JP2005170928A - Lyposome-containing x ray-imaging agent and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an X ray-imaging agent having a high safety, aiming at the stability of a lyposome and preservation stability of an iodine-based compound, also improving the staying property in blood of the imaging agent and achieving a good targeting. <P>SOLUTION: This X ray-imaging agent contains the lyposome containing together with a phospholipid, at least 1 kind of a compound selected from compounds each having a polyalkylene oxide group in its molecule and sterols in its lipid membrane, and containing a water soluble and nonionic iodine-based compound in an aqueous phase within a membrane of the liposome. The imaging agent contains the iodine-based compound and preparation assistants in both of the aqueous phase within the lyposome membrane and an aqueous medium in which the lyposome is dispersed, and each of their concentrations at the inside and outside of the membrane is substantially the same. Since the imaging agent is produced by a supercritical carbon dioxide method, it does not substantially contain toxic solvent such as a chlorine-based solvent. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an X-ray contrast agent and a method for producing the same, and more particularly to an X-ray contrast agent containing a liposome encapsulating a contrast substance therein and a method for producing the same.

  X-ray simple imaging and CT imaging (computer tomography) occupy the core of today's diagnostic imaging. Since so-called hard tissues such as bones and teeth absorb X-rays well, a high-contrast image can be easily obtained. On the other hand, since a difference in X-ray absorption is small between soft tissues, it is difficult to obtain a high contrast image. In such a case, a contrast agent is generally used to obtain an image with high contrast.

  Most of the X-ray contrast agents currently in practical use use a compound containing a triiodophenyl group and water-solubilized as a contrast substance. These contrast agents are administered to luminal sites such as blood vessels, ureters, oviducts, and the like, and are used for diagnosis of luminal shape, stenosis, and the like. However, since these compounds are rapidly excreted from the luminal site without interacting with the tissue or diseased site, they are not useful for the purpose of diagnosing the tissue or diseased site, particularly cancerous tissue, in more detail. . Therefore, there is a demand for an X-ray contrast agent that selectively accumulates in a target tissue or diseased site and provides an image that can be distinguished from the surrounding or other sites with a clear contrast. With such a contrast medium for X-ray examination, even a fine cancer tissue can be detected with high accuracy. Examination methods developed for tumor detection include not only X-ray imaging methods but also other methods with different measurement principles. Among them, MRI (magnetic resonance imaging) has a limit in terms of sensitivity to accurately image cancer tissues, especially minute cancer tissues, and PET (positron emission tomography) has a limitation in exposure and operating costs. None of them are general, and both require large-scale equipment and expensive equipment, so far they are not widely used inspection methods.

  International publications WO98 / 46275, WO95 / 31181, WO94 / 19025, WO96 / 28414, WO96 / 00089, US Pat. A method for imaging a tumor, a liver, a spleen image, an adrenal cortex, an arteriosclerotic lesion, a blood vessel pool, a lymphatic system, etc. by dispersing in water under water is disclosed. In these methods, a contrast agent is atomized to increase the residence time in the body and selectively contrast the diseased part. However, the formulation methods proposed for that purpose are not sufficient in contrast efficiency and selectivity. Furthermore, since the iodine compound to be used is hydrophobic, there is also a problem that the rate of discharge from the body after imaging is slow and the burden on the patient is large.

On the other hand, as a method for making a contrast agent into a fine particle form, a method of encapsulating a contrast-enhancing compound in a liposome composed of a lipid similar to a biological membrane and considered to be highly safe due to its low antigenicity has been studied. For example, in International Publications WO88 / 09165, WO89 / 00988, WO90 / 07491, JP07-316079 and JP2003-5596, liposomes containing an ionic or nonionic contrast agent are proposed. In Patent No. 2619037 (Patent Document 1), a water-soluble iodine compound is encapsulated in a liposome, and the organ selectivity is increased by increasing the amount of iodine compound in the liposome to 1.5 to 6 g / g relative to the amount of lipid. I am trying to have it. However, the selectivity for tumors was low. This is presumed to be due to the fact that the liposome particle size was substantially larger, 0.5 to 1.0 μm. Particles with a size of around 0.1 μm are preferred for accumulation in tumor tissue, but it is difficult to reduce the particle size by the above-mentioned manufacturing method, and the ratio of iodine compound to the amount of lipid is small when the particle size is small. Get smaller. In these methods, despite the use of liposomes that are highly safe as materials and have appropriate degradability in vivo, organic solvents such as chloroform, A chlorinated solvent such as dichloromethane is used (for example, see Patent Document 2). Therefore, it has not been put into practical use because the remaining solvent is toxic.

  On the other hand, lipid-soluble drugs are easily encapsulated in liposomes, but the encapsulated amount depends on other factors and is not necessarily so much. A drug that is a water-soluble electrolyte can be encapsulated in the aqueous phase inside the liposome through the interaction between the charge of the drug and the charge of the charged lipid, but if the drug is a water-soluble non-electrolyte, such means should be used. It cannot be taken. Regarding X-ray contrast agents, it is generally desirable to encapsulate nonionic iodine compounds that are substantially less toxic than ionic contrast compounds in liposomes, but this is not easy for the reasons described above. Furthermore, the liposomes that are formed tend to be multi-layered, and the efficiency is poor because the encapsulation rate of the iodine compound is low. As means for efficiently encapsulating such a water-soluble non-electrolyte in the liposome, there are a reverse phase evaporation method and an ether injection method. However, since an organic solvent is used, there still remains a safety problem.

In JP-A-2003-119120 (Patent Document 3), a cosmetic containing a liposome and an external preparation for skin are disclosed.
A method of producing using supercritical carbon dioxide is disclosed, and an example of producing an external preparation for skin in which a hydrophilic medicinal component or a lipophilic medicinal component is encapsulated in liposomes is shown. However, although examples of water-soluble electrolytes have been shown as hydrophilic medicinal ingredients, it was unclear whether water-soluble non-electrolytes could be efficiently encapsulated in liposomes by this method.

Even if the contrast material is successfully encapsulated inside the liposome, the problem of leakage to the outside over time or the situation where the liposome itself becomes unstable must be considered. Furthermore, it has been pointed out that even when liposomes are administered in vivo, many of them are trapped in the reticuloendothelial tissues such as the liver and spleen, so the desired effect cannot be obtained (Cancer Res., 43, 5328 (1983))
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Japanese Patent No. 2619037 Japanese Patent Laid-Open No. 7-316079 Japanese Patent Laid-Open No. 2003-119120

  An object of the present invention is to provide an X-ray contrast agent having high delivery efficiency and selectivity by encapsulating a contrast substance in liposomes. More specifically, an X-ray contrast medium excellent in tumor visualization that can encapsulate a water-soluble and nonionic iodide compound in a liposome without using a toxic organic solvent and can detect minute cancer tissues. An object of the present invention is to provide a manufacturing method thereof.

The present invention for solving the above problems has the following configuration.
The X-ray contrast medium of the present invention includes a liposome containing a water-soluble and nonionic iodine-based compound,
Liposomes formed without using chlorinated solvents by mixing phospholipids with supercritical carbon dioxide or subcritical carbon dioxide and bringing water-soluble and nonionic iodine compounds into contact with the phospholipids And
Moreover, it is characterized by not containing a chloro solvent.

The present invention includes a liposome containing a water-soluble and nonionic iodine-based compound,
Liposomes formed without using chlorinated solvents by mixing phospholipids with supercritical carbon dioxide or subcritical carbon dioxide and bringing water-soluble and nonionic iodine compounds into contact with the phospholipids And
Moreover
It is also an X-ray contrast agent characterized by containing no chlorinated solvent in excess of 10 μg / L.

It is preferable that at least 80% of the total amount of liposomes is a single membrane liposome.
The iodo compound has at least one 2,4,6-triiodophenyl group.

The average particle size of the liposome is 0.05 to 0.5 μm.
The average particle diameter of the liposome is preferably 0.05 to 0.2 μm.
The average particle size of the liposome is more preferably 0.11 to 0.13 μm.

  The liposome may be a polyethylene glycol (PEG) -modified liposome containing 0.1 to 30% by mass of polyethylene glycol having 10 to 3500 oxyethylene units with respect to the lipid constituting the liposome.

The liposome is preferably passed through a filtration membrane having a pore size of 0.1 to 0.4 μm.
The liposome is characterized by containing the iodine compound in a weight ratio of 1 to 10 with respect to the liposome membrane lipid.

The liposome preferably contains the iodine compound in a weight ratio of 3 to 8 with respect to the liposome membrane lipid.
More preferably, the liposome contains the iodine compound in a weight ratio of 5 to 8 with respect to the liposome membrane lipid.

