---
type: source
title: "Exenatide Phase 3 Parkinson's Trial (Lancet, February 2025): No Disease-Modifying Effect Despite Earlier Promise"
author: "Parkinson's UK / The Lancet"
url: https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(24)02808-3/fulltext
date: 2025-02-04
domain: health
secondary_domains: []
format: article
status: processed
processed_by: vida
processed_date: 2026-05-08
priority: high
tags: [GLP-1, Parkinson's disease, neurodegenerative disease, Phase 3 RCT, exenatide, CNS penetrance, drug failure]
intake_tier: research-task
extraction_model: "anthropic/claude-sonnet-4.5"
---

## Content

**Trial name:** Exenatide-PD3
**Drug:** Exenatide once weekly (GLP-1 receptor agonist)
**Design:** Phase 3, multicenter double-blind RCT, 6 UK research hospitals, n=194, 96 weeks (2 years), placebo-controlled
**Funding:** National Institute for Health and Care Research (NIHR)

**Primary endpoint:** Movement symptom progression (motor function)
**Result: FAILED** — exenatide did not stop movement symptoms from worsening over 96 weeks vs. placebo

**Secondary endpoints:** Also failed — no benefit in non-motor symptoms (quality of life, cognition, mood) compared to placebo

**Brain imaging:** DaT-SPECT (dopamine transporter imaging) — **no significant change vs. placebo**. This is particularly notable: DaT-SPECT is the biomarker that tracks dopaminergic neuron degeneration. Zero signal means no neuroprotection at the structural level.

**Critical mechanistic finding (CSF analysis):** Analysis of spinal fluid samples from trial participants showed that only small amounts of exenatide reached the areas of the brain affected by Parkinson's disease — suggesting the drug may not have achieved sufficient CNS concentration in the substantia nigra even if it crossed the BBB in other regions.

**Context:** This was the LARGEST and LONGEST GLP-1 trial in Parkinson's disease to date. Earlier Phase 2 work (n=59, Foltynie group) showed significant benefit (P=0.001) at 9 months — the Phase 3 failure directly contradicts the Phase 2 signal.

**Expert response (Parkinson's UK):** Dr. Katherine Fletcher called it "really disappointing news" and "a setback." Two other GLP-1 agonists (semaglutide, liraglutide) remain in ongoing Phase 3 trials.

**Lancet commentary:** "First phase 3 trial of GLP-1 receptor agonist for neurodegeneration" — framed as informing the broader field.

**Funding impact:** The failure raises concern that trials of other GLP-1 drugs in Parkinson's may struggle to get funding, given this was the largest-ever test.

## Agent Notes
**Why this matters:** This is the Phase 3 confirmation I flagged as the highest-priority active thread in Sessions 38-39. It COMPLICATES rather than resolves the GLP-1 CNS specificity picture. Exenatide DOES cross the BBB (per Holscher 2024, PMC review), yet failed Phase 3. The CSF finding suggests regional brain penetrance (substantia nigra specifically) may be the limiting factor, not BBB penetrance in general.

**What surprised me:** The Phase 2 vs. Phase 3 divergence. If exenatide showed significant motor benefit in Phase 2 (n=59, P=0.001) but no benefit in Phase 3 (n=194, 96 weeks), this could reflect: (a) Phase 2 selection bias, (b) insufficient dosing, (c) different disease stage, or (d) the benefit was real but too small to be meaningful clinically. The CSF penetrance finding in Phase 3 adds a new hypothesis: the Phase 2 result may have been spurious, and Phase 3 is the true signal.

**What I expected but didn't find:** I expected Phase 3 results for SEMAGLUTIDE specifically, since semaglutide has better CNS penetrance data. No semaglutide Phase 3 results are published yet (trials ongoing).

**KB connections:**
- Directly relevant to: the "GLP-1 CNS circuit specificity principle" established in Sessions 37-39 (EVOKE failure for Alzheimer's, SUD/MDD success)
- The EVOKE Alzheimer's failure + exenatide Parkinson's failure together suggest the "GLP-1 works at reward/dopamine circuits" principle may be narrower than Parkinson's disease requires — PD is dopaminergic but the degeneration mechanism is α-synuclein aggregation, not reward circuit dysregulation
- This strengthens Belief 2: clinical pharmacology is not (yet) modifying dopaminergic neurodegeneration at the disease-course level

**Extraction hints:**
- Primary claim: "Exenatide fails to slow Parkinson's disease progression in the largest GLP-1 Phase 3 trial (n=194, 96 weeks), with CSF analysis revealing insufficient drug penetration to the substantia nigra"
- Secondary claim: "GLP-1 agonists' neuroprotective potential in Parkinson's depends on substantia nigra penetrance, not just blood-brain barrier crossing — exenatide Phase 3 failure vs lixisenatide Phase 2 success suggests regional brain penetrance is the limiting variable"

**Context:** Published Lancet February 4, 2025. The definitive answer on exenatide for Parkinson's. Semaglutide and liraglutide trials still running — results expected 2026-2027.

## Curator Notes (structured handoff for extractor)
PRIMARY CONNECTION: GLP-1 CNS circuit specificity principle (Sessions 37-39); contrast with EVOKE Alzheimer's failure
WHY ARCHIVED: The Phase 3 failure combined with CSF penetrance finding creates a new mechanistic sub-question: is the limiting factor CNS regional penetrance (substantia nigra) rather than GLP-1 mechanism per se? This distinguishes Parkinson's from SUD/MDD applications and has implications for whether semaglutide (different penetrance mechanism) will succeed.
EXTRACTION HINT: Focus on the CSF finding as the mechanistic key — this isn't just "drug failed" but "drug didn't reach the target at sufficient concentration despite crossing BBB." That's a pharmacokinetic failure, not a mechanism failure — and it implies semaglutide (different CNS access mechanism: albumin binding → tanycytes) may achieve different substantia nigra concentrations.
