WO2003086609A1

WO2003086609A1 - Oil-filled nanocapsules from microemulsions utilizing cross-linkable surfactants

Info

Publication number: WO2003086609A1
Application number: PCT/US2003/010423
Authority: WO
Inventors: Royale S. Underhill; Aleksa V. Jovanovic; Randolph S. Duran
Original assignee: University Of Florida
Priority date: 2002-04-05
Filing date: 2003-04-04
Publication date: 2003-10-23
Also published as: AU2003220670A1

Abstract

A method of forming an oil-in-water microemulsion composed of nanocapsules includes mixing at least one oil and at least one surfactant to water or a water based solution, wherein the oil forms a plurality of oil droplets in the water or water based solution, the surfactant forming a coating layer on the oil droplets. A polymerizable material can then be added to form a cross linked outer layer on the coating layer. The cross linked layer stabilizes the microemulsion. A biocompatible layer can be used as a final coating layer.

Description

OIL-FILLED NANOCAPSULES FROM MICROEMULSIONS UTILIZING CROSS-LINKABLE SURFACTANTS

Field of the Invention

The invention relates to microemulsions, more particularly oil-in-water microemulsions having one or more coating layers on the oil droplets. Background The development of microemulsion technology has enabled formation of improved dispersions for some materials. Microemulsions are thermodynamically stable dispersions of one liquid phase into another, stabilized by an interfacial film of surfactant. The dispersion may be either an oil-in-water or water-in-oil dispersion. Microemulsions are typically clear solutions, as the droplet diameters are approximately 100 nanometers or less. The interfacial tension between the two phases is generally extremely low.

Drug toxicity a major health problem having an estimated yearly cost in excess of $10 billion/year. Drug overdoses can potentially cause death or permanent injury, such as brain damage. There are no known antidotes for most of toxic drugs.

Core-shell particle systems are promising candidates for the removal of drugs from the bloodstream. There are generally currently two methods for preparing core-shell particle systems, templating and self-assembly. Certain core-shell particle systems prepared by self-assembly have been already proven as promising drug uptake candidates. Oil-in-water microemulsions can utilize self-assembly to form transparent, isotropic solutions which are thermodynamically stable. Oil droplets less than 250 nm in diameter can be compartmentalized in a continuous domain of water. The oil droplets can be stabilized by a layer of surfactant on the surface at the interface between the oil droplets and the water. However, this arrangement is highly susceptible to droplet destabilization from certain environmental changes, including pH and temperature. Accordingly, stability problems have limited the potential utility of oil-in-water emulsions.

SUMMARY A method of forming an oil-in-water microemulsion includes the steps of mixing at least one oil and at least one surfactant to water or a water based solution, wherein the oil forms a plurality of oil droplets in the water or water based solution. The surfactant forms a surfactant layer which coats the oil droplets. A polymer shell layer is then formed on the oil droplets. The method can include the step of polymerizing the surfactant layer to form at least a portion of the polymer shell layer.

A cross-linking agent can be added, wherein the cross-linking agent reacts with the surfactant layer or a product derived from surfactant to provide at least a portion of the shell layer. The cross-linking agent can be a surfactant.

The surfactant can be lecithin, C₆₄H₁₂ 0₂₆₃ (Tween™) or OTMS. The oil can vary from 0.5% to 20 % weight by weight (w/w) of the microemulsion. A weight ratio of the surfactant to the oil can be from 0.2 to 5 (w/w). The size of the coated oil droplets included the polymer shell can range from approximately 80 nm to 250 nm. The polymer shell layer can include silicon, such as from Si-O-Si layers. For applications relating to people or animals, the method can include the step of forming a biocompatible layer, wherein the shell layer includes a biocompatible outer shell layer, such as PEO, polylactic acid or chitosane. DNA, RNA, antibodies, proteins, enzymes, cells or cell components, or biomimetics can be added to the mixing step for inclusion in any portion of the coated oil droplets.

