Production of recombinant DTctGMCSF fusion toxin in a baculovirus expression vector system for biotherapy of GMCSF-receptor positive hematologic malignancies

The fusion toxin DT(ct)GMCSF has been constructed by genetically replacing the native receptor-binding domain of diphtheria toxin (DT) with human granulocyte-macrophage colony stimulating factor (GMCSF). This recombinant fusion toxin preserves the catalytic (c) and membrane translocation (t) domains of DT and includes a sterically neutral peptide linker separating the toxin and growth factor domains. Previous work using DTct-GMCSF produced in Escherichia coli has shown that this chimeric toxin is selectively cytotoxic to GMCSF receptor (R)-positive acute myeloid leukemia (AML) cells both in vitro and in vivo. Its clinical development has been hampered due to very low expression levels, requirements for solubilization with guanidine hydrochloride and subsequent refolding, and concerns about bacterial endotoxin contamination. These difficulties prompted us to investigate the utility of a baculovirus/ insect cell expression system for the production of DTct-GMCSF. Here, we report that a soluble form of DT(ct)G-GMCSF can be produced in the baculovirus expression vector system (BEVS) and purified to homogeneity by column chromatography. The BEVS-derived DT(ct)G-MCSF fusion toxin caused apoptotic death in GMCSF-R-positive human AML cells at nanomolar concentrations. In contrast to the 100 mu g/L yields of pwified DT(ct)GMCSF obtained from E. coli, the BEVS allows us to routinely generate 8-10 mg/L of purified DT(ct)GMCSF. This increased capacity provided by the BEVS for the production of DT(ct)GMCSF makes it now possible to obtain sufficient quantities to carry out preclinical and clinical trials. To our knowledge, this is the first report of the successful utilization of the BEVS for producing a therapeutic fusion toxin. (C) 1998 Academic Press.