BCMA (B-cell maturation antigen) is a protein found on the surface of certain immune cells that has become one of the most important therapeutic targets in the treatment of multiple myeloma, a blood cancer. It acts as a survival signal for plasma cells, the immune cells responsible for producing antibodies. Because myeloma cells are cancerous plasma cells covered in BCMA, treatments that lock onto this protein can destroy cancer cells while largely sparing the rest of the body.
What BCMA Does in the Body
BCMA sits on the surface of antibody-producing cells called plasma cells and plasmablasts. Its job is to receive survival signals from two partner molecules (commonly referred to as APRIL and BAFF) that tell the plasma cell to keep living and keep making antibodies. Without BCMA, long-lived plasma cells in the bone marrow would die off, and the body would lose some of its long-term immune memory.
What makes BCMA useful in medicine is where it’s found and, just as importantly, where it isn’t. BCMA appears almost exclusively on antibody-secreting cells. It’s largely absent from blood stem cells, most other immune cells, and non-blood tissues. Research using fluorescent tracking in animal models has confirmed that BCMA expression increases as plasma cells mature, marking a distinct step in their final development within bone marrow. Small numbers of BCMA-positive cells also appear in the spleen, lymph nodes, and the lining of the small intestine, but all of them are antibody-secreting cells.
Why BCMA Matters in Multiple Myeloma
Multiple myeloma is a cancer of plasma cells. Because these malignant cells retain the characteristics of the healthy cells they came from, they are covered in BCMA. In fact, myeloma cells tend to overexpress BCMA compared to normal plasma cells, which makes the protein a reliable beacon for targeted therapies. Its minimal presence on other cell types means treatments aimed at BCMA are less likely to cause widespread collateral damage to healthy tissue.
BCMA also works as a monitoring tool. Myeloma cells shed pieces of BCMA into the bloodstream, creating what’s called soluble BCMA (sBCMA). Healthy individuals have sBCMA levels around 20.8 ng/mL, while newly diagnosed myeloma patients average roughly 676 ng/mL, over 30 times higher. These blood levels correlate with how much cancer is present in the bone marrow. Among newly diagnosed patients who responded to treatment, 89% showed at least a 50% drop in sBCMA by their fourth week of therapy. Patients whose sBCMA dropped by 50% or 90% from their baseline had significantly better progression-free survival, making sBCMA a practical way to track whether treatment is working. This is especially valuable in patients whose myeloma doesn’t secrete the usual proteins that doctors traditionally use for monitoring.
BCMA-Targeted Treatments
Three main categories of therapy use BCMA as their target. Each works differently, but all exploit the same principle: find BCMA on the cell surface and kill the cell carrying it.
CAR-T Cell Therapy
CAR-T therapy involves collecting a patient’s own immune cells (T cells), genetically engineering them in a lab to recognize BCMA, and infusing them back into the patient. These modified T cells then hunt down and destroy BCMA-carrying myeloma cells. Two CAR-T products targeting BCMA have received FDA approval. Idecabtagene vicleucel (brand name Abecma) was approved in March 2021, followed by ciltacabtagene autoleucel (Carvykti) in February 2022. In the pivotal trial for the first of these, 85% of patients responded to treatment, with 45% achieving a complete response. Median progression-free survival was 11.8 months.
Bispecific Antibodies
Bispecific antibodies are lab-made proteins designed to grab two things at once: BCMA on the myeloma cell and a molecule called CD3 on the patient’s own T cells. By physically bridging the two cells together, the antibody activates the T cell and directs it to kill the myeloma cell. Teclistamab (Tecvayli) became the first bispecific BCMA-targeting therapy to receive FDA approval, in October 2022. Unlike CAR-T therapy, bispecific antibodies are an off-the-shelf treatment that doesn’t require weeks of manufacturing personalized cells.
Antibody-Drug Conjugates
An antibody-drug conjugate, or ADC, is essentially a guided missile. It pairs an antibody that recognizes BCMA with a potent cell-killing chemical, connected by a stable molecular linker. When the antibody portion binds to BCMA on a myeloma cell, the entire package gets pulled inside the cell. Once inside, the cell’s own recycling machinery breaks the linker and releases the toxic payload, which disrupts the cell’s internal scaffolding and triggers cell death. The most studied ADC in this category, belantamab mafodotin, also blocks the survival signals that myeloma cells normally receive through BCMA, adding a second mechanism of action on top of its direct cell-killing ability.
Who Is Eligible for BCMA Therapy
BCMA-targeted treatments are currently reserved for patients with relapsed or refractory multiple myeloma, meaning the cancer has come back after treatment or stopped responding to it. Most approved therapies require that patients have already tried at least four prior lines of treatment, typically including the three main drug classes used in myeloma: proteasome inhibitors, immunomodulatory agents, and anti-CD38 antibodies. In clinical trials, many patients had received a median of five to seven prior treatment regimens before enrolling. These are therapies for patients who have run out of conventional options, though ongoing research is testing them earlier in the disease course.
Side Effects of BCMA Therapies
The most significant side effect across CAR-T and bispecific antibody therapies is cytokine release syndrome (CRS), an inflammatory reaction that occurs when large numbers of immune cells activate at once. Symptoms range from fever and chills to dangerously low blood pressure in severe cases. CRS occurred in about 58% of patients receiving idecabtagene vicleucel, 95% of those receiving ciltacabtagene autoleucel, and 72% of those on teclistamab. The reassuring detail is that severe CRS (grade 3 or higher) was rare across all three treatments, occurring in 0% to 4% of patients in major trials.
A smaller percentage of patients experience neurological side effects, including confusion, difficulty speaking, or tremors. This occurred in roughly 7% of idecabtagene vicleucel patients and 22% of ciltacabtagene autoleucel patients, though again, severe cases were uncommon. Other risks include prolonged drops in blood cell counts and increased susceptibility to infections, both of which require close monitoring in the weeks and months following treatment.
Antibody-drug conjugates carry a different side effect profile. Belantamab mafodotin notably causes eye-related toxicity, including blurred vision and changes to the cornea, which requires regular eye exams during treatment. It does not cause cytokine release syndrome or neurotoxicity.
BCMA Beyond Cancer
Because BCMA sits on all antibody-producing plasma cells, not just cancerous ones, researchers have begun testing BCMA-targeted therapies in autoimmune diseases where the immune system produces harmful antibodies that attack the body’s own tissues. Early results published in the New England Journal of Medicine showed that teclistamab, the same bispecific antibody approved for myeloma, led to clinical and blood-marker improvements in 10 patients with severe autoimmune diseases including rheumatoid arthritis and vasculitis that had not responded to standard treatments. By eliminating the plasma cells responsible for producing disease-driving antibodies, BCMA-targeted therapy could eventually offer a new approach for conditions like lupus and myasthenia gravis.