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Claims  |
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We claim:
1. A delivery vehicle for an active ingredient, the delivery vehicle having
a size from about 20 nm to about 100 nm in diameter and comprising a lipid
material enclosing an amphiphile-associated substrate in the essential
absence of a solution-phase inner volume, which includes:
an active ingredient phase,
an amphiphilic material capable of associating with the active ingredient
and having a polar hydrophilic portion and a lipophilic portion, the polar
hydrophilic portion of the amphiphilic material being associated with the
active ingredient phase to form the amphiphile-associated substrate; and
a single, biocompatible encapsulating monolayer surrounding the
amphiphile-associated substrate, comprising molecules having lipophilic
tails associated with the lipophilic portion of the amphiphile-associated
substrate.
2. The delivery vehicle of claim 1 in which the active ingredient phase
comprises a solid, particulate phase.
3. The delivery vehicle of claim 1 or 2 wherein the hydrophilic portion
comprises a carboxylic, hydroxyl, amino, phosphato or sulfato group.
4. The delivery vehicle of claim 1 or 2 wherein the lipophilic portion
comprises an aliphatic hydrocarbon, cycloaliphatic hydrocarbon,
aromatic-substituted aliphatic hydrocarbon, cycloaliphatic-substituted
aliphatic hydrocarbon or polyoxyethylene group.
5. The delivery vehicle of claim 4 wherein the amphiphilic material
comprises a member selected from the group consisting of fatty acids,
phospholipids, diglycerides, triglycerides, alcohols, amines, phosphates
and sulfates.
6. The delivery vehicle of claim 5 wherein the lipophilic portion comprises
an aliphatic chain of from 10 to 28 carbons in length.
7. The delivery vehicle of claim 1 or 2 wherein the biocompatible
encapsulating layer comprises a phospholipid material.
8. The delivery vehicle of claim 7 wherein the amphiphilic material
comprises a fatty acid or a phosphatidic acid.
9. A delivery vehicle for magnetite, the delivery vehicle having a size of
from about 20 nm to about 100 nm in diameter and comprising a lipid
particle enclosing a magnetite amphiphile-associated substrate in the
essential absence of a solution-phase inner volume, which includes:
a magnetite active ingredient phase,
an amphiphilic material capable of associating with the active ingredient
and having a polar hydrophilic portion and a lipophilic portion, the polar
hydrophilic portion being associated with the magnetite active ingredient
phase to form the magnetite amphiphile-associated substrate, and
a single, biocompatible encapsulating monolayer surrounding the magnetite
amphiphile-associated substrate, comprising molecules having lipophilic
tails associated with the lipophilic portion of the magnetite
amphiphile-associated substrate.
10. A process for preparing a delivery vehicle for an active ingredient,
the delivery vehicle having a size of from about 20 nm to about 10,000 nm
in diameter, the process comprising:
forming an initial aqueous dispersion consisting essentially of an
association between a particulate active ingredient which is a solid in
the aqueous dispersion and an amphiphilic material consisting of molecules
having a polar head group and a hydrophobic tail group, in an aqueous
phase to form an amphiphile-associate substrate in the aqueous phase, the
polar head groups being associated with the solid, particulate active
ingredient, and
encapsulating the amphiphile-associated substrate by then adding a material
capable of forming a biocompatible encapsulating layer for the substrate,
the layer comprising molecules having a lipophilic tail group associated
with the hydrophobic tail group of the amphiphile-associated substrate, to
form a delivery vehicle enclosing the amphiphile-associate substrate in
the essential absence of a solution-phase inner volume.
11. The process of claim 10 wherein the polar head group comprises a
carboxylic, hydroxyl, amino, phosphato or sulfato group.
12. The process of claim 10 wherein the hydrophobic tail group comprises an
aliphatic hydrocarbon, cycloaliphatic hydrocarbon, aromatic-substituted
aliphatic hydrocarbon, cycloaliphatic-substituted aliphatic hydrocarbon,
or polyoxyethylene group.
