Impact of Multiple Negative Charges on Blood Clearanceand Biodistribution Characteristics of 99mTc-Labeled DimericCyclic RGD Peptides

摘要

This study sought to evaluate the impact of multiple negative charges on blood clearance kinetics and biodistribution properties of ^99mTc-labeled RGD peptide dimers. Bioconjugates HYNIC-P6G-RGD2 and HYNIC-P6D-RGD2 were prepared by reacting P6G-RGD2 and P6D-RGD2, respectively, with excess HYNIC-OSu in the presence of diisopropylethylamine. Their IC50 values were determined to be 31 ± 5 and 41 ± 6 nM, respectively, against ¹²⁵I-echistatin bound to U87MG glioma cells in a whole-cell displacement assay. Complexes [^99mTc(HYNIC-P6G-RGD2)(tricine)(TPPTS)] (^99mTc-P6G-RGD2) and [^99mTc(HYNIC-P6D-RGD2)(tricine)(TPPTS)] (^99mTc-P6D-RGD2) were prepared in high radiochemical purity (RCP > 95%) and specific activity (37–110 GBq/μmol). They were evaluated in athymic nude mice bearing U87MG glioma xenografts for their biodistribution. The most significant difference between ^99mTc-P6D-RGD2 and ^99mTc-P6G-RGD2 was their blood radioactivity levels and tumor uptake. The initial blood radioactivity level for ^99mTc-P6D-RGD2 (4.71 ± 1.00%ID/g) was ∼5× higher than that of ^99mTc-P6G-RGD2 (0.88 ± 0.05%ID/g), but this difference disappeared at 60 min p.i. ^99mTc-P6D-RGD2 had much lower tumor uptake (2.20–3.11%ID/g) than ^99mTc-P6G-RGD2 (7.82–9.27%ID/g) over a 2 h period. Since HYNIC-P6D-RGD2 and HYNIC-P6G-RGD₂ shared a similar integrin α_vβ₃ binding affinity (41 ± 6 nM versus 31 ± 5 nM), the difference in their blood activity and tumor uptake is most likely related tothe nine negative charges and high protein binding of ^99mTc-P6D-RGD₂. Despite its low uptake in U87MG tumors, thetumor uptake of ^99mTc-P6D-RGD₂ was integrinα_vβ₃-specific. SPECT/CT studieswere performed using ^99mTc-P6G-RGD₂ in athymicnude mice bearing U87MG glioma and MDA-MB-231 breast cancer xenografts.The SPECT/CT data demonstrated the tumor-targeting capability of ^99mTc-P6G-RGD₂, and its tumor uptake depends onthe integrin α_vβ₃ expression levelson tumor cells and neovasculature. It was concluded that the multiplenegative charges have a significant impact on the blood clearancekinetics and tumor uptake of ^99mTc-labeled dimeric cyclicRGD peptides.