An oil in water microemulsion includes water or a water based solution, at least one oil, at least one surfactant coating layer, wherein the oil forms a plurality of oil droplets in the water or water based solution. The surfactant forms a surfactant layer which coats the oil droplets. A polymer shell layer is disposed on the oil droplets. The surfactant layer can be polymerizable, wherein polymerization of the surfactant layer provides at least a portion of the polymer shell layer. The polymer shell layer can include silicon. For applications relating to people or animals, the polymer shell layer preferably includes a biocompatible surface layer, such as PEO, polylactic acid or chitosane.

The invention can be used for a variety of applications. Exemplary applications include drug detoxification, environmental remediation and pesticide poisoning therapy. BRIEF DESCRIPTION OF THE DRAWINGS

A fuller understanding of the present invention and the features and benefits thereof will be accomplished upon review of the following detailed description together with the accompanying drawings, in which: FIG. 1 illustrates a schematic representation of a nanocapsule according to the invention, the nanocapsule cut open to reveal a plurality of absorbed lipophilic molecules absorbed within its oil core.

FIGS. 2(A)-(D) illustrates a hydrophilic surfactant, a hydrophobic surfactant, an oil and a polymerizable surfactant, respectively. FIGs. 3(A) and (B) each show a step in a two step siloxane polycondensation chemistry reaction.

FIG. 4 illustrates a reaction to form a Si-O-Si containing shell on nanocapsules.

FIG. 5(A)-(D) illustrate the dependence of nanocapsule size on the concentration of the various nanocapsule components.

FIG. 6(A) and (B) illustrate nanocapsule size as determined by quasi- elastic light scattering.

FIG. 7(A)-(C) illustrate nanocapsule size as determined by transmission electron microscopy (TEM). FIG.8 illustrates the three dimensional shape of nanocapsules determined using atomic force microscopy.

FIG. 9(A) and (B) illustrate nanocapsule size of PEO coated nanocapsules. FIG. 10(A) and (B) illustrate nanocapsule size distribution of PEO coated nanocapsules.

FIG. 11(A)-(B) illustrate nanocapsule uptake rates of a lipohilic compound as a function of the concentration of the lipophilic compound. FIG. 12(A)-(C) illustrate uptake rates of test drugs for various nanocapsule formulations.

DETAILED DESCRIPTION OF THE INVENTION

It has been discovered that polymerizing the surface of oil droplets can substantially stabilize oil-in-water emulsions. Although discussed in terms of oil- in water emulsions, the invention can also be used to stabilize water-in-oil emulsions as well. In all cases where data is shown, the model drug is bupivacaine and the model compound is quinoline. However, the invention is in now way limited to this specific drug or specific compound. Oil-in-water microemulsions can be prepared by mixing at least one oil and at least one surfactant to water or a water based solution, wherein the oil forms a plurality of oil droplets in the water or water based solution. Ethyl butyrate is an example of an oil that can be used. The surfactant forms a coating layer on the oil droplets. The surfactant layer can be cross linkable surfactant which can be polymerized. A shell layer can formed on the surface of the surfactant layer, such as through reaction of the surfactant with a suitable cross linking agent.

Figure 1 shows a cut-open nanocapsule 100 including a Si-O-Si (silica) containing outer layer 105, the outer layer 105 disposed on a surfactant coating layer 1 1 0. The surfactant coating layer 1 10 is disposed on an oil core 1 1 5. The oil core 1 1 5 is depicted having a plurality of absorbed lipophilic molecules 1 20 trapped within.

The surfactant layer 1 10 can be a hydrophilic, hydrophobic or a polymerizable surfactant layer. For example, the surfactant layer 1 10 can be selected from TWEEN-80™ (hydrophilic), lecithin (hydrophobic), and octadecyltrimethoxysilane (OTMS) or alkoxysilane (polymerizable). Depending on the application, in the case the surfactant layer 1 10 is itself polymerizable and is capable of forming a skin layer sufficiently robust for a given application, an outer layer 105 may not be necessary.

