99m Tc-Labeled Diarylpyrazoles for Single-Emission Computer Tomography Imaging of Neurotensin Receptor-Positive Tumors: A Comparative Preclinical Study

Background/Objectives: Neurotensin receptors (NTSRs), members of the G protein-coupled receptor (GPCR) family, have been found to be overexpressed in several types of human cancers, including breast, colon, lung, liver, prostate, and pancreatic cancer. In particular, NTSR1 is overexpressed in at least 75% of pancreatic ductal adenocarcinomas. The aim of the present study was the development and evaluation of new 99m Tc-labeled nonpeptide NTSR1-antagonists for SPECT imaging of NTSR-positive tumors. Methods: Multistep syntheses of NTSR1 antagonist derivatives were performed following our previously described procedure. Two different chelating strategies were applied for 99m Tc radiolabeling to provide the [ 99m Tc]Tc-HYNIC complex [ 99m Tc]1 and the [ 99m Tc]Tc-tricarbonyl complex [ 99m Tc]2 . Receptor binding assays were performed using hNTSR1-expressing CHO cells. Radiochemical yields (RCYs) were determined by radio-HPLC. For [ 99m Tc]1 and [ 99m Tc]2 , log D 7.4 , plasma protein binding, stability in human plasma and serum, and cellular uptake in HT-29 cells were determined. Biodistribution studies and small animal SPECT studies were performed in HT-29 tumor-bearing nude mice. Results: The radiosynthesis of [ 99m Tc]1 (log D 7.4 = −0.27) and [ 99m Tc]2 (log D 7.4 = 1.00) was successfully performed with RCYs of 94–96% (decay-corrected). Both radioligands were stable in human serum and plasma, showed plasma protein binding of 72% ( [ 99m Tc]1 ) and 82% ( [ 99m Tc]2 ), and exhibited high and specific uptake in HT-29 cells. Biodistribution studies in HT-29 tumor-bearing mice showed a higher tumor accumulation of [ 99m Tc]1 compared to [ 99m Tc]2 (8.8 ± 3.4 %ID/g vs. 2.7 ± 0.2 %ID/g at 2 h p.i.). [ 99m Tc]2 showed exceptionally high intestinal accumulation (49 ± 22 %ID/g at 1 h p.i.) and was therefore considered unfavorable. In the SPECT/CT imaging of HT-29 tumor xenografts, [ 99m Tc]1 showed a higher NTSR1-specific tumor uptake than [ 99m Tc]2 at all time points after tracer injection, with 12 ± 2.8 %ID/g for [ 99m Tc]1 vs. 3.1 ± 1.1 %ID/g for [ 99m Tc]2 at 4 h p.i. and adequate tumor-to-background ratios. Conclusions: In particular, the [ 99m Tc]Tc-HYNIC ligand ( [ 99m Tc]1 ) showed promising preclinical results, being a potential candidate for SPECT imaging and, therefore, appropriate for translation into the clinic.