Phase II Trial Shows Ambroxol Safe but Ineffective for Parkinson's Disease Dementia Cognitive Symptoms
Key Insights
A 52-week phase II study of 54 patients found that ambroxol treatment at doses up to 1,050 mg daily was safe and well-tolerated in Parkinson's disease dementia (search) patients.
Despite achieving therapeutic drug concentrations in plasma and cerebrospinal fluid, ambroxol failed to demonstrate significant improvements in cognitive function compared to placebo.
The study documented higher rates of gastrointestinal adverse events in ambroxol-treated patients, leading to treatment withdrawal in 26% of the drug group versus 12.5% in placebo.
A single-center phase II clinical trial has demonstrated that ambroxol, despite being safe and well-tolerated, fails to improve cognitive function in patients with Parkinson's disease dementia (search) (PDD). The 52-week randomized controlled study included 54 PDD patients and represents a significant setback for this repurposed drug approach to treating cognitive decline in Parkinson's disease (search).
Study Design and Patient Population
The trial enrolled patients who were at least 51 years of age, had Parkinson's disease (search) for at least one year before cognitive impairment (search) onset, presented with mild to moderate dementia (search), and were maintained on stable medications. Participants were randomly assigned to three groups: low-dose ambroxol at 525 mg per day (n=8, mean age 78.8 years, all male), high-dose ambroxol at 1,050 mg per day (n=22, mean age 70.7 years, 86.4% male), or placebo (n=24, mean age 72.7 years, 79.2% male).
The primary efficacy outcomes were measured using the Alzheimer Disease Assessment Scale–cognitive subscale version 13 (ADAS-Cog-13) and Clinician's Global Impression of Change (CGIC), standard assessments for cognitive function in dementia (search) studies.
Safety Profile and Adverse Events
The study documented a total of 365 adverse events (AEs), with 193 events (53%) occurring in the pooled ambroxol group and 172 events (47%) in the placebo group. Most adverse events were classified as mild to moderate in severity. However, treatment discontinuation rates were notably higher in the ambroxol group, with eight patients (26%) withdrawing from the drug group compared to three patients (12.5%) in the placebo group.
Gastrointestinal adverse events emerged as a particular concern, occurring more frequently in ambroxol-treated patients than in the placebo group (12% versus 5%). This finding suggests a dose-dependent gastrointestinal tolerability issue that could limit the drug's clinical utility even if efficacy were demonstrated.
Pharmacokinetic and Biomarker Results
The study achieved meaningful drug exposure levels, with mean high-dose ambroxol concentrations reaching 7.48 μM in plasma and 0.73 μM in cerebrospinal fluid at the end of the titration period. These concentrations indicate successful central nervous system penetration, addressing a key requirement for treating neurological conditions.
Importantly, the study demonstrated target engagement through increased β-glucocerebrosidase (search) enzyme levels. At week 26, mean β-glucocerebrosidase levels were significantly higher in the ambroxol group compared to placebo (12.45 versus 8.50 nmol/h/mg; p=0.05). This finding confirms that ambroxol successfully modulated its intended biological target, making the lack of clinical efficacy particularly noteworthy.
Efficacy Outcomes
Despite achieving therapeutic drug concentrations and demonstrating target engagement, statistical analyses comparing the high-dose ambroxol group with placebo revealed no evidence of differences between groups on primary or secondary outcomes. When the high-dose ambroxol group was specifically compared with the placebo group, no significant differences were observed in any efficacy measures.
This negative result is particularly significant given that ambroxol has been investigated as a potential treatment for Parkinson's disease dementia (search) based on its ability to enhance β-glucocerebrosidase (search) activity, an enzyme implicated in the pathogenesis of Parkinson's disease (search) and related disorders.