Intestinal absorption of fluorescently labeled nanoparticles.

Characterization of intestinal absorption of nanoparticles is critical in the design of noninvasive anticancer, protein-based, and gene nanoparticle-based therapeutics. Here we demonstrate a general approach for the characterization of the intestinal absorption of nanoparticles and for understanding the mechanisms active in their processing within healthy intestinal cells. It is generally accepted that the cellular processing represents a major drawback of current nanoparticle-based therapeutic systems. In particular, endolysosomal trafficking causes degradation of therapeutic molecules such as proteins, lipids, acid-sensitive anticancer drugs, and genes. To date, investigations into nanoparticle processing within intestinal cells have studied mass transport through Caco-2 cells or everted rat intestinal sac models. We developed an approach to visualize directly the mechanisms of nanoparticle processing within intestinal tissue. These results clearly identify a mechanism by which healthy intestinal cells process nanoparticles and point to the possible use of this approach in the design of noninvasive nanoparticle-based therapies. FROM THE CLINICAL EDITOR: Advances in nanomedicine have resulted in the development of new therapies for various diseases. Intestinal route of administration remains the easiest and most natural. The authors here designed experiments to explore and characterize the process of nanoparticle transport across the intestinal tissue. In so doing, further insights were gained for future drug design.

Silica nanoparticles to control the lipase-mediated digestion of lipid-based oral delivery systems.

We investigate the role of hydrophilic fumed silica in controlling the digestion kinetics of lipid emulsions, hence further exploring the mechanisms behind the improved oral absorption of poorly soluble drugs promoted by silica-lipid hybrid (SLH) microcapsules. An in vitro lipolysis model was used to quantify the lipase-mediated digestion kinetics of a series of lipid vehicles formulated with caprylic/capric triglycerides: lipid solution, submicrometer lipid emulsions (in the presence and absence of silica), and SLH microcapsules. The importance of emulsification on lipid digestibility is evidenced by the significantly higher initial digestion rate constants for SLH microcapsules and lipid emulsions (>15-fold) in comparison with that of the lipid solution. Silica particles exerted an inhibitory effect on the digestion of submicrometer lipid emulsions regardless of their initial location, i.e., aqueous or lipid phases. This inhibitory effect, however, was not observed for SLH microcapsules. This highlights the importance of the matrix structure and porosity of the hybrid microcapsule system in enhancing lipid digestibility as compared to submicrometer lipid emulsions stabilized by silica. For each studied formulation, the digestion kinetics is well correlated to the corresponding in vivo plasma concentrations of a model drug, celecoxib, via multiple-point correlations (R(2) > 0.97). This supports the use of the lipid digestion model for predicting the in vivo outcome of an orally dosed lipid formulation. SLH microcapsules offer the potential to enhance the oral absorption of poorly soluble drugs via increased lipid digestibility in conjunction with improved drug dissolution/dispersion.

Hybrid lipid-silica microcapsules engineered by phase coacervation of Pickering emulsions to enhance lipid hydrolysis.

We report on the fabrication of dry hybrid lipid-silica microcapsules for enhanced lipid hydrolysis using Pickering emulsion templates formed by interfacial nanoparticle-emulsifier electrostatic interaction. The microcapsules are produced by controlled precipitation of emulsion droplets by oppositely charged silica nanoparticles at room temperature. Microcapsule formation is driven by the interfacial structure of the initial Pickering emulsion, which is in turn controlled by the nanoparticle to lipid ratio. In the region of charge reversed, precipitated and aggregated droplets, droplet-nanoparticle networks have been identified by freeze-fracture SEM imaging. The microcapsules have diameters in the range 20-50 mum and contain approximately 65% oil distributed within an internal matrix structure composed of a labyrinth of interconnected pores approximately 20-100 nm. Pore distribution and diameters depend on the silica to nanoparticle ratio that in turn determines droplet coating and stability. The microcapsules facilitate enhanced lipid hydrolysis kinetics, i.e. their pseudo first-order rate constant for lipid hydrolysis is approximately 3 times greater than for equivalent submicron lipid droplets. This behaviour is attributed to the increased oil surface area within the microcapsule due to the specific porous structure that causes rapid release of submicron and micron size oil droplets. The simple route for fabrication of porous microcapsule morphologies may present new opportunities for applications in encapsulation, delivery, coatings, and catalysis.

