Comparative analgesic efficacy of nimesulide and diclofenac gels after topical application on the skin

Nimesulide is a newer non-steroidal anti-inflammatory drug (NSAID) with selective cyclo-oxygenase (COX)-2 blocking property and has demonstrated a potent analgesic and anti-inflammatory activity on oral and rectal administration. However, the Cmax through both these routes is reached only after 3 h of administration. Dose-dependent gastrointestinal side effects also limit the concentration of drug that can be achieved at the site of inflammation when administered through these routes. The present study was conducted to evaluate the antinociception induced by a new gel formulation of nimesulide when applied on the skin. Efficacy of topical nimesulide gel 1% (w/w) was studied on mice in the acetic-acid-induced writhing, tail flick latency (TFL) test and formalin-induced pain models. The antinociceptive effect of nimesulide was compared to diclofenac gel (1% w/w). Both the drugs induced dose-dependent analgesia with peak effect seen between 90 and 120 min after treatment. Greater antinociceptive effect (expressed as percent maximum possible effect) was seen in the writhing test than in the TFL test, indicating the peripheral action of both drugs. Nimesulide evidenced significant protection in the first phase of formalin-induced pain indicating modulation of peripheral nociceptors unlike other conventional NSAIDs. This suggests that COX-2 may be a primary contributor to afferent evoked increase in prostanoid-mediated changes in pain processing. Antinociception seen following drug application on the skin was lower than that seen on intraperitoneal administration, indicating localised action following topical application. The findings of the present study suggest that the transgel formulation of nimesulide provides significant analgesic activity when applied topically.