A common diabetes drug treating Parkinson’s disease

A new investigational drug, originally developed for type 2 diabetes, is being readied for human clinical trials in search of the world's first treatment to impede Parkinson's disease progression.

Parkinson's (PD) is the second most common neurodegenerative disorder. The connection between type 2 diabetes (T2DM) and PD was discovered in 1993, when PD patients with co-existing T2DM had worse motor symptoms and response to therapy. Dopaminergic neurons promote eating behaviour in hypoglycaemic states, mediated via insulin receptors in the substantia nigra, because dopaminergic neuronal loss affects glycaemic control. Thus, T2DM patients are more likely to acquire PD than people without diabetes. Excess glucose in the brain, as found in uncontrolled T2DM, may interact randomly with surrounding proteins and interfere with their function. 

These interactions also result in toxic end products promoting inflammation and α-synuclein clustering, both of which are PD characteristics. Over a 12-year period, retrospective data (N=8,190,323) showed that T2DM responders had considerably greater PD rates when compared to those without diabetes. The rise was significantly more pronounced among individuals with complex T2DM and those aged 25-44.  

Exenatide: Overview and Mechanism of Action

Exenatide is a synthetic form of exendin-4, a naturally occurring protein identified in the saliva of the Gila monster (poisonous lizard endemic to the Southwest US) by Dr. Eng in the early 1990s. In humans, the chemical is produced after a meal to increase insulin production, decreasing blood sugar. GLP-1 degrades fast in humans, and its benefits are short-lived. However, investigations have shown effects of exendin-4 continue longer in people. 

This finally led to FDA clearance in 2005, when the product was sold as ByettaTM. Its current indications are for the treatment of balancing glucose levels in T2DM with or without additional oral hypoglycemic medications. This glycaemic control is an analogue of human GLP-1, used in T2DM treatment, either alone or in conjunction with other antidiabetic medications. 

Exendin-4's neuroprotective characteristics may aid in rescuing degenerating cells and neuron protection. Because T2DM and PD are linked, researchers want to explore its effectiveness as a PD therapy. Patients treated with exenatide for one year (in addition to standard medication) experienced less deterioration in motor symptoms when tested without medication compared to the control group. 

Research on Exenatide as a Potential Parkinson's Disease Therapy

21 patients with intermediate PD were assessed over a 14-month period, and their progress was compared to 24 other people with Parkinson's who served as controls. Exenatide was well accepted by participants, albeit some individuals complained about weight loss. Significantly, exenatide-treated participants improved their PD movement symptoms, while the control patients continued to deteriorate. 

The researchers investigate exenatide, a possible PD therapy, in an upcoming clinical study, lending support to the repurposing of diabetes drugs for Parkinson's patients. This research adds to the evidence for a phase 3 clinical trial of exenatide for PD patients. 

Data on 100,288 T2DM revealed that people using two types of diabetic medications, GLP-1 agonists and DPP4-inhibitors, were less likely to be diagnosed with Parkinson's up to 3.3 years follow-up. Those who used GLP-1 agonists were 60% less likely to acquire PD than those who did not. The results revealed that T2DM had a higher risk of Parkinson's than those without diabetes, although routinely given medicines, GLP-1 agonists, and DPP4-inhibitors seemed to reverse the association. 

Furthermore, a 2-year follow-up research indicated individuals previously exposed to exenatide displayed a substantial improvement in their motor characteristics 12 months after they ceased taking the medication. However, this experiment was an open-label research so the gains may be explained by a placebo effect. 

The research adds to the evidence that exenatide may assist to prevent or treat PD, perhaps by altering the course of the illness rather than just lowering symptoms. Other risk factors for PD should be considered by clinicians when prescribing T2DM drugs, although further study is required to clarify clinical significance. 

Findings from Clinical Trials and Studies

Based on these findings, the UCL team broadened their investigation and conducted a more extensive, double-blind, placebo-controlled experiment. The findings establish the groundwork for a new generation of PD medicines, but they also confirm the repurposing of a commercially existing therapy for this illness. Patients were randomly randomised (1:1) to receive exenatide 2 mg or placebo subcutaneous injections once weekly in addition to their current medication for 48 weeks, followed by a 12-week washout period. Web-based randomisation was used, with a two-stratum block design depending on illness severity. Treatment allocation was concealed from both patients and investigators. 

The main outcome was the adjusted difference in the motor subscale of the Movement Disorders Society Unified Parkinson's Disease Rating Scale after 60 weeks in the realistically defined off-medication condition. Six major adverse events occurred in the exenatide group and two in the placebo group, but none were deemed to be connected to the research treatments in either group. It is unclear if exenatide alters the underlying illness mechanism or causes long-term clinical consequences. 

