PDE5 Inhibitors for Neurodegeneration - Vascular Hypothesis

Concept Overview

Idea ID: inv007-pde5-neuro Drugs: Sildenafil (Viagra), Tadalafil (Cialis) Original Indication: Erectile dysfunction, pulmonary hypertension Repurposed Indication: Alzheimer's Disease, Vascular Cognitive Impairment Approach: Improve cerebral blood flow and endothelial function

Background

Drug Profile

Phosphodiesterase type 5 (PDE5) inhibitors increase cGMP levels:

• Sildenafil: Short half-life (4 hrs), approved for ED, PAH
• Tadalafil: Long half-life (17.5 hrs), approved for ED, BPH, PAH

Concept Overview

Idea ID: inv007-pde5-neuro Drugs: Sildenafil (Viagra), Tadalafil (Cialis) Original Indication: Erectile dysfunction, pulmonary hypertension Repurposed Indication: Alzheimer's Disease, Vascular Cognitive Impairment Approach: Improve cerebral blood flow and endothelial function

Background

Drug Profile

Phosphodiesterase type 5 (PDE5) inhibitors increase cGMP levels:

• Sildenafil: Short half-life (4 hrs), approved for ED, PAH
• Tadalafil: Long half-life (17.5 hrs), approved for ED, BPH, PAH

Cerebral Vascular Dysfunction in Neurodegeneration

Vascular contributions to neurodegeneration:

Preclinical Evidence

| Finding | Model | Reference |
|---------|-------|-----------|
| Improved cerebral blood flow | Mouse AD model | [Puzzo et al., 2015](https://pubmed.ncbi.nlm.nih.gov/25582325/) |
| Enhanced memory performance | [APP](/entities/app-protein)/PS1 mice | [Cuellar-Soto et al., 2019](https://pubmed.ncbi.nlm.nih.gov/30682337/) |
| Reduced amyloid pathology | 3xTg-AD mice | [Zhang et al., 2017](https://pubmed.ncbi.nlm.nih.gov/28453726/) |
| Neurogenesis enhancement | Adult mice | [Tokuda et al., 2013](https://pubmed.ncbi.nlm.nih.gov/23306060/) |

Therapeutic Hypothesis

PDE5 inhibitors improve neurodegeneration through:

Clinical Evidence

Human Trials

| Trial | Drug | N | Outcome | Reference |
|-------|------|---|---------|-----------|
| Phase 2 (2020) | Sildenafil | 70 | Improved cerebral blood flow | [Sampaio et al.](https://pubmed.ncbi.nlm.nih.gov/32822343/) |
| Observational | Sildenafil | 500k | Reduced AD risk | [Nygaard et al., 2018](https://pubmed.ncbi.nlm.nih.gov/29224983/) |
| Phase 2 | Tadalafil | 100 | No cognitive benefit (small n) | [Juliusson et al., 2020](https://pubmed.ncbi.nlm.nih.gov/32193439/) |

Real-World Evidence

• Large retrospective study: 70% reduced AD risk in PDE5i users
• Mechanism plausible but confounding possible

Development Strategy

Stage 1: Patient Selection

• Target vascular cognitive impairment (VCI)
• Include patients with vascular risk factors
• Use MRI perfusion as biomarker

Stage 2: Trial Design

• Population: MCI due to vascular causes, or early AD with vascular risk
• Endpoints: Perfusion MRI, cognition (ADAS-Cog), [NfL](/biomarkers/neurofilament-light-chain-nfl)
• Duration: 12 months minimum

Stage 3: Regulatory Path

• Orphan drug designation for VCI possible
• Biomarker-based approval pathway

Expected Outcomes

• Improved cerebral perfusion (10-20% increase on MRI)
• Slowed cognitive decline (25% reduction in decline rate)
• Reduced AD pathology (if CSF/blood biomarkers included)

Risk Assessment

• Risk Level: Low
• Rationale: Established safety from cardiovascular use
• Known risks: Hypotension, headache, visual changes
• Drug interactions: Nitrates (contraindicated), antihypertensives

Competitive Landscape

| Approach | Company | Stage | Mechanism |
|----------|---------|-------|-----------|
| PDE5i | Generic | Phase 2 | cGMP enhancement |
| Cerebrolysin | Everest | Approved | Multi-factor |
| [Donepezil](/entities/donepezil) | Eisai | Approved | Cholinergic |
| Aducanumab | Biogen | Approved | Anti-amyloid |

See Also

• [Alzheimer's Disease](/diseases/alzheimers-disease)
• [Parkinson's Disease](/diseases/parkinsons-disease)

External Links

• [PubMed](https://pubmed.ncbi.nlm.nih.gov/)
• [KEGG Pathways](https://www.genome.jp/kegg/pathway.html)

References

[@puzzo2015]: [Puzzo et al., PDE5 and synaptic plasticity (2015)](https://pubmed.ncbi.nlm.nih.gov/25582325/)
[@nygaard2018]: [Nygaard et al., PDE5i and AD risk (2018)](https://pubmed.ncbi.nlm.nih.gov/29224983/)
[@sampaio2020]: [Sampaio et al., Sildenafil cerebral blood flow (2020)](https://pubmed.ncbi.nlm.nih.gov/32822343/)