Researchers Discover That One of Medicine’s Most Common Lab Tests May Warn of Rare But Serious Side Effects Weeks Before They Occur

As reported on Inside Precision Medicine, CAR-T cell therapy has revolutionized treatment for multiple myeloma, a blood cancer that was once invariably fatal for most patients. The therapy works by genetically engineering a patient’s own immune cells to recognize and attack cancer, often producing dramatic remissions. But success has come with an unsettling mystery: a small subset of patients develops severe neurological complications that can persist long after the therapy’s immediate side effects have resolved. Now, researchers at Memorial Sloan Kettering Cancer Center may have found the answer in one of the simplest tests available—a routine blood count that could potentially predict these serious complications weeks in advance.

CAR-T Therapy: A Game-Changer With Hidden Costs

To understand the significance of this discovery, it helps to appreciate what CAR-T cell therapy has accomplished. Multiple myeloma, a cancer of plasma cells in the bone marrow, remains incurable for many patients despite conventional treatments. The introduction of CAR-T cell therapy—in which T cells are extracted from a patient, genetically modified to target cancer cells, and then infused back into the patient—has transformed outcomes.

One such therapy, ciltacabtagene autoleucel (cilta-cel), marketed as Carvykti, is BCMA-directed, meaning it targets a specific protein found on myeloma cells. The results have been remarkable, producing “deep and sometimes long-lasting remissions” in patients with relapsed or refractory multiple myeloma (RRMM)—cases where the cancer has returned or stopped responding to standard treatments.

However, CAR-T therapy carries risks. The most well-known are cytokine release syndrome (CRS) and immune effector cell-associated neurotoxicity syndrome (ICANS), acute side effects that typically emerge shortly after infusion and are often manageable with supportive care and medications. But a troubling subset of patients experiences something different: neurological complications that emerge weeks after infusion, long after the acute toxicities have resolved—and these delayed complications can prove far more intractable.

The Mystery: Why Some Patients Develop Delayed Neurological Toxicity

Among the patients receiving cilta-cel, approximately 11% develop what researchers call non-ICANS neurotoxicities (NINTs)—neurological problems that emerge after the expected acute toxicity window. These conditions include cranial nerve palsies, Guillain-Barré syndrome (an autoimmune attack on nerve cells), peripheral neuropathy, and even movement disorders characterized by parkinsonian symptoms.

The clinical presentations vary. “While most cranial nerve palsies resolved with corticosteroid treatment, all three patients who developed movement disorders continued to experience symptoms at last follow-up,” according to the research published in Science Translational Medicine. For patients who have finally achieved remission from their cancer, the emergence of debilitating neurological symptoms represents a cruel trade-off.

Until now, clinicians had no reliable way to predict which patients would develop these delayed complications. That changed with the Memorial Sloan Kettering study of 109 patients treated with cilta-cel.

The Discovery: A Blood Test That Predicts Neurological Risk

The breakthrough came from examining something clinicians check routinely: absolute lymphocyte count (ALC)—simply the number of lymphocytes (a type of white blood cell) present in a patient’s blood. Among the 109 patients studied, 12 (11%) developed neurological toxicities at a median of 21 days after therapy infusion.

The researchers found a striking pattern. Patients whose peak ALC exceeded 3.2 × 10³/µL—a threshold that can be determined from a standard blood test—were dramatically more likely to develop neurotoxicity. Specifically, “12 of 32 patients (37.5%) experienced NINTs” among those with elevated ALC, “whereas none of the 77 patients below that threshold did.”

This is a remarkably clean association: patients below the threshold had zero neurological complications, while more than one-third of those above it developed serious neurological problems.

The finding was independently validated in a second cohort of 50 additional patients. Again, “40% of patients with elevated ALC developed neurotoxicity compared with just one patient below the cutoff.” The consistency across two patient populations strengthens confidence in the result.

Understanding the Mechanism: Which CAR-T Cells Cause the Problem?

The key insight from further analysis is that elevated lymphocyte counts reflected robust CAR-T cell expansion—essentially, the therapy was working very well in terms of generating CAR-T cells. Yet high CAR-T expansion alone didn’t fully explain the toxicity. Not all patients with strong CAR-T cell responses developed neurological complications.

