Detailed in vitro study of the photosensitization reaction of extracellular Talaporfin sodium in rat myocardial cells.

Background And Objective: We proposed a new non-thermal treatment for tachyarrhythmia that employs an extracellular photosensitization reaction. Oxygen depletion may easily occur in in vitro studies of this reaction because the photosensitizing agent is often highly concentrated in such studies. The aim of the current study was to examine the progress of the extracellular photosensitization reaction and the photocytotoxicity of extracellular Talaporfin sodium on myocardial cells for application in tachyarrhythmia therapy.

Materials And Methods: Photosensitization reactions were performed in single wells of 96-well plates; Talaporfin sodium solution concentrations from 5 to 40 µg/ml, radiant exposures up to 40 J/cm(2) , and irradiance of 0.29 W/cm(2) from a continuous wave (CW) red diode laser (wavelength: 663 nm) were used. We measured transient changes of temperature, photosensitizer fluorescence, dissolved oxygen pressure, and photosensitizer solution absorbance to monitor the progress of the photosensitization reaction in the system during laser irradiation. Rat myocardial cells were cultured in 96-well plates, and the drug-light interval was set to 15 minutes. We used a WST assay to measure cell lethality 2 hours after laser irradiation.

Results: A strong photosensitization reaction occurred several seconds after initiation of laser irradiation; this initial reaction depended upon dissolved oxygen. A gentler continuous photosensitization reaction followed the initial reaction, and was associated with temperature increases of less than 10°C. The oxygen pressure was kept in approximately 40 mmHg of the myocardial tissue oxygen pressure in the gentle photosensitization reaction phase. At radiant exposures from 10 to 40 J/cm(2) , a photosensitizer concentration of approximately 15 µg/ml was the threshold for myocardial cell necrosis in this in vitro system. The dependencies of photocytotoxicity on radiant exposure were separated into two distinct groups based on the molecular density ratio between Talaporfin sodium and albumin.

Conclusions: This in vitro system for the extracellular photosensitization reaction may reflect the situation in live myocardial tissue. We found that the extracellular photosensitization reaction progressed in two distinctive phases; the first phase depended upon dissolved oxygen, and the second upon the molar density ratio between Talaporfin sodium and albumin. Cell lethality due to the extracellular photosensitization reaction was influenced by both of these factors in our in vitro system. We suggest that a photosensitizer concentration of 25 µg/ml might be necessary to treat myocardial tissue with therapies involving the extracellular photosensitization reaction.