Inhibition of protein synthesis in small cell lung cancer cells induced by the diphtheria toxin-related fusion protein DAB389 GRP.

DAB389 GRP is composed of the catalytic and transmembrane domains of diphtheria toxin fused to gastrin-releasing peptide (GRP). DAB389 GRP is selectively targeted to, and inhibits protein synthesis in, cell lines expressing GRP receptors. Protein synthesis in 5'ET4 cells (BALB/3T3 fibroblasts transfected with the gene encoding the GRP receptor) was inhibited by 50% in the presence of 20 pM DAB389 GRP (IC50, 20 pM). DAB389 GRP did not inhibit protein synthesis in untransfected BALB/3T3 cells. A second neuropeptide-conjugated toxin, DAB389 SP, directed to cells expressing substance P receptors, was not cytotoxic to 5'ET4 cells, nor was DAB389 GRP cytotoxic to substance P receptor-bearing cells. DAB389 GRP cytotoxic effects were receptor specific and were inhibited either by excess GRP or anti-GRP antibody. Cytotoxicity was mediated by passage through an acidic vesicle, because addition of 10 microM chloroquine to the reaction inhibited cytotoxicity. DAB389 GRP and DAB389 SP were tested on a number of tumor cell lines. DAB389 GRP inhibited protein synthesis in AR42J rat pancreatic acinar cells and HuTu 80 human duodenal adenocarcinoma cells with IC50s of 65 and 200 pM, respectively. DAB389 SP had an IC50 of 9.5 pM for the AR42J cells and 12 nM for the HuTu 80 cell line. A number of small cell lung cancer cell (SCLC) lines were tested, and the IC50 for DAB389 GRP ranged from 1.1 to 85 nM. Sensitivity to DAB389 GRP appeared to be based on receptor number and receptor type (i.e., GRP or neuromedin B preferring). SCLC cells were also sensitive to DAB389 SP, with IC50s ranging from 2.4 to 11.5 nM. These results suggest that a potential use exists for diphtheria-based fusion toxins as therapeutic agents for treatment of SCLC and other neuropeptide receptor-bearing cancers.