Abstract:
Methods are disclosed for the use of apatite chromatography, particularly without reliance upon phosphate gradients, for fractionation or separation of phosphorylated and nonphosphorylated biomolecules. Integration of such methods into multi-step procedures, with other fractionation methods are additionally disclosed.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. provisional applications Ser. No. 61/011,513 filed Jan. 18, 2008; 61/062,663 filed Jan. 28, 2008; 61/069,859 filed Mar. 19, 2008; 61/070,841 filed Mar. 27, 2008; 61/135,787 filed Jul. 24, 2008; 61/189,467 filed Aug. 20, 2008, each of which are expressly incorporated herein by reference in their entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates in certain embodiments to methods for enhancing the fractionation or purification of phosphorylated and nonphosphorylated biomolecules by apatite chromatography in the presence of one or more of borate compounds, sulfate compounds, monocarboxylate compounds, and/or in the presence of calcium compounds. In certain embodiments, the invention may permit more effective removal of phosphorylated contaminants from nonphosphorylated products. In other embodiments, the invention may permit more effective removal of nonphosphorylated contaminants from phosphorylated products. In these or other embodiments, the invention may improve pH control during fractionation. 
       BACKGROUND OF THE INVENTION 
       [0003]    Hydroxyapatite [HA] is a crystalline mineral of calcium phosphate with a structural formula of Ca 10 (PO 4 ) 6 (OH) 2 . Fluorapatite may be prepared by fluoridating hydroxyapatite, creating a mineral with the structural formula Ca 10 (PO 4 ) 6 F 2 . Protein-reactive sites on both minerals include pairs of positively charged calcium ions (C-sites) and triplets of negatively charged phosphate groups (P-sites). C-sites interact with proteins via HA calcium chelation by protein carboxyl clusters. C-sites interact with phosphorylated solutes such as DNA, endotoxin, phosphoproteins, and lipid enveloped viruses via HA calcium coordination by solute phosphate residues. Calcium chelation and coordination are sometimes referred to as calcium affinity. P-sites interact with proteins via phosphoryl cation exchange with positively charged protein amino acid residues (Gorbunoff,  Analytical Biochemistry  136 425 (1984); Kawasaki,  J. Chromatography  152 361 (1985)). Hydroxyapatite is most commonly eluted with phosphate gradients. The strong calcium affinity of phosphate suspends calcium chelation and coordination interactions, while its ionic character suspends phosphoryl cation exchange interactions. Some applications elute hydroxyapatite with combinations of phosphate and chloride salts. Chlorides preferentially elute the phosphoryl cation exchange interaction while having relatively little effect on calcium affinity interactions. (Gagnon et al,  Bioprocess International,  4(2) 50 (2006)). 
         [0004]    Native hydroxyapatite and fluorapatite can be converted to calcium-derivatized forms by exposure to soluble calcium in the absence of phosphate. (Gorbunoff,  Anal. Biochem.,  136 425 (1984)). This converts P-sites into secondary C-sites, abolishing phosphoryl cation exchange interactions, increasing the number of C-sites, and fundamentally altering the selectivity of the apatite support. Small alkaline proteins typified by lysozyme (13.7-14.7 Kda, pI 10.7) and ribonuclease (14.7 kDa, pI 9.5-9.8) fail to bind to calcium-derivatized apatites, but most other proteins bind so strongly that even 3 M calcium chloride is inadequate to achieve elution (Gorbunoff). Other chloride salts also fail to achieve elution. Calcium-derivatized apatites are restored to their native forms by exposure to phosphate buffer, at which point they may be eluted by methods commonly applied for elution of native apatite supports. 
         [0005]    The effects of different salts on the selectivity of a given apatite are unpredictable. For example, in the absence of phosphate, sodium chloride is unable to elute most IgG monoclonal antibodies from native hydroxyapatite, even at concentrations in excess of 4 moles per liter (Gagnon et al, 2006,  Bioprocess International,  4(2)50). This implies extremely strong binding. In exclusively phosphate gradients, IgG is typically one of the latest eluting proteins, usually requiring 100-150 mM phosphate. This also implies strong binding. When eluted with a combination of lower concentrations of both salts, such as 0.25 M sodium chloride and 50 mM phosphate however, IgG is one of the earliest eluting proteins. Other paradoxes reinforce the point: increasing the sodium chloride concentration in the presence of phosphate, which causes IgG to bind less strongly, has the opposite effect on DNA (Gagnon et al, 2005,  Bioprocess International,  3(7) 52-55). Additionally,  Bovine  serum albumin (BSA) elutes at about 100 mM phosphate without respect to sodium chloride concentration; and lysozyme elutes at a higher phosphate concentration than BSA in the absence of sodium chloride but fails to bind in the presence of 1 M sodium chloride. 
         [0006]    Ammonium sulfate, sodium sulfate, and other sulfate salts are commonly used for precipitation of proteins, or to cause proteins to bind to hydrophobic interaction chromatography media. They can also be used to enhance binding with biological affinity chromatography media such as protein A, and have even been reported to cause proteins to bind to ion exchangers (Gagnon, 1996,  Purification Tools for Monoclonal Antibodies , ISBN 0-9653515-9-9; Mevarech et al, 1976,  Biochemistry,  15, 2383-2387; Leicht et al, 1981,  Anal. Biochem.,  114, 186-192; Arakawa et al, 2007,  J. Biochem. Biophys. Met.,  70, 493-498). Sulfates have occasionally been reported for elution of ion exchangers at low concentrations for research applications but are seldom exploited in preparative applications due to concerns over protein precipitation (Kopaciewicz et al, 1983,  J. Chromatogr.,  266 3-21; Gooding et al, 1984,  J. Chromatogr.,  296, 321-328; Rounds et al, 1984,  J. Chromatogr.,  283 37-45). None of these methods is an appropriate model for apatites because none of them exploits calcium affinity for binding. 
         [0007]    Several authors have concluded that, “The presence of . . . (NH 4 ) 2 SO 4  seems not to affect the elution [of hydroxyapatite].” (Karlsson et al, 1989, in  Protein Purification: Principles, High Resolution Methods, and Applications , Chapter 4, ISBN 0-89573-122-3). Even this reference mentions the application of sulfate strictly in the context of phosphate gradients. In the rare cases where alternatives to phosphate as a primary eluting salt have been discussed in the literature, suggestions have included calcium chloride, citrate and fluoride salts, but without mention of sulfates (Gagnon, 1996; Karlsson et al, 1989; Gorbunoff). Other publications indicate that sulfate salts in particular should be unsuitable as primary eluting agents for hydroxyapatite because “ . . . SO 3 H do[es] not form complexes with calcium.” (Gorbunoff). 
