METHOD FOR THE IN VITRO DETERMINATION OF UVA PROTECTION PROVIDED BY SUNSCREEN PRODUCTS. Edition of 2007a

Transcription

1 METHOD FOR THE IN VITRO DETERMINATION OF UVA PROTECTION PROVIDED BY SUNSCREEN PRODUCTS Edition of 2007a

2 METHOD FOR THE IN VITRO DETERMINATION OF UVA PROTECTION PROVIDED BY SUNSCREEN PRODUCTS Guideline 2007a Prepared by the COLIPA In vitro Photoprotection Methods Task Force 1

3 INTRODUCTION Sun protection products have long protected against sunlight-induced erythema with the level of performance indicated by the sun protection factor (SPF). However, since the SPF number is influenced primarily by UVB wavelengths, it is not necessarily a sufficient indicator of a sunscreen product s protection against UVA exposure. In recent years, the harmful effects of the UVA wavelengths of sunlight have been more thoroughly established. With this understanding arose the need, not only for sun protection products that were effective against the UVA wavelengths, but also for a common test method for measuring UVA protection levels. In seeking to develop an additional method for the separate assessment of a sunscreen product s ability to protect against UVA exposure, it was agreed that an in vitro methodology should be the goal, provided that such an in vitro method could be shown to correlate with similar data derived from human volunteer methods of determining UVA protection. In order to achieve this correlation, it was necessary for the test procedure to also take into account any photo-instability in the tested sunscreen product. The method described in the following sections arises from considerable technical discussion and practical investigation of the dynamics of measuring UVA protection in vitro. It is supported by extensive ring testing, which established a meaningful correlation with UVA protection factors determined on human volunteers using the previously published in vivo Persistent Pigment Darkening (PPD) method. The guideline contains detailed advice on the technical requirements of the instrumentation and of the procedures used in the method. These sections should be strictly observed to ensure the correct and accurate determination of UVA protection. OBJECTIVE The new guideline aims to provide a validated in vitro method to determine the UVA protection of sunscreen products. Specifications and descriptions, based on experimental data, are given to enable determination of UVA protection in a reproducible manner, in any laboratory. In order to determine skin relevant UVA protection parameters the method has been created to yield values which correlate with data achieved with the in vivo PPD method ( Persistent Pigment Darkening ). Recommendation on use: This in vitro method has been developed and validated primarily for liquid and emulsion-type sun protection products. It may be possible to use it for some powder containing products; however this should be validated on an individual product basis. 2

4 PRINCIPLE It is the aim of this proposed protocol to provide an in vitro test to measure UVA protection provided by sun care preparations. The method provides in vitro UVA protection factors (UVAPF), which are shown to correlate well with in vivo UVA protection factors determined by the (in vivo) PPD method, the latter being considered as a reference. The test is based on the assessment of UV-transmittance through a thin film of sunscreen sample spread on a roughened substrate, before and after exposure to a controlled dose of UV radiation from a defined UV source. Due to the current lack of inter-laboratory reproducibility of absolute in vitro UV protection factor measurements, each set of sunscreen transmission data is adjusted by first converting to absorbance data (before and after UV exposure) and then by multiplying by the same coefficient. The coefficient is iteratively determined from the non-exposed sample s absorbance data to provide a calculated in vitro SPF value equal to the labelled (in vivo) SPF. The sunscreen sample is exposed to an irradiation dose proportional to the initial UVA protection factor UVAPF 0, calculated from the adjusted absorbance data of the non exposed sample. The final in vitro UVA protection factor (UVAPF) is calculated from the adjusted absorbance data of the UV exposed sample. DEFINITION OF TERMS In vitro UVA protection factor (UVAPF) The protection performance of a suncare product against UVA radiation, calculated from the measured in vitro transmittance after irradiation and weighted with the PPD action spectrum and with the standard output spectrum of a UVA solar simulator. In vitro UVA protection factor before UV exposure (UVAPF 0 ) In vitro UVA Protection Factor measured before sample UV exposure. It is derived from the transmittance curve of the unexposed sample, weighted with the PPD action spectrum and with the standard output spectrum of a UVA solar simulator, after adjustment to the labelled (in vivo) SPF. In vitro Sun Protection Factor (SPF in vitro ) The protection performance of a suncare product against erythema-inducing radiation, calculated from the measured in vitro transmittance and weighted with the erythema action spectrum and with a standard output spectrum of a UV solar simulator used for SPF testing. 3

