The molecular weight of traditional antibody is about 150kDa, and its size is 14.2 nm ×  8.2 nm  ×  3.8 nm, so that the penetration of solid tumor is poor, and the curative effect of solid tumor is not ideal. The molecular weight of nano antibody (heavy chain variable region VHH) from camels and sharks is about one tenth of that of traditional antibody, which shows the hope of solid tumor treatment. This article summarizes eight advantages of nanoantibody and six applications in tumor diagnosis

1.What is sdAb

There is a natural heavy chain antibody with light chain deletion in camels. Cloning its variable region can obtain a single domain antibody consisting only of heavy chain variable region, called VHH (variable domain of heavy chain of heavy chain antibody), also known as a single domain antibody (sdAb), which is the smallest functional antigen-binding segment.

2.The difference between sdAb and ordinary antibody

In addition to producing monoclonal antibodies, infected or immunized camels or alpacas also secrete a unique and heavy chain only antibody, both of which have the ability to bind to antigens. The heavy chain antibody lacks all the CH1 regions of the common antibody, and CDR1 and CDR3 are longer than VH, which to some extent makes up for the deficiency of the decrease of antigen binding force caused by the loss of light chain. 

Advantages of sdAb：

1）Stable structure and high temperature resistance; The sdAb has only one domain and does not contain the traditional connecting peptide sequence. In addition to the conserved disulfide bonds in the domain, the CYS in VHH CDR3 can also form disulfide bonds with the CYS in CDR1 or FR2. These increased sequences and loop structures expand the area of antibody binding with antigen and the diversity of antibody, and at the same time lead to its very stable structure, which can withstand high temperature and harsh extreme environment.

2）The complement reaction caused by Fc segment is avoided: the sdAb has no traditional Fc segment, thus avoiding the complement reaction caused by Fc segment.

3）sdAbs are easy to carry out gene manipulation. Thus, univalent, bivalent, bispecific and multivalent antibodies can be formed, and fusion proteins can also be formed for targeted treatment.

4）sdAb has weak immunogenicity and good compatibility with human body. Compare the VH germline sequence with the germline sequence of the unimodal camel. The former is about 50 species, and the latter is about 40 species. The VHH germline gene sequence of camel is highly homologous with the human VH3 family sequence, so it is relatively simple to humanize VHH.

5）sdAb has small molecular weight and simple structure: it is encoded by a single gene, so it can be expressed in large quantities in bacteriophages, bacteria and yeast.

6）sdAbs have a strong and fast penetration ability, which is conducive to their entry into solid tumors and play a role. At the same time, because they can penetrate the blood-brain barrier, they provide a new method for brain drug delivery.

Conclusion: Compared with ordinary antibody, sdAb has small molecular weight, simple structure, easy genetic modification, small volume, good antigenic specificity, strong tissue penetration, high stability, and broad application prospects in disease diagnosis and treatment.

3.Progress in clinical research of sdAb

Since the first discovery of heavy-chain antibodies in 1993, their antigen-binding domain, sdAbs, has garnered significant attention. Research publications and reports on sdAbs have grown exponentially. In 2018, the first sdAb drug, Cablivi (caplacizumab), was approved by the European Medicines Agency (EMA), followed by approvals from the FDA and Health Canada for the treatment of adult patients with acquired thrombotic thrombocytopenic purpura (aTTP). In October 2021, Suzhou Alphamab's Envafolimab® became the world's first subcutaneously administered PD-L1 inhibitor, allowing patients to complete dosing in 30 seconds without intravenous infusion, significantly reducing administration time. In February 2022, Carvykti (ciltacabtagene autoleucel), a BCMA CAR-T product developed by Janssen and Legend Biotech, received FDA approval for the treatment of adult patients with relapsed or refractory multiple myeloma (MM). In September 2022, Ozoralizumab, developed by Taisho Pharmaceutical, was approved by Japan's PMDA for the treatment of rheumatoid arthritis. Ozoralizumab, originally developed by Ablynx, is a humanized TNFα nanobody.

 In addition, numerous sdAb drugs are currently in various stages of development worldwide, with broad indications, significant innovation, and promising prospects for drug development.