Figure 2(A)-(D) illustrate examples of the structure of a selected hydrophilic surfactant (Tween™), a hydrophobic surfactant (lecithin), an oil (ethyl butyrate) and a polymerizable surfactant (OTMS), respectively. TWEEN-80™ and the family of TRITON X™ compounds. TWEEN-80™ is manufactured by the ICI group of Companies, Newcastle, DE. TWEEN 80™ is polyoxyethylene sorbitan monooleate, and has the following synonyms: TWEEN 80-1™, polyoxyethylene sorbitol ester; polysorbate 80 and PEG (20) sorbitan monooleate. This material has the molecular formula C₆₄H₁₂₄0₂₆₃ and a corresponding molecular weight of 131 03 amu. Preferred surfactants have reactive headgroups which are cross-linked to form a three (3) dimensional network at the surface of the oil droplet. The coating layer can be modified, for example, by reacting a small silicon containing molecule, such as an alkoxysilane, to build a thicker shell, the shell including polysiloxane. The polysiloxane layer can be chemically bound to surfactant layer species which include hydroxyl groups, such as OTMS.

The oil filled nanocapsules 100 have the potential for removing hydrophobic compounds from an aqueous environment by absorbing the hydrophobic compounds inside the oil core region 1 1 5. Studies performed have shown that the nanocapsules are able to remove drugs and drug mimics from a saline solution and reverse poisonings due to pesticides. According, the invention has potential applications in drug and pesticide detoxification therapy. The nanocapsules 100 preferably include oil cores 1 1 5 and a Si-O-Si

(silica) based shell. The naocapsules 100 are dispersible in water and water- based systems such as saline and blood plasma. The oil core 1 1 5 can capture (e.g. absorb) hydrophobic compounds and sequester the hydrophobic compounds inside the oil droplet 1 20, thus removing or preventing entry of the hydrophobic compound from an aqueous phase, such as blood or saliva. The various layers comprising nanocapsule 1 00 can be doped with a variety of molecules, including biomolecules. For example, at least one of DNA, RNA, antibodies, proteins, enzymes, cells or cell components, and biomimetics can be added to one or more of the particle layers (105, 1 10) as well as to the oil core 1 1 5.

Magnetic particles (not shown) can also be included in the nanocapsules. Magnetic particles can permit the nanocapsules to be steered into desired locations or flow paths using a suitable applied magnetic field. The nanocapsules have potential applications beyond drug detoxification therapy. Nanocapsules 100 may find use in contaminated- water remediation, and as vessels to solubilize water-insoluble components such as heterogeneous catalysts. As a drug detoxification therapy, the nanocapsules offer a number of significant benefits. First, there is no current effective therapy for most drugs. Second, the nanocapsules can be provided suspended in saline solution which allow inexpensive manufacture and will facilitate stocking in emergency rooms. Third, the manufacture of nanocapsules according to the invention is cost effective because the components are generally readily available and inexpensive. Finally, the coated nanocapsules are quite stable.

Nanocapsules can be formed in the following exemplary way. A microemulsion can be formed by first dissolving a hydrophilic (e.g. Tween 80™) or hydrophobic (e.g. lecithin) surfactant in a saline solution. An oil is then added to swell the surfactant micelles and then the solution is heated. OTMS (a polymerizable surfactant) can then be added. OTMS gets intercalated in the surface of the microemulsion droplets. The microemulsion can then be further stabilized by forming a shell layer containing Si-O-Si layers around the oil droplets. For example, the shell layer can be formed by performing siloxane chemistry through a two step polycondensation reaction, the respective steps shown in FIGs. 3(A) and (B), respectively.

The condensed OTMS skin layer formed on the surface of the nanocapsules may not be robust enough for certain applications. Thus, if needed to improve the stability of the nanocapsules toward a given environment, a cross-linking agent, such as TMOS (tetramethoxysilane), can be added. By reacting TMOS with the hydroxyl groups provided by OTMS, additional Si-O-Si layers are added to thicken the shell around the nanoparticles. FIG. 4 shows the applicable reaction between OTMS and TMOS.

