using a 90% confidence interval, based on the lower and upper endpoints of the confidence interval for the ratios of the geometric means (log transformed) being within the 0.80-1.25 equivalence criterion.

RESULTS

Nineteen subjects, ten male and nine female, aged 21-34 years completed both treatment phases of the study. The Ritalin LA formulation displayed a distinctly biphasic pharmacokinetic profile, with mean initial peak plasma concentration of 7 microg/L at an average of 2.1 hours after administration and a second peak of 9.3 microg/L occurring at 5.6 hours. In contrast, the profile of the Concerta formulation rapidly reached an initial plateau concentration of 3.4 microg/L at 3.3 hours after administration and a second mean plateau concentration of 5.9 microg/L approximately 6 hours after administration. Substantially more MPH was absorbed from Ritalin LA than from Concert over the first 4 hours; the respective AUC(4) values were 18.5 and 9.3 microg x h/L (p < 0.001). The overall extent of absorption of MPH was similar between the two formulations. Oral clearance was identical between the two dosage forms.

CONCLUSIONS

The Ritalin LA formulation exhibited more rapid initial absorption and reached significantly higher peak plasma concentrations compared with the Concerta formulation, although the oral bioavailability of MPH was similar between the two formulations. The Ritalin LA capsule demonstrated a distinctly bimodal plasma concentration-time profile. MPH plasma concentrations resulting from Concerta reached a peak at 6 hours. These results indicate that the recommended starting dose of the Ritalin LA 20 mg capsule formulation provides more rapid absorption and higher peak plasma concentrations than the recommended 18 mg starting dose of the Concerta formulation.

Tests for individual and population bioequivalence based on generalized p-values

The U.S. Food and Drug Administration (FDA) has proposed new regulations that address the 'prescribability' and 'switchability' of new formulations of already-approved drugs. These new criteria are known, respectively, as population and individual bioequivalence. Two methods have been proposed in the bioequivalence literature for assessing population and individual bioequivalence that calculate an upper 95 per cent confidence bound for the bioequivalence criterion in question, and then test bio-equivalence by comparing this bound to the limit established by the FDA. In this paper we propose applying the generalized test function (GTF) methodology of Tsui and Weerahandi (Journal of the American Statistical Association 1989; 602-607) to this problem to produce tests based on a generalized p-value (GPV). This methodology allows us to construct hypothesis tests in the presence of nuisance parameters. Using simulation we show that these tests perform well in comparison to the confidence interval methods and have superior power for assessing population bioequivalence.

Pharmacokinetic and pharmacodynamic drug interactions in the treatment of attention-deficit hyperactivity disorder

The psychostimulants methylphenidate, amphetamine and pemoline are among the most common medications used today in child and adolescent psychiatry for the treatment of patients with attention-deficit hyperactivity disorder. Frequently, these medications are used in combination with other medications on a short or long term basis. The present review examines psychostimulant pharmacology, summarises reported drug-drug interactions and explores underlying pharmacokinetic and pharmacodynamic considerations for interactions. A computerised search was undertaken using Medline (1966 to 2000) and Current Contents to provide the literature base for reports of drug-drug interactions involving psychostimulants. These leads were further cross-referenced for completeness of the survey. Methylphenidate appears to be more often implicated in pharmacokinetic interactions suggestive of possible metabolic inhibition, although the mechanisms still remain unclear. Amphetamine was more often involved in apparent pharmacodynamic interactions and could potentially be influenced by medications affecting cytochrome P450 (CYP) 2D6. No published reports of drug interactions involving pemoline were found. The alpha2-adrenergic agonists clonidine and guanfacine have been implicated in several interactions. Perhaps best documented is their antagonism by tricyclic antidepressants and phenothiazines. In additional, concurrent beta-blocker use, or abrupt discontinuation, can lead to hypertensive response. Although there are few published well-controlled interaction studies with psychostimulants and alpha2-adrenergic agonists, it appears that these agents may be safely coadministered. The interactions of monoamine oxidase inhibitors with psychostimulants represent one of the few strict contraindications.

Individual bioequivalence revisited

For decades, the establishment of bioequivalence has generally relied on the comparison of population averages between the test and reference formulations. In the early 1990s, individual bioequivalence was proposed to ensure that an individual could be switched from the reference product to the test product with unchanged efficacy and safety. Since 1997, the US Food and Drug Administration (FDA) has published three guidance documents on the proposed criterion and statistical methodology for the individual bioequivalence approach. From a scientific stand-point, the individual bioequivalence criterion appears to offer several advantages for some drug products compared with the average criterion. It allows comparison of intraindividual variances, scaling the bioequivalence criterion to the reference variability and detection of an important subject-by-formulation interaction if it exists. Based on these considerations, the FDA has recently recommended replicate study designs for modified release dosage forms and highly variable drug products. The new criterion also promotes inclusion of a heterogeneous population of volunteers in bioequivalence studies. Despite all the advantages of the individual bioequivalence approach, questions remain on the optimal use of replicate study designs and the proposed criterion for evaluation of bioequivalence between formulations. In the finalised guidance documents, therefore, the FDA maintains the average bioequivalence criterion while allowing other criteria under certain circumstances. Collection and analysis of bioequivalence data from replicate study designs may permit further assessment and resolution of these questions.

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.