Patent 6,669,962

Polysaccharide-based shells are provided having use for forming water-dispersible microcapsular delivery systems for both hydrophilic and lipophilic drugs, pharmaceuticals, cosmetics and other active agents. These shells are prepared by intimately blending a solubilized polysaccharide with a lipophilic material to produce spherical droplets of the lipophilic material coated with the polysaccharide, diluting the emulsion with a solvent, and isolating the polysaccharide shells from the diluted emulsion.

SUMMARY OF THE INVENTION

We have now discovered a method for making microscopic-sized polysaccharide-based shells having use for forming water-dispersible microcapsular delivery systems for both hydrophilic and lipophilic drugs, pharmaceuticals, cosmetics and other active agents.

This invention is based on the discovery that when a completely solubilized natural polysaccharide and a water-immiscible material are intimately combined under high shear conditions, it is possible to isolate discrete polysaccharide-coated lipophilic droplets without the use of cross-linking agents. These droplets are conveniently separated from the predominantly-polysaccharide fraction of the jet-cooked dispersion by diluting the dispersion with water and then either centrifuging the diluted dispersion or allowing it to stand for a period of time sufficient for the layers to separate under the force of gravity according to their relative densities. Polysaccharide-coated lipophilic droplets are then isolated from either the surface of the dispersion or from the denser layer that settles to the bottom. The isolated droplets may be further purified by washing with water. The polysaccharide coating that surrounds each droplet is stable and prevents the droplets from coalescing when they are isolated, washed, and used for various end-use applications. The polysaccharide coating also causes these droplets to disperse instantly with minimum agitation when they are diluted with water. Moreover, if a volatile lipophilic material is used, hollow spheres can be recovered by permitting the lipophile to evaporate.

In accordance with this discovery, it is an object of this invention to provide new and novel compositions of matter comprised of microscopic-sized, fully biodegradable, natural polysaccharide-based shells useful as microcapsular delivery systems.

It is also an object of the invention to provide micron-sized lipophilic droplets, each droplet being coated with a thin layer of polysaccharide.

Another object of this invention to provide a process for the preparation of the aforementioned lipophile/polysaccharide compositions.

A further object of the invention is to provide injectable drugs and pharmaceuticals comprising aqueous dispersions of the aforementioned microcapsular delivery systems comprising polysaccharide-based shells containing hydrophilic or lipophilic active agents.

Other objects and advantages of the invention will become apparent from the following discussion.

DETAILED DESCRIPTION OF THE INVENTION

The preferred polysaccharide for use in the invention is starch. Starch is a high molecular weight polymer composed of repeating 1,4-alpha-D-glucopyranosyl units (anhydroglucose units or AGU) and is typically a mixture of linear and branched components. The linear component, amylose, has a molecular weight of several hundred thousand; while the molecular weight of the branched amylopectin is on the order of several million. Although normal cornstarch contains about 25% amylose, cornstarch varieties are available commercially that range in amylose content from 0% (waxy cornstarch) to about 70% (high-amylose cornstarch).

Starch is isolated from the seeds and tubers of living plants as granules that typically range from about 5 to 40 microns in diameter, depending upon the plant source. It is well known that starch, as isolated in its native state, is insoluble in water at room temperature. When a water suspension of granular starch is heated, granules slowly take up water with limited swelling. Then, at a definite temperature (typically about 65-70^oC.) the granules swell rapidly and irreversibly, as areas of crystallinity within the granule are lost, and hydrogen bonds are broken. The temperature at which this phenomenon occurs is commonly referred to as the gelatinization temperature.

Near the gelatinization temperature, a measurable percentage of the starch, in particular the amylose component, becomes soluble and diffuses out of the granule matrix and into the surrounding water. At temperatures greater than about 70^oC., a greater percentage of the starch becomes soluble, and granules become highly swollen and partially disrupted. At temperatures of about 90-100^oC., a viscous dispersion of starch in water is obtained. However, despite this outward appearance of solubility, starch is only partially water soluble and exists largely as highly swollen granules and granule fragments that are easily separable from starch solution, for example, by centrifugation. When cornstarch is heated in water to about 95^oC., only about 25% of the starch actually dissolves, the remainder being present as swollen granules and granule fragments.

