Preliminary Evaluations of [11C]Verubulin: Implications for Microtubule Imaging With PET

Preclinical Evaluation of Novel PET Probes for Dementia

The development of novel PET imaging agents that selectively bind specific dementia-related targets can contribute significantly to accurate, differential and early diagnosis of dementia causing diseases and support the development of therapeutic agents. Consequently, in recent years there has been a growing body of literature describing the development and evaluation of potential new promising PET tracers for dementia. This review article provides a comprehensive overview of novel dementia PET probes under development, classified by their target, and pinpoints their preclinical evaluation pathway, typically involving in silico, in vitro and ex/in vivo evaluation. Specific target-associated challenges and pitfalls, requiring extensive and well-designed preclinical experimental evaluation assays to enable successful clinical translation and avoid shortcomings observed for previously developed 'well-established' dementia PET tracers are highlighted in this review.

Radiosynthesis, Preclinical, and Clinical Positron Emission Tomography Studies of Carbon-11 Labeled Endogenous and Natural Exogenous Compounds

The presence of positron emission tomography (PET) centers at most major hospitals worldwide, along with the improvement of PET scanner sensitivity and the introduction of total body PET systems, has increased the interest in the PET tracer development using the short-lived radionuclides carbon-11. In the last few decades, methodological improvements and fully automated modules have allowed the development of carbon-11 tracers for clinical use. Radiolabeling natural compounds with carbon-11 by substituting one of the backbone carbons with the radionuclide has provided important information on the biochemistry of the authentic compounds and increased the understanding of their in vivo behavior in healthy and diseased states. The number of endogenous and natural compounds essential for human life is staggering, ranging from simple alcohols to vitamins and peptides. This review collates all the carbon-11 radiolabeled endogenous and natural exogenous compounds synthesised to date, including essential information on their radiochemistry methodologies and preclinical and clinical studies in healthy subjects.

Preliminary mechanistic insights of a brain-penetrant microtubule imaging PET ligand in a tau-knockout mouse model

BACKGROUND

Microtubules (MTs) are critical for cell structure, function, and survival. MT instability may contribute to Alzheimer's disease (AD) pathogenesis as evidenced by persistent negative regulation (phosphorylation) of the neuronal microtubule-associated protein tau. Hyperphosphorylated tau, not bound to MTs, forms intraneuronal pathology that correlates with dementia and can be tracked using positron emission tomography (PET) imaging. The contribution of MT instability in AD remains unknown, though it may be more proximal to neuronal dysfunction than tau accumulation. Our lab reported the first brain-penetrant MT-based PET ligand, [11C]MPC-6827, and its PET imaging with this ligand in normal rodents and non-human primates demonstrated high brain uptake and excellent pharmacokinetics. Target engagement and mechanism of action using in vitro, in vivo, and ex vivo methods were evaluated here.

METHODS

In vitro cell uptake assay was performed in SH-SY5Y neuronal cells with [11C]MPC-6827, with various MT stabilizing and destabilizing agents. To validate the in vitro results, wild type (WT) mice (n = 4) treated with a brain-penetrant MT stabilizing drug (EpoD) underwent microPET/CT brain imaging with [11C]MPC-6827. To determine the influence of tau protein on radiotracer binding in the absence of protein accumulation, we utilized tau knockout (KO) mice. In vivo microPET imaging, ex vivo biodistribution, and autoradiography studies were performed in tau KO and WT mice (n = 6/group) with [11C]MPC-6827. Additionally, α, β, and acetylated tubulin levels in both brain samples were determined using commercially available cytoskeleton-based MT kit and capillary electrophoresis immunoblotting assays.

RESULTS

Cell uptake demonstrated higher radioactive uptake with MT destabilizing agents and lower uptake with stabilizing agents compared to untreated cells. Similarly, acute treatment with EpoD in WT mice decreased [11C]MPC-6827 brain uptake, assessed with microPET/CT imaging. Compared to WT mice, tau KO mice expressed significantly lower β tubulin, which contains the MPC-6827 binding domain, and modestly lower levels of acetylated α tubulin, indicative of unstable MTs. In vivo imaging revealed significantly higher [11C]MPC-6827 uptake in tau KOs than WT, particularly in AD-relevant brain regions known to express high levels of tau. Ex vivo post-PET biodistribution and autoradiography confirmed the in vivo results.

CONCLUSIONS

Collectively, our data indicate that [11C]MPC-6827 uptake inversely correlates with MT stability and may better reflect the absence of tau than total tubulin levels. Given the radiotracer binding does not require the presence of aggregated tau, we hypothesize that [11C]MPC-6827 may be particularly useful in preclinical stages of AD prior to tau deposition. Our study provides immediate clarity on high uptake of the MT-based radiotracer in AD brains, which directly informs clinical utility in MT/tau-based PET imaging studies.