Rheological properties of reversible thermo-setting in situ gelling solutions with the methylcellulose–polyethylene glycol–citric acid ternary system

The composition of vehicle on the reversible sol-gel transition temperature in a ternary system made up of methylcellulose (MC), polyethylene glycol (PEG), and citric acid (SC) was investigated. The properties of the in situ gelling system were estimated by rheological measurement. When PEG (4000) concentration was varied from 0% to 10% while MC (SM25) and SC concentrations were kept constant at 1.5% and 3.5%, respectively, the reversible sol-gel transition temperature lowered from 38 °C to 26 °C with increasing PEG concentration. However, the extent of lowering in temperature was not influenced by the molecular weight of PEG. The reversible sol-gel transition temperature shifted towards the lower temperature with increasing MC concentration, and towards the higher temperature with decreasing pH. Comparison of rheological properties between the present thermo-setting in situ gelling solution and a conventional one, such as gellan gum solution or Poloxamer 407 solution, revealed that the present solution radically differed from the conventional solutions in the incipient gelling mechanism. These findings suggest that the ternary system in this study would be useful as a drug delivery system for instillation of drugs into the eye.