A noninvasive PET imaging method that measures granzyme B (image), a protein released by immune cells to kill cancer cells, was able to distinguish mouse and human tumors that responded to immune checkpoint inhibitors from those tumors that did not respond early in the course of treatment. The results were published in the May 2017 issue of Cancer Research. The article is titled “Granzyme B PET Imaging as a Predictive Biomarker of Immunotherapy Response.” The senior author was Umar Mahmood, MD, PhD, Professor of Radiology at Harvard Medical School and Director of the Division of Precision Medicine at the Athinoula A. Martinos Center for Biomedical Imaging in Massachusetts General Hospital (MGH), Boston. Although immunotherapies, such as checkpoint inhibitors, have revolutionized cancer treatment, they only work in a minority of patients, which means that most patients receiving this treatment will not benefit but still have the increased risk of side effects, besides losing time that they could spend on other therapies, Dr. Mahmood explained. Response to immunotherapy often cannot be measured effectively at early time points by traditional imaging techniques that measure tumor size, such as CT and MRI scans, or those that measure tumor glucose uptake, such as FDG PET, because these techniques cannot distinguish a nonresponding tumor from a tumor that is responding to immunotherapy but appears to grow in size because it is filled with immune cells and accompanied by increased glucose uptake, Dr. Mahmood noted. Tissue biopsies can be unreliable because of tumor heterogeneity and constant changes in the levels of the biomarker proteins measured.
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