Acyclovir bioavailability in human skin

Reverse Iontophoresis: Noninvasive Assessment of Topical Drug Bioavailability

Assessing drug disposition in the skin after the application of a topical formulation is difficult. It is hypothesized that reverse iontophoresis (RI), which can extract charged/polar molecules for monitoring purposes, may provide a noninvasive approach for the assessment of local drug bioavailability. The passive and RI extraction of salicylic acid (SA) and nicotine (NIC) from porcine skin in vitro was assessed after a simple solution of the former and a transdermal patch of the latter had been applied for 24 and 8 h, respectively. Immediately after this "passive skin loading", the amount of drug in the stratum corneum (SC) and "viable" tissue (VT) was measured either (a) after tape-stripping and subsequent solvent extraction of both skin layers or (b) following RI extraction over 4 h. Parallel experiments were then performed in vivo in healthy volunteers; in this case, the VT was not sampled and the skin loading period for NIC was only 4 h. RI extraction of both drugs was significantly higher (in vitro and in vivo) than that achieved passively, and the cumulative RI extraction profiles as a function of time were mathematically analyzed using a straightforward compartmental model. Best-fit estimates of drug amounts in the SC and VT (ASC,0 and AVT,0, respectively) at the end of "loading" and two first-order rate constants describing transfer between the model compartments were then determined. The in vitro predictions of ASC,0 and AVT,0 were in excellent agreement with the experimental results, as was the value of the former in vivo. The rate constants derived from the in vitro and in vivo results were also similar. In summary, the results provide proof-of-concept that the RI method has the potential to noninvasively assess relevant metrics of drug bioavailability in the skin.

Detailed pharmacokinetic characterization of advanced topical acyclovir formulations with IVPT and in vivo Open Flow Microperfusion

Successful treatment of herpes simplex viruses is currently limited by a lack of effective topical drugs. Commonly used topical acyclovir products only reduce the duration of lesions by a few days. Optimizing topical formulations to achieve an enhanced acyclovir solubility and penetration could increase the efficacy of topically applied acyclovir, but new formulations need to show reliable acyclovir delivery into at least the epidermis/dermis and need to provide sustained acyclovir release for extended time periods. The aim of this study was to compare pharmacokinetic data from in vitro permeation testing (IVPT) and preclinical dermal open flow microperfusion (dOFM) experiments regarding the penetration behavior of different acyclovir formulations relative to the reference product Zovirax® 5% cream. Four test formulations that delivered the best penetration data in IVPT were further tested using continuous dOFM in vivo dermal sampling. The use of dOFM identified one of the four tested formulations to perform significantly better than the other three tested formulations and the reference product. In vivo dOFM data showed differences in the dermal acyclovir concentration that had not been detected by using IVPT. Improved acyclovir delivery to the dermis was likely achieved by the new formulation that uses a much lower drug load compared to the reference product. This optimized formulation was able to achieve a dermal concentration similar to oral application and can thus provide the opportunity of more efficacious topical HSV-1 treatment with less side effects than oral systemic treatment.

Identification of critical formulation parameters affecting the in vitro release, permeation, and rheological properties of the acyclovir topical cream

To understand effects of formulation variables on the critical quality attributes (CQA) of acyclovir topical cream, this study investigated effects of propylene glycol (PG), poloxamer, and sodium lauryl sulfate (SLS) concentrations, acyclovir particle size, and formulation pH of the acyclovir cream. Fifteen formulations were prepared and characterized for rheological properties, particle size distribution, drug release and in vitro skin permeation. Drug distribution between various phases of the cream was determined. The concentration of soluble acyclovir in the aqueous phase was determined as a surrogate of the equilibrium with other acyclovir species in the cream. The interaction among effects of the formulation variables on the amount of acyclovir retained by skin was also evaluated. The results showed that PG significantly (p < 0.05) increased the yield stress, viscosity, drug concentration in the aqueous phase, and drug release. The PG and SLS significantly (p < 0.05) increased acyclovir retention by skin samples. Particle size of acyclovir inversely affected the drug release. This study revealed that the employed concentrations of PG and SLS and particle size of the dispersed acyclovir are critical formulation variables that should be carefully controlled when developing acyclovir topical creams with desired performance characteristics.

Sulforaphane-Loaded Ultradeformable Vesicles as A Potential Natural Nanomedicine for the Treatment of Skin Cancer Diseases

Sulforaphane is a multi-action drug and its anticancer activity is the reason for the continuous growth of attention being paid to this drug. Sulforaphane shows an in vitro antiproliferative activity against melanoma and other skin cancer diseases. Unfortunately, this natural compound cannot be applied in free form on the skin due to its poor percutaneous permeation determined by its physico-chemical characteristics. The aim of this investigation was to evaluate ethosomes^® and transfersomes^® as ultradeformable vesicular carriers for the percutaneous delivery of sulforaphane to be used for the treatment of skin cancer diseases. The physico-chemical features of the ultradeformable vesicles were evaluated. Namely, ethosomes^® and transfersomes^® had mean sizes <400 nm and a polydispersity index close to 0. The stability studies demonstrated that the most suitable ultradeformable vesicles to be used as topical carriers of sulforaphane were ethosomes^® made up of ethanol 40% (w/v) and phospholipon 90G 2% (w/v). In particular, in vitro studies of percutaneous permeation through human stratum corneum and epidermis membranes showed an increase of the percutaneous permeation of sulforaphane. The antiproliferative activity of sulforaphane-loaded ethosomes^® was tested on SK-MEL 28 and improved anticancer activity was observed in comparison with the free drug.

