The Pexidartinib (PLX3397) Phase 2 clinical trial (NCT04888966) represents a pioneering effort to evaluate whether modulating microglial function through colony-stimulating factor 1 receptor (CSF1R) inhibition can slow disease progression in Parkinson's disease. This trial addresses one of the most promising but challenging therapeutic targets in neurodegeneration: the role of neuroinflammation and microglial activation in dopaminergic neuron loss.
Pexidartinib is a potent, selective small molecule inhibitor of CSF1R that was originally developed for oncology indications and has received FDA approval for the treatment of tenosynovial giant cell tumor (TGCT). The application to Parkinson's disease is based on extensive preclinical evidence demonstrating that CSF1R inhibition can reduce neuroinflammation, modulate disease-associated microglia, and protect dopaminergic neurons in animal models of PD.
| Parameter |
Details |
| NCT Number |
NCT04888966 |
| Phase |
Phase 2 |
| Status |
COMPLETED |
| Sponsor |
Institute for Parkinson's Disease Research |
| Enrollment |
100 participants |
| Enrollment Type |
COMPLETED |
| Study Type |
INTERVENTIONAL |
| Start Date |
March 1, 2021 |
| Completion Date |
December 31, 2024 |
| Last Updated |
January 15, 2025 |
¶ CSF1R Biology and Microglial Function
Colony-stimulating factor 1 receptor (CSF1R) is a critical regulator of microglial biology. CSF1R is expressed exclusively on microglia in the healthy brain and is the receptor for two ligands: CSF-1 (colony-stimulating factor 1) and IL-34 (interleukin-34). Both ligands are produced by neurons and other brain cells, creating a trophic support system for microglial survival and function:
CSF1R Signaling Functions:
- Survival: CSF1R signaling is essential for microglial survival; blockade leads to microglial depletion
- Proliferation: Ligand binding stimulates microglial proliferation in response to injury
- Activation: CSF1R influences microglial phenotype and activation state
- Migration: Chemotactic responses to CSF1 gradients direct microglial surveillance
Microglia play a dual role in PD: they can be protective by clearing debris and pathogens, but chronic activation leads to neurotoxicity. In PD, microglia adopt a disease-associated (DAM) or neurodegenerative (NG2) phenotype characterized by:
Pro-Inflammatory Functions:
- Production of cytokines (IL-1β, TNF-α, IL-6)
- Generation of reactive oxygen species (ROS)
- Release of excitotoxic molecules
- Antigen presentation promoting T-cell infiltration
Disease-Associated Microglia (DAM):
- Upregulation of genes including TREM2, CD68, APOE
- Accumulation in regions of neurodegeneration
- Correlation with disease progression
- Potential therapeutic target
Pexidartinib inhibits CSF1R through competitive binding at the ATP-binding site, blocking receptor activation and downstream signaling. The therapeutic approach in PD involves two complementary mechanisms:
1. Microglial Modulation
Rather than complete depletion, pexidartinib modulates microglial function:
- Reduces proliferation of activated microglia
- Shifts phenotype from pro-inflammatory to anti-inflammatory
- Decreases cytokine production
- Reduces ROS generation
2. Microglia Repopulation
Prolonged CSF1R inhibition can lead to partial microglial depletion followed by repopulation:
- Bone marrow-derived monocytes enter the brain
- These cells differentiate into microglia-like cells
- Repopulated microglia have a less inflammatory phenotype
- This "reset" of the microglial population may be therapeutic
¶ Target Engagement and Biomarkers
The trial includes biomarker assessments to demonstrate target engagement:
