US20180100176A9

US20180100176A9 - Cell expression system

Info

Publication number: US20180100176A9
Application number: US15/148,930
Authority: US
Inventors: Noelle Sunstrom
Original assignee: Neuclone Biologics Pty Ltd
Current assignee: Neuclone Biologics Pty Ltd
Priority date: 2013-03-08
Filing date: 2016-05-06
Publication date: 2018-04-12
Anticipated expiration: 2034-02-17
Also published as: JP6464100B2; PL2964768T3; EP2964768A1; US20160017281A1; EP2964768A4; US20220002776A1; EP2964768B1; US20160244799A1; CA2904146A1; US20180195102A1; CA2904146C; KR20150125994A; US20200040371A1; US9404078B2; WO2014134657A1; AU2014225272A1; JP2016510592A; MY186731A; AU2016200361B2; AU2014225272B2

Abstract

An expression system for expressing a protein comprising: a eukaryotic host cell carrying a dihydrofolate reductase (DHFR) deficiency; and an expression vector, the expression vector encoding the human growth hormone gene; a expression vector, the expression vector comprising: a eukaryotic selectable marker including a minimal SV 40 early promoter driving expression of a sequence encoding dihydrofolate reductase for complementing the DHFR deficiency in the host cell; a prokaryotic selectable marker conveying Ampicillin resistance to a prokaryotic host cell; a prokaryotic Origin of Replication; a plurality of multiple cloning sites (MCS); and at least one protein expression module comprising: a Simian Vacuolating Virus 40 (SV40) early promoter, inclusive of its 72 bp enhancer repeats; and a rabbit β-globin intron sequence being separable from a SV40 p A sequence by a first multiple cloning site, for receiving a coding sequence and expressing a desired protein therefrom.

Description

TECHNICAL FIELD

The present invention relates to expression systems, in particular the invention relates to expression systems for the production of biological therapeutics.

BACKGROUND

Expression systems for the production of biological therapeutics or biopharmaceuticals, such as recombinant proteins, generally consist of a nucleic acid vector construct encoding the desired recombinant therapeutic and a chosen host cell. The vector is introduced into the host cell and the endogenous cell machinery is utilised for the production of the desired therapeutic i.e. the desired recombinant protein. The intricacies in establishing an efficient and reliable expression system for the production of approvable biological therapeutics are manifold. However, well-established expression systems may provide cost-effective alternatives for the production of pharmaceutical products otherwise difficult to obtain.
Efficiency of the system itself depends on a large variety of factors including the design of the vector and the choice of host cell. The strategic combination of regulatory elements, selection markers and stability elements within the vector sequence have to balance simple manipulation and application of the vector with high yield production of the desired biological therapeutic. Determining a cell's suitability to act as host cell in such an expression system is primarily governed by the need to maximise compatibility between the endogenous cell machinery and the regulatory elements present in the vector, while keeping potential adventitious contaminants in the final product minimal. Further, availability, cost and acceptability for regulatory approval of any therapeutic produced by the system, have to be considered.
Nucleic acid vectors used in expression systems comprise plasmids, cosmids, Yeast Artificial Chromosomes (YACs), Bacterial Artificial Chromosomes (BACs), retroviral. adenoviral and lentiviral vectors. These vectors differ in many characteristics, such as their capacity to accommodate different sized nucleic acid inserts. their most efficient introduction method into the host cell and specifically in their utilisation of the endogenous cell machineries of different types of host cells to ensure sufficient expression of the desired protein.
Regulatory elements commonly present in such expression vectors influence transcription. translation as well as protein synthesis of selection markers and of sequences encoding the desired biological therapeutic. Such regulatory elements include, but are not limited to, promoters, terminators, modifiers, insulators, spacers, regulatory protein binding sites, introns, inducers, etc.
Known promoters include constitutively active promoters such as the thymidine kinase (TK) promoter, the actin promoter, the glyceraldehyde 3-phosphate dehydrogenase (GAPDH) promoter, the simian vacuolating virus 40 (SV40) early promoter, the cyclin T1 promoter, the RNA polymerase III U3 promoter, the cyclophillin promoter, the cytomegalovirus (CMV) promoter, the Autographa californica nuclear polyhedrosis virus (AcNPV) P10 promoter and the β3-galactosyltransferase 5 (β3GAL-T5) promoter.
Known promoters also include inducible promoters such as the heat shock protein 70 (HSP70) promoter (stress induced), the heat shock protein 90 (HSP90) promoter (stress induced), the alcoholdehydrogenase I (alcA) promoter (alcohol induced), the activating copper-metallothionein expression (ACE1) promoter (metal induced), the small subunit of ribulose-1,5-bisphophate-carboxylase (SSU1) promoter (light induced), the hypoxia induced factor 1α (hif1α) promoter (hypoxia induced), the inducer of meiosis 2 (IME2) promoter (starvation induced), the glucocorticoid receptor (hormone induced), the estrogen receptor (hormone induced) and the ecdysone receptor (hormone induced).
Further, cell type/tissue specific promoters, such as the nkx2.5 promoter (heart cells), the islet 1 promoter (pancreatic cells), the MyoD promoter (muscle cells), the cluster of differentiation 2 (CD2) promoter (T-cells) and the collagen II promoter (cartilage), are known to change their level of activity in response to cell type specific stimuli or to progression through developmental stages.
Known terminator elements, such as the RNA Polymerase II terminator, the small nucleolar RNA 13 (snR13) terminator, the bovine growth hormone (BGH) terminator, the simian virus 40 (SV 40) terminator and the thymidine kinase (TK) terminator, may provide suitable polyadenylation signals.
Known modifier and insulator elements include the tetracycline operator/receptor (tetO/tetR) system, the upstream activating sequence of the galactose dependent GAL4 transcription factor (GAL4 UAS), the adenovirus early region B1 TATA box, binding sites for the herpes simplex virus (HSV) regulatory protein VP16, the 5′HS4 chicken β-globin insulator, the paternally expressed gene 3 (Peg3) insulator and the sea urchin arylsulfatase (ARS) gene insulator.
While many attempts have been made to establish efficient and reliable expression systems for the production of approvable biological therapeutics, problems relating to low yield and adventitious contamination of the produced biopharmaceuticals remain. Choosing the most effective combination of suitable regulatory elements from the plethora of options, such that the system conveys stability and the highest degree of compatibility with the endogenous host cell machinery, poses a major challenge in the field.
Obtaining regulatory approval for a biopharmaceutical product poses a further challenge. Regulatory approval involves determination of the safety and efficacy of the pharmaceutical product prior to marketing. The process of gaining regulatory approval for innovator drugs is very time consuming and expensive. However, once approved, these drugs may be very profitable. particularly when they are marketed under exclusivity rights such as patent protection.
The profitability of innovator drug's market may provide a substantial incentive to exploit this market once patent rights have expired. Following patent expiry, innovator drugs can be marketed as generic drugs or biosimilars for drugs produced by recombinant DNA technology. Generic versions of blockbuster biopharmaceuticals near patent expiry include Epogen (erythropoietin, EPO) and Neupogen (granulocyte colony stimulating factor, G-CSF). The approval of a follow-on version of Pfizer's Genotropin (recombinant human growth hormone) seems to indicate a change in a landscape where previously, biopharmaceuticals enjoyed immunity from competition even after expiration of their patent protection. At present, there are over 80 generic versions of biopharmaceuticals in development (Datamonitor 2010).
Biosimilars ideally are bioequivalents of the innovator drugs and, as such, the path to regulatory approval for biosimilars is in theory less arduous than for the original innovator drug as the clinical data establishing safety and efficacy have been carried out.
Approval of generic biopharmaceuticals is dependent on comparable dosage form, strength, route of administration, quality. performance characteristics and intended use compared with approved biopharmaceuticals (that is, the reference listed drugs). For example, under the United States Food and Drug Administration (FDA), approval for a generic drug involves an “Abbreviated New Drug Application” (ANDA) which generally does not include pre-clinical and clinical data to establish safety and effectiveness. Approval also involves a bioequivalence review, which establishes that the proposed generic drug is bioequivalent to the reference-listed drug. This bioequivalency is based upon a demonstration that the rate and extent of absorption of the active ingredient in the generic drug fall within the scope of the parameter of the reference listed drug.
Importantly, there is a chemistry/microbiology review process that provides an assurance that the generic drug will be manufactured in a reproducible manner under controlled conditions to ensure that the drug will perform in a safe and acceptable manner.
Although guidelines for the approval of biosimilar drugs exist, there is uncertainty in regard to the practicalities of regulatory approval of biosimilars. Much of the uncertainty is driven by the lack of a clear practical and detailed regulatory pathway for the approval of such drugs and the scientific debate over product comparability and interchangeability. The uncertainties resulting from the manufacture of biosimilar drugs under conditions different than those used by the innovator suggest that it may be impossible to develop a true “generic” version of a biotechnology drug. Indeed, regulatory authorities in Europe and the US have shunned the use of the term “biogeneric”, preferring the nomenclature “biosimilar” and “follow-on biologicals”.
Many quality concerns for expression system-derived biopharmaceuticals have originated from the presence of adventitious contaminants or from the properties of the host cells used to prepare the product. Several of these products have also had quality concerns regarding the expression vector of the system. It is well established that cell properties and events linked to cell culture can affect resultant product quality and safety. Effective quality control of recombinant products requires appropriate controls on all aspects of handling the cell and cell culture. This is particularly relevant to the development of biosimilars.
Previously, Chinese Hamster Ovary (CHO) cells were modified and engineered to produce insulin. However these CHO cells were unable to express biological active insulin and thus the patents associated with this type or method of CHO cell modification were not commercial exploited. Fully functional insulin was not produced in CHO cells due to cryptic splicing of the insulin gene message by the translational machinery In the CHO cell.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

