Assay of topically administered ibuprofen using a model of post-injury hypersensitivity

Objective: To develop a reliable assay for quantifying the analgesic efficacy of non-steroidal anti-inflammatory drugs (NSAIDs) using a model that is accepted as a paradigm of clinical pain. Subjects: Fifteen normal subjects, all of whom were volunteers from medical school staff, took part in the study. Methods: Capsaicin (20 μl) in solution (0.03 mg/ml) was applied to the volar surface of the forearm, and the skin was maintained at a constant temperature using a thermal stimulator. The magnitude of the surrounding area of mechanical allodynia to a brush stimulus (i.e. a clinical correlate of tenderness to touch) was assessed. Under double-blind, placebo-controlled conditions, the test was repeated using skin previously treated with ibuprofen gel or placebo. Results: A close linear relationship was observed between skin temperature over a range of 30 °C to 40 °C and the area of capsaicin-induced allodynia. Ibuprofen gel significantly reduced (P < 0.004) the area of touch-evoked allodynia at a constant skin temperature of 40 °C. Conclusions: The thermal-facilitated adaptation of the capsaicin model described in this study represents an inexpensive and reliable assay for the effects of topical formulations of NSAID upon mechanical sensitivity. As such, it is a potential alternative to many clinical studies in which inherent confounding and bias can preclude a meaningful conclusion. Received: 13 September 1999 / Accepted in revised form: 14 March 2000