Compartmental Targeting for mTHPC-Based Photodynamic Treatment In Vivo: Correlation of Efficiency, Pharmacokinetics, and Regional Distribution of Apoptosis

Purpose

The present study investigates the efficacy of compartmental targeting in grafted tumors treated by meta-tetra(hydroxyphenyl)chlorin (mTHPC)-mediated photodynamic therapy (PDT). The therapeutic efficacy was, furthermore, related to a regional photoinduced distribution of apoptosis and an mTHPC biodistribution profile.

Methods and Materials

Mice bearing EMT6 tumors were subjected to a single irradiation (10 J/cm²) of red laser light (652 nm) at different intervals after a single- (0.3 mg/kg or 0.15 mg/kg) or double-intravenous (2 × 0.15 mg/kg) injection(s) of mTHPC. Efficiency of the treatment was evaluated by monitoring tumor regrowth. mTHPC pharmacokinetics were assessed by high-performance liquid chromatography analysis of excised organs. The regional distribution of apoptosis in tumor sections was investigated with a newly developed colabelling immunohistochemistry technique.

Results

A fractionated double-injection protocol of mTHPC with 24 h and 3 h drug-light intervals (DLI) yielded 100% tumor cure, with tumors presenting a massive apoptosis of neoplastic cells along with a distortion of vessels. The best efficiency for a single injection (0.3 mg/kg) was about 54% tumor cure and corresponded to a DLI of 3 h. At this DLI, tumors showed apoptosis of endothelial cells in residual vessels. Concentrations of mTHPC observed in plasma and tumor for the fractionated injection were not statistically different and were less than the total drug dose in each compartment.

Conclusions

The present work suggests that clinical PDT protocols with mTHPC could be greatly improved by fractionation of the drug administration. Time points should be chosen based on the intratumoral spatiotemporal drug distribution.

Apoptosis, Compartmental targeting, mTHPC, Photodynamic therapy