Multiple nivolumab-induced CNS demyelination with spontaneous resolution in an asymptomatic metastatic melanoma patient

Anti-PD-1 cancer immunotherapy induces central nervous system immune-related adverse events by microglia activation

Cancer treatment with anti-PD-1 immunotherapy can cause central nervous system immune-related adverse events (CNS-irAEs). The role of microglia in anti-PD-1 immunotherapy-induced CNS-irAEs is unclear. We found that anti-PD-1 treatment of mice caused morphological signs of activation and major histocompatibility complex (MHC) class II up-regulation on microglia. Functionally, anti-PD-1 treatment induced neurocognitive deficits in mice, independent of T cells, B cells, and natural killer cells. Instead, we found that microglia mediated these CNS-irAEs. Single-cell RNA sequencing revealed major transcriptional changes in microglia upon anti-PD-1 treatment. The anti-PD-1 effects were mediated by anti-PD-1 antibodies interacting directly with microglia and were not secondary to peripheral T cell activation. Using a proteomics approach, we identified spleen tyrosine kinase (Syk) as a potential target in activated microglia upon anti-PD-1 treatment. Syk inhibition reduced microglia activation and improved neurocognitive function without impairing anti-melanoma effects. Moreover, we analyzed CNS tissue from a patient cohort that had received anti-PD-1 treatment. Imaging mass cytometry revealed that anti-PD-1 treatment of patients was associated with increased surface marker expression indicative of microglia activation. In summary, we identified a disease-promoting role for microglia in CNS-irAEs driven by Syk and provide an inhibitor-based approach to interfere with this complication after anti-PD-1 immunotherapy.

Neurologic Complications of Cancer Immunotherapy

OBJECTIVE

Immunotherapeutic approaches have revolutionized cancer treatment with immune checkpoint inhibitors and adoptive T-cell therapy now approved to treat a variety of solid and hematologic malignancies. This article summarizes the distinctive neurologic side effects of these therapies as well as their management.

LATEST DEVELOPMENTS

Neurologic immune-related adverse events are rare but potentially serious complications of immune checkpoint inhibitors. Both peripheral and central nervous system disorders have been described, often necessitating a pause or cessation of immunotherapy. Immune effector cell-associated neurotoxicity syndrome is a potentially serious complication of chimeric antigen receptor T-cell therapy. While symptoms may be mild and self-limited, delirium, encephalopathy, seizures, focal neurologic deficits, and fulminant cerebral edema can be seen. Close neurologic monitoring is imperative. The mainstay of treatment for neurologic complications includes high-dose corticosteroids, although other immunomodulatory strategies may be used in severe or refractory cases.

ESSENTIAL POINTS

The spectrum of neurologic complications of cancer immunotherapy is broad, encompassing both central and peripheral nervous system disorders, indolent as well as fulminant clinical presentations, and wide-ranging severity with variable response to treatment. Early identification and multidisciplinary management are crucial to balance neurologic recovery and antitumor control.

Imaging assessment of toxicity related to immune checkpoint inhibitors

In recent years, a wide range of cancer immunotherapies have been developed and have become increasingly important in cancer treatment across multiple oncologic diseases. In particular, immune checkpoint inhibitors (ICIs) offer promising options to improve patient outcomes. However, a major limitation of these treatments consists in the development of immune-related adverse events (irAEs) occurring in potentially any organ system and affecting up to 76% of the patients. The most frequent toxicities involve the skin, gastrointestinal tract, and endocrine system. Although mostly manageable, potentially life-threatening events, particularly due to neuro-, cardiac, and pulmonary toxicity, occur in up to 30% and 55% of the patients treated with ICI-monotherapy or -combination therapy, respectively. Imaging, in particular computed tomography (CT), magnetic resonance imaging (MRI), and 2-deoxy-2-[¹⁸F]fluoro-D-glucose positron emission tomography/computed tomography (¹⁸F-FDG-PET/CT), plays an important role in the detection and characterization of these irAEs. In some patients, irAEs can even be detected on imaging before the onset of clinical symptoms. In this context, it is particularly important to distinguish irAEs from true disease progression and specific immunotherapy related response patterns, such as pseudoprogression. In addition, there are irAEs which might be easily confused with other pathologies such as infection or metastasis. However, many imaging findings, such as in immune-related pneumonitis, are nonspecific. Thus, accurate diagnosis may be delayed underling the importance for adequate imaging features characterization in the appropriate clinical setting in order to provide timely and efficient patient management. ¹⁸F-FDG-PET/CT and radiomics have demonstrated to reliably detect these toxicities and potentially have predictive value for identifying patients at risk of developing irAEs. The purpose of this article is to provide a review of the main immunotherapy-related toxicities and discuss their characteristics on imaging.

