Effective drug delivery by PEGylated drug conjugates

The current review presents an update of drug delivery using poly(ethylene glycol) (PEG), that focuses on recent developments in both protein and organic drugs. Certainly the past 10 years has resulted in a renaissance of the field of PEG drug conjugates, initiated by the use of higher molecular weight PEGs (M(w)>20,000), especially 40,000 which is estimated to have a plasma circulating t(1/2) of approximately 10 h in mice. This recent resuscitation of small organic molecule delivery by high molecular weight PEG conjugates was founded on meaningful in vivo testing using established tumor models, and has led to a clinical candidate, PEG-camptothecin (PROTHECAN), an ester based prodrug currently in phase II trials. Additional applications of high molecular weight PEG prodrug strategies to amino containing drugs are presented: similar tripartate systems based on lower M(w) PEG and their use with proteins is expounded on. The modification of a benzyl elimination tripartate prodrug specific for mercaptans is presented, and its successful application to 6-mercaptopurine giving a water soluble formulation is discussed. Recent novel PEG oligonucleotides and immunoconjugates are also covered. Clinical results of FDA approved PEGylated proteins are also presented.     

Dextran cross-linked gelatin microspheres as a drug delivery system.

This paper describes the use of oxidized dextran as a cross-linker for the preparation of gelatin microspheres. Microspheres were obtained by a thermal gelation method and their dissolution kinetic was examined. In order to find evidence of sugar mediated cross-linking, swelling tests and gelatin microspheres dissolution experiments were performed. The obtained results indicated that oxidized dextran can form a cross-linked gelatin network which can reduce the dissolution of gelatin. More interestingly, gelatin microspheres treated by both native and oxidized dextran slow down, even if to a different extent, the release of the antitumor drug TAPP-Br used as a model compound. Taken together, our results suggest that oxidized dextran could be an interesting means to cross-link gelatin microspheres allowing the use of this delivery formulation for controlled release of drugs.     

Synthesis of dendrimers and drug-dendrimer conjugates for drug delivery

Dendrimers constitute a class of polymers that possess a well-defined structure allowing the precise control of size, shape and terminal group functionality. Their utility has been explored for a wide range of pharmaceutical applications. There is growing interest in the design and synthesis of novel biocompatible dendrimers and a number of novel dendrimer architectures are discussed in this review. Recently, there has also been interest in the design of drug-dendrimer prodrugs and several of these systems are described, with emphasis on how the properties of such carriers may be tailored via surface engineering.     

Targeted lipid based drug conjugates: a novel strategy for drug delivery

A majority of studies involving prodrugs are directed to overcome low bioavailability of the parent drug. The aim of this study is to increase the bioavailability of acyclovir (ACV) by designing a novel prodrug delivery system which is more lipophilic, and at the same time site specific. In this study, a lipid raft has been conjugated to the parent drug molecule to impart lipophilicity. Simultaneously a targeting moiety that can be recognized by a specific transporter/receptor in the cell membrane has also been tethered to the other terminal of lipid raft. Targeted lipid prodrugs i.e., biotin-ricinoleicacid-acyclovir (B-R-ACV) and biotin-12hydroxystearicacid-acyclovir (B-12HS-ACV) were synthesized with ricinoleicacid and 12hydroxystearicacid as the lipophilic rafts and biotin as the targeting moiety. Biotin-ACV (B-ACV), ricinoleicacid-ACV (R-ACV) and 12hydroxystearicacid-ACV (12HS-ACV) were also synthesized to delineate the individual effects of the targeting and the lipid moieties. Cellular accumulation studies were performed in confluent MDCK-MDR1 and Caco-2 cells. The targeted lipid prodrugs B-R-ACV and B-12HS-ACV exhibited much higher cellular accumulation than B-ACV, R-ACV and 12HS-ACV in both cell lines. This result indicates that both the targeting and the lipid moiety act synergistically toward cellular uptake. The biotin conjugated prodrugs caused a decrease in the uptake of [(3)H] biotin suggesting the role of sodium dependent multivitamin transporter (SMVT) in uptake. The affinity of these targeted lipid prodrugs toward SMVT was studied in MDCK-MDR1 cells. Both the targeted lipid prodrugs B-R-ACV (20.25 ± 1.74 μM) and B-12HS-ACV (23.99 ± 3.20 μM) demonstrated higher affinity towards SMVT than B-ACV (30.90 ± 4.19 μM). Further, dose dependent studies revealed a concentration dependent inhibitory effect on [(3)H] biotin uptake in the presence of biotinylated prodrugs. Transepithelial transport studies showed lowering of [(3)H] biotin permeability in the presence of biotin and biotinylated prodrugs, further indicating a carrier mediated translocation by SMVT. Overall, results from these studies clearly suggest that these biotinylated lipid prodrugs of ACV possess enhanced affinity towards SMVT. These prodrugs appear to be potential candidates for the treatment of oral and ocular herpes virus infections, because of higher expression of SMVT on intestinal and corneal epithelial cells. In conclusion we hypothesize that our novel prodrug design strategy may help in higher absorption of hydrophilic parent drug. Moreover, this novel prodrug design can result in higher cell permeability of hydrophilic therapeutics such as genes, siRNA, antisense RNA, DNA, oligonucleotides, peptides and proteins.     

