Effects of vasoactive drugs on transdermal lidocaine iontophoresis

Current advances in transdermal delivery of drugs for Alzheimer's disease

Alzheimer's disease (AD) is a common, progressive, fatal neurodegenerative disorder, which will play an increasingly important role both socially and financially in the aging populations. Treatments for AD show modest improvements in cognition and global functioning among patients. Furthermore, the oral administration of treating AD has had some drawbacks that decrease the medication adherence and efficacy of the therapy. Transdermal drugs are proposed as an alternative remedy to overcome the disadvantages of current pharmaceutical dosage options for this chronic disorder. They could have different strengths, such as offering a stable diffusion of active substance, avoiding the first pass metabolism, and reducing system adverse reactions. This article reviews the technical principles, novel techniques of transdermal delivery drug, and prospects for future development for the management of cognitive and behavioral dysfunctions in AD patients.

Modeling the human skin barrier--towards a better understanding of dermal absorption

Many drugs are presently delivered through the skin from products developed for topical and transdermal applications. Underpinning these technologies are the interactions between the drug, product and skin that define drug penetration, distribution, and elimination in and through the skin. Most work has been focused on modeling transport of drugs through the stratum corneum, the outermost skin layer widely recognized as presenting the rate-determining step for the penetration of most compounds. However, a growing body of literature is dedicated to considering the influence of the rest of the skin on drug penetration and distribution. In this article we review how our understanding of skin physiology and the experimentally observed mechanisms of transdermal drug transport inform the current models of drug penetration and distribution in the skin. Our focus is on models that have been developed to describe particular phenomena observed at particular sites of the skin, reflecting the most recent directions of investigation.

Transdermal delivery by iontophoresis

Recently there has been an increased interest in using iontophoretic technique for the transdermal delivery of medications, both ionic and nonionic. This article is an overview of the history of iontophoresis and factors affecting iontophoretic drug transfer for the systemic effects and laws for development of Transdermal delivery system are discussed.

Transdermal fentanyl patches in small animals

Fentanyl citrate is a potent opioid that can be delivered by the transdermal route in cats and dogs. Publications regarding transdermal fentanyl patches were obtained and systematically reviewed. Seven studies in cats and seven studies in dogs met the criteria for inclusion in this review. Dogs achieved effective plasma concentrations approximately 24 hours after patch application. Cats achieved effective plasma concentrations 7 hours after patch application. In dogs, transdermal fentanyl produced analgesia for up to 72 hours, except for the immediate 0- to 6-hour postoperative period. In cats, transdermal fentanyl produced analgesia equivalent to intermittent butorphanol administration for up to 72 hours following patch application.

A geometrical model of dermal capillary clearance

A new microscopic model is developed to describe the dermal capillary clearance process of skin permeants. The physiological structure is represented in terms of a doubly periodic array of absorbing capillaries. Convection-dominated transport in the blood flow within the capillaries is coupled with interstitial diffusion, the latter process being quantified via a slender-body-theory approach. Convection across the capillary wall and in the interstitial phase is treated as a perturbation which may be added to the diffusive transport. The model accounts for the finite permeability of the capillary wall as well as for the geometry of the capillary array, based on realistic values of physiological parameters. Calculated dermal concentration profiles for permeants having the size and lipophilicity of salicylic acid and glucose illustrate the power and general applicability of the model. Furthermore, validation of the model with published in vivo experimental results pertaining to human skin permeation of hydrocortisone is presented. The model offers the possibility for in-depth theoretical understanding and prediction of subsurface drug distribution in the human skin following topical application, as well as rates of capillary clearance into the systemic circulation. A simpler approach that treats the capillary bed as a homogeneously absorbing zone is also employed. The latter may be used in conjunction with the capillary exchange model to estimate measurable dermal transport and clearance parameters in a straightforward manner.

