For efficient photodynamic treatment of wound infection, a photosensitizer must be distributed in the whole infected
tissue region. To ensure this, depth profiling of a photosensitizer is necessary in vivo. In this study, we applied
photoacoustic (PA) imaging to visualize the depth profile of an intravenously injected photosensitizer in rat burn models.
In burned tissue, pharmacokinetics is complicated; vascular occlusion takes place in the injured tissue, while vascular
permeability increases due to thermal invasion. In this study, we first used Evans Blue (EB) as a test drug to examine the
feasibility of photosensitizer dosimetry based on PA imaging. On the basis of the results, an actual photosensitizer,
talaporfin sodium was used. An EB solution was intravenously injected into a rat deep dermal burn model. PA imaging
was performed on the wound with 532 nm and 610 nm nanosecond light pulses for visualizing vasculatures (blood) and
EB, respectively. Two hours after injection, the distribution of EB-originated signal spatially coincided well with that of
blood-originated signal measured after injury, indicating that EB molecules leaked out from the blood vessels due to
increased permeability. Afterwards, the distribution of EB signal was broadened in the depth direction due to diffusion.
At 12 hours after injection, clear EB signals were observed even in the zone of stasis, demonstrating that the leaked EB
molecules were delivered to the injured tissue layer. The level and time course of talaporfin sodium-originated signals
were different compared with those of EB-originated signals, showing animal-dependent and/or drug-dependent
permeabilization and diffusion in the tissue. Thus, photosensitizer dosimetry should be needed before every treatment to
achieve desirable outcome of photodynamic treatment, for which PA imaging can be concluded to be valid and useful.