Platelet monoamine oxidase inhibition by deprenyl and tranylcypromine: implications for clinical use

Tranylcypromine in mind (Part I): Review of pharmacology

It has been over 50 years since a review has focused exclusively on the monoamine oxidase (MAO) inhibitor tranylcypromine (TCP). A new review has therefore been conducted for TCP in two parts which are written to be read preferably in close conjunction: Part I - pharmacodynamics, pharmacokinetics, drug interactions, toxicology; and Part II - clinical studies with meta-analysis of controlled studies in depression, practice of TCP treatment, place in therapy. Pharmacological data of this review part I characterize TCP as an irreversible and nonselective MAO-A/B inhibitor at low therapeutic doses of 20mg/day with supplementary norepinephrine reuptake inhibition at higher doses of 40-60mg/day. Serotonin, norepinephrine, dopamine, and trace amines, such as the "endogenous amphetamine" phenylethylamine, are increased in brain, which leads to changes in neuroplasticity by e.g. increased neurotrophic growth factors and translates to reduced stress-induced hypersecretion of corticotropin releasing factor (CRF) and positive testing in animal studies of depression. TCP has a pharmacokinetic half-life (t_1/2) of only 2h which is considerably lower than for most other antidepressant drugs. However, a very long pharmacodynamic half-life of about one week is found because of the irreversible MAO inhibition. New studies show that, except for cytochrome P450 (CYP) 2A6, no other drug metabolizing CYP-enzymes are inhibited by TCP at therapeutic doses which defines a low potential of pharmacokinetic interactions in the direction from TCP to other drugs. Insufficient information is available, however, for plasma concentrations of TCP influenced by comedication. More quantitative data are also needed for TCP metabolites such as p-hydroxytranylcypromine and N-acetyltranylcypromine. Pharmacodynamic drug interactions comprise for instance severe serotonin toxicity (SST) with serotonergic drugs and hypertensive crisis with indirect sympathomimetics. Because of the risk of severe food interaction, TCP treatment remains beset with the need for a mandatory tyramine-restricted diet. Toxicity in overdose is similar to amitriptyline and imipramine according to the distance of therapeutic to toxic doses. In conclusion, TCP is characterized by an exceptional pharmacology which is different to most other antidepressant drugs, and a more special evaluation of clinical efficacy and safety may therefore be needed.

Multiple-dose pharmacokinetics of selegiline and desmethylselegiline suggest saturable tissue binding

The goal of this study was to examine the multiple-dose pharmacokinetics of selegiline and its metabolites desmethylselegiline, 1-methamphetamine, and 1-amphetamine after oral administration of selegiline HCl. Twelve healthy volunteers received 10 mg of selegiline HCl once daily for 8 days. The pharmacokinetic profiles of selegiline and the metabolites were examined from serum samples for 24 hours (i.e., the dosing interval, tau) on days 1, 4, and 8. The results indicated significant apparent accumulation of selegiline and desmethylselegiline during the 8-day period of selegiline administration. The AUC tau S of selegiline and desmethylselegiline were increased 2.7 fold (p < 0.001) and 1.5 fold (p < 0.001), respectively, from day 1 to day 8. However, the half-lives of selegiline (range, 1.5-3.5 h) and desmethylselegiline (range, 3.4-5.3 h) were found to be relatively short. Accordingly, the short half-lives of these compounds failed to predict the apparent accumulation. With both of the 1-amphetamine metabolites of selegiline, steady state was reached by day 4. We suggest that the most likely explanation for the apparent accumulation of selegiline and desmethylselegiline was the saturation of the MAO-B binding sites in tissues, although decreased first-pass metabolism of selegiline cannot be ruled out. The observed increase in selegiline and desmethylselegiline concentrations on multiple dosing is not likely to significantly increase the pharmacodynamic effect or adverse effects of selegiline compared with what has been found after a single 10-mg dose.

