Drug release retarding effect

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... 

Incorporation of surfactants in HPMC compressed matrices has been shown to cause retardation of drug release [110-112]. The first mechanism postulated was that anionic surfactants were capable of binding to nonionic polymers to increase the viscosity [110], More recently, drug-surfactant ionic interaction has been put forward instead, resulting in the formation of a complex with low solubility in water [111,112]. Note that such an effect can unintentionally occur upon putting some surfactant into the release medium in order to increase wettability. 

Prolonged-release (or slow-release) preparations are useful for water-soluble drugs with short biological half-lives. Hydrophobic CyDs, such as ethylated CyDs, with low aqueous solubilities have been demonstrated to work as slow-release carriers of water-soluble drugs such as isosorbide dinitrate, diltiazem hydrochloride, and 5-fluorouracil [19, 38, 39]. For example, 15 has the most prominent retarding effect for water-soluble molsidomine and diltiazem hydrochloride, after oral administration in dogs. The release of isosorbide dinitrate from 16 film was retarded and the plasma drug level after topical application of the film to rat abdominal skin was maintained at 100 ng mL for about 10 h [41]. 

Pariot N, Edwards-Levy F, Andry M-C, Levy M-C. Cross-linked P-cyclodextrin microcapsules. II. Retarding effect on drug release through semi-permeable membranes. Int J Pharm. 2002 232(1-2) 175-181. 

Because of their unique layered structure and highly tunable chemical composition based on different metal species and interlayer anions, LDHs have many interesting properties, such as unique anion-exchanging ability, easy synthesis, high bond water content, memory effect, nontoxicity, and biocompatibility. Based on these properties, LDHs are considered as very important layered crystals with potential applications in catalysis [6], controlled drugs release [7], gene therapy [8], improvement of heat stability and flame retardancy of polymer composites [9], controlled release or adsorption of pesticides [10], and preparation of novel hybrid materials for specific applications, such as visible luminescence [11], UV/photo stabilization [12], magnetic nanoparticle synthesis [13], or wastewater treatment [14]. 

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