A receptor-binding radiopharmaceutical for imaging of traumatic brain injury in a rodent model: [^99mTc]Tc-tilmanocept

Introduction: Blood-brain barrier (BBB) disruption and subsequent neuro-inflammation occur following traumatic brain injury (TBI), resulting in a spectrum of human nervous system disorders. [^99mTc]Tc-tilmanocept is a receptor-binding radiopharmaceutical FDA-approved for sentinel lymph node mapping. We hypothesize that after an intravenous (i.v.) injection, [^99mTc]Tc-tilmanocept, will traverse a disrupted BBB and bind to CD206-bearing microglial cells.

Methods: Age-matched mice were divided into three groups: 5-days post TBI (n = 4), and 5-days post sham (n = 4), and naïve controls (n = 4). IRDye800CW-labeled [^99mTc]Tc-tilmanocept (0.15 nmol per gram body weight) and FITC-labeled bovine serum albumin (FITC-BSA) were injected (i.v.) into each mouse. Mice were imaged with a high-resolution gamma camera for 45 min. Immediately after imaging, the brains were perfused with fixative, excised, imaged with a fluorescence scanner, assayed for radioactivity, and prepared for histology.

Results: In vivo nuclear imaging, ex vivo fluorescence imaging, ex vivo gamma well counting, and histo-microscopy demonstrated enhanced tilmanocept uptake in the TBI region. The normalized [^99mTc]Tc-tilmanocept uptake value from nuclear imaging and the maximum pixel intensity from fluorescence imaging of the TBI group (1.12 ± 0.12 and 2288 ± 278 a.u., respectively) were significantly (P < 0.04) higher than the sham group (0.64 ± 0.28 and 1708 ± 101 a.u., respectively) and the naive group (0.76 ± 0.24 and 1643 ± 391 a.u., respectively). The mean [^99mTc]Tc-tilmanocept scaled uptake in the TBI brains (0.058 ± 0.013%/g) was significantly (P < 0.010) higher than the scaled brain uptake of the sham group (0.031 ± 0.011%/g) and higher (P = 0.04) than the uptake of the naïve group (0.020 ± 0.002%/g). Fluorescence microscopy demonstrated increased uptake of the IRDye800CW-tilmanocept and FITC-BSA in the TBI brain regions.

Conclusion: [^99mTc]Tc-tilmanocept traverses disrupted blood-brain barrier and localizes within the injured region. ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE: [^99mTc]Tc-tilmanocept could serve as an imaging biomarker for TBI-associated neuroinflammation and any disease process that involves a disruption of the blood-brain barrier.