A Phase-2B Double-Blind Randomized International Prospective Trial of Inebilizumab in NMDAR Encephalitis: The ExTINGUISH Trial

Abstract

Background and Objectives

The lack of approved therapies for N-methyl-d-aspartate receptor (NMDAR) encephalitis has contributed to substantial variability in treatment. Inebilizumab is a humanized anti-CD19 monoclonal antibody that can be administered intravenously. Inebilizumab may be an efficacious treatment for patients with NMDAR encephalitis, with the potential to achieve early robust and sustained suppression CD19⁺ plasmablasts, some plasma cells, and NMDAR autoantibodies, with the potential to improve short-term and long-term outcomes in patients with NMDAR encephalitis.

Methods

The ExTINGUISH trial is a multisite, phase 2B, randomized, double-blind, placebo-controlled trial designed to evaluate the safety and efficacy of inebilizumab 300 mg for the acute treatment of participants with moderate-to-severe NMDAR encephalitis. ExTINGUISH will randomize 116 patients across the United States and Europe. Participants will receive standard first-line immunotherapies (intravenous steroids and intravenous immunoglobulins and/or plasma exchange) before randomization (1:1). In addition, cyclophosphamide rescue therapy will be offered to participants with persistent moderate-to-severe disease 6 weeks after randomization. Primary outcomes will be measured 16 weeks from randomization using the change in the adapted ExTINGUISH modified Rankin scale and accepted safety measures in 32 weeks. Secondary outcomes will be measured up to 96 weeks and include comprehensive neuropsychological tests, bedside cognitive screening tools, and quality-of-life/functional indices. Clinical data will be combined with blood and CSF biomarkers of immune activation to inform putative biologic contributors to outcomes (exploratory outcomes). Study operations are supported by the National Institute of Neurological Disorders and Stroke–supported Network for Excellence in Neuroscience Clinical Trial (NeuroNEXT) infrastructure.

Discussion

The ExTINGUISH trial will determine the safety and efficacy of inebilizumab as a treatment of NMDAR encephalitis while evaluating the utility of clinical, cognitive, and quality-of-life outcome measures and supporting standardized collection and measurement of biofluid biomarkers. These innovations will inform development of future treatments and trials for patients with NMDAR encephalitis and other types of autoimmune encephalitis. The ExTINGUISH trial opened to enrollment of adult patients (older than 18 years) in 2022 and pediatric patients (older than 12 years) in 2024; enrollment is ongoing.

Introduction

Anti-N-methyl-D-aspartate receptor (NMDAR) encephalitis is a life-threatening, antibody-mediated autoimmune disorder of the central nervous system. Although NMDAR encephalitis was initially believed to affect only women of childbearing age, it is now recognized as one of the most common forms of antibody-mediated encephalitis affecting individuals across the age spectrum, with a median age at symptomatic onset of 19 years, and a 4:1 female preponderance.^1–4 Approximately one quarter of cases in female patients are associated with ovarian teratoma, with the highest prevalence of tumor-associated cases in patients of reproductive age.^1,4,5 Adult patients typically present with a viral-like prodrome, with emotional and behavioral disturbances evolving over days to weeks, including agitation, apathy, depression, cognitive impairment, catatonic features, or psychosis. Patients may develop an increasing lack of responsiveness to their environment, with accompanying movement disorders and seizures. Autonomic instability is common, necessitating cardiorespiratory support in severe cases.³ IgG antibodies to the GluN1 subunit of the N-methyl-d-aspartate (NMDA) receptor are present in the CSF of all patients with NMDAR encephalitis and the serum of >80% of these patients.^5–9

No therapies are approved by the US Food and Drug Administration (FDA) or European Medicines Agency (EMA) for the treatment of NMDAR encephalitis. Current treatment approaches are informed by small prospective series,¹⁰ retrospective data,^1–3,11 and expert consensus^4,12—class III-IV evidence that is subject to high degrees of bias.^13,14 Most treatment approaches advocate for prompt tumor removal when present, followed by early initiation of immunomodulatory therapies.^{1–4,10,12–14} First-line therapies most commonly include high-dose intravenous corticosteroids (typically provided 1 gram/d for 3–5 days) and IV immunoglobulins (IVIg) (dose range, 1–2 g/kg divided across 2–5 days). Plasma exchange may also be used (typically 5 cycles).^4,15 Almost half of patients (47%) continue to experience substantial disability 4 weeks after these first-line therapies,³ justifying the use of second-line immunotherapies, most commonly including rituximab (375 mg/m² weekly × 4), with or without intravenous cyclophosphamide (750 mg/m² given with the first dose of rituximab).^15,16 Although variability in practice persists,⁴ this approach has become the de facto standard of care for the treatment of NMDAR encephalitis specifically and autoimmune encephalitis more broadly.⁴ Since the publication of the largest retrospective cohort supporting this approach in 2013,^3,4 patients with NMDAR encephalitis continue to be recognized earlier and treated sooner, with more and more patients receiving second-line therapies, including rituximab.⁴ Despite the apparent advances in diagnosis and treatment, outcomes in recovering patients have not changed significantly since 2013.⁴ These observations emphasize the need for novel approaches to the treatment of NMDAR encephalitis, backed by high-quality evidence from appropriately designed randomized controlled trials.

