Effect of the combination of organic and inorganic filters on the Sun Protection Factor (SPF) determined by in vitro method

Abstract

This paper describes the effect on Sun Protection Factor (SPF) of the combination of inorganic and organic filters in sunscreen products as determined by an in vitro method. O/W emulsions containing inorganic filters, such as titanium dioxide and zinc oxide, combined with 18 EU-authorized UV-B organic filters were tested. SPF measurements were carried out using a spectrophotometer equipped with an integrating sphere. This study observed a synergic effect when titanium dioxide was combined with either anisotriazine or octyldimethylPABA. The combination of zinc oxide with 11 UV-B organic filters also exhibited a similar synergy; however, the measured SPF was systematically lower than the protection factor achieved with titanium dioxide.

Full text

International Journal of Pharmaceutics 340 (2007) 1–5
Rapid communication
Effect of the combination of organic and inorganic filters on the Sun
Protection Factor (SPF) determined by in vitro method
S. El-Boury, C. Couteau, L. Boulande, E. Paparis, L.J.M. Coiffard∗
Universit´e de Nantes, Nantes Atlantique Universit´es, LPiC, SMAB, EA2160, Faculty of Pharmacy, 1 rue G. Veil - BP 53508, Nantes F-44000, France
Received 22 March 2007; received in revised form 10 May 2007; accepted 12 May 2007
Available online 26 May 2007
Abstract
This paper describes the effect on Sun Protection Factor (SPF) of the combination of inorganic and organic filters in sunscreen products
as determined by an in vitro method. O/W emulsions containing inorganic filters, such as titanium dioxide and zinc oxide, combined with 18
EU-authorized UV-B organic filters were tested. SPF measurements were carried out using a spectrophotometer equipped with an integrating
sphere.
This study observed a synergic effect when titanium dioxide was combined with either anisotriazine or octyldimethylPABA. The combination of
zinc oxide with 11 UV-B organic filters also exhibited a similar synergy; however, the measured SPF was systematically lower than the protection
factor achieved with titanium dioxide.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Inorganic; Organic; Filter; Combination; Synergy; SPF in vitro
1. Introduction
UV constitutes about 10% of the total solar radiation. There
are two categories of UV radiation: UV-A and UV-B. UV-A
(320–400 nm) has lower energy than UV-B, however, it pene-
trates more deeply and does not burn skin as easily. The 95% of
UV radiation is UV-A. UV-B (290–320 nm) radiation has higher
energy and penetrates only the outer skin layers, but induces skin
burns more easily (Hoffman et al., 2000). UV radiation has both
positive and negative effects. Positive effects of UV radiation
include warmth, light, photosynthesis in plants and vitamin D
synthesis in the skin (UV light converts a cholesterol derivative
into previtamin D3) (Holick et al., 1980). Overexposure to UV
radiation is the primary environmental risk factor in the devel-
opment of UV-related adverse health conditions, which include
diseases of the eye (Sliney, 2001), immune suppression (Norval,
2006) and skin cancers. Exposure to UV radiation appears to be
the most important environmental factor in the development of
skin cancer (Hussein, 2005). The increase in skin cancer has
resulted from an increased outdoor leisure time and a decrease
∗Corresponding author. Tel.: +33 2 40 41 28 73; fax: +33 2 40 41 29 87.
E-mail address: laurence.coiffard@univ-nantes.fr (L.J.M. Coiffard).
in the amount of protective clothing worn outdoors (Vanquerp
et al., 1999; Marks, 1999; Couteau et al., 2001; Morison, 2003).
Sun Protection Factor (SPF) is the universal indicator for
describing the efficiency of sunscreen products. It is the ratio
of the least amount of ultraviolet energy required to produce a
minimal erythema on sunscreen protected skin to the amount of
energy required to produce the same erythema on unprotected
skin (FDA, 1978). In this way, SPF indicates the ability of a sun-
screen product to reduce UV-induced erythema. It is measured
by both in vivo (Colipa method) and in vitro methods (Groves et
al., 1979). It is recommended to use sunscreen products with an
SPF of 15 or higher. This paper describes the study of the effect
of the combination of organic and inorganic UV filter substances
on the SPF of topically applied sunscreen formulations, using
an in vitro method.
2. Materials and methods
2.1. Materials
Tables 1 and 2 present the filters (organic and inorganic) and
their characteristics. Dimethicone (Abil®WE 09) was obtained
from Goldschmidt (Montigny-le-Bretonneux, France). Cetiol®
HE, stearic acid, glycerin, parabens and triethanolamin (TEA)
0378-5173/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ijpharm.2007.05.047

