Pathfinder: An experimental medicine study of the 18 kD translocator protein as a novel neuroimmunodulatory target for multiple sclerosis

Multiple sclerosis (MS) involves inflammation in the brain caused by lymphocytes that that circulate in the blood, by macrophages derived from another set of blood cells and by microglia, which are the brain's own resident inflammatory cells. Although treatments developed to date have focused on better controlling lymphocyte activity, microglia (and macrophages, which share some key features of microglia) also play a major role in the inflammation associated with relapses. They also may be important in driving the neurodegeneration associated with progressive forms of the disease. Our group has discovered, for example, that disability in secondary progressive MS is greater in people with higher levels of microglial activation in the brain. However, activated microglia are not directly modulated by current approved medicines for MS.

The "18kD Translator Protein" or "TSPO" is a natural molecule richly expressed in microglia and macrophages. Studies by many scientists using cells in a dish or animal models of diseases suggest that chemicals (e.g., some related to common anti-anxiety drugs like valium) binding to TSPO reduce inflammation. This suggests that TSPO may provide a new way of modulating inflammation associated with MS and, as molecules developed to interact with TSPO all can penetrate the brain, have a unique action on microglia. This possibility has never been tested in human studies.

To address this, we are proposing a comprehensive series of experiments in humans and with human cells or tissue that are intended to evaluate TSPO as a novel immune modulatory target for treatment of people with MS. We are focusing on secondary progressive MS because it is a serious, progressive form of the disease associated with high levels of brain microglial activation that cannot be well treated with current approved medicines. Our study will involve studies of brain tissue (donated to the MS Society Brain Bank by people with MS) to see whether TSPO is increased in microglia that are producing the kinds of chemical signals that damage nerve cells and myelin. We will study the inflammatory responses of human microglia grown in a dish when they are treated with molecules that modulate TSPO functions. Finally, under very carefully controlled conditions, we will administer the safe TSPO binding molecule XBD173 to healthy volunteers and to people with MS daily for a week to understand whether immune responses really can be changed with administration of this kind of molecule. Multiple measures of inflammation in the blood and in the brain will be used. Relatively new magnetic resonance imaging (MRI) and positron emission tomography (PET) brain scanning methods will be further validated for this application and then used as an additional tool in the healthy volunteers and people with MS to provide information more directly about the effects on brain inflammation.

Our interdisciplinary clinical study will bring together a broad range of experts in drug development for MS, neuroimmunology and neuroimaging. It will provide a first test of the hypothesis that a TSPO binding molecule can modulate the inflammatory responses of microglia in humans, help us to discover how to monitor any effects that such molecules might be having and begin to tell us how they work.

We do not believe that XBD173 itself can be used as a treatment, but many other molecules have been developed that should work in a similar way. If our study is successful, we intend to select and further develop one of these other molecules for use as a potential combination treatment with more traditional MS drugs. Together, a molecule modulating microglia and one of the currently approved disease modifying drugs that has a major effect on lymphocytes could provide a more powerful way of treating not just relapses, but also the progressive forms of MS.

Within several years of diagnosis, disability progression in the absence of relapses is observed in most people with multiple sclerosis (secondary progressive MS, SPMS). Current disease-modifying therapies have had little impact on SPMS, therapy for which is a major unmet medical need in neurology. We (and others) hypothesise that therapeutic down- regulation of chronically activated, pro-inflammatory brain microglia will slow or stop the neurodegeneration responsible for progressive forms of MS. However, safe and effective, targeted immunotherapy for microglia has not been identified to test this hypothesis in the human disease.
The 18kd Mitochondrial Translocator Protein (TSPO) is highly expressed in microglia and macrophages. There is an extensive literature documenting the innate immune modulatory effects mediated by TSPO in isolated cells and in a variety of acute and more chronic animal disease models. We hypothesise that TSPO ligands constitute a novel class of immune modulators acting primarily through effects on pro-inflammatory microglia (and macrophages). We propose now to test this more fully with human experimental medicine for the first time.
Previously, we discovered the genetic basis for variable TSPO interactions of ligands in the human population and have extensive experience with the pharmacology of this target. We now will test for immunomodulation by a TSPO ligand, XBD173, in humans, in people with MS and in human immune cells.
While there are 11 licensed medicines currently available for treating relapses in MS, these disease-modifying drugs primarily target the adaptive immune system (T- and B-cells). With validation of TSPO as a novel immunomodulatory target through our experimental medicine studies, we intend to progress towards development of novel TSPO ligands (with improved pharmacological characteristics relative to those available now) for therapeutic modulation of innate immune responses in progressive forms of MS.

TSPO radioligands are used with PET as measures of CNS inflammation. Results from our experiments (particularly the neuropathological studies) will contribute to enhance confidence in the interpretation of TSPO PET in patients with MS. Overall, our project will contribute to better understanding and validation a novel therapeutic target for modulation of brain microglial inflammation contributing to neurodegeneration in MS. A broader future impact can be expected, as high TSPO expression and evidence for pathological microglial activation also has been found in Alzheimer's disease, Parkinson's disease, traumatic brain injury and other disorders.

The applicants are experienced and committed to impact focused research. For example, the PI was in the pharmaceutical industry for almost 9 years leading groups in a variety of areas, including development of a medicine for MS. Nicholas was one of the co-PIs who designed and implemented the recent Simvastatin trial for secondary progressive MS, one of the first positive progressive MS studies in recent years and perhaps the first adequately powered Phase II MS "success" with a low cost, a re-purposed drug. As an academic and as Chief Scientific Officer of Imanova, Ltd., Gunn regularly engages with industry for experimental medicine and early phase trials in this area.

A preliminary patent review has not identified current patents covering inflammatory modulation by TSPO ligands. With validation of TSPO as a target for MS, we would work first to identify or discover a molecule with most ideal pharmacological characteristics. Multiple approaches will be employed, including new molecule development, working collaboratively with resources of the Imperial College Drug Discovery and Development Unit. Search for the new molecules can be accelerated using data from the current (wide) range of ligands, a recent crystal structure of TSPO and insight into molecular mechanism of action derived from this study.

With identification of a novel molecule, we then would hope to complete a proof of principle, e.g., through the MRC DPFS programme, to establish value for a specific molecule and then seek external partnership(s) for its further clinical development. Imperial College's technology commercialization company, Imperial Innovations (which listed on the London Alternative Exchange and has raised more than £350 million in funding for new enterprises), will support the investigators in protecting IP, defining the "critical path" in development of the business case for development and in seeking partnerships to enable it.

While not intended to be exhaustive, additional aspects of our project also have potential for impact:

-Detailed characterization of TSPO in the ex vivo brain and the relationship between its expression and inflammatory pathology will have immediate impact on the development of novel TSPO PET ligands for medical or drug development applications (e.g., GE Healthcare)

-Additional characterization of expression differences between people with MS and healthy volunteers and the relationship between expression changes in peripheral blood and inflammatory state will provide new marker candidates for inflammation or disease state. Potential impact from this could arise from development of a simple 'omics based test for spin out as a diagnostic/monitoring tool, e.g., based on the new "lab on a chip" technology developed by one of Imperial's "spin outs" (DNA Electronics Ltd., founded by Prof. C. Toumazou).

-Always, an integral aspect of this work will involve engaging people with MS and healthy volunteers. The project will raise awareness of therapeutic concepts and new potential paths to development.