The use of xanthan gum in an ophthalmic liquid dosage form: rheological characterization of the interaction with mucin.

The development of an ocular dosage form containing xanthan gum and capable of interacting with mucin in the precorneal area is a challenge. The polymer concentration that can be applied is restricted because of the limited patient acceptability of highly viscous preparations. The precorneal mucin concentration is low and the high ionic strength of the lachrymal fluid forces xanthan gum in an ordered structure, less capable of interacting through heterotypic junctions. Intrinsic viscosity measurements and shear rheometry are used to investigate the effect of several factors (polymer concentration, additional boiling or sonication step to the preparation procedure) on the physicochemical properties of xanthan gum and the degree of interaction with a low (8%, w/v) and high (16.0%, w/v) concentrated mucin dispersion. Independent of the preparation procedure applied, a xanthan gum concentration of 1.0% (w/v) is required to obtain a measurable interaction with mucin. If an extra boiling or sonication step is added to the standard preparation procedure, the minimum mucin concentration necessary to achieve formation of heterotypic junctions is decreased. Only by sonication of the highly concentrated xanthan gum dispersion is the viscosity decreased to a level that is tolerable and comfortable to the patient. The findings of the present study clearly demonstrate that a significant interaction between a tolerable and comfortable ocular dosage form containing xanthan gum, and mucin 8% (w/v), is feasible after sonication of a highly concentrated polymer dispersion.