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.

Cooling a nanomechanical resonator with quantum back-action

Quantum mechanics demands that the act of measurement must affect the measured object. When a linear amplifier is used to continuously monitor the position of an object, the Heisenberg uncertainty relationship requires that the object be driven by force impulses, called back-action. Here we measure the back-action of a superconducting single-electron transistor (SSET) on a radio-frequency nanomechanical resonator. The conductance of the SSET, which is capacitively coupled to the resonator, provides a sensitive probe of the latter's position; back-action effects manifest themselves as an effective thermal bath, the properties of which depend sensitively on SSET bias conditions. Surprisingly, when the SSET is biased near a transport resonance, we observe cooling of the nanomechanical mode from 550 mK to 300 mK--an effect that is analogous to laser cooling in atomic physics. Our measurements have implications for nanomechanical readout of quantum information devices and the limits of ultrasensitive force microscopy (such as single-nuclear-spin magnetic resonance force microscopy). Furthermore, we anticipate the use of these back-action effects to prepare ultracold and quantum states of mechanical structures, which would not be accessible with existing technology.

Skin penetration and distribution of polymeric nanoparticles

Encapsulation using nanoparticulate systems is an increasingly implemented strategy in drug targeting and delivery. Such systems have also been proposed for topical administration to enhance percutaneous transport into and across the skin barrier. However, the mechanism by which such particulate formulations facilitate skin transport remains ambiguous. In this study, confocal laser scanning microscopy (CLSM) was used to visualize the distribution of non-biodegradable, fluorescent, polystyrene nanoparticles (diameters 20 and 200 nm) across porcine skin. The surface images revealed that (i) polystyrene nanoparticles accumulated preferentially in the follicular openings, (ii) this distribution increased in a time-dependent manner, and (iii) the follicular localization was favoured by the smaller particle size. Apart from follicular uptake, localization of nanoparticles in skin "furrows" was apparent from the surface images. However, cross-sectional images revealed that these non-follicular structures did not offer an alternative penetration pathway for the polymer vectors, whose transport was clearly impeded by the stratum corneum.

Passive skin penetration enhancement and its quantification in vitro

The poor penetration of drugs into the skin (and, partially, the permeation across the stratum corneum) often limits the efficacy of topical formulations. Basically, skin penetration can be enhanced by the following strategies: (i) increasing drug diffusivity in the skin; (ii) increasing drug solubility in the skin, and/or (iii) increasing the degree of saturation of the drug in the formulation. In this article, we review the literature with respect to: (i) chemical penetration enhancers, which have been shown to influence the diffusivity and/or solubility of the drug in the skin and (ii) supersaturated formulations, in which the degree of saturation of the drug is increased compared to conventional formulations. In addition, three different in vitro methods, specifically, classic diffusion cell studies, attenuated total-reflectance-Fourier transform infrared spectroscopy, and tape stripping in conjunction with an appropriate analytical technique, are considered, emphasizing their application to obtain quantitative values for skin transport parameters and to separate the kinetic or thermodynamic effects of an enhancement strategy.

Visualization of skin penetration using confocal laser scanning microscopy

The use of skin as an alternative route for administering systemically active drugs has attracted considerable interest in recent years. However, the skin provides an excellent barrier, which limits the number of drug molecules suitable for transdermal delivery. Thus, in order to improve cutaneous delivery, it is necessary to adopt an enhancement method, either (i) passively using novel formulations, e.g. microemulsions, liposomes, and colloidal polymeric suspensions, or more conventional skin permeation enhancers, or (ii) with a physical approach, such as, iontophoresis, sonophoresis or electroporation. Although there has been much progress, the precise modes of action of the different techniques used are far from well-understood. The objective of this review, therefore, is to evaluate how confocal laser scanning microscopy may contribute to the determination of the mechanisms of diverse skin penetration enhancement strategies.

