Plasma distribution of tetraphenylporphyrin derivatives relevant for Photodynamic Therapy: importance and limits of hydrophobicity.

In the course of a Photodynamic Therapy (PDT) protocol, disaggregation of the sensitizer upon binding to plasma proteins and lipoproteins is one of the first steps following intravenous administration. This step governs its subsequent biodistribution and has even been evoked as possibly orientating mechanism of tumor destruction. It is currently admitted as being mainly dependent on sensitizer's hydrophobicity. In this context, as far as glycoconjugation of meso-tetraphenylporphyrin (TPP) macrocycle, a promising strategy to improve targeting of retinoblastoma cells confers to the sensitizer an amphiphilic character, we have studied the effect of this strategy on binding to plasma proteins and lipoproteins. With the exception of the majoritary protein binding (more than 80%) of more hydrophilic para-tetraglycoconjugated derivatives, high density lipoproteins (HDL) appear as main plasma carriers of the other amphiphilic glycoconjugated photosensitizers. This HDL-binding is a combined result of binding affinities (logKa ranging from 4.90 to 8.77 depending on the carrier and the TPP derivative considered) and relative plasma concentrations of the different carriers. Evaluation of binding affinities shows that if hydrophobicity can account for LDL- and HDL-affinities, it is not the case for albumin-affinity. Molecular docking simulations show that, if interactions are mainly of hydrophobic nature, polar interactions such as hydrogen bonds are also involved. This combination of interaction modalities should account for the absence of clear relationship between albumin-affinity and hydrophobicity. Taken together, our findings clarify the importance, but also the limits, of hydrophobicity's role in structure-plasma distribution relationship.
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