Relationship between systemic drug absorption and gastrointestinal transit after the simultaneous oral administration of carbamazepine as a controlled-release system and as a suspension of 15N-labelled drug to healthy volunteers

Assessment of Biopharmaceutical Performance of Supersaturating Formulations of Carbamazepine in Rats Using Physiologically Based Pharmacokinetic Modeling

There is an overgrowing emphasis on supersaturating drug delivery systems (SDDS) with increase in number of poorly water-soluble compounds. However, biopharmaceutical performance from these formulations is limited by phase transformation to stable crystalline form due to their high-energy physical form. In the present study, in vitro kinetic solubility in water and dissolution in biorelevant medium integrated with in silico physiologically based pharmacokinetic (PBPK) modeling was used to predict biopharmaceutical performance of SDDS of poorly water-soluble compound, carbamazepine (CBZ). GastroPlus™ with advanced compartmental absorption and transit model was used as a simulation tool for the study. Wherein, the model was developed using physicochemical properties of CBZ and disposition parameters obtained after intravenous administration of CBZ (20 mg/kg) into Sprague-Dawley (SD) rats. Biorelevant medium was selected by screening different dissolution media for their capability to predict oral plasma concentration-time profile of marketed formulation of CBZ. In vivo performance of SDDS was predicted with the developed model and compared to observed plasma concentration-time profile obtained after oral administration of SDDS into SD rats (20 mg/kg). The predictions, with strategy of using kinetic solubility and dissolution in the selected biorelevant medium, were consistent with observed biopharmaceutical performance of SDDS. Additionally, phase transformation of CBZ during gastrointestinal transit of formulations was evaluated and correlated with in vivo dissolution deconvoluted by Loo-Reigelman analysis.

Applications of stable isotopes in clinical pharmacology

This review aims to present an overview of the application of stable isotope technology in clinical pharmacology. Three main categories of stable isotope technology can be distinguished in clinical pharmacology. Firstly, it is applied in the assessment of drug pharmacology to determine the pharmacokinetic profile or mode of action of a drug substance. Secondly, stable isotopes may be used for the assessment of drug products or drug delivery systems by determination of parameters such as the bioavailability or the release profile. Thirdly, patients may be assessed in relation to patient-specific drug treatment; this concept is often called personalized medicine. In this article, the application of stable isotope technology in the aforementioned three areas is reviewed, with emphasis on developments over the past 25 years. The applications are illustrated with examples from clinical studies in humans.

Pharmacokinetic variability of long-acting stimulants in the treatment of children and adults with attention-deficit hyperactivity disorder

Methylphenidate- and amfetamine-based stimulants are first-line pharmacotherapies for attention-deficit hyperactivity disorder, a common neurobehavioural disorder in children and adults. A number of long-acting stimulant formulations have been developed with the aim of providing once-daily dosing, employing various means to extend duration of action, including a transdermal delivery system, an osmotic-release oral system, capsules with a mixture of immediate- and delayed-release beads, and prodrug technology. Coefficients of variance of pharmacokinetic measures can estimate the levels of pharmacokinetic variability based on the measurable variance between different individuals receiving the same dose of stimulant (interindividual variability) and within the same individual over multiple administrations (intraindividual variability). Differences in formulation clearly impact pharmacokinetic profiles. Many medications exhibit wide interindividual variability in clinical response. Stimulants with low levels of inter- and intraindividual variability may be better suited to provide consistent levels of medication to patients. The pharmacokinetic profile of stimulants using pH-dependent bead technology can vary depending on food consumption or concomitant administration of medications that alter gastric pH. While delivery of methylphenidate with the transdermal delivery system would be unaffected by gastrointestinal factors, intersubject variability is nonetheless substantial. Unlike the beaded formulations and, to some extent (when considering total exposure) the osmotic-release formulation, systemic exposure to amfetamine with the prodrug stimulant lisdexamfetamine dimesylate appears largely unaffected by such factors, likely owing to its dependence on systemic enzymatic cleavage of the precursor molecule, which occurs primarily in the blood involving red blood cells. The high capacity but as yet unidentified enzymatic system for conversion of lisdexamfetamine dimesylate may contribute to its consistent pharmacokinetic profile. The reasons underlying observed differential responses to stimulants are likely to be multifactorial, including pharmacodynamic factors. While the use of stimulants with low inter- and intrapatient pharmacokinetic variability does not obviate the need to titrate stimulant doses, stimulants with low intraindividual variation in pharmacokinetic parameters may reduce the likelihood of patients falling into subtherapeutic drug concentrations or reaching drug concentrations at which the risk of adverse events increases. As such, clinicians are urged both to adjust stimulant doses based on therapeutic response and the risk for adverse events and to monitor patients for potential causes of pharmacokinetic variability.

