Russell O. Potts

Evidence that oleic acid exists in a separate phase within stratum corneum lipids

Oleic acid is known to be a penetration enhancer for polar to moderately polar molecules. A mechanism related to lipid phase separation has been previously proposed by this laboratory to explain the increases in skin transport. In the studies presented here, Fourier transform infrared spectroscopy (FT-IR) was utilized to investigate whether or not oleic acid exists in a separate phase within stratum corneum (SC) lipids. Per-deuterated oleic acid was employed allowing the conformational phase behavior of the exogenously added fatty acid and the endogenous SC lipids to be monitored independently of each other. The results indicated that oleic acid exerts a significant effect on the SC lipids, lowering the lipid transition temperature (Tm) in addition to increasing the conformational freedom or flexibility of the endogenous lipid alkyl chains above their Tm. At temperatures lower than Tm, however, oleic acid did not significantly change the chain disorder of the SC lipids. Similar results were obtained with lipids isolated from the SC by chloroform:methanol extraction. Oleic acid, itself, was almost fully disordered at temperatures both above and below the endogenous lipid Tm in the intact SC and extracted lipid samples. This finding suggested that oleic acid does exist as a liquid within the SC lipids. The coexistence of fluid oleic acid and ordered SC lipids, at physiological temperatures, is consistent with the previously proposed phase-separation transport mechanism for enhanced diffusion. In this mechanism, the enhanced transport of polar molecules across the SC can be explained by the formation of permeable interfacial defects within the SC lipid bilayers which effectively decrease either the diffusional path length or the resistance, without necessarily invoking the formation of frank pores.

A predictive algorithm for skin permeability: the effects of molecular size and hydrogen bond activity.

AbstractPurpose. To develop a predictive algorithm of nonelectrolyte transport through skin based upon a partitioning-diffusion model. Methods. Drug permeability is described by a partitioning-diffusion equation. Through free-energy relationships, partitioning is related to the drugs molecular volume (MV), and hydrogen bond donor (Hd) and acceptor (Ha) activity. Diffusion is related to the drugs MV using a theory of diffusion through lipid lamellae based on free-volume fluctuations within the lipid domain. These two explicit descriptions are combined to give an equation describing permeability in terms of the permeants physical properties. The aqueous permeability coefficients of 37 nonelectrolytes through human epidermis were evaluated as a function of these physical properties using a multiple regression analysis. Results. The results of the regression analysis show that 94% of the variability in the data can be explained by a model which includes only the permeants MV, Hd and Ha. These results further provide an algorithm to predict skin permeability based upon the values of these parameters. In addition, the relative contribution of various chemical functional groups (e.g., -COOH) is derived, and can be used to predict skin transport from drug structure alone. Conclusions. A biophysically relevant model of drug transport through human skin is derived based solely on the physical properties of the drug. The model provides an algorithm to predict permeability from the drugs structure and/or physical properties. Moreover, the model is applicable to a number of lipid barrier membranes, suggesting a common transport mechanism in all.

Mechanism of oleic acid-induced skin penetration enhancement in vivo in humans

Abstract The outermost layer of mammalian skin, the stratum corneum (SC), by virtue of its unique architecture, presents a significant barrier to the transdermal delivery of drugs. Penetration enhancers such as oleic acid (OA), which increase skin permeability, appear to act selectively on the extracellular lipids representing the principal regulatory channel for the penetration of small molecules. In vitro studies investigating the mode of action of OA, have generated two mechanistic scenarios, which may account for the action of this enhancer; (a) lipid fluidization, and (b) lipid phase separation. In the studies presented here, attenuated total reflectance infrared spectroscopy was used to determine the mode of action of OA in vivo, in man. The use of perdeuterated OA ( [2H]OA) enabled the behaviour of endogenous lipids to be observed independently to that of the exogenously applied enhancer as a result of their spectrally distinct methylene group vibrations. Human forearm was treated topically with 1 ml of either (a) a solution of 5% (v/v) [2H] OA in ethanol, or (b) ethanol alone, for a period of 16 h. After removal of the delivery system, the SC at the application site was progressively removed by adhesive tape-stripping, while sequential IR spectra were obtained at each newly exposed surface. In this way, we were able to monitor (a) the distribution profile of [2H] OA across the SC, (b) the conformational order of the SC lipids as a function of depth, and (c) the phase behaviour of the enhancer in the SC. Our results indicate that [2H ] OA induces lipid disordering only in the superficial layers of the SC, albeit of a smaller magnitude than that associated with a gel to liquid crystalline conformational change. Additionally, [2H] OA was found to exist in a liquid phase at all levels of the SC spectroscopically examined. These results suggest, therefore, that OA-induced skin penetration enhancement results from a mechanism involving both SC lipid fluidization and phase separation, with the latter probably predominating.

Clinical evaluation of the GlucoWatch® biographer: a continual, non-invasive glucose monitor for patients with diabetes ☆

A device providing frequent, automatic, and non-invasive glucose measurements for persons with diabetes has been developed: the GlucoWatch biographer. This device extracts glucose through intact skin via reverse iontophoresis where it is detected by an amperometric biosensor. The biographer can provide glucose readings every 20 min for 12 h. The performance of this device was evaluated in two large clinical studies in a controlled clinical environment (n=231), and the home environment (n=124). Accuracy of the biographer was evaluated by comparing the automatic biographer readings to serial finger-stick blood glucose (BG) measurements. Biographer performance was comparable in both environments. Mean difference between biographer and finger-stick measurements was -0.01 and 0.26 mmol l(-1) for the clinical and home environments, respectively. The mean absolute value of the relative difference was 1.06 and 1.18 mmol l(-1) for the same studies. Correlation coefficient (r) between biographer and finger-stick measurements was 0.85 and 0.80 for the two studies. In both studies, over 94% of the biographer readings were in the clinically acceptable A+B region of the Clarke Error Grid. A slight positive bias is observed for the biographer readings at low BG levels. Biographer accuracy is relatively constant over all rates of BG changes, except when BG decreases more than 10 mmol l(-1) h(-1), which occurred for only 0.2% of points in the home environment study. Biographer precision, as measured by CV%, is approx. 10%. Skin irritation, characterized by erythema and edema, was either non-existent or mild in >90% of subjects and resolved in virtually all subjects without treatment in several days.

Oleic Acid: Its Effects on Stratum Corneum in Relation to (Trans)Dermal Drug Delivery

Calorimetric studies with porcine stratum corneum (SC) have shown that the lipid phase transitions associated with the intercellular bilayers are markedly affected by treatment with oleic acid. Specifically, the transition temperatures (Tm) and cooperativity are reduced, whereas no effect was observed on the endotherm associated with keratin denaturation, suggesting that oleic acid primarily affects the SC lipids. The decrease in the lipid-associated Tms was further correlated with the amount of oleic acid taken up by the SC. Parallel experiments with silastic implied that the uptake is dependent on the thermodynamic activity of oleic acid in the vehicle itself. The in vitro transport of Piroxicam across human and hairless mouse skin (HMS) was significantly enhanced by oleic acid, as a function of the extent of oleic acid uptake, with an attendant change in Tm. These results emphasize the role of SC lipids in percutaneous absorption. Transport also depended on the donor concentration of ionized drug suggesting that the enhanced transport mechanism cannot be accounted for solely on the principles of the classical pH-partition hypothesis. Accordingly, a model of skin permeability enhancement involving solid-fluid phase separation within the SC lipids is proposed for oleic acid, consistent with the existing phospholipid literature. In conjunction with the use of oleic acid as an enhancer, very soluble hydrophilic salts were recognized as key factors in attaining maximum delivery. Oleic acid uptake, lipid ΔTm, and enhanced drug flux were all found to correlate, exhibiting a bell-shaped curve as a function of the ethanol vehicle concentration. Therefore, uptake and/or DSC experiments are useful for formulating enhanced topical delivery systems.

Measurement of glucose in diabetic subjects using noninvasive transdermal extraction

Results from the Diabetes Care and Complications Trial show that tight blood glucose control significantly reduces the long-term complications of diabetes mellitus1. In that study, frequent self-testing of glucose and insulin administration resulted in a significant reduction in long-term complications. This protocol, however, also resulted in a threefold increase in the frequency of hypoglycaemic incidents. Currently, self-testing requires a drop of blood for each measurement. The pain and inconvenience of self-testing, along with the fear and danger of hypoglycaemia has led to poor patient acceptance of a tight control regimen, despite the clear long-term advantages. A continuously worn, noninvasive method to periodically measure glucose would provide a convenient and comfortable means of frequent self-testing2,3. A continuously worn device could also alert the user of low glucose levels, thereby reducing the incidence of hypoglycaemia4,5. Guy et al. demonstrated a noninvasive method to transport glucose through the skin using low-level electrical current6,7. To provide a quantitative measurement, the flux of glucose extracted across the skin must correlate with serum glucose in a predictive manner. The results presented here show a quantitative relationship between serum and transdermally extracted glucose in diabetics.

