Photodynamic therapy in fibrosarcoma BALB/c animal model: Observation of the rebound effect

In vivo spectrofluorometric analysis during photodynamic therapy (PDT) is a fundamental tool to obtain information about drug bleaching kinetics. Using a portable spectrofluorometer with an excitation source emitting at 400 nm wavelength and a spectral analyzer ranging from 500 nm to 800 nm, the evolution of the meta-tetra(hydroxyphenyl) chlorin (m-THPC) photosensitizer fluorescence spectrum at the tumoral tissue of BALB/c murines with fibrosarcoma located at their flank was followed up. Ex vivo fluorescence measurements of the tumor and skin were also performed with the aim of better characterizing the in vivo signal at different parts of the tumor. PDT was performed employing a LED 637 nm light source. Fluorescence at different parts of the tumor and at the tail and armpit of mice was measured immediately after injection and followed daily. The average fluorescence intensity in the tumor reached a maximum after 24-72 h. Subsequently, illuminations 24, 48, 72 and 96 h post-injection were performed, and the fluorescence was measured immediately before and after each illumination. Eventually, 24 h post-illumination, the fluorescence at certain parts of the tumor increased in comparison with that measured immediately after illumination. This effect, named “rebound effect”, was due to the new local accumulation of the drug, and was used to perform a second illumination on some mice to increase the amount of photodynamic reaction and significantly improve the PDT outcome. These results are encouraging to optimize PDT in the proposed animal model, thinking about the possible translation to humans.