A method for embolization treatment was developed in which microspheres with novel properties are administered in a mammal. The microspheres are made using a novel process that results in microspheres with new combined properties of high density, low fracture, high swell capacity, rapid swell, and deformability following swell. These microspheres form occlusions with high durability, withstanding over 100 mm Hg (13.3 kPa) of pressure.

Description of the Invention

SUMMARY OF THE INVENTION

The present invention provides a method for embolization in a mammal comprising administering microspheres prepared by a process comprising the following steps a) forming a first solution comprising: (i) water; (ii) at least one water miscible monomer selected from the group consisting of acrylic acid, methacrylic acid, salts of acrylic acid and methacrylic acid, acrylamide, methacrylamide, N-substituted acrylamides, N-substituted methacrylamides, 2-acryloylethane-sulfonic acid, 2-methacryloylethane-sulfonic acid, salts of 2-acryloylethane-sulfonic acid and 2-methacryloylethane-sulfonic acid, styrene-sulfonic acid, salts of styrene-sulfonic acid, 2-hydroxyethyl acrylate, and 2-hydroxyethyl methacrylate, provided that: (A) if said monomer is acrylamide, methacrylamide, N-substituted acrylamides, 2-hydroxyethyl acrylate, or 2-hydroxyethyl methacrylate, said monomer is used in combination with at least one other monomer selected from subgroup 1 consisting of: acrylic acid, methacrylic acid, salts of acrylic acid and methacrylic acid, 2-acryloylethane-sulfonic acid, 2-methacryloylethane-sulfonic acid, salts of 2-acryloylethane-sulfonic acid and 2-methacryloylethane-sulfonic acid, styrene-sulfonic acid, and salts of styrene-sulfonic acid; (B) if said first solution contains at least one monomer from subgroup 2 consisting of acrylic acid, methacrylic acid, salts of acrylic acid and methacrylic acid, acrylamide, methacrylamide, N-substituted acrylamides, N-substituted methacrylamides, 2-hydroxyethyl acrylate, and 2-hydroxyethyl methacrylate, but does not contain a monomer selected from subgroup 3 consisting of 2-acryloylethane-sulfonic acid, 2-methacryloylethane-sulfonic acid, salts of 2-acryloylethane-sulfonic acid and 2-methacryloylethane-sulfonic acid, styrene-sulfonic acid, and salts of styrene-sulfonic acid, then the pH of the first solution is at least about 3; or (C) if said first solution contains at least one monomer from subgroup 3 consisting of 2-acryloylethane-sulfonic acid, 2-methacryloylethane-sulfonic acid, salts of 2-acryloylethane-sulfonic acid and 2-methacryloylethane-sulfonic acid, styrene-sulfonic acid, and salts of styrene-sulfonic acid, then the pH of the first solution is less than about 3; (iii) a crosslinking agent that is miscible in the first solution in less than or equal to about 5 Mol %, relative to total moles of monomer and crosslinking agent, said crosslinking agent being selected from the group consisting of N,N'-methylene-bis-acrylamide, N,N'-methylene-bis-methacrylamide, N-methylolacrylamide, N-methylolmethacrylamide, glycidyl acrylate, glycidyl methacrylate, polyethylene glycol diacrylate, polyethylene glycol dimethacrylate, polyvalent metal salts of acrylic acid and methacrylic acid, divinyl benzene phosphoacrylates, divinylbenzene, divinylphenylphosphine, divinyl sulfone, 1,3-divinyltetramethyldisiloxane, 3,9-diviny1,2,4,8,10-tetraoxaspiro[5,5]undecane, phosphomethacrylates, ethylene glycol diglycidyl ether, glycerin triglycidyl ether, glycerin diglycidyl ether, and polyethylene glycol diglycidyl ether; (iv) a water soluble protecting colloid; (v) an emulsifier; and (vi) a low temperature aqueous soluble azo initiator; b) forming a second solution comprising at least one substantially chlorinated hydrocarbon of less than 6 carbon units, provided that the chlorinated hydrocarbon is not a halogenated aromatic hydrocarbon, and an organic soluble protecting colloid; c) forming a first suspension with agitation comprising the first and second solutions at a temperature below the initiation temperature of the azo initiator of (a); d) increasing the temperature of the agitating first suspension to a temperature at which the low temperature aqueous soluble azo initiator is activated; e) agitating the first suspension until it forms a second suspension comprising a gelatinous precipitate suspended in an organic liquid phase, wherein microspheres are formed; f) allowing the second suspension to cool to a temperature that is at or below about 30 .degree. C. while agitating the second suspension; g) washing the second suspension at least once with a dehydrating solvent wherein water is removed from the microspheres forming a microsphere preparation; h) recovering the microsphere preparation; and i) drying the microsphere preparation to form a free-flowing microsphere powder.

DETAILED DESCRIPTION

The present invention provides a method for embolization in which microspheres having properties of general consistency in size and shape, high density, low fracture, high swell capacity, rapid swell, and deformability following swell are administered to a mammal. The microspheres may be administered in a medium that allows a limited amount of swell. This limited swell allows the microspheres to pass through small diameter catheters due to their deformability. These microspheres are made using a process which is simple, consistent, and produces microspheres with these properties at a high yield. The process for microsphere preparation makes use of a water soluble, low temperature-active azo initiator in an aqueous solution of monomer, crosslinking agent, and emulsifier. A chlorinated organic medium is used in forming a suspension with the aqueous solution. The aqueous solution and organic medium both additionally include protecting colloids. The aqueous solution and organic medium, as well as the mixture of the two, are initially held below the initiation temperature of the azo initiator. The organic medium, which may comprise a chloroform and methylene chloride mixture, has a high enough boiling temperature that the aqueous soluble azo initiator can be activated to cause polymerization producing microspheres.

