Quantification of insulin release from implantable polymer-based delivery systems and augmentation of therapeutic effect with simultaneous release of somatostatin

Materials for diabetes therapeutics

This review is focused on the materials and methods used to fabricate closed-loop systems for type 1 diabetes therapy. Herein, we give a brief overview of current methods used for patient care and discuss two types of possible treatments and the materials used for these therapies-(i) artificial pancreases, comprised of insulin producing cells embedded in a polymeric biomaterial, and (ii) totally synthetic pancreases formulated by integrating continuous glucose monitors with controlled insulin release through degradable polymers and glucose-responsive polymer systems. Both the artificial and the completely synthetic pancreas have two major design requirements: the device must be both biocompatible and be permeable to small molecules and proteins, such as insulin. Several polymers and fabrication methods of artificial pancreases are discussed: microencapsulation, conformal coatings, and planar sheets. We also review the two components of a completely synthetic pancreas. Several types of glucose sensing systems (including materials used for electrochemical, optical, and chemical sensing platforms) are discussed, in addition to various polymer-based release systems (including ethylene-vinyl acetate, polyanhydrides, and phenylboronic acid containing hydrogels).

Generation of Monodisperse Protein Nanoparticles by Electrospray Drying

The feasibility of producing relatively monodisperse and biologically active protein particles by electrospray drying is demonstrated. The process entails dissolving dry powder in an electrosprayable solution. The solution is then dispersed and, after solvent evaporation, dry residues can be collected on suitable deposition substrates. The process was demonstrated in the case of insulin. Particles were sized visually, using a scanning electron microscope (SEM), and aerodynamically, using an inertial impactor. When electrosprays of nearly saturated solutions were operated in the stable cone-jet mode, impactor data showed that the particle average aerodynamic diameter ranged from about 88 to 110 nm in diameter and the distributions were quasi-monodisperse with relative standard deviation estimated at approximately 10%. SEM observations for the same conditions showed average particle dimensions ranging from 98 to 117 nm, with predominantly doughnut shapes. Smaller particles can be generated by decreasing the insulin concentration and/or by spraying smaller liquid flow rates. The biological activity of the electrospray-processed insulin samples was confirmed by comparing binding properties on insulin receptors against a control sample. Although the maximum production rate for monodisperse insulin nanoparticles from a single cone-jet is low, at about 0.23 mg/hour, overall production can be increased by multiplexing the device with microfabrication techniques.

Remotely triggerable drug delivery systems

Triggerable drug delivery systems enable on-demand controlled release profiles that may enhance therapeutic effectiveness and reduce systemic toxicity. Recently, a number of new materials have been developed that exhibit sensitivity to visible light, near-infrared (NIR) light, ultrasound, or magnetic fields. This responsiveness can be triggered remotely to provide flexible control of dose magnitude and timing. Here we review triggerable materials that range in scale from nano to macro, and are activated by a range of stimuli.

Optimal control of blood glucose: the diabetic patient or the machine?

In this issue of Science Translational Medicine, El-Khatib et al. describe a "closed-loop" bihormonal artificial pancreas, designed to avert episodes of low blood sugar in patients with insulin-dependent diabetes. We discuss the benefits and challenges of therapy directed at tight control of blood glucose and ask whether this and similar technological breakthroughs can address as yet unanswered questions in the biology of diabetes.

H NMR Investigation of Thermally Triggered Insulin Release from Poly(N-isopropylacrylamide) Microgels

We describe investigations of insulin release from thermoresponsive microgels using variable temperature (1)H NMR. Microgel particles composed of poly(N-isopropylacrylamide) were loaded with the peptide via a swelling technique, and this method was compared to simple equilibrium partitioning. Variable temperature (1)H NMR studies suggest that the swelling loading method results in enhanced entrapment of the peptide versus equilibrium partitioning. A centrifugation-loading assay supports this finding. Pseudo-temperature jump (1)H NMR measurements suggest that the insulin release rate is partially decoupled from microgel collapse. These types of direct release investigations could prove to be useful methods in the future design of controlled macromolecule drug delivery devices.

Lipid microparticles as a parenteral controlled release device for peptides

To investigate the potential of physiological lipids as an alternative to synthetic polymeric materials such as poly(lactide-co-glycolide), peptide-containing glyceryl tripalmitate microparticles were prepared. A modified solvent evaporation method and a melt dispersion technique without the use of organic solvent were employed. Thymocartin (TP-4), an immunomodulating tetrapeptide and insulin were chosen as model peptides and incorporated as a solid or dissolved in 100 microl aqueous solution. The resulting microparticles were characterized with respect to particle size and morphology, biocompatibility, drug content (encapsulation efficiency) and in vitro release behavior. Electron spectroscopy for chemical analysis was used to investigate the adsorption of the model peptides to the lipid matrix material. The modified solvent evaporation as well as the melt dispersion method were suitable for the preparation of microparticles in the size range of 20-150 microm with an acceptable yield. The biocompatibility of the glyceryl tripalmitate microparticles after implantation into NMRI-mice was comparable to poly(lactide-co-glycolide) microparticles. The encapsulation efficiency for both model peptides was high (>80%) even at high theoretical loadings when the peptide was incorporated as a solution with the melt dispersion technique. The in vitro release behavior was substantially influenced by the physicochemical properties of the model peptides used in this study.

Development and characterization of lipid microparticles as a drug carrier for somatostatin

Somatostatin, a therapeutic peptide with a high therapeutical potential but a very short biological half-live was encapsulated within microparticles by a modified solvent evaporation method and a melt dispersion method without the use of organic solvent. As the use of synthetic polymer matrix materials often goes along with detrimental effects on incorporated peptides, we investigated the potential of physiological lipids such as glyceryl tripalmitate (Dynasan 116) as an alternative matrix material. The two preparation methods were evaluated with respect to surface topography, particle size distribution, encapsulation efficiency, in-vitro release behavior and modification of the resulting microparticles. Microparticles with a suitable particle size distribution for i.m. or s.c. injection could be prepared with both methods. The encapsulation efficiency of the peptide into glyceryl tripalmitate microparticles was substantially influenced by the preparation method and the physical state of the peptide to be incorporated. The melt dispersion technique and the incorporation of the drug as an aqueous solution gave the best results with actual drug loadings up to 9% and an encapsulation efficiency of approximately 90%. Microparticles prepared by the melt dispersion technique crystallized in the unstable alpha-modification. The peptide was released almost continuously over 10 days with no burst effect, 20-30% of the incorporated somatostatin was not released in the monitored time period.