Matching the Decay Half-Life with the Biological Half-Life: ImmunoPET Imaging with ⁴⁴Sc-Labeled Cetuximab Fab Fragment

Abstract

Scandium-44 (t½ = 3.9 h) is a relatively new radioisotope of potential interest for use in clinical positron emission tomography (PET). Herein, we report for the first time the room temperature radiolabeling of proteins with 44Sc for in vivo PET imaging. For this purpose, the Fab fragment of Cetuximab, a monoclonal antibody that binds with high affinity to epidermal growth factor receptor (EGFR) was generated and conjugated with N-[(R)-2-Amino-3-(p-isothiocyanato-phenyl) propyl]-trans-(S,S)-cyclohexane-1,2-diamine-N,N,N′,N″,N″-pentaacetic acid (CHX-A″-DTPA). The high purity of Cetuximab-Fab was confirmed by SDS-PAGE and mass spectrometry. The potential of the bioconjugate for PET imaging of EGFR expression in human glioblastoma (U87MG) tumor-bearing mice was investigated after 44Sc labeling. PET imaging revealed rapid tumor uptake (maximum uptake of ~12 %ID/g at 4 h post-injection) of 44Sc-CHX-A″-DTPA-Cetuximab-Fab with excellent tumor-to-background ratio, which might allow for same day PET imaging in future clinical studies. Immunofluorescence staining was conducted to correlate tracer uptake in the tumor and normal tissues with EGFR expression. This successful strategy for immunoPET imaging of EGFR expression using 44Sc-CHX-A″-DTPA-Cetuximab-Fab can make clinically translatable advances to select the right population of patients for EGFR-targeted therapy and also monitor the therapeutic efficacy of anti-EGFR treatments.

ImmunoPET of tissue factor expression in triple-negative breast cancer with a radiolabeled antibody Fab fragment

“…The clearance varies with different Fab fragments, possibly dependent on their size, structure, and hydrohilicity. In this study, ALT-836-Fab was cleared predominantly by the liver, whereas other antibody fragments, such as TRC105-Fab and Cetuximab-Fab, were mainly cleared by kidney [19, 35]. Therefore, the types of Fabs should be correctly selected for different cancers.…”

In vivo imaging with antibodies and engineered fragments

“…Efforts have been made to improve upon the chelating molecule (Deri et al, 2014; Guérard et al, 2014, 2013; Price et al, 2014). Scandium-44 (t 1/2 = 3.9 h) continues to be explored in conjunction with intermediate-sized fragments such as Fabs (Chakravarty et al, 2014), and gallium-68 (t 1/2 = 68 min) is gaining popularity as a label for smaller fragments such as nanobodies and affibodies (Honarvar et al, 2014; Xavier et al, 2013). 99m Tc (t 1/2 = 6.0 hr) continues to be the most commonly used SPECT radionuclide for a variety of uses in the clinic including conventional bone scans.…”

Cyclotron production of 44Sc: From bench to bedside

“…In a number of preclinical in vivo studies 44 Sc was successfully employed in combination with DOTA-derivatized biomolecules, including dimeric cyclic RGD (arginine-glycineaspartic acid) peptides [10], bombesin analogs [11], puromycin [12], folic acid conjugates [8] and antibody fragments [13]. The 44 Sc activity which was necessary for the performance of these in vivo studies was very low and, hence, a production yield of <350 MBq 44 Sc always sufficient.…”