The iodine compound encapsulated in the liposome is 5 to 30% by mass of the total iodine compound in the X-ray contrast medium.
The liposome has a phospholipid having a polyalkylene oxide group in its lipid membrane, and cholesterol, dihydrocholesterol, cholesterol ester, phytosterol, sitosterol, stigmasterol, campesterol, cholestanol, lanosterol, 2,4-dihydrolanosterol, 1 It contains at least one compound selected from -O-sterol glucoside, 1-O-sterol maltoside and 1-O-sterol galactoside, and contains the iodo compound in the aqueous phase in the lipid membrane. It is said.

The liposome preferably contains a polyalkylene oxide-modified phospholipid in its lipid membrane, and contains the iodo compound in the aqueous phase within the lipid membrane.
The liposome may contain a block copolymer of polyethylene oxide and polypropylene oxide in the lipid membrane, and the iodine compound in the aqueous phase in the lipid membrane.

  The liposome contains the iodine compound and a formulation aid in both the aqueous phase in the lipid membrane and the aqueous medium in which the liposome is dispersed, and the respective concentrations are the same inside and outside the liposome membrane. It is characterized by.

The formulation aid is preferably a compound selected from at least one water-soluble amine buffer and chelating agent.
More preferably, the water-soluble amine buffer is trometamol.

Further, the chelating agent is preferably EDTANa ₂ -Ca.
[Detailed Description of the Invention]
Hereinafter, the present invention will be described in detail in the order of contrast compound, liposome, and X-ray contrast agent.
Contrast Compound The water-soluble iodine-based compound in the present invention is not particularly defined as long as it has a contrast property, regardless of whether it is ionic or non-ionic. In general, a nonionic iodine-based compound has a lower osmotic pressure and is more desirable than an ionic iodine-based compound. As the water-soluble nonionic iodide compound, a nonionic iodide compound containing at least one 2,4,6-triiodophenyl group, for example, containing phenyl iodide is particularly suitable.

Specific examples of such nonionic iodide compounds include iohexol, iopentol, iodixanol, iopromide, iotrolane, iomeprol, N, N′-bis [2-hydroxy-1- (hydroxymethyl) -ethyl] -5 [[(2-hydroxy-1-oxopropyl) -amino] -2,4,6-triiodo-1,3-benzene-dicarboxamide (iopamidol); metrizamide and the like.

Other iodo compounds include: diatrizoic acid; diatrizoate sodium; meglumine diatrizoate; acetolizoic acid and its soluble salts; diprotrizoic acid; iodamide, iodopamide sodium, meglumine iodipamide, iodohippuric acid and its solubles Salt; iodomethamic acid; iodopyrracetoiodo-2-pyridone-N-acetic acid, 3,5-
Diiodo-4-pyridone-N-acetic acid (iodopyracet); diethylammonium salt of said acid; iotalamic acid; metrizoic acid and its salts; Iopodate sodium and other similar iodinated compounds.

  These compounds may be used alone or in combination of two or more. Moreover, it is not limited to the illustration. In addition, in this specification, a compound may be mentioned including the salt, hydrate, etc. other than a free form.

  Examples of the iodine compound suitable for the X-ray contrast medium of the present invention include iomeprol, iopamidol, iotrolan, and iodixanol which are highly hydrophilic and do not increase the osmotic pressure even at a high concentration. In particular, dimer nonionic iodine compounds such as iodolan and iodixanol have the advantage of further reducing the osmotic pressure because the total number of moles is low even when a contrast agent having the same iodine concentration is prepared.

The concentration of the water-soluble iodide compound in the contrast agent of the present invention can be arbitrarily set based on various factors such as the nature of the contrast compound, the intended route of administration of the preparation, and clinical indicators. The amount of the iodine compound encapsulated in the liposome is typically 5 to 95% by weight, preferably 5 to 90% by weight, more preferably 5 to 70% by weight of the total iodine compound in the X-ray contrast medium. is there. In particular, in order to sufficiently prevent the destabilization of the liposome encapsulating the iodine compound over time, the amount of the iodine compound encapsulated in the liposome is 5 to 30 of the total iodine compound in the X-ray contrast medium. It is desirable that the content is 5% by mass, preferably 5 to 25% by mass, and more preferably 5 to 20% by mass. In the X-ray contrast medium, if the ratio of the iodine compound encapsulated in the liposome is 5 to 30% by mass (preferably 5 to 25% by mass), the remaining 70 to 95% by mass (preferably 75 to 95). The amount that flows out to the aqueous dispersion outside the liposome in which the mass%) is present is substantially negligible. Therefore, destabilization due to the osmotic pressure effect of the liposome encapsulating the iodine compound can be sufficiently prevented, and the retention stability of the contrast substance in the liposome is improved.
Liposome The X-ray contrast agent of the present invention is used in the form of being encapsulated in a liposome as a microcarrier in order to selectively and efficiently deliver the contrast compound to a target site such as a target organ or tissue. The contrast agent of the present invention improves the retention in blood by using liposomes with improved blood stability, and achieves efficient drug delivery and targeting. In order to produce an EPR (Enhanced permeability and retention) effect that is particularly effective for obtaining excellent tumor visualization, it is necessary to stabilize the liposome structure and retain the encapsulated substance. Further, it is required to have characteristics such as stability in blood and retention in blood after improving the efficiency of carrying as a carrier.

  The X-ray contrast agent of the present invention can realize the targeting function by appropriately designing the particle size (particle size) of the liposome encapsulating the contrast compound and its bilayer. Both passive targeting and active targeting are considered. The former can control the in vivo behavior through adjustment of liposome particle size, lipid composition, charge, and the like. Adjustment to align the liposome particle size in a narrow range is easily performed based on the method described later. The design of the liposome membrane surface can impart desired properties by changing the type and composition of phospholipids and coexisting substances.

  The adoption of active targeting that allows for higher levels of accumulation and selectivity of contrast agents should also be considered. As an example, introducing a polyalkylene oxide polymer chain or polyethylene glycol (PEG) on the liposome surface is extremely beneficial because the induction process to the target site can be controlled. Liposomes suitable for the X-ray contrast medium of the present invention are modified by adding polyalkylene oxide or PEG to the surface thereof, resulting in a further increase in blood retention and phagocytosis of reticuloendothelial cells such as the liver. Liposomes that are difficult to be treated. X-ray contrast agents that have not reached cancerous tissues, diseased sites, etc. are not accumulated in normal sites, but are decomposed and excreted from the body before the side effects occur. This can be achieved by appropriately controlling the stability in relation to the extracorporeal discharge time when designing the liposome. Since the contrast material uses a water-soluble iodine compound, it is rapidly excreted in the urine via the kidney. Therefore, adverse effects caused by staying in the body and delayed side effects can be prevented.

The design and production method of the liposome used as the carrier of the contrast compound in the present invention will be described in detail below.
Liposomes are usually formed from lipid bilayer membranes. Therefore, in the present specification, the liposome membrane may be referred to as a lipid membrane. In general, phospholipids and / or glycolipids are preferably used as components of the lipid membrane.

  Preferred neutral phospholipids in the liposome of the present invention include lecithin, lysolecithin and / or hydrogenated products and hydroxide derivatives obtained from soybeans, egg yolks and the like.

Other phospholipids may be derived from egg yolk, soy or other animals or plants, or semi-synthetic phosphatidylcholine, phosphatidylserine, phosphatidylinositol, phosphatidylglycerol, phosphatidylethanolamine, sphingomyelin, synthetically obtained phosphatidic acid, dipalmitoylphosphatidylcholine (DPPC), distearoylphosphatidylcholine (DSPC), dimyristolphosphatidylcholine (DMPC), dioleylphosphatidylcholine (DOPC), dipalmitoylphosphatidylglycerol (DPPG), distearoylphosphatidylserine (DSPS), distearoylphosphatidylglycerol (DSPG), Dipalmitoylphosphatidylinositol (DPPI),
Examples include distearoyl phosphatidylinositol (DSPI), dipalmitoyl phosphatidic acid (DPPA), and distearoyl phosphatidic acid (DSPA).

These phospholipids are usually used alone, but may be used in combination of two or more. However, when two or more kinds of charged phospholipids are used, it is desirable to use them between negatively charged phospholipids or between positively charged phospholipids from the viewpoint of preventing liposome aggregation. When neutral phospholipids and charged phospholipids are used in combination, the weight ratio is usually 200: 1 to 3: 1, preferably 100: 1 to 4: 1, and more preferably 40: 1 to 5: 1.

  Examples of glycolipids include glycerolipids such as digalactosyl diglyceride and galactosyl diglyceride sulfate, galactosylceramide, galactosylceramide sulfate, lactosylceramide, sphingoglycolipids such as ganglioside G7, ganglioside G6, and ganglioside G4.