13. The process of claim 12 wherein the amphiphilic material comprises a
member selected from the group consisting of fatty acids, phospholipids,
diglycerides, triglycerides, alcohols, amines, phosphates and sulfates.
14. The process of claim 13 wherein the hydrophobic tail group comprises an
aliphatic chain of from 10 to 28 carbons in length.
15. The process of claim 10 wherein the amphiphilic material comprises a
fatty acid or a phosphatidic acid.
16. The process of claim 10 wherein the outer biocompatible encapsulating
layer comprises a phospholipid material.
17. A process for preparing a delivery vehicle for an active ingredient,
the delivery vehicle having a size of from about 20 nm to about 10,000 nm
in diameter, the process comprising:
forming an initial aqueous dispersion consisting essentially of an
association between a solid, particulate active ingredient and an
amphiphilic material consisting of molecules having a polar head group and
a hydrophobic tail group, in an aqueous phase to form an
amphiphile-associate substrate in the aqueous phase, the polar head groups
being associated with the solid, particulate active ingredient, and
encapsulating the amphiphile-associated substrate within a single
encapsulating layer, which layer further constitutes an outer
biocompatible encapsulating layer, by then adding a material capable of
forming a biocompatible encapsulating layer for the substrate, the layer
comprising molecules having a lipophilic tail group associated with the
hydrophobic tail group of the amphiphile-associated substrate, to form a
delivery vehicle enclosing the amphiphile-associate substrate in the
essential absence of a solution-phase inner volume.
18. The process of claim 10, 15 or 17 wherein said delivery vehicle has a
size of from about 20 nm to about 100 nm in diameter.
19. A process for preparing a delivery vehicle for a magnetite active
ingredient, the delivery vehicle having a size of from about 20 nm to
about 10,000 nm in diameter, the process comprising:
forming an initial aqueous dispersion consisting essentially of an
association between a solid, particulate active ingredient, which active
ingredient comprises magnetite, and an amphiphilic material consisting of
molecules having a polar head group and a hydrophobic tail group, in an
aqueous phase to form a magnetite amphiphile-associated substrate in the
aqueous phase, the polar head groups being associated with the solid,
particulate magnetite active ingredient, and
encapsulating the magnetite amphiphile-associate substrate by then adding a
material capable of forming a biocompatible encapsulating layer for the
substrate, the layer comprising molecules having a lipophilic tail group
associated with the hydrophobic tail group of the magnetite
amphiphile-associated substrate, to a form a delivery vehicle enclosing
the amphiphile-associated substrate in the essential absence of a
solution-phase inner volume.
20. The process of claim 19 including encapsulating the magnetite
amphiphile-associated substrate within a single encapsulating layer, which
layer further constitutes said outer biocompatible encapsulating layer.
21. The process of claim 19 wherein the amphiphilic material comprises a
fatty acid or a phosphatidic acid. |
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Claims |
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Description |
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FIELD OF INVENTION
This invention relates to active ingredient delivery vehicle compositions
which incorporate an outer biocompatible encapsulating layer, an inner
amphiphilic active ingredient-associated layer and an encapsulated
material constituting an active ingredient. Also provided are methods for
the production of such compositions. These compositions are suitable for
solubilizing aqueous-insoluble or aqueous-soluble active ingredients into
solvents of interest, and in particular for solubilizing active
ingredients for in vivo delivery to bodily tissue or other bodily systems.
Specific targeting or delivery of this composition to particular tissues,
organs or cells is achieved, as well as extended circulation and serum
stability. Active ingredients suitable for use herein include
superparamagnetic and ferromagnetic materials such as magnetite for, e.g.,
nuclear magnetic resonance imaging, halogenated compounds for, e.g., x-ray
contrast imaging, radioisotopic compounds for radiographic purposes, other
diagnostic agents, and therapeutic agents, including proteins, enzymes,
antineoplastics, antifungals, etc.
