Carbon monoxide (CO) releasing compound therapy represents an emerging neuroprotective approach for neurodegenerative diseases that exploits the endogenous gasotransmitter's anti-inflammatory, anti-apoptotic, and mitochondrial protection properties. CO is one of three primary gasotransmitters in the human body (alongside nitric oxide [NO] and hydrogen sulfide [H₂S]) and plays crucial roles in cellular signaling, mitochondrial function, and stress response[@corm_neuro_2023].
The therapeutic potential of CO-releasing molecules (CORMs) spans multiple neurodegenerative conditions including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease. Unlike exogenous CO gas administration, CORMs deliver controlled amounts of CO to achieve therapeutic effects without toxicity[@co_brain_2024].
CO is produced endogenously through heme oxygenase (HO) enzyme-mediated degradation of heme:
Heme + O₂ + NADPH → Biliverdin + CO + Fe²⁺ + NADP⁺
Two HO isoforms exist with distinct physiological roles:
| Enzyme | Gene | Regulation | Primary Function |
|---|---|---|---|
| HO-1 | HMOX1 | Inducible (stress, oxidative) | Cytoprotection, stress response |
| HO-2 | HMOX2 | Constitutive | Homeostatic heme degradation |
HO-1 (also known as HSP32) is highly inducible by oxidative stress, heat shock, inflammatory stimuli, and hypoxia. Its upregulation represents a conserved cellular protective response. HO-2 is constitutively expressed in neurons and maintains baseline CO production for physiological signaling[@ho1_neuro_2023].
CO exerts multiple protective effects in the nervous system:
CORMs are designed to release CO in a controlled manner, avoiding the toxicity associated with high CO concentrations. Several classes have been developed:
| CORM | Class | CO Release Profile | Half-life | Key Features |
|---|---|---|---|---|
| CORM-2 | Ruthenium carbonyl | Light-triggered, slow | Variable | First-generation CORM |
| CORM-3 | Ruthenium carbonyl | Water-soluble, fast | ~3 min | Aqueous solubility |
| CORM-401 | Manganese carbonyl | Mitochondria-targeted | ~30 min | Targets mitochondrial CO release |
| ALF-186 | Metal carbonyl | Slow, sustained | ~6 hours | Veterinary use approved |
| ALF-494 | Iron carbonyl | Medium release | ~2 hours | Novel iron-based CORM |
| DI-1 | Photoactivatable | Light-triggered | Variable | Spatial/temporal control |
CORM-2 (dicobalt octacarbonyl) was one of the first CORMs developed:
Mechanism: Releases CO upon exposure to light or certain chemical conditions.
Advantages:
Limitations:
CORM-2 has demonstrated neuroprotective effects in multiple models, including reduction of infarct size in stroke models and protection against β-amyloid toxicity[@corm_neuro_2023].
CORM-3 (tricarbonylchloro(glycinato)ruthenium(II)) is a water-soluble CORM:
Advantages:
Limitations:
CORM-3 has shown particular promise in PD models, protecting dopaminergic neurons from 6-OHDA and MPTP toxicity[@corm_pd_2023].
CORM-401 is a mitochondria-targeted CORM:
Mechanism:
Advantages:
Limitations:
CORM-401 has shown efficacy in models where mitochondrial dysfunction is central, including PD and ALS[@co_mito_2023].
ALF-494 is a novel iron-based CORM:
Advantages:
Limitations:
ALF-494 represents the next generation of CORMs with improved safety profiles.
CO modulates neuroinflammation through multiple pathways:
p38 MAPK Pathway: CO activates p38 MAPK signaling, which suppresses pro-inflammatory cytokine production (TNF-α, IL-1β, IL-6) while promoting anti-inflammatory mediators (IL-10)[@corm_neuro_2023].
NF-κB Inhibition: CO inhibits NF-κB nuclear translocation, reducing expression of inflammatory genes. This effect is mediated through activation of p38 MAPK and upregulation of HO-1.
Microglial Modulation: CO shifts microglial activation from pro-inflammatory (M1) to anti-inflammatory (M2) phenotype, promoting tissue repair and reducing chronic neuroinflammation.
CO protects neurons from apoptotic cell death through:
Bcl-2 Family: CO upregulates anti-apoptotic Bcl-2 and Bcl-xL while inhibiting pro-apoptotic Bax translocation to mitochondria[@corm_apoptosis_2022].
Caspase Inhibition: CO inhibits caspase-3 activation through the MAPK pathway, blocking the execution phase of apoptosis.
