They mainly function by either directly targeting cytolytic immune cells such as NK cells and cytotoxic T lymphocytes (CTLs) to kill cancer cells or by inhibiting immuno-suppressive immune cells including regulatory T cells (Tregs) and type II macrophages to revert tumor microenvironment (TME). applications in solid tumor treatment. == 1. Introduction == Traditional interventions for cancer treatment include surgery, chemotherapy, and radiotherapy. These methods are often poorly targeted and can cause damage to normal cells while killing tumor cells, thus leading to a spectrum of side effects. With the increasing understanding of tumor occurrence and progression, the significance and necessity of targeted therapy for cancer have been well recognized. Monoclonal antibodies (mAbs) are powerful tools in targeted tumor therapy and have been used in clinics for decades1,2. They can inhibit tumor cell proliferation by blocking the transmission of related signaling molecules such as epidermal growth factor receptor (EGFR), human epidermal growth factor receptor 2 (HER2) or can directly induce tumor cell death through cytolytic immune cell engagement3. To date, numerous mAbs have been approved by the US Food and Drug Administration (FDA) for the treatment of different tumors. Many of them, such as the anti-vascular endothelial growth factor (VEGF) antibody bevacizumab, anti-EGFR antibody cetuximab, and anti-HER2 antibody trastuzumab, have been applied to the clinic for targeted tumor therapy and achieved superior effects4,5. Sugammadex sodium However, certain inherent structural properties limit the potency of mAbs and traditional antibody-derived biologics for tumor therapy, especially in solid tumors with dense tissues6. The large size of mAbs (about 150 kDa) constrains their access to the cryptic epitopes in dense tumor tissues and can potentially compromise the therapeutic effectiveness7,8. Additionally, the long half-life (from several days up to 4 weeks) of mAbs may lead to safety concerns, further limiting their applications in some fields like imaging or radio-immunotherapy9. The discovery of naturally occurring heavy chain antibodies (hcAbs) in camelids by Hamers R. and his team in 1993 has opened new avenues for the development of anti-tumor therapeutics10. They analyzed serum samples from Arabian camels and found a kind of immunoglobulin G (IgG) lacking light chains. These heavy-chain-only antibodies have a molecular weight of about 90 kDa and show excellent binding affinity to their targets. The antigen recognition part of hcAbs belongs to a single variable domain, called VHH, or nanobody (Nb)6. Later, Flajnik and co-workers also found a class of heavy-chain-only antibodies in cartilaginous fish named immunoglobulin new antigen receptor (IgNAR)11. The variable domain of IgNAR is also referred to as VNAR. In general, VHH, Nb, and VNAR are collectively referred to as sdAb. Compared with mAbs, sdAbs have a number of superior properties, making them an ideal choice for medical applications12. Firstly, they have outstanding physical properties such as good solubility and high thermal resistance, which are mainly attributed to their single-domain nature and a greater amount of hydrophilic amino acids on framework region two (FR2)13,14,15. Besides, the small size of sdAbs endow them with unique binding attributes. For example, the complementary determining region three (CDR3) loops of camelid VHHs are typically much longer than those of human VH domains, which may contribute to the increased interaction with the antigen16. Besides, sdAbs can bind to unique epitopes that are inaccessible by traditional antibodies17,18,19. This is mainly due to the extended CDR3 of sdAbs can form a finger-like extension that allows for binding to cavities and clefts of the target antigen20,21,22. Moreover, the long CDR3 of PKBG sdAbs may lower the binding entropy with antigens by forming a short helix23,24,25. Recent research has proven that sdAbs can across the bloodbrain barrier (BBB) more efficiently Sugammadex sodium than traditional antibodies, offering a good opportunity Sugammadex sodium for Sugammadex sodium the treatment of brain diseases26,27,28. Although with distinctive properties, the animal origin of sdAbs may introduce potential safety problems, hindering their clinical applications. Therefore, Sugammadex sodium performing humanization has become a mainstream approach to solve.