💊 MAO-B Inhibitors

Monoamine oxidase B (MAO-B) inhibitors are a class of drugs used in the treatment of Parkinson's disease. They selectively inhibit the MAO-B enzyme.

MAO-B inhibitors (e.g., selegiline, rasagiline) selectively and irreversibly inhibit the enzyme monoamine oxidase B in the brain. This enzyme is responsible for the breakdown of dopamine. By inhibiting MAO-B, these drugs increase the availability of dopamine in the synaptic cleft, thereby improving motor symptoms in Parkinson's disease.

MAO-B inhibitors are used as monotherapy in early Parkinson's disease to provide symptomatic relief and potentially delay the need for levodopa. They can also be used as adjunctive therapy with levodopa in advanced disease to reduce 'off' time and improve motor fluctuations. They do not have a strong symptomatic effect on their own.

Absolute contraindications include hypersensitivity to the drug. They are contraindicated with pethidine and other opioids due to the risk of severe serotonin syndrome. Co-administration with other MAO inhibitors (including non-selective ones), tricyclic antidepressants, or selective serotonin reuptake inhibitors (SSRIs) is also contraindicated due to the risk of serotonin syndrome.

Common adverse effects are generally mild and include nausea, dizziness, headache, and insomnia (particularly with selegiline if taken late in the day). When used with levodopa, they can exacerbate levodopa-induced dyskinesias or hallucinations. Serotonin syndrome is a serious but rare adverse effect if co-administered with serotonergic drugs.

Patients should be monitored for the development or worsening of dyskinesias, especially when used in combination with levodopa. Psychiatric status should be assessed for hallucinations or confusion. Blood pressure should be monitored, although the risk of hypertensive crisis with selective MAO-B inhibitors is lower than with non-selective MAO inhibitors.

Crucially, always check for concurrent use of pethidine, SSRIs, tricyclic antidepressants, or other MAO inhibitors, as these combinations are contraindicated due to the risk of serotonin syndrome. Counsel patients to report any unusual symptoms like confusion, agitation, or muscle rigidity. Advise on taking selegiline earlier in the day to avoid insomnia.

MAO-B inhibitors are selective for MAO-B, which primarily metabolises dopamine, thus avoiding the 'cheese effect' associated with non-selective MAO inhibitors. They are useful for mild symptoms in early Parkinson's or to extend the effects of levodopa. The risk of serotonin syndrome with certain drug combinations is a critical safety point.

SBAs often focus on the drug interactions of MAO-B inhibitors, particularly the contraindication with pethidine and the risk of serotonin syndrome with SSRIs or tricyclic antidepressants. Questions might also test their role in early Parkinson's disease as monotherapy versus adjunctive therapy. The selective nature of MAO-B inhibition is a key concept.