Acetylation of phenelzine

N-acetyltransferase: the practical consequences of polymorphic activity in man

Over the years, numerous studies have supported the premise that individuals possessing the "slow acetylator" phenotype are more at risk from developing drug side-effects. Most prominent amongst these reports are those concerned with hepatotoxicity and peripheral neuropathy following treatment with isoniazid, lupus-like symptoms during procainamide therapy and experiencing hypersensitivity reactions to the various sulphonamide derivatives. Similarly, "slow acetylators" undergoing heavy exposure to arylamines and related carcinogens are more likely to develop bladder cancer. Contrariwise, there appears a slight risk of "rapid acetylators" developing pancreatic tumours.Other therapeutic agents for which polymorphic N-acetylation plays a minor role in their metabolism have been investigated but any impact of this metabolic difference on clinical efficacy or associated toxicity is still under question. In the search for clues as to the underlying aetiology, patient groups with many disease states have been examined for association with differences in N-acetylation and the majority have provided data that could be interpreted as equivocal. Studies have given contradictory, often opposing, results, calculated risk factors that are (perhaps) just significant but certainly not high, and patients within the cohorts who are always exceptions. Undoubtedly, other as yet unappreciated factors are at play.

Tissue levels and some pharmacological properties of an acetylated metabolite of phenelzine in the rat

The metabolic generation of N2-acetylphenelzine by rats treated with phenelzine, and the activity of this metabolite as an inhibitor of monoamine oxidase enzymes in vivo were confirmed. The isomeric amide N1-acetylphenelzine was not a metabolic product of phenelzine and also did not inhibit monoamine oxidase enzymes. Levels of N2-acetylphenelzine in rat blood, after treatment with a dose (0.1 mmol.kg-1) of N2-acetylphenelzine sufficient to inhibit monoamine oxidase enzymes but not to increase brain levels of dopamine or noradrenaline, were higher than those generated metabolically from a higher dose (0.38 mmol.kg-1) of phenelzine which did increase brain levels of these biogenic amines. Metabolically derived N2-acetylphenelzine, therefore, probably does not contribute in any significant way to monoamine oxidase inhibition by phenelzine.

DNA alterations induced by the carbon-centered radical derived from the oxidation of 2-phenylethylhydrazine

The possible significance of carbon-centered radicals in hydrazine-induced carcinogenesis is explored by studies of the interaction between the 2-phenylethyl radical and DNA. The radical is efficiently generated during oxidation of phenelzine (2-phenylethylhydrazine) promoted by oxyhemoglobin or ferricyanide, as demonstrated by spin-trapping experiments and analysis of the reaction products. In the ferricyanide promoted oxidation, ethylbenzene formation accounts for about 40% of the initial drug concentration, from 5 to 100 mM phenelzine. By contrast, product formation in the presence of oxyhemoglobin depends on the enzyme concentration due to the fact that the prosthetic heme is destroyed during catalytic turnover. Covalent binding of the 2-phenylethyl radical to oxyhemoglobin is demonstrated by experiments with 2-[3H]phenelzine, where tritium incorporation to the protein is inhibited by the spin-trap, alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone. The 2-phenylethyl radical is also able to alkylate DNA as suggested by electrophoretic studies with plasmid DNA, and proved by experiments with 2-[3H]-phenelzine. The carbon-centered radical has a preference for attacking guanine residues as demonstrated by the use of sequencing techniques with 32P-DNA probes. The results indicate that the 2-phenylethyl radical is an important product of phenelzine oxidation and that this species can directly damage protein and DNA.

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.

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.

Clinical and pharmacokinetic factors affecting response to phenelzine

Sixty patients, 30 with depressive neurosis, 15 with anxiety neurosis and 15 with phobic anxiety states, were treated with the monoamine oxidase inhibitor, phenelzine, in two different dosage schedules for four weeks. All patients received an initial dose of 15 mg daily, increasing to 30 mg daily between the third and seventh day, but subsequently, using double-blind procedure, one group tooke the commonly prescribed dose of 45 mg daily and the other took 90 mg daily. Acetylator status was independently determined before the start of treatment. Each diagnostic group showed a similar response to treatment, but patients taking the higher dose improved significantly more than those taking normal dosage, and the rate of improvement, measured by weekly self-ratings, was also more rapid with higher dosage. Acetylator status did not affect clinical response. The results suggest that dosage is more important in determining clinical response to phenelzine in neurotic disorder than specific diagnosis or acetylator status.