Direct photodynamic therapy for vulnerable plaque: Investigation of light dosimetry for depth control

Photodynamic therapy (PDT) mechanism with high-intensity pulsed laser excitation has not been well understood. We think complete understanding of this unknown effect in PDT leads perfect treated depth control at various lesions. To realize the depth controlled PDT for atheromatous plaque therapy with a fibrous cap intact and surrounding damage free, we studied PDT against murine macrophage-like cells in vitro with the second-generation chlorin photosensitizer manufactured by Photochemical Co. Ltd. (Okayama Japan). The relation between the excitation conditions (pulse energy density and repetition rate) and PDT photocytotoxicity was examined in vitro. The XeCl excimer laser pumped dye laser (wavelength: 669±3 nm, pulse duration: 7ns in FWHM) was used with the pulse energy density from 1.2 to 9.5 mJ/cm², and the pulse repetition rate from 5 to 80 Hz. Under higher pulse energy density condition, no significant PDT photocytotoxicity was obtained. We examined the photobleaching of the protein containing photosensitizer medium solution, which is considered to correlates with the generation of singlet oxygen. Under higher pulse energy condition, the photobleaching efficiency decrease was observed and the measured PDT effect decrease in terms of laser pulse energy density could be explained by the photobleaching. We measured the oxygen partial pressure in photosensitizer medium solution immediately after the laser exposure. The decrease of oxygen partial pressure, i.e., the amount of the oxygen consumption during the laser exposure was observed 46 mmHg under the excitation condition of the pulse energy density of 9.5 mJ/cm², the total fluence of 5 J/cm², the repetition rate of 80Hz, and correlated with the bleaching efficiency 87% under the same condition. We calculated cell death distribution in depth direction based on measured photocytotoxicity under various pulse energy densities. The possibility of depth controlled PDT for safety atheromatous plaque therapy was suggested by the PDT effect alteration depending on pulse energy density.