The possibility of manipulating the size of the particles by changing the concentration of the respective nanocapsule components was investigated. The results are shown in FIGs. 5(A)-(D).

Nanoparticles formed using the invention were characterized. The characterization included the size and position of the core and overall assembly of the nanocapsules. For size determination, quasi-elastic light scattering was used as shown in FIGs. 6(A)-(B) . It revealed that the size of the nanocapsules are broadly distributed, but most were in the range of from 80-1 20 nm. Transmission Electron Microscopy (TEM) was used for the confirmation of the size and position of the oil core within the nanocapsule as shown in FIG. 7(A)-(C). The TEM images also confirmed the size of the nanocapsules to generally be 100 nm or less.

Atomic Force Microscopy (AFM) was used to further investigate the 3D appearance of the nanocapsules. The AFM results are shown in FIG. 8. Since a siloxane surface is neither generally biologically compatible nor tolerated by most bloodstream proteins, the introduction of biologically compatible (biocompatible) moiety on the surface of the nanocapsule can be used for applications involving animals or people. For example, a polyethylene oxide (PEO) covered nanocapsule is expected to be "stealth"

with respect to plasma proteins. PEO coverage of the nanocapsules did not measurably increase the particle size as shown in FIGs. 9(A) and (B) and

FIGs. 10(A) and (B) . Some biologically compatible alternatives to PEO for coating the surface of the nanocapsules include polylactic acid, and

chitosane.

The uptake capability of the nanocapsules was evaluated. The ability of the nanocapsules to absorb lipophilic compounds into their core and the kinetics of the uptake was determined. HPLC (high-performance-liquid

chromatography) was used in drug binding experiment. As shown in FIGs.

1 1 (A) and (B), it was found that 4-14% of the model drug was removed from

saline solution and the uptake does not significantly depend on the

concentration of the model drug.

The uptake can be improved in case of formulation with a PEO shell as shown in FIG. 1 2(A)-(C) . Quinoline was chosen as a model compound as it emulates the properties of certain target drugs. Quinoline absorption was monitored at 295 nm. Once it is inside the particle, absorption is quenched,

due to the Si0₂ layer on the surface of the particle which acts as a filter for that radiation. Example

The present invention is further illustrated by the following specific

example relating to formation of nanocapsules. This example is provided for

illustration only and is not to be construed as limiting the scope or content of

the invention in any way. Oil filled nanocapsules were formed by mixing 0.23g lecithin, 0.38g oil (e.g. ethyl-butyrate), 0.23g OTMS, 2.13g Tween-80™ and 25.3g saline. The above chemicals were heated (70 C) and stirred for 80 hours. Then, 1 g of HEPES (4-(2-hydroxyethyl)-piperazine-1 -ethane-sulfonic acid) buffer and 0.4g of (0.5 M) HCI were added. The mixture was then stirred at room temperature for 25 min.

The pH was adjusted to 7 to 7.4. 0.043g of tetramethoxysilane (TMOS) was then added to thicken the Si-O-Si shell around the nanoparticles. The solution was stirred at room temperature for 30 hours, then dialyzed for 24h (with 4 changes of the dialysis water). The solution was then filtered then diluted 2 fold.

In separate beaker, 0.45g of 2-(methoxy(polyethylenoxy)propyl) trimethoxysilane, 4.5ml of saline and 0.45g of 0.5M HCI were added, then stirred. After 1 5 min, 3.2g of the polyethyleneoxyl (PEO derivative) solution was added to the nanoparticles solution, and then stirred. After an additional 10 min., 1 .6g of the PEO derivative solution was added and after 5 more minutes the remainder of the PEO derivative solution was added to form a PEO shell on the nanocapsules. The resulting solution was stirred for 30h. Then pH was then adjusted to 7 to 7.4. The nanocapsule solution was then dialyzed for 24h (with 4 changes of the dialysis water). 1 g of HEPES buffer was added and the pH adjusted to 7 to7.4. Finally, the solution was filtered to remove large dust particulates and yield the PEO coated nanoparticles. [0001] It is to be understood that while the invention has been described in conjunction with the preferred specific embodiments thereof, that the foregoing description as well as the examples which follow are intended to illustrate and not limit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains.

