Complement System Therapeutics for Neurodegeneration

Latest Data Snapshot (March 2026)

Data refreshed: 2026-03-14 PT from ClinicalTrials.gov

Metric	Value
Total Clinical Trials	28
Active Trials (Recruiting/Active)	8 (29%)
Phase 1 Trials	5
Phase 2 Trials	3
Phase 3 Trials	0

Overview

flowchart TD
    COMPLEMENT["COMPLEMENT"] -->|"activates"| ASTROCYTES["ASTROCYTES"]
    COMPLEMENT["COMPLEMENT"] -->|"activates"| C1Q["C1Q"]
    COMPLEMENT["COMPLEMENT"] -->|"activates"| Als["Als"]
    COMPLEMENT["COMPLEMENT"] -->|"activates"| Complement["Complement"]
    COMPLEMENT["COMPLEMENT"] -->|"activates"| MICROGLIA["MICROGLIA"]
    COMPLEMENT["COMPLEMENT"] -->|"therapeutic target"| Als["Als"]
    COMPLEMENT["COMPLEMENT"] -->|"therapeutic target"| Complement["Complement"]
    COMPLEMENT["COMPLEMENT"] -->|"activates"| Aging["Aging"]
    COMPLEMENT["COMPLEMENT"] -->|"activates"| NEUROINFLAMMATION["NEUROINFLAMMATION"]
    COMPLEMENT["COMPLEMENT"] -->|"activates"| Inflammation["Inflammation"]
    COMPLEMENT["COMPLEMENT"] -->|"activates"| Alzheimer["Alzheimer"]
    COMPLEMENT["COMPLEMENT"] -->|"activates"| Neurodegeneration["Neurodegeneration"]
    COMPLEMENT["COMPLEMENT"] -->|"associated with"| Complement["Complement"]
    COMPLEMENT["COMPLEMENT"] -->|"regulates"| Complement["Complement"]
    style complement fill:#4fc3f7,stroke:#333,color:#000

The complement system is a critical component of the innate immune system that plays a dual role in neurodegeneration - both in protective immune surveillance and in driving pathological neuroinflammation. Therapeutic targeting of complement proteins represents a growing area of interest for Alzheimer’s disease, Parkinson’s disease, and other neurodegenerative disorders

The complement system has emerged as a key link between neuroinflammation and protein aggregation in neurodegenerative diseases. While complement proteins play essential roles in synaptic pruning during development and immune defense, dysregulated complement activation can drive pathology through microglial activation, synapse loss, and chronic neuroinflammation.

Investment Context

The complement therapeutics space has seen significant investment following:

Success in other indications: C5 inhibitors (eculizumab, ravulizumab) are approved for rare diseases
Alzheimer’s genetics: TREM2 variants highlight microglial pathways linked to complement
Anti-CD20 de-risking: Similar immune modulation approach validated in MS

Key challenges remain:

BBB penetration: Most complement inhibitors are large biologics
Infection risk: Complement inhibition increases susceptibility to infections
Timing: Optimal patient selection (early vs. late disease) unclear

Key Complement Targets

Target	Role in Neurodegeneration	Therapeutic Approach	Development Stage
C1q	Synaptic pruning, complement activation	Antibody blockade	Preclinical
C3	Central complement node	C3 inhibitor (Pegcetacoplan)	Phase 2
C5	Complement terminal pathway	Eculizumab, Ravulizumab	Approved (other), Phase 2/3
C5aR1	Neuroinflammation receptor	C5a receptor antagonists	Preclinical
C1QA	Synaptic elimination	Genetic studies	Research

Clinical Pipeline

C3 Inhibitors

Pegcetacoplan (Apellis): intravitreal formulation approved for geographic atrophy; systemic formulation in trials for AD^{1Pegcetacoplan in Alzheimer's Disease (ClinicalTrials.gov)Open reference}

C5 Inhibitors

Eculizumab (Alexion): Approved for PNH and aHUS; being explored for ALS
Ravulizumab (Alexion): Long-acting C5 inhibitor; Phase 2 trials in ALS

Emerging Programs

Nodthera: NLRP3 inflammasome + complement dual approach
Roche: Anti-C1q antibodies in development for CNS indications

Companies Active in Complement

Alexion (AstraZeneca): C5 inhibitors (eculizumab, ravulizumab)
Apellis Pharmaceuticals: C3 inhibitor (pegcetacoplan)
Roche/Genentech: Anti-C1q program
NodThera: NLRP3/complement dual inhibitors

Gap Analysis

Underrepresented Areas

C1q blockade: Early but promising; no CNS clinical trials yet
Oral small molecules: Limited options for chronic dosing
Brain-penetrant agents: Critical gap for complement inhibition
Combination approaches: Complement + other mechanisms (TREM2, tau)
Biomarkers: Need for complement activation markers in CSF/blood

Opportunities

Genetic validation from AD genetics (C1QA, C1QB variants)
Link to TREM2 pathway provides biological rationale
Synaptic protection as potential disease-modifying mechanism

Conclusion

Complement therapeutics represent a promising but challenging area in neurodegeneration R&D. While the biological rationale is strong, BBB penetration and infection risk remain key hurdles. The field awaits late-stage clinical data to validate this approach.