Synergistic cytotoxicity of a HER2-targeted antibody-colchinol methyl ether conjugate & an endosome disruptive peptide
Antibody-drug conjugates are an increasingly important class of cancer therapeutics.
These agents generally bind a specific receptor on cell surfaces, undergo receptor-mediated endocytosis, and are delivered into the endosomal-lysosomal system, where the environment in the lumen of these organelles facilitates cleavage of an attached cytotoxin. However, in some cell types, cleavage of the linker between the antibody and the drug may be inefficient, limiting release of the toxic agent. To control the release of small molecules from antibodies, we hypothesized that antibodies conjugated to a membrane-impermeable cytotoxin via a disulfide might synergize with a non-toxic peptide that facilitates the escape of small molecules from endosomes. To test this hypothesis, we conjugated a cell-impermeable anionic derivative of the tubulin-binding cytotoxin colchinol methyl ether to the HER2-targeting antibody trastuzumab (Herceptin) via a disulfide. When this antibody engages HER2 on the surface of SKBR3 breast cancer cells, and undergoes endocytosis, the impermeable cytotoxin becomes trapped in early endosomes, rendering it non-toxic. In contrast, co-administration of this conjugate with a non-toxic cholesterol-linked endosome disruptive peptide released the small molecule into the cytoplasm, producing a powerful synergistic effect. This co-administration resulted in IC50 values for the antibody conjugate of less than one nanomolar, enhancing toxicity by over 1000-fold. This approach has potential for enhancing the therapeutic efficacy of antibody-drug conjugates.