Treatment of epilepsy in postmenopausal women

Menopausal women represent a rapidly growing proportion of the population. Epidemiologic evidence for the increased incidence of epilepsy in elderly patients suggests that with global aging of the population, there is likely to be a growing need for healthcare workers to manage seizures in older women. Unfortunately, there has been relatively little scientific investigation into the unique concerns of postmenopausal woman with epilepsy. There is some evidence that women may experience increased seizure activity during the menopausal transition owing to the effects of estrogen and progesterone on neuronal excitability. During perimenopause and menopause, use of hormone-replacement therapy can also worsen seizure control. Menopausal women are particularly vulnerable to osteoporosis and fragility fractures, both of which demonstrate increased risk following exposure to antiepileptic drugs. Optimization of epilepsy therapy to avoid both seizures and falls caused by antiepileptic drug-induced imbalance is crucial in order to minimize fracture risk in this group of women. Elderly patients are more susceptible to adverse medication side effects owing to drug interactions and the physiologic changes of aging that result in altered drug pharmacokinetics.

Oral osmotically driven systems: 30 years of development and clinical use

The number of marketed oral osmotically driven systems (OODS) has doubled in the last 10 years. The main clinical benefits of OODS are their ability to improve treatment tolerability and patient compliance. These advantages are mainly driven by the capacity to deliver drugs in a sustained manner, independent of the drug chemical properties, of the patient's physiological factors or concomitant food intake. However, access to these technologies has been restricted by the crowded patent landscape and manufacturing challenges. In this review article, we intend to give an overview of the OODS development in the last 30 years, detailing the technologies, specific products and their clinical use. General guidance on technology selection is described in light of the recent advances in the field. The clinical performance of these technologies is also discussed, with a focus on food effects and the in vivo-in vitro correlation. Special attention is paid to safety given the controversial case study of Osmosin. Overall, oral osmotically driven systems appear to be a promising technology for product life-cycle strategies.

Extended-release formulations for the treatment of epilepsy

This review analyses the concept of extended-release (ER) formulations in epilepsy and evaluates ER formulations of carbamazepine, valproic acid and a modified-release (MR) formulation of oxcarbazepine. ER formulations are usually designed to reduce dose frequency and maintain relatively constant or flat plasma drug concentration. It is questionable whether flat plasma concentrations of an antiepileptic drug (AED) improve antiepileptic efficacy compared with fluctuating plasma concentrations. More certainly, they minimise concentration-related adverse effects, and the dosing flexibility and consistency of plasma concentrations may simplify the management of antiepileptic drug therapy. Neurologists would like ER formulations that can be administered once- and/or twice-daily to tailor therapy for the individual patient; however, switching dosage schedules from multiple dosages per day to once daily, although more convenient, will not generally improve therapeutic coverage (maintenance of effective drug concentration in biological fluids and tissue). Pharmacokinetically, the impact of a missed dose is greater the larger the dose and the less frequent the administration. Therefore, the risk of breakthrough seizure is higher during AED once-daily administration than twice-daily administration. Consequently, the increased compliance observed with fewer dosages per day should be weighed against the impact or forgiveness of omitted dose(s) and the shorter 'forgiveness' period associated with once-daily administration. Currently, the trend is to treat patients with epilepsy with ER formulations because of the better compliance, convenience and flat plasma concentration versus time curve. Thus, it seems that the term 'flatter is better' for AED plasma profiles has precipitated in the last 10-15 years among neurologists and epilepsy caregivers, and is being promoted by marketing forces of pharmaceutical companies. Data from the literature support the trend to treat epileptic patients with twice-daily administration of the existing ER formulations of valproic acid and carbamazepine, and oxcarbazepine-MR; however, the author of this article is not convinced that these ER formulations can guarantee a complete therapeutic coverage throughout the 24-hour dosing interval following once-daily administration. Epilepsy is a single-episode disease, and the convenience and possible better compliance associated with once-daily administration must be weighed against the shorter 'forgiveness' period and possible higher risk of breakthrough seizure due to sub-therapeutic plasma levels and/or omitted doses. Data suggest just a small difference in compliance between once- and twice-daily administration, with no significant difference in efficacy. Therefore, the increased compliance following once-daily administration may be counter-productive in minimising the occurrence of sub-therapeutic drug concentrations. Weighing up the advantages and disadvantages for once- versus twice-daily administration of ER formulations in epilepsy leads to a conclusion in favour of twice-daily administration.

