Photosensitizers (porphyrins) attack cancer cells and during photoactivation contribute to the generation of reactive oxygen species that damage various structures of cancer cells, which leads to their destruction. Porphyrins reach tumors through the blood and modelling of conditions for the transfer of porphyrins to the tumor (binding to blood proteins, the influence of pH and salt composition of the medium) is one of the most important problem of photodynamic therapy (PDT) of tumor. Ceruloplasmin (CP) is an important copper-containing plasma protein that actively binds photosensitizers (PS) and can play a certain role in the transfer of photosensitizers to the tumor. Changes in pH, as well as of salt composition in the tumor tissue and its cells can cause modifications in the complexes [CP + PS] and a possible separation of the PS from the complex can change the photodynamic process of tumor destruction. In this paper, it was shown that: 1) with a change in the pH of the medium from neutral to acidic, as well as with a change in the salt composition of the medium, significant conformational changes of ceruloplasmin and the transition of photosensitizers to the surface (for cationic porphyrins) or to the inside of the protein macromolecule (for chlorin e6) occur, with a partial separation of photosensitizers from protein; 2) CP can form complexes with photosensitizers and be an active carrier of PSs in the blood.
It has been demonstrated that chlorin e6 form molecular complex with polyvinylpyrrolidone. The binding constant and
the number of binding sites have been determined as Κ= (2.5 ± 0.5)•104 M-1 and N = 2.1 ± 0.5, respectively. Complex in comparison with free chlorin e6 possesses increased fluorescence quantum yield, triplet and singlet state lifetimes, slightly modified intersystem crossing quantum yield, and also more intensive and red shifted absorption band locating in the "phototherapeutic window" of biological tissues. The photophysical properties of chlorin e6 incorporated into the polymer are insensitive to pH. The formation of the complex leads to the disruption of chlorin e6 aggregates at acidic pH.
Binding to polymer only slightly improves photophysical properties of photosensitizer and should not provoke fundamental changes in PDT efficiency.