We have made an anti-mesothelin immunotoxin in which the major human B cell epitopes have been silenced or removed. Roche has taken a license to this invention and manufactured a new immunotoxin named RG7787 that has an anti-mesothelin Fab fused to a mutated form of PE. RG7787 has good anti-tumor activity in animal models and when combined with a Taxane produces complete remissions in these models. RG7787 (LMB-100) has entered clinical trials now ongoing at CCR NCI. We have also initiated a program to identify and remove human specific T cell epitopes. We have identified all the major T cell epitopes in PE38 and using alanine-scanning mutagenesis identified amino acids essential for T cell activation. Using this information we have constructed LMB-T20, which contains six mutations in domain III (R427A, F443A, L477H, R494A, R505A and L552E) and a deletion of all the epitopes in domain II. LMB-T20 has low immunogenicity, is very active on mesothelin-expressing cancer cells and has good anti-tumor activity in mice. It is a good candidate for clinical development We have also made an immunotoxin (LMB-T14) that has both B and T cell epitopes suppressed. We are carrying out experiments in which the goal is to identify compounds now in clinical use that increases immunotoxin activity. In collaboration with scientists at Roche we have found that Actinomycin D synergizes with anti-mesothelin immunotoxins to kill a variety of mesothelin expressing cancers both in cell culture and in mice. We have also found the protein kinase inhibitor H89 enhances killing by immunotoxins.
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