Assessment of the Airway Smooth Muscle Relaxant Effect of Drotaverine

Background: Drotaverine, a type 4 cyclic nucleotide phosphodiesterase (PDE4) inhibitor, blocks the degradation of 3',5'-cyclic adenosine monophosphate. However, published receptor binding data showed that drotaverin also binds to the L-type voltage-operated Calcium Channel (L-VOCC). Based on these molecular mechanisms of action, a direct and indirect (by blocking the constrictor response) relaxant effect on airway smooth muscle can be predicted, which has not yet been assessed.

Summary: Accordingly, drotaverine and reference agents were tested both on the histamine-, methacholine-, or KCl-induced contraction response and on precontracted guinea pig tracheal preparations. It was found that drotaverine not only relaxed the precontracted tracheal preparations but also decreased mediator-induced contraction. These effects of drotaverine were concentration dependent, with a significantly higher potency on the KCl-induced response, than on either the histamine or methacholine induced one. A similar result was noted for nifedipine. The PDE inhibitor, theophylline, also relaxed the precontracted preparations but was ineffective on the mediator-induced contraction in a physiologically relevant concentration range. Moreover, theophylline did not show selectivity and was the least potent relaxant among the 3 tested molecules. Key Message: These results show that drotaverine is a more potent airway smooth muscle relaxant molecule than theophylline. This enhanced potency on relaxation and inhibition of the constrictor response, at least partly, may be explained by the combined L-VOCC blocking and PDE inhibitory potential of drotaverine.