Stratum Corneum Sampling to Assess Bioequivalence between Topical Acyclovir Products

PURPOSE

To examine the potential of stratum corneum (SC) sampling via tape-stripping in humans to assess bioequivalence of topical acyclovir drug products, and to explore the potential value of alternative metrics of local skin bioavailability calculable from SC sampling experiments.

METHODS

Three acyclovir creams were considered in two separate studies in which drug amounts in the SC after uptake and clearance periods were measured and used to assess bioequivalence. In each study, a "reference" formulation (evaluated twice) was compared to the "test" in 10 subjects. Each application site was replicated to achieve greater statistical power with fewer volunteers.

RESULTS

SC sampling revealed similarities and differences between products consistent with results from other surrogate bioequivalence measures, including dermal open-flow microperfusion experiments. Further analysis of the tape-stripping data permitted acyclovir flux into the viable skin to be deduced and drug concentration in that 'compartment' to be estimated.

CONCLUSIONS

Acyclovir quantities determined in the SC, following a single-time point uptake and clearance protocol, can be judiciously used both to objectively compare product performance in vivo and to assess delivery of the active into skin tissue below the barrier, thereby permitting local concentrations at or near to the site of action to be determined.

Nanoemulsion-Loaded Hydrogels for Topical Administration of Pentyl Gallate

The aim of this study is to describe the development of nanoemulsion-loaded hydrogels to deliver pentyl gallate (PG), a gallic acid n-alkyl ester, through the skin. PG is an antioxidant agent; however, it seems to be a promising agent for herpis labialis treatment. Aristoflex AVC^® and chitosan were used as gelling agents for nanoemulsion thickening. The developed formulations presented suitable PG content (94.4-100.3% w/w), nanometric droplet sizes (162-297 nm), high zeta potentials, and a non-Newtonian pseudoplastic behavior. Both vehicles neither enhanced PG penetration nor delayed its release from the nanoemulsion. Formulations remained physically stable at 8°C during 3 months of storage.

Enhanced acyclovir delivery using w/o type microemulsion: preclinical assessment of antiviral activity using murine model of zosteriform cutaneous HSV-1 infection

The present study was aimed to develop and evaluate a microemulsion-based dermal drug delivery of an antiviral agent, acyclovir. A water-in-oil microemulsion was prepared using isopropyl myristate, Tween 20, Span 20, water and dimethylsulphoxide. It was characterized for drug content, stability, globule size, pH, viscosity and ex vivo permeation through mice skin. In vivo antiviral efficacy of optimized formulation was assessed in female Balb/c mice against herpes simplex virus-I (HSV-I)-induced infection. It was observed that optimized formulation when applied 24-h post-infection could completely inhibit the development of cutaneous herpetic lesions vis-à-vis marketed cream.

Topical iontophoretic delivery of ionizable, biolabile aciclovir prodrugs: A rational approach to improve cutaneous bioavailability

The objective was to investigate the topical iontophoretic delivery of a series of amino acid ester prodrugs of aciclovir (ACV-X, where ACV=aciclovir and X=Arg, Gly, Ile, Phe, Trp and Val) as a means to enhance cutaneous delivery of ACV. The newly synthesized prodrugs were characterized by (1)H NMR and high resolution mass spectrometry. Analytical methods using HPLC-UV were developed for their quantification and each method was validated. Investigation of solution stability as a function of pH showed that all ACV-X prodrugs were relatively stable in acid conditions at pH 2.0 and pH 5.5 for up to 8h but susceptible to extensive hydrolysis at pH 7.4 and under alkaline conditions (pH 10). No ACV-X hydrolysis was observed after contact for 2h with the external surface of porcine stratum corneum. However, there was significant hydrolysis following contact with the dermal surface of dermatomed porcine skin, in particular, for ACV-Arg. Passive transport of ACV and ACV-X prodrugs from aqueous solution after 2h was below the limit of detection. Iontophoresis of ACV at 0.5 mA/cm(2) for 2h led to modest ACV skin deposition (QDEP,ACV) of 4.6 ± 0.3 nmol/cm(2). In contrast, iontophoresis of ACV-X prodrugs under the same conditions produced order of magnitude increases in cutaneous deposition of ACV species, that is, QDEP,TOTAL=QDEP,ACV+QDEP,ACV-X. QDEP,TOTAL for ACV-Gly, ACV-Val, ACV-Ile, ACV-Phe, ACV-Trp and ACV-Arg was 412.8 ± 44.0, 358.8 ± 66.8, 434.1 ± 68.2, 249.8 ± 81.4, 156.1 ± 76.3, 785.9 ± 78.1 nmol/cm(2), respectively. The extent of bioconversion of ACV-X to ACV in the skin was high and the proportion of ACV present ranged from 81% to 100%. The skin retention ratio, a measure of the selectivity of ACV species for deposition over permeation after iontophoretic delivery of ACV-X prodrugs, was dependent on both the rate of transport and the susceptibility to hydrolysis of the prodrugs. Skin deposition of ACV and its six prodrugs were investigated further as a function of current density (0.125, 0.25 and 0.5 mA/cm(2)); the effect of duration of current application (5, 10, 30, 60 and 120 min) was evaluated using ACV-Arg and ACV-Ile. Iontophoresis of ACV-Arg and ACV-Ile at 0.25 mA/cm(2) for only 5 min resulted in the deposition of appreciable amounts of ACV (36.4 ± 5.7n mol/cm(2) and 40.3 ± 6.1 nmol/cm(2), respectively), corresponding to supra-therapeutic average concentrations in skin against HSV-1 or HSV-2. The results demonstrated that cutaneous bioavailability of ACV could be significantly improved after short-duration iontophoresis of ionizable, biolabile ACV-X prodrugs.

