ImmunoPET Imaging of CD146 Expression in Malignant Brain Tumors

Recently, the overexpression of CD146 and its potential as a therapeutic target in high-grade gliomas, the most lethal type of brain cancer, was uncovered. In this study, we describe the generation of 89Zr-Df-YY146, a novel Zr-89-labeled monoclonal antibody (mAb) for the targeting and quantification of CD146 expression in a mouse model of glioblastoma, using noninvasive immunoPET imaging. YY146, a high affinity anti-CD146 mAb, was conjugated to deferoxamine (Df) for labeling with the long-lived positron emitter Zr-89 (t(1/2): 78.4 h). In vitro assays, including flow cytometry, immunofluorescence microscopy, and Western blot, were performed with two glioblastoma cell lines, U87MG and U251, to determine their CD146 expression levels. Also, YY146 and Df_YY146s CD146-binding affinities were compared using flow cytometry. In vivo CD146-targeting of Zr-89-Df-YY146 was evaluated by sequential PET imaging, in athymic nude mice bearing subcutaneously implanted U87MG or U251 tumors. CD146 blocking, ex vivo biodistribution, and histological studies were carried out to confirm Zr-89-Df-YY146 specificity, as well as the accuracy of PET data. In vitro studies exposed elevated CD146 expression levels in U87MG cells, but negligible levels in U251 cells. Flow cytometry revealed no differences in affinity between YY146 and Df-YY146. Zr-89 labeling of Df-YY146 proceeded with excellent yield (similar to 80%), radiochemical purity (>95%), and specific activity (similar to 44 GBq/mu mol). Longitudinal PET revealed prominent and persistent Zr-89-Df-YY146 uptake in mice bearing U87MG tumors that peaked at 14.00 +/- 3.28%ID/g (n = 4), 48 h post injection of the tracer. Conversely, uptake was significantly lower in CD146-negative U251 tumors (5.15 +/- 0.99%ID/g, at 48 h p.i.; n = 4; P < 0.05). Uptake in U87MG tumors was effectively blocked in a competitive inhibition experiment, corroborating the CD146 specificity of Zr-89-Df-YY146. Finally, ex vivo biodistribution validated the accuracy of PET data and histological examination successfully correlated tracer uptake with in situ CD146 expression. Prominent, persistent, and specific uptake of Zr-89-Df-YY146 was observed in brain tumors, demonstrating the potential of this radiotracer for noninvasive PET imaging of CD146 expression. In a future clinical scenario, Zr-89-Df-YY146 may serve as a tool to guide intervention and assess response to CD146-targeted therapies.