The compositions of this invention are preferably prepared from unmodified starches obtained from cereal grains, such as corn, wheat and rice, or from root crops, such as potato and tapioca. Modified starches may also be used to obtain certain properties not obtainable with unmodified starches. An unmodified starch is one that has not been altered by chemical treatment or reduced in molecular weight by reaction with acids or enzymes. Use of unmodified starches for the preparation of compositions for injection or ingestion into the human body is preferred over modified starches; because unmodified starches have never been treated with potentially toxic chemicals.

Although any available starch variety is suitable for the preparation of these compositions, it is well known that differences in branching and molecular weight can cause differences in starch properties, which can lead to differences in the thickness, rigidity, water solubility and water swellability of the starch layers that surround the lipophilic droplets.

Starches having amylose and amylopectin components in various proportions may be used. Examples of these are waxy cornstarch having an amylose content of essentially 0%, normal cornstarch having an amylose content of approximately 25% and high amylose cornstarch varieties having amylose contents greater than 25%. Mixtures of these various starches can also be used. Since the ratio of amylose to amylopectin determines the rheology and gelling properties of starch solutions as well as the physical properties of the resulting dispersions and gels, the ratio of amylose to amylopectin will be a major factor in determining the thickness and physical properties of the starch coating. Although starch is preferably used in the preparation of these compositions, one may also use cereal flour, which is comprised largely of starch, but also contains the protein components of the cereal grain.

Examples of other natural polysaccharides for use herein include dextran, cellulose, and hydrocolloid gums of plant or microbial origin that have an inherent solubility, or can be physically altered to have a solubility, approximating that of the sheared starches described, below. Such gums include guar, locust bean, and xanthan. Of course, various mixtures of starch and these polysaccharides can also be used. The ensuing discussion will principally make reference to starch as the polysaccharide, with the understanding that aforementioned alternative polysaccharides could replace or be blended with the starch.

As previously stated, the shells of this invention consist essentially of non-cross-linked polysaccharide. This means that the structural component, or backbone, of the shell, that is the material constituting the rigid sphere, forms without the use of cross-linking agents. The term "cross-linking agent" is used herein in its usual sense to refer to any relatively short chain reactive chemical agent useful for covalently bonding polysaccharide molecules to one another. However, it is contemplated that groups, such as mannose or other sugars can be appended from the polysaccharide molecule for the purpose of targeting drugs as described in further detail, below.

The terms "water-immiscible material" and "lipophilic material" are used herein synonymously. In their broadest definitions, these terms are intended to encompass any organic compound that is largely insoluble in water, such as lipids, fats, oils, resins, rosins, silicones, and long chain ethers, alcohols, aldehydes, ketones, carboxylic acids, aliphatic and aromatic hydrocarbons, organic amines, organic polymers, and the like.

Although any lipid, fat, oil or substantially water-insoluble organic compound may be used for the preparation of the compositions of this invention, the particular lipophilic material used will depend upon the end-use application for the final product. For example, compositions prepared for the delivery of drugs and pharmaceuticals will contain bioactive ingredients, either by themselves or dissolved or dispersed within a second lipophilic phase, such as vegetable oil or vitamin E.

Lipophilic materials and/or other active ingredients that are either volatile or that might decompose under the high temperatures of the jet cooking process may be added with high-shear mixing to a jet cooked solution of starch. Jet cooking starch with a heat-stable lipophile prior to addition of the bioactive ingredient facilitates the incorporation of the bioactive component into the formulation. The jet cooked dispersion may also be dried (for example, by drum-drying) and the dried product later redispersed in water, just prior to addition of the bioactive ingredient. In this embodiment of the invention, the active component must be added while the starch dispersion is in a non-retrograded form. It is also essential that the active component be blended into the dispersion under conditions of high shear and turbulence, commensurate with that occurring within the jet cooker itself. On a laboratory scale, a Waring.RTM. blender provides sufficient mechanical shear to provide the intimate mixing needed. It is envisioned that a colloid mill could also be used for this purpose. We have discovered that starch can be locked into a non-retrograded form by drying the hot starch-containing dispersion shortly after it exits the steam jet cooker (for example, by drum drying). It is then possible to redisperse the dried product in water and introduce the active ingredient under high-shear conditions to form a final dispersion comparable to that produced by co-jet cooking.

