Module 4.3.1 (Pharmacology of Drugs for Parkinsons) Flashcards

1
Q

What is parkinson’s disease?

A

Parkinson’s disease results from the degeneration of dopaminergic neurons that arise in the substantia nigra and project to other structures in the basal ganglia.

> It is characterized by two major pathologic processes: (a) premature selective loss of dopamine neurons; (b) the accumulation of Lewy bodies, composed of α-synuclein, which become misfolded and accumulate in multiple systems of patients with Parkinson disease.

> Early non-motor features such as depression, sleep disturbances, constipation and anosmia may precede the diagnosis

Basal Ganglia include the

  • striatium
  • substantia nigra
  • globus pallidus
  • subthalamus

> imbalance between dopamine and ACh

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2
Q

In normal striatum, neurons that release dopamine will do what?

A

inhibit neurons that release GABA (an inhibitory transmitter)

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3
Q

In normal striatum, neurons that release AcH, will?

A

Excite the neurons that release GABA

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4
Q

Therefore what happens in the absence of dopamine?

A

The excitatory influence of ACh goes unopposed ⇒ excessive stimulation of neurons that release GABA

  • Overactivity of the GABAminergic neurons contributes to the motor Sx that characterise PD
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5
Q

Outline which drugs are as used in pakrinsons disease for the following:

A) Dopamine agonists

B) Anticholinergics

C) Monoamine oxidase type B inhibitors

D) Other drugs

A

A)

  • Apomorphine „
  • Bromocriptine „
  • Cabergoline „
  • Pramipexole „
  • Ropinirole „
  • Rotigotine

B)

  • Benzatropine
  • Trihexyphenidyl (also known as benzhexol)

C)

  • Rasagiline
  • Safinamide
  • Selegiline
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6
Q

How does levodopa work? Where is it absorbed from?

A

Levodopa increases the concentration of dopamine in the brain

  • Absorbed from dudodenum
  • Protein in food reduces absorption
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7
Q

What is the 2 metabolism pathways of levodopa?

A

Metabolism by 2 pathways in peripheral tissue:

  • Converted to dopamine by dopa decarboxylase
  • Metabolised to 3-O-methyldopa by catechol-O- methyltransferase (COMT)

> large first pass

> only about 1% of administred dose of levodopa reaches brain tissue

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8
Q

dopamine doesn’t cross BBB, but precursor levodopa is transported into CNS and converted to dopamine in brain. Need large doses of levodopa because much of drug is decarboxylated to dopamine in the periphery ⇒ SE eg N, V, cardiac arrhythmias & hypotension. Therefore what is levodopa given with? What is the advantages of this?

A

Hence levodopa is given with a dopa decarboxylase inhibitor:

  • Carbidopa
  • Benserazide

decreases the dose of levodopa needed and decreases the severity of SE

  • Dopa decarboxylase inhibitor does not cross BBB
  • ↓ the metabolism of levodopa in the GIT & peripheral tissues –> increase availability of levodopa to the CNS

> Another advantage of combining carbidopa with Levodopa is the reduced risk of cardiac
abnormalities. Dopamine can have a direct affect on beta-adrenoreceptors that can
result in cardiac arrhythmias. With less peripheral dopamine there are less issues
with cardiac stimulation.

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9
Q

What are the peripheral and CNS adverse effects of levodopa?

A

Peripheral effects

  • Anorexia, N & V –> Due to stimulation of the CTZ of the medulla.
  • Orthostatic hypotension.
  • Adrenergic action on the iris causes mydriasis
  • Saliva & urine may ⇒ a brownish color bec of the melanin pigment produced from catecholamine oxidation

CNS effects

  • Visual & auditory hallucinations
  • Abnormal involuntary movements (dyskinesias)
  • Episodes of sudden unpredictable loss of mobility (‘off’ effects)
  • Mood changes
  • Depression
  • Psychosis
  • Anxiety
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10
Q

What are the drug interactions for levodopa with the following:

A) Dopamine antagonists

B) Dopamine agonists

C) Non-selective MAOI

D) Antacids

E) Methyldopa

A

A)

  • Antipsychotics, metoclopramide, prochlorperazine
  • Antagonise levodopa activity & ↓ its therapeutic effect

B)

  • ↑ adverse effects of levodopa

C)

  • Inhibit the breakdown of dopamine and may cause hypertensive crisis in pts receiving levodopa

D)

  • May ↓ the absorption of levodopa from controlled release products eg Madopar HBS, Sinemet CR –> space dose apart at least 2 hours

E)

  • ↑ effect of levodopa
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11
Q

MOA of amantadine?

A
  • ↑ dopamine release from nigrostriatal neurons and blocks cholinergic receptors
  • Acts as a N-methyl-D-aspartate (NMDA) antagonist in the glutamatergic pathway from subthalamic nucleus to globus pallidus.
  • Has antiviral activity against some strains of influenza A

> Prevents release of viral RNA into host cell.

