Photoacoustic Imaging Quantifies Drug Release from Nanocarriers via Redox Chemistry of Dye‐Labeled Cargo

We report a new approach to monitor drug release from nanocarriers via a paclitaxel–methylene blue conjugate (PTX‐MB) with redox activity. This construct is in a photoacoustically silent reduced state inside poly(lactic‐co‐glycolic acid) (PLGA) nanoparticles (PTX‐MB@PLGA NPs). During release, PTX‐MB is spontaneously oxidized to produce a concentration‐dependent photoacoustic signal. An in vitro drug‐release study showed an initial burst release (25 %) between 0–24 h and a sustained release between 24–120 h with a cumulative release of 40.6 % and a 670‐fold increase in photoacoustic signal. An in vivo murine drug release showed a photoacoustic signal enhancement of up to 649 % after 10 hours. PTX‐MB@PLGA NPs showed an IC50 of 78 μg mL−1 and 44.7±4.8 % decrease of tumor burden in an orthotopic model of colon cancer via luciferase‐positive CT26 cells. A covalently linked paclitaxel–methylene blue conjugate (PTX‐MB) is used for real‐time monitoring of drug release from nanocarriers via photoacoustic imaging. The PTX‐MB remained in acoustically silent form when encapsulated in poly(lactic‐co‐glycolic acid) (PLGA) nanoparticles. After release, PTX‐MB instantly oxidized to the photoacoustically active form to report quantity and biodistribution.