[99mTc]Tc-HYNIC-RM2: A potential SPECT probe targeting GRPR expression in prostate cancers

by Selleckchem | Jan 12 2024

Introduction: Elevated density of gastrin releasing peptide receptors (GRPR) in prostate cancer has led to exploration of several radiolabeled peptides for imaging and staging of the disease. The GRPR antagonist peptide RM2 has been successfully conjugated with several chelators and radiolabeled with gallium-68. The goal of this study was to synthesize a 99mTc-labeled probe and investigate its potential for SPECT imaging of prostate cancer. Towards this HYNIC-RM2 peptide conjugate was synthesized, radiolabeled with 99mTc and evaluated in GRPR-positive PC3 tumor xenografts.

Methods: HYNIC-RM2 was manually synthesized by standard Fmoc solid phase strategy and radiolabeled with 99mTc. In vitro cell studies were performed in GRPR-positive human prostate carcinoma (PC3) cells. Metabolic stability studies of [99mTc]Tc-HYNIC-RM2 were performed in normal mice in the presence as well as absence of neutral endopeptidase (NEP) inhibitor, phosphoramidon (PA). Biodistribution and imaging studies of [99mTc]Tc-HYNIC-RM2 were performed in SCID mice bearing PC3-xenograft.

Results: [99mTc]Tc-HYNIC-RM2 exhibited high binding affinity in low nanomolar range (Kd = 1.83 ± 0.31 nM). Metabolic stability studies in mice indicated that in the absence of PA, radiolabeled peptide was about 65 % intact in the blood at 15 min p.i., whereas proportion of intact radiolabeled peptide was enhanced to 90 % on co-administration of PA. Biodistribution studies in PC3 tumor bearing mice demonstrated high tumor uptake (8.02 ± 0.9%ID/g and 6.13 ± 0.44%ID/g at 1 h and 3 h p.i.). Co-administration of PA with the radiolabeled peptide resulted in further enhancement of tumor uptake (14.24 ± 0.76 % ID/g and 11.71 ± 0.59%ID/g at 1 h and 3 h p.i.). SPECT/CT images of [99mTc]Tc-HYNIC-RM2 could clearly visualize the tumor. Significant (p < 0.001) reduction in the tumor uptake with a co-injected blocking dose of unlabeled peptide ascertained the GRPR specificity of [99mTc]Tc-HYNIC-RM2.

Conclusion: Encouraging results obtained in biodistribution and imaging studies indicate the potential of [99mTc]Tc-HYNIC-RM2 for further exploration as GRPR targeting agent.

Comments:

This study aimed to develop and evaluate a 99mTc-labeled probe, specifically HYNIC-RM2, for potential SPECT imaging of prostate cancer, targeting the elevated density of gastrin releasing peptide receptors (GRPR) in the disease. The methodology involved synthesizing the HYNIC-RM2 peptide conjugate, radiolabeling it with 99mTc, and assessing its performance in GRPR-positive PC3 tumor xenografts.

The manual synthesis of HYNIC-RM2 followed the standard Fmoc solid-phase strategy, and subsequent radiolabeling with 99mTc was executed. In vitro studies using GRPR-positive human prostate carcinoma (PC3) cells showcased high binding affinity in the low nanomolar range, demonstrating a Kd value of 1.83 ± 0.31 nM.

Metabolic stability studies in mice revealed that the radiolabeled peptide's integrity in the blood at 15 minutes post-injection increased from about 65% to 90% on co-administration of the neutral endopeptidase (NEP) inhibitor, phosphoramidon (PA). Biodistribution studies in mice bearing PC3 tumors demonstrated substantial uptake of the radiolabeled peptide in the tumor at 1 and 3 hours post-injection, with further enhancement noted upon co-administration of PA.

SPECT/CT imaging of [99mTc]Tc-HYNIC-RM2 successfully visualized the tumor, and the specificity of the probe to GRPR was confirmed through a significant reduction in tumor uptake when a blocking dose of unlabeled peptide was co-injected (p < 0.001).

Overall, the study's findings from biodistribution and imaging studies are promising, indicating the potential of [99mTc]Tc-HYNIC-RM2 as a candidate for further exploration as a GRPR targeting agent in the context of prostate cancer imaging and staging.