Asymmetrical flow field-flow fractionation with on-line detection for drug transfer studies: a feasibility study

Knowledge about drug retention within colloidal carriers is of uppermost importance particularly if drug targeting is anticipated. The aim of the present study was to evaluate asymmetrical flow field-flow fractionation (AF4) with on-line UV/VIS drug quantification for its suitability to determine both release and transfer of drug from liposomal carriers to a model acceptor phase consisting of large liposomes. The hydrophobic porphyrin 5,10,15,20-tetrakis(4-hydroxyphenyl)21H,23H-porphine (p-THPP), a fluorescent dye with an absorbance maximum in the visible range and structural similarity to the clinically used photosensitizer temoporfin, was used as a model drug, and two types of large liposomes were studied as a potential model acceptor phase. Efficiency of separation of small donor from large acceptor liposomes by AF4 was evaluated in dependence on the injected lipid mass using two different channel geometries. Drug quantification by on-line absorbance measurements was established by comprehensive evaluation of the size-dependent turbidity contribution in on-line UV/VIS detection and by comparison with off-line results obtained for the respective dye-loaded donor formulations (dissolved in methanol). Due to distinct differences in size, the acceptor liposomes (mean diameters ∼300-400 nm) could efficiently be separated from the donor liposomes (mean diameter ∼70 nm) with less than 4 % of p-THPP detected in the overlap region between both vesicle populations. Whereas p-THPP could accurately be determined in the fraction of small vesicles, on-line quantification was impaired in the fraction of the large acceptor liposomes due to the pronounced contribution of turbidity (about 80 % of total UV/VIS extinction signal). The AF4-based release/transfer approach suggested here was found repeatable and robust. The employed combination of AF4 with multi-angle laser light scattering furthermore provided detailed size information of the eluting sample and thus allowed to detect instabilities and/or interactions between the donor and acceptor liposomes. Drug quantification by on-line absorbance measurements was found feasible for the chosen model drug, but careful (re-)evaluation of turbidity effects is crucial for other drug and carrier combinations.