Abstract
Introduction: [(227)Th]Th-3,2-HOPO-MSLN-mAb, a mesothelin (MSLN)-targeted thorium-227 therapeutic conjugate, is currently in phase I clinical trial; however, direct PET imaging using this conjugate is technically challenging. Thus, using the same MSLN antibody, we synthesized 3,2-HOPO and deferoxamine (DFO)-based zirconium-89 antibody conjugates, [(89)Zr]Zr-3,2-HOPO-MSLN-mAb and [(89)Zr]Zr-DFO-MSLN-mAb, respectively, and compared them in vitro and in vivo. Methods: [(89)Zr]Zr-3,2-HOPO-MSLN-mAb and [(89)Zr]Zr-DFO-MSLN-mAb were evaluated in vitro to determine binding affinity and immunoreactivity in HT29-MSLN and PDX (NCI-Meso16, NCI-Meso21) cells. For both the zirconium-89 conjugates, in vivo studies (biodistribution/imaging) were performed at days 1, 3, and 6, from which tissue uptake was determined. Results: Both the conjugates demonstrated a low nanomolar binding affinity for MSLN and >95% immunoreactivity. In all the three tumor types, biodistribution of [(89)Zr]Zr-DFO-MSLN-mAb resulted in higher tumor uptake(15.88-28-33%ID/g) at all time points compared with [(89)Zr]Zr-3,2-HOPO-MSLN-mAb(7-13.07%ID/g). [(89)Zr]Zr-3,2-HOPO-MSLN-mAb femur uptake was always higher than [(89)Zr]Zr-DFO-MSLN-mAb, and imaging results concurred with the biodistribution studies. Conclusions: Even though the conjugates exhibited a high binding affinity for MSLN, [(89)Zr]Zr-DFO-MSLN-mAb showed a higher tumor and lower femur uptake than [(89)Zr]Zr-3,2-HOPO-MSLN-mAb. Nevertheless, [(89)Zr]Zr-3,2-HOPO-MSLN-mAb could be used to study organ distribution and lesion uptake with the caveat of detecting MSLN-positive bone lesions. Clinical trial (NCT03507452).