Biological activity of insulin in GMO gels and the effect of agitation

Glyceryl monooleate (GMO)-water cubic phase gel was previously shown to protect insulin from agitation induced aggregation. However, it is not known if insulin is biologically active in the gel and what effect agitation has on insulin in the gel. Therefore, the objective was to determine the stability of insulin in cubic phase gel in terms of its biological activity in a suitable animal model such as Sprague-Dawley rats. Effect of agitation on biological activity of insulin in cubic phase GMO gel was determined by subcutaneous injections of the agitated and non-agitated gels to two groups of previously fasted rats and measuring the effect on their blood glucose levels. Two groups of rats administered with agitated insulin solution and normal saline were used as controls. The biological activity of insulin was evaluated by comparing AAC (area above the blood glucose level-time curve, in %-h), C-max (maximum % decrease in blood glucose levels) and t(max) (time required to attain C-max in h) values for the four groups of rats. Since cubic phase gel is highly viscous, therapeutic equivalency of insulin in the lamellar phase gel, which converts in situ into cubic phase gel, was compared to insulin solution with normal saline as the control, using AAC, C-max and t(max) of the blood glucose profile. Insulin was biologically active in both agitated and non-agitated gels; however, upon agitation, insulin in solution totally lost its hypoglycemic activity. Agitation of insulin in the cubic phase gel was seen to have very little deleterious effect on its biological activity. Insulin in the lamellar phase gel was not only biologically active but also therapeutically equivalent to insulin solution based on AAC (327.9 +/- 100.8 and 431.7 +/- 113.3), C-max (57.1 +/- 7.0 and 70.2 +/- 5.5) and t(max) (2.8 +/- 0.7 and 4.0 +/- 1.7) for the lamellar phase gel and insulin solution, respectively (no significant difference, P > 0.05). In summary, GMO cubic phase gel protected insulin from agitation induced aggregation, and insulin was biologically active in the gel. (C) 1999 Elsevier Science B.V. All rights reserved.