Monoamine oxidase type B (MAO-B) inhibitors are a class of drugs widely used in the treatment of [Parkinson's disease](/diseases/parkinsons-disease) that work by blocking the enzyme responsible for the breakdown of [dopamine](/entities/dopamine) in the brain. Three MAO-B inhibitors are currently approved for clinical use: selegiline (first generation), rasagiline (second generation), and safinamide (third generation)[@finberg2020]. Among all classes of anti-parkinsonian drugs, MAO-B inhibitors have demonstrated the greatest neuroprotective potential in preclinical studies, through mechanisms including reduction of [oxidative stress](/mechanisms/oxidative-stress), inhibition of dopamine metabolism, and stimulation of neurotrophic factor release[@tan2024].
Mechanism of Action
Dopamine Metabolism
Dopamine released from nigrostriatal terminals in the [striatum](/brain-regions/striatum) is metabolized through two main pathways:
Monoamine oxidase type B (MAO-B) inhibitors are a class of drugs widely used in the treatment of [Parkinson's disease](/diseases/parkinsons-disease) that work by blocking the enzyme responsible for the breakdown of [dopamine](/entities/dopamine) in the brain. Three MAO-B inhibitors are currently approved for clinical use: selegiline (first generation), rasagiline (second generation), and safinamide (third generation)[@finberg2020]. Among all classes of anti-parkinsonian drugs, MAO-B inhibitors have demonstrated the greatest neuroprotective potential in preclinical studies, through mechanisms including reduction of [oxidative stress](/mechanisms/oxidative-stress), inhibition of dopamine metabolism, and stimulation of neurotrophic factor release[@tan2024].
Mechanism of Action
Dopamine Metabolism
Dopamine released from nigrostriatal terminals in the [striatum](/brain-regions/striatum) is metabolized through two main pathways:
MAO-B catalyzes the first step in the primary degradation pathway. This reaction generates hydrogen peroxide (H₂O₂) as a byproduct, which contributes to oxidative stress and nigrostriatal neurodegeneration[@naoi2024].
MAO-B is located predominantly on the outer mitochondrial membrane of [astrocytes](/cell-types/astrocytes) and serotonergic neurons in the brain. Its expression increases with age and in neurodegenerative conditions, making it both a therapeutic target and a potential contributor to disease progression[@finberg2020].
Mermaid diagram (expand to render)
Landmark Clinical Trials
DATATOP Trial (Selegiline)
The Deprenyl and Tocopherol Antioxidative Therapy of Parkinsonism (DATATOP) trial was the first large multicenter study examining early selegiline treatment in Parkinson's disease[@finberg2020]:
Design: 800 patients with early, untreated PD randomized to selegiline (10 mg/day), tocopherol (vitamin E, 2000 IU/day), both, or placebo
Primary endpoint: Time to requiring [levodopa](/therapeutics/levodopa) therapy
Results: Selegiline delayed the need for levodopa by approximately 9 months. Tocopherol showed no benefit
Controversy: The observed benefit was debated as being purely symptomatic rather than neuroprotective, as the drug washout period (2 months) was shorter than the time required for full recovery of MAO-B activity after irreversible inhibition
ADAGIO Trial (Rasagiline)
The Attenuation of Disease Progression with Azilect Given Once-daily (ADAGIO) trial tested whether rasagiline had disease-modifying effects using a delayed-start design[@szk2022]:
Design: 1,176 patients with early PD randomized to rasagiline 1 mg/day or 2 mg/day (early start) vs. placebo for 36 weeks then rasagiline (delayed start) for 36 weeks
Results: The 1 mg/day group met all three hierarchical endpoints, consistent with a disease-modifying effect. The 2 mg/day group did not, creating an unresolved dose-response paradox
Significance: First trial to use delayed-start design in PD; results remain debated
SETTLE Trial (Safinamide)
Design: 549 mid-to-late PD patients on levodopa randomized to safinamide (50–100 mg/day) or placebo for 24 weeks
