The role of vehicles in the percutaneous absorption of corticosteroids. An experimental and clinical study

Advanced topical formulations (ATF)

Topical formulations aim to target the skin for a variety of cosmetic, protective or therapeutic needs. Despite the use of creams and ointments over the millennia, the bioavailability of actives from topical preparations remains quite low, often not exceeding 1-2% of the applied dose. In this review we examine the reasons underlying the poor performance of topical preparations. We also outline a rational approach, based on Fick's laws of diffusion, to develop advanced topical formulations. Methodologies which are currently used in research and development are critically examined and the importance of understanding the fate of the vehicle as well as the active is emphasised. Advanced topical formulation development will also be facilitated by emerging and sophisticated analytical techniques that are able to probe real time delivery of actives to the skin. A good understanding of the underlying physical chemistry of both the formulation and the skin is crucial in the development of optimised topical products.

Solute-vehicle-skin interactions in percutaneous absorption: the principles and the people

An appreciation of solute-vehicle-skin interactions underpins our current understanding of the processes of percutaneous absorption as well as in the prediction of the extent of absorption. This understanding has been reached through principles developed and validated over the last century through the work of a number of authors, including Dale Wurster, Takeru Higuchi, Irvin Blank, Robert Scheuplein, Gordon Flynn, Boyd Poulsen and Tom Franz, as well as by many scientists from my and younger generations. Their work has led to an appreciation of the rate-limiting steps in percutaneous penetration, the role played by the physicochemical properties of the solute, vehicle and skin and the variability that may arise from using various experimental/mathematical/pharmacokinetic models to quantify absorption as well as enabling the prediction of local and systemic efficacy and toxicity. In addition, unexpected behaviour may result from non-ideality in solute-vehicle-skin effects, including dehydration, chemical enhancement, supersaturation, metabolism, sequestration and vascular effects, including those of nanosystems on the local vasculature. In general, in vitro skin penetration profiles are predictive of in vivo profiles but a number of exceptions also exist.

The skin-blanching assay

The skin-blanching assay is used for the determination and bioequivalence of dermatologic glucocorticoids (GCs). The exact mechanism of the production of blanching is not fully understood, but it is considered that local vasoconstriction of the skin microvasculature and the consequent blood-flow reduction cause this phenomenon. Several factors influence skin blanching, including drug concentration, duration of application, nature of vehicle, occlusion, posture and location. The intensity of vasoconstriction can be measured in several ways: visual or quantitative methods, such as reflectance spectroscopy, thermography, laser Doppler velocimetry and chromametry. In literature, contradicting results in the correlation of the skin-blanching assay with different tests to determine GC sensitivity have been reported, limiting its clinical usefulness.

Percutaneous absorption in man: in vitro-in vivo correlation

AIMS

To examine the existing literature to determine the degree to which percutaneous absorption data obtained using the excised human skin model match those obtained from living man.

METHODS

The scientific literature was reviewed to collect data on compounds whose percutaneous absorption through human skin had been measured under both in vitro and in vivo conditions. The in vitro-in vivo (IVIV) correlation was evaluated by computing the in vitro/in vivo ratio using total absorption (percent of applied dose) as the metric for comparison.

RESULTS

A total of 92 data sets were collected from 30 published studies. The average IVIV ratio across all values was 1.6, though for any single data set there could be a nearly 20-fold difference between the in vitro and in vivo values. In 85% of the cases, however, the difference was less than 3-fold. The correlation was significantly improved when data were excluded from studies in which the protocols for both studies were not fully harmonized. For harmonized data sets the average IVIV ratio was 0.96 and there was a less than 2-fold difference between the in vitro and in vivo results for any one compound, with IVIV ratios ranging from 0.58 to 1.28. The dominant factors leading to exclusion of data were the use of skin from different anatomical sites and vehicles of differing composition.

CONCLUSIONS

Percutaneous absorption data obtained from the excised human skin model closely approximate those obtained from living man when the two study protocols are appropriately matched.

Assessment of topical corticosteroid activity using the vasoconstriction assay in healthy volunteers

OBJECTIVE AND DESIGN

The aim of the study was to evaluate the vasoconstrictive activity of four new galenic preparations of hydrocortisone and to compare it with concentration-equivalent reference preparations. The study comprised two study phases: the pilot study phase and the main study phase. During open, nonrandomized pilot study, the optimal administration period was tested. The main study was performed in a randomized, double-blind intraindividual comparative design.