BACKGROUND
Phospholipid micellar particles in the form of unilamellar or multilamellar
vesicles, also known as liposomes, have been used in a number of contexts
as vehicles for the solubilization and delivery of active ingredient
materials. Liposomes have proven in some cases to be highly advantageous
in in vivo delivery systems in terms of biological compatibility, ability
to isolate and solubilize otherwise insoluble and/or toxic active
ingredients and ability selectively to deliver active ingredients to
specific tissues or systems of interest.
Efforts have been made to solubilize ferromagnetic materials in liquid
carriers in order to achieve ferromagnetic fluids since at least the early
1960's. One such example is that of magnetite, a ferromagnetic material of
formula Fe.sub.3 O.sub.4 often formed by precipitation from alkaline
solution of iron (II) and iron (III) chlorides. Examples of such
precipitation methods include those described in Mann et al J.C.S. Chem.
Comm. 1979, pp. 1067-1068, Khalafalla et al., IEEE Trans. Magnetics, Vol.
MAG-16, No. 2, pp. 178-183 (March 1980) and Molday et al., J.
Immunological Methods, Vol. 52, pp. 353-367 (1982). The ability of
magnetite to act as a T.sub.2 relaxation enhancer in nuclear magnetic
resonance has been recognized in the literature. See Ohgushi et al , J
Magn. Res., Vol 29, pp. 599-601 (1978).
A number of successful techniques for solubilizing magnetite have been
developed, but none prior to the present invention has been demonstrated
as being suitable for in vivo use as a delivery vehicle for magnetite
having extended circulation time, serum stability and biocompatibility.
For example, particulate magnetite, whether uncoated or with coatings
known in the prior art, is typically removed from the blood within a very
short time, usually in less than one hour and in many cases within five
minutes. Moreover, lack of proper solubilization of such particles may
lead to aggregation in the body and resultant deleterious effects.
Solubilization of magnetite in non-aqueous solution has been achieved by
ball-milling the material in the presence of a surfactant such as oleic
acid, by peptization into the desired solvent with a surfactant, and by
related methods. In this regard, see Charles et al., IEEE Trans.
Magnetics, Vol. MAG-16, No. 2, pp. 172-177 (March 1980), Khalafalla et
al., U.S. Pat. No. 3,764,540 (1973), and Reimers et al., U.S. Pat. No.
3,843,540 (1974). Characteristic of such non-aqueous, non-polar solvent
suspensions of magnetite are vehicles comprising a monolayer coating of
surfactant with the polar head thereof associated with the ferrite surface
and the lipophilic hydrocarbon tail thereof exposed outwardly to achieve
compatibility with the non-polar carrier solvent. Such compositions are
not suitable for solubilization in the aqueous environment of the body.
Aqueous or polar solvent suspensions of magnetite have also been achieved.
Monolayer surfactant coats of dodecylamine or dodecanoic acid on magnetite
have been shown to yield dispersions of the ferromagnetic material, the
latter surfactant giving a dilution-stable dispersion. Khalafalla et al.,
IEEE Trans. Magnetics, Vol. MAG-16, No. 2, pp. 178-183 (March 1980).
Aqueous ferrofluids using petroleum sulfonates as dispersing agents have
been decribed. Kelley, U.S. Pat. No. 4,019,994 (1977). The structure of
such monolayer surfactant-coated particles is similar to that of the
non-aqueous solubilized magnetite particles discussed above, with
prevention of aggregation but retention of water solubility being achieved
by virtue of shorter (less hydrophobic) hydrocarbon tails exposed to the
solvent phase.
Stable aqueous suspensions of magnetite particles have also been achieved
using ionic and non-ionic surfactants to produce a surface double layer.
