Inhibition of monoamine oxidase by clorgyline analogues

N-Methyl-N-propargyl-3-(4-phenoxy)phenoxypropylamine, an analogue of the MAO-A-selective irreversible inhibitor clorgyline in which the 2,4-dichloro- substitution in clorgyline was replaced by a 2-H atom and a 4-phenoxy group, has been synthesised and assessed as an inhibitor of monoamine oxidase (MAO). This compound proved to be a time-dependent irreversible inhibitor of both MAO-A and -B. However, unlike clorgyline, it was selective towards MAO-B, both in its initial, non-covalent, binding to the enzyme and as an irreversible inhibitor. In order to assess the influence of side-chain length on inhibitory potency, analogues were synthesised in which the side-chain was reduced to 2 CH2 units (N-methyl-N-propargyl-2- (4-phenoxy)phenoxyethylamine) or increased to 4 CH2 units (N-methyl-N-propargyl-4-(4- phenoxy)phenoxybutylamine). Both these compounds were also time-dependent irreversible inhibitors with selectivity towards MAO-B. In the case of the initial, non-covalent, inhibition all these compounds were competitive inhibitors of MAO-A, with respect to the amine substrate, and the affinity for inhibitor binding increased with carbon chain length. In contrast the compounds were all mixed inhibitors of MAO-B. The competitive element of this inhibition (measured by Kis) was similar for the 2 and 3 carbon-chain compounds but decreased markedly when the chain-length was increased to 4 carbons. The uncompetitive inhibition (measured by Kii) decreased as the carbon chain-length was increased from 2 to 3, but there was no significant further change when the length was increased to 4 carbons. The time-dependent irreversible inhibition (measured as the IC50 values after 60 min enzyme-inhibitor preincubation) showed that the potency towards MAO-A increased when the side-chain length was increased from 2 to 3 carbons but that there was no significant difference between the 3 and 4 carbon-chain compounds. In the case of MAO-B inhibition, the 2 and 3 carbon-chain compounds had similar inhibitory potencies but this increased substantially when the chain length was increased to 4 carbons. The significance of the inhibitory behaviour of these compounds is discussed in terms of the structure-activity relationships of mechanism-based irreversible MAO inhibitors.