Preclinical evaluation of DOTAGA.Glu.(FAPI)2 and DO3A.Glu.(FAPI)2 as theranostics with human dosimetry extrapolation to lutetium-177 and terbium-161 analogs.

Purpose
This study aims to assess DOTAGA.Glu.(FAPI)2 and DO3A.Glu.(FAPI)2, specifically engineered as precursors for the development of theranostic FAPI-targeted radioligands.Methods
DOTAGA.Glu.(FAPI)2 and DO3A.Glu.(FAPI)2 were radiolabeled with gallium-68 and lutetium-177, followed by in vitro (lipophilicity, protein binding, saturation, internalization and externalization) studies on FAP+ CAFs. In vivo (biodistribution, metabolic stability, blood kinetics, PET/SPECT/CT imaging) and ex vivo, (autoradiography, immunohistochemistry) conducted on PC3-mice. Murine dosimetry data were extrapolated to human estimates.Results
All radioligands achievied > 98% radiochemical purity, demonstrating high FAP affinity (Kd:0.7-0.9 nM) and rapid internalization in CAFs, with differences in lipophilicity and serum protein binding. In vivo studies, for [68Ga]Ga-DOTAGA.Glu.(FAPI)2 and [68Ga]Ga-DO3A.Glu.(FAPI)2 showed high and sustained tumor uptake up to 3 h p.i. (18-19%I.A./g). For [177Lu]Lu-DOTAGA.Glu.(FAPI)2 and [177Lu]Lu-DO3A.Glu.(FAPI)2 tumor uptake was 16.2 ± 2.5 and 15 ± 1.2% IA/g at 4 h p.i., reaching 5.1 ± 0.1 and 2.8 ± 0.4%IA/g at 48 h, respectively. All radioligands exhibited low blood retention levels. PET/SPECT/CT imaging confirmed high tumor-to-background ratios. Uptake patterns correlate well with autoradiography images of heterogeneous FAP distribution in PC3-mice, while the detection of both murine and human FAP in PC3-tumors was demonstrated through immunohistochemistry. The extrapolated human absorbed dose estimates (Gy/GBq) for [177Lu]Lu-DOTAGA.Glu.(FAPI)2 were generally higher across most organs compared to [177Lu]Lu-DO3A.Glu.(FAPI)2. Human extrapolation of the 161Tb-labeled radioligands delivered on average ~ 38% higher absorbed doses in tissues as compared to their 177Lu-labeled counterparts.Conclusion
These results support the potential clinical translation of DOTAGA.Glu.(FAPI)2 and DO3A.Glu.(FAPI)2, as promising candidates for precise diagnosis and treatment of FAP-expressing malignancies.