Formulation and process factors influencing product quality and in vitro performance of ophthalmic ointments

Owing to its unique anatomical and physiological functions, ocular surface presents special challenges for both design and performance evaluation of the ophthalmic ointment drug products formulated with a variety of bases. The current investigation was carried out to understand and identify the appropriate in vitro methods suitable for quality and performance evaluation of ophthalmic ointment, and to study the effect of formulation and process variables on its critical quality attributes (CQA). The evaluated critical formulation variables include API initial size, drug percentage, and mineral oil percentage while the critical process parameters include mixing rate, temperature, time and cooling rate. The investigated quality and performance attributes include drug assay, content uniformity, API particle size in ointment, rheological characteristics, in vitro drug release and in vitro transcorneal drug permeation. Using design of experiments (DoE) as well as a novel principle component analysis approach, five of the quality and performance attributes (API particle size, storage modulus of ointment, high shear viscosity of ointment, in vitro drug release constant and in vitro transcorneal drug permeation rate constant) were found to be highly influenced by the formulation, in particular the strength of API, and to a lesser degree by processing variables. Correlating the ocular physiology with the physicochemical characteristics of acyclovir ophthalmic ointment suggested that in vitro quality metrics could be a valuable predictor of its in vivo performance.

Quantification of uptake and clearance of acyclovir in skin layers

BACKGROUND

Quantification of drug uptake and clearance in the skin layers could provide better insight into the skin kinetics of dermatological formulations aimed for deeper skin tissues. This study assessed the skin kinetics of acyclovir in different skin layers following topical application on the abdominal region of Wistar rats.

METHODS

In vivo skin pharmacokinetics parameters were determined by two different protocols such as post drug load assessment and subsequent drug load assessment following topical application of 500 mg of cream formulation containing 5% (w/w) of acyclovir.

RESULTS

Topical application of acyclovir exhibited concentration gradient between the skin layers (stratum corneum > viable epidermis > dermis) which were inconsistent over the time-course of the study. The rate and extent of drug reaching target site (basal epidermis) was relatively low. The drug uptake and clearance profiles were found to be distinct in all the three skin layers suggesting no drug concentration correlation (P<0.05) between skin layers. Drug concentration in the viable epidermis continued to increase even after termination of therapy (Tmax=4 h) and then declined rapidly. The availability of acyclovir in the target was comparatively low (approximately 0.4% of the applied dose) although an order of magnitude higher percentage was determined in the stratum corneum.

CONCLUSIONS

The data observed in this study demonstrates low skin uptake and rapid clearance of acyclovir in the target site. Further, the methodology employed can be useful for studying other topical antiviral agents as well as for optimizing formulations for drugs (such as acyclovir) that may enhance their efficacy.

Enhanced dermal delivery of acyclovir using solid lipid nanoparticles

The present investigation was enthused by the possibility to develop solid lipid nanoparticles (SLNs) of hydrophilic drug acyclovir (ACV) and evaluate their potential as the carrier for dermal delivery. ACV-loaded SLNs (ACV-SLNs) were prepared by the optimized double emulsion process using Compritol 888 ATO as solid lipid. The prepared SLNs were smooth and spherical in shape with average diameter, polydispersity index, and entrapment efficiency of 262 ± 13 nm, 0.280 ± 0.01, and 40.08 ± 4.39% at 10% (w/w) theoretical drug loading with respect to Compritol 888 ATO content. Differential scanning calorimetry and powder X-ray diffraction pattern revealed that ACV was present in the amorphous state inside the SLNs. In vitro skin permeation studies on human cadaver and Sprague-Dawley rat skin revealed 17.65 and 15.17 times higher accumulation of ACV-SLNs in the dermal tissues in comparison to commercially available ACV cream after 24 h. Mechanism of topical permeation and dermal distribution was studied qualitatively using confocal laser scanning microscopy. While free dye (calcein) failed to penetrate skin barrier, the same encapsulated in SLNs penetrated deeply into the dermal tissue suggesting that pilosebaceous route was followed by SLNs for skin penetration. Histological examination and transdermal epidermal water loss measurement suggested that no major morphological changes occurred on rat skin surface due to the application of SLNs. Overall, it was concluded that ACV-loaded SLNs might be beneficial in improving dermal delivery of antiviral agent(s) for the treatment of topical herpes simplex infection.

