In Vitro and In Vivo Evaluation of [99mTc(CO)3]-Radiolabeled ErbB-2-Targeting Peptides for Breast Carcinoma Imaging

ErbB-2 is a type 1 receptor tyrosine kinase over-expressed on ~30% of breast cancers and is an attractive target for the development of new diagnostic and therapeutic agents. In this study, an ErbB-2-targeting peptide, KCCYSL, previously isolated using bacteriophage display, was radiolabeled with [99mTc(H2O)3(CO)3]+ and examined for breast cancer in vitro cell binding and in vivo biodistribution and breast cancer imaging propensities. KCCYSL peptide was synthesized with the chelates diaminopropionc acid (DAP), Nα-histidinyl acetic acid [(NαHis)Ac], and 4-Ala-1,2-3-Triazol-1-acetic acid [(Ala-Triazol)Ac] at its amino-terminus via a gly-ser-gly (GSG) spacer and radiolabeled with [99mTc(H2O)3(CO)3]+. Radiolabeled peptide binding to cultured human MDA-MB-435 breast carcinoma cells was examined. Biodistribution and single photon emission computed tomography (SPECT)/CT imaging of the radiolabeled peptides were evaluated in female SCID mice bearing human MDA-MB-435 breast tumors. Results demonstrated that 99mTc(CO)3-DAP-GSG-KCCYSL, 99mTc(CO)3-(NαHis)Ac-GSG-KCCYSL and 99mTc(CO)3- (Ala-Triazol)Ac-GSG-KCCYSL were stable and bound to MDA-MB-435 cells. In vivo biodistribution studies revealed that tumor uptake of 99mTc(CO)3-DAP-GSG-KCCYSL was 1.67 ±0.16, 1.25 ±0.61, 0.88 ±0.12, 0.30 ±0.06 % ID/g at 1, 2, 4, and 24 h post injection, respectively. Tumor uptake of 99mTc(CO)3-(NαHis)Ac-GSG-KCCYSL was 0.76 ±0.13, 0.75 ±0.40, 0.33 ±0.08, 0.16 ±0.02 % ID/g at 1, 2, 4, and 24 h post injection, respectively. Tumor uptake of 99mTc(CO)3-(Ala- Triazol)Ac-GSG-KCCYSL was 1.15 ±0.12, 0.63 ±0.09, 0.30 ±0.02, 0.09 ±0.02 % ID/g at 1, 2, 4, and 24 h post injection, respectively. SPECT/CT studies showed tumor selective uptake of the peptides in the tumor-bearing mice. Specific uptake was confirmed by competitive receptor blocking studies. 99mTc(CO)3-DAP-GSG-KCCYSL and 99mTc(CO)3-(Ala-Triazol)Ac-GSG-KCCYSL may be better as imaging agents due to their higher tumor to non-target uptake ratios than 99mTc(CO)3-(NαHis)Ac-GSG-KCCYSL.