2S. El-Boury et al. / International Journal of Pharmaceutics 340 (2007) 1–5
Table 1
Characteristics of the organic sunscreens investigated
INCI name Suppliers λmax (nm) Solubility Maximum concentration
authorized (%)
PABA Merck, Fontenay sous Bois, France 290.0 Hydrosoluble 5
Homosalate Merck, Fontenay sous Bois, France 306.0 Liposoluble 10
Oxybenzone BASF, Levallois-Perret, France 287.5 Liposoluble 10
Phenylbenzimidazole sulfonic acid Merck, Fontenay sous Bois, France 305.5 Hydrosoluble after
neutralisation with NaOH
8
Octocrylene BASF, Levallois-Perret, France 304.0 Liposoluble 10
Octylmethoxycinnamate BASF, Levallois-Perret, France 310.0 Liposoluble 10
PEG-25 PABA BASF, Levallois-Perret, France 307.0 Hydrosoluble 10
Isoamyl p-methoxycinnamate Symrise, Neuilly sur Seine, Paris 310.0 Liposoluble 10
Octyltriazone BASF, Levallois-Perret, France 314.5 Liposoluble 5
Diethylhexylbutamidotriazone Cr´
eations couleur, Dreux, France 310.5 Liposoluble 10
4-Methylbenzylidene camphor Merck, Fontenay sous Bois, France 301.0 Liposoluble 4
3-Benzylidene camphor Unipex, Rueil Malmaison, France 291.05 Liposoluble 2
Octylsalicylate Alzo, Helsinki, Finland 306.0 Liposoluble 5
OctyldimethylPABA Merck, Fontenay sous Bois, France 312.0 Liposoluble 8
Benzophenone-5 BASF, Levallois-Perret, France 287.5 Hydrosoluble 5
Methylene bis-benzotriazolyl
tetramethylbutylphenol
Ciba, Grenzach-Wyhlen, Germany 305.5 Hydrosoluble 10
Anisotriazine Ciba, Grenzach-Wyhlen, Germany 310.0 Liposoluble 10
Polysilicone 15 Roche, Fontenay sous Bois, France 312.5 Liposoluble 10
Table 2
Characteristics of the inorganic sunscreens investigated
INCI Name (Trade name) Suppliers Solubility Maximum concentration
authorized (%)
Titanium dioxide, hydrated silica, aluminium hydroxide,
dimethicone/methicone copolymer (T-Lite SFS)
BASF, Levallois-Perret, France Liposoluble 25
Zinc oxide, diphenyl capryl methicone (Z-Cote Max) BASF, Levallois-Perret, France Liposoluble –
were purchased from Cooper (Melun, France). Xanthan gum
(Keltrol®BT) was obtained from Kelco (Lille Skensved, Den-
mark). Polymethylmethacrylate (PMMA) plates were purchased
from Helioscience (Creil, France). Powder-free latex finger cots
were obtained from Cooper (Melun, France).
2.2. Preparation of sunscreen creams
Each O/W emulsion was prepared in the laboratory by com-
bining organic and inorganic filters, in the highest EU-authorized
concentration, to a basic formula (Table 3) were manufactured by
the authors. Hydrophilic-phase and oil-phase were heated sepa-
rately to between 78 and 82 ◦C, until the ingredients of each part
were solubilized. Next, the oily preparation was added slowly to
the hydrophilic preparation with constant stirring (Yellow line
OST basic mixer, IKA, Staufen, Germany). It was necessary to
continue stirring until the resulting emulsion was cooled to room
temperature (20 ◦C). In addition, sunscreen agents were incor-
porated at various concentrations into this emulsion. A filterless
cream was used as a blank (Couteau et al., in press-a,b).
2.3. Study of effectiveness
Thirty milligrams of precisely weighed product were spread
across the entire surface (25 cm2) of a polymethylmethacry-
late (PMMA) plates using a cot-coated finger. Plates have both
a smooth and a rough surface. The roughness was measured
between 5 and 10␮. After spreading, 15 mg of the product
remained on the finger cot. The SPF of the creams was then
measured in vitro. Three plates were prepared for each product
to be tested and nine measurements were performed on each
plate. Transmission measurements between 290 and 400 nm
Table 3
Composition of the emulsion
Ingredients Percent by weight
Abil®WE 09 (polyglyceryl-4 isostearate;
cetyl PEG/PPG-10/1 dimethicone; hexyl
laurate)
5
Paraffin oil 12
Cetiol®HE (PEG-7 glyceryl cocoate) 5
Butylhydroxytoluene 0.01
Stearic acid 5
Eumulgin®B1 (Ceteareth-12) 1.5
Eumulgin®B2 (Ceteareth-20) 1.5
Glycerine 4
Sodium propylparaben 0.05
Sodium methylparaben 0.1
Keltrol®BT (xanthan gum) 0.9
TEA 0.3
Distilled water qsp 100.0

