Objectives

• This study investigated the pharmacokinetics of fibroblast activation protein inhibitors (FAPI) radiopharmaceuticals for pan-cancer treatment.
• It hypothesized that adjusting the administered radiotracer dose could enhance tumor-to-background ratios.
• A dose-escalation study with PC3 xenografts was performed, using doses between 10 and 1500 pmol.
• The study found that increasing the dose from 10 to 600 pmol significantly reduced blood uptake and enhanced tumor uptake.
• All radiotracers showed peak efficacy at 350–600 pmol, with altered pharmacokinetics beyond 600 pmol.
• Human studies with [⁶⁸Ga]Ga-DOTA.SA.FAPI confirmed reduced off-target uptake, aligning with preclinical findings.

Methodology

• Biodistribution studies were conducted in PC3 xenograft mice with varying doses (10-1500 pmol) of five different FAPI radiotracers.
• PET/CT imaging was performed to visualize radiotracer distribution.
• In vivo selectivity was assessed by co-injecting radiotracers with inhibitors of FAP, PREP, and DPP4.
• Metabolite analysis via RP-HPLC was used to determine the stability of radiotracers in vivo.
• FAP expression in organs was quantified using qPCR.
• Circulating FAP (sFAP) levels were measured in mouse and human blood samples via ELISA.

Results

• Increasing the dose from 10 to 600 pmol significantly reduced blood uptake (from ~9% IA/g to ~1% IA/g for monomers and ~20% IA/g to 2-5% IA/g for dimers) and enhanced tumor uptake (peak 12-19% IA/g at 350-600 pmol).
• Tumor-to-blood and tumor-to-pancreas ratios improved with increased dose, with monomers showing higher contrast than dimers.
• Administering 1000 pmol reduced tumor uptake by factors of 0.3-0.9, depending on the radiotracer.
• qPCR confirmed FAP expression in various organs, with highest levels in bone, tumor, pancreas, salivary glands, and bone marrow.
• Human studies showed significantly decreased SUVmax values in pancreas (p<0.001) and salivary glands (p<0.003) with higher doses of [⁶⁸Ga]Ga-DOTA.SA.FAPI.

Discussions

• The study provides a comprehensive analysis of FAPI-radiotracer pharmacokinetics, but the mechanism behind increased pancreatic uptake when co-injecting PREP and DPP4 inhibitors remains unclear and requires further investigation.
• While the study identifies optimal doses for maximizing tumor uptake and minimizing off-target uptake, it would be beneficial to explore the impact of these findings on therapeutic efficacy in future studies.
• The study uses wild-type cell lines, which may have lower FAP-receptor density compared to transfected cell lines. It would be valuable to compare results with other models.
• The observation that sFAP levels are not correlated with tumor FAP overexpression is interesting. Further investigation into the source and regulation of sFAP could provide valuable insights.

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Reference: Improved FAPI radiopharmaceutical pharmacokinetics from the perspectives of a dose escalation study