99mTc radiotracers based on human GRP(18-27): synthesis and comparative evaluation

Area of Science:

• Nuclear medicine and molecular imaging.
• Radiopharmaceutical chemistry and drug development.
• Oncology and cancer diagnostics.

Background:

• Gastrin-releasing peptide receptors (GRPRs) are overexpressed in various human tumors, making them attractive targets for molecular imaging and therapy.
• Radiolabeled peptide analogs, particularly bombesin (BBN) analogs, have shown promise for targeting GRPRs.
• The development of novel radiotracers based on human GRP sequences offers potential for improved tumor targeting and detection.

Purpose of the Study:

• To synthesize and evaluate a series of novel technetium-99m (99mTc) labeled peptide analogs based on the human GRP(18-27) sequence.
• To investigate the impact of specific amino acid substitutions (Gly24 and Met27) on the biological characteristics of these radiotracers.
• To compare the performance of these analogs in vitro and in vivo for GRPR-positive tumor models.

Main Methods:

• Synthesis and purification of (99m)Tc-labeled GRP(18-27) analogs (SARNCs) using solid-phase peptide synthesis and HPLC.
• In vitro evaluation including competition binding assays in GRPR-positive PC-3 cell membranes and cellular internalization studies.
• In vivo assessment of metabolic stability in mice and biodistribution studies in mice bearing PC-3 xenografts, including GRPR blockade experiments.

Main Results:

• The synthesized SARNC analogs exhibited high affinity for GRPRs, with IC50 values ranging from 0.3 nM to 9.3 nM.
• (99m)Tc-SARNCs demonstrated specific internalization into PC-3 cells, with (99m)Tc-SARNC5 showing the fastest rate.
• The analogs displayed varying metabolic stability, and all effectively localized in GRPR-positive xenografts, with (99m)Tc-SARNC6 and (99m)Tc-SARNC2 showing the highest tumor-to-nontumor ratios.

Conclusions:

• Single and double amino acid substitutions in the (99m)Tc-SARNC1 motif significantly influence GRPR affinity, internalization, and in vivo stability.
• Improved analogs were identified, enhancing the pharmacokinetic profiles of the resulting radiopeptides.
• These findings support the potential clinical applicability of radioligands based on human GRP sequences for detecting and potentially treating GRPR-expressing tumors.