Such structures involve an inner layer of amphiphilic molecules coated on
the magnetite particle as in the monolayer case, and an outer surfactant
layer oriented with lipophilic tails disposed inwardly and hydrophilic
heads exposed outwardly to the aqueous/polar solvent. The inner layer
frequently is composed of oleic acid. Materials used as outer surfactants
include fatty acids and their salts, long chain ethers or esters, and
alkylaromatics such as alkylaryl polyethers. Examples of such bilayer
compositions are given in Shimoiizaka, Japanese Patent No. 51-44580 (1976)
and Sambucetti, IEEE Trans. Magnetics, Vol. MAG-16, No. 2, pp. 364-367
(March 1980). The outer layer surfactants which have thus far been shown
to be useful in solubilizing magnetite particles are not, however,
suitable for in vivo use inasmuch as they are themselves toxic and are,
moreover, rapidly broken down in the blood environment potentially to
allow harmful aggregation of the encapsulated materials.
Alternate means of preparing magnetite for in vivo administration include
attachment of the particles to micrometer-sized carbohydrate matrices
(Olsson et al., Proc. Soc. Magn. Res. Med., p. 889 (4th Ann. Mtg. Aug.
1985) and Olsson et al., Magn. Res. Imaging, Vol. 4, No. 2, pp. 142-143
(1986)) and coating of magnetite with the mucopolysaccharide chitosan (Yen
et al., U.S. Pat. No. 4,285,819 (1981)). It is believed that such
compositions, although possibly stable in serum, would quickly be removed
from circulation by the reticuloendothelial system. Magnetically
localizable polymerized liposomes containing pharmaceuticals and a ferrite
material have been described in Chang, U.S. patent application Ser. No.
714,411 (March 12, 1985) now U.S. Pat. No. 4,652,257. In addition, the
encapsulation of magnetite within the enclosed volume of a single bilayer
phosphatidylcholine vesicle and a proposal for use in nuclear magnetic
resonance spectroscopy is disclosed in Mann et al., J.C.S. Chem. Comm.
1979, pp. 1067-1068 (1979). The utility and safety of such a vesicle in
this regard is not demonstrated. Furthermore, the composition described
would have limited in vivo stability, making it undesirable as an imaging
agent. In contrast, the delivery vehicles of the present invention have
high stability in serum at 37.degree. C., are capable of extended
circulation time, and are biocompatible.
The problems inherent in achieving a solubilized form of magnetite suitable
for in vivo use are often applicable to other active ingredients. In
particular, such ingredients may be particulate, aqueous-insoluble or
toxic in nature, or it may be useful or necessary to deliver them to
specific bodily sites. Furthermore, prior art delivery vehicles frequently
do not have sufficient serum-stability to achieve optimal results in a
safe manner.
Accordingly, the present invention addresses the need to develop improved
compositions and methods capable of safely and specifically delivering
active ingredients, such as therapeutic agents or diagnostic agents,
including magnetic or other imaging agents, to the body in amounts
effective to achieve beneficial results.
SUMMARY OF THE INVENTION
The present invention relates to biologically compatible compositions
capable of delivering soluble or insoluble active ingredients within
living systems. The compositions include an active ingredient and a first
layer comprising an amphiphilic material capable of encapsulating or
associating with the solid active ingredient through association of the
polar head of the amphiphile molecule(s) with the active ingredient. A
second outer layer comprises a material, such as, for example, a
phospholipid, capable of encapsulating or associating with the
amphiphile-coated structure in a manner which renders the delivery vehicle
as a whole biocompatible. An appropriate "biocompatible" delivery vehicle
will be non-toxic and non-immunogenic to the recipient, both as an intact
composition and as breakdown products, if any. Thus, the encapsulating
outer layer in the intact composition must present a biocompatible
"surface" to the recipient, and is preferably composed of a material which
would itself be biocompatible if the delivery vehicle were broken down in
the body. In the case of a phospholipid outer layer, the lipophilic tails
of the phosphoglyceride associate with the lipophilic tail(s) of the
amphiphile, thus stabilizing the amphiphile-active ingredient structure
within a phospholipid layer. The exposed polar heads of the
phosphoglycerides allow solubilization of the composition in the in vivo
environment. Such a composition is biologically compatible by virtue of
the phosphoglyceride nature of the exposed surface, and is moreover highly
stable in serum and capable of extended circulation in the body.