Inebilizumab is a humanized, affinity-optimized, IgG1 kappa monoclonal antibody (mAb) that binds to the B-cell–specific surface antigen CD19, resulting in the depletion of CD19–positive B cells and circulating CD20-negative plasmablasts and some plasma cells. Inebilizumab is FDA and EMA approved for the treatment of patients with neuromyelitis optica spectrum disorder—an autoantibody-mediated disease of the central nervous system.¹⁷

The Encephalitis Treatment with Inebilizumab for NMDAR and GUIdance on Serum + CSF biomarkers to predict Health outcomes (ExTINGUISH) trial is the first NIH-funded randomized clinical trial for patients with NMDAR encephalitis. The ExTINGUISH trial will systematically evaluate the safety and efficacy of inebilizumab in 116 participants with acute NMDAR encephalitis treated with first-line therapies in addition to inebilizumab or placebo.

Methods

Study Organization and Trial Governance

The ExTINGUISH trial is a randomized, placebo-controlled, phase 2B clinical trial designed to determine the safety and efficacy of inebilizumab in adult and pediatric patients with new-onset NMDAR encephalitis and to inform clinical and biofluid markers of disease progression and treatment response. Study design and conduct are supported by the Network for Excellence in Neuroscience Clinical Trials (NeuroNEXT).¹⁸

Study Funding

The ExTINGUISH trial (NeuroNEXT NN111) is funded by NIH/National Institute of Neurological Disorders and Stroke (NINDS) U01NS120901, with additional unrestricted support (including funding for European sites) from Amgen (Thousand Oaks, CA). Amgen is the holder of the intellectual property rights to inebilizumab. The support provided by Amgen is classified as investigator-sponsored study support. The ExTINGUISH study design team maintains full responsibility for initiating, developing, designing, and conducting the trial and holds the IND for this study, independent of Amgen.¹⁹

Standard Protocol Approvals and Registrations

The Mass General Brigham sIRB approved the study protocol for all US sites. Current Protocol Version: v10.0, Approved Protocol Date: Dec 17, 2024. European site operations are conducted under oversight of the Erasmus Medical Center Ethics Committee (Rotterdam, Netherlands). Trial Registration: Clinicaltrials.gov Identifier: NCT04372615. Registered on May 4th, 2020. Sponsor of IND/IDE: Stacey L. Clardy, MD, PhD, cross-referenced to inebilizumab (formerly known as MEDI-551).

Overall Trial Design

The ExTINGUISH trial is a multicenter, randomized, double-blind, placebo-controlled, parallel-group study that aims to randomize 116 participants, aged 12 years or older during the enrollment period. The primary objective of the ExTINGUISH trial is to evaluate the safety and efficacy of inebilizumab administered as 300 mg intravenous (IV) infusion on day 1 (randomization) and day 15, compared with placebo, in participants with new diagnoses of NMDAR encephalitis. Trial participants must have acute NMDAR encephalitis with moderate-to-severe impairment, as defined by an encephalitis-optimized ExTINGUISH modified Rankin scale (mRS) between 3 and 5 at randomization (eAppendix 1). The study includes a 14-day screening phase during which participants are assessed for eligibility (Table 1). First-line treatments may have been administered before the 14-day screening phase (up to 90 days before randomization), including high-doses of intravenous corticosteroids and either IVIg and/or plasmapheresis (participants who receive corticosteroids, IVIg, and plasmapheresis are still considered eligible). The treatment phase of the ExTINGUISH trial begins at randomization. The treatment phase is a 15-day period beginning with 1:1 randomization of participants to receive either inebilizumab 300 mg IV or IV placebo (normal saline) on day 1 and day 15. Randomized participants should all receive IVIg at day 22 of the study. Efficacy is assessed using the encephalitis-optimized ExTINGUISH mRS at week 16 and semistructured clinical assessments. Secondary outcomes measurement will be completed in week 24 with follow-up continues at standard intervals until week 96 (Figure).

Table 1.

Eligibility Criteria

Inclusion criteria	Exclusion criteria
• Age ≥12y and mRS ≥3 at screening. Minimum weight 40 kg for ages 12–18 y • Diagnosis of NMDAR encephalitis, requiring  Subacute change in mental status consistent with autoimmune encephalitis and  NMDAR IgG Ab confirmed in CSF by a study-specified laboratory • Participant received at least 3 d of methylprednisolone 1,000 mg IV within 90 d of randomization, and  IVIg (1.2–2 g/kg), and/or  plasmapheresis (5 or 6 exchanges) • Participant willing to forego other immunomodulatory therapies	• Past NMDAR encephalitis within 5 y • Untreated NMDAR encephalitis for ≥3 mo before screening • Exposure to a B-cell-depleting therapy (e.g., rituximab) within 6 months of randomization • Recent history (last 5 y) of herpes simplex virus encephalitis or known central nervous system demyelinating disease • Exposure to other immunomodulatory therapies within 3 mo of randomization (beyond corticosteroids, IVIg, plasmapheresis) • Any condition that would interfere with the evaluation or administration of the investigational agent or study conduct

Abbreviations: IVIg = IV immunoglobulin; mRS = modified Rankin scale (mRS); NMDAR = N-methyl-d-aspartate receptor; SAE = serious adverse event.

