Comparative in vitro percutaneous absorption of p-substituted phenols through rat skin using static and flow-through diffusion systems

Flow-through versus static in vitro percutaneous penetration at 50 years: Possible relevance for bioequivalence

OBJECTIVE

Compare the relevance of flow-through versus static diffusion cells data as relates to bioequivalence.

METHODS

Search was conducted on PubMed and Google Scholar. Keywords utilized: static cells, flow-through cells, percutaneous permeation, percutaneous absorption, dermal absorption, and types of permeation.

RESULTS

Fifteen articles were identified with no consistent significant differences between flow-through and static diffusion cells identified; any differences could exist for two main reasons. (1) Sampling time differences and (2) physical chemistry (lipophilic vs hydrophilic) of the penetrant examined.

CONCLUSION

Even though there was no consistent significant difference observed, labs have generally adapted to the method they regularly use, which is usually stated in their respective articles. Well-designed multicentered prospective comparative experiments should clarify potential advantages and disadvantages for each. For flow-through systems, the flow rate that most approximates to comparable in vivo data for animals and humans may be preferable.

Oral uptake of nanoparticles: human relevance and the role of in vitro systems

Nanoparticles (NPs) present in environment, consumer and health products, food and medical applications lead to a high degree of human exposure and concerns about potential adverse effects on human health. For the general population, the exposure through contact with the skin, inhalation and oral uptake are most relevant. Since in vivo testing is only partly able to study the effects of human oral exposure, physiologically relevant in vitro systems are being developed. This review compared the three routes taking into account the estimated concentration, size of the exposed area, morphology of the involved barrier and translocation rate. The high amounts of NPs in food, the large absorption area and the relatively high translocation rate identified oral uptake as most important portal of entry for NPs into the body. Changes of NP properties in the physiological fluids, mechanisms to cross mucus and epithelial barrier, and important issues in the use of laboratory animals for oral exposure are mentioned. The ability of in vitro models to address the varying conditions along the oro-gastrointestinal tract is discussed, and requirements for physiologically relevant in vitro testing of orally ingested NPs are listed.

In vitro human skin penetration model for organophosphorus compounds with different physicochemical properties

A flow-through diffusion cell was validated for in vitro human epidermal penetration studies of organophosphorus compounds (OPCs) applied by infinite dosing. By testing OPCs with similar molecular weight but different physicochemical properties, it was shown that hydrophilic and lipophilic properties are major determinants for the penetration rate. Lipophilic OPCs displayed maximum cumulative penetration in the 20-75% agent concentration range whereas the hydrophilic OPCs displayed maximum cumulative penetration at 10 or 20% agent concentration. Low penetration was observed for all agents at 1% agent concentration or when applied as neat agents. The impact of the receptor solution composition was evaluated by comparing the penetration using receptor solutions of different ratios of ethanol and water. For diluted OPCs, a high concentration of ethanol in the receptor solution significantly increased the penetration compared to lower concentrations. When OPCs were applied as neat agents, the composition of the receptor solution only affected the penetration for one of four tested compounds. In conclusion, the flow-through diffusion cell was useful for examining the penetration of OPCs through the epidermal membrane. It was also demonstrated that the penetration rates of OPCs are strongly influenced by dilution in water and the receptor fluid composition.

Design, formulation and optimization of valsartan transdermal gel containing iso-eucalyptol as novel permeation enhancer: preclinical assessment of pharmacokinetics in Wistar albino rats

OBJECTIVE

The aim of this study was to develop and optimize a transdermal gel formulation of valsartan using Box-Behnken design and to evaluate it for pharmacokinetic study.

METHODS

The independent variables were Carbopol 940 (X1), PEG 400 (X2) and ethanol (X3) while valsartan flux (Y1), Tlag (Y2) and gel viscosity (Y3) were the dependent variables. Iso-eucalyptol was added in all gel formulations as permeation enhancer except for control gel.

RESULTS

It was observed that the permeation rate of valsartan significantly increased in direct proportion to the ethanol concentration, but significantly decreased in direct proportion to polymer concentration. Lag time and viscosity decreased in reverse proportion to ethanol concentration. The optimized valsartan gel formulation (VGF-OPT) yielded flux of 143.27 ± 7.11 µg/cm(2)/h and 27.55 ± 2.51 µg/cm(2)/h across rat and human cadaver skin, respectively. In vivo pharmacokinetic study of VGF-OPT-transdermal therapeutic system containing iso-eucalyptol showed a significant increase in the bioavailability (2.52 times) compared with oral formulation of valsartan by virtue of better permeation through Wistar rat skin.

CONCLUSION

It was concluded that the developed transdermal gel accentuates the flux of valsartan and could be used as an antihypertensive dosage form for effective transdermal delivery of valsartan.

Quantitative structure-permeability relationships (QSPRs) for percutaneous absorption

Quantitative structure-permeability relationships (QSPRs) have been derived by many researchers to model the passive, diffusion-controlled, percutaneous penetration of exogenous chemicals. Most of these relationships are based on experimental data from the published literature. They indicate that molecular size (as molecular weight) and hydrophobicity (as the logarithm of the octanol-water partition coefficient; log k(ow)) are the main determinants of transdermal penetration. This article reviews the current state of the art in QSPRs for absorption of chemicals through the skin, and where this technology can be exploited in future research. The main shortfalls in QSPR models result from inconsistency and error of the experimental values used to derive them. This is probably caused by the manner in which they employ data from a variety of sources and, in some cases, slightly different experimental protocols. Further, most current models are based on data generated from either aqueous or ethanolic solution, where each penetrant is present at its saturated solubility or a fraction of its saturated solubility. No models currently account for the influences of formulation upon percutaneous penetration. Current QSPR models provide a significant tool for assessing the percutaneous penetration of chemicals. They may be important in determining the bioavailability of a range of topically applied exogenous chemicals, and in issues of dermal toxicology and risk assessment. However, their current use may be limited by their lack of applicability across different formulation types. As a consequence, their true value may be to make predictions within specific formulation types, as opposed to a general model based on a range of formulation types. In addition, the endpoint of models may be inappropriate for specific applications other than the systemic delivery of topically applied chemicals.