Neurological adverse events of immune checkpoint blockade: from pathophysiology to treatment

PURPOSE OF REVIEW

We review the recent advances in neurological toxicities of immune checkpoint inhibitors, with a focus on underlying pathophysiologic mechanisms and the implications on their therapeutical management.

RECENT FINDINGS

A growing number of cancer patients benefit from immune checkpoint agents and oncologists are increasingly confronted with these novel autoimmune syndromes. During the last years, further progresses have occurred in this field, notably in the identification of specific clinical patterns, such as the association of myasthenic syndrome with myositis and myocarditis, and polyradiculoneuropathies accompanied by cerebrospinal fluid lymphocytic pleocytosis. In addition, recent immune-histological studies improved the understanding of the pathophysiologic mechanisms behind immune-related neurotoxicities.

SUMMARY

Neurological toxicity is rare compared with other organs and systems, but its potential morbidity and mortality requires a prompt management. If there is a consensus for steroids as a first-line treatment, no exhaustive clinical data exist for other treatments. Recent advances in the knowledge of pathophysiological mechanisms (behind these toxicities) should be taken into account for the management of these patients. Drugs targeting T-cell mediated inflammation should be preferred in patients who are refractory to steroids, whereas therapies targeting humoral mechanisms should be considered in specific cases associated with autoantibodies such as immune-related myasthenic syndrome.

Neurologic Complications of Immune Checkpoint Inhibitors

The clinical use of cancer immunotherapy with immune checkpoint inhibitors has transformed the management of cancer and added another effective treatment option for different types of malignancies. The blockade of immune checkpoint pathways triggers an enhanced immune response leading to cancer regression but may also lead to autoimmune toxicities or immune-related adverse events, which may involve skin, endocrine, respiratory, gastrointestinal or neurologic manifestations. Clinically relevant neurologic complications involving the central and/or peripheral nervous system affect up to 1% of patients treated with immune checkpoint inhibitors and may be associated with significant morbidity and mortality. Common neurologic complications include aseptic meningitis and encephalitis, hypophysitis, myasthenia, myositis and neuropathies. Neurologic immune-related adverse events after immune checkpoint inhibition should be distinguished from cancer progression or other complications of cancer therapy (e.g. infections). The treatment of neurologic complications may include holding or withdrawing cancer immunotherapy, anti-inflammatory and immunosuppressive therapies with corticosteroids and steroid-sparing agents, immunomodulation with intravenous immune globulin or plasmapheresis and symptomatic treatment (e.g. antiepileptic medications, pain medications).

Enhancing T Cell Chemotaxis and Infiltration in Glioblastoma

Glioblastoma is an immunologically 'cold' tumor, which are characterized by absent or minimal numbers of tumor-infiltrating lymphocytes (TILs). For those tumors that have been invaded by lymphocytes, they are profoundly exhausted and ineffective. While many immunotherapy approaches seek to reinvigorate immune cells at the tumor, this requires TILs to be present. Therefore, to unleash the full potential of immunotherapy in glioblastoma, the trafficking of lymphocytes to the tumor is highly desirable. However, the process of T cell recruitment into the central nervous system (CNS) is tightly regulated. Naïve T cells may undergo an initial licensing process to enter the migratory phenotype necessary to enter the CNS. T cells then must express appropriate integrins and selectin ligands to interact with transmembrane proteins at the blood-brain barrier (BBB). Finally, they must interact with antigen-presenting cells and undergo further licensing to enter the parenchyma. These T cells must then navigate the tumor microenvironment, which is rich in immunosuppressive factors. Altered tumoral metabolism also interferes with T cell motility. In this review, we will describe these processes and their mediators, along with potential therapeutic approaches to enhance trafficking. We also discuss safety considerations for such approaches as well as potential counteragents.