Figure.

Study Time Line

Participants who remain moderately to severely impaired (mRS ≥3) at Week 6 may receive rescue therapy with cyclophosphamide (750 mg/m² IV) at the discretion of the treating team. Additional doses of cyclophosphamide may be provided every 30 (±2) days for participants with persistent disability (mRS ≥3). Participants who do not qualify for rescue therapy (mRS <3) or who opt not to receive it at Week 6, if participant meeting rescue therapy criteria, may be reconsidered for rescue therapy at any time point after Week 6, at the discretion of the treating team.

Inebilizumab Dose Rationale

A 300-mg dose of inebilizumab administered IV on Day 1 and Day 15 was selected for this study to achieve and maintain complete and persistent peripheral B-cell depletion for at least 6 months. The second dose of inebilizumab administered on Day 15 is timed to deplete newly circulating B cells mobilized from lymphoid and other tissues. This regimen is supported by data from phase 1 studies of inebilizumab in multiple sclerosis and scleroderma, in which B-cell depletion at lower dose levels was less complete and durable than at the 300-mg dosage level.^19,20 Further support comes from efficacy and safety demonstrated in participants with neuromyelitis optica spectrum disorder (NMOSD) enrolled in phase 1 and phase 3 (N-Momentum study) clinical trials.¹⁷ The phase 3 N-Momentum study fixed dose of 300 mg inebilizumab given on Day 1 and Day 15 depleted peripheral blood B cells to <1 cell/uL and maintained B-cell suppression for 28 weeks. Exposure-response analysis of primary and key secondary end points demonstrated that the 300-mg dose resided at the efficacy plateau, minimizing the impact of pharmacokinetic variability (NCT02200770).¹⁷ The safety profile of inebilizumab 300 mg was acceptable.^17,19,20

Inebilizumab Rationale for NMDAR Encephalitis

A recent study on the long-term efficacy and safety of inebilizumab in NMOSD demonstrated that inebilizumab treatment resulted in a robust depletion of CD20-positive B cells, which was maintained throughout ≥4 years of follow-up (and potentially longer).²¹ The high potential for a monophasic illness course in NMDAR encephalitis,^3,4,22–24 reported response rate after treatment with de facto first-line and second-line therapies,^{3,4,23,25–29} and potential protracted effects of inebilizumab suggests that patients may not require prolonged/extended administration of immunotherapeutics.³

IV Globulin (IVIg) Use Rationale

On Day 22 from randomization, all ExTINGUISH participants will receive a 3-day course of IVIg given as 0.4 g/kg daily as the standard of care to (1) ensure sustained benefit of this first-line immunotherapeutic, (2) avoid treatment-related fluctuation in trial participants (including those randomized to placebo), and (3) reduce the risk of infection associated with hospitalization and immunosuppression in acutely ill patients.^11,30,31

Placebo Justification for NMDAR Encephalitis

There are no FDA-approved therapies for the treatment of NMDAR encephalitis. The use of a placebo in this trial design is justified by the need to assess inebilizumab monotherapy and to maintain blinding (participant, site, investigator, or sponsor) to assigned treatment during the randomized-control phase, limiting bias. Equipoise is supported through the requirement for sufficient first-line therapies before randomization, the provision of maintenance IVIg at Day 22 to study participants, and the integration of opportunities for rescue therapy (with cyclophosphamide) for participants who do not respond by Week 6.

Relapse for NMDAR Encephalitis

Relapses are reported in 8%–36.4% of patients after the first presentation with NMDAR encephalitis.^3,22–24,29 The cause of NMDAR relapses are unknown. Putative patient and disease-specific factors associated with higher odds of relapse identified in retrospective studies include sex (female), patients without a detected and removed teratoma at onset, abnormal brain MRI during the initial disease course, and lack of second-line therapy (rituximab and cyclophosphamide).^3,16,32–34 Clinical features of relapse may include speech disorders, psychiatric symptoms, disturbances of consciousness or attention, and seizures. Less commonly, relapses present with isolated atypical symptoms/signs suggestive of brainstem-cerebellar involvement (ataxia, diplopia, dysphagia, and tremor).^3,35 Relapse identification and adjudication are critical for patient safety and for the integrity of the ExTINGUISH trial. Participants and care partners are encouraged to contact the study site at the first symptom or sign of a potential relapse. The investigator (or their designee) should promptly review symptoms and signs and solicit other relevant information to determine whether an urgent relapse evaluation is warranted. Relapse of NMDAR encephalitis should be considered in any patient with new or recurrent psychiatric or neurologic symptoms and signs, not explained by other causes. Rescue therapy can be given if a relapse is confirmed by the medical safety monitor after an in-person (off-schedule) relapse evaluation by the local study team.

Rescue Therapy for NMDAR Encephalitis

To ensure study equipoise, rescue therapy (cyclophosphamide 750 mg/m² IV followed by additional doses every 30 (±2) days) will be considered for participants with moderate-to-severe (mRS ≥3) disability 6 weeks after randomization or confirmed NMDAR encephalitis relapse. Retrospective cohort studies suggest that additional treatment with second-line immunotherapies (i.e., rituximab, cyclophosphamide, or both) was associated with lower mRS scores in patients who did not demonstrate sustained improvement at least 4 weeks after treatment with first-line therapies and lower frequency of relapse (among the patients who do experience relapse).^3,25,35 Accordingly, inclusion of options for rescue therapy is important to support and maintain clinical equipoise and to promote recruitment and retention in the ExTINGUISH trial.

