[Control of photocoagulation intensity by thermo-induced release of a fluorescent marker encapsulated in liposomes: study of an in vivo vascular model].

PURPOSE

To evaluate the feasibility of thermal damage assessment of blood vessels by using laser-induced release of liposome-encapsulated dye.

METHODS

A skin flap window model of aluminium was implanted on the loose skin on the back of adults Golden hamsters to expose skin blood vessels in vivo. Thermosensitive liposomes (DSPC) loaded with 5,6-Carboxyfluorescein were injected together with a specific Indocyanine green (ICG) formulation (O/W emulsion) in order to enhance diode laser absorption. Photocoagulations were then performed on the vessels with a diode laser (lambda = 810 nm, P = 0.8W, phi = 1.3 mm, 1 to 6s). Fluorescence measurements were realized with an ultra high sensitivity intensified camera (Hamamatsu Argus 50 imaging system).

RESULTS

Two different fluorescence intensity curves corresponding to the variability of absorption of the targets were observed. Variability was related to the amount of ICG. For each curve, 3 zones were identified: (i) for fluences ranging from 60 +/- 20 J/cm2 to 110 +/- 20 J/cm2 a transient intravascular fluorescence was observed only for the loser pulses targeted on the vessels, (ii) for fluences ranging from 110 +/- 20 J/cm2 to 190 +/- 20 J/cm2 a permanent fluorescent spot limited to the vessel was observed for the laser pulses targeted on the vessels; for the laser pulses targeted on the skin a transient low fluorescence circular spot was observed. For this fluence range a selective photocoagulation of a vessel was performed. (iii) for fluences ranging from 190 +/- 20 J/cm2 to 300 +/- 20 J/cm2 persistent intense fluorescence spots were observed on both skin and vessels. This type of fluorescence was related to an overdosage.

CONCLUSION

These results are in fair agreement with the data of the literature about liposomes and with the data we obtained in a previous study on a vascular model. This study demonstrates the interest of a laser-induced release of liposome-encapsulated dye for a real-time quantification of thermal damage. Such a method could be useful for laser photocoagulation in ophthalmology for indications such as choroidal neovessels where the production of a precise thermal damage is required.