Akt Pathway: CO activates PI3K/Akt signaling, which promotes neuronal survival through phosphorylation of BAD and activation of downstream effectors.
The HO-1/CO system creates a beneficial antioxidant response:
HO-1 Induction: CO itself induces HO-1 expression, creating a positive feedback loop for antioxidant protection[@ho1_neuro_2023].
Ferritin Sequestration: While heme degradation releases iron (potentially pro-oxidant), this is rapidly sequestered by ferritin, which is also induced by CO. This coupling ensures antioxidant benefits outweigh pro-oxidant effects.
Nrf2 Activation: CO activates Nrf2 (Nuclear factor erythroid 2-related factor 2), the master regulator of antioxidant response genes, leading to upregulation of HO-1, NQO1, GCLM, and other antioxidant enzymes[@co_nrf2_2024].
CO preserves mitochondrial function through:
Membrane Potential: CO preserves mitochondrial membrane potential (ΔΨm) and inhibits mitochondrial permeability transition pore (mPTP) opening[@co_mito_2023].
Complex IV: CO binds to cytochrome c oxidase (Complex IV), modulating its activity in a protective manner at low concentrations.
PGC-1α: CO promotes mitochondrial biogenesis through PGC-1α (PPARGC1A) activation, supporting generation of new healthy mitochondria.
CO induces autophagy through:
mTOR Inhibition: CO inhibits mTORC1 signaling, relieving inhibition of autophagy initiation[@co_autophagy_2024].
LC3 Conversion: CO promotes LC3-I to LC3-II conversion, enhancing autophagosome formation.
Aggregate Clearance: CO-induced autophagy may enhance clearance of toxic protein aggregates including amyloid-β, tau, and α-synuclein.
CO deficiency has been documented in AD patients, with reduced HO-1 activity and CO levels in the brain. CORM therapy addresses multiple hallmarks of AD pathology:
Amyloid pathology: CORMs reduce amyloid-β aggregation and toxicity through:
Tau pathology: CO modulates tau phosphorylation through:
Synaptic dysfunction: CO enhances synaptic plasticity:
Mitochondrial function: CO improves energy metabolism:
Preclinical evidence: In 5xFAD and APP/PS1 mice, CORM-3 treatment reduced cortical amyloid plaque burden, improved performance in Morris water maze, and preserved hippocampal synaptic density[@corm_ad_2022].
Mitochondrial dysfunction is central to PD pathogenesis, making CORMs particularly relevant:
Dopaminergic neuron protection: CORMs protect against:
Mitochondrial quality control: CORMs enhance:
Neuroinflammation: CORMs modulate:
α-Synuclein pathology: CORMs may reduce:
Preclinical evidence: In MPTP-treated mice and 6-OHDA-lesioned rats, CORM-3 protected dopaminergic neurons, improved behavioral scores, and preserved striatal dopamine content[@corm_pd_2023].
ALS involves multiple pathological mechanisms that CORMs can address:
Motor neuron protection: CORMs have shown:
Glial modulation: CORMs affect:
SOD1 models: In SOD1-G93A transgenic mice:
Energy metabolism: CORMs support:
CO plays roles in HD through several mechanisms:
Mitochondrial dysfunction: CO improves:
Transcriptional dysregulation: CO modulates:
Excitotoxicity: CO protects against:
Autophagy: CO enhances:
As of 2026, CORM therapy for neurodegeneration remains in preclinical development:
| CORM | Company/Group | Indication | Development Stage |
|---|---|---|---|
| CORM-2 | Academic groups | Various | Preclinical |
| CORM-3 | Academic groups | PD, AD | Preclinical |
| CORM-401 | Academic groups | Mitochondrial disorders | Preclinical |
| ALF-494 | Academic groups | Preclinical | Early preclinical |
Challenges to clinical translation:
Clinical trials to watch:
| Property | CO-Releasing Molecules | H₂S Donors | NO Donors |
|---|---|---|---|
| Primary targets | HO-1, CO sensors | CBS, CSE, KATP channels | sGC, NOS |
| BBB penetration | Moderate | Good | Good |
| Clinical stage | Preclinical | Preclinical | Approved (nitroglycerin) |
| Dosing frequency | Daily | Daily | As needed |
| Major toxicity | High doses: tissue hypoxia | High doses: respiratory depression | Hypotension |
CORMs offer unique advantages:
The field of CORM therapy for neurodegeneration requires: