The Design of Experiment Approach, Rheology for Optimization of a Topical Anti-inflammatory and Analgesic Cream

Background: A capsaicin cream was formulated by optimizing the rheological stability, the release behavior of the drug, and the pharmacological effect. Objectives: This study aimed to: (a) apply the Design of Experiment approach to study the rheological stability and release behaviors of a drug (capsaicin) from a formulated oil-in-water cream and (b) investigate the skin irritation and anti-inflammatory and analgesic effects of the optimized cream. Methods: The cream prepared by the emulsification method was optimized using the central composite design, and then the pharmacological effect in experimental animals was determined using Complete Freund's adjuvant (CFA). Results: The effects of a permeation enhancer (X1), Vaseline (X2), and surfactants (X3) on the fluctuation of the ratio of the viscous modulus (G ') to elastic modulus (G') (tan δ) after three cycles of cooling-heating (10-40°C), flux, and skin deposition of capsaicin after 8 h on mouse skin were statistically analyzed and optimized. The final obtained CAP-cream did not cause irritation in the rabbit model and produced comparable anti-inflammatory and analgesic effects to the reference product (Voltaren® emulgel). Conclusion: This study successfully integrated the DoE approach, rheological science, and pharmacological studies to develop a stable and highly effective semi-solid product containing capsaicin.