Phenelzine has also been linked to vitamin B6deficiency.[22] Transaminases such as GABA-transaminase have been shown to be dependent upon vitamin B6[23] and may be involved in a potentially related process, since the phenelzine metabolite phenylethylidenehydrazine (PEH) is a GABA transaminase inhibitor. Both phenelzine and vitamin B6 are rendered inactive upon these reactions occurring. The pyridoxine form of B6 is recommended for supplementation, since this form has been shown to reduce hydrazine toxicity from phenelzine and, in contrast, the pyridoxal form has been shown to increase the toxicity of hydrazines.[24]
Phenelzine and its metabolites also inhibit at least two other enzymes to a lesser extent, of which are alanine transaminase (ALA-T),[26] and γ-aminobutyric acid transaminase (GABA-T),[27] the latter of which is not caused by phenelzine itself, but by a phenelzine metabolite phenylethylidenehydrazine (PEH). By inhibiting ALA-T and GABA-T, phenelzine causes an increase in the alanine and GABA levels in the brain and body.[28] GABA is the major inhibitory neurotransmitter in the mammaliancentral nervous system, and is very important for the normal suppression of anxiety, stress, and depression. Phenelzine's action in increasing GABA concentrations may significantly contribute to its antidepressant, and especially, anxiolytic/antipanic properties, the latter of which have been considered superior to those of other antidepressants. As for ALA-T inhibition, though the consequences of disabling this enzyme are currently not well understood, there is some evidence to suggest that it is this action of the hydrazines (including phenelzine) which may be responsible for the occasional incidence of hepatitis and liver failure.[29]
Phenelzine has also been shown to metabolize to phenethylamine (PEA).[30] PEA acts as a releasing agent of norepinephrine and dopamine, which occurs in a similar manner to amphetamine by being taken up into vesicles, displacing and causing the release of those monoamines, and reversing monoamine flux through their respective transporters via TAAR1 agonism (though with markedly shorter pharmacokinetics).[31]
Like many other antidepressants, phenelzine usually requires several weeks of treatment to achieve full therapeutic effects. The reason for this delay is not fully understood. Still, it is believed to be due to many factors, including achieving steady-state levels of MAO inhibition and the resulting adaptations in mean neurotransmitter levels, the possibility of necessary desensitization of autoreceptors which generally inhibit the release of neurotransmitters like serotonin and dopamine, and also the upregulation of enzymes such as serotonin N-acetyltransferase. Typically, a therapeutic response to MAOIs is associated with an inhibition of at least 80-85% of monoamine oxidase activity.[32]
Pharmacokinetics
Phenelzine is administered orally in the form of phenelzine sulfate[4] and is rapidly absorbed from the gastrointestinal tract.[33] The time to peak plasma concentration is 43 minutes, and the half-life is 11.6 hours.[34] Unlike most other drugs, phenelzine irreversibly disables MAO. As a result, it does not necessarily need to be present in the blood at all times for its effects to be sustained. Because of this, upon phenelzine treatment being ceased, its effects typically do not wear off until the body replenishes its enzyme stores, a process which can take as long as 2–3 weeks.[4]
Phenelzine is metabolized primarily in the liver, and its metabolites are excreted in the urine. Oxidation is the primary routine of metabolism, and the major metabolites are phenylacetic acid and parahydroxyphenylacetic acid, recovered as about 73% of the excreted dose of phenelzine in the urine over 96 hours after single doses. Acetylation to N2-acetylphenelzine is a minor pathway.[35][36] Phenelzine may also interact with cytochrome P450 enzymes, inactivating these enzymes through the formation of a heme adduct.[37] Two other minor metabolites of phenelzine, as mentioned above, include phenylethylidenehydrazine and phenethylamine.[38]
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^Votoček E, Leminger O (1932). "Sur la β-phenoéthylhydrazine" [On the [preparation and properties of] β-phenoethylhydrazine]. Collection of Czechoslovak Chemical Communications (in French). 4: 271–281. doi:10.1135/cccc19320271.
^Vallejo J, Gasto C, Catalan R, Salamero M (November 1987). "Double-blind study of imipramine versus phenelzine in Melancholias and Dysthymic Disorders". The British Journal of Psychiatry. 151 (5): 639–642. doi:10.1192/bjp.151.5.639. PMID3446308. S2CID145651628.
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^Buigues J, Vallejo J (February 1987). "Therapeutic response to phenelzine in patients with panic disorder and agoraphobia with panic attacks". The Journal of Clinical Psychiatry. 48 (2): 55–59. PMID3542985.
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^Walsh BT, Gladis M, Roose SP, Stewart JW, Stetner F, Glassman AH (May 1988). "Phenelzine vs placebo in 50 patients with bulimia". Archives of General Psychiatry. 45 (5): 471–475. doi:10.1001/archpsyc.1988.01800290091011. PMID3282482.
^Frank JB, Kosten TR, Giller EL, Dan E (October 1988). "A randomized clinical trial of phenelzine and imipramine for posttraumatic stress disorder". The American Journal of Psychiatry. 145 (10): 1289–1291. doi:10.1176/ajp.145.10.1289. PMID3048121.
^Vallejo J, Olivares J, Marcos T, Bulbena A, Menchón JM (November 1992). "Clomipramine versus phenelzine in obsessive-compulsive disorder. A controlled clinical trial". The British Journal of Psychiatry. 161 (5): 665–670. doi:10.1192/bjp.161.5.665. PMID1422616. S2CID36232956.
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