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Biomedical subjects

R H Guy

Publications and source records attributed to R H Guy.

At least 55 records · Page 3Linked to original sources
PubMed1998

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.

2-Propanol↗
PubMed1998

Transdermal iontophoresis: modulation of electroosmosis by polypeptide.

The objective of this research was to further evaluate the relative importance of electrorepulsion and electroosmosis to the mechanism of enhanced transport across the skin during iontophoresis. Specifically, the impact of iontophoresing into the skin positively and negatively charged polypeptides (poly-L-lysines and poly-L-glutamic acids, respectively) on the membrane's permselectivity and hence on the quantity and direction of electroosmotic flow, was examined. Experiments were performed in vitro at pH 7.4 using conventional methodology; electroosmosis during the iontophoresis of the polypeptides into and across the skin was tracked in the usual way via the movement of the polar, uncharged, non-metabolizable marker, D-mannitol. Electrotransport of the cationic polypeptides attenuated electroosmotic flow in the normal anode-to-cathode direction; the degree of inhibition was correlated both with the initial concentration of poly-L-lysine in the anodal chamber and with the molecular weight of the polypeptide employed (from 1 to 25 kilodaltons). Iontophoresis of the anionic poly-L-glutamic acids from the cathode provoked a slight increase in electroosmotic flow in the 'reverse' direction (i.e. from the receptor phase beneath the skin towards the cathode chamber located on the epidermal side of the membrane); this effect, however, was much less dramatic than that produced in the opposite sense by the cationic polypeptides. The results suggest that driving large positively-charged polypeptide molecules into the skin leads to neutralization of the membrane's negativity, a subsequent loss of permselectivity and a concomitant attenuation of electroosmosis in the conventional anode-to-cathode direction. Presumably, the relatively poor iontophoretic permeability of these species (which becomes more and more evident with increasing molecular weight) results in a sufficiently important association of the polypeptide with the skin during the period of current passage. Much less significant effects are realized by the cathodal iontophoresis of poly-anions due to the difficulty of 'pushing' negative ions into an already negatively-charged membrane.

Administration, Cutaneous↗
PubMed1998

Transdermal drug delivery. Clinical aspects.

The delivery of drugs into and through the skin is a recognized and effective means of therapy for dermatologic, regional, and systemic disease. The selection of drug candidates and the rational design of suitable formulations depends upon the biological make-up of the skin's barrier, and the physiochemical interactions between the membrane, the delivery system technology, and the active agent. This article summarizes the state of the art and examines more recent developments that are the subject of considerable research at this time. In addition, the potential to use the skin as a portal for noninvasive clinical chemistry (e.g., for glucose monitoring in diabetics) is discussed.

Administration, Cutaneous↗
PubMed1998

Ion mobility across human stratum corneum in vivo.

The aim of this study was to develop methods to determine ionic transport parameters, in particular ionic mobilities across human stratum corneum (SC) in vivo. It has been shown previously that the SC, a structurally heterogeneous biomembrane, behaves as a homogeneous barrier to water transport; that is, water diffusivity does not vary as a function of position within the SC; in this work, therefore, the question posed was whether ion motion behaved similarly. Low-frequency impedance measurements (1.61 Hz) reported on the decrease of SC impedance as the barrier was progressively removed by serial adhesive tape stripping. This corresponded to an increase in ion mobility of approximately 2 orders of magnitude across the SC (from the external surface to the interior). Therefore, an algorithm was developed from the absolute impedance data to calculate ion mobility as a function of position within (i.e., depth into) the SC. The mobilities deduced from the algorithm correlated well with water permeability across the SC. The data presented here are thought to be the first measurements of ionic mobility across human skin in vivo.

Adult↗
PubMed1998

Iontophoretic permselectivity of mammalian skin: characterization of hairless mouse and porcine membrane models.

PURPOSE: To evaluate the transport number of Na+, and the isoelectric point, of two skin membranes frequently used for iontophoretic in vitro research. METHODS: Na+ transport numbers were determined by the Hittorf method or by the measurement of membrane potential. The skin isoelectric point was deduced from the electroosmosis of mannitol (a polar non-electrolyte) as a function of pH. RESULTS: The Na+ transport number across porcine skin, like that for hairless mouse, indicated a modest cation permselectivity. Consistent with this observation, the isoelectric points of porcine and hairless mouse skin were determined to be in the ranges of 3.5-3.75 and 4.5-4.6, respectively. That is, at physiological pH, both of these model membranes supports a net negative charge. CONCLUSIONS: The permselective properties of porcine and hairless mouse skin are similar (but with the porcine membrane having apparently fewer basic or more weakly-acidic groups than that of the mouse) and consistent with the characteristics, which have been deduced elsewhere, of human skin.

Animals↗
PubMed1998

Determination of the pH gradient across the stratum corneum.