Cyclophosphamide is a cytotoxic alkylating agent that targets hematopoietic cells, including B and T cells. Cyclophosphamide is routinely used in the treatment of autoimmune neurologic disorders (including NMDAR encephalitis), demonstrating broad immunosuppressive effects and good CSF penetration.^3,36,37 For these reasons, cyclophosphamide was selected as rescue therapy in the ExTINGUISH trial. The doses of cyclophosphamide used during the intervention phase of the ExTINGUISH trial for NMDAR encephalitis treatment (<4 grams/m2) do not generally approach those considered to have a significant impact on fertility in the available literature (primarily related to cyclophosphamide use in the treatment of rheumatologic and childhood malignancies) or as calculated by the commonly used Cyclophosphamide Equivalent Dose (CED) calculator as borrowed from the Childhood Cancer Survivor Survey.³⁸ As noted above, rituximab is commonly used as a second-line therapy for patients with NMDAR encephalitis. Rituximab is a B-cell–depleting monoclonal antibody, similar to inebilizumab. The potential for overlap in mechanism of action (limiting efficacy) between inebilizumab and rituximab and potential safety risk arising from profound/prolonged B-cell suppression from exposure to 2 B-cell–suppressing therapies precluded selection of rituximab (or other B-cell–depleting therapy) as rescue therapy for participants enrolled in the ExTINGUISH trial.

Study Objectives and End Points

The ExTINGUISH trial is designed to evaluate the safety and efficacy of inebilizumab in participants with acute moderate-to-severe NMDAR encephalitis. The ExTINGUISH trial aims to assess the difference in mRS at 16 weeks between NMDAR encephalitis participants treated with standard first-line immunomodulatory therapies and either inebilizumab (investigational agent) or placebo, while also evaluating the safety of inebilizumab in these participants (Table 2).

Table 2.

ExTINGUISH Trial End Points and Measures

Endpoints	Measures
Primary end points
Clinical disability Safety	• Change of the ExTINGUISH mRS from randomization to Week 16, adjusted for use of rescue therapy • Number of treatment-emergent adverse events and treatment-emergent serious adverse events from randomization to wk 32
Secondary end points
Clinical disability Safety	• Time to ExTINGUISH mRS <2 (continuous measure) • Change in Clinical Assessment Scale in Autoimmune Encephalitis (CASE) Score corrected from baseline value to Week 24 (Weeks 6 and 16) • ExTINGUISH mRS at wk 6, as measured by proportional odds logistic regression/shift analysis • Change of the ExTINGUISH mRS from randomization to wk 16, adjusted for use of rescue therapy (adults only) • Number of participants with any treatment-emergent AE and SAE from randomization to wk 32 (adults only)
Rescue therapy	• Use of rescue therapy according to the protocol-defined criteria at Week 6 (ExTINGUISH mRS ≥3)
Cognitive outcome	• Cognitive outcome at 24-wk measure by Montreal Cognitive Assessment (MoCA),e9 Frontal Assessment Battery (FAB),e10 Repeatable Battery for the Assessment of Neuropsychological Status (RBANS),e11 and Trial Making Test (TMT)e12
Global impression change	• Change of Clinician Global Impression of Change (CGI-I)e13 and Caregiver Global Impression of Change (CaGI)e14
Survival analysis	• Kaplan-Meier survival analyses, where events include death from any cause, with data right-censored up to 96 wk
Exploratory end points
Clinical disability	• Change of the ExTINGUISH mRS from randomization to wk 16, adjusted for use of rescue therapy (pediatric only) • Number of participants with any treatment-emergent AE and SAE from randomization to wk 32 (pediatric only) • Change in mRS and Karnofsky Performance Scale (KPS)e15 score from baseline through wk 96 • Change in CASE from baseline to wk 96
Change in biomarker	• Change in CSF and serum NMDAR antibody titers, oligoclonal band, cytokines, and inebilizumab pharmacokinetics/ pharmacodynamics from baseline throughout study assessments period
Disease relapse	• Proportion of participants who experience disease relapse, as defined by the protocol, up to Week 32
Cognitive outcomes	• Montreal Cognitive Assessment (MoCA) and Frontal Assessment Battery (FAB) out to wk 24 in comparison with wk 6 and 16
Quality-of-life assessments	• Barthel index (BI)e16,e17 to wk 96 • Short-form-36 version 2 (SF-36)e18 to wk 96 • Beck Depression Inventory-II (BDI-II) to wk 96
Health care utilization	• Hospitalization/ICU care
Clinical pharmacology	• Inebilizumab pharmacokinetics • Inebilizumab pharmacodynamics, as measured by counts of CD19+ and CD20+ cells

Abbreviations: mRS = modified Rankin scale (mRS); NMDAR = N-methyl-d-aspartate receptor; SAE = serious adverse event.

