Lectin-mediated drug delivery: the second generation of bioadhesives

Oral Delivery of Peptide Drugs

A wide variety of peptide drugs are now produced on a commercial scale as a result of advances in the biotechnology field. Most of these therapeutic peptides are still administered by the parenteral route because of insufficient absorption from the gastrointestinal tract. Peptide drugs are usually indicated for chronic conditions, and the use of injections on a daily basis during long-term treatment has obvious drawbacks. In contrast to this inconvenient and potentially problematic method of drug administration, the oral route offers the advantages of self-administration with a high degree of patient acceptability and compliance. The main reasons for the low oral bioavailability of peptide drugs are pre-systemic enzymatic degradation and poor penetration of the intestinal mucosa. A considerable amount of research has focused on overcoming the challenges presented by these intestinal absorption barriers to provide effective oral delivery of peptide and protein drugs. Attempts to improve the oral bioavailability of peptide drugs have ranged from changing the physicochemical properties of peptide molecules to the inclusion of functional excipients in specially adapted drug delivery systems. However, the progress in developing an effective peptide delivery system has been hampered by factors such as the inherent toxicities of absorption-enhancing excipients, variation in absorption between individuals, and potentially high manufacturing costs. This review focuses on the intestinal barriers that compromise the systemic absorption of intact peptide and protein molecules and on the advanced technologies that have been developed to overcome the barriers to peptide drug absorption.

Nanoscale technology of mucoadhesive interactions

Nanoscale analysis may be used to design new types of mucoadhesive polymers. Understanding of the surface interactions between hydrophilic polymer surfaces and mucins can lead to improved adhesive bonding by hydrogen bonding. Alternatively, decoration of a mucoadhesive polymer surface with tethers of linear and block copolymers containing neutral or ionizable structures provides increased interdigitation and interpenetration with the mucus. Finally, formation of micro- or nanopatterns on these surfaces can lead to promising new systems of oral delivery applications.

Lectin-functionalized poly (lactide-co-glycolide) nanoparticles as oral/aerosolized antitubercular drug carriers for treatment of tuberculosis

OBJECTIVES

This study was carried out to explore lectin-functionalized poly (lactide-co-glycolide) nanoparticles (PLG-NPs) as bioadhesive drug carriers against tuberculosis (TB), in order to reduce the drug dosage frequency of antitubercular drugs and thus improve patient compliance in TB chemotherapy.

METHODS

Wheat germ agglutinin (WGA)-coated PLG-NPs were prepared by a two-step carbodiimide procedure. This formulation was administered to guinea pigs through the oral/aerosol route for a detailed pharmacokinetic and chemotherapeutic evaluation. Immunological or hepatotoxic effects of WGA lectin, if any, were also determined.

RESULTS

WGA-functionalized PLG-NPs were in the size range of 350-400 nm, with binding of 3-3.5 microg of WGA/mg of PLG-NPs and drug encapsulation efficiency of 54%-66%. Upon administration of lectin-coated PLG-NPs through the oral/aerosol route, the presence of drugs in plasma was observed for 6-7 days for rifampicin and 13-14 days for isoniazid and pyrazinamide. However, upon administration of uncoated PLG-NPs (oral/aerosolized) rifampicin was detectable in plasma for 4-6 days, whereas isoniazid and pyrazinamide were detectable for 8-9 days. All three drugs were present in lungs, liver and spleen for 15 days. Administration of WGA-coated PLG-NPs caused a significant (P < 0.001) increase in the relative bioavailability of antitubercular drugs. Chemotherapeutic studies revealed that three doses of oral/nebulized lectin-coated nanoparticles fortnightly could yield undetectable mycobacterial colony forming units (cfu); this was achievable with 45 doses of oral free drugs.

CONCLUSION

WGA-functionalized PLG-NPs could be potential drug carriers for antitubercular drugs through the oral as well as aerosol route for effective TB control.

