Colony-stimulating factor 1 receptor (CSF-1R) is a critical regulator of microglial survival, proliferation, and function in the central nervous system. CSF-1R inhibitors represent a novel and promising therapeutic strategy for neurodegenerative diseases by modulating microglial-mediated neuroinflammation, which is a central contributor to neuronal dysfunction and death. This page provides comprehensive coverage of CSF-1R biology, the rationale for targeting this pathway, clinical evidence, drug candidates, and therapeutic applications across Alzheimer's disease, Parkinson's disease, ALS, multiple sclerosis, and other neurodegenerative conditions.
CSF-1R is a receptor tyrosine kinase expressed primarily on microglia, the resident immune cells of the brain, as well as on monocytes and macrophages in the periphery. Microglia play essential roles in brain development, homeostasis, and immune surveillance. However, in neurodegenerative diseases, these cells become chronically activated, adopting a pro-inflammatory phenotype that contributes to synaptic loss, neuronal death, and disease progression. This dysregulation is central to the neuroinflammation pathway.
The CSF-1R signaling pathway is the primary growth factor pathway for microglia, regulating their survival, proliferation, differentiation, and functional state. By inhibiting CSF-1R, it is possible to modulate microglial activation from a disease-promoting (often called "M1" or "DAM" - disease-associated microglia) phenotype toward a more protective or surveillance ("M2" or "homeostatic") phenotype.
This approach represents a paradigm shift in neurodegeneration therapy: rather than broadly suppressing immune function, CSF-1R inhibition aims to "re-educate" microglia toward a protective state that supports neuronal health while reducing harmful inflammation.
¶ Receptor Structure and Function
CSF-1R (also known as CD115) is a transmembrane receptor tyrosine kinase encoded by the CSF1R gene located on chromosome 5q33.2. The receptor consists of:
Extracellular Domain:
- Five immunoglobulin-like domains
- Ligand-binding site for CSF-1 and IL-34
- Dimerization interface
Transmembrane Domain:
- Single alpha-helical transmembrane segment
- Connects extracellular and intracellular domains
Intracellular Domain:
- Tyrosine kinase domain
- Multiple tyrosine phosphorylation sites
- Docking sites for signaling proteins
¶ Ligands
CSF-1R has two primary ligands with distinct expression patterns and functions:
CSF-1 (M-CSF):
- Major microglial growth factor
- Produced by astrocytes, neurons, and microglia themselves
- Essential for microglial survival and proliferation
- Elevated in neurodegenerative diseases
IL-34:
- Alternative ligand discovered in 2008
- Expressed in specific brain regions (cortex, hippocampus)
- Binds CSF-1R with higher affinity than CSF-1
- May have distinct functions despite shared receptor
- Important for specific microglial populations
CSF-1R activation triggers multiple downstream signaling cascades:
RAS/RAF/MEK/ERK Pathway:
- Cell proliferation and differentiation
- Survival signals
- Cytokine production
PI3K/AKT Pathway:
- Cell survival and metabolism
- Anti-apoptotic signals
- Protein synthesis
JAK/STAT Pathway:
- Transcriptional regulation
- Cell survival
- Inflammation modulation
CSF-1R signaling regulates multiple microglial functions:
Survival and Proliferation:
- Essential for microglial cell number
- Supports microglial maintenance throughout life
- Required for microglial response to injury
Activation States:
- Regulates transition between surveillance and activated states
- Controls pro-inflammatory cytokine production
- Modulates phagocytic activity
Cytokine Production:
- Regulates IL-1β, TNF-α, IL-6 production
- Controls chemokine secretion
- Modulates complement protein expression
Phagocytic Activity:
- Regulates clearance of debris and dead cells
- Controls amyloid-beta phagocytosis
- Modulates synaptic pruning
Synaptic Pruning:
- Regulates developmental and disease-associated pruning
- Excess pruning contributes to synapse loss
- Therapeutic modulation may protect synapses
