Molar Dose Optimization for Accurate Pharmacokinetic and Biodistribution Evaluation of FAP-Targeted Radiopharmaceuticals in a Mouse Syngeneic Tumor Model

Most studies on fibroblast activated protein (FAP)-targeted radiopharmaceuticals focus on administered radioactivity, often overlooking the impact of a molar dose on tumor-targeting and off-target accumulation. Here, we investigate the effect of molar dose on biodistribution and pharmacokinetics using [⁶⁸Ga]Ga-FAPI-04 PET and systematically evaluated two FAP-targeted dimers, DOTAGA.(SA.FAPi)₂ and DOTAGA.Glu.(FAPi)₂, in a 4T1 syngeneic tumor model. Dynamic PET imaging confirmed a clear molar dose-dependent effect on tumor uptake, tumor-to-organ ratios, and organ pharmacokinetics with lower molar doses prolonging tumor retention. Comparative analyses across multiple molar doses revealed that DOTAGA.Glu.(FAPi)₂ achieved comparable tumor uptake to DOTAGA.(SA.FAPi)₂ but exhibited significantly reduced liver accumulation. An optimal molar dose range of 8-32 nmol/kg was identified, balancing maximal tumor uptake with reduced off-target exposure. At this optimized dose, [¹⁷⁷Lu]Lu-DOTAGA.Glu.(FAPi)₂ demonstrated therapeutic efficacy in 4T1 tumor-bearing mice with limited systemic toxicity. These results establish molar dose optimization as a broadly applicable strategy for accurately evaluating and comparing FAP-targeted radiopharmaceuticals and provide a methodological framework to guide future preclinical development and translational studies.