Parkinson’s Disease Drugs: Classification, Mechanism, Uses & Side Effects

Parkinson’s disease is a progressive neurodegenerative disorder that affects movement. It is associated with the degeneration of dopamine-producing neurons in the substantia nigra of the brain. This leads to reduced dopamine activity in the basal ganglia and causes symptoms such as resting tremor, muscle rigidity and bradykinesia.

Drugs used in Parkinson’s disease aim to improve dopamine activity, stimulate dopamine receptors, reduce dopamine breakdown or modify other neurotransmitter pathways.

Understanding the classification, mechanism of action, therapeutic uses and adverse effects of antiparkinsonian drugs is important for GPAT, RRB Pharmacist, DSSSB Pharmacist and other pharmacy competitive examinations.

Pathophysiology of Parkinson’s Disease

Dopamine is an important neurotransmitter involved in the regulation of movement.

In Parkinson’s disease, dopaminergic neurons in the substantia nigra pars compacta progressively degenerate. This reduces dopamine availability in the striatum and disrupts the basal ganglia circuits responsible for controlling movement.

Important symptoms

• Resting tremor
• Muscle rigidity
• Bradykinesia (slowness of movement)
• Postural and gait disturbances
• Shuffling gait
• Reduced facial expression

Important concept: Reduced dopamine activity contributes to the motor symptoms of Parkinson’s disease. Relative cholinergic overactivity is also relevant to the pharmacological treatment of tremor.

📝 Classification of Drugs Used in Parkinson’s Disease

Antiparkinsonian drugs can be classified into the following major groups.

A. Dopamine precursor

• Levodopa

Levodopa crosses the blood–brain barrier and is converted into dopamine in the brain.

B. Peripheral DOPA decarboxylase inhibitors

• Carbidopa
• Benserazide

These drugs reduce the peripheral conversion of levodopa into dopamine and are administered with levodopa.

C. Dopamine receptor agonists

• Pramipexole
• Ropinirole
• Rotigotine
• Apomorphine
• Bromocriptine

These drugs stimulate dopamine receptors and mimic some actions of dopamine.

D. Monoamine oxidase-B (MAO-B) inhibitors

• Selegiline
• Rasagiline
• Safinamide

These drugs inhibit dopamine breakdown by MAO-B and can help improve symptom control.

E. Catechol-O-methyltransferase (COMT) inhibitors

• Entacapone
• Opicapone
• Tolcapone

These drugs inhibit COMT-mediated levodopa metabolism and can prolong levodopa’s therapeutic effect.

F. Other drugs

Amantadine: Has several actions affecting dopaminergic and glutamatergic neurotransmission. It is particularly useful for levodopa-induced dyskinesia in selected patients.

Anticholinergic drugs:

• Trihexyphenidyl
• Benztropine

These reduce central muscarinic acetylcholine activity and may improve tremor in selected patients.

Adenosine A2A receptor antagonist:

• Istradefylline

This drug can be used as an adjunct to levodopa in selected patients experiencing OFF episodes.

⚕️ Mechanism of Action of Antiparkinsonian Drugs

1. Levodopa

Levodopa is a precursor of dopamine and is one of the most effective medicines for improving the motor symptoms of Parkinson’s disease.

Mechanism of action:

• Crosses the blood–brain barrier.
• Is converted into dopamine by aromatic L-amino acid decarboxylase.
• Increases dopamine availability in the brain.
• Improves bradykinesia and rigidity and may improve tremor.

Levodopa is usually combined with carbidopa or benserazide to reduce peripheral conversion and improve the amount of levodopa available to reach the brain.

Important adverse effects: Nausea, orthostatic hypotension, dyskinesia, hallucinations and motor fluctuations.

2. Carbidopa

Carbidopa inhibits peripheral DOPA decarboxylase.

Mechanism of action:

• Reduces the conversion of levodopa into dopamine outside the brain.
• Increases the proportion of administered levodopa available to enter the central nervous system.
• Reduces several peripheral dopaminergic adverse effects, such as nausea and vomiting.

Exam point: Carbidopa does not significantly cross the blood–brain barrier at therapeutic doses.

3. Dopamine receptor agonists

Examples include pramipexole, ropinirole, rotigotine and apomorphine.

Mechanism of action:

• Directly stimulate dopamine receptors.
• Mimic dopamine activity without requiring conversion into dopamine.
• Improve selected motor symptoms.

Important adverse effects: Nausea, dizziness, orthostatic hypotension, hallucinations, excessive daytime sleepiness and impulse-control disorders.

4. MAO-B inhibitors

Examples include selegiline, rasagiline and safinamide.

Mechanism of action:

• Inhibit monoamine oxidase-B.
• Reduce dopamine breakdown in the brain.
• Help prolong dopaminergic activity.

They may be used alone in selected patients or as adjuncts to levodopa, depending on the drug and clinical situation.

Important adverse effects: Headache, nausea, dizziness and, in some cases, insomnia or hallucinations. Interactions with other medicines must be considered.

