Characterization of in Vitro Transdermal Iontophoretic Delivery of Insulin

Controlled Transdermal Iontophoresis of Insulin from Water-Soluble Polypyrrole Nanoparticles: An In Vitro Study

The iontophoresis delivery of insulin (INS) remains a serious challenge due to the low permeability of the drug through the skin. This work aims to investigate the potential of water-soluble polypyrrole nanoparticles (WS-PPyNPs) as a drug donor matrix for controlled transdermal iontophoresis of INS. WS-PPyNPs have been prepared via a simple chemical polymerization in the presence of sodium dodecyl sulfate (SDS) as both dopant and the stabilizing agent. The synthesis of the soluble polymer was characterized using field emission scanning electron microscopy (FESEM), dynamic light scattering (DLS), fluorescence spectroscopy, and Fourier transform infrared (FT–IR) spectroscopy. The loading mechanism of INS onto the WS-PPyNPs is based on the fact that the drug molecules can be replaced with doped dodecyl sulfate. A two-compartment Franz-type diffusion cell was employed to study the effect of current density, formulation pH, INS concentration, and sodium chloride concentration on anodal iontophoresis (AIP) and cathodal iontophoresis (CIP) of INS across the rat skin. Both AIP and CIP delivery of INS using WS-PPyNPs were significantly increased compared to passive delivery. Furthermore, while the AIP experiment (60 min at 0.13 mA cm^–2) show low cumulative drug permeation for INS (about 20.48 µg cm⁻²); the CIP stimulation exhibited a cumulative drug permeation of 68.29 µg cm⁻². This improvement is due to the separation of positively charged WS-PPyNPs and negatively charged INS that has occurred in the presence of cathodal stimulation. The obtained results confirm the potential applicability of WS-PPyNPs as an effective approach in the development of controlled transdermal iontophoresis of INS.

Accelerating transdermal delivery of insulin by ginsenoside nanoparticles with unique permeability

Diabetes is a metabolic disease caused by insufficient insulin secretion, action or resistance, in which insulin plays an irreplaceable role in the its treatment. However, traditional administration of insulin requires continuous subcutaneous injections, which is accompanied by inevitable pain, local tissue necrosis and hypoglycemia. Herein, a green and safe nanoformulation with unique permeability composed of insulin and ginsenosides is developed for transdermal delivery to reduce above-mentioned side effects. The ginsenosides are self-assembled to form shells to protect insulin from hydrolysis and improve the stability of nanoparticles. The nanoparticles can temporarily permeate into cells in 5 min and promptly excrete from the cell for deeper penetration. The insulin permeation is related to the disorder of stratum corneum lipids caused by ginsenosides. The skin acting as drug depot mantains the nanoparticles released continuously, therefore the body keeps euglycemic for 48 h. Encouraged by its long-lasting and effective transdermal therapy, ginsenosides-based nano-system is expected to deliver other less permeable drugs like proteins and peptides and benefit those who are with chronic diseases that need long-term medication.

Advances in transdermal insulin delivery

Insulin therapy is necessary to regulate blood glucose levels for people with type 1 diabetes and commonly used in advanced type 2 diabetes. Although subcutaneous insulin administration via hypodermic injection or pump-mediated infusion is the standard route of insulin delivery, it may be associated with pain, needle phobia, and decreased adherence, as well as the risk of infection. Therefore, transdermal insulin delivery has been widely investigated as an attractive alternative to subcutaneous approaches for diabetes management in recent years. Transdermal systems designed to prevent insulin degradation and offer controlled, sustained release of insulin may be desirable for patients and lead to increased adherence and glycemic outcomes. A challenge for transdermal insulin delivery is the inefficient passive insulin absorption through the skin due to the large molecular weight of the protein drug. In this review, we focus on the different transdermal insulin delivery techniques and their respective advantages and limitations, including chemical enhancers-promoted, electrically enhanced, mechanical force-triggered, and microneedle-assisted methods.