Compositions of the invention are prepared by initially combining starch and lipophilic material in amounts ranging from about 5 parts to about 900 parts of lipophile, by weight, per 100 parts of starch (about 5-90% of the combined starch/lipophilic material composition on a dry weight basis). The upper practical limit for the lipophilic material content of the final composite composition is usually dictated by the point at which the lipophilic material begins to separate from the recovered product. For most embodiments envisioned herein, the upper limit of lipophilic material would not exceed 65 parts lipophilic material per 100 parts by weight of the starch (40%). Preferred compositions are comprised of about 20 parts to 50 parts of lipophilic material per 100 parts by weight of starch (17-33%).

The usual and most common method for preparing the compositions of this invention is to first prepare a blend of starch, lipophile and water by rapidly stirring together the components of the mixture at or near room temperature. When the stirrer is stopped, these mixtures tend to separate rapidly into an upper lipophilic phase and a lower phase that consists essentially of starch granules and water. The pH of the dispersion is typically in the 5-7 range, but may be optionally adjusted to any desired range by addition of acid, base or buffer system. It is well known that the properties of cooked starch are highly dependent upon the pH during cooking. As the pH is reduced to a value lower than about 4, starch will suffer increasing amounts of hydrolytic degradation, which will affect the formation and properties of the starch coating. At a sufficiently low pH, total hydrolysis of the polysaccharide occurs to yield glucose and other water-soluble sugars. The concentration of starch in water is typically about 10-15%, by weight; however, the upper limit is variable and is dictated by the desired viscosity of the cooked dispersion.

Cooking is preferably carried out with an excess steam jet cooker (see R. E. Klein and D. L. Brogly, Pulp and Paper, Vol. 55, pp. 98-103, May 1981) under conditions of intense turbulence and mechanical shear necessary to attain complete disruption of starch granules and complete solution in water of both the amylose and amylopectin components of starch. These components dissolve by virtue of not only the high temperature of the jet-cooking process, but also the shear-induced cleavage and rupture of starch molecules, especially those having the highest molecular weight. This intense mechanical shear not only facilitates the total and complete solubility of starch in water, but also lowers the apparent viscosity of the starch solution, as compared to either thermal steam jet cooking or conventional batch cooking. We believe that the spontaneous formation of starch coatings or shells around the individual lipophilic droplets is due to the fact that (1) starch is rendered totally soluble and is reduced in molecular weight by the cooking process and (2) the intense mixing and turbulence that takes place at the high temperatures and pressures of the cooking process converts the oil component into micron-sized droplets that become intimately mixed with starch solution. The exact mechanism by which a stable starch coating forms around each lipophilic droplet is unknown at the present time. No emulsifying or dispersing agents are used in the preparative process. Starch coatings are not formed during the high preparative mixing process. Rather, these coatings form spontaneously around each lipophilic droplet during the high temperature high shear cooking or blending step; and they remain intimately associated with the droplets when they are isolated, washed and purified.

Although jet cooking conditions may be widely varied by one skilled in the art, conditions are typically those cited in U.S. Pat. No. 5,676,994. Preferred cooking conditions are in the range 130^o-150^oC. (20-50 psig) within the hydroheater portion of the cooker, with a steam line pressure of 65-70 psig entering the cooker. Steam pressure as the hot dispersion leaves the cooker results in an immediate temperature drop in the cooked dispersion to 100^oC.

Dispersions produced by this cooking process will contain droplets of lipophilic material with diameters ranging from less than 1 micron to about 30 microns. Typically, about 95% of these droplets will be under 10 microns in diameter. For purposes of this invention, droplets in this size range will be considered as being "micron-size". Droplet size can be controlled by varying the steam pressure and temperature used in the jet cooking process. For example, higher mechanical shear within the hydroheater, and thus a smaller average droplet size, can be achieved by increasing the steam line pressure entering the cooker. This increases the amount of excess steam passing through the hydroheater and thus increases the intensity of mixing during the jet cooking process. Subjecting cooked dispersions to repeated passes through the steam jet cooker will also decrease the average droplet size. Droplets are stabilized by the formation of extremely thin layers of starch that surround, or coat, each lipophilic droplet.