Monomer and Crosslinking Agent

Monomers that may be used in the present process for preparing microspheres are water miscible monomers including, but not limited to, acrylic acid, methacrylic acid, salts (such as sodium and ammonium) of acrylic acid and methacrylic acid, acrylamide, methacrylamide, N-substituted acrylamides, N-substituted methacrylamides, 2-acryloylethane-sulfonic acid, 2-methacryloylethane-sulfonic acid, salts of 2-acryloylethane-sulfonic acid and 2-methacryloylethane-sulfonic acid, styrene-sulfonic acid, salts of styrene-sulfonic acid, 2-hydroxyethyl acrylate, and 2-hydroxyethyl methacrylate. Monomers may be used singly or in combinations as co-monomers. Monomers that perform well as single monomer components (subgroup 1) include acrylic acid, methacrylic acid, salts (such as sodium and ammonium) of acrylic acid and methacrylic acid, 2-acryloylethane-sulfonic acid, 2-methacryloylethane-sulfonic acid, salts of 2-acryloylethane-sulfonic acid and 2-methacryloylethane-sulfonic acid, styrene-sulfonic acid, and salts of styrene-sulfonic acid. Preferably, the following monomers are used as co-monomers with at least one of the monomers from subgroup 1: acrylamide, methacrylamide, N-substituted acrylamides, N-substituted methacrylamides, 2-hydroxyethyl acrylate, and 2-hydroxyethyl methacrylate. Most useful in producing microspheres for medical applications are monomers having biocompatibility such as acrylic acid, methacrylic acid, salts of acrylic acid and mcthacylic methacrylic acid, 2-hydroxyethyl acrylate and 2-hydroxyethyl methacrylate, and combinations thereof. In one embodiment the monomer is a combination comprising acrylic acid and at least one monomer from the group of sodium acrylate, 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, styrene sulfonic acid, and the sodium salt of styrene sulfonic acid. In another embodiment, the monomer is styrene sulfonic acid or a combination comprising styrene sulfonic acid and the sodium salt of styrene sulfonic acid.

Many of these monomers are liquids which are miscible with water. For monomers that are solids, an aqueous solution of the monomer may be prepared, and this monomer solution is miscible with water. Acid monomers and salts of monomers may be combined to adjust the pH of a monomer solution. It is particularly useful to partially neutralize an acid monomer, thereby providing a mixture of acid monomer and monomer salt. Acid monomers that may be used are, for example, acrylic acid, methacrylic acid, 2-acryloylethane-sulfonic acid, 2-methacryloylethane-sulfonic acid, styrene-sulfonic acid, and combinations thereof. A monomer prior to partial neutralization is referred to as an initial monomer. An acid monomer is typically partially neutralized using a base. Suitable bases include, but are not limited to, sodium hydroxide, potassium hydroxide, ammonium hydroxide, lithium hydroxide and combinations thereof. Bases containing divalent cations, such as calcium hydroxide and barium hydroxide may also be used; however, they are preferably used in combination with a base containing monovalent cations because divalent cations have a strong tendency to induce ionic crosslinking, which could severely alter the desirable properties of the microspheres. For some applications it may be desirable to substitute a portion of the base with barium hydroxide (Ba(OH).sub.2) to introduce a radio-opaque element, which makes the resulting microspheres amenable to x-ray imaging. Barium hydroxide may be used in a ratio of up to about 1:1 by weight of Ba(OH).sub.2 to NaOH, to produce a combination salt that includes barium salt. Alternatively, a barium monomer salt may be included in a monomer combination.

A crosslinking agent that is miscible with an aqueous monomer solution is copolymerized with the monomer in the present process. Examples of crosslinking agents that may be used include, but are not limited to, N,N'-methylene-bis-acrylamide, N,N'-methylene-bis-methacrylamide, N-methylolacrylamide, N-methylolmethacrylamide, glycidyl acrylate, glycidyl methacrylate, polyethylene glycol diacrylate and polyethylene glycol dimethacrylate (which are most useful with hydrophobic monomers), polyvalent metal salts of acrylic acid and methacrylic acid, divinyl benzene phosphoacrylates, divinylbenzene, divinylphenylphosphine, divinyl sulfone, 1,3-divinyltetramethyldisiloxane, 3,9-divinyl-2,4,8,10-tetraoxaspiro[5,5]undecane, phosphomethacrylates, and polyol polyglycidyl ethers such as ethylene glycol diglycidyl ether, glycerin triglycidyl ether, glycerin diglycidyl ether, and polyethylene glycol diglycidyl ether, and combinations thereof. The amount of crosslinking agent included for copolymerization may vary and is inversely related to the amount of swell capacity in the microspheres produced using the present process. Different amounts of crosslinking agent result in swelling capacities over a range of about 1.5 grams of water per gram of microspheres to over 100 grams of water per gram of microspheres. Generally useful is an amount of crosslinking agent that results in microspheres with a swell capacity of at least about 50 grams of water per gram of microspheres. Particularly useful is an amount of crosslinking agent that results in microspheres with a swell capacity of at least about 70 grams of water per gram of microspheres. The exact amount of crosslinking agent needed will vary depending on the specific agent used and can be readily determined by one skilled in the art. The amount of crosslinking agent is calculated as Mol % (mole percent) based on the sum of the moles of monomer and moles of crosslinking agent. Thus, the Mol % is calculated as moles of crosslinking agent/(moles of monomer+moles of crosslinking agent). For example, 4.0 Mol % of N,N'-methylenebisacrylamide with respect to moles of acrylic acid monomer+sodium acrylate+crosslinking agent produces microspheres with a swell of about 50 grams of water per gram of microspheres, 2.9 Mol % of N,N'-methylenebisacrylamide produces microspheres with a swell of about 70 grams of water per gram of microspheres, and 2.3 Mol % of N,N'-methylenebisacrylamide produces microspheres with a swell of about 107 grams of water per gram of microspheres. Preferably, the Mol % of crosslinking agent is equal to or less than about 5 Mol %, preferably, equal to or less than about 4 Mol %, more preferably about 0.08 Mol % to about 4 Mol %, most preferably about 0.08 Mol % to about 2.3 Mol % relative to total moles of monomer and crosslinking agent. Microspheres with very high swell (i.e., over 250 grams of water per gram of microspheres) can be prepared using a hydrophilic monomer such as sodium acrylate, a low amount of crosslinking agent (e.g., 0.083 Mol % of N,N'-methylenebisacrylamide), with low temperature drying conditions, as described in Example 35 below.

First Solution

A monomer and crosslinking agent as described above are prepared in an aqueous solution, together with additional components, which is herein called the "first solution". The monomer is generally included at about 0.5% to about 30% as weight percent of the first solution. Monomer weight percents of about 15% to about 25% and about 20% to about 25% are particularly useful in the process of the invention. If a combination of monomers is used in the process, the total amount of all the monomers is about 0.5% to about 30%, in addition from about 15% to about 25%, and in addition from about 20% to about 25%, as weight percent of the first solution.

