Carboxylesterase-Activated NIR-II Thioxanthene Probe for Enhanced Photoacoustic Imaging and Type I Phototherapy of Hepatocellular Carcinoma

Abstract To address the bottlenecks of poor tumor specificity and compromised photodynamic therapeutic efficacy caused by the hypoxic microenvironment in solid hepatocellular carcinoma (HCC), a water-soluble NIR-II thioxanthene probe with galactose targeting and carboxylesterase (CE)-activatable property was synthesized via esterification in this work. The galactose ligand specifically recognized the asialoglycoprotein receptor (ASGPR) on HCC, and the intramolecular ester bond served as the specific cleavage site for CE. In vitro spectroscopic characterizations verified that CE-mediated hydrolysis released the parent fluorophore SOH from SP, which triggered a time- and concentration-dependent “off-on” optical response. After CE activation, the generation efficiencies of O2•–, •OH and total reactive oxygen species (ROS) enhanced by 5.2-, 4.5- and 4.8-fold, respectively, and the photothermal conversion efficiency (PCE) increased from 33.5% to 54.6%. Molecular simulations and quantum chemical calculations demonstrated that CE cleavage unlocked the intramolecular charge transfer (ICT) channel and boosted the intersystem crossing (ISC) rate by 141 times, which facilitated the production of type I ROS via electron transfer and well adapted to the hypoxic milieu of HCC. Cellular assays validated that SP exclusively induced mitochondria-mediated apoptosis in CE-overexpressing HepG2 cells with excellent specificity. In vivo experiments on HCC-bearing mice confirmed that SP was activated by endogenous CE to achieve enhanced photoacoustic (PA) imaging, and SP remarkably suppressed tumor proliferation with outstanding biocompatibility under 808 nm laser irradiation. This probe integrated NIR-II PA imaging and hypoxia-tolerant synergistic phototherapy and offered a high-performance molecular tool for precise and noninvasive theranostics of hepatocellular carcinoma.