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

Targeted thermosensitive liposomes: an attractive novel approach for increased drug delivery to solid tumors

INTRODUCTION

Currently available chemotherapy is hampered by a lack in tumor specificity and resulting toxicity. Small and long-circulating liposomes can preferentially deliver chemotherapeutic drugs to tumors upon extravasation from tumor vasculature. Although clinically used liposomal formulations demonstrated significant reduction in toxicity, enhancement of therapeutic activity has not fully met expectations.

AREAS COVERED

Low drug bioavailability from liposomal formulations and limited tumor accumulation remain major challenges to further improve therapeutic activity of liposomal chemotherapy. The aim of this review is to highlight strategies addressing these challenges. A first strategy uses hyperthermia and thermosensitive liposomes to improve tumor accumulation and trigger liposomal drug bioavailability. Image-guidance can aid online monitoring of heat and drug delivery and further personalize the treatment. A second strategy involves tumor-specific targeting to enhance drug delivery specificity and drug internalization. In addition, we review the potential of combinations of the two in one targeted thermosensitive-triggered drug delivery system.

EXPERT OPINION

Heat-triggered drug delivery using thermosensitive liposomes as well as the use of tumor vasculature or tumor cell-targeted liposomes are both promising strategies to improve liposomal chemotherapy. Preclinical evidence has been encouraging and both strategies are currently undergoing clinical evaluation. A combination of both strategies rendering targeted thermosensitive liposomes (TTSL) may appear as a new and attractive approach promoting tumor drug delivery.

Paramagnetic liposomes for molecular MRI and MRI-guided drug delivery

Liposomes are a versatile class of nanoparticles with tunable properties, and multiple liposomal drug formulations have been clinically approved for cancer treatment. In recent years, an extensive library of gadolinium (Gd)-containing liposomal MRI contrast agents has been developed for molecular and cellular imaging of disease-specific markers and for image-guided drug delivery. This review discusses the advances in the development and novel applications of paramagnetic liposomes in molecular and cellular imaging, and in image-guided drug delivery. A high targeting specificity has been achieved in vitro using ligand-conjugated paramagnetic liposomes. On targeting of internalizing cell receptors, the effective longitudinal relaxivity r1 of paramagnetic liposomes is modulated by compartmentalization effects. This provides unique opportunities to monitor the biological fate of liposomes. In vivo contrast-enhanced MRI studies with nontargeted liposomes have shown the extravasation of liposomes in diseases associated with endothelial dysfunction, such as tumors and myocardial infarction. The in vivo use of targeted paramagnetic liposomes has facilitated the specific imaging of pathophysiological processes, such as angiogenesis and inflammation. Paramagnetic liposomes loaded with drugs have been utilized for therapeutic interventions. MR image-guided drug delivery using such liposomes allows the visualization and quantification of local drug delivery.

Phospholipid vesicle stability and temporal variations in acyl chain organization

We report on our investigation of the long-term dimensional stability and acyl chain organization of unilamellar vesicles containing 1,2-dimyristoyl-sn-phosphatidylcholine (DMPC) and DMPC with cholesterol. Vesicles of 100% DMPC, 80% DMPC/20% cholesterol and 70% DMPC/30% cholesterol, formed by extrusion in aqueous buffer solution (pH 8) were shown to remain dimensionally stable for periods in excess of 600 h by dynamic light scattering (DLS) measurements. The rotational diffusion dynamics of perylene confined in the vesicle acyl chain region revealed structural evolution that was dependent on vesicle composition. Re-extrusion of the vesicles caused no changes in the average diameter or size distribution, but did give rise to diminished organization in the lipid acyl chain region for DMPC vesicles. Cholesterol-containing vesicles exhibited somewhat less pronounced change in organization on re-extrusion, suggesting a structurally mediating role for cholesterol.

SPECT/CT imaging of temperature-sensitive liposomes for MR-image guided drug delivery with high intensity focused ultrasound

The goal of this study was to investigate the blood kinetics and biodistribution of temperature-sensitive liposomes (TSLs) for MR image-guided drug delivery. The co-encapsulated doxorubicin and [Gd(HPDO3A)(H₂O)] as well as the ¹¹¹In-labeled liposomal carrier were quantified in blood and organs of tumor bearing rats. After TSL injection, mild hyperthermia (T=42 °C) was induced in the tumor using high intensity focused ultrasound under MR image-guidance (MR-HIFU). The biodistribution of the radiolabeled TSLs was investigated using SPECT/CT imaging, where the highest uptake of ¹¹¹In-labeled TSLs was observed in the spleen and liver. The MR-HIFU-treated tumors showed 4.4 times higher liposome uptake after 48 h in comparison with controls, while the doxorubicin concentration was increased by a factor of 7.9. These effects of HIFU-treatment are promising for applications in liposomal drug delivery to tumors.

Novel approaches to treatment of hepatocellular carcinoma and hepatic metastases using thermal ablation and thermosensitive liposomes

Because of the limitations of surgical resection, thermal ablation is commonly used for the treatment of hepatocellular carcinoma and liver metastases. Current methods of ablation can result in marginal recurrences of larger lesions and in tumors located near large vessels. This review presents a novel approach for extending treatment out to the margins where temperatures do not provide complete treatment with ablation alone, by combining thermal ablation with drug-loaded thermosensitive liposomes. A history of the development of thermosensitive liposomes is presented. Clinical trials have shown that the combination of radiofrequency ablation and doxorubicin-loaded thermosensitive liposomes is a promising treatment.

Hyperthermia-induced drug targeting

INTRODUCTION

Specific delivery of a drug to a target site is a major goal of drug delivery research. Using temperature-sensitive liposomes (TSLs) is one way to achieve this; the liposome acts as a protective carrier, allowing increased drug to flow through the bloodstream by minimizing clearance and non-specific uptake. On reaching microvessels within a heated tumor, the drug is released and quickly penetrates. A major advance in the field is ThermoDox® (Celsion), demonstrating significant improvements to the drug release rates and drug uptake in heated tumors (∼ 41°C). Most recently, magnetic resonance-guided focused ultrasound (MRgFUS) has been combined with TSL drug delivery to provide localized chemotherapy with simultaneous quantification of drug release within the tumor.

AREAS COVERED

In this article the field of hyperthermia-induced drug delivery is discussed, with an emphasis on the development of TSLs and their combination with hyperthermia (both mild and ablative) in cancer therapy. State-of-the-art image-guided heating technologies used with this combination strategy will also be presented, with examples of real-time monitoring of drug delivery and prediction of efficacy.

EXPERT OPINION

The specific delivery of drugs by combining hyperthermia with TSLs is showing great promise in the clinic and its potential will be even greater as the use of image-guided focused ultrasound becomes more widespread - a technique capable of penetrating deep within the body to heat a specific area with improved control. In conjunction with this, it is anticipated that multifunctional TSLs will be a major topic of study in this field.