Imaging CAR T cell therapy with PSMA-targeted positron emission tomography

Chimeric antigen receptor (CAR) T cell therapy for hematologic malignancies is fraught with several unknowns, including number of functional T cells that engage target tumor, durability and subsequent expansion and contraction of that engagement, and whether toxicity can be managed. Non-invasive, serial imaging of CAR T cell therapy using a reporter transgene can address those issues quantitatively. We have transduced anti-CD19 CAR T cells with the prostate-specific membrane antigen (PSMA) because it is a human protein with restricted normal tissue expression and has an expanding array of positron emission tomography (PET) and therapeutic radioligands. We demonstrate that CD19-tPSMA^(N9del) CAR T cells can be tracked with [¹⁸F]DCFPyL PET in a Nalm6 model of acute lymphoblastic leukemia. Divergence between the number of CD19-tPSMA^(N9del) CAR T cells in peripheral blood and bone marrow and those in tumor was evident. These findings underscore the need for non-invasive repeatable monitoring of CAR T cell disposition clinically.

Positron emission tomography imaging analysis of G2A as a negative modifier of lymphoid leukemogenesis initiated by the BCR-ABL oncogene

G2A is a lymphocyte-expressed G protein-coupled receptor whose genetic ablation results in the development of autoimmunity. Using HSV-TK reporter gene directed positron emission tomography (PET), we demonstrate that prior to any indication of the onset of illness, mice transplanted with BCR-ABL transduced G2A-deficient bone marrow harbor expanded populations of leukemic cells compared to recipients of wild-type bone marrow. The target cell type and anatomical locations of leukemia development are indistinguishable in animals transplanted with G2A+/+ or G2A-/- cells. Shorter disease latency in the G2A-deficient background is associated with an increased rate of cellular expansion. PET can be successfully applied to the temporal and spatial analysis of Bcr-Abl driven leukemic progression and should have utility for the study of other leukemias and lymphomas.

The detection of virally induced tumors by 131I- and 125I-labeled syngeneic monoclonal antibodies

The binding of the syngeneic monoclonal antibodies IC5F5 and 4D2B4 to Rauscher virus-induced myeloid leukemic (RMB-1) cells was analyzed in vivo in tumor-bearing BALB/c mice. To verify it these antibodies bind specifically to RMB-1 cells, purified antibodies were iodinated with the isotopes 125I and 131I. Mice previously inoculated with tumor cells were injected with these labeled monoclonal antibodies and the plasma clearance and the tissue distribution were determined. The clearance in tumor-bearing animals was faster than in control mice. The tissue distribution was corrected for nonspecific accumulation by scoring for an unrelated antibody. Calculation of a localization index showed that IC5F5 binds at least 4.5 times more specifically to tumor cells than to other tissues. A preferential localization of radioactivity in s.c. tumor tissue was seen in the scanning of animals injected with 131I-labeled antibodies. The most direct proof of specific binding was observed in autoradiograms of animals treated with 125I-labeled antibodies. Small islands of tumor cells in the livers of mice inoculated i.v. had a high density of grains compared to other tissues and also compared to tumor cells in mice treated with unrelated monoclonal antibodies. These results show efficient targeting of these monoclonal antibodies and make immunotherapy of these myeloid leukemic cells possible.

Nuclear magnetic resonance and gamma camera tumor imaging using gadolinium-labeled monoclonal antibodies

Chelate-derivatized monoclonal antibody labeled with paramagnetic gadolinium-3+ ion has been evaluated as a tumor-specific contrast-enhancing agent in nuclear magnetic resonance imaging in the Rauscher murine erythroleukemia system. With 10(-7) M concentrations of Gd3+ delivered to the tumor target, a small but reproducible difference in proton relaxation times (T1S) was observed in excised tumors. Nuclear magnetic resonance imaging of animals, however, failed to show significant contrast enhancement of the tumor; by comparison, gamma camera images with 153Gd-labeled specific antibody did permit clear tumor visualization without subtraction. The potential use of monoclonal antibodies in tumor imaging appears to be far greater in gamma camera and positron imaging than in nuclear magnetic resonance imaging.

Tumor imaging with radioactive metal chelates conjugated to monoclonal antibodies

High-resolution gamma camera images of mouse erythroid tumors were obtained by use of leukemia cell-specific monoclonal antibodies labeled with bifunctional radioactive metal chelates. Small tumors (200 to 300 milligrams) were visible without subtraction or enhancement 1 to 5 hours after injection of antibody. Chelate-derivitized monoclonal antibodies permit targeting of a broad spectrum of radioisotopes, including those that are optimum for agamma for gamma camera imaging or positron tomography, as well as those that are tumoricidal.