Biomedical subjects

Daniel W Pack

Publications and source records attributed to Daniel W Pack.

At least 19 recordsLinked to original sources

PubMed2006

Use of thermodynamic parameters for design of double-walled microsphere fabrication methods.

Double-walled microspheres (DWMS), with drug localized to the particle core, present a promising route for control of drug release rates, for example, by varying the degradation rate or erosion mechanism of the polymer used to form the shell or the thickness of the shell. DWMS are often difficult to fabricate, however. Thermodynamic descriptions for polymer-polymer immiscibility, drug distribution between phases and polymer-solution spreading coefficient provide predictions of appropriate solvents and polymer concentrations for efficiently producing well-formed DWMS. As an example, thermodynamic parameters for a polyphosphoester/poly(D,L-lactide-co-glycolide) (PLG) DWMS system, encapsulating piroxicam, have been calculated and the predictions tested experimentally. Appropriate choices of solvents and initial polymer concentrations resulted in DWMS with the desired polyphosphoester shells and piroxicam located selectively in PLG cores.

Chromatography, Gel↗

PubMed2006

Acetylation of polyethylenimine enhances gene delivery via weakened polymer/DNA interactions.

We previously reported that gene delivery efficiency of 25-kDa, branched polyethylenimine (PEI) increased upon acetylation of up to 43% of the primary amines with acetic anhydride. In the present work, we investigated the effects of further increasing the degree of acetylation and elucidated the source of the higher gene delivery efficiency. Despite reduced buffering capacity, gene delivery activity continued to increase (up to 58-fold in HEK293) with acetylation of up to 57% of primary amines but decreased at higher degrees of acetylation. Characterization of polymer-DNA interactions showed that acetylated polymers bind less strongly to DNA. Further, a fluorescence resonance energy transfer assay showed that increasing acetylation causes polyplexes to unpackage inside cells to a higher degree than polyplexes formed with unmodified PEI. Overall, the data suggest that the increased gene delivery activity may be attributable to an appropriate balance between polymer buffering capacity and strength of polymer/DNA interactions.

Acetylation↗

PubMed2005

In vitro degradation of polyanhydride/polyester core-shell double-wall microspheres.

Double-wall microspheres (DWMS), comprising distinct polymer core and shell phases, are useful and interesting for controlled-release drug delivery. In particular, the presence of a surface-eroding polymer core may be expected to limit water penetration and, therefore, delay degradation of the core phase and drug release. In this study, solid microspheres and DWMS were fabricated using a surface-eroding polymer (poly[1,6-bis(p-carboxyphenoxy)hexane]; PCPH) and a bulk-eroding polymer (poly(D,L-lactide-co-glycolide); PLG). Erosion of the particles was observed by optical and electron microscopy, while polymer degradation was followed by gel permeation chromatography, during incubation in buffer at 37 degrees C. Degradation and erosion were very different depending on which polymer formed the particle shell. Nevertheless, the relatively thin (approximately 5 microm) PCPH shells could not prevent water penetration, and the PLG cores completely eroded by 6 weeks of incubation.

Chromatography, Gel↗

PubMed2005

Modeling small-molecule release from PLG microspheres: effects of polymer degradation and nonuniform drug distribution.

Modeling release of small molecules from degradable microspheres is important to the design of controlled-release drug delivery systems. Release of small molecules from poly(d,l-lactide-co-glycolide) (PLG) particles is often controlled by diffusion of the drug through the polymer and by polymer degradation. In this study, a model is developed to independently determine the contributions of each of these factors by fitting the release of piroxicam from monodisperse 50-microm microspheres made with PLG of different initial molecular weights. The dependence of the drug diffusivity on polymer molecular weight was determined from in vitro release of piroxicam from monodisperse 10-microm PLG microspheres, and the polymer degradation rate was experimentally measured using gel permeation chromatography. The model also incorporates the effect of nonuniform drug distribution within the microspheres, which is obtained from confocal fluorescence microscopy. The model results agree well with experiments despite using only one fit parameter.

Lactic Acid↗

PubMed2005

Cyclodextrin-polyethylenimine conjugates for targeted in vitro gene delivery.

