In terms of drug discovery, proteins can not only serve as drug targets such as ion channels, receptors, enzymes, and transporters, but can also have therapeutic potential, such as antibodies, clotting factors, hormones, growth factors, enzymes, and antimicrobial peptides. In 1982, the first recombinant protein drug, recombinant human insulin, was launched, followed by important drugs such as recombinant human growth hormone and various recombinant human cytokines. The research and development of these recombinant protein drugs promoted the rapid development of biological drugs.

In recent years, with the development of emerging disciplines such as genomics, proteomics, and bioinformatics, the continuous improvement of various recombinant protein expression systems has accelerated the expression of recombinant proteins in prokaryotic, eukaryotic, and plant expression systems, and the study of biological activity.

Prokaryotic Protein Expression System

About 30% of recombinant protein drugs, including partial insulin, growth hormone, interferon beta and interleukin, approved by the World Food and Drug Administration are expressed in prokaryotic protein expression systems. For example, the vaccine antigens in Pfizer’s Trumenba® and GSK’s Bexsero® were produced in E. coli. E.coli is the most mature and widely used prokaryotic protein expression system and is mostly used for proteins whose molecular weight is less than 100kD and the post-translational modification does not affect the expression of specific structures or biological activity. However, E. coli expression systems generally do not provide post-translational modifications (PTMs), which may affect the nature of the immune response and properties such as protein solubility and stability, thereby affecting the function of protein drugs. Therefore, factors affecting the expression of recombinant proteins in E. coli have been continuously explored.

Eukaryotic Protein Expression System

Eukaryotic protein expression systems mainly include yeast expression systems, insect cell expression systems and mammalian cell expression systems.

Yeast protein expression systems are also widely used because of its rapid growth and ease of genetic manipulation. Unlike E. coli, yeast can secrete recombinant proteins outside the cell, which makes the downstream purification process simpler and cheaper. It also contains some post-translational modifications that are helpful for the correct folding of recombinant proteins. For example, Lundbeck’s Eptinezumab is a humanized monoclonal antibody that binds to calcitonin gene-related peptide (CGRP) ligands and blocks its binding to the receptor. Regarding hepatitis B vaccines such as Engerix-B®, Recombivax HB, and HeplisAV-B, these recombinant antibodies and recombinant hepatitis B surface antigen (HBsAg) were produced by yeast expression systems.

Mammalian cell protein expression systems are the most commonly used in the production of recombinant protein drugs, such as enzymes, antibodies, and vaccine antigens. Although more expensive, they are favored for their natural structure, post-translational modifications (PTM), cofactors, and molecular chaperones that enable correct protein folding and efficient production. Moreover, the protein glycosylation degree is high and uniform, which effectively improves drug efficacy while reducing drug side effects. For glycoprotein recombinant protein drugs, most production is performed in mammalian cell expression systems. Mammalian cell lines include CHO (Chinese hamster ovary cells), HEK293 (human embryonic kidney cells), HT-1080 (human fibrosarcoma cells), and BHK (Suckling hamster kidney cell), among which more than 60% of antibody drugs were developed using a CHO cell line.

Insect cell protein expression systems are also commonly used in the drug development of recombinant proteins. For example, Cervarix, a cervical cancer subunit vaccine approved for general medical use in 2007, is produced in a baculovirus-based system. Compared with mammalian cells expression systems, insect cell expression systems also contain post-translational modifications. The expressed recombinant proteins are generally well-folded, soluble, and can reach higher densities in a shorter time, thus enabling the expression of membrane proteins and protein kinases. However, N-glycosylation of baculovirus-infected insect cells are not the same as N-glycosylation in higher eukaryotes, so this system may not be the best choice if complex glycan is required to maintain the function of recombinant proteins.

Highly-purified recombinant protein products are identical or highly similar to human endogenous proteins and can avoid immune response. However, about 30% of recombinant drugs are modified by genetic engineering or post-translational modification by other means. The purpose of changing the protein structure is to optimize its pharmacokinetics without weakening its biological function or generating new antigenicity.

About Synbio Technologies

Synbio Technologies has rich experience in the field of recombinant protein expression. We have established four protein expression systems based on E. coli, Pichia pastoris, insects, and mammalian cells and have successfully delivered more than 1,000 kinds of recombinant protein/antibody products annually with a success rate of more than 95%.

Synbio Technologies’s E. coli protein expression system is divided into three service types:
1. Guaranteed prokaryotic protein expression service, delivered with requested yield and purity.
2. Customized protein expression and purification process development, break the bottleneck of your exploration with our technology, platform, and experience.
3. Industrial-grade recombinant protein production, using fermenters to provide you with gram-level protein products to ensure the stability of your downstream experiments.

Pollet J, Chen WH, Strych U. Recombinant protein vaccines, a proven approach against coronavirus pandemics. Adv Drug Deliv Rev. 2021;170:71-82. doi:10.1016/j.addr.2021.01.001

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