Discoveries into molecular mechanisms, risk factors — especially genetic — and advances in potential and repurposed therapies for Parkinson’s disease over the last 20 years are reason to believe that major breakthroughs await the next two decades, a review article by two researchers states.
The review article, “Therapies to Slow, Stop, or Reverse Parkinson’s Disease” was published in a supplement of the Journal of Parkinson’s Disease.
The development of better laboratory models, especially animal models that capture the slowly progressive nature of Parkinson’s, together with data resulting from scientific research and early clinical trials “strongly justifies sending this message of hope,” the authors write, explaining that the mechanisms underlying this neurodegenerative disease are gradually being deciphered.
The researchers, Tom Foltynie at University College London and J. William Langston at Stanford University, highlighted possible therapies that are most likely to emerge as disease-modifying treatments for Parkinson’s, despite the considerable challenges that remain in bringing a treatment successfully through a clinical study.
Based on the knowledge that mutations in the LRRK2 gene are one of the most common genetic causes of Parkinson’s disease, researchers have focused on therapies that can inhibit (block) LRRK2. But these efforts have been hindered by lung complications (lung toxicity) in primates exposed to inhibitor candidates, and scientists are exploring more selective ways of delivering such medications to avoid toxicity.
Questions also remain as to whether the brain is the prime target for LRRK2 activity, with some evidence pointing to the gut as well.
Treatments targeting the GBA gene, which encodes an enzyme called beta-glucocerebrosidase, may be relevant for people with sporadic forms of the disease in whom low levels of beta-glucocerebrosidase have been observed. This enzyme plays an important role in the mobilization and processing of alpha synuclein, which is low in GBA mutation carriers.
Ambroxol, an approved treatment for respiratory diseases associated with sticky or excessive mucus, is known to boost beta-glucocerebrosidase activity. However, it remains to be determined if Parkinson’s patients can tolerate the dose required for this therapy to reach the central nervous system. Other molecules that work in the body in ways similar to Ambroxol have been identified.
Since most available Parkinson’s therapies aim to ease motor symptoms, targeting non-motor features like cognition, speech, gait, balance difficulties and autonomic failure (or problems with regulating blood pressure and other process controlled by the autonomic nervous system) is important, given that many of these may precede motor onset. This could allow treatments to be started earlier, possibly delaying or preventing the onset of motor symptoms.
One approach to slowing disease progression gaining interest is that of “repurposing” medications already approved for diseases other than Parkinson’s. Preclinical studies found that type 2 diabetes medications — scientifically known as glucagon-like peptide 1 (GLP-1) receptor agonists — protect against alpha-synuclein-induced neurodegeneration. Various ongoing Phase 2 trials are assessing the effect of various GLP-1 receptor agonists (liraglutide, lixisenatide and semaglutide ) in Parkinson’s disease patients — NCT03659682, NCT03439943, NCT02953665). Plans for a Phase 3 trial of exenatide, another GLP-1 agonist, are underway.
Medicines used to treat primary biliary cirrhosis (an autoimmune disease of the liver; ursodeoxycholic acid), chronic myelocytic leukemia (nilotinib) and asthma (salbutamol and clenbuterol) also hold promise for Parkinson’s as they seem to contribute to nerve cell survival, eliminate toxic alpha-synuclein buildup, and modulate alpha-synuclein production, respectively.
Various studies have linked alpha-synuclein-induced neuroinflammation to Parkinson’s disease. As such, immunomodulatory therapies can be a treatment option. Evidence suggests a person’s immune system can react to toxic forms of alpha-synuclein and trigger an inflammatory reaction, which can speed disease progression. Azathioprine and sargramostim, both immunomodulatory medications, are being considered as potential candidates for slowing Parkinson’s progression.
A link between metabolism products generated by gut bacteria and brain inflammation has also been identified, and scientists might look to manipulate the gut microbiome — the trillions of microorganisms and their genetic material that live in the intestinal tract — in Parkinson’s patients, study the effects of such manipulation on the neurodegeneration process.
Lastly, the authors highlighted the possible use of nanoparticles in the disease context, as these molecules have been shown to block the formation of toxic alpha-synuclein clusters and actively work against their aggregation. In theory, nanotechnology might hold the potential to accurately target Parkinson’s-related neuropathology.
“We now have better understanding of the processes involved in PD [Parkinson’s disease] degeneration and can therefore have greater confidence that laboratory data and positive results from early clinical trials will ultimately translate to therapies that slow down PD progression,” Foltynie and Langston said in a news release.
“There are currently no drugs that have been proven to slow down PD progression. Demonstrating that one or several of the candidate approaches is successful will lead to a frameshift in patient care,” they added. “Useful cooperation and coordination between investigators around the globe are significantly accelerating the path towards discovering agents that may slow, stop, or even reverse the progression of PD.”
Their review concluded by stressing the possible importance of combination treatments in future clinical trials.
“It is tempting to speculate that the future patient may be recruited into research reminiscent of the current state of play in HIV/cancer fields, e.g., where following genotyping/ microbiome testing, they are either given the curative enzyme corrective therapy or randomised to receive combination therapies rather than any/each of these alone,” they wrote.
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