FIELD OF THE INVENTION
The invention relates to culture methods for the production of erythropoiesis stimulating protein. The disclosure further provides an improved serum-free medium that comprises non-toxic concentrations of divalent or trivalent metal ions for production of erythropoiesis stimulating protein.
BACKGROUND OF THE INVENTION
Erythropoiesis stimulating protein, such as erythropoietin and analogs of erythropoietin are glycoprotein hormones that are the principle homeostatic regulators of red blood cell production. Natural erythropoietin, or EPO, is produced by the kidney, but its high demand for therapeutic purposes has led to large scale production by recombinant DNA methods. Purified recombinant human EPO is administered to human patients for the treatment of medical indications associated with inadequate red blood cell supply, e.g., anemia and chronic renal failure. Recombinant human erythropoietin (rHuEPO) and its analog darbepoetin alfa are used clinically to treat anemia and increase red blood cell production in various conditions, such as perisurgery, chronic renal failure, side effects of HIV or HCV treatment, and side effects of cancer chemotherapy.
The recombinant expression and production of glycoproteins, such as EPO and analogs of EPO, is complicated by the need for both high level expression, as well as appropriate post translation processing of these glycoproteins. An important post-translational processing that occurs subsequent to cellular protein expression is glycosylation, wherein glycans are enzymatically attached to the polypeptide backbone. Protein glycosylation is of two general types: O-linked glycosylation, wherein the glycan group is attached to the hydroxyl oxygen of serine or threonine
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side chains; and N-linked glycosylation, wherein the glycan group is attached to amide nitrogen of aspargine side chains. The composition of the sugars that makeup the glycan group can affect the physico-chemical properties, isoform composition and biological activities of glycoproteins. For instance N-acetylneuraminic acid (hereinafter referred to as sialic acid) is one of the most commonly present sugars in the glycan moiety of glycoproteins. Being negatively charged, the number of sialic acid groups present in a glycoprotein glycan affects the isoelectric point of the glycoprotein: more the number sialic acids, lower the isoelectric point. Darbepoetin alfa, a novel glycosylation analog of rHuEPO contains five N-linked carbohydrate chains and up to 22 sialic acids. In contrast, recombinant human EPO has 3 N-linked carbohydrate chains and a maximum of 14 sialic acids (Egrie JC., and Browne JK., Br. J. cancer 2001; 84, 3-10 and Elliot S, et.al., Blood 2000;96:8 2a). As a result of the additional glycosylation, darbepoetin has decreased receptor-binding activity, but exhibits a three-fold longer serum half-life and increased in vivo activity as compared to recombinant human EPO.
rHuEPO produced in Chinese hamster ovary (CHO) cells can exhibit a variable extent of glycosylation and sialylation. (Takeuchi et al., 1989 PNAS 86(20):7819-22, Zanette et al., 2003 Journal of Biotechnology 101(3):275-287). Given that sialylation of erythropoietin is necessary for its in vivo bioactivity, consistency in glycosylation and higher levels of sialylation of rHuEPO and its analogs are desirable attributes when producing these recombinant proteins for therapeutic purposes. Thus there is a need for methods that improve glycosylation and sialylation of these glycoproteins in cell cultures.
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The literature describes methods for increasing glycosylation of glycoproteins by culturing cells expressing the same in media comprising metal ions. US Patent No 6528286 discloses a process for the preparation of glycoprotein in which the sialic acid content in glycoprotein is increased by addition of copper ion to the cell culture. However, the patent does not describe the use of divalent manganese ions or trivalent ferric ions.
US Patent Application Publication No. 20070161084 describes a method for producing erythropoietic compositions with some increase in sialylation by culturing cell lines expressing an erythropoietic protein in medium comprising 0.01 to 50 µM concentration of manganese. However, the application reports increase in the toxicity of cells at concentrations of manganese greater than 50 µM.
The novelty and inventiveness of the present invention lie in the effect of higher concentrations of trivalent ferric ions in the cell culture medium in increasing the levels of low pI isoforms of erythropoiesis-stimulating proteins.
