Spurred by recent progress in materials chemistry and drug delivery, stimuli-responsive devices that deliver a drug in spatial-, temporal- and dosage-controlled fashions have become possible. Implementation of such devices requires the use of biocompatible materials that are susceptible to a specific physical incitement or that, in response to a specific stimulus, undergo a protonation, a hydrolytic cleavage or a (supra)molecular conformational change. In this Review, we discuss recent advances in the design of nanoscale stimuli-responsive systems that are able to control drug biodistribution in response to specific stimuli, either exogenous (variations in temperature, magnetic field, ultrasound intensity, light or electric pulses) or endogenous (changes in pH, enzyme concentration or redox gradients).

Stimuli-responsive nanocarriers for drug delivery

Liposomes — microscopic phospholipid bubbles with a bilayered membrane structure — have received a lot of attention during the past 30 years as pharmaceutical carriers of great potential. More recently, many new developments have been seen in the area of liposomal drugs — from clinically approved products to new experimental applications, with gene delivery and cancer therapy still being the principal areas of interest. For further successful development of this field, promising trends must be identified and exploited, albeit with a clear understanding of the limitations of these approaches.

Recent advances with liposomes as pharmaceutical carriers.

The EPR effect: Unique features of tumor blood vessels for drug delivery, factors involved, and limitations and augmentation of the effect.

Liposomes, sphere-shaped vesicles consisting of one or more phospholipid bilayers, were first described in the mid-60s. Today, they are a very useful reproduction, reagent, and tool in various scientific disciplines, including mathematics and theoretical physics, biophysics, chemistry, colloid science, biochemistry, and biology. Since then, liposomes have made their way to the market. Among several talented new drug delivery systems, liposomes characterize an advanced technology to deliver active molecules to the site of action, and at present, several formulations are in clinical use. Research on liposome technology has progressed from conventional vesicles to ‘second-generation liposomes’, in which long-circulating liposomes are obtained by modulating the lipid composition, size, and charge of the vesicle. Liposomes with modified surfaces have also been developed using several molecules, such as glycolipids or sialic acid. This paper summarizes exclusively scalable techniques and focuses on strengths, respectively, limitations in respect to industrial applicability and regulatory requirements concerning liposomal drug formulations based on FDA and EMEA documents.

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Liposome: classification, preparation, and applications

Among several promising new drug-delivery systems, liposomes represent an advanced technology to deliver active molecules to the site of action, and at present several formulations are in clinical use. Research on liposome technology has progressed from conventional vesicles ("first-generation liposomes") to "second-generation liposomes", in which long-circulating liposomes are obtained by modulating the lipid composition, size, and charge of the vesicle. Liposomes with modified surfaces have also been developed using several molecules, such as glycolipids or sialic acid. A significant step in the development of long-circulating liposomes came with inclusion of the synthetic polymer poly-(ethylene glycol) (PEG) in liposome composition. The presence of PEG on the surface of the liposomal carrier has been shown to extend blood-circulation time while reducing mononuclear phagocyte system uptake (stealth liposomes). This technology has resulted in a large number of liposome formulations encapsulating active molecules, with high target efficiency and activity. Further, by synthetic modification of the terminal PEG molecule, stealth liposomes can be actively targeted with monoclonal antibodies or ligands. This review focuses on stealth technology and summarizes pre-clinical and clinical data relating to the principal liposome formulations; it also discusses emerging trends of this promising technology.

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Stealth liposomes: review of the basic science, rationale, and clinical applications, existing and potential.

Hyaluronic acid for anticancer drug and nucleic acid delivery.

PEGylation is one of the most successful strategies to improve the delivery of therapeutic molecules such as proteins, macromolecular carriers, small drugs, oligonucleotides, and other biomolecules. PEGylation increase the size and molecular weight of conjugated biomolecules and improves their pharmacokinetics and pharmacodinamics by increasing water solubility, protecting from enzymatic degradation, reducing renal clearance and limiting immunogenic and antigenic reactions. PEGylated molecules show increased half-life, decreased plasma clearance, and different biodistribution, in comparison with non-PEGylated counterparts. These features appear to be very useful for therapeutic proteins, since the high stability and very low immunogenicity of PEGylated proteins result in sustained clinical response with minimal dose and less frequent administration. PEGylation of liposomes improves not only the stability and circulation time, but also the 'passive' targeting ability on tumoral tissues, through a process known as the enhanced permeation retention effect, able to improve the therapeutic effects and reduce the toxicity of encapsulated drug. The molecular weight, shape, reactivity, specificity, and type of bond of PEG moiety are crucial in determining the effect on PEGylated molecules and, at present, researchers have the chance to select among tens of PEG derivatives and PEG conjugation technologies, in order to design the best PEGylation strategy for each particular application. The aim of the present review will be to elucidate the principles of PEGylation chemistry and to describe the already marketed PEGylated proteins and liposomes by focusing our attention to some enlightening examples of how this technology could dramatically influence the clinical application of therapeutic biomolecules.

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Franco Dosio

Papers

Stealth liposomes: review of the basic science, rationale, and clinical applications, existing and potential.

Hyaluronic acid for anticancer drug and nucleic acid delivery.

PEGylation of proteins and liposomes: a powerful and flexible strategy to improve the drug delivery.

Preparation, characterization and properties of sterically stabilized paclitaxel-containing liposomes

Preparation, characterization, cytotoxicity and pharmacokinetics of liposomes containing docetaxel