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12
Q

AE (CIR) of Amantadine

A

Common

  • N, V, dry mouth, constipation
  • Livedo reticularis

Infrequent

  • Urinary retention

Rare

  • Confusion
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13
Q

What are two monoamine oxidase type B inhibitors? What is the MOA?

A

Rasagiline

Selegiline

  • Selectively & irreversibly inhibit MAO Type B (which metabolizes dopamine)
  • Do not inhibit MAO type A
  • By ↓ metabolism of dopamine, selegiline & rasagiline ↑ dopamine levels in the brain
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14
Q

AE (CIR) and can tyramine reach food be eaten with MAO-B inhibitors?

A

Common

  • N, V „ Orthostatic hypotension „ Headache „ Dyskinesia „ Accidental injury „ Arthralgia „ Rash
  • Rasagiline can cause anorexia, weight loss.

Infrequent/rare

  • Hallucinations „
  • Arrhythmias

Note

  • Unlike other MAOIs, MAO-B inhibitors rarely cause hypertension when tyramine-rich food are eaten (and drug used at recommended doses)
  • MAOI dietary restrictions apply if moclobemide + MAOB inhibitor used concurrently
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15
Q

What is Catechol-O-methyltransferase (COMT)?

A

COMT is an enzyme present at adrenergic & dopaminergic synapses

  • Normally, methylation of levodopa by COMT to 3-O-methyldopa is a minor pathway.
  • BUT when peripheral dopamine decarboxylase activity is inhibited by carbidopa, a significant concn of 3-O-methyldopa is formed that competes with levodopa for active transport into the CNS –> so we dowan this therefore we use COMT inhibitors
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16
Q

What is the MOA of COMT inhibitors? name of drug?

A

Inhibition of COMT by entacapone

  • ↓ plasma concn of 3-O-methyldopa
  • ↑ central uptake of levodopa,
  • Greater concns of brain dopamine

Has been shown to ↓reduce Sx of “wearing-off” phenomena (seen in patients on levodopa–carbidopa)

17
Q

What are the common and rare AE of COMT inhibitors? When is it CI?

A

Common

  • „ N, V, dry mouth „ Dyskinesia „ Hallucinations „ Paranoia „ Discoloured urine–> reddish-brown

Rare

  • Skin, hair or nail discolouration.
  • Increase in liver enzymes

CI

  • Phaeochromocytoma –> tumour of medually of adrenal gland
  • Hx of NMS
  • Use with MAOI
  • Hepatic impairment
18
Q

How do dopamine receptor agonists work? What are the two types, include drugs for each one.

A

Directly activate dopamine receptors in the striatum

  • Work mainly by activating D2 receptors.

Ergot –derivatives

  • Bromocriptine „
  • Cabergoline

Nonergot drugs

  • Apomorphine
  • Pramipexole
  • Ropinirole
  • Rotigotine.

> The non-ergot derivatives are highly selective for dopamine receptors. The ergot derivatives activate serotonergic & α-adrenergic receptors also ∴ ↑ SE

19
Q

SE of dopamine receptor agonis

A

Similar to those of levodopa

Nausea

  • Occurs in as many as 50% when first treated
  • Mainly results from stimulation of dopamine receptors in the vomiting center located in the medulla.

Dose-related CNS effects:

  • Confusion, dyskinesias, sedation, vivid dreams, hallucinations

Adverse effects

Common

  • N, V, abdominal pain, constipation „
  • Headache, dizziness „
  • Orthostatic hypotension „
  • Dyskinesia „
  • Psychiatric disorders including impulse control disorders

Rare

  • Fainting „
  • Alopecia
20
Q

For dopamine receptor agonist, apomorphine, what are some PK and SE?

A
  • Highly emetogenic - requires pretreatment with domperidone to ↓ N & V
  • Highly lipophilic but undergoes extensive first pass

> ineffective orally

> Injectable dopamine agonist

>Half-life ∼ 40 min

Common SE

  • Eosinophilia
  • Injection site reactions (including subcutaneous nodules, skin necrosis)
21
Q

What are some common AE of pramipexole and rotigotine? –> dopamine receptor agonists

A

Pramipexole common AE

  • Amnesia „ Visual disturbances „ Paradoxical worsening of RLS

Rotigotine common AE:

  • Application site reactions „ Insomnia „ Paradoxical worsening of RLS
22
Q

What are examples of anticholinergics/antimuscarinics? What do they cause? CI in? SE?

A

Benzhexol = trihexyphenidyl „ Benztropine „ Biperiden = decrease amount of acetylcholine

Cause

  • Mood changes „
  • Xerostomia (dryness of the mouth) „
  • Visual problems

> interfere with GI paralysis

CI

  • GI obstruction „
  • Urinary obstruction „
  • Myasthenia gravis

SE

Similar to those caused by high doses of atropine eg

  • Pupillary dilation, confusion, hallucination, sinus tachycardia, urinary retention, constipation, and dry mouth.