Plant-based sterols and some sterol derivatives show many attrac- tive properties and uses, particularly in medicine. These compounds have been found to have therapeutic effects on the function of the heart, kidneys and prostate, lipid metabolism, infections and some forms of cancer, for ex- ample. The use of plant sterols in lowering blood cholesterol level is particu- larly interesting. For example, the cholesterol-lowering effect of ß-sitosterol has been known since the 1950's. The effect is based on the fact that-sitosterol inhibits the absorption of cholesterol in the intestines. Consequently,p- sitosterol has been used for decades as a drug and natural product to lower blood cholesterol level. In addition to p-sitosterot, p-sitostano) has also been found to have similar or even more advantageous properties. The greatest problem in using these compounds and plant sterols in general is, however, that they are poorly soluble in oils and fats and completely insoluble in water.

The hydroxylic group contained in sterols allow them to form esters with fatty acids, carboxylic acids, carboxylic acid anhydrides or carboxylic acid chlorides, for example. Such ester derivatives have been prepared in order to modify the solubility of sterols in oil, fat and water.

The solubility in oils/fats has been improved by transesterifying plant sterols with fatty acids. For example, WO 92/19640 (Raision Margariini Oy) discloses a cholesterol-lowering agent containing a fatty acid ester of - sitostanol or a mixture of such fatty acid esters. The p-sitostanot fatty acid es- ter used in the composition is prepared by a transesterification reaction using the methyl ester of rapeseed oil and sitostanol, for example, as the starting material. However, the water solubility of plant sterols cannot be improved in this manner.

EP 0,007,474 B1 (Roecar Holdings) discloses an attempt to im- prove the water solubility of sterols by forming dihemiesters and trihemiesters, e. g. disuccinates and trisuccinates, of sterol glycosides and salts thereof.

These sterol derivatives are prepared by a reaction of sterol glycoside and carboxylic acid anhydride. The compounds are stated to be therapeutically

active in preventing infections and as cancer drugs, for example. These com- pounds have, however, the disadvantage that they are not stable in aqueous solutions; furthermore, due to the preparation process, they are not always necessarily completely non-toxic.

Polar sterol derivatives are surfactants that lower the surface ten- sion and accumulate to phase boundaries. Hence, the function thereof as emulsifiers, dispersing and solubilizing agents and formers of various liquid crystal structures (liposomes, for example) is due to the unique molecular structure of these compounds.

Steroid esters are commonly used in cosmetics. They are used in hair cosmetics and skincare. For example, esters of 2-ethyl hexane acid, stearic acid and oleic acid with cholesterol are used in lipsticks, eye shadows and in skin creams and emulsions. Steroid esters are also used in hair care products. The use of steroid esters is based on their good emulsifying ability and good affinity with skin. Steroid esters act as additives that form boundaries between different phases, generally enabling mixtures containing different phases and materials to be stable.

Steroid esters are also used in the manufacture of liquid crystals and biodegradable polymers. Also in these applications, the steroids act as substances combining different phases, enabling stability of the mixtures.

BRIEF DESCRIPTION OF THE INVENTION An object of the invention is thus to provide novel, non-toxic plant sterol derivatives that consist of naturally occurring compounds and are hy- drophilic, thus having a higher solubility in polar solutions, such as alcohol so- lutions, and particularly in aqueous solutions. The objects of the invention are achieved by a product and a process that are characterized by what is dis- closed in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

In accordance with the invention, the hydrophilicity and water solu- bility of plant sterols were improved by esterifying a plant sterol, such as- sitosterol, with a fruit acid, such as citric acid, to give a hydrophilic ester of the plant sterol and the fruit acid. The products thus obtained are non-toxic and consist of plant-based compounds found as such in many foodstuffs, thereby being highly suitable for use in pharmaceuticals and foodstuffs. In the present invention, the term'hydrophilicity'refers to the ability of the compound of the

invention to form hydrogen bonds with the surrounding solution and thus the tendency of the compound to be more soluble than a sterol molecule with one OH group.

DETAILED DESCRIPTION OF THE INVENTION The invention relates to a sterol derivative which is an ester of a plant sterol and a fruit acid.

