Fibroblast Activation Protein (FAP) targeting homodimeric FAP inhibitor radiotheranostics: a step to improve tumor uptake and retention time.

Several radiopharmaceuticals targeting fibroblast activation protein (FAP) based on the highly potent FAP inhibitor UAMC1110 are currently under investigation. Pre-clinical as well as clinical research exhibited the potential of these imaging agents. However, the monomeric small molecules seemed to have a short retention time in the tumor in combination with fast renal clearance. Therefore, our strategy was to develop homodimeric systems having two FAP inhibitors to improve residence time and tumor accumulation. The homodimers with two squaramide coupled FAP inhibitor conjugates DOTA.(SA.FAPi)₂ and DOTAGA.(SA.FAPi)₂ were synthesized and radiochemically evaluated with gallium-68. [⁶⁸Ga]Ga-DOTAGA.(SA.FAPi)₂ was tested for its in vitro stability, lipophilicity and affinity properties. In addition, human PET/CT scans were performed for [⁶⁸Ga]Ga-DOTAGA.(SA.FAPi)₂ with a head-to-head comparison with [⁶⁸Ga]Ga-DOTA.SA.FAPi and [¹⁸F]FDG. Labeling with gallium-68 demonstrated high radiochemical yields. Inhibition measurements revealed excellent affinity and selectivity with low nanomolar IC₅₀ values for FAP. In PET/CT human studies, significantly higher tumor uptake as well as longer tumor retention could be observed for [⁶⁸Ga]Ga-DOTAGA.(SA.FAPi)₂ compared to [⁶⁸Ga]Ga-DOTA.SA.FAPi. Therefore, the introduction of the dimer led to an advance in human PET imaging indicated by increased tumor accumulation and prolonged retention times in vivo and thus, the use of dimeric structures could be the next step towards prolonged uptake of FAP inhibitors resulting in radiotherapeutic analogs of FAP inhibitors.