Over the past 30 years, traditional monoclonal antibodies have occupied an important position in the development of therapeutic biomolecule drugs. In 1993, Belgian scientists published a paper in Nature, discovering for the first time in the alpaca body a naturally occurring variable region of heavy-chain antibody (HcAb) lacking light chain with a molecular weight of only 15 kDa, which is 1/10 of the molecular weight of traditional antibodies. It is also the smallest known natural antibody, and it is named a single domain antibody (sdAb).
Due to its unique advantages of a small structural domain, good stability, flexible route of administration, high solubility, and microbial expression, sdAb has gradually become an emerging force in the new generation of therapeutic biopharmaceuticals and clinical diagnostic reagents. In addition, sdAb is more likely to recognize antigens that cannot be captured by traditional antibodies and has better tissue penetration into tumor tissues and across the blood-brain barrier. sdAb, with its excellent characteristics, has been widely used in the fields of scientific research, diagnosis, and therapeutic treatment.
In September 2018, Caplacizumab, the world's first sdAb drug, was approved for marketing by the FDA, a milestone event that further boosted sdAb R&D and industrialization fever.
Highlights
* sdAb is easily captured on magnetic beads by His tagging, and the antigen-binding domain is exposed to the liquid surface, which contributes to antigen capture.
* sdAb has a higher binding surface capacity and a lower non-specific binding rate. It can be eluted under mild conditions, and its high stability allows for its reuse.
* sdAb serves as a crystallization chaperone to lock proteins in a specific conformation, which is used in protein crystallization studies.
* sdAb is expressed intracellularly and forms binders directly to antigens for the development of new research tools in molecular and developmental biology.
* sdAb is expressed intracellularly. For example, sdAb-based systemic immunolabeling visualizes systemic neuronal projections in adult mice to assess the effects of trauma on the central nervous system.
Uses
sdAb captures trace antigens with high density and firmly binding to solid-phase carriers, reduces non-specific binding of Fc, increases sensitivity and specificity of immunological assays, and is preferred for the development of in vitro diagnostic methods. For example, sdAb targeting EGFR has been used in the diagnosis of breast, ovarian, and prostate cancer. ELISA-based methods can detect body fluid antigens. Combined with molecular imaging technology, it can be used for target discovery, effectiveness evaluation, etc. sdAb can also be used as a non-invasive molecular imaging tracer to study the disease process. sdAb, as a probe for novel biosensors with high sensitivity, can be used in the fields of pharmaceuticals, environment, and food analysis.
Of course, the wider application of sdAb is in the fields of disease treatment and drug discovery. sdAb's own high affinity and specificity make it easier to bind to the receptor target on cancer cells with low immunogenicity. Modification of sdAb into drug carriers can achieve active targeting of sdAb-based anticancer drugs to tumor tissues and increase the accumulation of drugs in tumors. sdAb-conjugate drugs utilize sdAb-specific targeting to deliver drugs to the tumor site, interfering with DNA replication in the nucleus and inhibiting the proliferation of cancer cells. The sequence of sdAb is more similar to the human heavy chain variable region, and the humanization process is simpler. CAR-T with humanized sdAb as the targeting structural domain has been used in several clinical studies.
Although sdAb has obvious advantages, it still faces challenges in practical application, especially in preparation. Phage display technology is commonly used to construct sdAb libraries to screen and enrich specific sdAb, and the preparation process is relatively complex and requires more optimization. There are various biotech companies specializing in sdAb development currently.