Wim E. Hennink

Novel crosslinking methods to design hydrogels

Hydrogels are presently under investigation as matrices for the controlled release of bioactive molecules, in particular pharmaceutical proteins, and for the encapsulation of living cells. For these applications, it is often required that the gels degrade under physiological conditions. This means that the originally three-dimensional structure has to disintegrate preferably in harmless products to ensure a good biocompatibility of the hydrogel. In this overview, different chemical and physical crosslinking methods used for the design of biodegradable hydrogels are summarized and discussed. Chemical crosslinking is a highly versatile method to create hydrogels with good mechanical stability. However, the crosslinking agents used are often toxic compounds, which have been extracted from the gels before they can be applied. Moreover, crosslinking agents can give unwanted reactions with the bioactive substances present in the hydrogel matrix. Such adverse effects are avoided with the use of physically crosslinked gels.

Reduction-sensitive polymers and bioconjugates for biomedical applications.

Reduction-sensitive biodegradable polymers and conjugates have emerged as a fascinating class of biomedical materials that can be elegantly applied for intracellular triggered gene and drug delivery. The design rationale of reduction-sensitive polymers and conjugates usually involves incorporation of disulfide linkage(s) in the main chain, at the side chain, or in the cross-linker. Reduction-sensitive polymers and conjugates are characterized by an excellent stability in the circulation and in extracellular fluids, whereas they are prone to rapid degradation under a reductive environment present in intracellular compartments such as the cytoplasm and the cell nucleus. This remarkable feature renders them distinct from their hydrolytically degradable counterparts and extremely intriguing for the controlled cytoplasmic delivery of a variety of bioactive molecules including DNA, siRNA, antisense oligonucleotide (asODN), proteins, drugs, etc. This review presents recent advances in the development of reduction-sensitive biodegradable polymers and conjugates, with particular focus on the up-to-date design and chemistry of various reduction-sensitive delivery systems including liposomes, polymersomes, polymeric micelles, DNA containing nanoparticles, polyion complex micelles, nano- and micro-gels, nanotubes, and multi-layered thin films. It is evident that reduction-sensitive biodegradable polymers and conjugates are highly promising functional biomaterials that have enormous potential in formulating sophisticated drug and gene delivery systems.

Cationic Polymer Based Gene Delivery Systems

Gene transfer to humans requires carriers for the plasmid DNA which canefficiently and safely carrythe gene into the nucleus of the desired cells. A series of chemically differentcationic polymers arecurrently being investigated for these purposes. Although many cationic polymersindeed condense DNAspontaneously, which is a requirement for gene transfer in most types of cells,the physicochemical andbiopharmaceutical behavior of the current generation of polyplexes severelylimits an efficient genetransfer in vitro and especially in vivo. This papersummarizes recent physicochemical and biologicalinformation on polyplexes and aims to provide new insights with respect to thistype of gene deliverysystem. Firstly, the chemical structure of frequently studied cationic polymersis represented. Secondly,the parameters influencing condensation of DNA by cationic polymers aredescribed. Thirdly, the surfaceproperties, solubility, aggregration behavior, degradation and dissociation ofpolyplexes are considered.The review ends by describing the in vitro and in vivo genetransfection behavior of polyplexes.

Drug targeting to tumors: principles, pitfalls and (pre-) clinical progress

Many different systems and strategies have been evaluated for drug targeting to tumors over the years. Routinely used systems include liposomes, polymers, micelles, nanoparticles and antibodies, and examples of strategies are passive drug targeting, active drug targeting to cancer cells, active drug targeting to endothelial cells and triggered drug delivery. Significant progress has been made in this area of research both at the preclinical and at the clinical level, and a number of (primarily passively tumor-targeted) nanomedicine formulations have been approved for clinical use. Significant progress has also been made with regard to better understanding the (patho-) physiological principles of drug targeting to tumors. This has led to the identification of several important pitfalls in tumor-targeted drug delivery, including I) overinterpretation of the EPR effect; II) poor tumor and tissue penetration of nanomedicines; III) misunderstanding of the potential usefulness of active drug targeting; IV) irrational formulation design, based on materials which are too complex and not broadly applicable; V) insufficient incorporation of nanomedicine formulations in clinically relevant combination regimens; VI) negligence of the notion that the highest medical need relates to metastasis, and not to solid tumor treatment; VII) insufficient integration of non-invasive imaging techniques and theranostics, which could be used to personalize nanomedicine-based therapeutic interventions; and VIII) lack of (efficacy analyses in) proper animal models, which are physiologically more relevant and more predictive for the clinical situation. These insights strongly suggest that besides making ever more nanomedicine formulations, future efforts should also address some of the conceptual drawbacks of drug targeting to tumors, and that strategies should be developed to overcome these shortcomings.

Protein Instability in Poly(Lactic-co-Glycolic Acid) Microparticles

In this review the current knowledge of protein degradation during preparation, storage and release from poly(lactic-co-glycolic acid) (PLGA) microparticles is described, as well as stabilization approaches. Although we have focussed on PLGA microparticles, the degradation processes and mechanisms described here are valid for many other polymeric release systems. Optimized process conditions as well as stabilizing excipients need to be used to counteract several stress factors that compromise the integrity of protein structure during preparation, storage, and release. The use of various stabilization approaches has rendered some success in increasing protein stability, but, still, full preservation of the native protein structure remains a major challenge in the formulation of protein-loaded PLGA microparticles.

