Preclinical Characterization of a Stabilized Gastrin-Releasing Peptide Receptor Antagonist for Targeted Cancer Theranostics

Radiolabeled gastrin-releasing peptide receptor (GRPR) antagonists have shown great promise for the theranostics of prostate cancer; however, their suboptimal metabolic stability leaves room for improvements. It was recently shown that the replacement of Gly(11) with Sar(11) in the peptidic [D-Phe(6),Leu(13)-NHEt,des-Met(14)]BBN(6–14) chain stabilized the [(99m)Tc]Tc-DB15 radiotracer against neprilysin (NEP). We herein present DOTAGA-PEG(2)-(Sar(11))RM26 (AU-RM26-M1), after Gly(11) to Sar(11)-replacement. The impact of this replacement on the metabolic stability and overall biological performance of [(111)In]In-AU-RM26-M1 was studied using a head-to-head comparison with the unmodified reference [(111)In]In-DOTAGA-PEG(2)-RM26. In vitro, the cell uptake of [(111)In]In-AU-RM26-M1 could be significantly reduced in the presence of a high-excess GRPR-blocker that demonstrated its specificity. The cell uptake of both radiolabeled GRPR antagonists increased with time and was superior for [(111)In]In-AU-RM26-M1. The dissociation constant reflected strong affinities for GRPR (500 pM for [(111)In]In-AU-RM26-M1). [(111)In]In-AU-RM26-M1 showed significantly higher stability in peripheral mice blood at 5 min pi (88 ± 8% intact) than unmodified [(111)In]In-DOTAGA-PEG(2)-RM26 (69 ± 2% intact; p < 0.0001). The administration of a NEP inhibitor had no significant impact on the Sar(11)-compound (91 ± 2% intact; p > 0.05). In vivo, [(111)In]In-AU-RM26-M1 showed high and GRPR-mediated uptake in the PC-3 tumors (7.0 ± 0.7%IA/g vs. 0.9 ± 0.6%IA/g in blocked mice) and pancreas (2.2 ± 0.6%IA/g vs. 0.3 ± 0.2%IA/g in blocked mice) at 1 h pi, with rapid clearance from healthy tissues. The tumor uptake of [(111)In]In-AU-RM26-M1 was higher than for [(111)In]In-DOTAGA-PEG(2)-RM26 (at 4 h pi, 5.7 ± 1.8%IA/g vs. 3 ± 1%IA/g), concordant with its higher stability. The implanted PC-3 tumors were visualized with high contrast in mice using [(111)In]In-AU-RM26-M1 SPECT/CT. The Gly(11) to Sar(11)-substitution stabilized [(111)In]In-DOTAGA-PEG(2)-(Sar(11))RM26 against NEP without negatively affecting other important biological features. These results support the further evaluation of AU-RM26-M1 for prostate cancer theranostics after labeling with clinically relevant radionuclides.