Ambroxol as a Novel Disease Modifying Treatment for Lewy Body Dementia

The increasing prevalence of dementia is a serious threat to our medical system and our society. About 500,000 Canadians are affected with dementia, and this number will rise to more than 1 million in the next 20 years. Dementia already costs our economy 15 billion dollars per year. While much of the focus of dementia is on Alzheimer's disease, autopsy studies suggest that up to 30% of dementia is due to diseases caused by abnormal alpha-synuclein accumulation (Synucleinopathies). In healthy brains, alpha-synuclein plays a number of important roles, especially in the process by which brain cells (neurons) communicate. However, when alpha-synuclein abnormally accumulates into clumps inside the neurons it forms Lewy bodies. Eventually, as a result, brain neurons will die causing widespread damage to specific brain regions.

Until the 1980's, cortical Lewy bodies were thought to be relatively rare. However, with improved alpha-synuclein immunostaining techniques, Lewy body dementias are now recognized as the second most common neurodegenerative dementia, after Alzheimer's disease. Lewy body-related disorders include idiopathic Parkinson's disease (PD), Parkinson's disease dementia (PDD) and Lewy Body Dementia (LBD). In LBD, alpha- synuclein accumulation is found in the brainstem, limbic and neocortical regions, giving rise to autonomic, cognitive and motor impairments.

Cognitive Symptoms: Progressive cognitive decline typically begins early in the course of the disease in advance of parkinsonism, but by consensus may follow the development of motor signs up to 1 year. Impaired cognitive domains include executive and visual-spatial functions, attention and short-term memory. For example, patients may have difficulty multi-tasking, following conversations, episodes of staring and perturbed flow of ideas. Regarding short-term memory, patients with LBD experience impairment in memory retrieval, which can be improved by cueing, which is in contrast to memory encoding seen in AD. The early presence of recurrent visual hallucinations is also common in patients with LBD and therefore diagnostically useful. Later in the course of the disease, delusions can be present giving rise to paranoia.

Motor symptoms: Parkinsonian motor signs in LBD are often symmetric, with bradykinesia and gait impairments being more common than resting tremor. Importantly, patients with LBD will often show limited or no response to typical Parkinson's disease pharmacological intervention such as levodopa/carbidopa. LBD patients however do show reduced dopamine transporter activity on single-photon emission computed tomography (SPECT) or positron emission tomography (PET) imaging.

Other Associated symptoms can include: loss of olfaction, autonomic dysfunction (i.e. neurogenic orthostatic hypotension, constipation, neurogenic urinary frequency and urgency), high sensitivity to medications and rapid eye movement sleep behaviour disorder typically reported by a sleep partner as kicking, punching, yelling and acting out their dreams.

Mechanism for targeting Lewy bodies: A potential target for disease-modifying treatment is the enzyme β-Glucocerebrosidase (GCase; gene name GBA1). Glucocerebrosidase (GCase) is a degradative enzyme that resides in a subcellular compartment called the lysosome, and cleaves a neutral glycolipid, glucocerebroside, present in the plasma membrane of most cells. GCase is intimately linked with Parkinson's disease - a Lewy body-related disorder. Being an "asymptomatic carrier" of a GCase mutation is currently the highest genetic risk factor for Parkinson's disease, Parkinson's disease dementia and Lewy Body Dementia, with some studies suggesting up to 1/3 of patients carry mutations. Reductions in GCase activity most likely also play a role in sporadic Parkinson's disease, as these patients have lower levels of GCase in their brain and cerebrospinal fluid, even when they do not carry a mutant GCase allele. 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. Moreover, reducing GCase genetically or pharmacologically 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. Taken together, these findings suggest that increasing GCase levels could be a therapy that addresses the underlying pathophysiology of Lewy body-related disorders such as Lewy Body dementia to modify the course of disease progression.

Background on Ambroxol Studies: Ambroxol is an expectorant that has been available over the counter in more than 50 countries for over 30 years. The Mahuran Lab identified Ambroxol by screening a library of compounds as an agent that stabilizes wild-type (normal) GCase. By stabilizing GCase, Ambroxol is able to markedly increase GCase protein and activity in normal and Gaucher disease fibroblasts at doses of 10 µM. Ambroxol can also increase GCase in normal mouse neuronal cultures to more than 150% of normal at a dose of 30 µM. Ambroxol has good lipophilicity (cLogP = 2.8) and low polar surface area (PSA 58 Å2), predicting good CNS penetration. Unpublished studies performed by ExSAR corporation demonstrate that in single and multiple dose experiments in rats, Ambroxol crossed rapidly into the brain and exhibited brain to plasma concentration ratios of greater than 10 indicating outstanding CNS penetration. In pilot studies, Ambroxol was effective at improving GCase function in humans. In a trial aimed at 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. Ambroxol has also been administered to three Japanese Gaucher disease patients with severe neurological disease, at 1000-3000 mg/day for 12-31 months. These patients had improvements in seizure frequency and neurological symptoms; one patient regained the ability to sit unsupported and to walk. Importantly, in a recent study of Ambroxol in 18 Parkinson's disease patients, the authors observed a 35% increase in CSF GCase protein levels at a daily dose of 1260mg with no serious adverse events.

Safety: Ambroxol has an excellent safety record, and has been studied in >15,000 patients in more than 100 trials. Ambroxol is sold over the counter in much of the world as an expectorant at doses of 75-120 mg/day. Ambroxol is considered so safe that it is approved for intravenous use in pregnant women at a dose of 1000 mg/day IV (15 mg/kg) to improve fetal lung maturation before preterm delivery. Clinical trials in more than 390 pregnant women have been performed using doses up to 3000 mg in one day and 1300 mg/day for up to 33 days. Critically ill neonates have also been given doses as high as 30 mg/kg for respiratory distress. The fact that Ambroxol has been used at very high doses in pregnant women and neonates suggests that these doses are safe.

Project Summary: This is a proof of principle randomized, placebo-controlled, double-blind study investigating whether the medication Ambroxol (dose: 1350mg/day) is safe, well tolerated and will raise GCase levels in the CSF and plasma of patients diagnosed with probable Lewy Body Dementia (LBD). Additionally, increased levels of the enzyme β-glucocerebrosidase (GCase) and lower the levels of the protein α-synuclein have both been shown to improve cognition in mouse models. Therefore, as secondary aim will be to investigate whether Ambroxol will improve cognitive symptoms in patients diagnosed with Lewy Body Dementia (LBD). This will be a 52-week trial of Ambroxol in 15 patients diagnosed with probable LBD. Patients will undergo clinical, neuropsychological and neuroimaging assessment throughout the study to assess changes. Ambroxol has never been examined in LBD patients; however, trials of pharmacological chaperone therapy have been suggested in a recent review in the journal "The Proceedings of the National Academy of Sciences". A successful outcome of this trial will greatly accelerate the development of therapeutics for neurodegenerative disease. This proposal outlines a completely novel pharmacological target for LBD, namely the GCase enzyme. The investigators also propose a completely novel therapy using the drug Ambroxol, an agent considered safe enough to give to pregnant women, which has improved GCase function in pilot studies in humans. This strategy could stabilize the underlying pathology of Lewy Body Dementia; it might allow patients to get better. Furthermore, repurposing an existing medication with excellent safety record will greatly shorten the time to bring this therapy to general use, allowing us to leapfrog the normally decades-long drug development process.