The pH of the first solution may vary and is a factor in the swell capacity of the microspheres prepared in the process of the invention. The useful pH range of the first solution also depends on the particular monomer or combination of monomers used. If the first solution contains at least one monomer from subgroup 2 consisting of acrylic acid, methacrylic acid, salts of acrylic acid and methacrylic acid, acrylamide, methacrylamide, N-substituted acrylamides, N-substituted methacrylamides, 2-hydroxyethyl acrylate, and 2-hydroxyethyl methacrylate, but does not contain a monomer from subgroup 3 consisting of 2-acryloylethane-sulfonic acid, 2-methacryloylethane-sulfonic acid, salts of 2-acryloylethane-sulfonic acid and 2-methacryloylethane-sulfonic acid, styrene-sulfonic acid, and salts of styrene-sulfonic acid, then the pH of the first solution is at least about 3, preferably between about 3.5 and about 10, more preferably between about 5 and about 9, to produce microspheres with a high swell capacity. For example, a mixture of acrylic acid and sodium acrylate at a pH of between about 3.5 and about 10, and a 2 to 5 Mol % of N,N'-methylenebisacrylamide crosslinking agent (with respect to the monomer), when used in the process of the invention, produces microspheres with a swell capacity of at least about 80 grams of water per gram of microspheres. If the first solution contains at least one monomer from subgroup 3 consisting of 2-acryloylethane-sulfonic acid, 2-methacryloylethane-sulfonic acid, salts of 2-acryloylethane-sulfonic acid and 2-methacryloylethane-sulfonic acid, styrene-sulfonic acid, and salts of styrene-sulfonic acid, then the pH of the first solution is less than about 3 to produce highly swellable microspheres (see Examples 36-38).

The pH of the first solution may be adjusted in any number of ways. For example, if the monomer is prepared as a monomer solution, as described above, the pH of the monomer solution will govern the pH of the first solution. In the case of an acid monomer, the pH of the monomer solution is related to the amount of base or monomer salt added to the acidic monomer solution. Alternatively, the pH of the first solution may be adjusted as required by the addition of acid or base after all the components have been added.

Included in the "first solution" is a component that can modify the viscosity of an aqueous solution to provide a surface tension that allows droplet formation in the aqueous/organic suspension that is formed during the present microsphere preparation process. This component is referred to herein as a "protecting colloid". A variety of natural and synthetic compounds that are soluble in aqueous media may be used as a protecting colloid including cellulose derivatives, polyacrylates (such as polyacrylic acid and polymethacrylic acid), polyalkylene glycols such as polyethylene glycol, partially hydrolyzed polyvinyl alcohol and other polyols, guar gum, and agar gum. Particularly useful are cellulose ethers such as methyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, ethylhydroxyethyl cellulose, hydroxypropyl cellulose, ethyl cellulose, and benzyl cellulose; as well as cellulose esters such as cellulose acetate, cellulose butylate, cellulose acetate butylate, cellulose propionate, cellulose butyrate, cellulose acetate propionate, cellulose acetate butyrate, and cellulose acetate phthalate. The amount of the protecting colloid in the first solution is sufficient to reduce microdroplet coalescence in the aqueous/organic suspension, and is generally between about 0.1% and about 3% by weight % of the first solution. Preferred is methyl cellulose at about 0.5% to about 0.6% by weight.

An emulsifier is included in the first solution to promote the formation of a stable emulsion on addition of the first solution to an organic second solution (described below). Any emulsifier which stabilizes the aqueous/organic emulsion may be used. Suitable emulsifiers include, but are not limited to, alkylaryl polyether alcohols such as the Triton.TM. X nonionic surfactants commercially available from Union Carbide (Danbury, CN). These products generally contain mixtures of polyoxyethylene chain lengths and include, for example, Triton.RTM. X-100: polyoxyethylene(10) isooctylphenyl ether; Triton.RTM. X-100, reduced: polyoxyethylene(10) isooctylcyclohexyl ether; Triton.RTM. N-101, reduced: polyoxyethylene branched nonylcyclohexyl ether; Triton.RTM. X-114: (1,1,3,3-tetramethylbutyl)phenyl-polyethylene glycol; Triton.RTM. X-114, reduced: polyoxyethylene(8)isooctylcyclohexyl ether; Triton.RTM. X-405, reduced: polyoxyethylene(40)isooctylcyclohexyl ether; and Triton.TM. X-405: polyoxyethylene(40)isooctylphenyl ether, 70% solution in water. Particularly suitable is Triton.TM. X-405, 70 wt % solution, which is an alkylaryl polyether alcohol preparation having an average of at least about 30 ethylene oxide units per ether side chain. Typically, the emulsifier in the first solution is used at a concentration of about 1% to about 10% by weight % of the first solution.

In addition, the first solution includes a polymerization initiator. The initiator used in the process of the invention is a water soluble azo initiator which has a low temperature of activation. Azo initiators are substituted diazo compounds that thermally decompose to generate free radicals and nitrogen gas. The temperature of activation of the azo initiator used is low enough so that the boiling point of an organic second solution (described below) is above the azo initiator activation temperature. Examples of suitable low temperature water soluble azo initiators include, but are not limited to, 2,2'-azobis(2-amidinopropane)dihydrochloride; 4,4'-azobis(4-cyanopentanoic acid); and 2,2'-azobis(2-[2-imidazolin-2-yl])propane dihydrochloride. A particular azo initiator, having a particular activation temperature, is used with an organic second solution composition (described below) at a temperature and with a reaction time period that is effective in initiating polymerization. Most effective is use of an azo initiator at a temperature that is close to its optimal activation temperature and which is also below the boiling temperature of the organic second solution. However, an azo initiator may be used at a temperature that is lower than its optimal activation temperature in order to stay below the boiling temperature of the organic second solution, but this will require a longer reaction time for polymerization. A particularly suitable azo initiator has an activation temperature that is less than about 53.degree. C. and this azo initiator is used with an organic second solution having a boiling temperature of about 55.degree. C. A particularly suitable azo initiator is VA-044.TM. (2,2'-azobis(2-[2-imidazolin-2-yl])propane dihydrochloride, commercially available from Wako Pure Chemical Industries, Ltd., Richmond, Va.) having an activation temperature of between 51.degree. C. and 52.degree. C.

The azo initiator has advantages over other initiators such as persulfates and hydroperoxides. The azo initiator is effective when used in very low amounts, in contrast to other initiators. The azo initiator is used at about 0.1% to 1.0% by weight % of monomer. Preferably about 0.5% azo initiator is used. The low level of azo initiator results in very low levels of initiator contamination in the polymerized hydrogel as compared to contamination resulting from use of other initiators. In addition, there is no metal contamination resulting from the azo initiator, while other initiators typically include metal catalysts that do leave metal contamination in the polymerized product. In addition, other typical initiators are sensitive to oxygen, and, therefore, solutions in contact with these initiators must be de-aerated. The remaining oxygen content of the de-aerated solutions is variable, leading to inconsistency in the microsphere forming process. With use of an azo initiator, no de-aeration is required, which reduces the complexity of solution preparation for use in the microsphere formation process and increases the consistency of microsphere preparation. In addition persulfate initiators generally give more inconsistent conversion and yields of microspheres than azo initiators.

