Synergistic Cytotoxicity of a Toxin Targeting the Epidermal Growth Factor Receptor and the Glycosylated Triterpenoid SO1861 in Prostate Cancer

Treatment of advanced prostate cancer lacks specificity and curative intent. Therefore, the need for new targeted therapeutic approaches is high. In the present study, we generated the new targeted toxin EGF-PE24mutΔREDLK binding to the epidermal growth factor receptor (EGFR) on the surface of prostate cancer cells. It consists of the human epidermal growth factor (EGF) as binding domain and a de-immunized variant of Pseudomonas Exotoxin A (PE), called PE24mutΔREDLK, as toxin domain. The toxin domain contains a deletion of the C-terminal KDEL-like motif REDLK to prevent its transport from sorting endosomes via the KDEL receptor mediated pathway into the cytosol, where it can inhibit cellular protein biosynthesis and induce apoptosis. Indeed, REDLK deletion resulted in a strong decrease in cytotoxicity of the targeted toxin in prostate cancer cells compared to the parental targeted toxin EGF-PE24mut. However, addition of the plant glycosylated triterpenoid SO1861, which is known to mediate the release of biomolecules from endolysosomal compartments into the cytosol, resulted in an up to almost 7,000-fold enhanced synergistic cytotoxicity. Moreover, combination of PE24mutΔREDLK with SO1861 led to a cytotoxicity that was even 16- to 300-fold enhanced compared to that of EGF-PE24mut. Endolysosomal entrapment of the non-toxic targeted toxin EGF-PE24mutΔREDLK followed by activation through enhanced endosomal escape therefore represents a new promising approach for the future treatment of advanced prostate cancer with high efficacy and diminished side effects.