Synthesis of new bifunctional chelators for conjugation to vector molecules for tumor targeting

Problem: The goal of this study is to prepare novel chelators suitable for conjugation to vector molecules which can be labeled by yttrium or copper in order to achieve high specific activities and to improve pharmacokinetics. In this context, new water soluble bifunctional DOTA- and bis (2-pyridylmethyl)triazacyclocyclononane (DMPTACN)-based chelators have been synthesized and conjugated to the monoclonal antibody Cetuximab which binds to HER2 of the epithelial growth factor receptor (EGFR) family which is over-expressed by various tumors.

Material and Method: Both chelators have been conjugated to Cetuximab via thiourea-bridging. Radiolabeling of DOTA derivatives has been performed in aqueous ammonium acetate solution at r.t. using ⁸⁶YCl₃ or ⁹⁰YCl₃. Radiolabeling of DMPTACN conjugates with ⁶⁴Cu was achieved in MES buffer solution at 50°C using ⁶⁴CuCl₂. The affinity of the bioconjugates towards EGFR was determined by ELISA.

Results: The ELISA test showed that the affinity of the bioconjugates has decreased compared to native Cetuximab. A chelator/antibody molar ratio of 4 was achieved as determined by MALDI-TOF-MS for the DOTA-Cetuximab conjugate. Radiolabeling of DOTA-conjugates with ⁸⁶Y and ⁹⁰Y at 37°C requires optimization to improve radiochemical yield. DMPTACN-Cetuximab conjugates can be rapidly labeled with 64Cu under mild conditions in almost quantitative yield.

Conclusions: DMPTACN- and DOTA-ligands are attractive bifunctional chelating agents which can be conjugated to vector molecules for PET-imaging and radiotherapy. In the near future, the work with the ligands investigated will be extended using the pre-labeling approach.