S. El-Boury et al. / International Journal of Pharmaceutics 340 (2007) 1–5 3
Table 4
Combination of UV-B filters and titanium dioxide
Filter (INCI name) SPF (filter) (mean ±S.D.) SPF (filter + titanium
dioxide combination)
(mean ±S.D.)
Increase or decrease of SPF
compared to predicted SPF
(SPF units)
PABA 5.48 ±0.62 41.04 ±6.05 –
Homosalate 4.25 ±0.96 38.09 ±3.27 −4
Oxybenzone 5.10 ±0.57 39.07 ±4.11 −4
Phenylbenzimidazole sulfonic
acid
13.39 ±1.60 49.37 ±11.07 –
Octocrylene 9.40 ±1.42 43.42 ±3.79 –
Octylmethoxycinnamate 12.09 ±1.20 53.12 ±4.69 –
PEG-25 PABA 4.09 ±0.56 35.87 ±3.08 −6
Isoamyl p-methoxycinnamate 13.49 ±1.90 52.84 ±5.85 –
Octyltriazone 12.54 ±2.15 36.57 ±3.67 −14
Diethylhexylbutamidotriazone 10.73 ±1.44 47.27 ±3.89 –
4-Methylbenzylidene camphor 6.44 ±0.88 43.38 ±2.99 –
3-Benzylidene camphor 2.84 ±0.47 33.47 ±4.03 −7
Octylsalicylate 2.89 ±0.37 38.81 ±4.13 −
OctyldimethylPABA 8.98 ±0.81 53.55 ±4.07 +7
Benzophenone-5 5.59 ±0.88 35.77 ±3.61 −7
Methylene
bisbenzotriazolyltetramethyl
butylphenol
6.68 ±1.80 19.50 ±4.03 −25
Anisotriazine 29.63 ±4.19 73.06 ±4.96 +6
Polysilicone 15 4.25 ±0.95 38.77 ±4.32 –
were carried out using a spectrophotometer equipped with an
integrating sphere (UV Transmittance Analyzer UV1000S, Lab-
sphere, North Sutton, US). The SPF were carried out according
to the following equation:
SPF =400
290EλSλΔλ
400
290EλSλTλΔλ
(1)
where Eλis CIE erythemal spectral effectiveness, Sλis solar
spectral irradiance and Tλis spectral transmittance of the
sample (Ferrero et al., 2003; Villalobos-Hernandez and M¨
uller-
Goymann, 2007).
3. Results and discussion
The SPF of the cream containing 25% titanium dioxide or
25% zinc oxide was, respectively, 37.65 ±3.90 and 7.14 ±1.22.
Table 5
Combination of UV-B filters and zinc oxide
Filter (INCI name) SPF (filter) (mean ±S.D.) SPF (filter +zinc oxide
combination)
(mean ±S.D.)
Increase or decrease of SPF
compared to predicted SPF
(SPF units)
PABA 5.48 ±0.62 10.94 ±1.22 –
Homosalate 4.25 ±0.96 11.94 ±2.25 –
Oxybenzone 5.10 ±0.57 13.42 ±1.61 –
Phenylbenzimidazole sulfonic
acid
13.39 ±1.60 24.76 ±3.82 +4
Octocrylene 9.40 ±1.42 25.74 ±2.57 +9
Octylmethoxycinnamate 12.09 ±1.20 26.63 ±2.98 +7
PEG-25 PABA 4.09 ±0.56 15.06 ±3.18 +4
Isoamyl p-methoxycinnamate 13.49 ±1.90 29.07 ±3.56 +8
Octyltriazone 12.54 ±2.15 25.88 ±2.94 +6
Diethylhexylbutamidotriazone 10.73 ±1.44 49.28 ±4.37 +31
4-Methylbenzylidene camphor 6.44 ±0.88 15.16 ±2.06 –
3-Benzylidene camphor 2.84 ±0.47 12.72 ±1.77 +3
Octylsalicylate 2.89 ±0.37 9.08 ±1.40 –
OctyldimethylPABA 8.98 ±0.81 28.51 ±2.94 +12
Benzophenone-5 5.59 ±0.88 15.28 ±1.42 +3
Methylene bis-benzotriazolyltetra
methyl butylphenol
6.68 ±1.80 12.92 ±1.90 –
Anisotriazine 29.63 ±4.19 36.89 ±3.29 –
Polysilicone 15 4.25 ±0.95 15.55 ±1.37 +4

