Physiological and Pharmacokinetic Factors Affecting Performance of Sustained Release Dosage Forms

Design of controlled release delivery systems using a modified pharmacokinetic approach: a case study for drugs having a short elimination half-life and a narrow therapeutic index

The objectives of peroral controlled release drug delivery systems (CRDDS) are to maintain therapeutically effective plasma drug concentration levels for a longer duration thereby reducing the dosing frequency and to minimise the plasma drug concentration fluctuations at steady state by delivering drug in a controlled and a reproducible manner. Drug delivery rate, duration of delivery and the dosing interval are the target features for any temporal CRDDS. The classical pharmacokinetic model for designing CRDDS [Drug Dev. Ind. Pharm. 15 (1989) 1073] assumes the time of drug delivery (t(del)) to be less than the dosing interval. However, termination of drug release from such a CRDDS at t(del) and/or a declining drug input function towards the terminal phase of t(del) from a first order kinetic CRDDS can have severe implications on plasma drug concentration and steady state fluctuations for a drug with very short half-life. A case study is presented in this paper, wherein by means of theoretical calculations using a classical pharmacokinetic approach, it is shown that a first order kinetic CRDDS for hypothetical drugs with short elimination half-life and different pharmacokinetic conditions would result in sub-therapeutic plasma concentrations at least for some time during the dosing interval at steady state. In order to avoid sub-therapeutic plasma drug concentrations a modification in classical pharmacokinetic model is proposed and discussed through theoretical calculations for different hypothetical pharmacokinetic situations and a practical single dose pharmacokinetic study with a first order kinetic CRDDS for nifedipine (a short half-life drug; about 2h). It is shown that improved therapeutic efficacy could be expected from a CRDDS developed based on proposed modification in the classical pharmacokinetic model.

Microspheres as floating drug-delivery systems to increase gastric retention of drugs

Gastric emptying is a complex process, which is highly variable and makes in vivo performance of the drug-delivery systems uncertain. In order to avoid this variability, efforts have been made to increase the retention time of the drug-delivery systems for more than 12 h. The floating or hydrodynamically controlled drug-delivery systems are useful in such applications. The present review addresses briefly the physiology of the gastric emptying process with respect to floating drug-delivery systems. In recent years, the multiparticulate drug-delivery systems are used in the oral delivery of drugs. One of the approaches toward this goal is to develop the floating microspheres so as to increase the gastric retention time. Such systems have more advantages over the single-unit dosage forms. The development of floating microspheres involves different solvent evaporation techniques to create the hollow inner core. The present review addresses the preparation and characterization of the floating microspheres for the peroral route of administration of the drug.

Intragastric positioning of two concurrently ingested pharmaceutical matrix dosage forms

A triple nuclide scintigraphic technique is presented for the precise and lasting intragastric monitoring in man of two concurrently ingested hydrophilic matrix pharmaceutical forms. After in vitro testing of various labels, ion exchange resin-coupled 201T1- and 111In-oxinate were selected for their release characteristics. 99mTc-pertechnetate oral solution was used to selectively delineate the stomach. This technique was validated on healthy volunteers. It allowed measurement of the effects on gastric residence time of galenic parameters (size, density of matrices), as well as of physiological parameters such as subject posture.

Food and fasting absorption of a single dose of a sustained release theophylline sprinkle formulation in children

The bioavailability and absorption pattern of theophylline from a single dose of a slow release theophylline sprinkle product (Somophyllin) were investigated in 10 asthmatic children both in fasting conditions and after a standardized breakfast. Theophylline given intravenously was used as a reference. The fasting absorption of Somophyllin was rather fast with peak serum theophylline levels 3-5 h (mean 3.7 h) after dosing. Food produced a small but statistically significant reduction in the rate of absorption of theophylline, so that the mean time to peak serum theophylline level was 5.6 h (range 4-8 h) after food. In no case was there any important difference between the absorption profiles on the test days, and the bioavailability was complete after both fasted and fed intake of the product (92.5% and 105%, respectively). It is suggested that to obtain the optimum absorption profile children should take Somophyllin with food rather than between meals.

Influence of food on the absorption of theophylline from a sustained release formulation (Somophyllin)

The bioavailability and absorption pattern of theophylline from a sustained theophylline sprinkle product (Somophyllin) were investigated in ten healthy adult volunteers both in fasting conditions and after a standardized solid meal. Theophylline given intravenously was used as a reference. The only effect of the meal was a statistically significant reduction in the rate of absorption of theophylline during the first 2 hr after medication. After that time food had no effect upon the absorption characteristics of the preparation and the bioavailability was complete after both fasted and fed intake of the product (96.1% and 105.9% respectively).

The effect of diet upon serum concentrations of theophylline

Eight patients with airways obstruction were investigated to determine if dietary components had any significant effect upon serum concentrations of theophylline when taking a slow release formulation. A high carbohydrate, low protein diet, given for 1 week, resulted in the area under the concentration time curve for 12 h being 33.3% greater than a high protein low carbohydrate diet given under the same conditions. In contrast to two previous studies no significant effect upon the elimination half-life was detected after changing from one diet to the other and an effect of dietary components upon absorption and/or distribution needs to be considered. Morning and afternoon trough levels were significantly different from each other on both diets suggesting that theophylline kinetics are different at night. Individualising of theophylline dosing is particularly needed when treating patients on a varied dietary intake.