Development of performance matrix for generic product equivalence of acyclovir topical creams

The effect of process variability on physicochemical characteristics and in vitro performance of qualitatively (Q1) and quantitatively (Q2) equivalent generic acyclovir topical dermatological creams was investigated to develop a matrix of standards for determining their in vitro bioequivalence with reference listed drug (RLD) product (Zovirax®). A fractional factorial design of experiment (DOE) with triplicate center point was used to create 11 acyclovir cream formulations with manufacturing variables such as pH of aqueous phase, emulsification time, homogenization speed, and emulsification temperature. Three more formulations (F-12-F-14) with drug particle size representing RLD were also prepared where the pH of the final product was adjusted. The formulations were subjected to physicochemical characterization (drug particle size, spreadability, viscosity, pH, and drug concentration in aqueous phase) and in vitro drug release studies against RLD. The results demonstrated that DOE formulations were structurally and functionally (e.g., drug release) similar (Q3) to RLD. Moreover, in vitro drug permeation studies showed that extent of drug bioavailability/retention in human epidermis from F-12-F-14 were similar to RLD, although differed in rate of permeation. The results suggested generic acyclovir creams can be manufactured to obtain identical performance as that of RLD with Q1/Q2/Q3.

Block copolymer stabilized nonaqueous biocompatible sub-micron emulsions for topical applications

Polyethylene glycol (PEG) 400/Miglyol 812 non-aqueous sub-micron emulsions were developed due to the fact that they are of interest for the design of drug-loaded biocompatible topical formulations. These types of emulsions were favourably stabilized by poly (2-vinylpyridine)-b-poly (butadiene) (P2VP-b-PBut) copolymer with DPBut>DP2VP, each of these sequences being well-adapted to the solubility parameters of PEG 400 and Miglyol 812, respectively. This type of block copolymers, which might limit the Ostwald ripening, appeared to be more efficient stabilizers than low molecular weight non-ionic surfactants. The emulsion characteristics, such as particle size, stability and viscosity at different shear rates were determined as a function of the phase ratio, the copolymer concentration and storage time. It was further shown that Acyclovir, as a model drug of low water solubility, could be incorporated into the PEG 400 dispersed phase, with no significant modification of the initial emulsion characteristics.

Development of novel microemulsion-based topical formulations of acyclovir for the treatment of cutaneous herpetic infections

The present investigation aims at developing microemulsion-based formulations for topical delivery of acyclovir. Various microemulsions were developed using isopropyl myristate/Captex 355/Labrafac as an oil phase, Tween 20 as surfactant, Span 20 as cosurfactant, and water/dimethylsulfoxide (1:3) as an aqueous phase. Transcutol, eucalyptus oil, and peppermint oil were used as permeation enhancers. In vitro permeation studies through laca mice skin were performed using Franz diffusion cells. The optimum formulation containing 2.5% Transcutol as the penetration enhancer showed 1.7-fold enhancement in flux and permeation coefficient as compared to marketed cream and ointment formulation. In vivo antiviral studies were performed in female Balb/c mice against induced herpes simplex virus I infection. A single application of microemulsion formulation containing 2.5% Transcutol given 24 h post-injection resulted in complete suppression of development of herpetic skin lesions.

Acyclovir delivery systems

Herpes viruses (herpes simplex, varicella zoster, cytomegalovirus) are the main cause of a wide variety of human infections. Although the development of successful antiviral agents against infections caused by herpes viruses had been slow until the last decade, the production of delivery systems for acyclovir are a promising alternative. The present review summarizes the principal advances made in developing carriers for the delivery of acyclovir by different routes of administration.

Transdermal drug delivery systems: skin perturbation devices

Human skin serves a protective function by imposing physicochemical limitations to the type of permeant that can traverse the barrier. For a drug to be delivered passively via the skin it needs to have a suitable lipophilicity and a molecular weight < 500 Da. The number of commercially available products based on transdermal or dermal delivery has been limited by these requirements. In recent years various passive and active strategies have emerged to optimize delivery. The passive approach entails the optimization of formulation or drug carrying vehicle to increase skin permeability. However, passive methods do not greatly improve the permeation of drugs with molecular weights >500 Da. In contrast, active methods, normally involving physical or mechanical methods of enhancing delivery, have been shown to be generally superior. The delivery of drugs of differing lipophilicity and molecular weight, including proteins, peptides and oligonucletides, has been shown to be improved by active methods such as iontophoresis, electroporation, mechanical perturbation and other energy-related techniques such as ultrasound and needleless injection. This chapter details one practical example of an active skin abrasion device to demonstrate the success of such active methods. The in vitro permeation of acyclovir through human epidermal membrane using a rotating brush abrasion device was compared with acyclovir delivery using iontophoresis. It was found that application of brush treatment for 10 s at a pressure of 300 N m(-2) was comparable to 10 min of iontophoresis. The observed enhancement of permeability observed using the rotating brush was a result of disruption of the cells of the stratum corneum, causing a reduction of the barrier function of the skin. However, for these novel delivery methods to succeed and compete with those already on the market, the prime issues that require consideration include device design and safety, efficacy, ease of handling, and cost-effectiveness. This chapter provides a detailed review of the next generation of active delivery technologies.

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.

Topical Iontophoresis of Valaciclovir Hydrochloride Improves Cutaneous Aciclovir Delivery

PURPOSE

To investigate the topical iontophoresis of valaciclovir (VCV) as a means to improve cutaneous aciclovir (ACV) delivery.

METHODS

ACV and VCV electrotransport experiments were conducted using excised porcine skin in vitro.