  As a liposome membrane component, other substances may be added as needed in addition to the above lipids. Examples include sterols that act as membrane stabilizers, such as cholesterol, dihydrocholesterol, cholesterol esters, phytosterols, sitosterol, stigmasterol, campesterol, cholestanol, lanosterol or 2,4-dihydrolanosterol. Further, sterol derivatives such as 1-O-sterol glucoside, 1-O-sterol maltoside or 1-O-sterol galactoside have been shown to be effective in stabilizing liposomes (Japanese Patent Laid-Open No. 5-245357). Of these, cholesterol is particularly preferred.

The amount of sterols used is desirably 0.05 to 1.5 parts by weight, preferably 0.2 to 1 part by weight, and more preferably 0.3 to 0.8 parts by weight with respect to 1 part by weight of phospholipid. If the amount is less than 0.05 parts by weight, the stabilization by the sterols that improve the dispersibility of the mixed lipid is not exhibited, and if it is more than 2 parts by weight, the formation of liposomes is inhibited, or even if formed, it becomes unstable.

  Cholesterol in the liposome membrane can also serve as an anchor for introducing polyalkylene oxide. Specifically, a cholesterol to be included in the membrane as a liposome membrane constituent may be bonded with a polyalkylene oxide group at the end thereof via a linker as necessary. As the linker, a short-chain alkylene group, oxyalkylene group, or the like is used. Japanese Patent Application Laid-Open No. 09-3093 discloses a novel cholesterol derivative in which various functional substances can be efficiently immobilized by covalent bonding at the end of a polyoxyalkylene chain, and can be used as a liposome-forming component. Is disclosed.

Other examples of compounds that can be added include dialkyl phosphates such as dicetyl phosphate, which is a negatively charged substance, and aliphatic amines such as stearylamine, as compounds that give a positive charge.
In the present invention, a phospholipid or compound having a polyalkylene oxide (PAO) group or a similar group may be used as a component of the liposome membrane depending on the intended purpose of the X-ray contrast agent. As a method for solving the problem of being trapped in reticuloendothelial cells and the problem of instability of liposome itself such as disintegration and aggregation, a polyethylene glycol (PEG) chain that is a polymer chain on the surface of the liposome, Attempts have been made to introduce — (CH ₂ CH ₂ O) _n —H (for example, JP-A-1-249717, FEBS letters, 268, 235 (1990)).

A new function can be imparted to the liposome by attaching a polyalkylene oxide chain (polyoxyalkylene chain) or a PEG chain to the surface of the liposome. For example, PEGylated liposomes can be expected to have an effect of being hardly recognized by the immune system (in a so-called “stealthed” state). Alternatively, it has been clarified that liposomes have a tendency to be hydrophilic, thereby increasing blood stability and maintaining the concentration in blood over a long period of time (Biochim. Biophys. Acta., 1066, 29-36 (1991) ). In addition, a method of incorporating a polyalkylene oxide-modified phospholipid into a liposome lipid membrane in order to improve the blood retention of liposomes has been disclosed (Japanese Patent Laid-Open No. 2002-37833). Such liposomes have also been shown to have improved stability over time.

Using these properties, organ specificity can be imparted to the X-ray contrast medium. Specifically, since lipid components are easily stored in the liver, it is desirable not to use PEG or to use liposomes having a low PEG content when selective imaging of the liver is intended. Further, when the particle size is increased to 0.2 μm or more, the possibility of rapid uptake by the phagocytosis of liver Kupffer cells increases and it accumulates at the site of the liver. In imaging of liver cancer, since the cancer tissue has fewer Kupffer cells than the normal tissue, the amount of contrast agent liposome taken up is relatively small, and the contrast becomes clear. The same applies to the spleen.

  On the other hand, in the case of contrast imaging of other organs, the introduction of PEG makes it possible to steal the liposome and make it difficult to collect in the liver and the like, and therefore the use of PEGylated liposome is recommended. Since the hydration layer is formed by the introduction of PEG, the liposome is stabilized and the retention in blood is also improved. By appropriately changing the length of the oxyethylene unit of PEG and the ratio of introduction, the function can be adjusted. As PEG, polyethylene glycol having 10 to 3500 oxyethylene units is preferred. The amount of PEG used is 0.1 to 30% by mass, preferably about 1 to 15% by mass, based on the lipid constituting the liposome.

  A known technique can be used for PEGylation of the liposome. An anchor to which PEG is bound (for example, cholesterol) may be mixed with a phospholipid that is a membrane component to prepare a liposome, and activated PEG may be bound to the anchor. In addition, since the polyethylene glycol group introduced into the liposome surface does not react with the “functional substance” described later, it is difficult to immobilize the “functional substance” on the liposome surface. Alternatively, liposomes can be prepared by binding PEG, which is further modified to the PEG tip, to phospholipid and including this as a liposome constituent.

Instead of the PEG, various known polyalkylene oxide groups,-(AO) _n -Y, may be introduced on the liposome surface. Here, AO represents an oxyalkylene group having 2 to 4 carbon atoms, and n is an average added mole number of the oxyalkylene group. Y represents a hydrogen atom, an alkyl group or a functional functional group.

  Examples of the oxyalkylene group having 2 to 4 carbon atoms (represented by AO) include, for example, oxyethylene group, oxypropylene group, oxytrimethylene group, oxytetramethylene group, oxy-1-ethylethylene group, oxy-1,2 -A dimethylethylene group etc. are mentioned. These oxyalkylene groups are groups obtained by addition polymerization of alkylene oxides such as ethylene oxide, propylene oxide, oxetane, 1-butene oxide, 2-butene oxide, and tetrahydrofuran.

n is a positive number of 1 to 2000, preferably 10 to 500, and more preferably 20 to 200.
When n is 2 or more, the types of oxyalkylene groups may be the same or different. In the latter case, it may be added in a random manner or a block shape. In the case of imparting hydrophilicity to the polyalkylene oxide chain, AO is preferably an ethylene oxide added alone, and in this case, n is preferably 10 or more. When different types of alkylene oxides are added, it is desirable that ethylene oxide is added in an amount of 20 mol% or more, preferably 50 mol% or more. When imparting lipophilicity to the polyalkylene oxide chain, the number of added moles other than ethylene oxide is increased. For example, a liposome containing a block copolymer of polyethylene oxide and polypropylene oxide is a preferred embodiment of the present invention.

  Y is a hydrogen atom, an alkyl group or a functional functional group. Examples of the alkyl group include an aliphatic hydrocarbon group having 1 to 5 carbon atoms which may be branched. The functional functional group is for attaching a “functional substance” such as sugar, glycoprotein, antibody, lectin, cell adhesion factor to the end of the polyalkylene oxide chain. For example, amino group, oxycarbonylimidazole group, N- A reactive functional group such as a hydroxysuccinimide group may be mentioned.

  In addition to the effect of introducing a polyalkylene oxide chain, a liposome with a polyalkylene oxide chain immobilized with a “functional substance” attached to the tip is a “functional substance” that is not obstructed by the polyalkylene oxide chain. Functions such as specific organ directivity, cancer tissue directivity, etc. are sufficiently exerted as a “recognition element”.

A phospholipid or compound having a polyalkylene oxide group can be used alone or in combination of two or more. The content thereof is 0.001 to 50 mol%, preferably 0.01 to 25 mol%, more preferably 0.1 to 10 mol%, based on the total amount of the liposome membrane-forming component. If it is less than 0.001 mol%, the expected effect becomes small.

  A known technique can be used for introducing the polyalkylene oxide chain into the liposome membrane. For example, an anchor (such as cholesterol) to which a polyalkylene oxide is bonded may be mixed with a phospholipid that is a membrane component to prepare a liposome, and the activated polyalkylene oxide may be bonded to the anchor. In such a method, it is necessary to perform a multi-step chemical reaction on the surface of the liposome membrane after preparing the liposome, which limits the introduction amount of the above-mentioned “functional substance” to a low level, and prevents the side reaction caused by the reaction. There are also problems such as contamination of products and impurities, and significant damage to the liposome membrane. As another production method, liposomes may be prepared by including a phospholipid polyalkylene oxide (PEO) derivative or the like in the raw material phospholipids in advance. A modified phospholipid represented by the following formula suitable for the method has been proposed (Japanese Patent Laid-Open No. 7-165770).

(Wherein R ¹ and R ² are alkyl groups having ² to 29 carbon atoms, M is a hydrogen atom or an alkali metal such as sodium or potassium, and AO, n and Y are as described above.)

