Determination of analgesic interaction between acetaminophen and d-propoxyphene obtained by means of the surface of synergistic interaction.

The analgesic effects of acetaminophen, p.o., (ACE) and d-propoxyphene, s.c., (PROP) administered either separately or in 24 different combinations were determined in a model of pain-induced functional impairment in the rat. This allowed us to detect the analgesic interaction profile of the combinations. Furthermore, we set out to determine the optimal degree of enhancement obtained with a specific combination of the above drugs by means of the surface of synergistic interaction (SSI) of the combinations. This parameter was calculated from the total analgesic effect produced by the combination after having subtracted the analgesic effect produced by each drug alone. The ED50s for ACE and PROP were 286.1 +/- 1.4 mg/kg and 66.3 +/- 1.2 mg/kg, respectively. Over the dose ranges used, the analgesic activities of both ACE and PROP tended to be smaller than those of their respective combinations. Furthermore, 11 combinations showed various degrees of enhancement (p<0.01), while the others (13) exhibited additive analgesic effects. The combination of ACE (562.3 mg/kg) and PROP (56.2 mg/kg) produced the maximum analgesic effect. However,5 combinations of ACE with PROP (177.8-56.2, 316.2-10.0, 316.2-17.8, 316.2-56.2 and 562.3-10.0 mg/kg) produced the highest enhancement. The SSI clearly showed which combination of these analgesic drugs produced the highest degree of enhancement in the rat. This study shows that a specific combination ratio of analgesic drugs can produce optimum enhancement of their analgesic effects.