In vivo quantification of photosensitizer concentration using fluorescence differential path-length spectroscopy: Influence of photosensitizer formulation and tissue location

In vivo measurement of photosensitizer concentrations may optimize clinical photodynamic therapy (PDT). Fluorescence differential path-length spectroscopy (FDPS) is a non-invasive optical technique that has been shown to accurately quantify the concentration of Foscan® in rat liver. As a next step towards clinical transla- tion, the effect of two liposomal formulations of mTHPC, Fospeg® and Foslip®, on FDPS response was investigated. Furthermore, FDPS was evaluated in target organs for head-and-neck PDT. Fifty-four healthy rats were intrave- nously injected with one of the three formulations of mTHPC at 0.15 mgkg −1 . FDPS was performed on liver, ton- gue, and lip. The mTHPC concentrations estimated using FDPS were correlated with the results of the subsequent harvested and chemically extracted organs. An excellent goodness of fit (R 2 ) between FDPS and extraction was found for all formulations in the liver (R 2 ¼ 0.79). A much lower R 2 between FDPS and extraction was found in lip (R 2 ¼ 0.46) and tongue (R 2 ¼ 0.10). The lower performance in lip and in particular tongue was mainly attributed to the more layered anatomical structure, which influences scattering properties and photosensitizer dis-