853P - PRAME expression and ImmTAC TCR bispecific sensitivity in acute myeloid leukaemia in the presence and absence of the hypomethylating agent decitabine

Background

ImmTAC molecules are high affinity TCR-anti-CD3 bispecific fusion proteins currently being tested in solid tumour trials, with the gp100-directed ImmTAC tebentafusp demonstrating survival benefit in metastatic uveal melanoma (1). The cancer testis antigen PRAME is expressed in multiple cancer types including malignant haematological cells and can be increased by treatment with hypomethylating agents (HMA), which are used clinically in AML (2). These findings prompted us to test leukaemia cell sensitivity to PRAME-directed ImmTAC in vitro and its potential regulation by HMA.

Methods

Public bulk and single-cell RNAseq datasets were analysed for PRAME expression and extended by in-house analyses. Cancer cell lines, healthy donor PBMC, and the hypomethylating agent decitabine were sourced commercially, and a PRAME-specific ImmTAC was used for IFNg, Granzyme B and killing assays.

Results

Multiple AML cohorts identified 24-29% frequency of detectable PRAME transcripts across all disease subtypes by bulk RNAseq, and scRNAseq analyses confirmed expression in AML blasts. ImmTAC induced robust T cell activation towards multiple PRAME+ haem cancer cell lines (2xAML, 1xHL, 1xCLL), and subsequent ImmTAC-dependent killing of AML cells was confirmed by Annexin V stain. Treatment of PRAMEneg AML cells with clinically relevant levels of decitabine (300nM) induced sustained expression of both PRAME transcript and protein, and PRAME upregulation in response to HMA therapy was confirmed in AML patient samples. Notably, decitabine turned unresponsive AML cells into efficient mediators of PRAME ImmTAC-dependent T cell activation (IFNg EC₅₀ 137 pM) and killing activity (Granzyme B EC₅₀ 113 pM).

Conclusions

These results demonstrate that PRAME ImmTAC can redirect T cells against PRAME+ AML cells. HMA treatment augments PRAME expression in PRAMEneg AML lines, resulting in robust ImmTAC-dependent T cell activation. This preclinical study suggests that a PRAME-directed ImmTAC may have anti-cancer activity in AML patients.1. Piperno-Neumann et al. AACR 2021 Abstract #5342; 2. Atanackovic et al. Am. J. Hematol. 86:918-922, 2011.

Clinical trial identification

Editorial acknowledgement

Legal entity responsible for the study

Immunocore Holdings PLC.

Funding

Immunocore FIMM.

Disclosure

C. Britton-Rivet, J. Houghton, R. Khanolkar, M. Patel, T. Aleksic, A. Benlahrech, D.M. Gascoyne: Financial Interests, Full or part-time Employment: Immunocore. All other authors have declared no conflicts of interest.