Oversight
The MG-001 part 3 trial was performed in accordance with the principles of the Declaration of Helsinki and the International Council for Harmonization E6 guidelines for Good Clinical Practice. The trial was approved by regulatory authorities in each country (USA, Canada and Türkiye), the central and local institutional review boards in the USA (Western Institutional Review Board-Copernicus Group, WCG IRB, Puyallup, WA), and the ethics committee at each site in other countries (the research ethics boards at the University of Toronto and the University of Alberta, as well as the ethics committee at Istanbul University). Oversight of the study was provided by an independent study monitoring committee comprising a neurologist, a hematologist specializing in cell therapy and a statistician. The committee met at least annually to review the safety data. All participants provided written informed consent before any study-related activities.
Trial design
MG-001 part 3 was a phase 2b, randomized, double-blind, placebo-controlled trial of Descartes-08 in patients with MGFA class II–IV gMG, conducted in academic medical centers and community neurology clinics in North America and Türkiye (ClinicalTrials.gov identifier: NCT04146051). All eligible participants underwent leukapheresis and had a Descartes-08 lot and an acellular placebo lot manufactured under Good Manufacturing Practice conditions. Participants were then randomly assigned in a 1:1 ratio to receive intravenous treatment with Descartes-08 or placebo, administered once weekly for 6 weeks as an intravenous infusion over 20 min. The placebo was matched to Descartes-08 in appearance and supplied in identical containers. Randomization was performed centrally using a computer-generated permuted-block scheme without stratification. Block sizes were varied and concealed from site personnel to maintain unpredictability in allocation. The randomization list was accessible only to the unblinded pharmacy or designated unblinded study staff responsible for preparing the study infusions; investigators, participants, outcome assessors and all other study personnel remained blinded to the treatment assignment throughout the trial. The random allocation sequence was generated by the study statistician and implemented centrally at the autologous lot manufacturing stage by sponsor staff who were otherwise uninvolved in the study conduct. The dose of Descartes-08 was 52.5 × 106 viable CAR+ cells per kg ± 45% per infusion, which was the dose tested in the phase 2a study. Participants who underwent apheresis but did not have the minimum number of cells to reach the required dose were not randomized; they received Descartes-08 under an open-label protocol and were not included in this analysis. Diphenhydramine (or an equivalent) and acetaminophen were administered 30 min before each infusion as premedications. Blinding of study personnel, clinic staff and participants during infusion was maintained using opaque coverings for the infusion bag and tubing, which were identical between Descartes-08 and placebo. Participants were observed for 1 h after each infusion.
Blinded follow-up occurred at 2 weeks (month 2) and 6 weeks (month 3) after the last infusion. Participants who demonstrated a ≥3-point worsening of the MG-ADL score from baseline or showed signs of an impending myasthenic crisis could receive rescue therapy, which, for those randomized to placebo, included Descartes-08.
Both the study teams and participants were blinded to the treatment assignment through the month 3 visit. After the primary endpoint assessment at month 3, participants randomized to placebo had the option to cross over to Descartes-08, which was administered under an open-label protocol. All participants were followed up for 12 months.
Part 3 was added to the MG-001 study protocol in amendment v3.0 (August 1, 2022). Major amendments include v3.2 (October 17, 2023), which increased the enrollment cap to 50; v3.3 (January 8, 2024), which specified a 5-point reduction in the MGC score at month 3 as the primary endpoint; and v4.2 (June 19, 2024), which defined the primary efficacy population and outlined the use of the estimand framework for intercurrent events.
Participants
All participants were required to meet the following inclusion criteria: ≥18 years of age; a diagnosis of gMG, defined as MGFA clinical class III or IV (part 1; dose escalation) or class II–IV (parts 2–4) at the time of screening; use of concomitant immunosuppressive drugs deemed necessary by the investigator; daily dose of corticosteroids of ≤40 mg per day of prednisone equivalent, with a stable dose for a minimum of 4 weeks before the baseline visit; for the seropositive cohort only: MG-specific antibody titer must be above the reference laboratory’s upper limit of normal and documented within 10 years of screening (patients who were MuSK or LRP4 autoantibody-positive were allowed to be enrolled in parts 1 and 2 only and were not eligible for this trial); for the seronegative cohort only: unequivocal response to cholinesterase inhibitors and abnormal repetitive nerve stimulation or increased jitter (participants must not have another neuromuscular disease that may cause increased jitter and must have a negative congenital myasthenic syndrome panel); participants must be willing to return for all study visits; participants must be able to provide written informed consent; women of reproductive potential must agree to use highly effective birth control from screening through 14 days after the last dose of Descartes-08 (women of childbearing potential are defined as women who have reached menarche and who have not been postmenopausal for at least 24 consecutive months; that is, have had menses within the preceding 24 months or have not undergone a sterilization procedure, such as hysterectomy, tubal ligation or bilateral oophorectomy); and participants must have an MG-ADL total score of ≥6.
