Development of forskolin microemulsion formula and its irritation test on rabbits

INTRODUCTION: This study aims to develop a microemulsion formula that can increase the solubility and stability of forskolin and is safe for topical use
METHODS: The materials used for the development of the microemulsion formula were triglyceride oil, nonionic surfactants, and polyethylene glycol for cosurfactants which were selected based on the results of the forskolin solubility test using high performance liquid chromatography. Microemulsion was formulated by phase titration method. Formula stability was determined on storage for 90 days at refrigerator, room temperature, and accelerated stability test by determining globule size, forskolin concentration, and pH. The safety of using microemulsion was determined by skin irritation tests on albino rabbits.
RESULTS: The optimum microemulsion formula consisted of Maisine CC, polyoxyethylene sorbitan 20, and polyethylene glycol 400 with a ratio of 4: 25: 5 w/v which increased the solubility of forskolin the most, namely 2.19 mg.mL-1. Based on globule size (<50nm), forskolin concentration (2 mg.mL-1), and pH (6.0-6.35), the formula was stable in refrigerator storage and room temperature, but unstable in the accelerated stability test (40ºC) starting on day 21. This optimum formula shows a primary irritation index of 0.11 which is categorized as very weak irritation and can be ignored.
DISCUSSION AND CONCLUSION: The microemulsion prepared by the phase titration method containing Maisine CC, polyoxyethylene sorbitan 20 and polyethylene glycol 400 (4: 25: 5, w/v) as a base and 0.2% forskolin, is stable in refrigerator storage and room temperature. This microemulsion is very mild or negligible.