Immunotoxin and bcl-2 inhibitor combination therapy targeting chondroitin sulfate proteoglycan 4.

74 Background: Immunotoxins (ITs) are a class of bifunctional chimeric proteins composed of an antibody fragment linked to a toxin. When ITs internalize into target cells, they induce protein synthesis inhibition and apoptosis. While ITs are highly specific and potent, the efficacy of IT-based therapies in some tumor cells is limited by hyperactive anti-apoptotic pathways and inefficient translocation of ITs from the endoplasmic reticulum to the cytosol. Therefore, to improve the efficacy of IT-based therapies, we evaluated a dual-pathway therapy that combines an IT with the ABT-737, ABT-263, or ABT-199 small molecule Bcl-2 inhibitor. Methods: The immunotoxin 9.2.27-PE38KDEL (9.2.27-IT) was generated by fusing a truncated mutant form of Pseudomonas exotoxin A to a single-chain variable fragment antibody. It targets human chondroitin sulfate proteoglycan 4 (CSPG4), an antigen highly expressed in a variety of cancer cells. We screened and identified 3 human glioblastoma xenografts, 3 human melanoma cell lines, and 5 human breast cancer cell lines resistant to the 9.2.27-IT despite their high levels of cell surface expression of CSPG4 (IC50 of IT alone was >100 ng/ml in all cell lines except for one melanoma cell line). In vitro cytotoxicity of the 9.2.27-IT —alone or in combination with the individual Bcl-2 inhibitors ABT-737, ABT-263, or ABT199—was assessed. Concentrations of ABT analogues were chosen so that ABT alone did not induce cytotoxicity. Results: The treatment groups that responded to the combination therapy yielded IC50 values ranging from 0.04 – 9 ng/ml for glioblastoma xenografts, 0.21-15 ng/ml for melanoma cell lines, and 4.5-50 ng/ml for breast cancer cell lines. The most potent combination group showed >1000 fold improvement of IC50 compared to using the immunotoxin alone. ABT-737 produced the strongest synergistic effects among the ABT analogues. Preliminary results from in vivo studies further demonstrated that this approach engendered a synergistic response and delayed tumor growth in immunotoxin-resistant mouse tumor models. Conclusions: This new combinatorial approach will potentially help to overcome immunotoxin resistance in cancer patients and provide better therapeutic outcomes.