Many human gene therapies will require cell-specific targeting. Though recombinant viruses are much more efficient than nonviral vectors, the latter, especially polymers, have the advantage of being targetable via conjugation of cell-specific ligands, including sugars, peptides, and antibodies, which can be covalently attached to the polymer using a variety of chemistries. Cyclodextrin, which forms inclusion complexes with small hydrophobic molecules, has been incorporated into a gene-delivery polymer and may provide a facile and versatile attachment site for targeting ligands. Polyethylenimine (PEI) was derivatized with beta-cyclodextrin on approximately 10% of the polymer's amines (termed CD-PEI). Human insulin was also derivatized with a hydrophobic palmitate group (pal-HI), which could anchor the protein to CD-PEI/DNA polyplexes. CD-PEI was essentially nontoxic to HEK293 cells at concentrations optimal for gene delivery and mediated nearly 4-fold higher gene expression than unmodified PEI, which is relatively toxic to these cells. More importantly, addition of the pal-HI to CD-PEI enhanced gene expression by more than an order of magnitude compared to unmodified PEI, either with or without the pal-HI. Because of the relative ease with which CD-binding moieties may be attached to various types of ligands, CD-PEI may be a generally useful material for testing novel cell-specific targeting compounds.

Cell Line↗

PubMed2005

Small-molecule release from poly(D,L-lactide)/poly(D,L-lactide-co-glycolide) composite microparticles.

Addition of biodegradable polymer shells surrounding polymeric, drug-loaded microparticles offers the opportunity to control drug release rates. A novel fabrication method was used to produce microparticles with precise control of particle diameter and the thickness of the polymer shell. The effect of shell thickness on release of a model drug, piroxicam, has been clearly shown for 2- to 15-microm thick shells of poly(D,L-lactide) (PDLL) surrounding a poly(D,L-lactide-co-glycolide) (PLG) core and compared to pure PLG microspheres loaded with piroxicam. Furthermore, the core-shell microparticles are compared to microspheres containing blended polymers in the same mass ratios to demonstrate the importance of the core-shell morphology. Combining PDLL(PLG) microcapsules of different shell thicknesses allows nearly constant release rates to be attained for a period of 6 weeks.

Chemistry, Pharmaceutical↗

PubMed2005

Design and development of polymers for gene delivery.

The lack of safe and efficient gene-delivery methods is a limiting obstacle to human gene therapy. Synthetic gene-delivery agents, although safer than recombinant viruses, generally do not possess the required efficacy. In recent years, a variety of effective polymers have been designed specifically for gene delivery, and much has been learned about their structure-function relationships. With the growing understanding of polymer gene-delivery mechanisms and continued efforts of creative polymer chemists, it is likely that polymer-based gene-delivery systems will become an important tool for human gene therapy.

Animals↗

PubMed2004

Three-month, zero-order piroxicam release from monodispersed double-walled microspheres of controlled shell thickness.

Double-walled microspheres represent an increasingly important class of drug delivery devices that provide enhanced control of drug delivery schedules. Clearly, the overall particle size and shell thickness are important parameters in modulating the drug release rates. Precision particle fabrication technology has been used to fabricate double-walled microspheres of predefined uniform diameters of 40-60 microm exhibiting a poly(D,L-lactide-co-glycolide) (PLG) core and poly(L-lactide) (PL) shell of controllable thickness from approximately 2 to 10 microm. The release of a model small-molecule drug, piroxicam, from uniform microspheres of pure PLG and PL is compared to the release from double-walled microspheres exhibiting different PL shell thicknesses. The presence of the PL shell enveloping a PLG core essentially eliminated the initial "burst" of piroxicam that was observed when the drug was released from pure PLG microspheres. In addition, increasing the PL shell thickness shifted the release profile from a biphasic shape for pure PLG microspheres to zero-order piroxicam release over 3 months for the thickest (approximately 10 microm) PL shell.

Drug Delivery Systems↗

PubMed2004

Uniform double-walled polymer microspheres of controllable shell thickness.

