Understanding Clinical Trial Phases

For patients and families following the research pipeline, understanding what each phase means helps set realistic expectations:

• Phase 1: Tests safety and dosing in a small number of participants (20-80). The primary question to be answered is: Is this safe? Success rate for proceeding to Phase 2: approximately 60-70%.

• Phase 2: Tests efficacy in a larger group (100-300). The primary question to be answered is: Does this work? Success rate for proceeding to Phase 3: approximately 30-40%.

• Phase 3: Confirms efficacy and safety in a large population (300-3,000+). The primary question here is: Is this effective and safe enough for FDA approval? Success rate for approval: approximately 50-60%.

• FDA Review: After successful Phase 3, the sponsor submits a New Drug Application (NDA) or Biologics License Application (BLA) to the FDA. NDA’s are for chemicall synthesed drugs which are relatively simple while BLA’s are for more complicated bilogical products, typically made from living material.

  Review typically takes 10 to 12 months.

Overall, the probability that a drug entering Phase 1 clinical trials moving into Phase 3 and ultimately receiving FDA approval is approximately 10 to 15 percent. For neurological diseases, the rate is historically even lower. This context is important for maintaining realistic expectations while remaining informed and hopeful.

Upcoming Trial Milestones

Trial / Drug	Milestone	Expected Date
Tavapadon	FDA approval decision	Quarter 3 (Q3) 2026 (expected)
Solengepras (ARISE)	Phase 3 data	End of Q3 2026
GT-02287	Phase 1b extension data	September 2026
BEACON (BIIB122, LRRK2 carriers)	Phase 2a data	H1 2027
AB-1005 (REGENERATE-PD)	Phase 2 primary completion (estimated)	2028
SLEIPNIR platform	Recruitment begins	2026
Prasinezumab Phase 3 (PARAISO)	Phase 3 primary completion	June 2029

Phase 3 Clinical Trials

Phase 3 trials are large-scale studies (typically hundreds to thousands of participants) designed to confirm efficacy and safety. Success in Phase 3 is required for FDA approval.

Tavapadon

Tavapadon is a novel oral, once-daily dopamine receptor agonist that selectively targets D1 and D5 receptors. This distinguishes it from existing dopamine agonists (pramipexole, ropinirole, rotigotine), which primarily target D2 and D3 receptors. AbbVie submitted an NDA to the FDA on September 26, 2025, based on positive results from the Phase 3 TEMPO clinical trial program.

The TEMPO trials demonstrated statistically significant improvements on the Movement Disorder Society – Unified Parkinson’s Disease Rating Scale (MDS-UPDRS), increased “on” time without troublesome dyskinesia in patients with motor fluctuations, and a favorable long-term safety profile. If approved, tavapadon would be the first new oral dopamine agonist approved for Parkinson's disease in over a decade.

Solengepras (Cerevance) — Non-Dopaminergic Adjunctive

• ARISE Phase 3 trial: Evaluating solengepras (formerly CVN424, a GPR6 inhibitor) as an adjunctive treatment for motor fluctuations in Parkinson's disease. Cerevance completed enrollment (about 341 participants) in May 2026.

• Data expected: End of Q3 2026.

Prasinezumab (Roche/Genentech) — Anti-Alpha-Synuclein Antibody

Prasinezumab is a monoclonal antibody designed to bind and clear aggregated forms of alpha-synuclein — the protein that forms toxic Lewy bodies in Parkinson's disease. It represents the most advanced attempt at disease modification through immunotherapy.

• Phase IIb PADOVA results (June 2025): In 586 participants treated for 18+ months, the trial narrowly missed its primary endpoint of time to confirmed motor progression (hazard ratio 0.84, p=0.0657). However, a pre-specified subgroup of patients already on levodopa showed a significant reduction in motor progression risk (HR=0.79, p=0.04). Safety was favorable with no new safety signals across 900+ participants.

• Phase 3 PARAISO initiated November 2025: about 900 participants, 2-year treatment period, with primary completion expected in June 2029.

• Significance: If successful, prasinezumab would be the first therapy proven to slow the underlying progression of Parkinson's disease — a fundamental advance in treatment.

Bemdaneprocel (BlueRock/Bayer) — Stem Cell Therapy

Bemdaneprocel is the most advanced stem cell therapy for Parkinson's disease. It consists of dopamine-producing neurons derived from human embryonic stem cells, surgically implanted into the brain to replace neurons lost to the disease.

• Phase 3 exPDite-2 trial: First patient treated September 2025. This is the first large-scale global cell therapy trial for Parkinson's disease.

• Significance: If successful, cell replacement could fundamentally change the treatment of Parkinson's disease by restoring dopamine production rather than merely supplementing it with medication.

Sleipnir

Sleipnir is the eight-legged horse ridden by the god Odin in Norse mythology. He is the son of the god Loki (in the form of a mare) and the stallion Svadilfari who belonged to the jötunn that built the walls of Asgard.

Sleipnir is always depicted as incredibly swift and the "best of all horses", symbolized by his eight legs that carry a rider anywhere in the nine realms of Norse cosmology in record time. Since he was born of two supernatural entities, he possesses the power to move easily between realms, leading to his name (pronounced Slayp-near), which means "the sliding one".

One of the ongoing challenges for Parkinson’s research is determining whether treatments are reaching the brain and if they are having their intended effect there. Many late-stage trials that failed to meet their primary endpoints have found evidence of low brain penetrance, indicating there was not enough of the drug in the brain to have a therapeutic effect. Given the amount of time and resources it takes to conduct a clinical trial, it is vital that we have a clear understanding of how a drug performs in the body early in the process so that we know which treatments have the best chance of success.

SLEIPNIR is an upcoming multi-arm clinical trial platform that aims to address this, hence the name. SLEIPNIR will test up to three potentially disease-modifying treatments for Parkinson’s simultaneously against one placebo group. Led by Professor Charalampos Tzoulis at Haukeland University Hospital and the University of Bergen, the team will assess whether the drugs reach the brain and interact with their intended targets by looking at blood and cerebrospinal fluid – the fluids surrounding the brain and spinal cord – samples. They will also monitor the safety and tolerability of these therapies.

SLEIPNIR will also support the wider trial landscape. By assessing multiple drugs simultaneously in a smaller, shorter time frame, it is hoped that SLEIPNIR will reduce the risk of larger clinical trials failing. This will accelerate the process of identifying promising treatments that can then be fed into larger, late-stage clinical trial platforms.