SUMMARY

Means for Solving the Problem

A first aspect of the present invention may relates to novel expression systems and methods of employing an expression system according to the invention that may, in certain embodiments, increase the yield and decrease the cost of manufacture.
This invention relates to methods of production of recombinant biosimilars, bio-pharmaceuticals and other desirable proteins, polypeptides and peptides using mammalian cell cultures. In particular, the methods of the invention involve the use of specially bioengineered mammalian cell lines for the production of complex proteins in low cost media. These cell lines have the acquired ability for autonomous growth in cheap. reproducible, fully-defined protein-free medium, with the cells expressing and secreting its growth factor requirements.
Accordingly, in a first aspect, the present invention provides an expression system for expressing a protein comprising:
a eukaryotic host cell carrying a dihydrofolate reductase (DHFR) deficiency an expression vector, the expression vector encoding the human growth hormone gene;
an expression vector, the expression vector comprising:
a eukaryotic selectable marker downstream of for expression of a sequence encoding dihydrofolate reductase for complementing the DHFR deficiency in the host cell;
a prokaryotic selectable marker conveying Ampicillin resistance to a prokaryotic host cell;
a prokaryotic Origin of Replication
a plurality of multiple cloning sites (MCS); and
at least one protein expression module comprising:
a Simian Vacuolating Virus 40 (SV40) early promoter, inclusive of its 72 bp enhancer repeats; and a rabbit β-globin intron sequence being separable from a SV40 polyadenylation sequence by a first multiple cloning site, for receiving a coding sequence and expressing a desired protein therefrom.
Preferably, the expression system further comprises a second protein expression module, the second protein expression module including: a Cyotomegalovirus promoter being separable from a SV40 polyadenylation sequence by a second multiple cloning site for co-expression of at least two proteins from the expression modules.
Preferably, the protein is the subject of a request for regulatory approval and wherein the host cell is subjected to a plurality of predetermined manipulations such that the host cell expresses said protein; and wherein information is recorded on each manipulation and each manipulation is carried out in a manner which prevents contact of the host cell with a contaminating agent; and wherein the information is used to generate a history record of the host cell for inclusion in a submission to a regulatory agency involved in assessing the safety and efficacy of drugs thereby expediting regulatory approval of the protein.
The predetermined manipulations preferably comprise:

- (i) ligating the coding sequence encoding the desired protein into the expression vector to produce a recombinant vector;
- (ii) introducing the recombinant vector into the host cell; and
- (iii) culturing the host cell under conditions such that the protein is expressed by the host cell.

Preferably the recombinant vector is at least in part incorporated into the genome of the host cell. Step (iii) includes growing the host cell in a medium that contains no animal or plant derived proteins or peptides and no undefined hydolysates or lysates thereby reducing contact of the host cell with a contaminating agent.
In a particularly preferred embodiment, the host cell is a Chinese Hamster Ovary (CHO) DG44 cell.
In certain preferred embodiments, the host CHO DG44 expresses a growth hormone. In certain preferred embodiment the growth hormone is human growth hormone. In certain preferred embodiments, the protein may be a biosimilar drug.
In a second aspect, the present invention provides a method of managing the development of a protein expressed by the expression system according to the first aspect wherein the protein is the subject of a request for regulatory approval, the method comprising the steps of:

- a. subjecting the host cell to a plurality of predetermined manipulations such that the cell expresses the protein;
- b. recording information on each manipulation wherein each manipulation is carried out in a manner which prevents contact of the host cell with a contaminating agent;
- c. using the information to generate a history record of the host cell for inclusion in a submission to a regulatory agency involved in assessing the safety and efficacy of drugs; and
- d. including the history record in the submission to the regulatory agency for regulatory approval of the product.