Influence of Polymorphic Form, Morphology, and Excipient Interactions on the Dissolution of Carbamazepine Compacts

To gain a deeper understanding of the behavior of carbamazepine (CBZ) and CBZ dihydrate (DH) compacts during in vitro dissolution tests various factors were investigated: hydrate formation of CBZ, crystal morphology, surface area, and excipient influence. Dissolution tests were performed in three different dissolution media: distilled water, hydroxypropyl methylcellulose (HPMC), and polyethylene glycol (PEG) solutions. For the CBZ compacts, the dissolution rate of CBZ in water was fastest (0.338 mg L(-1) min(-1)). With increasing ability of the excipients to inhibit the hydration of CBZ (PEG < HPMC), surprisingly the dissolution rate of CBZ compacts decreased: PEG solution (0.314 mg L(-1) min(-1)) > HPMC solution (0.257 mg L(-1) min(-1)). This implies that DH formation resulted in an apparent increase in the dissolution rate rather than slowing it down. For the DH compacts, the dissolution rate in water (0.055 mg L(-1) min(-1)) was slower than that of PEG and HPMC solutions (0.174 and 0.178 mg L(-1) min(-1), respectively). The contact angle measurements showed a significantly higher value in water (61.0 degrees) than in PEG and HPMC solutions (44.8 degrees and 43.1 degrees, respectively). Although the dissolution of CBZ and DH compacts in various dissolution media are complex processes, the influence and relative importance of these factors were clearly detected providing better understanding of the dissolution behavior of the drug.

Antiepileptic Drug Formulation and Treatment in the Elderly: Biopharmaceutical Considerations

The pharmacokinetics of antiepileptic drugs (AEDs) determine their effectiveness in the treatment of patients with epilepsy. Given the likelihood of comorbid medical conditions that require polytherapy, as well as the normal physiological changes associated with aging, an understanding of AED pharmacokinetics and pharmacodynamics in the elderly patient is critical. There is a relative sparsity of data regarding changes in the oral absorption patterns of AEDs that may accompany aging. Therefore, the objective of this chapter is to discuss fundamental principles related to oral drug absorption, and to discuss their potential impact on AED treatment in the older patient. Although most drugs are absorbed via the diffusion process, active transport also plays a role in absorption. While the gastrointestinal tract shows remarkable resilience during aging, physiological changes that influence oral and esophageal function, gastric pH, gastric emptying rates, and intestinal transit times do occur. Oral administration of AEDs may be affected by changes associated with aging, including altered oral protective reflexes, xerostomia, thickening of the esophageal smooth muscle layer, reduced contraction velocity and duration, altered esophageal emptying rates, and enteric plexus neuron reduction. Gastric acid secretion is similar between older and younger patients, but older patients require more time to return to baseline gastric pH values and have prolonged gastric emptying rates compared to younger patients. Elderly patients may similarly have reduced numbers of myenteric neurons, decreased postprandial contractions, reduced frequency of migrating motor complex, and diminished rectal compliance as well as reduced sphincter tones. All of these effects observed in the aging patient, in turn, produce numerous opportunities for changes in AED absorption, particularly for those agents demonstrating poor water solubility or variable absorption patterns.

