[Micelle formation as the origin of the cut-off phenomenon in a homologous series of local anesthetics].

On the example of the surface anaesthetic homologous series of 4-alkoxybenzoic-diethylaminoethylester-HCl (parethoxycaine) the so-called "cut off"-effect was studied. The "cut off"-effect is the phenomenon, that a weaker efficiency or an inefficiency is observed on the higher homologues of therapeutic substances. It is demonstrated that the "cut off"-effect is referred to the micellisation and aggregation of the higher homologues of the local anaesthetics in aqueous solution. This has the consequence, that the higher homologues cannot diffuse on the pain receptors and so the efficiency is reduced. By the addition of urea the aggregation will be reduced and the chain length-efficiency curve will take a different course.