SUMMARY OF INVENTION The present invention provides a process for increasing the levels of low pl isoforms of darbepoetin alfa, comprising culturing Chinese hamster ovary cell lines expressing said darbepoetin alfa, in a medium comprising trivalent ferric ions (Fe+3) having concentrations in a range of 9 µM to 40 µM wherein the darbepoetin alfa so obtained demonstrates an increase in low pl isoforms in comparison to darbepoein alfa obtained from cells cultured in absence of said concentration of iron (Fe+3) as determined by isoelectric focusing.
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The culture medium is optionally serum-free and may comprise one or more supplementary amino acids selected from the group consisting of asparagine, aspartic acid, cysteine, cystine, isoleucine, leucine, tryptophan, or valine. The host cells may be any mammalian cell lines e.g. any CHO cell lines, and may be grown in any suitable cell culture systems such as in roller bottles.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 illustrates an IEF gel, performed as described in example 2 of harvest of cells cultured in medium devoid of either manganese ions or trivalent ferric ions (example 1A).
Lanes 1-6 are different harvest batches. Lane 7 is a sample of Aranesp® used as control. The isoforms of Aranesp® have a pI range of 3-3.9 (see Franc oise Lasne et al., Analytical Biochemistry 311 (2002) 119–126).
Figure 2 illustrates an IEF Gel gel, performed as described in example 2 of harvest of cells cultured in medium supplemented with 100 µM manganese chloride but devoid of trivalent ferric ions. Lane 1 is Aranesp® and lane 2-9 is the Darbepoetin alfa samples showing low pI isoforms expressed in the presence of manganese chloride.
Figure 3 illustrates an IEF gel, performed as described in example 2 showing the low pI isoforms of Darbepoetin alfa. Lane 1 is harvests of cells grown in the presence of 80 µM manganese and Lane 3 is with 38 µM ferric ammonium citrate, were subjected to purification by methods disclosed in International Application No. PCT/US2009/048239 and incorporated
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herein by reference. Purified samples were then analysed by IEF. Aranesp® (Lane 2) was used as a control.
Figure 4 illustrates a profile showing the viable cell count and percentage viability against age. Line A represents the VCC and percentage viability of cells cultured in the absence of manganese chloride. Line B represents the VCC and percentage viability of cells cultured in the presence of manganese chloride. Lines A and B show similar viable cell count pattern indicating higher concentration of manganese chloride does not effect the viable cell count and hence is not cytotoxic.
DETAILED DESCRIPTION OF THE INVENTION
The term "isoform", as used herein, refers to proteins with identical amino acid sequence that differs with respect to charge and therefore isoelectric point, as a result of differences in glycosylation, acylation, deamidation or sulfation.
The “isoelectric point” or “pI” is the pH at which a particular molecule or surface carries no net electrical charge. The "pI" of a polypeptide refers to the pH at which the polypeptide's positive charge balances its negative charge. The pI can be estimated by various methods known in the art, e.g., from the net charge of the amino acid and/or sialic acid residues on the polypeptide or by using isoelectric focusing, chromatofocussing, etc.
The “low pI isoforms” refer to isoforms with the pI of about 5 or less.
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“Non toxic amount or levels” refers to the concentration of metal ions (manganese or trivalent ferric ions) in the cell culture media at which there is no reduction in the cell viability, cell growth, or protein production as compared to media devoid of metal ions.
Erythropoiesis-stimulating proteins, such as human erythropoietin (EPO) and analogs of erythropoietin are glycoprotein hormones that are the principle homeostatic regulators of red blood cell production and are used in the treatment of anemia caused by chronic renal failure.
