Influence of coating and core modifications on the in vitro release of methylene blue from ethylcellulose microcapsules produced by pan coating procedure

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Abstract
Core pellets containing methylene blue, a highly water-soluble drug, lactose and polyvinylpyrrolidone (PVP) were produced and coated by pan technology with ethylcellulose 7 cp. Scanning electron microscopy revealed that larger microcapsules were aggregates of smaller microcapsules which remained intact during the dissolution testing. The coating appeared porous with rough and smooth areas. Polymer disc studies confirmed a lack of swelling of the ethylcellulose used when in contact with the dissolution medium. Modifications of the coating applied included addition of hydroxypropylmethylcellulose (a cofilm former), diethyl phthalate, di-n-butylphthalate and castor oil (plasticizers), talc (an extender and anti-tack agent) and paraffin wax and hydrogenated castor oil (waxy sealants) to the coating solution. Most products had released 50% drug after about 20 min dissolution treatment although the inclusion of the waxy sealants in the coating markedly retarded release during the first 60 min. In contrast, the inclusion of hydrogenated castor oil or polyethylene powder in the core, retarded dissolution of the drug over 3 h but tended to produce a coarser product. Other modifications of the core were addition of hydroxypropylmethylcellulose, carnauba wax-beeswax (1:1) and carnauba wax, with little retardation of drug release. The mechanism of drug release from the microcapsules was complex involving mainly diffusion.