Poly(ADP-ribose) polymerases (PARPs) are a family of enzymes involved in DNA repair, cellular stress responses, and cell death pathways. In neurodegenerative diseases, overactivation of PARP (particularly PARP1) leads to excessive NAD+ depletion, energy failure, and programmed cell death. PARP inhibitors, originally developed for cancer, represent a repurposing opportunity for neuroprotection by preventing NAD+ exhaustion and promoting DNA repair in [neurons](/entities/neurons) and glia.
Mechanism of Action
Pathological Context
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Overview
Mermaid diagram (expand to render)
Poly(ADP-ribose) polymerases (PARPs) are a family of enzymes involved in DNA repair, cellular stress responses, and cell death pathways. In neurodegenerative diseases, overactivation of PARP (particularly PARP1) leads to excessive NAD+ depletion, energy failure, and programmed cell death. PARP inhibitors, originally developed for cancer, represent a repurposing opportunity for neuroprotection by preventing NAD+ exhaustion and promoting DNA repair in [neurons](/entities/neurons) and glia.
Mechanism of Action
Pathological Context
PARP1 is rapidly activated by DNA strand breaks that accumulate in neurons due to oxidative stress, mitochondrial dysfunction, and aging[@berger1985]. While PARP-mediated DNA repair is essential, overactivation leads to:
NAD+ depletion: PARP consumes NAD+ to synthesize poly(ADP-ribose), depleting cellular energy stores
ATP exhaustion: NAD+ depletion triggers ATP consumption in a futile cycle attempting to restore NAD+
AIF translocation: Severe PARP activation triggers [apoptosis](/entities/apoptosis)-inducing factor (AIF) translocation to nucleus
Neuroinflammation: PARP activation in glial cells promotes inflammatory responses
Therapeutic Strategy
Primary Mechanism: PARP inhibitors (particularly PARP1-selective) prevent excessive PARP activation, preserving NAD+ and ATP levels during DNA damage stress[@moroni2008].
Secondary Mechanism: By maintaining NAD+ pools, PARP inhibitors support sirtuin (SIRT1) activity, promoting cellular stress resistance and mitochondrial function.
Tertiary Mechanism: Some PARP inhibitors also enhance DNA repair fidelity, potentially preventing the accumulation of deleterious mutations in neurons.
Rubric Scores
| Dimension | Score | Rationale | |-----------|-------|-----------| | Novelty | 6 | Established drug class (cancer); repurposing for neurodegeneration is emerging | | Mechanistic Rationale | 8 | Strong preclinical data; addresses energy crisis and DNA repair | | Addresses Root Cause | 7 | Targets oxidative stress response and energy failure; complementary to other approaches | | Delivery Feasibility | 7 | Many PARP inhibitors have good oral bioavailability; some CNS penetration demonstrated | | Safety Plausibility | 7 | Well-characterized safety profile from oncology; hematological effects need monitoring | | Combinability | 8 | Synergistic with NAD+ precursors, sirtuin activators, and mitochondrial protectants | | Biomarker Availability | 7 | NAD+ levels, DNA damage markers, PAR levels can be measured | | De-risking Path | 8 | Multiple PARP inhibitors already approved; clear regulatory path | | Multi-disease Potential | 8 | Strong rationale for AD, PD, ALS, stroke, and traumatic brain injury | | Patient Impact | 7 | Addresses fundamental cellular energetics; broad applicability |
Total Score: 72/100
Preclinical Evidence
PARP Inhibition Models
Stroke models: PARP inhibitors significantly reduce infarct size and improve functional outcomes in rodent stroke models[@endres1997]
PD models: Protect dopaminergic neurons in MPTP and 6-OHDA models[@chiu2008]
AD models: Reduce DNA damage, improve cognition in [APP](/entities/app-protein)/PS1 mice[@strosznajder2012]
ALS models: Delay disease progression in SOD1 mice
Key Mechanisms
NAD+ preservation: PARP inhibitors prevent the dramatic NAD+ depletion seen in neurodegeneration
Mitochondrial protection: Maintain mitochondrial function and ATP production
Anti-apoptotic: Prevent AIF-mediated cell death pathway
Anti-inflammatory: Reduce microglial activation and neuroinflammation
Clinical Data
Olaparib: Approved for multiple cancers; extensive safety data available
Niraparib, rucaparib, talazoparib: Additional approved agents with varying CNS penetration
Clinical Development Status
Ongoing Trials
Phase 2 in ALS: Multiple PARP inhibitors in early clinical testing
Phase 2 in PD: Evaluating neuroprotective effects
Preclinical: New PARP1-selective inhibitors with optimized CNS penetration
Repurposing Opportunities
Olaparib: Good safety profile; potential for repositioning
Niraparib: Strong preclinical data in neurodegeneration models
Novel PARP1-selective: In development for CNS indications
Development Pathway
Phase 2 Repurposing (Months 1-18)
Identify PARP inhibitor with optimal CNS penetration
Conduct Phase 2 in early AD or PD patients
Establish biomarker endpoints (NAD+, DNA damage markers)
Go/No-Go: Demonstrate target engagement and safety
Phase 3 Development (Months 18-36)
Registrational trial design based on Phase 2
Accelerated approval pathway using biomarker endpoints
[Berger NA, Poly(ADP-ribose) in the cellular response to DNA damage (1985)](https://pubmed.ncbi.nlm.nih.gov/2937105/)
[Moroni F, Poly(ADP-ribose)polymerase 1 (PARP-1) and its therapeutic effects in neurological diseases (2008)](https://pubmed.ncbi.nlm.nih.gov/18992319/)
[Endres M, Wang ZQ, Namura S, Waeber C, Moskowitz MA, Ischemic brain injury is mediated by the activation of poly(ADP-ribose) polymerase (1997)](https://pubmed.ncbi.nlm.nih.gov/9413751/)
[Chiu PY, Leung HY, Siu AW, Poon MK, Ko KM, Schisandrin B protects against PAR-4 and AIF translocation in 6-hydroxydopamine-treated PC12 cells (2008)](https://pubmed.ncbi.nlm.nih.gov/18404466/)
[Strosznajder JB, Czapski GA, Adamczyk A, Strosznajder RP, Poly(ADP-ribose) polymerase-1 in Alzheimer's disease (2012)](https://pubmed.ncbi.nlm.nih.gov/22895052/)