SUBJECTS

Twenty male and female volunteers with healthy skin who responded to topically applied clobetasol-17-propionate before entering the trial participated in this study.

TREATMENT

All subjects received the same treatments. The test preparations new galenic formulation (NGF) hydrocortisone 0.25% cream, NGF hydrocortisone acetate 0.25% cream, NGF hydrocortisone 0.5% cream, and NGF hydrocortisone 1.0% cream were compared with the respective reference preparations Soventol hydrocortisone (hydrocortisone acetate 0.25%), Hydroderm HC 0.5% cream (hydrocortisone 0.5%), Hydrogalen cream (hydrocortisone 1.0%) and placebo (vehicle of test preparations).

METHOD

The topical preparations were applied occlusively for 2 h (pilot study) or 24 h (main study) on outlined areas (5 x 5 cm with a distance of 3 cm) of both forearms (4 areas for each). Assessment of vasoconstriction was performed before treatment, and 0.5, 4, 6 and 24 h after treatment (observation period) using a subjective rating scale (OLSEN vasoconstriction score) and measuring the colorimetric parameter a* (redness) by use of the Chroma-Meter (Minolta company, Ahrensburg, Germany).

RESULTS

A significant vasoconstriction (positive blanching effect) was measured by use of chromametry for test preparations hydrocortisone 0.25% cream, hydrocortisone 0.5% cream, hydrocortisone 1.0% cream and for the reference preparation Hydrogalen cream compared to placebo 30 min after the end of treatment. In contrast, the reference preparations Soventol hydrocortisone and Hydroderm HC 0.5% did not differ significantly from placebo 30 min after treatment. No statistically significant effect of all formulations was observed 4-24 h after treatment in comparison with placebo.

CONCLUSIONS

The vasoconstrictive efficacy of test preparations was mostly stronger than the concentration-equivalent reference preparations. This effect was achieved by use of new galenics of test preparations resulting in enhanced skin penetration and improved efficiency. No unwanted side effects were observed during the course of the study despite increased efficacy of the topically applied test preparations.

Skin permeation: The years of enlightenment

Considerable advances in our understanding of the mechanisms by which drugs permeate the skin barrier have been made over the past 60 years. The key publications, which have influenced the field of skin permeation research are highlighted in the present review. The methodologies commonly employed for estimation of skin permeability are discussed as are the mechanisms proposed for skin permeation. The principal findings from the commonly employed in vitro and in vivo models are considered as well as the applications of enhancers and surfactants for optimization of skin delivery. As these studies presaged the emergence of transdermal drug delivery research in the 1970s, early approaches to model and predict dermal and transdermal absorption are also outlined. The published work on skin permeability in this period embodies the fundamental literature sources for consultation by scientists new to and currently engaged in transdermal drug delivery.

Formulation factors affecting release of drug from topical vehicles. II. Effect of solubility on in vitro delivery of a series of n-alkyl p-aminobenzoates

The major influence on the rate of drug transfer out of its vehicle and into the skin is the thermodynamic activity of the drug within its formulation. This study addresses certain thermodynamic dependencies of topical delivery in a model system. Prototypical water-in-oil (W/O) and oil-in-water (O/W) emulsions and their component phases are used as the test vehicles, polydimethylsiloxane is the membrane, and three homologous n-alkyl p-aminobenzenzoate esters are the test permeants. In an emulsion, the interaction of the compound between the water and oil phase can determine the extent of lowering of the thermodynamic activity in the external phase in contact with the membrane. The emulsifiers (surfactants) impact strongly on partitioning and permeation as a result of the extra solubilizing capacity contributed by the surfactant micelles. The lower flux in the aqueous phase of the O/W emulsion is the result of micellar solubilization, and this solubilization increased with increasing ester chain length. Solubilization is also an influence in nonaqueous phases, but permeant hydrophobicity is without specific influence; therefore, transport become less dependent upon the structure of the compound.

