29th Annual Meeting of the American Society of Photobiology

Downtown Marriot

Chicago, Il.

Photosensitized Inactivation of Melanoma Cells by Photothermal Pathways: Mechanistic Studies and Phototherapeutic Applications

Abstract-
Photothermal (PT) sensitization of a biological system consists in a highly localized hyperthermal effect (greater than 100 ^oC above the basal value) which causes important chemical (bond cleavage) and mechanical (ultrasonic shock) damage. This requires (a) an absorber decaying from the electronically excited states primarily by non-radiative pathways; (b) irradiation with lasers operated in a pulsed regime at high fluence rates. Both Cu(II)-hematoporphyrin (CuHp) and Ni(II)-octabutoxy-napthalocyanine (NiNc) were found to act as efficient PT sensitizers against amelanotic and melanotic melanoma cells irradiated with 532 nm from a Q-switched Nd:YAG laser (10 ns pulses, 250 mJ/pulse, 10 Hz) or, respectively, 850 nm from a Q-switched Ti:sapphire laser (30 ns pulses, 200 mJ/pulse, 10 Hz). Using 5-10 M CuHp or NiNc concentrations, a decrease in cell survival greater than 98% is obtained upon 1-2 min. irradiation. The process is not of photodynamic type since photoexcited CuHp and NiNc generate no triplet signal or singlet oxygen, no cell inactivation is obtained upon prolonged cw irradiation with both dyes, and the efficiency of the pulsed laser-promoted cell killing is independent of the presence of oxygen. Thus, the mechanism of action of PT sensitizers appears to be markedly different from that typical of photodynamic sensitizers: electron microscopy analysis of PT-sensitized cells shows an extensive photodamage which is completed immediately after the end of irradiation and involves the appearance of large vacuoles throughout the cytoplasm; moreover, the efficiency of the photoprocess is enhanced by the formation of large aggregates of the PT sensitizer in specific subcellular sites (eg., Golgi apparatus, rough endoplasmic reticulum). Preliminary in vivo studies with mice bearing a s.c. transplanted amelanotic or pigmented melanoma and injected with NiNc (3 mg/kg) indicate that PT sensitization can be utilized for tumor therapy, and a synergistic action between the exogenously added NiNc and an endogenous PT sensitizer, i.e. melanin, is possible. At present, we are investigating the parameters which optimize PT sensitization, as well as the possibility of a combined application of PDT and PTT.

Keywords: Photosensitization, Photothermal, Pulsed lasers