Codeine did not increase analgesic efficacy of coxibs in contrast to that of paracetamol or ibuprofen: isobolographic analysis in mice.

OBJECTIVE

There is good evidence that opioids can potentiate analgesic activity of some older non-opioid analgesics (such as paracetamol or ibuprofen) but it is not known whether this also holds true for newer non-opioid analgesics that selectively inhibit cyclooxygenase 2 (coxibs). This study was undertaken to determine the nature of the interaction between codeine and celecoxib or etoricoxib in peritoneal irritation-induced visceral pain in mice. For comparison, interactions of codeine with paracetamol and ibuprofen were also tested using the same method.

METHODS

A small volume of a weak acetic acid (0.6%) was injected into the peritoneal cavity and the number of writhes (contractions of abdominal muscles) was counted. All drugs were given orally. Their interaction was characterized using isobolographic analysis.

RESULTS

Codeine, etoricoxib, celecoxib, ibuprofen and paracetamol all independently produced dose-dependent suppression of writhing. The isobolographic analysis carried out using equipotent dose ratios showed that the interactions between codeine and etoricoxib or celecoxib were sub-additive or additive, respectively. This was in contrast to combinations of codeine with ibuprofen or paracetamol, which were supra-additive. Interaction indexes γ, determined as a ratio between experimental and theoretical ED50 values of the mixture, were as follows: 2.7 for codeine + etoricoxib, 0.62 for codeine + celecoxib, 0.43 for codeine + ibuprofen and 0.33 for codeine + paracetamol.

CONCLUSIONS

These and other results suggest that opioids do not seem to potentiate analgesic effects of selective COX-2 inhibitors, in contrast to nonselective COX inhibitors or paracetamol.