Molecular diagnostic and drug delivery agents based on aptamer-nanomaterial conjugates

Recent progress in an emerging area of designing aptamer and nanomaterial conjugates as molecular diagnostic and drug delivery agents in biomedical applications is summarized. Aptamers specific for a wide range of targets are first introduced and compared to antibodies. Methods of integrating these aptamers with a variety of nanomaterials, such as gold nanoparticles, quantum dots, carbon nanotubes, and superparamagnetic iron oxide nanoparticles, each with unique optical, magnetic, and electrochemical properties, are reviewed. Applications of these systems as fluorescent, colorimetric, magnetic resonance imaging, and electrochemical sensors in medical diagnostics are given, along with new applications as smart drug delivery agents.     

Dextran hydrogels for colon-specific drug delivery. V. Degradation in human intestinal incubation models

In the present study degradation of dextran hydrogels, potential drug carriers for colon-specific drug delivery, was studied in simulated small intestinal juices as well as in a human colonic fermentation model. Dextran hydrogels were shown to be stable when incubated at 37°C with the small intestinal enzymes amyloglucosidase, invertase and pancreatin. After a 24 h incubation, less than 3.3% of free glucose was released. However, the hydrogels were still intact as measured by the dry weight remaining. The fermentation of dextran hydrogels and several mono- and polysaccharides to short-chain fatty acids (SCFA) was investigated after anaerobic incubation in a human colonic fermentation model at 37°C for 0–72 h. In addition, the dextranase activity of the incubations was determined. The amounts and ratios of SCFA formed varied considerably in relation to the type of substrate fermented (glucose, maize starch, potato starch, cellulose, soluble dextran and dextran hydrogels). Detailed SCFA analysis demonstrated that fermentable saccharides resulted in an increased SCFA production, in contrast to the metabolic inert polysaccharide, cellulose. The hydrogels were found to be completely degraded in the human colonic fermentation model. An increased crosslinking density or a decreased degree of hydration resulted in a lower degradability. The pH of the incubations were found to be inversely proportional to the SCFA production as a result of the increased acid formation.     

Potential of transferrin and transferrin conjugates of liposomes in drug delivery and targeting

