Abstract 2650: Triggered content release from optimized stealth thermosensitive liposomes using mild hyperthermia
Authors: Li Li , ten Hagen Timo L.M. , Schipper Debby , Wijnberg Tom M. , van Rhoon Gerard , Eggermont Alexander M.M. , Lindner Lars H. , Koning Gerben A. 
Published: February 2011

Abstract
Liposomes are potent nanocarriers to deliver chemotherapeutic drugs to tumors. However, the inefficient drug release hinders their application. Liposomes around 100 nm can extravasate through leaky microvasculature and accumulate in tumor tissue. Local hyperthermia (HT) augments the permeability of tumor microvasculature, resulting in enhanced intratumoral liposomal drug accumulation. Subsequently, thermosensitive liposomes (TSL) release their contents at elevated temperature. The combination of stealth TSL with phase transition temperature at 41 – 43 °C and mild local HT offers a promising clinical advance. This study identified thermal dose for liposome extravasation through tumor vessels and the optimum DSPE-PEG2000 concentration for triggered drug release from TSL.
TSL were prepared with DSPE-PEG2000 concentration from 1 to 10 mol% and size of around 80 nm. Quenched carboxyfluorescein (CF) in the aqueous phase represented encapsulated drugs. In vitro temperature/time-dependent CF release and TSL stability in serum were quantified by fluorometry. In vivo CF release was observed by confocal microscopy in dorsal skin flap window chamber models implanted with human BLM melanoma. A circular resistive electric heating coil provided local HT. In vivo liposome extravasation was also studied in window chamber models of syngeneic B16 melanoma in mice with constitutive vascular endothelial cell expression of an eNOS-Tag-GFP fusion protein.
The study of liposome extravasation through tumor microvasculature illustrated a thermal dose of 41 °C for 30 min tuned up the leakiness of tumor vessels towards liposomes of around 80 nm. A significant amount of liposomes escaped from circulation and intratumorally accumulated in the interstitial space.
The in vitro temperature-dependent CF release indicated in vitro CF release increased with increasing DSPE-PEG2000 density. Six mol% and higher DSPE-PEG2000 caused content leakage at physiological temperature. Thus the optimum DSPE-PEG2000 concentration was 5 mol% (p &lt; 0.05). These TSL released over 60 % of CF in one minute, and released almost 100 % of CF in one hour at 42 °C with encapsulation efficiency of 0.08 (mole/mole of CF/lipid). Content retention in 90 % serum at 37 °C for one hour was 96.6 %. In vivo optical intravital imaging showed immediate massive CF release above 41 °C.
Incorporation of 5 mol% of DSPE-PEG2000 in the liposomal bilayer optimized stealth TSL content release triggered by HT (&gt; 41 °C). In vivo tumor models confirmed in vitro TSL release characteristics. The superior application of TSL and mild HT will achieve both maximal intratumoral liposomal drug accumulation and rapid triggered drug release to aid in liposomal chemotherapy in clinical practice.
Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2650.