Design, Preclinical Evaluation, and Clinical Translation of ⁶⁸Ga-FAPI-LM3, a Heterobivalent Molecule for PET Imaging of Nasopharyngeal Carcinoma.

Abstract

Extensive research has been conducted on radiolabeled fibroblast activation protein (FAP) inhibitors (FAPIs) and p-Cl-Phe-cyclo(d-Cys-Tyr-d-4-amino-Phe(carbamoyl)-Lys-Thr-Cys)d-Tyr-NH₂ (LM3) peptides for imaging of FAP and somatostatin receptor 2 (SSTR2)-positive tumors. In this study, we designed and synthesized a FAPI-LM3 heterobivalent molecule radiolabeled with ⁶⁸Ga and evaluated its effectiveness in both tumor xenografts and patients with nasopharyngeal carcinoma (NPC). <b>Methods:</b> The synthesis of FAPI-LM3 was based on the structures of FAPI-46 and LM3. After radiolabeling with ⁶⁸Ga, its dual-receptor-binding affinity was evaluated in vitro and in vivo. Preclinical studies, including small-animal PET and biodistribution evaluation, were conducted on HT-1080-FAP and HT-1080-SSTR2 tumor xenografts. The feasibility of ⁶⁸Ga-FAPI-LM3 PET/CT in a clinical setting was evaluated in patients with NPC, and the results were compared with those of ¹⁸F-FDG. <b>Results:</b> ⁶⁸Ga-FAPI-LM3 showed high affinity for both FAP and SSTR2. The tumor uptake of ⁶⁸Ga-FAPI-LM3 was significantly higher than that of ⁶⁸Ga-FAPI-46 and ⁶⁸Ga-DOTA-LM3 in HT-1080-FAP-plus-HT-1080-SSTR2 tumor xenografts. In a clinical study involving 6 NPC patients, ⁶⁸Ga-FAPI-LM3 PET/CT showed significantly higher uptake than did ¹⁸F-FDG in primary and metastatic lesions, leading to enhanced lesion detectability and tumor delineation. <b>Conclusion:</b> ⁶⁸Ga-FAPI-LM3 exhibited FAPI and SSTR2 dual-receptor-targeting properties both in vitro and in vivo, resulting in improved tumor uptake and retention compared with that observed with monomeric ⁶⁸Ga-FAPI and ⁶⁸Ga-DOTA-LM3. This study highlights the clinical feasibility of ⁶⁸Ga-FAPI-LM3 PET/CT for NPC imaging.