What ESO-T01 Tells Us About the Next Frontier in Multiple Myeloma

For over a decade, the transformative potential of CAR-T therapy has been constrained by a fundamental bottleneck: manufacturing. Leukapheresis, centralized production timelines, lymphodepletion, and inpatient monitoring collectively introduce delays, costs, and clinical risk—particularly for patients with aggressive disease who may deteriorate before infusion. Against this backdrop, the study by An and colleagues published in Nature Medicine represents not an incremental improvement, but a conceptual shift: the in vivo generation of CAR-T cells, bypassing the traditional manufacturing paradigm entirely.¹

In this phase 1 study (NCT06791681), five heavily pretreated patients with relapsed or refractory multiple myeloma received a single intravenous infusion of ESO-T01—a nanobody-directed, immune-shielded lentiviral vector encoding a humanized anti-BCMA CAR—at a dose of 0.2 × 10⁹ transduction units. Notably, no leukapheresis or lymphodepleting chemotherapy was required. The vector selectively targeted circulating T cells, reprogramming them in vivo to express the CAR construct, after which these cells expanded and mediated antitumor activity.¹

Despite the small cohort, the efficacy signal was striking: 4 of 5 patients achieved objective responses, including three stringent complete remissions, with MRD negativity at 10⁻⁵ sensitivity in all evaluable responders by day 60.¹ While durability remains uncertain—median follow-up was limited to 6 months, and the trial was terminated early in 2025—the biologic proof of concept is compelling and carries implications that extend well beyond this initial cohort.

The Problem ESO-T01 Is Designed to Solve

The logistical burden of conventional CAR-T therapy is clinically consequential. In the KarMMa trial (NCT03361748), a subset of patients experienced disease progression during the manufacturing window prior to receiving idecabtagene vicleucel (Abecma).² Similarly, real-world data for axicabtagene ciloleucel (Yescarta) demonstrate vein-to-vein times of approximately 4 to 5 weeks, often necessitating bridging therapy with associated risks.³ For patients with rapidly progressive disease, this delay can preclude treatment altogether.

These challenges are further compounded by inequities in access. CAR-T delivery remains largely confined to specialized centers with the infrastructure required for manufacturing coordination, lymphodepletion, and toxicity management. Patients in community settings or resource-limited regions face disproportionate barriers, and even in well-resourced systems, manufacturing failures—though uncommon—remain a concern.⁴

ESO-T01 directly addresses these structural limitations. A single, off-the-shelf intravenous vector—administered without conditioning or apheresis—represents a fundamentally different operational model.¹ If validated, this approach could enable decentralized, scalable CAR-T delivery, expanding access far beyond current constraints.

Vector Engineering: Precision by Design

The ESO-T01 platform reflects deliberate engineering to overcome key barriers to in vivo gene delivery.⁵ An anti-TCR nanobody (VHH) directs the lentiviral vector selectively to circulating T cells, while a modified VSVG envelope reduces off-target tropism and immunogenicity. Surface expression of CD47 minimizes phagocytic clearance, prolonging vector persistence and enhancing transduction efficiency. MHC class I knockout reduces immune-mediated elimination, and a T-cell–specific promoter restricts CAR expression to appropriately transduced cells. The BCMA CAR construct itself is derived from the clinically validated PRG1801 VHH backbone.¹

This design translated into highly selective T-cell transduction, with minimal off-target modification of other immune populations. CAR expression in non-T cells remained below 1% across all patients, a critical safety observation given the theoretical risks of unintended immune activation.¹

Efficacy in Context

The response rates observed—four of five patients responding, including three stringent complete remissions—must be interpreted cautiously given the small sample size and limited follow-up. However, they are biologically meaningful.

In vivo–generated CAR-T cells demonstrated expansion kinetics similar to conventional products, peaking around day 14 and transitioning toward memory phenotypes over time.¹ The achievement of MRD negativity at 10⁻⁵ sensitivity aligns with the depth of response seen with approved BCMA-directed therapies.⁶^,⁷

Prior clinical evidence for in vivo CAR-T approaches in myeloma has been limited. Xu et al. reported early feasibility in The Lancet,⁸ but the current study provides more robust mechanistic and biomarker data, including detailed analyses of CAR expansion, immune phenotypes, cytokine dynamics, and off-target transduction.¹

Safety: Interpreting a Novel Profile

The safety profile of ESO-T01 introduces new considerations. No dose-limiting toxicities were observed, and CAR-T expansion occurred without lymphodepletion.¹ Cytokine release syndrome (CRS) developed in 4 of 5 patients—3 grade 3 and 1 grade 2—and was managed with standard interventions including corticosteroids and tocilizumab (Actemra).¹ One patient experienced grade 1 ICANS.

A distinctive feature was the biphasic CRS pattern: an early inflammatory response attributed to innate immune activation by the viral vector, followed by a second phase associated with CAR-T expansion and tumor engagement.¹ This differs from the single-phase CRS typically seen with conventional CAR-T therapies and may necessitate adapted monitoring and management strategies.