5. COMT inhibitors

Examples include entacapone, opicapone and tolcapone.

Mechanism of action:

• Inhibit catechol-O-methyltransferase.
• Reduce levodopa metabolism.
• Prolong levodopa’s effect when used in appropriate treatment regimens.

These medicines are generally used as adjuncts to levodopa rather than as stand-alone treatment for Parkinson’s motor symptoms.

Important adverse effects: Dyskinesia, nausea and diarrhea. Tolcapone carries an important risk of severe liver injury and requires specific safety precautions.

6. Amantadine

Amantadine has multiple pharmacological actions, including effects on glutamatergic NMDA receptor activity and dopaminergic neurotransmission.

It can help reduce levodopa-induced dyskinesia in selected patients.

Important adverse effects: Confusion, hallucinations, ankle edema and livedo reticularis, a mottled skin discoloration.

7. Anticholinergic drugs

Examples include trihexyphenidyl and benztropine.

Mechanism of action:

• Block central muscarinic acetylcholine receptors.
• Reduce cholinergic activity.
• May improve tremor in selected patients.

These drugs have limited benefits for some other motor symptoms and can cause significant adverse effects, especially in older adults.

Important adverse effects: Dry mouth, blurred vision, constipation, urinary retention, memory impairment and confusion.

🩺Therapeutic Uses

The choice of drug depends on symptoms, age, other medical conditions, adverse-effect risks and treatment response.

📍 Important Adverse Effects

Levodopa

• Nausea and vomiting
• Orthostatic hypotension
• Dyskinesia
• Hallucinations
• Wearing-off and ON–OFF motor fluctuations

Dopamine agonists

• Nausea
• Dizziness and low blood pressure on standing
• Hallucinations
• Excessive daytime sleepiness
• Impulse-control disorders

MAO-B inhibitors

• Headache
• Nausea
• Dizziness
• Insomnia with some agents
• Potential drug interactions

COMT inhibitors

• Diarrhea
• Increased dyskinesia when combined with levodopa
• Nausea
• Potential liver toxicity with tolcapone

Amantadine

• Ankle edema
• Hallucinations
• Confusion
• Livedo reticularis

Anticholinergics

• Dry mouth
• Blurred vision
• Constipation
• Urinary retention
• Confusion and memory impairment

Important Drug Interactions

Levodopa and non-selective MAO inhibitors: The combination can cause dangerous blood-pressure reactions and is generally avoided unless managed under specific medical guidance.

Levodopa and dopamine-blocking drugs: Some antipsychotics and antiemetics can reduce the therapeutic effect of dopaminergic treatment or worsen parkinsonian symptoms.

Levodopa and iron supplements: Iron can reduce levodopa absorption when taken together. Appropriate administration timing should be discussed with a pharmacist or clinician.

Levodopa and dietary protein: Large amounts of dietary protein can interfere with levodopa absorption or transport in some patients. Dietary changes should be individualized rather than made without clinical guidance.

COMT inhibitors and levodopa: These drugs can increase levodopa exposure and may increase dyskinesia, sometimes requiring clinician-directed adjustment of levodopa treatment.

📑Important Exam Points

• Levodopa is a dopamine precursor.
• Carbidopa inhibits peripheral DOPA decarboxylase.
• Pramipexole and ropinirole are dopamine receptor agonists.
• Selegiline and rasagiline inhibit MAO-B.
• Entacapone inhibits COMT.
• Amantadine can help reduce levodopa-induced dyskinesia.
• Trihexyphenidyl is an anticholinergic drug that may improve tremor.
• Tolcapone has an important risk of severe liver injury.
• Dopamine agonists can cause impulse-control disorders and excessive sleepiness.
• Anticholinergic drugs can cause urinary retention, blurred vision and confusion.

📋Frequently Asked Questions

1. Which drug is commonly combined with levodopa?

Carbidopa or benserazide is combined with levodopa to inhibit its peripheral conversion into dopamine.

2. Which drugs inhibit MAO-B?

Selegiline, rasagiline and safinamide are examples of MAO-B inhibitors.

3. Which drug class prolongs the action of levodopa by inhibiting COMT?

COMT inhibitors, such as entacapone and opicapone, can prolong levodopa’s effect.

4. Which drug is useful for levodopa-induced dyskinesia?

Amantadine is used to help reduce levodopa-induced dyskinesia in selected patients.

5. Which anticholinergic drugs are used in Parkinson’s disease?

Trihexyphenidyl and benztropine are examples. Their use is limited by adverse effects, particularly in older adults.

🎯 Conclusion

Antiparkinsonian drugs improve symptoms by increasing dopamine availability, stimulating dopamine receptors, reducing dopamine breakdown or modifying other neurotransmitter pathways.

For pharmacy examinations, focus on drug classification, the mechanism of levodopa and carbidopa, MAO-B and COMT inhibitors, characteristic adverse effects and important drug interactions.

Educational disclaimer: This article is intended for pharmacy education and examination preparation. Drug selection and treatment changes should be guided by an appropriately qualified healthcare professional.