A brief report on incidence, radiographic feature and prognostic significance of brain MRI changes after anti-PD-1/PD-L1 therapy in advanced non-small cell lung cancer

INTRODUCTION

Neurologic immune-related adverse events (nirAEs) are uncommon but potentially lethal complications of immune checkpoint inhibitor (ICI) treatment. However, the incidence, radiographic features and prognostic significance of brain magnetic resonance imaging (MRI) changes after ICI treatment remain largely unknown.

METHODS

Consecutive patients with advanced non-small cell lung cancer (NSCLC) at three participating institutions receiving anti-PD-1/PD-L1 therapy from June 2017 to September 2020 were screened, and those who received brain MRI within 6 weeks before ICI initiation and at least one follow-up brain MRI after ICI treatment were included. Serial brain MRI images were independently reviewed by two experienced radiologists.

RESULTS

With a median follow-up of 13.2 months, 27 (20.0%) of the 135 enrolled patients developed certain kind of brain MRI aberration. The 1-, 2- and 3-year cumulative incidence of brain MRI aberration was 17.1%, 36.3% and 52.2%, respectively. Brain MRI aberration indicative of stroke, mimicking typical white matter lesions and presenting as T2-hyperintensity suggestive of CNS vasculitis or encephalitis, was documented in 11, 9 and 4 patients, respectively. Patients with brain MRI aberration had higher clinical benefit rate (p = 0.030), longer progression-free survival (p = 0.015) and a tendency of improved overall survival (p = 0.054).

CONCLUSIONS

Brain MRI aberrations developed after ICI treatment are not uncommon, and their manifestations vary a lot. Patients developing brain MRI aberrations tended to have better prognosis, which needed to be further investigated.

Drug-related demyelinating syndromes: understanding risk factors, pathophysiological mechanisms and magnetic resonance imaging findings

Some drugs and medications can precipitate immune system deregulations, which might be confused with recurrent demyelinating diseases, such as multiple sclerosis (MS) and neuromyelitis optica spectrum disorders (NMO), exacerbations of an existing disease, neoplastic lesions or other conditions. In this narrative review we describe some of the most relevant drugs and medications associated with iatrogenic demyelination. The anthelminthic agent levamisole is a frequent cocaine adulterant and can precipitate an exacerbated immune response attacking the central nervous system (CNS). High-efficacy multiple sclerosis (MS) drugs might induce a selective CNS immunosuppression, making it susceptible for opportunistic infections that course with demyelination, such as progressive multifocal leukoencephalopathy. Sometimes, the interruption of a high-efficacy drug to treat MS can induce a rapid CNS reentry of lymphocytes, exacerbating demyelinating processes and triggering rebound syndromes. Furthermore, selective cytokines inhibition, such as anti-TNFα agents, might induce an imbalance between cell death and proliferation inducing a paradoxical increase of CNS tumor necrosis factor (TNF), affecting the activity of lymphocytes, microglia and macrophages, triggering aberrant inflammation and demyelination. Immune checkpoint inhibitors are a new class of antineoplastic drugs that enhance the immune response against tumor cells by an upregulation of T-cell activity. However, this hyperactivation of the immune system might be associated with induction of unwanted autoimmune responses. In this paper we review the risk factors, the possible pathological mechanisms and the magnetic resonance imaging (MRI) findings of these drug-related demyelinating syndromes.

Consensus disease definitions for neurologic immune-related adverse events of immune checkpoint inhibitors

