The observed effects render COE-labels questionable for in vivo studies using CEPT-rich species. Here, choline labelled (14)C-DPPC was found to be the most promising alternative. Bilayer composition has a high influence on stability and drug release of a liposomal formulation in human plasma.

Liposomal delivery constitutes a promising approach for i.v. administration of temoporfin (mTHPC) because lipid membranes can host these drug molecules. This study investigates the transfer and release of mTHPC to plasma proteins and stability of various liposomal formulations. To this end, we employed traces of radioactive markers and studied the effects of fatty acid chain length and the degree of saturation in the lipophilic tail, addition of cholesterol and PEGylation of the membrane surface and different drug-to-lipid ratios (DLRs). Liposomes were incubated in human plasma for various incubation times. Drawn samples were separated by asymmetrical flow field-flow fractionation (AF4). Drug was recovered in four fractions identified as albumin, high-density lipoprotein (HDL), low-density lipoprotein (LDL) and liposomes. Our results suggest that mTHPC fits best into fluid, unmodified bilayers when the drug-to-lipid ratio is low. Membrane rigidification as well as the presence of cholesterol and PEGyated lipids reduced the ability of the membrane to accommodate the drug but simultaneously improved the vesicle stability in plasma. Both mechanisms jointly affect the total degree of mTHPC release. We analyzed our data using a kinetic model that suggests the drug to be associated with the host membrane in two distinct states of which only one interacts directly with the plasma compartment.

The effect of plasma on the retention of lipophilic drugs by liposomes as affected by the liposome fluidity was investigated. Liposomes consisted of eggPC/eggPG (9:1 w/w) or DPPC/DPPG (9:1 w/w) with 10 wt% incorporated 14C‐labeled Temoporfin (mTHPC) and trace amounts of 3H‐labeled cholesteryloleylether (COE) as lipid marker. Liposomal formulations were incubated at different concentrations in 50% human plasma for 48 h and the mixtures fractionated using gel permeation chromatography, separating liposomes and (lipo)proteins (albumin, HDL, LDL) from each other. The DPPC/DPPG liposomes retained 75–90% of the COE label at 48 h, depending on the liposome concentration, while the EPC/EPG liposomes retained only 5% of this label. By contrast, for the case of the mTHPC label, the EPC/EPG liposomes retained 24% at 48 h. whereas the DPPC/DPPG liposomes retained only 15%. In all cases, HDL was the predominant acceptor of both drug and COE label, closely followed by LDL. Albumin participated only marginally in the transfer process. It can be concluded, that EPC/EPG liposomes can retain lipophilic drugs better than DPPC/DPPG liposomes. Given a lifetime of 8 h for liposomes in vivo, 40% of the drug stays in the liposomes, whereas only 25% is retained for DPPC/DPPG liposomes at sufficient liposome concentration. Transfer of drug from liposomal formulations to human plasma was measured after separation of plasma components by GPC. Incubation of mTHPC loaded liposomes with human plasma results in a transfer of the drug and phospholipids mostly to HDL, followed by LDL, whereas albumin is only marginally involved in the transfer process. In comparison to a fluid liposomal formulation composed of eggPC/eggPG (9:1 w/w), rigid DPPC/DPPG (9:1 w/w) liposomes show less interactions with plasma proteins but a faster drug release.