Individuals who met any of the following criteria were excluded from participation in this study: major chronic illness that was not well managed at the time of study entry and, in the opinion of the investigator, may increase the risk to the patient; intravenous infusion of immunoglobulin or plasma exchange within 4 weeks before the baseline (day 1) visit; treatment with rituximab or ocrelizumab within 12 months before the baseline (first infusion) visit; treatment with calcineurin inhibitors (for example, tacrolimus, cyclosporine, cyclophosphamide), FcRn antagonists and/or other biologics within 3 weeks before the planned leukapheresis and within 8 weeks before the baseline (first infusion) visit; initiation of eculizumab treatment within 8 weeks before the baseline (first infusion) visit (patients who had been receiving eculizumab for more than 8 weeks and met other criteria for enrollment were eligible for treatment on the trial); sexually active female patients of childbearing age who were pregnant based on a serum pregnancy test, lactating or not using an acceptable birth control method (combined estrogen and progestogen-containing hormonal contraception associated with inhibition of ovulation (oral, intravaginal or transdermal); progestogen-only hormonal contraception associated with inhibition of ovulation (oral, injectable or implantable); an intrauterine device; an intrauterine hormone-releasing system; bilateral tubal occlusion; and a vasectomized partner). Male patients were to agree to effective contraception (for example, condoms (male or female) with or without a spermicidal agent, a diaphragm or cervical cap with spermicide, or an intrauterine device) from screening through 14 days after the last dose of Descartes-08. True heterosexual sexual abstinence was considered an acceptable form of contraception. Periodic abstinence and withdrawal were not considered acceptable methods of contraception. Other exclusion criteria: abnormal prothrombin time/international normalized ratio or partial thromboplastin time increased by >1.5-fold or the patient is on anticoagulation therapy (except in cases of elevated partial thromboplastin time with documented lupus anticoagulant, in patients who had been on stable doses of anticoagulation therapy for more than 6 months following a venous thromboembolism diagnosis, or in patients on stable doses of anticoagulation therapy for at least 8 weeks after an atrial fibrillation diagnosis; these conditions will not be exclusionary unless, in the investigator’s opinion, they make participation in the study unsafe); absolute neutrophil count <1,000 cells per microliter; hemoglobin <8.0 g dl−1; platelets <50,000 cells per mm3; alanine transaminase and/or aspartate transaminase levels more than three times above upper limit of normal; creatinine clearance less than 30 ml min−1; history of primary immunodeficiency or organ or allogeneic bone marrow transplant; seronegativity for hepatitis B surface antigen; seronegativity for hepatitis C antibody (if a hepatitis C antibody test is positive, patients must be tested for the presence of viremia by reverse transcription followed by PCR and must be negative for hepatitis C virus RNA); history of positive human immunodeficiency virus (HIV) or positive HIV at screening; active tuberculosis or a positive QuantiFERON test at screening; any other laboratory abnormality that, in the opinion of the investigator, may jeopardize the individual’s ability to participate in the study; any active significant cardiac or pulmonary disease (patients with asthma and chronic obstructive pulmonary disease controlled with inhaled medications were allowed); a history of malignancy that required treatment in the past 3 years, except for successfully treated squamous cell and/or basal cell carcinoma of the skin and/or breast or colon cancer that was surgically removed and did not require adjuvant chemotherapy or radiotherapy; treatment with any investigational agent within 2 weeks of screening or five half-lives of the investigational drug (whichever is longer); receipt of a live vaccination within 4 weeks before baseline (day 1) or intent to receive live vaccination during the study (mRNA-based vaccines such as those against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are not considered live; likewise, the Janssen COVID-19 vaccine is not live); a history of significant recurrent infections or any active infection that may interfere with the patient’s participation in the study in the opinion of the investigator; and any known psychiatric illness that may interfere with the patient’s participation in the study in the opinion of the investigator.
Permitted concomitant medications for gMG included pyridostigmine, corticosteroids (equivalent to ≤40 mg of prednisone per day), azathioprine, mycophenolate mofetil and complement inhibitors, provided the dose was stable for at least 8 weeks before the first infusion. No dosing changes were allowed for concomitant MG-specific medications during the study, other than corticosteroids. The dose of corticosteroids was not permitted to be increased, but it could be tapered at the site investigator’s discretion after month 6.
Randomization and blinding
Following the successful manufacturing of the autologous product, participants enrolled in part 3 were randomized 1:1 to receive either Descartes-08 or placebo. Randomization was performed centrally using a computer-generated permuted-block scheme without stratification (Mathematica v13.0, Wolfram Research). Block sizes were varied and concealed from site personnel to maintain unpredictability in allocation. The randomization list was accessible only to the unblinded pharmacy or designated unblinded study staff responsible for preparing the study infusions; investigators, participants, outcome assessors and all other study personnel remained blinded to the treatment assignment throughout the trial. The lack of stratification by site, combined with the small sample size and a higher-than-anticipated rate of enrolled participants not being randomized, likely led to unequal allocation between the active group and the placebo group.
Participants, study team members and all sponsor staff involved in the treatment or clinical evaluation of the patients, as well as in the review or analysis of data, were blinded to the treatment assignment until after the primary endpoint assessment. The bag and tubing containing the final infusion product were covered with an opaque, light-protective cover to ensure the blinding of participants and study team members.