ExTINGUISH mRS

The mRS is a 7-point measure, validated for the assessment of poststroke motor outcomes, with high inter-rater reliability.^39–42 Scoring options span from 0 (no symptoms) to 6 (death). The mRS has been extensively applied as a measure of outcomes in patients with NMDAR encephalitis included in retrospective and prospective cohort studies and case series^3,5,15,43 and patients with other types of autoimmune encephalitis.^31,44,45 The ExTINGUISH mRS was adapted by the protocol coinvestigators and NeuroNEXT ExTINGUISH study central team to more adequately capture disability attributed to acute NMDAR encephalitis, recognizing the frequent contribution of emotional, behavioral, and cognitive impairment to disability in patients with NMDAR encephalitis (nonmotor outcomes).¹ The ExTINGUISH mRS captures both motor and emotional disturbances effecting NMDAR patients’ degree of disability or dependence in daily activities (eAppendix 1). The changes to the ExTINGUISH mRS formalize practical applications of the mRS in retrospective studies that justify the use of the mRS as an outcome measure in autoimmune encephalitis.^{3,5,6,15,25,46–49,e1}

The ExTINGUISH mRS is performed by the assigned clinical study site principal investigators (CSSPIs), who are board-certified physicians from participating sites. Before assigning mRS in ExTINGUISH, CSSPIs must complete an online ExTINGUISH trial-specific case-based assessment, with passing score >80%. CSSPIs are also asked to review specific scoring guidelines provided by the protocol principal investigators, and the DCC also offers a scoring algorithm for guiding CSSPIs in the adjudication of the ExTINGUISH mRS.

During adjudication of the baseline ExTINGUISH mRS, if CSSPIs and the DCC algorithm disagree, sites are advised to call the ExTINGUISH on-call hotline to consult with on-call study staff to identify any discrepancies between the mRS algorithm form and clinician impression of the mRS. Subsequent to the ascertainment of the baseline mRS, any discrepancies are adjudicated by a medical safety monitor (MSM). The MSM can review any unaligned ExTINGUISH mRS scores by review of additional study or medical records to ensure all scores consistently and accurately reflect participant status. To reduce inconsistency ratings for ExTINGUISH mRS, all participating clinical sites are encouraged by the protocol principal investigators to designate 1 mRS rater for each participant throughout the participant’s trial assessment period.

Competency and consistency in the application of the ExTINGUISH designated mRS is maintained through continuous teaching and assessment incorporated within monthly investigator calls, hosted by the protocol principal investigators (Drs. Clardy, Day, and Titulaer) and attended by CSSPIs. These calls include discussions and reviews of challenging cases and interactive mRS case-based assessments. Respondent data are routinely analyzed by the NeuroNEXT DCC team and used to develop training materials deployed in future investigator calls.

The primary efficacy comparison measure using the ExTINGUISH mRS is a composite end point of participant ranks based on the change in mRS at 16 weeks, use of rescue therapy, and time to achieve the final mRS score. The change in mRS between baseline and 16 weeks will be calculated as a difference. Owing to the anticipated benefit of rescue therapy, a penalty of 1.5 points in mRS change will be applied to participants who require rescue treatment. The rationale for the 1.5-point penalty is to account for the anticipated gain of approximately 1 point on the mRS because of rescue therapy (to differentiate between that and the effect of the investigational drug) and to adequately represent the need for rescue treatment as a failure in the rankings without potentially inflating the effect of the investigational drug with a higher penalty. There will also be a 3-point penalty applied to those who die within the 16-week visit window. For participants with remaining ties in mRS change scores after penalties, rank order will be allocated based on time (in days) to reach the final mRS score, with earlier improvement leading to a better rank. If ties remain after using time as a tiebreaker, those participants will be assigned the average rank of all the participants who are still tied.

The primary safety analysis will assess inebilizumab safety by conducting a comparison of the percentage of participants with treatment-emergent adverse events (AEs) and serious adverse events (SAEs) between the 2 treatment groups from baseline to week 32. Each study participant will be classified as having experience and treatment-emergent AE. The percentage of participants who experience any TEAE in each treatment group will be compared using a logistic regression model, with adjustment for the stratification variables.

Secondary Outcomes

Priority secondary outcomes include clinical outcomes captured by the time to ExTINGUISH mRS ≤2 (continuous measure), as evaluated by a certified CSSPI blinded to treatment status, and the CASE score. The CASE score is a 9-item score developed to rate the severity of autoimmune encephalitis. Following the established rubric,^e2 patients with encephalitis are directly evaluated by clinicians across 9 domains commonly affected/involved in patients with autoimmune encephalitis, including seizures, memory dysfunction, psychiatric symptoms, consciousness, language problems, dyskinesia/dystonia, gait instability, ataxia, brainstem dysfunction, and motor weakness. Patients are assigned a score of 0–3 on each domain reflecting domain-specific level of impairment (roughly corresponding to no [0], mild [1], moderate [2], and severe [3] impairment). Scores are summed across domains to yield the CASE score, ranging from 0 (no autoimmune encephalitis-associated impairment) to 27 points (severe impairment). In ExTINGUISH, the CASE is performed by CSSPIs who are required to complete CASE training provided by the protocol principal investigators.