RESULTS

While the charged nature of the prodrug, VCV, enabled it to be more efficiently iontophoresed into the skin than the parent molecule, ACV, only the latter was detectable in the receptor chamber, suggesting that VCV was enzymatically cleaved into the active metabolite during skin transit. Iontophoresis of VCV was significantly more efficient than that of ACV; the cumulative permeation of ACV after 1, 2 and 3 h of VCV iontophoresis at 0.5 mA cm(-2) and using an aqueous 2 mM (approximately 0.06%) formulation was 20+/-10, 104+/-47 and 194+/- 82 microg cm( -2), respectively (cf. non-quantifiable levels, 0.1 and 1.0+/-0.7 microg cm(-2) after ACV iontophoresis).

CONCLUSIONS

These delivery rates provide ample room to reduce either current density or the duration of current application. Preliminary in vitro data serve to emphasize the potential of VCV iontophoresis to improve the topical therapy of cutaneous herpes simplex infections and merit further investigation to demonstrate clinical efficacy.

Topical iontophoretic administration of acyclovir for the episodic treatment of herpes labialis: a randomized, double-blind, placebo-controlled, clinic-initiated trial

BACKGROUND

Multiple studies of the use of acyclovir for the treatment of herpes labialis have suggested that the nominal efficacy of the topical formulation is the result of inadequate penetration of the drug into the target site of infection, the basal epidermis.

METHODS

We developed a low-voltage, wireless, hand-held, computer-controlled, iontophoretic applicator to enhance the skin penetration of topical acyclovir in the treatment of herpes labialis. We performed a multicenter, placebo-controlled, clinic-initiated, pilot trial of a single, topical, iontophoretic application of 5% acyclovir cream for the episodic treatment of herpes labialis among 200 patients with an incipient cold sore outbreak at the erythema or papular/edema lesion stage.

RESULTS

The median classic lesion healing time (aborted lesions were assigned a value of 0 h) was 1.5 days shorter for the active treatment group than for the vehicle group (113 h vs. 148 h; P = .02). In the subgroup of patients who presented with lesions in the erythema stage, the median classic lesion healing time was 3 days shorter for the acyclovir group, compared with the control group (49 h vs. 120 h; P < .03), and the acyclovir group tended to have more aborted lesions than did the control group (46% vs. 24%; P = .10).

CONCLUSIONS

Single-dose topical iontophoresis of acyclovir appears to be a convenient and effective treatment for cold sores and merits further clinical investigation.

Kinetics of finite dose absorption through skin 2: Volatile compounds

A diffusion model to account for the disposition of an arbitrary dose of a (potentially) volatile compound applied to skin from a volatile vehicle is presented. In its most general form, the model allows for variable diffusivity of the permeant in the stratum corneum (SC) and must be solved numerically. However, for permeants having a constant diffusivity, absorption, and evaporation is characterized in terms of four dimensionless parameters-a reduced time tau, a fractional deposition depth in the SC f, a ratio of membrane capacity for the permeant to the applied dose beta, and a ratio of evaporative mass transfer coefficient to diffusive permeability chi. An important combination of these parameters arises as the reduced dose M(r) = (fbeta)(-1). Two cases are distinguished. In Case 1, corresponding to M(r) < or = 1, the dose is less than that required to saturate the upper layers of the SC, and the shape of the absorption and evaporation profiles is independent of the dose. Analytical solutions to Case 1 may be derived for arbitrary initial distributions of the permeant; the solution for a square wave is presented. In Case 2, corresponding to M(r) > 1, absorption and evaporation approach steady-state values as the dose is increased. Numerical evaluations of this behavior are shown. Limiting behavior for the case of a highly volatile solvent applied to skin is discussed. A companion paper discusses the application of the model to the absorption and evaporation of benzyl alcohol from human skin in vitro.

The Influence of Iontophoresis on Acyclovir Transport and Accumulation in Rabbit Ear Skin

PURPOSE

The aim of this work was to explore the effect of iontophoresis on acyclovir (ACV) accumulation and permeation. In particular, the objectives were to check the efficacy of the transport mechanisms, electromigration and electrosmosis, on drug accumulation.

METHODS

Permeation experiments were performed in vitro, using rabbit ear skin as barrier, from donor solutions at pH 3.0, 5.8, and 7.4. At the end of the experiments, drug accumulation in epidermis and dermis was measured. Anodal and cathodal iontophoresis were applied at pH 3.0, whereas only anodal iontophoresis was used at pH 5.8 (current densities 0.06--0.50 mA/cm(2)) and 7.4.

RESULTS

Cathodal iontophoresis was more efficient than anodal iontophoresis on ACV permeation across the skin at pH 3.0. At pH 5.8, ACV flux and accumulation increased with current density during anodal iontophoresis. At pH 7.4, anodal iontophoresis produced a remarkable increase of flux and a modest increase of accumulation. Overall, anodal flux increased as the pH of the donor solution was increased as a result of the increase of the skin net negative charge.

CONCLUSIONS

From the results obtained in the present work, it can be concluded that iontophoresis application increases ACV flux and, to a limited extent, accumulation in the skin.

The porcine snout--an in vitro model for human lips?

