The Relationship of Phthalocyanine 4 (Pc 4) Concentrations Measured Noninvasively to Outcome of Pc 4 Photodynamic Therapy in Mice

The ability to noninvasively measure photosensitizer concentration at target tissues will allow optimization of photodynamic therapy (PDT) and could improve outcome. In this study, we evaluated whether preirradiation tumor phthalocyanine 4 (Pc 4) concentrations, measured noninvasively by the optical pharmacokinetic system (OPS), correlated with tumor response to PDT. Mice bearing human breast cancer xenografts were treated with 2 mg kg−1 Pc 4 iv only, laser irradiation (150 J cm−2) only, Pc 4 followed by fractionated irradiation or Pc 4 followed by continuous irradiation. Laser irradiation treatment was initiated when the tumor to skin ratio of Pc 4 concentration reached a maximum of 2.1 at 48 h after administration. Pc 4 concentrations in tumor, as well as in Intralipid in vitro, decreased monoexponentially with laser fluence. Pc 4‐PDT resulted in significant tumor regression, and tumor response was similar in the groups receiving either fractionated or continuous irradiation treatment after Pc 4. Tumor growth delay following Pc 4‐PDT correlated with OPS‐measured tumor Pc 4 concentrations at 24 h prior to PDT (R2 = 0.86). In excised tumors, OPS‐measured Pc 4 concentrations were similar to the HPLC‐measured concentrations. Thus, OPS measurements of photosensitizer concentrations can be used to assist in the scheduling of Pc 4‐PDT.