Investigation of ALLO-316: A Fratricide-Resistant Allogeneic CAR T Targeting CD70 As a Potential Therapy for the Treatment of AML
Authors: Srinivasan Surabhi , Tan Nguyen , Cheng Hsin-Yuan , Zhang Yi , Tacheva-Grigorova Silvia , Van Blarcom Tom , Sommer Cesar , Nguyen Duy , Sasu Barbra , Panowski Siler 
Published: November 2020

CD70, a member of the TNF superfamily, is a type II transmembrane glycoprotein that interacts with its receptor (CD27) to promote survival of primed T cells and leads to formation of effector and memory T cells. Expression of CD70 in normal tissues is restricted to activated T and B lymphocytes and mature dendritic cells. CD70 is also widely expressed in various malignancies, including renal cell carcinoma (RCC) and acute myeloid leukemia (AML). The restricted expression pattern of CD70 in normal tissues makes it an attractive target for cancer therapeutics.
Adoptive transfer of T cells expressing chimeric antigen receptors (CARs) is an exciting new therapeutic modality showing great promise in hematologic malignancies. Approval of two CD19-targeting autologous CAR Ts, Kymriah® and Yescarta®, has been followed with promising results from BCMA autologous CAR T clinical trials, showing that activity can extend to other targets. We have previously described the functional screening of a library of anti-CD70 scFv-based CARs and the identification of lead CD70 allogeneic CAR T cells (AlloCAR TTM) with robust activity against RCC cell lines both in vitro and in vivo. Here, we evaluate the anti-tumor activity and safety of a lead CD70 AlloCAR T (ALLO-316) for the treatment of AML.
CD70 expression was evaluated and detected on three AML cell lines and in six primary AML patient samples, with 5/6 patient samples showing expression on 24%-99% of cells. CD70 expression will be profiled in a broader subset of AML patients and preliminary data will be presented. Despite the expression of CD70 on activated T cells it was possible to generate CD70 AlloCAR T cells. No CD70 expression was observed on CAR T cells after generation, suggesting either cells are succumbing to fratricide or are being "masked" by the CAR. CD70 was also not detected on Jurkat cells expressing CARs and this data, in combination with results showing CAR expression is protective when overexpressed in RCC cells support the phenomenon of "masking". Cellectis' TALEN® gene-editing was used to inactivate the TRAC and CD52 loci with the intent to minimize the risk of graft-versus-host disease and to confer resistance to ALLO-647, an anti-CD52 antibody that can be used as part of the conditioning regimen to deplete host alloreactive immune cells potentially leading to increased persistence and efficacy of the infused allogeneic cells. ALLO-316 cells were highly effective at lysing CD70-expressing target cells and eliminated greater than 99% of cells at the high effector to target (E:T) ratio and were unable to lyse AML cells in which CD70 was knocked out. Moreover, ALLO-316 cells were able to kill primary AML blasts with CD70 expression ex vivo. An orthotopic in vivo model utilizing the AML cell line MV4-11 was developed and anti-tumor activity was observed.
In addition to evaluating efficacy against AML cell lines and tumors we also explored potential toxicity liabilities related to ALLO-316 treatment. Previous studies have reported that certain AML tumor antigens can also be expressed on normal hematopoietic progenitors and such expression could potentially lead to toxicity with targeted therapeutics. No detectable CD70 was observed by flow cytometry on purified CD34+ cells from 14 healthy donors. Taken together, our results support clinical development of CD70 AlloCAR T therapy for the treatment of AML.

Disclosures
Srinivasan: Allogene Therapeutics: Current Employment. Tan:Allogene Therapeutics: Current Employment. Cheng:Allogene Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company. Zhang:Allogene Therapeutics: Current Employment, Current equity holder in publicly-traded company. Tacheva-Grigorova:Allogene Therapeutics: Current Employment, Current equity holder in publicly-traded company. Van Blarcom:Allogene Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company. Sommer:Allogene Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company. Nguyen:Allogene Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company. Sasu:Allogene Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company. Panowski:Allogene Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company.