Photostabilization of Retinol and Retinyl Palmitate by Ethylhexyl Methoxycrylene

Jan 1, 2011 | Contact Author | By: Craig Bonda and Jean Zhang, The HallStar Company
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Title: Photostabilization of Retinol and Retinyl Palmitate by Ethylhexyl Methoxycrylene
retinoidsx ethylhexyl methocycrylenex UV/visual spectrax fluorescencex photodegradationx stabilityx
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Keywords: retinoids | ethylhexyl methocycrylene | UV/visual spectra | fluorescence | photodegradation | stability

Abstract: This study examines the photostability of retinol and retinyl palmitate, finding they break down rapidly when exposed to UV radiation in the 290–400 nm range. This severely reduces their concentrations in finished formulations. However, when combined with ethylhexyl methoxycrylene, these retinoids are shown to be protected against photodegradation, thus preserving their concentrations both during the manufacturing process and following application to the skin.

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C Bonda and J Zhang, Photostabilization of Retinol and Retinyl Palmitate by Ethylhexyl Methoxycrylene, Cosmet & Toil 126 (1) 40 (2011)

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Retinoids are a class of chemical compounds that are similar in structure to or derived from vitamin A. Retinoids serve many important and diverse functions throughout the body, including roles in vision,1 regulation of cell proliferation and differentiation,2 growth of bone tissue,3, 4 immune functions,5 and as activators of tumor suppressor genes.6 These compounds are also being investigated as preventive agents for skin cancer.7

The basic structure of a retinoid compound consists of four isoprenoid units joined head-to-tail with a cyclic group on one end, a polyene side chain, and a functional group on the other end.8 The conjugation of the polyene side chain—i.e. alternating single and double bonds—is responsible for the color of retinoids, which are typically yellow, orange or red, as well as their ability to act as chromophores. Variations in the side chains and polar end groups of these compounds lead to different classes of retinoids. Retinoids are classified into three generations (see Table 1),9 the first of which contains naturally occurring retinoids, though all are also produced synthetically. Both first- and second-generation retinoids are ligands for (bind to) several retinoid-binding proteins, called receptors, found in serum, cytoplasm and nuclei. Third- generation retinoids are less flexible than first- and second-generation retinoids and interact with fewer retinoid receptors. The body stores the majority of its retinoid reserves in the liver, mostly as retinyl esters of palmitic, stearic, oleic and linoleic acids.10

Once retinol, or vitamin A, has been taken up by a cell, it can be oxidized to retinal, which is further oxidized to retinoic acid.11 Retinoic acid acts as a ligand for both the RAR and retinoid X receptors (RXR). Retinol appears to function in maintaining skin health and is being used in an increasing number of consumer skin care products for treating aged skin and wrinkles, presumably by stimulating new collagen production.

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Table 2. Test formulations for photostability of a) 0.1% retinol and b) 0.25% retinyl palmitate

Table 2. Test formulations for photostability of a) 0.1% retinol and b) 0.25% retinyl palmitate

Similarly, three formulations containing 0.25% retinyl palmitate, alltrans- retinol ester of palmitic acid, were prepared as traditional o/w emulsions (see Table 2).

Table 3. Chromatographic separation conditions and pump programs

Table 3. Chromatographic separation conditions and pump programs

Chromatographic separation was achieved using a C18 column under the conditions summarized in Table 3.

Table 1. Examples of retinoid compounds

Table 1. Examples of retinoid compounds

Retinoids are classified into three generations (see Table 1).

Figure 3. Results of the qualitative fluorescence quenching experiment

Figure 3. Results of the qualitative fluorescence quenching experiment; the bottom three spots contain the negative controls—i.e. solutions of 0.2% retinol in ethyl acetate with (from left) 0%, 1% and 2% of a non-photoactive diluent.

The bottom three spots contain the negative controls—i.e. solutions of 0.2% retinol in ethyl acetate with (from left) 0%, 1% and 2% of a non-photoactive diluent. The top three test spots are shown (from left) with 0%, 1% and 2% EHMC.

Figure 7. Results of the UV transmittance studies on formulations containing either 0.1% retinol or 0.25% retinyl palmitate

Figure 7. Results of the UV transmittance studies on formulations containing either 0.1% retinol or 0.25% retinyl palmitate;

Formulations with no EHMC lost 87% and 93% of their peak UV absorbances after irradiation with 5 MED, whereas the formulas containing 4% EHMC showed little if any loss of peak absorbance after 5 MED.

Figure 8: The UV absorbances of 0.10% retinol formulations before irradiation (dark blue, dashed line) and after 5 MED (red and green)—approx. the amount of UVR in one hour’s worth of mid-summer sun.

