Cutaneous responses to topical methyl nicotinate in human forearm and vulvar skin

Effect of L-4-Thiazolylalanine (Protinol™) on skin barrier strength and skin protection

OBJECTIVES

Skin barrier properties are critical for maintaining epidermal water content, protecting from environmental factors and providing the first line of defense against pathogens. In this study, we investigated the non-proteinogenic amino acid L-4-Thiazolylalanine (L4) as a potential active ingredient in skin protection and barrier strength.

METHODS

L4 on wound healing, anti-inflammatory and anti-oxidant properties were evaluated using monolayers and 3D skin equivalents. The transepithelial electrical resistance (TEER) value was used in vitro as a strong indicator of barrier strength and integrity. Clinical L4 efficacy was assessed for the evaluation of the skin barrier integrity and soothing benefits.

RESULTS

In vitro treatments of L4 show beneficial effects in wound closure mechanism, and we demonstrate that L4 anti-oxidant benefits with markedly increased HSP70 and decreased reactive oxygen species production induced by UVs exposure. Barrier strength and integrity were significantly improved by L4, confirmed clinically by an increase in 12R-lipoxygenase enzymatic activity in the stratum corneum. In addition, soothing benefits of L4 have been shown clinically with the decrease in redness after methyl nicotinate application on the inner arm and the significant reduction of the erythema and the skin desquamation on the scalp.

CONCLUSION

L4 delivered multiple skin benefits by strengthening the skin barrier, accelerating the skin repair process as well as soothing the skin and the scalp with anti-inflammaging effects. The observed efficacy validates L4 as a desirable skincare ingredient for topical treatment.

Topical Corticosteroids in the Treatment of Vulvar Lichen Sclerosus: A Review of Pharmacokinetics and Recommended Dosing Frequencies

Topical corticosteroids are often utilized as the first-line treatment for vulvar lichen sclerosus (VLS). However, there is wide variability in dosing regimens, as well as a lack of consensus on maintenance dosing. Available guidelines on dosing frequency and regimen continuation for VLS are based on clinical expert opinion and do not necessarily reflect the pharmacokinetics of topical corticosteroids. Over the past few decades, there have been many advances in the techniques used to measure the local and systemic absorption of topical corticosteroids. These techniques have led to a greater understanding of the pharmacokinetics and bioavailabilities of these medications. However, it is not clear how this new information has been applied in evaluating dosing regimens and commonly cited risks when considering short- and long-term use in different vulvar dermatoses. This purpose of this review is to evaluate the available evidence on pharmacokinetics, absorption rates, and concentration levels of topical corticosteroids in lesional and nonlesional skin. Additionally, the evidence regarding commonly cited risks of topical corticosteroid use, including dermal thinning, adrenal suppression, systemic immunosuppression, and tachyphylaxis are reviewed. Differences in the effects of topical corticosteroids on the varied tissues of the vulva are specifically explored. Finally, these considerations are applied to evaluate the current treatment guidelines for VLS to provide direction in determining an evidenced-based dosing regimen and to inform future research in this area. Mautz TT, Krapf JM, Goldstein AT. Topical Corticosteroids in the Treatment of Vulvar Lichen Sclerosus: A Review of Pharmacokinetics and Recommended Dosing Frequencies. Sex Med Rev 2021;XX:XXX-XXX.

Ex vivo penetration analysis and anti-inflammatory efficacy of the association of ferulic acid and UV filters

BACKGROUND

Unprotected chronic exposure to ultraviolet (UV) radiation generates many harmful effects to human skin and sunscreens are essential to health, however, traditional products do not provide enough protection against cutaneous oxidative stress, a process amplified by UV radiation. Therefore, the development of multifunctional photoprotective formulations seems to be a more efficacious approach, since these enable the absorption/reflection of UV radiation and maintain the cutaneous homeostasis.

OBJECTIVES

In the present study, ferulic acid (FA), a well-known antioxidant, has been combined with two UV filters, bemotrizinol and ethylhexyl triazone, and the safety and efficacy of this formulation has been assessed combining ex vivo and in vivo methods.

