18F, 64Cu, and 68Ga Labeled RGD-Bombesin Heterodimeric Peptides for PET Imaging of Breast Cancer

Radiolabeled RGD (Arg-Gly-Asp) and bombesin (BBN) radiotracers that specifically target integrin alpha(v)beta(3) and gastrin releasing peptide receptor (GRPR) are both promising radiopharmaceuticals for tumor imaging. We recently designed and synthesized a RGD-BBN heterodimeric peptide with both RGD and BBN motifs in one single molecule. The F-18-labeled RGD-BBN heterodimer exhibited dual integrin alpha(v)beta(3) and GRPR targeting in a PC-3 prostate cancer model. In this study we investigated whether radiolabeled RGD-BBN tracers can be used to detect breast cancer by using microPET. Cell binding assay demonstrated that the high GRPR expressing breast cancer cells typically express low to moderate level of integrin a,03, while high integrin alpha(v)beta(3) expressing breast cancer cells have negligible level of GRPR. We labeled RGD-BBN heterodimer with three positron emitting radionuclides F-18, Cu-64, and Ga-68 and investigated the corresponding PET radiotracers in both orthotopic T47D (GRPR(+)/low integrin alpha(v)beta(3)) and MDA-MB-435 (GRPR(-)/integrin alpha(v)beta(+)(3)) breast cancer models. The three radiotracers all possessed in vitro dual integrin a alpha(v)beta(3) and GRPR binding affinity. The advantages of the RGD-BBN radiotracers over the corresponding BBN analogues are obvious for imaging MDA-MB-435 (GRPR(-)/integrin alpha(v)beta(+)(3)) tumor. F-18-FB-PEG(3)-RGD-BBN showed lower tumor uptake than Cu-64-NOTA-RGD-BBN and Ga-68-NOTA-RGD-BBN but was able to visualize breast cancer tumors with high contrast. Synthesis of Cu-64-NOTA-RGD-BBN and Ga-68-NOTA-RGD-BBN is much faster and easier than F-18-FB-PEG(3)-RGD-BBN. Cu-64-NOTA-RGD-BBN showed prolonged tumor uptake but also higher liver retention and kidney uptake than 68Ga-NOTA-RGD-BBN and F-18-FB-PEG(3)-RGD-BBN. Ga-68-NOTA-RGD-BBN possessed high tumor signals but also relatively high background uptake compared with the other two radiotracers. In summary, the prosthetic labeling groups, chelators, and isotopes all have a profound effect on the tumor targeting efficacy and in vivo kinetics of the RGD-BBN tracers for dual integrin and GRPR recognition. Further development of suitably labeled RGD-BBN tracers for PET imaging of cancer is warranted.