Second Solution

An organic solution acts as a dispersion medium in the process of microsphere preparation, and is herein called the "second solution". The second solution comprises at least one substantially chlorinated hydrocarbon of less than 6 carbon units, excluding halogenated aromatic hydrocarbons. A substantially chlorinated hydrocarbon may be a hydrocarbon that is at least 50% chlorinated, as well as a fully chlorinated hydrocarbon. Particularly suitable is a chlorinated solvent that readily dissolves ethyl cellulose to a homogeneous solution, boils above at least about 50.degree. C. and has a density able to support microsphere formation in aqueous/organic suspension. A particularly useful organic medium in the process of microsphere preparation is a mixture containing chloroform and methylene chloride. Methylene chloride alone does not have a high enough boiling temperature to allow the use of a low temperature aqueous azo initiator. Chloroform alone is not sufficient to support microsphere formation. The combination of chloroform and methylene chloride provides an organic solution which has a boiling temperature allowing use of a low temperature aqueous azo initiator and which supports microsphere formation in the aqueous/organic suspension. Chloroform and methylene chloride may be used in volume ratios between about 20:1 and about 1:20. More suitable is a chloroform and methylene chloride solution with a volume ratio between about 5:1 and 1:5. Particularly suitable is a volume ratio of 3:1 chloroform:methylene chloride solution which has a boiling temperature of about 53.degree. C.

Additionally, other solvents or solvent mixtures may be used in combination with a substantially chlorinated hydrocarbon such as methylene chloride. For example, it may be desirable to substitute for chloroform in the chloroform-methylene chloride mixtures described above because of the health hazards of chloroform. Suitable solvent or solvent mixtures to substitute for chloroform may be selected by matching the Hansen solubility parameters (Hansen, Hansen Solubility Parameters, A User's Handbook, CRC Press LLC, Boca Raton, Fla., 2000) of particular solvent or solvent mixtures to those of chloroform. The Hansen solubility parameters are an extension of the Hildebrand solubility parameters. According to Hansen, "the basis for the Hansen Solubility Parameters (HSP) is that the total energy of vaporization of a liquid consists of several individual parts, that arrive from (atomic) dispersion forces, (molecular) permanent dipole-permanent dipole forces and (molecular) hydrogen bonding (electron exchange)." Materials having similar HSP have high affinity for each other. The basic equation for the HSP is that the total cohesion energy, E, must be the sum of the individual energies: E=E.sub.D+E.sub.p+E.sub.H Where E.sub.D is the Hansen dispersion cohesion energy, E.sub.P is the Hansen polarity cohesion energy, and E.sub.H is the Hansen hydrogen bonding cohesion energy. Dividing this expression by the molar volume, gives the total Hildebrand solubility parameter as the sum of the squares of the Hansen components: .delta..sup.2=.delta..sub.D.sup.2+.delta..sub.P.sup.2+.delta..sub.H.sup.2 Chloroform has a Hansen dispersion of 17.8, Hansen polarity of 3.1 and Hansen hydrogen bonding of 5.7 in units of the square root of megapascals (mPa.sup.1/2). A software program (Molecular Modeling Pro Plus, ChemSW, Fairfield, Calif.) is available to calculate the Hansen solubility parameters from molecular structure. Preferred solvent mixtures have a sum of the differences (in absolute value) in Hansen solubility parameters relative to the Hansen solubility parameters of chloroform of less than about 0.21. A sample calculation of the sum of the differences in Hansen solubility parameters for a mixture of 30 vol % (percent by volume) ethyl heptanoate and 70 vol % phenethyl acetate relative to chloroform is shown in Table A (see Original Patent). Suitable solvent mixtures are given in Table B (see Original Patent).

In one embodiment, the second solution comprises a combination of a solvent mixture of 30 vol % ethyl heptanoate (CAS No. 106-30-9) and 70 vol % phenethyl acetate (CAS No. 103-45-7), with methylene chloride in a volume ratio of about 20:1 to about 1:20, in addition about 5:1 to about 1:5, and further in addition of about 3:1.

The second solution also comprises a viscosity modifying component that provides a surface tension that allows droplet formation in the aqueous/organic suspension formed during the present microsphere preparation process. This viscosity modifying component is again called a "protecting colloid". A variety of natural and synthetic compounds soluble in organic media may be used as a protecting colloid, including, but not limited to, cellulose derivatives, polyacrylates (such as polyacrylic acid and polymethacrylic acid), polyalkylene glycols such as polyethylene glycol, partially hydrolyzed polyvinyl alcohol and other polyols, guar gum, and agar gum. Particularly useful are cellulose ethers such as methyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, ethylhydroxyethyl cellulose, hydroxypropyl cellulose, ethyl cellulose, and benzyl cellulose; as well as cellulose esters such as cellulose acetate, cellulose butylate, cellulose acetate butylate, cellulose propionate, cellulose butyrate, cellulose acetate propionate, cellulose acetate butyrate, and cellulose acetate phthalate. The amount of the protecting colloid in the organic second solution is sufficient to reduce microdroplet coalescence in the aqueous/organic suspension, and is generally between about 0.5% and about 5% by weight % of the organic second solution. Particularly suitable is ethyl cellulose at about 1.5% by weight.

Process for Microsphere Preparation

The first solution and the second solution are combined with agitation to form a first suspension. The second solution is used in an amount that is adequate to form a good suspension, while the amount may be as great as is practical. Generally the volume ratio of second to first solutions is in the range of about 10:1 to about 2:1. Preferably the volume ratio of second to first solutions is in the range of about 6:1 to about 4:1.

The first and second solutions may be combined in any order. Specifically, the first solution can be added to the second solution, the second solution can be added to the first solution, or the two solutions can be combined simultaneously. Preferably, the first solution is added to the second solution. During the combination of the first and second solutions, the resulting mixture is agitated at a rate capable of forming a uniform suspension from the two solutions. Agitation may be by any method which thoroughly mixes the two solutions, such as shaking or stirring. Typically, the second solution is stirred in a container while the first solution is poured into the same container. The combined first and second solution is agitated at a temperature that is below the azo initiation temperature (and above the freezing point of the solution) to form a uniform, first suspension. Generally the temperature is below about 50.degree. C., and more typically is below about 40.degree. C. A temperature that is below about 30.degree. C. is preferred. Typically the first suspension is stirred at about 100 to 600 rpm, depending on the size of the container, at room temperature for about one-half to one hour.

