Rational Design and Comparison of Novel ^99mTc-Labeled FAPI Dimers for Visualization of Multiple Tumor Types

Recognizing the significance of SPECT in nuclear medicine and the pivotal role of fibroblast activation protein (FAP) in cancer diagnosis and therapy, this study focuses on the development of ^99mTc-labeled dimeric HF₂ with high tumor uptake and image contrast. The dimeric HF₂ was synthesized and radiolabeled with ^99mTc in one pot using various coligands (tricine, TPPTS, EDDA, and TPPMS) to yield [^99mTc]Tc-TPPTS-HF₂, [^99mTc]Tc-EDDA-HF₂, and [^99mTc]Tc-TPPMS-HF₂ dimers. SPECT imaging results indicated that [^99mTc]Tc-TPPTS-HF₂ exhibited higher tumor uptake and tumor-to-normal tissue (T/NT) ratio than [^99mTc]Tc-EDDA-HF₂ and [^99mTc]Tc-TPPMS-HF₂. Notably, [^99mTc]Tc-TPPTS-HF₂ exhibited remarkable tumor accumulation and retention in HT-1080-FAP and U87-MG tumor-bearing mice, thereby surpassing the monomeric [^99mTc]Tc-TPPTS-HF. Moreover, [^99mTc]Tc-TPPTS-HF2 achieved acceptable T/NT ratios in the hepatocellular carcinoma patient-derived xenograft (HCC-PDX) model, which provided identifiable contrast and imaging quality. In conclusion, this study presents proof-of-concept research on ^99mTc-labeled FAP inhibitor dimers for the visualization of multiple tumor types. Among these candidate compounds, [^99mTc]Tc-TPPTS-HF₂ showed excellent clinical potential, thereby enriching the SPECT tracer toolbox.