Nanoparticle coated submicron emulsions: sustained in-vitro release and improved dermal delivery of all-trans-retinol.

PURPOSE: The aim of this research is to investigate the dermal delivery of all-trans-retinol from nanoparticle-coated submicron oil-in-water emulsions as a function of the initial emulsifier type, the loading phase of nanoparticles, and the interfacial structure of nanoparticle layers. METHODS: The interfacial structure of emulsions was characterized using freeze-fracture-SEM. In-vitro release and skin penetration of all-trans-retinol were studied using Franz diffusion cells with cellulose acetate membrane, and excised porcine skin. The distribution profile was obtained by horizontal sectioning of the skin using microtome-cryostat and HPLC assay. RESULTS: The steady-state flux of all-trans-retinol from silica-coated lecithin emulsions was decreased (up to 90%) and was highly dependent on the initial loading phase of nanoparticles; incorporation from the aqueous phase provided more pronounced sustained release. For oleylamine emulsions, sustained release effect was not affected by initial location of nanoparticles. The skin retention significantly (p < or = 0.05) increased and was higher for positive oleylamine-stabilised droplets. All-trans-retinol was mainly localized in the epidermis with deeper distribution to viable skin layers in the presence of nanoparticles, yet negligible permeation (approximately 1% of topically applied dose) through full-thickness skin. CONCLUSIONS: Sustained release and targeted dermal delivery of all-trans-retinol from oil-in-water emulsions by inclusion of silica nanoparticles is demonstrated.

Nanoparticle layers controlling drug release from emulsions.

The influence of interfacial layers of silica nanoparticles on the release kinetics of a model lipophilic drug (di-butyl-phthalate (DBP)) from polydimethylsiloxane droplets in water is reported. The nanoparticle layers are formed by self-assembly from solution and their structure is controlled by nanoparticle hydrophobicity and the solution conditions. For DBP loading levels resulting in released concentrations below the solubility limit, release is rapid from uncoated droplets whereas significant sustained release is facilitated by rigid interfacial layers of hydrophobic silica nanoparticles. Activation energies for release are in the range 580-630kJmol(-1), which is ten times greater than for barriers introduced by typical polymeric stabilisers. In contrast, at higher DBP loading levels (total concentration greater than the solubility level), both hydrophilic and hydrophobic nanoparticle layers increase the rate and extent of dissolution compared with uncoated droplets and pure DBP solutions. Nanoparticle layers are shown to significantly influence the release kinetics of lipophilic drugs from oil in water emulsions: either sustained or enhanced release properties can be introduced depending on the nanoparticle layer type and drug loading level. Thus, nanoparticle layers may be engineered to facilitate a range of release behaviours and offer great potential in the delivery of poorly soluble drugs.

Nanoparticle encapsulation of emulsion droplets.

The overall aim of this study is to coat emulsion droplets with nanoparticles using a simple heterocoagulation process in aqueous dispersion and determine: the adsorption behavior and interfacial layer microstructure, droplet physical stability against flocculation and coalescence, and the release profile of a model lipophilic molecule (dibutylphtalate (DBP)) from within the droplets. Polydimethylsiloxane (PDMS) droplets were used as a model emulsion due to their colloidal stability in the absence of added stabilisers. Aerosil type silica nanoparticles with different hydrophobicity levels were used as the model nanoparticles. The adsorption behavior of silica nanoparticles at the droplet-water interface was studied using adsorption isotherms and SEM imaging. Adsorption of hydrophilic nanoparticles is weakly influenced by pH, but significantly influenced by salt addition, whereas for hydrophobically modified nanoparticles a balance of hydrophobic and electrostatic forces controls adsorption over a wide range of pH and salt concentrations. The coalescence kinetics (determined under coagulation conditions at high salt concentration) and the physical structure of coalesced droplets were determined from optical microscopy. Adsorbed layers of hydrophilic nanoparticles introduced a barrier to coalescence of approximately 1kT and form kinetically unstable droplet networks at high salt concentrations. The highly structured and rigid adsorbed layers significantly reduce coalescence kinetics. Significant sustained release of DBP can be achieved using rigid layers of hydrophobic silica nanoparticles at the interface. Activation energies for release are in the range 580-630 kJ mol(-1), 10 times higher than for barriers introduced by Pluronic stabilisers.

