Development of non-invasive technology for islet imaging is a major yet unattained goal for monitoring islet cell mass during the progression of diabetes, for determining the efficacy of therapeutics, and the survival of islets following curative transplantation. Imaging of islets within the pancreas is particularly difficult since the pancreas is adjacent to organs where imaging agents clear through or into, leading to high background and low target to non-target (T/NT) ratios. Investigators have attempted to improve the T/NT ratios using directly labeled antibodies specific for beta cells but have observed high background of the labeled antibody in non-target organs adjacent to pancreas. We now have available three novel technologies to overcome these obstacles. First, we have developed a novel pretargeting technology that lowers the background observed using directly labeled antibodies. Second, we now have available human-specific islet antibodies. Third, we have developed unique human islet engrafted immunodeficient mouse models that will allow in vivo imaging of human islets in the absence or during alloimmune destruction. Applying our novel pretargeting technology we developed for imaging tumors, we will determine if we can successfully image human islets following islet transplantation into immunodeficient mice. Antibodies specific to human islets will be conjugated to a synthetic oligomer (MORF) and injected into immunodeficient mice transplanted with a human insulin-producing cell line or primary human islets. Following clearance of the unbound MORF-antibody from the circulation, a radiolabeled complementary oligomer (cMORF) that specifically binds to the MORF on the antibody will be injected. We have documented that labeled cMORF clears within 3 h through the kidney and does not accumulate within liver or intestine, leading to a very low background of the label in non-target organs. Using this new technology, we will image a human insulin-producing cell line and then primary human islets transplanted intrasplenically or intrapancreatically in mice and follow their loss during alloimmune destruction by a human immune system. The increase in T/NT ratios attained using our pretargeting strategy, particularly in organs surrounding the pancreas, will enhance greatly our ability to image human islets in vivo. We have validated the conjugation of MORF to antibodies and established the protocols for cMORF labeling with SPECT imaging nuclides 99mTc and 111In. Applying our novel technology to non-invasive human islet cell imaging will provide a critical tool for evaluating islet cell mass in healthy high-risk individuals, efficacy of intervention therapies, and islet cell survival in ongoing clinical islet transplantation trials. The conjugation of MORF to antibody, in vitro evaluation of conjugated antibodies, radiolabeling of cMORF, and imaging with SPECT will be assumed by the Biomarker lab of Dr. Liu and his colleagues. Selection of antibodies and appropriate models for imaging human islets will be provided by Dr. Greiner. Combining these diverse but complementary areas of expertise uniquely positions us to use novel new technologies for developing a non-invasive approach for imaging beta cells in vivo.
Type 1 diabetes mellitus (T1DM) is characterized by and Type 2 diabetes mellitus (T2DM) is also involved in the loss of insulin-producing beta cells. The only cure for T1DM presently available is islet transplantation. However, during the development or progression of T1DM and T2DM and following islet transplantation, there is currently no non-invasive method available to measure islet cell mass. We have assembled a collaborative team to address this critical issue. Dr. Liu, a radiochemist with expertise in imaging, and Dr. Greiner, an immunologist with expertise in humanized animal, will test a novel strategy developed in the Biomarker Development Lab of Dr. Liu and his colleagues for measuring beta cell mass in human islet transplanted mice and in mice undergoing human islet allograft rejection. This approach, if successful, will permit the evaluation of the progression of the disease process, the effectiveness of intervention strategies, and the survival of transplanted human islets using non-invasive imaging technology prior to expression of overt hyperglycemia.
