Pharmacokinetics and Clinical Pharmacology of Selegiline. In: Szelenyi I, editor. Inhibitors of Monoamine Oxidase B. Basel: Birkhäuser; 1993. pp. 201-13. [DOI: 10.1007/978-3-0348-6348-3_10]

Physiologically Based Pharmacokinetic Modeling of Transdermal Selegiline and Its Metabolites for the Evaluation of Disposition Differences between Healthy and Special Populations

A physiologically based pharmacokinetic (PBPK) model of selegiline (SEL), and its metabolites, was developed in silico to evaluate the disposition differences between healthy and special populations. SEL is metabolized to methamphetamine (MAP) and desmethyl selegiline (DMS) by several CYP enzymes. CYP2D6 metabolizes the conversion of MAP to amphetamine (AMP), while CYP2B6 and CYP3A4 predominantly mediate the conversion of DMS to AMP. The overall prediction error in simulated PK, using the developed PBPK model, was within 0.5-1.5-fold after intravenous and transdermal dosing in healthy and elderly populations. Simulation results generated in the special populations demonstrated that a decrease in cardiac output is a potential covariate that affects the SEL exposure in renally impaired (RI) and hepatic impaired (HI) subjects. A decrease in CYP2D6 levels increased the systemic exposure of MAP. DMS exposure increased due to a reduction in the abundance of CYP2B6 and CYP3A4 in RI and HI subjects. In addition, an increase in the exposure of the primary metabolites decreased the exposure of AMP. No significant difference between the adult and adolescent populations, in terms of PK, were observed. The current PBPK model predictions indicate that subjects with HI or RI may require closer clinical monitoring to identify any untoward effects associated with the administration of transdermal SEL patch.

MAO-inhibitors in Parkinson's Disease

Monoamine oxidase inhibitors (MAO-I) belong to the earliest drugs tried in Parkinson's disease (PD). They have been used with or without levodopa (L-DOPA). Non-selective MAO-I due to their side-effect/adverse reaction profile, like tranylcypromine have limited use in the treatment of depression in PD, while selective, reversible MAO-A inhibitors are recommended due to their easier clinical handling. For the treatment of akinesia and motor fluctuations selective irreversible MAO-B inhibitors selegiline and rasagiline are recommended. They are safe and well tolerated at the recommended daily doses. Their main differences are related to (1) metabolism, (2) interaction with CYP-enzymes and (3) quantitative properties at the molecular biological/genetic level. Rasagiline is more potent in clinical practise and has a hypothesis driven more favourable side effect/adverse reaction profile due to its metabolism to aminoindan. Both selegiline and rasagiline have a neuroprotective and neurorestaurative potential. A head-to head clinical trial would be of utmost interest from both the clinical outcome and a hypothesis-driven point of view. Selegiline is available as tablet and melting tablet for PD and as transdermal selegiline for depression, while rasagiline is marketed as tablet for PD. In general, the clinical use of MAO-I nowadays is underestimated. There should be more efforts to evaluate their clinical potency as antidepressants and antidementive drugs in addition to the final proof of their disease-modifying potential. In line with this are recent innovative developments of MAO-I plus inhibition of acetylcholine esterase for Alzheimer's disease as well as combined MAO-I and iron chelation for PD.

In vitro formation of selegiline-N-oxide as a metabolite of selegiline in human, hamster, mouse, rat, guinea-pig, rabbit and dog