         [0008]    Borate salts have been likewise overlooked. Borate is occasionally used in the field of chromatography as a buffering agent at pH values from about 8.8 to 9.8 (pK ˜9.24). It is also used infrequently at alkaline pH to modify the charge characteristics of cis-diol compounds to selectively enhance their retention on anion exchangers. In contrast to phosphates, chlorides, and sulfates, all of which exhibit molar conductivities of about 90 mS/cm, a 1 M solution of borate at pH 7 has a molar conductivity of about 9 mS. 
         [0009]    Acetates have been compared to chlorides for hydroxyapatite separation of IgG from aggregates and were found to support inferior fractionation (Gagnon et al, Practical issues in the industrial use of hydroxyapatite for purification of monoclonal antibodies, Poster, 22 nd  national meeting of the American Chemical Society, San Francisco, Sep. 10-14, 2006 &lt;http://www.validated.com/revalbio/pdffiles/ACS_CHT — 02.pdf&gt;. Monocarboxylic acid salts have been otherwise neglected, and the elution potential of monocarboxylic zwitterions totally so. 
         [0010]    Hydroxyapatite is used for purification of a wide variety of biomolecules, including proteins, phosphoproteins, carbohydrates, polynucleotides, and viral particles. The column is usually equilibrated and the sample applied in a buffer that contains a low concentration of phosphate. Adsorbed biomolecules are usually eluted in an increasing gradient of phosphate salts. Alternatively, some biomolecules may be eluted in an increasing gradient of chloride salts, but both elution formats impose disadvantages on purification procedures. The high phosphate concentration in which antibodies elute in phosphate gradients has strong buffer capacity that may interfere with subsequent purification steps. The high conductivity at which some biomolecules elute in chloride gradients may also interfere with downstream steps. Both situations require either that the eluted biomolecule be diluted extensively, or that it be buffer-exchanged, for example by diafiltration, in order to modify the conditions to render the preparation suitable for application to a subsequent purification step. Dilution and buffer exchange have a negative impact on process economics. As a result, apatite chromatography steps are often placed at the end of a purification process. This tends to eliminate them from consideration as capture steps. It also discourages the use of HA as an intermediate step. A further disadvantage of chloride gradients is that the application of chloride to hydroxyapatite causes an uncontrolled reduction of pH. Acidic pH causes destruction of hydroxyapatite and risks adverse affects to biomolecules bound to it. 
         [0011]    Another limitation of hydroxyapatite with biomolecule purification is that the binding capacity for some biomolecules is reduced at elevated conductivity values. This strongly reduces its versatility since the salt concentration of cell culture supernatants and biomolecule-containing fractions from purification methods such as ion exchange and hydrophobic interaction chromatography, confers sufficient conductivity to reduce the binding capacity of hydroxyapatite to such an extent that it may not be useful for a particular application. This disadvantage can be overcome by diafiltration or dilution of the sample prior to its application to the hydroxyapatite column, but as noted above, these operations increase the expense of the overall purification process. Alternatively, the disadvantage can be ameliorated by using a larger volume of hydroxyapatite, but this increases process expense by requiring larger columns and larger buffer volumes. It also causes the antibody to elute in a larger volume of buffer, which increases overall process time in the subsequent purification step. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention in certain embodiments relates to methods of fractionating or purifying a desired biomolecule from an impure preparation by contacting said preparation with a native or calcium-derivatized apatite chromatography support, then eluting the support in the presence of an ionic species which is a sulfate, borate, monocarboxylic organic acid salt or monocarboxylic zwitterion. In certain embodiments the ionic species is the primary eluting ion in the eluent. In certain embodiments the eluent is substantially free of phosphate as an eluting ion. 
         [0013]    In certain embodiments of the inventions, a method for purifying a biomolecule from an impure preparation is provided wherein the impure preparation is contacted with an apatite chromatography support in either the calcium derivatized form or in its native form and the apatite support is converted to the other form prior to elution of the biomolecule. 
         [0014]    In certain embodiments of the invention, the desired biomolecule to be fractionated or purified is a biomolecule other than an antibody or antibody fragment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Advantages of some embodiments of the invention include the following: 1) Calcium-derivatized apatites support higher binding capacity than native hydroxyapatite for most biomolecules, even at high conductivity values, thereby making apatite chromatography more effective as a capture method, or as an intermediate fractionation step following high-salt elution from another fractionation step such as ion exchange or hydrophobic interaction chromatography; 2) Calcium-derivatized apatites also produce unique selectivities that may enable effective biomolecule fractionation, including removal of aggregates, in situations where native apatites fail to do so; 3) biomolecules may be bound to a native apatite support which is then converted to the calcium-derivatized form to achieve a particular selectivity for elution or; 4) biomolecules may be bound to a calcium-derivatized apatite support which is them converted to the native form for elution. 5) Sulfate, borate, and certain monocarboxylic acids or zwitterions are able to elute biomolecules from apatite supports in the absence of phosphate; 6) Elution in the presence of sulfate, borate, and certain monocarboxylic acids or zwitterions produces unique selectivities that permit effective fractionation of biomolecules that may not be adequately served by elution with phosphate or by combinations of phosphate and chloride; 7) Borate permits elution of biomolecules at low conductivity values, and does so without imposing significant buffer capacity at neutral pH, thereby facilitating use of the eluted biomolecule in subsequent ion exchange chromatography steps without the necessity for intervening steps such as diafiltration; 8) Borate and certain monocarboxylic acids or zwitterions create an increase in pH on contact with apatites which can be used to counteract the effect of chlorides on pH, thereby attenuating or eliminating the pH reduction that otherwise accompanies the introduction of chlorides; 9) Sulfate differentially enhances the retention of phosphorylated biomolecules, thereby enhancing their separation from non-phosphorylated biomolecules. 
