Timothy J. Barnes

Oxidized mesoporous silicon microparticles for improved oral delivery of poorly soluble drugs.

Surface functionalized mesoporous silicon (pSi) microparticles are reported as a solid dispersion carrier for improving dissolution and enhancing the orally administered pharmacokinetics (fasted rat model) of indomethacin (IMC), employed as a model poorly soluble BCS type II drug. IMC was loaded via immersion/solvent evaporation onto the thermally oxidized pSi particles, which provide a stable hydrophilic matrix with a nanoporous structure. The solid state properties of IMC loaded pSi were characterized by Fourier transform infrared spectroscopy, X-ray powder diffraction, differential scanning calorimetry and thermogravimetric analysis. IMC molecules are encapsulated in a noncrystalline state due to geometric confinement in the nanopores; stability of the noncrystalline state has been demonstrated for several months under accelerated storage conditions. The pSi carrier facilitates accelerated immediate release of IMC and enhanced oral delivery performance in comparison with crystalline indomethacin and Indocid i.e. a 4-times reduction on T(max), a 200% increase on C(max) and a significant increase in bioavailability. The in vitro-in vivo correlation is discussed based on the noncompartment model and gives insight into the delivery mechanism for the pSi carrier.

Mesoporous silicon: a platform for the delivery of therapeutics

Nanostructuring materials can radically change their properties. Two interesting examples highlighted here are nanoscale porosity inducing biodegradability, and nanoscale confinement affecting the physical form of an entrapped drug. Mesoporous silicon is under increasing study for drug-delivery applications, and is the topic of this review. The authors focus on those properties of most relevance to this application, as well as those recent studies published on small molecule and peptide/protein delivery.

Silica nanoparticle coated liposomes: A new type of hybrid nanocapsule for proteins

A hybrid silica-liposome nanocapsule system containing insulin has been developed and the encapsulation, protection and release properties are evaluated. The formulation strategy is based on using insulin-loaded 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and cholesterol liposomes as a template for the deposition of inert silica nanoparticles. The influence of formulation and process variables on particle size, zeta potential and liposome entrapment of insulin is reported. The ability to protect against lipolytic degradation and sustain insulin release in vitro in simulated GI conditions is also reported. Depending on the concentration and charge ratio of liposomes and silica nanoparticles, nanoparticle coated liposomes with varied size and zeta potential were obtained with an insulin entrapment efficiency of 70%. The silica nanoparticle coating protected liposomes against degradation by digestive enzymes in vitro; the release rate of insulin from silica coated liposomes was reduced in comparison to uncoated liposomes. Thus the liposomal release kinetics and stability can be controlled by including a specifically engineered nanoparticle layer. Silica nanoparticle-liposomes hybrid nanocapsules show promise as a delivery vehicle for proteins and peptides.

Surface chemistry of porous silicon and implications for drug encapsulation and delivery applications

Porous silicon (pSi) has a number of unique properties that appoint it as a potential drug delivery vehicle; high loading capacity, controllable surface chemistry and structure, and controlled release properties. The native Si(y)SiH(x) terminated pSi surface is highly reactive and prone to spontaneous oxidation. Surface modification is used to stabilize the pSi surface but also to produce surfaces with desired drug delivery behavior, typically via oxidation, hydrosilylation or thermal carbonization. A number of advanced characterization techniques have been used to analyze pSi surface chemistry, including X-ray photoelectron spectroscopy and time of flight secondary ion mass spectrometry. Surface modification not only stabilizes the pSi surface but determines its charge, wettability and dissolution properties. Manipulation of these parameters can impact drug encapsulation by altering drug-pSi interactions. pSi has shown to be a successful vehicle for the delivery of poorly soluble drugs and protein therapeutics. Surface modification influences drug pore penetration, crystallinity, loading level and dissolution rate. Surface modification of pSi shows great potential for drug delivery applications by controlling pSi-drug interactions. Controlling these interactions allows specific drug release behaviors to be engineered to aid in the delivery of previously challenging therapeutics. Within this review, different pSi modification techniques will be outlined followed by a summary of how pSi surface modification has been used to improve drug encapsulation and delivery.

PAMAM Dendrimer Interactions with Supported Lipid Bilayers: A Kinetic and Mechanistic Investigation

The interaction kinetics of polyamidoamine (PAMAM) dendrimers with supported lipid bilayers of 1,2-sn-glycero-dimyristoylphosphocholine prepared by the vesicle deposition has been probed by optical waveguide lightmode spectroscopy and atomic force microscopy (AFM). In particular, the influence of PAMAM dendrimer generation (G2, G4, and G6) and concentration (1 to 100 nM) on the levels of adsorption and lipid bilayer removal have been determined as a function of time; hence interaction kinetics and mechanisms have been further elucidated. Dendrimer interaction kinetics with the lipid bilayer are concentration dependent in a complex manner, with net bilayer removal at 1 and 100 nM and net adsorption at 10 nM; these effects are irrespective of dendrimer generation. The pseudo first order rate constant for bilayer removal (at 1 and 100 nM) follows the order G6 > G4 > G2. In contrast, the pseudo first order rate constant for adsorption at 10 nM follows the order G2 > G4 > G6. AFM has confirmed expansion of lipid bilayer defects, hole formation, and adsorption to the bilayer or bilayer defects, and their concentration and generation dependence. These findings have implications when designing dendrimers for specific biopharmaceutical activities, e.g., as drugs, drug delivery vehicles, transfection agents, or antimicrobials.

