Impact of plasma protein binding on cargo release by thermosensitive liposomes probed by fluorescence correlation spectroscopy

Thermosensitive liposomes (TSLs) whose phase-transition temperature (T_m) lies slightly above body temperature are ideal candidates for controlled drug release via local hyperthermia. Recent studies, however, have revealed disruptive shifts in the release temperature T_r in mouse plasma, which are attributed to undefined interactions with blood proteins. Here, we study the effects of four major plasma proteins – serum albumin (SA), transferrin (Tf), apolipoprotein A1 (ApoA1) and fibrinogen (Fib) – on the temperature-dependent release of fluorescein di-β-D-galactopyranoside (FDG) from TSLs. The amount of fluorescein released was quantified by fluorescence correlation spectroscopy (FCS) after hydrolysis of FDG with β-galactosidase (β-Gal). This approach is more sensitive and thus superior to previous release assays, as it is impervious to the confounding effects of Triton on conventional fluorescence measurements. The assay determines the molar release ratio, i.e. the number of molecules released per liposome. We show that shifts in the T_r of release do not reflect protein affinities for the liposomes derived from adsorption isotherms. We confirm a remarkable shift in induced release towards lower temperatures in the presence of mouse plasma. In contrast, exposure to rat or human plasma, or fetal bovine serum (FBS), has no effect on the release profile.