Assay of amoxicillin sustained release from matrix tablets containing different proportions of Carbopol 971P NF.

The sustained release of amoxicillin is desired because of its short biological half-life. Particularly to treat Helicobacter pylori infections, the sustained release is desired to be confined to the upper gastrointestinal tract. In vitro dissolution of amoxicillin has been evaluated utilizing a direct UV-absorption method. However, UV-absorption has been reported as not useful to determine amoxicillin in acidic dissolution test medium. To clarify the suitability of the assay method, the stability and dissolution behavior of amoxicillin sustained release tablets was determined by HPLC, iodometric titration and UV-absorption. Stability of amoxicillin studied under dissolution test conditions of pH 1.2, 37 degrees C and 50 rpm and determined by HPLC and titration showed considerable degradation of amoxicillin. On the other hand, the UV-absorption increased progressively as amoxicillin degradation proceeded. Amoxicillin release curves determined by different analytical methods show different release profiles, which can be corrected for amoxicillin degradation and change in the UV-absorption to produce similar dissolution results. Release curves determined by UV-absorption and obtained from tablets containing 1017 mg amoxicillin trihydrate and Carbopol 971P NF in a range from 180 to 680 mg showed increasing values of the exponent indicative of the release mechanism (n) and decreasing release constant values (k) as the matrix polymer content increased. The release constant (k) and the exponent (n) were found to be logarithmically related.