In vivo methods for the assessment of topical drug bioavailability

This paper reviews some current methods for the in vivo assessment of local cutaneous bioavailability in humans after topical drug application. After an introduction discussing the importance of local drug bioavailability assessment and the limitations of model-based predictions, the focus turns to the relevance of experimental studies. The available techniques are then reviewed in detail, with particular emphasis on the tape stripping and microdialysis methodologies. Other less developed techniques, including the skin biopsy, suction blister, follicle removal and confocal Raman spectroscopy techniques are also described.

Enhancement of Topical Delivery from Biodegradable Nanoparticles

PURPOSE

To determine whether and how encapsulation of lipophilic compounds in polymeric nanoparticles is able to improve topical delivery to the skin.

METHODS

The penetration of octyl methoxycinnamate (OMC; Parsol MCX), a highly lipophilic sunscreen, into and across porcine ear skin in vitro was investigated, subsequent to encapsulation in poly(epsilon-caprolactone) nanoparticles, using tape-stripping. Confocal laser scanning microscopy (CLSM) was used to visualize the distribution of nanoparticles, charged with Nile red (NR), a lipophilic and fluorescent dye.

RESULTS

Quantification of OMC in the skin using tape-stripping demonstrated that nanoparticulate encapsulation produced a 3.4-fold increase in the level of OMC within the stratum corneum (SC), although the use of nanoparticles did not appear to increase skin permeation (it was not possible to detect OMC in the receiver compartment after 6 h). The confocal images showed that the fluorescence profile observed in the skin after application of NR-containing nanoparticles was clearly different from that seen following application of NR dissolved in propylene glycol. Two hours postapplication of NR-containing nanoparticles, fluorescence was perceptible at greater depths (up to 60 microm) within the skin.

CONCLUSIONS

i) Nanoparticulate encapsulation of OMC increased its "availability" with the SC. ii) The altered distribution of NR when delivered via nanoparticles was due, at least in part, to its altered thermodynamic activity (relative to that in propylene glycol) and, as a result, an increase in its partition coefficient into the SC.

Pig ear skin ex vivo as a model for in vivo dermatopharmacokinetic studies in man

OBJECTIVE

The objective was to investigate pig ear skin as a surrogate for human skin in the assessment of topical drug bioavailability by sequential tape-stripping of the stratum corneum (SC). The potential benefits of ex vivo investigations are manifold: ethical approval is not required, multiple replicate experiments are more easily performed, and toxic compounds can be evaluated.

MATERIALS AND METHODS

Ex vivo experiments on isolated pig ears were compared with in vivo studies in human volunteers. Four formulations, comprising the model drug, ibuprofen, in different propylene glycol (PG)-water mixtures (25:75, 50:50, 75:25 and 100:0), were compared.

RESULTS

Derived dermatopharmacokinetic parameters characterizing the diffusion and partitioning of the drug in the SC ex vivo were consistent with those in vivo following a 30-minute application period. Further, the non-steady-state ex vivo results could be used to predict the in vivo concentration profile of the drug across the SC when a formulation was administered for 3 h (i.e., close to steady-state).

CONCLUSIONS

Taken together, the results obtained suggest that pig ear skin ex vivo has promise as a tool for topical formulation evaluation and optimization.

Computer-aided design (CAD) of Mn(II) complexes: superoxide dismutase mimetics with catalytic activity exceeding the native enzyme

