Development of At-211-labeled cancer therapeutic compounds targeting fibroblast activation protein

Introduction: Recently, PET tracers based on fibroblast activation protein inhibitors have been developed and have succeeded in clearly imaging various types of cancer. In addition, FAPI-derivatives labeled with therapeutic radionuclides such as ¹⁷⁷Lu and ²²⁵Ac have been also reported. However, since FAPI derivatives are rapidly excreted from cancer tissues, it would be inefficient to use therapeutic radionuclides with long half-lives such as ¹⁷⁷Lu and ²²⁵Ac. Therefore, we considered that if FAPI is labeled with a therapeutic radionuclide with short-half-life, it can irradiate cancer cells efficiently even with a short residence time in cancer tissues, and a high therapeutic effect can be expected. In this study, we designed and synthesized FAPI derivatives labeled with ²¹¹At, an alpha-particle emitting radionuclide with a short half-life, using neopentyl glycol scaffold developed in our laboratory that can stably retain ²¹¹At in vivo.

Methods: Based on the structure of [⁶⁸Ga]Ga-FAPI-02, we designed radiolabeled FAPI derivatives in which the neopentyl glycol scaffold and FAP biding moiety were linked via two or three molecules of D-glutamic acid (Glu2 and Glu3) as hydrophilic linker. For radiolabeling, we first used ¹²⁵I, which is easy to handle, was used instead of ²¹¹At. The biodistribution of these radiolabeled compounds in normal mice and tumor-bearing mice were evaluated.

Results: ¹²⁵I-labeled Glu2 was obtained with a radiochemical yield of 46% and a radiochemical purity of over 95%. When ¹²⁵I-labeled Glu2 was administered to normal mice, it was rapidly cleared from the blood and most organs like [⁶⁷Ga]Ga-FAPI-02, but accumulation in intestine was observed over time. Considering that the accumulation in intestine is due to the hydrophobicity of the compound, we synthesized an ¹²⁵I-labeled PAPI derivative with three Glu molecules (Glu3) to increase its hydrophilicity and evaluated its pharmacokinetics. As a result, the accumulation of ¹²⁵I-labeled Glu3 in intestine was reduced compared with that of ¹²⁵I-labeled Glu2. When ¹²⁵I-labeled Glu3 was administered to tumor-bearing mice, accumulation in the tumor was observed.

Conclusions: ¹²⁵I-labeled Glu3 was considered a useful radiolabeled FAPI-derivative because it showed low accumulation in the intestine and accumulation in tumor tissue. Therefore, the application of the neopentyl-conjugated FAPI derivatives via Glu3 linker to ²¹¹At is expected to be a useful radiotherapeutic agent. Currently, application of ²¹¹At is under investigation.