Simultaneous direct and indirect assessments of singlet oxygen generation during vascular-targeted photodynamic therapy with thrombin molecular beacon.

Vascular-targeted photodynamic therapy (V-PDT) offers a precise therapeutic approach for treating diseases associated with abnormal vasculature. The therapeutic efficacy of type II V-PDT mainly relies on the vascular response to singlet oxygen (¹O₂), which could convert prothrombin into thrombin and then result in vasocontraction and subsequent tissue ischemia. In this study, the photosensitizer pyropheophorbide-a (Pyro) was conjugated with the fluorescent molecule 5-carboxy-X-rhodamine (Rox) through a thrombin-cleavable peptide, forming a thrombin-activated molecular beacon Pyro-thrombin-cleavable peptide-Rox (PPR). Furthermore, a novel multimodal imaging system was developed for simultaneously imaging near-infrared (NIR) ¹O₂ luminescence at around 1270 nm and Rox fluorescence, which could be used to directly and indirectly assess the ¹O₂ generation during V-PDT for an in vivo model, respectively. It was found that the vasoconstrictions are positively correlated with both ¹O₂ luminescence intensity and Rox fluorescence intensity, respectively. For this, the PPR could serve as a therapeutic PS and as an indirect indicator for ¹O₂ generation during V-PDT, which has the advantage of higher sensitivity compared to the direct measurement of ¹O₂ luminescence.