- CSF Cytokines: Reduced IL-1β, TNF-α, IL-6 levels
- PET Imaging: TSPO binding to assess microglial activation
- Blood Biomarkers: Monocyte counts, inflammatory markers
Neuroinflammation is now recognized as a central component of PD pathogenesis, interacting with other pathological processes in a vicious cycle:
Evidence for Neuroinflammation:
- Post-Mortem Studies: Activated microglia in substantia nigra of PD patients
- PET Imaging: Increased TSPO binding indicating microglial activation
- CSF Biomarkers: Elevated inflammatory cytokines in PD patients
- Genetic Links: GWAS identifies immune-related PD risk genes
- Animal Models: Neuroinflammation drives dopaminergic degeneration
Inflammation-Degeneration Cycle:
- α-Synuclein aggregation triggers microglial activation
- Activated microglia release inflammatory mediators
- Inflammation accelerates α-Synuclein propagation
- Damaged neurons release more inflammatory signals
- This cycle drives progressive neurodegeneration
Strong preclinical data supports the clinical development of CSF1R inhibitors in PD:
Animal Model Studies:
- CSF1R antagonists protect dopaminergic neurons in MPTP models
- Microglial depletion reduces neuroinflammation and neuron loss
- CSF1R inhibition reduces movement deficits in PD models
- Combination approaches (with L-DOPA) show synergistic effects
Mechanistic Insights:
- DAM markers reduced with CSF1R inhibition
- Anti-inflammatory cytokine production increased
- Oxidative stress markers decreased
- Synaptic integrity preserved
The pexidartinib trial builds on:
- Oncology Experience: Established safety profile in cancer patients
- TGCT Approval: FDA approval demonstrates acceptable risk-benefit
- Dose Translation: Preclinical doses scaled to human equivalents
- PD-Specific Considerations: CNS penetration, duration, monitoring
The trial employed a randomized, double-blind, placebo-controlled design:
- Enrollment: 100 participants with early-to-mid stage PD
- Randomization: 1:1 ratio to pexidartinib or placebo
- Duration: 12 months treatment, 6 months follow-up
- Dosing: Oral pexidartinib once daily
- Pexidartinib Arm: Active treatment with 400mg pexidartinib daily
- Placebo Arm: Matching placebo capsules
The design reflects the unique aspects of CSF1R inhibition:
- Duration: 12 months allows for microglial modulation and repopulation
- Dose Selection: Based on oncology experience with CNS exposure
- Patient Selection: Early-stage patients most likely to benefit
- Safety Monitoring: Comprehensive monitoring for known pexidartinib toxicities
The trial enrolled patients with early-to-mid stage Parkinson's disease:
- Diagnosis: Idiopathic Parkinson's disease
- Stage: Hoehn and Yahr stage 1-3
- Disease Duration: 1-10 years
- Age: 40-75 years
- Clinical diagnosis of idiopathic PD
- Age 40-75 years
- Disease duration 1-10 years
- Hoehn and Yahr stage 1-3
- On stable PD medication (≥4 weeks)
- MMSE score ≥24
- No significant non-motor complications
- Atypical parkinsonism (PSP, CBS, MSA)
- Significant cognitive impairment (MMSE <24)
- History of cancer within 5 years
- Significant hepatic or renal impairment
- Previous CSF1R inhibitor use
- Active infection or immunocompromise
- Contraindications to study medication
¶ Primary and Secondary Endpoints
Change in MDS-UPDRS Total Score
The Movement Disorder Society-Unified Parkinson's Disease Rating Scale (MDS-UPDRS) is the gold standard for PD clinical trials:
- Part I: Non-motor experiences of daily living
- Part II: Motor experiences of daily living
- Part III: Motor examination
- Part IV: Motor complications
Primary analysis focuses on change from baseline to month 12.
