Stability of methimazole in poloxamer lecithin organogel to determine beyond-use date.

The purpose of this stability study was to determine the beyond-use date of the active ingredient methimazole extemporaneously prepared for transdermal absorption to treat feline hyperthyroidism. The methimazole was prepared in poloxamer lecithin organogel and subjected to a three-month stability study. It was determined that under room temperature conditions (25 degrees Celsius +/- 2 degrees/60% RH +/- 5%) the methimazole concentration remained above 90% after 62 days. An accelerated condition of 35 degrees Celsius and refrigerated condition of 5 degrees Celsius were also employed to see how temperature affected the stability of the drug. Under all three conditions the methimazole concentration dropped below 90% after day 62. It was observed that the refrigerated conditions appeared to be least stable both physically and chemically. A control sample was also prepared, stored, and packaged similarly to the sample preparations. As a result, the beyond-use date was determined to be 60 days at room temperature. The drug preparation should not be stored in a refrigerated condition.