Claims

CLAIMS We claim: 1 . A method of forming an oil-in-water microemulsion, comprising the steps of: mixing at least one oil and at least one surfactant to water or a water based solution, wherein said oil forms a plurality of oil droplets in said water or water based solution, said surfactant forming a surfactant layer coating said oil droplets to form coated oil droplets, and forming a polymer shell layer on said oil droplets.

2. The method of claim 1 , further comprising the step of polymerizing said surfactant layer to form at least a portion of said polymer shell layer.

3. The method of claim 1 , further comprising the step of adding a cross- linking agent, wherein said cross-linking agent reacts with said surfactant layer or a product from a reaction of said surfactant to provide at least a portion of said shell layer.

4. The method of claim 1 , wherein said cross-linking agent comprises a surfactant.

5. The method of claim 1 , wherein said oil varies from 0.5% to 20% of the oil, weight by weight of said microemulsion.

6. The method of claim 1 , wherein said surfactant is at least one selected from the group consisting of lecithin, C₆₄H₁₂₄0₂₆₃ and OTMS.

7. The method of claim 1 , wherein a weight ratio of said surfactant to said oil is from 0.2 to 5 (w/w).

8. The method of forming an oil in water microemulsion of claim 1 , wherein a size of said coated oil droplets including said polymer shell ranges from approximately 80 nm to 250 nm.

9. The method of claim 1 , wherein said polymer shell layer includes silicon.

10. The method of 9, wherein said silicon is provided by Si-O-Si layers.

1 1 . The method of claim 1 , further comprising the step of forming a biocompatible layer, wherein shell layer includes a biocompatible outer shell layer.

12. The method of claim 1 1 , wherein said biocompatible layer is at least one selected from the group consisting of PEO, polylactic acid and chitosane.

1 3. The method of claim 1 , wherein at least one of DNA, RNA, antibodies, proteins, enzymes, cells or cell components, and biomimetics are added in said mixing step.

14. An oil in water microemulsion, comprising: water or a water based solution; at least one oil; at least one surfactant coating layer, wherein said oil forms a plurality of oil droplets in said water or water based solution, said surfactant forming a surfactant layer coating said oil droplets, and a polymer shell layer disposed on said oil droplets.

1 5. The microemulsion of claim 14, wherein said surfactant layer is polymerizable, wherein polymerization of said surfactant layer provides at least a portion of said polymer shell layer.

1 6. The oil in water microemulsion of claim 14, wherein said surfactant layer and said polymer shell layer comprise different layers.

17. The oil in water microemulsion of claim 16, wherein said polymer shell layer includes silicon.

1 8. The oil in water microemulsion of claim 1 7, wherein said polymer shell layer includes a biocompatible surface layer.

1 9. The oil in water microemulsion of claim 1 8, wherein said biocompatible surface layer includes at least one selected from the group consisting of PEO, polylactic acid and chitosane.

20. The microemulsion of claim 14, wherein an average size of said oil droplets including said surfactant layer and said polymer shell layer ranges from 80 nm to 250 nm.

21 . The oil in water microemulsion of claim 1 8, wherein said microemulsion includes at least one of DNA, RNA, antibodies, proteins, enzymes, cells or cell components, and biomimetics.

22. A method if using oil filled nanocapsules, comprising the steps of: providing an oil in water microemulsion including water or a water based solution, at least one oil, at least one surfactant coating layer, wherein said oil forms a plurality of oil droplets in said water or water based solution, said surfactant forming a surfactant layer coating said oil droplets, and a polymer shell layer disposed on said oil droplets, wherein said using comprises at least one selected from the group consisting of drug detoxification, environmental remediation and pesticide poisoning therapy.