Background The cancer/testis antigen PRAME exemplifies an ideal TCR-T cell therapy target due to its high expression in multiple malignancies and its absence in normal tissues. Initially identified in metastatic cutaneous melanoma,¹ PRAME is highly expressed in various additional solid tumors including lung, head & neck, and ovarian cancers. PRAME plays a pivotal role in multiple cellular processes and has been demonstrated to exhibit protumorigenic function primarily through inhibition of retinoic acid receptor signaling.² Targeting of PRAME in solid tumors, particularly when performed as part of a TCR-T multiplexing strategy, represents a promising therapeutic approach in the treatment of many cancer indications

Methods We discovered TCRs specific for 5 different A*02:01-restricted PRAME-derived epitopes using TScan’s proprietary ReceptorScan platform. Using an activation-based screening technology termed ActivScan, we identified the most functional TCRs from a library of 1300 PRAME-specific TCRs to select for TCRs with greatest avidity and expression. These highly active TCRs were examined for their cytotoxic function using a panel of PRAME-expressing A*02:01-positive cell lines. Lead TCRs were assessed for potential off-target reactivity using our proprietary SafetyScan platform, in which off-target recognition of antigens derived from all proteins that comprise the human proteome is evaluated. Safety was further confirmed by examining alloreactivity to high-frequency class I HLAs and by testing TCR reactivity to a panel of normal primary human cells. Lastly, we tested TCR-T cells for their ability to control tumor growth in vivo using PRAME-expressing xenograft mouse models.

Results We screened 871 million naïve CD8⁺ T cells from 16 unique healthy donors in ReceptorScan to identify 5706 TCRs specific for 5 PRAME epitopes. PRAME_425–434-specific TCRs demonstrated superior recognition of a PRAME-expressing cell line compared to all other PRAME epitopes tested. Following selection of high-expressing and high avidity PRAME_425–434-specific TCRs in ActivScan, TCRs were evaluated for their cytotoxic function, and two TCRs compared favorably to a clinical-stage benchmark TCR with respect to cytotoxicity, cytokine release, and T cell proliferation. Safety assessment demonstrated that few off-target peptides were recognized by lead TCRs, minimal alloreactivity was observed to 110 allotypes tested, and no reactivity to normal primary human cells was found. PRAME_425–434-specific TCR-T cells were also able to control tumor growth in vivo following infusion into immunodeficient mice implanted with PRAME-expressing xenografts.

Conclusions Based on its demonstrated activity in vitro and in vivo, this autologous TCR-T cell therapy candidate, TSC-203-A02, has been advanced to IND-enabling studies.

References

1. Ikeda H, Lethe B, Lehmann F, van Baren N, Baurain JF, de Smet C, et al. Characterization of an antigen that is recognized on a melanoma showing partial HLA loss by CTL expressing an NK inhibitory receptor. Immunity. 1997;6(2):199–208.

2. Epping MT, Wang L, Edel MJ, Carlee L, Hernandez M, Bernards R. The human tumor antigen PRAME is a dominant repressor of retinoic acid receptor signaling. Cell. 2005;122(6):835–47.