Parkinson’s disease
What is Parkinson’s Disease?
Disease definition & pathophysiology
Parkinson’s disease (PD) is a progressive neurodegenerative disorder characterized by motor symptoms such as slowness of movement (bradykinesia), resting tremor, rigidity, and impaired balance (postural instability). It results primarily from loss of dopamine-producing neurons in a small midbrain region (substantia nigra), leading to dopamine deficiency in the deep brain structures that control movement (basal ganglia). Lewy bodies (aggregates of α-synuclein) are pathologic hallmarks contributing to neuronal dysfunction and death.
Commonly used Pharmacodynamic (PD) Models
- Indirect response or turnover models describing dopamine depletion and symptomatic relief by dopaminergic agents.
- Disease progression models, such as the Unified Parkinson’s Disease Rating Scale (UPDRS) progression models.
- Exposure-response models linking levodopa or dopamine agonist concentrations to motor-symptom improvement or involuntary movement (dyskinesia) development.
Patient characteristics
Typical patient population
Typically affects individuals older than 60 years, though early-onset PD (<50 years) accounts for approximately 5–10% of cases. Slightly higher prevalence observed in males compared to females, with global prevalence around 1–2% among elderly populations.
Risk factors & disease progression indicators
Risk factors include older age, family history, exposure to neurotoxins, and possibly head trauma. Disease progression indicators include worsening motor symptoms, onset of motor fluctuations (wearing-off phenomena), dyskinesias, and non-motor symptoms such as cognitive decline, depression, and problems with automatic body functions (autonomic dysfunction).
Diagnosis & biomarkers
Key Clinical Biomarkers
Diagnosis relies heavily on clinical assessment (presence of cardinal motor symptoms and response to dopaminergic therapy). Imaging biomarkers (e.g., DaTSCAN/SPECT) can confirm dopamine transporter deficiency. MRI may exclude other causes but does not confirm PD.
Disease Severity Classification
Severity often assessed using clinical rating scales, notably the Unified Parkinson’s Disease Rating Scale (UPDRS):
- Mild (early disease): UPDRS motor score ≤ 20
- Moderate: UPDRS motor score ~21–40
- Severe (advanced): UPDRS motor score >40
Hoehn and Yahr staging is also commonly used, ranging from stage 1 (mild, one-sided involvement) to stage 5 (severe, wheelchair-bound or bedridden).
How can Parkinson’s Disease be treated?
Treatment aim (PD-targets)
Treatment aims at restoring dopamine signaling, improving motor function, and reducing symptoms. PD-targets primarily include dopamine synthesis, metabolism, and receptor activation.
Common Drug Classes & Regimens
Anti-parkinson drugs are ATC N04, within N: Nervous system.
Dopamine replacement therapy (first-line treatment)
- Levodopa (N04BA01; with carbidopa or benserazide, N04BA02):
- MoA: Levodopa is converted to dopamine in the brain, restoring dopaminergic neurotransmission; peripheral decarboxylase inhibitors (carbidopa/benserazide) minimize peripheral side effects.
Dopamine agonists (alternative first-line or adjunctive)
- Pramipexole (N04BC05), ropinirole (N04BC04), rotigotine (N04BC09):
- MoA: Directly activate dopamine receptors (D2/D3), reducing motor symptoms and potentially delaying levodopa-induced dyskinesias.
Monoamine Oxidase B inhibitors (adjunctive or mild cases)
- Selegiline (N04BD01), rasagiline (N04BD02), safinamide (N04BD03):
- MoA: Inhibit dopamine metabolism, prolonging dopamine action and enhancing levodopa effectiveness.
Catechol-O-methyltransferase (COMT) inhibitors (adjunctive therapy)
- Entacapone (N04BX02), opicapone (N04BX04), tolcapone (N04BX01):
- MoA: Reduce peripheral breakdown of levodopa, prolonging its availability and reducing motor fluctuations.
Dose Adjustments
Dose initiation and titration are individualized based on therapeutic response and tolerability. Renal impairment requires dose reduction for dopamine agonists. Elderly or cognitively impaired patients often require lower doses due to increased sensitivity to side effects.
Commonly used PK-models
- Population PK models describing levodopa pharmacokinetics, accounting for covariates such as age, body weight, renal function, and concomitant medications.
- Compartmental models (one- or two-compartment) used for dopamine agonists and levodopa.
- PK-PD models linking drug exposure to improvement in UPDRS scores or motor complications (e.g., dyskinesias, motor fluctuations).
Clinical studies
The European Medicines Agency (EMA) guideline sets the trial design by disease stage and treatment aim. In early disease, before levodopa is started, efficacy should be shown in double-blind trials of at least 6 months against placebo and a standard drug, with the improvement in motor symptoms as the main efficacy variable. Accepted, validated scales for motor function are part II (activities of daily living) and part III (motor examination) of the UPDRS. In patients on levodopa with less predictable motor fluctuations, the new drug is added to levodopa in three-arm trials of at least 3 months, and the main efficacy variable is the decrease in periods of poor symptom control (OFF periods), recorded by the patients in diaries. In untreated patients, trials aimed at slowing disease progression should be randomized, placebo-controlled and probably up to 24 months long, with the change in UPDRS or the time until levodopa or a dopamine agonist is needed as the endpoint. The guideline recommends expressing efficacy in clinically interpretable terms, as success or failure (responders) or as the time to an event [1].
Clinical pharmacology studies
The guideline notes that there is no specific human pharmacodynamic model for anti-Parkinson drugs, and it considers the apomorphine (N04BC07) and levodopa tests, used as models of the response to single doses of the new drug in advanced disease, too insensitive [1].