New Mn(II) macrocyclic pentaamine complexes derived from the biscyclohexyl-pyridine complex, M40403 ([manganese(II)dichloro[(4R,9R,14R,19R)-3,10,13,20,26-pentaazatetracyclo[20.3.1.0.(4,9)0(14,19)]hexacosa-1(26),-22(23),24-triene]]), are described here. The complex M40403 was previously shown to be a superoxide dismutase (SOD) catalyst with rates for the catalytic dismutation of superoxide to oxygen and hydrogen peroxide at pH = 7.4 of 1.2 x 10(+7) M(-1) s(-1).(1) The use of the computer-aided design paradigm reported previously for this class of Mn(II) complexes(2,3) led to the prediction that the 2S,21S-dimethyl derivative of M40403 should possess superior catalytic SOD activity. The synthesis of this new macrocyclic Mn(II) complex, [manganese(II)dichloro[2S, 21S-dimethyl-(4R,9R,14R,19R)-3,10,13,20,26-pentaazatetracyclo[20.3.1.0.(4,9)0(14,19)]hexacosa-1(26),22(23),24-triene]], 5, was accomplished via a high yield template condensation utilizing the linear tetraamine, N,N'-Bis[(1R,2R)-[2-(amino)]cyclohexyl]-1,2-diaminoethane, 1, 2,6-diacetylpyridine, and MnCl(2) to form the macrocyclic diimine complex, 2, which then is reduced. The two other possible dimethyl diastereomers of 5 (2R,21R-dimethyl,3, and 2R,21S-dimethyl, 6) were also prepared via reduction of the diimine complex 2. Two of these complexes, 3 and 5, were characterized by X-ray structure determination confirming their absolute stereochemistry as 2R,21R-dimethyl and 2S,21S-dimethyl, respectively. The results of the MM calculations which predict that the 2S,21S-dimethyl complex, 5, should be a high activity catalyst and that the 2R,21R-dimethyl complex, 3, should have little or no catalytic activity are presented. The catalytic SOD rates for these complexes are reported for each of these complexes and a correlation with the modeling predictions is established showing that 2R,21R-complex, 3, has no measurable catalytic rate, while the 2R,21S complex, 6, is identical to M40403, and the 2S,21S- complex, 5, possesses a very fast rate at pH = 7.4 of 1.6 x 10(+9) M(-1) s(-1) exceeding that of the native mitochondrial MnSOD enzymes.

In vivo assessment of enhanced topical delivery of terbinafine to human stratum corneum

PURPOSE

The objective of this study was to evaluate, using attenuated total reflectance Fourier transform infrared spectroscopy, the stratum corneum (SC) bioavailability of terbinafine (TBF) following topical treatment with four different formulations.

METHODS

Four skin sites on the ventral forearms of five healthy volunteers were treated for 2 h using one of four formulations based on a vehicle consisting of 50% ethanol and 50% isopropyl myristate. Three of these formulations included a percutaneous penetration enhancer: either 5% oleic acid, 10% 2-pyrrolidone or 1% urea. The SC concentration profile of TBF was measured by repeated infrared spectroscopic measurements while sequentially stripping off the layers of this barrier membrane with adhesive tape. This method was validated by HPLC analysis of TBF extracted from the stripped tapes. Transepidermal water loss (TEWL) measurements were also performed, to permit facile estimation of SC thickness.

RESULTS

The SC concentration profiles of TBF were fitted to the appropriate solution of Fick's second law of diffusion, thereby allowing determination of the characteristic diffusion and partitioning parameters of the permeating drug. This analysis enabled the efficacies of the different formulations tested to be compared to the no-enhancer control. While it was found that the formulation containing 5% oleic acid significantly enhanced the SC availability of TBF, the other formulations did not improve the apparent drug delivery.

CONCLUSIONS

A facile and minimally invasive methodology to evaluate an important aspect of topical drug bioavailability has been described. The analytical methods used (infrared spectroscopy and HPLC) allow estimates of both relative and absolute drug bioavailability in the SC and may be useful, therefore, in the critical determination of bioequivalence between topical formulations.

Effect of ethanol and isopropyl myristate on the availability of topical terbinafine in human stratum corneum, in vivo

PURPOSE

The objective of this study was to determine the availability of the topical drug terbinafine (TBF) in human stratum corneum (SC) in vivo following its administration in formulations containing isopropyl myristate and ethanol.

METHODS

The ventral forearms of human volunteers were treated for 4 h with TBF, at a concentration equal to 1/4 saturation, in isopropyl myristate (IPM), in ethanol (EtOH) and in 50:50 v/v IPM/EtOH. At the end of the application period, the treated sites were carefully cleaned of excess vehicle and the SC was progressively removed by sequential tape stripping. TBF was quantified in the SC by: (a) extraction of the tape strips and subsequent HPLC analysis; and (b) attenuated total reflectance infrared spectroscopy (ATR-FTIR) of each sequentially exposed SC surface during the tape stripping procedure.

