How Does the Encapsulation of Porphyrinic Photosensitizers into Polymer Matrices Affect Their Self-Association and Dynamic Properties?

Photodynamic therapy (PDT) with porphyrinic photosensitizers largely relies on efficient drug formulations to prevent porphyrin aggregation and to enhance water solubility and stability in physiologic environments. In this study, we compare two polymeric carrier systems, polyvinylpyrrolidone (PVP) and block copolymer micelles (BCMs) formed by the poloxamer Kolliphor P188 (KP), for their encapsulation efficiencies of porphyrin (xPP) and chlorin e6 (xCE) derivatives. Monomerization, loading efficiency, and dynamic properties were examined by 1 H NMR spectroscopy chemical shift titration, DOSY, and T2 relaxation time measurements. Binding affinity was determined by UV/Vis spectroscopy. Both PVP and KP-BCMs were well suited to disaggregate and encapsulate amphiphilic xCE, whereas they were less efficient for the xPP compounds. PVP exhibited higher monomerization efficiency than KP-BCMs. Significant differences were found in the dynamic behavior of the carriers. PVP formed rather stable complexes with the porphyrinic compounds, whereas a dynamic equilibrium between free and bound porphyrins was found to exist in the presence of KP-BCMs. This may have a considerable impact on the pharmacokinetic properties of the corresponding delivery systems.