         [0016]    In certain embodiments the ionic species is borate. In certain embodiments the borate is sodium borate or potassium borate. In certain such embodiments the primary eluting ion is borate. In certain embodiments the borate is present at a pH where the borate lacks substantial buffering capacity; in certain such embodiments the pH is less than 8.7. In certain other embodiments the borate is present at greater than 50 mM and at a pH where the borate has substantial buffering capacity; in certain such embodiments the pH is 8.7 or greater. 
         [0017]    In certain embodiments the ionic species is sulfate. In certain embodiments the sulfate is sodium or potassium sulfate. In certain embodiments the sulfate is the primary eluting ion. 
         [0018]    In certain embodiments the ionic species is a monocarboxylic acid salt. In certain such embodiments the monocarboxylate acid anion is formate, acetate, lactate, succinate, pyruvate, gluconate, glucuronate or proprionate. In certain embodiments the monocarboxylate is the primary eluting ion. 
         [0019]    In still other embodiments the ionic species is a monocarboxylic zwitterion. In certain such embodiments the monocarboxylate zwitterion is glycine, proline, lysine or histidine. 
         [0020]    In some embodiments, the biomolecule preparation may be applied to the apatite chromatography support under conditions that permit binding of the desired biomolecule and contaminants, with purification being achieved subsequently by application of an elution gradient. This mode of chromatography is often referred to as bind-elute mode. 
         [0021]    In some embodiments, the impure biomolecule preparation may be applied to the apatite chromatography support under conditions that prevent binding of the desired biomolecule, while binding contaminants. This mode of application is often referred to as flow-though mode. Bound contaminants may be removed subsequently from the column by means of a cleaning step. 
         [0022]    Suitable apatite chromatography supports include native hydroxyapatite, calcium-derivatized hydroxyapatite, native fluorapatite, and calcium-derivatized fluorapatite. 
         [0023]    In certain embodiments, elution may be achieved exclusively by means of increasing the concentration of the ionic species such as borate, sulfate, or monocarboxylic acids or zwitterions. In certain of such embodiments such elution is achieved with a single ionic species as the eluting ion, e.g., borate or sulfate. 
         [0024]    In some embodiments, elution may be achieved by borate in combination with calcium, magnesium, phosphate, sulfate, chloride, monocarboxylic acids or zwitterions, arginine, glycine, urea, or nonionic organic polymers. 
         [0025]    In some embodiments, elution may be achieved by sulfate in combination with calcium, magnesium, phosphate, borate, chloride, monocarboxylic acids or zwitterions, arginine, glycine, urea, or nonionic organic polymers. 
         [0026]    In some embodiments, elution may be achieved by monocarboxylic acids or zwitterions in combination with calcium, magnesium, phosphate, borate, sulfate, chloride, arginine, glycine, urea, or nonionic organic polymers. 
         [0027]    In certain embodiments, the method for purifying a biomolecule from an impure preparation containing said biomolecule includes the steps of (a) contacting the impure preparation with an apatite chromatography support, wherein the apatite chromatography support is in a calcium-derivatized form when it is contacted with the biomolecule and (b) substantially converting the calcium-derivatized apatite chromatography support to its native form prior to eluting the biomolecule. In certain such embodiments the biomolecule is eluted with phosphate as the primary eluting ion. 
         [0028]    In certain embodiments, the method for purifying a non-aggregated biomolecule from an impure preparation containing said biomolecule involves the steps of (a) contacting the impure preparation with an apatite chromatography support, wherein the apatite chromatography support is in its native form when it is contacted with the biomolecule and (b) substantially converting the native form apatite chromatography support to a calcium-derivatized form prior to eluting the biomolecule. In certain such embodiments the conversion of the apatite chromatography support to the calcium derivatized form causes elution of the biomolecule of interest. 
         [0029]    In certain embodiments, phosphorylated biomolecules of interest are separated by a method of the invention from non-phosphorylated biomolecules. In other embodiments, non-phosphorylated biomolecules of interest are separated by a method of the invention from phosphorylated biomolecules. 
         [0030]    Embodiments of the invention may be practiced in combination with one or more other purification methods, including but not limited to size exclusion chromatography, protein A and other forms of affinity chromatography, anion exchange chromatography, cation exchange chromatography, hydrophobic interaction chromatography, mixed mode chromatography, and various filtration methods. It is within the ability of a person of ordinary skill in the art to develop appropriate conditions for these methods and integrate them with the invention herein to achieve purification of a particular antibody or antibody fragment. 
         [0031]    Terms are defined so that the invention may be understood more readily. Additional definitions are set forth throughout this disclosure. 
         [0032]    “Apatite chromatography support” refers to a mineral of calcium and phosphate in a physical form suitable for the performance of chromatography. Examples include but are not limited to hydroxyapatite and fluorapatite. This definition is understood to include both the native and calcium-derivatized forms of an apatite chromatography support. 
         [0033]    “Salt” refers to an aqueous-soluble ionic compound formed by the combination of negatively charged anions and positively charged cations. The anion or cation may be of organic or inorganic origin. Anions of organic origin include but are not limited to acetate, lactate, malate, and succinate. Anions of inorganic origin include but are not limited to chloride, borate, sulfate, and phosphate. Cations of organic origin include but are not limited to arginine and lysine. Cations of inorganic origin include but are not limited to sodium, potassium, calcium, magnesium, and iron. 
         [0034]    “Borate” refers to ionic compounds of boron and oxygen such as, but not limited to boric acid, sodium borate, and potassium borate. 
         [0035]    “Phosphate” refers to salts based on phosphorus (V) oxoacids such as, but not limited to, sodium phosphate and potassium phosphate. 
         [0036]    “Sulfate” refers to salts based on sulfur (VI) oxoacids such as, but not limited to sodium sulfate and ammonium sulfate. 
         [0037]    “Chloride” refers to salts such as, but not limited to sodium chloride and potassium chloride. 
         [0038]    “Monocarboxylic acid salt” or “Monocarboxylate” refers to organic acid salts having a single carboxylic acid moiety including but not limited to the sodium or potassium salts of formic, acetic, propionic, lactic, pyruvic, gluconic, or glucuronic acid. 
         [0039]    “Monocarboxylic zwitterion” refers to a molecule containing a single carboxyl moiety and at least one moiety with a positive charge. Suitable examples include but are not limited to the amino acids glycine, proline, lysine, and histidine. 