RESULTS

The concentration profile of TBF in the SC (i.e. drug concentration as a function of depth in the membrane) was fitted to the appropriate solution of Fick's second law of diffusion, allowing thereby the drug's SC/vehicle partition coefficient (K) and characteristic diffusion parameter (D/L(2), where D is the diffusivity of TBF in the SC of thickness L) to be deduced.

CONCLUSIONS

While D/L(2) for TBF derived from the three vehicles remained essentially constant, the drug's partitioning into the SC was significantly higher from formulations containing ethanol. Both the semi-quantitative infrared data and the more rigorous HPLC results supported these deductions.

Ibuprofen transport into and through skin from topical formulations: in vitro-in vivo comparison

The goal was to compare ibuprofen transport into and through skin in vivo in man and in vitro (across silicone membranes and freshly excised pig skin) from four marketed formulations. Ibuprofen gels were administered in vivo for 30 minutes. The stratum corneum (SC) at the application site was then tape-stripped, quantified gravimetrically, and extracted for drug analysis. Together with concomitant transepidermal water loss measurements, SC drug concentration-depth profiles were reproducibly determined and fitted mathematically to obtain a partition coefficient, a first-order rate constant related to ibuprofen diffusivity, and the total drug amount in the SC at the end of the application. All derived parameters were consistent across formulations. Ibuprofen permeation data through both silicone membrane and pig ear skin were also fitted to yield partitioning and diffusion parameters. The former revealed that ibuprofen partitioned differently from the gels into this model barrier. Across pig skin, however, better correlation with in vivo results was found. The dermatopharmacokinetic approach, using SC tape-stripping, offers a valid method to assess equivalency between topical drug formulations. In vitro experiments must be extrapolated cautiously to the clinic, especially when complex interactions between real formulations, which deliver both drug and excipients, and the skin occur.

Skin permeability enhancement by low frequency sonophoresis: lipid extraction and transport pathways

The objective of this study was to shed light on the mechanism(s) by which low-frequency ultrasound (20 KHz) enhances the permeability of the skin. The physical effects on the barrier and the transport pathway, in particular, were examined. The amount of lipid removed from the intercellular domains of the stratum corneum following sonophoresis was determined by infrared spectroscopy. Transport of the fluorescent probes nile red and calcein, under the influence of ultrasound, was evaluated by laser-scanning confocal microscopy. The results were compared with the appropriate passive control data and with data obtained from experiments in which the skin was exposed simply to the thermal effects induced by ultrasound treatment. A significant fraction ( approximately 30%) of the intercellular lipids of the stratum corneum, which are principally responsible for skin barrier function, were removed during the application of low-frequency sonophoresis. Although the confocal images from the nile red experiments were not particularly informative, ultrasound clearly and significantly (again, relative to the corresponding controls) facilitated transport of the hydrophilic calcein via discrete permeabilized regions, whereas other areas of the barrier were apparently unaffected. Lipid removal from the stratum corneum is implicated as a factor contributing the observed permeation enhancement effects of low-frequency ultrasound. However, microscopic observations imply that sonophoresis induces localized (aqueous?) permeation pathways at discrete sites.

Emerging strategies for the transdermal delivery of peptide and protein drugs

Transdermal delivery has been at the forefront of research addressing the development of non-invasive methods for the systemic administration of peptide and protein therapeutics generated by the biotechnology revolution. Numerous approaches have been suggested for overcoming the skin's formidable barrier function; whereas certain strategies simply act on the drug formulation or transiently increase the skin permeability, others are designed to bypass or even remove the outermost skin layer. This article reviews the technologies currently under investigation, ranging from those in their early-stage of development, such as laser-assisted delivery to others, where feasibility has already been demonstrated, such as microneedle systems, and finally more mature techniques that have already led to commercialisation (e.g., velocity-based technologies). The principles, mechanisms involved, potential applications, limitations and safety considerations are discussed for each approach, and the most advanced devices in each field are described.