METHODS

Skin permeation assays were performed by applying the formulation in the volar forearm of participants, after which consecutive samples of the stratum corneum were collected by tape stripping, and the quantification of FA, bemotrizinol and ethylhexyl triazone was performed by high-performance liquid chromatography (HPLC). Also, the FA anti-inflammatory action in combination with the UV filters was probed through a method employing Laser Doppler flowmetry to measure the vasodilatory response to methyl nicotinate topical application.

RESULTS

Skin permeation assay was able to characterize the penetration depth of each substance. It should also be noted that a specific HPLC analytical method was developed in this study to enable the rapid simultaneous quantification of the three substances. Results from Laser Doppler flowmetry showed that the FA was able to mitigate the vasodilatory response.

CONCLUSIONS

FA proved to be a valuable resource in a multifunction sunscreen, not only providing an increase in the SPF of sunscreens, previously published, but also decreasing the extent of inflammation.

Assessment of the variation of skin barrier function with anatomic site, age, gender and ethnicity

The skin is the largest organ of the human body and its functions include protection, thermoregulation sensation and secretion. Significant advances in our understanding of how the morphology and physiology of the skin contribute to the skin's barrier role have been achieved in recent years. The aim of this review is to summarize the principal approaches which have been used to assess variation in skin barrier function with anatomic site, age, gender, and ethnicity. The methods discussed include trans-epidermal water loss (TEWL) measurement, assessment of corneocyte size, response to vasoactive compounds and attenuated total reflectance Fourier transform infrared (ATR-FTIR) interrogation of skin. The utility of the various methods is considered and the most important findings in the literature to date are highlighted.

Transfer of the experimental methodology described in the FDA guidance for corticosteroid bioequivalence testing to pharmacodynamic effects caused by nicotinates

BACKGROUND

The measurement of the pharmacodynamic response allows the noninvasive quantification of cutaneous drug penetration.

AIMS

The objective of this study was to investigate whether the experimental methods described in the US Food and Drug Administration Guidance for Industry "Topical Dermatologic Corticosteroids: In vivo Bioequivalence" may be transferred to other response parameters such as skin redness and surface temperature.

METHODS

Drug penetration experiments with methyl nicotinate in two different lipophilic vehicles were performed according to the FDA guidance for corticosteroid bioequivalence testing measuring the cutaneous erythema and skin temperature response.

RESULTS

The guidance methodology was transferred to the response parameters redness and temperature. Bioequivalence testing was feasible with these response parameters.

CONCLUSIONS

An open one-compartment model could only be confirmed for skin redness data by a compartmental analysis of response vs. time profiles. The obtained temperature data can neither be described by an open one-compartment nor by a two-compartment model. A correlation between skin color and skin surface temperature could not be found.

Time of Erythema Onset after Application of Methyl Nicotinate Ointments as Response Parameter: Influence of Penetration Kinetics and Enhancing Agents

The time of erythema onset may be used as a response parameter for quantification of the cutaneous erythema response induced by methyl nicotinate. The vehicles light mineral oil (LMO; test) and medium chain triglycerides (MCT; standard) were compared with regard to the pharmacodynamic response. Moreover, the influence of penetration enhancers on the time of erythema onset was investigated under zero order penetration kinetics. The enhancers dimethyl sulfoxide, diethylene glycol monoethyl ether and three different glycerides in different concentrations were added to MCT as a standard vehicle. All preparations were applied to the forearms of volunteers under infinite dose conditions at different thermodynamic drug activity levels (0.2-3.2% of the saturation level) and different drug concentrations (0.051-0.816%), respectively. Different penetration kinetics do not influence data of erythema onset, as these data are comparable to those obtained under finite dose conditions (first order penetration kinetics). With regard to the penetration enhancers, a significantly enhanced penetration of methyl nicotinate could be observed only for diethylene glycol monoethyl ether and dimethyl sulfoxide. However, no significant difference between light mineral oil and MCT could be found with regard to penetration enhancement. The time of erythema onset is an easy and efficient parameter for quantification of the pharmacodynamic response caused by nicotinates.

