Evaluation of the α-synuclein PET radiotracer (d3)-[11C]MODAG-001 in pigs.

BACKGROUND: A positron emission tomography (PET) radiotracer to neuroimage α-synuclein aggregates would be a crucial addition for early diagnosis and treatment development in disorders such as Parkinson's disease, where elevated aggregate levels are a histopathological hallmark. The radiotracer (d3)-[11C]MODAG-001 has recently shown promise for visualization of α-synuclein pre-formed fibrils (α-PFF) in rodents. We here test the radiotracer in a pig model where proteins are intracerebrally injected immediately before scanning. Four pigs were injected in one hemisphere with 150 µg α-PFF, and in the other hemisphere, either 75 µg α-PFF or human brain homogenate from either dementia with Lewy bodies (DLB) or Alzheimer's disease (AD) was injected. All pigs underwent one or two (d3)-[11C]MODAG-001 PET scans, quantified with the non-invasive Logan graphical analysis using the occipital cortex as a reference region. RESULTS: The α-PFF and AD homogenate injected brain regions had high uptake of (d3)-[11C]MODAG-001 compared to the occipital cortex or cerebellum. BPND values in 150 µg α-PFF injected regions was 0.78, and in the AD homogenate injected regions was 0.73. By contrast, the DLB homogenate injected region did not differ in uptake and clearance compared to the reference regions. The time-activity curves and BPND values in the 150 µg and 75 µg injected regions of α-PFFs show a dose-dependent effect, and the PET signal could be blocked by pretreatment with unlabeled MODAG-001. CONCLUSION: We find that both α-PFF and AD brain homogenates give rise to increased binding of (d3)-[11C]MODAG-001 when injected into the pig brain. Despite its limited specificity for cerebral α-synuclein pathology, (d3)-[11C]MODAG-001 shows promise as a lead tracer for future radiotracer development.

An in vivo Pig Model for Testing Novel Positron Emission Tomography Radioligands Targeting Cerebral Protein Aggregates.

Positron emission tomography (PET) has become an essential clinical tool for diagnosing neurodegenerative diseases with abnormal accumulation of proteins like amyloid-ß or tau. Despite many attempts, it has not been possible to develop an appropriate radioligand for imaging aggregated α-synuclein in the brain for diagnosing, e.g., Parkinson's Disease. Access to a large animal model with α-synuclein pathology would critically enable a more translationally appropriate evaluation of novel radioligands. We here establish a pig model with cerebral injections of α-synuclein preformed fibrils or brain homogenate from postmortem human brain tissue from individuals with Alzheimer's disease (AD) or dementia with Lewy body (DLB) into the pig's brain, using minimally invasive surgery and validated against saline injections. In the absence of a suitable α-synuclein radioligand, we validated the model with the unselective amyloid-ß tracer [11C]PIB, which has a high affinity for ß-sheet structures in aggregates. Gadolinium-enhanced MRI confirmed that the blood-brain barrier was intact. A few hours post-injection, pigs were PET scanned with [11C]PIB. Quantification was done with Logan invasive graphical analysis and simplified reference tissue model 2 using the occipital cortex as a reference region. After the scan, we retrieved the brains to confirm successful injection using autoradiography and immunohistochemistry. We found four times higher [11C]PIB uptake in AD-homogenate-injected regions and two times higher uptake in regions injected with α-synuclein-preformed-fibrils compared to saline. The [11C]PIB uptake was the same in non-injected (occipital cortex, cerebellum) and injected (DLB-homogenate, saline) regions. With its large brain and ability to undergo repeated PET scans as well as neurosurgical procedures, the pig provides a robust, cost-effective, and good translational model for assessment of novel radioligands including, but not limited to, proteinopathies.