Active ingredients suitable for use in the compositions of the present
invention are characterized in that they are capable of being effectively
encapsulated as an aggregate by the amphiphilic layer, or associated on a
molecular level with one or more amphiphile molecules. Microcrystalline
structures, such as that of magnetite, are suitable active ingredients, as
are radionuclides, x-ray contrast imaging agents, and the like.
Therapeutic drug agents, such as the antifungal drugs amphotericin B and
miconazole and the chemotherapeutic drugs bisanthrene and cisplatin may
also be successfully encapsulated or associated with amphiphiles. The
active ingredient phase may range in size from individual molecules to
aggregates or particles 60 nm or more in diameter. In the case of
individual molecules of active ingredient, each molecule may be associated
with as few as one amphiphilic molecule.
The amphiphile material will be characterized in that it will be capable of
being encapsulated by the outer biocompatible material layer. The
particular amphiphile type most useful in a given formulation will depend
on the nature of the active ingredient and the surrounding biocompatible
material. Saturated or unsaturated fatty acids having from 10 to 28
carbons in the hydrophobic chain are particularly useful, with myristic
acid (fourteen carbon chain length) being especially preferred in the case
of the active ingredient magnetite. Dialkoylphosphatidic acids are also
useful amphiphiles. Both palmitic acid and distearoyl phosphatidic acid
have been shown to be effective amphiphiles in the case of the active
ingredient amphoteracin B.
Where a phospholipid-encapsulated delivery vehicle is used, targeting of
specific organs, tissues, cells or other systems in the body may be
achieved in a manner similar to that seen with liposomes previously known
in the art. Thus, specific cells such as tumor cells, or specific organs
such as the liver or spleen, may be selectively targeted for delivery of
spectrometric, radiometric, medicinal or other agents. Moreover, due to
the effective isolation of the active ingredient which may be achieved by
virtue of the encapsulating amphiphilic and phospholipid layers, any
toxicity of the active ingredient may be reduced and/or targeted to
specific sites in the body. The delivery vehicle exhibits extended
circulation time and stability in serum at 37.degree. C.
Accordingly, the present invention in one aspect provides novel and useful
compositions capable of safely delivering active ingredients to the human
or other mammalian bodies in amounts effective to achieve beneficial
results. It should be recognized, however, that in vivo administration is
not the only mode contemplated, and that the advantages inherent in the
present invention, including enhanced solubilization of active
ingredients, may be utilized in in vitro or other non-living systems or
applications.
In another aspect, the present invention provides compositions for
delivering magnetic, radiometric, x-ray contrast or other biological
imaging agents to the body. In particular, the compositions are useful in
delivering ferromagnetic agents such as magnetite for the purpose of
nuclear magnetic resonance imaging.
In another aspect, the compositions provide means for delivering
therapeutic agents in a safe manner and in effective quantities.
The present invention also provides methods for making the compositions
disclosed herein.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an enlarged diagrammatic representation of a typical liposomal
delivery vehicle.
FIG. 2 is an enlarged diagramatic representation of a delivery vehicle of
the present invention, wherein an active ingredient phase is associated
with amphiphilic molecules and incorporated into the intra-bilayer region
of an encapsulating material.
FIG. 3 is an enlarged diagrammatic representation of a delivery vehicle of
the present invention, wherein single-molecular or small particulate
phases of active ingredient are associated with single molecules of
amphiphilic material and incorporated into the intra-bilayer region of an
encapsulating material.
FIG. 4 is an enlarged diagrammatic representation of a delivery vehicle of
the present invention, wherein an active ingredient phase is associated
with amphiphilic molecules and encapsulated within a monolayer of an
encapsulating material.
FIG. 5 is a graphical representation showing one correlation between active
ingredient (magnetite) incorporation or NMR relaxation rate enhancement
and amphiphile formulation concentration.