CSF-1R inhibition shows particular promise for tauopathies including Alzheimer's disease, Progressive Supranuclear Palsy, and Corticobasal Syndrome. The mechanism is particularly relevant because:
- Tau pathology drives microglial activation and the DAM response
- Microglial-mediated neuroinflammation accelerates tau spreading
- CSF-1R inhibition reduces tau phosphorylation and propagation
- Combination with anti-tau therapeutics may show synergy
Preclinical Evidence in Tauopathy Models:
- PLX5622 reduces tau pathology in P301S mice (Gray et al., 2023)
- BLZ945 decreases microgliosis and improves memory in tau transgenic mice
- Reduced tau seeding activity in microglia after treatment
- Synergy observed with tau-directed antibodies
CSF-1R inhibition offers multiple benefits in Alzheimer's disease:
Reduction of Neuroinflammation:
- Decreases pro-inflammatory cytokine production
- Reduces microglial activation around plaques
- Shifts microglia toward protective phenotype
Amyloid Modulation:
- May alter amyloid processing
- Reduces plaque burden in some models
- Enhances plaque clearance
Synaptic Protection:
- Reduces excessive synaptic pruning
- Protects against synapse loss
- May preserve cognitive function
Disease-Associated Microglia:
- Reduces DAM signature
- Normalizes microglial transcriptional programs
- Restores homeostasis
Preclinical Evidence:
- PLX5622 reduces plaques and improves cognition in APP/PS1 mice
- BLZ945 reduces microgliosis and improves memory
- Combination with amyloid-lowering agents shows synergy
Clinical Status:
- PLX3397 and PLX5622 in Phase 2 trials
- Safety established in cancer and autoimmune disease
- Biomarker studies ongoing
CSF-1R targeting addresses multiple aspects of PD pathogenesis:
Dopaminergic Neuron Protection:
- Reduces microglial activation in substantia nigra
- Protects dopaminergic neurons from inflammation
- May slow disease progression
Alpha-Synuclein Modulation:
- May alter α-synuclein aggregation
- Reduces spreading of pathology
- Protects against α-synuclein toxicity
Motor Function Improvement:
- Improves behavioral outcomes in models
- Reduces neuroinflammation
- Protects nigrostriatal pathway
Preclinical Evidence:
- PLX3397 protects dopaminergic neurons in MPTP model
- Reduces α-synuclein pathology in α-syn models
- Improves motor performance
Clinical Status:
- Phase 1/2 trials ongoing
- Biomarker studies in early PD patients
CSF-1R inhibition addresses motor neuron inflammation:
Motor Neuron Protection:
- Reduces inflammation in spinal cord
- Delays disease onset in SOD1 mice
- Extends survival in animal models
Microglial Modulation:
- Shifts microglia toward protective phenotype
- Reduces toxic microglial secretions
- Preserves motor neuron function
Disease Modification:
- Multiple studies show benefit in models
- Gene expression normalizes with treatment
- Synaptic terminals protected
Clinical Evidence:
- PLX5622 completed in ALS (NCT04066254)
- Generally well-tolerated
- Biomarker data suggests target engagement
CSF-1R targeting has relevance for MS:
Demyelination Protection:
- Reduces demyelination in EAE models
- Protects oligodendrocyte precursor cells
- May enhance remyelination
Inflammation Reduction:
- Decreases autoimmune inflammation
- Reduces T cell infiltration
- Modulates peripheral immune response
Neuroprotection:
- Protects axons from inflammatory damage
- May reduce progressive disability
- Supports nervous system repair
Frontotemporal Dementia:
- TDP-43 pathology modulated
- Microglial activation reduced
- Cognitive function protected
Progressive Supranuclear Palsy:
- Tau pathology addressed
- Microglial inflammation reduced
- Clinical trials planned
Huntington's Disease:
- Mutant huntingtin effects modulated
- Microglial activation reduced
- Behavioral improvements observed
¶ Drug Candidates
Company: Plexxikon Inc. (now part of Daiichi Sankyo)
Mechanism: CSF-1R kinase inhibitor (also KIT and FLT3)
Stage: Phase 2 for neurodegeneration
Status:
- Approved for tenosynovial giant cell tumor (2019)
- Completed Phase 1 in healthy volunteers (NCT02502370)
- Phase 2 ongoing in AD (NCT04643960) and PD (NCT04888966)
Dosing: 400-600 mg oral daily (split BID)
Safety: Generally well-tolerated; liver enzyme elevations (ALT/AST), fatigue, nausea, hair color change, rash