Expanding the US Food and Drug Administration-approved indications for immune checkpoint inhibitors in patients with cancer has resulted in therapeutic success and immune-related adverse events (irAEs). Neurologic irAEs (irAE-Ns) have an incidence of 1%-12% and a high fatality rate relative to other irAEs. Lack of standardized disease definitions and accurate phenotyping leads to syndrome misclassification and impedes development of evidence-based treatments and translational research. The objective of this study was to develop consensus guidance for an approach to irAE-Ns including disease definitions and severity grading. A working group of four neurologists drafted irAE-N consensus guidance and definitions, which were reviewed by the multidisciplinary Neuro irAE Disease Definition Panel including oncologists and irAE experts. A modified Delphi consensus process was used, with two rounds of anonymous ratings by panelists and two meetings to discuss areas of controversy. Panelists rated content for usability, appropriateness and accuracy on 9-point scales in electronic surveys and provided free text comments. Aggregated survey responses were incorporated into revised definitions. Consensus was based on numeric ratings using the RAND/University of California Los Angeles (UCLA) Appropriateness Method with prespecified definitions. 27 panelists from 15 academic medical centers voted on a total of 53 rating scales (6 general guidance, 24 central and 18 peripheral nervous system disease definition components, 3 severity criteria and 2 clinical trial adjudication statements); of these, 77% (41/53) received first round consensus. After revisions, all items received second round consensus. Consensus definitions were achieved for seven core disorders: irMeningitis, irEncephalitis, irDemyelinating disease, irVasculitis, irNeuropathy, irNeuromuscular junction disorders and irMyopathy. For each disorder, six descriptors of diagnostic components are used: disease subtype, diagnostic certainty, severity, autoantibody association, exacerbation of pre-existing disease or de novo presentation, and presence or absence of concurrent irAE(s). These disease definitions standardize irAE-N classification. Diagnostic certainty is not always directly linked to certainty to treat as an irAE-N (ie, one might treat events in the probable or possible category). Given consensus on accuracy and usability from a representative panel group, we anticipate that the definitions will be used broadly across clinical and research settings.

Central nervous system injury from novel cancer immunotherapies

PURPOSE OF REVIEW

Neurotoxicity from antineoplastic treatment remains a challenge in oncology. Cancer treatment-induced central nervous system (CNS) injury can be therapy-limiting, severely disabling, and even fatal. While emerging cancer immunotherapies have revolutionized oncology during the past decade, their immunomodulatory properties can cause immune-related adverse effects (IRAE) across organ systems, including the nervous system. Central neurologic IRAEs from chimeric antigen receptor T cells (CAR-T) and immune checkpoint inhibitors (ICPI) are challenging complications of such therapies.We aim to provide clinicians with a comprehensive review of the relevant forms of CAR-T and ICPI-associated CNS toxicity, focusing on clinical features of such complications, diagnostic workup, predictive biomarkers, and management considerations in affected patients.

RECENT FINDINGS

Unique forms of CAR-T and ICPI-related CNS toxicity have been characterized in the recent literature. CAR-T-related neurotoxicity is common and clinically well delineated. ICPI-related CNS toxicity is relatively rare but includes a heterogenous spectrum of severe and diagnostically challenging conditions. While putative risk factors, neurotoxicity biomarkers, imaging correlates and treatment strategies have been put forward, development of tailored diagnostic and management consensus guidelines awaits further clinical investigation.

SUMMARY

As CAR-T and ICPI become more widely adopted, early recognition, documentation, and management of immunotherapy-related CNS toxicity are of paramount importance in the clinical setting.

Neurological complications of immune checkpoint inhibitor cancer immunotherapy

Neurological autoimmunity is increasingly recognized as a complication of immune checkpoint inhibitor (ICI) cancer immunotherapy. ICIs act by enhancing endogenous anti-tumor immune responses and can also lead to autoimmunity affecting all organs. ICI-related neurological autoimmunity is rare, most often manifests with neuromuscular involvement and more rarely affects the central nervous system. Neurological complications often often present in the first three months of ICI treatment but can also appear after ICI discontinuation. These can occur in patients with tumors not traditionally associated with paraneoplastic neurological autoimmunity, such as melanoma and renal-cell carcinoma and should be suspected when a new neurological symptoms present while on ICI and cannot be explained by disease progression or as a consequence of metabolic dysfunction. Treatment consists of ICI discontinuation or withdrawal depending on the severity with or without immunosuppression. Generally, improvement is observed depending on the patient's baseline characteristics and neurological presentation.

