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

Abstract

Positron emission tomography (PET) has become an essential clinical tool for diagnosing neurodegenerative diseases with abnormal accumulation of proteins like amyloid-β 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-β tracer [11C]PIB, which has a high affinity for β-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.

Pretargeted imaging beyond the blood–brain barrier

“…For example, intracerebral injection does not create a large pool of blood circulating vector which reacts with administered Tz and prevents a significant fraction of it from reaching the brain. 60 A detailed description of the model and its validation is reported in ESI† Section S9. In brief, Long–Evans rats were injected with TCO-decorated PeptoBrush (100 μg polymer, 15 nmol TCO in 4 μL of 10 mM phosphate buffered saline at pH 7.2) into the right striatum.…”

Selecting the Best Animal Model of Parkinson’s Disease for Your Research Purpose: Insight from in vivo PET Imaging Studies

“…One such model, the preformed α-syn fibril rodent model of PD, has shown a 3-fold increase of phosphorylated α-syn in motor cortex 6-22 weeks post-PFF injection and spreading of α-syn aggregates [ 247 ], and would be appropriate for the evaluation of chronic changes and correlation with symptom severity. An alternative approach for the initial screening of lead compounds would be the acute local injection of fibrils into the cortex rather than the nigrostriatal system, an approach currently being trialed in Danish farm pigs [ 251 ]. This rapid approach would allow a fast screen of potential ligands, but has the disadvantage of the ligands not being trialled in a classical PD-associated brain regions or in a relevant progressive model.…”