MicroPET Imaging of Integrin αvβ3 Expressing Tumors Using 89Zr-RGD Peptides

Purpose: The dimeric transmembrane integrin, alpha(v)beta(3), is a well-investigated target by different imaging modalities through suitably labeled arginine-glycine-aspartic acid (RGD) containing peptides. In this study, we labeled four cyclic RGD peptides with or without PEG functional groups: c(RGDfK) (denoted as FK), PEG(3)-c(RGDfK) (denoted as FK-PEG(3)), E[c(RGDfK)](2) (denoted as [FK](2)), and PEG(4)-E[PEG(4)-c(RGDfK)](2) (denoted as [FK](2)-3PEG(4)), with Zr-89 (t (1/2) = 78.4 h), using the chelator desferrioxamine-p-SCN (Df) for imaging tumor integrin alpha(v)beta(3). Methods: The Df conjugated RGD peptides were subjected to integrin alpha(v)beta(3) binding assay in vitro using MDA-MB-435 breast cancer cells. The Zr-89-labeled RGD peptides were then subjected to small animal positron emission tomography (PET) and direct tissue sampling biodistribution studies in an orthotopic MDA-MB-435 breast cancer xenograft model. Results: All four tracers, Zr-89-Df-FK, Zr-89-Df-FK-PEG(3), Zr-89-Df-[FK](2), and Zr-89-Df-[FK](2)-3PEG(4), were labeled in high radiochemical yield (89 +/- 4%) and high specific activity (4.07-6 MBq/mu g). Competitive binding assay with I-125-echistatin showed that conjugation of the RGD peptides to the Df chelator did not have significant impact on their integrin alpha(v)beta(3) binding affinity and the dimeric peptides were shown to be more potent than the monomers. In agreement with binding results, tumor uptake of Zr-89-Df-[FK](2) and Zr-89-Df-[FK](2)-3PEG(4) was significantly higher (4.32 +/- 1.73%ID/g and 4.72 +/- 0.66%ID/g, respectively, at 2 h post-injection) than the monomers Zr-89-Df-FK and Zr-89-Df-FK-PEG(3) (1.97 +/- 0.38%ID/g and 1.57 +/- 0.49%ID/g, respectively, at 2 h post-injection). Out of the four labeled peptides, Zr-89-Df-[FK](2)-3PEG(4) gave the highest tumor-to-background ratio (18.21 +/- 2.52 at 2 h post-injection and 19.69 +/- 3.99 at 4 h post-injection), with the lowest uptake in metabolic organs. Analysis of late time points biodistribution data revealed that the uptake in the tumor was decreased, along with increase in the bone, which implies decomplexation of Zr-89-Df. Conclusion: Efficient radiolabeling of peptides with an appropriate chelator such as Df-RGD with Zr-89 was observed. The Zr-89 radiolabeled peptides provided high-quality and high-resolution microPET images in xenograft models. Zr-89-Df-[FK](2)-3PEG(4) demonstrated the highest tumor-to-background ratio of the compounds tested. Preparation of Zr-89 peptides to take advantage of the longer half-life is unwarranted due to the relatively rapid clearance from the tumor region of peptide tracers prepared for this study and the increased uptake in the bone of transchelated Zr-89 with time (2.0 +/- 0.36%ID/g, 24 h post-injection).