Failure of topical acyclovir in ointment to penetrate human skin

Topical acyclovir (ACV) in polyethylene glycol (PEG) ointment has been disappointing in the treatment of recurrent herpes simplex virus infections in immunocompetent patients. To investigate the possible role of poor drug delivery from this formulation, we studied the penetration of ACV through excised human skin from three vehicles; PEG ointment, modified aqueous cream, and dimethyl sulfoxide. A second antiviral agent, idoxuridine, was studied in the same formulations, and drug delivery through excised guinea pig skin was also assessed for comparison. The delivery of ACV from PEG ointment was very slow for both human and guinea pig skin (drug flux, 0.055 and 0.047 microgram/cm2 per h, respectively). Formulation of ACV in modified aqueous cream and in dimethyl sulfoxide resulted in an 8- and 60-fold increase, respectively, in the flux of ACV through human skin. Idoxuridine behaved similarly to ACV in the three vehicles. The poor clinical results seen with topical use of ACV ointment may be due in part to retarded drug delivery from this formulation.

Topical glucocorticoids and suppression of contact sensitivity. A mouse bioassay of anti-inflammatory effects

A mouse model for assessment of the anti-inflammatory effect of topical glucocorticoids is described. Mice, sensitized to picryl chloride, had both ears painted with the sensitizer followed 2 hours later by the application of the topical steroid to one ear and the corresponding vehicle to the other. The contact sensitivity reaction, measured by swelling of the ear, was recorded 24 hours later. Dilutions of the steroid formulations inhibited the ear swelling in a manner related to dose-response. Suppression of the contact sensitivity reaction of the vehicle-treated ear as well was regarded as a systemic effect of the glucocorticoid. There seems to be a good correlation between the efficacy of the topical steroids assessed in this mouse model and the vasoconstrictor test on intact human skin.

On the bioavailability of topical formulations of clindamycin hydrochloride

The role of the vehicle in the percutaneous absorption of clindamycin hydrochloride has been studied. Fourteen vehicles have been examined for their ability (1) to adequately solubilize clindamycin and (2) to effect penetration of the drug into the skin. The solubility of clindamycin was found to be good in ten of the fourteen vehicles studied, with a drug concentration of greater than 1.0% being achieved. A vehicular water content of greater than 20% or the presence of another suitable cosolvent appeared to be necessary for adequate solubilization of drug. Percutaneous absorption varied greatly among the vehicles, ranging from 0.7% to 12.9% of the applied dose in 24 hours. The vehicles effecting the greatest clindamycin absorption contained the penetration enhancers dimethylsulfoxide or N-methyl-2-pyrrolidone. Overall, the vehicles studied varied a hundredfold with respect to clindamycin bioavailability. Correlation of these data with other literature data suggests a possible relationship between clindamycin bioavailability and clinical efficacy in the treatment of acne and, therefore, argues against extemporaneous compounding of this drug in the absence of bioavailability data.

Comparative study of the vasoconstrictor activity of halopredone acetate in a modified McKenzie test

The vasoconstrictor activity of four steroids, administered in solution and in the commercially available form, were compared in healthy volunteer subjects. Evaluation was based on conventional visual observations and photometric measurement of reflectance. Statistical analysis showed that halopredone acetate had less vasoconstrictor action on healthy skin than fluocinolone acetonide, beta-methasone valerate and hydrocortisone acetate. The halopredone acetate results were identical to those of th excipient, although it was found that halopredone acetate cream (Topicon) had a marked anti-inflammatory effect in animals and on human dermatoses. The validity of the McKenzie test, as an unequivocal screening procedure for the potency of topical steroids is discussed, and the accuracy and precision of photometric evaluation is emphasized.

A comparative clinical evaluation of a new topical steroid 'halcinonide' and hydrocortisone in steroid-responsive dermatoses

Fifty patients with symmetrical, bilateral lesions of psoriasis, eczematous dermatitis, atopic dermatitis, or neurodermatitis participated in a double-blind paired comparison study in which 0-1% halcinonide (in a cream formulation containing also neomycin and nystatin) was applied to the lesions on one side of the body and 1% hydrocortisone cream to those on the opposite side for two to three weeks. The number of excellent responses to therapy showed the halcinonide combination to be significantly superior (p less than 0-01) to the control cream in all diagnostic categories if considered collectively, and in psoriasis if the responses were grouped according to diagnosis. No adverse reactions occurred during the trial.

