Hydrophilic gel-forming matrix systems are widely used in oral controlled-release dosage forms. Hydrophilic polymer hydrates form a viscous gel layer around the tablet surface, and drug release is controlled in a sustained manner by diffusion through a gel layer and the erosion of the gel (1). Drug solubility, however, greatly affects the rate of diffusion and erosion. It is especially difficult to control the release rate of highly water-soluble and insoluble drugs (2–4). Moreover, various gastrointestinal factors such as ionic strength and mechanical destructive force also affect the drug-release rate (5–8). To solve these problems or predict these influences, many studies have examined factors affecting drug release.
Materials. Model drugs. Ethenzamide (ETZ), acetaminophen (APAP), and sodium salicylic acid (SSA) were used as model drugs and may be characterized as water-insoluble, moderately water-soluble, and highly water-soluble drugs, respectively. All drugs were purchased from Yoshitomi Pharmaceutical (Tokyo).
Matrix excipients. HS was prepared by Asahi Kasei Chemicals (Tokyo). Hydroxypropyl methylcellulose [(HPMC) Metolose 90SH-100SR, Metolose 90SH-4000SR, Metolose 10000SR] was purchased from Shin-Etsu Chemical (Tokyo).
Other excipients. Partially pregelatinized corn starch (Starch 1500) was purchased from Colorcon (West Point, PA). Fully pregelatinized corn starch (Amycol C) and fully pregelatinized potato starch (Amycol HF) were purchased from Nippon Starch Chemical (Osaka, Japan). Microcrystalline cellulose (MCC), Ceolus KG-802, was manufactured by Asahi Kasei Chemicals. PEG (Macrogol 6000) was supplied by Sanyo Chemical (Kyoto, Japan). Sorbitol (Sorbitol SP) was purchased from Kowa Pharmaceutical (Nagoya, Japan).
Evaluation of properties of modified starches and HPMC. Average particle diameter. Measurement samples of 5 g were sieved for 5 min using an air-jet JIS sieve of 20, 38, 75, and 150 µm-mesh. The weight percent remaining on each sieve after sieving was calculated, and the particle diameter was calculated as the cumulative weight percent of 50%.
Water-soluble content. Sample dispersions of 1% were prepared at 20 °C and were centrifuged for 15 min at 5000 G. The supernatant was dried at 105 °C until a constant weight was reached. The dry weight was defined as the amount of soluble content and was expressed as a percentage of the initial weight of the samples.
Viscosity. The viscosity of 2% sample dispersions was determined at 25 °C with a rotary viscometer (TVB-10, Toki Sangyo, Tokyo) using an M1 body.
Preparation of matrix tabletsA model drug, a matrix excipient, and other excipients were physically admixed. The mixtures of 180 mg were compressed using an 8.0-mm diameter round-faced punch at compression pressure of 60 MPa.
Drug-release study. The dissolution tests were carried out at 37 °C ± 0.5 °C using a USP Type 1 apparatus or a USP Type 2 apparatus rotating from 50 to 200 rpm. The test media consisted of 900 mL of second fluid of the Japanese Pharmacopoeia (14th edition) (JP-2, pH 6.8, and ionic strength 0.14 M) and McIlvaine buffer (pH 7.2 and ionic strength 0.39 M).
Stability test under accelerated conditions. Dissolution profiles and the yellowness index (YI) of HS and HPMC matrix tablets were evaluated after storage at 40 °C and 75% relative humidity (RH) in sealed glass bottles, and at 60 °C in polyamine–polyethylene bags. The YI was measured by a spectrophotometer (SE200, Nippon Denshoku, Tokyo).