Descartes-08
(2026)Objective
To evaluate the efficacy and safety of Descartes-08, an autologous BCMA-directed mRNA CAR T cell therapy, compared to placebo in patients with generalized myasthenia gravis (gMG).
Study Summary
• A third of all treated patients and over half of biologic-naive patients achieved minimum symptom expression (MG-ADL <=1) by month 6, maintained through month 12, without additional therapy.
• Descartes-08 was well tolerated with no grade 4/5 events; infusion-related reactions were the most common adverse event, and no B cell depletion or hypogammaglobulinemia was observed.
Intervention
Descartes-08: autologous BCMA-directed mRNA CAR T cell therapy (52.5 x 10^6 viable CAR+ cells/kg per infusion), administered as six once-weekly IV infusions over 20 minutes in an outpatient setting without preconditioning chemotherapy
Inclusion Criteria
Age >=18 years; generalized MG (MGFA class II-IV); MG-ADL total score >=6; on concomitant immunosuppressive therapy; prednisone equivalent <=40 mg/day (stable >=4 weeks); AChR autoantibody-positive, LRP4-positive, or triple seronegative (MuSK-positive excluded)
Study Design
Arms: Descartes-08 vs. placebo (acellular, appearance-matched)
Patients per Arm: Descartes-08: 15 (efficacy); 20 (safety); Placebo: 11 (efficacy); 16 (safety)
Outcome
• Mean change from baseline in MGC/MG-ADL/QMG at month 4: -7.1/-5.5/-4.8 (Descartes-08), with improvements sustained or deepening through month 12 (QMG -6.0 at month 12)
• Minimum symptom expression (MG-ADL <=1): achieved by 33.3% by month 6, all maintained through month 12; 55.6% of biologic-naive patients achieved MSE
Bottom Line
Six once-weekly infusions of Descartes-08 significantly improved MG composite scores at 3 months compared to placebo in patients with gMG, with 66.7% of treated patients achieving a meaningful clinical response. Responses deepened over time, were sustained through 12 months, and a third of all patients achieved minimum symptom expression. The therapy was well tolerated without lymphodepletion, B cell depletion, or hypogammaglobulinemia.
Major Points
- Phase 2b randomized double-blind placebo-controlled trial (MG-001 part 3); NCT04146051
- Descartes-08 uses mRNA rather than viral vector to encode the CAR, resulting in transient expression without need for preconditioning chemotherapy, enabling outpatient administration
- Primary endpoint met: MGC responders (>=5-point reduction) at month 3: 66.7% (10/15) vs. 27.3% (3/11); p = 0.0472
- AChR autoantibody-positive subgroup: 63.6% vs. 12.5% MGC responders at month 3 (p = 0.0258)
- Responses deepened over 12 months: mean MG-ADL change -5.5 at month 4, maintained at -4.8 at month 12; QMG reached maximum improvement of -6.0 at month 12
- 83% of treated patients achieved sustained clinically meaningful response at month 12
- Minimum symptom expression (MSE; MG-ADL <=1) achieved by 33.3% of Descartes-08 patients by month 6, all sustained through month 12
- Biologic-naive patients showed superior response: 88.9% MGC responders at month 3 and 55.6% achieving MSE by month 6, maintained through month 12
- Median prednisone equivalent dose decreased by 55% in the Descartes-08 group at 12 months (20 mg to 9 mg); no participant required rescue escalation
- Primary outcome (MG-ADL) was changed mid-study to MGC before database lock, based on MGFA Task Force recommendations; this is a notable methodologic concern
- No grade 4 or 5 adverse events; no B cell depletion, no hypogammaglobulinemia, no cases of cytokine release syndrome meeting formal criteria
- Infusion-related reactions (IRRs) were the most common AE: 80% in Descartes-08 vs. 56.3% in placebo; mostly grade 1-2, managed outpatient
- HSV reactivation occurred in 3 Descartes-08 patients (15%), all with prior labial herpes on concomitant azathioprine
- Notable placebo response, particularly in triple seronegative participants; a transient leukapheresis effect could not be excluded
- Confirmatory phase 3 trial (AURORA, NCT06799247) enrolling AChR-positive patients
Study Design
- Study Type
- Phase 2b randomized double-blind placebo-controlled trial
- Sample Size
- 36
Limitations & Criticisms
- Primary endpoint was changed mid-study (before database lock) from MG-ADL to MGC score, raising concerns about post-hoc endpoint modification even if pre-specified in amendment v3.3
- Small sample size (n=26 in mITT) with only 80% power to detect the primary outcome, making secondary outcome comparisons purely descriptive and precluding formal statistical testing
- Efficacy population restricted to academic center participants, excluding community clinic enrollees, which limits generalizability and introduced unequal allocation between arms
- Lack of randomization stratification by site contributed to imbalances in baseline characteristics (disease duration longer and thymoma rate higher in placebo group)
- Notable imbalances in concomitant medications: more nonsteroidal immunosuppressants in placebo, more complement inhibitors and corticosteroids in Descartes-08 group
- Significant placebo response, particularly in triple seronegative participants (overrepresented at 23.1%), limiting interpretation of treatment effect
- Transient immunosuppressive effect of leukapheresis on the placebo group cannot be excluded as a contributor to placebo response
- Protocol prohibited changes in nonsteroidal immunosuppressants throughout the study, potentially limiting the observed steroid-sparing effect and preventing drug-free remission
- IRRs (fever, chills, myalgias) were more common in the Descartes-08 group and may have introduced unblinding bias in the assessment of MG severity scales
- Absence of crossover data (placebo to Descartes-08) in the current report limits comparison of outcomes
- AChR autoantibody testing by radioimmunoassay may be insufficiently sensitive to detect changes in antibody subtype or epitope specificity
- Sponsor (Cartesian Therapeutics) was involved in study design, execution, and analysis; multiple authors hold equity in Cartesian Therapeutics