4S. El-Boury et al. / International Journal of Pharmaceutics 340 (2007) 1–5
Fig. 1. Decrease of effectiveness (%) for the combination between titanium
dioxide and methylene bis-benzotriazolyl tetramethylbutylphenol (MBBTP),
octyltriazone (OT), benzophenone-5 (BZ-5), 3-benzylidene camphor (3-BC),
PEG-25 PABA, benzophenone-3 (BZ-3) and homosalate (HMS).
We noted a clear superiority of TiO2over ZnO in terms of effec-
tiveness. A previous study established that SPF is a function
of filter concentration (Couteau et al., in press-a,b). There-
fore, by knowing the equation SPF = f(c) for each filter and
each separately added screen, it will be possible to predict
the SPF of sun creams combining both filter and screens.
We expect manufacturers to question the relevance of all of
these combinations, a query that will be answered by this
paper.
The effect of the combinations was evaluated statistically
by a Student’s t-test (N= 27; p< 0.05) (Tables 4 and 5). A
combination was considered relevant if the SPF of the cream
combining filter and screen was higher or equal to the SPF
obtained separately, filter only or screen only. On the other
hand, we considered a combination to be irrelevant if the SPF
of the combination remains inferior to the expected result. In
9 out of 18 trials, the creams formulated with TiO2revealed
a purely additive effect. Seven creams turned out to be less
promising than predicted (with a loss of SPF compared to
predicted results between 4 and 25) (Fig. 1). We found two
synergistic combinations worth noting: the cream formulated
with TiO2and anisotriazine resulted in a SPF value of about
70 (an increase of 6 SPF units). The second interesting com-
bination was obtained with octyldimethylPABA (an SPF about
55). The increase was about 7 SPF units. So it is possible to
predict the SPF of all the combinations between the various
molecules.
In a large majority of the cases (11 out of 18), a combina-
tion with zinc oxide was more promising because it generated
more synergy (Fig. 2). In terms of an increase in SPF protection,
two combinations are particularly worth mentioning: the com-
bination with diethylhexylbutamidotriazone (an increase of 31
SPF units) and the combination with octyldimethylPABA (an
increase of 12 SPF units).
The formulated creams made with zinc oxide turned out
to be more reliable than those made with titanium dioxide in
the sense that there was no unexpected loss of SPF compared
with the predicted results. It will be necessary, however, to
Fig. 2. Increase of effectiveness (%) for the combination between zinc oxide
and diethylhexylbutamidotriazone (DHBT), octyldimethylPABA (OD-PABA),
octocrylene (OCT), isoamyl p-methoxycinnamate (IMC), octylmethoxycin-
namate (OMC), polysilicone-15 (P-15), PEG-25 PABA, octyltriazone (OT),
3-benzylidene camphor (3-BC), phenylbenzimidazole sulfonic acid (PBSA) and
benzophenone-5 (BZ-5).
further investigate the use of titanium dioxide because high
SPF (70 for example) products can be created with it; these
high values cannot be attained with zinc oxide (maximum
SPF of 49). By referencing Tables 4 and 5 of this paper as
well as the linear curves (SPF = f(c)) established in an ear-
lier study (Couteau et al., in press-a,b), it is possible to select
filter–screen combinations in function of a desired protection
level.
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