A method for fabricating uniform double-walled microspheres with controllable size and shell thickness has been developed. The method, based on previous work to fabricate uniform microspheres, employs multiple concentric nozzles to produce a smooth coaxial jet comprising an annular shell and core material, which is acoustically excited to break up into uniform core-shell droplets. The orientation of the jets, material flow rates, and rate of solvent extraction are controlled to create uniform and well-centered "double-walled" microspheres exhibiting a controllable shell thickness. Double-walled microspheres were fabricated with different arrangements of bulk-eroding poly(D,L-lactide-co-glycolide) (PLG) and surface-eroding poly[(1,6-bis-carboxyphenoxy) hexane] (PCPH). Variation of the fabrication parameters allowed complete encapsulation by the shell phase, including the efficient formation of a PCPH shell encapsulating a PLG core. Utilizing this technology, double-walled microsphere shell thickness can be varied from <2 microm to tens of microns while maintaining complete and well-centered core encapsulation for double-walled microspheres near 50 microm in overall diameter.

Microspheres↗

PubMed2004

Microsphere size, precipitation kinetics and drug distribution control drug release from biodegradable polyanhydride microspheres.

A thorough understanding of the factors affecting drug release mechanisms from surface-erodible polymer devices is critical to the design of optimal delivery systems. Poly(sebacic anhydride) (PSA) microspheres were loaded with three model drug compounds (rhodamine B, p-nitroaniline and piroxicam) with a range of polarities (water solubilities). The drug release profiles from monodisperse particles of three different sizes were compared to release from polydisperse microspheres. Each of the model drugs exhibited different release mechanisms. Drug distribution within the polymer was investigated by laser scanning confocal microscopy and scanning electron microscopy. Rhodamine, the most hydrophilic compound investigated, was localized strongly toward the microsphere surface, while the much more hydrophobic compound, piroxicam, distributed more evenly. Furthermore, all three compounds were most uniformly distributed in the smallest microspheres, most likely due to the competing effects of drug diffusion out of the nascent polymer droplets and the precipitation of polymer upon solvent extraction, which effectively "traps" the drug in the polymer matrix. The differing drug distributions were manifested in the drug release profiles. Rhodamine was released very quickly independent of microsphere size. Thus, extended release profiles may not be obtainable if the drug strongly redistributes in the microspheres. The release of p-nitroaniline was more prolonged, but still showed little dependence on microsphere size. Hence, when water-soluble drugs are encapsulated with hydrophobic polymers, it may be difficult to tailor release profiles by controlling microsphere size. The piroxicam-loaded microspheres exhibit the most interesting release profiles, showing that release duration can be increased by decreasing microsphere size, resulting in a more uniform drug distribution.

Anhydrides↗

PubMed2004

Controlling surface nano-structure using flow-limited field-injection electrostatic spraying (FFESS) of poly(D,L-lactide-co-glycolide).

Improved control of surface micro- and nano-structure may lead to enhanced performance of degradable biomedical devices such as surgical dressings, vascular grafts, tissue engineering scaffolds, sutures, and structures for guided tissue regeneration. An electrohydrodynamic method called flow-limited field-injection electrostatic spraying (FFESS) has been developed as an improved technique for the controlled deposition of polymeric material. Injecting charge using a nano-sharpened tungsten needle in a process called field ionization can efficiently induce an ionic state in a solution of poly(D,L-lactide-co-glycolide) increasing its capacity to carry charge. As a result, sprays have been produced that are finer and more precisely controlled than sprays produced by conventional electrospraying techniques, which employ hypodermic needles as the spray nozzle. Here, the effect of FFESS variables including applied voltage, polymer solution flow rate, and solvent properties (surface tension, viscosity, vapor pressure) on spray performance have been qualitatively evaluated. Under certain conditions, increasing the applied voltage produced an increasingly rough surface morphology. Similarly, by reducing solvent surface tension and increasing solvent vapor pressure, more distinct surface structures could be formed including uniform nanoparticles. Working ranges of the important parameters for the production of specific structure types such as smooth or porous surfaces, non-woven or melded fibers, and distinct or melded nanoparticles have been defined. FFESS technology provides a simple yet powerful technique for fabricating biomedical devices with a precisely defined nano-structure potentially capable of utilizing a broad range of biocompatible polymeric materials.

