A burden too high for CAR T cells in CLL

In this issue of Blood, Davids et al 1 demonstrate that chimeric antigen receptor (CAR) T-cell efficacy in chronic lymphocytic leukemia (CLL) is tightly constrained by tumor burden, with responses largely restricted to patients with low circulating disease.In their phase 1 ZUMA-8 study, Davids et al evaluated brexucabtagene autoleucel, a CD19-directed CAR T-cell product, in patients with relapsed or refractory CLL.Manufacturing was consistently successful, and toxicity remained within acceptable limits.Clinical activity, however, was modest.The key signal came from patients with low tumor burden: all 3 in this subgroup responded, 2 with lasting remissions.By contrast, patients with high lymphocyte counts or recent ibrutinib exposure showed minimal expansion and few responses. 1 These findings underscore that, in CLL, to a far greater extent than in other B-cell malignancies, tumor burden remains a central barrier to CAR T-cell efficacy.Expansion and clinical benefits appear to depend less on product characteristics than on the ability of CAR T cells, to interact effectively with tumor cells in vivo.