In the present invention, plant sterols refer to plant-based sterols (including the saturated forms thereof, i. e. stanols) that can be separated from plants. These also include sterols of the plant sterol type synthetically pre- pared from steroid precursors. In the sterols of the invention, the OH group of a sterol is preferably at the 3 position, preferably also at the P position of the steroid nucleus. The numbering of the steroid nucleus is shown in the follow- ing: Plant sterols include all wood-based sterols, for example. Such wood-based sterols are typically found in the neutral fraction in pulping, and they include-sitosterol and campesterol and the saturated forms thereof, i. e.

P-sitostanol and campestanol, as well as stigmasterol. The structures of these wood-based sterols are shown in the following: ß-Sitostanol

ß-Sitosterol

Campestanol Campesterol

Stigmasterol Other suitable plant sterols further include ergosterol, desmosterol, sargasterol, brassicasterol and fucosterol and the saturated forms thereof. The structure of these compounds are shown in the following:

Ergosterol Desmosterol

Sargasterol

Fucosterol

Brassicasterol The plant steroi is most preferably ß-sitosterol, stigmasterol or ß- sitostanol.

In the present invention, fruit acids refer to organic carboxylic acids that can be separated from fruit or the like. These also include corresponding fruit acids that can be synthetically or biochemically, particularly enzymatically, prepared. Typically, the fruit acids used according to the invention are hy- droxycarboxylic acids. Particularly preferable are a-hydroxy carboxylic acids (they contain a hydroxyl group in the carbon atom adjacent to the carboxylic group). Typically, the hydroxy carboxylic acids and a-hydroxy carboxylic acids used according to the invention are saturated, aliphatic carboxylic acids, pref- erably lower aliphatic carboxylic acids. In this connection, the term'lower're- fers to carboxylic acids containing 10 carbon atoms at most, particularly pref- erably 6 carbon atoms at most. The acids are thus preferably C2-C10-carboxylic acids, particularly preferably C2-C6-carboxylic acids. The acids can be mono-, di-or tricarboxylic acids, preferably tricarboxylic acids, and they may contain more than one hydroxyl group. Typical a-hydroxy carboxylic acids used in ac- cordance with the invention are citric acid, maleic acid, tartaric acid, lactic acid and glycolic acid.

When the fruit acid part of the compound contains more than one carboxylic acid group (di-and tricarboxylic acids), the remaining free carbox- ylic acid group (s) of the sterol ester formed can be as ester, free acid or salt.

Preferably, the ester is then an alkyl ester, preferably a lower alkyl ester (C,- C, 0-alkyl ester), such as a methyl ester, ethyl ester, or propyl ester (isopropyl or n-propyl). The salt is preferably an alkali salt or alkali earth salt, particularly preferably a potassium or sodium salt.

The invention further relates to a process of preparing plant sterol fruit acid esters, the process being characterized by esterifying the plant sterol and the fruit acid by a transesterification reaction.

In the transesterification reaction, the starting sterol is transesteri- fied with the ester of the fruit acid, the ester typically being in the form of an alkyl ester, preferably a lower alkyl ester (C,-C,o-alkyl ester), such as a methyl ester, ethyl ester or propyl ester. The propyl ester can be an n-propyl ester or an isopropyl ester.

The following shows by way of example the transesterification reac- tion of ß-sitostanol with a trimethyl citrate to give a monoester of ß-sitostanol and trimethyl citrate as the final product:

Typically, the transesterification is conducted at a temperature of 50 to 230°C, preferably at a temperature of 120 to 180°C. The reaction is typically conducted in a vacuum, preferably at a pressure of 1 to 900 mbar, using an

inert protective gas, such as nitrogen. The reaction can also be conducted ei- ther in a vacuum or in a protective gas.

The transesterification is conducted in the presence of a catalyst, which is preferably an alkali metal alkoxide, i. e. a compound of an alkali metal and alcool, particularly potassium methoxide, potassium ethoxide, potassium n-propoxide, potassium i-propoxide, potassium i-butoxide, potassium n- butoxide, potassium t-butoxide, sodium methoxide, sodium ethoxide, sodium n-propoxide, sodium i-propoxide, sodium n-butoxide or sodium t-butoxide. The amount of catalyst used is typically 0.05 to 5.0% by weight of the total amount of the starting materials.

