Optimisation of carbomer viscous eye drops: an in vitro experimental design approach using rheological techniques.

The optimisation of the in vitro interaction between several poly(acrylic acid) derivatives (Carbopol 1342P NF, Carbopol 974P and Carbopol 980 NF) and mucin was performed by an analysis technique combining oscillatory shear rheology and experimental design in order to improve the formulation of carbomer viscous eye drops.First, standard oscillation procedures were used to characterise the polyacrylic acid and mucin dispersions, and to investigate the influence of several polymer-related factors (concentration, preparation, type of polymer used) on the rheological properties. Second, an experimental plan design was developed to investigate the effect of polymer-related factors on the mucoadhesive indexes (MAI(G') and MAI(G")) which were calculated using the viscoelastic data obtained from polymer/mucin, polymer/tearfluid and mucin/tearfluid mixtures. Optimal mucoadhesive interactions were determined based on the experimental design results. Finally, the optima were fully characterised rheologically to further verify the mucoadhesive capacity. The main conclusion is that the factor influencing most explicitly the mucoadhesive interaction of the viscous eye drop is the mucin concentration and neither the type of polyacrylic acid, nor its concentration.