Polymer and particle adsorption at the PDMS droplet-water interface.

Polymer and particle adsorption at the polydimethylsiloxane (PDMS) droplet-water interface has been investigated. Adsorption isotherms and adsorbed layer structure are reported for a range of PEO-PPO-PEO block copolymers, and hydrophilic and hydrophobic silica "nanoparticles". The influence of solution conditions on the adsorption behaviour has indicated the thermodynamics of polymer-droplet and particle-droplet interactions. The influence of droplet cross-linking (deformability) has indicated the role of interfacial penetration in controlling adsorption at the droplet-water interface. The plateau adsorbed amount (Gamma(max)) and adsorbed layer thickness (delta(max)) of PEO-PPO-PEO copolymers are dependent on the copolymer structure and the level of cross-linking within droplets. For a wide range of copolymer structures, Gamma(max) values are in the range 2 to 20 mg m(-2). For delta(max), values range from 2 to 20 nm and are directly proportional to the PEO block length. Droplet cross-linking significantly reduces Gamma and delta values; this is considered to be due to the influence of interfacial penetrability on the adsorbed copolymer conformation. Hydrophilic silica particles adsorb onto PDMS droplets with plateau surface coverages that correspond to their hard sphere radius+double layer thickness, i.e. lateral silica-silica interactions control particle packing. Free energies of adsorption (DeltaG(ads)) are concurrent with a physical adsorption mechanism. Surface coverages, DeltaG(ads) and particle packing at the interface are only weakly influenced by pH, but are significantly influenced by salt addition. Droplet cross-linking reduced particle adsorption only at higher salt concentrations; this was attributed to the increased likelihood of silica particles wetting PDMS. Freeze fracture SEM revealed that individual silica particles are adsorbed at the droplet interface with negligible interfacial aggregation. Densely packed adsorbed particle layers are only observed when the double layer thickness is a few nanometers. Adsorption of hydrophobic particles at the PDMS droplet-water interface is more pronounced (greater adsorbed amounts and DeltaG(ads) values) than for hydrophilic particles and displays a pH dependency in line with 'DLVO behaviour'. The surface coverage values correspond to multiple close packed layers and are significantly influenced by droplet cross-linking, conferring extensive interfacial penetration (confirmed by SEM). Densely packed adsorbed particle layers with interfacial aggregation are observed over a wide range of solution conditions. Interfacial particle saturation occurred at a salt concentration two orders of magnitude less than the critical coagulation concentration (ccc) for silica in water. This phenomenon was observed for both liquid and cross-linked PDMS droplets indicating that particle interaction through the water phase plays a decisive role in particle packing at the interface. SEM indicated the presence of a rigid interfacial crust layer at the salt concentration corresponding to interfacial saturation and a multi-layered interfacial particle wall at salt concentrations >/= ccc. The PDMS droplets under consideration, having inherent colloid stability in the absence of added stabilisers, are an excellent model system for characterising polymer and particle adsorption at the droplet-water interface. The insight gained concerning adsorption thermodynamics at the droplet-water interface is not available from more conventional emulsions.

Solid self-emulsifying phospholipid suspension (SSEPS) with diatom as a drug carrier.