The morphology and histology of test sites commonly used to study the penetration of lip products differ significantly from those of the human lip itself. The aim of this study was to investigate whether the porcine snout could serve as an equivalent in vitro model for human lips. The lips of human test subjects and biopsies of porcine snout tissue were compared using histological and microscopic techniques. Using a dermatological laser scanning microscope, the penetration of topically applied fluorescent sodium fluorescein was investigated in vivo on human lips and in vitro on the porcine snout. Biopsies from the in vitro experiments were studied using fluorescence microscopy. Some parts of the porcine snout show a similar morphology and histology as human lips. The stratum corneum (SC) and the epidermis of the porcine snout are thicker than those of human tissue. Both in vivo and in vitro, the topically applied fluorescent dye was detected only on the skin surface and within the uppermost SC layer. These results indicate that porcine snout can be used as an in vitro model for human lips in penetration studies. Both human and porcine tissues exhibit an efficient barrier against the penetration of topically applied substances.

In vitro delivery of novel, highly potent anti-varicella zoster virus nucleoside analogues to their target site in the skin

PURPOSE

To determine the in-vitro dermal delivery of a new class of lipophilic, highly potent and uniquely selective anti-VZV nucleoside analogues in comparison with aciclovir.

METHODS

Three test compounds (Cf1698, Cf1743, Cf1712) and aciclovir were formulated into propylene glycol/aqueous cream BP formulations and finite doses applied to full-thickness pig ear skin for 48 hours in vertical Franz-type diffusion cells. Receptor phase samples were taken at specific intervals to determine permeation, and depth profiles were constructed following tape stripping and membrane separation.

RESULTS

All three test compounds reached the target basal epidermis in concentrations suggesting they would be highly efficacious in reducing viral load. Furthermore, the data showed that each of the test compounds would perform in a far superior manner to aciclovir, the current treatment of choice.

CONCLUSIONS

The dermatomal site of viral replication during secondary infection--the basal epidermis--was successfully targeted. Topical delivery of these compounds is highly promising as a new first line treatment of VZV infections. By attacking the virus at the first sign of reactivation, it is proposed that the extent of damage caused by the virus would be significantly lowered, thereby limiting the extent and severity of post-herpetic neuralgia.

The Mystical Effects of Dermatological Vehicles

Topical treatment of the skin is as old as the evolution of man. Instinctively, we try to treat a skin injury or irritation with cooling or soothing substances. Even animals lick their wounds, trusting instinctively in the healing power of saliva. When did this archaic pattern of treatment take the gigantic leap from folk medicine to modern drug therapy? This text illustrates the evolution of topical dermatological vehicles, their application (guidelines) and future use. In particular, a phenomenon that has so far been ignored in product development and clinical testing is the vehicle metamorphosis. In clinical and experimental situations, most dermatological vehicles undergo considerable changes after they have been removed from the primary container and are applied to the skin. Subsequently, the initial structural matrix, and the quantitative composition of the vehicle, will most likely change during and after the mechanical shear associated with application of the product and/or evaporation of ingredients. This natural, but highly dynamic process will generate mini-environments for the active moiety that are difficult to predict and that are crucial to the fate of the active moiety. Despite the reasonable wishes of formulators, clinicians, patients and customers, there are still no universal vehicles. Each drug, at each concentration, requires a different vehicle for optimized therapy. Stability and compatibility of excipients and active moiety are crucial for any commercially available pharmaceutical or cosmetic formulation, together with local and systemic safety of all components. Nonetheless, more diverse and molecularly complex classes of new dermatological vehicles are continuously being researched and refined. The scientific progress has been remarkable when one considers the simple emulsion mixtures that were commonplace in dermatological therapy and still persist to this day in commercial products. It is to be hoped that the result of these research endeavors will be the emergence of more innovative topical formulations, applying engineered bioavailability control systems, with broader applications in topical therapeutic and cosmetic vehicles.

Pharmacokinetics of acyclovir in rabbit skin after i.v.-bolus, ointment, and iontophoretic administrations

The aim of this study was to characterize and compare the pharmacokinetics of acyclovir (ACV) in skin and plasma after iontophoresis, i.v.-bolus, and ointment administrations in rabbit. On five occasions, each separated by at least 1-week washout, rabbits received a 10 mg/kg dose of ACV as i.v.-bolus, ACV iontophoresis for 1 h at different current densities (100, 200, 300 microA/cm2) or a commercially available ointment for two hours. Blood samples were collected serially up to 6 h. Skin ACV concentrations were monitored via microdialysis using linear microdialysis probes (1 cm window). Cathodic iontophoresis was performed using commercially available patches (10 cm2 contact area). Following i.v.-bolus, C(max) in skin occurred with a delay of 38 +/- 4 min compared with plasma. No quantifiable concentration of ACV was detected in the skin on passive drug delivery. Following iontophoresis, skin exposure to ACV was 40, 22, and 11% of that following i.v.-bolus. Conversely, systemic exposure to ACV was negligible and plasma concentrations were below the limit of quantification at any time-point. In skin dialysate, C(max), AUC, and half-life increased with current density. During ointment application, ACV in dermis was detectable only for the first 30 min thereafter ACV skin concentrations were below the LOQ (30 ng/ml).