Novel strategies for treatment-resistant depression

This article describes a highly selective constellation of the more unique strategies for managing the treatment-resistant patient. In light of the often-devastating toll that treatment-resistant depression takes on an individual's life, it behooves us to continue the search for more effective strategies for those patients that fail more traditional interventions. As each successive move down the treatment algorithm flowchart becomes necessary, the risk/benefit ratio may shift toward less well-substantiated, but still biologically informed, strategies. Although some of the more unusual treatments described in this article represent minimally charted territories, the more promising techniques are deserving of further careful exploration.

The effect of L-deprenyl on behavior, cognitive function, and biogenic amines in the dog

Behavioral and pharmacological effects of oral administration of L-deprenyl in the dog are described. Spontaneous behavior is unaffected at doses below 3 mg/kg while at higher doses there was stereotypical responding. There was evidence of improved cognitive function in animals chronically treated with a 1 mg/kg dose but the effectiveness varied considerably between subjects. Chronic administration produced a dose dependent inhibition in brain, kidney and liver monoamine oxidase B, and had no effect on monoamine oxidase A. There were also dose dependent increases in brain phenylethylamine and in plasma levels of amphetamine. Dog platelets did not have significant levels of MAO-B. Brain dopamine and serotonin metabolism were unaffected by L-deprenyl at doses up to 1 mg/kg. It appears that for the dog, deamination of catecholamines is controlled by MAO-A. Nevertheless, it is suggested that L-deprenyl serves as a dopaminergic agonist, and there is also evidence that it affects adrenergic transmission. These catecholaminergic actions may account for the effects of L-deprenyl on behavior and cognitive function.

Changes in spontaneous behavior in the dog following oral administration of L-deprenyl

An open-field activity test was developed for studying the effect of a single oral dose (range of 0.1-5 mg/kg) of L-deprenyl on spontaneous behavior in the dog. A computer program was used to quantify observations of locomotor activity, directed sniffing, urination, grooming, inactivity, jumping, rearing, and vocalization during a 10-min baseline and posttreatment session. Three dose-dependent behavioral changes were observed: an overall decrease in directed sniffing, an increase in total locomotor activity in females, and a decrease in frequency of urination in males. These effects were only seen at the dose levels of 2 mg/kg or higher. Computer-assisted tracings of behavioral patterns showed increased stereotypical behavior and decreased exploratory behavior at the high-dose levels. These behavioral effects are most likely due to either increased levels of phenylethylamine resulting from inhibition of monoamine oxidase B and/or the production of amphetamines as a result of the metabolism of L-deprenyl.

The new generation of monoamine oxidase inhibitors

Irreversible and unspecific inhibitors of MAO were the first modern antidepressants, but after an initial success they fell into discredit due to adverse side effects. In the past two decades interest in MAO inhibitors has been renewed because of progress in basic research, a milestone being the finding that there are two subtypes of MAO, MAO-A and MAO-B. These are distinct proteins with high amino acid homology, coded by separate genes both located on the short arm of the human chromosome X. The enzyme subforms show different substrate specificities in vitro and different distributions within the central nervous system and in peripheral organs. In the central nervous system of man MAO-A seems to be mainly involved in the metabolism of 5 HT and noradrenaline, whereas 2-phenylethylamine and probably dopamine are predominantly deaminated by MAO-B. In the intestinal tract tyramine is mainly metabolized by MAO-A. These characteristics indicate distinct physiological functions of the two MAO-subforms. Several irreversible and reversible non-hydrazine inhibitors with relative selectivities for one of the MAO-subforms have been developed. They belong to various chemical classes with different modes of enzyme inhibition. These range from covalent mechanism based interaction (e.g. by propargyl- and allylamine derivatives) to pseudosubstrate inhibition (e.g. by 2-aminoethyl-carboxamides) and non-covalent interaction (e.g. by brofaromine, toloxatone and possibly moclobemide). The most important pharmacological effects of the new types of MAO inhibitors are those observed in neuropsychiatric disorders. The inhibitors of MAO-A show a favorable action in various forms of mental depression. The drugs seem to have about the same activity as other types of antidepressants, including tricyclic and related compounds as well as classical MAO inhibitors. The onset of action of the MAO-A inhibitors is claimed to be relatively fast. Other possible indications of these drugs include disorders with cognitive impairment, e.g. dementia of the Alzheimer type. In subjects with Parkinson's disease the MAO-B inhibitor L-deprenyl exerts a L-dopa-sparing effect, prolongs L-dopa action and seems to have a favorable influence regarding on-off disabilities. The action is in general transitory (months to several years). In addition L-deprenyl has been shown to delay the necessity for L-dopa treatment in patients with early parkinsonism. Whether the drug influence the progression of the disease is still a matter of debate. L-deprenyl also appears to have some antidepressant effect (especially in higher doses) and to exert a beneficial influence in other disorders, e.g. dementia of the Alzheimer type.(ABSTRACT TRUNCATED AT 400 WORDS)

