Two-step photoactivation of porphyrins with nanosecond red pulses

A new method of laser-induced photochemotherapy is proposed which involves the formation of cytotoxic photoproducts of porphyrins via a two-step excitation. The excitation is provided by two excimer-pumped dye lasers tuned at 631 and 700 nm to ensure good penetration in tissues. The two pulses have time durations of 5 ns. The pulse at 631 nm raises the porphyrin to its S1 state and that at 700 nm is delayed by 16 ns with respect to the first one to induce a resonant absorption from the T1 state when it is highly populated. The formation of photoproducts was observed for both hematoporphyrin in methanol and for hematoporphyrin derivative in water. Preliminary experiments performed on groups of 4 mice each, bearing a murine fibrosarcoma tumor (MS-2), show a better therapeutic efficiency of this technique as compared to that based on cw irradiation at 631 nm.