Preclinical Testing of a Translocator Protein Ligand for the Treatment of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal paralytic disease that affects adults. There is, at present, no effective treatment for this devastating disorder. Most cases of ALS occur spontaneously (sporadic ALS), although sometimes it can be inherited. Mutations in the antioxidant enzyme superoxide dismutase-1 (SOD1) are a cause of inherited ALS, and animals that express mutated SOD1 mimic the hallmarks of ALS, including the loss of the specific brain cells, motor neurons. The search for agents that can slow or even stop ALS has always been paramount. However, this quest has been hampered by a lack of screenable disease models with clear relevance to the human condition and no known existing agents that prevent motor neuron loss. Previously, our group and others made the following groundbreaking observations: motor neurons can be taken from mouse brains or spinal cords; and, when these motor neurons are placed in a dish with non-neuronal supportive brain cells called astrocytes from animals that express mutant SOD1, the motor neurons die in a strikingly similar fashion to how they die in ALS. Recently, even more fascinatingly, we demonstrated that astrocytes taken directly from sporadic ALS patient fresh autopsied tissues are killing motor neurons, recapitulating in a dish with human cells the most common form of this human disease. Our preliminary data demonstrated that the demise of motor neurons in both these mouse and human ALS models can be prevented by the addition of a small molecule named PK11195 (PK), which binds to the translocator protein (TSPO) known to be involved in cholesterol metabolism, brain inflammation, and mechanisms of cell death. This striking prevention of motor neuron degeneration in our models of both familial and sporadic ALS strongly suggest that PK could be a drug candidate for the vast majority of ALS forms, but also its efficacy on both mouse and human cells predicts for a more reliable clinical application to humans compared to previous targets identified solely with mouse cells. Together with our promising preliminary data, the fact that PK freely permeates the blood-brain barrier and is already approved for imaging in humans to follow brain inflammation makes it an ideal candidate for preclinical testing for ALS.

The objective of our research is twofold. First in Specific Aim 1, we propose to elucidate whether the protective molecule PK is acting on motor neurons directly to prevent their death or alternatively is acting on astrocytes to suppress their toxicity to motor neurons. This work is essential to discover the mechanism of PK protective effect and build-up on this knowledge for future drug development of molecules even more efficient than PK itself. In addition, we will investigate whether PK beyond preventing the death of the motor neurons is also preserving the integrity of their long processes that are traveling the body to contract our muscles. This point is very important as these long processes named axons are also degenerating in ALS, causing the progressive paralysis of patients. Accordingly, the preservation of axons is an equally important target for achieving real clinical motor benefit. Second, our study will rigorously assess the therapeutic value of PK in the best preclinical animal model of ALS available so far. For this, in the first phase of Specific Aim 2 we will confirm which doses of PK we will need to administrate orally to the ALS mice to reach brain and spinal cord levels allowing PK to effectively bind on TSPO. We will also make sure that the mice are tolerating the drug treatment well. Then, based on the optimal conditions of PK administration, the second phase of Specific Aim 2 will determine whether PK can mitigate the expression and progression of the paralysis in the transgenic mutant SOD1 mouse model of ALS using a comprehensive set of motor tests and morphological investigations to assess the protection of motor neurons and the preservation of their axons. All of the techniques, reagents, and expertise necessary for the performance of this project are available in the laboratories of the research teams.

The clinical benefit of this project would be to prevent motor neuron loss and paralysis and provide patients with a significantly improved quality of life through better motor function as well as the potential for a longer life due to the prevention of motor neuron death. Risks for this project are mitigated by using an already highly developed drug (near Investigational New Drug-ready). The assembly of a research team with many years of preclinical animal model experience that encompasses all of the methodologies that will be employed in this project makes us well-positioned to execute this project. Indeed, the combination of the Motor Neuron Center's world-renowned expertise in ALS and motor neuron biology coupled with the long-standing expertise of the Guilarte group that has been a pioneer in the validation and application of TSPO as a biomarker of brain injury and inflammation in animal models and in humans make this team uniquely qualified to direct this collaborative translational research program. We are confident that the comprehensive set of investigations that we are proposing can play a decisive role in identifying a brain penetrant molecule with protective properties that are directly relevant to ALS. The present proposal may thus have far-reaching implications for the treatment of ALS and may be useful for other neurodegenerative diseases such as Parkinson and Alzheimer's disease in which glial inflammation is also a prominent feature.