In a third aspect, the present invention provides a method of expediting regulatory approval of a protein expressed by the expression system according to the first aspect the method comprising:

- b. subjecting the host cell to a plurality of predetermined manipulations such that the host cell expresses the product;
- c. recording information on each manipulation wherein each manipulation is carried out in a manner which prevents contact of the host cell with a contaminating agent;
- d. using the information to generate a history record of the host cell for inclusion in a submission to a regulatory agency involved in assessing the safety and efficacy of drugs; and
- e. including the history record in the submission to the regulatory agency for regulatory approval of the protein.

Preferably, step (a) of the methods according to the second and third aspect comprises

The recombinant vector is preferably at least in part incorporated into the genome of the host cell.
Preferably, step (iii) of the methods according to the second and third aspect includes growing the host cell in a medium that contains no animal or plant derived proteins or peptides and no undefined hydrolysates or lysates thereby reducing contact of the host cell with a contaminating agent.
In a particularly preferred embodiment of the methods according to the second and third aspect, the protein is a biosimilar drug.
Another aspect of the present invention may also provide a method for producing a desired recombinant protein, polypeptide or peptide comprising the step of: culturing a mammalian host cell in culture medium, wherein said host cell includes:

- (i) at least one introduced DNA sequence encoding a protein, polypeptide and/or peptide factor(s) required for growth of the host cell in said culture medium. expressibly linked to a constitutive promoter (e.g. CMV and SV40 promoters) The invention thereby enables the use of low cost, protein/serum-free medium by utilising a host cell which is able to produce the protein, polypeptide and/or peptide growth factor(s) required for its growth in such medium. The culture medium used in the method of the invention is, therefore, preferably serum-free or otherwise free of protein, polypeptide and/or peptide growth factor(s) necessary for the growth of the particular host cell type. However, methods wherein the culture medium includes one or more of the required growth factor(s) and the host cell itself expresses one or more of the same and/or other required growth factor(s), is also to be regarded as falling within the scope of the invention.

The mammalian host cell may be any of those commonly used in the art for expressing recombinant proteins, polypeptides or peptides. For example, the host cell may be a Chinese Hamster Ovary (CHO) cell such as CHO-K1, CHO-DG44 DHFR- and CHO-S. These include both adherent and suspension cell lines. Also other cell lines described within the embodiments of the present invention may also be used or preferred.
The introduced DNA sequence(s) may be present on plasmids or otherwise integrated into the host cell chromosomes (e.g. by homologous recombination).
The DNA sequence(s) encoding the protein, polypeptide and/or peptide factor(s) required for growth of the host cell, may be selected from DNA sequences encoding human Growth Hormone (hGH), modified hGH and other growth factors and mixtures thereof. Where the host cell is CHO it is preferable that the host cell includes DNA sequences encoding human Growth Hormone (hGH).
In the context of the present invention, the words “comprise”, “comprising” and the like are to be construed in their inclusive, as opposed to their exclusive, sense, that is in the sense of “including, but not limited to”.
In the context of the present invention, the words “comprise”, “comprising” and the like are to be construed in their inclusive, as opposed to their exclusive, sense, that is in the sense of “including, but not limited to”.
The invention is to be interpreted with reference to the at least one of the technical problems described or affiliated with the background art. The present aims to solve or ameliorate at least one of the technical problems and this may result in one or more advantageous effects as defined by this specification and described in detail with reference to the preferred embodiments of the present invention.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a plasmid vector map entitled “pNAS-hGH” for the high level expression human growth hormone (hGH) inserted within a pNAS vector with use with NeuCHO cell line;

FIG. 1B is an expression vector map entitled “pNeu” used for the high level expression of a single chain protein in CHO DG44 cells;

FIG. 1C is an expression vector map entitled “pNeu-IRES-DHFR” used for the high level expression of a single chain protein in CHO DG44 cells. A dicistronic expression cassette with recombinant gene in 1^stcistron followed by DHFR gene in 2^ndcistron;

FIG. 1D is an expression vector map entitled “pNeuMAB” used for the high level expression of heavy and light antibody chains and/or recombinant monoclonal antibodies;

FIG. 1E is an expression vector map entitled “pNeuMAB-IRES-DHFR” used for the high level expression of heavy and light antibody chains;

FIG. 1F is an expression vector map entitled “pNeuMAB-IRES-DHFR (CMV)” used for the high level expression of heavy and light antibody chains;

FIG. 1G is a single chain expression vector map entitled “pMAB LC (IRES-DHFR)” used for expression of light chains (LC);

FIG. 1H is a single chain expression vector map entitled “pMAB HC” used for expression of heavy chains (HC);

FIG. 2. depicts a growth chart demonstrating viable cell density plotted against time in respect of various cultures and cells used. Growth of DG44 Cell Lines expressing hGH compared to the Parental DG44 Cell Line and a DG44 Cell Line expressing the IGF-1 gene;

FIG. 3. depicts a graph comparing the integral of viable cell densities against time for various preferred organisms and culture; and

FIG. 4. depicts a comparison chart showing the relative expression levels of proteins from either CHO DG44 cells or NeuCHO cell lines.

This specification also includes the following genetic sequence information relating to expression vectors:
Sequence No. 1 depicts the preferred coding sequence for the expression vector pMAB HC;
Sequence No. 2 depicts the preferred coding sequence for the expression vector pMAB LC(ires-dhfr);
Sequence No. 3 depicts the preferred coding sequence for the expression vector pNAS-hGH;
Sequence No. 4 depicts the preferred coding sequence for the expression vector pNeu;
Sequence No. 5 depicts the preferred coding sequence for the expression vector pNeu-IRES-DHFR;
Sequence No. 6 depicts the preferred coding sequence for the expression vector pNeuMAB;
Sequence No. 7 depicts the preferred coding sequence for the expression vector pNeuMAB-IRES-DHFR (CMV);
Sequence 8 depicts the preferred coding sequence for the expression vector pNeuMAB-IRES-DHFR;
Please note that in this specification Sequence No. is same and the equivalent term to SEQ ID NO.

