Development and Biodistribution of a Nerve Growth Factor Radioactive Conjugate for PET Imaging.

Abstract

The purpose of these studies was to develop a nerve growth factor (NGF) radiometal-chelator conjugate to determine the biodistribution and brain uptake of NGF by positron emission tomography/computerized tomography (PET-CT). Purified NGF from llama seminal plasma was conjugated with FITC, and the chelator NOTA or DFO. NGF conjugates were evaluated for bioactivity. NOTA- and DFO-conjugated NGF were radiolabeled with gallium-68 or zirconium-89 ([68Ga]GaCl3, half-life = 68min; [89Zr]Zr(oxalate)4, half-life = 3.3days). [89Zr]Zr-NGF was evaluated for biodistribution (0.5, 1, or 24h), PET imaging (60min), and brain autoradiography in mice. Cell-based in vitro assays confirmed that the NGF conjugates maintained NGF receptor-binding and biological activity. Zirconium-89 and gallium-68 radiolabeling showed a high efficiency; however, only[89Zr]Zr-NGF was stable in vitro. Biodistribution studies showed that, as with most small proteins < 70kDa, [89Zr]Zr-NGF uptake was predominantly in the kidney and was cleared rapidly with almost complete elimination of NGF at 24h. Dynamic PET imaging from 0-60min showed a similar pattern to ex vivo biodistribution with some transient liver uptake. Interestingly, although absolute brain uptake was very low, at 24h after treatment, cerebral cortex uptake was higher than any other brain area examined and blood. We conclude that conjugation of DFO to NGF through a thiourea linkage allows effective radiolabeling with zirconium-89 while maintaining NGF bioactivity. Following intravenous administration, the radiolabeled NGF targets non-neuronal tissues (e.g., kidney, liver), and although absolute brain uptake was very low, the brain uptake that was observed was restricted to the cortex.