In-Labeled DOTA-conjugated 6-aminohexanoic linker-containing variant of anti-epidermal growth factor receptor 2 Affibody Z (ABY-003)

An Affibody molecule is a chain of 58 amino acids (~6.5 kDa) that contains a modified B domain of the staphylococcal protein A and can be obtained through chemical synthesis or produced in bacteria with the use of recombinant technology (1). Because of their small size and high chemical and thermal stability, these molecules are used as radiolabeled probes for the targeted detection and treatment of malignant tumors as discussed elsewhere (1-3). The epidermal growth factor receptor 2 (HER2) is considered to be an important Affibody target because it is believed to promote the development of the malignant phenotype, it plays a role in the development of resistance to anticancer drugs and radiation therapy, it is overexpressed in several different cancer tumor cells, and it often indicates a poor prognosis for the patient (4). Several radionuclide-labeled Affibodies and their derivatives, such as Z, have been evaluated to detect tumors expressing HER2 in preclinical studies as discussed by Tolmachev (5). Earlier, a derivative of the Z Affibody, designated ABY-002, which has the radiometal chelator 1,4,7,10-tetraazacyclododecane-,',,'-tetraacetic acid (DOTA) linked directly to the N-terminal valine of the molecule was constructed, radiolabeled with In or Ga, and evaluated for the imaging of malignant tumors that express HER2 (6). Although radiolabeled ABY-002 (both with In or Ga) could detect these tumors under conditions, the Z Affibody derivative exhibited a reduced affinity for HER2 ( = 65 pM) compared to the parent Affibody ( = 22 pM). From this study, it was concluded that the reduced affinity of ABY-002 for the receptor is probably due to a steric hindrance from the DOTA moiety, which is located very close to the HER2 binding site of the molecule. Tolmachev et al. decided to introduce a linker between the DOTA moiety and the HER2 binding sequence and evaluate whether this modification reduced or abolished the steric obstruction that may reduce the affinity of the Affibody for the receptor (7). The investigators placed an aliphatic linker (6-aminohexanoic acid) between DOTA and the valine on the N-terminal of ABY-002, labeled the modified Affibody (ABY-003) with In ([In]-ABY-003), and evaluated the biodistribution and tumor detection properties of [In]-ABY-003 in mice bearing HER2-expressing LS174T cell xenograft tumors (a human colon adenocarcinoma cell line showing a moderate expression level of HER2) or SKOV-3 cell xenograft tumors (a human ovarian carcinoma cell line showing a high expression level of HER2) (7).