Biocompatible Materials↗

PubMed2004

Partial acetylation of polyethylenimine enhances in vitro gene delivery.

PURPOSE: Polyethylenimine (PEI) is a highly effective gene delivery vector, but because it is an off-the shelf material, its properties may not be optimal. To investigate the effects of the protonation properties of the polymer, we generated PEI derivatives by acetylating varying fractions of the primary and secondary amines to form secondary and tertiary amides, respectively. METHODS: Reaction of PEI with increasing amounts of acetic anhydride at 60 degrees C for 4.5 h yielded polymers with 15%, 27%, and 43% of the primary amines modified with acetyl groups. Polymer-DNA complexes were characterized by dynamic light scattering and zeta potential measurements. Cytotoxicity of the polymers was assessed by XTT assay for metabolic activity, and gene delivery efficiency was determined as the relative expression of a luciferase gene in MDA-MB-231 and C2C12 cell lines. RESULTS: Acetylation of PEI decreased the "physiological buffering capacity," defined as the moles of protons absorbed per mole of nitrogen on titration from pH 7.5 to 4.5, from 0.29 mol H+/mol N to 0.17 mol H+/mot N, 0.12 mol H+/mol N, and 0.090 mol H+/mol N for PEI-Ac15, PEI-Ac27, and PEI-Ac43, respectively. In addition, acetylation decreased the zeta potential of polyplexes from 14 mV to 8-11 mV and increased the polyplex diameter by two- to threefold. Surprisingly, acetylation had a negligible effect on cytotoxicity of the polymers and increased gene delivery effectiveness by up to 21-fold compared to unmodified PEI, both in the presence and absence of serum. CONCLUSIONS: Reduction of the buffering capacity of PEI greatly enhanced the gene delivery activity of the polymer. The mechanism is not yet understood, but the enhancement may be caused by more effective polyplex unpackaging, altered endocytic trafficking, and/or increased lipophilicity of acetylated PEI-DNA complexes. Future studies will address these possibilities in more detail.

Acetic Anhydrides↗

PubMed2004

Timing is everything.

Explore the source record for details and available documents.

Adjuvants, Immunologic↗

PubMed2004

Microspheres for controlled release drug delivery.

Controlled release drug delivery employs drug-encapsulating devices from which therapeutic agents may be released at controlled rates for long periods of time, ranging from days to months. Such systems offer numerous advantages over traditional methods of drug delivery, including tailoring of drug release rates, protection of fragile drugs and increased patient comfort and compliance. Polymeric microspheres are ideal vehicles for many controlled delivery applications due to their ability to encapsulate a variety of drugs, biocompatibility, high bioavailability and sustained drug release characteristics. Research discussed in this review is focused on improving large-scale manufacturing, maintaining drug stability and enhancing control of drug release rates. This paper describes methods of microparticle fabrication and the major factors controlling the release rates of encapsulated drugs. Furthermore, recent advances in the use of polymer microsphere-based systems for delivery of single-shot vaccines, plasmid DNA and therapeutic proteins are discussed, as well as some future directions of microsphere research.

Animals↗

PubMed2003

PLG microsphere size controls drug release rate through several competing factors.

PURPOSE: Although the rate of drug release from poly(D,L-lactide-co-glycolide) (PLG) microspheres is often modulated by changing fabrication conditions or materials, the specific factors directly controlling the release profiles are often unclear. We have fabricated uniform rhodamine- and piroxicam-containing microspheres, 10 to 100 microm in diameter, to better understand how microsphere size controls drug release. METHODS: Drug distribution within the microspheres was examined using confocal fluorescence microscopy. The rate of polymer degradation was determined as the change in molecular weight, measured by gel permeation chromatography, during in vitro degradation experiments. Further, changes in the surface and interior morphology of the particles during in vitro degradation were investigated by scanning electron microscopy. RESULTS: Microsphere size greatly affected drug distribution. Small (approximately 10-microm) microspheres showed an essentially uniform drug distribution. Larger (approximately 100-microm) microspheres showed redistribution of drug to specific regions of the microspheres. Rhodamine partitioned to the surface and piroxicam partitioned to the interior of large PLG microspheres. Further, the rate of polymer degradation increased with microsphere size, possibly the result of a more acidic interior caused by increased accumulation of hydrolyzed polymer products in larger particles. Finally, larger microspheres developed a more porous interior structure during the drug release. CONCLUSION: Microsphere size affects drug release not only through changes in diffusion rates but also through secondary effects including drug distribution in the particle, polymer degradation rate, and microsphere erosion rates.

