The substance P receptor antagonist CP-99,994 reduces acute postoperative pain

Animal studies suggest that substance P, a peptide that preferentially activates the neurokinin-1 (NK1) receptor, is involved in pain transmission, with particular importance in pain after inflammation.

The analgesic efficacy of CP-99,994, a NK1 receptor antagonist, was compared with ibuprofen and placebo in 78 subjects undergoing third molar extraction. The initial 60 subjects randomly received 1 of 3 possible treatments in a double-blind fashion before oral surgery: 750 microg/kg CP-99,994 infused intravenously over 5 hours on a tapering regimen starting 2 hours before surgery, 600 mg oral ibuprofen 30 minutes before surgery, or placebo. In a second study, 18 subjects were randomized to the same regimens starting 30 minutes before surgery to maximize the amount of CP-99,994 circulating during pain onset.

In the first study, ibuprofen significantly reduced pain, as measured by visual analog scale, from 90 to 240 minutes postoperatively compared with placebo. CP-99,994 produced analgesia that was significant at 90 minutes (P < 0.01 compared with placebo), but not at subsequent time points. In the second study, ibuprofen and, to a lesser extent, CP-99,994 significantly suppressed pain in comparison to placebo at 60, 90, and 120 minutes (P < 0.05). The incidence of side effects was similar across groups.

This replicate demonstration that a NK1 receptor blocker relieves clinical pain supports the hypothesis that substance P contributes to the generation of pain in humans. The reduction in postoperative pain at doses not producing side effects suggests that NK1 antagonists may be clinically useful.