The primary efficacy and safety analyses will also be repeated using the subgroup of adult-only participants. Additional secondary outcomes are outlined in Table 2 and include the ExTINGUISH mRS at week 6; the need for rescue therapy (beyond 6 weeks from randomization); cognitive outcomes assessed using the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) and Trial Making Test (TMT); global impression of change measures (Change of Clinician Global Impression of Change [CGI-I] and Caregiver Gobal Impression of Change [CaGI]); and survival analysis.

Exploratory (Tertiary) Outcomes

Exploratory (tertiary) outcomes examine multiple factors that may inform mechanisms of NMDAR encephalitis, predictors of treatment response, or the design of future clinical trials in NMDAR encephalitis and related conditions. Prespecified exploratory outcomes in the ExTINGUISH trial incorporate measures in biofluid biomarkers (blood and CSF), clinical changes, quality-of-life measurements, health care utilization, and clinical pharmacology. Biofluid biomarkers will consider changes in NMDAR antibody titers, cytokines, inflammatory markers, oligoclonal bands, and potential antibodies against the investigational agent, from baseline to Week 16 and beyond. Clinical changes from baseline through Week 96 for ExTINGUISH mRS, KPS, and CASE will be measured along with the proportion of participants who experience a disease relapse from baseline to Week 32. Cognitive changes will be measured by the changes in MoCA and FAB score out to Week 24 in comparison with (Weeks 6 and 16). Quality-of-life measures (BI, SF-36, CGI-I, CaGI-I, and BDI-II) will be measured from baseline up to Week 96. Health care utilization will be measured with the number of NMDAR encephalitis–related days of hospitalization and intensive care unit care, up to 96 weeks (Table 2). Clinical pharmacology will be examined through inebilizumab pharmacokinetics and inebilizumab pharmacodynamics, as measured by counts of CD19⁺ and CD20⁺ cells. The primary and safety analyses will also be repeated with a subgroup of pediatric-only participants.

Intervention

Participants who meet eligibility criteria will be randomized to receive either inebilizumab 300 mg IV on days 1 and 15 or placebo (matching IV bags).

Treatment Allocation

Eligible participants are randomly allocated 1:1 to receive either inebilizumab or placebo. Randomization is stratified by baseline ExTINGUISH mRS (3–4 vs 5) and population (adult vs pediatric) to balance the severity of disease, as well as pediatric from adult participants, between treatment and control arms of the protocol. A randomization sequence will be generated for each stratum using a permuted block design, whereby the allocation of participants to the experimental or control condition will be balanced within a block and within strata. Blocks of sizes 4 and 6 will be randomly determined. Participants are randomized through a web-based application after confirmation of eligibility and completion of required procedures.

Blinding Assignment

ExTINGUISH trial investigators, coordinators, participants, sponsor, trial support/design team, and medical safety monitor (MSM) are blinded to treatment assignment. This will be accomplished using a matching IV placebo (saline). Upon randomization, a kit number will be generated by the ExTINGUISH Interactive Response Technology (IRT) system. Unblinded pharmacists at the study sites will then use this kit number to prepare either the investigational product or the matching placebo, based on the participant’s treatment assignment. This system ensures that the clinical team remains blinded to the treatment allocation while maintaining the integrity of the trial. Only unblinded trial monitors and unblinded pharmacist at the central and site pharmacy have access to the treatment assignments and medication kit ID numbers. All other study personnel and participants are blinded to treatment assignments. To prevent potential unblinding, peripheral B-cell level testing is prohibited for all participants at the site level throughout the study period, and all central research laboratory results related to B-cell levels are blinded to investigators, coordinators, participants, the sponsor, the trial support/design team, and the MSM.

Procedures for Unblinding

If unblinding is necessary and authorized by the trial MSM, the unblinded staff will provide the unblinded code to the investigator. The investigator must document the reasons for unblinding in the participant’s source documents but should not divulge the participant’s treatment assignment to any individuals except those individuals involved in the direct care of the participant. The date and reasons for breaking the blind must be submitted to the central study team within 24 hours. The central study team will work with the protocol principal investigators on behalf of the study team to report to the sIRB and DSMB.

Sample Size Calculation

Power calculations are based on a simulated sample set derived from the original 2013 retrospective NMDAR encephalitis cohort.³ Using this primary end point, and allowing a dropout rate of 25%, 58 participants will need to be randomized to treatment and 58 participants to placebo to identify a significant difference (α = 0.05) in the primary outcome at 16 weeks with 80% power (β = 0.2). Specifically, group sample sizes of 58 and 58 achieve 99% power to detect an effect as great as 0.9, 90% if the effect is 0.6, and 74% if the effect is as low as 0.5 when the null-hypothesized mean difference is 0.0 using a two-sided Mann-Whitney U or Wilcoxon rank-sum test at a 0.05 significance level (α). These results are based on 2000 Monte Carlo samples from the normal distribution.

Recruitment

NMDAR encephalitis is a rare neurologic disorder, and this study aims to enroll 116 participants. The ExTINGUISH team and sites are actively working to enroll eligible participants to reach this enrollment goal. The central NN111 team makes efforts to provide consent and other trial materials for non-English speakers. The study has made significant efforts to enroll a diverse population to date, emphasizing the importance of diversity, equity, inclusion, and accessibility (DEIA) in ensuring that the trial results are representative of various demographic groups and health disparities are addressed.