Lidocaine iontophoresis mediates analgesia in lateral epicondylalgia treatment

BACKGROUND AND PURPOSE

Iontophoresis transcutaneously delivers anti-inflammatory and analgesic drugs for the treatment of musculoskeletal dysfunction. Lidocaine is a local anaesthetic with analgesic but no anti-inflammatory properties. The purpose of this investigation was to examine the clinical use of lidocaine iontophoresis-mediated analgesia in a larger treatment algorithm for five patients with lateral humeral epicondylalgia.

METHOD

The investigation was a case series design of five subjects, aged 52 (+/- 6) years, with epicondylalgia of 12-393 days' duration. At each treatment session, the patients received cryotherapy, cross-fibre massage and passive stretch. Between sessions analgesia was provided by an 80 mA-min low-current, long-duration lidocaine iontophoresis (LI) over a 24-hour period. Patients were treated on an every-other-day basis for a total of three treatment sessions. Clinical improvements were determined by triplicate measurements of dolorimetric force over the affected epicondyle prior to treatment 1 (baseline), prior to sessions 2 and 3, and one week following the last session.

RESULTS

Patients demonstrated an increasing tolerance to dolorimetric force application prior to the next session. The force values prior to session 2 (3.1 (+/- 1.1) Newton (N)) and one week following the third session (3.4 (+/- 0.5) N) were significantly improved from the baseline values (2.1 (+/- 0.9) N).

CONCLUSIONS

Pain associated with lateral epicondylalgia decreased, and function improved in all patients at the final measurement. One patient returned during the 90-day follow-up period to seek additional medical attention. This investigation documents the potential for analgesia provided by LI in the rehabilitation process of musculoskeletal dysfunction.

Compartmental Modeling of Transdermal Iontophoretic Transport II: In Vivo Model Derivation and Application

PURPOSE

This study was aimed to develop a family of compartmental models to describe in a strictly quantitative manner the transdermal iontophoretic transport of drugs in vivo. The new models are based on previously proposed compartmental models for the transport in vitro.

METHODS

The novel in vivo model considers two separate models to describe the input into the systemic circulation: a) constant input and b) time-variant input. Analogous to the in vitro models, the in vivo models contain four parameters: 1) kinetic lag time (tL), 2) steady-state flux during iontophoresis (JSS), 3) skin release rate constant (KR), and 4) passive flux in the post-iontophoretic period (Jpas). The elimination from the systemic circulation is described by a) the one-compartment and b) the two-compartment pharmacokinetic models. The models were applied to characterize the observed plasma concentration vs. time data following single-dose iontophoretic delivery of growth hormone-releasing factor (GRF) and R-apomorphine. Moreover, the models were also used to simulate the observed plasma concentration vs. time profiles following a two-dose transdermal iontophoretic administration of alniditan.

RESULTS

The time-variant input models were superior to the constant input models and appropriately converged to the observed data of GRF and R-apomorphine allowing the estimation of JSS, KR, and Jpas. In most cases, the values of tL were negligible. The estimated JSS and the in vivo flux profiles of GRF and R-apomorphine were similar to those obtained using the deconvolution method. The two-dose iontophoretic transport of alniditan was properly simulated using the proposed time-variant input model indicating the utility of the model to predict and to simulate the drug transport by a multiple-dose iontophoresis. Moreover, the use of the compartmental modeling approach to derive an in vitro-in vivo correlation for R-apomorphine was demonstrated. This approach was also used to identify the optimum in vitro model that closely mimics the in vivo iontophoretic transport of R-apomorphine.

CONCLUSIONS

The developed in vivo models demonstrate their consistency and capability to describe the in vivo iontophoretic drug transport. This compartmental modeling approach provides a scientific basis to examine in vitro-in vivo correlations of drug transport by iontophoresis.