Drug absorption in the elderly: biopharmaceutical considerations for the antiepileptic drugs

The management of antiepileptic drug (AED) pharmacokinetics remains a challenge in the treatment of patients with epilepsy. Drug characteristics, such as protein binding, mechanisms of drug elimination, and the potential for pharmacokinetic/pharmacodynamic interactions, are important considerations for drug selection and may help determine overall effectiveness. In elderly patients with epilepsy, the likelihood of polytherapy, along with physiological changes associated with aging, can make pharmacokinetic issues even more significant. One aspect of pharmacokinetics that has received less attention is the process of oral drug absorption. Aging can have variable effects on the gastrointestinal system. Some of these physiological changes have the potential to impact absorption patterns of some medications, including AEDs. Altered oral protective reflexes, xerostomia, and delayed esophageal emptying in elderly patients may complicate oral administration of some medications. Altered gastric pH could modify drug absorption, and modified gastric emptying rates can influence the bioavailability of some AEDs. Finally, intestinal transit times may be slower in elderly patients compared to younger patients, possibly altering the absorption of some AEDs. These age-related physiological changes that may affect AED pharmacokinetics should be considered when treating elderly patients with epilepsy.

Patterns of care, outcomes, and direct health plan costs of antiepileptic therapy: a pharmacoeconomic analysis of the available carbamazepine formulations

BACKGROUND

Although generic formulations of immediate-release carbamazepine (IR-CBZ) are available, extended-release delivery systems may offer important advantages, including the convenience of less-frequent administration and smaller peak-to-trough serum carbamazepine (CBZ) fluctuations.

OBJECTIVE

The aim of this study was to compare the patterns of pharmacotherapy, rates of adverse events, and the utilization costs among patients treated with the available CBZ formulations (ie, generic and branded IR-CBZ, and extended-release CBZ (ER-CBZ) capsules [Carbatrol, Shire US Inc., Wayne, Pennsylvania] and tablets [Tegretol-XR, Novartis Pharmaccuticals Corporation, East Hanover, New Jersey]).

METHODS

Data were retrieved from the PharMetrics patient-centric database (which contains integrated claims data for almost 36 million unique patients from 61 US health plans) for patients who were diagnosed with epilepsy and initiated CBZ between July 1999 and June 2001. Patient demographic and clinical characteristics, adverse events, discontinuations, CBZ therapy switches, and utilization and costs for related care subsequent to treatment initiation were recorded. Annual rates of adverse events and discontinuations were calculated, and the risks of these events were compared across treatment groups.

RESULTS

Data were gathered for 1737 patients. The branded CBZ group was demographically and clinically different than the other groups (ie, migraine and cerebral palsy prevalence) and therefore was excluded from event-risk analyses. Results of the proportional hazards regression analysis indicated that Tegretol-XR patients were more likely to experience common central nervous system (CNS)-related adverse events relative to Carbatrol (hazard ratio, 1.67; P = 0.043). A lower percentage of subjects switched off ER-CBZ relative to IR-CBZ (Carbatrol, 5.2%; Tegretol-XR, 5.7%; generic IR-CBZ, 13.0%; branded IR-CBZ, 16.7%). Differences in mean payments for epilepsy-related health care services at 1 year among Carbatrol, Tegretol-XR, and branded or generic CBZ did not reach statistical significance.

CONCLUSIONS

Among the available CBZ formulations, Carbatrol was associated with a lower incidence of common CNS adverse events. ER-CBZ formulations were also associated with reduced likelihood of therapy discontinuation or switching CBZ medications, relative to patients taking generic IR-CBZ, in this retrospective data analysis.