Specific examples include polyethylene oxide (PEO) derivatives such as phosphatidylethanolamine, such as distearoyl phosphatidylethanolamine polyethylene oxide (DSPE-PEO). Furthermore, JP-A-2002-37883 discloses a high-purity polyalkylene oxide-modified phospholipid for producing a water-soluble polymer-modified liposome with increased blood retention. It is described that when a polyalkylene oxide-modified phospholipid having a low monoacyl body content is used in producing such a liposome, the temporal stability of the liposome dispersion is good.

Various methods have been proposed so far for preparing liposomes. When the production method is different, the shape and characteristics of the final liposome are often significantly different (Japanese Patent Laid-Open No. 6-80560). Therefore, the production method is appropriately selected according to the desired form and characteristics of the liposome. In general, liposomes are prepared by dissolving and mixing lipid components such as phospholipids, sterols, and lecithins in containers together with organic solvents such as chloroform, dichloromethane, ethyl ether, carbon tetrachloride, ethyl acetate, dioxane, and THF. It is prepared by. In particular, chlorinated solvents are often used. Such preparations of liposomes always contain an organic solvent. At present, in order to remove these remaining organic solvents, a multi-step process and a long-time treatment are required. Such residual organic solvents, particularly chlorinated organic solvents, are concerned about adverse effects on living bodies, such as side effects.

In order to prepare the liposome used in the present invention, a liposome preparation method using supercritical carbon dioxide or subcritical carbon dioxide, which can avoid the above-mentioned problems since an organic solvent, particularly a chlorinated organic solvent is not used, is utilized. Carbon dioxide has a critical temperature of 31.1 ° C and a critical pressure of 75.3 kg / cm ^{2, which} is relatively easy to handle. It is preferable for the reason. Furthermore, the liposome produced by this method has various preferable characteristics and advantages for encapsulating the iodine compound of the X-ray contrast agent as described later.

When producing liposomes using supercritical carbon dioxide or subcritical carbon dioxide, it is necessary to dissolve, disperse or mix the lipid membrane components in carbon dioxide in the supercritical state (including subcritical state). Become. At that time, it is desirable to use one or more alcohols such as lower alcohols, glycols and glycol ethers as a solubilizing agent in order to further improve the solubility of the lipid membrane component. For example, alcohol is 0.1% of supercritical carbon dioxide.
It is good to use as a co-solvent in the ratio of -10 mass%, Preferably, 1-8 mass%. Among these, a more preferable dissolution aid is ethanol from the viewpoint of safety.

The temperature of carbon dioxide in the supercritical state (including subcritical state) used in the production method of the present invention is usually 25 to 200 ° C, preferably 31 to 100 ° C, and more preferably 35 to 80 ° C. Suitable pressure is usually 50 to 500 kg / cm ^2, preferably 100 to 400 kg / cm ^2, particularly preferably in the range of 90~150 kg / cm ^2.

A preferred method for preparing liposomes used in the X-ray contrast medium of the present invention is specifically performed as follows. Liquid carbon dioxide is added to the pressure vessel to bring it to the supercritical or subcritical state under the preferred pressure and temperature described above. Phospholipids and lipid membrane stabilizing substances are dissolved or dispersed as liposome membrane components in supercritical (or subcritical) carbon dioxide. As the membrane lipid component, the phospholipid is preferably selected from at least one selected from cationic phospholipids, polyalkylene oxide-modified phospholipids, compounds having a polyalkylene oxide group, compounds having a polyethylene glycol group, sterols, and glycols. Mix with compound and dissolve. Alternatively, liquid carbon dioxide is added to a pressure vessel to which these compounds have been added in advance, and then the temperature and pressure are adjusted and mixed in a supercritical state. Subsequently, an aqueous solution containing a nonionic iodine-based compound and, if necessary, a formulation aid described later, is added to the supercritical carbon dioxide containing the produced phospholipid and lipid membrane stabilizing substance. The side to be added and the side to be added may be reversed. When the inside of the system is decompressed and carbon dioxide is discharged, an aqueous dispersion in which liposomes encapsulating an iodine compound are dispersed is generated. In this case, an iodine compound may be contained in an aqueous phase other than the inside of the liposome. Since the aqueous solution is also encapsulated inside the liposome, the iodine compound exists mainly in the aqueous phase inside the liposome in addition to the external aqueous phase of the liposome, and is in a so-called “encapsulation” state. Further, the liposome is passed through a filtration membrane having a pore size of 0.1 to 0.4 μm. Subsequently, the X-ray contrast medium of the present invention is prepared through a preparation process such as sterilization and packaging.

  It has been shown that liposome preparation methods using supercritical carbon dioxide or subcritical carbon dioxide have higher liposome production rates, encapsulation rates of encapsulated substances, and retention rates of encapsulated substances in liposomes than conventional methods. (See Patent Document 3 above). Furthermore, the present invention, which can be applied on an industrial scale, can efficiently encapsulate a nonionic and water-soluble substance in a liposome without using an organic solvent, is the X-ray imaging of the present invention. It is a useful method for the manufacture of the agent. “Substantially” means that the upper limit of the concentration in the contrast agent is 10 μg / L.

The liposome contained in the X-ray contrast medium of the present invention is preferably a substantially monolayer liposome, that is, a liposome composed of a membrane (unilamellar vesicle) having a phospholipid bilayer as one layer. Here, “substantially” means that the liposome is composed of a phospholipid bilayer in which the replica is generally recognized as one layer in the following transmission electron microscope (TEM) observation by the Freeze fracture replica method. It means that That is, the observed particle trace on the carbon film is determined as a single film when there is no level difference, and the case where two or more levels are recognized is determined as a “multilayer film”. In the X-ray contrast medium of the present invention, such a single membrane liposome contains at least 80%, preferably 90% or more of the total amount of liposomes contained in the contrast medium.

Such unilamellar liposomes can be efficiently produced by the above-described supercritical carbon dioxide or subcritical carbon dioxide as a solvent for lipids and using a water phase separation method. On the other hand, in the conventional method for preparing liposomes, liposomes composed of multilamellar vesicles (MLV) of various sizes and forms are often present in a considerable proportion. Therefore, in order to increase the ratio of single membrane liposomes, further operations such as irradiation with ultrasonic waves or passing through a filter having a fixed pore size many times are required. Single-membrane liposomes also have the advantage that the dose of liposomes, in other words, the dose of lipids does not increase compared to MLV.

Single membrane liposomes, particularly large unilamellar veislcles (LUV), have the advantage of providing a large encapsulation capacity compared to multilamellar liposomes. The liposome used for the contrast agent of the present invention is located between LUV having a particle size of 0.2 to 1 μm and SUV (Small unilamellar vesicles) which are small unilamellar liposomes having a particle size of less than 0.05 μm. For this reason, the holding volume is also larger than that of SUV, and the trapping efficiency of the water-soluble iodine-based compound, in other words, the encapsulation efficiency, is remarkably excellent as described later. In addition, unlike MLV and LUV, they are not taken up by reticuloendothelial cells and rapidly disappear from the bloodstream.

However, even in the case of a single-film liposome having good encapsulation efficiency of an iodine compound, the stability of the liposome is lowered when the weight of the encapsulated iodine compound is relatively large. In particular, a weak tendency was observed for a sudden change in ionic strength. The liposome of the contrast agent of the present invention is adjusted to a relatively small particle size, and the liposome membrane contains a compound having a polyalkylene oxide group (for example, phospholipid) and / or sterols to stabilize the lipid membrane. ing. As a result, such liposomes were found to be stable against salt shock.

The size and distribution of the liposome particles are closely related to the high blood retention, targeting properties, and delivery efficiency aimed by the X-ray contrast medium of the present invention. The particle size (particle size) is measured by freezing a dispersion containing liposomes containing an iodine-based compound, then depositing carbon on the crushed interface, and observing the carbon with an electron microscope (freeze-crushing TEM method). Can do. Here, the “average particle diameter” refers to a simple average of the observed diameters of 20 contrast agent particles.
The adjustment of the particle size can be carried out according to the formulation or process conditions. For example, when the above supercritical pressure is increased, the liposome particle size formed is reduced. In order to make the particle size distribution of the liposome to be produced in a narrower range, filtration may be performed with a polycarbonate membrane, a cellulose membrane, or the like. In this case, by passing through an extruder equipped with a filter having a pore size of 0.1 to 0.4 μm as a filtration membrane, it is possible to efficiently prepare liposomes having an optimal size that is 0.5 μm or less as the average particle size of a single membrane. The extrusion filtration method is described in, for example, Biochim. Biophys. Acta 557, 9 (1979). By incorporating such an “extrusion” operation, in addition to the above sizing, it is possible to adjust the concentration of iodine compounds existing outside the liposome, exchange the liposome dispersion, and remove undesirable substances. There is also.