The objective of this work was to determine the pH gradient profile across hairless mouse stratum corneum (SC) using ratiometric laser scanning confocal microscopy imaging and direct pH measurement with a flat surface electrode. Dual-emission ratiometric imaging used the fluorophore, carboxy seminaphthorhodafluor-1, which displays a pH-dependent shift in its emission spectrum. The assay developed was unsuccessful, however, because (i) the pKa of seminaphthorhodafluor (approximately 7.5) makes it insufficiently sensitive to the normal pH gradient (4-7.4) across mammalian SC, and (ii) the unexpectedly high buffering capacity of the skin precluded a meaningful calibration of the system. In the second method, pH measurements with a flat surface electrode were recorded as the hairless mouse SC was progressively tape stripped. In the superficial SC, the pH remained relatively constant (approximately 5.9); further removal of the barrier resulted in a steady increase in pH to approximately 7. In conclusion, improved methodology is clearly required to characterize precisely the pH profile across the SC.

Animals↗
PubMed1998

Visualization and quantitation of iontophoretic pathways using confocal microscopy.

Laser scanning confocal microscopy (LSCM) has been used to visualize and quantitate the penetration of a model, anionic, fluorescent compound (calcein) along the iontophoretic transport pathways within hairless mouse skin. The LSCM technique permits optical sectioning of full-thickness, unfixed tissue, thereby avoiding poor image quality due to blurring from out-of-focus fluorescence, and obviating artifactual redistribution of the permeant during processing. Simple measurements of the approximate flux of the probe across hairless mouse skin were also made using standard in vitro diffusion cell methodology and a fluorometric assay. Most importantly, LSCM imaging strategies were developed to overcome depth-dependent sensitivity problems. These visualization studies showed that iontophoresis of calcein into hairless mouse skin enhanced delivery, particularly via follicular structures, to significant depths into the barrier. Nonfollicular transport was also apparent, especially at more superficial levels. Quantitative analyses of the LSCM images showed that, although significant nonfollicular transport occurs, the efficiency of the follicular pathway, when the relative surface area is taken into account, is considerable. Overall, therefore, this work contributes significantly to the ultimate goal of fully comprehending the mechanism(s) of iontophoretic drug delivery across the skin.

Animals↗
PubMed1998

Development of skin barrier function in premature infants.

Histologic analysis suggests that epidermal development is complete in utero at approximately 34 wk gestational age. Infants born more prematurely have elevated rates of both transepidermal water loss and transcutaneous heat loss, and have difficulty maintaining homeostasis. The underdeveloped integument is also a portal of entry for infection and the percutaneous uptake of toxins. Previous measurements of transepidermal water loss have suggested that, regardless of gestational age, competent barrier function is attained within 2-4 wk postnatal age. In this study we have utilized another noninvasive biophysical technique, low frequency impedance spectroscopy, to complement transepidermal water loss measurements. We present longitudinal data from infants ranging from 23 to 32 wk gestational age. The results suggest that, for ultra-low birth weight infants (23-25 wk gestational age), the complete development of a fully functional stratum corneum can require significantly longer than 4 wk. In contrast, the data from the older infants suggest that a postnatal existence period of 2-4 wk may not be necessary to attain functional maturity, because infants born at 30 and 32 wk gestational age were found to have barrier function comparable with that of adults.

Body Water↗
PubMed1998

Iontophoresis--recent developments.

This paper addresses the development and optimization of the technology of iontophoresis as a non-invasive approach to transdermal diagnosis and therapy. The rationale for this work is that the skin offers a unique and easily accessible body surface through which drugs can be delivered and general clinical monitoring information can be extracted.

Administration, Cutaneous↗
PubMed1997

Characterization of the permeability barrier of human skin in vivo.

Attenuated-total-reflectance Fourier-transform-infrared spectroscopy has been used to rapidly and noninvasively quantify in vivo the uptake of a chemical into the outermost, and least permeable, layer of human skin (the stratum corneum). The objective of the experiment was to develop a general model to predict the rate and extent of chemical absorption for diverse exposure scenarios from simple, and safe, short-duration studies. Measurement of the concentration profile of the chemical in the stratum corneum, and analysis of the data using the unsteady-state diffusion equation, enabled estimation of the permeability coefficient and calculation of the time required to achieve maximal transdermal flux. Validation of the spectroscopic technique employed was established, and quantitation of chemical uptake into the stratum corneum was confirmed independently using trace amounts of radiolabeled chemical in conjunction with liquid scintillation counting and accelerator mass spectrometry. The results presented have pharmacological and toxicological implications, as the technology lends itself both to the prediction of transdermal drug delivery, and the feasibility of this route of administration, and to the assessment of risk after dermal contact with toxic chemicals.

Absorption↗
PubMed1997

Transdermal therapy and diagnosis by iontophoresis.

