Development of a non-invasive approach to image human islet cell mass within the pancreas or following islet transplantation is of considerable interest for the diagnosis and prognosis of pre-diabetes or ongoing rejection of transplanted islets and for determining the efficacy of therapeutic interventions 1–8
. However, two major difficulties must first be overcome. First, islets of Langerhans comprise only 1–2% of the normal human pancreas and each islet contains 40–70% beta cells. Thus the beta cell mass within a healthy pancreas constitutes only a small percentage of the total pancreas mass, and in diabetic patients this percentage is even lower. In islet transplant recipients, the preferred engraftment approach is via the intraportal venous route 9
, which leads to islets scattered through the hepatic vascular tissue. Fortunately, the current non-invasive imaging modalities that include magnetic resonance imaging (MRI) 10, 11
, nuclear imaging by either positron emission tomography (PET) or single photon emission computed tomography (SPECT) all exhibit extreme detection sensitivity favorable to the detection of deeply seated organs such as the pancreas or liver. Secondly, the pancreas is surrounded anatomically by the liver, spleen, and intestines that often accumulate high levels of imaging agents following intravenous administration. The accumulation of the radioactive tracer in these and other surrounding non-target tissues can reduce target/non-target ratios and therefore interfere with pancreas imaging.
Previous attempts using nuclear imaging for detection of the pancreatic islets have mainly focused on radiolabeled small molecules directly targeting the vesicular monoamine transporter-type 2 receptor (VMAT2R) and radiolabeled peptides directly targeting the glucagon-like peptide-1 receptor (GLP-1R) 12–17
. In addition to the common challenges of imaging islet cell mass, each of these approaches has its own caveats. Apart from beta cells, VMAT2R is also expressed in cells of the central nervous system, in chromaffin cells of the adrenal medulla, and in enterochromaffin-like cells of the stomach 18
. Nonspecific accumulation in the pancreas is also a concern 18, 19
. In the case of the GLP-1R targeting peptides, the target level fluctuates with the concentration of blood glucose and therefore may be an unreliable indicator of beta cell mass. A third approach using directly labeled antibodies specific for beta cells is under consideration 20, 21
, but like all labeled large proteins, intact antibodies accumulate slowly in their target and clear slowly from surrounding tissues, often leading to unfavorable target/non-target ratios. Smaller single chain antibodies displaying faster pharmacokinetics are now under development to address the slow target accumulation and tissue clearance 22
Pretargeting is an attractive alternative that addresses the slow pharmacokinetics and unfavorable target/non-target ratios observed in the direct targeting approach using labeled antibodies. Pretargeting increases target to non-target ratios and has been successful for imaging tumors 23–27
and infection 28, 29
. We have validated a novel pretargeting strategy using a complementary pair of phosphorodiamidate morpholino oligomers (MORF/cMORF) in mouse tumor models 30–35
, showing both high sensitivity and specificity and favorable target to non-target ratios.
In the present report, we designed an imaging approach to extend the benefits of MORF/cMORF pretargeting experienced in tumor imaging to a non-cancerous tissue, in this case, islets. We used the human islet cell-specific antibody HPi1 36
to target human islets and a human insulinoma cell line (betalox5) 37, 38
following their transplantation into NOD-scid IL2rγnull
mice. We now report that the pretargeting MORF conjugated antibody specifically binds to human islet cells and the labeled cMORF specifically binds to the pretargeting MORF-antibody both in vitro and in vivo. We observed that our pretargeting strategy readily allows non-invasive imaging of human islets and betalox5 cells transplanted into immunodeficient mice.