From moclobemide to Ro 19-6327 and Ro 41-1049: the development of a new class of reversible, selective MAO-A and MAO-B inhibitors.

This study describes the serendipitous discovery of moclobemide, a short-acting MAO-A inhibitor which is in an advanced stage of clinical development as an antidepressant. The short duration of action of this MAO inhibitor containing a morpholine ring moiety is due to the complete reversibility (probably by metabolism of the inhibitory molecular species) of MAO-A inhibition. Since moclobemide is much more effective in vivo than expected from its in vitro activity, investigations to identify a possible metabolite(s) more active as MAO-A inhibitor than the parent compound were carried out. The study of the MAO inhibitory characteristics of several known and putative moclobemide metabolites did not allow the identification of a potent MAO-A inhibitor but led to the discovery of Ro 16-6491, a potent MAO-B inhibitor of novel chemical structure. Systematic chemical modification of the aromatic ring system of Ro 16-6491 finally provided Ro 19-6327 and Ro 41-1049 which are highly selective and reversible inhibitors of MAO-B and MAO-A, respectively. Tritiated derivatives of Ro 19-6327 and Ro 41-1049 were used in binding studies to elucidate their mechanisms of action and to study their cellular distribution by quantitative enzyme radioautography.

Molecular neuroanatomy of MAO-A and MAO-B.

A selective, quantitative and high resolution technique (in vitro and in vivo enzyme radioautography) has been used to reveal the tissue distribution and abundance of MAO-A and MAO-B in the central nervous system and peripheral organs in the rat. The in vitro approach was also used to map the enzymes in human post-mortem brain. Furthermore, using in situ hybridization histochemistry, locus coeruleus and raphé neurons in the human brain were found to code for MAO-A and MAO-B respectively and not vice versa.