Towards the Next Generation CAR T Cells with TEGs: In Vivo Efficacy — Toxicity Profile in Xenografts of Primary Human AML Disease and Healthy Bone Marrow

T.S. and I.J.G. contributed equally</span

Background

γδT cells mediate essential cancer immune surveillance by sensing transformation-dependent metabolic changes through their γδT cell receptors (TCRs). This process is independent ofMHC-restricted antigen presentation, making these innate-like receptors valuable additions to current treatment strategies. The γδTCR alone is instrumental to distinguish between healthy and leukemic stem cells. This concept led to the development of next generation CAR T cells, so-called TEGs: αβT cells Engineered to express a defined γδTCR. A particular γ9δ2TCR, isolated from "clone 5", has been selected as the candidate for clinical testing (TEG001). A purification strategy was introduced that combined maximal interference of γδTCR chains with endogenous TCR chains with a GMP-grade anti-αβTCR-bead-based depletion strategy to obtain a pure population of engineered immune cells (Straetemans et al. Clin Can Res 2015). TEG001 cells showed a strong and broad recognition of hematological malignancies against both cell lines and primary AML in vitro and limited the tumor outgrowth in cell line-based mouse xenograft models (Marcu-Malina et al. Blood 2011, Gruender et al. Blood 2012). To further prepare clinical implementation of this novel strategy we here investigated the in vivo efficacy and toxicity profile against primary leukemic blasts and healthy hematopoietic cells, respectively.

Results

In order to evaluate the biodistribution and safety profile of TEGs we developed patient derived xenograft (PD-X) in vivo models by establishing primary malignant AML blasts, which tested positive in an in vitro assay for recognition by TEGs, in NSG mice. In addition, healthy stem cells from human cord-blood were in parallel engrafted in a separate set of NSG mice. This allowed us to study the homing capacity, tumor-targeting efficacy and off-target toxicity of TEGs by following the outgrowth of malignant and healthy human cell fractions in the peripheral blood. Healthy hematological cellular compartments remained unharmed by TEGs, in spite of effector cell persistence for over 50 days. In contrast, AML blasts could be eliminated as measured by frequencies of blasts in peripheral blood.

Conclusion

Within the limitations of humanized PD-X models, TEGs target acute myeloid leukemia but do neither interfere with engraftment of hematopoietic progenitors nor harm matured subsets of the hematopoiesis. In addition, no additional signs of off-target toxicity were observed in mice. TEGs are a promising addition to the currently available immune therapeutic strategies as they target cancer as a metabolic disorder. A GMP-compliant production procedure has now been established in order to evaluate TEG001 in a phase I open label dose escalation study to assess the safety of TEGs in patients with primary refractory or relapsed acute myeloid leukemia as well as patients with multiple myeloma.