Fatty diacid-modified FAP-targeted NIR fluorescent probes with modulated pharmacokinetics for enhanced tumor imaging

Near-infrared (NIR) fluorescent probes targeting fibroblast activation protein (FAP) have emerged as a promising approach for tumor imaging; however, rapid systemic clearance of small-molecule probes often limits tumor-to-background contrast and narrows the effective imaging window. Here we report a fatty diacid-based albumin-binding strategy to modulate the pharmacokinetics of FAP-targeted NIR fluorescent probes. By incorporating dodecanedioic acid (C12D) or hexadecanedioic acid (C16D) into a sulfo-Cy7-labeled FAP-targeting scaffold through an Ado₂-Glu (AG) linker, we evaluated the impact of diacid chain length on tumor accumulation and background clearance. In a U87MG xenograft model, fatty diacid modification prolonged systemic circulation and enhanced tumor-associated fluorescence compared with Cy7-FAPi. Notably, the longer-chain probe Cy7-FAPi-AG-C16D produced higher absolute tumor fluorescence but showed slower background clearance, whereas Cy7-FAPi-AG-C12D achieved superior tumor-to-background contrast with the highest tumor-to-muscle ratio at 24 h (T/M ratio = 31.3 ± 7.6). These results demonstrate that fatty diacid modification provides a practical strategy to tune the exposure-clearance balance of small-molecule FAP-targeted probes, enabling high-contrast tumor imaging within clinically relevant time windows.