Percutaneous Penetration of Methyl Nicotinate from Ointments Using the Laser Doppler Technique: Bioequivalence and Enhancer Effects

Laser Doppler flowmetry (LDF) may be used to quantify erythema response as a result of an increased cutaneous microcirculation induced by methyl nicotinate (MN). Bioequivalence of a test and a standard preparation (vehicles: light mineral oil and medium chain triglycerides, respectively) was confirmed according to the pilot study of the FDA Guidance for Industry "Topical dermatologic corticosteroids: In Vivo bioequivalence" applying the staggered application and synchronized removal method for one defined concentration. Furthermore, the influence of penetration enhancers (5% w/w Dimethylsulfoxide (DMSO) and 10% w/w diethylene glycol monoethyl ether) on MN penetration was investigated. It was shown that DMSO and diethylene glycol monoethyl ether altered cutaneous microcirculation and thus MN penetration in comparison to the standard formulation. However, true penetration enhancement could only be proved with diethylene glycol monoethyl ether resulting from an improved drug solubility in the skin which was confirmed by attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR). Increased MN penetration by DMSO was only caused by thermodynamic effects, i.e. a decreased drug solubility in the vehicle.

Kinetics of blood flow after topical application of benzyl nicotinate on different anatomic sites

Cutaneous characteristics, e.g., thickness of the SC and density of follicles, affect the penetration of topically applied substances. In the present study, the penetration of benzyl nicotinate, causing a vasodilation, was studied on three anatomic sites (forearm, forehead and calf) differing in their skin characteristics. Therefore, the blood flow of the superficial dermal plexus and that of the larger capillaries in the deeper skin layers was simultaneously measured using a laser Doppler flowmeter. In addition, the cutaneous temperature and redness were determined as a function of time. These four biological reactions were measured in turn on a skin area treated with a gel containing benzyl nicotinate and on an untreated control area. The highest basal levels were observed on the forehead. Topical application of benzyl nicotinate resulted in an increase in each biological response. Compared to the other sites, the maximal values were reached earliest on the forehead, which also showed the fastest decrease. No significant differences were obtained comparing the kinetic data of the calf with that of the forearm. The results indicate an additional contribution by the numerous vellus hair follicles of the forehead to the penetration and exposure of the drug.

Sensory, clinical and physiological factors in sensitive skin: a review

Certain individuals experience more intense and frequent adverse sensory effects than the normal population after topical use of personal care products, a phenomenon known in popular usage as sensitive skin. Consumer reports of sensitive skin are self-diagnosed and often not verifiable by objective signs of physical irritation. Companies who manufacture cosmetic and personal care products are challenged to provide safe products to an audience with tremendous differences in skin type, culture and habits. This review examines the still incomplete understanding of this phenomenon with respect to aetiology, diagnosis, appropriate testing methods, possible contributing host factors such as, sex, ethnicity, age, anatomical site, cultural and environmental factors, and the future directions needed for research.

Establishing a minimal erythema concentration of methyl nicotinate for optimum evaluation of anti-inflammatories

Topical administration of chemicals such as methyl nicotinate that induce erythema have been employed to measure the effectiveness of formulations containing anti-inflammatory agents. Prior studies have utilized a single concentration of methyl nicotinate, between 36.5 and 100 mM, for all test subjects in evaluations of topical formulations. However, individuals have different thresholds of response to methyl nicotinate; thus, a single concentration may not be appropriate for all individuals and could result in the apparent lack of anti-inflammatory activity of the formulation being evaluated. In the current study, we evaluated the use of a minimal erythema concentration (MEC) of methyl nicotinate, defined as the lowest concentration that produces a complete and even erythema at the test site, compared with a 36.5-mM concentration of methyl nicotinate. Hydroalcoholic gels containing the nonsteroidal anti-inflammatory drug ibuprofen were compared with placebo. Diffuse reflectance spectroscopy was employed to measure differences in cutaneous inflammatory response between the control (placebo)-treated group and the ibuprofen-treated group. When chemical erythema was induced using an MEC of methyl nicotinate, greater reductions in erythema were seen in ibuprofen-treated sites compared with sites treated with a 36.5-mM concentration of methyl nicotinate. In conclusion, for an accurate assessment method of erythema induced by methyl nicotinate, consideration should be given to determining the extent of response of an erythema-producing agent on an individual basis. An MEC of methyl nicotinate should be determined and employed for each individual to obtain more consistent and reliable efficacy results of anti-inflammatory activity.