Darbepoetin alfa, an EPO analog with five N-linked carbohydrate chains and up to 22 sialic acids, exhibits a three-fold longer serum half-life and increased in vivo activity as compared to recombinant human EPO. Compositions of erythropoiesis stimulating proteins, such as darbepoetin alfa comprise a mixture of various isoforms that are known to differ in their extent of glycosylation and sialylation. The low pI isoforms of rHuEPO and darbepoetin alfa, as result of higher of sialic acid content, exhibit much higher specific activity as compared to isoforms of higher pI (having low sialic acid content) (e.g. Imai et al., "Physicochemical and Biological Characterization of Asialoerythropoeitin," Eur. J. Biochem. 194 (2), pages 457-462 (1990), 457-462;and EP-A-0 428 267). Thus, isoforms with higher sialic acid content and lower pI are of greater therapeutic value. For example, Aranesp® (Amgen Corporation), the approved and marketed form of darbepoietin alpha, comprises essentially low pI isoforms of the protein, in the pI range, 3.0-3.9 (see Franc Lasne et al., "Detection of isoelectric profiles of erythropoetin in urine:differentiation of natural and administered recombinant hormones," Analytical Biochemistry, Vol 311 (2), pages 119-126, (2002)
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Exemplary sequences, manufacture, purification and use of darbepoetin and other erythropoietin analogs are described in a number of patent publications, including Strickland et al., WO 91/05867, Elliott et al., WO 95/05465, Egrie et al., WO 00/24893, and Egrie et al. WO 01/81405, each of which is incorporated herein by reference in its entirety.
The present invention provides a cell culture medium comprising high levels of trivalent ferric ions (Fe3+) which is effective in increasing the levels of low pI isoforms of erythropoiesis stimulating protein without affecting the viability of the cell culture.
The addition of high concentrations of divalent manganese ions and/or trivalent ferric ions to the culture medium results in an increase in the number and yield of low pI isoforms of erythropoiesis-stimulating molecules, such as human erythropoietin, or its biologically active variants, derivatives (including chemically modified derivative), or analogs such as darbepoetin.
U.S. Patent Application Publication No.US20070161084 discloses the use of Mn+2 ions in the culture medium at concentrations of 0.01 µM – 50 µM to improve the yield of erythropoiesis stimulating protein. However toxicity and poor protein yield is reported at concentrations of Mn+2 greater than 50 µM. In contrast, the present invention discloses a process of increasing the levels of low pI isoforms of erythropoiesis stimulating protein by growing cell cultures expressing erythropoiesis stimulating proteins in a culture medium comprising Mn+2 without any concomitant decrease in cell viability.
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The present invention provides a process for increasing the levels of low pl isoforms of darbepoetin alfa, comprising culturing Chinese hamster ovary cell lines expressing said darbepoetin alfa, in a medium comprising trivalent ferric ions (Fe+3) having concentrations in a range of 9 µM to 40 µM wherein the darbepoetin alfa so obtained demonstrates an increase in low pl isoforms in comparison to darbepoein alfa obtained from cells cultured in absence of said concentration of iron (Fe+3) as determined by isoelectric focusing.
In embodiments, the concentration of ferric ions (Fe+3) in the culture medium ranges from about 9 µM to 40 µM, or from about 20 µM to 40 µM, or is about 38 µM.
Examples of salts of trivalent ferric ions include, but not limiting to, ferric ammonium citrate (FAC), ferric ammonium oxalate, ferric ammonium fumarate, ferric ammonium malate, ferric ammonium succinate, ferric citrate and ferric nitrate monohydrate.
In another embodiment, the invention provides a culture medium comprising salts of trivalent ferric ions at high non-toxic concentrations which are effective in increasing the levels of low pI isoforms of erythropoiesis stimulating protein and but does not reduce the cell viability, cell growth or protein.
Without wishing to be bound by any particular theory, the use of the selenium, poloxamers, or both in a cell culture medium may prevent adverse effects of high concentrations of Fe+3 ions on the cells, thereby maintaining cell viability.
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Thus, in embodiments, the cell culture media comprise selenium, or salts of selenium such as sodium selenite. In embodiments, the cell culture media comprise a poloxamer such as Pluronic® F68. In embodiments, the cell culture media comprise sodium selenite and a poloxamer such as Pluronic® F 68.
