Synthesis and Evaluation of a ^99mTc-Labeled Fibroblast Activation Protein (FAP)-Specific Radiotracer for SPECT...

Fibroblast activation protein (FAP), which is overexpressed in malignant epithelial tumors, represents a key therapeutic and diagnostic target. This study reports the development and evaluation of a novel FAP-targeted SPECT radiotracer, [^99mTc]4. The precursor was synthesized and labeled with ^99mTc using a one-pot labeling procedure, yielding a radiochemical purity exceeding 95%. The tracer exhibited high hydrophilicity (log P = -1.366 ± 0.004) and demonstrated excellent stability in PBS and serum over 7 h. In vitro, [^99mTc]4 showed specific uptake and rapid internalization in FAP-positive cells, which could be effectively blocked by competitive FAP inhibitors. In vivo SPECT/CT imaging and biodistribution studies in U87MG tumor-bearing mice revealed modest and specific tumor uptake, which was significantly reduced by co-injection with FAPI-04, confirming FAP-mediated targeting. These findings indicate that [^99mTc]4 is a highly promising candidate for noninvasive SPECT imaging of FAP expression in tumors, providing a valuable tool for tumor diagnosis and therapeutic monitoring.