Various techniques may be used to isolate starch-coated lipophilic droplets from jet cooked dispersions. In one such technique (which will be referred to as "hot-dilution"), the hot, jet cooked dispersion is diluted immediately after cooking with several volumes of hot (95^o-100^oC.) water, and the diluted dispersion is then allowed to cool to room temperature. In another isolation technique (referred to as "cold-dilution"), the hot, jet cooked dispersion is first allowed to cool to room temperature, and the cooled dispersion is then diluted with several volumes of unheated water. In still another isolation technique, the hot, jet cooked dispersion is dried (for example, by drum drying), and the dried solid is then redispersed in water. It was particularly surprising to discover that the starch coatings around the droplets withstood the severity of treatment imparted by the hot dilution and drum drying. The amount of water used for dilution or redispersion should be sufficient to reduce the viscosity of the dispersion to a point whereby the spheres attain sufficient mobility in the dispersion to be readily isolated from starch that is not organized into spherical structures. Typically, the starch/lipophile dispersion is diluted with water about 15-fold to 25-fold. Diluted dispersions are then either centrifuged or allowed to stand for several hours. A layer comprised of starch-coated lipophilic droplets rises to the surface of the dispersion, because of the lower density of the starch-coated droplets relative to water; and this layer is then separated from the rest of the dispersion. In addition to the low-density fraction that rises to the surface, a high density fraction is often observed at the bottom of the dispersion. Although this fraction may be comprised largely of retrograded starch, it can also contain starch-coated lipophilic droplets. The factor that determines whether starch-coated droplets rise to the surface or settle to the bottom under the force of gravity is their cumulative density, which is governed by the weight ratio of lipophile-to-starch. For example, thin starch coatings (thin-walled spheres) on relatively large, low density lipophilic droplets yield coated droplets that rise to the surface; because they have a cumulative density lower than that of water. Conversely, thick starch coatings (thick-walled spheres) on relatively small droplets yield high-density droplets that settle on standing. Specific starch/lipophile ratios can be selected by adjusting the density of the aqueous phase such that both bouyant and sedimented combinations are excluded from the middle phase containing coated droplets of desired lipophile to starch ratio.

The thickness of the starch coating, and thus the cumulative density of coated lipophilic droplets, are influenced by several factors, some of which are: (1) The method used to isolate coated droplets, i.e., whether the jet cooked dispersion is subjected to hot-dilution, cold dilution, or drying followed by redispersion in water. Cold-dilution tends to produce a thicker starch coating; because starch is still at a high concentration when the cooked dispersion is cooled. Hydrogen-bonding and retrogradation therefore take place rapidly to form a rigid gel that is not easily dispersed or dissolved when the cooled dispersion is diluted. In the hot-dilution process, dissolved starch is diluted with hot water before gelling takes place, and a thin starch coating is therefore produced. (2) The ratio of amylose to amylopectin. Amylose solutions form gels more rapidly and at higher temperatures than amylopectin solutions and also produce gels that are more rigid. Amylose-containing starch will thus surround the lipophilic droplets with a thicker, more rigid and more water-insoluble gel coating. (3) The intensity of mixing during the dilution process. High-speed, high-shear mixing can strip away and disperse more of the surrounding gelled starch layer, thus producing a thinner starch coating. Typically, these starch layers have a thickness of less than one micron, and are often on the order of 0.1 micron in thickness. For most applications, the thickness of the shell wall should not exceed about 25% of the shell diameter.

After separation from the main body of the dispersion, the resulting free-flowing, non-coalesced, starch-coated lipophilic droplets may be freed of excess starch by water washing. The starch coating is not removed by the washing procedure. The starch coating that surrounds each lipophilic droplet prevents individual droplets from adhering to each other and coalescing, even under the high gravitational fields provided by a high-speed centrifuge. Moreover, the starch coating allows a concentrated dispersion of separated lipophilic droplets to be redispersed in water with only minimal agitation.

