Schedule-dependent interaction between Doxorubicin and mTHPC-mediated photodynamic therapy in murine hepatoma in vitro and in vivo

Purpose: To evaluate cytotoxic and antitumor effects of a conventional anticancer drug Doxorubicin (Dox) and photodynamic therapy (PDT) mediated by a promising photosensitizer of second generation meta-tetra (3-hydroxyphenyl)-chlorin (mTHPC) in combination. Methods: Murine hepatoma MH-22A was used for investigation in vitro and in vivo. In vitro, the cells were incubated with 0.15 μg/ml mTHPC for 18 h and exposed to light from LED array (λ= 660±20 nm) at 0.6–2.4 kJ/m2. 0.05–0.2 μg/ml Dox was administered either 24 h prior to or immediately after light exposure (Dox→PDT or PDT + Dox, respectively). The cytotoxicity was tested by staining with crystal violet. The character of the combined effect was assessed by multiple regression analysis. In vivo, the antitumor activity was estimated by monitoring the tumor volume over time, in mice transplanted subcutaneously with MH-22A and treated with Dox and/or PDT. For PDT, mice were exposed to light from diode laser (λ=650±2 nm) at 12 kJ/m2 following 24 h after administration of 0.15 mg/kg mTHPC. A 3 mg/kg Dox was administered either within 15 min prior to mTHPC or within 15 min after light exposure (Dox→PDT or PDT + Dox, respectively). Results: Both in vitro and in vivo, the combination of mTHPC-mediated PDT and Dox was evaluated to be more effective than each treatment alone. In vitro, the difference between cell viability curves after photodynamic treatment as a single modality and after combination of photodynamic treatment with Dox was statistically significant under most of the applied conditions (P≤0.02). In the case of PDT + Dox, the combination had an additive character, and the sequence Dox→PDT caused a sub-additive interaction. In vivo, both regimens of combination were more effective in inhibiting tumor growth than any single treatment (P<0.09). The antitumor activity of PDT + Dox regimen was more prominent than that of Dox→PDT; however, significance of the difference was not high (P=0.08). Conclusions: These results indicate that Dox potentiates therapeutic efficacy of mTHPC-mediated PDT and vice versa, and the degree of potentiation is influenced by the combination schedule: administration of Dox immediately after light exposure is preferable to administration of Dox at 24 h prior to light exposure.