It is well established in the litrature, that selegiline is metabolised to its N-dealkylated metabolites, N-desmethylselegiline, methamphetamine and amphetamine. However, most studies on selegiline metabolism did not characterize the species differences in the formation of the metabolites. Therefore, in this study, we investigated the in vitro metabolism of selegiline in liver microsomes of different species. In addition, to the previously well-characterized metabolites, selegiline-N-oxide (selegiline-NO) was found to be formed as a metabolite of selegiline in rat liver microsomal preparation. The results of experiments with liver microsomes from other species indicated species differences in the rate and extent of formation of selegiline-NO. The dog and hamster liver microsomal preparations were the most active in terms of selegiline-NO production, whereas little selegiline was metabolized to its N-oxide in human liver microsomes. When selegiline-NO was incubated with rat liver microsomes, no metabolism occurred. When a short incubation time was applied in selegiline expriments no increase in the amount of selegiline-NO was detected. Accordingly, it was clear that selegiline was not metabolized to the N-dealkylated or N,N-bis-dealkylated compounds via selegiline-NO. Studies with different isoenzyme inhibitors indicated that the formation of selegiline-NO might be catalyzed at least partly by cytochrome P450 (CYP) 2D6 and CYP3A4. With the exception of hamster microsomes in the microsomal preparations in vitro, the formation of the R,S-stereoisomer of selegiline-NO was preferred.

Gas-chromatographic study on the stereoselectivity of deprenyl metabolism

The metabolism and urinary elimination of both (-)-deprenyl and (+)-deprenyl have been studied. Gas-chromatographic analysis with mass specific detection indicated that the metabolism of (-)-deprenyl results in a large excess of methamphetamine compared to amphetamine, while the metabolism of (+)-deprenyl gave nearly equal amounts of amphetamine and methamphetamine. A novel deprenyl metabolite, phenylacetone, was also identified in our studies.

Autonomic nervous system function in narcolepsy

A disturbance of the autonomic nervous system (ANS) in narcolepsy has been suggested, based on abnormalities on pupillometry, ejaculatory and cardiovascular function. The ANS function was studied by measuring the variation in the heart rate and blood pressure during provocations, using the following tests: deep breathing test, Valsalva test and Orthostatic test for heart rate reactivity measurements, and Orthostatic test for blood pressure control. Each test session gave seven variables, and these were compared to age-adjusted reference values in healthy normals. In 22 unmedicated narcoleptics (median age 50.5 y, range 18-70 y) the results did not differ from these. Seventeen of the patients were included in a controlled stimulant medication trial (selegiline 10-40 mg daily), and they showed no significant changes in the ANS variables except for a dose-dependent rise in heart ratio (placebo 1.32+/-1-0.13 and 40 mg 1.14+/-0.05; mean+/-SD) and a decrease in systolic blood pressure (placebo 5.8+/-9.7 and 40 mg 30.1+/-21.5 mmHg) on Orthostatic test. Although blood pressure decreases >/=30 mmHg (maximally 72 mmHg) occurred in 9 patients, they were asymptomatic. These changes are considered primarily to reflect the known characteristic of monoamine oxidase inhibitors to cause postural hypotonia. Abnormalities using these methods were not found, thus supporting the view that cardiovascular reflex abnormalities would be characteristic of narcolepsy.

SPECT imaging of dopamine and serotonin transporters with [123I]beta-CIT. Binding kinetics in the human brain

Single photon emission computerized tomography (SPECT) studies in non-human primates have previously shown that the cocaine derivative [123I]-2-beta-carbomethoxy-3-beta-(4-iodophenyl)-tropane ([123I]beta-CIT) labels dopamine transporters in the striatum and serotonin transporters in the hypothalamus-midbrain area. Here, we report on the regional kinetic uptake of [123I]beta-CIT in the brain of 4 normal volunteers and 2 patients with Parkinson's disease. In healthy subjects striatal activity increased slowly to reach peak values at about 20 hours post injection. In the hypothalamus-midbrain area peak activities were observed at about 4 hours with a slow decrease thereafter. Low activity was observed in cortical and cerebellar areas. The striatal to cerebellar ratio was about 4 after 5 hours and 9 after 20 hours. In 2 patients with idiopathic Parkinson's disease striatal activity was markedly decreased while the activity in hypothalamus-midbrain areas was only mildly diminished. Uptake into cortical and cerebellar areas appeared to be unchanged in Parkinson's disease. Consequently, in Parkinson's disease the striatal to cerebellar ratio was decreased to values around 2.5 after 20 hours. These preliminary methodological studies suggest that [123I]beta-CIT is a useful SPECT ligand for studying dopamine and possibly also serotonin transporters in the living human brain.