Analgesic effect of percutaneously absorbed non-steroidal anti-inflammatory drugs: an experimental study in a rat acute inflammation model

BACKGROUND: External medication that is absorbed percutaneously may be used to reduce inflammation and relieve pain from acute injuries such as ankle sprains and bruises. The plaster method of percutaneous absorption for non-steroidal anti-inflammatory drugs (NSAIDs) was established in Japan in 1988. However, due to the possibility of a placebo effect, the efficacy of this method remains unclear. This experimental study was conducted to control for the placebo effect and to study the efficacy of the plaster method in relieving pain by using a rat model of inflammation. METHODS: Male Wistar-Imamichi rats were used. A yeast suspension was injected into the right hind paw to induce inflammation. A sheet (2.0 × 1.75 cm) containing the drug was adhered to the inflamed paw. Five treatment groups were used, and each sheet contained a single drug: loxoprofen sodium (loxoprofen-Na) (2.5 mg); felbinac (1.75 mg); indomethacin (1.75 mg); ketoprofen (0.75 mg); or base only (control, 0 mg). Mechanical pain threshold, expression of c-Fos in the dorsal horn, and amount of prostaglandin (PG) E(2 )in the inflamed paw were evaluated. RESULTS: Pain threshold increased after treatment, and was significantly increased in the loxoprofen-Na group compared with the control group (p < 0.05). Amounts of PGE(2 )were significantly decreased in the loxoprofen-Na and indomethacin groups compared with the control group (p < 0.05). Expression of c-Fos was significantly decreased in the loxoprofen-Na group compared with the control group (p < 0.05). CONCLUSION: Percutaneously absorbed NSAIDs have an analgesic effect, inhibit expression of c-Fos in the dorsal horn, and reduce PGE(2 )in inflamed tissue, indicating the efficacy of this method of administration for acute inflammation and localized pain.