The lectin-cell interaction and its implications to intestinal lectin-mediated drug delivery

Based on the fact that oligosaccharides encode biological information, the biorecognition between lectinised drug delivery systems and glycosylated structures in the intestine can be exploited for improved peroral therapy. Basic research revealed that some lectins can mediate mucoadhesion, cytoadhesion, and cytoinvasion of drugs. Entering the vesicular pathway by receptor mediated endocytosis, part of the conjugated drug is accumulated within the lysosomes. Additionally, part of the drug is supposed to be transported across the epithelium. Moreover, factors probably adversely influencing feasibility of the concept such as toxicity, immunogenicity, and intestinal stability of plant lectins are discussed. As exemplified by lectin-grafted prodrug and carrier systems, this strategy is expected to improve absorption and probably bioavailability of poorly absorbable drugs, peptides and proteins as well as therapeutic DNA.

Mechanistic study of the uptake of wheat germ agglutinin-conjugated PLGA nanoparticles by A549 cells

The purpose of this study was to evaluate the extent and mechanism of uptake of wheat germ agglutinin-conjugated PLGA nanoparticles by A549 cells. PLGA nanoparticles of 150 nm were prepared by a solvent diffusion method and covalently conjugated to FITC-WGA (fWGA) or FITC-bovine serum albumin (fBSA) by a two-step carbodiimide method. Uptake of fWGA-PLGA and fBSA-PLGA nanoparticles by confluent A549 cells was quantified by fluorometry. A549 cellular uptake of fWGA-PLGA nanoparticles at 2 h, 37 degrees C was 5.02-fold that of fBSA-PLGA nanoparticles at a loading concentration of 2.65 mg/mL. The difference in uptake between the two types of nanoparticles was increased to 7.84-fold at a higher loading concentration of 5.3 mg/mL, but was reduced to 2.07-fold by lowering the uptake temperature to 4 degrees C. Coincubation with 5 mg/mL of unlabeled WGA negated the differential uptake of fWGA-PLGA nanoparticles at 4 degrees C, suggesting that the nanoparticles interacted with a specific WGA-binding receptor on the cell membrane. Internalization of the fWGA-PLGA nanoparticles by the A549 cells was confirmed by confocal microscopy. Filipin (1 microg/mL), a known inhibitor of caveolae, reduced the 1-h uptake of the nanoparticles by 75%. Surface modification of PLGA nanoparticles with WGA significantly enhanced its endocytosis by A549 cells by a receptor-mediated, caveola-dependent pathway.

Amphiphilic copolymers of epsilon-caprolactone and gamma-substituted epsilon-caprolactone. Synthesis and functionalization of poly(D,L-lactide) nanoparticles

Fully biodegradable and surface-functionalized poly(D,L-lactide) (PLA) nanoparticles have been prepared by a co-precipitation technique. Novel amphiphilic random copolyesters P(CL-co-gammaXCL) were synthesized by controlled copolymerization of epsilon-caprolactone and epsilon-caprolactone substituted in the gamma-position by a hydrophilic X group, where X is either a cationic pyridinium (gammaPyCL) or a non-ionic hydroxyl (gammaOHCL). Nanoparticles were prepared by co-precipitation of PLA with the P(CL-co-gammaXCL) copolyester from a DMSO solution. Small amounts of cationic P(CL-co-gammaPyCL) copolymers are needed to quantitatively form stable nanoparticles (ca. 10 mg/ 100 mg PLA), although larger amounts of non-ionic P(CL-co-gammaOHCL) copolymers are needed (> or = 12.5 mg/ 100 mg PLA). Copolymers with a low degree of polymerization (ca. 40) are more efficient stabilizers, probably because of faster migration towards the nanoparticle-water interface. The nanoparticle diameter decreases with the polymer concentration in DMSO, e.g. from ca. 160 nm (16 mg/ml) to ca. 100 nm (2 mg/ml) for PLA/P(CL-co-gammaPyCL) nanoparticles. Migration of the P(CL-co-gammaXCL) copolyesters to the nanoparticle surface was confirmed by measurement of the zeta potential, i.e. ca. +65 mV for P(CL-co-gammaPCL) and -7 mV for P(CL-co-gammaOHCL). The polyamphiphilic copolyesters stabilize PLA nanoparticles by electrostatic or steric repulsions, depending on whether they are charged or not. They also impart functionality and reactivity to the surface, which opens up new opportunities for labelling and targeting purposes.

