*4.3. Fab-Based Probes*

Fab is characterized by a light chain and a heavy chain of an immunoglobulin, containing variable regions, constant domain of the light chain (CL), and first constant domain of the heavy chain (CH1) [135]. The Fab, therefore, takes the specificity of the immunoglobulin. Unlike the traditional antibodies (produced from mammalian cells), Fabs could be generally and easily produced from bacteria cells, like *E. coli* [136]. One drawback of Fab is the limited retention on the antigen and rapid clearance [137]. Some Fabs have been discovered for the treatment of TC (targeting cluster of differentiation 276 [CD276] [138], etc.), and some publications reported the potential value of Fab as diagnostic probes targeting Galectin-3 [125,139,140].

[ 89Zr]Zr-DFO-αGal3-Fab-PAS200, an imunoPET probe fused with 200 Pro, Ala, and Ser residues (PAS200) and conjugated with [89Zr]Zr-deferoxamine ([89Zr]Zr-DFO), is a recently

reported Fab-based probe derived from the rat anti-Gal3 mAb. Similar to the full-size [ 89Zr]Zr-labeled Gal-3 mAb (mentioned in Section 4.1.3) [125], the [89Zr]Zr-DFO-αGal3-Fab-PAS200 can bind the Gal-3 well [139] (Figure 12). Unlike the uptake of full-size 89Zr-labeled Gal-3 mAb which lasts over five days after injection, the [89Zr]Zr-DFO-αGal3-Fab-PAS200 was supposed to have a shorter lasting time, but the exact time was undetermined [125]. Research concerning a head-to-head comparison between the anti-Gal3 IgG probe and the corresponding fragment probe is lacking.

**Figure 12.** PET/CT images of mice at 24 h after intravenous injections. (**A**) Injection with 3 MBq of [89Zr]Zr-DFO-αGal3-Fab-PAS200. (**B**) Co-injection of 3 MBq of [89Zr]Zr-DFO-αGal3-Fab-PAS200 and 1000-fold of nonradioactive aGal3-Fab-PAS200 (for blocking). (**C**) Control. Color scale bars: 3.3–7.8%ID/g. Reproduced with permission from [139], copyright 2020 Mary Ann Liebert, Inc.