Lactic Acid↗

PubMed2002

Precise control of PLG microsphere size provides enhanced control of drug release rate.

An important limitation in the development of biodegradable polymer microspheres for controlled-release drug delivery applications has been the difficulty of specifically designing systems exhibiting precisely controlled release rates. Because microparticle size is a primary determinant of drug release, we developed a methodology for controlling release kinetics employing monodisperse poly(D,L-lactide-co-glycolide) (PLG) microspheres. We fabricated 20-, 40- and 65-microm diameter rhodamine-containing microspheres and 10-, 50- and 100-microm diameter piroxicam-containing microspheres at various loadings from 1 to 20%. In vitro release kinetics were determined for each preparation. Drug release depended strongly on microsphere diameter with 10- and 20-microm particles exhibiting concave-downward release profiles while larger particles resulted in sigmoidal release profiles. Overall, the rate of release decreased and the duration increased with increasing microsphere size. Release kinetics from mixtures of uniform microspheres corresponded to mass-weighted averages of the individual microsphere release kinetics. Appropriate mixtures of uniform microspheres were identified that provided constant (zero-order) release of rhodamine and piroxicam for 8 and 14 days, respectively. Mixing of uniform microspheres, as well as control of microsphere size distribution, may provide an improved methodology to tailor small-molecule drug-release kinetics from simple, biodegradable-polymer microparticles.

Biocompatible Materials↗

PubMed2002

On the kinetics of polyplex endocytic trafficking: implications for gene delivery vector design.

Synthetic gene therapy vectors must be designed to safely and efficiently escort DNA from outside the cell to the nucleus and to overcome several physical barriers that are obstacles to internalization, escape from endocytic vesicles, movement through the cytoplasm, and transport into the nucleus. By providing a firm foundation for polymer design, a quantitative understanding of polymer-mediated gene delivery mechanisms may allow more efficient and timely design of new vectors. We have used a flow-cytometry-based assay for investigating endocytic trafficking by quantitation of polyplex pH microenvironments. We investigated polyethylenimine (PEI)- and poly-L-lysine (PLL)-DNA trafficking, with and without the endosomotropic agent chloroquine. PLL-DNA complexes appear to be localized to early endocytic vesicles and are not trafficked to lysosomes. Further, chloroquine appears to facilitate PLL-mediated gene delivery by a mechanism other than buffering of endocytic compartments. Additionally, PEI does not appear to buffer endocytic compartments, but requires exposure to an acidic environment for efficient gene delivery.

Fluorescent Dyes↗

PubMedDate not supplied

A degradable polyethylenimine derivative with low toxicity for highly efficient gene delivery.

Routine clinical implementation of human gene therapy awaits safe and efficient gene delivery methods. Polymeric vectors hold promise due to the availability of diverse chemistries, potentially providing targeting, low immunogenicity, nontoxicity, and robustness, but lack sufficient gene delivery efficiency. We have synthesized a versatile group of degradable polycations, through addition of 800-Da polyethylenimine (PEI) to small diacrylate cross-linkers. The degradable polymers reported here are similar in structure, size (14-30 kDa), and DNA-binding properties to commercially available 25-kDa PEI, but mediate gene expression two- to 16-fold more efficiently and are essentially nontoxic. These easily synthesized polymers are some of the most efficient polymeric vectors reported to date and provide a versatile platform for investigation of the effects of polymer structure and degradation rate on gene delivery efficiency.

Cell Line, Tumor↗