Translational molecular imaging and drug development in multiple sclerosis

Abstract

Multiple sclerosis (MS) is a chronic inflammatory neurodegenerative disorder that typically affects young adults and is primarily characterized by demyelinating lesions in the central nervous system (CNS). According to the Revised McDonald Criteria, the clinical diagnosis of MS can be established based on a combination of clinical observations, the presence of focal lesions in at least two distinct CNS areas on magnetic resonance imaging (MRI) and the detection of specific oligoclonal bands in the cerebrospinal fluid. Conventional MRI remains a cornerstone of MS diagnosis and disease monitoring, providing high-resolution assessments of lesion burden and brain atrophy. In addition, advanced MRI methods are increasingly applied in research settings to probe myelin integrity, iron deposition, and biochemical changes, with the potential to complement established diagnostic workflows in the future. Despite remarkable advances in the management of MS over the past two decades, complex differential diagnoses and the lack of effective imaging tools for therapy monitoring remain major obstacles, thus channeling the development of innovative molecular imaging probes that can be harnessed in clinical practice. Indeed, positron emission tomography (PET) has a significant potential to advance the contemporary diagnosis and management of MS. Given the solid body of evidence implicating myelin dysfunction in the pathophysiology of MS, myelin-targeted imaging probes have been developed, and are currently under clinical evaluation for MS diagnosis and therapy monitoring. In parallel, ligands for the 18 kDa translocator protein (TSPO) and the cannabinoid receptor type 2 (CB2R) have been employed to capture neuroinflammatory processes by visualizing microglial activation, while other tracers allow the assessment of synaptic integrity across various disease stages of MS. Further, PET probes have been employed to delineate the role of activated microglia and facilitate the assessment of synaptic dysfunction across all disease stages of MS. This review discusses the challenges and opportunities of translational molecular imaging by highlighting key molecular concepts that are currently leveraged for diagnostic imaging, patient stratification, therapy monitoring and drug development in MS. Moreover, we shed light on potential future developments that hold promise to advance our understanding of MS pathophysiology, with the ultimate goal to provide the best possible patient care for every individual MS patient.

Keywords: demyelination; drug development; multiple sclerosis; neuroinflammation; positron emission tomography (PET); tracer development; translational molecular imaging.

Figure 1

Clinical presentation and biomarker-guided diagnosis of multiple sclerosis (MS). A. The clinical manifestation of MS depends on the location and severity of lesions, involving a myriad of possible symptoms that affect several organs and ultimately lead to disability. B. Diagnostic imaging with T1-weighted (left panel), T2-weighted (middle panel) MRI and 18F-florbetapir PET (right panel) can be used to detect damaged white matter lesions (yellow arrows). Upper and lower rows represent examples from two distinct patients with relapsing-remitting MS according to the revised McDonalds criteria . Figure 1B, Adapted with permission from Elsevier, Zhang et al., doi: 10.1016/j.eclinm.2021.100982, copyright 2021. C. The diagnosis of MS is supported by laboratory findings, including the presence of specific oligoclonal bands in the CSF following lumbar puncture. Abbreviations: CSF, cerebrospinal fluid; IgG, immunoglobulin G; MRI, magnetic resonance imaging; PET, positron emission tomography; PL, plasma.

Figure 2

Simplified model of the pathophysiology in multiple sclerosis (MS). Pathophysiological hallmarks include (1) an impaired blood-brain barrier function, leading to infiltration of the central nervous system (CNS) by lymphocytes and macrophages. (2) Crosstalk between T and B lymphocytes prompts (3) the production of antibodies against oligodendrocyte antigens. (4) T cells interact with microglia, prompting (5) microglial activation and cytokine release. (6) Chronic inflammation ultimately leads to (6) axonal demyelination and the formation of glial scars by reactive astrocytes that are believed to exert protective functions by isolating damaged tissue. Macrophages are recruited to MS lesions, contributing to the phagocytosis of myelin debris - a process that triggers the release of pro-inflammatory cytokines and reactive oxygen species (ROS). (7) Axonal degeneration in MS lesions is driven by (8) activated microglial cells and macrophages.

