Anticalins directed against the fibronectin extra domain B as diagnostic tracers for glioblastomas

The standard of care for diagnosis and therapy monitoring of gliomas is magnetic resonance imaging (MRI), which however, provides only an indirect and incomplete representation of the tumor mass, offers limited information for patient stratification according to WHO‐grades and may insufficiently indicate tumor relapse after antiangiogenic therapy. Anticalins are alternative binding proteins obtained via combinatorial protein design from the human lipocalin scaffold that offer novel diagnostic reagents for histology and imaging applications. Here, the Anticalins N7A, N7E and N9B, which possess exquisite specificity and affinity for oncofetal fibronectin carrying the extra domain B (ED‐B), a well‐known proangiogenic extracellular matrix protein, were applied for immunohistochemical studies. When investigating ED‐B expression in biopsies from 41 patients with confirmed gliomas of WHO grades I to IV, or in non‐neoplastic brain samples, we found that Anticalins specifically detect ED‐B in primary glioblastoma multiforme (GBM; WHO IV) but not in tumors of lower histopathological grade or in tumor‐free brain. In primary GBM samples, ED‐B specific Anticalins locate to fibronectin‐rich perivascular areas that are associated with angiogenesis. Anticalins specifically detect ED‐B both in fixed tumor specimen and on vital cells, as evidenced by cytofluorometry. Beyond that, we labeled an Anticalin with the γ‐emitter 123I and demonstrated specific binding to GBM‐tissue samples using in vitro autoradiography. Overall, our data indicate that ED‐B specific Anticalins are useful tools for the diagnosis of primary GBM and related angiogenic sites, presenting them as promising tracers for molecular tumor imaging. What's new? Today the standard of care for diagnosis of gliomas and therapy monitoring is magnetic resonance imaging (MRI), but it only offers limited clinical information. Here, the authors investigated the potential of engineered non‐Ig binding proteins (Anticalins) directed against the fibronectin extra‐domain B (ED‐B) as molecular imaging tracers to diagnose glioblastomas. They found that Anticalins specifically detect ED‐B in primary glioblastoma multiforme and not in tumors of lower histopathological grade or in tumor‐free brain. Anticalins fulfil major requirements to become biomolecular tracers that may in the future improve the classification and localization of primary brain tumors through molecular imaging approaches.