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.