Electrical Properties of Skin at Moderate Voltages: Contribution of Appendageal Macropores

The electrical properties of human skin in the range of the applied voltages between 0.2 and 60 V are modeled theoretically and measured experimentally. Two parallel electric current pathways are considered: one crossing lipid-corneocyte matrix and the other going through skin appendages. The appendageal ducts are modeled as long tubes with distributed electrical parameters. For both transport systems, equations taking into account the electroporation of lipid lamella in the case the lipid-corneocyte matrix or the walls of the appendageal ducts in the case of the skin appendages are derived. Numerical solutions of these nonlinear equations are compared with published data and the results of our own experiments. The current-time response of the skin during the application of rectangular pulses of different voltage amplitudes show a profound similarity with the same characteristics in model and plasma membrane electroporation. A comparison of the theory and the experiment shows that a significant (up to three orders of magnitude) drop of skin resistance due to electrotreatment can be explained by electroporation of different substructures of stratum corneum. At relatively low voltages (U < 30 V) this drop of skin resistance can be attributed to electroporation of the appendageal ducts. At higher voltages (U > 30 V), electroporation of the lipid-corneocyte matrix leads to an additional drop of skin resistance. These theoretical findings are in a good agreement with the experimental results and literature data.

Macro- and molecular physical-chemical considerations in understanding drug transport in the stratum corneum

Abstract Traditional permeability studies of full-thickness skin have implied molecules permeated through the skin by various polar or nonpolar pathways depending on the hydrophilicity or lipophilicity of the permeant. However, relatively little is known about the structure of stratum corneum, even though it is considered the primary barrier in transdermal permeation of most permeants. Current macroscopic (permeability, differential scanning calorimetry) and molecular (Fourier transform infrared spectroscopy) investigations of the stratum corneum and its components imply hydration and temperature enhanced permeability of lipophilic molecules through the stratum corneum is associated with transitions involving the hydrocarbon chains of the lipid components.

Glucose monitoring by reverse iontophoresis

Glucose can be extracted through intact skin by electro‐osmotic flow (a process called ‘reverse iontophoresis’) upon the application of a low‐level electrical current. Recently we have combined iontophoretic extraction with an in situ glucose sensor in a device called the GlucoWatch® biographer. Clinical results with this device show close tracking of blood glucose over a range of 2.2 to 22.2 mmol/l for up to 12 h using a single blood glucose value as calibration. The biographer readings lag behind blood glucose values by an average of 18 min. An analysis of data from 92 diabetic subjects in a controlled clinical setting shows a linear relationship (r=0.88) between GlucoWatch biographer readings and blood glucose. The mean absolute relative difference between the two measurements was 15.6% and more than 96% of the data fell in the (A+B) regions of the Clarke error grid. Similar results have been obtained from subjects using the GlucoWatch biographer in an uncontrolled home environment.

The GlucoWatch biographer: a frequent automatic and noninvasive glucose monitor.

The GlucoWatch® (Cygnus, Inc, Redwood City, CA, USA) biographer provides automatic, frequent and noninvasive blood glucose measurements for up to 12 h. The device extracts glucose through intact skin where it is measured by an amperometric biosensor. Clinical trials in a variety of environments have shown that the biographer provides accurate and precise glucose measurements when compared with serial fingerstick blood glucose measurements. Mean difference between these measurements was 0.26 mmol/L in the home environment (r = 0.80). Over 94% of biographer readings were in the clinically acceptable A+B region of the Clarke Error Grid. A slight positive bias is observed for the biographer readings at low glucose levels. Biographer precision, as measured by coefficient of variation (CV)%, is approximately 10%. The low glucose alert function of the biographer was able to detect up to 75% of hypoglycaemic episodes with a low false alert level. Skin irritation, characterized by erythema and oedema was either nonexistent or mild in over 87% of subjects and resolved in virtually all subjects without treatment in several days. The GlucoWatch® biographer has been shown to be a safe and effective method to track glucose level trends and patterns, which should enable improved glycaemic control for many patients.