Neurological complications of cancer immunotherapy

Immunotherapy has emerged as a powerful therapeutic approach in many areas of clinical oncology and hematology. The approval of ipilimumab, a monoclonal antibody targeting the immune cell receptor CTLA-4, has marked the beginning of the era of immune checkpoint inhibitors. In the meantime, numerous antibodies targeting the PD-1 pathway have expanded the class of clinically approved immune checkpoint inhibitors. Furthermore, novel antibodies directed against other immune checkpoints are currently in clinical evaluation. More recently, bispecific antibodies, which link T cells directly to tumor cells as well as adoptive T cell transfer with immune cells engineered to express a chimeric antigen receptor, have been approved in certain indications. Neurological complications associated with the use of these novel immunotherapeutic concepts have been recognized more and more frequently. Immune checkpoint inhibitors may cause various neurological deficits mainly by alterations of the peripheral nervous system's integrity. These include radiculopathies, neuropathies, myopathies as well as myasthenic syndromes. Side effects involving the central nervous system are less frequent but may result in severe clinical symptoms and syndromes. The administration of chimeric antigen receptor (CAR) T cell is subject to rigorous patient selection and their use is frequently associated with neurological complications including encephalopathy and seizures, which require immediate action and appropriate therapeutic measures. Close clinical monitoring for neurological symptoms is key for early recognition of immunotherapy-related side effects. Comprehensive diagnostic work-up and adequate therapeutic measures are essential to avoid further clinical deterioration and residual neurological deficits.

Primary and secondary central nervous system mature T- and NK-cell lymphomas

Primary central nervous system (CNS) mature T- and NK-cell lymphomas are rare, only comprising 2% to 3% of all primary CNS lymphomas. Among them, peripheral T-cell lymphoma, not otherwise specified, anaplastic large cell lymphoma (ALCL), and extranodal NK/T-cell lymphoma (ENKTL) are the commonly reported histological subtypes. Secondary CNS T-cell lymphoma generally affects about 5% of patients with T- or NK-cell lymphoma, with some exceptions. Acute and lymphomatous subtypes of adult T-cell leukemia/lymphoma (ATLL) have high risk of CNS progression, may affect up to 20% of patients; ALK-positive ALCL with extranodal involvement >1 also has high risk of CNS progression. However, the impact and the optimal methodology of CNS prophylaxis remain unclear in systemic T-cell lymphomas. There are little data on the treatment strategy of primary and secondary CNS T-cell lymphoma. Treatment strategy derived from B-cell CNS primary lymphoma is generally used; this includes induction therapy with high-dose methotrexate-based regimens, followed by high-dose chemotherapy with autologous stem cell transplant in fit patients. There are unmet needs for patients who are not fit for intensive chemotherapy. The prognosis after CNS progression in T-cell lymphoma is dismal with the median overall survival of less than 1 year. New agents targeting T-cell lymphomas are emerging and should be tested in patients with mature T- and NK-cell lymphoma who suffer from CNS involvement.

The potential for immune checkpoint modulators in cerebrovascular injury and inflammation

Introduction: Neuroinflammation has been linked to poor neurologic and functional outcomes in many cerebrovascular disorders. Immune checkpoints are upregulated in the setting of traumatic brain injury, intracerebral hemorrhage, ischemic stroke, central nervous systems vasculitis, and post-hemorrhagic vasospasm, and are potential mediators of pathologic inflammation. Burgeoning evidence suggests that immune checkpoint modulation is a promising treatment strategy to decrease immune cell recruitment, cytokine secretion, brain edema, and neurodegeneration.Areas covered: This review discusses the role of immune checkpoints in neuroinflammation, and the potential for therapeutic immune checkpoint modulation in inflammatory cerebrovascular disorders. A search of Pubmed and clinicaltrials.gov was performed to find relevant literature published within the last 50 years.Expert opinion: The clinical success of immune-activating checkpoint modulators in human cancers has shown the immense clinical potential of checkpoint-based immunotherapy. Given that checkpoint blockade can also precipitate a pathologic pro-inflammatory or autoimmune response, it is plausible that these pathways may also be targeted to quell aberrant inflammation. A limited but growing number of studies suggest that immune checkpoints play a critical role in regulating the immune response in the central nervous system in a variety of contexts, and that immune-deactivating checkpoint modulators may be a promising treatment strategy for acute and chronic neuroinflammation in cerebrovascular disorders.

Renal cell carcinoma with central nervous system demyelination caused by nivolumab

INTRODUCTION

Central nervous system demyelination caused by immune checkpoint inhibitors is a very rare condition.

CASE PRESENTATION

A 65-year-old man who received nivolumab for renal cell carcinoma developed abnormal behavior, such as disagreeable speech and sudden anger. Brain-enhanced magnetic resonance imaging revealed multiple lesions with partial contrast effects in the cerebral white matter. We tentatively diagnosed demyelination caused by nivolumab, and performed steroid pulse therapy twice. After that, his symptoms improved. For the next 2 years, his symptoms did not recur, nor did his cancer progress.

