Monoamine oxidase inhibitors values

Use of monoamine oxidase inhibitors (MAOI) antidepressant blood levels to monitor patients with depression is of no clinical value. 

Gander GA The clinical value of monoamine oxidase inhibitors and tricyclic antidepressants inccmbinatiorL//irCon r(1966) 122,336-43. 

Section B (Fig. 17) shows some commonly used drugs which are electroactive. The a-methyltryptamine was detected at + 0.8 V by Marsden (1980) in 5-HT studies. Several monoamine oxidase inhibitors are electroactive, but there is so far no evidence that they can be detected in vivo and, even if they are, it is at potentials sufficiently positive not to interfere with most studies. Chlorpromazine and haloperidol have solution Ep values about + 0.6 V and + 0.7 V, respectively. Clozapine oxidizes at about + 0.4 V. Since neuroleptics (and other drugs) are extensively metabolized by the time they appear in significant concentrations in the CNS, the potentials of the parent compounds may not be too relevant—but no data are available for metabolites. In any event, there is no evidence that any of these drugs are detected in CNS voltammetry. 

Monoamine oxidase exists in two forms, MAOa and MAOb. The former is more active against NA and 5-HT than it is against DA, which is a substrate for both, even though, like S-phenylethylamine, it is more affected by MAOb. H seems likely that MAOb is the dominant enzyme in human brain and inhibitors of it, such as selegiline, have some value in the treatment of Parkinson s disease by prolonging the action of the remaining endogenous DA as well as that formed from administered levodopa. 

Fig. 1. Influence of substituents in the aromatic ring on the selectivity of 3-(2-aminoethoxy)-1,2-benzisoxazole derivatives as monoamine oxidases (MAO) A or B inhibitors. Ratio of selectivity was calculated from IC50 values of MAO A and B determined by an in vitro assay of mouse brain mitochondria using 5-hydroxy tryptamine (5-HT) and 2-phenylethylamine (PEA) as specific substrates, respectively (see Ref. [9]). Smaller values indicate that inhibitors are MAO A selective [71].

The side effects and cardiotoxicity of the tricyclic antidepressants have been discussed in detail elsewhere in this volume and, while there is ample evidence of their therapeutic efficacy, it seems difficult to justify their use, particularly in a group of patients who are most vulnerable to their detrimental side effects. Of the newer antidepressants, the reversible inhibitors of monoamine oxidase type A such as moclobemide may also be of value in the elderly depressed patient, particularly in those patients who fail to respond to the amine uptake inhibitor type of antidepressant. 

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