Stereochemical Optimization and Tumor-Model-Dependent Performance of [18F]FBFP for Sigma-1 Receptor-Targeted Tumor PET Imaging

Abstract Chirality plays a critical role in ligand–receptor interactions, yet its impact on sigma-1 receptor (σ1R)-targeted positron emission tomography (PET) imaging remains insufficiently explored. Herein, we systematically investigated the stereochemical effect of σ1R-targeted radioligands using Rac-, (R)-(−) and (S)-(+)-[18F]FBFP for tumor PET imaging. Racemic and enantiomerically pure [18F]FBFP were successfully prepared with high radiochemical purity (>98%), molar activities of 289–934 GBq/μmol, and excellent in vitro stability (>95% intact at 4 h) using KOTf-assisted radiofluorination. Western blotting (WB), immunohistochemistry (IHC), and immunofluorescence (IF) confirmed high σ1R expression in U87MG cells and tumor tissues, with predominant intracellular localization. Cellular uptake and blocking studies in U87MG cells confirmed specific σ1R-mediated binding of Rac-, (R)-, and (S)-[18F]FBFP, with SA4503 eliciting marked reductions of 80%, 82%, and 75%, respectively, at 30 min. In vivo biodistribution and PET imaging in subcutaneous U87MG tumor-bearing mice demonstrated clear tumor visualization, high early tumor uptake SUVmax > 2 at 10 min), and favorable tumor-to-brain and tumor-to-muscle ratios. Notably, (R)-[18F]FBFP exhibited the highest tumor uptake at 30 min (SUVmax = 1.92 vs 1.55 and 1.67 for Rac- and (S)-[18F]FBFP, respectively) and the most sustained tumor retention. Both enantiomers clearly visualized orthotopic U87MG gliomas, with (R)-[18F]FBFP providing more favorable delayed imaging performance. Further evaluation across tumor types showed clear visualization of OSRC-2 xenografts and a negative imaging profile for LN229 gliomas against a background of high physiological brain uptake. Integrated analysis of σ1R expression and CD31-positive vascularization suggested that in vivo tumor uptake depended not only on receptor expression but also on tumor vascularization, tracer delivery, and model-specific microenvironmental features. Our results highlight the importance of stereochemical optimization and tumor biological context in σ1R-targeted PET imaging and identify (R)-[18F]FBFP as a promising radioligand for precision brain and renal tumor imaging.