As described above, sizing of the particle size is important for imparting passive targeting ability to the liposome. Japanese Patent No. 2619037 describes that the exclusion of liposomes with a particle size of 3 μm or more avoids inconvenient retention in lung capillaries. However, liposomes with a particle size range of 0.15-3 μm are not necessarily tumorigenic.

The average particle size of the liposome in the contrast agent of the present invention is usually 0.05 to 0.5 μm, preferably 0.05 to 0.3 μm, more preferably 0.05 to 0.2 μm, and particularly preferably 0.05 to 0.13 μm. X
Depending on the purpose of the line imaging, the average particle size may be set appropriately. For example, for the purpose of selective imaging of a tumor part, 0.11 to 0.13 μm is particularly preferable. By making the average particle size of the liposomes in the range of 0.1 to 0.2 μm, more preferably 0.11 to 0.13 μm, the X-ray contrast agent can be selectively concentrated on the cancer tissue. This is known as the “EPR effect”. The pores of the neovascular wall in the solid cancer tissue are abnormally large compared to the pore size of the capillary wall window (fenestra) of normal tissue, less than 0.03 to 0.08 μm, and the size of the molecule is about 0.1 μm to about 0.2 μm. But it leaks out of the blood vessel wall. That is, the EPR effect is due to the fact that the neovascular wall in cancer tissue is more permeable than the microvascular wall in normal tissue.

The contrast agent leaking from the hole in the blood vessel wall does not return to the blood vessel again because the lymphatic vessel is not sufficiently developed around the cancer cell, and remains in the place for a long time. Since the EPR effect is a passive transport using the bloodstream, the retention in the blood must be improved as a requirement for its effective expression. That is, it is required that contrast agent particles (liposome particles encapsulating iodine compounds) stay in the blood for a long time and pass through blood vessels near cancer cells many times. Since the X-ray contrast agent of the present invention is not particularly large particles, it is difficult to be captured by reticuloendothelial cells. Liposomes behave like erythrocytes, and are not rapidly excreted via the kidneys. If they are stealthed, they are phagocytosed by reticuloendothelial cells. But stays relatively long in the bloodstream. The EPR effect inevitably increases the transferability of the contrast compound to the target organ or tissue, and achieves selective concentration and accumulation of the contrast agent in the cancer tissue. An increase in the tumor cell / normal cell accumulation ratio of the contrast compound enhances the contrast performance of the X-ray contrast agent. Such improvement in tumor visibility makes it possible to even discover micrometastatic cancers that have been difficult to detect.
X-Ray Contrast Agent In the X-ray contrast agent of the present invention, the efficiency of retention of the contrast material by the liposome is improved through stabilization of the liposome structure and stabilization of the inclusion of the contrast material.

  The contrast agent of the present invention preferably contains at least an iodine compound and a formulation aid in both the aqueous phase inside the membrane of the liposome and the aqueous medium in which the liposome is dispersed. Concentration is substantially the same. Here, “substantially” means that the concentrations are almost the same in a normal case.

The “formulation aid” is added together with the contrast material at the time of formulation, and various materials are used based on the conventional contrast agent production technique. Specifically, various physiologically acceptable buffers, chelating agents such as EDTANa ₂ -Ca and EDTANa ₂ and, if necessary, α-tocopherol as an osmotic pressure regulator, stabilizer and antioxidant , Ascorbic acid, viscosity modifier and the like. Preferably, both a water-soluble amine buffer and a chelating agent are included. As the pH buffer, amine-based buffers and carbonate-based buffers are preferably used, and amine-based buffers are more preferable, and trometamol is desirable among them. Chelating agent is preferably, EDTANa ₂ -Ca.

An aqueous medium is a water-based solvent that dissolves iodine compounds, formulation aids, and the like.
In addition to at least an iodine-based compound, an aqueous solution other than the aqueous phase (encapsulated aqueous solution) inside the liposome membrane (ie, an aqueous medium in which the liposome is dispersed) is used as a formulation aid (for example, a water-soluble amine buffer, Chelating agents). Therefore, there is no significant osmotic pressure difference between the inside and outside of the membrane, thereby maintaining the structural stability of the liposomes.

The required delivery amount of iodine to the target organ that allows good contrast performance of X-ray contrast has been elucidated (for example, Japanese Patent No. 2619037). When an iodine compound is encapsulated in a microcarrier called a liposome as in the present invention, the weight of the membrane lipid of the liposome must be considered in addition to the delivery efficiency and retention stability of the contrast medium. As the membrane lipid weight of the liposome increases, the viscosity of the contrast agent increases. As the amount of the iodine compound enclosed in the liposome, the iodine compound is 1 to 10, preferably 3 to 8, more preferably 5 to 8 with respect to the liposome membrane lipid in the aqueous solution sealed in the liposome. It is desirable to contain by weight ratio.

When the weight ratio of the iodo-based compound encapsulated in the liposome aqueous phase is less than 1, it is necessary to inject a relatively large amount of lipid, resulting in poor contrast material delivery efficiency. According to the description of Japanese Patent No. 2619037, even if the ratio is 1, it was regarded as a high value from the technical level at that time. Since the viscosity of the X-ray contrast medium depends on the amount of lipid in the liposome, the superiority of the single membrane liposome having excellent retention volume and encapsulation efficiency is clear. On the other hand, when the encapsulated weight ratio of the iodine compound to the liposome membrane lipid exceeds 10, the liposome becomes structurally unstable, and the diffusion and leakage of the iodine compound outside the liposome membrane is injected during storage or in vivo. Inevitable even after being done. Also, it is described that even if 100% encapsulation is achieved immediately after the liposome suspension drug is manufactured and separated, the encapsulated components will decrease as soon as possible based on destabilization due to the osmotic pressure effect. (Japanese National Publication No. 9-505821).
The X-ray contrast agent produced by the method of the present invention is usually 30 to 500 mgI / mL, preferably 100 to 500 mgI / mL, as the iodine content, usually in the assumed dosage of 10 to 300 mL of the preparation solution. More preferably, it is in the range of 150 to 300 mg I / mL. The viscosity of the preparation solution of the present invention (when measured by the Oswald method) is 20 mPa · s or less, preferably 18 mPa · s or less, more preferably 15 mPa · s or less at 37 ° C.

From the above, an example of a more desirable embodiment in which the tumor visualization and the contrast medium retention of liposomes are improved,
A liposome having a lipid membrane, the liposome containing a compound having a polyalkylene oxide group in the lipid membrane (for example, a polyalkylene oxide-modified phospholipid), a compound selected from sterols, and in the lipid membrane The aqueous phase contains an iodo compound. Further, the liposome is a single membrane liposome having an average particle size of 0.05 to 0.2 μm, containing the iodine compound at a weight ratio of 3 to 8 with respect to the liposome membrane lipid, and encapsulated in the liposome. The iodine compound is 5 to 25 of the total iodine compound in the X-ray contrast medium.
It is an X-ray contrast agent characterized by being mass%.

  The X-ray contrast agent of the present invention is in the form of a solution or suspension that is isotonic with respect to the osmotic pressure in the body so that the liposomes contained after administration are stably maintained in the body for the required time. It is desirable to prepare with. As a medium for such a solution or suspension, water, a buffer solution such as Tris-HCl buffer solution, phosphate buffer solution, citrate buffer solution and the like can be used.

The pH range of the solution or suspension is preferably 6.5 to 8.5, more preferably 6.8 to 7.8 at room temperature. When the X-ray contrast material is a water-soluble iodine-based compound having multiple hydroxyl groups, a preferred buffer is a buffer having a negative temperature coefficient as described in US Pat. No. 4,278,654. The amine-based buffer has a property that satisfies such a requirement, and tris (TRIS) is particularly preferable. This type of buffer has a low pH at the autoclave temperature, which increases the stability of the X-ray contrast agent in the autoclave, while returning to a physiologically acceptable pH at room temperature. Therefore, in order to produce a sterile contrast medium for injection, it is extremely convenient that the liposome preparation can be sterilized by autoclave, and storage stability and the like can be ensured. However, filtration sterilization is good for liposomes to which autoclaving cannot be applied.

  To obtain an isotonic solution or suspension, the contrast agent is dissolved or suspended in the medium at a concentration that provides the isotonic solution. For example, if the contrast compound is poorly soluble and cannot provide an isotonic solution alone, other non-toxic water-soluble substances such as sodium chloride will form an isotonic solution or suspension. Sugars such as salts, mannitol, glucose, sucrose, and sorbitol may be added to the medium.