Targeted drug delivery has gained recognition in modern therapeutics and attempts are being made to explore the potential of cell biology-related bioevents in the development of specific and target-oriented systems. In connection to modern therapeutic systems, most of the emphasis has been laid upon the bioconjugated drug delivery systems. Bioconjugates involve the linking of two or more molecules to form a novel complex having the combined properties of its individual components. The nature of the linking agent between the pharmacologic agent and the delivery-augmenting moiety dictates the degree of successful delivery and its outcome. The component for the bioconjugated drug delivery includes receptors and ligands, where the receptors act as molecular targets or portals whereas ligands with receptors provide selective and specific trafficking towards the targeting site. Recently, a number of bioconjugated systems have been discovered for the site-specific presentation and delivery of various bioactive substances using biorelevant ligands, including antibodies, glycoprotein, viral proteins, and molecules of endogenous origin. In this review, the potential of transferrin (Tf) and Tf conjugates of liposomes in site-specific drug delivery systems are discussed. Tf is an abundant component of serum with the capacity to bind and transport iron, while Tf receptor (TfR), a dimeric transmembrane glycoprotein, is present on the surface of the most proliferating, higher eukaryotic cells. Tf expression is also found in nonproliferating tissues, such as hepatocytes, tissue macrophages, pituitary cells, pancreatic islet cells, and the endothelium of brain capillaries. Tumor cells frequently carry elevated numbers of TfRs compared with corresponding normal cells, and reduced serum levels of Tf are often observed in patients with tumors. In the past, various strategies have been developed, which include coupling of the liposomal surface with Tf by using various linking agents. Low-molecular weight drugs and proteins as well as liposomes can be linked with Tf. The Tf-coupled vesicular system is physicochemically stable in the bioenvironment and is site-specific. The aim of coupling liposomes with Tf is to improve the physical and biochemical stability of liposomes and make them appropriate for targeting specific organs and cells. Tf may be widely applied either as a carrier or targeting ligand in the active targeting of anticancer agents, proteins, and genes to primarily proliferating malignant cells that overexpress TfRs. Tf has been used as a molecular conjugate to deliver DNA to erythroleukemic, lung, and liver cell lines. Tf can also be modified with the positive charge N-acylurea groups to make them suitable for electrostatic binding of DNA, in order to achieve a well defined DNA-binding ligand for receptor-mediated gene transfer. Association of Tf with lipoplexes, in particular the negatively charged ternary complexes, significantly overcomes the inhibitory effect of serum and facilitates efficient transfection in many cell lines, including HeLa, K-562 cells, and lung carcinoma cells Calu-3 and H-292 cells. Tf-lipoplex has demonstrated high efficiency in tumor-targeted gene delivery and long-term therapeutic accuracy in systemic p53 gene therapy for both human head and neck cancer and prostate cancer. Tf and Tf-coupled liposomal drug delivery systems may prove particularly valuable to enable the use of a drug that seems to be ineffective or toxic if delivered systematically. The delivery of drugs to the brain has been particularly challenging because of the presence of the blood-brain barrier, which restricts the passage of most therapeutic agents into the brain. Therefore, active targeting of the brain is crucial for effective treatment of brain diseases. The anti-TfR antibody, such as OX26, when coupled with therapeutic agents, has shown potential in drug and gene delivery to the brain.     

Responsive polymer conjugates for drug delivery applications: recent advances in bioconjugation methodologies

Abstract

Protein–polymer conjugates have achieved tremendous attention in the last few years, since their importance in diverse fields including drug delivery, biotechnology and nanotechnology. Over the past few years, numerous chemical strategies have been developed to conjugate different synthetic polymers onto proteins and great progress has been made. Currently, there are a handful of therapeutic polymer conjugates that have been approved by the FDA, while many hundreds of products are under extensive clinical trials and preclinical development phases. In this way, the development of novel techniques for conjugation, especially living radical polymerisation (LRP) has greatly enhanced the potential to broaden the scope of therapeutic conjugates. As a consequence, versatile techniques have developed, such as the ‘grafting from’ approach, which allows modifications of biomacromolecules at the atomic level, and subsequently preparing well-defined stimuli-responsive conjugates. These strategies present a unique perspective for therapy expansion of a new generation of ‘smart’ products with proprieties that can be finely controlled and tuned rather than just enhanced. This article highlights recent advances in the synthesis and application of protein–polymer conjugates by controlled radical polymerisation techniques, with special emphasis on stimuli-responsive conjugates on new applications in biomedical and pharmaceutical areas.     

Biopharmaceutical aspects of a chemical approach to drug delivery: macromolecule-drug conjugates

Some recent advances pertaining to the biopharmaceutical aspects of drug delivery are briefly given, followed by a review of the current status of the application of soluble macromolecular carriers for drug delivery. The dextran conjugates were selected as model compounds to demonstrate the implications of the biopharmaceutical approach in the design, evaluation, and delivery of macromolecular conjugates.     

