Chimeric antigen receptor (CAR)-T cell therapy has transformed pediatric oncology by producing high remission rates and potent effects in CD19+ B-cell
malignancies. This scenario is ideal as CD19 expression is homogeneous and human blood provides a favorable environment for CAR-T cells to thrive and destroy
cancer cells (along with normal B cells). Yet, CAR-T cell therapies for solid
tumors remain challenged by fewer
tumor targets and poor CAR-T cell performances in a hostile tumor microenvironment. For
acute myeloid leukemia and childhood solid
tumors such as
osteosarcoma, the primary treatment is systemic
chemotherapy that often falls short of expectation especially for relapsed and refractory conditions. We aim to develop a CAR-T adaptor molecule (CAM)-based
therapy that uses a bispecific small-molecule
ligand EC17,
fluorescein isothiocyanate (
FITC) conjugated with
folic acid, to redirect
FITC-specific CAR-T cells against
folate receptor (FR)-positive
tumors. As previously confirmed in rodents as well as in human clinical studies, EC17 penetrates solid
tumors within minutes and is retained due to high affinity for the FR, whereas unbound EC17 rapidly clears from the blood and from receptor-negative tissues. When combined with a rationally designed CAR construct, EC17 CAM was shown to trigger CAR-modified T cell activation and cytolytic activity with a low FR threshold against
tumor targets. However, maximal cytolytic potential correlated with (i) functional FR levels (in a semi-log fashion), (ii) the amount of effector cells present, and (iii)
tumors' natural sensitivity to T cell mediated killing. In
tumor-bearing mice, administration of EC17 CAM was the key to drive CAR-T cell activation, proliferation, and persistence against FR+ pediatric hematologic and solid
tumors. In our modeling systems,
cytokine release syndrome (CRS) was induced under specific conditions, but the risk of severe CRS could be easily mitigated or prevented by applying intermittent dosing and/or dose-titration strategies for the EC17 CAM. Our approach offers the flexibility of
antigen control, prevents T cell exhaustion, and provides additional safety mechanisms including rapid reversal of severe CRS with intravenous
sodium fluorescein. In this paper, we summarize the translational aspects of our technology in support of clinical development.