Comparison of biological properties of 99mTc-labeled cyclic RGD Peptide trimer and dimer useful as SPECT radiotracers for tumor imaging

Abstract

Introduction: This study sought to evaluate a ^99mTc-labeled trimeric cyclic RGD peptide (^99mTc-4P-RGD₃) as the new radiotracer for tumor imaging. The objective was to compare its biological properties with those of ^99mTc-3P-RGD₂ in the same animal model.

Methods: HYNIC-4P-RGD₃ was prepared by reacting 4P-RGD₃ with excess HYNIC-OSu in the presence of diisopropylethylamine. ^99mTc-4P-RGD₃ was prepared using a kit formulation, and evaluated for its tumor-targeting capability and biodistribution properties in the BALB/c nude mice with U87MG human glioma xenografts. Planar and SPECT imaging studies were performed in athymic nude mice with U87MG glioma xenografts. For comparison purpose, ^99mTc-3P-RGD₂ (a α_vβ₃-targeted radiotracer currently under clinical evaluation for tumor imaging in cancer patients) was also evaluated in the same animal models. Blocking experiments were used to demonstrate the α_vβ₃ specificity of ^99mTc-4P-RGD₃.

Results: ^99mTc-4P-RGD₃ was prepared with >95% RCP and high specific activity (~200GBq/μmol). ^99mTc-4P-RGD₃ and ^99mTc-3P-RGD₂ shared almost identical tumor uptake and similar biodistribution properties. ^99mTc-4P-RGD₃ had higher uptake than ^99mTc-3P-RGD₂ in the intestines and kidneys; but it showed better metabolic stability. The U87MG tumors were clearly visualized by SPECT with excellent contrast with ^99mTc-4P-RGD₃ and ^99mTc-3P-RGD₂.

Conclusion: Increasing peptide multiplicity from 3P-RGD₂ to 4P-RGD₃ offers no advantages with respect to the tumor-targeting capability. ^99mTc-4P-RGD₃ is as good a SPECT radiotracer as ^99mTc-3P-RGD₂ for imaging α_vβ₃-positive tumors.

Keywords: (99m)Tc-trimeric cyclic RGD peptide; SPECT; integrin α(v)β(3); tumor imaging.