Synthesis, hydrolysis, and skin retention of amino acid esters of α-tocopherol

doi:10.1002/jps.21608

Journal of Pharmaceutical Sciences

Volume 98, Issue 7, July 2009, Pages 2364-2376

https://doi.org/10.1002/jps.21608 Get rights and content

ABSTRACT:

The aim of this work was to synthesize new pro-vitamins of α-tocopherol (VE) able to release another moiety such as an amino acid, in order to obtain a combined antioxidant and moisturizing effect upon topical application. The new derivatives were characterized and tested for sensitivity to chemical and enzymatic hydrolysis. Lipophilicity was estimated using Log capacity factor and skin retention was determined in vitro, using rabbit ear skin as barrier. Five molecules were synthesized using L-proline, L-serine, L-tyrosine, L-asparagine, and L-citrulline as amino acidic moiety. All pro-vitamins showed similar or lower lipophilicity than α-tocopheryl acetate (VEAc), taken as reference, and similar stability in aqueous solutions. All pro-vitamins showed to be sensitive to enzymatic hydrolysis. None of the pro-vitamins crossed the skin in significant amounts, whereas they accumulated into the skin, in both the dermis and the epidermis. They are more hydrophilic and more water-soluble than the currently used acetate. © 2008 Wiley-Liss, Inc. and the American Pharmacists Association.

Section snippets

INTRODUCTION

Tocopherols are a class of lipophilic compounds, globally known as vitamins E. They exist as four homologous, α, β, γ, and δ, which differ in the number and position of the methyl groups in the chroman ring.¹ α-Tocopherol (VE) is the most active lipophilic antioxidant in biological membranes² and, together with ascorbic acid, constitutes the “antioxidant network” that protect skin against oxidative damage.³ Moreover α-tocopherol plays different roles in the maintenance of skin physiological

EXPERIMENTAL

α-Tocopherol (m.w. 430.72, yellow oil), α-tocopheryl acetate, as well reactants and catalysts, were purchased by Sigma (Sigma Chemical, St. Louis, MO), while all amino acids and anhydrous solvents were obtained by Fluka (Fluka Chemie, Buchs, Switzerland). All other solvents were of analytical grade and were purchased by Riedel-de Haen (Seelze, Germany).

Dimethyl-β-cyclodextrin (DM-β-CD) was purchased from Wacker-Chemie (Milan, Italy), while polysorbate 80 was obtained from A.C.E.F. (Fiorenzuola

Solubility, Stability, and Lipophilicity

Since the solubility of pro-vitamins and VE in pH 8.0 PBS, at room temperature, resulted very low (lower than the limit of quantification of the analytical method), DM-β-CD 5% was added as solubilizing agent, to guarantee sink conditions in permeation experiments.²² The solubility values obtained in the presence of cyclodextrin are reported in Table 1. The citrulline derivative (7) was the most soluble, showing a value more than 15 times higher that VE and almost 40 times higher than VEAc.

CONCLUSIONS

Five new conjugates of α-tocopherol with amino acids were prepared and tested. The amino acids used were L-proline, L-citrulline, L-asparagine, L-tyrosine, and L-serine.

The new pro-vitamins synthesized were sensitive to enzymatic hydrolysis, in particular their half-life was inversely proportional to their lipophilicity as estimated by the log capacity factor. The in vitro skin retention and metabolism of the pro-vitamins demonstrated that all of them accumulated into the skin in significant

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Citation Excerpt :
The sensitivity of α-tocopheryl acetate and the α-tocopheryl asparagine ester to hepatic hydrolysis were previously verified using the porcine liver esterase (Marra et al., 2009). The half-life of commercial α-tocopheryl acetate was established as equal to ca. 1 h, and the α-tocopheryl asparagine ester has proved as significantly more stable against hepatic enzymes with the half-life of 6 h (Marra et al., 2009). In order to avoid an extensive liver metabolism and to achieve high concentrations of the VE esters in plasma, the parenteral formulations of the VE esters were proposed (dos Santos et al., 2012; Weber et al., 2002).
Tocopherols and tocotrienols belong to the family of vitamin E (VE) with the well-known antioxidant properties. For certain α-tocopherol and γ-tocotrienol derivatives used as the lead compounds in this study, antitumor activities against various cancer cell types have been reported. In the course of the last decade, structural analogs of VE (esters, ethers and amides) with an enhanced antiproliferative and proapoptotic activity against various cancer cells were synthesized. Within the framework of this study, seven amino acid esters of α-tocopherol (4a–d) and γ-tocotrienol (6a–c) were prepared using the EDC/DMAP reaction conditions and their ability to inhibit proliferation of the MCF-7 and MDA-MB-231 breast cancer cells and the A549 lung cancer cells was evaluated. Compound 6a showed an activity against all three cell lines (IC₅₀: 20.6 μM, 28.6 μM and 19 μM for the MCF-7, MDA-MB-231 and A549 cells, respectively), while compound 4a inhibited proliferation of the MCF-7 (IC₅₀ = 8.6 μM) and A549 cells (IC₅₀ = 8.6 μM). Ester 4d exerted strong antiproliferative activity against the estrogen-unresponsive, multi-drug resistant MDA-MB-231 breast cancer cell line, with IC₅₀ value of 9.2 μM. Compared with the strong activity of compounds 4a, 4d and 6a, commercial α-tocopheryl succinate and γ-tocotrienol showed only a limited activity against all three cell lines, with IC₅₀ values > 50 μM. Investigation of the cell cycle phase distribution and the cell death induction confirmed an apoptosis of the MDA-MB-231 cells treated with 4d, as well as a synergistic effect of 4d with the known anticancer drug doxorubicin. This result suggests a possibility of a combined therapy of breast cancer in order to improve the therapeutic response and to lower the toxicity associated with a high dose of doxorubicin. The stability study of 4d in human plasma showed that ca. 83% initial concentration of this compound remains in plasma in the course of six hours incubation. The ligand based virtual screening of the ChEMBL database identified new compounds with a potential antiproliferative activity on MCF-7 and on multi-drug resistant MDA-MB 231 breast cancer cells.
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Research ArticlesSynthesis, hydrolysis, and skin retention of amino acid esters of α-tocopherol

ABSTRACT:

Section snippets

INTRODUCTION

EXPERIMENTAL

Solubility, Stability, and Lipophilicity

CONCLUSIONS

J Invest Dermatol

J Photochem Photobiol B

J Control Release

J Control Release

Biochim Biophys Acta

J Invest Dermatol

J Pharm Sci

J Pharm Sci

Bioorg Med Chem Lett

The location and function of vitamin E in membranes (review)

Mol Membr Biol

The antioxidant network of the stratum corneum

Curr Probl Dermatol

Determination of alpha-tocopherol in plasma by high performance liquid chromatography with fluorescence detection and stability of alpha-tocopherol under different conditions

Arzneimittelforschung

Research Articles
Synthesis, hydrolysis, and skin retention of amino acid esters of α-tocopherol