The agitation of the first suspension allows formation of small droplets in the suspension. The size of the forming droplets, and therefore the size of the microspheres that are produced, is related to the rate of agitation. As the agitation is reduced, droplets coalesce. Agitation is maintained at a rate sufficient to reduce droplet coalescence allowing the formation of micron sized microspheres. For example, for the formation of microspheres in the size range of 40 to 500 microns, stirring is typically about 150-250 rpm when using a one liter container. The optimum agitation rate for any particular system will depend on many factors, including the particular monomer, crosslinking agent, and solvent system used, the geometry of the container, the geometry of the agitator, and the desired microsphere properties for the intended application. For example, the size of the microspheres depends on the agitation rate. In general, larger microspheres are obtained at lower agitation rates. The agitation rate for any given conditions can be readily optimized by one skilled in the art using routine experimentation.

After the formation of the first suspension, a low level of heat is applied such that the temperature of the first suspension is brought to a temperature that is below the boiling temperature of the first solution, and below or at the boiling temperature of the second solution. Typically the temperature is between about 50.degree. C. and 55.degree. C., depending on the mixture of the second solution. It is preferred to bring the temperature of the first suspension made with a chloroform and methylene chloride ratio of about 3:1 to about 51.degree. C. to 52.degree. C. At this temperature the low temperature azo initiator is activated. The first suspension is agitated until it forms a second suspension comprising a precipitate of gelatinous microspheres in the suspending medium, which is predominantly an organic liquid phase. The gelatinous precipitate appears as a milky material which falls out of the suspension. Additionally, a white foam may be seen on top of the second suspension. Typically stirring of the first suspension to form the second suspension at the elevated temperature is for about 8-10 hours. The second suspension is agitated for another period of time at room temperature to ensure that the polymerization and microsphere formation is complete. During this time the second suspension cools to a temperature which is easily handled. Generally this is at or below about 30.degree. C. Room temperature, typically at about 25.degree. C., is conveniently used. Typically stirring remains at about 150-250 rpm, when using a one liter container, for about 8-14 hours.

Agitation is ceased, allowing the formed microspheres to settle to the bottom of the container. Removing the water from these hydrogel microspheres may be accomplished by washing with a dehydrating solvent such as methanol, ethanol, or acetone. Particularly useful is methanol, which is added, and the mixture is optionally agitated gently for about an hour to allow good solvent exchange. The microspheres are then recovered by a method such as by decanting or filtering, and may be washed a second time with methanol and again recovered. With removal of the water, the microspheres change in appearance from milky and gelatinous to hard and opaque white. The microspheres finally may be washed in ethanol, which is desirable for removal of residual methanol, particularly for microsphere use in medical applications. The washed microspheres in ethanol form one type of microsphere slurry. The microspheres optionally may be dried to form a powder of microspheres. Drying rids the microspheres of remaining washing solvent and additional water. Drying may be by any standard method such as using air, heat, and/or vacuum. Particularly useful is drying under vacuum in a vacuum oven set at about 20.degree. C. to about 100.degree. C. with a nitrogen purge. The use of lower drying temperatures requires longer drying times. For preparation of highly swellable microspheres, drying at room temperature (i.e., about 20.degree. C. to about 25.degree. C.) under vacuum with a nitrogen purge is preferred (see Example 34). A small amount of water generally remains in the microspheres after drying. The amount of remaining water may be about 1% to 10% of the microsphere total weight. The resulting microsphere preparation, though retaining a small amount of water in the microspheres, flows when tilted or swirled in a container and thus forms a free-flowing microsphere powder.

Microsphere Physical Properties

Microspheres prepared according to the present process are substantially spherical. A population of microspheres has sphericity measurements centered near about 95%, within a range of about 80% to about 100%. The population may include some individual microspheres which have a lower sphericity measurement, while maintaining the high sphericity measurement for the population as a whole.

The microspheres are in the size range of about 10 to about 730 microns in diameter, in addition from about 14 to about 730 microns in diameter. A prevalence of the microspheres are in the size range of about 25 to about 250 microns in diameter, as seen when analyzing a small sample size of microspheres. A heterogeneous size mixture of microspheres may be separated into microsphere samples of specific size ranges, if desired, for specific applications. Microspheres may be separated by methods such as fluidized bed separation and sieving, also called screen filtering. Particularly useful is sieving through a series of sieves appropriate for recovering samples containing microspheres of desired sizes. For example, separate samples of microspheres may be obtained using a series of sieves with mesh sizes of 35 to 400 microns. Separate microsphere samples may be obtained that have diameters ranging between about 30 and about 44 microns; about 115 and about 165 microns; about 180 and about 330 microns; and with size ranges also falling between and outside of these exemplary groups. These samples of size separated microspheres exemplify the production of microsphere preparations having a predominant size ranging between about 30 microns and 600 microns in diameter and including microspheres in a size range that is generally within +/-30% of the median for about 90% of the sample. Microsphere preparations may be produced having microspheres in a size range that is generally within +/-20% of the median for about 90% of the sample.

The microspheres prepared according to the present process have a high density, yet a high capacity for swell. The microspheres have low porosity, especially as compared to the microspheres described in U.S. Pat. No. 6,2184,40, as viewed by scanning electron microscopy (SEM). The microspheres of U.S. Pat. No. 6,218,440 have cavities joined by interconnecting pores wherein at least some of the cavities at the interior of the material communicate with the surface of the material. These microspheres have pores throughout and a rough porous surface as well. The microspheres prepared by the present process have by comparison a relatively small number of voids embedded within a solid material. Generally, although not invariably, these voids are closed cell voids that are not interconnected to each other or to the surface of the microsphere. The surface of the present microspheres is generally smooth and rounded, although some surface imperfections may be present. The porosity of microspheres can also be assessed by density measurements. One preparation of microspheres prepared by the present process had a bulk density of 0.68 g/cm.sup.3 (Example 6). It is expected that microspheres produced by the present process will have a bulk density of at least about 0.5 g/cm.sup.3. In contrast, the bulk density of microspheres prepared according to one method in the prior art (U.S. Pat. No. 6,218,440 Example 2) was measured to be 0.182 g/cm.sup.3 (see present Example 6). Individual microsphere density of the present microspheres is between about 0.9 g/cm.sup.3 and about 2 g/cm.sup.3, while the individual density of microspheres prepared according to one method in the prior art (U.S. Pat. No. 6,218,440 Example 2) was measured to be 0.8 g/cm.sup.3 or less (see present Example 6).