  The X-ray contrast medium of the present invention is administered to a subject parenterally, specifically intravascularly, preferably intravenously, as an injection or infusion, and imaged by X-ray irradiation. The dose is in accordance with a conventional iodine-based contrast agent. The total iodine amount in the liposome or the sum of the iodine amount outside the liposome and the total iodine amount outside the liposome may be the same as the conventional dosage.

  The X-ray contrast agent of the present invention contains at least an iodine compound and a formulation aid both in the aqueous phase inside the liposome membrane and in the aqueous medium in which the liposome is dispersed. Are substantially the same concentration. This means that the same iodine compound coexists in different forms in the same X-ray contrast medium. It can be pointed out that such an aspect of contrast material may be advantageous in the diagnostic examination as follows. That is, by using the X-ray contrast medium of the present invention, the difference in the in vivo diffusion time between the contrast material that is not encapsulated and the contrast material that is encapsulated in the liposome is different from that of the distribution behavior over time. May provide useful diagnostic information.

In actual diagnostic examinations, the use of the X-ray contrast medium of the present invention in an X-ray imaging apparatus combined with a computed tomography apparatus (CT) is also expected to exhibit the contrast agent performance more effectively. Is done. For example, when a computed tomography (CT) diagnosis of a liver tumor is performed, a contrast medium that is not encapsulated in liposomes immediately after administration freely passes through the gap between the liver sinusoids and reaches the liver parenchymal cells. The image density of liver tissue increases, but this decreases rapidly. This decrease in concentration is compensated by subsequent reinforcement from the liposomal contrast agent to maintain a high concentration of contrast material over time. Since this released non-encapsulated contrast material shows a difference in distribution behavior, a detailed analysis of the tissue state becomes possible.
[The invention's effect]
The X-ray contrast medium according to the present invention contains unilamellar liposomes having a uniform average particle diameter, and preferably contains a compound having a polyalkylene oxide group and / or a sterol in the lipid membrane. Further, both the aqueous phase inside the liposome membrane and the aqueous medium in which the liposome is dispersed contain at least an iodine compound and a formulation aid, and preferably the concentrations thereof are substantially the same inside and outside the membrane. is there. Based on these, the stability of the liposome structure and the retention stability of the encapsulated substance are improved.

  The contrast medium of the contrast agent of the present invention has a good retention in blood and exhibits an EPR effect. As a result, it selectively concentrates and accumulates at a target disease site or tissue, particularly a cancer tissue. After imaging, the iodine compound is water-soluble and will eventually be excreted outside the body. Furthermore, the difference in the in vivo diffusion time between the non-encapsulated contrast material and the contrast material encapsulated in liposomes provides images with different distribution behavior over time and provides useful diagnostic information.

The production of the liposome employs a method in which phospholipids are prepared by dissolving them in supercritical carbon dioxide, so that it is not necessary to use a toxic solvent, particularly a highly toxic chlorinated solvent.
The X-ray contrast agent according to the present invention can be used in a smaller amount than conventional X-ray contrast agents, and toxicity and side effects are much reduced. Therefore, the burden on the patient receiving the administration is small.

〔Example〕
Hereinafter, the present invention will be described more specifically based on examples. The present invention is not limited in any way by such examples.
〔test〕
It was examined by a transmission electron microscope (TEM) by the size and structure of the liposome in contrast agent form and particle size preparing liposomes freeze fracturing. That is, the liposome dispersion is rapidly frozen in liquid nitrogen and crushed in a frozen state to expose the internal structure of the liposome. The crushed surface was vapor-deposited with carbon, and the formed carbon film was observed with a transmission electron microscope.

The average particle diameter was a simple average of the observed diameters of 20 contrast agent particles. The structure of the liposome particles was determined as “single film” when there was no step in the traces of the particles left on the observed carbon film, and “multilayer film” when two or more steps were observed. Twenty particles were observed, and those having a single membrane structure of 80% or more were substantially determined as single membrane liposomes.
Quantitative Determination of Iodine Compound Iodine The liposome dispersion was dialyzed against isotonic saline, ethanol was added after the dialysis was completed to destroy the liposome, and the amount of iodine compound in the liposome was determined by measuring absorbance.

Dipalmitoyl phosphatidylcholine (DPPC) and carbon dioxide were charged into a stainless steel autoclave together with ethanol, and the autoclave was stirred at 60 ° C. and 300 kg / cm ² to dissolve DPPC in supercritical carbon dioxide. While stirring this supercritical carbon dioxide solution, iomeprol solution (816.5 mg of iomeprol was heated and dissolved in water for injection, and 20 mM
Ascorbic acid was added and dissolved so that 1 mg of trometamol was further added and dissolved. The pH was adjusted to physiological pH with dilute hydrochloric acid. Finally, a solution prepared by adding water for injection to 1.0 mL was continuously injected with a metering pump. Thereafter, the system was depressurized to discharge carbon dioxide, and a dispersion of liposomes containing iomeprol was obtained.

Further, the obtained dispersion was put in a glass vial and autoclaved at 121 ° C. for 20 minutes to obtain a contrast agent.
The particle size of the liposomes in the obtained contrast agent was measured by freeze-fracture TEM method. The average particle size of the liposomes in the contrast medium was 0.13 μm. Further, according to the morphology observation by the same method, the completed liposome was a monolayer liposome.

  The contrast agent obtained in Example 1 was diluted with an isotonic glucose solution to a concentration of 50 mg iodine / mL. When this solution was intravenously injected into rats, it was observed in X-ray images that it was concentrated especially in the liver. As time passed, the contrast level in the liver decreased in parallel with all other organ contrast levels, indicating that most of iomeprol was excreted in the urine.

The contrast agent obtained in Example 1 was diluted with an isotonic glucose solution to a concentration of 50 mg iodine / mL. This solution was injected intravenously into rats with multiple liver cancer metastases. The tumor metastasis portion had a high contrast level, and a tumor with a diameter of about 5 mm was observed. Furthermore, the decrease in the level of contrast enhancement of the tumor was later than that of other organs over time.

A contrast agent was prepared in the same manner as in Example 1 except that liposomes having an average particle size of 0.07 μm, 0.18 μm, and 0.22 μm were prepared by controlling the pressure and temperature.

A contrast agent was prepared in the same manner as in Example 1 except that 40 g of PEG having 2000 oxyethylene units was added. The average particle size was 0.12 μm.

  The contrast agents prepared in Example 4 and Example 5 were evaluated in the same manner as in Example 2. It was observed in the X-ray image that the contrast agent prepared in Example 4 was concentrated and distributed in the liver after injection regardless of the particle diameter. The contrast agent produced in Example 5 was less concentrated on the liver than the contrast agent produced in Example 4.

The contrast agents prepared in Examples 4 and 5 were evaluated in the same manner as in Example 3.
In contrast agents containing liposomes having an average particle size of 0.07 μm, the contrast level of tumors was higher than that of Example 3 in contrast to the contrast agent of Example 3.

In contrast agents containing liposomes having an average particle size of 0.18 μm, the contrast level of the tumor was reduced as compared with the contrast agent of Example 3, but the reduction rate of the contrast level was slow.
In contrast agents containing liposomes having an average particle size of 0.22 μm, the contrast level of the tumor was significantly reduced as compared with the contrast agent of Example 3.

The contrast agent produced in Example 5 had a higher tumor partial contrast level than that of Example 3, and the rate of decrease in contrast level was slow.
[Comparative Example 1]
In a conventional liposome preparation method, phospholipids and the like were dissolved in an organic solvent instead of supercritical carbon dioxide to prepare a dispersion containing liposomes, and an X-ray contrast medium was prepared. The completed liposomes were not monolayer liposomes. As a result of evaluation in the same manner as in Example 3, the tumor partial contrast level was significantly reduced as compared with the contrast agent of Example 3.

  As is apparent from the results, the X-ray contrast agent prepared by dissolving phospholipids or the like in supercritical carbon dioxide is compared with the conventional X-ray contrast agent prepared by dissolving in an organic solvent. Good sex.

A mixture of 0.04 g of dipalmitoyl phosphatidylcholine (DPPC), 1.2 mg “Pluronic F-88” (block copolymer of polyethylene oxide and polypropylene oxide) and 0.9 g of ethanol was charged into a stainless steel autoclave. Heat to 0 ° C., then add 13 g of liquid carbon dioxide. The pressure inside the autoclave was increased from 50 kg / cm ² to 200 kg / cm ² and the inside of the autoclave was stirred to dissolve DPPC in supercritical carbon dioxide. While stirring this supercritical carbon dioxide solution, it contains 647 mg / mL of iohexol solution (iodine content: 300 mg / mL), 1.21 mg / mL of trometamol, and 0.1 mg / mL of calcium disodium edetate. 5 g of a solution whose pH was adjusted to around 7 with sodium oxide was continuously injected with a metering pump. Thereafter, the system was depressurized to discharge carbon dioxide, and a liposome dispersion containing iohexol was obtained. The dispersion obtained by repeating the same operation several times was heated to 60 ° C. and filtered under pressure with a cellulose filter manufactured by Advantech Co., Ltd., 0.1 μm. Liposomes obtained by pressure filtration with a 0.1 μm filter have a particle size of 0.15 μm or less. The obtained contrast agent was designated as Sample 1.

