Neurologic Pharmacology Overview

Levodopa / Carbidopa

Mechanism

  • Levodopa: Converts to dopamine in the substantia nigra, enhancing dopaminergic activity.
  • Carbidopa: Inhibits the conversion of levodopa to dopamine outside the brain (peripheral conversion), allowing for a smaller dose of levodopa while maximizing central nervous system (CNS) effects and minimizing peripheral side effects.

Therapeutic Use

  • Primary treatment for Parkinson's Disease (PD).
  • Relieves motor manifestations including:
      - Tremor
      - Rigidity
      - Bradykinesia
      - Postural instability

Adverse Reactions

  • Common:
      - Nausea
      - Vomiting
      - Darkening of urine and sweat (benign finding)
  • Other:
      - Orthostatic hypotension
      - Dyskinesias, tremors, twitching
      - Psychosis, hallucinations, paranoia (rare)
      - Random "on-off" episodes: Sudden worsening of PD symptoms throughout the day.

Nursing Interventions

  • Monitor darkening of urine/sweat and reassure that it is normal.
  • Assess for orthostatic hypotension; implement fall prevention measures.
  • For dyskinesias, consider administration of amantadine, adjust dosing frequency, or add adjunct medications like pramipexole or selegiline.
  • Observe for psychosis; administer second-generation antipsychotics as needed (e.g., quetiapine), avoid first-generation antipsychotics as they can decrease levodopa efficacy.

Administration

  • Start at low doses and titrate slowly to mitigate adverse effects.
  • Full therapeutic effect may take up to 6 months.
  • Immediate-release formulation: Onset within 30 minutes, duration shorter.
  • Extended-release formulation: Lasts 4-6 hours, onset may take up to 2 hours in the morning.

Client Teaching

  • Administer with food if nausea occurs, but avoid high-protein meals which can decrease absorption.
  • Inform that dark urine/sweat is normal and harmless.
  • Rise slowly to prevent dizziness from orthostatic hypotension.
  • Report dyskinesias, hallucinations, or loss of efficacy to the provider.
  • Avoid vitamin B6 supplements which can impair levodopa efficacy.

Contraindications

  • Angle-closure glaucoma
  • History of melanoma
  • Severe psychological issues (psychosis, suicidal thoughts)

Precautions

  • In older adults
  • Patients with renal, hepatic, respiratory, or endocrine disorders
  • Patients with wide-angle glaucoma
  • Those with peptic ulcer disease or psychiatric disorders (depression, bipolar disorder)

Interactions

  • First-generation (typical) antipsychotics + Vitamin B6 → decrease levodopa/carbidopa action
  • MAO inhibitors (MAOIs) within 2 weeks → risk of hypertensive crisis
  • High-protein meals → decrease absorption of levodopa; should be consumed with low-protein meals.
  • Anticholinergic medications → increase levodopa/carbidopa efficacy.

Direct Dopamine Agonist

Drug Class

  • Prototype Drugs: Pramipexole, Ropinirole, Bromocriptine.

Mechanism

  • Binds directly to dopamine receptors (D2/D3), mimicking the effect of dopamine without necessitating conversion.
  • Inhibits monoamine oxidase B (MAO-B) to prevent dopamine breakdown, extending its activity.
  • Inhibits catechol-O-methyltransferase (COMT) to reduce peripheral breakdown of levodopa.

Therapeutic Use

  • Primarily for Parkinson's Disease and Restless Leg Syndrome (RLS).

Adverse Reactions

  • Nausea and GI distress.
  • Drowsiness/sudden sleep attacks (uncontrolled onset of sleep).
  • Muscle weakness.
  • Orthostatic hypotension.
  • Dyskinesias.

Nursing Interventions

  • Monitor for nausea, advising food intake if necessary.
  • Safeguard against sudden drowsiness.
  • Monitor muscle strength and cardiac status.
  • Observe for abnormal involuntary muscle movements.

Administration

  • For RLS: administer 2-3 hours before bedtime.
  • Increase doses gradually (allow 5-7 days between adjustments).
  • Taper off slowly if discontinuing (over one week).

Client Teaching

  • Advise against driving or alert activities if drowsy.
  • Avoid alcohol and CNS depressants to mitigate adverse effects.
  • Report any instances of muscle weakness.

Contraindications

  • Allergy to pramipexole or ropinirole.
  • Pregnancy and lactation.

Precautions

  • Renal dysfunction (dose adjustments may be warranted).
  • Pre-existing compulsive behaviors (gambling or hypersexuality at higher doses).
  • Older adults.

Interactions

  • Cimetidine can increase pramipexole levels; use cautiously.
  • Metoclopramide and phenothiazine antipsychotics may decrease therapeutic effect.
  • Alcohol and CNS depressants heighten the risk of adverse effects (like sleep attacks).

Monoamine Oxidase Type B (MAO-B) Inhibitor

Drug Class

  • Prototype Drugs: Selegiline (Eldepryl, Zelapar).

Mechanism

  • Selectively inhibits MAO-B, slowing dopamine breakdown, helping to sustain its levels in the brain and extending the efficacy of levodopa/carbidopa.
  • Used adjunctively to reduce the necessary dose of levodopa.

Therapeutic Use

  • Treats Parkinson's disease as adjunctive therapy and major depression via transdermal patches.

Adverse Reactions

  • Insomnia, notably with late administration.
  • Hypertension at elevated doses or due to interactions.
  • Irritation of the oral mucous membranes with orally disintegrating tablets (ODT).

Nursing Interventions

  • Monitor for adverse effects, especially blood pressure changes.
  • Review concurrent medications thoroughly for interactions.
  • Regularly inspect the tongue and oral cavity for irritation if on ODT.

