Toward understanding the high PDT efficacy of chlorin e6–polyvinylpyrrolidone formulations: Photophysical and molecular aspects of photosensitizer–polymer interaction in vitro

It is recognized that chlorin e6–polyvinylpyrrolidone (Ce6–PVP) formulations are characterized by a high efficacy in photodynamic therapy of malignant tumors. Currently, a commercially available formulation of this type is Photolon® (Fotolon®) with Ce6:PVP = 1:1 (w/w) and the weight-average molecular weight of PVP is 1.2 × 104. To gain a better understanding of the role played by PVP in Ce6–PVP formulations, we carry out experiments on IR and UV–VIS absorption, steady-state and time-resolved fluorescence, time-resolved triplet–triplet absorption, octanol–water partitioning, and solubility of chlorin e6 in buffer solutions at pH 6.3, 7.4, and 8.5 in presence of PVP with Ce6:PVP ratios ranging from 1:0 to 1:1000 (w/w) for PVP samples with weight-average molecular weights of 8 × 103, 1.2 × 104, and 4.2 × 104. We show that Ce6 interacts with PVP by forming molecular complexes via hydrophobic interactions and determine the Ce6–PVP binding constant, as well as the mean number of PVP monomers per binding site. We find that complexation of Ce6 with PVP prevents Ce6 aggregation in aqueous media and leads to an enhancement of Ce6 fluorescence quantum yield, while keeping the quantum yield of the intersystem crossing essentially unchanged. Possible scenarios of how the presence of PVP can favorably affect the PDT efficacy of chlorin e6 in Ce6–PVP formulations are discussed.