IN VITRO PENETRATION OF PETROLATUM IN STRATUM CORNEUM 255 all time points is summarized in Table I. The average total recovery was between 92% and 103% (range: 82–129%). DISCUSSION APPLYING AND RINSE-OFF METHOD A previous study investigated t he penetrat ion of petrolatum into the str atum corneum after skin application (6). The differences between the previous Franz diffusion cell and tape-stripping study and the present study include the methodology (with and without simulated rinsing) and the use of the bodywash formulation. In the present study, the bodywash formulation was applied to the in vitro skin and spread in circular motions using a smooth ground fl at surface glass rod. A rinse-off protocol was then applied to remove the bodywash from the in vitro skin surface. Nguyen et al. (7) studied the meth- ods to simulate rubbing of a gel topical formulation on skin and found that using the vertical glass rod method resulted in spreading of the gel to the entire area and facili- tated a rapid onset of product delivery at 4 h. The glass rod method was the method of choice for applying topical formulations in skin penetration studies in vitro. Using the present method of bodywash application and rinsing, a signifi cant portion of petrola- tum was deposited on the skin. Particularly, the rinse-off data in Table I show that approximately 1/3 of the petrolatum in the bodywash formulation was deposited on the skin after the rinse-off protocol. Approximately 1/3 of the petrolatum in the bodywash remaining on the skin (~1/9 of the total petrolatum in the bodywash) was removed by the fi lter wipes at the end of the penetration study. The relatively large amount of pet- rolatum deposited on the skin surface after rinsing (i.e., ~1/3 of the dose of petrolatum) could lead to better petrolatum penetration into the stratum corneum with the body- wash formulation. PENETRATION OF PETROLATUM INTO THE STRATUM CORNEUM In a previous study w ith a heat-separated human epidermal membrane, when petrola- tum was directly deposited on the skin surface using a volatile solvent, hexane, a trace amount of petrolatum ( 2% applied dose) was found to penetrate the membrane into the receptor chamber of the diffusion cell at 72 h postapplication and no detectable penetration of petrolatum was found in the receptor chamber when split-thickness skin was used ( 0.05% applied dose, estimated from the detection limit) (6). This result indicates that petrolatum could penetrate across the epidermis but not the split-thick- ness dermis, possibly because of the high lipophilicity of petrolatum. This also suggests that petrolatum likely would not penetrate across the dermis and enter the systemic circulation from each application under the in vivo condition. Petrolatum is a mixture of C12–C85 long-chain aliphatic hydrocarbons (8). It was suggested that highly lipo- philic compounds such as petrolatum can accumulate in the stratum corneum (9). It was also suggested that the shorter chain hydrocarbons in petrolatum were able to dis- solve and penetrate into the deeper layers of the epidermis, whereas the longer chain hydrocarbons were “trapped” in the upper layers. 14 C-dotriacontane is a C32 alkane and is one of the main components of petrolatum (6). It was used as the radiolabeled probe
JOURNAL OF COSMETIC SCIENCE 256 (i.e., a model permeant) of the petrolatum in the present study. The study results show that dotriacontane was mainly found in the upper layers of the stratum corneum (e.g., Tapes 1 and 2) and was traceable in the stratum corneum deeper layers (e.g., in 12 tape- stripping study, Figure 3). The petrolatum detected in the stratum corneum from tape- stripping could be related to the ridges and furrows (10) and/or desquamated layers of the skin. A comparison of these tape-stripping results with those from petrolatum de- position using a volatile solvent in a previous study (6) shows that the amounts of petrolatum (as % applied dose) penetrated into the stratum corneum from the solvent deposition method were ~5× larger than those from the bodywash. The difference is likely attributed to the (a) different dosing methods, (b) different amounts of petrola- tum deposited on the skin surface possibly relating to the dosing methods, (c) different data analyzes including the number of compartments used in the calculations, and/or (d) skin-to-skin variability. Despite these differences, when the data were compared based on the total amounts of petrolatum penetrating the stratum corneum, taking into account the differences due to water rinsing, skin wiping, and donor chamber re- covery wash, the two skin deposition methods provided similar % amount penetration profi les in the stratum corneum. In addition to the present in vitro study, previous clinical data have suggested that the penetration of petrolatum into the stratum corneum is related to the effect of glycerol monooleate (11). Glycerol monooleate is a glyceryl fatty acid ester that has a cis C=C double bond at the C9 position. It is widely used in cosmetic products as an emulsifi er and absorption enhancer. The Food and Drug Administration (FDA) has classifi ed this ingredient as generally recognized as safe in the FDA Inactive Ingredient Guide. It is considered nontoxic, biodegradable, and biocompatible (12). The study concluded that petrolatum bodywash with glyceryl monooleate clinically improved skin appearance, skin surface barrier function, and hydration in humans compared with bodywashes without glyceryl monooleate (11). The results in the present study support a possible relationship between petrolatum skin penetration and the observed clinical benefi ts, and the increase in petro- latum penetration depth by glyceryl monooleate in the stratum corneum (6) could be one of the factors leading to these benefi ts. Further studies will be needed to explore the rela- tionship and mechanism of petrolatum and glyceryl monooleate. CONCLUSION The bodywash formulation with petrolatum could provide signifi cant deposition and penetration of petrolatum into the stratum corneum (~0.03–0.05 mg/cm2 from 5 μL bodywash formulation) at 1–72 h postapplication. Specifi cally, petrolatum was observed to deposit from the bodywash when applied on the split-thickness skin with simulated rinsing. The petrolatum then penetrated into the stratum corneum and was detected at the depth of 12 tape-stripping and further in the epidermis. A CKNOWLEDGMENTS T his study was fi nancially supported by Procter & Gamble Co. (Mason, Ohio). The authors are grateful to Dr. Gerald B. Kasting for his invaluable discussion and expert guidance.
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