The average particle size of the liposomes in the contrast agent was 0.12 μm, which was substantially a single-membrane liposome. The ratio of the iodine amount of the iodine compound encapsulated in the liposome to the lipid weight of the sample was 5.6.

Except for adjusting the pressure to the high pressure side, adjusting the pressure reduction rate, and changing the amount of phospholipid used, in the same manner as in Example 8, pressure-filtered with an Advantech cellulosic filter, 0.1 μm. A liposome dispersion was obtained and used as sample 2. The liposomes in Sample 2 were substantially single-membrane liposomes as observed by TEM by freeze crushing. Results such as average particle size are shown in Table 1.

Except for adjusting the pressure to the high pressure side, adjusting the pressure reduction rate, and changing the amount of phospholipid used, in the same manner as in Example 8, pressure-filtered with an Advantech cellulosic filter, 0.1 μm. A liposome dispersion was obtained and used as sample 3. The liposomes in Sample 3 were substantially single-membrane liposomes as observed by TEM by freeze crushing. Results such as average particle size are shown in Table 1.

Except for adjusting the pressure to the high pressure side, adjusting the pressure reduction rate, and changing the amount of phospholipid used, in the same manner as in Example 8, pressure-filtered with an Advantech cellulosic filter, 0.1 μm. A liposome dispersion was obtained and used as sample 4. The liposomes in Sample 4 were substantially single-membrane liposomes as observed by TEM by freeze crushing. Results such as average particle size are shown in Table 1.

Except for adjusting the pressure to the high pressure side, adjusting the pressure reduction rate, and changing the amount of phospholipid used, in the same manner as in Example 8, it was filtered under pressure with a cellulose filter manufactured by Advantech, 0.2 μm. A liposome dispersion was obtained and used as Sample 5. The liposomes in Sample 5 were substantially single-membrane liposomes as observed by TEM by freeze crushing. Results such as average particle size are shown in Table 1.
[Comparative Example 2]
Using a conventional liposome preparation method, phospholipids and the like were dissolved in an organic solvent, and the amount of phospholipid used was changed to prepare a dispersion containing liposomes. Thereby, two types of X-ray contrast agents (samples 6 and 7) were prepared. The completed liposomes were not monolayer liposomes. Results such as average particle size are shown in Table 1.

A cell suspension of VX2 carcinoma was implanted subcutaneously in the rabbit. Example 8 2 weeks after transplantation
The contrast medium obtained in (1) was injected intravenously and observed with an X-ray image after injection. The contrast level decreased with time, but the contrast level decreased slowly for the transplanted part.

Similarly, the contrast medium samples 2 to 5 prepared in Examples 9 to 12, the contrast medium samples 6 and 7 prepared in Comparative Example 2, and the sample 2B of Example 2 of Patent No. 2619037 were reproduced. The same evaluation was performed on the sample (Sample 8, Comparative Example 3). The results are shown in Table 1.

Table 1 shows the evaluation for cancer contrast as follows.
◎ Decrease in contrast level of transplanted part is clearly slower than other parts.
○ The decrease in contrast level in the transplanted part is somewhat slower than in other parts.

[Delta] Decrease in the contrast level of the transplanted part is unchanged compared to the other parts.
× The increase in the contrast level of the transplanted portion is small.

As can be seen from Table 1, the contrast enhancement of Sample 8 with an average liposome particle size of more than 0.5 μm was significantly reduced compared to the other samples. Samples 1 to 5 prepared by dissolving phospholipids or the like in supercritical carbon dioxide have better cancer contrast than samples 6 to 8 prepared by dissolving them in an organic solvent.

Dipalmitoyl phosphatidylcholine (DPPC) 0.04g and Adeka Pluronic F-88
(Block copolymer of polyethylene oxide and polypropylene oxide) A mixture of 1.2 mg and ethanol 0.9 g was charged into a stainless steel autoclave, the inside of the autoclave was heated to 60 ° C., and then 13 g of liquid carbon dioxide was added. The pressure inside the autoclave was increased from 50 kg / cm ² to 200 kg / cm ² and the inside of the autoclave was stirred to dissolve DPPC in supercritical carbon dioxide. While stirring this supercritical carbon dioxide solution, 647 mg / mL of iohexol solution (iodine content 300 mg / mL), 1.21 mg / mL of trometamol, disodium calcium edetate (ED
(TANa ₂ -Ca) 0.1 mg / mL, 5 g of a solution whose pH was adjusted to around 7 with appropriate amounts of hydrochloric acid and sodium hydroxide were continuously injected with a metering pump. Thereafter, the system was depressurized to discharge carbon dioxide, and a liposome dispersion containing iohexol was obtained. The resulting dispersion was heated to 60 ° C. and filtered under pressure with a cellulose filter manufactured by Advantech, 0.1 μm. The obtained contrast agent was designated as Sample 9.

As a result of conducting the above test on the liposomes in sample 9, the average particle size was 0.12 μm, which was substantially a single membrane liposome. The ratio of the iodine amount of the iodine compound encapsulated in the liposome to the lipid weight of the sample 9 was 5.6.

  By adjusting the pressure to the high pressure side, adjusting the pressure reduction rate, using a cellulose filter, 0.1 μm, 0.2 μm and 0.4 μm, and changing the amount of phospholipid used, the same as above Liposome dispersions were obtained and used as Samples 10 to 15.

In addition, a mixture of 0.04 g of dipalmitoylphosphatidylcholine (DPPC), 0.01 g of cholesterol, 1.2 mg of Pluronic F-88 (polyethylene oxide and polypropylene oxide block copolymer) and 0.9 g of ethanol in an autoclave was charged into a stainless steel autoclave. Sample 16 was produced in the same manner as in Sample 9 except for the above. Sample 17 was obtained in the same manner as Sample 16, except that Pluronic F-88 was not used.

Further, Sample 18 was produced in the same manner as Sample 9, except that Pluronic F-88 was not used. Samples 9-18 were all single membrane liposomes.
Table 2 shows the measurement results such as the average particle size of the liposomes in each sample.

Samples 9 to 18 obtained in Example 14 were added to physiological saline, sealed, heated to 42 ° C., and left for 3 days. The obtained sample and the sample before heating were observed with an optical microscope.
In all of the samples before heating, fine particles were not observed with an optical microscope. However, after heating, in the sample 18, fine particles of several μm that are considered to be a coalescence of liposome particles were observed. In Sample 17, a small number of fine particles that seemed to be a combination of liposome particles were observed. When the sample was further heated for 4 days, Samples 9 to 15 were slightly observed to have fine particles that were considered as coalesced liposome particles. However, no change was observed for Sample 16.

  As is apparent from this result, stability can be improved by using a contrast agent containing a liposome using a block copolymer of polyethylene oxide and polypropylene oxide or cholesterol.

Samples 9 to 18 obtained in Example 14 were diluted with isotonic glucose solution to a concentration of 50 mg iodine / mL. When this solution was intravenously injected into rats, it was observed in X-ray images that Samples 17 and 18 were concentrated and distributed especially in the liver.

  As is clear from this result, selective contrast of the liver is made possible by using a contrast agent containing liposomes that does not use a block copolymer of polyethylene oxide and polypropylene oxide.

A cell suspension of VX2 carcinoma was implanted subcutaneously in the rabbit. Two weeks after transplantation, samples 9 to 18 obtained in Example 14 were intravenously injected into rabbits and observed by X-ray images after injection. When Samples 9, 10, 11, 12, 14, and 16 were used, the contrast level at the transplanted part of the rabbit was decreased more slowly than Samples 13, 15, 17, and 18.