DESCRIPTION OF THE INVENTION

Preferred embodiments of the invention will now be described with reference to the accompanying drawings and non-limiting examples.
It has been found that events during the culture of a cell may contribute significantly to the assessment of the risks associated with the use of that particular cell for production of proteins and more particularly proteins for therapeutic use.
Diligent records of all manipulations including the history of a cell throughout development, extending to the parental cell line from which it was derived. may contribute to the quality and safety of the final product.
In one scenario, such information may be important for gaining regulatory approval of protein therapeutics expressed from a cell. In particular, biosimilars present unique issues. These issues include demonstrating that immunogenicity of the biosimilar has not been altered with respect to the reference listed drug, as well as ensuring that there are no undetected differences in the product that may potentially impact the safety and efficacy of the drug. Resolving such issues would be problematic without conducting extensive clinical trials. As such, it is likely that any application for a biosimilar would be required to demonstrate that there are no clinically meaningful differences in safety, purity and potency between the biosimilar and the reference listed drug. Moreover, an application for a biosimilar would need to provide evidence that the biosimilar has “profound similarity” (as it is impractical to demonstrate identical biological products) and that the biosimilar will produce the same clinical result as the reference listed drug in any given patient.
In order to gain regulatory approval, traditional generic manufacturers are required to demonstrate their drug is chemically identical to the referenced listed drug and exhibit the same properties in the human body as the original drug. In regard to biosimilars. it was previously not possible to readily demonstrate that a second-source biologic drug is unequivocally identical to an innovator drug due to the complexities of the synthesis of the drugs in potentially disparate biological systems. As such, biosimilars may exhibit slightly different properties to the original drugs that may necessitate abbreviated clinical trials in order to gain regulatory approval.
In the context of the present invention, the term “contaminating agent” refers to any agent that can potentially compromise regulatory approval of a product by a regulatory agency. Such agents may include but are not limited to adventitious agents such as viruses, bacteria, fungi and mycoplasma or proteins there from.
As used herein the term “cell expressed product” refers to any product produced by the cell, including but not limited to proteins. peptides, glycoproteins, carbohydrates, lipids, glycolipids and nucleic acids.
The term “regulatory approval” in so far as it relates to a product defined in the context of this specification, refers to approval from a regulatory authority which permits marketing of the product.
The term “safety and effectiveness studies” refers to any studies conducted on a product that assess the safety and efficacy of that product for human and/or animal administration.
The term “clinical trials” refers to studies involving either animal or humans designed assess the safety and/or efficacy of a product for a therapeutic application.
The term “abbreviated safety effectiveness studies and/or abbreviated clinical trials” refers to studies carried out on a drug which does not involve complete phase I, II and III clinical trials. Such studies may include a bioequivalence review and a chemistry/microbiology review as defined by the US Food and Drug Administration (FDA).
The term “biosimilar drug” and “biosimilar” refer to a bioequivalent pharmaceutical of a drug in which patent protection has expired and where the previously protected drug has regulatory approval. In particular, this includes products prepared in cell culture by recombinant DNA technology. The term “biosimilar drug” and “biosimilar” as used herein is equivalent to the terms “follow-on biologicals” or “biosimilars”.
The term “protein” refers to a “complete” protein as well as fragments. derivatives or homologs or chimeras thereof comprising one or more amino acid additions, deletions or substitutions, but which substantially retain the biological activity of the complete protein.
The embodiments of the present invention will now be described by reference to the following non-limiting examples.

Example 1

Construction of pNeu and pNAS vectors for high level expression of recombinant therapeutic protein
The vector pNeu was designed for high-level expression of single chain peptides for the production of therapeutic proteins. The vector facilitates the insertion DNA sequences into a convenient multiple cloning site for expression in CHO cells. See Table 1 and FIG. 1A for a description of the vector and its component features.
The 5026 bp vector encodes essential coding and regulatory sequences for the efficient expression of the recombinant gene as well as essential sequences for the selection and propagation of the plasmid in bacteria. It was designed for chemical synthesis and is void of nonessential and redundant sequences that are common components in commercial expression vectors. This allows for ease of genomic insertion with less likelihood of deletion of sequences during plasmid propagation resulting in loss of expression. The multiple cloning site encodes a minimum of two unique restriction sites for rapid gene cloning.

Example 2

Synthesis and Cloning of Human Growth Hormone (hGH) cDNA into pNAS

The amino acid sequence encoding for hGH was subjected to bioinformatic analysis through proprietary third party software by GENEART AG, Regensburg Germany. Codon options were utilized to maximize expression by improving mRNA maintenance and the exploitation of available tRNA pools in CHO cells. RNA and codon optimization was performed on the coding sequences. The gene was analysed with respect to splice site recognition, mRNA stability, presence of ribosomal entry sites, mRNA secondary structures, self-homology for the purpose of increasing gene expression in CHO cells. The hGH gene was cloned into pNAS using AgeI and EcoRV restriction sites using methods well known in the art.

