Skin whitening product efficacy testing examined

“Skin whitening”, “skin lightening”, “skin brightening” – these terms are used interchangeably1 when describing the claim for what is in effect a benefit of skin care products associated with attempting to arrest or reverse the effects of actinic light damage associated with time and sun exposure.

While local terms do appear to vary from country to country, “skin whitening” has been negatively associated with products which were previously marketed for the purpose of reducing the overall colour of the skin, utilising actives such as hydroquinone which can cause permanent depigmentation and sensitivity to sunlight. In most countries these are now considered to be therapeutics and banned from open sale. “Brightening” more or less describes the overall end effect of evening the skin appearance or tone. For the purposes of this article, I will use the term “lightening”, interpreted as implying a focus on treating deeply pigmented areas. The skin lightening properties of any personal care product must obviously be designed into the formulation. As they are invariably a function of one or more “active” ingredients, this is the first place to look when deciding on the claim support protocol. Typically, the providers of these active raw materials provide claims supported by evidence of proven or potential efficacy. Some examples of these are set out in Table 1. What is obvious from these claims is that they are mostly too scientific for the consumer level, that they are based on in vitro bio-chemical pathway experiments and that they imply therapeutic definition. For the marketer of a skin lightening preparation, this type of claim substantiation testing is probably not appropriate. It is important to recognise that there are two ways to approach efficacy for these types of formulation – treatment and prevention. Claims also fall into the general classification of high level, low level or simple puffery. When briefing the test laboratory for preparation of an appropriate test protocol, the easiest start point is to draft an intended label. This will provide a much closer match for eventual claim support than simply requesting “to see if it works”. Some claims will be beyond substantiation. Consider for example “strongest formula available over the counter” or “stabilises the immune system of the skin”. In general though, anything relating to visual change, the primary performance expectation of the consumer, will be measurable. Treatment testing will imply the use of human test subjects, an in-use study over a reasonably extended time, and the measurement of skin colour change. Prevention testing is more likely to require in vitro testing, with the common focus being on challenging the product with ultraviolet light.

Treatment testing

As the consumer’s expectation is the reduction in skin coloration of overly pigmented areas, such as age spots and blotches, the most appropriate measurement is associated with defining colour change. For this purpose, a chromameter is used, typically in combination with a computer. The instrument most commonly used is a hand-held spectrophotometer such as supplied by Konica Minolta. The instrument works by projecting tri-stimulus light2 (similar to an overhead projector) and recording the colour values into an interpretable scale – L*a*b*. These values are derived from the C.I.E. standardised three-dimensional colour space model, where L* is the “grey” scale i.e. whiteness – blackness, a* is the redness – greenness value and b* is the yellowness – blueness (see Fig. 1). This is not dissimilar to the wheel for colour selection in computer software such as Microsoft Word. For the L*a*b* scale, the L* value to a large degree represents the lightening (clarity) measurement we are looking for in the test study, but it is modified by the a* and b* values according to skin type as seen in the background colour of an adjoining non-pigmented skin area. In general L* decreases while blueness b* and redness a* increase with chronological age.3 Typically, the values obtained will form the quantifiable values for expressing change from the start point of the study. Table 2 shows an example. Protocol parameters which must be considered for any study are test subject age group, ethnicity and study duration. While the target market might be, for instance, age group 25 to 65, the most measurable change will typically be in the older segment of this group. So, is the objective to prove how effective the product is, or to show it works for a 25 year old as well as a 65 year old? The client will need to decide this. What ethnicity mix should be selected? Once again, this will depend on the balance between marketing and scientific considerations. Certainly, Asian skin tends to express age spots as blotching, particularly on the cheeks, while Caucasian skin expresses a more speckled appearance of very small blemishes. For darkly pigmented skin, it is much more difficult to measure change as the initial difference from the background is lower to begin with. While short-term studies can be conducted, the slow acting nature of nontherapeutic formulations usually means that a study over 4, 8 or 12 weeks will be needed in order to show statistically significant change, for example 90% of users showed more than 55% reduction in intensity of deeply pigmented spots after 60 days of use. The preferred season for performing the testing is the northern or southern hemisphere winter, where actinic light is at its lowest. This gives the best opportunity to make the measurements without the interference of sun tan, which will cause changes to the colour of the background unpigmented reference skin areas. As it is unethical for the clinical tester to restrict the test participants from using their usual sun protection practices during the course of the study, this should be taken into account in preparation of the protocol. Standard colour swatches, such as the widely available PMS colour charts can be useful in expressing the colour of skin. They can be used for smaller pigmented areas below the minimum diameter limits of the chromameter. However, the matching is highly subjective and subject to the discriminatory abilities of the technician. Photography is a very useful and convenient adjunct to colour measurement. Systems such as Visia provide highly reproducible mapping of progression during treatment testing.

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