Alternating-pulse iontophoresis for targeted cutaneous anesthesia

In studies of sensory contributions to motor control, it may be advantageous to temporarily reduce the sensitivity of specific sensory systems. This article details a method for non-invasively inducing cutaneous anesthesia, leaving proprioceptive and motor functions intact. This method, called alternating-pulse iontophoresis, differs from conventional direct-current (DC) iontophoretic drug delivery in that adjacent drug delivery electrodes are stimulated out-of-phase. The total current delivered at any instant is then less than that produced during a comparable DC application, while the uniformity of drug delivery is expected to improve. Effective delivery of local anesthetics to the cutaneous foot soles by alternating-pulse iontophoresis was demonstrated using cutaneous pressure sensory threshold levels (STL's) assessed with Semmes-Weinstein monofilaments (arbitrary units of perceived force, or a.u.). Thirteen of 16 healthy subjects achieved a level of anesthesia greater than or equal to that normally associated with clinical peripheral sensory neuropathy. Average STL's measured prior to the anesthesia procedure were 4.00 a.u. ( approximately 10 mN). Immediately following the procedure, STL's were elevated to an average of 5.40 a.u. ( approximately 246 mN) and averaged 4.97 a.u. ( approximately 92 mN) after 50 min of standing. A number of research and clinical applications for this technique are suggested.

Transdermal penetration of vasoconstrictors--present understanding and assessment of the human epidermal flux and retention of free bases and ion-pairs

PURPOSE

As reductions in dermal clearance increase the residence time of solutes in the skin and underlying tissues we compared the topical penetration of potentially useful vasoconstrictors (VCs) through human epidermis as both free bases and ion-pairs with salicylic acid (SA).

METHODS

We determined the in vitro epidermal flux of ephedrine, naphazoline, oxymetazoline, phenylephrine, and xylometazoline applied as saturated solutions in propylene glycol:water (1:1) and of ephedrine, naphazoline and tetrahydrozoline as 10% solutions of 1:1 molar ratio ion-pairs with SA in liquid paraffin.

RESULTS

As free bases, ephedrine had the highest maximal flux, Jmax = 77.4 +/- 11.7 microg/cm2/h, being 4-fold higher than tetrahydrozoline and xylometazoline, 6-fold higher than phenylephrine, 10-fold higher than naphazoline and 100-fold higher than oxymetazoline. Stepwise regression of solute physicochemical properties identified melting point as the most significant predictor of flux. As ion-pairs with SA, ephedrine and naphazoline had similar fluxes (11.5 +/- 2.3 and 12.0 +/- 1.6 microg/cm2/h respectively), whereas tetrahydrozoline was approximately 3-fold slower. Corresponding fluxes of SA from the ion-pairs were 18.6 +/- 0.6, 7.8+/- 0.8 and 1.1 +/- 0.1 respectively. Transdermal transport of VC's is discussed.

CONCLUSIONS

Epidermal retention of VCs and SA did not correspond to their molar ratio on application and confirmed that following partitioning into the stratum corneum, ion-pairs separate and further penetration is governed by individual solute characteristics.

Failure to detect dexamethasone phosphate in the local venous blood postcathodic lontophoresis in humans

STUDY DESIGN

A single-blind, 2-factor (4 treatments by 8 time points) repeated-measures study design.

OBJECTIVE

To analytically determine dexamethasone and dexamethasone phosphate concentrations in plasma derived from proximal effluent venous blood, following cathodic iontophoresis.

METHODS AND MEASURES

Six volunteers received the following dexamethasone phosphate (2.5 ml, 4 mg/ml) treatments to their wrists on separate occasions: cathodic iontophoresis (4 mA, 10 minutes or 4 mA, 20 minutes), passive application (10 or 20 minutes). Plasma samples from the ipsilateral antecubital vein were obtained 10 minutes prior to and half way through the treatment (5 or 10 minutes), at the end of the treatment (10 or 20 minutes), and posttreatment (15, 30, 60, 90, and 120 minutes). The present investigation examined: (1) the sensitivity and linearity of extraction and analysis of dexamethasone and dexamethasone phosphate; (2) the necessity for determining both; and (3) the plasma levels from proximal effluent venous blood following cathodic iontophoresis.

RESULTS

The aggregate (n = 18) of the 6-point standard curves were linear for dexamethasone (r > 0.974) and dexamethasone phosphate (r > 0.829). In vitro dephosphorylation of dexamethasone phosphate to dexamethasone occurred in plasma at 37 degrees C and during freeze-thaw. Measurable dexamethasone or dexamethasone phosphate concentrations were absent at all time points and under all conditions in the human subjects.

