Shramm PA, Ancheta L, Higgins D, Lappi DA (2017) A rapid, pH-sensitive screening method to detect internalization of cell surface markers for development of antibody-based pharmaceuticals to treat brain tumors. Neuroscience 2017 Abstracts 566.24 / H7. Society for Neuroscience, Washington, DC.
Summary: Some of the most potent treatments for cancers have been antibodies to cell surface proteins that cause tumor cell proliferation. Examples are cetuximab (antigen: EGFR) approved for colorectal cancer and Trastuzumab (ERBB2) for breast cancer. These antibodies have more than one effect on the cancer cell, but one of the most important is that, upon binding to the cell surface antigen, the complex is internalized by so-called antibody mediated internalization. As such, the mitogenic cell surface protein no longer plays a role in cancer cell division. Despite the blood brain barrier challenging systemic treatment for brain tumors, intracerebroventricular injection can produce similar results. For example, Gholamin et al., (Sci Transl Med 9:381, 2017) and Kang et al. (Sci Rep 6:34922, 2016) reported down-regulation of brain tumor mitogenic agents through antibody-mediated endocytosis. The quick and efficient screening of antibodies that internalize effectively is vital for determining suitability of an antibody as a therapeutic targeting agent. Here we describe a method for the efficient determination of internalization of cell surface molecules by antibodies using a pH-dependent fluorescent reporter cross-linked to a secondary antibody in a plate-based assay with visualization of internalization in hours. This conjugate is comprised of an affinity-purified monovalent secondary antibody against both the heavy and light chain of human or mouse IgG and is conjugated to a pH -dependent fluorescent reporter. The fluorescence from this reporter increases intensity as the pH of its surroundings becomes more acidic, as evident when exposed to the environment inside a cell (thereby providing evidence of internalization). A successful assay protocol has been developed to provide an EC50 by way of a fluorescence-detecting plate reader, which could be used to explore antibody candidates as therapeutics in a quick and reproducible manner.
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