FIG. 6 is a graphical representation showing the correlation between NMR
relaxation rate enhancement and incorporated active ingredient (magnetite)
concentration.
FIG. 7 is a graphical representation showing the time dependence of NMR
T.sub.2 relaxation rate enhancement in various biodistribution systems.
FIG. 8 is a graphical representation showing the time dependence of NMR
T.sub.1 relaxation rate enhancement in various biodistribution systems.
FIG. 9 is a graphical representation showing the dose dependence of NMR
T.sub.2 relaxation rate enhancement in various biodistribution systems.
FIG. 10 is a graphical representation showing the dose dependence of NMR
T.sub.1 relaxation rate enhancement in various biodistribution systems.
FIG. 11 is a graphical representation showing one correlation between NMR
relaxation rate enhancement and amphiphile chain length.
FIG. 12 is a graphical representation showing the time dependence of fungal
proliferation in the presence of an active ingredient (miconazole)
delivery vehicle of the present invention.
DETAILED DESCRIPTION
The present invention provides compositions for the encapsulation and
improved delivery in in vivo systems of aqueous-soluble or
aqueous-insoluble active ingredients. The compositions exhibit the
advantages associated with liposomal delivery vehicles or vesicles by
virtue of their phospholipid outer coating, although the compositions need
not be liposomal per se in structure, nor consist of phospholipid. The
delivery vehicles are capable of incorporating and delivering active
ingredients of poor solubility, or which cause irritation when
administered by conventional means, or which are to be delivered by an
altered biodistribution scheme.
Active ingredients susceptible to encapsulation within the vesicular
delivery vehicles of the present invention include a broad range of
therapeutic and diagnostic agents. Appropriate therapeutic agents include
analgesics, antibacterials, antibiotics, antifungal agents,
anti-inflammatory agents, antineoplastics, antiparasitics, antiviral
agents, cardiovascular preparations, cell receptor binding molecules,
neurotransmitters, ophthalmologicals, polysaccharides and proteins
including enzymes, hormones, glycoproteins, immunomodulators, etc.
Appropriate diagnostics-related active ingredients include those useful
for angiography, CT scan imaging, nuclear magnetic resonance imaging,
radiography, X-ray contrast imaging, ultrasound, etc. Particular
diagnostic active ingredients include superparamagnetic and ferromagnetic
materials such as magnetite, halogenated compounds, radioisotopic
compounds, fluorescent compounds and dyes.
Referring to FIG. 1, a typical unilamellar liposome vesicle has a
phospholipid bilayer with an enclosed inner volume surrounded by the
bilayer. Active ingredients may be incorporated into and encapsulated
within this inner volume, surrounded by what will be referred to herein as
an "encapsulating layer," which in this case is a phospholipid bilayer. A
polar solution in this region is compatible with the stable liposome
structure because of attractive interactions between the inner polar
solvent and the inwardly-oriented polar portions of the phospholipid
molecules of the inner liposomal layer. Alternately, lipophilic active
ingredients may be incorporated into and encapsulated within the
hydrophobic intra-bilayer region of the liposome (that is, within the
encapsulating layer).