Sheddable Coatings for Long-Circulating Nanoparticles

Nanoparticles, such as liposomes, polymeric micelles, lipoplexes and polyplexes are frequently studied as targeted drug carrier systems. The ability of these particles to circulate in the bloodstream for a prolonged period of time is often a prerequisite for successful targeted delivery. To achieve this, hydrophilic ‘stealth’ polymers, such as poly(ethylene glycol) (PEG), are used as coating materials. Such polymers shield the particle surface and thereby reduce opsonization by blood proteins and uptake by macrophages of the mononuclear phagocyte system. Yet, after localizing in the pathological site, nanoparticles should deliver their contents in an efficient manner to achieve a sufficient therapeutic response. The polymer coating, however, may hinder drug release and target cell interaction and can therefore be an obstacle in the realization of the therapeutic response. Attempts have been made to enhance the therapeutic efficacy of sterically stabilized nanoparticles by means of shedding, i.e. a loss of the coating after arrival at the target site. Such an ‘unmasking’ process may facilitate drug release and/or target cell interaction processes. This review presents an overview of the literature regarding different shedding strategies that have been investigated for the preparation of sterically stabilized nanoparticulates. Detach mechanisms and stimuli that have been used are described.

In situ gelling hydrogels for pharmaceutical and biomedical applications.

Since Wichterle et al. introduced hydrogels as novel materials possibly suitable for a variety of biomedical applications, hydrogel research has become a fast-developing and exciting research field. The soft and hydrophilic nature of hydrogels makes them particularly suitable as protein delivery system or as cell-entrapping scaffold in tissue engineering. Traditional hydrogels were formed by chemical crosslinking of water-soluble polymers or by polymerization (of mixtures) of water-soluble monomers. Because of incompatibility of these crosslinking methods with fragile molecules like pharmaceutical proteins and living cells, in recent years research interest has been focused on hydrogels that gel spontaneously under physiological conditions. In these systems, hydrogel formation occurs in situ, at the site of injection, without the aid of potentially toxic or denaturizing crosslinking agents. This review provides an overview of in situ gelling systems and their potential in biomedical applications. Both photopolymerizable as well as self-assembling hydrogels, based on either chemical crosslinks or physical interactions will be addressed.

Chitosan-based delivery systems for protein therapeutics and antigens

Therapeutic peptides/proteins and protein-based antigens are chemically and structurally labile compounds, which are almost exclusively administered by parenteral injections. Recently, non-invasive mucosal routes have attracted interest for administration of these biotherapeutics. Chitosan-based delivery systems enhance the absorption and/or cellular uptake of peptides/proteins across mucosal sites and have immunoadjuvant properties. Chitosan is a mucoadhesive polysaccharide capable of opening the tight junctions between epithelial cells and it has functional groups for chemical modifications, which has resulted in a large variety of chitosan derivatives with tunable properties for the aimed applications. This review provides an overview of chitosan-based polymers for preparation of both therapeutic peptides/protein and antigen formulations. The physicochemical properties of these carrier systems as well as their applications in protein and antigen delivery through parenteral and mucosal (particularly nasal and pulmonary) administrations are summarized and discussed.

Cyclodextrin-Based Polymeric Materials: Synthesis, Properties, and Pharmaceutical/Biomedical Applications

This review describes the synthesis, properties, and, in particular, biomedical and pharmaceutical applications of an upcoming class of polymeric networks and assemblies based on cyclodextrins (CDs). CDs are cyclic oligosaccharides composed of alpha-1,4-coupled d-glucose units, which contain a hydrophobic internal cavity that can act as a host for various, generally lipophilic, guest molecules. Because of this unique physicochemical property, commonly referred to as inclusion complex formation, CDs have often been used to design polymeric materials, such as hydrogels and nanoparticles. Polymeric systems based on CDs exhibit unique characteristics in terms of mechanical properties, stimuli-responsiveness, and drug release characteristics. In this contribution, first, an outline is given of covalently cross-linked polymeric networks in which CD moieties were structurally incorporated to modulate the network strength as well as the complexation and release of low molecular weight drugs. Second, physically assembled polymeric systems are discussed, of which the formation is accomplished by inclusion complexes between polymer-conjugated CDs and various guest molecule-derivatized polymers. Due to their physical nature, these polymeric systems are sensitive to external stimuli, such as temperature changes, shear forces and the presence of competing CD-binding molecules, which can be exploited to use these systems as injectable, in situ gelling devices. In recent years, many interesting CD-containing polymeric systems have been described in literature. These systems have to be optimized and extensively evaluated in preclinical studies concerning their safety and efficacy, making future clinical applications of these materials in the biomedical and pharmaceutical field feasible.

Tumour-targeted nanomedicines: principles and practice.

Drug targeting systems are nanometre-sized carrier materials designed for improving the biodistribution of systemically applied (chemo)therapeutics. Various different tumour-targeted nanomedicines have been evaluated over the years, and clear evidence is currently available for substantial improvement of the therapeutic index of anticancer agents. Here, we briefly summarise the most important targeting systems and strategies, and discuss recent advances and future directions in the development of tumour-targeted nanomedicines.