The fruit acid used as the second starting material in the trans- esterification is preferably in the form of an alkyl ester, preferably a lower alkyl ester (C1-C10-alkyl ester). The alkyl esters of fruit acids are commercially avail- able but they can also be prepared by known esterification processes by es- terifying the fruit acid by methanol, ethanol or propanol (n-or i-propanol), for example, to give the methyl ester, ethyl ester and propyl ester, respectively, of the fruit acid.

The boiling points of the fruit acid alkyl esters are usually above 200°C. Consequently, temperatures over 200°C can be used in the trans- esterification. The compounds of the invention can be prepared also at tem- peratures below 100°C. A preferred temperature used in the transesterification is, however, 120 to 180°C.

In the transesterification, alcohol is released, and the removal of the alcohol from the reaction mixture improves the yield. Consequently, the reac- tion is typically conducted in a vacuum reactor in an inert protective gas. It is also feasible to remove the alcohol formed from the reaction mixture entirely in a vacuum or in a flowing protective gas. Nitrogen is preferably used as the protective gas, and the pressure can be 1 to 900 mbar.

The transesterification can be conducted without a solvent or in a solvent. Suitable solvents are inert, such as hexane, cyclohexane and toluene.

Compounds are preferably prepared without a solvent as a melt reaction or with an ester of the fruit acid acting as the solvent of the sterols.

The transesterification can be conducted in a batch reactor or a continuous reactor. The reaction is preferably conducted in a stirred reactor.

It was unexpectedly found that the compounds of the invention could be prepared by the transesterification reaction under mild reaction con-

ditions with harmless and non-toxic chemicals or with chemicals that become non-toxic upon degradation. Unexpectedly, fruit acids, such as citric acid, were found to remain stable in the reaction conditions used.

When the fruit acid used as the starting material is in the form of al- kyl ester, in the transesterification an ester of sterol and fruit acid is formed in which the potential free carboxylic acid groups (in the case of di-and tricar- boxylic acids) are as an alkyl ester. Such form of an alkyl ester is as such soluble in esters and fruit acid oils. The alkyl ester form of the compound of the invention is thus as such suitable for use in products containing fruit acid esters and fruit acid oils.

The solubility of the aforementioned sterol fruit acid monoester in the form of an alkyl ester can be further improved by saponifying the alkyl es- ter groups of the compound to the form of an alkali metal salt. The saponifica- tion can be conducted in a solvent containing an alkali, such as sodium hy- droxide or potassium hydroxide. Suitable solvents are alcools and hydrocar- bon solvents. Suitable alcools include ethanol, propanols and butanols, for example, and suitable hydrocarbon solvents include hexane, cyclohexane and toluene, for example. A compound in the alkali salt form has a particularly high solubility in aqueous solutions and other polar solutions.

The salt groups of the sterol and fruit acid monoester in the alkali salt form can be further transformed into free acids. This is conducted by an acid hydrolysis in which a homogenous acid (such as mineral acid or hydro- chloric acid) or a heterogeneous acid (for example an ion exchange resin, such as a strong cation exchange resin) can be used.

The compounds of the invention can also be prepared by direct esterification of fruit acids with sterols. Direct esterification can be conducted as a melt reaction or in a solvent. Either a homogenous acid catalyst (such as mineral acid or p-toluene sulphonic acid) or a heterogeneous acid catalyst (such as sulphonated polystyrene divinyl benzene) can be used as the cata- lyst.

The compounds of the invention can also be prepared by esterifying sterols by means of acid chlorides or acid anhydrides that are more reactive than carboxylic acids, and by acylation reactions between the sterols them- selves.

The invention also relates to a composition containing esters of plant sterols and esters of fruit acids or mixtures thereof. The invention also

relates to compositions further containing one or more fruit acids and/or alkyl esters thereof, preferably lower alkyl esters.

The invention further relates to the use of plant sterol fruit acid es- ters as cholesterol-lowering agents and pharmaceuticals, and as an additive in foodstuffs, pharmaceuticals, cosmetics and polymers. The invention also re- lates to cholesterol-lowering agents, pharmaceuticals, foodstuffs, cosmetics and polymers containing the plant sterol fruit acid esters of the invention.