A mixture of 0.032 g of dipalmitoyl phosphatidylcholine (DPPC), 0.008 g of cholesterol, and 1.2 mg of Pluronic F-88 (block copolymer of polyethylene oxide and polypropylene oxide) by Adeka and 0.9 g of ethanol was placed in a stainless steel autoclave. Heat to 60 ° C., then add 13 g of liquid carbon dioxide. The pressure inside the autoclave was increased from 50 kg / cm ² to 200 kg / cm ² and the inside of the autoclave was stirred to dissolve DPPC in supercritical carbon dioxide. While stirring this supercritical carbon dioxide solution, it contains 647 mg / mL of iohexol solution (iodine content 300 mg / mL), 1.21 mg / mL of trometamol, and 0.1 mg / mL of calcium disodium edetate (EDTANa ₂ -Ca). Then, 5 g of a solution whose pH was adjusted to around 7 with appropriate amounts of hydrochloric acid and sodium hydroxide was continuously injected with a metering pump. Thereafter, the system was depressurized to discharge carbon dioxide, and a liposome dispersion containing iohexol was obtained. The resulting dispersion was heated to 60 ° C. and filtered under pressure with a cellulose filter manufactured by Advantech, 0.1 μm. The obtained contrast agent was designated as Sample 19.

The average particle diameter of the liposomes in the sample 19 determined by the above method was 0.12 μm, and it was substantially a monolayer liposome. The ratio of the iodine amount of the iodine compound contained in the liposome to the lipid weight of the sample was 3.7, and the ratio of the iodine compound in the liposome among all the iodine compounds was 23% by mass. .

  Adjust the pressure to the high pressure side, adjust the pressure reduction speed, use a cellulose filter, 0.1 μm, 0.2 μm, and 0.4 μm, and change the amount of phospholipid used. Thus, a dispersion of liposomes was obtained, and these were designated as Samples 20 to 25. The liposomes in Samples 20-25 were substantially monolayer liposomes. Table 3 shows the average particle size of the liposomes in these samples, the ratio of the iodine amount of the iodine compound contained in the liposome to the lipid weight, and the ratio of the iodine compound in the liposome to the total iodine compound.

  Further, Sample 26 was prepared in the same manner as in Sample 19, except that 0.024 g of dipalmitoyl phosphatidylcholine (DPPC) and 0.006 g of cholesterol were used to adjust the pressure and the pressure reduction rate.

Sample 27 was prepared in the same manner as in sample 19, except that 0.048 g of dipalmitoylphosphatidylcholine (DPPC) and 0.012 g of cholesterol were used to adjust the pressure and the pressure reduction rate.

The liposomes in samples 26 and 27 were substantially monolayer liposomes. Table 3 shows the average particle size of the liposomes in these samples, the ratio of the iodine amount of the iodine compound contained in the liposome to the lipid weight, and the ratio of the iodine compound in the liposome to the total iodine compound.

Samples 29 to 37 were prepared in the same manner as samples 19 to 27 except that calcium edetate disodium (EDTANa ₂ -Ca) was not used. The liposomes in samples 29-37 were substantially monolayer liposomes. Table 3 shows the average particle size of the liposomes in these samples, the ratio of the iodine amount of the iodine compound contained in the liposome to the lipid weight, and the ratio of the iodine compound in the liposome to the total iodine compound. According to this, calcium edetate 2
It was almost the same as when sodium was used.

  Samples 39 to 47 were prepared in the same manner as Samples 19 to 27 except that trometamol was not used. The liposomes in Samples 39-47 were substantially monolayer liposomes. Table 3 shows the average particle size of the liposomes in these samples, the ratio of the iodine amount of the iodine compound contained in the liposome to the lipid weight, and the ratio of the iodine compound in the liposome to the total iodine compound. According to this, it was almost the same as the case of using trometamol.

  For samples 19-27, 29-37, 39-47, the resulting liposome dispersion was dialyzed with isotonic saline, and the concentrations of the iodine-based compound and formulation aid in the aqueous medium in which the liposomes were dispersed, Comparative samples 119 to 127, 129 to 137, and 139 to 147 having lower concentrations than the aqueous phase in the liposome membrane were prepared. Table 3 shows the average particle size of the liposomes in these samples, the ratio of the iodine amount of the iodine compound contained in the liposome to the lipid weight, and the ratio of the iodine compound in the liposome to the total iodine compound.

Stability Evaluation The sample obtained in Example 18 was added to physiological saline, sealed, heated to 42 ° C. and left for 14 days. The obtained sample and the sample before heating were observed with an optical microscope.

In all of the samples before heating, fine particles were not observed with an optical microscope, but after heating, for some samples, fine particles of several μm that were considered to be a coalescence of liposome particles were observed. Stability was evaluated according to the following criteria. That is,
5: No coalesced particles 4: Very few coalesced particles.
3: Coherent particles are clearly observed.
2: There are many coalesced particles.
1: There are very many coalesced particles.
The results are shown in Table 3.

  As is clear from Table 3, when there is a difference in the water-soluble components inside and outside the liposome, the stability of the liposome decreases. Further, the stability of a sample passing through a filtration membrane having a pore diameter of 0.1 to 0.4 μm is improved even with the same particle diameter.

  On the other hand, when there is no difference in the water-soluble component inside and outside the liposome, the stability is further improved if a water-soluble amine buffer or chelating agent is present.

Claims (20)

Including liposomes containing water-soluble and nonionic iodo compounds,
Liposomes formed without using chlorinated solvents by mixing phospholipids with supercritical carbon dioxide or subcritical carbon dioxide and bringing water-soluble and nonionic iodine compounds into contact with the phospholipids And
And the X-ray contrast agent characterized by not containing a chloro solvent. Including liposomes containing water-soluble and nonionic iodo compounds,
Liposomes formed without using chlorinated solvents by mixing phospholipids with supercritical carbon dioxide or subcritical carbon dioxide and bringing water-soluble and nonionic iodine compounds into contact with the phospholipids And
Moreover
An X-ray contrast agent characterized by containing no chlorinated solvent in excess of 10 μg / L.   2. The X-ray contrast medium according to claim 1, wherein at least 80% of the total amount of the liposomes is a single membrane liposome. 2. The X-ray contrast medium according to claim 1, wherein the iodine compound has at least one 2,4,6-triiodophenyl group.   The X-ray contrast medium according to claim 1, wherein the liposome has an average particle size of 0.05 to 0.5 μm.   6. The X-ray contrast medium according to claim 5, wherein the liposome has an average particle size of 0.05 to 0.2 [mu] m. The X-ray contrast medium according to claim 6, wherein the liposome has an average particle size of 0.11 to 0.13 μm.   2. The liposome is a polyethylene glycol (PEG) -modified liposome comprising 0.1 to 30% by mass of polyethylene glycol having 10 to 3500 oxyethylene units based on lipids constituting the liposome. The X-ray contrast agent as described in 1. above.   2. The X-ray contrast medium according to claim 1, wherein the liposome is passed through a filtration membrane having a pore size of 0.1 to 0.4 μm.   The X-ray contrast medium according to claim 1, wherein the liposome contains the iodine compound in a weight ratio of 1 to 10 with respect to the liposome membrane lipid.   11. The X-ray contrast medium according to claim 10, wherein the liposome contains the iodine compound in a weight ratio of 3 to 8 with respect to the liposome membrane lipid.   12. The X-ray contrast medium according to claim 11, wherein the liposome contains the iodine compound in a weight ratio of 5 to 8 with respect to the liposome membrane lipid.   11. The X-ray contrast medium according to claim 10, wherein the iodine compound encapsulated in the liposome is 5 to 30% by mass of the total iodine compound in the X-ray contrast medium.   The liposome has a phospholipid having a polyalkylene oxide group in its lipid membrane, and cholesterol, dihydrocholesterol, cholesterol ester, phytosterol, sitosterol, stigmasterol, campesterol, cholestanol, lanosterol, 2,4-dihydrolanosterol, 1 It contains at least one compound selected from -O-sterol glucoside, 1-O-sterol maltoside and 1-O-sterol galactoside, and contains the iodo compound in the aqueous phase in the lipid membrane. The X-ray contrast agent according to claim 1. 2. The X-ray contrast imaging according to claim 1, wherein the liposome contains a polyalkylene oxide-modified phospholipid in its lipid membrane, and contains the iodine compound in an aqueous phase in the lipid membrane. Agent.   2. The liposome according to claim 1, wherein the lipid membrane contains a block copolymer of polyethylene oxide and polypropylene oxide, and the iodine compound is contained in an aqueous phase in the lipid membrane. The X-ray contrast agent as described.   The liposome contains the iodine compound and formulation aid in both the aqueous phase in the lipid membrane and the aqueous medium in which the liposome is dispersed, and the respective concentrations are the same inside and outside the liposome membrane. The X-ray contrast medium according to claim 1, wherein:   18. The X-ray contrast medium according to claim 17, wherein the formulation aid is a compound selected from at least one of a water-soluble amine buffer and a chelating agent.   19. The X-ray contrast medium according to claim 18, wherein the water-soluble amine buffer is trometamol. 19. The X-ray contrast medium according to claim 18, wherein the chelating agent is EDTANa ₂ —Ca.