Synthesis and evaluation of ^99mTc-N4-tyrosine for breast cancer imaging

Objectives This study was aimed to develop an efficient approach to synthesize ^99mTc-O-[3-(1,4,8,11-tetraazabicyclohexadecane)-propyl]-tyrosine (^99mTc-N4-tyrosine), a novel amino acid-based radiotracer, and evaluate its potential in breast cancer gamma imaging.

Methods Precursor N4-tyrosine was synthesized by reacting N-BOC O-bromopropyl tyrosine methyl ester with N4-oxalate, followed by deprotection. Its structure was confirmed by NMR, HPLC and mass spectra. To assess the cellular uptake kinetics, the %uptake of ^99mTc-N4-tyrosine was measured and compared with that of ^99mTc-N4 and the clinical gold standard ¹⁸F-FDG in the rat breast cancer cell line 13762 at 0-4 h. Tissue distribution of ^99mTc-N4-tyrosine was determined in normal rats up to 4 h after injection. To investigate whether ^99mTc-N4-tyrosine can differentiate tumor from inflammation, tumor-to-inflammation ratios were determined in the rats carrying both tumor and turpentine-induced inflammation. Planar imaging of ^99mTc-N4-tyrosine in tumor-bearing rats was performed at 0.5-4 h after injection.

Results The total synthesis yield of N4-tyrosine was 38%. It was successfully labeled with ^99mTc in a high radiochemical purity >95% (by TLC and HPLC). Cellular uptake of ^99mTc-N4-tyrosine was much higher than that of ^99mTc-N4 and ¹⁸F-FDG in rat breast tumor cells. Tissue uptake and dosimetry estimation in normal rats revealed that ^99mTc-N4-tyrosine could be safely administered to humans. Planar imaging in breast tumor-bearing rats showed that ^99mTc-N4-tyrosine had high tumor-to-muscle uptake ratios and could clearly detect breast tumors. In addition, ^99mTc-N4-tyrosine had more accumulation in breast tumor tissues than in chemical-induced inflammatory tissues.

Conclusions We have successfully established a procedure for efficient synthesis of ^99mTc-N4-tyrosine. Our in vitro and in vivo data collectively suggest that ^99mTc-N4-tyrosine has great potential in breast tumor diagnostic imaging given its high tumor uptake