The somatostatin receptor 2 antagonist ⁶⁴Cu-NODAGA-JR11 outperforms ⁶⁴Cu-DOTA-TATE in a mouse xenograft model

<div><p>Copper-64 is an attractive radionuclide for PET imaging and is frequently used in clinical applications. The aim of this study was to perform a side-by-side comparison of the <i>in vitro</i> and <i>in vivo</i> performance of ⁶⁴Cu-NODAGA-JR11 (NODAGA = 1,4,7-triazacyclononane,1-glutaric acid,4,7-acetic acid, JR11 = p-Cl-Phe-cyclo(D-Cys-Aph(Hor)-D-Aph(cbm)-Lys-Thr-Cys)D-Tyr-NH₂), a somatostatin receptor 2 antagonist, with the clinically used sst2 agonist ⁶⁴Cu-DOTA-TATE ((TATE = D-Phe-cyclo(Cys-Tyr-D-Trp-Lys-Thr-Cys)Thr). <i>In vitro</i> studies demonstrated K_d values of 5.7±0.95 nM (Bmax = 4.1±0.18 nM) for the antagonist ^64/natCu-NODAGA-JR11 and 20.1±4.4. nM (Bmax = 0.48±0.18 nM) for the agonist ^64/natCu-DOTA-TATE. Cell uptake studies showed the expected differences between agonists and antagonists. Whereas ⁶⁴Cu-DOTA-TATE (the agonist) showed very effective internalization in the cell culture assay (with 50% internalized at 4 hours post-peptide addition under the given experimental conditions), ⁶⁴Cu-NODAGA-JR11 (the antagonist) showed little internalization but strong receptor-mediated uptake at the cell membrane. Biodistribution studies of ⁶⁴Cu-NODAGA-JR11 showed rapid blood clearance and tumor uptake with increasing tumor-to-relevant organ ratios within the first 4 hours and in some cases, 24 hours, respectively. The tumor washout was slow or non-existent in the first 4 hours, whereas the kidney washout was very efficient, leading to high and increasing tumor-to-kidney ratios over time. Specificity of tumor uptake was proven by co-injection of high excess of non-radiolabeled peptide, which led to >80% tumor blocking. ⁶⁴Cu-DOTA-TATE showed less favorable pharmacokinetics, with the exception of lower kidney uptake. Blood clearance was distinctly slower and persistent higher blood values were found at 24 hours. Uptake in the liver and lung was relatively high and also persistent. The tumor uptake was specific and similar to that of ⁶⁴Cu-NODAGA-JR11 at 1 h, but release from the tumor was very fast, particularly between 4 and 24 hours. Tumor-to-normal organ ratios were distinctly lower after 1 hour. This is indicative of insufficient <i>in vivo</i> stability. PET studies of ⁶⁴Cu-NODAGA-JR11 reflected the biodistribution data with nicely delineated tumor and low background. ⁶⁴Cu-NODAGA-JR11 shows promising pharmacokinetic properties for further translation into the clinic.</p>