         [0040]    “Nonionic organic polymer” refers to any uncharged linear or branched polymer of organic composition. Examples include, but are not limited to, dextrans, starches, celluloses, polyvinylpyrrolidones, polypropylene glycols, and polyethylene glycols of various molecular weights. Polyethylene glycol has a structural formula HO—(CH 2 —CH 2 —O) n —H. Examples include, but are not limited to, compositions with an average polymer molecular weight ranging from 100 to 10,000 daltons. The average molecular weight of commercial PEG preparations is typically indicated by a hyphenated suffix. For example, PEG-600 refers to a preparation with an average molecular weight of about 600 daltons. 
         [0041]    “Buffering compound” refers to a chemical compound employed for the purpose of stabilizing the pH of an aqueous solution within a specified range. Phosphate is one example of a buffering compound. Other common examples include but are not limited to compounds such as acetate, morpholinoethanesulfonic acid (MES), Tris-hydroxyaminomethane (Tris), and hydroxyethylpiperazinesulfonic acid (HEPES). 
         [0042]    “Buffer” refers to an aqueous formulation comprising a buffering compound and other components required to establish a specified set of conditions to mediate control of a chromatography support. The term “equilibration buffer” refers to a buffer formulated to create the initial operating conditions for a chromatographic operation. “Wash buffer” refers to a buffer formulated to displace unbound contaminants from a chromatography support. “Elution buffer” refers to a buffer formulated to displace the one or more biomolecules from the chromatography support. 
         [0043]    “Biomolecule” refers to any molecule of biological origin, composite, or fragmentary form thereof. 
         [0044]    “Phosphorylated biomolecule” refers to any biomolecule, composite or fragmentary form thereof that includes at least one phosphate residue. Phosphorylation may be natural or induced by chemical modification. Examples include but are not limited to nucleotides, polynucleotides, DNA, RNA, endotoxins, lipid enveloped virus, phosphoproteins, phosphopeptides, phosphorylated amino acids, lipoproteins (where the lipid moiety is phosphorylated), phospholipids, glycophosphates, and glycophospholipids. 
         [0045]    “Nonphosphorylated biomolecule” refers to any biomolecule, composite or fragmentary form thereof that is devoid of phosphate residues. Examples include but are not limited to proteins, peptides, amino acids, lipids, and carbohydrates. Examples of proteins include but are not limited to antibodies, enzymes, growth regulators, and clotting factors. 
         [0046]    “Biomolecule preparation” refers to any composition containing a biomolecule which is desired to be fractionated from contaminants. 
         [0047]    “Antibody” refers to any immunoglobulin or composite form thereof. The term may include, but is not limited to polyclonal or monoclonal antibodies of the classes IgA, IgD, IgE, IgG, and IgM, derived from human or other mammalian cell lines, including natural or genetically modified forms such as humanized, human, single-chain, chimeric, synthetic, recombinant, hybrid, mutated, grafted, and in vitro generated antibodies. “Antibodies” may also include composite forms including but not limited to fusion proteins containing an immunoglobulin moiety. 
         [0048]    “Antibody fragment” refers to any antibody fragment such as Fab, F(ab′) 2 , Fv, scFv, Fd, mAb, dAb or other compositions that retain antigen-binding function. Antibody fragments may be derived from human or other mammalian cell lines, including natural or genetically modified forms such as humanized, human, chimeric, synthetic, recombinant, hybrid, mutated, grafted, and in vitro generated, from sources including but not limited to bacterial cell lines, insect cell lines, plant cell lines, yeast cell lines, or cell lines of other origin. Antibody fragments may also be derived by controlled lysis of purified antibody with enzymes such as, but not limited to ficin, papain, or pepsin. 
         [0049]    As it relates to the invention herein, the term “bind-elute mode” refers to an operational approach to chromatography in which the buffer conditions are established so that the desired biomolecule and contaminants bind to the column upon application, with fractionation being achieved subsequently by modification of the buffer conditions. 
         [0050]    As it relates to the invention herein, the term “flow-through mode” refers to an operational approach to chromatography in which the buffer conditions are established so that the desired biomolecule flows through the column upon application while contaminants are selectively retained, thus achieving their removal. 
         [0051]    “Analytical application” refers to a situation in which the invention is practiced for the purpose of identifying and or determining the quantity of the desired molecule in particular preparation, in order to obtain information pertinent to research, diagnosis, or therapy. 
         [0052]    “Preparative application” refers to a situation in which the invention is practiced for the purpose of purifying intact non-aggregated antibody for research, diagnostic, or therapeutic applications. Such applications may be practiced at any scale, ranging from milligrams to kilograms of antibody per batch. 
       Materials 
       [0053]    1. Apatite Chromatography Support 
         [0054]    Various apatite chromatography supports are available commercially, any of which can be used in the practice of this invention. These include but are not limited to hydroxyapatite and fluorapatite. “Ceramic” hydroxyapatite (CHT™) or “ceramic” fluorapatite (CFT™) refer to forms of the respective minerals in which nanocrystals are aggregated into particles and fused at high temperature to create stable ceramic microspheres suitable for chromatography applications. Commercial examples of ceramic hydroxyapatite include, but are not limited to CHT Type I and CHT Type II. Commercial examples of fluorapatite include, but are not limited to CFT Type II. Unless specified, CHT and CFT refer to roughly spherical particles of any diameter, including but not limited to 10, 20, 40, and 80 micron. HA Ultrogel™ refers to a product comprising microfragments of non-ceramic hydroxyapatite embedded in porous agarose microspheres. 
         [0055]    The choice of hydroxyapatite or fluorapatite, the type, and average particle diameter suitable for a particular fractionation can be determined through experimentation by the skilled artisan. 
         [0056]    The invention may be practiced in a packed bed column, a fluidized/expanded bed column containing the hydroxyapatite or fluorapatite, and/or a batch operation where the hydroxyapatite or fluorapatite is mixed with the solution for a certain time. 
         [0057]    Certain embodiments employ CHT or CFT packed in a column. 
         [0058]    Certain embodiments employ CHT or CFT, packed in a column of about 5 mm internal diameter and a height of about 50 mm, for evaluating the effects of various buffer conditions on the binding and elution characteristics of a particular antibody preparation of antibody fragment preparation. 