CONCLUSIONS

These results demonstrate the sensitivity of the current assay and the need for evaluating both forms of the drug, as in vitro dephosphorylation results in the presence of dexamethasone and dexamethasone phosphate in samples. Absence of measurable dexamethasone or dexamethasone phosphate in the proximal effluent venous blood may require re-evaluation of the extent of drug delivery during the clinical iontophoresis of dexamethasone phosphate.

Potential and problems of developing transdermal patches for veterinary applications

A new frontier in the administration of therapeutic drugs to veterinary species is transdermal drug delivery. The primary challenge in developing these systems is rooted in the wide differences in skin structure and function seen in species ranging from cats to cows. The efficacy of a transdermal system is primarily dependent upon the barrier properties of the targeted species skin, as well as the ratio of the area of the transdermal patch to the species total body mass needed to achieve effective systemic drug concentrations. A drug must have sufficient lipid solubility to traverse the epidermal barrier to be considered for delivery for this route. A number of insecticides have been developed in liquid "pour-on" formulations that illustrate the efficacy of this route of administration for veterinary species. The human transdermal fentanyl patch has been successfully used in cats and dogs for post-operative analgesia. The future development of transdermal drug delivery systems for veterinary species will be drug and species specific. With efficient experimental designs and available transdermal patch technology, there are no obvious hurdles to the development of effective systems in many veterinary species.

Diffusion modeling of percutaneous absorption kinetics. 1. Effects of flow rate, receptor sampling rate, and viable epidermal resistance for a constant donor concentration

A diffusion model for the percutaneous absorption of a solute through the skin is developed for the specific case of a constant donor concentration with a finite removal rate from the receptor due to either perfusion rate or sampling. The model has been developed to include a viable epidermal resistance and a donor-stratum corneum interfacial resistance. Numerical inversion of the Laplace domain solutions were used for simulations of solute flux and cumulative amount absorbed and to model specific examples of percutaneous absorption. Limits of the Laplace domain solutions were used to define the steady-state flux, lag time, and receptor concentration. Steady-state approximations obtained from the solutions were used to relate the steady-state flux and the effective permeability coefficient to the viable epidermis resistance, a donor-stratum corneum interfacial resistance, receptor removal rate, and partitioning between the receptor and donor phases. The lag time was shown to be dependent on these parameters and on the volume of the receptor phase. It is concluded that curvilinear cumulative amount and flux-time profiles are dependent on the processes affecting percutaneous absorption, the shapes of the profiles reflecting the processes most determining transport.

Lateral iontophoretic solute transport in skin

PURPOSE

The lateral iontophoretic transport of three solutes (sodium, ethanolamine, lidocaine) from an active electrode through skin and other tissues to an indifferent electrodes was investigated.

METHODS

Anodal epidermal iontophoresis was carried out on an in vivo rat model using constant direct current of 0.38 mA/cm2. Cells were fixed on the epidermis of anesthetized rats at distances of adjacent, 3 cm and 7 cm apart. After iontophoresis, tissues were dissected at I cm intervals between the electrodes. Concentrations of the radiolabelled solutes in tissues were determined by liquid scintillation counting or gamma counting.

RESULTS

The concentration of each solutes in the epidermis, dermis and other tissues was found to decrease in an exponential manner with lateral distance from the active electrode to the indifferent electrode. The detectable lateral distance for ethanolamine and lidocaine was less than 2 cm from the donor sites, at which distance the concentrations were not significantly different to those found in the corresponding contralateral site. The lateral drift velocities for all solutes in the epidermis and dermis were consistent with diffusivities of the order of 10(-6) cm2/s. The drift velocity of sodium was greater than either lidocaine or ethanolamine.

CONCLUSIONS

The decline in solute concentration with lateral distance is mainly due to clearance from the site of application by the skin's microcirculation and decreases with distance from the active electrode until a baseline concentration, similar to the contralateral tissue concentration is reached.

