Dose-dependent competitive block by topical acetylsalicylic and salicylic acid of low pH-induced cutaneous pain

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Abstract
In a human acid pain model, which uses continuous intradermal pressure infusion of a phosphate-buffered solution (pH 5.2) to induce localized non-adapting pain, the flow was adjusted to result in constant pain ratings of about 20% or 50% on a visual analog scale (VAS). Six volunteers in each group participated in 4 different placebo-controlled double-blind cross-over studies to measure rapidly evolving cutaneous analgesia from topically applied new ointment formulations of acetylsalicylic acid (ASA) and salicylic acid (SA) as well as of commercial ibuprofen and benzocain creams. Similar, log-linear dose-response curves were found for both ASA and SA, significant in effect at 3 g/kg and higher drug contents and reaching saturation level at 15 or 30 g/kg, respectively, which, 20 min after application, caused a mean pain suppression of 95% using ASA and 80% using SA. Half-maximal effects were achieved using 3 g/kg ASA or 15 g/kg SA. The SA action was also clearly slower to develop. With an increased flow of the acidic buffer, producing lower effective tissue pH and more intense pain, the effect of ASA and SA decreased to 73% pain suppression. A competitive mechanism of both drug effects was suggested by the fact that, with 15 g/kg ASA and SA, pain reduction could be reversed by increasing the buffer flow by a factor of 1.75, on average. Commercial ibuprofen (50 g/kg) and benzocain creams (100 g/kg) were comparably as effective as ASA and SA, but the local anesthetic caused a loss of all cutaneous sensations while the touch threshold (von Frey) under the specific analgesics was the same as under the placebo ointment. Thus, topical applications of non-steroidal anti-inflammatory drugs (NSAIDS) dissolved in different ointment formulations have proven dose-dependently effective and specific in suppressing experimental acidotic pain by a local and competitive mechanism.