Masato Hatao

Algorithm for in vitro Sun Protection Factor Based on Transmission Spectrum Measurement with Concomitant Evaluation of Photostability

In thein vitro evaluation of Sun Protection Factor (SPF), the photostability of the ultraviolet (UV) filters can have a major impact, especially for high‐SPF formulations, but is generally not taken into consideration. In this study, we present a UV transmission spectrum measurement system utilizing a high‐sensitivity UV photomultiplier tube with concomitant evaluation of photostability. We have developed an algorithm to estimate SPF in vitro by converting the amount of UV light transmission through the sunscreen layer into cumulative relative erythema effectiveness to obtain one minimal erythema dose. Thus, the algorithm uses the same endpoint as in vivo SPF methods, but with a photomultiplier tube as the detector instead of skin. The values obtained showed an excellent correlation with in vivo SPF values, even for high‐SPF sunscreens exceeding SPF 50. This method should be suitable as an in vitro SPF testing method for regulatory purposes.

Influence of application amount on sunscreen photodegradation in in vitro sun protection factor evaluation: proposal of a skin-mimicking substrate.

Widely used polymethylmethacrylate substrates for in vitro sun protection factor (SPF) testing of sunscreens do not mimic the rough surface structure of skin, and in addition, sample loading is less than that used in in vivo SPF testing (2.00 mg cm−2). We have developed a skin‐mimicking substrate (SMS), which has furrows and ridges on its surface, like human skin. A comparison of the photodegradation profiles of sunscreens on commercially available substrates (including SMS) at the recommended application amounts, and on SMS at various application amounts showed that the photodegradation rate of photounstable sunscreen was dependent on the application amount being higher at lower application amounts. SMS at the recommended application amount of 2.00 mg cm−2 provided in vitro SPF values that were comparable with in vivo SPF values. Our results confirm that, in order to develop a reliable in vitro SPF method, which is consistent with in vivo SPF determination, it is important to use the same application amount of sample as in the in vivo method, in order to take proper account of sunscreen photostability.

Light Source Spectrum Strongly Influences the In Vitro Estimation of Sun Protection Factor

Irradiation from an artificial solar simulator that matches the relative cumulative erythema effectiveness (RCEE) of sunlight is used for in vivo sun protection factor (SPF) testing, whereas irradiation that matches the spectrum of natural sunlight (NS) is generally used for in vitro SPF testing. This study was designed to clarify whether this difference is important. Eight sunscreens spread on artificial substrates and several optical filters as sunscreen mimics were used. Their in vitro SPF values were evaluated using RCEE‐compliant and NS spectrum‐matching light sources in calculation. The calculated in vitro SPF values obtained using NS light (SPF [NS]) were lower than those obtained using RCEE light (SPF [RCEE]). The in vitro SPF (RCEE) values showed a better correlation and better agreement with in vivo SPF values, as compared with the in vitro SPF (NS) values. A marked difference between in vitro SPF values obtained with the two light sources in calculation was found for sunscreens showing low transmittance in the ultraviolet B region. To obtain in vitro SPF values that correspond well to in vivo SPF values measured with currently accepted methodology, it is important to use an RCEE‐compliant light source.

Response to Comments by Sayre, Dowdy and Stanfield on our Article Entitled “Algorithm for in vitro Sun Protection Factor Based on Transmission Spectrum Measurement with Concomitant Evaluation of Photostability.”Photochem. Photobiol. (2008)

We appreciate the interest shown by Drs. Sayre, Dowdy and Stanfield in our article ‘‘Algorithm for in vitro Sun Protection Factor Based on Transmission Spectrum Measurement with Concomitant Evaluation of Photostability,’’ published in the November–December 2008 issue (volume 84) of Photochemistry and Photobiology (1), and their recognition of the importance of in vitro SPF measurement taking photostability into account. Our response to their comment, ‘‘In-Vitro Sunscreen Transmittance with Concomitant Evaluation of Photostability: Evolution of a Method’’ (2), is as follows. First, we are pleased to note that they agree with our comment that it would be preferable to use an application dose of 2.00 mg cm, as in in vivo SPF measurement, stating that ‘‘the application amount for in vitro sunscreen evaluations should be 2 mg cm as used in the human SPF test, because photostability testing on thinner layers of sunscreen product, as many methods have utilized, may underestimate product photostability of the thicker films associated with human SPF testing.’’ Actually, we have already reported that in vitro SPF evaluation among various types of formulation with the application dose of 2 mg cm shows a good correlation with in vivo SPF value, in the IFSCC Congress 2009 in Barcelona (3). We think this good result is at least partly due to the development of the new substrate that mimics skin well; its roughness shows a value similar to that of back skin, and samples at this application dose can be uniformly applied. Regarding the use of relative intensity (a.u.), we also agree with their comment (in ‘‘Predicting SPF During Photodegradation’’) that ‘‘The resulting spectra should be transmitted irradiance in W cm nm, preferably plotted on a logarithmic y-axis scale to illustrate system dynamic range and noise equivalent irradiance detection limit.’’ However, at this point, we used relative intensity (a.u.). because in vitro SPF can be simply estimated from the relative value of a sample with respect to the reference. Calibration of the system to take account of the quantum efficiencies of detector and grating of the double monochromator, gain and so on would allow us to convert relative values to approximate absolute values. Sayre et al. suggested in the section ‘‘Key Information Needed’’ that further information was required about various aspects of the study, and we are happy to provide that as follows: