Bristol Myers Squibb disclosed on October 23, 2026, that a patient died during an early-stage clinical study of its experimental CAR-T cell therapy for multiple sclerosis (MS), raising new questions about safety as researchers investigate whether the treatment could offer longer-lasting control of the autoimmune disease.
The patient developed delayed motor symptoms after receiving zolacabtagene autoleucel, known as zola-cel, and subsequently died of septic pneumonia following additional medical treatment. The death occurred 44 days after the CAR-T infusion.
The company has not established a definitive cause for the fatal outcome. In its account to Reuters, Bristol Myers Squibb described the patient's previous treatments and subsequent complications as factors complicating its assessment.
The disclosure accompanied preliminary findings from the Phase 1 Breakfree-2 trial, presented at the MSToronto 2026 scientific meeting. Although the company reported encouraging signs of disease control, the treatment remains investigational, and its long-term safety and effectiveness have not been established.
What Happened to the Patient in the CAR-T Trial?
According to Bristol Myers Squibb's October 23 clinical update, the patient had highly active relapsing multiple sclerosis and experienced late-onset motor symptoms following zola-cel infusion.
The company classified those symptoms as separate from immune effector cell-associated neurotoxicity syndrome (ICANS), a recognized neurological complication of CAR-T treatment.
The symptoms did not resolve after intermittent corticosteroid treatment and intravenous immunoglobulin. Clinicians subsequently administered high-dose cyclophosphamide, a potent chemotherapy and immunosuppressive medication.
The patient developed septic pneumonia and died the following day, 44 days after receiving zola-cel. Bristol Myers Squibb reported that the fatal event occurred after the interim study data cutoff.
These events establish a serious clinical sequence, but not a definitive explanation of causation. The publicly disclosed information does not determine the relative contributions of the experimental CAR-T therapy, the patient's underlying condition, previous treatment, subsequent immunosuppression or other complications.
The company said the patient's CAR-T cell expansion was consistent with observations elsewhere in the study. That finding alone does not resolve the cause of the death.
How Does Experimental CAR-T Therapy for MS Work?
CAR-T therapy involves collecting a patient's T cells, modifying them to recognize a specific biological target, manufacturing the modified cells and infusing them back into the patient. Zola-cel targets CD19, a protein found on B cells involved in immune-system activity.
In multiple sclerosis, immune activity damages myelin, the protective covering surrounding nerve fibers. Researchers are investigating whether eliminating certain B cells could interrupt harmful autoimmune activity and allow the immune system to recover in a way that produces sustained disease control.
In Breakfree-2, participants undergo cell collection and manufacturing before receiving preparatory lymphodepleting treatment and a single zola-cel infusion.
The approach differs from conventional MS treatments that often require ongoing administration. However, the possibility of extended periods without disease-modifying medication remains a research objective rather than an established clinical benefit.
CAR-T therapies also carry potentially serious risks, including inflammatory reactions, neurological complications and vulnerability to infections. Those risks are particularly important when assessing an experimental intervention for a chronic autoimmune condition.
What Did the Early Trial Results Show?
The Breakfree-2 interim analysis included 66 people with multiple sclerosis: 30 with highly active relapsing disease and 36 with progressive forms of MS.
The study is a single-arm, dose-escalation Phase 1 trial, meaning all participants receive an investigational treatment rather than being randomly assigned to a comparison group. Its primary endpoint is safety, while measures of potential effectiveness are secondary.
According to the company's reported results:
- 28 of 66 patients (42.4%) experienced cytokine release syndrome, an inflammatory reaction associated with immune-cell activation. Most events were mild, and the company reported a median resolution time of two days.
- Five patients (7.6%) experienced ICANS. One patient with progressive MS developed Grade 4 ICANS, which the company said resolved within four days following treatment.
- All 60 patients receiving dose level 2 remained off disease-modifying therapies at their latest follow-up assessments. Nine had been followed for more than 12 months.
- 22 of 24 evaluable patients with progressive MS (approximately 92%) had stable or improved disability measurements after at least 26 weeks of follow-up.
- Six of seven evaluable patients with highly active relapsing MS met the study's criteria for no evidence of disease activity at or beyond 26 weeks.
The fatal septic pneumonia event was disclosed separately because it occurred after the interim data cutoff. Bristol Myers Squibb also stated that no additional Grade 5 events had been reported across its broader Phase 1 zola-cel autoimmune disease programs, comprising 190 treated patients.
That broader total includes patients outside the 66-person MS analysis and should not be interpreted as a separate estimate of mortality risk for people with multiple sclerosis.
Why the Results Cannot Yet Establish Long-Term Safety or Effectiveness
These findings provide early evidence of biological activity and possible clinical benefit, but several limitations remain.
First, without a randomized control group, researchers cannot reliably determine how much of the observed improvement or stability resulted from zola-cel rather than other factors.
Second, the efficacy results come from selected groups with different follow-up durations. Findings among seven, 24 or 32 evaluable participants cannot be treated as outcomes for the entire study population.
Third, remaining off disease-modifying medication is not equivalent to achieving a cure or demonstrating durable remission. Longer observation is required to assess disease recurrence, disability progression and delayed complications.
Finally, the serious neurological event and the subsequently disclosed death reinforce the importance of continued safety investigation. A single fatal event does not establish an overall treatment-related mortality rate, but its circumstances require careful assessment before broader development proceeds.
Why Were the Studies Paused, and What Happens Next?
Bristol Myers Squibb had voluntarily paused enrollment in its zola-cel development program before the fatal event, following the identification of inflammatory and neurological safety concerns.
The company told Reuters that the patient's death occurred after the pause decision and was not the reason for that earlier action.
The concerns emerged during a period of increased scrutiny of experimental CAR-T treatments for autoimmune diseases. In September, Novartis also suspended several studies of a different investigational CAR-T therapy following three patient deaths associated with severe immune complications. Those events involved a separate treatment program and should not be treated as evidence of the cause of the Bristol Myers Squibb patient's death.
Bristol Myers Squibb says it is working with the U.S. Food and Drug Administration toward restarting its zola-cel studies and is targeting early 2027 for renewed enrollment.
As of October 24, the company's Breakfree-2 trial listing identifies the study as active but not recruiting. This means the study remains ongoing, although it is not accepting new participants; it does not establish that regulators have authorized enrollment to restart.
The central questions now concern the cause of the fatal event, whether additional safeguards can address the observed complications, and whether future controlled studies can demonstrate benefits that justify the treatment's risks.
Until those questions are answered through further clinical investigation and regulatory review, zola-cel remains an experimental approach to multiple sclerosis rather than an established alternative to approved treatments.
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