         [0059]    Certain embodiments employ CHT or CFT, packed in columns of any dimensions required to support preparative applications. Column diameter may range from 1 cm to more than 1 meter, and column height may range from 5 cm to more than 30 cm depending on the requirements of a particular application. 
         [0060]    Appropriate column dimensions can be determined by the skilled artisan. 
         [0061]    2. Biomolecule Preparations 
         [0062]    Biomolecule preparations to which the invention can be applied may include unpurified or partially purified biomolecules from natural, synthetic, or recombinant sources. Unpurified preparations may come from various sources including, but not limited to, plasma, serum, ascites fluid, milk, plant extracts, bacterial lysates, yeast lysates, or conditioned cell culture media. Partially purified preparations may come from unpurified preparations that have been processed by at least one chromatography, precipitation, other fractionation step, or any combination of the foregoing. The chromatography step or steps may employ any method, including but not limited to size exclusion, affinity, anion exchange, cation exchange, protein A affinity, hydrophobic interaction, immobilized metal affinity chromatography, or mixed-mode chromatography. The precipitation step or steps may include salt or PEG precipitation, or precipitation with organic acids, organic bases, or other agents. Other fractionation steps may include but are not limited to crystallization, liquid:liquid partitioning, or membrane filtration. 
       B. Description of the Method 
       [0063]    In preparation for contacting the biomolecule preparation with the apatite support, it is usually necessary to equilibrate the chemical environment inside the column. This is accomplished by flowing an equilibration buffer through the column to establish the appropriate pH, conductivity, concentration of salts; and/or the identity, molecular weight, and concentration of nonionic organic polymer. 
         [0064]    The equilibration buffer for applications conducted in bind-elute mode may include phosphate salts at a concentration of about 5-50 mM, or calcium salts at a concentration of about 2-5 mM, but not mixtures of phosphate and calcium. It may optionally include a nonionic organic polymer at a concentration of about 0.01-50%, and a buffering compound to confer adequate pH control. Buffering compounds may include but are not limited to MES, HEPES, BICINE, imidazole, and Tris. The pH of the equilibration buffer for hydroxyapatite may range from about pH 6.5 to pH 9.0. The pH of the equilibration buffer for fluorapatite may range from about pH 5.0 to 9.0. 
         [0065]    In one embodiment, the equilibration buffer contains sodium phosphate at a concentration of about 5 mM at a pH of 6.7, in the presence or absence of MES or Hepes at a concentration of about 20-50 mM. 
         [0066]    In one embodiment, the equilibration buffer contains a calcium salt at a concentration of about 2.5 mM, in the presence of Hepes at a concentration of about 20-50 mM and a pH of about 7.0. 
         [0067]    The biomolecule preparation may also be equilibrated to conditions compatible with the column equilibration buffer before the invention is practiced. This consists of adjusting the pH, concentration of salts, and other compounds. 
         [0068]    After the column and biomolecule preparation have been equilibrated, the biomolecule preparation may be contacted with the column. Said preparation may be applied at a linear flow velocity in the range of, but not limited to, about 50-600 cm/hr. Appropriate flow velocity can be determined by the skilled artisan. 
         [0069]    In one embodiment of the bind-elute mode, a column equilibrated in phosphate to obtain a particular binding selectivity during column loading may be switched to calcium to obtain a particular elution selectivity. Or the opposite may be performed, with a column equilibrated to calcium to obtain a particular binding selectivity, and then switched to phosphate to obtain a particular elution selectivity. 
         [0070]    In one embodiment of the flow-through mode, non-aggregated biomolecule flows through the column and is collected, while aggregated biomolecule binds to the column. The biomolecule preparation is followed with a wash buffer, usually of the same composition as the equilibration buffer. This displaces remaining non-aggregated biomolecule from the column so that it can be collected. Retained aggregates may optionally be removed from the column with a cleaning buffer of about 500 mM sodium phosphate, among others. 
         [0071]    In one embodiment of an application conducted in bind-elute mode, some combination of unwanted biomolecules, intact non-aggregated biomolecule, and aggregated biomolecule bind to the column. The biomolecule preparation is followed with a wash buffer, usually of the same composition as the equilibration buffer. This removes unretained contaminants from the column. Unwanted biomolecule fragments may be selectively displaced by a wash buffer that removes fragments without removing intact non-aggregated biomolecule. Intact non-aggregated biomolecule is then eluted from the column under conditions that leave aggregated biomolecule bound to the column. Retained aggregates may optionally be removed from the column with a cleaning buffer of about 500 mM sodium phosphate, among others. 
         [0072]    In one embodiment of the bind-elute mode, the wash buffer may have a formulation different than the equilibration buffer. 
         [0073]    After use, the apatite column may optionally be cleaned, sanitized, and stored in an appropriate agent. 
         [0074]    The invention may be practiced in combination with other purification methods to achieve the desired level of biomolecule purity. The invention may be practiced at any point in a sequence of 2 or more purification methods. 
       C. EXAMPLES 
       [0075]    Considerable variation in chromatographic behavior is encountered from one biomolecule preparation to another. This includes variation in the composition and proportion of undesired biomolecule contaminants, intact biomolecule, biomolecule fragments, and biomolecule aggregates, as well as variation in the individual retention characteristics of the various constituents. This makes it necessary to customize the buffer conditions to apply the invention to its best advantage in each situation. This may involve adjustment of pH, the concentration salts, the concentration of buffering components, and the content of nonionic organic polymer. Appropriate levels for the various parameters and components can be determined systematically by a variety of approaches. The following examples are offered for illustrative purposes only. 
         [0076]    Example 1 . Dynamic binding capacity comparison of native and calcium-derivatized hydroxyapatite. A column of hydroxyapatite, CHT Type II, 40 micron, 5 mm diameter, 50 mm height, was equilibrated at a linear flow rate of 300 cm/hr with 20 mM Hepes, 3 mM CaCl 2 , pH 6.7. A sample of protein A purified IgG monoclonal antibody was applied to the column by in-line dilution at a proportion of 1 part antibody to 4 parts equilibration buffer. Dynamic breakthrough capacity at 5% breakthrough was 114 mg/mL of hydroxyapatite. The experiment was repeated with an equilibration buffer of 20 mM Hepes, 3 mM CaCl 2 , 1 M NaCl, pH 6.7. Dynamic capacity at 5% breakthrough was 43 mg/mL. The experiment was repeated with an equilibration buffer of 5 mM sodium phosphate, pH 6.7. Dynamic capacity at 5% breakthrough was 29 mg/mL. The experiment was repeated with an equilibration buffer of 5 mM sodium phosphate, 1 M NaCl, pH 6.7. Dynamic capacity at 5% breakthrough was 3 mg/mL. This example illustrates the dramatic improvement in antibody binding capacity that is achieved by calcium derivatized apatite. It will be recognized by the skilled practitioner that a similar benefit may be obtained by substituting magnesium for calcium. 