Synthesis of cytoadhesive poly(methylmethacrylate) for applications in targeted drug delivery

The objective of this work was to incorporate cytoadhesive properties into poly(methylmethacrylate) (PMMA) for potential applications in highly localized tissue-specific drug delivery. First, the PMMA was chemically modified by aminolysis to yield amine-terminated surfaces. X-ray photoelectron microscopy confirmed the presence of surface nitrogen entities, and the distribution of amine groups was found to be relatively uniform, as characterized by atomic force microscopy. The availability of these groups for attachment of biologically active molecules was characterized by fluorescence microscopy after immobilization of avidin-FITC. To render the PMMA cytoadhesive, avidin molecules were conjugated to the amine-terminated surfaces with a hydroxy-succinimide-catalyzed carbodiimide reagent and biotin-labeled lectins (tomato, which binds selectively to Caco-2 cells, and peanut, an unrelated lectin) subsequently were attached utilizing avidin-biotin chemistry. Cytoadhesive activity was evaluated by characterizing the interactions between microfabricated PMMA particles and Caco-2 monolayers. After 15-, 30-, 60-, and 120-min incubation periods, the tomato lectin-conjugated PMMA showed a two to sixfold increase in Caco-2 cell recognition over control particles. Furthermore, the stability of the cytoadhesive PMMA interactions appeared to be three to seven times greater than that of the control surfaces. These findings demonstrate that cytoadhesive properties of modified PMMA, making this novel bioactive polymer very promising for applications in targeted drug delivery.

Bioadhesive poly(methyl methacrylate) microdevices for controlled drug delivery

Oral delivery is the preferred route of drug administration. However, the breakdown of molecules and low levels of absorption in the gastrointestinal system render the oral delivery of proteins and peptides ineffective. Bioadhesive delivery devices can be used to circumvent these problems by protecting the drug from gastrointestinal denaturation, localizing and prolonging a drug at a specific target site, and maintaining direct contact with the intestinal cells, thereby increasing the drug concentration gradient. Microfabrication technology may offer some potential advantages over conventional delivery technologies. The benefits of microfabrication include the ability to tailor the size, shape, reservoir volume, and surface characteristics of the drug delivery vehicle. In this study, bioadhesive properties were introduced to microfabricated poly(methyl methacrylate) (PMMA) microdevices by attachment of lectins, a group of proteins capable of specifically targeting cells in the gastrointestinal tract. In this process, the PMMA microdevices were chemically modified by aminolysis to yield amine-terminated surfaces. Avidin molecules were covalently bound to the surface of the particles using a hydroxysuccinimide catalyzed carbodiimide reagent and then incubated in an aqueous solution of biotinylated lectin. The lectin-modified microdevices were examined in vitro in terms of their bioadhesive characteristics.

Microfabricated drug delivery systems: from particles to pores

Microfabrication techniques which permit the creation of therapeutic delivery systems that possess a combination of structural, mechanical, and perhaps electronic features may surmount challenges associated with conventional delivery of therapy. In this review, delivery concepts are presented which capitalize on the strengths of microfabrication. Possible applications include micromachined silicon membranes to create implantable biocapsules for the immunoisolation of pancreatic islet cells-as a possible treatment for diabetes-and sustained release of injectable drugs needed over long time periods. Asymmetrical, drug-loaded microfabricated particles with specific ligands linked to the surface are proposed for improving oral bioavailability of peptide (and perhaps protein) drugs. In addition, microfabricated drug delivery systems ranging from transdermal microneedles to implantable microchips will be discussed.

