Design, Synthesis, and Preclinical Evaluation of ⁸⁹Zr/¹⁷⁷Lu-FAPI-Tz: A Potential Theranostic Pair Targeting Fibroblast Activation Protein

Fibroblast activation protein (FAP)-targeted radioligands have recently emerged as attractive tumor imaging agents. However, the therapeutic applicability of most FAP ligands has been impeded by their short tumor retention. In this study, a tetrazine (Tz)-modified FAPI derivant DOTA-FAPI-Tz was synthesized and radiolabeled with ¹⁷⁷Lu and ⁸⁹Zr to produce ⁸⁹Zr-FAPI-Tz and ¹⁷⁷Lu-FAPI-Tz with high radiochemical purity. Cellular uptake, internalization, efflux, and affinity experiments were performed using the U87MG-FAP cell line (glioma) to evaluate the <i>in vitro</i> FAP-targeting efficacy of the prepared radiotracers. In addition, micro-PET imaging, <i>ex vivo</i> biodistribution, and <i>in vivo</i> anticancer investigations were performed to evaluate the tumor-targeting ability, pharmacokinetic profile, and therapeutic effect of ⁸⁹Zr/¹⁷⁷Lu-FAPI-Tz. The results show that ⁸⁹Zr-FAPI-Tz and ¹⁷⁷Lu-FAPI-Tz demonstrate satisfactory <i>in vitro</i> stability, while ¹⁷⁷Lu-FAPI-Tz has a reduced hydrophilicity compared to ¹⁷⁷Lu-FAPI-04. Consistent with the performance of ¹⁷⁷Lu-FAPI-04, ¹⁷⁷Lu-FAPI-Tz presents rapid and specific FAP-targeting capability but a more significant time-dependent decrease in cellular retention. ⁸⁹Zr-FAPI-Tz and ¹⁷⁷Lu-FAPI-Tz alike display fast tumor localization, showing relatively low radioactivity accumulation in normal organs. Consequently, high-contrast PET images and favorable tumor-to-organ ratios can be obtained. Furthermore, ¹⁷⁷Lu-FAPI-Tz exhibits an effective antitumor efficacy and a satisfactory safety profile in U87MG-FAP tumor-bearing mice. In conclusion, ⁸⁹Zr-FAPI-Tz and ¹⁷⁷Lu-FAPI-Tz are a promising radiopharmaceutical pair for FAP-targeted theranostics, with remarkable tumor accumulation and a favorable pharmacokinetic profile.