Example 3

Construction of pNeuMAB Vector for Expression of Recombinant Monoclonal Antibody

The pNeuMAB vector was designed for the cloning and expression of recombinant monoclonal antibodies. The DNA encoding heavy and light chains are configured in the vector as two distinct and tandem transcription units. See Table 1 and FIG. 1B for a description of the vector and its component features.
Synthesis of cDNA Encoding Heavy Chain and Light Chain of an Antibody—Infliximab
The amino acid sequence encoding the heavy chain (HC) and light chain (LC) of the monoclonal antibody, Infliximab were subjected to bioinformatic analysis through proprietary third party software by GENEART AG, Regensburg Germany. Codon options were utilized to maximize expression by improving mRNA maintenance and the exploitation of available tRNA pools in CHO cells. RNA and codon optimization was performed on the coding sequences. The genes were analysed with respect to splice site recognition, mRNA stability, presence of ribosomal entry sites, mRNA secondary structures, self-homology for the purpose of increasing gene expression in CHO cells.
Cloning Gene Encoding Heavy Chain of Infliximab
The synthetic gene encoding for the heavy chain of Infliximab was assembled from synthetic oligonucleotides and/or PCR products. The fragment was cloned into pGA14 (ampR) using AscI and PacI restriction sites. The plasmid DNA was purified (Pure Yield™ Plasmid Midiprep, Promega) from transformed bacteria and concentration determined by UV spectroscopy. The final construct was verified by sequencing. The sequence congruence within the used restriction sites was 100%. The synthetic cDNA sequence encoding heavy chain of Infliximab was designed to incorporate unique restriction sites Age I and Eco RV at the 5′ and 3′ ends respectively for directional cloning into the first multiple cloning site of NeuClone's antibody expression vector, pNeuMAB digested with the same restriction sites.
Cloning Gene Encoding Light Chain of Infliximab
The synthetic gene encoding the light chain of Infliximab was assembled from synthetic oligonucleotides and/or PCR products. The fragment was cloned into pGA18 (ampR) using AscI and PacI restriction sites. The plasmid DNA was purified (Pure Yield™ Plasmid Midiprep, Promega) from transformed bacteria and concentration determined by UV spectroscopy. The final construct was verified by sequencing. The sequence congruence within the used restriction sites was 100%. The synthetic cDNA sequence encoding light chain of Infliximab incorporates the unique restriction sites Sal I and Mlu I at the 5′ and 3′ ends respectively for directional cloning into the second multiple cloning site of NeuClone's antibody expression vector, pNeuMAB digested with the same restriction sites.
Generation of NeuCHO
Transfection of DG44 Cells with pNAS-hGH
One of the preferred methods by which the expression vector encoding human growth hormone into the host CHO DG44 cell line and the status of the rDNA within the host (copy number. etc.) is as follows. Briefly, a total of 1.5×10e7 cells were transfected with 1.8 ug of linearized plasmid DNA together with 15 ul of FreeStyle MAX Reagent (Invitrogen) in a volume of 30 ml. The transfected cell cultures were incubated at 37 C. 8% CO2 on an orbital shaker platform. At 48 hours post transfection the cells were cultured in hypoxanthne- and thymidine-deficient, medium supplemented with Gentamycin at a final concentration of 500 ug/ml for selection of uptake of plasmid DNA. Clones were selected by limiting dilution cloning. Several single clones arising from a single cell were expanded and cell lines were characterised for production of human growth hormone. Resulting clones were examined for growth properties in comparison to the standard CHO DG44 cell line.
Transfection of NeuCHO with pNeuMAB Encoding Infliximab Genes
Linearized plasmid pNeuMAB DNA encoding Infliximab genes was used to transfect NeuCHO cell cultures At 48 hours post transfection the cells were cultured into hypoxanthne- and thymidine-deficient, medium to select for cells expressing the DHFR gene. A stable cell population was then subjected to subsequent stepwise increasing methotrexate (MTX) concentration (50-, 100-, 200-, 400-, 800 nM, 1 uM) in order to amplify template DNA copy number and gene expression. Clones were selected by limiting dilution cloning. Clones with high level expression of infliximab protein were scaled up for protein production.

Example 4

Cell Banking

A critical part of quality control involves the full characterization of cells. The cell banks are examined for adventitious agents (viral, bacterial, fungal and mycoplasmal). Documentation describing the type of banking system used, the size of the cell bank(s) the container (vials. ampoules and closure system used, the methods used for preparation of the cell bank(s) including the cryoprotectants and media used, and the conditions employed for cryopreservation and storage are provides.
The procedures used to avoid microbial contamination and cross-contamination by other cell types present in the laboratory, and the procedures that allow the cell bank containers to be traced are all made available. This includes a description of the documentation system as well as that of a labelling system which can withstand the process of preservation, storage, and recovery from storage without loss of labelling information on the container.
It is essential that production is based on a well-defined master and working cell bank system. During the establishment of the banks no other cell lines are handled simultaneously in the same laboratory suite or by the same persons. The origin, form, storage, use and expected duration at the anticipated rate of use are described in full for all cell banks.
The following table identifies some of the components and features of the various expression vectors using with either CHO DG44 or NeuCHO cell lines. The data has been divided into three tables for purposes of presentation in this patent specification.

TABLE 1

		pNeu-IRES-
Feature	pNeu	DHFR	pNAS	pNeuMAB

Multiple	One multiple	One multiple	One multiple	Two multiple
cloning site	cloning site for	cloning site for	cloning site for	cloning sites for
	insertion of	insertion of	insertion of	insertion of
	expression unit	expression unit	expression unit	expression units
	coding for	coding for	coding for	coding for heavy
	single chain	single chain	single chain	and light chains
	protein	protein	protein	of a monoclonal
				antibody
Strong	The SV40	The SV40	The CMV early	The SV40 virus
promoter/enhancer	virus early	virus early	promoter/enhancer	early
combination	promoter/enhancer	promoter/enhancer	drives	promoter/enhancer
			expression of	drives
			each	expression of
			transcription	the first and 2nd
			unit	transcription
				unit
Intron/intervening	The intron	The intron		The intron
sequence	sequence II	sequence II		sequence II from
	from rabbit	from rabbit		rabbit beta
	beta globin	beta globin		globin gene is
	gene is located	gene is located		located
	downstream of	downstream of		downstream of
	the promoter	the promoter		the promoter
	providing for	providing for		providing for
	increased	increased		increased
	expression and	expression and		expression and
	mRNA	mRNA		mRNA stability
	stability of the	stability of the		of the first
	transcription	transcription		transcription
	unit	unit		unit
Internal		For the
Ribosome		expression of
Entry Site		DHFR gene
(IRES)		downstream of
		2^nd
		transcription
		unit ensuring
		high level
		expression of
		2^ndcistron in
		cells growing
		in the presence
		of
		methotrexate
Polyadenylation	A strong	A strong	A strong	A strong
signal	polyadenylation	polyadenylation	polyadenylation	polyadenylation
	signal from	signal from	signal from S40	signal from S40
	S40 virus for	S40 virus for	virus for	virus is for
	efficient	efficient	efficient	efficient
	expression of	expression of	expression of	expression of
	recombinant	recombinant	recombinant	each
	gene.	gene.	gene	recombinant
				gene
DHFR gene	Auxotrophic	Auxotrophic		Auxotrophic
	selection in HT	selection in HT		selection in HT
	negative media	negative media		negative media
	eliminates the	eliminates the		eliminates the
	need to	need to		need to maintain
	maintain	maintain		selection
	selection	selection		pressure using
	pressure using	pressure using		antibiotics.
	antibiotics.	antibiotics.		Amplification of
	Amplification	Amplification		gene copy
	of gene copy	of gene copy		number is
	number is	number is		accomplished by
	accomplished	accomplished		the addition of
	by the addition	by the addition		methotrexate to
	of	of		the culture
	methotrexate	methotrexate		media. The
	to the culture	to the culture		murine DHFR
	media. The	media. The		gene is driven
	murine DHFR	murine DHFR		by a minimal
	gene is driven	gene is driven		SV40 early
	by a minimal	by a minimal		promoter
	SV40 early	SV40 early		lacking the
	promoter	promoter		enhancer
	lacking the	lacking the		sequence
	enhancer	enhancer
	sequence.	sequence.
Ampicillin	For	For	For propagation	For propagation
resistance	propagation of	propagation of	of plasmid in	of plasmid in
gene	plasmid in	plasmid in	bacteria	bacteria
	bacteria	bacteria
Neomycin			For selection in
gene			mammalian
			cells