Endpoints
The primary endpoint was the proportion of participants who demonstrated a ≥5-point decrease in the MGC score at month 3 compared to baseline. The MGC scale is a ten-item, 60-point weighted instrument composed of both patient-reported and provider-assessed items, which are themselves components of two other scales (MG-ADL and QMG). MGC was recommended by the 2000 MGFA Task Force for MG trials34. A 3-point change in the MGC score is considered clinically meaningful.
Secondary efficacy endpoints were the mean change from baseline in MGC, MG-ADL, QMG and MG-QoL-15r scores at each postinfusion visit (months 2, 3, 4, 6, 9 and 12), as well as the proportion of participants with ≥2–8-point improvements in MG-ADL, QMG and MGC scores at each postinfusion visit. The MG-ADL scale is an eight-item, 24-point, patient-reported instrument that assesses the effects of gMG on daily functioning65. A 2-point change is considered clinically meaningful. The QMG scale is a standardized, 39-point scoring system consisting of 13 provider-assessed items, which include hand-grip strength and forced vital capacity66. A 3-point change is considered clinically meaningful. The MG-QoL-15r scale is a 15-item, 30-point quality-of-life, patient-reported instrument with no consensus on what constitutes a clinically meaningful change67.
Safety endpoints included the occurrence of AEs and SAEs from the time of apheresis through month 12, as well as laboratory values and vital signs. AEs were coded using CTCAE version 5.0. Serum samples collected during noninfusion visits (screening day, day 57, day 85) and preinfusion visits (day 1, day 29) were analyzed using the Meso Scale Discovery platform, in addition to serum immunoglobulin, vaccine titer and anti-AChR antibody testing in a CLIA-compliant laboratory. Cytokines were analyzed using the following panels: S-PLEX Proinflammatory Panel 1 (IFNγ, IL-10, IL-1β, IL-6, TNF), V-PLEX Proinflammatory Panel 1 (IL-8), V-PLEX Chemokine Gen B Panel 1 (MCP-1) and V-PLEX Cytokine Panel 1 (GM-CSF), all from Meso Scale Discovery.
Statistical analysis
We estimated that 15 participants per treatment group would provide 80% power to detect a difference of 47% in responders between Descartes-08 and placebo. Calculations assumed 87% responders in the Descartes-08 group and 40% responders in the placebo group, based on a two-sample proportion test for independent samples to detect a difference in proportions at the 0.05 (nondirectional) level of significance. The effect size estimate was based on the results of the phase 1b/2a study of Descartes-08 in gMG and historical placebo controls, and it was verified using Monte Carlo analyses of 100,000 trials to estimate the mean proportion of responders and the s.d. expected in the placebo group for a range of cutoff values (2–8 points) across all four scores (MG-ADL, MGC, QMG and MG-QoL-15r). To account for screen failures and missed randomization due to insufficient cells manufactured, we enrolled up to 50 participants.
Primary efficacy analyses were performed on a modified intention-to-treat (mITT) population, comprising all participants enrolled at an academic medical center who had at least one postbaseline follow-up. Baseline demographics and safety endpoints (that is, type and frequency of AEs) were analyzed in all enrolled participants using descriptive statistics. Categorical variables were expressed as percentages, while continuous variables were presented as means and s.d. values or medians and ranges for variables with a skewed distribution (including those with a mean-to-s.d. ratio of <2).
The primary endpoint—the response status at month 3 (day 85), in which responders were defined as those who had a reduction of 5 points or more in their MGC total score compared to baseline—was analyzed using a two-independent-sample proportion test at a two-sided 5% significance level; a Wald chi-squared test was used to determine whether there was a significant association between the treatment group and the response status. The primary estimand defined the population of interest as the mITT population, which included all participants who had at least one postbaseline follow-up measurement. Participants who required rescue medication before day 85 had their MGC scores set to missing; these were imputed using baseline observation carried forward, with the response status set to nonresponder. All other missing data were imputed using multiple imputation, assuming data were missing at random. This was not required for the participants in the mITT population, as the data were complete.
The percentage of responders in each treatment group, along with the difference in proportions between the treatment arms with the corresponding Wald 95% CI, was reported. Missing data were imputed using a fully conditional specification method, which included sex, age, race, treatment group and baseline scores. A total of 50 complete datasets were generated. PROC MI and PROC MIANALYZE were used to conduct multiple imputation and combine the results. Subgroup analyses were carried out to determine whether there was a difference in the response between participants who were anti-AChR antibody-positive and those who were anti-AChR antibody-negative, between those with early- and late-onset disease, between those with and without prior exposure to complement and FcRn inhibitors, and between those who did and did not develop a fever. Differences in the proportions of responders were determined within each treatment group. The difference in the proportion of responders was assessed, and the Wald chi-squared test for association between response status and subgroup was performed separately within each treatment group.
Statistical analyses were conducted using SAS (version 9.2 or higher) and Mathematica (version 11.0 or higher), where applicable.
Reporting summary
Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.