[^113mIn]In-RM2: a high potential agent for SPECT imaging of GRPR-expressing tumors

Gastrin-releasing peptide receptors (GRPRs) overexpressed in many cancers are known as promising biomarkers to target tumors such as prostate, breast, and lung cancers. As the early diagnosis of the cancers can serve for better treatment of the patients, [^113mIn]In-DOTA-Pip-D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2 ([^113mIn]In-RM2) was prepared using an in-house developed ¹¹³Sn/^113mIn generator. 0.05 M HCl was chosen as the best solution for the generator elution, and 3rd-6th fractions of the generator with the highest activity concentration were used. The chemical, radiochemical, and radionuclide purities of the eluted [^113mIn]InCl₃ were studied using Inductively Coupled Plasma Mass Spectrometry (ICP-MS), radio thin layer chromatography (RTLC), and gamma spectrometry methods, respectively. The radiolabeled peptide was prepared in optimal conditions and the radiochemical purity (RCP) was investigated by RTLC and high-performance liquid chromatography (HPLC) methods. After stability and lipophilicity assessments, the biodistribution of the final complex was checked in normal animals by imaging and scarification. [^113mIn]In-RM2 was prepared with RCP > 99% (RTLC and HPLC), and specific activity of 43.2 TBq/mmol at optimal labeling conditions. The complex was stable in human serum and PBS buffer for at least 3 h (RCP > 96%). The partition coefficient showed the hydrophilic nature of the complex which results in the fast blood clearance via urinary tract. The biodistribution studies was demonstrated high accumulation of [^113mIn]In-RM2 in GRPR-expressing tissues. The results showed [^113mIn]In-RM2 can be considered a high-potential agent for SPECT imaging of GRPR-expressing cancers.