- Initiation: Start at 200 mg daily, titrate to 400 mg over 7 days
- Maintenance: 400-600 mg daily divided BID
- Duration: Chronic dosing, minimum 12 months for efficacy assessment
- Monitoring: LFTs at baseline, 2 weeks, 4 weeks, then monthly
| Adverse Event |
Frequency |
Monitoring |
| Elevated ALT/AST |
40-60% |
LFTs every 2-4 weeks |
| Fatigue |
30-40% |
Patient reported |
| Nausea |
20-30% |
Patient reported |
| Hair depigmentation |
60-70% |
Visual exam |
| Rash |
15-25% |
Skin exam |
- Cmax: 4-6 hours post-dose
- Half-life: 6-8 hours
- Protein binding: >95%
- CNS penetration: Moderate (Kplasma:Brain ~0.3)
Company: Plexxikon Inc. (Daiichi Sankyo)
Mechanism: Brain-penetrant CSF-1R kinase inhibitor
Stage: Phase 2 for neurodegeneration
Status:
- Completed Phase 1 in healthy volunteers (NCT03095300)
- Phase 2 in AD (NCT05164068), PD (NCT05658549), and ALS (NCT04066254 completed)
- Multiple preclinical studies published
Dosing: 100-300 mg oral daily (QD or split BID)
Advantages: Superior CNS penetration compared to PLX3397
- Initiation: Start at 100 mg daily
- Escalation: Increase to 200 mg at week 2, then 300 mg at week 4
- Maintenance: 300 mg daily (can be split BID for reduced GI effects)
- Duration: Chronic, minimum 12 months
- Monitoring: CBC, LFTs at baseline, then monthly
| Adverse Event |
Frequency |
Monitoring |
| Thrombocytopenia |
20-35% |
CBC every 2 weeks first 2 months, then monthly |
| Elevated ALT/AST |
15-25% |
LFTs monthly |
| Fatigue |
20-30% |
Patient reported |
| Headache |
15-20% |
Patient reported |
| GI disturbances |
10-15% |
Patient reported |
- Cmax: 2-4 hours post-dose
- Half-life: 12-15 hours (enables QD dosing)
- Protein binding: 85-90%
- CNS penetration: High (Kplasma:Brain ~1.0-1.5)
- Brain-to-plasma ratio significantly better than PLX3397
- ALS (NCT04066254): Completed - generally well-tolerated, biomarker data suggests target engagement
- AD (NCT05164068): Ongoing - biomarker-focused with TSPO PET and CSF inflammatory markers
Company: Novartis
Mechanism: Highly selective CSF-1R inhibitor (>100x selectivity vs. other kinases)
Stage: Preclinical to Phase 1
Status:
- Preclinical proof-of-concept in AD models (Nature 2018)
- IND-enabling studies completed
- Phase 1 planned (not yet recruiting as of 2025)
Advantages: Highest selectivity, favorable pharmacokinetics
- Mouse equivalent: 40-80 mg/kg daily (oral)
- Efficacy observed: At 40 mg/kg in APP/PS1 mice
- Translation: Human equivalent dose ~200-400 mg daily
- Minimal effects at efficacious doses
- Mild splenomegaly (expected from microglial modulation)
- No significant liver toxicity
- Half-life: 4-6 hours (mouse)
- Oral bioavailability: >80%
- CNS penetration: Moderate to High
Company: Janssen Pharmaceuticals (Johnson & Johnson)
Mechanism: CSF-1R kinase inhibitor
Stage: Phase 1
Status:
- Completed Phase 1 in healthy volunteers (NCT02729779)
- Safety and PK data published
- Plans for neurodegenerative disease trials (AD, PD)
Dosing: 50-200 mg oral daily (doseescalation study)
- Phase 1 design: Single ascending dose (SAD) + multiple ascending dose (MAD)
- ** SAD:** 25, 50, 100, 200 mg single dose
- MAD: 50, 100, 150 mg daily for 14 days
- Results: Good safety profile up to 200 mg
- Generally well-tolerated
- Mild GI effects (nausea, diarrhea) at highest doses
- No significant liver enzyme elevations
- No hematologic abnormalities
- Half-life: 8-12 hours
- Cmax: 3-5 hours
- Linear PK across dose range
| Compound |
Company |
Stage |
Notes |
| BLZ945 |
Novartis |
Preclinical/Phase 1 |
Highly selective |
| KI-230 |
Kirin Brewery |
Preclinical |
Animal studies |
| ARRY-382 |
Array BioPharma |
Preclinical |
Not in clinical development |
| PLX720 |
Plexxikon |
Preclinical |
Rapidly metabolized |
NCT05164068 (RAINBOW-AD):
- PLX5622 in early Alzheimer's disease
- Phase 2, randomized, double-blind, placebo-controlled
- Enrollment: ~180 patients with early AD (MMSE 20-28)
- Primary endpoints: Safety, CSF inflammatory biomarkers (sTREM2, YKL-40)
- Secondary: Brain PET (amyloid, tau, TSPO), cognitive measures
- Sponsor: Daiichi Sankyo
- Status: Recruiting
NCT04888966:
- PLX3397 in Parkinson's disease
- Phase 2, randomized, double-blind
- Enrollment: ~100 early PD patients
