Background: Engineering a single organic-molecule-based nanoparticle integrating precise diagnosis and effective therapy is of great significance for cancer treatment and future clinical applications but remains a great challenge. The goal of this study is to explore small organic molecule-based nanoparticles with high photothermal conversion efficiency for photoacoustic imaging-guided therapy.

Methods: Heptacyclic B, O-chelated BODIPY structure (namely Boca-BODIPY) with strong near-infrared (NIR) absorption was designed as a theranostic agent through simply molecular engineering, in which heavy atoms and alkyl chains were introduced to promote its application for tumor theranostics. The Boca-BODIPY molecules are further encapsulated in reduced bovine serum albumin (BSA) through self-assembly.

Results: The BSA-Boca-BODIPY exhibited excellent biocompatibility, extraordinary stability and high photothermal conversion efficiency up to 58.7%. The nanoparticles could dramatically enhance photoacoustic contrast of the tumor region, and the signal-to-noise ratio was increased about 14 times at 10 h post intravenous injection in 4T1 tumor-bearing mice. In addition, the nanoassemblies can efficiently convert laser energy (808 nm, 0.75 w cm^-2, 5min) into hyperthermia for tumor ablation. Under the photoacoustic imaging-guided photothermal therapy (PTT), the 4T1 cancer cells were efficiently killed, no tumor recurrence and PTT-induced toxicity is observed.

Conclusions: Molecular engineering is a promising way to design organic-molecule-based nanoparticles for cancer theranostics. Other organic-molecule-based nanoparticles which show great promise for imaging-guided cancer precision therapy can be engineered through this method.