         [0077]    Example 2. Purification of a biomolecule from cell culture supernatant on native hydroxyapatite, eluted with a borate gradient. A column of hydroxyapatite, CHT Type I, 40 micron, 8 mm diameter, 50 mm height, was equilibrated at a linear flow rate of 300 cm/hr with 5 mM sodium phosphate, 20 mM Hepes, pH 7.0. A monoclonal antibody preparation consisting of a mammalian cell culture supernatant previously filtered through a membrane with porosity of about 0.22 μm, and diafiltered to about the same conditions as the equilibration buffer was applied to the column. The column was eluted with a linear gradient to 1 M sodium borate, 5 mM sodium phosphate, pH 7.0. The majority of contaminating proteins eluted before the antibody. Non-aggregated antibody eluted at an average conductivity of about 5 mS/cm. Aggregates eluted later. The column was cleaned with 500 mM sodium phosphate, pH 7.0. It will be recognized by the person of ordinary skill in the art that eluted antibody may be further purified by additional purification methods, and that the low conductivity and buffer capacity of the eluted antibody fraction will facilitate such methods. 
         [0078]    Example 3. Purification of an biomolecule from cell culture supernatant on native hydroxyapatite, eluted with a monocarboxylic acid (lactate) gradient. A column of hydroxyapatite, CHT Type I, 40 micron, 5 mm diameter, 50 mm height, was equilibrated at a linear flow rate of 600 cm/hr with 5 mM sodium phosphate, 20 mM Hepes, pH 7.0. 100 microliters of a monoclonal antibody preparation consisting of a mammalian cell culture supernatant previously filtered through a membrane with porosity of about 0.22 μm, was injected onto the column and the column washed with 2 column volumes of equilibration buffer. The column was eluted with a 20 column volume linear gradient to 1 M sodium lactate, 20 mM Hepes, pH 7.0. The majority of contaminating proteins eluted before the antibody and most of the remainder eluted later. Non-aggregated antibody eluted at an average conductivity of about 20 mS/cm. Aggregates eluted later. The column was cleaned with 500 mM sodium phosphate, pH 7.0. 
         [0079]    Example 4. Purification of a biomolecule from cell culture supernatant on native hydroxyapatite, eluted with a borate gradient. The same column was prepared with the same buffers but with a different IgG monoclonal antibody. The majority of contaminating proteins eluted as previously but only about 70% of the antibody eluted within the gradient, with the remainder eluting with the aggregate in the 500 mM phosphate cleaning step. The run was repeated but with 20 mM phosphate in the equilibration and elution buffers. Under these conditions, more than 80% of the antibody eluted within the gradient with the remainder eluting with the aggregate in the 500 mM phosphate cleaning step. The run was repeated but with 30 mM phosphate in the equilibration and elution buffers. Under these conditions, more than 90% of the antibody eluted within the gradient, while a small amount of antibody eluted with aggregates in the 500 mM phosphate cleaning step. This example illustrates one way to adapt the procedure to antibodies that may not elute fully within the gradient in the absence of phosphate, or at low phosphate concentrations. The phosphate concentration may be increased more if necessary. Alternatively or additionally, the borate concentration and/or pH of the eluting buffer may be increased. It will be recognized by the skilled practitioner that the low conductivity and buffering capacity of the borate-eluted product make it better suited for subsequent purification by cation exchange chromatography than elution in a sodium chloride gradient. It will be equally recognized that the substitution of borate with monocarboxylic acids or zwitterions with molar conductivities lower than sodium chloride may confer a similar benefit. 
         [0080]    Example 5. Purification of a biomolecule from cell culture supernatant on calcium derivatized hydroxyapatite, eluted with a borate gradient. A column of hydroxyapatite, CHT Type I, 40 micron, 8 mm diameter, 50 mm height, was equilibrated at a linear flow rate of 300 cm/hr with 2.5 mM calcium chloride, 20 mM Hepes, pH 7.0. A monoclonal antibody preparation consisting of cell culture supernatant previously filtered through a membrane with porosity of about 0.22 μm and diafiltered to about the same conditions as the equilibration buffer was applied to the column. The column was eluted with a linear gradient to 1 M sodium borate, 2.5 mM calcium chloride, 10% PEG-600, pH 7.0. The majority of contaminating proteins eluted before the antibody. Antibody aggregate eluted after non-aggregated antibody. The column was cleaned with 500 mM sodium phosphate, pH 7.0. PEG is known to have the general effect of enhancing the separation between fragments, intact antibody, and aggregates on hydroxyapatite. The skilled practitioner will recognize how to adjust the PEG concentration to optimize the results. 
         [0081]    Example 6. Biomolecule capture on calcium-derivatized hydroxyapatite and elution in a sulfate gradient. A column of hydroxyapatite, CHT Type II, 40 micron, 5 mm diameter, 50 mm height, was equilibrated at a linear flow rate of 300 cm/hr with 20 mM Hepes, 3 mM CaCl 2 , pH 6.7. Cell culture supernatant containing approximately 60 mg monoclonal IgG was equilibrated to 5 mM calcium by addition of 1 M calcium chloride at a proportion of 0.5%, then filtered to 0.22 microns. The sample was applied to the column. No antibody was detected in the flow-through. The column was washed with equilibration buffer, then eluted with a 20 column volume (CV) linear gradient to 20 mM Hepes, 3 mM CaCl 2 , 0.5 M sodium sulfate, pH 6.7. The antibody eluted in a single peak at about 0.25 M sodium sulfate. 