Effect of permeation enhancers on the iontophoretic transport of metoprolol tartrate and the drug retention in skin

Utilization of chemical penetration enhancers in conjunction with iontophoresis is regarded as the most effective method to enhance the passage of molecules across the skin barrier. A systematic approach to enhance the transdermal delivery of metoprolol tartrate and the subsequent release of the drug depot in the skin was investigated. Gel formulations with proximate viscosity were prepared and assessed for the effect of polymers (carbopol, hydroxypropyl methyl cellulose, and methyl cellulose), permeation enhancers (5% w/w, sodium lauryl sulfate (SLS), dimethyl formamide, n-methyl-2-pyrrolidone, and polyethylene glycol 400), and the combination approach (permeation enhancers with iontophoresis-0.5 mA/cm² on the drug delivery. The flux values observed in passive (4.59-5.89 µg/cm²/h) and iontophoresis (37.99-41.57 µg/cm²/h) processes revealed that the permeation of metoprolol was not influenced by the polymers studied, under similar conditions, and further studies were carried out using carbopol gel as a representative polymer. Appreciable enhancement (~5-fold) in drug delivery was observed with SLS in the passive process while the optimum iontophoretic delivery condition ensured better delivery (~7-fold). Combination of iontophoresis with SLS further enhanced the drug delivery (~9-fold) and leads to noticeable drug retention in the skin as well. Moreover, the drug retained in the cutaneous layer of the skin eventually released over a period of time (5 days) and followed a near first order profile. This study concludes that the combination of iontophoresis with SLS augmented the metoprolol delivery and rendered skin drug depot, which eventually released over a period of time.

Transdermal iontophoresis of insulin. Part 1: A study on the issues associated with the use of platinum electrodes on rat skin

We have studied the issues associated with the use of platinum electrodes for transdermal iontophoretic delivery of peptides, using insulin as a model peptide. Insulin permeation was studied using full-thickness rat skin by varying the donor solution pH as a function of electrode polarity. The stability of insulin under the iontophoretic conditions was studied using TLC, SDS-polyacrylamide gel electrophoresis and HPLC. Large pH shifts were observed during anodal iontophoresis (AI), when the donor solution pH was above the isoelectric point of insulin and in cathodal iontophoresis (CI), when the donor solution pH was below the isoelectric point of insulin. The direction and magnitude of electroosmotic flow was influenced by pH of the donor solution and the electrode polarity. On the other hand, the buffer used to maintain the pH governed the contribution of electrorepulsion to the overall transport of insulin. Electrochemical degradation of insulin was significant during Al at pH 7.4. Among the pH investigated, Al of insulin at pH 3.6 and Cl at pH 8.35 were better, as the pH shift was relatively less and electrochemically more stable during iontophoresis as compared with other pH. In summary, the pH shift caused by platinum electrodes had a significant influence on the permeation and stability of insulin.

Influence of electrical parameters in the iontophoretic delivery of a small peptide: in vitro studies using arginine–vasopressin as a model peptide

The present investigation was carried out to understand the influence of electrical parameters on iontophoretic transport of a small peptide like arginine-vasopressin (AVP). In vitro studies using rat skin were conducted to assess the effect of different current densities (CDs), durations, duty cycles and alternating polarity on vasopressin permeation. HPLC was used for ensuring electrochemical stability of the peptide whereas FT-IR and TGA were used to understand the biophysical changes caused in skin due to passage of current. Application of CD > 0.75 mA/cm(2) was found to compromise skin barrier integrity as well as electrochemical stability. Periodic current did not show any significant difference in permeation compared to continuous current. Alternating polarity was useful in reducing pH shift however, was less efficient compared to continuous direct current. FT-IR and TGA studies showed that skin hydration increased as a function of CD and duration and all the results could be explained on the basis of increased skin hydration.

Iontophoretic drug delivery

The composition and architecture of the stratum corneum render it a formidable barrier to the topical and transdermal administration of therapeutic agents. The physicochemical constraints severely limit the number of molecules that can be considered as realistic candidates for transdermal delivery. Iontophoresis provides a mechanism to enhance the penetration of hydrophilic and charged molecules across the skin. The principal distinguishing feature is the control afforded by iontophoresis and the ability to individualize therapies. This may become significant as the impact of interindividual variations in protein expression and the effect on drug metabolism and drug efficacy is better understood. In this review we describe the underlying mechanisms that drive iontophoresis and we discuss the impact of key experimental parameters-namely, drug concentration, applied current and pH-on iontophoretic delivery efficiency. We present a comprehensive and critical review of the different therapeutic classes and molecules that have been investigated as potential candidates for iontophoretic delivery. The iontophoretic delivery of peptides and proteins is also discussed. In the final section, we describe the development of the first pre-filled, pre-programmed iontophoretic device, which is scheduled to be commercialized during the course of 2004.