Recruitment for the first approved site commenced on March 28, 2022; the first participant was enrolled on March 30, 2022. Approval to enroll pediatric patients (age 12 years and older + weight kg) was received from the US FDA on June 9, 2023, and the ExTINGUISH trial protocol was modified on February 13, 2024. The ExTINGUISH trial was opened to pediatric enrollment on March 4, 2024.

Data Collection Method (Additional Information for Each Section is Available in eAppendix 2)

Plans for Assessment and Collection of Outcomes

Data are collected using a Good Clinical Practice–compliant electronic data capture (EDC) system across all ExTINGUISH trial sites.

Plans to Promote Participant Retention and Complete Follow-Up

To ensure participant retention, the study team will highlight the importance of follow-up visits and ensure participant safety through an intention-to-treat analysis, noting that withdrawal is possible at any time without affecting their care.

Data Management

Data collection forms were constructed prioritizing the use of the NINDS Common Data Elements.^e3 All study data will be collected using systems created by the central study team, with appropriate protections for patient confidentiality and data integrity.

Auditing

Quarterly remote monitoring and annual in-person monitoring will be conducted by the NeuroNEXT DCC to ensure the integrity, accuracy, and completeness of study data.

Safety Assessment

Safety oversight will be provided by an NINDS-approved MSM/DSMB and the NeuroNEXT sIRB. In addition to the safety end points outlined in Table 2, suicidality screening using the Columbia-Suicide Severity Rating Scale (C-SSRS) is also assessed at regular intervals.^e4

Adverse Event Reporting and Harms

Adverse events and serious adverse events will be determined by site investigators based on the Common Terminology Criteria for Adverse Events (CTCAE) v5.0^e5 and reported to MSM and DSMB.

Confidentiality

Participants will be assigned a study identifier number upon signing the informed consent form. Data entered in the electronic case report form will be associated with the study identifier number. Blood and CSF samples that are shipped to the central research laboratory will also have a study identifier number.

Participants will be invited to consent to participation in the optional biorepository study, permitting biofluid samples to be stored in the ExTINGUISH biorepository, located at the University of Utah (Salt Lake City, UT).

Statistical Method

Descriptive Statistics

Using CONSORT guidelines for randomized trials,^e6 data will be reported through descriptive statistics by randomized group (mean [± SD] or median [minimum, maximum] values for continuous variables and frequencies [%] for categorical variables).

Data Analysis

Analyses will be reported by intention-to-treat (ITT) group assignments for all primary, secondary, and safety outcomes. The α level for all primary efficacy and safety analyses will be set to 0.05. The Holm-Bonferroni procedure for multiple end points will be used to evaluate the secondary end points (excluding survival, which will be tested at the 0.05 level). To assess the sensitivity of the results, the primary efficacy analysis will be replicated using a per-protocol (PP) population, which includes the subset of all ITT participants who successfully received both cycles of assigned treatment and have no major protocol deviations due to protocol compliance.

The primary efficacy analysis will assess the change in ExTINGUISH mRS between baseline and 16 weeks, accounting for factors influencing the change such as use of rescue therapy and time to reach the final mRS. This analysis will be implemented using the Van Elteren test, an extension of the Wilcoxon rank-sum test adjusting for age strata. The secondary analysis will assess AEs and SAEs occurring within 32 weeks by comparing frequency between the 2 treatment groups using a logistic regression model, adjusted for baseline mRS and age group strata.

All models to assess the secondary end points will include adjustments for the baseline mRS and age group strata. The secondary end point of time from baseline to mRS ≤2 over the 16-week period will be compared using proportional hazards regression. The CASE score will be assessed as the rate of change over follow-up using a linear mixed-model approach. The mRS at 6 weeks will be compared using a proportional odds logistic regression. The use of rescue therapy will be compared using logistic regression. The RBANS and TMT measured at the 24-week follow-up will be compared using linear regression. The CGI-I and CaGI will be assessed using a repeated-measures mixed-effects cumulative logit model with proportional odds. Overall survival throughout the follow-up period will be assessed by proportional hazards regression. Subgroup analyses repeating the primary efficacy and safety analyses will be performed using an adults-only population.

Exploratory subgroup analysis will be conducted with the pediatric subgroup. Other exploratory analyses will examine changes in biomarkers, cognitive status, quality of life, health care utilization, and clinical pharmacology.

Trial Status

The ExTINGUISH trial is currently enrolling participants aged 12 years and older with new onset of NMDAR encephalitis within the prior 3 months under protocol version 10.0, approved on December 17, 2024. Initial trial site activation in the United States was achieved in April 2022, and the first participant was randomized in April 2022. Recruitment is currently projected to continue through the end of 2026. A summary of protocol changes is provided in eTable 1.

Dissemination Plan

Positive and negative results will be summarized by the ExTINGUISH protocol team and published in peer-reviewed international journals.

