Immuno PET imaging of CS1/SLAMF7 in multiple myeloma using 89Zr DFO elotuzumab

1068 Background: Multiple myeloma (MM) is a hematological malignancy caused by abnormal plasma cells. Despite recent advances in diagnosis and treatment strategies, MM patients frequently relapse and become refractory due to the genetic instability and heterogeneity of these malignant plasma cells. Molecular imaging of overexpressed proteins in MM cells can significantly impact patient management by improving diagnosis, therapy planning, and evaluation of therapy. CS1 is a cell surface glycoprotein that is selectively overexpressed in MM cells including patients with monoclonal gammopathy of undetermined significance (MGUS), smoldering multiple myeloma (SMM) and MM. Elotuzumab is an FDA-approved monoclonal antibody for targeting CS1. Here, we evaluated the potential of 89Zr labeled elotuzumab as a PET imaging agent for MM in pre-clinical MM mouse models. Methods: Elotuzumab was conjugated to DFO-Bz-NCS and radiolabeling of the conjugate was optimized with zirconium-89 (89Zr), a PET radiotracer with a 3.3 d half-life. Human myeloma cell line, MM.1S was modified to express click beetle red luciferase (C) and green fluorescent protein (GFP/G; MM.1S-CG). These modified MM.1S-CG cells were injected in NOD SCID Gamma (NSG) mice via tail vein. The in vitro binding affinity of radiolabeled elotuzumab to CS1 was confirmed by flow cytometry and cellular binding assays. The in vivo CS1 expression was evaluated by small animal 89Zr-DFO-elotuzumab/PET imaging and tissue biodistribution studies. Specificity of the radiotracer was evaluated by performing blocking studies and 89Zr-DFO-IgG was also evaluated as a non-specific control. Results: Flow cytometry studies confirmed high expression of CS1 in MM.1S-CG cells. Saturation binding assay with 89Zr-DFO-elotuzumab (>99% radiochemical purity) was performed and the dissociation constant (Kd) and receptor density (Bmax) values were 40.4 nmoles and 124 fmoles/mg of protein respectively. 89Zr-DFO-elotuzumab PET/CT imaging demonstrated high specificity to detect CS1 expressing myeloma cells in a disseminated MM model as compared to the gold standard 18F-FDG PET/CT. The standard uptake value (SUV) of 89Zr-DFO-elotuzumab in spine, pelvis and leg was 5.88, 5.46 and 7.81 as compared to the SUV of 18F-FDG - 1.25, 0.99 and 0.93 respectively. In vivo specificity and binding affinity of 89Zr-DFO-elotuzumab to CS1 was confirmed by PET imaging data that showed high tumor uptake of 89Zr-DFO-elotuzumab in tumor bearing tissues as compared to the mice injected with 89Zr-DFO-IgG. In vivo blocking was successfully achieved by using 200-fold excess of elotuzumab. Conclusions: The potential utility of 89Zr-DFO-elotuzumab/PET was investigated for evaluating the expression of CS1 antigen in MM mouse models. Preclinical and ex vivo data demonstrated high specificity of 89Zr-DFO-elotuzumab for MM cells as compared to the standard-of-care, 18F-FDG. In summary, 89Zr-DFO-elotuzumab/PET can potentially be utilized for stratifying patients for CS1 targeted immunotherapies.