The density, and porosity, of microspheres play a role in the durability of the microspheres. The microspheres prepared by the present process are highly durable in that the swelled microspheres have substantial resistance to fracture as they are passed through a small bore needle. Under the same conditions, swelled highly porous microspheres do fracture, and thus have low durability. For example, microspheres prepared by the present process that are swelled and passed through a 20 gauge needle maintain an average diameter similar to that of the starting sample, while the average diameter of microspheres prepared according to one method in the prior art (U.S. Pat. No. 6,218,440 Example 2) after passing through a 20 gauge needle is reduced by almost half indicating fracture of the particles.

As described above, the swell capacity (amount of water uptake) of microspheres prepared by the present process may vary depending on the amount of crosslinking agent added to the first solution. For example, crosslinking agent may be added in such an amount as to impart a swell capacity to the microspheres of about 50 grams of water per gram of microspheres, an amount to impart a swell capacity of about 70 grams per gram of microspheres, and alternatively an amount to impart a swell capacity of about 100 grams per gram of microspheres. Particularly suitable is a microsphere preparation having at least 70 grams of water uptake per gram of microsphere powder. Using the same amount of crosslinking agent, microspheres prepared according to one method in the prior art (U.S. Pat. No. 6,218,440 Example 2) had less than half of this swell capacity (see Example 6, Table 7 (see Original Patent)).

The microspheres made by the present process exhibit rapid swell. Individual microspheres can be seen to reach a maximum size within about 15 seconds of contacting the microspheres with water. Thus the individual microspheres reach their full swell capacity within about 15 seconds, and typically within about 10 seconds. A population of microspheres also has rapid swell as long as each microsphere has sufficient exposure to water. In general, when contacting a population of microspheres with water, those microspheres in the center of the population, or on the bottom of a container, do not have full exposure to water so that their swell time is longer. For example 1 gram of microspheres may reach 50% of full swell in 5 seconds and about 70% of full swell in 10 seconds with water exposure as described in the General Methods. Generally full swell is reached within 30 seconds for a population of microspheres under the described water contact conditions.

An additional attribute of the microspheres prepared by the present process is the capacity to deform following swell. When placed under pressure, the swelled microspheres do not maintain their substantially spherical shape, but compress in the axis of the pressure and expand in the axis that is perpendicular to the pressure. Thus environmental factors, such as pressure of a flowing medium or from the walls of an enclosing container, may cause deformation of the microspheres. In addition, pressure of individual microspheres next to each other may cause deformation. This ability to deform is thought to be imparted and enhanced through the closed cell void structure of the microspheres. While not wishing to be bound by theory, it is thought that the closed cell voids are able to compress allowing the swelled hydrogel in the microspheres to deform maximally.

This ability to deform allows the microspheres to take on a shape of a containing space, and to fill that space. Additionally, deformed microspheres have increased surface area contact with each other, as compared to the contact area between spherical beads. The increased surface area contact between the deformed microspheres provides a more compact structure than is achievable with non-deforming spherical microspheres. This compact structure provides high resistance to penetration. The deformability is highly desirable in some applications such as in embolization treatment, where the deformed, compact microspheres may provide strong blockage at target vascular sites. In a test system that uses a flexible, substantially nonexpendable tubing having an internal diameter of 1.58 mm, the swelled, deformed microspheres were able to form occlusions that withstood very high pressures. For example, 15 mg of dry microspheres, when fully swelled, formed microsphere occlusions that were not dislodged by water pressure less than about 114 mm Hg (15.2 kilopascals (kPa)). Starting with 18 mg of dry microspheres the occlusions formed were dislodged by water pressure at about 570 mm Hg (76.0 kPa), and starting with 20 mg of dry microspheres the occlusions formed withstood over 1,000 mm Hg of pressure (133 kPa).

Microsphere Properties Advantageous for Medical Applications

Microspheres prepared by the present process are biocompatible in that they lack cytotoxicity, are non-inflammatory, and are non-hemolytic. The microspheres have a swell response in whole blood that is similar to the swell response in water: achieving up to 100-fold swell within seconds. These properties allow the microspheres described herein to be used in medical applications, where advantage may be taken of their full swell potential. In addition, the resistance to fracture of the microspheres described herein makes them particularly suitable for embolization since resistance to fracture reduces the potential for effects such as embolism downstream of the target site, unwanted inflammatory response, exacerbation of clotting cascade, and loss of therapeutic occlusion.

The Microsphere Preparation

The present microsphere preparation is prepared according to the present process and contains swellable/deformable microspheres having the properties described herein. The microsphere preparation may be the direct product of the process prior to drying, where the microspheres form a microsphere slurry including extraction solvent. The additional drying step of the process produces the microsphere powder of the present invention. The microsphere powder may be made available for use as a powder or for addition of a liquid appropriate to the intended use. Addition of a liquid to the microsphere powder produces a microsphere slurry or microsphere suspension. Liquids used in a microsphere suspension may be any that are appropriate for the intended use. For example, a biocompatible liquid that controls swell is used to suspend microspheres for medical uses, such as tissue augmentation, wound treatment, and embolization. Typical swell-control biocompatible liquids include, for example, propylene glycol, dimethylsulfoxide (DMSO), Ethiodol.RTM., MD-76.RTM., and mineral oil. Ethiodol.RTM. and MD-76.RTM. are contrast agents typically used in medical intravascular arteriography or lymphography procedures. Ethiodol.RTM. contains iodine organically combined with ethyl esters of the fatty acids of poppyseed oil and is available from SAVAGE Laboratoriese.RTM. (Melville, N.Y.). MD-76.RTM. is an aqueous solution of diatrizoate meglumine (CAS No. 131-49-7, 66 wt %) and diatrizoate sodium (CAS No. 737-31-5, 10 wt %) buffered with monobasic sodium, with a pH of 6.5 to 7.7, having organically bound iodide to provide for radiological visualization. MD-76.RTM. is manufactured by Mallinckrodt Inc. (St. Louis, Mo.).

Embolization Suspension

A microsphere preparation made according to the present process is used to prepare a suspension for embolization treatment, herein called an "embolization suspension". Sterility is an important factor in embolization treatment. The described microsphere preparation process including a final ethanol wash, provides a sterilization treatment. Further sterilization may be performed by extending the ethanol wash for a long period of time, such as overnight. Sterility may be enhanced by using additional measures such as carrying out the process for making the microspheres in a sterile environment, and treating the microsphere preparation with UV light, ethylene oxide or gamma radiation, as is known to one skilled in the art.

