Efecto del bicarbonato de sodio sobre la flotación y la liberación controlada de metronidazol desde matrices de Methocel K4M y Carbopol 971P NF

In vitro dissolution of metronidazole (150 mg) from a controlled release floating tablet has been studied varying the proportion of sodium bicarbonate (BS) (8-24%) and the matrix polymer, Methocel K4M and Carbopol 971P NF (200 mg). All Methocel matrices floated more than 8 hours while Carbopol matrices floated from few minutes up to 4 hours. As well known, matrices hydration volume increases with time and reach a maximum, some times outside 8 hours, showing thereafter a declining behavior. Methocel matrices show greater hydration volumes and faster metronidazole dissolution compared to Carbopol matrices. Increasing proportions of BS in Methocel matrices produce decreasing release constant values (k) and increasing exponent (n) values, when using the equation: M_t/M_inf = k* tⁿ. Carbopol matrices show faster and entire water penetration while Methocel matrices hydrate rapidly only at the surface delaying the entire hydration and preserving for longer time the CO₂ bubbles. The effect of the gas bubbles is attributed to matrix expansion and to obstruction of the matrix available area for diffusion.