Drug absorption by the respiratory mucosa: cell culture models and particulate drug carriers

The inhalation route is of increasing interest for both local and systemic drug delivery, including macromolecular biopharmaceuticals, such as peptides, proteins, and gene therapeutics. In addition to appropriate aerosolization for deposition in relevant areas of the respiratory tract, therapeutic molecules may require an advanced carrier system for safe and efficient delivery to their target. Two approaches to obtain novel carrier systems for pulmonary drug delivery are large porous microparticles with a low aerodynamic diameter and lectin-functionalized liposomes. Epithelial cells of alveolar or bronchial origin, obtained either from patient material or from established cell lines, can be grown on permeable filter supports, resulting in polarized monolayers with functional intercellular junctions. With such in vitro models, transport of drugs into pulmonary epithelial cells and/or across the air-blood barrier, as well as the effect and efficacy of novel drug carrier systems can be systematically studied.

Bioadhesive microdevices with multiple reservoirs: a new platform for oral drug delivery

A variety of delivery systems have been devised, in recent years, to improve the oral bioavailability of drugs including enterically coated tablets, capsules, particles, and liposomes. Microfabrication technology may offer some potential advantages over conventional drug delivery technologies. This technology, combined with appropriate surface chemistry, may permit the highly localized and unidirectional release of drugs, permeation enhancers, and/or promoters. In this study, we demonstrate the fabrication of prototype reservoir-containing microdevices and a surface chemistry protocol that can be used to bind lectin via avidin-biotin interactions to these micromachined drug delivery vehicles. The use of microfabrication allows one to tailor the size, shape, reservoir volume, and surface characteristics of the drug delivery vehicle. In vitro studies show enhanced bioadhesion of these lectin conjugated silicon microdevices. This approach may be used to improve the absorption of pharmacologically active biopolymers such as peptides, proteins and oligonucleotides into circulation at targeted sites in the GI system via the creation of a robust hybrid organic/inorganic delivery system. This paper describes one of the first applications of microfabrication to oral drug delivery.

A quantitative evaluation of radiolabelled lectin retention on oral mucosa in vitro and in vivo

Previous work has identified lectins that bind to the cells present on the oral mucosa for their potential use as a means of retaining a drug delivery system on the mucosal surfaces of the mouth. In this study, a radiolabelling technique was developed to allow the quantification of lectin binding to human buccal cells in vitro, and the retention of the lectins in the oral cavity of a rat model in vivo. Lectins were labelled with 99mTc using a cyclic diethylene triamine pentaacetic acid conjugation technique. In the in vitro study, human buccal cells were obtained by scraping the inner surface of the cheek. The suspended cells were exposed to the labelled lectin solution for 30 min and after washing with buffer the activity associated with the cells determined. In the in vivo study, male Wistar rats were briefly anaesthetized during which 10 microl of a solution containing labelled lectin was applied into the buccal pouch. At set times the rats were killed and the lower buccal cavity mucosal tissue and tongue dissected out and monitored for bound lectin. The in vitro study indicated that the lectins from Arachis Hypogaea, Canavalia ensiformis and Triticum vulgaris bound to oral mucosal cells. The T. vulgaris lectin showed the greatest binding, calculated to be 6.77 x 10(9) molecules per cell. The in vivo retention of C. ensiformis and T. vulgaris lectins on rat oral mucosal tissue was also evident. The T. vulgaris lectin showed significantly higher levels of retained lectin after 30 min (29.54 +/- 4.20 microg SD) on the oral mucosal tissue and 28.37 microg (+/-2.13 SD) on the tongue and was still detected at similar levels after 2 h. These studies indicate that significant lectin binding to human buccal cells occurs in vitro and retention in an animal model occurs for over 2 h in vivo. The T. vulgaris lectin showed most promise for further work.

Mucoadhesive polymers with immobilized proteinase inhibitors for oral administration of protein drugs

A new approach to overcome the degradation of protein drugs by proteolytic enzymes and their targeting to the blood through the digestive apparatus was developed. The approach is based on the immobilization of drugs into the polymeric hydrogel containing glycoprotein--ovomucoid from duck egg whites. This glycoprotein inhibits the activity of proteolytic enzymes and acts as a biospecific ligand to lectins on the walls of the gastrointestinal tract.

