Comparison of benzoate- and dodecaborate-based linkers for attachment of radioiodine to HER2-targeting Affibody ligand

The use of radionuclide molecular imaging enables the selection of patients for treatment using molecular medicine. Preclinical studies have demonstrated that a novel low-molecular-weight affinity ligand, Affibody molecule ZHER2:342 can image the expression of HER2 with high sensitivity and specificity in tumour xenografts and has a potential for the selection of patients for treatment using Herceptin or other anti-HER2 medicine. In this study, we performed a comparative evaluation of two possible linkers for radioiodination of the Affibody molecule ZHER2:342, 4-iodobenzoate (PIB) and [4-isothiocyanatobenzyl)-amino]-undecahydro-closo-dodecaborate (DABI). It was shown that the use of DABI makes it possible to obtain radioiodinated ZHER2:342 with preserved capacity for selective binding to HER2-expressing cells. There was no difference between 125I-PIB-ZHER2:342 and 125I-DABI-ZHER2:342 in cellular retention of radioactivity after interrupted incubation with radiolabelled Affibody ligands. In vivo, the biodistribution of 125I-PIB-ZHER2:342 was characterized by a high tumour uptake at 4 h pi (12.7±4.6% IA/g) and a quick clearance from blood and normal organs. The tumour uptake of 125I-DABI-ZHER2:342 was appreciably lower (2.7±1.2% IA/g), and a high uptake of this conjugate in the liver was observed. A γ-camera experiment (at 6 h pi) demonstrated that the use of 125I-PIB-ZHER2:342 provided a much better contrast of imaging HER2-expressing xenografts than the use of 125I-DABI-ZHER2:342. In conclusion, 125I-PIB-ZHER2:342 is superior to 125I-DABI-ZHER2:342 as an agent for imaging HER2 expression in vivo.