Figure 3

Translational molecular imaging concepts in multiple sclerosis (MS). Selected biological targets and processes that have been leveraged for positron emission tomography (PET) imaging in MS. PET can be used to improve diagnostic imaging and to facilitate drug discovery & development, and several probes have been successfully developed to non-invasively visualize myelination/demyelination by myelin-binding tracers and probes that image the potassium (K+) channel, which is exposed upon demyelination. Neuroinflammation imaging has traditionally relied on the visualization of biological targets that are overexpressed in activated microglia, including the 18-kDa translocator protein (TSPO), reactive oxygen species (ROS), cyclooxygenase (COX) enzymes, the cannabinoid type 2 receptor (CB2), colony stimulating factor 1 receptor (CSF1R) and purinergic receptors, P2X7 and P2Y12. Attempts to visualize the adaptive immune response have channeled the development of PET tracer that bind to CD19 and CD20 on B cells as well as probes that preferentially accumulate in T cells via a retention mechanism that involves binding to deoxyguanosine and deoxycytidine kinases (dGK and dCK). Neurodegeneration can be visualized by targeting glucose metabolism and mitochondrial complex I (MC-I), while synaptic integrity can be assessed with probes targeting the synaptic vesicle glycoprotein 2A (SV2A) or GABA receptors.

Figure 4

Interplay between imaging and disease modifying therapies (DMTs) in multiple sclerosis (MS). A. Mechanistic landscape of current and emerging (in blue colour) DMTs in MS. Primary sites of action of established and emerging DMTs (highlighted in blue) are shown across the MS disease continuum, spanning peripheral immune modulation, leukocyte trafficking and central nervous system (CNS) infiltration, microglial modulation, and remyelination therapy. B cell-depleting therapies include approved anti-CD20 monoclonal antibodies such as ocrelizumab, ofatumumab, rituximab, and ublituximab, as well as investigational anti-CD19 approaches (inebilizumab and CD19 chimeric antigen receptor T (CAR-T) cells). Bruton's tyrosine kinase (BTK) inhibitors such as tolebrutinib, evobrutinib, and fenebrutinib modulate signaling in B cells and myeloid lineage cells and are currently in late-stage development. T cell-modulating agents, including interferon-β (IFN-β), glatiramer acetate, fumarates (e.g. dimethyl fumarate), and teriflunomide, act by modulating the balance of pro- and anti-inflammatory T cell subsets, limiting T cell activation, and reducing the proliferation of lymphocytes. Sphingosine-1-phosphate (S1P) receptor modulators - including fingolimod, siponimod, ozanimod and ponesimod - block lymphocyte egress from secondary lymphoid tissues, thereby limiting CNS infiltration, while natalizumab prevents immune cell entry by inhibiting α4β1 integrin-mediated adhesion at the blood-brain barrier (BBB). DMTs such as cladribine, alemtuzumab, and mitoxantrone exert broader cytotoxic effects on proliferating immune cells. Within the CNS, masitinib and ibudilast inhibit the activation of microglia and mast cells, while temelimab, a monoclonal antibody targeting the envelope protein of the human endogenous retrovirus-W (HERV-W ENV), reduces innate immune activation and hold potential to preserve myelin integrity. Clemastine fumarate, a muscarinic receptor antagonist, enhances remyelination by promoting differentiation of oligodendrocyte precursor cells (OPCs), while metformin restores the regenerative capacity of aged OPCs. Elezanumab, an anti-repulsive guidance molecule-a (anti-RGMa) antibody, facilitates axonal regeneration and remyelination, and BTK inhibitors may additionally modulate CNS-resident microglia and macrophages. Pathways that can be probed with molecular imaging are denoted by a red star. Collectively, the therapeutic landscape of MS is rapidly evolving toward CNS-penetrant, pathway-specific agents that not only suppress peripheral inflammation but also directly modulate CNS disease processes and promote tissue repair. B. Receptor occupancy of PIPE-307. PIPE-307 displays dose-dependent M1 receptor occupancy in mouse brain following oral administration, with an ED₅₀ of 0.4 mg/kg. Total and nonspecific binding were determined using 3H-PIPE-307 in forebrain and cerebellum, respectively. Time course studies indicate that 30 mg/kg achieves rapid, full occupancy with ~50% occupancy persisting at 16 h, while 3 mg/kg maintains ~50% occupancy for approximately 10 h. This study exemplifies how tritiated radioligands can be used to quantify ex vivo target engagement and inform dose selection during early drug development. Figure. 4B was reprinted with permission from National Academy of Sciences, Poon et al. copyright (2024) doi: 10.1073/pnas.2407974121. C. Brain distribution of TSPO ligand 18F-DPA-714. Following intravenous administration of 304 MBq 18F-DPA-714, brain radioactivity was visualized at early (0-10 min) and late (30-90 min) post-injection time points. Parametric distribution volume ratio (DVR) maps were generated using the cerebellum as a reference region. This study illustrates the use of PET imaging to quantify TSPO expression as a surrogate for microglial activation in humans. Red star highlights imaging targets leveraged in MS drug development. Figure. 4B was reprinted with permission from Elsevier, Arlicot et al. copyright (2012) doi: 10.1016/j.nucmedbio.2011.10.012.