Increase of the photosensitizing efficiency of the Bacteriochlorin a by liposome-incorporation

Abstract

To describe the action mechanisms of Bacteriochlorin a (BCA), a second generation photosensitizer, in phosphate buffer (PB) and in dimyristoyl phosphatidylcholine (DMPC) liposomes we carried out oxygen consumption and ESR measurements. In PB, where BCA was in a monomer-dimer equilibrium, our results suggested that the oxygen consumption was related to the BCA monomers concentration in solution. Incorporation of BCA in DMPC liposomes, by promoting the monomerization of BCA, increased 9-fold the oxygen consumption in comparison to the value in PB. The use of specific singlet oxygen quenchers (Azide and 9, 10-Anthracenedipropionic acid) in ESR and oxygen consumption experiments allowed us to assert that BCA was mainly a type II sensitizer when it was incorporated in DMPC. Finally, the cell survival of WiDr cells after a PDT treatment was measured for cells incubated with BCA in cell culture medium and cells incubated with BCA in DMPC. Irrespective of the dye concentration, the cell survival was lower when liposomes were used. This effect could be the result of a better BCA monomerization and / or a different BCA uptake in cells.

Determination of the singlet oxygen quantum yield of bacteriochlorin a: a comparative study in phosphate buffer and aqueous dispersion of dimiristoyl-l-α-phosphatidylcholine liposomes

“…3,4 Therefore, BCA is an attractive sensitizer since it has been demonstrated that BCA photosensitizes the production of 1 O 2 and that 1 O 2 is involved in killing cells in the presence of photoexcited BCA. 10, 12 Furthermore, oxygen consumption and electron paramagnetic resonance experiments seem to indicate that solubilization of BCA in the bilayers of DMPC liposomes leads to an increase of singlet oxygen production relative to the same value in phosphate buffer. This increase is concomitant with the monomerization of the dye.…”

“…Indeed, it has been shown that the survival of WiDr cells (derived from a primary adenocarcinoma of a rectosigmoid colon carcinoma) after PDT treatment was lower with a liposomal formulation of BCA than with treatment with BCA in cell culture medium. 12 Among the causes that could explain this effect, there are, of course, a higher singlet oxygen production but also other parameters like the modification of the cell uptake or location inside the cells when liposomes are used as drug carriers.…”

“…11 Oxygen consumption measurements carried out with a Clarke electrode in water and in liposome solutions indicate that the production of 1 O 2 is strongly influenced by the dye aggregation state since it seems that only the monomeric species is appreciably photoactive. 12 Indeed, monomerization of BCA by solubilization within liposomes increases the PDT activity and is accompanied by an enhancement of singlet oxygen production. 12 Electron paramagnetic resonance (EPR) experiments confirmed the latter results and, in addition, showed that the type I mechanism is not affected by the presence of liposomes.…”

Liposomal photosensitizers: potential platforms for anticancer photodynamic therapy

“…For instance, both phthalocyanines and chlorins are hydrophobic PSs that form aggregates in physiological solutions (16). When aggregated, the PSs are far less efficient in translating light energy into a chemical potential.…”

“…When aggregated, the PSs are far less efficient in translating light energy into a chemical potential. This happens because the effect of quenching is amplified in these aggregates, i.e., the aggregated molecule is not able to absorb light or, even though it absorbs light, the photoexcited PS decays to the ground-state before producing singlet oxygen (16). Moreover, PS aggregates also hamper parenteral administration (such as intravenous injection) and the delivery of the PS drugs to targeted tissues (6,7).…”