Proteins and cholesterol lipid vesicles are mediators of drug release from thermosensitive liposomes.

Thermosensitive liposomes (TSL) are a promising tool for triggered drug delivery in combination with local hyperthermia. Objective of this study was to investigate the influence of serum on TSL in more detail and to identify serum components which are responsible for increasing drug release. Four different formulations were investigated: DPPC/DSPC/1,2-dipalmitoyl-sn-glycero-3-phosphodiglycerol (DPPG(2)) 50/20/30 (mol/mol) (DPPG(2)-TSL); DPPC/DSPC/DPPG(2)/DSPE-PEG2000 50/15/30/5 (mol/mol) (DPPG(2)/PEG-TSL), DPPC/P-Lyso-PC/DSPE-PEG2000 90/10/4 (mol/mol) (PEG/Lyso-TSL), and DPPC/DSPC/DSPE-PEG2000 80/15/5 (mol/mol) (PEG-TSL). DPPG(2)-TSL was the only formulation which was unaffected by osmotic stress. All formulations tested were influenced by serum components but the susceptibility was depended on the lipid composition of the vesicle. Presence of albumin (HSA) or cholesterol-containing lipid vesicles (DPPC/Chol-LLV) increased the membrane permeability for all tested formulations at temperatures around and above T(m) in a concentration based manner. PEGylation was not able to prevent the observed effect. PEG-TSL and PEG/Lyso-TSL were more susceptible to DPPC/Chol-LLV than DPPG(2)-containing TSL. In contrast, immunoglobulin type G (IgG) affected only anionic formulations. The membrane of DPPG(2)-TSL and DPPG(2)/PEG-TSL was more susceptible toward IgG as compared to HSA. DPPG(2)-TSL and PEG/Lyso-TSL were differentially influenced by fetal calf serum (FCS). As DPPG(2)-TSL was stabilized by pre-incubation with FCS at 37°C, this was the opposite for PEG/Lyso-TSL which were destabilized under these conditions. Individual serum components were unable to mimic the complex situation in full serum. Hence, the use of plasma or serum is still inevitable to investigate stability and release properties of novel TSL formulations until all serum components have been identified that alter TSL integrity.