PNNL

Abstract

Integrins avß3 and avß5 play a critical role in tumor-induced angiogenesis and metastasis, and have become promising diagnostic indicators and therapeutic targets of tumors. Radiolabeled RGD peptides that are integrin-specific may be used for non-invasive imaging of integrin expression level as well as for integrin-targeted radionuclide therapy. We previously conjugated a series of mono- and dimeric RGD peptides with 1,4,7,10-tetraazacyclododecane-N, N’,N’’,N’’’-tetraacetic acid (DOTA) and labeled these with copper-64 for microPET imaging in various mouse xenograft models. The copper-64 tracers showed avß3-selective tumor uptake, but the magnitude of tumor uptake was relatively low, the tumor washout was rapid, and non-target organ/tissue retention was high. In this study we developed a tetrameric RGD peptide tracer 64Cu-DOTA-E{E[c(RGDfK)]2}2 for positron emission tomography (PET) imaging of integrin avß3 expression in a subcutaneous U87MG glioma xenograft model in female athymic nude mice. The RGD tetramer showed significantly higher integrin binding affinity than the corresponding mono- and dimeric RGD analogs, most likely due to polyvalency effect. The radiolabeled peptide showed rapid blood clearance (0.61 ± 0.01%ID/g at 30 min and 0.21 ± 0.01 %ID/g at 4 h postinjection (p.i.), respectively) and predominantly renal excretion. Tumor uptake was rapid and high and the tumor washout was slow (9.93 ± 1.05 %ID/g at 30 min p.i. and 4.56 ± 0.51 %ID/g at 24 h post-injection). The metabolic stability of 64Cu-DOTA-E{E[c(RGDfK)]2}2 was determined in mouse blood, urine, and liver and kidney homogenates at different times after tracer injection. The average fractions of intact tracer in these organs at 1 h were approximately 70, 58, 51 and 26 percent, respectively. Non-invasive microPET imaging studies showed significant tumor uptake and good contrast in the subcutaneous tumor-bearing mice, which agreed well with the biodistribution results. Integrin avß3 specificity was demonstrated by successful blocking of tumor uptake of 64Cu-DOTA-E{E[c(RGDfK)]2}2 in the presence of excess amount of c(RGDyK) at 1 h postinjection. The highest absorbed radiation doses determined for the human reference adult were received by the urinary bladder wall (0.263 mGy/MBq), kidneys (0.0298 mGy/MBq), and liver (0.0244 mGy/MBq). Assuming 0.5-g U87MG glioma tumors in man, we calculated an absorbed dose of 65.3 mGy/MBq (242 rad/mCi) following a single injection of 64Cu-DOTA-E{E[c(RGDfK)]2}2. In conclusion, the high integrin avidity and favorable biokinetics make 64Cu-DOTA-E{E[c(RGDfK)]2}2 a promising agent for peptide receptor radionuclide imaging therapy of integrin-positive tumors.