Micelle-like nanoassemblies based on polymer-drug conjugates as an emerging platform for drug delivery

INTRODUCTION: During the past decades, polymer-drug conjugates are one of the hottest topics in novel drug development fields. Amphiphilic polymer-drug conjugates in aqueous solution could form micelles or micelle-like nanoassemblies. Compared with polymer-drug conjugates and the micelles into which drugs are physically entrapped, micelles or micelle-like nanoassemblies based on polymer-drug conjugates bring several additional advantages, including increased drug-loading capacity, enhanced intracellular uptake, reduced systemic toxicity, and improved therapeutic efficacy. AREAS COVERED: This review focuses on recent progress achieved in the research field of micelles or micelle-like nanoassemblies based on polymer-drug conjugates. Firstly, properties of polymers, drugs, and linkers which could be used to build polymer-drug conjugate micelles or micelle-like nanoassemblies are summarized. Then, the characterization methods are described. Finally, the drug-targeting mechanisms are discussed. Micelles or micelle-like nanoassemblies based on polymer-drug conjugates as an emerging platform have the potential to achieve medical treatments with enhanced therapeutic effect. EXPERT OPINION: The application of micelles or micelle-like nanoassemblies based on polymer-drug conjugates may give new life to old active compounds abandoned due to their low solubility problems. For clinical application, there is a need to further optimize the properties of the polymer, drug, and linker.     

Poly(amidoamine) dendrimer-erythromycin conjugates for drug delivery to macrophages involved in periprosthetic inflammation

Erythromycin (EM), an antibiotic that has been used for infectious diseases, is now gaining attention because of its novel anti-inflammatory effects. We explore a dendrimer-EM nanodevice for sustained treatment of orthopedic inflammation. To sustain pharmacological activity, EM was conjugated to poly(amidoamine) dendrimer (PAMAM) through an ester bond. A bifunctional PAMAM dendrimer was prepared having neutral hydroxy and reactive amine groups on the surface and was reacted with EM prodrug (EM-2'-glutarate). The cytotoxicity, efficacy and antibacterial properties were evaluated on macrophages (RAW 264.7 cells) associated with periprosthetic inflammation. The conjugate is noncytotoxic and showed significant reduction of nitrite level (by 42% as compared with untreated cells and free EM). The zone of inhibition of the conjugate on bacterial growth at different concentrations showed similar activity compared to free EM. The anti-inflammatory properties of EM combined with the targeting potential of the dendrimer can lead to sustained and targeted intracellular delivery. FROM THE CLINICAL EDITOR: In this study, a specific dendrimer-erythromycin conjugate nanodevice is investigated for the treatment of periprosthetic inflammation. The anti-inflammatory properties of erythromycin combined with the targeting potential of the dendrimer can lead to sustained and targeted intracellular delivery.     

Recent development of poly(ethylene glycol)-cholesterol conjugates as drug delivery systems

Poly(ethylene glycol)-cholesterol (PEG-Chol) conjugates are composed of “hydrophilically-flexible” PEG and “hydrophobically-rigid” Chol molecules. PEG-Chol conjugates are capable of forming micelles through molecular self-assembly and they are also used extensively for the PEGylation of drug delivery systems (DDS). The PEGylated DDS have been shown to display optimized physical stability properties in vitro and longer half-lives in vivo when compared with non-PEGylated DDS. Cell uptake studies have indicated that PEG-Chol conjugates are internalized via clathrin-independent pathways into endosomes and Golgi apparatus. Acid-labile PEG-Chol conjugates are also able to promote the content release of PEGylated DDS when triggered by dePEGylation at acidic conditions. More importantly, biodegradable PEG-Chol molecules have been shown to decrease the “accelerated blood clearance” phenomenon of PEG-DSPE. Ligands, peptides or antibodies which have been modified with PEG-Chols are oftentimes used to formulate active targeting DDS, which have been shown in many systems recently to enhance the efficacy and lower the adverse effects of drugs. Production of PEG-Chol is simple and efficient, and production costs are relatively low. In conclusion, PEG-Chol conjugates appear to be very promising multifunctional biomaterials for many uses in the biomedical sciences and pharmaceutical industries.     

Polysaccharide-oligoamine based conjugates for gene delivery

This work describes a versatile and universal polycation system based on oligoamines grafted on natural polysaccharides that is capable of complexing various plasmids and administering them into various cells in high yield to produce a desired protein. These polycations are expected to better meet the requirements for effective complexation and delivery of plasmid or an antisense and to biodegrade into nontoxic components at a controlled rate. The developed biodegradable polycations are based on spermine, a natural tetramine, conjugated to dextran or arabinogalactan. These polycations were prepared by reductive amination of oxidized polysaccharides with the desired oligoamines. The Schiff base conjugates thus obtained were reduced to the stable amine conjugates by sodium borohydride. Over 300 different polycations were prepared starting from various polysaccharides and oligoamines, mainly oligoamines of two to four amino groups. Although most of these conjugates formed stable complexes with various plasmids as determined by turbidity experiments, only a few polycations were found to be active in transfecting cells. This work indicates that the structure of the polycation plays a significant role in the transfection activity of polycations.     