The plant sterol fruit acid esters of the invention can be used as cholesterol-lowering food supplements in various foodstuffs, such as cereals, dairy products, processed foods, candies and natural products. In foodstuffs, the compounds can also be used as antioxidants and markers. In beverages, the compounds can be used both as antioxidants and cholesterol-lowering agents in soft drinks, juices and sport drinks. In pharmaceuticals, the com- pounds can primarily be used as cholesterol-lowering agents but also as anti- atherosclerotic agents.

The compounds of the invention can also be used in cosmetics, such as make-up products, skin creams and mulsions and hair cosmetics.

Furthermore, the compounds of the invention can be used in biode- gradable polymers as agents improving the biodegradability.

Due to their surface-active properties, for example, the plant sterol fruit acid esters of the invention are useful as stabilizers in chemical composi- tions. In the present invention, chemical compositions typically refer to phar- maceuticals, cosmetics and polymer compositions. It was observed that when the plant sterol fruit acid esters of the invention were used, the homogeneity of pharmaceutical compositions, cosmetic compositions and polymer composi- tions was improved substantially.

The following examples illustrate the preparation of the compounds of the invention.

Example 1 An ester (plant sterol ester mixture) of citric acid and a wood-based plant sterol was prepared by a transesterification reaction as follows: A reactor was charged with 50.1 g of plant stanol mixture (containing 91.9% by weight of ß-sitostanol and 5.7% by weight of campesta- nol, manufactured by UPM-Kymmene Oyj, Lappeenranta) and 450.2 g of a triethyl ester of citric acid, i. e. triethyl citrate (commercial quality, purity > 98%

by weight). The nitrogen flush and heating of the reactor was initiated. Mixing was initiated and the mixing speed was adjusted to a value of 350 rpm. A small underpressure of 800 mbar was sucked into the reactor, whereafter the temperature was raised to 150°C. The reaction mixture was heated for 45 min before adding a catalyst. The catalyst added was sodium ethoxide (0.102 g).

After the addition of the catalyst, the reaction was continued for 6.5 h. The catalyst was decomposed by adding water into the reaction mixture, and the non-reacted plant stanol was extracted with hexane. The product obtained contained the desired reaction product and triethyl citrate. The reaction prod- uct was an ester of ß-sitostanol and triethyl citrate in which one of the acid groups of the citric acid forms an ester with p-sitostero ! and the two other groups are in the form of an ethyl ester. The reaction product also contained small amounts of a corresponding campestanol ester.

The mixture of the reaction product and triethyl citrate was saponi- fied in a KOH solution containing ethanol. The saponified mixture was highly soluble in aqueous solutions.

The non-saponified final product was analyzed by a mass specto- meter using direct ionization. The results of the analysis indicated that the product consisted of a compound with a mass number of 644 to 648. This cor- responds to the monoester of ß-sitostanol/ß-sitosterol and triethyl citrate (two free carboxylic groups of citric acid in ethyl form). The product also contained small amounts of a corresponding campestanol ester.

The hydrophilicity and water solubility of the product thus obtained were improved substantially.

Example 2 An ester (plant sterol ester mixture) of citric acid and a wood-based plant sterol was prepared by a transesterification reaction as follows: The trimethyl citrate used as the second reaction component was prepared from citric acid and methanol at 40°C using Amberlyst 15 cation ex- change resin (Rohm & Haas, the USA) as the catalyst. The reaction time was 24 h, whereafter the catalyst and methanol were removed from the reaction mixture.

For the transesterification, the reactor was charged with 41 g of a plant sterol mixture (containing 36% by weight of ß-sitostanol, 52% by weight of p-sitosteroi, 7% by weight of campesterol and, in addition, small amounts of

campestanol and stigmasterol, manufactured by UPM-Kymmene Oyj, Lap- peenranta) and 23 g of the trimethyl citrate prepared above. The reaction was conducted in a nitrogen protective gas at 160°C. The reaction time was 8 h and the mixing rate 500 rpm. 0.2 g of sodium ethoxide was used as the cata- lyst. The final product was analyzed by a mass spectometer using direct ioni- zation, whereby a molecule peak with a mass number of 616 to 620 was de- tected in the mass spectometer. This corresponds to the ester of p-sitostero) and trimethyl citrate in which one of the acid groups of the citric acid forms an ester with ß-sitosterol while the two other groups are in the form of a methyl ester. The product also contained some of a corresponding ß-sitostanol ester and small amounts of corresponding campesterol, campestanol and stigmas- terol esters.

