Tailoring Fibroblast-Activation Protein Targeting for Theranostics: A Comparative Preclinical Evaluation of the ⁶⁸Ga-...

FAP radiopharmaceuticals show promise for cancer diagnosis; however, their limited tumor residency hinders treatment. This study compared two FAPi derivatives, DOTA.SA.FAPi and DOTAGA.(SA.FAPi)₂, labeled with gallium-68 and lutetium-177, aiming to determine an optimum combination for creating theranostic pairs.

The radiotracers were studied for lipophilicity, binding to human serum proteins, and binding to human cancer-associated fibroblasts (CAFs) in vitro, including saturation and internalization/externalization studies. PET/SPECT/CT and biodistribution studies were conducted in PC3 and U87MG xenografts for [⁶⁸Ga]Ga-DOTA.SA.FAPi and [⁶⁸Ga]Ga-DOTAGA.(SA.FAPi)₂. [¹⁷⁷Lu]Lu-DOTA.SA.FAPi and [¹⁷⁷Lu]Lu-DOTAGA.(SA.FAPi)₂, were evaluated in PC3 xenografts. Biodistribution studies of [⁶⁸Ga]Ga-DOTA.SA.FAPi were performed in healthy male and female mice.

All radiotracers exhibited strong binding to FAP. Their internalization rate was fast while only [¹⁷⁷Lu]Lu-DOTAGA.(SA.FAPi)₂ was retained longer in CAFs. [⁶⁸Ga]Ga-DOTAGA.(SA.FAPi)₂ and [¹⁷⁷Lu]Lu-DOTAGA.(SA.FAPi)₂ displayed elevated lipophilicity and affinity for human serum proteins compared to [⁶⁸Ga]Ga-DOTA.SA.FAPi and [¹⁷⁷Lu]Lu-DOTA.SA.FAPi. In vivo studies revealed slower washout of [⁶⁸Ga]Ga-DOTAGA.(SA.FAPi)₂ within 3 h compared to [⁶⁸Ga]Ga-DOTA.SA.FAPi. The tumor-to-tissue ratios of [⁶⁸Ga]Ga-DOTAGA.(SA.FAPi)₂ versus [⁶⁸Ga]Ga-DOTA.SA.FAPi did not exhibit any significant differences. [¹⁷⁷Lu]Lu-DOTAGA.(SA.FAPi)₂ maintained a significant tumor uptake even after 96 h p.i. compared to [¹⁷⁷Lu]Lu-DOTA.SA.FAPi.

Dimeric compounds hold promise for therapy, while monomers are better suited for diagnostics. Finding the right combination is essential for effective disease management.