El carbopol (carbomer), es un polímero derivado del ácido acrílico, entrecruzado con alil sucrosa o alilpentaeritritol, es soluble en agua, alcohol y glicerina (Handbook of Pharmaceutical Excipients, 2000), es un ácido débil y posee un gran peso molecular (aproximadamente 3500000 daltons) (Jacques y col., 1997). Algunos de estos polímeros, como el 934 P o el 974 P, poseen la capacidad de sostener la cesión de las drogas dispersas en la matriz y son aptos para su administración por vía oral.
Su utilización como matriz para la liberación controlada de drogas, y sus propiedades mucoadhesivas han sido ampliamente estudiadas (Singla y col., 2000). Como estos compuestos son polianiónicos poseen la capacidad de formar complejos poliiónicos con otros policationes, como el chitosán (Wang y col., 1997). El hinchamiento del carbopol 934 P NF depende del pH del medio, proporciona una cesión sostenida del principio activo en medio gástrico, pero cede más rápidamente el fármaco en medios de pH más alto, es decir en el tracto intestinal (Kratz y col., 2000, Nakanishi y col., 1998, Slovin y col., 1997, Shin y col., 1996 y Meshali y col., 1996). Asimismo, estos compuestos han sido utilizados con éxito como agentes protectores de mucosa (Copeman y col., 1994).
Figura 8. Estructura química del ácido acrílico. Este compuesto es un ácido débil, que se ioniza a pH > 4,5, y puede formar puentes de hidrógeno con otras moléculas de ácido acrílico (Annaka y col., 1992).
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II.8. Direcciones de Internet.
- HAN, Sang-Mun, Ph. D. 's Chitin and Chitosan Sciences: http://www.user.chollian.net