Effects of sucrose oleate and sucrose laureate on in vivo human stratum corneum permeability

PURPOSE

The purpose of this work was to 1) investigate the effect of sucrose esters (sucrose oleate and sucrose laureate in water or in Transcutol, TC) on the stratum corneum (SC) barrier properties in vivo and 2) examine the impact of these surfactant-like molecules on the in vivo percutaneous penetration of a model penetrant 4-hydroxybenzonitrile (4-HB).

METHODS

Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and transepidermal water loss measurements were used to evaluate the sucrose oleate- and sucrose laureate-induced biophysical changes in SC barrier function in vivo. In addition. the effect of the enhancers on 4-HB penetration was monitored in vivo using ATR-FTIR spectroscopy in conjunction with tape-stripping of the treated site.

RESULTS

Treatment of the skin with 2% sucrose laureate or sucrose oleate in TC significantly increased the extent of 4-HB penetration relative to the control. Furthermore, when skin treated with these formulations was examined spectroscopically, the C-H asymmetric and symmetric stretching bands of the lipid methylene groups were characterized by 1) decreased absorbances and 2) frequency shifts to higher wavenumbers. These effects on the SC lipids and 4-HB penetration were more pronounced for sucrose laureate when combined with TC.

CONCLUSIONS

A combination of sucrose esters (oleate or laureate) and TC is able to temporally alter the stratum corneum barrier properties, thereby promoting 4-HB penetration. These molecules are worthy of further investigation as potential candidates for inclusion in transdermal formulations as penetration enhancers.

Dermatopharmacokinetic prediction of topical drug bioavailability in vivo

The overall goal of this study was to explore the potential of using stratum corneum (SC) tape-stripping, post-application of a topical drug formulation, to derive dermatopharmacokinetic parameters describing the rate and extent of delivery into the skin. Ibuprofen was administered in 75:25 v/v propylene glycol-water to the ventral forearms of human volunteers for periods ranging between 15 and 180 minutes. Subsequently, SC was tape-stripped, quantified gravimetrically, and extracted for drug analysis. Together with concomitant transepidermal water loss measurements, SC concentration-depth profiles of the drug were reproducibly determined and fitted mathematically. The SC-vehicle partition coefficient (K) and a first-order rate constant related to ibuprofen diffusivity in the membrane (D/L2, where L=SC thickness) were derived from data-fitting and characterized the extent and rate of drug absorption across the skin. Integration of the concentration profiles yielded the total drug amount in the SC at the end of the application period. Using K and D/L2 obtained from the 30-minute exposure, it was possible to predict ibuprofen uptake as a function of time into the SC. Prediction and experiment agreed satisfactorily suggesting that objective and quantitative information, with which to characterize topical drug bioavailability, can be obtained from this approach.

Validation of reflectance infrared spectroscopy as a quantitative method to measure percutaneous absorption in vivo

Attenuated total-reflectance infrared (ATR-IR) spectroscopy has been used to follow the penetration of a model compound (4-cyanophenol; CP) across human stratum corneum (SC) in vivo, in man. CP was administered for periods of 1, 2, or 3 hr, either (a) as a 10% (w/v) solution in propylene glycol or (b) in an identical vehicle which also contained 5% (v/v) oleic (cis-9-octadecenoic) acid. At the end of the treatment periods, SC at the application site was progressively removed by adhesive tape-stripping. Prior to the removal of the first tape-strip, and after each subsequent tape-strip, an ATR-IR spectrum of the treated site was recorded. The presence of CP, as a function of position in the SC, was monitored spectroscopically via the intense C = N stretching absorbance at 2230 cm-1. The absolute amount of CP, as a function of SC depth, was determined by "spiking" the applied solutions with 14C-labeled compound and subsequent liquid scintillation counting of the removed tape-strips. The presence of oleic acid in the applied formulation significantly increased the rate and extent of CP delivery as evaluated by either spectroscopy or radiochemical analysis. Furthermore, the ATR-IR and direct 14C analysis of CP as a function of SC position were highly correlated. These data strongly support, therefore, the validation of ATR-IR as a quantitative tool to assess percutaneous penetration in vivo.

