This study investigated the effects of formulation on the penetration and retention kinetics of budesonide through canine skin in vitro. Full thickness, thoracic, dog skin was mounted in Franz-type diffusion cells and randomly assigned to receive one of three 0.025% (0.25mg/mL) budesonide-containing formulations: Barazone (BZ, a novel formulation), isopropyl myristate (IPM) or propylene glycol (PG). At regular intervals over 84h, the amount of budesonide penetrating or retained within the skin was quantified using high performance liquid chromatography. The restricted (or residual) maximum likelihood mixed model predicted that the flux of budesonide from BZ was 9.2-fold (P<0.001) and 105-fold (P<0.001) greater than from IPM and PG, respectively. Similarly, the skin retention of budesonide from BZ was more than 3-fold (P<0.0001) and nearly 6-fold (P<0.0001) greater than from IPM and PG, respectively. This study has demonstrated that the formulation can greatly affect the skin penetration and retention of budesonide in dogs, and consequently could be selected to maximise drug concentration and retention at the site of action. This has the potential to improve the efficacy and safety of, and owner compliance with, topical glucocorticoid therapy in dogs.

SynopsisTwenty products, containing a radiolabelled form of each active in typical cosmetic formulations, were made and applied to female human epidermal membranes mounted in Franz diffusion cells for 48 h under ‘in use’ conditions. The products consisted of combinations of five formulations (a hydro‐alcoholic gel, an oil in water emulsion, a water in oil emulsion, a microemulsion and an oil) with four model drug actives (testosterone, hydrocortisone, 5‐fluorouracil and ketoconazole). Steady‐state flux appeared to be reached by 8 h and maintained for all products, other than for the microemulsions, consistent with the actives being present in the residual formulation on the skin at saturation. The recovery for each active at the end of the 48‐h study (from a series of stratum corneum tape strips, the remaining skin, cumulative amount penetrating into the receptor solution, product washed from the skin and on the donor chamber cap) ranged from 86.5% to 100.6%. The rank order of the fluxes for the actives from the hydro‐alcoholic gel is consistent with the known active molecular size and polarity determinants for maximum epidermal flux. Actives with similar steady‐state (maximum) fluxes from a range of formulations had retention in the stratum corneum and similar transport rate constants through the stratum corneum. The microemulsion formulation significantly enhanced both the stratum corneum steady‐state flux and transport rate constant for 5‐fluorouracil, hydrocortisone and testosterone. The penetration flux of each active could be related to its size and polarity and appeared maximal when the actives in the different cosmetic formulations applied to the skin under ‘in use’ conditions were likely to remain in the residual product on the skin as a saturated solution after solvent evaporation. Enhanced penetration fluxes can be achieved by formulation selection and an appropriate choice/mix of emollients/adjuvants. The principles described here provide a framework for understanding the delivery of cosmetic ingredients from various formulations.

The aim of this study was to compare the penetration of hydrocortisone through canine skin in vitro, when applied from six, commercially available, veterinary or human medical creams containing 0.5% hydrocortisone (HC) or hydrocortisone acetate (HCA). Full thickness skin was mounted in Franz-type diffusion cells and randomly divided for use between treatment groups of different HC or HCA formulations. The amount of HC penetrating the skin in the 30 hours following application of a cream was quantified using HPLC analyses. The rate of transderrnal penetration of HC from one cream was 1.7, 1.9, 13.1 and 20.6 times greater than from four others, respectively. This study has shown that the rate of penetration through dog skin can vary greatly from different formulations of the same

This study investigated the effects of allergic skin disease on the penetration kinetics of hydrocortisone through canine skin in vitro. Full-thickness lesional and nonlesional (normal) skin was removed from the dorsal lumbosacral and dorsocaudal thoracic regions, respectively, of five canine cadavers. The dogs were suspected of having flea allergy dermatitis based on their distribution and types of skin lesions. Nonlesional skin was confirmed to be histologically normal, and the histopathology of the lesional skin was consistent with allergic dermatitis. Excised skin was clipped, mounted in Franz-type diffusion cells, and the transdermal penetration of a saturated, radiolabelled hydrocortisone solution was measured over 30 h. When the penetration data for all five dogs were pooled, a restricted (or residual) maximal likelihood mixed model predicted that the permeability coefficient and pseudosteady-state flux of hydrocortisone was more than twice as great (95% confidence interval 1.55-2.71 times as great; P < 0.0001) through lesional compared with nonlesional skin. There was no significant difference in the lag time for hydrocortisone penetration through lesional compared with nonlesional skin of the dogs. This study has confirmed that the transdermal penetration of hydrocortisone may be altered, typically increased twofold, but could be as high as 10-fold, through lesional compared with nonlesional skin of dogs with suspected flea allergy dermatitis. This is likely to be affected by variables such as disease severity, concurrent infections and interindividual differences in skin characteristics.