Percutaneous penetration enhancement and its quantification

True penetration enhancing effects resulting from structural alterations of the barrier stratum corneum manifest themselves in an increase of the drug diffusion coefficient DB and/or of the drug solubility in the barrier csB. The quantification of enhancing effects on drug penetration is possible either by the direct determination of the drug fluxes or by an indirect determination through the measurement of the pharmacodynamic response. In both cases the thermodynamic drug activity has to be considered. In the case of pharmacodynamic measurements, enhancing effects may be determined from the horizontal distance of activity-response lines obtained without and with enhancer, respectively, i.e. the quotient of the drug concentrations that induce the same effect. The activity-standardized bioavailability factors fa obtained from the horizontal distances correspond to the enhancer-induced relative changes in the permeabilities PB, or more exactly in the product DB X csB. On the other hand, the vertical distance between the activity-response lines, i.e. the differences in the drug response after application of preparations with equal (even maximum) thermodynamic drug activities may be used to quantify penetration enhancing effects.

Electrically-assisted transdermal drug delivery

Electrically-assisted transdermal delivery (EATDD) is the facilitated transport of compounds across the skin using an electromotive force. It has been extensively explored as a potential means for delivering peptides and other hydrophilic, acid-labile or orally unstable products of biotechnology. The predominant mechanism for delivery is iontophoresis, although electroosmosis and electroporation have also been investigated. The focus of this review is to put these different mechanisms in perspective and relate them to the drug and skin model system being investigated.

Ultrastructural characterization of sulfur mustard-induced vesication in isolated perfused porcine skin

The isolated perfused porcine skin flap (IPPSF) is a novel alternative, humane in vitro model consisting of a viable epidermis and dermis with a functional microvasculature. For this study, 200 microliters of either 10.0, 5.0, 2.5, 1.25, 0.50, or 0.20 mg/ml of bis (2-chloroethyl) sulfide (HD) in ethanol or ethanol control was topically applied to a 5.0 cm2 dosing area of the IPPSF and perfused for 8 h with recirculating media. HD dermatotoxicity was assessed in the flap by cumulative glucose utilization (CGU), vascular resistance (VR), light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). HD produced a statistically significant dose relationship for gross blisters and microvesicles. The HD-treated IPPSFs were also characterized by a decrease in CGU and an increase in VR. Light microscopic changes included mild intracellular and slight intracellular epidermal edema, multifocal epidermal-dermal separation, and dark basal cells. Ultrastructural alterations consisted of cytoplasmic vacuoles, pyknotic basal cells, nucleolar segregation, and epidermal-dermal separation occurring between the lamina lucida and lamina densa of the basement membrane. The severity of these changes increased in a dose-dependent manner. Morphologically, the IPPSF appeared similar to human skin exposed to HD with the formation of macroscopic blisters and microscopic vesicles. In conclusion, the IPPSF appears to be an appropriate in vitro model with which to study the pathogenesis of vesicant-induced toxicity.

Anionic gels as vehicles for electrically-modulated drug delivery. I. Solvent and drug transport phenomena

PURPOSE

The purpose of this study was to elucidate the in vitro behavior of anionic gels as formulation matrices for electrically-modulated drug delivery. Agarose and combinations of agarose with other anionic polymers (carbomer 934P; xanthan gum) were selected and tested to evaluate their potential for drug delivery.

METHODS

Electrical current was applied by an automatic crossover power supply to minimize the current fluctuation. Hydrocortisone was selected as the model drug in order to minimize electrostatic interference with drug transport. Syneresis and drug migration were evaluated as a function of current application time and the intensity of electrical current.

RESULTS

The data show that electrical current strength and gellant content can affect both the syneresis and drug migration. A linear correlation was found between hydrocortisone loss and mass loss via the exudate. Moreover, in agarose-carbomer 934P gel systems, cumulative gel mass loss is a linear function of time at low intensities of electrical current (e.g., 0.5 mA and 1 mA). However, hydrocortisone distribution, after electrical application, is relatively asymmetric in those agarose-carbomer 934P gels (and in agarose-xanthan gum gels) in contrast to gel matrices containing only agarose.