We report the application of diatom as a solid carrier for water insoluble drugs applied in oral drug delivery system based on the self-emulsifying drug delivery system (SEDDS) caprylocaproyl macrogol-8 glycerides/lecithin/propylene glycol/caprylic/capric triglyceride. Diatoms are fossilized skeletons of photosynthetic algae with complex 3-dimensional (3D), porous structure consisting of amorphous silica, obtained by purification of diatomaceous earth. Different solid samples of carbamazepine (CBZ) suspension in SEDDS, called solid self-emulsifying phospholipid suspension (SSEPS), were prepared using two methods: adsorption of CBZ dispersion in SEDDS by gentle mixing with diatoms in mortar with pestle (Method A) or dispersion of diatoms in ethanol solution of CBZ and SEDDS components, followed by ethanol evaporation (Method B). Release rate of CBZ from SSEPS was significantly higher in comparison to pure drug, physical mixture of diatoms and CBZ as well as solid dispersion of pure CBZ and diatoms obtained by ethanol evaporation. The dissolution of CBZ from SSEPS sample prepared using method B was faster than from the sample prepared by the method A. Higher dissolution for sample prepared by the method B can be attributed to the partial adsorption (deeper localization) of liquid material inside the pores of diatoms. Upon storage of the samples under accelerated conditions (40°C and 70% RH) for 10 weeks no significant changes in CBZ crystallinity and dissolution was in case of SSEPS, contrary to solid dispersion with increased crystallinity, indicating that diatoms with adsorbed liquid CBZ-loaded SEPS can maintain initial CBZ characteristics.

Assembling nanoparticle coatings to improve the drug delivery performance of lipid based colloids.

Lipid based colloids (e.g. emulsions and liposomes) are widely used as drug delivery systems, but often suffer from physical instabilities and non-ideal drug encapsulation and delivery performance. We review the application of engineered nanoparticle layers at the interface of lipid colloids to improve their performance as drug delivery systems. In addition we focus on the creation of novel hybrid nanomaterials from nanoparticle-lipid colloid assemblies and their drug delivery applications. Specifically, nanoparticle layers can be engineered to enhance the physical stability of submicron lipid emulsions and liposomes, satbilise encapsulated active ingredients against chemical degradation, control molecular transport and improve the dermal and oral delivery characteristics, i.e. increase absorption, bioavailability and facilitate targeted delivery. It is feasible that hybrid nanomaterials composed of nanoparticles and colloidal lipids are effective encapsulation and delivery systems for both poorly soluble drugs and biological drugs and may form the basis for the next generation of medicines. Additional pre-clinical research including specific animal model studies are required to advance the peptide/protein delivery systems, whereas the silica lipid hybrid systems have now entered human clinical trials for poorly soluble drugs.

Solid-state nanoparticle coated emulsions for encapsulation and improving the chemical stability of all-trans-retinol.

Submicron oil-in-water (o/w) emulsions stabilised with conventional surfactants and silica nanoparticles were prepared and freeze-dried to obtain free-flowing powders with good redispersibility and a three-dimensional porous matrix structure. Solid-state emulsions were characterised for visual appearance, particle size distribution, zeta potential and reconstitution properties after freeze-drying with various sugars and at a range of sugar to oil ratios. Comparative degradation kinetics of all-trans-retinol from freeze-dried and liquid emulsions was investigated as a function of storage temperatures. Optimum stability was observed for silica-coated oleylamine emulsions at 4 °C in their wet state. The half-life of all-trans-retinol was 25.66 and 22.08 weeks for silica incorporation from the oil and water phases respectively. This was ∼4 times higher compared to the equivalent solid-state emulsions with drug half-life of 6.18 and 6.06 weeks at 4 °C. Exceptionally, at a storage temperature of 40 °C, the chemical stability of the drug was 3 times higher in the solid-state compared to the wet emulsions which confirmed that freeze-drying is a promising approach to improve the chemical stability of water-labile compounds provided that the storage conditions are optimised.

Poly(lactic-co-glycolic acid) as a particulate emulsifier.