The embolization suspension includes a biocompatible carrier. The carrier provides not only a medium to suspend and administer the microspheres, but also to control the swelling of the microspheres. Typically, the carrier used in the suspension has a low enough viscosity to allow delivery of the microspheres through small-bore needles and catheters, such as those of 20 gauge or 7 French (F) or smaller. A gauge measurement is used for needles, while a French measurement is used for catheters, both of which designate the outside diameter. The inside diameter of a needle or catheter is related to the outside diameter, but also depends on the thickness of the wall and so can vary between manufacturers. Thus precise measurements of the inside diameters of needles and catheters are not specified by the gauge or French unit. However, inside bore diameters of specific catheters and needles are known or can readily be obtained by one skilled in the art. Biocompatible carriers that limit swell of the microspheres, and thus are swell-control media, include the commonly used contrast agents Ethiodol.RTM. (SAVAGE Laboratories.RTM., Melville, N.Y.) and MD-76.RTM. (Mallinckrodt Inc., St. Louis, Mo.). In addition, swell may be controlled by salt concentration and ionic strength, as well as with pH. The organic polar solvent dimethylsulfoxide (DMSO) was found, as described in examples herein, to be a useful medium for controlling microsphere swelling, and for making an embolization suspension for administering the microspheres. Microspheres suspended in DMSO at concentrations between about 60% and 100% undergo appreciably no swell. Particularly suitable biocompatible carriers are those containing DMSO above about 60% concentration, those with an acidic pH, and contrast agents. The contrast agent MD-76.RTM. allows some swell, ranging between about 3.5.times. and about 7.5.times. original volume, and may be used as a swell-control medium. Different carriers may be mixed, such as combining a percentage of DMSO and a contrast agent to establish the desired amount of miscosphere swell (explained below) in the embolization suspension.

The microsphere concentration in the embolization suspension varies depending on the carrier used and the size catheter to be used for administering the suspension, which in turn depends on the size of the vasculature to be embolized. In addition, the size of the microspheres affects the concentration used, where samples of different sized microspheres may be prepared, for example by sieving, as described herein. For example, 250 mg/mL concentration of approximately 250 micron microspheres in DMSO (no swell) may be used with catheters of 6 F and larger. For delivery of high concentrations of microspheres with smaller catheters, such as 5 F and smaller, it may be desirable to have limited swell of the microspheres for administering the microsphere suspension. The limited swell may take place prior to or during the administering. The limited swell may be up to about 10.times. the original volume of the microspheres. Limited swell provides deformability of the microspheres which allows them to pass through small diameter catheters and needles. Limited swell may be achieved by methods such as adjusting the salt concentration, pH or DMSO concentration of the carrier, or with use of a contrast agent. For example, with about 50% or less DMSO concentration in the carrier, the microspheres begin to swell. In addition, the microspheres may swell to between about 3.5.times. and 7.5.times. the original volume in contrast agent. Passage through 5 F catheters may be achieved with suspensions containing, for example, 150 mg/mL of 250 micron microspheres in MD-76.RTM.. Also, embolizing suspensions containing 300 mg/mL of 50-150 micron microspheres in MD-76.RTM. can pass through a 5 F catheter. The specific size and concentration of microspheres, as well as the desired carrier, may be chosen by one skilled in the art for the particular embolization treatment to be performed.

Embolization Treatment

In the present method for embolization in a mammal, microspheres prepared by the present process are administered to the mammal. These microspheres may be delivered through smaller catheters than expected and form more durable occlusions than expected, as described herein. The specific method by which the microspheres are administered is not limiting and is performed as is known to one skilled in the art, for example as described in "Uterine Artery Embolization and Gynecologic Embolotherapy", Spies and Pelage, 2005 ISBN: 0-7817-4532-2 and in "Vascular and Interventional Radiology: Principles and Practice", Bakal et. al, 2002 ISBN: 0-86577-678-4. Administration of an embolization suspension containing microspheres prepared by the present process is generally by passage through a catheter or needle into the vasculature of the mammal such that the microspheres reach a target site. As the embolization suspension contacts the blood in the vasculature, the swellable/deformable microspheres swell and form an occlusion. The occlusion effectively blocks the blood flow distal to the occlusion site. The occlusion site may be any target site where, for medical treatment, it is desired to block the flow of blood. For example, the occlusion site may be in a blood vessel that feeds a tumor such as a uterine fibroid or a cancerous tumor, in an arteriovenous malformation, or in a blood vessel where the blood is not contained, such as in the case of a stomach ulcer or injury. Preoperative embolization may also be performed to stop blood flow to a region targeted for surgery.

The swellable/deformable microspheres, made by the present process, surprisingly were found to provide more readily administered and more durable occlusion than expected and as previously described in the art. The functional embolic load, which is defined as the amount of dry microsphere mass required to establish an occlusion when reconstituted in blood substitute in a fixed diameter tubing, is lower than expected. Thus smaller catheters than expected may be used to deliver an effective functional embolic load of the swellable/deformable microspheres, as compared to treatment with microspheres known in the art. The delivery of swellable/deformable microspheres in small catheters, such as 5 F and 3 F catheters, allows better placement of microspheres by using the smaller catheters to more closely reach a target site. Use of smaller catheters also reduces the risk of vasospasm, caused by catheter irritation, which can result in abortion of the procedure or spurious end-point occlusions that disintegrate after vasospasm resolution. The use of small catheters is also enabled by the deformability of the microspheres, as described herein. Also due to their deformability the microspheres are capable of deep vessel penetration prior to occlusion, as seen in the porcine renal vasculature embolization in the Examples, below. The deep penetration also leads to enhanced packing of microspheres within the vessels, leading to robust occlusions.

The microspheres prepared as described herein, producing the microsphere powder of the invention, form more durable occlusions than expected. This feature is most likely attributable to the swellable and deformable properties of the microspheres in the microsphere powder. The swelling of the microspheres within the elastic environment of the target artery creates a recoil force from the arterial wall which in turn deforms the microspheres, thereby creating a packed occlusion with maximal surface contact between deformed, swelled microspheres. The tight packing of the swelled, deformed microspheres was shown with in vivo studies of embolization in porcine renal vasculature. In addition, dislodging pressures were measured for swelled, deformed microsphere occlusions in vitro. The in vitro system used a 1.58 mm internal diameter tubing, which is of larger diameter than typical of target vessels and therefore more difficult to block. The tubing used for measuring the occlusion durability was flexible, and non-expandable at the test pressures used. Tubing such as Tygon.RTM. tube, specifically Beverage tubing (B-44-3) with an internal diameter of 1.58 mm, outer diameter of 4.76 mm and tubing wall thickness of 1.59 mm, is particularly suitable for the assay. Occlusions of microspheres prepared according to the present process withstand physiological blood pressures, and much higher pressures. For example, 15 mg (measured dry mass) of microspheres of size 250-500 micron diameter form occlusions that withstand on average about 125 mm Hg (16.7 kPa), 18 mg of microspheres of size 250-500 micron diameter form occlusions that withstand on average about 600 mm Hg (80.0 kPa), and 20 mg of size 250-500 micron diameter microspheres form occlusions that withstand on average over 1,000 mm of Hg (133 kPa) in pressure.