TABLE 2

	pNeuMAB-IRES-	pNeuMAB-IRES-DHFR
Features	DHFR	(CMV)	pMAB-LC (ires-dhfr)

Multiple cloning	Two multiple	Two multiple	One multiple cloning
site	cloning sites for	cloning sites for	site for insertion of
	insertion of	insertion of	Light chain gene
	expression units	expression units
	coding for heavy	coding for heavy
	and light chains	and light chains of
	of a monoclonal	a monoclonal
	antibody	antibody
Strong	The SV40 virus	The SV40 virus	The SV40 virus early
promote/enhancer	early	early	promoter/enhancer
combination	promoter/enhancer	promoter/enhancer	drives expression of
	drives	drives expression	LC gene
	expression of the	of the first gene
	first and 2nd	and the CMV
	genes	promoter drives
		expression of the
		2^ndgene.
Intron/intervening	The intron	The intron	The intron sequence II
sequence	sequence II from	sequence II from	from rabbit beta
	rabbit beta globin	rabbit beta globin	globin gene is located
	gene is located	gene is located	downstream of the
	downstream of	downstream of the	promoter providing
	the promoter	promoter providing	for increased
	providing for	for increased	expression and mRNA
	increased	expression and	stability of the
	expression and	mRNA stability of	transcription unit
	mRNA stability	the transcription
	of the	unit
	transcription unit
Intenal Ribosome	For the	For the expression	For the expression of
Entry Site (IRES)	expression of	of DHFR gene	DHFR gene
	DHFR gene	downstream of 2^nd	downstream of 2^nd
	downstream of	transcription unit	transcription unit
	2^ndtranscription	ensuring high level	ensuring high level
	unit ensuring high	expression of 2^nd	expression of 2^nd
	level expression	cistron in cells	cistron in cells
	of 2^ndcistron in	growing in the	growing in the
	cells growing in	presence of	presence of
	the presence of	methotrexate	methotrexate
	methotrexate
Polyadenylation	A strong	A strong	A strong
signal	polyadenylation	polyadenylation	polyadenylation signal
	signal from S40	signal from S40	from S40 virus for
	virus for efficient	virus for efficient	efficient expression of
	expression of	expression of	recombinant gene.
	recombinant	recombinant gene.
	gene.
DHFR gene	Auxotrophic	Auxotrophic	Auxotrophic selection
	selection in HT	selection in HT	in HT negative media
	negative media	negative media	eliminates the need to
	eliminates the	eliminates the need	maintain selection
	need to maintain	to maintain	pressure using
	selection pressure	selection pressure	antibiotics.
	using antibiotics.	using antibiotics.	Amplification of gene
	Amplification of	Amplification of	copy number is
	gene copy	gene copy number	accomplished by the
	number is	is accomplished by	addition of
	accomplished by	the addition of	methotrexate to the
	the addition of	methotrexate to the	culture media. The
	methotrexate to	culture media. The	murine DHFR gene is
	the culture media.	murine DHFR gene	driven by a minimal
	The murine	is driven by a	SV40 early promoter
	DHFR gene is	minimal SV40	lacking the enhancer
	driven by a	early promoter	sequence.
	minimal SV40	lacking the
	early promoter	enhancer sequence.
	lacking the
	enhancer
	sequence.
Ampicillin	For propagation	For propagation of	For propagation of
resistance gene	of plasmid in	plasmid in bacteria	plasmid in bacteria
	bacteria
Neomycin gene

TABLE 3

Features	pMAB-HC

Multiple cloning site	One multiple cloning site for insertion of
	Heavy chain gene. Vector is similar to pNAS
Strong promoter/enhancer	The CMV early promoter/enhancer drives
combination	expression of heavy chain gene
Intron/intervening sequence
Intenal Ribosome Entry Site
(IRES)
Polyadenylation signal	A strong polyadenylation signal from S40
	virus for efficient expression of recombinant
	gene.
DHFR gene
Ampicillin resistance gene
Neomycin gene	For selection in mammalian cells

