The Introduction of Small Molecule Antibody

Posted by beauty33 on August 18th, 2019

What is the small molecule antibody?

Figure1.Small molecule antibody

Because the binding sites of antibody molecules to antigens are limited to their variable regions, we can construct molecular fragments with small molecular weight and bind to antigens by genetic engineering technology. These small molecular fragments are called small molecular antibodies. Small molecule antibodies have the following advantages: (1) Prokaryotic expression can be carried out; (2) It is easy to cross blood vessel wall or tissue barrier; (3) There is no Fc segment, which can reduce the impact of Fc segment; (4) It is easy to carry out genetic engineering.

Classification of Small Molecular Antibodies

Common small molecule antibodies include Fab antibody, single chain Fv (scFv）fragment antibody, single domain antibody, double chain antibody, three chain antibody and micro-antibody.

1. Fab antibody

Fab antibody consists of a complete light chain and a heavy chain Fd segment connected by an interchain disulfide bond, and its molecular structure is relatively stable. It not only maintains the structure of Fv segment of natural antibody molecule, but also has strong penetration and low immunogenicity. Besides, it can be coupled with a variety of drugs and radioisotopes and used as a drug-oriented therapy, therapeutic carriers and imaging features. However, the expression of Fab antibody in prokaryotic cells is limited due to its double-chain structure, and the secretory expression of Fab antibody is affected by the low efficiency of protein molecules passing through the intima and folding. There was only one antigen binding site and the affinity with antigen was relatively low.

2. ScFv fragment antibody

ScFv is an antigen-binding Fv fragment, which is about 1/6 of the complete monoclonal antibody. It is the smallest form of antibody with biological function, which is confirmed by the correct folding and binding of VH and VL in the form of non-covalent bond.

3. Single domain antibody

The VH region of some antibodies can also bind to antigens and maintain the specificity of the complete antibody, which is called single domain antibody. Single domain antibodies have only 1/12 of the complete IgG molecule, which is smaller and easier to enter cells than other antibodies. In recent years, with the continuous development of antibody preparation technology, single domain antibodies are not only highly expressed in microorganisms, but also maintain a certain degree of solubility and stability. Therefore, they are more suitable for preparation by phage display or ribosome display technology. Moreover, they are easy to prepare and can be used to construct effective function and bind affinity antibodiesAll of which makes them have certain application prospects.

4. Bispecific antibodies

Bispecific antibodies are artificial antibodies containing two specific antigen binding sites. One site can bind to target cell surface antigens, and the other site can be coupled with loads such as toxins, enzymes, cytokines and radiotoxins, which can bridge the gap between target cells and functional molecules (cells) and stimulate directed immunity. Epidemic response has become a hotspot in the field of antibody engineering and has broad application prospects in the immunotherapy of tumors.

Preparation of Small Molecular Antibodies

At present, the expression systems commonly used to produce small molecule antibody fragments include E. coli expression system, yeast expression system, insect cell expression system and mammalian cell expression system. Their cost of expression increases in turn, but the accuracy and accuracy of post-translational processing decreases in turn. Because different small molecule antibodies have different primary structures, physical and chemical properties and biological activities. Therefore, the selection of expression system should be based on the type, nature, expression level and purity of the small molecule antibody to be expressed.

Clinical Application of Amall Molecular Antibodies

With the development of biotechnology, the preparation of small molecule antibodies is becoming easier by degrees. A series of characteristics of small molecule antibodies determine that they will have great potential in many fields of medicine, mainly in the diagnosis of diseases, the treatment of cancer diseases and anti-infection applications.

1. Application in disease diagnosis

With the continuous development of antibody technology, the application of radiolabeled antibodies is much more widely used in cancer imaging and diagnosis. Specific antibodies against tumor-associated antigens are injected into the human body after being labeled with radionuclides, which flow to the tumor tissues with blood and bind to the tumor-associated antigens, thus obtaining positive imaging of the tumor is called radioimmunoimaging. The characteristics of small molecule antibody, such as small molecular weight, strong tissue penetration and fast clearance, make it more suitable for radioimmunoimaging and cancer diagnosis.

2. Application in tumor guided therapy

The directed therapy of malignant tumors refers to the method of selectively delivering the active substances to the tumor site and directionally killing the tumor cells by connecting the specific oncophilic carriers with the active substances killing the tumor cells. In cancer therapy, bispecific antibodies can selectively deliver active substances to cancer cells against low levels of tumor-associated antigens.

3. Anti-infective effect of small molecule antibodies

At present, vaccines and antibiotics are most commonly used to prevent and treat infectious diseases, but for SARS, AIDS and other diseases, no effective treatment has been found, but antibody therapy can be chosen as the preferred treatment.

In addition, small molecule antibodies can also be used as enzyme-labeled antibodies, for clinical diagnosis and tumor imaging analysis. The scFv fragment antibody was linked with the genes of enzymes and proteins to construct a complex functional antibody gene. Enzyme-labeled antibodies for clinical diagnosis were directly separated by mature expression and purification technology, which made the diagnosis more convenient, rapid and suitable for clinical application.

Future

In recent years, the research on small molecule antibodies has made rapid progress, and has achieved certain results in basic theoretical research and clinical application. At present, small molecule antibodies prepared by genetic engineering antibody technology have been widely used in disease diagnosis, cancer treatment, anti-infection and radiography. However, the production, affinity and stability of the prepared small molecule antibodies have not yet met the requirements of clinical application. Immunogenicity of the antibodies is still the main obstacle to clinical treatment. How to obtain enough small molecule antibodies in quantity and quality is still an unsolved problem in technology. Therefore, how to increase its expression yield, enhance its stability and specificity, and improve the effectiveness and reliability of clinical application still need to do a lot of in-depth research and clinical trials. It is believed that with the continuous development of antibody technology and the deepening of basic research, the application of small molecule antibodies in the diagnosis and treatment of clinical diseases will be more extensive.