Effective Targeting of PRAME-Positive Tumors with Bispecific T Cell-Engaging Receptor (TCER^®) Molecules

T cell receptors (TCRs) naturally recognize human leukocyte antigen (HLA)-bound peptides derived from foreign and endogenous proteins regardless of their extracellular or intracellular location. Preferentially expressed antigen in melanoma (PRAME) has been shown to be expressed at high levels in a variety of cancer cells while being absent or present only at very low levels in normal adult tissues except testis. In contrast to most other cancer/testis antigens, PRAME is expressed not only in solid tumors but also in leukemia and myeloma cells.

Immunotherapy with bispecific T cell engagers has emerged as a novel and promising treatment modality for malignant diseases, however, antibody-based approaches (ie. blinatumomab) are restricted to few surface antigens such as CD19 or BCMA. Immatics has developed bispecific T cell-engaging receptors (TCER^®) that are fusion proteins consisting of an affinity-maturated TCR and a humanized T cell-recruiting antibody with an effector function-silenced IgG1 Fc part. TCER^® molecules confer extended half-life together with antibody-like stability and manufacturability characteristics. The molecular design allows for effective redirection of T cells towards target peptide-HLA selectively expressed in tumor tissues.

Here we present proof-of-concept data from a TCER^® program targeting a PRAME-derived peptide bound to HLA-A*02:01. We confirmed the abundant presence of the target peptide-HLA in several cancer indications and its absence in relevant human normal tissues by using the XPRESIDENT^® target discovery engine, which combines quantitative mass spectrometry, transcriptomics and bioinformatics. Yeast surface display technology was used to maturate the stability and affinity of a parental human TCR recognizing PRAME with high functional avidity and specificity. During maturation we applied XPRESIDENT^®-guided off-target toxicity screening, incorporating the world's largest normal tissue immunopeptidome database, to deselect cross-reactive candidate TCRs.

The maturated TCRs were engineered into the TCER^® scaffold and production in Chinese hamster ovary (CHO) cells generated highly stable molecules with low tendency for aggregation as confirmed during stress studies. Following TCR maturation, the TCER^® molecules exhibited an up to 10,000-fold increased binding affinity towards PRAME when compared to the parental TCR. The high affinity correlated with potent in vitro anti-tumor activity requiring only low picomolar concentrations of TCER^® molecules to induce half-maximal lysis of tumor cells expressing the target at physiological levels. Furthermore, using a tumor xenograft model in immunodeficient NOG mice, we could demonstrate significant growth inhibition of established tumors upon intravenous injection of TCER^® molecules. Pharmacokinetic profiling in NOG mice determined a terminal half-life of more than 4 days, compatible with a once weekly dosing regimen in patients. For the safety assessment, we measured killing of more than 20 different human normal tissue cell types derived from high risk organs. Notably, we could confirm a favorable safety window for selected TCER^® molecules, which induced killing of most normal tissue cells only at significantly higher concentrations than required for killing of tumor cells. To further support safety of TCER^® molecules, we also performed a comprehensive characterization of potential off-target peptides selected from the XPRESIDENT^® normal tissue database based on its high similarity to the sequence of the target peptide or based on data from alternative screening approaches.

In summary, the efficacy, safety and manufacturability data to be presented provide preclinical proof-of-concept for a novel bispecific T cell-engaging receptor (TCER^®) molecule targeting PRAME for treatment of various malignant diseases.