**11. Antibody fragments**

Advancement in protein sciences has enabled scientists to produce antibody fragments with a smaller size but the same efficacy. The ideal characteristics of an antibody fragment are discussed in **Figure 2**.

In the beginning, proteolysis was the method of choice to produce smaller antibody fragments [47]. These fragments had a molecular weight of around 54 kDa–100 kDa (Fab, Fab2). In the later stages, recombinant DNA technology was used to prepare univalent and bivalent fragments which had heavy and light chains of a variable section of antibody [48]. Such a structure was the smallest targeting unit to be generated. The two chains were joined with a flexible polypeptide linkage giving a 'single chain variable fragment' (scFv). It was convenient to use because of its small size and easy production.

**Figure 2.** *Different characteristics of antibody fragments.*

In the 1980s, researchers isolated and screened a heavy chain of the murine antibody for its binding to lysosomes [49]. It was called a single domain antibody' (dAb) as it contained only a heavy or light chain and had a meager molecular weight (15 kDa). However, it had drawbacks like poor solubility and aggregation, and a major issue was that the fragments did not retain the original's binding efficacy [49]. Components from animals such as camels, llamas, and fish such as sharks were used as more soluble, but they suffered from immunogenicity issues. Efforts like immunization and bioengineering to reduce agglomeration were carried out for effective use in therapy [50].

Later the above types were converted from univalent to multivalent through protein engineering, which was then used to target multiple entities at once [51]. These multivalent fragments show slower dissociation from the receptor and high functionality. One great example of multivalent fragments is a 'diabody' formed by linking light and heavy chain by a single chain variable fragment to be self-assembled into a dimer [52]. Diabodies have an advantage such as moderate molecular weight, multivalency which give them characteristics like improved penetration in tissue, rapid clearance. These diabodies bind to tumor antigen as well as to CD3 cells to kill tumors through T-cell mediated toxicity. The mini body is another type of synthesized antibody fragment, which is a pair of single chains of variable fragments interconnected by-CH3 bonds, and a variable region specific for any antigen is attached to this pair. Minibodies are more suitable for targeted radiotherapy because they show better uptake and are cleared faster as compared to other types of fragments and are cleared rapidly. In the structure of the Mini body, the single variable region can be replaced by a cytotoxic agent, radioisotope, for its delivery.

Nanobody is the shortest antibody fragment. It is isolated from camelid heavy chains of variable antibody region. It is produced by making phage viral coat cover the desired fragment. As these antibodies do not have light chains, they are structurally different than normal antibodies. They have a concave antigen-binding region larger than other antibodies. Hydrophilic structures replace the usual hydrophobic amino acid residue. Such adjustments allow antigen-binding property even in the absence of light region. Another specific property is its ability to cross blood–brain barrier.

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*Targeted Cancer Therapy Using Nanoparticles and Antibody Fragments*

Antibodies that bind to the target can be coupled with a radionuclide, fluorescent molecules to obtain images of cancer. Antibodies that have longer half-life need more time for imaging and may blur the image. These fragments have a short half-life and higher permeability, which allow easy detection. Techniques such as Positron emission tomography (PET), Computed tomography (CT), Single-photon emission computed tomography (SPECT) are now being performed where the

Nuclear imaging is essential for the detection of cancer. Using fragments reduces nuclear exposure to radiation. The bifunctional connector connects antibody fragment and radionuclide, and such complexity easily accumulates at the tumor and gives clear visualization [54]. This method can be used to measure the absorption of drugs and the expression of receptors. One issue with the complex is increased radiation in the kidney due to complex breakdown and retention of radionuclide in

It involves non-invasive assessment of disease and fluorescence imaging of tumors during surgery. This technique shows accurate and reliable results [55]. Heterodimeric antibodies are used to target two issues at once and have a stronger

Antibody fragments can be coupled with microbubbles, and it enhances the targeting efficiency [56]. Photoacoustic imaging also can be performed with antibody fragments to give high-resolution images. The laser causes expansion of tissues, and it produces sound waves, which later can be converted to images by the ultrasonic

The selection of antigen is most important for targeting. The target antigen is highly expressed in cancer cells but not on a normal cell. Single, as well as multiple targeting, can be achieved through the use of an engineered antibody. Multivalent antibodies not only block signaling but also overcome resistance through multiple

The delivery of anti-tumor drugs using antibody fragments is a common practice of targeting medicine to the tumor [58]. Such systems ensure accurate delivery and improve the pharmacokinetics of agents. Effector molecules such

*DOI: http://dx.doi.org/10.5772/intechopen.96550*

antibody fragments are employed [53].

**12.1 Cancer imaging**

**12.2 Nuclear imaging**

the kidney [37].

**12.3 Dual modality imaging**

affinity than homodimer.

**13. Application in cancer therapy**

**13.1 Intrinsic therapeutic effect**

**13.2 Targeted drug delivery**

**12.4 Other imaging**

transducer.

targets [57].

**12. Application of fragments in imaging**