5 Action spectrum, E(λ) for erythema or P(λ) for PPD The relative effects of individual spectral bands of an exposure source on a biological response. Plots of action spectra show the reciprocal of the dose (D) required to produce the photobiological effects at each wavelength. Data of the action spectra used herein are given in Appendix I. CIE: Commission Internationale de l Eclairage. Mean monochromatic absorbance A λ The sunscreen absorbance at wavelength λ is related to the sunscreen transmittance (T λ ) by: A λ = - log (T λ ) Transmittance (T λ ) being the fraction of incident irradiance transmitted by the sunscreen film. Irradiance Flux per unit area, units W m -2 ; always related to a defined range of wavelengths, for example from 290 to 400 nm for UVA + UVB irradiance or from 320 to 400 nm for UVA irradiance. Spectral irradiance I(λ) for SPF testing or PPD testing Irradiance per unit wavelength, I(λ), units W m -2 nm. MATERIALS AND INSTRUMENTATIONS Spectrophotometer (specifications) The primary waveband of interest is 290 to 400 nm: the spectrophotometer wavelength range must span it. The incremental wavelength step should be 1 nm. A spectrophotometer which uses monochromatic illumination and in which the transmitted radiation is not measured through a monochromator should employ a fluorescence rejection filter. The spectrophotometer input optics should be designed for diffuse illumination and/or diffuse collection of the transmitted irradiance through the roughened PMMA substrate, without and with the sunscreen layer spread on its surface. An integrating sphere is recommended and when used, smaller fractional port areas compared with total sphere wall area will lead to greater accuracy. In any case, the spatial response should be close to a cosine response (cosine error smaller than ± 5% for incident angles < 70 ). 4

6 To reduce the variability between readings and to compensate for the lack of uniformity in product layer, it is recommended that the area of each reading site should be at least 0.5 cm². The wavelength accuracy should be equal to or better than ± 2 nm, as checked using a mercury spectral standard lamp or a specially-doped filter with a regular xenon lamp. The ability of an instrument to accurately measure transmission or protection factors is limited by the sensitivity of the instrument. The minimum required dynamic range for this methodology is at least 2.2 absorbance units as determined according to Appendix II. The maximum measured absorbance should be no more than 90% of the dynamic range of the device used. As a consequence, the spectroradiometer could have a high stray-light rejection, which is provided by an adequate set up (e.g., double monochromator). The lamp that is used to measure the transmittance must emit a continuous spectrum of radiation over the range nm, and the level of irradiance should be sufficiently low, so that the photostability of the product is not unduly challenged (a xenon flash lamp is a convenient solution). Therefore the UV-dose during one measurement cycle should not exceed 0.2 J/cm². For every wavelength, the irradiance must be at least 100 times higher than the so-called irradiance which is measured while the lamp is switched off. 5

7 Monitoring of the UV spectrophotometer The spectrophotometer must be controlled at regular intervals (recommended at least every month) by measurements of reference materials. A twofold test is recommended: o o Monitoring the instrument s efficiency with PMMA standard plates (see Appendix II A, Use of the PMMA standard plates for instrument control purposes ) Controlling the wavelength accuracy with an approved standard material (recommended : Holmium perchlorate, see Appendix II B) UV source for pre-irradiation The spectral irradiance at the exposure plane of the artificial UV source that is used for irradiation should be as similar as possible to the irradiance at ground level under a standard zenith sun as defined by COLIPA (1994) or in DIN (1999). The UV irradiance must be within the following acceptance limits (measured at sample distance). Light source specifications as measured spectroradiometrically Total UV irradiance (290 to 400 nm) W/m 2 Irradiance ratio of UVA (320 to 400 nm) to UVB (290 to 320 nm) 8-22 The reference standard sun has a total irradiance of 51.4 to 63.7 W/m 2 (Colipa 1994 / DIN 67501) and a UVA to UVB irradiance ratio of 16.9 to The device should have the ability to maintain samples below 40 C (preferably by using cooling trays or/and ventilators). Example of an appropriate UV source: Long-arc xenon Atlas Suntest TM insolator, type CPS, CPS+ or XLS/XLS+, filtered with a coated quartz filter dish (Ref.: ) combined with UV special glass filter (Ref: ), allowing to supply VIS+UVA+UVB spectrum. Technical documentation is available for these insolators, with associated modifications over the years, to help the users in clearly identifying instrument models / filters and how to use them. The insolator will be regulated in intensity at a given power setting and stabilised in UV irradiance. Substrate plates, treated with sunscreen, should be placed on a non-reflecting surface during UV exposure. A water-cooled sample tray for effective cooling of the samples is available from ATLAS Material Testing Technology GmbH (Atlas No ). 6

