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Modified Bitter Yam (Dioscorea dumetorum) Starches as Immediate and Sustained Release Polymers in Bilayer Tablets of Metformin Hydrochloride

Adenike Okunlola

Bilayer tablets can control the delivery rate of either a single or two different active pharmaceutical ingredients, solving the problem of repetitive dosing associated with conventional tablets. The use of suitable excipients such as modified starches can control the release of drugs from the bilayer tablets while imparting mechanical strength. The aim of the research is to formulate bilayer tablets of metformin hydrochloride using bitter yam starches (Dioscorea dumetorum) modified by Carboxymethylation and acid hydrolysis as immediate and sustained release polymers respectively. The native and modified starches were characterized using morphology, FTIR, density and flow properties. Bilayer tablets of metformin hydrochloride were prepared using carboxymethylated (2.5 and 5% w/w) and acid-modified (10.0% w/w) bitter yam starches with Sodium Starch Glycolate (SSG) and Hydroxy Propyl Methyl Cellulose (HPMC) as standards. The bilayer tablets were evaluated for Crushing Strength (CS), Friability (FR), Disintegration Time (DT) and dissolution time, t90 (time taken for 90% drug release). The degree of substitution of carboxymethylated bitter yam starch was 0.38. The swelling of the starch increased with carboxymethylation but reduced with acid hydrolysis. Modification of starch resulted in improved flow. Bilayer tablets containing the modified starches gave lower friability and higher CSFR/DT ratio than those containing SSG and HPMC. The bilayer tablets showed initial burst release to provide the loading dose of drug followed by controlled release up to over 10 hours. The in vitro dissolution kinetics generally followed the Higuchi and Korsmeyer-Peppas models.