99mTc-labeled tetramer and pentamer of single-domain antibody for targeting epidermal growth factor receptor in xenografted tumors

The single-domain antibody EG2 can be fused with right-handed coiled-coil (RHCC) and human cartilage oligomeric matrix protein (COMP), to form the multivalent antibodies EG2-RHCC and EG2-COMP. We labeled these two antibodies with Tc-99m and assessed their targeting efficiency for epidermal growth factor receptor (EGFR). Cell binding, uptake, efflux, and blocking studies were performed with EGFR high- and/or low-expressing cells with Tc-99m-labeled EG2-RHCC or EG2-COMP. Single photon-emission computed tomography (SPECT) imaging and biodistribution studies were further carried out. Both Tc-99m-EG2-RHCC and Tc-99m-EG2-COMP can specially bind to EGFR in vitro. SPECT imaging showed that A431, which expresses high levels of EGFR, was clearly visible 6h after Tc-99m-EG2-COMP injection; however, it was not detectable after administration of Tc-99m-EG2-RHCC. Uptake of both antibodies by the non-EGFR-secreting OCM-1 tumors was low. EG2-COMP shows promise in identifying EGFR over-expression in tumors; however, EG2-RHCC may not be suitable for targeting EGFR in vivo.