Redistribution of meta-tetra(hydroxyphenyl)chlorin (m-THPC) from conventional and PEGylated liposomes to biological substrates.

We used the phenomenon of previously described photoinduced fluorescence quenching and fluorescence polarization to evaluate the transfer of meta-tetra(hydroxyphenyl)chlorin (m-THPC) from commercial high-drug load liposomes to plasma proteins and model membranes. Fluorescence quenching of m-THPC in liposomes by iodide indicates that part of m-THPC in PEGylated liposomes is localized in the PEG shell, while the rest is bound to the lipid bilayer. It was shown that the two molecule pools in the commercial PEGylated liposomal formulation Fospeg® condition the characteristics of the m-THPC release kinetics. A substantial percentage of m-THPC from Fospeg® is released much faster than from the conventional liposomal formulation Foslip®. Using the technique of resonance light scattering, it was shown that partial m-THPC aggregation is present in liposomes with very high drug loads, higher in PEGylated liposomes compared to conventional ones.

Unusual photoinduced response of mTHPC liposomal formulation (Foslip).

Liposomal formulations of meso-tetra(hydroxyphenyl)chlorin (mTHPC) have already been proposed with the aim to optimize photodynamic therapy. Spectral modifications of these compounds upon irradiation have not yet been investigated. The objective of this study was to evaluate photobleaching properties of mTHPC encapsulated into dipalmitoylphosphatidylcholine (DPPC) liposomes, Foslip. Fluorescence measurements in DPPC liposomes with different DPPC:mTHPC ratios demonstrated a dramatic decrease in fluorescence anisotropy with increasing local mTHPC concentration, thus suggesting strong interactions between mTHPC molecules in lipid bulk medium. Exposure of Foslip suspensions to small light doses (<50 mJ/cm(2)) resulted in a substantial drop in fluorescence, which, however, was restored after addition to the sample of a non-ionic surfactant Triton X-100. We attributed this behavior to photoinduced fluorescence quenching. This effect depended strongly on the molar DPPC:mTHPC ratio and was revealed only for high local mTHPC concentrations. The results were interpreted supposing energy migration between closely located mTHPC molecules with its subsequent dissipation by the molecules of photoproduct acting as excitation energy traps. We further assessed the effect of photoinduced quenching in plasma protein solution. Relatively slow kinetics of photoinduced Foslip response during incubation in the presence of proteins was attributed to mTHPC redistribution from liposomal formulations to proteins. Therefore, changes in mTHPC distribution pattern in biological systems would be consistent with changes in photoinduced quenching and would provide valuable information on mTHPC interactions with a biological environment.

Redistribution of Foscan from plasma proteins to model membranes.

Photodynamic therapy is a comparatively novel modality of tumours treatment that includes simultaneous action of photosensitizers, light and oxygen. Photosensitizer redistribution between plasma proteins and biomembranes define photosensitizers interaction with cells, their intracellular localization and kinetics of sensitizers accumulation in the tumour. Present study investigates the kinetics of Foscan release from plasma proteins to model membranes using fluorescence resonance energy transfer (FRET) from label, covalently bound to protein, to sensitizer. We have demonstrated very slow kinetics of Foscan release from protein complexes with rate constants of (1.7 +/- 0.1) x 10(-3) s(-1) for albumin and (1.6 +/- 0.3) x 10(-4) s(-1) for high-density lipoproteins (HDL). Foscan redistributes by both collision and diffusion-mediated transfer from complexes with HDL, with bimolecular rate constant k(out) = (8.8 +/- 1.4) x 10(-2) M(-1) s(-1). Thermodynamic considerations proposed that sensitizer release from HDL into the aqueous medium is unfavourable and collision mechanism appeared to be a preferred mode of transfer in biological environment. Slow rates of Foscan redistribution from plasma proteins should be considered while planning dosimetry protocol of Foscan-PDT.