The amounts of microspheres, prepared by the present process, that are capable of forming durable occlusions can easily be delivered through microcatheters. Suspensions of up to about 10 mg/mL of unswelled microspheres may be delivered through a 5 F catheter. Thus, less than 2 mL of a 10 mg/mL suspension of the present microspheres can form a durable occlusion as described above. The microspheres in a DMSO medium were also able to pass through a 3 F microcatheter when at a concentration of 10 mg/mL and 30 mg/mL.

Some typically used media, such as Ethiodol.RTM., are too viscous to pass through microcatheters. Thus alternative media with low viscosity, yet the ability to limit microsphere swell, are used in embolization suspensions delivered with microcatheters. The small volume of microspheres, prepared by the present process, that is delivered for forming an occlusion allows the use of suspension media that may cause ill effects when injected into a mammal in larger volumes. For example, media including more than 60% DMSO or media with a pH in the range of 2-3 may be used in embolization suspensions to control microsphere swelling. Several milliliters of these media may be administered to deliver an adequate amount of microspheres to form an occlusion. Additionally, for minimizing effects of potentially harmful media, a buffering solution may be administered prior to and after administering the embolization suspension. Phosphate buffered saline or bicarbonate buffer are examples of solutions that may be used in this manner.

In addition to forming an occlusion, the present microspheres used in embolization may also be prepared such they are able to deliver medications, such as pharmaceutical drugs or therapeutic agents. The medication may be loaded into the microspheres using various methods known in the art. For example, the microspheres may be imbibed with the medication by swelling the microspheres in a medium containing the medication and allowing it to soak into the microspheres. The microspheres may then be dried or deswelled by removing water by washing with a dehydrating solvent, as described above. Additionally, the medication may be coated onto the microspheres using methods such as spraying, immersion, and the like. The medication may also be directly incorporated into the microspheres during their preparation by adding the medication to the first solution. Following delivery of the microspheres containing the medication to the target site, the pharmaceutical drug or therapeutic agent is released over time as the microspheres are in contact with body fluids. For example, anti-cancer drugs may be delivered by microspheres forming an occlusion in proximity to a cancerous tumor. Delivery in embolizing microspheres of agents such as anti-angiogenic factors, anti-inflammatory drugs, analgesics, and local anesthetics provide additional treatment to the physical blockage of embolization. Additional therapeutic agents that may be delivered in the present microspheres are described in WO 01/72281, which is herein incorporated by reference.

Kits

Further provided is a kit including swellable/deformable microspheres prepared according to the process described herein, which may be in a slurry, a powder, or a suspension. The quantity of microspheres in the kit may vary depending on the specific intended application. For example, as described above, an occlusion may be formed by different quantities of the present microspheres such as 15 mg, 18 mg, or 20 mg. The quantity of microspheres will depend on factors such as the diameter of the site to be occluded and the pressure that the occlusion must withstand. Particularly useful is a kit containing swellable/deformable microspheres, prepared according to the process described herein, in a suspension and optionally a syringe and/or a catheter for use in administering the microspheres. The kit may also include a biocompatible carrier. Typically included in a kit are instructions for use of the components.
 

Claim 1 of 29 Claims

1. A method for embolization in a mammal comprising administering into the vasculature of said mammal microspheres prepared by a process comprising: a) forming a first solution comprising: (i) water; (ii) at least one water miscible monomer selected from the group consisting of acrylic acid, methacrylic acid, salts of acrylic acid and methacrylic acid, 2-hydroxyethyl acrylate, and 2-hydroxyethyl methacrylate, provided that: (A) if said monomer is 2-hydroxyethyl acrylate, or 2-hydroxyethyl methacrylate, said monomer is used in combination with at least one other monomer selected from subgroup 1 consisting of: acrylic acid, methacrylic acid, and salts of acrylic acid and methacrylic acid; (B) if said first solution contains at least one monomer from subgroup 2 consisting of acrylic acid, methacrylic acid, salts of acrylic acid and methacrylic acid, 2-hydroxyethyl acrylate, and 2-hydroxyethyl methacrylate, then the pH of the first solution is at least about 3; (C) if said first solution contains at least one monomer from subgroup 3 consisting of 2-acryloylethane-sulfonic acid, 2-methacryloylethane-sulfonic acid, salts of 2-acryloylethane-sulfonic acid and 2-methacryloylethane-sulfonic acid, styrene-sulfonic acid, and salts of styrene-sulfonic acid, then the pH of the first solution is less than 3; (iii) a crosslinking agent that is miscible in the first solution in less than or equal to about 5 mol %, relative to total moles of monomer and crosslinking agent, said crosslinking agent being selected from the group consisting of N,N' -methylene-bis-acrylamide, N,N' -methylene-bis-methacrylamide, N-methylolacrylamide, N-methylolmethacrylamide, glycidyl acrylate, glycidyl methacrylate, polyethylene glycol diacrylate, polyethylene glycol dimethacrylate, polyvalent metal salts of acrylic acid and methacrylic acid, divinyl benzene phosphoacrylates, divinylbenzene, divinylphenylphosphine, divinyl sulfone, 1,3-divinyltetramethyldisiloxane, 3,9-divinyl-2,4,8,10-tetraoxaspiro[5,5]undecane, phosphomethacrylates, ethylene glycol diglycidyl ether, glycerin triglycidyl ether, glycerin diglycidyl ether, and polyethylene glycol diglycidyl ether; (iv) a water soluble protecting colloid; (v) an emulsifier; and (vi) a low temperature aqueous soluble azo initiator; b) forming a second solution comprising at least one substantially chlorinated hydrocarbon of less than 6 carbon units, provided that the chlorinated hydrocarbon is not a halogenated aromatic hydrocarbon, and an organic soluble protecting colloid; c) forming a first suspension with agitation comprising the first and second solutions at a temperature below the initiation temperature of the azo initiator of (a); d) increasing the temperature of the agitating first suspension to a temperature at which the low temperature aqueous soluble azo initiator is activated; e) agitating the first suspension until it forms a second suspension comprising a gelatinous precipitate suspended in an organic liquid phase, wherein microspheres are formed; f) allowing the second suspension to cool to a temperature that is at or below about 30.degree. C. while agitating the second suspension; g) washing the second suspension at least once with a dehydrating solvent wherein water is removed from the microspheres forming a microsphere preparation; h) recovering the microsphere preparation; and i) drying the microsphere preparation to form a free-flowing microsphere powder.
 

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