Development of a population pharmacokinetic model for carbamazepine based on sparse therapeutic monitoring data from pediatric patients with epilepsy

BACKGROUND

Population models can be important extensions of therapeutic drug monitoring (TDM), as they allow estimation of individual pharmacokinetic parameters based on a small number of measured drug concentrations.

OBJECTIVE

This study used a Bayesian approach to explore the utility of routinely collected and sparse TDM data (1 sample per patient) for carbamazepine (CBZ) monotherapy in developing a population pharmacokinetic (PPK) model for CBZ in pediatric patients that would allow prediction of CBZ concentrations for both immediate- and controlled-release formulations.

METHODS

Patient and TDM data were obtained from a pediatric neurology outpatient database. Data were analyzed using an iterative 2-stage Bayesian algorithm and a nonparametric adaptive grid algorithm. Models were compared by final log likelihood, mean error (ME) as a measure of bias, and root mean squared error (RMSE) as a measure of precision.

RESULTS

Fifty-seven entries with data on CBZ monotherapy were identified from the database and used in the analysis (36 from males, 21 from females; mean [SD] age, 9.1 [4.4] years [range, 2-21 years]). Preliminary models estimating clearance (Cl) or the elimination rate constant (K(el)) gave good prediction of serum concentrations compared with measured serum concentrations, but estimates of Cl and K(el) were highly correlated with estimates of volume of distribution (V(d)). Different covariate models were then tested. The selected model had zero-order input and had age and body weight as covariates. Cl (L/h) was calculated as K(el) . V(d), where K(el) = [K(i) - (K(s) . age)] and V(d) = [V(i) + (V(s) . body weight)]. Median parameter estimates were V(i) (intercept) = 11.5 L (fixed); V(s) (slope) = 0.3957 L/kg (range, 0.01200-1.5730); K(i) (intercept) = 0.173 h(-1) (fixed); and K(s) (slope) = 0.004487 h(-1) . y(-1) (range, 0.0001800-0.02969). The fit was good for estimates of steady-state serum concentrations based on prior values (population median estimates) (R = 0.468; R(2) = 0.219) but was even better for predictions based on individual Bayesian posterior values (R(2) = 0.991), with little bias (ME = -0.079) and good precision (RMSE = 0.055).

CONCLUSIONS

Based on the findings of this study, sparse TDM data can be used for PPK modeling of CBZ clearance in children with epilepsy, and these models can be used to predict Cl at steady state in pediatric patients. However, to estimate additional pharmacokinetic model parameters (eg, the absorption rate constant and V(d)), it would be necessary to combine sparse TDM data with additional well-timed samples. This would allow development of more informative PPK models that could be used as part of Bayesian dose-individualization strategies.

The role of gamma-scintigraphy in oral drug delivery

The gastrointestinal tract is usually the preferred site of absorption for most therapeutic agents, as seen from the standpoints of convenience of administration, patient compliance and cost. In recent years there has been a tendency to employ sophisticated systems that enable controlled or timed release of a drug, thereby providing a better dosing pattern and greater convenience to the patient. Although much about the performance of a system can be learned from in vitro release studies using conventional and modified dissolution methods, evaluation in vivo is essential in product development. The non-invasive technique of gamma-scintigraphy has been used to follow the gastrointestinal transit and release characteristics of a variety of pharmaceutical dosage forms. Such studies provide an insight into the fate of the delivery system and its integrity and enable the relationship between in vivo performance and resultant pharmacokinetics to be examined (pharmacoscintigraphy).

Oral drug absorption studies: the best model for man is man!

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Factors influencing the bioavailability of peroral formulations of drugs for dogs

The oral route is presently the preferred route of drug delivery. Poor oral bioavailability results in variable concentrations of drugs in the plasma and variable pharmacological responses, in addition to higher product costs. The unique canine physiology, anatomy and biochemistry makes designing canine dosage forms a challenging exercise. This article reviews the physicochemical, physiological, pharmacokinetic, pharmacological and formulation factors that can influence the drug availability of the oral formulations in dogs in an effort to provide a source of data to aid development of canine drug products with superior bioavailability.