CONCLUSION

Demyelination caused by immune checkpoint inhibitors can be fatal and requires early diagnosis and treatment.

Central Nervous System Demyelination Associated With Immune Checkpoint Inhibitors: Review of the Literature

Immune checkpoint inhibitors (ICI) are a novel class of antineoplastic treatment that enhances immunity against tumors. They are associated with immune adverse events, and several neurological syndromes have been described, including multiple sclerosis and atypical demyelination. We performed a systematic literature review of case reports with neurological immune adverse events that presented with central nervous system demyelination, up to December 2019. We found 23 cases: seven with myelitis, four isolated optic neuritis, one neuromyelitis optica spectrum disorder, five multiple sclerosis, and six with atypical demyelination. Ipilimumab was the most frequently used ICI (11/23). The median time to develop symptoms from the onset of ICI was 6.5 weeks [range 1.0–43.0], and from last ICI dose was 14 days [range 0–161]. Anatomopathological examination was performed in four cases, with the finding of a T-cell mediated immune response. Outcomes were generally favorable after immunosuppression: 18 patients had improvement or a full recovery, three patients did not respond to treatment, three patients died, and in one, treatment was not reported. We describe the patients' clinical presentation, treatment administered, and outcomes. We further speculate on possible pathophysiological mechanisms and discuss potential treatments that may be worth investigating.

Neurologic Complications of Immune Checkpoint Inhibitors in Thoracic Malignancies

Immune checkpoint inhibitors (ICIs) have transformed the prognosis of cancers previously considered lethal. The spectrum of therapeutic indications is rapidly expanding, including the vast majority of thoracic malignancies. By enhancing the immune responses against cancer, the ICI treatments lead to the development of immune-related adverse events (irAEs) that may affect any organ. Severity varies from mild to fatal clinical manifestations. Neurologic involvement is relatively rare and highly heterogeneous, including central and peripheral nervous system diseases associated with neural-specific autoantibodies or not, central nervous system vasculitis, and granulomatous and demyelinating disorders. Symptoms often manifest within the first four cycles of treatment and can develop regardless of the class of ICI used. An unfavorable outcome is found in up to one-third of patients and is generally associated with the patients' clinical characteristics (e.g., age, coexistence of systemic adverse events), cancer type (e.g., lung cancer versus other), and specific clinical setting (e.g., ICI treatment in patients with preexisting paraneoplastic neurologic autoimmunity, ICI rechallenge after a first neurologic irAE). Diagnosis should be suspected in patients with new-onset neurologic symptoms while on ICI treatment which are not explained by metastatic disease or other metabolic/infectious disorders. Recommended treatment is based on clinical severity and consists of ICI discontinuation with or without immunosuppressive/immunomodulatory therapy, although alternative approaches are reasonable depending on cancer status (e.g., aggressive immunosuppression without discontinuing ICI in patients with initial cancer response). Early recognition and appropriate treatment of these neurologic irAEs are crucial for improved patient outcomes and therapeutic planning.

Exploring the VISTA of microglia: immune checkpoints in CNS inflammation

Negative checkpoint regulators (NCR) are intensely pursued as targets to modulate the immune response in cancer and autoimmunity. A large variety of NCR is expressed by central nervous system (CNS)-resident cell types and is associated with CNS homeostasis, interactions with peripheral immunity and CNS inflammation and disease. Immunotherapy blocking NCR affects the CNS as patients can develop neurological issues including encephalitis and multiple sclerosis (MS). How these treatments affect the CNS is incompletely understood, since expression and function of NCR in the CNS are only beginning to be unravelled. V-type immunoglobulin-like suppressor of T cell activation (VISTA) is an NCR that is expressed primarily in the haematopoietic system by myeloid and T cells. VISTA regulates T cell quiescence and activation and has a variety of functions in myeloid cells including efferocytosis, cytokine response and chemotaxis. In the CNS, VISTA is predominantly expressed by microglia and macrophages of the CNS. In this review, we summarize the role of NCR in the CNS during health and disease. We highlight expression of VISTA across cell types and CNS diseases and discuss the function of VISTA in microglia and during CNS ageing, inflammation and neurodegeneration. Understanding the role of VISTA and other NCR in the CNS is important considering the adverse effects of immunotherapy on the CNS, and in view of their therapeutic potential in CNS disease.