Lectin-mediated drug delivery: binding and uptake of BSA-WGA conjugates using the Caco-2 model

To examine whether the dietary lectin wheat germ agglutinin (WGA) can facilitate binding and uptake of protein drugs due to its cytoadhesive and cytoinvasive properties, conjugates were prepared by covalent coupling of fluorescein-labeled bovine serum albumin (F-BSA) to WGA using divinylsulfone for crosslinking. Increasing the molar ratio of F-BSA/WGA resulted in 2.6-8.7 times higher Caco-2 binding as compared with glycyl-F-BSA. About 75% of F-BSA-WGA were bound specifically to Caco-2 cells according to inhibition studies in presence of the complementary carbohydrate. The Caco-2 association of F-BSA-WGA was temperature-dependent indicating active uptake of membrane bound conjugate, which was confirmed by confocal microscopy. The conjugate accumulated within lysosomal compartments followed by proteolytic degradation of F-BSA-WGA 1-4 h after conjugate loading as observed by equilibrating the intracellular pH with monensin. Finally low molecular weight degradation products of the proteinaceous prodrug appear in the extracellular medium. Contrary to Caco-2 single cells, a minor part of the conjugate is degraded by brush border proteases already 30 min after exposure to Caco-2 monolayers. But most of the conjugate is taken up into differentiated cells and processed as in single cells. Though the enzymic barrier remains to be surmounted, WGA-mediated drug delivery is a promising strategy for peroral delivery of even high molecular weight drugs to overcome the mucosal barrier.     

Drug delivery systems based on sugar-macromolecule conjugates

The specificity of carbohydrate-protein interactions can greatly outstrip that of many other ligand-binding systems; such is the enormous density of information that sugars can convey. In addition, macromolecules allow for the fine-tuning of active drug delivery through their great ability to undergo site-specific modification and their inherent physicochemical properties. Once combined, these two factors suggest that sugar-macromolecule conjugates, targeted using endogenous carbohydrate binding proteins, are a promising route to the 'magic bullet'.     

Organogels in drug delivery

In the last decade, interest in physical organogels has grown rapidly with the discovery and synthesis of a very large number of diverse molecules, which can gel organic solvents at low concentrations. The gelator molecules immobilise large volumes of liquid following their self-assembly into a variety of aggregates such as rods, tubules, fibres and platelets. The many interesting properties of these gels, such as their thermoreversibility, have led to much excitement over their industrial applications. However, only a few organogels are currently being studied as drug/vaccine delivery vehicles as most of the existing organogels are composed of pharmaceutically unacceptable organic liquids and/or unacceptable/untested gelators. In this paper a brief overview of organogels is presented, followed by a more in-depth review of the gels that have been investigated for drug and/or vaccine delivery. These include microemulsion-based gels and lecithin gels (studied for transdermal delivery), sorbitan monostearate organogels and amphiphilogels (studied as vaccine adjuvants and for oral and transdermal drug delivery, respectively), gels composed of alanine derivatives (investigated as in situ forming gels) and Eudragit organogels (studied as a matrix for suppositories). Finally, pluronic lecithin organogels, descendents of lecithin gels but which are not really organogels, are briefly discussed for their interesting history, their root and the wide interest in these systems.     

BioMEMS in drug delivery

The drive to design micro-scale medical devices which can be reliably and uniformly mass produced has prompted many researchers to adapt processing technologies from the semiconductor industry. By operating at a much smaller length scale, the resulting biologically-oriented microelectromechanical systems (BioMEMS) provide many opportunities for improved drug delivery: Low-dose vaccinations and painless transdermal drug delivery are possible through precisely engineered microneedles which pierce the skin's barrier layer without reaching the nerves. Low-power, low-volume BioMEMS pumps and reservoirs can be implanted where conventional pumping systems cannot. Drug formulations with geometrically complex, extremely uniform micro- and nano-particles are formed through micromolding or with microfluidic devices. This review describes these BioMEMS technologies and discusses their current state of implementation. As these technologies continue to develop and capitalize on their simpler integration with other MEMS-based systems such as computer controls and telemetry, BioMEMS' impact on the field of drug delivery will continue to increase.