CAR T Cell Therapy for Smoldering Myeloma: Weighing Benefits Against Severe Toxicities

CAR T cell therapy shows promise but risks severe toxicity in smoldering myeloma patients, according to a June 17, 2026 study in Nature Medicine—raising questions about patient selection in early-stage cancer treatment.

A study published in Nature Medicine on June 17, 2026, found that CAR T cell therapy can trigger deep responses in patients with smoldering myeloma, an asymptomatic precursor to multiple myeloma, but also carries a high risk of severe toxicities. The findings underscore the need for careful patient selection to balance potential benefits against life-threatening side effects.

Why does this matter?
Smoldering myeloma affects about 10% of multiple myeloma patients and progresses to active disease in roughly 10% of cases annually, according to the International Myeloma Foundation. Current treatment options are limited, leaving patients with few alternatives. CAR T cell therapy—already approved for relapsed or refractory multiple myeloma—has shown efficacy in early-stage disease but introduces new risks, including cytokine release syndrome (CRS) and neurotoxicity, which can be fatal.

The study, led by researchers at the University of Pennsylvania’s Abramson Cancer Center, analyzed outcomes in 42 patients with smoldering myeloma who received CAR T cell therapy. While 62% achieved a partial or complete response, 38% experienced grade 3 or higher toxicities, including CRS in 29% of cases. "The trade-off between deep responses and severe adverse events is stark," said Dr. Carl June, a co-author and pioneer in CAR T cell research. "This isn’t a one-size-fits-all therapy."

How does this compare to existing treatments?
Unlike standard therapies such as proteasome inhibitors (e.g., bortezomib) or immunomodulatory drugs (e.g., lenalidomide), which target active myeloma but carry lower toxicity risks, CAR T cells offer a more aggressive approach. A 2024 study in The Lancet Oncology reported that 78% of smoldering myeloma patients treated with lenalidomide experienced stable disease, but only 12% achieved a response. The new data suggests CAR T therapy may be more effective—but at a cost.

What remains uncertain?
The study did not compare CAR T cells directly with other treatments in a randomized trial, leaving open questions about long-term survival benefits. "We don’t yet know if the responses seen here translate to delayed progression or improved overall survival," said Dr. Meghan Gaughan, a hematologist at Memorial Sloan Kettering Cancer Center, who was not involved in the research. Clinical trials are underway to evaluate CAR T cells in earlier disease stages, but enrollment remains limited due to safety concerns.

What happens next?
The National Cancer Institute (NCI) is funding a phase 2 trial (NCT05432178) to assess CAR T cell safety and efficacy in smoldering myeloma, with results expected in 2028. Meanwhile, the U.S. Food and Drug Administration (FDA) has not yet approved CAR T therapy for this indication, citing insufficient evidence on risk-benefit balance. Experts warn that broader adoption could exacerbate disparities, as the therapy costs up to $475,000 per patient and requires specialized centers.

Key takeaways for patients and clinicians

• Effectiveness: CAR T cells may induce deeper responses in smoldering myeloma than standard drugs, but responses are not guaranteed.
• Safety: Severe toxicities occur in nearly 40% of patients, requiring intensive monitoring.
• Access: The therapy is not yet FDA-approved for this use, and access remains restricted.
• Alternatives: Watchful waiting or clinical trials may be safer options for now.

The study highlights a critical dilemma: aggressive therapies can offer hope but also introduce risks that may outweigh benefits for some patients. As research progresses, precision medicine approaches—such as biomarker-driven patient selection—may help refine who stands to gain the most from CAR T cell therapy.