Transdermal iontophoresis of insulin. II. Physicochemical considerations

Transdermal iontophoresis is one of the potential enhancement strategies for the delivery of large and charged molecules. Insulin, a polypeptide of 6 kDa was used as a model for large peptides to understand the influence of peptide concentration, NaCl concentration, buffer type and its concentration on the transport efficiency of iontophoresis. Maximum enhancement was found at 3 mg/ml (75 IU/ml). The permeation of insulin was found to increase up to 0.05 M NaCl and decreased at higher concentrations of NaCl. The glucose permeation studies showed that permeation of insulin increased in the presence of NaCl due to ion induced convective flow. The flux enhancement of insulin in the presence of phthalate buffer was higher in comparison to citrate buffer, but the enhancement in these two buffers was the same in the presence of 0.05 M NaCl, which was also supported by a similar trend in conductivity values. However, the solution conductivity values did not reflect the influence of co-ions and counter ions on the transport of large peptides across the skin. Overall the findings revealed that the transport efficiency of large peptides like insulin may be improved by the optimisation of competing ions in solution.

Analysis of skin permeation-enhancing mechanism of iontophoresis using hydrodynamic pore theory

The effects of constant DC iontophoresis (0-1.5 mA/0.966 cm(2)) on the permeation of three hydrophilic compounds, antipyrine (ANP, M.W. 188.23), sucrose (SR, M.W. 342.30) and 1-kestose (KT, M.W. 506.73), through excised hairless rat skin were evaluated using hydrodynamic pore theory. The electro-osmotic flow caused by iontophoresis was measured using deuterium oxide (D(2)O). The penetration-enhancing mechanism of iontophoresis was found to increase solvent flow through electro-osmosis and pore enlargement and/or new pore production in the skin barrier, together with enhancement of electrochemical potential difference across the skin. These effects were closely related to the strength of the current applied. The electro-osmotic flow of D(2)O (J(D(2)O)) greatly enhanced the skin permeation clearance of all hydrophilic penetrants (CL(drug)). Pore production was classified into reversible and irreversible processes, which resulted from lower (0-0.5 mA/0.966 cm(2)) and higher (0.5-1. 5 mA/0.966 cm(2)) currents, respectively. Thus, the enhancing effects of iontophoresis on skin permeation of nonionic hydrophilic compounds can be explained by increase in pore size and higher solvent flow.

Pulsatile peptide release from multi-layered hydrogel formulations consisting of poly(ethylene glycol)-grafted and ungrafted dextrans

Multi-layered hydrogel formulations consisting of poly(ethylene glycol)-grafted dextran (PEG-g-Dex) and ungrafted Dex were investigated as a model of pulsatile drug release. In these formulations, it is considered that the grafted PEG domains act as a drug reservoir dispersed in the Dex matrix based on aqueous polymer two-phase systems. The formulations exhibited surface-controlled degradation by dextranase, and insulin release was observed in a pulsatile manner because of the multi-layered structure: PEG-g-Dex hydrogel layers containing insulin and insulin-free Dex hydrogel layers. Thus, it is suggested that the multi-layered hydrogel formulations using PEG-g-Dex and Dex are feasible for chronopharmacological drug delivery systems.

Transdermal iontophoretic delivery of bovine insulin and monomeric human insulin analogue

The present study was undertaken to explore the possibility of delivering bovine insulin in streptozocin (STZ)-induced diabetic rats by iontophoresis. Further, the effect of iontophoresis of monomeric human insulin analogue (r-DNA origin) on the plasma glucose level (PGL) of diabetic rats was studied. Iontophoresis of bovine insulin (10-200 IU/ml) was not effective in decreasing the PGL in untreated diabetic rats. Pretreatment of skin with oleic acid or menthol for 3 h followed by iontophoresis of bovine insulin also failed to produce a fall in PGL. Application of a depilatory cream for hair removal (24 h before the experiment), followed by iontophoresis of bovine insulin (10, 30 and 100 IU/ml) produced a concentration-dependent fall in PGL. Further, application of depilatory cream immediately before the experiment produced a substantial fall in PGL both by passive diffusion and iontophoresis. Depilatory cream might have drastically reduced the barrier function of skin such that conventional bovine insulin (dimer and hexamer) penetrates through the intact skin by iontophoresis and even by passive diffusion. Depilatory cream or the active components of depilatory cream may be useful as penetration enhancers for transdermal delivery of drugs especially macromolecules such as insulin. Iontophoresis of monomeric human insulin analogue (B9 Asp, B27 Glu) through intact skin (untreated) produced a significant fall in PGL in diabetic rats. Monomeric human insulin analogues which have low tendency to self aggregation may be promising candidates for the transdermal iontophoretic delivery of insulin.

