Fast and reliable in vivo diagnosis at an early stage of the disease, or monitoring treatment efficiency, remains a major challenge in cancer diagnosis. The ideal cancer-imaging agent should deliver an amount of label sufficient to detect the smallest metastases against a low-level of nonspecific noise background. In addition, it should localize promptly by penetrating the tumor, and unbound conjugate should clear rapidly from the system to reduce target-to-background ratios. Prolonged clearance kinetics have hampered the development of intact anti-bodies as imaging agents, despite their ability to effectively deliver radionuclides to tumor targets in vivo. Small antibody fragments have potential for good tumor targeting, as they penetrate rapidly and give high tumor-to-background ratios at very early time points after administration.
The superior penetration potential of sdAbs, due to their high-affinity target binding and fast clearance from the circulation of the excess of non-targeted nanobodies*, represents an ideal basis for imaging purposes. A nanobody against lysozyme, which was previously shown to inhibit lysozyme activity in vitro, effectively targeted bulky tumors and metastatic lesions transgenic for hen egg white lysozyme, whereas excess of this nanobody was rapidly cleared from the circulation. A few hours after administration, tumor-to-blood ratios of up to 10 could be demonstrated with radiola-belled nanobodies. Nanobody uptake by tumors matches well with the half-life of radionuclides, including 99mTc and 123I.
Besides their potential as imaging agents, sdAbs may offer added value to cancer diagnostic tests used at present. Recently, sdAbs have been generated that can discriminate between different isoforms of PSA. Remarkably, these nanobodies seem to sense or induce conformational changes on different PSA isoforms, a feature that may be exploited to discriminate different stages of prostate cancer. Moreover, llama-derived nanobodies were shown to be excellent tools in immunocytochemical, immunohistochemical and immunoblot analysis to detect oculopharyngeal muscular dystrophy
SdAbs in cancer therapeutic tools
Given their high affinity and specificity, the small size of nanobodies makes them particularly suitable for target in antigen in obstructed locations, such as tumors, where penetration into poorly vascularized tissue is crucial to the success of the drug. In addition, nanobodies have an extremely low immunogenic potential. In animal studies, repetitive administration of nanobodies does not yield any detectable humoral or cellular immune response. As a versatile building block for manifold constructs, sdAbs can easily be used as a delivery vehicle for a range of molecules that provide ancillary functions after target binding. This ‘magic bullet’ would be delivering the toxic payload to the tumor while minimizing the length of time that the toxic compound could cause damage to healthy cells.
Today, with the discovery of bacterial strains that specifically target tumors, and aided by genetic engineering, there is a new interest in the use of bacteria as tumor vectors. Clostridium, Salmonella and Bifidobacterium have been shown to preferentially replicate in solid tumors, and can be manipulated to transport and amplify genes encoding factors such as toxins, angiogenesis inhibitors and cytokines. To improve the tropism of these bacteria for tumors, retargeting molecules, such as antibodies, may be expressed on their surface. In this context, it has been shown that nanobody-based manifold constructs can be efficiently translocated to the bacterial cell surface, whereas scFv constructs failed to do so, probably reflecting the high stability of nanobodies and their low tendency to form aggregates.
Besides targeting to obstructed lesions, the delivery of therapeutics across the blood–brain barrier (BBB) remains a major challenge in developing treatments for neurological diseasesand cancers of the brain. Macromolecule delivery across the BBB using antibodies takes advantage of transporters or receptors selectively expressed on brain capillary endothelium, such as transferring-receptor, that undergo receptor-mediated transcytosis. sdAbs have been isolated that target the brain in vivo and transmigrate across human BBB endothelium.
The potential of sdAbs as intrabodies provides an attractive means for manipulating antitumor triggering agents when expressed in tumor cells. Such antibodies can neutralize or modify the activity of oncogenic molecules when addressed in specific subcellular compartments and/or they can be used to trigger antitumor immunity when expressed on the tumor cell surface.
Source: Nanobodies as novel agents for cancer therapy. Available from: https://www.researchgate.net/publication/8023065_Nanobodies_as_novel_agents_for_cancer_therapy