Effect of propylene glycol on ibuprofen absorption into human skin in vivo

The objective was to assess the impact of propylene glycol (PG), a common cosolvent in topical formulations, on the penetration of ibuprofen into human skin in vivo. Drug uptake into the stratum corneum (SC), following application of saturated formulations containing from 0 to 100% v/v PG, was assessed by tape-stripping. Dermatopharmacokinetic parameters, characterizing drug amount in and diffusivity through the SC, were derived. The solubility behavior of ibuprofen in PG-water mixtures was carefully evaluated, as were a number of other physical properties. Ibuprofen delivery depended on the level of PG in the vehicle, despite all formulations containing the drug at equal thermodynamic activity. PG appeared to alter the solubility of ibuprofen in the SC (presumably via its own uptake into the membrane), the effect becoming more important as the volume fraction of cosolvent in the formulation increased. In summary, tape-stripping experiments, with careful interpretation, can reveal details of a drug's bioavailability in the skin following topical application and may be used to probe the mechanism(s) by which certain excipients influence local drug delivery.

Transdermal Delivery of Cytochrome C—A 12.4 kDa Protein—Across Intact Skin by Constant–Current Iontophoresis

PURPOSE

To demonstrate the transdermal iontophoretic delivery of a small (12.4 kDa) protein across intact skin.

MATERIALS AND METHODS

The iontophoretic transport of Cytochrome c (Cyt c) across porcine ear skin in vitro was investigated and quantified by HPLC. The effect of protein concentration (0.35 and 0.7 mM), current density (0.15, 0.3 or 0.5 mA.cm(-2) applied for 8 h) and competing ions was evaluated. Co-iontophoresis of acetaminophen was employed to quantify the respective contributions of electromigration (EM) and electroosmosis (EO).

RESULTS

The data confirmed the transdermal iontophoretic delivery of intact Cyt c. Electromigration was the principal transport mechanism, accounting for approximately 90% of delivery; correlation between EM flux and electrophoretic mobility was consistent with earlier results using small molecules. Modest EO inhibition was observed at 0.5 mA.cm(-2). Cumulative permeation at 0.3 and 0.5 mA.cm(-2) was significantly greater than that at 0.15 mA.cm(-2); fluxes using 0.35 and 0.7 mM Cyt c in the absence of competing ions (J ( tot ) = 182.8 +/- 56.8 and 265.2 +/- 149.1 microg.cm(-2).h(-1), respectively) were statistically equivalent. Formulation in PBS (pH 8.2) confirmed the impact of competing charge carriers; inclusion of approximately 170 mM Na(+) resulted in a 3.9-fold decrease in total flux.

CONCLUSIONS

Significant amounts ( approximately 0.9 mg.cm(-2) over 8 h) of Cyt c were delivered non-invasively across intact skin by transdermal electrotransport.

Assessment of topical bioavailability in vivo: the importance of stratum corneum thickness

We present a method to determine the cutaneous bioavailability and hence to evaluate the bioequivalence of topically applied drugs in vivo. The procedure uses serial tape-stripping and transepidermal water loss measurements to quantify the thickness of the removed stratum corneum (SC) and to determine the intact membrane thickness. Following tape-stripping, the drug is extracted from the tapes and assayed, e.g., by HPLC. This provides a drug concentration profile as a function of the normalized position within the SC. The data are fitted to a solution of Fick's second law of diffusion in order to calculate characteristic membrane transport parameters. Integration of the concentration profile over the entire SC thickness, that is, the 'area-under-the-curve', provides a measure of the cutaneous bioavailability and hence can be used to assess the bioequivalence of topically applied drugs.