FIG. 2 depicts one form of the compositions of the present invention. In
this form, an active ingredient is associated with an encapsulating outer
layer through incorporation into and encapsulation within the hydrophobic
region within the membranal phospholipid bilayer of a liposome. In order
to achieve a stable structure of this type, including an association
between the active ingredient phase and the encapsulating layer of the
delivery vehicle, it is essential that the active ingredient be compatible
with this hydrophobic intra-bilayer region of the encapsulating layer. The
present invention achieves such compatibility by virtue of a layer, or
other active ingredient-associated group, of amphiphilic molecules applied
to the active ingredient in a manner which exposes the lipophilic portions
of the amphiphilic molecules outwardly to interact with the hydrophobic
intra-bilayer region. Such a structure comprising an active ingredient
particle and its associated amphiphilic molecules will be referred to
herein as an "Amphiphile-Associated Substrate," or "AAS". The active
ingredient in such a magnetite AAS is associated with and stabilized
within the intra-bilayer region by virtue of the intermolecular
compatibility and attractive forces between the lipophilic portions of the
phospholipid or other encapsulating molecules and the amphiphilic
molecules. This layer may constitute the outer encapsulating layer of the
delivery vehicle, in which case it will comprise a material which renders
the delivery vehicle as a whole biocompatible. Alternately, additional
encapsulating layers may exist, as in multilamellar vesicles, which may
themselves be associated with amphiphile-associated substrates. The outer
encapsulating layer of the delivery vehicle will comprise a material which
renders the delivery vehicle as a whole biocompatible, i.e., having an
acceptable level of non-toxicity and non-immunogenicity to the recipient.
(Of course, the active ingredient may be selected so as to exhibit
controlled toxicity, as, for example, to targeted cancer cells.) Thus, the
outer biocompatible encapsulating layer must present a non-toxic and
non-immunogenic "surface" to the recipient. Futhermore, if the delivery
vehicle is broken down within the body, the components thereof should be
non-toxic and non-immunogenic. It is therefore preferred that the
encapsulating layer or layers themselves be composed of a material which
is biocompatible upon breakdown in the body, if such occurs.
It is believed that the size of the active ingredient phase or AAS as shown
in FIG. 2 may vary significantly while still preserving a stable and
effective delivery vehicle. Thus, in the case of the active ingredient
magnetite, it has been shown that the diameter of the microcrystalline
magnetite particle or particles within the bilayer may be as much as two
or more times greater than the thickness of a phospholipid bilayer not
associated with any incorporated solid active ingredient, as for example
in the liposome of FIG. 1. For example, a liposome of a size useful for
delivery of active ingredients to bodily tissues or organs may have an
overall diameter of approximately 35 nm to 100 nm, preferably 50 nm to 80
nm, and a "nominal" phospholipid bilayer thickness of approximately 4 to 7
nm (as measured between opposing polar head groups in the membrane without
any active ingredient enclosed therein). It has been found that a
magnetite microcrystal having a mean diameter of approximately five to 20
nm, and most probably around 11 nm, may be associated with appropriate
amphiphilic molecules to form an AAS having a diameter of about 15 nm and
then encapsulated within the bilayer of a phospholipid vesicle with a
nominal bilayer thickness of only about 5 nm and an overall liposomal
diameter of about 60 nm. It is probable that the bilayer structure in such
a case must be capable of significant distortion in order to accomodate
active ingredients of such relatively large size. Nevertheless, it has
been shown in freeze fracture and negative stain electron microscopy
investigations relating to the present invention that such structures are
possible. Their effectiveness as delivery vehicles has also been
demonstrated.
Magnetite may be produced under a range of conditions by alkaline
precipitation from ferrous and ferric chloride solutions. For example,
immediately following precipitation, the magnetite may be heated; the
precipitate may also be allowed to settle in the presence or absence of a
magnetic field. Depending on the method of magnetite preparation,
maximizing final incorporation into delivery vehicles may require
differing conditions. The maximum incorporation achievable may also be
dependent on the magnetite preparation method.
It is often useful to maximize the amount of active ingredient associated
with a particular delivery vehicle in order to achieve appropriate levels
of activity, including magnetic, radiographic or other imaging activity as
well as drug therapeutic activity. Overall size of the delivery vehicle
must also be taken into consideration, especially where targeting of
delivery to particular regions or cells in the body is sought. It is
believed that liposomal and other phospholipid-related delivery agents
described herein may vary significantly in size while still yielding
effective activity results. Delivery vehicles having a diameter of
approximately 50 nm to 80 nm have proven to be particularly useful in the
case of targeting specific cells such as tumor cells. Moreover, nuclear
magnetic resonance imaging techniques usin | | |