CONCLUSIONS

In this study, the use of carbomer 934P in conjunction with agarose enables the formulator to achieve zero-order release with electrical application. Increased anisotropicity of a gel system due to the application of electrical current could alter the effectiveness of a drug delivery system.

Transdermal iontophoresis and solute penetration across excised human skin

The potential of transdermal iontophoresis to facilitate drug delivery was evaluated by studying the transport kinetics of model compounds salicylate (anion), phenylethylamine (cation), mannitol (polar neutral compound of low molecular weight), and inulin (polar neutral compound of high molecular weight) using an excised human skin model. The transport kinetics of solutes were determined across both intact and cellophane-tape-stripped dermatomed skin both in the presence and absence of applied current to probe the mechanisms of iontophoretic delivery. Iontophoresis effectively enhanced delivery of all compounds relative to passive transport. The skin is shown to be both ion selective and size selective. On the basis of results from present work and other studies, the "aqueous pathway" of iontophoretic transport is further reinforced.

Importance of dermal blood supply and epidermis on the transdermal iontophoretic delivery of monovalent cations

The effect of dermal blood flow on the transdermal iontophoresis of five monovalent cationic solutes has been investigated using an in vivo rat model. The iontophoretic flux of solutes from topical application sites was shown, using anesthetized and sacrificed rats, to be independent of the dermal blood supply. The presence of a viable blood supply significantly affected the extent of penetration of solutes into deeper underlying tissues during iontophoresis. Higher tissue concentrations of solutes were found in the upper tissue layers of sacrificed rats, with no blood supply, compared to those in anesthetized rats. In all animals the highest concentration of solute after 2 h iontophoresis was found in the epidermis. Iontophoretic application of solutes to skin with the epidermis removed resulted in significantly lower fluxes than from sites where the skin was intact. These findings suggest that local blood flow determines systemic and underlying tissue solute absorption but not epidermal penetration fluxes during iontophoretic delivery. Finally, the dependence of iontophoretic transport of a solute on the solute's size in vivo was similar to relationships previously reported for excised human skin studies.

A biophysically based dermatopharmacokinetic compartment model for quantifying percutaneous penetration and absorption of topically applied agents. I. Theory

We present a general comprehensive mathematical model to stimulate and predict percutaneous absorption and subsequent disposition of chemicals in vivo that is chiefly based on biophysical parameters estimated or measured with in vitro and ex vivo perfused skin preparations. Current physicochemical principles of drug diffusion and partitioning across the skin barrier, solute and solvent concentration dynamics, the influence of solute and solvent on the stratum corneum barrier, and dynamic vascular perfusion effects are integrated in this model. Such a comprehensive approach is necessary to achieve optimal biological relevance in a quantitative model of percutaneous absorption, particularly when a chemical is applied as a binary (solute and solvent) or more complex formulation or chemical mixture. The proposed model should have applications in (a) designing drugs and permeation enhancers for passive or active (e.g., electrically assisted) transdermal drug delivery, (b) assessing the systemic exposure of topical drugs used in dermatology, and (c) integration into other mathematical models being developed to assess the risk after topical exposure to mixtures of environmental pollutants. We also have included experimental data to provide a preliminary illustration of the performance of the model.

Detection of sulfur mustard bis (2-chloroethyl) sulfide and metabolites after topical application in the isolated perfused porcine skin flap

The purpose of this study was to develop an assay to study the flux of sulfur mustard (HD) through the skin and determine if metabolites are formed due to the epidermal metabolism of HD after topical exposure of the isolated perfused porcine skin flap (IPPSF) to 14C-HD. Four IPPSFs were topically dosed with 2.85 mg of 14C-HD in ethanol. Venous perfusate samples were collected and added to a 34% solution of NaCl and snap-frozen to inhibit the metabolism of HD until time for assay. Perfusate samples were extracted using a solid-phase extraction cartridge with ethyl acetate and then assayed using gas chromatography. Two of the 4 IPPSFs showed detectable levels of HD in the venous perfusate 15 min after dosing, with 1 of these 2 IPPSFs showing detectable levels of HD in the perfusate 2 hours after dosing. All 4 IPPSFS had no more than 3 metabolites of HD appearing in the perfusate throughout the 2 hr experiment, with one of the these metabolites identified as thiodiglycol. These experiments showed that little, if any, HD appears in the venous perfusate intact after percutaneous absorption and that epidermal metabolism of HD does occur to a significant degree in the IPPSF.