Implications and Future Directions 

Indeed, the UCL study showed that exenatide decreases deterioration compared to a placebo. However, participants reported no change in their quality of life. The study team would broaden their study to include a broader sample of people from several locations. Because PD proceeds slowly, longer-term trials might provide a better understanding of how exenatide works in these responders. Overall, findings suggest that gathering data on this class of medications should be the topic of additional inquiry to evaluate their potential. Exenatide is also being studied to see whether it might postpone the onset of levodopa-induced problems (e.g., dyskinesias). Furthermore, if exenatide works for Parkinson's, why not for other neurodegenerative illnesses (Alzheimer's, amyotrophic lateral sclerosis, Huntington's disease, multiple sclerosis) or neurological diseases (including cerebrovascular disorders, traumatic brain injury...)?

Exenatide has been FDA-approved for diabetes for many years and has a good track record, but it does have some adverse side effects in Parkinson's patients, namely gastrointestinal difficulties (nausea, constipation). Exenatide as a prospective PD therapy is an example of medication repurposing or repositioning, an essential method for bringing novel therapies to patients in a timely and cost-effectively. However, further research is required, so it will be many years before a new therapy is licenced and available. Drug repurposing, or using authorised medicines for one ailment to treat another, opens up new paths for Parkinson's therapeutic development. 

Conclusion

Exenatide shows potential as a therapy for Parkinson's disease (PD). Studies have shown that exenatide may help improve motor symptoms and slow down the progression of PD. However, further research and clinical trials are needed to fully understand its effectiveness and long-term effects. The findings also suggest that repurposing existing medications, like exenatide, could provide new avenues for developing PD therapies. While exenatide shows promise, it will likely be many years before it is licensed and widely available as a PD treatment.

PROJECT GALLERY IMAGES DESCRIPTION

Figure 1- The use of GLP-1 is beyond diabetes treatment. Nineteen clinical studies found that GLP-1 agonists can improve motor scores in Parkinson's Disease, improve glucose metabolism in Alzheimer's, and improve quality of. They can also treat chemical dependency, improve lipotoxicity, and reduce insulin resistance. However, adverse effects are primarily gastrointestinal. Thus, GLP-1 analogues may be beneficial for other conditions beyond diabetes and obesity.

Figure 2- Potent GLP-1 agonists suppress appetite through a variety of mechanisms, including delayed gastric emptying, increased glucose-dependent insulin secretion, decreased glucagon levels, and decreased food ingestion via central nervous system effects. Short-acting agents, including exenatide, primarily function by impeding gastric evacuation, thereby leading to a decrease in postprandial glucose levels. On the contrary, extended-release exenatide and other long-acting agonists (e.g., albiglutide, dulaglutide) exert a more pronounced impact on fasting glucose levels reduction via their mechanism of action involving the release of insulin and glucagon. The ineffectiveness of long-acting GLP-1 receptor agonists on gastric evacuation can be attributed to the development of tolerance to GLP-1 effects, which is regulated by parasympathetic tone alterations.

Figure 3- Illustrated is the cross-communication with insulin receptor signalling pathways and downstream effectors. Biomarkers can be derived from the formation and origin of extracellular vesicles, which indicate the initial inward budding of the plasma membrane. An early endosome is formed when this membrane fuses; it subsequently accumulates cytoplasmic molecules. As a consequence, multivesicular bodies are generated, which subsequently fuse with the plasma membrane and discharge their constituents into the extracellular milieu. Akt denotes protein kinase B; Bcl-2 signifies extracellular signal-related kinase; Bcl-2 antagonist of death; Bcl-2 extra large; Bcl-XL signifies Bcl-2; Bim signifies Bcl-2-like protein 11; cAMP signifies cyclic adenosine monophosphate; CREB signifies cAMP response element-binding protein; Erk1/2 signifies extracellular signal-related kinase IDE, insulin-degrading enzyme; IL-1α, interleukin 1α; IRS-1, insulin receptor signalling substrate 1; MAPK, mitogen-associated protein kinase; mTOR, mechanistic target of rapamycin; mTORC1, mTOR complex 1; mTORC2, mTOR complex 2; NF-kB, nuclear factor–κB; PI3-K, phosphoinositide 3-kinase; PKA, protein kinase; FoxO1/O3, forkhead box O1/O3, forkhead box O1/O3; GRB2, growth factor receptor-bound protein 2; GSK-3β, 

Written by Sara Maria Majernikova

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