Engineered antibodies: new possibilities for brain PET?

International audienceAlmost 50 million people worldwide are affected by Alzheimer's disease (AD), the most common neurodegenerative disorder. Development of disease-modifying therapies would benefit from reliable, non-invasive positron emission tomography (PET) biomarkers for early diagnosis, monitoring of disease progression, and assessment of therapeutic effects. Traditionally, PET ligands have been based on small molecules that, with the right properties, can penetrate the blood-brain barrier (BBB) and visualize targets in the brain. Recently a new class of PET ligands based on antibodies have emerged, mainly in applications related to cancer. While antibodies have advantages such as high specificity and affinity, their passage across the BBB is limited. Thus, to be used as brain PET ligands, antibodies need to be modified for active transport into the brain. Here, we review the development of radioligands based on antibodies for visualization of intrabrain targets. We focus on antibodies modified into a bispecific format, with the capacity to undergo transferrin receptor 1 (TfR1)-mediated transcytosis to enter the brain and access pathological proteins, e.g. amyloid-beta. A number of such antibody ligands have been developed, displaying differences in brain uptake, pharmacokinetics, and ability to bind and visualize the target in the brain of transgenic mice. Potential pathological changes related to neurodegeneration, e.g. misfolded proteins and neuroinflammation, are suggested as future targets for this novel type of radioligand. Challenges are also discussed, such as the temporal match of radionuclide half-life with the ligand's pharmacokinetic profile and translation to human use. In conclusion, brain PET imaging using bispecific antibodies, modified for receptor-mediated transcytosis across the BBB, is a promising method for specifically visualizing molecules in the brain that are difficult to target with traditional small molecule ligands

Genome-wide association study of corticobasal degeneration identifies risk variants shared with progressive supranuclear palsy

Corticobasal degeneration (CBD) is a neurodegenerative disorder affecting movement and cognition, definitively diagnosed only at autopsy. Here, we conduct a genome-wide association study (GWAS) in CBD cases (n = 152) and 3, 311 controls, and 67 CBD cases and 439 controls in a replication stage. Associations with meta-analysis were 17q21 at MAPT (P = 1.42 x 10(-12)),8p12 at lnc-KIF13B-1, a long non-coding RNA (rs643472;P = 3.41 x 10(-8)),and 2p22 at SOS1 (rs963731;P = 1.76 x 10(-7)). Testing for association of CBD with top progressive supranuclear palsy (PSP) GWAS single-nucleotide polymorphisms (SNPs) identified associations at MOBP (3p22;rs1768208;P = 2.07 x 10(-7)) and MAPT H1c (17q21;rs242557;P = 7.91 x 10(-6)). We previously reported SNP/transcript level associations with rs8070723/MAPT, rs242557/MAPT, and rs1768208/MOBP and herein identified association with rs963731/SOS1. We identify new CBD susceptibility loci and show that CBD and PSP share a genetic risk factor other than MAPT at 3p22 MOBP (myelin-associated oligodendrocyte basic protein)

Tauopathies. From molecule to therapy

Background. The microtubule-associated tau protein is the defining denominator of agroup of neurodegenerative diseases termed tauopathies. Objective. Provide atimely state of the art review on recent scientific advances in the field of tauopathies. Material and methods. Systematic review of the literature from the past 10years. Results. Tau proteins are increasingly being recognized as ahighly variable protein, underlying and defining aspectrum of molecularly defined diseases, with aclinical spectrum ranging from dementia to hypokinetic movement disorders. Genetic variation at the tau locus can trigger disease or modify disease risk. Tau protein alterations can damage nerve cells and propagate pathologies through the brain. Thus, tau proteins may serve both as aserological and imaging biomarker. Tau proteins also provide abroad spectrum of rational therapeutic interventions to prevent disease progression. This knowledge has led to modern clinical trials. Conclusion. The field of tauopathies is in a state of dynamic and rapid progress, requiring close interdisciplinary collaboration

Validation of the movement disorder society criteria for the diagnosis of 4‐repeat tauopathies

Background: The Movement Disorder Society criteria for progressive supranuclear palsy introduced the category “probable 4‐repeat (4R)‐tauopathy” for joint clinical diagnosis of progressive supranuclear palsy and corticobasal degeneration. / Objectives: To validate the accuracy of these clinical criteria for “probable 4R‐tauopathy” to predict underlying 4R‐tauopathy pathology. / Methods: Diagnostic accuracy for 4R‐tauopathies according to the established criteria was estimated retrospectively in autopsy‐confirmed patients with progressive supranuclear palsy and corticobasal degeneration (grouped as 4R‐tauopathies), and Parkinson's disease, multiple system atrophy, and frontotemporal lobar degeneration (grouped as non‐4R‐tauopathies). / Results: We identified 250 cases with progressive supranuclear palsy (N = 195) and corticobasal degeneration (N = 55) and with and non‐4R‐tauopathies (N = 161). Sensitivity and specificity of “probable 4R‐tauopathy” was 10% and 99% in the first year and 59% and 88% at final record. / Conclusions: The new diagnostic category “probable 4R‐tauopathy” showed high specificity and may be suitable for the recruitment of patients with progressive supranuclear palsy and corticobasal degeneration into therapeutic trials targeting 4R‐tauopathy. The low sensitivity underpins the need for diagnostic biomarkers

