Development of [⁶⁸Ga]Ga/[¹⁷⁷Lu]Lu-DOTA-NI-FAPI-04 Containing a Nitroimidazole Moiety as New FAPI Radiotracers with Improved Tumor Uptake and Retention

Fibroblast activation protein (FAP), which is overexpressed in cancer-associated fibroblasts (CAFs), represents a promising target for cancer diagnosis and therapy. Hypoxia is a common feature of solid tumors. A bivalent agent, DOTA-NI-FAPI-04 (1), was developed by incorporating hypoxia-sensitive nitroimidazole (NI) into the FAP-targeting agent FAPI-04. Compound 1 exhibited a strong FAP binding affinity with an IC₅₀ of 7.44 nM. Radiolabeled [⁶⁸Ga]Ga-1 and [¹⁷⁷Lu]Lu-1 demonstrated enhanced in vitro cell uptake. In vivo positron emission tomography/computed tomography (PET/CT) imaging showed that [⁶⁸Ga]Ga-1 displayed significantly higher specific uptake and retention in U87MG tumor-bearing mice compared to [⁶⁸Ga]Ga-FAPI-04 (SUV_avg: 7.87 vs 1.99% ID/mL at 120 min). Biodistribution studies confirmed superior tumor uptake of [⁶⁸Ga]Ga-1 (48.15 vs 5.72% ID/g at 120 min). Similarly, [¹⁷⁷Lu]Lu-1 exhibited higher tumor uptake than [¹⁷⁷Lu]Lu-FAPI-04 (50.75 vs 20.48% ID/g at 120 min). These preliminary results suggest that a nitroimidazole-containing bivalent-targeting agent, [⁶⁸Ga]Ga/[¹⁷⁷Lu]Lu-1, is a promising candidate for tumor theranostics.