The culture medium can also comprise any other nutrient ingredient known in the art, such as carbohydrates, including glucose, essential and non-essential amino acids, lipids and lipid precursors, nucleic acid precursors, vitamins, inorganic salts, trace elements including rare metals, and cell growth factors. The culture medium may be a chemically defined medium or may include serum, plant hydrolysates, or other undefined substances. The culture medium may be entirely serum-free or free of any animal-component. Exemplary medium used for culturing the cells include DMEM/F-12 media about 15.6 g/L(Gibco). DMEM medium comprises the following inorganic salts: Calcium Chloride, Cupric sulfate, Potassium Chloride, Magnesium Sulfate or Chloride, Sodium Chloride, Sodium Dihydrogen Phosphate, Sodium Bicarbonate, Zinc sulfate; the following amino acids L-Alanine, L-Arginine, L-Asparagine, L-Aspartic acid, L-Cysteine, L-Glutamic acid, L-Glutamine, Glycine, L-Histidine, L-Isoleucine, L-Leucine, L-Lysine, L-Methionine, L-Phenylalanine, L-Proline, L-Serine, L-Threonine, L-Tryptophan, L-Tyrosine, L-Valine; the following lipids and vitamins: Biotin, D-Calcium-Pantothenate, Choline Chloride, Folic Acid, myo-Inositol, Niacinamide, Nicotinamide, Pyridoxine, Riboflavin, Thiamine, Vitamin B12 (cobalamin), Thymidine, Linoleic Acid, Lipoic Acid; and other
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components including D-Glucose, Phenol Red, Hypoxanthine, Sodium Pyruvate, Putrescine, and HEPES. The culture medium may also include supplementary amino acids, e.g. asparagine, aspartic acid, cysteine, cystine, isoleucine, leucine, tryptophan, and valine. The medium may also comprise lipids and/or lipid precursors such as ethanolamine 5.1 µg/L, Soy Peptone 1.0 g/L, Wheat peptone 1 g/L, Fructose 1.0 g/L, sodium bicarbonate 1.2 g/L, Pluronic F-68 1.0 g/L, HyQ 35 mL/L, sodium selenite 5 µg/L, and insulin 5 mg/L. In a preferred embodiment, the addition of MnCl2 is done in after a period of rapid growth phase of about 9 days along with the HyQ feed of about 35 mL/L.
The following examples will further illustrate certain aspects and embodiments of the invention in greater detail and are not intended to limit the scope of the invention.
Example 1
Preparation of the clone
Darbepoetin alfa producing cell lines were made by transduction of the CHO-S parental cell line with expression retrovector encoding gene of darbepoetin alfa. Following transduction, the pooled population of cells expressed up to 36 µg/ml of darbepoetin alfa after 10-14 days in T-flasks. The pooled population of cells were diluted to very low cell density (1-3 viable cells/ 200 µl media) and plated in 96 well microtiter plates to establish clonal cell lines that originated from single cells. Clones were screened for darbepoetin alfa production and the clones with high productivity were selected for expression.
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Example 1A:
Expression of darbepoetin alfa in the absence of either manganese or trivalent ferric ions
The cells expressing darbepoietin alpha were expanded in spinners and seed reactor before being inoculated into the production reactor. PF CHO HyClone medium (Catalog # SH30333.03) was used for culturing the cells in spinner bottle. Table 1 shows the composition of PF CHO medium used for culturing cells. The pH of the medium was at 7 and cells from the master cell bank were inoculated at an initial cell count of about 0.2 million cells/mL. The spinner bottles were incubated in a 5% CO2 and at 37 °C for 72 h. After the spinner stages, cells were inoculated in a 6.5 L seed reactor containing 4 L SFM-6 medium (composition Table 2) at an initial cell density of 0.2 million cells/mL, the pH was maintained at 7, temperature at 37.0 °C and dissolved oxygen was maintained at 40%. After 72 h the seed reactor culture was aseptically harvested and the cells were transferred to an 11.5 L production reactor containing 10 L of SFM-6 medium at an initial cell density of 0.2 million cells/mL and fermentation continued under conditions maintained as in the seed reactor described above. The production culture was harvested after 12 days, supernatant collected and analyzed.