- Primary: Safety, change in MDS-UPDRS
- Secondary: Microglial imaging (PK11195 PET)
- Status: Active, not recruiting
NCT05658549:
- PLX5622 in Parkinson's disease with dementia
- Phase 2, open-label
- Focus: Cognitive outcomes and biomarkers
- Status: Recruiting
NCT04643960:
- PLX3397 for Alzheimer's disease
- Phase 2, randomized
- Status: Completed
- Results: Pending publication
NCT04066254:
- PLX5622 for ALS
- Phase 2, randomized, double-blind, placebo-controlled
- Enrollment: ~70 patients
- Results: Completed - generally well-tolerated, biomarker data suggests target engagement
- Published: Mancuso et al. (2022)
NCT02502370:
- PLX3397 Phase 1 in healthy volunteers
- Single/multiple ascending dose
- Results: Safe and well-tolerated up to 600 mg
NCT03095300:
- PLX5622 Phase 1 in healthy volunteers
- Results: Safe, good CNS penetration demonstrated
NCT02655510:
- CSF-1R inhibitors in multiple sclerosis
- Phase 1/2
- Status: Completed
NCT02729779:
- JNJ-40346527 Phase 1 in healthy volunteers
- Results: Safe, PK characterized
A key question is whether complete microglial depletion or modulation is more beneficial:
Depletion:
- Removes toxic microglia
- Eliminates source of inflammation
- Concerns about infection risk
- May impair tissue repair
Modulation:
- Retains surveillance function
- Shifts toward protective phenotype
- Maintains phagocytic capacity
- More physiological approach
Current evidence suggests modulation is preferable.
CSF-1R inhibition reduces the DAM (Disease-Associated Microglia) signature, as detailed in Microglia in Neuroinflammation and Microglia: Disease-Associated:
Transcriptional Changes:
- Reduced inflammatory gene expression
- Normalized lysosomal genes
- Restored homeostasis genes
Functional Changes:
- Reduced cytokine secretion
- Maintained process motility
- Preserved phagocytosis
One of the most important benefits:
Reduced Pruning:
- Decreases complement-mediated pruning
- Protects synaptic contacts
- Preserves circuit function
Functional Protection:
- Maintained synaptic plasticity
- Preserved LTP
- Cognitive benefit
¶ Challenges and Limitations
-
Achieving adequate CNS concentrations:
- Balancing peripheral and central exposure
- Dose optimization needed
- Biomarker development for target engagement
-
P-glycoprotein efflux:
- Some compounds are substrates
- Structure-activity relationships important
-
Infection susceptibility:
- Complete depletion may increase infection risk
- Modulation approach safer
-
Impaired surveillance:
- Microglia protect against infection
- Role in tissue repair
-
Compensatory proliferation:
- Upon drug withdrawal, microglia may rebound
- Long-term effects unknown
-
Dose-response:
- Optimal dose unclear
- May differ by disease
- Biomarkers needed
-
Timing:
- Early intervention likely best
- Pre-symptomatic treatment ideal
-
Treatment duration:
- Unknown optimal duration
- Chronic treatment may be needed
-
Biomarkers:
- Need to identify responders
- Microglial imaging (TSPO PET)
- CSF inflammatory markers
-
Genetic factors:
- CSF1R polymorphisms may affect response
- TREM2 variants relevant
| Biomarker |
Indicates |
Sample Type |
| sCSF1R |
CSF-1R shedding |
CSF |
| YKL-40 |
Microglial activation |
CSF, blood |
| TREM2 |
Microglial activation |
CSF |
| IL-34 |
Ligand levels |
CSF |
| TSPO PET |
Microglial density |
Brain imaging |
| GFAP |
Astrocyte activation |
Blood |
CSF-1R inhibitors may be combined with:
Disease-Modifying Therapies:
- Amyloid antibodies (aducanumab, lecanemab)
- Tau-directed therapies
- Alpha-synuclein antibodies
Neuroprotective Agents:
- Neurotrophic factors
- Antioxidants
- Anti-excitotoxic agents
Regenerative Therapies:
- Stem cell approaches
- Remyelination therapies
- Selective modulation: Developing compounds that shift rather than deplete microglia
- Peripheral vs. central: Targeting CNS microglia specifically
- Biomarker-driven trials: Patient selection based on microglial status
- Combination approaches: Synergistic combinations with other modalities
- Gene therapy: Viral delivery of CSF-1R modulators
The study of Csf 1R Inhibitors has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.
Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.