Targeted dermal delivery of highly potent anti-varicella zoster virus nucleoside analogues from saturated solutions and ethanolic oil-in-water creams

Varicella zoster virus (VZV) is responsible for causing chickenpox and shingles infections, the latter of which can lead to long-term post-herpetic neuralgia (PHN), the most common complication of VZV infections. A class of anti-VZV nucleoside analogues has been synthesised that shows up to 30,000 times the potency of aciclovir in vitro. The relatively high lipophilicities exhibited by the compounds led them to be selected for dermal delivery. The aim was to assess the relative penetration and permeation of the compounds into and through the skin, ideally targeting the region of skin in which the reactivated virus replicates. By targeting the skin it should be possible to reduce the viral load that causes damage to the nerves, thereby limiting zoster-associated pain, in particular PHN. Three compounds, as saturated solutions or as ethanol-based creams, were applied to full-thickness pig ear skin in Franz-type diffusion cells. An ethanolic and water receptor phases were compared. Samples of the receptor phase were taken at specific intervals, followed by tape stripping and separation of the remaining membrane at the end of the experiment. Analysis of the samples showed that all three compounds penetrated into the ethanolic receptor phase to a considerable degree, while only the least lipophilic compound entered the water receptor phase. The effects of the organic solvent in the receptor phase were visible in both the penetration and permeation of the compounds. All three compounds were distributed throughout the membrane in a manner that indicates that the site of viral replication in the skin is reached.

Acyclovir permeation through rat skin: mathematical modelling and in vitro experiments

The aim of this work is to characterise the skin permeation properties of a male rat by means of a purely diffusive mathematical model based on Fick's second law. Additionally, in the attempt of proposing a reliable tool allowing the skin permeability (or resistance) determination on the basis of experimental data, the model automatically accounts also for two typical experimental conditions. In particular, drug dissolution in the donor environment and receiver sampling technique (part of the receiver volume is withdrawn and immediately replaced by fresh solvent) are considered. The results of this characterisation are then compared with those coming from a common simplified approach. Acyclovir is chosen as model drug and a thermostatic (37 degrees C) Franz cell apparatus is used to perform permeation experiments. This study suggests that Acyclovir permeation through the rat skin can be well described by the proposed model and that some differences arise in the evaluation of the full-skin resistance performed by means of our model or the usual simpler approach.

Topical application of acyclovir-loaded microparticles: quantification of the drug in porcine skin layers

The goal of this work was to increase the amount of acyclovir (ACV) in the basal epidermis, site of Herpes virus simplex infections, using microparticles as carriers. Poly(D,L-lactic-co-glycolic acid) microparticles loaded with ACV were prepared using a solvent evaporation technique. ACV distribution into porcine skin after topical application of microparticles for 6, 24 and 88 h, was determined by horizontal slicing of the skin. An ACV suspension served for comparison. The results showed that, at 6 and 24 h, the quantity of the drug in the basal epidermis with the microparticles, is similar to that obtained with the ACV suspension. However, after 88 h, the ACV reservoir in the basal epidermis was higher with the microparticles compared with the control suspension. This could be explained by the controlled drug release produced by the vector in the basal epidermis. Besides, at 88 h the amount of ACV detected in the receptor chamber of the diffusion cells was much lower with the microparticles than with the suspension. This type of carrier can improve acyclovir topical therapy since it increases drug retention in the basal epidermis and consequently increases the time intervals between doses.

A double-blind, randomized study assessing the equivalence of valacyclovir 1000 mg once daily versus 500 mg twice daily in the episodic treatment of recurrent genital herpes. Genival Study Group

Valaciclovir is a prodrug of acyclovir with more favourable bioavailability. Twice daily oral administration of valacyclovir is recommended in patients with genital herpes. A double-blind, randomized, controlled, multicriteria equivalence trial was conducted to determine whether od treatment with valacyclovir 1000 mg is as effective as bd treatment with 500 mg in patients with recurrent genital herpes. A total of 922 immunocompetent outpatients were treated with either regimen for 5 days; treatment was self-initiated at the first symptoms of the next recurrence. The principal outcome measures were the percentage of lesions healed at day 6, time to healing, time to cessation of pain, discomfort or itching, the percentage of abortive episodes and safety. Equivalence was assessed by comparison of 80% confidence limits for each measure; the two regimens were regarded as equivalent if the lower confidence limit was higher than a pre-determined equivalence limit calculated to show a maximum 10% inferiority of valacyclovir 1000 mg od against valaciclovir 500 mg bd. Intention-to-treat analysis showed that the two treatments were equivalent for each outcome measure. Hence, it is concluded that valacyclovir 1000 mg od is as effective as 500 mg bd. as self-initiated therapy in patients with recurrent genital herpes.

Distribution of khellin in excised human skin following iontophoresis and passive dermal transport

The aim of this work was to study in vitro khellin distribution into human skin after passive or iontophoretic transport. The experiments were performed on excised human skin, using vertical Franz-type diffusion cells. The effects of current application and reservoir pH were studied. At the end of the experiments the skin was sliced thinly and the drug was extracted and analyzed by HPLC. The results showed that khellin is able to penetrate through stratum corneum, to reach basal epidermis and upper dermis. The application time proved to be an important parameter. Current application (30 min; 0.5 mA/cm(2)), with a donor at pH 7.0, favored khellin accumulation even if the drug is not ionized. On the contrary, the use of a formulation at pH 3.2 inhibited drug accumulation. Leaving the drug reservoir in contact with the skin for 30 min after current application led to a dramatic increase of khellin concentration. A combination of dermal iontophoresis and passive diffusion is then a useful technique to govern khellin distribution in the skin.