The death of one patient due to extramedullary disease–related spinal cord compression highlights the limitations of small phase 1 datasets.¹ While attributed to disease progression rather than treatment toxicity, such events underscore the need for cautious interpretation.

Key Unknowns

Several critical questions remain.

First, durability of response is unknown. While early MRD-negative remissions are encouraging, multiple myeloma remains a relapsing disease, and long-term persistence of in vivo–generated CAR-T cells has yet to be demonstrated.

Second, resistance mechanisms—particularly BCMA antigen loss—may mirror those observed with existing BCMA-directed therapies.⁹ The optimal sequencing of therapies post–in vivo CAR-T, including the role of alternative targets such as GPRC5D or FcRH5, remains undefined.

Third, patient-specific factors such as immune fitness, prior therapy, and T-cell quality may significantly influence in vivo transduction efficiency, but these variables cannot be adequately assessed in a five-patient cohort.

Finally, while the therapeutic model simplifies delivery, large-scale lentiviral vector manufacturing presents its own regulatory and economic challenges that must be addressed before widespread implementation.

A Glimpse of a New Therapeutic Paradigm

In vivo CAR-T generation joins a broader effort to simplify and expand access to cellular therapy. Allogeneic CAR-T, CAR-NK platforms, and non-viral delivery systems are all being explored to overcome current limitations.¹¹

What distinguishes ESO-T01 is its ability to generate persistent, memory-capable CAR-T cells directly within the patient—without preconditioning. The demonstration that endogenous T cells can be reprogrammed in situ to mount meaningful antitumor responses represents a foundational advance.¹

Even at this early stage, the implications are profound. For patients who currently lack access to CAR-T therapy due to logistical, geographic, or clinical barriers, an off-the-shelf in vivo approach could redefine the standard of care.

Whether this strategy will ultimately match or complement conventional CAR-T therapies depends on future studies—larger cohorts, optimized dosing strategies, and comparative trials. The field is not there yet. But the central message of the An et al. study is clear: the biology works—and that alone justifies urgent continued investigation.

References

1. An N, Wang D, Zhang P, et al. In vivo generation of anti-BCMA CAR-T cells in relapsed or refractory multiple myeloma: a phase 1 study. Nat Med. 2026. doi:10.1038/s41591-026-04244-6.
2. Munshi NC, Anderson LD Jr, Shah N, et al. Idecabtagene vicleucel in relapsed and refractory multiple myeloma. N Engl J Med. 2021;384:705-716. doi:10.1056/NEJMoa2024850.
3. Locke FL, Siddiqi T, Jacobson C, et al. Impact of vein-to-vein time in patients with R/R LBCL treated with axicabtagene ciloleucel. Blood Adv. 2025;9:2663-2676. doi:10.1182/bloodadvances.2024013656.
4. Amini L, Silber A, Schmidt A, et al. Preparing for CAR T cell therapy: patient selection, bridging therapies and lymphodepletion. Nat Rev Clin Oncol. 2022;19:342-355. doi:10.1038/s41571-022-00607-3.
5. Milani M, Annoni A, Bartolaccini S, et al. Phagocytosis-shielded lentiviral vectors improve liver gene therapy in nonhuman primates. Sci Transl Med. 2019;11:eaav7325. doi:10.1126/scitranslmed.aav7325.
6. Berdeja JG, Madduri D, Usmani SZ, et al. Ciltacabtagene autoleucel, a B-cell maturation antigen-directed CAR T-cell therapy in patients with relapsed or refractory multiple myeloma (CARTITUDE-1): a phase 1b/2 open-label study. Lancet. 2021;398:314-324. doi:10.1016/S0140-6736(21)00933-8.
7. Raje N, Berdeja J, Lin Y, et al. Anti-BCMA CAR T-cell therapy bb2121 in relapsed or refractory multiple myeloma. N Engl J Med. 2019;380:1726-1737. doi:10.1056/NEJMoa1817226.
8. Xu J, Wang Y, Gao X, et al. In-vivo B-cell maturation antigen CAR T-cell therapy for relapsed or refractory multiple myeloma. Lancet. 2025;406:228-231. doi:10.1016/S0140-6736(25)01030-X
9. Diorio C, Teachey DT, Grupp SA. Allogeneic chimeric antigen receptor cell therapies for cancer: progress made and remaining roadblocks. Nat Rev Clin Oncol. 2025;22:10-27. doi:10.1038/s41571-024-00959-y.
10. Nicolai CJ, Velazquez EF, Steinberg BE, et al. In vivo CAR T-cell generation in nonhuman primates using lentiviral vectors displaying a multidomain fusion ligand. Blood. 2024;144:977-987. doi:10.1182/blood.2024024523.
11. Marin D, Li Y, Basar R, et al. Safety, efficacy and determinants of response of allogeneic CD19-specific CAR-NK cells in CD19+ B cell tumors: a phase 1/2 trial. Nat Med. 2024;30:772-784. doi:10.1038/s41591-023-02785-8.