Ambroxol as a novel disease-modifying treatment for Parkinson’s disease dementia: protocol for a single-centre, randomized, double-blind, placebo-controlled trial

Parkinson’s disease dementia (PDD) is a highly prevalent evolution of Parkinson’s disease [1], believed to result from the aggregation of α-synuclein in subcortical and cortical neurons [2, 3]. Longitudinal studies have shown that by the 10-year follow up from diagnosis, 46% of individuals with Parkinson’s disease had progressed to PDD [4]. Importantly, decline in cognitive function contributes to decreased quality of life [5], increased risk of institutionalization [6] and mortality [7] in individuals with Parkinson’s disease. Therefore, it is not surprising that the development of new therapies to treat cognitive decline has been rated by individuals with Parkinson’s disease and caregivers as one of the top 10 priorities in Parkinson’s disease research [8].

Currently, the treatment for cognition in PDD includes the use of cholinesterase inhibitors in addition to dopaminergic medication [9]. Although somewhat effective in their role of alleviating symptoms, these therapies do not affect the underlying mechanisms leading to progression of the disease and patients get worse despite treatment. Thus, the development of disease-modifying treatments for PDD is critical.

A potential target for disease-modifying treatment is the enzyme β-Glucocerebrosidase (GCase; gene name GBA1). GCase is a degradative enzyme that resides in the lysosome and cleaves a neutral glycolipid, glucocerebrosidase, present in the membranes of most cells. Having mutations in both alleles of GBA1 results in Gaucher disease, which can manifest with hepatic, splenic, hematological and bone abnormalities (Type 1) or with a devastating neurological deterioration that can be rapid or chronic (Type 2 and 3, respectively). However, being a carrier for a pathological mutation in GBA1 is one of the strongest genetic risk factors for Parkinson’s disease, PDD and Lewy Body dementia. Parkinson’s disease patients bearing GBA1 mutation, tend to have earlier onset, a greater prevalence of cognitive decline as well as more severe cognitive impairment compared to those without the mutation [10]. In addition, reductions in GCase activity may play a role even in sporadic Parkinson’s disease, as these individuals have low levels of GCase in the brain and cerebrospinal fluid [11‐13].

Laboratory studies have demonstrated a direct link between GCase activity and α-synuclein accumulation. In cultured cells, loss of GCase results in α-synuclein accumulation and this process feeds back upon itself, with overexpression of α-synuclein further inhibiting GCase function, and increasing GCase expression reducing α-synuclein [14]. Moreover, reducing GCase genetically [15, 16] or pharmacologically [17] in animal studies results in increased α-synuclein aggregates. Remarkably, overexpressing GCase in the brain of a Parkinson’s disease mouse model reduces α-synuclein and improves cognition [15, 18]. Taken together, these findings suggest that increasing GCase levels could be a therapy that addresses the underlying pathophysiology of PDD to modify the course of disease progression.

Ambroxol is an expectorant that has been available for adults and children over the counter in more than 50 countries for over 30 years [19]. Ambroxol was identified by the Mahuran Lab in a High Throughput Screen as a pharmacological chaperone of GCase, an agent that stabilizes and increases the levels of GCase [20]. Pharmacological chaperones are small molecules that bind to proteins to stabilize them, increasing the number of protein molecules that fold and function correctly, and thereby reduce the amount of protein degraded. They have been proposed as therapies for a wide range of inborn errors of metabolism, a large fraction of which are caused by mutations which destabilize protein folding, leading to degradation [21‐23].

In addition, Ambroxol has good lipophilicity (cLogP = 2.8) and low polar surface area (PSA 58 Ų), predicting good CNS penetration. Ambroxol has an excellent safety record and has been studied in > 15,000 patients in more than 100 trials. While the normal expectorant dose in adults is in the range of 75–100 mg/day, doses of 1000 mg IV are used in pregnant women experiencing premature delivery to aid fetal lung maturation, and doses of 30 mg/kg in neonates for fetal respiratory distress syndrome [19].