The hydrophilicity and water solubility of the product of this example have also improved substantially.

Example 3 Esters of maleic acid, tartaric acid, lactic acid and glycolic acid of wood-based sterols can also be prepared in a similar manner using a trans- esterification reaction. Citric acid esters of ergosterol, desmosterol, sargas- terol, brassicasterol and fucosterol and esters of maleic acid, tartaric acid, lac- tic acid and glycolic acid can also be prepared in a similar manner.

Examples of use: Example 4 A skin cream containing the ester of ß-sitostanol and triethyl citrate was prepared as follows. An oil mixture containing 10.0% by weight of stearic acid, 8.0% by weight of olive oil and a small amount of tocopherol as an anti- oxidant was heated at 80°C. An aqueous solution containing 6.0% by weight of Na-lauryl-iminodipropionate and 4.0% by weight of the ester of p-sitostanot and triethyl citrate prepared in Example 1 was added to the mixture. After be- ing completely mixed, the product mixture was cooled to room temperature.

Example 5 (A) Preparation of a commonly known skin cream composition containing sterols.

Oil phase and aqueous phase having the following composition were used for preparing the cream: Oil phase Component Amount (% by weight) Liquid paraffin 53.0 White vaseline 10.0 Ester of cholesterol and 2-ethyl hexane acid 3.0 Polyoxyethylene (3)-stearyl ether 3.0 Polyoxyethylene (10)-oleyl ether 2.0 Polyoxyethylene (25)-octyl ether 1.0 Polyoxyethylene (4)-glyceryl ester distearate 1.5 Aqueous phase Amount (% by weight) Polyethylene glycol 800 0.5 Na-lauryl sulphate 0.2 Glycerin 0.1 Water 25.7 The oil phase was heated to 75°C and mixed to form homogeneous dispersion. The aqueous phase heated to 70°C was slowly added to the oil phase. The product mixture was completely mixed and cooled to room tem- perature.

(B) Preparation of a skin cream composition of the invention con- taining sterol fruit acid ester.

Oil phase and aqueous phase with the following composition were used for preparing the skin cream: Oil phase Component Amount (% by weight) Liquid paraffin 52.0 White vaseline 9.0 Polyoxyethylene (3)-stearyl ether 2.0

Polyoxyethylene (10)-oleyl ether 1.0 Polyoxyethylene (25)-octyl ether 0.5 Polyoxoethylene (4)-glyceryl ester distearate 0.5 Aqueous phase Ester of p-sitostano) and triethyl citrate 3.0 Polyethene glycol 800 0.5 Na-lauryl sulphate 0.2 Glycerin 0.1 Water 24.0 The oil phase was heated to 75°C and mixed to form a homogene- ous dispersion. The aqueous phase heated to 70°C was slowly added to the oil phase. The product mixture was completely mixed and cooled to room tem- perature.

When the plant sterol derivative of the invention was used, the product mixture obtained was more homogeneous, yielding a better affinity with skin than a composition containing known plant sterol derivatives.

Example 6 A polymer composition was prepared as follows.

An excess of p-vinylphenol/p-hydroxystyrene (0.93 g), an ester (4.0 g) of ß-sitostanol and triethyl citrate and anhydrous potassium carbonate (1.28 g) were mixed in 10 ml of dimethyl formamide under a nitrogen atmosphere at room temperature. The mixture was refluxed at 100°C for 3 h and cooled for 30 min. The monomer obtained after the purification was polymerized in tolu- ene at 100°C. The initiator used in the polymerization was AIBN. The polymer was purified by precipitation in methanol. When the fruit acid sterol ester of the invention was used, the structure of the polymer remained homogeneous without any discernible phase changes at the temperature of 80°C.

It is obvious to those skilled in the art that as the technology pro- gresses, the basic idea of the invention can be implemented in many ways.

The invention and its embodiments are thus not restricted to the examples described above but they can vary within the scope of the claims.