FIG. 1 A-H depicts a series of expression vector map relevant to expression using CHO DG44 cell lines or NeuCHO cell lines.
More specifically, FIG. 1A depicts an expression vector pNAS including hGH coding sequence for use in constructing the NeuCHO Cell Line from CHO DG44. Preferably, NeuCHO cell line is produced by the inclusion of the vector shown in FIG. 1A within a CHO-DG44 cell line. The genetic sequence for this expression vector has been submitted along with this application and is designated SEQ. ID No. 1.
It is noted that CHO DG44 cell line includes a relatively and fragile cell line which inherently has issues and problems in regard to long term viability, cell density and population stability.
In this preferred embodiment, the addition of hGH to the cell culture vector may lead to increases in cell density or production that were previously not realizable using previous techniques and sequences. Previously, growth factors such as IGF-1 and insulin were used to supplement CHO cells (such as DG44). However these previous methods lead to disappointing results in terms of cell viability and/or survival. In the present embodiments, the addition of hGH coding sequences to CHO cells allows for the excretions by the CHO-DG44 cells of hGH. This expression and secretion of hGH into the cell media leads to increase in cell survival of CHO cells.
Further the expression of hGH may also improve the robustness of the NeuCHO cell line as compared to other CHO cell lines.
More specifically, the addition of hGH expressing sequences to CHO-DG44 cells gave rise to a new cell line, NeuCHO cell line. The NeuCHO cell line may include any of the expression vectors described and shown in respect to FIG. 1 A-H.
The NeuCho cell line, deposited under the provisions of the Budapest Treaty with the Cell Bank Australia located at 214 Hawkesbury Rd, Westmead, NSW, 2145, Australia as of 4 Feb. 2013 and assigned accession no. CBA20130024, as is particularly suitable for use in pharmaceutical manufacture as described within the present application.
Preferably, the CHO-DG44 cell line was transfected with the pNAS-hGH vector to produce the NeuCHO cell line. Possible transfection methods include: standard methods described in the scientific literature including: calcium phosphate precipitation, PEI, Electroporation and lipofaction. It is generally noted that previous teachings in the field have often argued that it is preferred to deliver higher relative levels of DNA to cells by transfection to get better results. However, transfection methods delivering more DNA into the cell do not generate more stable or high producing cell lines, as the cell lines may become less stable and less robust.
FIG. 1B depicts an expression vector used for the expression of a single chain protein in CHO DG44 cells. Preferably, the recombinant gene expression may be driven by the SV40 early promoter/enhancer within the vector. The genetic sequence for this expression vector has been submitted along with this application and is designated SEQ. ID No. 2.
FIG. 1C depicts an expression vector used for the expression of a single chain protein in CHO DG44 cells. Preferably, a dicistronic expression cassette with a recombinant gene in the 1^stcistron followed by a DHFR gene in the 2^ndcistron is described in this example. The gene expression in this vector is preferably driven by SV40 early promoter or enhancer. The genetic sequence for this expression vector has been submitted along with this application and is designated SEQ. ID No. 3.
FIG. 1D depicts a pNeuMAB, which is a dual expression vector containing two cloning cassettes to insert heavy and light chain genes into a single vector. This expression vector has two gene expression cassettes for the insertion of multiple recombinant genes. Each cassette includes an SV40 early promoter and downstream poly A sequence. Each gene is driven by driven SV40 promoter without an enhancer sequence. This expression vector is suitable for expression of light and heavy chains expression of the antibodies. The genetic sequence for this expression vector has been submitted along with this application and is designated SEQ. ID No. 4.
FIG. 1E depicts a further expression vector, pNeuMAB-IRES-DHFR, for high level expression of heavy and light chains of a recombinant monoclonal antibody on a single vector driven one SV40 promoter and enhancer. This expression vector may be used for the expression of light and heavy antibody chains. This expression vector generally includes two gene expression cassettes for insertion of recombinant genes. Each cassette consists of SV40 early promoter/enhancer and downstream poly A sequence. Heavy chain and light chain are inserted in 1st and 2nd cassettes respectively. The 2nd cassette is dicistronic having light chain followed by DHFR downstream of IRES. The genetic sequence for this expression vector has been submitted along with this application and is designated SEQ. ID No. 5.
FIG. 1F depicts a further expression vector, pNeuMAB-IRES-DHFR-(CMV), for high level expression of heavy and light chains of recombinant monoclonal antibody on a single vector driven by CMV and SV40 promoters of heavy and light chains of antibodies respectively. The DHFR gene is driven by IRES downstream of light gene; for the expression of heavy and light chains of antibody in opposite orientations with respect to each other. Heavy chain is driven by CMV promoter whereas light chain is driven by SV40 promoter/enhancer. Light chain and DHFR gene have a dicistronic configuration with DHFR downstream of IRES. The genetic sequence for this expression vector has been submitted along with this application and is designated SEQ. ID No. 6.
FIG. 1G depicts a further expression vector, pMAB-LC (ires-dhfr), for expression of only light chains (LC) of antibodies. A discistronic cassette for cloning LC in 1st cistron and DHFR in 2nd cassette downstream of IRES. The vector is used in co-transfection with pMAB HC, which is the expression vector shown in FIG. 1H. The genetic sequence for this expression vector has been submitted along with this application and is designated SEQ. ID No. 7.
FIG. 1H depicts a further expression vector, pMAB-HC, for expression of only heavy chain (HC) of antibodies. Both pMAB-LC and pMAb-HC are co-transfected for expression of complete antibody. The genetic sequence for this expression vector has been submitted along with this application and is designated SEQ. ID No. 8.
A further graph is shown in FIG. 2. The graph of FIG. 2 represents the: Growth of DG44 Cell Lines expressing IGF-1 or hGH compared to the Parental DG44 Cell Line and a Mock Cell Line. A control (mock) cell line is derived from the Parental Cell Line which has been stably transfected with a DNA plasmid containing the selection marker but without the Gene Of Interest (GOI).
The preferred NeuCHO Cell Line demonstrates superior growth advantage compared to the original Parental DG44 Cell Line. In this example, the growth of NeuCHO cells demonstrates higher viable cell densities to that of a DG44 Cell Line expressing the IGF-1 gene.
This graph shows that when DG44 cells express human Growth Hormone, (Line Graphs E and F), the cells have a very high Maximum Viable Cell Density (up to 425%) compared to the untransfected DG44 Parental Cell Line, the Mock transfected Cell Line, and DG44 Cell Lines expressing high or Low IGF-1 protein.
The NeuCHO Cell Line has an Integral Cell Density of up to 3.67×10⁷cell/day/mL, which is 230% that of the Parental DG44 Cell Line, 1.57×10⁷cell/day/mL.
Also in FIG. 2. the Viable Cell Density is plotted on the Y-axis in cells/mL. and the number of days in culture is plotted on the X-axis. Six line graphs are shown in the figure, namely line graph A, B. C. D, E and F.
Line A represents the growth pattern of a parental DG44 cell line that is not transfected with DNA.
Line B represents the growth pattern of a parental DG44 cell line that was transfected with a DNA plasmid containing the selection marker but without the Gene Of Interest (GOI).
Line C represents the growth pattern of a parental DG44 cell line that was stably transfected with a DNA plasmid containing both the selection marker and the Gene Of Interest (GOI). The GOI here is Insulin-like growth factor 1 (IGF-1).
Line D represents the growth pattern of a parental DG44 cell line that was stably transfected with a DNA plasmid containing both the selection marker and the Gene Of Interest (GOI). The GOI here is Insulin-like growth factor 1 (IGF-1).
Line E represents the growth pattern of a parental DG44 cell line that was stably transfected with a DNA plasmid containing both the selection marker and the Gene Of Interest (GOI). The GOI here is human Growth Hormone (hGH).
Line F represents the growth pattern of a parental DG44 cell line that was stably transfected with a DNA plasmid containing both the selection marker and the Gene Of Interest (GOI). The preferred GOI in this example is human Growth Hormone (hGH).
In FIG. 3, a further graph is depicted comparing the integral of viable cell densities (IVCD) of NeuCHO with the standard CHO DG44 cells. This figure demonstrates the difference in the Integral of Viable Cell Densities achieved with the parent cell line NeuCHO compared to parental CHO.
The NeuCHO cell line is superior in growth capabilities and this translates into a more efficient production process which can minimize costs by having higher productions rates, fewer production runs, thus lower productions costs, lower Cost of Goods (COGS).
Growth and productivity of NeuCHO cell line expressing a recombinant mAB.
The preferred NeuCHO cell line demonstrates high titre of mAB x compared to traditional CHO expression system.
FIG. 5 demonstrates in graphical form that NeuCHO cells may have greater stable transfection efficiency than CHO cells (such CHO DG44 cells). Cells (NeuCHO and CHO) were transfected with DNA encoding mAB ‘x’ prior to selection and single cell cloning from a stable pool. The data is shown in FIG. 5 and demonstrates that stable transfection of NeuCHO cells results in a greater number of clones with high productivity than that of standard CHO cells. The graph shows the levels of various protein expressed in relative quantities at a given time.
NeuCHO cells have an integral of viable cell density that is about 230% greater than CHO DG44 cell lines. CHO DG44 cell lines expressing insulin like growth factor 1 (IGF-1) do not demonstrate the ability to grow to high cell densities as NeuCHO cell lines may generally achieved. NeuCHO cells have a generally greater transfection efficiency than CHO DG44 cells. The survival rate of transfected NeuCHO cells is generally greater then transfected CHO DG44 cells. Additionally, transfection of NeuCHO cells may result in a greater number of clones with a higher productivity than that of standard CHO DG44 cells.
Preferably, the expression system and vectors described herein may be able to allow or facilitate CHO cells such NeuCHO or CHO DG44 cells to produce desired proteins suitable for pharmaceutical preparation including, but not limited to: Infliximab tumour necrosis factor (referred to as Remicab™); Adalimumab tumour necrosis factor (referred to as Humira™); Etanercept tumour necrosis factor (referred to as Enbrel™); Rituximnab CD20 (referred to as Rituxan™ & MabThera™); Bevacizumab vascular endothelial growth factor (referred to as Avastin™) Trastuzumab HER2 (referred to as Herceptin™); Ranibizumab vascular endothelial growth factor (referred to as Lucentis™); Cetuximab epidermal growth factor receptor (referred as Erbitux™); Erythropoietin α; Interferon α-Pegylated interferon alfa-2a; Interferon α-Pegylated interferon alfa-2b and hGH.
NeuCHO cells when used as feeder layer may also increase efficiency of single cell cloning. NeuCHO cells were seeded in single wells of microtitre plates prior to single cell cloning of a stable transfected pool. Secretion of human growth hormone secreted from NeuCHO cells results in an increased survival rate of single cells following Limiting Dilution Cloning.
Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. in keeping with the broad principles and the spirit of the invention described herein.
The present invention and the described preferred embodiments specifically include at least one feature that is industrial applicable.

