The Rate a Drug is Released in the Body

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Rate of drug release at any given location within gastrointestinal tract depends on the mechanism by which the drug released from pellets and local environment within git. In order to predict in vivo behavior of dosage forms, knowledge of release mechanism and physiological conditions of git should be understood.
Type of coating and method of applying coating will determine dominant mechanism of drug release from coated pellets. Release kinetic is predominantly determined by the behavior of coating material under physiological conditions of git. Multiparticulates coated for extended release purpose; release the drug by eighter diffusion or erosion or combination of these mechanisms.
Pellets coated with polymer not soluble under gi conditions releases drug by 4 different mechanisms:
1. Solution/ diffusion through the continuous plasticized polymer phase
This mechanism is based on assumption that the polymer forms a continuous phase in which the plasticizer & other additives are homogeneously dispersed. When polymer film is formed, it contains molecular sized openings between crosslinked chains of polymer. Plasticizer & other additives affects wetting of these openings as well as interchain dimensions by changing the crosslinking properties of polymer chains which are critical for diffusion of drug. Mostly, drug molecules diffuse through these openings in a process known as hindered molecular diffusion. It is an activated process wherein thermal fluctuations of chain segments
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