Phenazone potentiates the local anaesthetic effect of lidocaine in mice.

To justify the inclusion of phenazone, independently of its anti-inflammatory properties, in combination with a local anaesthetic, such as lidocaine, in some ear drop medications, we have studied the effect of this compound on the local anaesthetic activity of lidocaine in an animal model, that of sciatic nerve blockade in mice. Lidocaine and phenazone were tested alone and in combination at various concentrations. The local anaesthetic activity was estimated as the loss of motor activity of the hindlimb after topical injection of the drugs in the region of the sciatic nerve. Lidocaine, at concentrations ranging from 0.03 to 0.25%, induced a concentration-dependent anaesthetic effect. Phenazone alone had no effect at 0.25-1%. When combined, the two compounds acted synergistically. The local anaesthesia induced by lidocaine plus phenazone was significantly more intense and longer lasting than that induced by lidocaine alone. Phenazone enhanced the potency of lidocaine in this animal model. It is suggested that the potentiated local anaesthetic effect of the combination may be partly due to enhanced local bioavailability of lidocaine. Copyright 2000 Academic Press.