Results: Safinamide significantly increased daily "on" time by 1.42 hours vs. 0.57 hours for placebo, decreased "off" time, and improved UPDRS motor scores
Long-term extension (MOTION): Sustained benefits observed over 2 years
Neuroprotective Potential
MAO-B inhibitors have consistently demonstrated the greatest neuroprotective potential among all classes of anti-parkinsonian drugs[@tan2024]:
Reduction of oxidative stress: By inhibiting MAO-B, these drugs decrease the production of H₂O₂ generated during dopamine metabolism
Neurotrophic factor induction: Stimulate production and release of [GDNF](/entities/gdnf) and [BDNF](/proteins/bdnf), which support dopaminergic neuron survival
Anti-inflammatory effects: Reduce microglial activation and pro-inflammatory cytokine release in the striatum
Glutamate modulation (safinamide): Inhibits excessive glutamate release, counteracting [excitotoxicity](/mechanisms/excitotoxicity) in the basal ganglia motor circuit[@tan2024]
Effects on Non-Motor Symptoms
Recent systematic reviews have highlighted MAO-B inhibitor benefits beyond motor symptoms[@giossi2022]:
Quality of life: Rasagiline and safinamide significantly improved quality of life measures (PDQ-39) compared to placebo
Depression: Safinamide demonstrated improvements in depression scores, potentially through its dual dopaminergic/glutamatergic mechanism
Pain: Safinamide reduced musculoskeletal and neuropathic pain in PD patients
Fatigue: Both rasagiline and safinamide showed benefits for fatigue
Sleep: Mixed results; selegiline may worsen insomnia due to amphetamine metabolites, while rasagiline and safinamide have neutral to beneficial effects
Cognition: Some evidence for cognitive benefit, particularly with rasagiline, possibly through noradrenergic and serotonergic effects[@arajo2024]
Safety and Drug Interactions
Common Side Effects
Selegiline: Insomnia, nausea, orthostatic hypotension, confusion, hallucinations (especially in elderly)
Safinamide: Dyskinesia (when added to levodopa), nausea, falls, insomnia
Drug Interactions
Tyramine-containing foods: At therapeutic doses, MAO-B inhibitors have minimal risk of the "cheese effect" (hypertensive crisis), unlike non-selective MAO inhibitors. Risk increases only at supratherapeutic doses that inhibit MAO-A[@finberg2020].
Applications Beyond Parkinson's Disease
MAO-B inhibitors are being investigated for other neurodegenerative conditions[@tan2024]:
[Alzheimer's disease](/diseases/alzheimers-disease): MAO-B activity is elevated in astrocytes surrounding amyloid plaques; selegiline and rasagiline have shown modest cognitive benefits in AD trials. Novel MAO-B inhibitors that also target amyloid are in development
[ALS](/diseases/amyotrophic-lateral-sclerosis): Rasagiline demonstrated a potential disease-modifying effect in ALS clinical trials, possibly through anti-apoptotic mechanisms
[Huntington's disease](/mechanisms/huntingtons-disease-pathway): Preclinical evidence suggests neuroprotective effects in striatal neurons
Depression: Selegiline transdermal patch (Emsam) is FDA-approved for major depressive disorder
Traumatic brain injury: Neuroprotective potential under investigation
Naoi M, Maruyama W, Shamoto-Nagai M, An overview of the role of monoamine oxidase-B in Parkinson's Disease: implications for neurodegeneration and therapy (2024)
Szökő É, Tábi T, Riederer P, Vécsei L, Magyar K, A critical appraisal of MAO-B inhibitors in the treatment of Parkinson's Disease (2022)
Giossi R, Carrara F, Calogero AM, et al, Effects of MAO-B inhibitors on non-motor symptoms and quality of life in Parkinson's Disease: a systematic review (2022)
Araújo B, Silva AR, Torres T, et al, Effects of MAO-B inhibitors on life quality of Parkinson's Disease patients: a systematic review and meta-analysis (2024)
Zhang Y, et al, Safety comparisons among monoamine oxidase inhibitors against Parkinson's Disease using FDA adverse event reporting system (2023)
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