The starch coating is in the form of a continuous layer or shell that surrounds each lipophilic droplet. These starch coatings may be seen by: (1) Adding iodine/KI solution to an aqueous dispersion of coated droplets. This treatment causes the starch coating to assume the blue color characteristic of the amylose/iodine complex. Iodine-stained starch coatings may then be seen with a light microscope. (2) Dehydrating a concentrated aqueous dispersion of starch-coated droplets by freeze drying on a glass microscope slide. This treatment yields an oily residue. When this residue is diluted with a drop of the same lipophile used in the original preparation, the starch coatings appear as spherical shells when viewed with a phase-contrast light microscope. (3) Adding a water dispersion of coated droplets to ethanol. This treatment dissolves the lipophilic moiety and also causes dehydration of the starch coating. The resulting starch coatings or shells may then be observed in ethanol dispersion with a light microscope. (4) Using supercritical carbon dioxide to critical-point dry an ethanolic dispersion of the starch coatings or shells. This critical-point drying technique minimizes distortion of the starch moiety, and the coatings or shells may then be examined with a scanning electron microscope.

By virtue of the shells consisting essentially of starch, they are considered to be both digestible and biodegradable. By "digestible" it is meant that the shells or coatings can be broken down in the digestive systems of mammals or other organisms that produce amylases in the digestive tract. By "biodegradable" it is meant that the shells or coatings are metabolized in the target organism or are readily broken down to harmless byproducts by microorganisms prevalent in the environment.

The coatings or shells of starch that surround these lipophilic droplets cause them to disperse instantly in water with gentle agitation, and the microscopic size of the droplets makes them suitable for numerous practical applications.

One such application is as a delivery system for drugs and pharmaceuticals, primarily by means of injection. Many drugs and pharmaceuticals are lipophilic in nature and can be combined with starch according to the process of this invention to yield starch-coated droplets. High-melting drugs and pharmaceuticals may be first dissolved in non-toxic, liquid oils, for example, vegetable oil or Vitamin E.

If a drug or pharmaceutical is dissolved or dispersed within the lipophilic component of these polysaccharide-coated droplets, the surrounding polysaccharide coating or shell can be used to target these drug-containing droplets and thus deliver the drug to specific sites within the body. An example of this technique is the targeting and delivery of drugs used for chemotherapy to specific tumor sites. Since targeting is known to be directed by the chemical structure of sugar units surrounding or chemically attached to the drug, the products of this invention are especially useful for drug targeting, since the structure of the polysaccharide coating or shell can be easily altered as previously described by: (1) replacing or blending starch with other polysaccharides (e.g., dextran or a galactomannan such as guar or locust bean gum) or (2) reacting sugars, such as mannose, with the polysaccharide coating or shell. The use of dextrans as drug carriers has been reviewed (L. Molteni, in Drug Carriers in Biology and Medicine, G. Gregoriadis, ed., Academic Press, New York, 1979, pp 107-125). The starch coating can also serve as a reactive site for the attachment of targeting proteins that can direct a bioactive agent to specific areas within the body. The term "bioactive agent" is used herein to refer to any compound or agent that would have a physiological effect on a living organism.

The coated droplets of the invention may be stored in aqueous suspension, or alternatively as a dry product that is suspended in a pharmaceutically acceptable vehicle or carrier immediately prior to administration. The terms "administer", "administration" and the like are used herein in reference to any form of parenteral introduction, such as by intravenous, subcutaneous or intramuscular injection. Other therapeutic modes of administering the compositions of this invention include oral administration and application to the skin surface. The coated droplets may also be formulated with adjuvants conventional in the art for the treatment of humans and animals. Starch hydrolysis modifying agents may also be included, which are effective for increasing or decreasing the rate at which the active agent is released through the shell.

Other applications of a non-medical nature for these starch-coated lipophilic droplets will become obvious to those skilled in the art. For example, coated droplets can be used to deliver essential oils and flavoring materials in food products, and to deliver lipophilic materials and other active components in cosmetic preparations. These droplets can also be used as delivery agents for waxes and other lipophilic materials in water-based coating formulations.

In one embodiment of the invention, a volatile lipophilic material is selected as a transient carrier for an active agent that is deposited on the inner surface of the starch shells after the lipophilic material is allowed to evaporate or is drawn off under vacuum. Also, a hydrophilic agent or material can be absorbed into hollow shells remaining after evaporation of a volatile lipophile.

When desired, these starch-coated lipophilic droplets can be made to coalesce and separate from water by removing the starch coating with a starch-degrading enzyme, for example, alpha-amylase.

Claim 1 of 19 Claims

We claim:

1. A composition consisting of discrete spherical shell and optionally material encased in said shell selected from the group consisting of lipophilic material and bioactive material, wherein the structural component of said shell consists essentially of non-cross-linked jet-cooked starch.

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