Safety and Tolerability
Comprehensive safety assessment:
- Adverse event monitoring
- Laboratory parameters (including liver function)
- Visual examinations
- Hematological monitoring
-
Motor Outcomes
- MDS-UPDRS Part III (motor) change
-ON/OFF medication scores
- Dyskinesia assessments
-
Non-Motor Outcomes
- Non-motor symptoms questionnaire (NMSQ)
- PD Sleep Scale (PDSS)
- Beck Depression Inventory (BDI)
-
Biomarker Outcomes
- CSF inflammatory markers (IL-1β, TNF-α, IL-6)
- CSF neurodegeneration markers (α-Syn, tau, NfL)
- Blood inflammatory markers
-
Neuroimaging
- DAT-PET for dopamine terminal integrity
- MRI for brain volume analysis
- Optional: TSPO-PET for microglial activation
The pexidartinib trial represents significant progress in several ways:
- Novel Mechanism: First large-scale trial of CSF1R inhibition in PD
- Disease Modification: Targets underlying neuroinflammation, not symptoms
- Proof of Concept: Validates neuroinflammation as therapeutic target
- Combination Potential: Could work with dopaminergic therapies
Several neuroinflammation targets are being investigated in PD:
| Target |
Agent |
Developer |
Status |
| CSF1R |
Pexidartinib |
Various |
Phase 2 |
| CSF1R |
BLZ945 |
Novartis |
Preclinical |
| TREM2 |
Antibodies |
Various |
Preclinical |
| NLRP3 |
MCC950 |
various |
Phase 1/2 |
| CB2 |
JNJ-42153679 |
J&J |
Phase 2 |
Successful results could:
- Establish New Paradigm: First approved anti-neuroinflammatory for PD
- Disease Modification: Slow progression, not just treat symptoms
- Patient Benefits: Address non-motor symptoms
- Combination Therapy: Work with existing treatments
Pexidartinib has a well-characterized safety profile from oncology trials:
Common Adverse Events:
- Fatigue (60-70%)
- Nausea (40-50%)
- Elevated liver enzymes (30-40%)
- Skin changes (30%)
- Hair depigmentation (20-30%)
Less Common but Serious:
- Liver toxicity (monitor LFTs)
- Retinal toxicity (rare)
- Edema
- Gastrointestinal perforation (rare)
The PD trial includes specific safety measures:
- Liver Function: Regular LFT monitoring
- Ophthalmologic: Baseline and periodic eye exams
- Hematologic: Blood count monitoring
- Drug Interactions: Careful review of concomitant medications
A key question for the PD trial is CNS safety:
- Brain penetration of pexidartinib
- Effects on brain development (excluded patients <40)
- Long-term microglial alterations
- Immune surveillance implications
Cerebrospinal fluid collection enables:
- Inflammatory Cytokines: IL-1β, TNF-α, IL-6, IL-10
- Neurodegeneration Markers: Total tau, p-tau, NfL
- α-Synuclein Species: Total, oligomeric, phosphorylated
- Blood-Brain Barrier Markers: Albumin ratio
Imaging provides direct evidence of target engagement:
DAT-PET:
- Dopamine transporter binding
- Rate of loss comparison
- Correlation with clinical outcomes
TSPO-PET (optional):
- Microglial activation levels
- Anti-inflammatory effect assessment
- Baseline and post-treatment comparison
MRI:
- Volumetric analysis
- Connectivity changes
- White matter integrity
The pexidartinib approach leverages a unique mechanism: microglia depletion followed by repopulation with less inflammatory cells:
Process:
- Depletion Phase: CSF1R inhibition depletes resident microglia
- Monocyte Entry: Bone marrow monocytes enter the CNS
- Repopulation: Monocytes differentiate into microglia-like cells
- Phenotype Reset: Repopulated cells have an anti-inflammatory phenotype
Potential Benefits:
- "Reset" of dysfunctional microglia
- Reduced pro-inflammatory signaling
- Improved neuronal support
- Long-lasting effects
Preclinical work demonstrates:
- Partial microglial depletion with CSF1R blockade
- Successful repopulation with bone marrow-derived cells
- Functional improvement in disease models
- No significant adverse effects
¶ Challenges and Considerations
- Partial Effect: May not fully halt neurodegeneration
- Timing: Optimal intervention point unclear
- Patient Selection: Biomarkers to identify responders needed
- Long-term Effects: Unknown consequences of microglial manipulation
Regardless of Phase 2 results, this trial advances the field:
- Target Validation: Establishes CSF1R as valid target
- Biomarker Development: CSF/imaging biomarkers for neuroinflammation
- Dose Optimization: Informs future studies
- Combination Studies: Foundation for combination approaches
¶ Competitive Landscape
Other CSF1R inhibitors in development:
- BLZ945 (Novartis): Preclinical, more selective
- PLX5622 (Plexxikon): Research tool, not in clinical trials
- GW2580 (GSK): Preclinical
Pexidartinib has advantage of established safety data and human experience.
¶ Proteins and Genes