PURPOSE: The aim of the present study was to optimize and simplify photodynamic therapy using a new liposomal formulation of the photosensitizer meta-(tetrahydroxyphenyl)chlorin [m-THPC (Foscan); liposomal m-THPC (Fospeg)] and to reduce systemic reactions to the photosensitizer. EXPERIMENTAL DESIGN: To examine the pharmacokinetics of liposomal m-THPC, we determined tissue and plasma variables in feline patients with spontaneous squamous cell carcinoma. In vivo fluorescence intensity measurements of tumor and skin were done with a fiber spectrophotometer after i.v. injection of m-THPC or liposomal m-THPC in 10 cats. Blood samples, drawn at several time points after photosensitizer administration, were analyzed by high-performance liquid chromatography. RESULTS: None of the liposomal m-THPC-treated cats showed side effects during or after drug injection. Fluorescence intensities, fluorescence ratios (tumor fluorescence divided by skin fluorescence), and bioavailability in the tumor were 2 to 4 times higher with liposomal m-THPC compared with m-THPC. Liposomal m-THPC concentration in the tumor increased constantly to reach a maximum at 4 hours after injection. Plasma concentration and bioavailability were approximately 3 times higher with liposomal m-THPC compared with m-THPC measured at the time points of highest plasma concentration. The distribution half-life was shorter with liposomal m-THPC, resulting in maximal tumor accumulation up to 5.5 times earlier. Maximal tumor accumulation and maximal fluorescence ratio with liposomal m-THPC occurred at the same time point, indicating maximal selectivity. In both groups, all cats responded to therapy. CONCLUSIONS: Liposomal m-THPC was well tolerated by all cats and seems to have superior pharmacokinetic properties compared with m-THPC. The efficacy of the drug warrants further study.