Antibody-Based PET Imaging of Misfolded Superoxide Dismutase 1 in an Amyotrophic Lateral Sclerosis Mouse Model

Amyotrophic lateral sclerosis (ALS) is a rare neurodegenerative disease characterized by motor neuron loss in the motor cortex, brain stem, and spinal cord. Mutations in the superoxide dismutase 1 (SOD1) gene, resulting in misfolding of its protein product, are a common cause of ALS. Currently, there is no approved ALS diagnostic tool. Here, we present the development of a PET radiotracer, [⁸⁹Zr]Zr-desferoxamine (DFO)-α-miSOD1, targeting selectively misfolded SOD1 (misSOD1). Methods: DFO-α-miSOD1 was prepared by conjugating α-miSOD1 antibody with DFO and labeled with ⁸⁹Zr. A longitudinal imaging study was performed to identify the optimal mouse age and time after administration of [⁸⁹Zr]Zr-DFO-α-miSOD1 for the detection of misSOD1 aggregation in transgenic mice overexpressing misSOD1 and in wild-type mice. Subsets of mice were either coinjected with an excess of α-miSOD1 or imaged with deglycosylated [⁸⁹Zr]Zr-DFO-α-miSOD1 to assess target specificity. The internal radiation dose for [⁸⁹Zr]Zr-DFO-α-miSOD1 was estimated by extrapolating data from mouse biodistribution experiments. Results: Imaging with [⁸⁹Zr]Zr-DFO-α-miSOD1 was optimal in 136-d-old transgenic mice on day 10 after administration. Significant accumulation of [⁸⁹Zr]Zr-DFO-α-miSOD1 was detected in the spinal cord and cartilage of ALS transgenic mice compared with the wild-type mice (P = 0.01). The radiotracer accumulation is selective and blockable with an excess of α-miSOD1. Deglycosylated [⁸⁹Zr]Zr-DFO-α-miSOD1 results in high-contrast detection of misSOD1 but is prone to aggregation. The dosimetry for [⁸⁹Zr]Zr-DFO-α-miSOD1 is comparable to that for other ⁸⁹Zr-based tracers currently used in humans. Conclusion: This work thus establishes that [⁸⁹Zr]Zr-DFO-α-miSOD1 PET can detect misSOD1 in transgenic mice, paving the way for application in early diagnosis of ALS and therapeutic monitoring.