A review of the pharmacology of selegiline

Selegiline (1-deprenyl) is an irreversible inhibitor of monoamine oxidase (MAO) type B. Because in the human brain, dopamine is metabolised mainly by MAO-B, selegiline increases dopamine content in the central nervous system. Besides the inhibition of MAO-B, selegiline also inhibits the uptake of dopamine and noradrenaline into presynaptic nerve and increases the turnover of dopamine. Thanks to these properties, selegiline significantly potentiates the pharmacological effects of levodopa. These favourable characteristics have been applied in the treatment of Parkinson's disease using selegiline both with levodopa and alone. Unlike earlier MAO-inhibitors, selegiline does not potentiate the hypertensive effects of tyramine. This is due to the selectivity to MAO-B, leaving intestinal MAO-A intact, and also due to the fact that selegiline inhibits the uptake of tyramine into neurons. Selegiline can prevent the parkinsonism caused by MPTP in animals; similar findings have been reported with other toxins like 6-OHDA and DSP-4, that destroys noradrenergic nuclei. Furthermore, selegiline reduces oxidative stress caused by degradation of dopamine and increases free radical elimination by enhancing superoxide dismutase and catalase activity. These findings may be important when considering the possible neuroprotective effects of selegiline. Besides the basic pharmacology also the interactions and pharmacokinetics of selegiline are reviewed in this article.

Relationship between tyramine potentiation and monoamine oxidase (MAO) inhibition: comparison between moclobemide and other MAO inhibitors

The pharmacodynamic properties of moclobemide, a reversible inhibitor of MAO-A (RIMA), were compared with the properties of other reversible as well as older irreversible MAO inhibitors in human subjects. All the substances supposed to have MAO-A-inhibitory activity, with the exception of toloxatone, were shown by the decrease in plasma DHPG or MHPG levels to cause inhibition ranging between 50% and 85%. Toloxatone and low doses of deprenyl (a MAO-B inhibitor) caused 20% and 17% inhibition respectively; higher doses of deprenyl, however, strongly inhibited MAO-A. MAO-B inhibition was confirmed for all nonselective and selective MAO-B inhibitors. Moclobemide and clorgyline were found to be the most highly selective MAO-A inhibitors, although both also inhibited 30% of platelet MAO-B activity. Potentiation of the tyramine pressor effect is mainly influenced by the irreversibility and degree of MAO-A inhibition. Tyramine sensitivity was raised (a factor of 10-30) by all irreversible MAO inhibitors in doses inhibiting MAO-A; it diminished with increasing reversibility. In therapeutic doses, moclobemide potentiated the intravenous tyramine pressor effect 3 times less than the old irreversible MAO inhibitors; with the highest therapeutic dose, the tyramine sensitivity factor for moclobemide is only one-seventh to one-tenth that of tranylcypromine or phenelzine. Duration of action is obviously also closely related to the reversibility of inhibition: it ranged from up to 2 days with high doses of moclobemide to 3 weeks with tranylcypromine; clorgyline and phenelzine have been shown to maintain their action for several months. The new generation of RIMAs represents a significant progress in safety.(ABSTRACT TRUNCATED AT 250 WORDS)