Claims

The claims defining the invention are as follows:

1. A method for producing a desired recombinant polypeptide comprising culturing a mammalian host cell expressing a desired recombinant polypeptide in the presence of a human Growth Hormone (hGH) or modified hGH, wherein expression of the hGH or modified hGH enhances survival and/or cell density and/or cell viability of the mammalian host cell expressing the desired recombinant polypeptide.

2. The method according to claim 1, wherein the mammalian host cell co-expresses the human Growth Hormone (hGH) or modified hGH and the desired recombinant polypeptide.

3. The method according to claim 1, wherein the hGH is constitutively expressed in the mammalian host cell.

4. The method according to claim 1, wherein the mammalian host cell is a CHO cell.

5. The method according to claim 4, wherein the CHO cell is selected from the group consisting of: a CHO-K1, a CHO-DG44 and a CHO-S cell.

6. The method according to claim 1, wherein culturing is performed in a suspension culture.

7. The method according to claim 1, wherein culturing is performed in an adherent culture.

8. The method according to claim 1, wherein the desired recombinant polypeptide is a biosimilar of a recombinant protein.

9. The method according to claim 8, wherein the recombinant protein is selected from the group consisting of: Infliximab, Adalimumab, Etanercept, Rituximab, Bevacizumab, Trastuzumab, Ranibizumab, Cetuximab, Erythropoietin alpha, Interferon alpha, Interferon alpha 2a and Interferon alpha 2b.

10. The method according to claim 1, wherein the expression of the hGH or modified hGH increase cell density and viability of the host mammalian cell compared to a mammalian cell supplemented with insulin-like growth factor 1 (IGF-1).

11. A recombinant mammalian host cell for producing a desired recombinant polypeptide, wherein a mammalian host cell co-expresses:

i) a human Growth Hormone (hGH) or modified hGH, and

ii) a desired recombinant polypeptide,

wherein expression of the hGH or modified hGH enhances survival of the mammalian host cell and wherein the desired recombinant polypeptide is selected from the group consisting of: Infliximab, Adalimumab, Etanercept, Rituximab, Bevacizumab, Trastuzumab, Ranibizumab, Cetuximab, Erythropoietin alpha, Interferon alpha, Interferon alpha 2a and Interferon alpha 2b.

12. The recombinant mammalian host cell according to claim 11, wherein the hGH is constitutively expressed in the mammalian host cell.

13. The recombinant mammalian host cell according to claim 11, wherein the mammalian host cell is a CHO cell.

14. An expression system for producing a desired recombinant polypeptide comprising:

an expression vector comprising a nucleotide sequence encoding a human Growth Hormone (hGH) or modified hGH;

an expression vector comprising a nucleotide sequence encoding a desired recombinant polypeptide; and

a mammalian host cell used to co-express the hGH or modified hGH and a desired recombinant polypeptide.

15. The expression system according to claim 14, wherein the mammalian host cell is a CHO cell.

16. The expression system according to claim 15, wherein the CHO cell is selected from the group consisting of: a CHO-K1, a CHO-DG44 and a CHO-S cell.

17. The expression system according to claim 16, wherein the CHO cell has a dihydrofolate reductase (DHFR) deficiency.

18. The expression system according to claim 14, wherein the desired recombinant polypeptide is a biosimilar of a recombinant protein.

19. The expression system according to claim 18, wherein the recombinant protein is selected from the group consisting of: Infliximab, Adalimumab, Etanercept, Rituximab, Bevacizumab, Trastuzumab, Ranibizumab, Cetuximab, Erythropoietin alpha, Interferon alpha, Interferon alpha 2a and Interferon alpha 2b.

20. The expression system of claim 18, wherein the recombinant protein is selected from the group consisting of: an antibody that binds tumour necrosis factor, an antibody that binds to a vascular endothelial growth factor, an antibody that binds to HER2, and an antibody that binds to an epidermal growth factor receptor.