A biotinylated aloe emodin-based anthraquinone bioconjugate with enhanced cellular uptake and photodynamic activity

Natural products represent a rich and versatile source of structurally diverse and biologically active molecules, providing valuable scaffolds for the development of anticancer therapies. Among them, anthraquinones such as aloe emodin (AE) have attracted attention due to their broad therapeutic activity; however, their photodynamic properties remain comparatively underexplored, particularly in breast cancer models. Herein, we report the synthesis and evaluation of a novel biotinylated aloe emodin derivative (AE-B7), designed to enhance cellular uptake in breast cancer cells for photodynamic therapy (PDT). The bioconjugate was obtained via Steglich esterification and characterized by NMR spectroscopy and mass spectrometry. Photophysical studies demonstrated that AE-B7 preserves the photophysical properties of the anthraquinone core, which involves excited-state intramolecular proton transfer (ESIPT) and the formation of two tautomeric species. In vitro assays in MCF-7 breast cancer cells revealed a marked improvement in photodynamic activity, with the IC₅₀ decreasing from 3.59 μM for AE to 0.23 μM for AE-B7 under blue-light irradiation, while no detectable dark toxicity was observed within the tested concentration range (IC₅₀ > 100 μM for AE and > 1 μM for AE-B7). This enhanced photodynamic activity was associated with a 4.3-fold increase in cellular uptake of AE-B7 compared with AE. Moreover, competition with excess free biotin significantly reduced AE-B7 phototoxicity, supporting the involvement of biotin-mediated uptake pathways. Consistently, DCFH-DA fluorescence microscopy revealed a more pronounced intracellular reactive oxygen species (ROS) response for AE-B7 than for AE under irradiation. ROS scavenging and quenching experiments further supported a predominant contribution of Type I photodynamic processes to photodynamic cell death in breast cancer cells. These findings highlight the potential of rational functionalization of natural-product scaffolds with targeting moieties to enhance the cellular uptake and photodynamic performance of anthraquinone-based photosensitizers.