Anti-Parkinson's Drugs I

Introduction and Overview

  • Speaker: Yi Xiang

  • Affiliation: VA Brain Rehabilitation Research Center and adjunct faculty at the Department of Pharmacology, University of Florida.

Lecture Structure

  • Topic: Drugs for Parkinson's Disease, specifically in two parts.

  • Focus for this session: First part on anti-Parkinson's drugs.

  • Learning Objectives: Four key objectives outlined in the presentation, but not detailed in this lecture. It's recommended for students to review these learning objectives before exams to grasp the fundamental concepts better.

Overview of Parkinson's Disease

  • Definition: Parkinson's disease is the second most common neurodegenerative disease, following Alzheimer's Disease. This means that it affects the brain over time, causing various problems with movement and coordination.

  • Incidence: There are over 60,000 new cases of Parkinson's disease diagnosed every year in the United States. This statistic highlights how prevalent the disease is, affecting many individuals and their families.

  • Demographics: The disease primarily affects older adults, typically those aged 60 years or older. Although it can occur at younger ages, it's more common in the elderly population, indicating that age is a significant risk factor.

Etiology

  • Idiopathic Cases: Over 85% of Parkinson's disease cases are idiopathic, meaning that the exact cause is unknown. This is a significant challenge for researchers and healthcare providers trying to understand and treat the disease.

  • Genetic Cases: Approximately 10% of cases are linked to genetic factors. This involves changes in certain genes, like the:

    • Parkin gene: Related to the function of neurons, which are the cells affected in Parkinson's.

    • PINK gene: Involved in protecting brain cells.

    • Synuclein gene: Associated with a protein that can build up in the brain and is thought to play a role in the disease.

Risk Factors

  • Age: The primary risk factor for developing Parkinson's disease is getting older. As people age, they are more likely to develop various health conditions, including this one.

  • Genetic Influences: Having a family history of Parkinson's can contribute to an individual’s risk of developing the disease. This suggests that genetics plays a role in the disease's onset.

  • Environmental Exposures: Certain drugs and toxins have been linked to an increased risk of Parkinson's disease. These include:

    • MPTP: This is a synthetic opioid that can damage the brain.

    • Pesticides: Some chemicals like Rotenone found in pesticides are suspected to increase risk, possibly due to their toxic effects on neuronal health.

Neuropathology of Parkinson's Disease

  • Primary Issue: The main problem in Parkinson's disease is the loss of dopamine neurons in the substantia nigra, a specific area of the midbrain responsible for movement control.

  • Substantia Nigra Characteristics: This region contains dense neuromelanin, giving it a dark appearance in healthy brains. Neuromelanin is a type of pigment that can provide insight into neuronal health.

  • Post-Mortem Observations: Studies of brains from patients who had Parkinson's show a significant reduction in the dark appearance of the substantia nigra, which indicates severe loss of dopaminergic neurons.

  • Dopamine Neurons Loss: By the time Parkinson's is diagnosed, patients often lose up to 80% of their dopamine neurons, indicating that substantial damage has already occurred.

Role of Dopamine

  • Function: Dopamine is a critical neurotransmitter, which is a chemical messenger in the brain. It plays a crucial role in the nigrostriatal pathway that connects the substantia nigra to the striatum, a part of the brain involved in movement and coordination.

  • Significance: Dopamine modulates the circuitry between different brain regions, essential for tasks like planning and executing voluntary movements.

Clinical Symptoms of Parkinson's Disease

  • Motor Symptoms: These include:

    • Bradykinesia: This is the slowness of movement noticeable when starting, continuing, or stopping movement.

    • Resting Tremors: Uncontrollable shaking happens while at rest, which is often one of the first signs of the disease.

    • Rigidity: Stiffness that results in limited range of motion. It can manifest as cogwheel rigidity, where movements feel jerky.

    • Postural Instability: Problems with maintaining posture can lead to falls, especially as the disease progresses. This symptom usually becomes more pronounced in later stages.

  • Non-Motor Symptoms: These symptoms often occur alongside motor symptoms and can include anxiety, depression, dementia, and autonomic dysfunction (issues with the automatic functions of the body like heart rate and digestion).

Treatment Landscape

  • Current State: There currently is no cure for Parkinson's disease. Treatments available mainly focus on managing symptoms — particularly the motor ones — rather than stopping the disease from progressing.

  • Dopamine Replacement Therapy: This is an essential strategy to help counteract the loss of dopamine function in individuals with Parkinson's.

Dopamine Biosynthesis

  • Starting Material: The process of making dopamine begins with the amino acid Tyrosine, which is an essential building block for many proteins in the body.