Iontophoresis, the use of an electric current to drive charged molecules across the skin, has the potential to expand the feasible range of drugs for transdermal administration significantly. This method of delivery is being examined carefully with respect to higher-molecular-weight therapeutics (in particular, peptides and small proteins), which cannot be absorbed following oral administration and for which, at this time, an invasive injection remains the only option. In addition, the procedure of so-called 'reverse' iontophoresis would appear to represent a truly noninvasive approach for diagnostic monitoring of blood chemistry.

Administration, Cutaneous↗
PubMed1997

Iontophoretic transport pathways: dependence on penetrant physicochemical properties.

The objective of this work was to investigate how the preferred iontophoretic transport pathways of a molecule depend on its physicochemical properties. Laser scanning confocal microscopy (LSCM) was used to visualize in hairless mouse skin the distribution of two fluorescent penetrants: calcein, a multiply charged (-4), hydrophilic molecule; and nile red, a lipophilic, neutral compound. Iontophoresis and passive delivery of nile red showed that the percutaneous transport of this compound occurred via (inter- and intracellular) pathways that were clearly distinct from those followed by calcein. Although the distribution of nile red was influenced somewhat by the passage of current relative to the passive control, there was relatively little enhancement of the penetration of this compound into the skin. Calcein, on the other hand, did not passively enter the skin. However, with iontophoresis, greatly enhanced transport, with an important contribution from follicular structures, was observed. Sequential (dual) transport of the two fluorophores illustrated clearly the different pathways followed and reflected the transport and visualization studies of the individual species. It may be concluded, therefore, that the iontophoretic pathways followed across the skin are dictated by the physicochemical properties of the penetrant and by its affinity for the different environments available.

Animals↗
PubMed1997

Iontophoresis of poly-L-lysines: the role of molecular weight?

PURPOSE: (1) To determine the extent of iontophoretic transport as a function of molecular weight (MW) of the penetrant; and (2) to visually and quantitatively characterize the iontophoretic transport pathways (follicular (F) versus nonfollicular (NF) of the fluorescently-labeled poly-L-lysines employed. METHODS: A series of fluorescently-labeled poly-L-lysines (FITC-PLLs) [4 KDa, 7 KDa and 26 KDa] were used to study the extent and distribution of iontophoretic skin penetration as a function of MW using laser scanning confocal microscopy (LSCM). RESULTS: It was found that, relative to the passive controls, and under the electrical conditions considered, iontophoresis greatly enhanced the penetration of the 4 KDa analog, slightly elevated the delivery of the 7 KDa FITC-PLL, but had no effect on the transport of the larger 26 KDa FITC-PLL. Quantitative analyses of LSCM images revealed that iontophoresis increased transport via F pathways only slightly more than that through NF pathways for the 4 KDa and 7 KDa FITC-PLL molecules. CONCLUSIONS: It is visually apparent that the iontophoretic transport pathways taken are importantly determined by the physicochemical properties (including size and charge) of the penetrant. The results presented here demonstrate an inverse dependence of iontophoretic delivery upon the MW of the penetrant.

Animals↗
PubMed1997

Iontophoretic delivery across the skin: electroosmosis and its modulation by drug substances.

PURPOSE: The long-term objective of this research is to understand how the efficiency of iontophoresis depends upon the structural and physicochemical properties of the administered drug. Specifically, the ability of certain drug species to alter the permselective properties of the skin was examined. METHODS: Using conventional in vitro methodology, the inhibition of electroosmotic flow induced by the iontophoresis of five different beta-blockers (of varying lipophilicity) was examined. The concomitant electrotransport of the most lipophilic species (propranolol) and the convective movement of solvent in the anode-to-cathode direction were measured. In addition, the possibility that electroosmosis might be augmented by the delivery of anionic drugs was also considered. RESULTS: Iontophoresis of lipophilic, cationic beta-blockers caused a concentration-dependent inhibition of conventional electroosmosis. The most hydrophilic analogs elicited no effect. As a result of this charge neutralization phenomenon, the optimal concentration for propranolol iontophoresis was significantly less than the maximum achievable in aqueous solution. Only a very modest improvement in convective solvent flow was induced by the cathodal iontophoresis of anionic compounds. CONCLUSIONS: The permselectivity of the skin can be altered by drugs which are positively charged and which possess a significant, adjacent hydrophobic surface. The latter seems able to "anchor" the molecule in the skin and the counter charge to the membrane's negative character ensures a tight association. Both lipophilicity and a positive charge are essential-without either, the phenomenon is not observed. The conformational flexibility of the drugs studied to-date, however, prevents unambiguous conclusions about the three-dimensional nature of the putative "binding site".

Adrenergic beta-Antagonists↗
PubMed1997

The effect of current on skin barrier function in vivo: recovery kinetics post-iontophoresis.