Iontophoresis and electroporation: comparisons and contrasts

The techniques of iontophoresis and electroporation can be used to enhance topical and transdermal drug delivery. Iontophoresis applies a small low voltage (typically 10 V or less) continuous constant current (typically 0.5 mA/cm2 or less) to push a charged drug into skin or other tissue. In contrast, electroporation applies a high voltage (typically, ?100 V) pulse for a very short (micros-ms) duration to permeabilize the skin. This electric assistance of drug delivery across skin will expand the scope of transdermal delivery to hydrophilic macromolecules such as the drugs of biotechnology. These two techniques differ in several aspects such as the mode of application and pathways of transport but can be used together for effective drug delivery. Iontophoresis is already used clinically in physical therapy clinics and is close to commercialization for development of a systemic delivery patch with miniaturized circuits and similar in overall size to a passive patch. The use of electroporation for drug delivery is relatively new and is being actively researched.

Iontophoresis of monomeric insulin analogues in vitro: effects of insulin charge and skin pretreatment

The aim of this study was to investigate the influence of association state and net charge of human insulin analogues on the rate of iontophoretic transport across hairless mouse skin, and the effect of different skin pretreatments on said transport. No insulin flux was observed with anodal delivery probably because of degradation at the Ag/AgCl anode. The flux during cathodal iontophoresis through intact skin was insignificant for human hexameric insulin, and only low and variable fluxes were observed for monomeric insulins. Using stripped skin on the other hand, the fluxes of monomeric insulins with two extra negative charges were 50-100 times higher than that of hexameric human insulin. Introducing three additional charges led to a further 2-3-fold increase in flux. Wiping the skin gently with absolute alcohol prior to iontophoresis resulted in a 1000-fold increase in transdermal transport of insulin relative to that across untreated skin, i.e. to almost the same level as stripping the skin. The alcohol pretreatment reduced the electrical resistance of the skin, presumably by lipid extraction. In conclusion, monomeric insulin analogues with at least two extra negative charges can be iontophoretically delivered across hairless mouse skin, whereas insignificant flux is observed with human, hexameric insulin. Wiping the skin with absolute alcohol prior to iontophoresis gave substantially improved transdermal transport of monomeric insulins resulting in clinically relevant delivery rates for basal treatment.

Electroresponsive pulsatile depot delivery of insulin from poly(dimethylaminopropylacrylamide) gel in rats

We describe a model for pulsatile drug delivery with an electroresponsive polymer that is stimulated by an externally applied electrical field. Insulin loaded in an electroresponsive poly(dimethylaminopropylacrylamide) (PDMAPAA) gel was administered as a subcutaneous depot in rats. The gel induced a pulsatile plasma glucose decrease in correspondence to stimulation with a constant current of 1.0 mA (0.36 mA/cm2). The first drop occurred at 0.5 h after a 1-min application of current at 0 h and the second drop occurred at 3 h after a 10-min application of current at 2 h. Calculation of pharmacological bioavailability showed that the gel released 0.12% of the loaded insulin after these two stimuli. This in vivo study demonstrates the feasibility of this pulsatile delivery system. The mechanism of insulin release from the electroresponsive PDMAPAA gel is associated with electrokinetic flow of solvated insulin with water; that is, transportation process of counterions (electrophoresis) and water molecules (electroosmosis) in the crosslinked polyelectrolyte gel network.

Transdermal iontophoretic delivery of enkephalin formulated in liposomes

Transdermal iontophoretic transport of a liposomal formulation of [Leu5]enkephalin, across human cadaver skin, was investigated. Franz (vertical) cells were supplied with 0.5 mA/cm2 current density via silver/silver chloride electrodes from a Scepter power supply. Enkephalin spiked with [3H]enkephalin was transported across skin from anode or cathode, depending on the charge on the molecule. Liposomes or their constituents were shown to penetrate into the skin. Enkephalin, when delivered iontophoretically at its isoelectric point, from liposomes carrying positive or negative charge on their surface, resulted in permeation of radioactivity which was same or less than that of the controls when analyzed by liquid scintillation counting. When analyzed by radiochromatography detector on HPLC, degradation of enkephalin during transport was observed, with several degradation peaks in the chromatogram. The degradation was less in liposome formulations, as compared to controls. This is the first report of the combined use of liposomes and iontophoresis for transdermal delivery.