Comparison of blood flow to the cutaneous temperature and redness after topical application of benzyl nicotinate

The topical application of drugs, such as nicotinates, affects cutaneous blood flow. Such a biological response, which is dependent on the drug and the individual, can be measured noninvasively using laser Doppler flowmetry. We illustrate the kinetics of vasodilation caused by topically applied benzyl nicotinate using a new frequency-selective laser Doppler flowmeter. This flowmeter measures the blood flow in the superficial dermal plexus and the deeper lying larger capillaries simultaneously and indirectly by determining the flow velocity. Both sets of data are compared with the skin temperature and redness. Four biological parameters are measured consecutively on a skin area treated with gel containing benzyl nicotinate and on an untreated control area. A linear relationship between both blood flows is observed. However, no correlation is obtained between the microcirculation with either the cutaneous temperature or the redness. These results indicate the transport of the drug in the blood from the upper to the deeper capillaries. Cutaneous temperature and redness are unsuitable parameters to measure the kinetics of the blood flow after topical application of drugs.

Transepidermal Water Loss Dynamics of Human Vulvar and Thigh Skin

Refinement in procedures to assess skin surface water loss (SSWL) dynamics of the vulvar skin on a large sample of subjects (60) is described and compared to another semi-occluded skin site, the inner thigh. Vulvar SSWL significantly decreased over a 30-min period from 46.2 +/- 2.6 (SE) to 24.7 +/- 1.6 g m(-2) h (p < 0.001). The inner thigh, another semi-occluded region, showed no similar pattern for SSWL (6.2 +/- 0.3 to 6.6 +/- 0.5 g m(-2) h), and the values were significantly less than those for vulvar skin. There was no significant effect of age, body mass index or atopic status on vulvar SSWL.

A new technique to map vulva microcirculation using laser Doppler perfusion imager

The location of the vulva has had limited technologic progress in the assessment of the skin microcirculation and its application to clinical practice. Our group previously demonstrated increased perfusion in vulvar cancer compared to adjacent uninvolved skin with the laser Doppler flowmetry (LDF). The LDF is severely limited by its low spatial resolution ( approximately 1 mm(2)) and pressure involved in positioning of the probe, which may affect value of the underlying tissue perfusion. Topographic perfusion mapping of the whole vulvar skin using LDF is also time consuming and is not clinically practical. We describe for the first time the application of the novel laser Doppler perfusion imager (LDPI) to map vulvar skin blood flow and give example in two cases with well-defined vulvar pathology-psoriasis and lichen sclerosus with invasive neoplasia-and discuss the potential of LDPI to study vulvar skin blood flow.

Topical methyl nicotinate-induced skin vasodilation in diabetic neuropathy

OBJECTIVE

To evaluate the vasodilation induced by topical application of methyl nicotinate (MN) and to compare it with the vasodilatory response to acetylcholine (ACh) and sodium nitroprusside (SNP) in healthy subjects and diabetic neuropathic patients.

RESEARCH DESIGN AND METHODS

Ten diabetic patients with peripheral neuropathy (DN) and 10 age- and sex-matched healthy control subjects (C) were enrolled. The vasodilatory response to topical application of 1% MN and a placebo emulsion at the forearm and dorsum of the foot skin at 5, 15, 30, 60 and 120 min was measured using Laser Doppler Perfusion Imaging. The vasodilatory response to iontophoresis of 1% ACh and 1% SNP solutions was also evaluated.

RESULTS

The maximal vasodilatory response to ACh, SNP and MN was similar at the forearm and foot level in the diabetic patients. In the control group, the responses to MN, ACh and SNP were similar on the forearm but in the foot, the MN vasodilatory response was higher when compared to the ACh and SNP responses. MN-related vasodilation was present 5 min after the application, reached its peak at 15-30 min and declined to pre-application levels 120 min afterward.

CONCLUSIONS

Topical application of MN at the forearm and foot levels of diabetic neuropathic patients results in skin vasodilation that is comparable to the maximal vasodilation that can be induced by iontophoresis of ACh or SNP and lasts for less than 2 h. Further studies will be required to explore the potential of MN to increase blood flow and to prevent diabetic foot problems in clinical practice.