Synthesis and Testing of a Binary Catalytic System for Imaging of Signal Amplification in Vivo

We report a binary targeted enzymatic system that is composed of two covalent monoclonal antibody conjugates for specific labeling of cellular targets in vivo. The system utilizes low-molecular weight peroxidase-reducing substrates synthesized by linking 5-hydroxytryptamine (serotonin) with DTPA (5HT-DTPA) for magnetic resonance and radionuclide imaging or with Cy5.5 for near-infrared optical imaging. Initially, the conjugation reaction conditions were optimized to achieve a low level of antiepidermal growth factor receptor (EGFR) antibody (EMD 72000) modification with the N-hydroxysuccinimide ester of 4-hydrazinonicotinate acetone hydrazone (SANH), yielding mAb-HNH conjugate. The resultant modified antibodies were incubated with the periodate-oxidized peroxidase (HRP) or 4-formylbenzoyl-conjugated glucose oxidase (GO), followed by the purification of the resultant mAb-enzyme conjugates by size-exclusion HPLC. The conjugates were further characterized by electrophoresis and were tested by cross-titration on A431 EGFR+ squamous carcinoma or SW620 adenocarcinoma cells (negative control). The conjugates at the optimized concentration ratios were further tested using near-infrared fluorescence microscopy in the presence of Cy5.5 monocarboxy-5-hydroxytryptamide. Further in vitro experiments demonstrated that (1) antibody binding was specific and could be inhibited by free antibody; (2) both antibody conjugates exhibited high enzymatic activity after the binding to the cells; (3) 111In-labeled 5-HT-DTPA was avidly binding to EGFR-positive cells only if both HRP- and GO-conjugates were bound to the cells. The conjugates were tested in vivo using a SPECT imaging experiment, which demonstrated the accumulation of 111In-labeled 5-HT-DTPA substrate at the site containing both conjugates.