Acetylator status and inhibition of platelet monoamine oxidase following treatment with phenelzine

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

The application of pharmacogenetics holds great promise for individualized therapy. However, it has little clinical reality at present, despite many claims. The main problem is that the evidence base supporting genetic testing before therapy is weak. The pharmacology of the drugs subject to inherited variability in metabolism is often complex. Few have simple or single pathways of elimination. Some have active metabolites or enantiomers with different activities and pathways of elimination. Drug dosing is likely to be influenced only if the aggregate molar activity of all active moieties at the site of action is predictably affected by genotype or phenotype. Variation in drug concentration must be significant enough to provide "signal" over and above normal variation, and there must be a genuine concentration-effect relationship. The therapeutic index of the drug will also influence test utility. After considering all of these factors, the benefits of prospective testing need to be weighed against the costs and against other endpoints of effect. It is not surprising that few drugs satisfy these requirements. Drugs (and enzymes) for which there is a reasonable evidence base supporting genotyping or phenotyping include suxamethonium/mivacurium (butyrylcholinesterase), and azathioprine/6-mercaptopurine (thiopurine methyltransferase). Drugs for which there is a potential case for prospective testing include warfarin (CYP2C9), perhexiline (CYP2D6), and perhaps the proton pump inhibitors (CYP2C19). No other drugs have an evidence base that is sufficient to justify prospective testing at present, although some warrant further evaluation. In this review we summarize the current evidence base for pharmacogenetics in relation to drug-metabolizing enzymes.

Effect of a novel monoamine-oxidase inhibitor, moclobemide on the sensitivity to intravenous tyramine and norepinephrine in humans

This study compared the effects of moclobemide (Ro11-1163), a selective and reversible inhibitor of monoamine-oxidase (MAOI) type A and phenelzine, an irreversible non-selective MAOI, on the pressor responses to IV tyramine and norepinephrine. Because of the reversibility of this inhibition, the pressor effect of tyramine was expected to be minimal. Twelve healthy men participated in this randomized double-blind, placebo-controlled, crossover study. Volunteers began with oral treatment of moclobemide (100 mg TID) or phenelzine (15 mg TID) for 1 week followed by placebo treatment for 2 weeks and then moclobemide or phenelzine treatment for another week. The tyramine and norepinephrine challenge tests were conducted at baseline and then at weekly intervals, for a total of five challenges. The average tyramine dose that was required to increase systolic blood pressure by 25 mm Hg (PD25) was 1.6 +/- 0.2 mg after moclobemide treatment, which was lower (P less than .01) than the baseline value of 3.6 +/- 0.7 mg and that after phenelzine (3.0 +/- 0.5 mg) treatment. Moclobemide did not influence norepinephrine sensitivity. In conclusion, moclobemide mildly decreased the sensitivity to IV tyramine as compared with placebo and phenelzine.

The present status of monoamine oxidase inhibitors

The present status of monoamine oxidase inhibitors in the treatment of depression is reviewed. With adequate doses they are effective antidepressants, but dosages have in the past been too low. Provided proper dietary precautions are taken, the incidence of fatality from dietary interactions is very small and should not deter doctors from using these drugs, especially in those depressed patients who do not respond to tricyclic-type antidepressants. The present status of combining monoamine oxidase inhibitors with tricyclics is discussed, as are the newer specific inhibitors particularly clorgyline and deprenyl.

Platelet MAO, personality and response to lithium prophylaxis

Patients with bipolar affective disorder and who were maintained on long lithium prophylaxis were evaluated retrospectively regarding response to lithium. Good responders to lithium prophylaxis were compared to non-responders and to normal controls on platelet MAO activity, and on psychological variables (Eysenck's extraversion, neuroticism, psychoticism and lie scale; Foulds' extrapunitiveness, intropunitiveness and dominance). Lithium responders, non-responders and normal controls had similar platelet MAO activity. There was a tendency for higher plasma lithium levels to be associated with higher platelet MAO activity in male patients but not in females. Lithium non-responders exhibited significant correlations between their platelet MAO activity and their test scores on psychoticism, and extraversion. Lithium non-responders, compared to responders and normals, showed abnormal personality profiles on neuroticism, and dominance.

Influence of acetylator phenotype on antidepressant effects of phenelzine

Acetylator phenotype, determined by blood levels after sulphadimidine, was related to outcome in a controlled trial of phenelzine in patients with depression and mixed anxiety depression. Slow acetylators showed more improvement and greater phenelzine-placebo differences than did fast acetylators, at two, four and six weeks. A tendency towards faster acetylation was observed in subjects with previous increased alcohol intake. Acetylator phenotype was not related to other symptom, diagnostic or history data.