8 Monitoring of the UV source for pre-irradiation The emission of the solar simulator will be checked (at least) annually for compliance with the given acceptance limits by a suitably qualified expert. The inspection should be conducted with a spectroradiometer that has been calibrated against an internationally accepted calibration standard (e.g. institution that is certified by the European co-operation for accreditation EA). Between two inspections the emission of the solar simulator will be monitored radiometrically (e.g. with an integrated UV meter). In parallel with the control of the UV source spectrum, the radiometer(s) and UVA cell(s), usually employed for controlling or adjusting the irradiance of the UV source at sample level and calculating the UV doses, will be calibrated in terms of UVA irradiance (W.m-² UVA, nm) for the same UV source spectrum, according to the Colipa recommendations given in the Guideline "Monitoring of UV light sources" (2007). Substrate / Plate The substrate / plate is the material to which the sun care product is applied. It must be UV-transparent, non-fluorescent (i.e. no detectable fluorescence when exposed under UVR and measured with the spectrophotometer), photostable and inert towards all ingredients of the preparations to be tested. Furthermore, to enable the application of stable thin films of different emulsion types in a skin-like manner, the substrate should have a textured upper surface. For this method, PMMA plates (polymethylmethacrylate Plexiglas ) with one side of the substrate roughened, have proven to be satisfactory. The preparation of these plates is described in Appendix III. The size of the substrate should be chosen such that the application area is not less than 16 cm² - a square shape is preferred (e.g. 50 x 50 x 2.5 mm). The qualitative transmission spectrum of an emulsion, applied to the rough side of the plates, strongly depends on the degree of roughness and the properties of the base material. Furthermore the degree of roughness will affect the optical properties of the roughened material itself. The recommended PMMA plates have been characterized by coating 20 plates with 15µl of glycerine and measuring transmission levels at four different wavelengths in the range from 290 to 400nm to determine reference values. The characteristic transmission values are given in Appendix III. 7

9 METHOD General procedure Step 1: Step 2: In vitro transmission measurement of the sunscreen product spread on PMMA plate, prior to any UV irradiation. Acquisition of initial UV transmission spectrum with A 0 (λ) data. Mathematical adjustment of the initial UV spectrum using coefficient C (see calculation below) to achieve an in vitro SPF (0% UV dose) equal to the labelled SPF (in vivo). Initial UVAPF 0 is calculated using A 0 (λ) and C. Step 3: A single UV dose D is calculated, proportional to UVAPF 0. Step 4: Step 5: Step 6: UV exposure of the same sample as in step 1, according to the calculated UV dose D. In vitro transmission measurement of the sunscreen product after UV exposure. Acquisition of second UV spectrum with A(λ) data. Mathematical adjustment of the second spectrum (following UV exposure) according to the same C coefficient, previously determined in step 2. Calculation of the in vitro UVA protection factor UVAPF after irradiation using A(λ) and C. Transmission measurements through the untreated plate It is first necessary to determine the transmission of UV radiation through the reference plate. Prepare a 100% transmission reference sample by spreading a few microliters of glycerine or another appropriate UV-transparent substance on the roughened side of a substrate plate. Choose an amount of glycerine such that the entire surface is just completely covered (approximately 15µl for a 50 x 50 mm plate). Any excess of glycerine should be avoided. Sample application Sunscreen product is applied to the roughened PMMA plate (roughened side uppermost) by weight, at an application rate of 0.75 mg/cm² (actual quantity applied to the plate). To ensure dose accuracy / repeatability, the application area should be not less than 16 cm². A square form of the substrate is recommended to enable optimal application. The sunscreen is applied as a large number of small droplets of approximate equal volume, distributed evenly over the whole surface of the plate. A positivedisplacement automatic pipette can be used for this purpose. To ensure the correct application rate, it is recommended that a method of validating the amount of 8