         [0082]    Example 7. Biomolecule capture on calcium-derivatized hydroxyapatite, conversion to native hydroxyapatite, and elution in a phosphate gradient. A column of hydroxyapatite, CHT Type II, 40 micron, 5 mm diameter, 50 mm height, was equilibrated at a linear flow rate of 300 cm/hr with 20 mM Hepes, 3 mM CaCl 2 , pH 6.7. Cell culture supernatant containing monoclonal approximately 40 mg IgG was equilibrated to 5 mM calcium by addition of 1 M calcium chloride at a proportion of 0.5%, then filtered to 0.22 microns. The sample was applied to the column. No antibody was detected in the flow-through. The column was washed with 5 mM sodium phosphate, 20 mM MES, pH 6.7, then eluted with a 20 CV linear gradient to 300 mM phosphate, pH 6.7. The antibody eluted in a single peak at about 165 mM sodium phosphate. This example illustrates the use of calcium-derivatized hydroxyapatite to obtain high binding capacity, followed by conversion to and elution from native hydroxyapatite. 
         [0083]    Example 8. Intermediate purification of a biomolecule by binding in the presence of calcium, conversion to native apatite, and elution in a sodium chloride gradient. A column of hydroxyapatite, CHT Type II, 40 micron, 5 mm diameter, 50 mm height, was equilibrated at a linear flow rate of 300 cm/hr with 20 mM Hepes, 3 mM CaCl 2 , pH 6.7. Approximately 50 mg of protein A purified monoclonal IgG was equilibrated to 5 mM calcium by addition of 1 M calcium chloride at a proportion of 0.5%, then filtered to 0.22 microns. The sample was applied to the column. No antibody was detected in the flow-through. The column was washed with 20 mM Hepes, 10 mM sodium phosphate, pH 6.7, then eluted with a 20 CV linear gradient to 20 mM Hepes, 10 mM phosphate, 1 M sodium chloride, pH 6.7. The antibody eluted in a single peak at 0.6 M sodium chloride, followed by a well-separated aggregate peak. 
         [0084]    Example 9. Unwanted fragment and aggregate removal from a partially purified biomolecule on native hydroxyapatite, eluted with a borate gradient. A column of hydroxyapatite, CHT Type I, 40 micron, 8 mm diameter, 50 mm height, was equilibrated at a linear flow rate of 300 cm/hr with 5 mM sodium phosphate, 20 mM Hepes, pH 7.0. A monoclonal antibody preparation previously purified by protein A affinity chromatography was applied to the column. The column was eluted with a linear gradient to 1 M sodium borate, 5 mM sodium phosphate, 20 mM Hepes, pH 7.0. The majority of fragments eluted before the antibody. Antibody aggregates and other contaminating proteins eluted after non-aggregated antibody. The column was cleaned with 500 mM sodium phosphate, pH 7.0. 
         [0085]    Example 10. Bind-elute mode, comparison of biomolecule elution in phosphate and sulfate gradients. A column of hydroxyapatite, CHT Type II, 40 micron, 5 mm diameter, 50 mm height, was equilibrated at a linear flow rate of 200 cm/hr with 20 mM Hepes, 3 mM CaCl 2 , pH 6.7. Cell supernatant containing a monoclonal IgM antibody was applied to the column. The column was eluted with a 20 CV linear gradient to 20 mM Hepes, 3 mM CaCl 2 , 1.0 M sodium sulfate, pH 6.7. The center of the IgM peak eluted about 415 mM sodium sulfate. DNA eluted at 855 mM sulfate under these conditions. IgM aggregates did not elute within the sulfate gradient and were removed in a subsequent wash step with 500 mM phosphate. The experiment was repeated except that the column was equilibrated with 10 mM sodium phosphate pH 6.7 and eluted with a 20 CV linear gradient to 500 mM sodium phosphate, pH 6.7. The center of the IgM peak eluted at about 207 mM phosphate, essentially co-eluting with DNA as revealed by its elution at 205 mM phosphate. IgM aggregates were only partially eliminated. This example again illustrates the dramatic difference of selectivity between sulfate and phosphate gradients, specifically and dramatically highlights how sulfate gradients are more effective for removal of DNA from IgM preparations, and specifically illustrates the superior ability of sulfate gradients to eliminate aggregates. It will also be apparent to the skilled practitioner that these results illustrate the ability of the method to eliminate nonphosphorylated contaminants, such as proteins, from a preparation of a phosphorylated biomolecule, such as DNA. 
         [0086]    Example 11. Improved pH control by the application of borate. A column of hydroxyapatite was equilibrated to 5 mM sodium phosphate, pH 7.0. A gradient step of 0.5 M sodium chloride, 5 mM sodium phosphate, pH 7.0 was applied to the column. This caused the pH to drop to about pH 5.9. The column was re-equilibrated to 5 mM phosphate pH 7.0. A gradient step of 0.5 mM sodium chloride, 5 mM sodium phosphate, 50 mM sodium borate, pH 7.0 was applied to the column. Column pH dropped only to pH 6.7. It will be understood by the skilled practitioner that the same approach can be used to control pH in any situation where the introduction of an eluting agent causes an unacceptable reduction of pH, and that the borate concentration can be adjusted to achieve the desired degree of pH control. Like borate, the application of lactate to an equilibrated apatite support causes an increase in pH, which can likewise be exploited to manage uncontrolled pH reduction caused by chlorides. The skilled practitioner will recognize that other monocarboxylic acids or zwitterions may be substituted to produce a similar effect. 
         [0087]    Example 12. Biomolecule fractionation with a borate gradient. A column of hydroxyapatite, CHT Type I, 40 micron, 8 mm diameter, 50 mm height, was equilibrated at a linear flow rate of 300 cm/hr with 5 mM sodium phosphate, 20 mM Hepes, pH 7.0. A Fab preparation from papain digestion of an IgG monoclonal antibody was applied to the column. The column was eluted with a linear gradient to 1 M sodium borate, 5 mM sodium phosphate, 20 mM Hepes, pH 7.0. The majority of contaminating Fc fragments eluted before the Fab. Intact antibody eluted after the Fab. The column was cleaned with 500 mM sodium phosphate, pH 7.0. 
         [0088]    Example 13. Flow-through purification of a biomolecule on calcium-derivatized apatite. A column of hydroxyapatite, CHT Type I, 20 micron, 5 mm diameter, 50 mm height, was equilibrated with 10 mM sodium Hepes, 2.5 mM calcium chloride, pH 7.0 at a linear flow rate of 300 cm/hr. Calcium chloride was added to a Fab digest to a final concentration of 2.5 mM, then loaded onto the column. The Fab was unretained and flowed through the column at a purity of about 95%. 
