Multiple myeloma (MM) is a plasma B-cell hematologic cancer that causes significant skeletal morbidity. Despite improvements in survival, heterogeneity in response remains a major challenge in MM. Cluster of differentiation 38 (CD38) is a type II transmembrane glycoprotein over-expressed in myeloma cells and is implicated in MM cell signaling. Daratumumab is US Food and Drug Administration approved high-affinity monoclonal antibody targeting CD38 that is clinically benefiting refractory MM patients. Here, we evaluated [89Zr]Zr-DFO-daratumumab positron emission tomography/computed tomography (PET/CT) imaging in MM tumor models. Methods: Daratumumab was conjugated to desferrioxamine-p-benzyl-isothiocyanate (DFO-Bz-NCS) for radiolabeling with Zr-89. Chelator conjugation was confirmed by electrospray ionization-mass spectrometry (ESI-MS), and radiolabeling was monitored by instant thin-layer chromatography. Daratumumab was additionally conjugated to Cyanine5 (Cy5) dye for cell microscopy. In vitro and in vivo evaluation of [89Zr]Zr-DFO-daratumumab was performed using CD38+ human myeloma MM1.S-luciferase (MM1.S) cells. Cellular studies determined the affinity, immunoreactivity, and specificity of [89Zr]Zr-DFO-daratumumab. CD38low 5TGM1-luciferase (5TGM1) murine MM cells served as negative controls. [89Zr]Zr-DFO-daratumumab PET/CT small animal imaging was performed in severe combined immunodeficient (SCID) mice bearing solid and disseminated MM tumors. Tissue biodistribution (7 days after tracer administration, 1.11 MBq/animal, n = 4-6/group) was performed in wild-type and MM1.S tumor-bearing SCID mice. Results: Specific activity of 55.5 MBq/nmol (0.37 MBq/µg) was reproducibly obtained with [89Zr]Zr-DFO-daratumumab. Flow cytometry confirmed CD38 expression (>99%) on the surface of MM1.S cells. Confocal microscopy with daratumumab-Cy5 demonstrated specific cell binding. A dissociation constant, Kd: 3.3 nM (± 0.58) and receptor density, Bmax: 10.1 fmol/mg (±0.64) was obtained with saturation binding assay. [89Zr]Zr-DFO-daratumumab/PET demonstrated specificity and sensitivity for detecting CD38+ myeloma tumors of variable sizes (8.5 to 128 mm3 ) with standardized uptake values ranging from 2.1 to 9.3. Discrete medullar lesions, confirmed by bioluminescence images, were efficiently imaged with [89Zr]Zr-DFO-daratumumab/PET. Biodistribution at seven days post administration of [89Zr]Zr-DFO-daratumumab showed prominent tumor uptake (27.7 ± 7.6 %ID/g). In vivo blocking was achieved with a 200-fold excess of unlabeled daratumumab. Conclusion: [89Zr]Zr-DFO- and Cy5-daratumumab demonstrated superb binding to CD38+ human MM cells and significantly low binding to CD38low MM cells. Daratumumab bio-conjugates are being evaluated for image guided delivery of therapeutic radionuclides.
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