Evaluation of a novel diffusion cell for in vitro transdermal permeation: effects of injection height, volume and temperature

The objective of this study was to evaluate the performance of a new, compact, dynamic diffusion cell for in vitro transdermal permeation. These so-called Kelder-cells were developed as an automated alternative to the static Franz diffusion cells. The new cells were used in combination with the ASPEC-system (automatic sample preparation with extraction columns) which was initially designed for the automation of solid-phase extractions. Three variables were tested to optimize the performance of the new cell system: injection height into the inlet compartment, volume flowing through the receptor compartment and temperature. Experiments were performed using the tritium labelled anticholinergic [3H]dexetimide permeating through an artificial membrane (Silastic). The injection height of the needle into the inlet compartment of the cell should be programmed at -34 mm to ensure complete air tightness, thus forcing the buffer to flow through the cell. The volume of buffer flow through the receptor compartment is important in maintaining sink conditions: a volume of 117 microliters was chosen to replace the total content of the cell (84 microliters) every 2 min. The temperature was precisely controlled in a thermostatic cabinet to minimize variations in experimental conditions. For [3H]dexetimide, an increase in temperature of 20 degrees C reduced the lag time by a factor of approximately two, however the influence on the flux was negligible. The data for the Kelder-cells were comparable with static Franz diffusion cells at a pseudo-steady state, however Kelder-cells have the advantage of automatic sampling, continuous replacement of the receptor solution, and unattended operation over at least 24 h.

Percutaneous penetration of 2-phenoxyethanol through rat and human skin

2-Phenoxyethanol applied in methanol was absorbed (64 +/- 4.4% at 24 hr) through unoccluded rat skin in vitro in the static diffusion cell with ethanol/water as receptor fluid. By comparison (43 +/- 3.7% in 24 hr) was absorbed in the flow-through diffusion system with tissue culture medium as receptor fluid. 2-Phenoxyethanol applied in methanol was absorbed (59.3 +/- 7.0% at 6 hr) through unoccluded human skin in vitro in the flow-through diffusion cell with tissue culture medium. With both unoccluded cells, 2-phenoxyethanol was lost by evaporation but occlusion of the static cell reduced evaporation and increased total absorption to 98.8 +/- 7.0%. Skin, post mitochondrial fraction, metabolized phenoxyethanol to phenoxyacetic acid at 5% of the rate for liver. Metabolism was inhibited by 1 mM pyrazole, suggesting involvement of alcohol dehydrogenase. However, first-pass metabolism of phenoxyethanol to phenoxyacetic acid was not detected during percutaneous penetration through viable rat skin in the flow-through system. First-pass metabolism in the skin does not therefore have an influence on systemic availability of dermally absorbed phenoxyethanol. These measures of phenoxyethanol absorption through rat and human skin in vitro agree well with those obtained previously in vivo.

In vivo disposition of p-substituted phenols in the young rat after intraperitoneal and dermal administration

The objective of this study was to examine the 120-hr disposition of phenol and four p-substituted congeners after ip and dermal administration in the 29-day-old female rat. The dermal absorption was very high (66-80% of the dose) for phenol, cyanophenol, heptyloxyphenol and nitrophenol, but minimal for hydroxybenzoic acid (2%). The major portion of the dose for all of the phenols not absorbed dermally in 24 hr was washed from the skin. Only minor amounts (1-2%) were detected in the treated skin at 120 hr. Urinary excretion was the predominant means of elimination for these phenols and occurred primarily within 24 hr after dermal and ip administration. However, the excretion of heptytoxyphenol after administration by both routes differed from that of the other compounds, with more of it detected in the faeces. The profile of metabolites in urine (collected at 12-24 hr) from the animals dermally treated with phenol, cyanophenol, heptyloxyphenol and nitrophenol showed only peaks that eluted earlier than the parent compound, which suggests that conjugates or more polar metabolites were formed and excreted. The difference in dermal absorption between hydroxybenzoic acid and the other phenols may be due to potential ionization of the p-substituted carboxylic acid group of hydroxybenzoic acid. This suggests that, at least for the phenols examined in this study, physicochemical characteristics other than just lipophilicity can affect in vivo dermal absorption.

Disposition of phenol in rat after oral, dermal, intravenous, and intratracheal administration

The absorption and elimination of [14C]-phenol (63.5 nmol) after oral, dermal, intratracheal, or intravenous administration in rat was rapid and extensive. Urinary elimination of radioactivity predominated, with a range of 75-95% of the dose detected in urine by 72 h post-exposure. Washing the dermal site 72 h post-exposure removed 14% of the dose. Two per cent of the dose was detected in the skin. The urinary metabolites at 4 and 8 h after administration by the four routes included phenyl sulphate and lower amounts of phenyl glucuronide. Phenol was poorly retained in the body after administration by the four routes. Phenol remaining in the body was widely distributed, with accumulation primarily in the liver, lung, and kidney.