Time-targeted therapy (TTT): proof of principle using doxorubicin and radiation in a cultured cell system.

We test the hypothesis that a pulse of the anti-cancer agent doxorubicin renders cells more sensitive to ionizing radiation in a strongly time-specific, dose-specific manner. We have treated cultured cells from a human tumor line, HepG2, with graded doses of two agents: doxorubicin (Dox) and ionizing radiation (XR), delivered in sequence-specific, time-specific, dose-specific patterns. We observe a strong increase in cell killing, up to 120 fold, between pulsed treatment with Dox followed exactly 4 hours later with acute XR. This effect is more pronounced for larger doses of irradiation (>7.5 Gy). These data demonstrate proof of principal in a model system that timing between agents may be an exceptionally important variable in using combined, multi-modal therapy in the treatment of cancer. Since the levels of Dox needed to induce substantial increases in cellular radiosensitivity may be achievable in vivo, we suggest that this phenomenon, which we refer to as time-targeted therapy (TTT), be considered for translation into the clinic if preclinical studies identify appropriate timing and toxicity for combined of Dox XR can be overcome.