Discussion

The ExTINGUISH trial is a first-in-disease prospective, randomized, placebo-controlled, multisite clinical trial supported by both NINDS and Amgen as an investigator-initiated study to evaluate the safety and efficacy of inebilizumab in patients with new presentations of moderate-to-severe NMDAR encephalitis. The ExTINGUISH trial will also inform selection of clinical, cognitive, and quality-of-life outcome measures that may inform outcome measures in future trials. The ExTINGUISH trial sample size will provide an opportunity to conduct validation studies of disease progression for the primary and secondary outcomes measures. Defining validated, robust clinical measures for NMDAR encephalitis that are sensitive and reliable will result in more nuanced NMDAR encephalitis outcome measures and provide disease-specific validated assessments for future clinical trials.

Recruitment of adequate numbers of trial-ready participants with NMDAR encephalitis presented the greatest challenge in past studies of NMDAR encephalitis.⁴³ To address this challenge, the ExTINGUISH trial leveraged the NeuroNEXT Network infrastructure, including NINDS-appointed clinical sites across the United States. NeuroNEXT infrastructure provides centralized ethics oversight, master clinical trial agreements, and additional centralized resources to support investigator-initiated trials (e.g., clinical design advice, regulatory, project, and budget management and manages site contracts, sIRB, central laboratory, and central pharmacy, statistical and trial design support, data management and analysis, and study monitoring).^18,e7 NeuroNEXT sites are housed within academic medical centers. Each institution agrees to participate in the single IRB before involvement in NeuroNEXT. Selection of ExTINGUISH trial sites was informed by site-specific surveys capturing interest, capabilities, and anticipated recruitment (based on the number of potentially eligible patients encountered within the preceding 24 months) across the NeuroNEXT Network.

The ExTINGUISH trial has adopted a multifaceted approach to ensure the inclusion of a diverse demographic of patients with NMDAR encephalitis. This strategy involves a broad geographic distribution of study sites, including locations in Europe, to enhance participant representation. In addition, the trial specifically aims to include patients aged 12–18 years, recognizing that approximately 40% of patients affected by NMDAR encephalitis fall within this age group.^3,e8 By focusing on diverse patient populations, the trial aims to generate findings that are more generalizable and applicable across different demographics. The ExTINGUISH trial opened to enrollment of adult patients (18 years and older) in 2022. Access to capable trial-ready sites with relevant expertise and established referral networks spanning broad catchment areas supported rapid activation and expansion of ExTINGUISH sites, with 26-sites onboarded/open to recruitment by November 2023. Permission to enroll pediatric patients (12 years and older) was granted in 2024 necessitating onboarding/opening of additional adult and pediatric-ready sites through 2024; enrollment is ongoing. Recruitment is further supported through partnership with key European sites, prioritizing inclusion of experts at centers with established expertise in diagnosis and treatment of patients with NMDAR encephalitis and broad catchment areas. These sites include the Erasmus MC (Rotterdam, Netherlands) and Hospital/IDIBAPS (Barcelona, Spain): quaternary care centers that have made substantial contributions to the worldwide understanding of the presenting symptoms and signs, paraclinical and laboratory findings that support diagnoses, approach to treatment, and outcomes in patients with NMDAR encephalitis.^1,3,5,7

The ExTINGUISH trial is a pivotal study designed to evaluate the safety and efficacy of inebilizumab for treating patients with acute NMDAR encephalitis who experience moderate-to-severe disability after standard first-line immunotherapies. The trial’s design reflects the input of patients and international disease experts, ensuring that it maintains equipoise while aiming to achieve primary and secondary outcomes that are clinically meaningful and objectively relevant to patients with NMDAR encephalitis. Leveraging the infrastructure of the NeuroNEXT Network, the trial benefits from centralized ethics oversight, statistical design and analysis, site monitoring, and additional resources that support investigator-initiated trials. By incorporating exploratory outcomes, including biofluid biomarkers (blood and CSF), clinical changes, quality-of-life assessments, and health care utilization metrics, the ExTINGUISH trial aims to inform future treatments and trials for patients with NMDAR encephalitis and other forms of autoimmune encephalitis.

Most second-line treatments of NMDAR encephalitis focus on targeting circulating B cells, which vary in effectiveness and brain penetrance. There remains a lack of consensus regarding the optimal timing, dosage, and administration routes for these candidate therapies, underscoring the need for high-quality evidence to guide clinical practice. Inebilizumab, a promising humanized monoclonal antibody against the B-cell surface antigen CD19, has shown safety and efficacy in treating neuromyelitis optica spectrum disorder, another antibody-mediated central nervous system disorder. Unlike other off-label B-cell–depleting therapies, such as rituximab, inebilizumab not only targets CD20⁺ B cells but also effectively depletes CD20⁻plasmablasts and some plasma cells, leading to a robust and sustained suppression of B-cell activity.

The findings from the ExTINGUISH trial are expected to have immediate implications for patient care and will inform the design and implementation of future clinical trials targeting autoimmune encephalitis, ultimately contributing to improved treatment strategies and outcomes for affected patients.

Glossary

CaGI

Caregiver global impression of change

CGI-I

Change of Clinician global impression of change

EMA

European Medicines Agency

FDA

Food and Drug Administration

IVIg

IV immunoglobulins

mRS

modified Rankin scale

NINDS

National Institute of Neurological Disorders and Stroke

NMDAR

N-methyl-d-aspartate receptor