¹⁸F-PI2620 Tau-PET in progressive supranuclear palsy: A multi-center evaluation

Background: Progressive supranuclear palsy (PSP) is a 4-repeat (4R) tauopathy and region-specific tau deposits establish the neuropathological diagnosis of “definite PSP” post mortem. Future interventional trials against tau in PSP would strongly benefit from biomarkers to validate the specific presence of the target and to monitor the target’s magnitude during therapy. First generation tau positron-emission-tomography (PET) ligands such as 18F-THK5351 or 18F-AV1451 were somewhat able to distinguish PSP patients from healthy controls (HC) or from patients with other neurodegenerative diseases, but relevant fractions of the PET signal in PSP may have been related to concomitant monoamine-oxidase (MAO) increases. The novel second generation tau-PET ligand 18F-PI2620 proved absent off-target binding to MAO and high affinity to 3/4R tau in Alzheimer’s disease (AD). The aim of this multicenter-evaluation was to investigate 18F-PI2620 in patients with suspected 4R tau pathology in clinically diagnosed PSP. Methods: Seventeen patients (70±7 y, n=8 female) with probable or possible PSP Richardson syndrome according to MDS-PSP criteria underwent 18F-PI2620 PET at four different centers together with ten HC and seven disease controls (Multi-system atrophy, Parkinson’s disease, and AD). PET scans were acquired 0-60 min p.i. followed by coregistration to a 18F-PI2620 template in the MNI space. Standardized uptake value ratios (SUVr) of predefined brain regions in the basal ganglia were generated using cerebellar scaling of a 30-60 min p.i. frame after inspection of the full dynamic range. SUVr data were compared between PSP, HC, and disease controls by an ANOVA including Bonferroni post hoc correction. Statistical parametric mapping (SPM, V12) was performed between PSP and HC (t-test). SUVr and SPM data were corrected for different centers. Additionally, disease severity measured by the PSP rating scale (PSPRS) was correlated with PET findings. An in vitro pilot autoradiography using 18F-PI2620 incubation of brain slices was performed for the globus pallidus of a single PSP patient and compared to the cortical binding in a specimen from an AD patient. Results: Our study indicates significantly elevated mean 18F-PI2620 SUVr in PSP patients (PSPRS: 40±17; range 13-71) in the globus pallidus (1.34±0.16; p = 0.001; d = 1.68) and the substantia nigra (1.33±0.14; p = 0.003; d = 1.49) when compared to HC (1.12±0.09 / 1.15±0.08). Disease controls showed a similar signal in the globus pallidus (1.11±0.06; p = n.s.) and a slight elevation in the substantia nigra (1.23±0.09; p = n.s.) when compared to HC. A voxel-wise analysis SPM revealed elevated 18F-PI2620 uptake in the globus pallidus, the substantia nigra as well as in the frontal and parietal cortex (all p 0.2). Subjects with low disease severity (PSPRS ≤ 30; n=4) already had a significantly elevated 18F-PI2620 uptake in the globus pallidus when compared to HC (1.38±0.13 vs. 1.12±0.09; p = 0.001; d = 2.32). Preliminary in vitro autoradiography showed distinguishable 18F-PI2620 binding in the globus pallidus of a PSP patient which was however far lower when compared to cortical binding in AD. Conclusions: The results of this preliminary multi-center evaluation indicate a value of 18F-PI2620 to diagnose and differentiate suspected PSP patients in vivo. The magnitude of tracer binding between patients seems to be variably expressed but not correlated with disease severity. These results indicate that 18F-PI2620 may show potential as a biomarker to assess tau pathology in PSP patients and that it may be helpful to establish earlier and more reliable diagnosis of PSP

Clinical diagnosis of progressive supranuclear palsy: the movement disorder society criteria

Background: PSP is a neuropathologically defined disease entity. Clinical diagnostic criteria, published in 1996 by the National Institute of Neurological Disorders and Stroke/Society for PSP, have excellent specificity, but their sensitivity is limited for variant PSP syndromes with presentations other than Richardson's syndrome. Objective: We aimed to provide an evidence- and consensus-based revision of the clinical diagnostic criteria for PSP. Methods: We searched the PubMed, Cochrane, Medline, and PSYCInfo databases for articles published in English since 1996, using postmortem diagnosis or highly specific clinical criteria as the diagnostic standard. Second, we generated retrospective standardized clinical data from patients with autopsy-confirmed PSP and control diseases. On this basis, diagnostic criteria were drafted, optimized in two modified Delphi evaluations, submitted to structured discussions with consensus procedures during a 2-day meeting, and refined in three further Delphi rounds. Results: Defined clinical, imaging, laboratory, and genetic findings serve as mandatory basic features, mandatory exclusion criteria, or context-dependent exclusion criteria. We identified four functional domains (ocular motor dysfunction, postural instability, akinesia, and cognitive dysfunction) as clinical predictors of PSP. Within each of these domains, we propose three clinical features that contribute different levels of diagnostic certainty. Specific combinations of these features define the diagnostic criteria, stratified by three degrees of diagnostic certainty (probable PSP, possible PSP, and suggestive of PSP). Clinical clues and imaging findings represent supportive features. Conclusions: Here, we present new criteria aimed to optimize early, sensitive, and specific clinical diagnosis of PSP on the basis of currently available evidence