We examined uptake of the model therapeutic agent, minoxidil, into appendages, stratum corneum (SC), and through human skin, under the influence of different vehicles. Quantitative estimation of therapeutic drug deposition into all three areas has not previously been reported. Finite doses of minoxidil (2%, w/v) in formulations containing varying amounts of ethanol, propylene glycol (PG), and water (60:20:20, 80:20:0, and 0:80:20 by volume, respectively) were used. Minoxidil in SC (by tape stripping), appendages (by cyanoacrylate casting), and receptor fluid was determined by liquid scintillation counting. At early times (30 min, 2 h), ethanol-containing formulations (60:20:20 and 80:20:0) caused significantly greater minoxidil retention in SC and appendages, compared to the formulation lacking ethanol (0:80:20). A significant increase in minoxidil receptor penetration occurred with the PG-rich 0:80:20 formulation after 12 h. We showed that deposition of minoxidil into appendages, SC, and skin penetration into receptor fluid were similar in magnitude. Transport by the appendageal route is likely to be a key determinant of hair growth promotion by minoxidil.

Objectives The development of methods to predict the transport of molecules across biological membranes, without the need for time-consuming collection of experimental data, is a rapidly growing science. The use of structural characteristics of molecules has been investigated to predict the maximum transport rates of molecules across skin epidermal and intestinal membranes, known as maximum flux and maximum absorbable dose, respectively, although different approaches have been used. The aim of the present study was to determine whether the relationship between polar surface area and number of rotatable bonds of molecules and their permeability through intestinal membranes could be applied to the permeation of solutes through the epidermis following topical application.Methods We used a published dataset of human epidermal maximum flux values for 182 solutes and stepwise regression to determine relationships between structural predictors and maximum membrane transport rates.Key findings Results showed that diffusion processes occurring across intestinal and skin epidermal membranes cannot be estimated by the same solute molecular properties, as different combinations of partitioning and diffusion processes appear to be dominating in each type of membrane. The basis of these differences in terms of molecular weight dependence and the usefulness of polar surface area are discussed.Conclusions Based on available literature, we concluded that transdermal penetration is poorly predicted by parameters derived from intestinal or Caco-2 model membranes. While this approach may be useful for small sets of structurally related compounds, it appears to have limited value for screening and selection of novel structures in the pharmaceutical industry.

For many years the barrier function of the skin has been considered to reside in the unique structural properties of the stratum corneum. However, this is only part of the story, as the skin’s defense against its environment is biological as well as physical. The body’s innate immune system evolved to first recognize pathogens and then eradicate them, a basic survival mechanism to prevent the host being overrun with microbial infections. Over 10 years ago, we were introduced to the concept that perhaps peptides with antimicrobial activities were also naturally present in the skin of humans (1).

Burn tissue sites are a potential source of bacteremia during debridement surgery. Burn injury is likely to affect the distribution of antibiotics to tissues, but direct evidence of this is lacking. The aim of this study was to directly evaluate the influence of burn trauma on the distribution of cephalothin to peripheral tissues. We used subcutaneous microdialysis techniques to monitor interstitial fluid concentrations of cephalothin in the burnt and nonburnt tissues of adult patients with severe burns following parenteral administration of 1 g cephalothin for surgical prophylaxis. Analogous simultaneous studies conducted with healthy adult volunteers provided reference tissue concentration data. Equivalent tissue exposures were seen for burn and nonburn sites, giving overall median interstitial cephalothin concentrations (from 0 to 240 min) of 2.84 mg/liter and 3.06 mg/liter, respectively. A lower overall median interstitial cephalothin concentration of 0.54 mg/liter was observed for healthy individuals, and the patient nonburnt tissue and volunteer control tissue cephalothin concentrations exhibited significantly different data distributions (P < 0.001; Kolmogorov-Smirnov nonparametric test). The duration of tissue residence for cephalothin was longer for burn patients than for healthy volunteers. The results demonstrate the potential fallibility of using healthy population models to extrapolate tissue pharmacodynamic predictions from plasma data for burn patients.

ABSTRACT Cephalothin (cefalotin) pharmacokinetics were evaluated for nine severely burned patients (42% ± 9% mean burn areas) and five healthy volunteers by using non-plasma-protein-bound concentration-time profiles. Burn patients gave increased mean residence times (36%) and reduced total clearances (25%). Mean residence times and distribution volumes increased between 1 and 4 days posttrauma, suggesting that burn patient pharmacokinetics change during the initial fluid resuscitation phase of treatment.

This study investigated the effects of common skin surface preparations on the penetration kinetics of hydrocortisone through canine skin. Thoracic skin from five dogs was clipped of hair, divided between five treatment groups and prepared as follows: shaved (S); tape-stripped with adhesive bandage (TS); cleaned with aqueous chlorhexidine (Aq-C); cleaned with alcoholic chlorhexidine (Al-C); or allocated to the control group and had no further preparation performed (C). The skin samples were mounted in Franz-type diffusion cells and transdermal hydrocortisone penetration was measured over 30h. The pseudo-steady-state flux (J(SS)) of hydrocortisone through S, Al-C, Aq-C and TS skin was, respectively, 2.3 (P=0.021), 2.2 (P=0.037), 2.0 (P=0.070) and 1.5 (P=0.351) times greater than through the control skin, but there were no significant differences in the lag times (t(lag)) for hydrocortisone penetration between the groups. The study has shown that some skin surface preparations can significantly increase the subsequent penetration of hydrocortisone through canine skin in vitro.