10 product applied should be adopted (e.g., weighing the pipette before and after dispensing the product and / or weighing the plate before and after applying the product). It is essential to limit possible product evaporation during the weighing process. After application (and check-weighing, if employed), the sunscreen product is spread immediately over the whole surface using light strokes with a fingertip presaturated with the product. Spreading should be completed in a two-phase process. First, the product should be distributed over the whole area as quickly as possible (less than 30 seconds) without pressure. Then the sample should be rubbed into the rough surface using pressure. The second phase should also take 20 to 30 seconds. This treated sample should then be allowed to equilibrate for at least 15 minutes in the dark at ambient temperature to help facilitate formation of a standard stabilised product film. Transmission measurements through the product-treated plate The product-treated plate is placed in the light-path of the measurement device s UV source and a mean value for transmission of UV radiation through the sample (by measuring monochromatic transmission on different subsites within the plate or by measuring the whole plate) is determined for each wavelength, from 290 to 400 nm, with 1-nm steps. Number of determinations Spread the sunscreen onto at least three PMMA plates for each sunscreen sample. Each plate should be measured at a number of different sites to ensure that a total area of at least 2 cm² is measured. The single spot area should exceed 0.5 cm², i.e. if the spot area is 0,6 cm² then at least 4 measurements on different areas are required so that the total measurement area exceeds 2 cm². UV exposure For exposure in a UV source as defined above, the incident irradiance should be measured in the plane of the treated plate surface. If a radiometer is used to make this measurement, the irradiance reading may need to be corrected using a correction factor. The correction factor can be derived from a spectrophotometric measurement of the real irradiance. The CF determined in this way should be added into cell E27of the Excel calculation spreadsheet. Samples should be maintained below 40 C. The PMMA substrate plates should be firmly supported by a device which can easily be cooled (e.g. an extruded polystyrene block or any similar device). The device should be of sufficient size to contain all the PMMA plates being used and should have a dark background behind 9

11 each plate to reduce the risk of any back exposure. The UV source should not be turned off while placing samples under the lamp. Attention: Personnel working with this irradiator system must be protected adequately against UV rays (e.g. glasses, gloves, etc.). No bare skin to be exposed to Xenon light. If a Suntest TM is used as an appropriate UV source, place the Suntest case on two 15 cm high blocks on the bench and remove the bottom plate of the Suntest. To avoid switching off the bulb of the Suntest during measurements, keep the door closed (security switch) and place the exposure plate in the light beam passing under the Suntest case, using an adjustable stand to maintain the plate in contact with the Suntest case. Transmission measurements after UV exposure It is recommended to perform transmission measurement after UV exposure on the same spots as measured before. CALCULATIONS The UVAPF of a product is the mean of the UVAPF s derived from individual plates. Calculation of the in vitro SPF (SPF in vitro ) where: E(λ) = Erythema action spectrum (CIE- 1987) (see Appendix I) I(λ) = Spectral irradiance received from the UV source (SSR for SPF testing) (see Appendix I) A 0 (λ) = Mean monochromatic absorbance of the test product layer before UV exposure dλ = Wavelength step (1 nm) 10

12 Calculation of the adjusted in vitro SPF (SPF in vitro,adj ) and determination of the coefficient of adjustment C. C is the coefficient of adjustment, iteratively determined to adjust the calculated in vitro SPF value to the labelled (in vivo) SPF value. It is recommended that C falls within a range between 0,8 and 1,2. Where: E(λ),I(λ),A 0 (λ) and dλ are defined in equation (1). Calculation of UVAPF 0 UVAPF 0 is calculated for each plate individually. where: P(λ) = PPD action spectrum (see Appendix I) I(λ) = Spectral irradiance received from the UV source (UVA nm for PPD testing) A 0 (λ) = Mean monochromatic absorbance of the test product layer before UV exposure C = Coefficient of adjustment previously determined in equation (2) dλ = Wavelength step (1 nm) Calculation of UVA dose D for sample irradiation The single UVA dose D is deduced from the UVAPF value. The sample is exposed 0 to full spectrum UV radiation but the dose is being defined by the UVA content. D = UVAPF D J/cm² (4)

13 D 0 is unit UVA dose per unit UVAPF 0, to be applied with the UV source spectrum, experimentally determined to achieve a fair correlation between in vitro UVAPF and in vivo PPD values. D 0 value has been optimised, according to the entire set of data from a round robin test performed by Colipa and is fixed at 1.2 J.cm-² UVA. Calculation of UVAPF of plates after UV irradiation of the sample Where: P(λ),I(λ),C and dλ are defined in equation (3). A(λ) is the mean monochromatic absorbance of the test product layer after UV exposure. UVAPF of the plate is calculated from the mean absorbance value from all individual spots. If the coefficient of variation of maf between spots exceeds 50% then the plate should be rejected and a new plate prepared. Calculation of UVAPF UVAPF of the product is the mean of the UVAPF s of at least three individual plates If the coefficient of variation between the UVAPF s of the individual plates exceeds 20% then further plates have to be measured until the coefficient of variation requirement is reached. Ratio SPF / UVAPF Calculation Using the in vivo sun protection factor (SPF label) and the in vitro UVA protection factor UVAPF, calculate the ratio which characterizes the relationship between the sun protection factor SPF in vivo and the UVA protection. 12