New formulations of drugs in epilepsy

The advent of numerous new treatment options in epilepsy therapy over the last decade is enabling a more flexible and individualized approach to patients with seizures. For some patients, these products offer added efficacy, reduction of troublesome side effects associated with standard anticonvulsants, and control over acute seizure exacerbations. This review profiles new formulations of anti-epileptic drugs. Tegretol-XR (TXR) and Carbatrol (CBTL), two extended-release preparations of carbamazepine (CBZ), which allow twice daily administration, minimising drug toxicity and improving efficacy. Topiramate sprinkles and lamotrigine chewable dispersible tablets allow easier administration in children. The rectal gel preparation of diazepam (Diastat) is useful for parents of patients with acute seizure exacerbations. Intravenous valproate (Depacon) and fosphenytoin (Cerebyx) provide parenteral treatment of acute seizures, without sedation or significant peripheral venous side effects. All of these new formulations expand treatment options for patients with epilepsy, who will benefit from them.

Pharmacokinetic considerations in gastrointestinal motor disorders

Although it has been recognised that alterations in gastrointestinal motility, whether induced by physiological or pathological processes, have significant effects on the pharmacokinetics of orally administered drugs, this subject has received inappropriately little attention. Studies relating to this topic have focused on healthy volunteers and animals and have largely been confined to the effects of single drug doses. There is limited information about the effects of disease on pharmacokinetics under steady-state conditions. Changes in gastrointestinal motility may affect the pharmacokinetics of orally administered drugs by altering the rate of delivery, bioavailability or mucosal absorption of the drug. In general the rate of absorption and time taken to achieve maximal plasma concentrations for well absorbed drugs may be modified by changes in gastrointestinal motility, but overall bioavailability is not usually affected. In these cases the therapeutic and clinical effects of the alteration in pharmacokinetics will, therefore, depend on which parameters are important for the action of the drug. For poorly absorbed drugs both the rate of absorption and bioavailability are likely to be altered by changes in gastrointestinal motility. However, the complex effects of food and disease, as well as the properties and formulation of any drug (solubility, ease of dispersion, delayed release formulation) often make the prediction of the magnitude, or even the direction, of any effect difficult to predict. Drugs with direct effects on gastrointestinal motility may influence their own patterns of absorption. In patients with gastrointestinal motility disorders, drugs administered in a controlled release formulation, or those with poor bioavailability, are most likely to have a poorly predictable therapeutic effect. Care should be taken to ensure that the formulation of the drug, its timing of administration in relation to meals and the use of coadministered drugs optimise, or at least ensure consistent absorption.

Stable isotope methodology for studying the performance of metoprolol Oros tablets in comparison to conventional and slow release formulations

Metoprolol Oros tablets were designed to deliver their drug content as a constant rate over a period of time longer than that currently recorded with slow-release dosage forms. The bioavailability of 7/95, 14/190 and 21/285 Oros tablets was compared to that of either 100 mg conventional or 200 mg slow-release Lopresor tablets in 3 two-period change over experiments. In each experiment, 6 healthy volunteers received intravenous deuterated metoprolol concomitantly with one of the Oros systems or with one of the other two formulations. The Oros tablets gave rise to lower and steady plasma levels of metoprolol over 24 h than the other formulations. Their mean absorption time was around 3 times longer than that of the slow-release tablets. The amount of the drug absorbed unchanged was linearly related to the dose. The influence of the gastrointestinal transit time on the performance of Oros tablet was limited. These studies demonstrated the value of the stable isotope methodology in bioavailability assessment for drugs presenting a high inter-subject variability in their plasma clearance such as metoprolol.

Novel oral drug formulations. Their potential in modulating adverse effects

The rationale for specialised oral formulations of drugs include prolongation of effect for increased patient convenience and reduction of adverse effects through lowered peak plasma concentrations. Local and systemic adverse effects due to high concentrations of drug can be minimised by the use of controlled release delivery systems. Local effects in the gastrointestinal (GI) tract from the release of irritant drug molecules can also be reduced, but the gastric damage caused by nonsteroidal anti-inflammatory drugs (NSAIDs) is only partially relieved by formulation approaches because of the involvement of systemic factors in the aetiology of GI adverse events. The advantages for each drug class must be examined. Newer dosage forms include: (i) osmotic pumps and zero order kinetics systems to control the release rate of the drug; (ii) bioadhesive systems and gastric retention devices to control GI transit; (iii) bioerodible hydrogels; (iv) molecular carrier systems (e.g. cyclodextrin-encapsulated drugs) to modulate local toxicity in the GI tract; (v) externally activated systems; and (vi) colloidal systems such as liposomes and microspheres. There is evidence for improved tolerability for a variety of drugs administered in novel delivery systems. However, the evidence for improved tolerability is complicated by the potential bias in adverse reaction reporting systems, and a lack of studies directly comparing conventional and modified release preparations. The technology now available to produce delivery systems which not only release drugs in a controlled and predetermined fashion, but which can also target to regions of the GI tract such as the colon, should allow greater control of therapy and potentially might minimise patient variables. However, the problem of variable GI transit times still eludes solution. Systems which rely on time to release drug might be more vulnerable to patient-to-patient variability than those which respond to local environments. The effect of food intake is more apparent on single-unit, nondisintegrating dosage forms, although of course none so far are immune from influence. The risk of new adverse effects resulting from such positional therapy with novel delivery devices must be considered. Understanding the mechanisms of induction of individual adverse effects can lead to advances in modes of delivery to decrease the potential for adverse reactions and events while maintaining therapeutic efficacy. Increased compliance can led to increased therapeutic control and hence safety. Each system has to be considered on its merits. No generalisations can be made, although invariably the modulation of high peak plasma concentrations diminishes adverse effects due to rapid absorption.

Localization of drug release sites from an oral sustained-release formulation of 5-ASA (Pentasa) in the gastrointestinal tract using gamma scintigraphy

Release of 5-ASA from a sustained release formulation (Pentasa, Ferring A/S, Copenhagen, Denmark) was monitored with plasma sampling for up to 24 hours in nine volunteers under both fasted and fed conditions. Drug absorption was correlated with location of the sustained-release microgranules in the gastrointestinal tract by gamma scintigraphy. Disintegration of the labeled tablet preparation occurred in the stomach within 20 minutes and acetylated 5-ASA was detectable in the plasma less than 60 minutes after ingestion. No significant differences were detected in either transit times through the small intestine, peak plasma acetylated 5-ASA concentration or lag time to absorption between fasted and fed individuals. Peak plasma concentration of acetylated 5-ASA usually occurred when the microgranules were present in the small intestine or ascending colon. The pharmacoscintigraphic study confirmed that 5-ASA release from the formulation occurred throughout the gastrointestinal tract, and that food effects on the in vivo behavior of the preparation were minimal.

Gamma scintigraphy in the evaluation of pharmaceutical dosage forms

Gamma-scintigraphy is applied extensively in the development and evaluation of pharmaceutical drug delivery systems. It is used particularly for monitoring formulations in the gastrointestinal and respiratory tracts. The radiolabelling is generally achieved by the incorporation of an appropriate technetium-99m or indium-111 labelled radiopharmaceutical into the formulation. In the case of complex dosage forms, such as enteric-coated tablets, labelling is best undertaken by the addition of a non-radioactive tracer such as samarium-152 oxide or erbium-170 oxide followed by neutron activation of the final product. Systems investigated include tablets and multiparticulates for oral administration, enemas and suppositories, metered dose inhalers and nebulisers, and nasal sprays and drops. Gamma-scintigraphy provides information on the deposition, dispersion and movement of the formulation. The combination of such studies with the assay of drug levels in blood or urine specimens, pharmacoscintigraphy, provides information concerning the sites of drug release and absorption. Data acquired from the scintigraphic evaluation of pharmaceutical dosage forms are now being used increasingly at all stages of product development, from the assessment of prototype delivery systems to supporting the product licence application.