Side-by-side comparison of the two widely studied GRPR radiotracers, radiolabeled NeoB and RM2, in a preclinical setting

Introduction NeoB and RM2 are the most investigated gastrin-releasing peptide receptor (GRPR)–targeting radiotracers in preclinical and clinical studies. Therefore, an extensive side-by-side comparison of the two radiotracers is valuable to demonstrate whether one has advantages over the other. Accordingly, this study aims to compare the in vitro and in vivo characteristics of radiolabeled NeoB and RM2 to guide future clinical studies. Method The stability of the radiolabeled GRPR analogs was determined in phosphate buffered saline (PBS), and commercially available mouse and human serum. Target affinity was determined by incubating human prostate cancer PC-3 cells with [ 177 Lu]Lu-NeoB or [ 177 Lu]Lu-RM2, + / − increasing concentrations of unlabeled NeoB, RM2, or Tyr 4 -bombesin (BBN). To determine uptake and specificity cells were incubated with [ 177 Lu]Lu-NeoB or [ 177 Lu]Lu-RM2 + / − Tyr 4 -BBN. Moreover, in vivo studies were performed to determine biodistribution and pharmacokinetics. Finally, radiotracer binding to various GRPR-expressing human cancer tissues was investigated. Results Both radiotracers demonstrated high stability in PBS and human serum, but stability in mouse serum decreased substantially over time. Moreover, both radiotracers demonstrated high GRPR affinity and specificity, but a higher uptake of [ 177 Lu]Lu-NeoB was observed in in vitro studies. In vivo, no difference in tumor uptake was seen. The most prominent difference in uptake in physiological organs was observed in the GRPR-expressing pancreas; [ 177 Lu]Lu-RM2 had less pancreatic uptake and a shorter pancreatic half-life than [ 177 Lu]Lu-NeoB. Furthermore, [ 177 Lu]Lu-RM2 presented with a lower tumor-to-kidney ratio, while the tumor-to-blood ratio was lower for [ 177 Lu]Lu-NeoB. The autoradiography studies revealed higher binding of radiolabeled NeoB to all human tumor tissues. Conclusion Based on these findings, we conclude that the in vivo tumor-targeting capability of radiolabeled NeoB and RM2 is similar. Additional studies are needed to determine whether the differences observed in physiological organ uptakes, i.e., the pancreas, kidneys, and blood, result in relevant differences in organ absorbed doses when the radiotracers are applied for therapeutic purposes.