Pharmacologic modulation of the cutaneous vasculature in the isolated perfused porcine skin flap

The isolated perfused porcine skin flap (IPPSF), an ex vivo model system used in cutaneous toxicology and pharmacology, is capable of assessing the percutaneous absorption of vasoactive compounds. However, the vascular responses of the IPPSF to classical pharmacologic agents have not been calibrated. The ability of acetylcholine, nitroglycerin, tolazoline, and norepinephrine to affect vasculature resistance and glucose utilization was investigated in the IPPSF. Norepinephrine infusions between 10(-7) and 10(-5) M increased vascular resistance in a dose-dependent manner; half-maximal (EC50) and maximal responses occurred at 3.18 x 10(-6) and 10(-5) M, respectively. In non-preconstricted flaps, neither acetylcholine, nitroglycerin, nor tolazoline vasodilated the IPPSF; however, acetylcholine, nitroglycerin, and tolazoline each lowered vascular resistance in a dose-dependent manner in norepinephrine-preconstricted flaps. Maximal relaxation was induced at 10(-4), 10(-6), and 5 x 10(-5) M, by tolazoline, acetylcholine, and nitroglycerin, respectively, whereas the EC50 values were 2.88 x 10(-7), 1.35 x 10(-8), and 1.72 x 10(-7) M, respectively. In flaps pretreated with norepinephrine and methylene blue (a potential blocker of edothelium-derived relaxing factor), no concentration of acetylcholine, and only the highest concentration of nitroglycerin, lowered vascular resistance. In non-preconstricted flaps, glucose utilization decreased in norepinephrine-infused flaps, increased in nitroglycerin- and tolazoline-infused flaps, and was biphasic in acetylcholine-infused flaps. These results indicate that the IPPSF responds to pharmacologic agents in a manner similar to classic in vitro and in vivo models. Thus, the IPPSF would be a relevant model for investigating the delivery and/or toxicity of pharmacologically active compounds.

Model describing transdermal iontophoretic delivery of lidocaine incorporating consideration of cutaneous microvascular state

A three-compartment pharmacokinetic model describing percutaneous absorption of iontophoretically driven topically applied lidocaine in the isolated perfused porcine skin flap is presented. Delivery from the active (drug-dosed) electrode to skin is estimated as a ramp input profile. Model parameters were estimated separately for dosing (4 h current-on) and washout (4 h current-off) periods in experiments with coadministered vasoactive drugs [tolazoline (vasodilator) and norepinephrine (vasoconstrictor)] and controls (lidocaine alone). The model presented was able to predict 8-h lidocaine absorptions and compartmental mass profiles for each of the three treatments, was able to document vascular effects of co-iontophoresed vasoactive compounds, and gives insight into the factors that modulate cutaneous disposition of iontophoretically delivered lidocaine in a biologically relevant model approximating in vivo delivery.

Topical penetration of piroxicam is dependent on the distribution of the local cutaneous vasculature

The mechanism of the topical delivery of piroxicam, a nonsteroidal antiinflammatory drug, has been controversial as to whether systemic absorption is required for topical efficacy. This study, using in vivo pigs treated with topical 3H-piroxicam gel, was designed to assess the role of systemic absorption on its delivery to deep tissues. Further, the role of the structure of the cutaneous vasculature (e.g., direct cutaneous or musculocutaneous) was studied. Finally, piroxicam delivery was measured using in vitro diffusion cells with pig skin obtained from the same sites to determine inherent permeability independent of vascular anatomy. These studies showed that penetration of the radiolabel occurred in subcutaneous and muscle tissue only under the dosed sites and not at the remote sites, ruling out systemic absorption as a prerequisite for local delivery. Tissue penetration in vivo was enhanced at the musculocutaneous compared to the direct cutaneous sites. In contrast, in vitro flux was identical in skin harvested from the two vascular sites, suggesting that the vasculature plays a pivotal role in deep tissue penetration of piroxicam. In conclusion, local delivery of topical drugs occurs independent of systemic absorption and the nature of the cutaneous vasculature at different sites must be taken into consideration for optimal delivery.