Polymer-cysteamine conjugates: new mucoadhesive excipients for drug delivery?

In the present study, the features of two new thiolated polymers--the so-called thiomers--were investigated. Mediated by a carbodiimide cysteamine was covalently attached to sodium carboxymethylcellulose (Na-CMC) and neutralised polycarbophil (Na-PCP). Depending on the weight-ratio polymer to cysteamine during the coupling reaction, the resulting CMC-cysteamine conjugate and PCP-cysteamine conjugate showed in maximum 43 +/- 15 and 138 +/- 22 micromole thiol groups per g polymer (mean +/- S.D.; n=3), respectively, which were used for further characterisation. Tensile studies carried out with the CMC-cysteamine conjugate on freshly excised porcine intestinal mucosa displayed no significantly (P<0.01) improved mucoadhesion, whereas, the mucoadhesive properties of the PCP-cysteamine conjugate were increased 2.5-fold compared with the unmodified polymer. The swelling behaviour of the CMC-cysteamine conjugate was uninfluenced by the covalent attachment of the sulfhydryl compound. In contrast the swelling behaviour of the PCP-cysteamine conjugate was improved significantly (P<0.01) versus unmodified PCP. Furthermore, in aqueous solutions the disintegration time of tablets based on the CMC- and PCP-cysteamine conjugates was prolonged 1.5 and 3.2-fold, respectively, in comparison to tablets containing the corresponding unmodified polymers. According to these results, especially the PCP-cysteamine conjugate represents a promising new pharmaceutical excipient for various drug delivery systems.

Lectin-sugar interaction. Calculated versus experimental binding energies

Although a steadily increasing number of protein--ligand docking experiments have been performed successfully, there are only few studies concerning protein--sugar interactions. In this study, we investigate the interaction of wheat germ agglutinin (WGA) with N-acetylglucosamine and a number of its derivatives and predict the binding free energies using flexible docking techniques. To assess the quality of our predictions, we also determined those binding free energies experimentally in cell-binding studies. The predicted binding site, ligand orientation, and details of the binding mode are in perfect agreement with the known crystal structure of WGA with a sialoglycopeptide. Furthermore, we obtained an excellent linear correlation of our predicted binding free energies with both our own data and experimental data from the literature [Monsigny, M., Roche, A.C., Sene, C., Maget Dana, R. & Delmotte, F. (1980) Eur. J. Biochem. 104, 147-153.]. In both cases, predicted energies were within 1.0 kJ x mol(-1) of the experimental value. These results illustrate the usefulness of docking-based methods for the qualitative and quantitative prediction of protein--carbohydrate interactions. The insights gained from such theoretical studies may be used to complement the results from the still scarce crystal structures.

Preparation of poly(D,L-lactide) nanoparticles assisted by amphiphilic poly(methyl methacrylate-co-methacrylic acid) copolymers

When co-precipitated with amphiphilic copolymers from DMSO, poly(D,L-lactide) (PLA) can be readily converted into stable sub-200 nm nanoparticles by addition of an aqueous phase, free of any polymeric stabilizers such as poly(vinyl alcohol) or Poloxamer. In this work, the ability of random poly(methyl methacrylate-co-methacrylic acid) copolymers (PMMA-co-MA) to stabilize PLA nanoparticles was demonstrated, and the properties of PLA/PMMA-co-MA nanoparticles were investigated. When co-precipitated with PMMA-co-MA, PLA was totally converted into nanoparticles using a polymer concentration in DMSO (Cp) below 17.6 mg ml(-1), and a PMMA-co-MA proportion above a critical value depending on the content of MA repeating units (X). For instance, the lowest PMMA-co-MA proportion required was 0.9 mg mg(-1) PLA for X = 12%, and 0.5 mg mg(-1) PLA for X = 25% (for C(PLA) = 16 mg ml(-1) DMSO). The nanoparticle diameter was essentially independent of X, the proportion of PMMA-co-MA, and the PLA molecular weight, except for oligomers for which the nanoparticle diameter was smaller. It decreased when the organic phase was diluted (126 +/- 13 nm for Cp = 17.6 mg ml(-1), and 81 +/- 5 nm for C(P) = 5.6 mg ml(-1)). The time-dependence of the stability and the degradation of PLA/PMMA-co-MA nanoparticles was discussed. One of the main advantages of this technique is the ability to control surface properties and to bring functional groups to otherwise non-functionalized PLA nanoparticles. To illustrate this, a conjugate of PMMA-co-MA25 and biotin was synthesized, and used to prepare biotinylated nanoparticles that could be detected by fluorescence and transmission electron microscopy after infiltration into ligatured rat small intestine.

