In multiple myeloma, survival has been very significantly improved by immunomodulatory drugs, proteasome inhibitors, and CD38-targeting antibodies. Despite these advances, multiple myeloma, which is characterized by malignant proliferation of clonal plasma cells in bone marrow, remains an incurable plasma cell disorder with near-certain relapse after successful treatment. Prognosis for patients who develop triple-class refractory disease is poor, with less than 1-year survival. The substantial unmet therapeutic need extends further to other poor survival multiple myeloma populations that include newly diagnosed patients with high cytogenic risk profiles and those with early relapse after first-line therapy. For all of these, interest in drugs with novel mechanisms of action is naturally high.
More specific, less toxic
Post allogeneic hematopoietic stem-cell transplantation and donor lymphocyte infusion sustained remissions reflect a graft-versus-myeloma effect mediated by donor T cells.1 The substantial morbidity and mortality associated with graft-versus-host disease and opportunistic infections, however, have spurred searches for alternative, more specific, and less toxic T-cell therapies with stronger antitumor activity.
Chimeric antigen receptors (CARs)
In CAR T-cell therapies for multiple myeloma, autologous T cells are harvested from the patient and reprogrammed to target multiple myeloma cells through the introduction of genes that encode CARs, which are fusion proteins coupling an antigen-recognition moiety and a transmembrane-spanning element to a T-cell activation domain (typically CD3 zeta [CD247]). The T cells are then expanded and reinfused to the patient following a lymphodepletion regimen. Five strategies using autologous CAR T cells are currently approved for diffuse large B-cell lymphomas, acute lymphoblastic leukemia, multiple myeloma, and other hematologic malignancies. Notably, in patients with heavily pretreated multiple myeloma, CAR T cells have demonstrated impressive activity.
BCMA-targeting CAR T cells
The B-cell maturation antigen (BCMA; TNFRSF17), which plays an important role in the survival of long-lived plasma cells in bone marrow, is an attractive target for CAR T-cell therapy because it is expressed on normal and malignant plasma cell surfaces and by mature B cells. When ligands (TNFSF 13B/TNFSF13) bind to BCMA expressed on multiple myeloma cell surfaces, survival and proliferation pathways and drug resistance are activated.
High-quality responses have been demonstrated in several trials of anti-BCMA CAR T cells, which kill multiple myeloma cell lines and primary multiple myeloma cells through degranulation of T cells and lysis of tumor cells, even those with low BCMA expression. Based on efficacy in triple-class exposed multiple myeloma that compared favorably to conventional care with improved health-related quality of life, the U.S. Food and Drug Administration gave breakthrough designation to ciltacabtagene autoleucel in December 2019 and approval for idecabtagene vicleucel in March 2021.
Idecabtagene vicleucel
Idecabtagene vicleucel expresses a murine BCMA-targeting single-chain variable fragment with a 4-1BB costimulatory motif. The phase 2 KarMMa study2 evaluated idecabtagene vicleucel (target dose of 450 × 106 CAR T cells; range 150 × 106 to 450 × 106) activity in 128 patients with triple-class exposed multiple myeloma. Partial responses or better were observed in 94 of 128 patients (73%) (95% confidence interval, 66-81); 42 (33%) had a complete response or better (95% CI, 25-41), with a median progression-free survival of 8.8 months (95% CI, 5.6-11.6). Outcomes were improved in the highest fixed-dose group, with partial response or better in 81% (44 of 55), complete response or better in 39% (21), and median overall survival of 12.1 months (95% CI, 8.8-12.3). Patients with high-risk cytogenetic profiles, extramedullary disease, and high tumor burden also had deep and durable responses. Outcomes were less favorable in patients with revised International Staging System stage 3 disease.