In vitro/vivo SPF Correlation and Repeatability According to Substrate

Sep 1, 2013 | Contact Author | By: Sébastien Miksa, Dominique Lutz and Céline Guy, HelioScreen Labs
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Title: In vitro/vivo SPF Correlation and Repeatability According to Substrate
substratex variabilityx correlationx in vitro SPFx sunscreen productsx
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Keywords: substrate | variability | correlation | in vitro SPF | sunscreen products

Abstract: This work evaluates the impact of three different substrates on in vitro SPF measurements, and defines experimental conditions to improve their correlation with in vivo values. Evaluations of 32 products, shown here, led the authors to conclude that molded substrates improved repeatability and correlation with in vivo SPF values.

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S Miksa, D Lutz and C Guy, In vitro/vivo SPF Correlation and Repeatability According to Substrate, Cosm & Toil 128(9) 648 (2013)

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Editor’s note: This article is the second in a four-part series considering the effects of test variables on SPF results. Here, the authors assess substrate choice; the first, published in July 2013, considered changes in substrate surface temperature. The third will consider an automated approach to controlling several test variables and will appear in the October 2013 issue.

Sun protection is currently a topic of concern due to the correlation between UV radiation and the incidence of skin cancer and skin aging. The determination of the level of protection a sunscreen product provides against ultraviolet (UV) radiation, expressed by SPF and UVA-PF, is historically based on in vivo methods and recent standards that have been published.1, 2 Further, for various reasons, in vitro methods are replacing in vivo methods; therefore, the reliability of in vitro evaluations is based solely on correlations with previous in vivo indexes. Test endpoints are determined in laboratory conditions and are made as realistic as possible, but reproducibility is the main factor in ensuring the reliability of the target for the in vitro method. To ensure inter laboratory reproducibility with the in vivo method, requirements include: a balanced volunteer panel, careful application, a controlled light source and a reproducible minimal erythema dose (MED) reading. In addition, one must consider the in vivo/in vitro correlation and repeatability—key parameters of which several previous studies have identified, including: quantity of product;3 spreading protocol; UV spectrum; temperature on the surface substrate;4 substrate properties such as surface energy5 or topographic parameters.6

Concerning this last parameter, different substrates have been proposed in recent years, including transparent tapea;7 quartz plates and skin equivalentsb, and polymethyl methacrylate (PMMA) plates. Initially, PMMA plates were produced using an uncontrolled sand blasting process, but this led to variability and poor reproducibility. However, when produced via a molded process, the resulting plates offer high reproducibility for both roughness and topographic parameters.6 For these reasons, Cosmetics Europe, formerly COLIPA, and ISO recommended plates with strictly controlled topographic parameters in their most recent methods and standards.8, 9

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Table 1. Liquids used for surface energy determination

Table 1. Liquids used for surface energy determination

Here, the contact angle of water and diiodo-methane were measured by means of drop shape analysis system by placing a 5-µL drop of each on the substrate; the characteristics of these liquids are listed in the Table 1.

Table 2. Tested sunscreens products

Table 2. Tested sunscreens products

For all products, several measurements by different institutes had previously been taken, and in vivo SPF values ranged from 9 to 85 (see Table 2).

Table 3. Topography results according to substrates

Table 3. Topography results according to substrates

The surface topography of substrates was measured to compare topography parameters, the results of which are summarized in Table 3.

Table 4. Surface images according to substrates

Table 4. Surface images according to substrates

Surface images according to substrates are shown in Table 4.

Table 5. In vivo and in vitro SPF values according three different substrates; mean, SD, CV% on 18 values

Table 5. In vivo and in vitro SPF values according three different substrates; mean, SD, CV% on 18 values

Next, the in vitro SPFs for all 32 products were evaluated on the three different substrates.

Table 6. Interpretation of in vitro SPF coefficient of variation

Table 6. Interpretation of in vitro SPF coefficient of variation

The detailed data and description of box plots parameters are shown in Table 6.

Table 7. Correlation results between in vivo and in vitro SPF values according to different substrates

Table 7. Correlation results between in vivo and in vitro SPF values according to different substrates

Detailed results of linear regression, correlation coefficient and RMSE are summarized in Table 7.

Figure 1. Surface energy according substrates

Figure 1. Surface energy according substrates

The surface energies with dispersive and polar fractions of surface roughness also were calculated.

Figure 2. Transmittance spectrum according to substrates

Figure 2. Transmittance spectrum according to substrates

The absorbance spectrum assessment of substrates was performed using white petroleum; results are shown in Figure 2.

Figure 3. Box plots of in vitro SPF variability according to substrates

Figure 3. Box plots of in vitro SPF variability according to substrates

Concerning repeatability of results, a box plot graphic, also referred to as a box and whisker diagram, shown in Figure 3, represents the in vitro SPF variations according to substrate.

Figure 4. In vivo/in vitro correlation for MPP substrate

Figure 4. In vivo/in vitro correlation for MPP substrate

Results shown in Figure 4, for the MPP substrate, led to a correlation coefficient of 0.693, which indicates the best correlation with in vivo SPF values compared to other substrates.

Figure 5. In vivo/in vitro correlation for MSSP substrate

Figure 5. In vivo/in vitro correlation for MSSP substrate

From results obtained with the MSSP or SPP substrates (see Figures 5 and 6), the correlation coefficients were close but a little lower than the MPP substrates, at respectively 0.624 and 0.629.

Figure 6. In vivo / in vitro correlation for SPP substrate

Figure 6. In vivo / in vitro correlation for SPP substrate

From results obtained with the MSSP or SPP substrates (see Figures 5 and 6), the correlation coefficients were close but a little lower than the MPP substrates, at respectively 0.624 and 0.629.

Figure 7. In vivo/In vitro correlation according to the three different substrates

Figure 7. In vivo/In vitro correlation according to the three different substrates

Correlation results with midday midsummer sunlight for the three different substrates are shown in Figure 7.

Footnotes (CT1309 Miksa)

a Transpore is a product of 3M, www.3m.com.
b Vitroskin is a product of IMS Inc., www.ims-usa.com.
c Helioplate HD6 PMMA plates are manufactured by HelioScreen, http://helioscreen.fr/en.
d Skin-mimicking substrate PMMA plates are manufactured by Shiseido Irica Technology Inc.
e Sandblasted PMMA plates are manufactured by EuroPlast, www.europlast.fi/fi/europlast.html.
fThe Drop Shape Analysis System DSA10 Mk2 is manufactured by KRÜSS GmbH, www.kruss.de/en/home.html.
g Milli-Q is a device from Merck Millipore, and
h Diiodo-methane was obtained from Merck Schuchardt OHG, www.millipore.com/index.do.
j The Altisurf 500 lab workstation is manufactured by Altimet, www.altimet.fr/fr/index.htm.
k The Mountain AltiMap Premium Microtopography software module is a product of Digital Surf, www.digitalsurf.fr/en/index.html.
m The Labsphere UV-2000S transmittance analyzer, and
n The Ultraviolet Transmittance Analyzer Performance Validation Standards are manufactured by Labsphere Inc., www.labsphere.com.
p Helioplate HD0 PMMA plates are manufactured by HelioScreen, http://helioscreen.fr/en.

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