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 in order to prevent porphyrin aggregation and 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 1H NMR spectroscopic 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 the amphiphilic xCE while they were less efficient for the xPP compounds. PVP exhibited increased monomerization efficiency compared to KP-BCMs. Significant differences were found in the dynamic behaviour of the carriers. PVP forms rather stable complexes with the porphyrinic compounds, while a dynamic equilibrium between free and bound porphyrins exists in the presence of KP-BCMs. This may have a considerable impact on the pharmacokinetic properties of the corresponding delivery systems.