An Experimental Study Of A Novel Myocardial Perfusion Imaging Agent Characterized By The First Extraction Rate Of High Myocardium And A Novel RGD Imaging Agent

Part ⅠImpact of Boronate Capping Groups on Biological Characteristics of Novel 99mTc(Ⅲ) Complexes [99mTcCl(CDO)(CDOH)2B-R](CDOH2= Cyclohexanedione Dioxime)Objective:This study sought to explore the impact of boronate groups on heart uptake and myocardial retention of novel 99mTc(Ⅲ) complexes [99mTcCl(CDO)(CDOH)2B-R] (99mTc-ISboroxime:R= isoxazol-4-yl (IS); 99mTc-MPboroxime:R= N-methylpyridinium (MP); 99mTc-PAboroxime:R= pyrazol-3-yl (PA); 99mTc-PYboroxime:R= pyridin-3-yl (PY); and 99mTc-5Uboroxime:R= uracil-5-yl(5U)).Methods:Biodistribution and imaging (planar and SPECT) studies were carried out using Sprague-Dawley (SD) rats. Planar image quantification was performed to compare their myocardial retention and liver clearance kinetics. SPECT/CT MPI imaging was performed in normal pigs with 99mTc-PAboroxime.Results:All new 99mTc radiotracers [99mTcCl(CDO)(CDOH)2B-R] (R= IS, MP, PA, PY and 5U) were prepared in high yield and high radiochemical purity (RCP= 90-98%). They were all stable in the kit mixture for> 6 h. It was found that their heart retention and liver clearance curves were best fitted to the bi-exponential decay function. The initial heart uptake at 0-1 min after injection followed the general ranking order of 99mTc-ISboroxime (4.98 ± 1.05% ID)～99mTc-Teboroxime (4.56 ± 0.91% ID)～99mTc-PAboroxime (4.03 ± 1.23% ID)～99mTc-PYboroxime (4.07 ± 0.80% ID)> 99mTc-5Uboroxime (3.24 ± 0.67% ID)> 99mTc-MPboroxime (2.53 ± 0.65% ID). The fast-phase retention time followed the general order of 99mTc-PAboroxime (3.21 ± 0.29 min)> 99mTc-Teboroxime (1.63 ± 0.40 min)～99mTc-PYboroxime (1.57 ± 0.29 min)～99mTc-ISboroxime (1.55 ± 0.32 min)> 99mTc-MPboroxime (0.68 ±0.16 min)> 99mTc-5Uboroxime (0.33 ±0.11 min).99mTc-PAboroxime (3.05 ± 1.10% ID/g) and 99mTc-ISboroxime (3.75 ± 0.68% ID/g) had the initial heart uptake very close to that of 99mTc-Teboroxime (3.30 ± 0.50% ID/g), but the myocardial retention time of 99mTc-PAboroxime was significantly longer than that of 99mTc-ISboroxime and 99mTc-Teboroxime. Even though the best time window is 0-5 min for SPECT image acquisition, high quality SPECT images could be readily obtained during over 30 min post-injection of 99mTc-PAboroxime in the SD rats. This statement was supported by the results from SPECT/CT studies in normal pigs.Conclusion:It was concluded that boronate groups had significant impact on the heart uptake, myocardial retention, and liver clearance of 99mTc(III) complexes [99mTcCl(CDO)(CDOH)2B-R].99mTc-PAboroxime has the heart uptake comparable to that of 99mTc-Teboroxime; but its myocardial retention time is significantly longer, which makes it possible to image the heart during the first 30 min with 99mTc-PAboroxime using both standard and specialized cardiac SPECT cameras.Part IIImpact of Multiple Negative Charges on Blood Clearance and Biodistribution Characteristics of 99mTc-Labeled Dimeric Cyclic RGD PeptidesObjective:This study sought to evaluate the impact of multiple negative charges on blood clearance kinetics and biodistribution properties of 99mTc-labeled RGD peptide dimers.Methods: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. They were evaluated in athymic nude mice bearing U87MG glioma xenografts for their biodistribution. SPECT/CT studies were performed using 99mTc-P6G-RGD2 in athymic nude mice bearing U87MG glioma and MDA-MB-231 breast cancer xenografts.Results: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). 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 αvβ3 binding affinity (41 ± 6 nM versus 31 ± 5 nM), the difference in their blood activity and tumor uptake is most likely related to the nine negative charges and high protein binding of 99mTc-P6D-RGD2. Despite its low uptake in U87MG tumors, the tumor uptake of 99mTc-P6D-RGD2 was integrin αvβ3-specific. The SPECT/CT data demonstrated the tumor-targeting capability of 99mTc-P6G-RGD2, and its tumor uptake depends on the integrin αvβ3 expression levels on tumor cells and neovasculature.Conclusion:It was concluded that the multiple negative charges have a significant impact on the blood clearance kinetics and tumor uptake of 99mTc-labeled dimeric cyclic RGD peptides.