Figure 8: The UV absorbances of 0.10% retinol formulations before irradiation (dark blue, dashed line) and after 5 MED (red and green)—approx. the amount of UVR in one hour’s worth of mid-summer sun.

The formulation with 4% EHMC (blue) retains 100% of its UV absorbance, indicating little if any loss of retinol. The formulation without EHMC lost 86% of its UV absorbance.

Figure 2. The UV/vis spectra of retinol (red) measured at 10 ppm in methanol and retinyl palmitate (blue), measured at 10 ppm in tetrahydrofuran

Figure 2. The UV/vis spectra of retinol (red) measured at 10 ppm in methanol and retinyl palmitate (blue), measured at 10 ppm in tetrahydrofuran

The molecular structure and the two isomers of EHMC; both isomers are always present in some proportion and account for the two peaks in the HPLC chromatograms. determined by spectrophotometerc (see Figure 2).

Figure 9: The UV absorbances of 0.25% retinyl palmitate formulations before irradiation (dark blue, dashed line) and after 5 MED (red and green—approx. the amount of UVR in one hour’s worth of mid-summer sun.

Figure 9: The UV absorbances of 0.25% retinyl palmitate formulations before irradiation (dark blue, dashed line) and after 5 MED (red and green—approx. the amount of UVR in one hour’s worth of mid-summer sun.

The formulation with 4% EHMC (blue) lost about 1% of its UV absorbance, indicating little if any loss of retinyl palmitate. The formulation without EHMC lost 93% of its UV absorbance.

Figure 4. Results of the HPLC studies on formulations containing retinol and retinyl palmitate

Figure 4. Results of the HPLC studies on formulations containing retinol and retinyl palmitate

The results of the HPLC studies of the three formulations containing 0.1% retinol are summarized graphically in Figure 4, and portions of the chromatograms of two of the irradiated samples appear in Figures 5a and b.

Figure 5. Portions of the HPLC chromatograms from the retinol studies showing the relative heights of the peaks for BHT (blue), used as the internal standard, and retinol (orange)

Figure 5. Portions of the HPLC chromatograms from the retinol studies showing the relative heights of the peaks for BHT (blue), used as the internal standard, and retinol (orange)

a) The formulation without EHMC lost 79% of its retinol, while b) the formulation with 4% EHMC lost less than 5% of its retinol. The E and Z isomers of EHMC are in yellow. Other peaks represent the excipients used to make the formulations.

Figure 6. Portions of the HPLC chromatograms from the retinyl palmitate studies

Figure 6. Portions of the HPLC chromatograms from the retinyl palmitate studies

This figure shows the relative areas of the peaks for tocopherol (green), used as the internal standard, and retinyl palmitate (orange), after exposure to 5 MED; formulation a) without EHMC lost 65% of its retinyl palmitate, while formulation b) with 4% EHMC lost less than 5%; the peaks for the E and Z isomers of EHMC are shown in yellow.

Figure 1. The molecular structure and the two isomers of EHMC; both isomers are always present in some proportion and account for the two peaks in the HPLC chromatograms.

Figure 1. The molecular structure and the two isomers of EHMC; both isomers are always present in some proportion and account for the two peaks in the HPLC chromatograms.

Considering these instability issues, the authors sought to determine whether ethylhexyl methoxycrylene (EHMC), an effective photostabilizer for certain organic UV filters (see Figure 1), could perform the same photostabilizing function for topically applied retinoids.

Footnotes (CT1101 C. Bonda)

a Retinol (99%) and retinyl palmitate (18,000,000 USP Units/G) were obtained from Sigma-Aldrich.
b Solastay S1 (INCI: Ethylhexyl Methoxycrylene) is a product of The HallStar Company.
c The Varian Cary 50 CONC UV/Vis spectrophotometer was used for this analysis.
d Whatman Catalog #4420221 TLC plates were used for this study.
e The Spectroline Model ENF-240C lamp was used for this study.
f The model D70 camera used for this study is manufactured by Nikon.
g The Hewlett Packard Series 1100 HPLC System was used for this analysis.
h ChemStation for LC 3D is produced by Agilent Technologies.
j The Apollo C18 column (4.6 x 150 mm, with a 5-μm particle size) was used for this assessment.
k The Model 16S Solar Simulator with a PMA 2105 UV-B DCS Detector controlled by a PMA 2100 microprocessor-based controller used for this study is a device from Solar Light Co.
m The Labsphere UV-2000S UV Transmittance Analyzer was used for this assessment.
n The Helioplate HD6 PMMA plates used for this study are manufactured by HelioScreen Labs.

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