14 TEST REPORT The test report on the determination of the ratio SPF / UVAPF of a suncare product should contain at least the following information: a) Detailed description of the instruments used b) Detailed information on the substrate (manufacturer, batch code and reference date) c) Information on the exact quantity applied d) Individual data of the transmittance measurements for the sample and reference e) Statement of the measured, labelled in vivo sun protection factor f) Constant C g) UVA dose used h) Statement of the values calculated for the UVAPF and the ratio SPF/UVAPF for each suncare product i) Statistical data (e.g. number of measurements, standard deviation) 13

15 APPENDIX I PPD and erythema action spectra and UVA and UV-SSR spectral irradiances Wavelength nm PPD ction spectrum Erythema action spectrum UV-SSR source W.m -2 nm -1 UVA radiation Source W.m -2 nm E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-06 14

16 Wavelength nm PPD ction spectrum Erythema action spectrum UV-SSR source W.m -2 nm -1 UVA radiation Source W.m -2 nm E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-04 15

17 Wavelength nm PPD ction spectrum Erythema action spectrum UV-SSR source W.m -2 nm -1 UVA radiation Source W.m -2 nm E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-04 16

18 Wavelength nm PPD ction spectrum Erythema action spectrum UV-SSR source W.m -2 nm -1 UVA radiation Source W.m -2 nm E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-05 17

19 APPENDIX II A) Use of the PMMA standard plates with UV filter for instrument control purposes The calibration plates with UV filter (Schönberg GmbH supplier order number 951) are cut from a large sheet of a standard cast, UV-stabilised Plexiglas (helping ensure the same optical properties for each plate). The plates are made in a way as to match the absorption spectra of a range of common sunscreens. The casting process enables a very homogeneous distribution of UV absorbing material, relative to a manually applied film of a test emulsion. The plates are roughened on one side by an industrial sandblasting procedure (glasspearls, µm, 30 cm, 6 bar) in a very standardised manner. Because of their stable and standardised absorption and diffuse-scattering properties, they are very suitable as reference emulsions to check and compare instruments used for in-vitro determination of UV protection, for intra- as well as inter-laboratory purposes. The first extinction-measurement through one plate should be made in the same way as with real emulsion films, i.e., the scattering surface should be orientated to the incident beam with the back of the polished plate positioned as near as possible to the aperture of the integrating sphere: The second measurement should be done with two plates, orientated with the roughened surfaces toward one another, to ensure that the position of the scattering surface is identical to that of the first measurement. Ideally, the UV attenuation (absorption and scattering) measured through the two plates should be exactly equal to twice the UV attenuation of a single plate. (upper spectrum in diagram, below): Practically, many spectrophotometers, equipped with an integration device, will exhaust their linear measurement range at a certain degree of attenuation (middle spectrum in diagram below). The points, where the flanks of the upper and the middle spectra diverge (arrows), determine the instrument's upper range of measurement in the UVA- and UVB-region. 18

20 Note: When measuring with a glycerine covered PMMA-plate as a reference, as described in the method above, the instrument s efficiency around 300nm will decrease by approx. 0.2 absorption units. Summing it up, several pieces of data can be derived from these two fast and simple measurements: wavelength comparability between different instruments (measurement 1) quantitative comparability, measuring a Standard sunscreen (measurement 1) dynamic linear range of the instrument (measurement 2) The plates can also be used for routinely checking the instrument s performance. 19

21 B) Use of the holmium perchlorate for instrument control purposes The positions of the six relevant spectral bands of a Holmium perchlorate solution, rounded to integer wavelengths, are listed in the table below. Each certified standard is provided with its special calibration table. Peak Peak Peak Peak Peak Peak According to the European Pharmacopoeia the deviations of the measured bands from the reference values in the UV range should not exceed 1nm for any spectrophotometer. As an example, a measured calibration spectrum is shown in the graphic below. 20

22 APPENDIX III Characteristics of the PMMA substrate plates The mean optical transmission through the substrate material treated with glycerin should be as follows (N=20): Wavelength range (nm) Minimum transmission (%) Maximum transmission (%) Micro topography measurements can be achieved to characterize the plate surface roughness. Contact or non-contact systems are available to provide accurately a surface roughness value (e.g., Sa in µm, according to EUR EN). The roughness value of the PMMA plates used for the validation of the present method (Schönberg GmbH supplier order reference: Suntest) is close to Sa = 2 µm. For such a low roughness value, a sunscreen application rate of 0.75 mg / cm 2 is the quantity needed in order to not exceed a spectral absorbance 2,0 in the range 290 nm λ 400 nm. 21

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