Table 1: Composition of PF-CHO medium
PF-CHO main powder
6.0 g
PF-CHO base powder
10.4 g
L-Glutamine
0.58 g
*Pluronic® F-68
1.0 g
sodium bicarbonate
2.0 g
pH
7.0
*PLURONIC® F 68 is a copolymer of ethylene oxide and propylene oxide, having an average molecular weight of 8400, and is sold by BASF (Germany).
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Table 2: Composition of SFM-6 medium
DMEM/F-12”
15.6 g
MEM amino acids solution (50X)
10 mL
MEM vitamin solution (100 X)
10 mL
TE 30 (1000X)
1 mL
Sodium selenite
5 µg,
Ethanolamine
5.1 µg
Soy Peptone
1.0 g
Wheat peptone
1.0 g
Fructose
1.0 g
Sodium bicarbonate
1.2 g
Pluronic® F-68
1.0 g
HyQ
35 ml
Insulin
5.0 mg
Example 1B:
Expression of darbepoetin alfa in the presence of manganese
The culture medium in the seed reactor stage and production stage were supplemented with manganese chloride to a final concentration of 80 to 100 µM. All other conditions were maintained as described in example 1A. The cell viability so obtained was compared to the viability obtained in example 1A (see Figure 3 and Figure 4).
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Example 1C:
Expression of darbepoetin alfa in the presence of trivalent ferric ions
The culture medium in the seed reactor stage and production stage were supplemented with ferric ammonium citrate to a final concentration of 38 µM. All other conditions were maintained as described in example 1A.
Example 2:
Isoform analysis by IEF
Harvests from examples 1A, 1B and 1C were analyzed by IEF for the expression of darbepoetin alfa. The gel for IEF was prepared using water, urea, 30 % acrylamide, and ampholyte (pH range 2-4 and 3-10). The above components were mixed gently and 10 % w/v ammonium per sulphate and TEMED were added to mixture and the mixture was casted in gel sandwich apparatus (BIORAD Mini Protean Cell) and fitted with a comb. The gel was allowed to polymerize for 45 minutes at room temperature. A small amount of this sample was mixed with equal volume of sample buffer (glycerol, ampholyte and Milli Q water) and loaded into the gel. The gel was then placed in BIORAD Mini Protean Cell assembly and filled with Cathode buffer (25 mM sodium hydroxide) and Anode buffer (25 mM ortho phosphoric acid) in separate compartments. Samples were run at 200 V constant voltage for 1.5 h for pre-focusing of ampholytes at room temperature and then the voltage was increased to 400 V and run for next 1.5 h at room temperature. After the run, the gel was carefully removed and stained using either a Coomassie stain or Silver stain. The samples were analyzed (Figure 2 and Figure 3).
We Claim:
1. A process for increasing the levels of low pl isoforms of darbepoetin alfa, comprising culturing Chinese hamster ovary cell lines expressing said darbepoetin alfa in a medium comprising trivalent iron ions having a concentration in a range of 9 pM to 40 pM, wherein the darbepoetin alfa so obtained demonstrates an increase in low pl isoforms in comparison to darbepoetin alfa obtained from cells cultured in absence of trivalent iron ions as determined by isoelectric focusing.
2. A process as claimed in claim 1, wherein the concentration of trivalent iron ions is preferably 9 pM to 40 pM, more preferably 20 pM to 40 pM, and particularly 38 pM.
3. A process as claimed in claim 1, comprising culturing cell lines expressing said darbepoetin alfa in a medium comprising non-toxic amounts of trivalent iron ions.
4. A process as claimed in claims 1 to 3, wherein the trivalent iron ions are derived from salts of iron.
5. A process as claimed in claims 1-4, wherein the trivalent iron ions are derived from ferric ammonium citrate.
6. A process as claimed in claims 1 to 5, wherein the medium further comprises selenium or a salt of selenium.
7. A process as claimed in claim 6, wherein the salt of selenium is sodium selenite.
8. A process as claimed in claims 1 to 7, wherein the medium further comprises a poloxamer.
9. A process as claimed in claims 1 to 8, wherein a medium further comprises selenium or a salt of selenium, and a poloxamer.