PO-418 ImmTAC® molecules: cancer treatment through soluble, high affinity T cell receptors

Abstract

Introduction Tumour associated antigen (TAA) peptides presented on cancer cells in complex with human leukocyte antigen (HLA) molecules provide a marker for targeting by cytotoxic T cells. The ability of T cells to identify aberrant peptides presented on malignant cells is achieved through a membrane-anchored a/b heterodimer called the T cell receptor (TCR). Whilst patients frequently mount anti-tumour T cell responses, often these are insufficient to clear the tumour. This can be attributed to the low level expression of tumour peptides on cancer cells and to the low binding affinity of TCRs targeting cancer antigens. To overcome the target recognition limitations of the naturally occurring T cell pool, we have developed a new class of soluble bi-specific molecules called ImmTAC (immune mobilising monoclonal TCRs against cancer) which combine a tumour targeting domain, an affinity-engineered TCR and a T cell-activating effector function, an anti-CD3 scFv. Material and methods T cell clones are isolated from human blood, which harbour wild-type TCRs recognising peptides derived from TAAs. The TCR α- and β- chains are sequenced and a stabilising non-native inter-chain disulphide bond is introduced. The TCR chains are expressed as inclusion bodies in Escherichia coli and are refolded in vitro to produce soluble and stable protein. The binding affinity is determined by surface plasmon resonance (SPR) and is typically in the range ~1–400 mM with half-lives of 1–10 s. For effective tumour targeting, the TCR is engineered using phage display to enhance affinity for cognate antigen by a million-fold or greater. To generate an ImmTAC molecule, the soluble TCR is fused to a humanised anti-CD3 scFv effector function and potency and specificity is assessed in vitro against HLA-relevant antigen positive cancer cell lines and cultured primary cells isolated from normal human tissues. Results and discussions Our lead clinical candidate, IMCgp100, targets the HLA-A*02/gp100280–288 epitope expressed at the surface of melanoma cells. Using directed molecular evolution and phage display targeting the complementarity-determining regions (CDRs) we have produced a bispecific reagent with picomolar binding affinity and a binding half-life of several hours. IMCgp100 has entered pivotal monotherapy trials for the treatment of patients with metastatic uveal melanoma. Conclusion The ImmTAC technology platform is a new class of bi-specific biologic based on T cell receptors. IMCgp100 is the first ImmTAC in clinical trials to treat advanced melanoma.