Zirconium-89 slow-offrate modified aptamers for PET imaging

1126 Objectives Oligonucleotide aptamers are novel binding reagents comprised of short strands of DNA or RNA that bind to non-nucleotide targets. Our team has made progress towards engineering aptamer reagents with very slow offrates, utilizing non-natural nucleotide bases to increase hydrophobic aptamer-target interactions. We developed an Epidermal Growth Factor Receptor (EGFR) targeted slow offrate modified aptamer (SOMAmer reagent) derivatized with the radionuclide 89Zr for in vivo PET imaging of EGFR-overexpressing neoplasms. Methods A SOMAmer was generated against EGFR. The SOMAmer was labeled with fluorophores on its 5’ end for fluorescence imaging and flow cytometry, or an amine functional group on the 5’ end to facilitate conjugation to desferroxamine (DFO). A nonbinding control SOMAmer was made with a scrambled sequence. SOMAmers were radiolabeled with 89Zr and analyzed by size exclusion chromatography. Results The EGFR SOMAmer binds specifically to EGFR-expressing cells in vitro in frozen sections of normal skin, cell culture staining, and via flow cytometry (Figure 1). 89Zr-labeled SOMAmers demonstrated significantly increased binding to human U118 EGFR+ glioblastoma cells (1680(200)) counts (mean(sd)) compared to C6 rat glioma cells lacking human EGFR expression in vitro (660(290)) counts (background (290(55) counts, comparison p Conclusions We have demonstrated EGFR-specific binding of a selective SOMAmer in vitro, with an indication that this selectively remains upon 89Zr-labeling. Further efforts will aim to reduce nonspecific binding by purifying the reagent and optimizing the synthetic route with the goal of evaluating the SOMAmer for in vivo imaging of EGFR-expressing neoplasms. Research Support 2013 Bracco Diagnostics Inc./RSNA Research Resident Grant #RR1314 Radiotracer work funded by: US Department of Energy (DOE) Somalogic, Inc. provided the EGFR Somamer