Effect of charge and molecular weight on transdermal peptide delivery by iontophoresis

PURPOSE

The study was conducted to investigate the impact of charge and molecular weight (MW) on the iontophoretic delivery of a series of dipeptides.

METHODS

Constant current iontophoresis of lysine and 10 variously charged lysine- and tyrosine-containing dipeptides was performed in vitro.

RESULTS

Increasing MW was compensated by additional charge; for example, Lys (MW = 147 Da, +1) and H-Lys-Lys-OH (MW = 275 Da, +2) had equivalent steady-state fluxes of 225 +/- 48 and 218 +/- 40 nmol cm(-2) h(-1), respectively. For peptides with similar MW, e.g., H-Tyr-D-Arg-OH (MW = 337 Da, +1) and H-Tyr-D-Arg-NH(2) (MW = 336 Da, +2), the higher valence ion displayed greater flux (150 +/- 26 vs. 237 +/- 35 nmol cm(-2) h(-1)). Hydrolysis of dipeptides with unblocked N-terminal residues, after passage through the stratum corneum, suggested the involvement of aminopeptidases. The iontophoretic flux of zwitterionic dipeptides was less than that of acetaminophen and dependent on pH.

CONCLUSIONS

For the series of dipeptides studied, flux is linearly correlated to the charge/MW ratio. Data for zwitterionic peptides indicate that they do not behave as neutral ("charge-less") molecules, but that their iontophoretic transport is dependent on the relative extents of ionization of the constituent ionizable groups, which may also be affected by neighboring amino acids.

In vivo infrared spectroscopy studies of alkanol effects on human skin

Many studies investigate the permeation of actives through the skin and ignore the role of excipients. The solvents used in formulations will undoubtedly penetrate the skin where they can have a number of effects. They can extract skin lipids, they can alter the fluidity of the lipids and they can alter the polarity of the skin. The degree to which they do this and the depth into the skin where this occurs will depend on the uptake kinetics. The problem is to distinguish the different effects. Using ATR-FTIR and deuterated materials this can be achieved in vivo. The aim of the present study was to study the higher alkanols (hexanol, octanol, decanol) in vivo using a combination of ATR-FTIR spectroscopy and tape stripping. Studies conducted in vivo using deuterated vehicles confirmed the lipid extraction effects of d-hexanol and d-octanol, whereas d-decanol did not change skin lipid content. The uptake of d-decanol was higher than for the other vehicles consistent with previous observations on mouse skin for alkanols of increasing chain length. In general, solvent uptake was proportional to the induced shift in the C-H stretching frequency. Lipid disorder was induced by all vehicles studied in vivo and was proportional to the amount of vehicle present in the skin.

Skin permeation enhancement by sucrose esters: A pH-dependent phenomenon

The purpose of the present study was to evaluate the effect of sucrose esters (particularly, sucrose laureate and sucrose oleate in Transcutol) on the percutaneous penetration of a charged molecule as a function of ionization. We have investigated the influence of these sucrose esters on the in vitro diffusion profiles of lidocaine hydrochloride, a weak ionizable base (pKa=7.9), at different pH values, using porcine ear skin as the barrier membrane. As expected, lidocaine flux in the absence of an enhancer, increased from pH 5 to 9 with a corrresponding increase in the level of the unionized base. However, when skin was pretreated with 2% laureate in Transcutol (2% L-TC), drug permeation was higher at pH 5.0 and 7.0 than at 9.0. A different trend was observed in experiments with 2% oleate in Transcutol (2% O-TC), where skin flux was maximal at a more basic pH, when the degree of ionization is low. The results suggest that sucrose laureate enhances the penetration of the ionized form of the drug (12-fold greater flux relative to control), whereas sucrose oleate is more effective in promoting permeation of the unionized species. The structural properties of the sucrose esters as well as the degree of ionization of the drug are important characteristics affecting the transdermal flux of lidocaine.