Although Ambroxol was originally identified as a drug which stabilizes GCase bearing disease-causing mutations which increase its degradation, it is also able to increase the levels of normal GCase [20, 24]. As such, it can markedly increase GCase protein and activity in both normal and Gaucher disease fibroblasts (bearing GCase mutations) at concentrations in the micromolar range [20, 24]. Ambroxol treatment has also been shown to improve lysosomal biochemistry overall by indirect activation of a lysosomal master regulatory gene/transcription factor called TFEB and improved clearance of α-synuclein [25]. More recently, Ambroxol was found to raise GCase levels and lower α-synuclein in induced pluripotent stem cell (iPSC)-derived dopaminergic neurons from GBA1 mutation patients [26].

In mice, oral Ambroxol for 12 days increased GCase activity in the brainstem, midbrain, cortex, and striatum of wild-type as well as transgenic mice carrying a L444P mutation (i.e. a GBA1 mutation) [27]. The same study found that transgenic mice overexpressing human α-synuclein treated with Ambroxol had a reduction in α-synuclein levels in the brainstem and striatum compared to untreated mice. Furthermore, Ambroxol reduced S129 phosphorylation of α-synuclein (suggested to play a critical role in α-synuclein aggregation and formation of Lewy bodies and neuritis) in the brainstem by 41% in treated mice compared to untreated mice. Ambroxol has also recently been demonstrated to increase GCase levels in non-human primates [28]. Together, these results support the potential for using Ambroxol as a disease-modifying treatment for synucleinopathies such as PDD.

In pilot studies in humans, Ambroxol was effective at improving GCase function [29, 30]. In a trial aimed at Type 1 (non-neurological) Gaucher disease, 12 patients received 150 mg/day for 6 months, and all but one had some measurable improvement. The best response was in the lightest patient (who received 3 mg/kg/day), suggesting that Ambroxol was under dosed in this study. Ambroxol has also been administered to five Japanese Gaucher disease patients with severe neurological disease, at 25 mg/kg/day or a maximum dose of 1300 mg/day for 6–48 months [29]. Despite their advanced disease, patients had improvements in neurological symptoms (decreased myoclonus and seizure frequency). Improvements in myoclonus allowed two patients to stand steadily, control their balance, and walk again. To our knowledge, no study has tested the effects of Ambroxol on people with Parkinson’s disease or PDD.

The main objectives of the present randomized controlled trial are 1) to demonstrate the safety and tolerability of Ambroxol in patients with PDD, 2) to examine the effects of Ambroxol on cognitive, biochemical, and neuroimaging measures, and 3) to acquire additional pharmacokinetic and pharmacodynamics data for use in future trials. It is hypothesized that Ambroxol will be safe and well tolerated in PDD, and may improve the course of cognitive impairment or motor function, or biomarkers of neurodegeneration in PDD by raising GCase levels in blood and CSF.

The study has been approved by Western University Health Science Research Ethics Board (Protocol ID: 105234) and Health Canada (Protocol ID: HC6–24-c181033) in November 2015. The trial has been registered on ClinicalTrials.​gov (ID: NCT02914366).

Cognitive decline and its implications to the life of individuals with PDD represent a significant burden to patients, caregivers, and the health care system. Current pharmacological therapies are helpful for cognitive symptoms in PDD but are unable to modify the course of disease progression, making treatment for cognition in PDD a major unmet need. Increasing levels of the GCase enzyme has been proposed as a potential therapy for slowing neurodegeneration in Parkinson’s disease, and consequently PDD. There is increasing evidence in cell culture and animal models of PD that Ambroxol can raise levels of GCase. Strikingly, increased levels of GCase were accompanied by decreased levels of α-synuclein aggregation in brain areas of mice treated with Ambroxol. The present trial is the first to test Ambroxol in human subjects with PDD. The effects of Ambroxol will be measured using standardized cognitive, functional, behavioural, biochemical, as well as brain imaging outcomes. Results from this clinical trial may have a large impact in the research of disease-modifying therapies for PDD and other synucleinopathies including Parkinson’s disease and Lewy Body dementia. Importantly, Ambroxol is a drug already commercially available in most of the world and has an excellent safety record. Thus, repurposing Ambroxol for the treatment of PDD may expedite a potential disease-modifying treatment and circumvent a lengthy drug development process.