Radio immunotherapeutic agents for targeting human epidermal growth factor receptor 2 overexpressing cancers

Development of radiolabeled antibody-based radiopharmaceuticals has seen a renaissance in the last decade. Monoclonal antibodies (mAbs) and their fragments are gaining attention especially in oncology as molecules specific to the cell surface antigens over expressed on tumors, immune cells and also for targets in the tumor microenvironment have been developed which can be radiolabeled for targeting cancers. Trastuzumab and pertuzumab are FDA approved monoclonal antibodies targeting HER2 receptors over expressed on breast cancers. Lu-177 (β- _max 498 KeV, t_1/26.7 d) is a promising radioisotope for development of radio immunotherapeutic agents. It has several advantages over other radionuclides for radioimmunotherapy (RIT) including relatively long physical half-life which appropriately matches with the biological half-life of intact mAbs. To demonstrate the potential of ¹⁷⁷Lu labeled trastuzumab/pertuzumab and their F (ab')₂ fragments for targeting HER2 receptors, p-SCN- Bn-CHX-A"-DTPA chelator was conjugated to antibodies and its fragments and subsequently labelling with¹⁷⁷Lu was optimized. The radiochemical purity (RCP) labeled antibody were assessed by chromatographic technique (SE-HPLC and PC). The radioimmuno conjugates were used for in-vitro binding affinity and specificity studies in SK-BR3, SK-OV-3 and MDA-MB-231 cell lines while in-vivo studies were conducted in tumor bearing female SCID mice. ¹⁷⁷Lu-labeled formulations could be prepared with >95% radiochemical purity (% RCP) as ascertained by chromatography techniques. In-vitro binding studies revealed high affinity and specificity of the formulations towards HER2 receptors. Strong synergy in binding of radiolabeled pertuzumab in presence of cold trastuzumab was observed. SPECT imaging in tumor bearing mice revealed specific tumor uptake and prolonged retention of the radioformulations in HER2 positive tumors. In-vivo synergy in binding and uptake was also confirmed by SPECT imaging studies. The results of the studies revealed that these radioformulations could serve as effective RIT agents for management of HER2 over expressing cancers. Further studies are underway to obtain the regulatory clearance for translating¹⁷⁷Lu labeled formulations to clinics. (author)