In vivo detection of inducible nitric oxide synthase in rodent gliomas

Increased iNOS expression is often found in brain tumors, such as gliomas. The goal of this study was to develop and assess a novel molecular MRI (mMRI) probe for in vivo detection of iNOS in rodent models for gliomas (intracerebral implantation of rat C6 or RG2 cells or ethyl nitrosourea-induced glioma). The probe we used incorporated a Gd-DTPA (gadolinium(III) complex of diethylenetriamine-N,N,N′,N″,N″-pentaacetate) backbone with albumin and biotin moieties and covalent binding of an anti-iNOS antibody (Ab) to albumin (anti-iNOS probe). We used mMRI with the anti-iNOS probe to detect in vivo iNOS levels in gliomas. Nonimmune normal rat IgG coupled to albumin–Gd-DTPA–biotin was used as a control nonspecific contrast agent. By targeting the biotin component of the anti-iNOS probe with streptavidin Cy3, fluorescence imaging confirmed the specificity of the probe for iNOS in glioma tissue. iNOS levels in glioma tumors were also confirmed via Western blots and immunohistochemistry. The presence of plasma membrane-associated iNOS in glioma cells was established by transmission electron microscopy and gold-labeled anti-iNOS Ab. The more aggressive RG2 glioma was not found to have higher levels of iNOS compared to C6. Differences in glioma vascularization and blood–brain barrier permeability between the C6 and the RG2 gliomas are discussed. In vivo assessment of iNOS levels associated with tumor development is quite feasible in heterogeneous tissues with mMRI.