Administration

  • Administer with food and preferably with levodopa/carbidopa.
  • ODT form should be placed on the tongue and allowed to dissolve without food/drink for 5 minutes.
  • Ensure the last daily dose is by noon to avoid insomnia.

Client Teaching

  • Notify provider of any new medications including herbal supplements due to potential interactions.
  • Avoid tyramine-rich foods despite lower risks compared to non-selective MAOIs.
  • Report any oral irritation if using ODT.

Contraindications

  • Hypersensitivity to selegiline.
  • Concurrent meperidine use due to the risk of serotonin syndrome.

Precautions

  • Monitor clients with peptic ulcer disease closely due to potential gastrointestinal effects.

Interactions

  • Meperidine and other opioids, along with MAOIs, can induce severe symptoms like high fever and hypertension.
  • Tyramine-rich foods may cause hypertension even though risks are less than with non-selective MAOIs.
  • Caution in the combination with antihypertensives and diuretics due to potential hypotension.

Cholinesterase Inhibitor

Drug Class

  • Prototype Drugs: Donepezil (Aricept).

Mechanism

  • Inhibits acetylcholinesterase, allowing increased levels of acetylcholine (ACh) at synapses, which can help cognitive functioning in Alzheimer's Disease (AD).

Therapeutic Use

  • Mild to moderate Alzheimer's disease to enhance cognition and behavioral symptoms.

Adverse Reactions

  • Common GI issues: nausea and diarrhea.
  • CNS effects: insomnia, dizziness, and headaches.
  • Bradycardia.

Nursing Interventions

  • Administer with food to alleviate GI distress.
  • Monitor weight since GI effects can lead to weight loss.
  • Check for signs of GI bleeding and monitor heart rate due to bradycardia.

Administration

  • Given once daily at bedtime to aid tolerance of GI side effects.

Client Teaching

  • Report signs of GI bleeding or CNS effects to the provider promptly.

Contraindications

  • Active GI bleeding and allergies to donepezil.
  • Not recommended for pediatric use.

Precautions

  • Caution with liver, renal, cardiac, GI, and pulmonary disorders.

Interactions

  • Escalation of GI bleeding risks if used with NSAIDs.
  • Anticholinergic drugs can decrease donepezil's effectiveness.

NMDA Receptor Antagonists

Drug Class

  • Prototype Drugs: Memantine (Namenda).

Mechanism

  • Blocks NMDA glutamate receptors, preventing excessive calcium influx into neurons, which reduces neuronal damage and excitotoxicity related to high levels of glutamate.

Therapeutic Use

  • Moderate to severe Alzheimer’s disease and mild to moderate vascular dementia.

Adverse Reactions

  • CNS symptoms, notably dizziness, headaches, and confusion.

Nursing Interventions

  • Monitor for CNS effects such as confusion and dizziness; fall prevention is critical.

Administration

  • Administer orally, without food considerations.
  • Can be combined with donepezil for enhanced effect.

Client Teaching

  • Instruct the client or caregiver to report CNS changes to the healthcare provider.

Contraindications

  • Renal failure and previous allergy to memantine.

Precautions

  • Monitor clients with liver disorders closely, and may require dosage adjustments in renal impairment.

Interactions

  • Avoid OTC antacids that alkalinize urine, as this can elevate memantine levels and risk toxicity.

Immunomodulator

Drug Class

  • Prototype Drugs: Interferon beta-1a, Interferon beta-1b.

Mechanism

  • Modulates the immune response to prevent leukocyte-induced damage to the myelin sheath in multiple sclerosis (MS) patients.

Therapeutic Use

  • Used for relapsing-remitting types of MS primarily.

Adverse Reactions

  • Flu-like manifestations (e.g., fever, chills, fatigue).
  • Myelosuppression (decreased blood cells).
  • Liver toxicity and injection site reactions.

Nursing Interventions

  • Begin treatment at low doses and monitor for side effects.
  • Pre-medicate with acetaminophen to mitigate flu-like symptoms.
  • Conduct regular CBC and liver function tests.

Administration

  • Administer subcutaneously or intramuscularly depending on the formulation, with strict timing for Beta-1b.

Client Teaching

  • Rotate injection sites and educate on how to manage injection-related effects.

Contraindications

  • Known allergies to beta interferons or mannitol.

Precautions

  • Caution in patients with existing liver conditions or mental health disorders.

Interactions

  • Concurrent use of immunosuppressants increases risks of myelosuppression.

Dangerous Drug Combinations to Memorize

  • Selegiline + Meperidine: Risk of hyperthermia, rigidity, and possible fatality.
  • Levodopa + MAOIs: Risk of hypertensive crisis if within 2 weeks.
  • Levodopa + First-Generation Antipsychotics: Decreases efficacy of levodopa.
  • Donepezil + Anticholinergics: Counteractive effects resulting in reduced donepezil efficacy.
  • Donepezil + NSAIDs: Increased risk of GI bleeding.
  • Interferon + Immunosuppressants: Increased myelosuppression risk.
  • Memantine + Urine alkalinizers: Increased risk of toxicity.

Special Considerations for Stopping or Continuing Medications

  • Pramipexole: Must taper over one week to avoid worsening PD symptoms.
  • Baclofen: Abrupt discontinuation can lead to life-threatening withdrawal seizures and hyperthermia.

Normal Findings to Communicate to Clients

  • Levodopa/carbidopa: Expect dark urine and sweat, which are harmless.
  • Interferon beta: Expect flu-like symptoms that should improve with continued treatment; pre-treat as necessary.
  • Pramipexole: Mild nausea is common, which typically diminishes over time.