The structure and stability of emulsions formed in the presence of nanoparticles of poly(lactic-co-glycolic acid) (PLGA) were characterised. From oil-water contact angles on PLGA films, it was deduced that particle surface hydrophobicity is linked to the oil phase polarity. Incorporation of polyvinyl alcohol molecules into the nanoparticle surfaces reduces the particle hydrophobicity sufficiently for oil-in-water emulsions to be preferentially stabilised. PLGA nanoparticles enhance the stability of emulsions formed from a wide range of oils of different polarities. The nanoparticle concentration was found to be a key parameter controlling the average size and coalescence stability of the emulsion drops. Visualisation of the interfacial structure by electron microscopy indicated that PLGA nanoparticles were located at the drop surfaces, evidence of the capacity of these particles to stabilise Pickering-type emulsions. These results provide insights into the mechanism of PLGA nanoparticle stabilisation of emulsions.

Interactions of hydrophilic silica nanoparticles and classical surfactants at non-polar oil-water interface.

The interfacial and bulk properties of submicron oil-in-water emulsions simultaneously stabilised with a conventional surfactant (either lecithin or oleylamine) and hydrophilic silica nanoparticles (Aerosil®380) were investigated and compared with emulsions stabilised by either stabiliser. Emulsions solely stabilised with lecithin or oleylamine showed poor physical stability, i.e., sedimentation and the release of pure oil was observed within 3 months storage. The formation and long-term stability of silica nanoparticle-coated emulsions was investigated as a function of the surfactant type, charge, and concentration; the oil phase polarity (Miglyol®812 versus liquid paraffin); and loading phase of nanoparticles, either oil or water. Highly stable emulsions with long-term resistance to coalescence and creaming were formulated even at low lecithin concentrations in the presence of optimum levels of silica nanoparticles. The attachment energy of silica nanoparticles at the non-polar oil-water interface in the presence of lecithin was significantly higher compared to oleylamine in line with good long-term stability of the former compared to the sedimentation and release of oil in the latter. The attachment energy of silica nanoparticles at the polar oil-water interface especially in the presence of oleylamine was up to five-times higher compared to the non-polar liquid paraffin. The interfacial layer structure of nanoparticles (close-packed layer of particle aggregates or scattered particle flocs) directly related to the free energy of nanoparticle adsorption at both MCT oil and liquid paraffin-water interfaces.

Silica-lipid hybrid microcapsules: influence of lipid and emulsifier type on in vitro performance.

This study reports on the physicochemical characterisation and in vitro investigations of macro-porous silica-lipid hybrid (SLH) microcapsules when formulated using various lipids: long-chain triglycerides (LCT), medium-chain triglycerides (MCT), medium-chain mono-, diglycerides (MCMDG); and emulsifiers: anionic lecithin and cationic oleylamine. For the lipophilic compound coumarin 102 (logP=4.09), a complete and immediate in vitro release was attained for the SLH microcapsules under simulated intestinal sink conditions. The in vitro digestion study of various types of SLH microcapsules demonstrates: (i) reduced variability and enhanced lipid digestibility for the MCMDG-based microcapsules (i.e. 90-100% lipolysis) in comparison with an equivalent lipid solution and emulsion (50-90% lipolysis); and (ii) more controllable digestion kinetics for the LCT-based microcapsules which produce a lipolysis rate higher than that of a lipid solution but lower than that of a lipid emulsion. The drug phase partition results show approximately 5- to 17-fold increase in the drug solubilisation degree resulting from the digestion of MCT and MCMDG-based microcapsules (116 µg/mL), and LCT-based microcapsules (416 µg/mL) in comparison with the blank micellar medium (24 µg/mL). In conclusion, the SLH microcapsules could be tailored to manipulate the digestion patterns of both medium- and long-chain lipids in order to maximise the drug solubilisation capacity.

Silica materials in drug delivery applications.