Objective. Our understanding of the differential effects of topically applied vehicles on solute partitioning and diffusion within the skin is presently limited. In this work, in vitro epidermal partitioning, penetration and multiphoton laser scanning microscopy (MPLSM) imaging studies were used to assess the distribution of 2-naphthol across human epidermis.Materials and Methods. Four commonly used liquid vehicles (100% water, 20% propylene glycol (PG)/water, 50% ethanol (EtOH)/water and 100% isopropyl myristate (IPM)) were used.Results and Discussion. The maximum flux and membrane retention of 2-naphthol from 50% EtOH/water was almost an order of magnitude or larger than from the other vehicles evaluated whereas IPM resulted in the highest membrane retention and lowest membrane penetration for 2-naphthol than other vehicles. MPLSM studies showed that 2-naphthol solute partitioned favourably into the intercellular lipids and that there was a vehicle-dependent uptake of 2-naphthol into corneocytes.Conclusions. The integrated evaluation using in vitro penetration, epidermal retention and MPLSM imaging has shown that vehicle effects on skin penetration occurs by an alteration in the distribution of solutes between the corneocytes and intercellular lipids in addition to the well known mechanisms of altered partitioning into the stratum corneum and enhanced epidermal diffusion.

This study investigated the effects of freezing canine skin on the penetration kinetics of hydrocortisone. Skin samples from three dogs were used for in vitro penetration studies commencing on the day of skin collection (fresh skin) and again after freezing at -20 degrees C for 1, 4, 8 and 12 months. When the data from the dogs was averaged, the pseudo-steady-state flux (Jss) of hydrocortisone through skin frozen for any duration was significantly (P < 0.023) greater than through fresh skin and there was a positive relationship (P < 0.007) between the length of freezing and DeltaJss. For all dogs, the lag times (tlag) calculated for hydrocortisone penetration were significantly (P < 0.029) shorter through skin that had been frozen, compared with fresh skin. However, the shapes of the permeation profiles of hydrocortisone appeared similar through the fresh and frozen dog skins and no differences were detected between the groups on histological examination. The results of this study have shown that freezing dog skin at -20 degrees C can significantly increase the transdermal penetration of hydrocortisone in vitro, and that the extent of this enhancement can increase with duration of freezing.

The effect of dermal clearance on epidermal concentrations of topically applied drugs is poorly understood but fundamental to absorption kinetics and efficacy. Previously generated data quantifying changes in flux and epidermal retention of a series of alcohols and steroids was used to relate solute physicochemical properties to changes in flux under conditions of infinite vasoconstriction (full-thickness skin) and infinite vasodilatation (epidermal membranes) using stepwise regression. Flux through full-thickness skin decreased with increasing lipophilicity, up to 45-fold for alcohols and 4-fold for steroids. This change in flux was not uniformly predictable for the solutes, with alcohols showing a strong dependency on molecular weight (MW), indicative of stratum corneum diffusion limited penetration. Epidermal retention of lipophilic solutes was found to increase up to sixfold in full-thickness skin, with changes predictable from a combination of log P and MW (correlation 0.898, p < 0.001). This study shows that drug epidermal retention and penetration is significantly affected by dermal clearance, however aspects of these effects may be able be predicted from solute physicochemical properties, though extrapolation of the results of this study to other solute-membrane combinations remains to be proven. These findings have significant implications in understanding the effects of erythema and vasoconstriction on topical drug efficacy.

The rate and regional differences for the penetration of fentanyl through equine skin was investigated in vitro using a commercial transdermal therapeutic system (TTS) or ‘patch’. Skin collected from the thorax, groin and leg (dorsal metacarpal) regions of five horses was placed in diffusion cells and a fentanyl TTS applied to each skin sample. Drug penetration through each skin sample over 48h measured using high performance liquid chromatography (HPLC). Cumulative penetration (μg/cm2) was plotted against time (h) and used to regress the steady state flux (μg/cm2/h) of fentanyl through each skin site. Results showed similar fluxes for both the thorax (2.32±0.17μg/cm2/h and groin (2.21±0.11 (μg/cm2/h) regions, but significantly lower flux (P=<0.05) for the leg region (1.56±0.120μg/cm2/h. Interestingly, there was a significantly longer lag time for the penetration of fentanyl through the groin region (7.87±0.51h) compared to the other two sites (5.66±0.97 h and 5.75±0.43h for the thorax and leg regions respectively). The results suggest that a fentanyl TTS applied to the leg region may have a small but significantly lower amount of fentanyl available systemically, compared to patches applied to the thorax or groin regions, which may affect the level of analgesia subsequently achieved in the horse.