PURPOSE: The objective of this study was to determine the extent to which current passage perturbed the skin's intrinsic permeability, and to quantify how quickly and to what extent the barrier properties recovered from the effects of iontophoresis. METHODS: Laser scanning confocal microscopy (LSCM) and impedance spectroscopy (IS) were employed, respectively, to visualize and quantify the recovery kinetics. RESULTS: LSCM images were obtained following passive calcein diffusion through pre-iontophoresed HMS skin in vivo that had been allowed to recover for progressively longer periods of time. IS was used to quantify the rate and extent of skin permeability recovery following current pretreatment. Impedance spectra were recorded 0, 3, 5, 7, 9 and 18 hrs after current termination. CONCLUSIONS: Enhanced calcein permeability as assessed by confocal microscopy persisted for up to 24 hrs following current passage. Consistent with these LSCM findings, IS indicated that the time required for the impedance of hairless mouse skin to return to pre-iontophoresis levels (following 2-hr current passage at 0.5 mA/cm2) was at least 18 hrs.

Animals↗
PubMed1996

Homogeneous transport in a heterogeneous membrane: water diffusion across human stratum corneum in vivo.

The objective of this study was to determine whether a structurally heterogeneous biomembrane, human stratum corneum (SC), behaved as a homogeneous barrier to water transport. The question is relevant because the principal function of the SC in vivo is to provide a barrier to the insensible loss of tissue water across the skin. Impedance spectra (IS) of the skin and measurements of the rate of transepidermal water loss (TEWL) were recorded sequentially in vivo in human subjects as layers of the SC were progressively removed by the serial application of adhesive tape strips. The low-frequency (< or = 100 rad s-1) impedance of skin was much more significantly affected by tape stripping than the higher frequency values; removal of the outermost SC layer had the largest effect. In contrast, TEWL changed little as the outer SC layers were stripped off, but increased dramatically when 6-8 microns of the tissue had been removed. It follows that the two noninvasive techniques probe SC barrier integrity in somewhat different ways. After SC removal, recovery of barrier function, as assessed by increasing values of the low-frequency impedance, apparently proceeded faster than TEWL decreased to the prestripping control. The variation of TEWL as a function of SC removal behaved in a manner entirely consistent with a homogeneous barrier, thereby permitting the apparent SC diffusivity of water to be found. Skin impedance (low frequency) was correlated with the relative concentration of water within the SC, thus providing an in vivo probe for skin hydration. Finally, the SC permeability coefficient to water, as a function of SC thickness, was calculated and correlated with the corresponding values of skin admittance derived from IS.

Adult↗
PubMed1996

Metabolic approaches to enhance transdermal drug delivery. 1. Effect of lipid synthesis inhibitors.

The intercellular domains of the stratum corneum, which contain a mixture of cholesterol, free fatty acids, and ceramides, mediate both the epidermal permeability barrier and the transdermal delivery of both lipophilic and hydrophilic molecules. Prior studies have shown that each of the three key lipid classes is required for normal barrier function. For example, selective inhibition of either cholesterol, fatty acid, or ceramide synthesis in the epidermis delays barrier recovery rates after barrier perturbation of hairless mouse skin in vivo. In this study, we investigated the potential of certain inhibitors of lipid synthesis to enhance the transdermal delivery of lidocaine or caffeine as a result of their capacity to perturb barrier homeostasis. After acetone disruption of the barrier, the extent of lidocaine delivery and the degree of altered barrier function paralleled each other. Moreover, the further alteration in barrier function produced by either the fatty acid synthesis inhibitor 5-(tetradecyloxy)-2-furancarboxylic acid (TOFA), the cholesterol synthesis inhibitor fluvastatin (FLU), or cholesterol sulfate (CS) resulted in a further increase in lidocaine absorption. Furthermore, coapplications of TOFA and CS together caused an additive increase in lidocaine uptake. Finally, a comparable increase in drug delivery occurred when the barrier was disrupted initially with DMSO instead of acetone; coapplications of TOFA and FLU together again delayed barrier recovery and increased drug delivery by about 8-fold vs delivery from a standard enhancing vehicle. Whereas these metabolic inhibitors also variably increased the octanol/water partitioning of the drugs studied (perhaps via complexion or pH alterations), physicochemical effects of the inhibitors alone did not alter drug uptake in intact skin; i.e., passive mechanisms alone cannot account for the net increase in drug delivery. Our results show that modulations of epidermal lipid biosynthesis, following application of conventional, chemical penetration enhancers, cause a further boost in drug delivery, attributable to the ability of these agents to alter both permeability barrier homeostasis and thermodynamics. This biochemical/metabolic approach provides a novel means to enhance transdermal drug delivery in conjunction with the concurrent or prior use of chemical enhancers.

Acetone↗