  • Enzyme 1: The enzyme Tyrosine Hydroxylase converts Tyrosine into L-DOPA, which is a direct precursor to dopamine.

  • Enzyme 2: Aromatic Amino Acid Decarboxylase (AAD), also known as DOPA Decarboxylase, further converts L-DOPA into dopamine.

  • Further Metabolism: Once made, dopamine is broken down by enzymes such as:

    • Catechol-O-Methyl Transferase (COMT): An enzyme that metabolizes dopamine in the brain.

    • Monoamine Oxidase Isoform B (MAO-B): Another enzyme that breaks down dopamine, which is significant since maintaining dopamine levels is crucial for managing symptoms.

Implications for Drug Therapy

  • Targeting Dopamine Biosynthesis: There are various points in the dopamine synthesis pathway that medications can target. For example:

    • Tyrosine Supplementation: Although it might seem effective, it's generally ineffective because the body can only utilize a certain amount; beyond that, it leads to saturation and competition for transport to the brain.

    • Dopamine Administration: This method is ineffective for treating Parkinson's because dopamine itself cannot cross the blood-brain barrier, which is a protective barrier that limits what substances can enter the brain.

    • Levodopa (L-DOPA): This is effective because it can cross the blood-brain barrier and is a precursor to dopamine, making it a cornerstone of Parkinson's treatment.

Issues with L-DOPA Treatment

  • First Pass Metabolism: Approximately 70% of the L-DOPA taken orally gets metabolized in the body before it reaches the blood. Only about 30% actually enters the bloodstream to reach the brain, causing inefficiencies in treatment.

  • Periphery Side Effects: The conversion of L-DOPA into dopamine outside the brain can lead to side effects such as nausea and vomiting, which can limit how much medication a patient can tolerate.

Carbidopa: The Solution

  • Role of Carbidopa: Carbidopa acts as an AAD inhibitor, meaning it prevents the premature conversion of L-DOPA into dopamine in the body outside the brain. This allows more L-DOPA to enter the central nervous system (CNS) effectively.

    • It also increases L-DOPA's half-life, allowing it to remain in the body longer, which helps in effectively treating symptoms.

    • By reducing side effects in the periphery (like nausea and low blood pressure), Carbidopa enhances the overall treatment experience for patients.

    • It can significantly reduce the required daily doses of L-DOPA by 70%–75%, making management easier.

  • Combination Therapy: The combination of L-DOPA and Carbidopa is known as Sinemet and is widely considered the gold standard for managing motor symptoms in Parkinson’s disease.

Variations of Sinemet

  • Controlled Release Formulations: These versions of the drug allow for extended action and less frequent dosing.

  • Sublingual Form (Pacopa): This form is for patients who have difficulty swallowing pills and can help ensure they still get their medication effectively.

  • Duopa: A gel formulation that is administered directly into the gastrointestinal (GI) tract for advanced Parkinson's patients who need steady medication.

Efficacy and Considerations

  • Patient Response: Some patients, particularly in earlier or middle stages of Parkinson's, respond very well to L-DOPA therapy, allowing them to maintain a relatively normal daily life.

  • Dosing and Diet: Traditionally, it was recommended not to take L-DOPA with protein-rich meals, as protein can interfere with the absorption of the drug into the bloodstream. However, recent studies indicate that individual responses can vary, so some patients may tolerate it better than others.

  • Long-term Treatment Issues: After 2-10 years of taking Sinemet, patients may start to experience fluctuations in their symptoms. This includes:

    • 'On and off' phenomena: Where a patient’s motor control can suddenly fluctuate between periods of good control (on) and periods of poor control (off).

    • Dyskinesia: This is a side effect that can manifest as uncontrollable movements, particularly if the medication has been used long-term.

Management of Side Effects

  • Drug Management: It's essential to adjust the Sinemet dosage and frequency of administration, and sometimes introduce other medications to help manage both dyskinesia and fluctuations in motor control.

  • Summary Points:

    • L-DOPA therapy aims to restore dopamine levels within the CNS to alleviate symptoms associated with Parkinson’s disease.

    • When combined with Carbidopa, treatment outcomes significantly improve, resulting in better overall quality of life for patients.

    • Additional enzymatic pathways, like those involving COMT and MAO-B, represent potential targets for future drug development to help protect and maintain dopamine levels after synthesis.

Conclusion

  • This lecture has explored various pharmacological strategies and interventions for managing Parkinson's disease, particularly focusing on L-DOPA replacement therapy as a primary treatment method.

  • The next session will continue the discussion on drugs relevant to Parkinson's Disease, particularly exploring the role of antipsychotics in treatment.