This led investigators to ask: what’s different about the CAR-T cells in patients who do develop neurotoxicity?

Using advanced single-cell techniques, researchers discovered that patients experiencing neurological toxicity preferentially expanded a specific population of CAR-T cells—memory-like CD4-positive T cells expressing high levels of IL-7 receptor alpha chain (IL-7Rα/CD127) and the anti-apoptotic protein BCL2. These cells displayed “transcriptional programs associated with persistence and inflammatory signaling,” distinguishing them from other CAR-T populations.

In other words, the problem isn’t CAR-T expansion per se, but rather expansion of specific long-lived CAR-T subsets that appear primed for persistent inflammatory activity. These cells could become both biomarkers for identifying at-risk patients and therapeutic targets for preventing toxicity.

The Treatment Dilemma: Better Efficacy Versus Neurological Risk

The research reveals an uncomfortable trade-off. Patients with higher lymphocyte expansion—the very patients at risk for neurological toxicity—also experienced longer progression-free survival. “The same robust CAR-T-cell proliferation associated with durable disease control also increases the risk of neurological toxicity,” the study notes.

This poses a clinical challenge: selectively suppressing the problematic CAR-T subsets while preserving the robust anti-cancer response. Fortunately, the research suggests several potential intervention strategies:

JAK/STAT Pathway Inhibition: Because IL-7Rα signaling operates through the JAK/STAT pathway, existing drugs targeting this pathway could potentially blunt neurotoxicity. Notably, ruxolitinib, a JAK inhibitor already approved for other blood cancers, has shown “anecdotal benefit in patients with cilta-cel-associated movement disorders” and could become a preventive strategy.

BCL2 Inhibition: Because the problematic CAR-T cells express high levels of BCL2 (a protein that prevents cell death), BCL2 inhibitors represent another potential therapeutic approach.

These options suggest that rather than broadly limiting CAR-T expansion—which would sacrifice anti-cancer benefit—future strategies could target specific neurotoxic CAR-T subsets while preserving the robust immune response that makes CAR-T therapy so effective.

A Simple Early Warning System

Perhaps most immediately practical is what this research enables clinically. “Peak ALC as an inexpensive, widely available biomarker” could allow clinicians to identify at-risk patients weeks before symptoms emerge. In practical terms, this means a simple blood test done routinely after CAR-T infusion—something already standard clinical care—could become an early warning system.

Patients identified as having elevated ALC could receive enhanced neurological monitoring before symptoms appear. More speculatively, they might eventually receive preemptive treatment to prevent neurotoxicity from developing. The key advantage: this intervention would happen before patients experience debilitating neurological complications like movement disorders or cranial nerve palsies.

Implications for Myeloma Care

For the multiple myeloma community, these findings are significant on several levels. CAR-T therapy represents one of the most effective treatments available for relapsed/refractory myeloma, offering hope where options were previously limited. However, that hope has been tempered by the specter of potentially irreversible neurological complications in a subset of patients.

The Memorial Sloan Kettering study doesn’t eliminate that risk, but it illuminates a pathway toward managing it. By identifying high-risk patients early and understanding the specific CAR-T populations responsible for neurotoxicity, clinicians gain the ability to act preemptively rather than reactively.

The study does have limitations—it was retrospective and involved a relatively small cohort. Prospective validation will be important. However, the authors note that “routine blood counts could become an early warning system that enables preemptive monitoring and targeted intervention while preserving the remarkable efficacy that has made cilta-cel one of the most effective therapies in multiple myeloma.”

The Broader Message

This research exemplifies modern precision medicine: identifying which patients face specific risks, understanding the biological mechanisms underlying those risks, and developing targeted interventions that preserve benefits while mitigating harms. For myeloma patients who have finally achieved remission from their cancer, the prospect of preventing debilitating neurological complications with something as simple as monitoring blood counts and, potentially, targeted medication represents genuine progress.

The next step is prospective validation in larger patient populations. If the findings hold, what was once an unpredictable and sometimes irreversible complication could become a manageable risk factor—allowing more patients to enjoy the life-saving benefits of CAR-T therapy without fear of hidden neurological dangers emerging weeks later.