A novel photoacoustic-fluorescent contrast agent for quantitative imaging of lymphatic drainage

In vivo near-infrared (NIR) photoacoustic imaging (PAI) studies using novel contrast agents require validation, often via fluorescence imaging. Bioconjugation of NIR dyes to proteins is a versatile platform to obtain contrast agents for specific biomedical applications. Nonfluorescent NIR dyes with higher photostability present advantages for quantitative PAI, compared to most fluorescent NIR dyes. However, they don't provide a fluorescence signal required for fluorescence imaging. Here, we designed a hybrid PA-fluorescent contrast agent by conjugating albumin with a NIR nonfluorescent dye (QC-1) and a visible spectrum fluorescent dye, a BODIPY derivative. The new hybrid tracer QC-1/BSA/BODIPY (QBB) had a low minimum detectable concentration (2.5 mu M), a steep linear range (2.4-54.4 mu M; slope 3.39 E -5), and high photostability. Tracer signal was measured in vivo using PAI to quantify its drainage from eye to the neck and its localization in the neck lymph node was validated with postmortem fluorescence imaging.