Nanobody: the "magic bullet" for molecular imaging?

Abstract

Molecular imaging involves the non-invasive investigation of biological processes in vivo at the cellular and molecular level, which can play diverse roles in better understanding and treatment of various diseases. Recently, single domain antigen-binding fragments known as 'nanobodies' were bioengineered and tested for molecular imaging applications. Small molecular size (~15 kDa) and suitable configuration of the complementarity determining regions (CDRs) of nanobodies offer many desirable features suitable for imaging applications, such as rapid targeting and fast blood clearance, high solubility, high stability, easy cloning, modular nature, and the capability of binding to cavities and difficult-to-access antigens. Using nanobody-based probes, several imaging techniques such as radionuclide-based, optical and ultrasound have been employed for visualization of target expression in various disease models. This review summarizes the recent developments in the use of nanobody-based probes for molecular imaging applications. The preclinical data reported to date are quite promising, and it is expected that nanobody-based molecular imaging agents will play an important role in the diagnosis and management of various diseases.

Keywords: Nanobody; arthritis; atherosclerosis; cancer; molecular imaging; positron emission tomography (PET).

Figure 1

A schematic representation of nanobody and antibody domains. Adapted from .

Figure 2

Nanobody-based imaging of EGFR expression. Transverse, coronal, and sagittal views of SPECT/CT images of mice bearing A431 tumor injected with ^99mTc-7C12 (A) or ^99mTc-7D12 (B). Images were acquired at 1 h after injection. Adapted from .

Figure 3

Nanobody-based imaging of HER-2 expression. (A) Transverse and coronal views of SPECT/CT images of HER-2 positive SKOV3 tumor-bearing mice at 1 h post-injection of ^99mTc-2Rs15d. (B) PET/CT images of rats bearing SKOV3 (left) or HER-2 negative MDA-MB-435D (right) tumor xenografts at 1 h post-injection of ⁶⁸Ga-2Rs15d. (C) In vivo optical imaging of SKBR3 tumor-bearing mice at 4 h post-injection of HER-2 specific (11A4, 18C3 or 22G12) or negative control (R2) nanobodies conjugated to IRDye800CW (abbreviated as IR). Tumors are indicated with red arrow and kidneys with green arrow. Adapted from , , .

Figure 4

Nanobody-based imaging of MMR expression. (A) Coronal and transverse views of SPECT/CT images of 3LL tumor-bearing wide-type (WT) or MMR knockout (MMR^-/-) mice at 3 h post-injection of ^99mTc-labeled BCII10 control nanobody (Nb) or anti-MMR (α-MMR) nanobody. (B) SPECT/CT images of mice showing signs of arthritis in both hind limbs at 3 h post-injection of ^99mTc-labeled α-MMR nanobody or BCII10 control nanobody. Tracer accumulation of ^99mTc-labeled α-MMR nanobody was evident in knees, ankles, and metatarsal joints (indicated by arrows), but not the control nanobody. Adapted from , .

Figure 5

Nanobody-based imaging of VCAM-1 expression. (A) SPECT/CT images of atherosclerotic lesions in ApoE-deficient mice injected with ^99mTc-cAbVCAM1-5 alone (no competition) or with a 100-fold excess of unlabeled cAbVCAM1-1 (competition). Competition resulted in significant decrease of tracer uptake in the liver, lymphoid tissues, and atherosclerotic lesions, thereby demonstrating VCAM-1 specificity. (B) Transverse B-mode ultrasound images of MC38 tumors overlaid with contrast-enhanced signal at 10 min post-injection of either μB-cAbVCAM1-5 or a non-targeting control μB-cAbGFP4. Adapted from , .