Drug release retardants

Drug release profiles from the tablets in various dissolution media are shown in Fig. 2. In all cases the release rates decreased initially from the control (distilled water) as electrolyte concentration increased, until a minimum release rate was obtained. As the electrolyte concentration further increased the release rates similarly increased until a burst release occurred. These initial decreases in release rates were probably coincident with a decrease in polymer solubility, in that as the ionic strength of the dissolution medium is increased the cloud point is lowered towards 37°C. It may be seen from Table 5 that minimum release rates occurred when the cloud point was 37°C. At this point the pore tortuosity within the matrix structure should also be at a maximum. It is unlikely to be an increase in viscosity that retards release rates since Ford et al. [1] showed that viscosity has little effect on release rates. Any reduction in hydration, such as that by increasing the concentration of solute in the dissolution media or increasing the temperature of the dissolution media, will start to prevent gelation and therefore the tablet will cease to act as a sustained release matrix. 

It was shown that liposomes, due to their structure, have a retarding effect on the incorporated drug release. In early studies, Knepp et al. reported that progesterone release from agarose gel was faster than from liposomes embedded in the gel [29]. This retarding release behavior from liposomes was further confirmed by a lower drug transport rate as compared to the gel measured across hairless mouse skin [30], Another study by Foldvari et al. [8] examined the... 

Modification of the temporal and spatial aspects of drug release using coating involves applying a layer or layers of retardant material between the drug and the elution/dissolution medium. If the coating material is... 

This kind of retarded drug release is employed when a rapid rise in blood levels of drug is undesirable, or when absorption is being slowed in order to prolong the action of drugs that have a short sojourn in the body. 

Fig. 20 Thymine-functionalized poly(styrene) for time-retarded drug release...

Enteric-coated systems (ECS) utilize polymeric coatings that are insoluble in the gastric media and therefore, prevent or retard drug release in the stomach. Various types of ionizable polymers are commercially available. They dissolve at various pH ranging between 4.8 and 7.2. ECSs are generally applicable to four major types of drugs ... 

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