Emilio Kyburz

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.

[3H]Ro 19-6327: A reversible ligand and affinity labelling probe for monoamine oxidase-B

This study demonstrated the existence of specific binding sites for [3H]Ro 19-6327 in human platelet membranes. This compound is a novel, time-dependent inhibitor of monoamine oxidase type B (MAO-B) and is structurally closely related to [3H]Ro 16-6491. The density of the sites labelled with high affinity by [3H]Ro 19-6327 was similar to that observed in previous studies with [3H]Ro 16-6491 as ligand. Binding was reversible at 20 degrees C and showed a relatively slow dissociation (t1/2 = 220 min). The dissociation rate was markedly decreased (t1/2 = greater than 24h) at 0 degrees C. MAO-B, but not MAO-A inhibitors, effectively prevented the binding of [3H]Ro 19-6327. Like [3H]Ro 16-6491, [3H]Ro 19-6327 is recognized as a substrate by MAO-B, being eventually deaminated by the enzyme. Since the deaminated aldehyde derivative of Ro 19-6327 did not inhibit MAO-B, a still unidentified reversible adduct, formed at the MAO-B active site, might explain the high potency and selectivity of [3H]Ro 19-6327. Incubation of the radioligand-enzyme complex from platelet and brain membranes with NaBH3CN and acetic acid (to pH 4.5) caused the irreversible incorporation of the radioactivity into a single polypeptide as shown by SDS-PAGE analysis. This polypeptide had a molecular weight identical to that of the MAO-B subunit, i.e. 58,000. The presence of unlabelled MAO-B inhibitors in the incubation mixture prevented the covalent incorporation of [3H]Ro 19-6327. The irreversible MAO-B inhibitor, [3H] pargyline, labelled a protein with a molecular weight identical to the protein labelled by [3H]Ro 19-6327.(ABSTRACT TRUNCATED AT 250 WORDS)

An efficient resolution of 3-PPP and assignment of the absolute configuration.

Abstract ( + )-3-PPP has been resolved by means of (+)- and (-)- 2,2′-(1,1′-binaphthyl)phosphoric acid (BNPPA) in high optical purity; (+)-3-PPP has been assigned the R configuraiton on the basis of a single crystal X-ray analysis.