The effect of clobetasone butyrate and other topical steroids on skin thickness of the domestic pig

A new glucocorticoid, clobetasone butyrate, has been shown in patients to have good topical anti-inflammatory activity and a minimal effect on hypothalamic-pituitary-adrenal function. Among a group of topically active corticosteroids, compared in a controlled study in the domestic pig, clobetasone butyrate is shown to cause less epidermal thinning than any of the others except only 1% hydrocortisone. This evidence of a lesser atrophogenic effect may indicate further dissociation of unwanted from wanted properties in clobetasone butyrate.

Percutaneous absorption of betamethasone-17, 21-dipropionate and salicylic acid during the treatment of psoriasis and eczema

The percutaneous absorption of betamethasone-17, 21-dipropionate and salicylic acid was studied after one and two weeks' treatment of psoriaris and eczema. The treated areas varied between 8 and 41 dm2. No change in the plasma cortisol levels was detectable by the routinely used laboratory method. The treatment gave no detectable salicylate concentrations in plasma.

Halcinonide, a new topical anti-inflammatory steroid

Halcinomide, a new topical fluorinated steroid was compared in a double-blind, simultaneous comparison with betamethasone valerate. Halcinoide was found to be superior in the treatment of psoriasis and equally effective as betamethasone valerate in the treatment of atopic dermatitis.

Effect of topical administration of antiinflammatory drugs to rats with adjuvant arthritis

Oral or intramuscular administration of antiinflammatory agents produces numerous undesirable side effects. This work explores the hypothesis that topical administration of such agents directly to the site of inflammation would have beneficial antiinflammatory effects. Topical administration of steroidal and nonsteroidal antiinflammatory drugs to rats with adjuvant arthritis was as effective as oral or intramuscular administration. Peak blood levels of radioactivity following administration of equal doses of hydrocortisone-3H were considerably lower after topical administration than after oral administration.

A new formulation of 0.1% hydrocortisone cream with vasoconstrictor activity and clinical effectiveness

A new formulation of hydrocortisone with vasoconstrictor activity has been shown to be effective clinically and was more effective in a 0.1% concentration than 1.0% Hydrocortisone Cream BPC. Its use should obviate the adverse effects of fluorinated steroids.

Absolute concentrations of dithranol and triacetyl-dithranol in the skin layers after local treatment: in vivo investigations with four different types of pharmaceutical vehicles

The pharmacokinetics of dithranol (anthralin) and its triacetate were investigated by employing a method which determines the quantity of the drug that penetrated into single layers of the human skin in vivo. For this purpose, tritium-labeled dithranol or triacetyl-dithranol was incorporated into four different ointments. The ointments were applied to the skin and biopsies were taken after 10, 30, 100, and 1000 min. The horny layer was removed before biopsy by Scotch tape stripping. The biopsies were sliced horizontally and the tritium determined in each sample. Dithranol as well as its triacetate penetrated best from more hydropholic ointments (Vaseline and aqueous wool-wax-alcohol ointment). From hydrophilic ointments (polyethylene glycol ointment and aqueous hydrophilic cream), only poor penetration was observed. Dithranol penetrated in far greater amounts than its triacetate, and the two compounds revealed fundamentally different penetration kinetics in epidermis and dermis. The data indicate that the triacetate was not split into its parent compound, dithranol, in substantial quantity, as the data obtained show the criteria of two independent substances.

A Comparison of Metosyn and Betnovate in the Treatment of Eczema and Psoriasis

A double-blind, paired-lesion comparison of fluocinonide 0·05% in a fatty alcohol propylene glycol base and betamethasone- 17-valerate 0·1% cream in the treatment of patients with eczema or psoriasis, is described. The fluocinonide preparation, ‘Metosyn’, was significantly superior to betamethasone- 17-valerate in the treatment of patients with eczema. Both topical steroids were equally effective in the treatment of psoriasis. The advantage of fluocinonide over betamethasone- 17-valerate was seen mainly in those patients who had been treated with betamethasone- 17-valerate preparations before the start of the investigation. Tolerance was equally good with both preparations.