A caspase-3-activatable bimodal probe for photoacoustic and magnetic resonance imaging of tumor apoptosis in vivo

Multimodal imaging probes have shown promise in both biomedical research and clinical diagnosis. However, the development of molecular probes capable of being switched on multimodality imaging signals in response to a biomarker of interest remains challenging. In this paper, we report a caspase-3-activatable small-molecule bimodal probe (Gd-IR780) for photoacoustic imaging (PAI) and magnetic resonance imaging (MRI) of tumor apoptosis via caspase-3-triggered intramolecular macrocyclization and in situ self-assembly process. Upon interaction with caspase-3, Gd-IR780 can be efficiently converted into a macrocyclic product, Gd-IR780-MC, which subsequently self-assembles into near-infrared absorptive and paramagnetic nanoparticles (Gd-IR780-NPs), allowing to concurrently switch on PAI (∼4.3-fold at 855 nm) and MRI (r1 relaxivity increases from 7.98 ± 0.03 mM−1 s−1 to 19.66 ± 0.7 mM−1 s−1 at 0.5 T) bimodal signals in caspase buffer. Noninvasive in vivo imaging results show that Gd-IR780 can enter apoptotic U87MG tumor tissues after systemic administration, and produce markedly enhanced PAI and MRI signals for high sensitivity and spatial-resolution visualization of caspase-3 activity in the doxorubicin-treated apoptotic U87MG tumor tissues. Gd-IR780 holds a good potential to report tumor apoptosis via combined PAI and MRI, which is beneficial for the early evaluation of antitumor efficacy in vivo.