In this review article we collect and analyse preparation, chemistry and properties of silica materials relevant for drug delivery applications. We review some of the most relevant milestones in the research of silica materials for implantable, oral, intravenous and dermal drug delivery systems. Preparation, chemistry and drug delivery characteristics of fumed silica nanoparticles (oral and dermal delivery route), silica xerogels (implant delivery), mesoporous silica materials (implant and oral delivery) and mesoporous silica spheres (intravenous delivery) with particular emphasis on their role in anticancer therapy and the design of stimuli responsive drug delivery systems are analysed. Recent progress in the research of silica materials for controlled drug delivery, namely, biocompatibility aspects, research on hybrid materials, anticancer and stimuli-responsive mesoporous silica materials are particularly emphasized.

Silica microcapsules from diatoms as new carrier for delivery of therapeutics.

AIM: This study explores the use of natural silica-based porous material from diatoms, known as diatomaceous earth, as a drug carrier of therapeutics for implant- and oral-delivery applications. MATERIALS & METHODS: To prove this concept, two drugs models were used and investigated: a hydrophobic (indomethacin) and hydrophilic (gentamicin). RESULTS & DISCUSSION: Results show the effectiveness of diatom microcapsules for drug-delivery application, showing 14-22 wt% drug loading capacity and sustained drug release over 2 weeks. Two steps in the drug release from diatom structures were observed: the first, rapid release (over 6 h is attributed to the surface deposited drug) and the second, slow and sustained release over 2 weeks with zero order kinetics. CONCLUSION: These results confirm that natural material based on diatom silica can be successfully applied as a drug carrier for both oral and implant drug-delivery applications, offering considerable potential to replace existing synthetic nanomaterials.

Controlled drug release from porous materials by plasma polymer deposition.

In this communication, we present a novel approach for control of drug release from porous materials. The method is based on deposition of a plasma polymer layer with controlled thickness which reduces a pore diameter and, hence, defines the rate of drug release.

Mechanistic insight into the dermal delivery from nanoparticle-coated submicron O/W emulsions.

The influence of silica nanoparticle coating of negatively and positively charged submicron emulsion oil droplets on the dermal delivery of a lipophilic fluorescent probe, acridine orange 10-nonyl bromide (AONB) using an ex vivo porcine skin model is reported. The skin retention and depth of the penetration of AONB significantly increased (p

Surface functionalisation of diatoms with dopamine modified iron-oxide nanoparticles: toward magnetically guided drug microcarriers with biologically derived morphologies.

Diatom silica microcapsules prepared by purification of diatomaceous earth (DE) were functionalised by dopamine modified iron-oxide nanoparticles, in order to introduce diatoms with magnetic properties. The application of magnetised diatoms as magnetically guided drug delivery microcarriers has been demonstrated.

Platforms for controlled release of antibacterial agents facilitated by plasma polymerization.

Bacterial infections present an enormous problem causing human suffering and cost burdens to the healthcare systems worldwide. Herein we present several versatile strategies for controlled release of antibacterial agents which include silver ions as well as traditional antibiotics. At the heart of these release platforms is a thin film deposited by plasma polymerization. The use of plasma polymerization makes these strategies applicable to the surface of many types of medical devices since the technique for deposition of a polymer film from plasma in practically substrate independent.

An oral delivery system for indomethicin engineered from cationic lipid emulsions and silica nanoparticles.

We report on a porous silica-lipid hybrid microcapsule (SLH) oral delivery system for indomethacin fabricated from Pickering emulsion templates, where the drug forms an electrostatic complex with cationic lipid present in the oil phase. Dry SLH microcapsules prepared either by spray drying (approximately 1-5 microm) or phase coacervation (20-50 microm) exhibit a specific internal porous matrix structure with pore diameters in the range of 20 to 100 nm. Dissolution studies under sink conditions and in the presence of electrolytes revealed a decreased extent of dissolution; this confirms the lipophilic nature the drug-lipid complex and its location in the oil phase. Orally dosed in-vivo studies in rats showed complete drug absorption and statistically higher fasted state bioavailability (F) (p<0.05) in comparison to aqueous suspensions and o/w submicron emulsions of indomethacin. It is postulated that the SLH microcapsules improve oral absorption via complete solubilisation of drug-lipid electrostatic complexes during enzymatic lipolysis in the GI track.