Polymers in drug delivery

Advances in polymer science have led to the development of several novel drug-delivery systems. A proper consideration of surface and bulk properties can aid in the designing of polymers for various drug-delivery applications. Biodegradable polymers find widespread use in drug delivery as they can be degraded to non-toxic monomers inside the body. Novel supramolecular structures based on polyethylene oxide copolymers and dendrimers are being intensively researched for delivery of genes and macromolecules. Hydrogels that can respond to a variety of physical, chemical and biological stimuli hold enormous potential for design of closed-loop drug-delivery systems. Design and synthesis of novel combinations of polymers will expand the scope of new drug-delivery systems in the future.

Lectin-functionalized liposomes for pulmonary drug delivery: effect of nebulization on stability and bioadhesion

The generation of respirable aerosols of a functionalized colloidal carrier has been investigated in this study. Lectin-functionalized liposomes, which proved to show improved cell association (using A549 cell line and primary human alveolar cells) even in the presence of a commercial lung surfactant preparation, have been developed. The stability of non-functionalized liposomes during nebulization using a jet nebulizer (Pari II provocation nebulizer, operated using an air flow of 30 l/min) was firstly investigated, and the experimental and formulation conditions were optimized and applied for the preparation of lectin-functionalized liposomes. The incorporation of cholesterol enhanced the stability of the liposomes during nebulization (from 15-20% leakage of a hydrophilic marker to 8% upon cholesterol incorporation) and upon incubation with lung surfactant preparation. Nebulization of the functionalized liposomes did not significantly influence their physical stability. Their enhanced cell binding capability (compared to non-functionalized liposomes) was also maintained. A drop in cell association compared to fresh functionalized liposomes was detected after nebulization, nevertheless, the binding was still significantly higher than that of the non-functionalized liposomes. The deposition of the liposomal preparation in lung periphery, proved by the deposition of the liposomal preparation on the lower stages of an ASTRA type cascade impinger and a mean median aerodynamic diameter (MMAD) of 2.85 microm, makes it a potential candidate as a macromolecule-drug carrier for local and/or systemic administration.

The interaction between wheat germ agglutinin and other plant lectins with prostate cancer cells Du-145

The bioadhesive properties of fluorescein-labeled plant lectins with different carbohydrate specificities were investigated by flow cytometry at 4 and 37 degrees C using Du-145 prostate cancer cells. At both temperatures the lectin association rate increased following the order: Dolichos biflorus agglutinin (DBA)<peanut agglutinin<Ulex europaeus isoagglutinin I<Lens culinaris agglutinin<Solanum tuberosum lectin << wheat germ agglutinin (WGA), reflecting the glycosylation pattern of Du-145 cells. Both, the BSA-binding capacity of the cells referring to nonspecific binding and inhibition studies using the complementary carbohydrate, assured specificity of the lectin-cell interactions except for DBA. The WGA-association rate of Du-145 cells was dependent on temperature indicative for cellular uptake of membrane-bound WGA. Intracellular enrichment of WGA was confirmed by confocal microscopy. As resulted from experiments in presence of ouabain active transport mechanisms were involved in cellular uptake of WGA. Equilibration of the intracellular pH with monensin pointed to accumulation of intracellular located WGA within acidic compartments of Du-145 cells such as the lysosomes or the trans-Golgi complex. Consequently the interaction of WGA with Du-145 cells at 37 degrees C is a one way process due to immediate active transport of membrane-bound lectin into acidic compartments of prostate cancer cells.