Thermal Oxidation for Controlling Protein Interactions with Porous Silicon

Thermal oxidation of porous silicon (pSi) has been used to control interactions with three proteins; lysozyme, papain, and human serum albumin (HSA) enabling the influences of protein structure, molecular weight, and charge to be elucidated. Adsorption behavior was assessed via adsorption isotherms while the structures of adsorbed proteins were investigated using a bioactivity assay, FTIR, and zeta potential. Time-of-flight secondary ion mass spectrometry was used to examine protein pore penetration. High protein adsorption onto unoxidized pSi (240-610 microg/m(2)) was attributed to predominately hydrophobic interactions which resulted in structural changes of the adsorbed proteins and significant loss of bioactivity. Thermal oxidation at 400 and 800 degrees C significantly reduced protein adsorption (80-485 microg/m(2)) by reducing hydrophobicity. Oxidation of pSi modified the protein adsorption mechanisms to solely electrostatic attraction for positively charged proteins and structural rearrangement for negatively charged proteins. Adsorption via electrostatic attraction preserved protein bioactivity and zeta potential, thus inferring a retention of their native structure. In contrast, the negative charge and globular structure of HSA resulted in a loss of structure. We have demonstrated that thermal oxidation of pSi can be used to control protein interactions, adsorbed structure, and bioactivity.

PEGylation of Porous Silicon Using Click Chemistry

Porous silicon has received considerable interest in recent years in a range of biomedical applications, with its performance determined by surface chemistry. In this work, we investigate the PEGylation of porous silicon wafers using click chemistry. The porous silicon wafer surface chemistry was monitored at each stage of the reaction via photoacoustic Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, whereas sessile drop contact angle and model protein adsorption measurements were used to characterize the final PEGylated surface. This work highlights the simplicity of click-chemistry-based functionalization in tailoring the porous silicon surface chemistry and controlling protein-porous silicon interactions.

Mechanistic Insight into Cell Growth, Internalization, and Cytotoxicity of PAMAM Dendrimers

We report on the role of PAMAM dendrimer concentration and generation (G2, G4, G6) on cell growth and cytotoxicity in HEK293T and HeLa cell lines and make comparisons with dendrimer-induced leakage from liposomes to probe the mechanisms in action. Specifically, we observed a striking transition from cell growth enhancement to a reduction in cell viability at a critical PAMAM dendrimer concentration, that is, approximately 500 nM. Confocal microscopy studies show evidence of a transition from cell membrane adhesion to cell internalization and cell nucleus interaction at equivalent dendrimer concentrations. A dendrimer concentration window of 500-700 nM was identified for effective cell internalization without significant cytotoxicity. Though liposome leakage correlated with cytotoxicity, no quantitative agreement was observed, that is, cells are 100 times (based on surface coverage) more resistant to dendrimers than liposomes. These findings have significant implications in the design of effective drug/gene delivery vehicles based on dendrimers.

Detecting the Presence of Denatured Human Serum Albumin in an Adsorbed Protein Monolayer Using TOF−SIMS

We demonstrate the application of time-of-flight secondary ion mass spectrometry (TOF-SIMS) in conjunction with multivariate statistics to differentiate trace levels of denatured proteins in adsorbed monolayers; specifically, human serum albumin (HSA) on oxidized silicon substrates. Subtle differences in protein conformation due to thermal denaturation of HSA, unable to be determined by dynamic light scattering nor circular dichroism, were differentiated by TOF-SIMS. The fragmentation pattern is highly sensitive to protein conformation, allowing assessment of relative amounts of proteins in mixtures and quantifying amounts of denatured protein in a sample. Discussion is presented on ascribing orientation and conformational differences between samples based upon TOF-SIMS spectra. This has implications for detecting denatured protein in biotechnology and medical applications.

Adsorption of Nonlamellar Nanostructured Liquid-Crystalline Particles to Biorelevant Surfaces for Improved Delivery of Bioactive Compounds

The adsorption of nanostructured lyotropic liquid-crystal particles, cubosomes and hexosomes, at surfaces was investigated for potential use in surface-specific agrochemical delivery. Adsorption of phytantriol (PHYT) and glyceryl monooleate (GMO)-based cubosomes and hexosomes, stabilized using Pluronic F127, at tristearin-coated (model leaf surface) and uncoated zinc selenide surfaces was studied using attenuated total reflectance Fourier transform IR (ATR-FTIR) spectroscopy, by quantifying the IR absorbance due to the lipid components of the particles over time. The delivery of an encapsulated hydrophobic model herbicide [dichlorodiphenyldichloroethylene (DDE)] was also examined on the model and real leaf surfaces. The adsorption behavior of the particles by ATR-FTIR was dependent on the internal nanostructure and lipid composition, with PHYT cubosomes adsorbing more avidly at tristearin surfaces than GMO-based cubosomes or hexosomes. There was a direct correlation between DDE associated with the surfaces and the particle adsorption observed in the ATR-FTIR study, strongly implicating particle adsorption with the delivery efficiency. Differences between the mode of interaction of the Pluronic stabilizer with the different lipids and particle nanostructures were proposed to lead to differences in the particle adsorption behavior.