         [0089]    Example 14. Biomolecule purification by application to native apatite, eluted by conversion to calcium-derivatized apatite. A column of hydroxyapatite, CHT Type I, 20 micron, 5 mm diameter, 50 mm height, was equilibrated with 5 mM sodium phosphate, 10 mM Hepes, pH 7.0 at a linear flow rate of 300 cm/hr. A Fab preparation was titrated to 5 mM phosphate and loaded onto the hydroxyapatite column. The Fab was retained and eluted with a step to 10 mM Hepes, 2.5 mM calcium chloride, pH 7.0. Purity was greater than 95%. 
         [0090]    Example 15. Bind-elute mode, comparison of elution of a phosphorylated biomolecule in phosphate and sulfate gradients. A column of hydroxyapatite, CHT Type II, 40 micron, 5 mm diameter, 50 mm height, was equilibrated at a linear flow rate of 300 cm/hr with 20 mM Hepes, 3 mM CaCl 2 , pH 6.7. A sample of DNA isolated from salmon sperm was applied to the column. The column was eluted with a 20 CV linear gradient to 20 mM Hepes, 3 mM CaCl 2 , 1.0 M sodium sulfate, pH 6.7. The center of the DNA peak eluted at about 855 mM sodium sulfate. The experiment was repeated except that the column was equilibrated with 10 mM sodium phosphate pH 6.7 and eluted with a 20 CV linear gradient to 500 mM sodium phosphate, pH 6.7. The center of the DNA peak eluted at about 205 mM sodium phosphate. This example illustrates the dramatic difference between selectivity of sulfate and phosphate gradients. It will be apparent to the skilled practitioner that these results also show the ability of sulfate gradients to achieve more effective removal of DNA than phosphate gradients. 
         [0091]    Example 16. Bind-elute mode, comparison of elution of a phosphorylated biomolecule in phosphate and sulfate gradients. A column of hydroxyapatite, CHT Type II, 40 micron, 5 mm diameter, 50 mm height, was equilibrated at a linear flow rate of 300 cm/hr with 20 mM Hepes, 3 mM CaCl 2 , pH 6.7. A sample of endotoxin prepared by phenol extraction from  Salmonella enterica  serotype  typhimurium  was applied to the column. The column was eluted with a 20 column volume (CV) linear gradient to 20 mM Hepes, 3 mM CaCl 2 , 1.0 M sodium sulfate, pH 6.7. A minor fraction of endotoxin eluted early in the gradient, followed by a DNA contaminant peak at 855 mM sodium sulfate. The majority of the endotoxin failed to elute and was removed from the column by cleaning it with 500 mM sodium phosphate, pH 6.7. The experiment was repeated except that the column was equilibrated with 10 mM sodium phosphate pH 6.7 and eluted with a 20 CV linear gradient to 500 mM sodium phosphate, pH 6.7. A minor fraction of the endotoxin, corresponding to the early eluting population in the sulfate gradient, failed to bind in phosphate and flowed through the column immediately upon application. The center of the primary endotoxin peak eluted at 85 mM sodium phosphate. This example illustrates the dramatic difference between selectivity of sulfate and phosphate gradients in general, specifically illustrates the ability of sulfate gradients to achieve unique separations among differentially phosphorylated biomolecules, and specifically illustrates that some phosphorylated biomolecules do not elute from at least some apatite chromatography supports in sulfate gradients conducted in the absence of phosphate. It will be apparent to the skilled practitioner that these results also show the ability of sulfate gradients to achieve more effective removal of endotoxin than phosphate gradients. 
         [0092]    Example 17. Enhanced fractionation of a phosphorylated protein from a nonphosphorylated protein by differential enhancement of the phosphorylated protein in a sulfate gradient. A column of hydroxyapatite, CHT Type I, 40 micron, 5 mm diameter, 50 mm height, was equilibrated at a linear flow rate of 600 cm/hr with 20 mM Hepes, pH 7.0. A purified monoclonal antibody (unphosphorylated) and a purified alpha-casein (polyphosphorylated) were applied, and the column was eluted in a 20 column volume linear gradient to 500 mM phosphate. The antibody eluted at 156 mM phosphate. Alpha-casein eluted at 223 mM phosphate. The experiment was repeated, except eluting the column with a 20 column volume linear gradient to 1 M ammonium sulfate, 20 mM Hepes, pH 7.0. The antibody eluted at 308 mM sulfate. Alpha-casein did not elute within the sulfate gradient. This shows that retention of the unphosphorylated protein was increased by about 97%, while the retention of alpha-casein was increased by at least 350%. 
         [0093]    It will be understood by the person of ordinary skill in the art how to optimize and scale up the results from experiments such as those described in the above examples. It will also be understood by such persons that other approaches to method development, such as but not limited to high-throughput robotic systems, can be employed to determine the conditions that most effectively embody the invention for a particular antibody. 
       D. Additional Optional Steps 
       [0094]    The present invention may be combined with other purification methods to achieve higher levels of purification, if necessary. Examples include, but are not limited to, other methods commonly used for purification of biomolecules, such as size exclusion chromatography, protein A and other forms of affinity chromatography, anion exchange chromatography, cation exchange chromatography, hydrophobic interaction chromatography, immobilized metal affinity chromatography, mixed mode chromatography, precipitation, crystallization, liquid:liquid partitioning, and various filtration methods. It is within the purview of one of ordinary skill in the art to develop appropriate conditions for the various methods and integrate them with the invention herein to achieve the necessary purification of a particular antibody. 
         [0095]    All references cited herein are incorporated herein by reference in their entirety and for all purposes to the same extent as if each individual publication or patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety for all purposes. To the extent publications and patents or patent applications incorporated by reference contradict the disclosure contained in the specification, the specification is intended to supersede and/or take precedence over any such contradictory material. 
         [0096]    All numbers expressing quantities of ingredients, chromatography conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and attached claims are approximations that may vary depending upon the desired performance sought to be obtained by the present invention. 
         [0097]    Many modifications and variations of this invention can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. The specific embodiments described herein are offered by way of example only and are not meant to be limiting in any way. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.