Transdermal iontophoretic peptide delivery: in vitro and in vivo studies with luteinizing hormone releasing hormone

Protein and peptide drugs are not orally active. Their large molecular size and charged character make them poor candidates for passive transdermal delivery. With an applied electromotive force, these drugs can be forced through the skin to be absorbed by the systemic circulation. The present study investigates the transdermal iontophoretic delivery of a peptide hormone in an in vitro model system, the isolated perfused porcine skin flap, as well as in vivo. It is shown that with knowledge of the systemic disposition of the drug, transdermal fluxes can be utilized to accurately predict in vivo serum concentrations. It is also shown that the iontophoretically delivered hormone retains both its immunologic and biologic activity.

Characterization of lewisite toxicity in isolated perfused skin

Lewisite (L) is a potent organic arsenical that causes rapid onset of pain and severe vesication on contact with epithelial tissues. The isolated perfused porcine skin flap (IPPSF) is an in vitro model that has shown potential as a model for cutaneous vesicant research. The objective of this study was to characterize IPPSF responses after topical exposure to six concentrations of L ranging from 0.07 to 5.0 mg/ml (n = 4/treatment plus controls). Biochemical markers of viability (glucose utilization (CGU) and lactate dehydrogenase (LDH) release), vascular resistance (VR), venous arsenic flux, and morphological parameters (light and electron microscopy) were evaluated. In addition, lewisite lesions were characterized at 1, 3, 5, and 8 hr after exposure (n = 4/time plus controls) using these morphological parameters, as well as enzyme histochemistry. Macroscopic and microscopic lesions caused by L exposure were dose related. Mild decreases in CGU were noted with the higher concentrations of L, while generally increased responses in LDH release and VR were seen. Marked increases in LDH activity were noted in the blister fluid of IPPSFs treated with 5.0 mg/ml of L. Also, significant cutaneous arsenic flux was noted at the 5.0 mg/ml dose of L. The formation of gross blisters, the location and characterization of epidermal-dermal junction separation, and the time course of lesion production paralleled the description of L-induced lesions in humans. The sensitivity of the IPPSF to L exposure and the similarity of lesions to those described for humans suggests that this model provides a relevant in vitro model with which to study mechanisms of chemical vesication and arsenic toxicity, as well as protective and therapeutic intervention for vesicant exposure.

Determination of lidocaine concentrations in skin after transdermal iontophoresis: effects of vasoactive drugs

The purpose of this study was to investigate the effect of vasoactive drugs on transdermal lidocaine inotophoresis by measuring the concentrations of radiolabeled lidocaine which has penetrated the skin. Previous studies had demonstrated that coinotophoresis of vasoactive drugs could modulate the transcutaneous flux of lidocaine and suggested that a dermal depot of lidocaine was involved. To address this, lidocaine hydrochloride (14C) was iontophoresed in vivo in anesthetized weanling pigs either alone or with the vasodilator tolazoline or the vasoconstrictor norepinephrine. Tissue cores under the active electrode were then collected, quick-frozen, and sectioned on a cryostat, and then the radioactivity was determined in each 40-microns section. Coiontophoresis with norepinephrine resulted in increased concentrations of lidocaine in skin up to a depth of 3 mm. These concentrations decreased to lidocaine-alone levels after a 4-hr washout. Tolazoline decreased tissue concentrations of lidocaine. Concentrations were intermediate when lidocaine alone was administered. These studies support the hypothesis that coiontophoresis of vasoactive drugs modulates the transdermal delivery of lidocaine, in part by altering the cutaneous "depot."