Synthesis and evaluation of a stable bacteriochlorophyll-analog and its incorporation into high-density lipoprotein nanoparticles for tumor imaging

The syntheses of novel near-infrared (NIR) dyes with excellent optical properties in biological tissues have driven the continued improvement of fluorescence imaging of deeply seated tumors. Bacteriochlorophyll a (Bchl), a dye synthesized by the phototrophic bacteria, R. sphaeroids, is particularly suited for deep tissue imaging due to its high absorbance coefficient and good fluorescence quantum yield in the NIR spectrum. However, obstacles that impede the development of this fluorophore are its poor stability and lack of tumor specificity. These issues ultimately limit its utility for tumor detection. Herein we describe a robust synthesis of a novel Bchl analog, bacteriochlorin e₆ bisoleate (BchlBOA), which is chemically stable, has excellent photophysical properties (ex, 752 nm; em, 762 nm) and is tailored for the incorporation into a tumor targetable high-density lipoprotein (HDL)-like nanoparticle (NP). Incorporating BchlBOA into HDL (HDL-BchlBOA) yielded 12 nm sized particles, corresponding well with the diameter of native HDL. Functional cell uptake studies showed that HDL-BchlBOA was taken up by cells expressing the HDL receptor, scavenger receptor B type I (SR-BI), and was inhibited by 25-fold excess native HDL. Furthermore, the NP was successfully detected in KB cancer cells both in vitro and in tumor xenografts. Taken together, these results demonstrate that we successfully synthesized and formulated a stable analog of Bchl that is capable of being incorporated within HDL-like NPs for tumor-targeted imaging.