Coating dissolution medium

A recent addition to General Chapter Dissolution <711> allows for the addition of enzyme to the dissolution medium. This is justified when gelatin-coated tablets or gelatin-encapsulated products exhibit a slow-down in the dissolution rate attributed to the crosslinking of the gelatin. The addition of enzyme will break up the crosslinking and therefore remove 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... 

Figure 3 Effect of the pH of the dissolution medium on the release of mesalazine tablets tested in pH 6.5 (open symbols) and 7.0 (closed symbols) mixed phosphate buffers. The mesalazine core tablets were coated with Eudragit LlOO-55 and Eudragit SlOO combinations of 1 4 (squares), 1 5 (triangles), and 0 1 (circles) ratios (w/w) and tested in 0.1 N HCl for 120 min prior to the buffer stage. The dissolution test was performed using United States Pharmacopoeia method B for extended-release tablets. The vertical bars indicate standard errors of the means (n = 6). (Adapted from Ref. 35.)...

Hypromellose or hydroxypropylmethylcellulose is a partly O-methylated and 0-(2-hydroxypropylated) cellulose [25]. It is used from 10% up to 80% w/w for controlled drug release in solid dosage form and from 2% to 20% w/w as coating solution for tablets and pellets [25,30,31 ]. Hydroxypropylmethylcellulose is a nonionic polymer with pH of a 2% w/w aqueous solution ranging from 5 to 8. It is soluble in water. Being nonionic, it will not form complexes with metallic salts or ionic compounds that can possibly lead to the precipitation of insoluble compounds [32]. Hydroxypropylmethylcellulose matrix hydrates and swells into a gel layer in the direction of matrix surfaces to core when it is contacted with the dissolution medium [14,20,33]. Erosion of gel takes place thereafter and may occur simultaneously with the subsequent phases of matrix hydration and swelling [14]. The swelling and erosion properties of a solid matrix made of... 

Close inspection of USP Apparatus 1 and 2 before use can help identify sources of error. Obviously, dimensions should be as specified. In cases of both baskets and paddles, shafts must be straight and true. The paddles are sometimes partially coated with Teflon. This coating can peel and partially shed from the paddle, causing flow disturbance of hydrodynamics within the vessel. Paddles can rust and become nicked or dented this can adversely affect dissolution hydrodynamics and be a source of contamination. Thorough cleaning of the paddles is also important, to preclude carry over of drug or medium. 

A survey of the German market showed that more than 50% of enteric formulations were coated with methacrylate copolymers, about 40% with cellulose derivatives, 5% with shellac, and 3% with other materials [1], Enteric coating materials (Table 1) are described in various publications [21, 22], In addition to polymers mentioned in Table 1, others are being studied (e.g., to obtain release at lower pH) [23], Polymers with a dissolution at lower pH are intended for the protection of drugs in acidic medium and not for the protection of the gastric mucosa. 

Typically, coatings (e.g., enteric coatings) are intended to delay the release of medication until the dosage form has passed through the acidic medium of the stomach. In vivo tests for delayed-release drug products are similar to those for extended-release drug products. In vitro dissolution tests for these products should document that they are stable under acidic conditions and that they release the drug only in a neutral medium (e.g., pH 6.8). 

Some problems associated with conventional LLC (e.g., the loss of the liquid stationary phase through dissolution in the mobile phase) may be obviated by chemically bonding the liquid stationary phase to the support medium. This type of liquid-liquid chromatography is designated bonded phase chromatography (BPC)(11). Since the properties of bonded phases may differ substantially from those of coated phases, BPC separation characteristics may differ from those of conventional LLC. Many phases have exhibited increased efficiency when bonded to the support medium. Most current reverse phase HPLC work involves the use of stationary phases bonded to microparticles. 

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