(467g) Imaging Agent Design for Type 1 Diabetes

Type 1 diabetes mellitus (T1DM), an organ-specific autoimmune disease, is characterized by the destruction of insulin-secreting beta cells in the pancreas, resulting in unregulated blood glucose levels. Due to the asymptomatic early stages of disease progression and difficulties visualizing the immune response, diabetes proves challenging to both study and treat. Recently, exendin-based derivatives have emerged as promising agents that rapidly target the GLP1-R expressed on beta cells for in vivo imaging and potential treatment. Here we use near-infrared (NIR) fluorophore-tagged exendin-based biologics to measure the effect of molecular weight on binding affinity to beta cell surface receptors, plasma clearance, permeability in the pancreas, and rate of endocytosis upon cell surface binding. Novel exendin-based probes were synthesized via copper-catalyzed click chemistry and multivalent linkers; NIR-tagged molecular weight variants include monomers (5 kDa), dimers (9 kDa), and trimers (13 kDa). In vitro assays with NIT-1 cells suggest all variants exhibit both specific beta cell surface labeling and high binding affinity. Surprisingly, the steric hindrance of multimers outweigh the avidity effects as shown by binding affinity values of 2 nM, 8 nM, and 16 nM for monomer, dimer, and trimer, respectively. In vivo studies indicate exendin dimers and trimers circulate longer in the plasma than the monomer in B6 mice while maintaining islet specificity. Lastly, rapid internalization of bound exendin is observed with NIT-1 cell lines followed by strong GLP1-R down regulation. Using experimentally determined values for binding, clearance, and internalization, the in vitro and in vivo data is incorporated in a pharmacokinetic model including vascular mass transfer coefficients and reaction diffusion equations to predict the optimal molecular weight for in vivo imaging and protein therapy of islets. A more comprehensive understanding of how intrinsic peptide and protein properties quantitatively impact in vivo transport and trafficking will allow for the design of more robust imaging agents and novel protein therapeutics for the diagnosis and treatment of this disease.