Extemporaneous compounding. The end of the road?

Extemporaneous compounding has long been a part of dermatology. It has served to produce niche therapies that otherwise would have been poorly treated with available drugs. Increasingly however, the unpredictable nature of compounded medications, both in effectiveness, as well as safety and stability of such products, has diminished the use of this approach. The increasing availability of new pharmaceutical drugs that fill these niches more effectively, coupled with economic and legal concerns over the practice of compounding make it a tradition with an increasingly limited role in dermatology today. It is safe to predict that in the near future, compounding will virtually disappear from dermatology, as it already has from virtually all other medical specialties.

Assay of acyclovir in human skin layers by high-performance liquid chromatography

This paper describes an assay procedure for acyclovir quantification in human skin after in vitro transdermal transport experiments. The procedure employs warm distilled water for acyclovir (ACV) extraction and high-performance liquid chromatography (HPLC) as analytical method. The procedure has good reproducibility, sensitivity and specificity, resulting in a reliable method for biopharmaceutical studies of ACV distribution in skin tissue. The chromatographic conditions set up, using distilled water as mobile phase, makes the analytical procedure simple and easy to perform.

Iontophoresis enhances the transport of acyclovir through nude mouse skin by electrorepulsion and electroosmosis

PURPOSE

Iontophoresis was employed for enhancing the transdermal delivery of acyclovir through nude mouse skin in vitro, with the aim of understanding the mechanisms responsible for drug transport, in order to properly set the conditions of therapeutical application.

METHODS

Experiments were done in horizontal diffusion cells, using as donor a saturated solution of acyclovir at two different pH values (3.0 and 7.4). Different electrical conditions (current density and polarity) were employed.

RESULTS

At pH 3.0, acyclovir anodal transport was due to electrorepulsion, since acyclovir was 20% in the protonated form. In acyclovir anodal iontophoresis at pH 7.4 the main mechanism involved was electroosmosis, since the drug was substantially unionized and the negative charge of the skin at this pH caused the electroosmotic flow to be from anode to cathode. In the case of cathodal iontophoresis at pH 3.0, acyclovir transport was enhanced approx. seven times, due to the presence of an electroosmotic contribution caused by the reversal of the charge of the skin. At pH 7.4 during cathodal iontophoresis acyclovir transport was not enhanced because the electroosmotic flow was in the opposite direction, compared to drug electric transport, i.e. anode to cathode. The increased skin permeability caused by current application was demonstrated to be less important than electrorepulsion and electroosmosis.

CONCLUSIONS

Anodal iontophoresis shows potential applicability for enhancing acyclovir transport to the skin, considering that both electric transport and electroosmosis can be used by appropriately setting the pH of the donor.

Determination of acitretin in the skin, in the suction blister, and in plasma of human volunteers after multiple oral dosing

Several HPLC methods for quantification of acitretin and its 13-cis isomer in biological fluids have been described. Only limited data are available on determination of this drug in skin samples. Our objective was to improve the sensitivity and selectivity of existing methods to measure drug in small skin samples from humans treated with acitretin. With a new optimized mobile phase [methanol: acetonitrile (7:3, v/v), purified water with 1.5% (v/v) acetic acid, mixed in a 85:15 ratio (v/v)] and a new internal standard (arotinoid ethyl sulfone), a limit of quantification of 1 ng/g tissue was reached. Nine male volunteers were given an oral daily dose of 50 mg acitretin for up to 28 days. Blood and skin samples (punch and shave biopsies, suction blister skin, and fluid) were taken at various time points during and after treatment. Drug concentration and metabolism in plasma and skin samples appeared to be linked in that the trans-isomer concentration was always higher than the cis-isomer concentration during dosing and 3 h after the last dose. However, 7 and 14 days after the last dose in plasma and in all tissue samples (except the shave biopsy), the all-trans-acitretin concentration rapidly decreased and approached the detection limit. In the shave biopsy, the all-trans-acitretin concentration remained higher than the 13-cis-acitretin concentration. Furthermore, the elimination of two isomers from the shave biopsy was delayed.(ABSTRACT TRUNCATED AT 250 WORDS)

Acyclovir: is it an effective virostatic agent for orofacial infections?

Oral and intravenous acyclovir formulations provide effective virostasis against many herpes viruses infections, especially severe herpes simplex or varicella-zoster infections in ambulatory and immunocompromised patients. The therapeutic virostatic efficacy of topical acyclovir formulations requires further development, however, especially for orolabial herpetic infections.

In vivo skin penetration of acitretin in volunteers using three sampling techniques

Etretinate and acitretin are given orally to treat psoriasis and various keratinization disorders. Acitretin, the main active metabolite of etretinate, has the pharmacokinetic advantage of being rapidly eliminated, but it shares etretinate's toxicologic profile. Thus a topical delivery of acitretin with no or reduced systemic adverse effects is desirable. To characterize the therapeutic potential of topically delivered acitretin, we quantitatively assessed its percutaneous penetration in healthy human volunteers. Additionally, three skin sampling techniques, the punch biopsy, the shave biopsy, and the suction blister technique, were validated to quantitate acitretin in the skin. The results suggest that topical delivery of acitretin renders skin concentrations which exceed those reported after oral administration of etretinate or acitretin. However, because of possible interlaminate drug contamination, drug localization within a particular skin compartment cannot be determined.