From frontotemporal lobar degeneration pathology to frontotemporal lobar degeneration biomarkers

Current Status of Clinical Trials on Tau Immunotherapies

Tau immunotherapies have advanced from proof-of-concept studies to over a dozen clinical trials for Alzheimer's disease (AD) and other tauopathies. Mechanistic studies in animal and culture models have provided valuable insight into how these therapies may work but multiple pathways are likely involved. Different groups have emphasized the importance of intracellular vs extracellular antibody-mediated clearance of the tau protein and there is no consensus on which pool of tau should ideally be targeted. Likewise, various normal and disease-selective epitopes are being targeted, and the antibody isotypes either favor phagocytosis of the tau-antibody complex or are neutral in that aspect. Most of the clinical trials are in early stages, thus their efficacy is not yet known, but all have been without any major adverse effects and some have reported target engagement. A few have been discontinued. One in phase I, presumably because of a poor pharmacokinetic profile, and three in phase II for a lack of efficacy although this trial stage is not well powered for efficacy measures. In these phase II studies, trials with two antibodies in patients with progressive supranuclear palsy or other primary tauopathies were halted but are continuing in patients with AD, and one antibody trial was stopped in early-stage AD but is continuing in moderate AD. These three antibodies have been reported to only work extracellularly and tau is not increased in the cerebrospinal fluid of primary tauopathies, which may explain the failures of two of them. In the discontinued AD trial, there are some concerns about how much of extracellular tau contains the N-terminal epitope that is being targeted. In addition, extracellular tau is only a small part of total tau, compared to intracellular tau. Targeting only the former may not be sufficient for functional benefits. Given these outcomes, decision makers within the pharmaceutical companies who green light these trials should attempt to target tau not only extracellularly but also intracellularly to increase their chances of success. Hopefully, some of the ongoing trials will provide some functional benefits to the large number of patients with tauopathies.

Autopsy-diagnosed neurodegenerative dementia cases support the use of cerebrospinal fluid protein biomarkers in the diagnostic work-up

Various proteins play a decisive role in the pathology of different neurodegenerative diseases. Nonetheless, most of these proteins can only be detected during a neuropathological assessment, although some non-specific biomarkers are routinely tested for in the cerebrospinal fluid (CSF) as a part of the differential diagnosis of dementia. In antemortem CSF samples from 117 patients with different types of neuropathologically confirmed neurodegenerative disease with dementia, we assessed total-tau (t-tau), phosphorylated-tau (181P) (p-tau), amyloid-beta (1–42) (Aβ42), TAR DNA binding protein (TDP)-43, progranulin (PGRN), and neurofilament light (NfL) chain levels, and positivity of protein 14-3-3. We found t-tau levels and the t-tau/p-tau ratios were significantly higher in prion diseases compared to the other neurodegenerative diseases. Statistically significant differences in the t-tau/Aβ42 ratio predominantly corresponded to t-tau levels in prion diseases and Aβ42 levels in AD. TDP-43 levels were significantly lower in prion diseases. Additionally, the TDP-43/Aβ42 ratio was better able to distinguish Alzheimer’s disease from other neurodegenerative diseases compared to using Aβ42 alone. In frontotemporal lobar degeneration, PRGN levels were significantly higher in comparison to other neurodegenerative diseases. There is an increasing need for biomarkers suitable for diagnostic workups for neurodegenerative diseases. It appears that adding TDP-43 and PGRN to the testing panel for neurodegenerative diseases could improve the resolution of differential diagnoses.

Tau immunotherapies: Lessons learned, current status and future considerations

The majority of clinical trials targeting the tau protein in Alzheimer's disease and other tauopathies are tau immunotherapies. Because tau pathology correlates better with the degree of dementia than amyloid-β lesions, targeting tau is likely to be more effective in improving cognition than clearing amyloid-β in Alzheimer's disease. However, the development of tau therapies is in many ways more complex than for amyloid-β therapies as briefly outlined in this review. Most of the trials are on humanized antibodies, which may have very different properties than the original mouse antibodies. The impact of these differences are to a large extent unknown, can be difficult to decipher, and may not always be properly considered. Furthermore, the ideal antibody properties for efficacy are not well established and can depend on several factors. However, considering the varied approaches in clinical trials, there is a general optimism that at least some of these trials may provide functional benefits to patients suffering of various tauopathies. This article is part of the special issue entitled 'The Quest for Disease-Modifying Therapies for Neurodegenerative Disorders'.

Frontotemporal dementia

Frontotemporal dementia (FTD) is the second commonest cause of young onset dementia. Our understanding of FTD and its related syndromes has advanced significantly in recent years. Among the most prominent areas of progress is the overlap between FTD, MND, and other neurodegenerative conditions at a clinicopathologic and genetic level. In parallel major advances in neuroimaging techniques, the discovery of new genetic mutations as well as the development of potential biomarkers may serve to further expand knowledge of the biologic processes at play in FTD and may in turn propel research toward identifying curative and preventative pharmacologic therapies. The aim of this chapter is to discuss the clinical, pathologic, and genetic complexities of FTD and related disorders.

Cerebrospinal Fluid Biomarkers for the Differential Diagnosis between Alzheimer's Disease and Frontotemporal Lobar Degeneration: Systematic Review, HSROC Analysis, and Confounding Factors

BACKGROUND

Differential diagnosis in dementia is at present one of the main challenges both in clinical practice and research. Cerebrospinal fluid (CSF) biomarkers are included in the current diagnostic criteria of Alzheimer's disease (AD) but their clinical utility is still unclear.

OBJECTIVE

We performed a systematic review of studies analyzing the diagnostic performance of CSF Aβ42, total tau (t-tau), and phosphorylated tau (p-tau) in the discrimination between AD and frontotemporal lobar degeneration (FTLD) dementias.

METHODS

The following electronic databases were consulted until May 2016: Medline and PreMedline, EMBASE, PsycInfo, CINAHL, Cochrane Library, and CRD. For the first-time in the field, a Hierarchical Summary Receiver Operating Characteristic (HRSOC) model was applied, which avoids methodological problems of meta-analyses based on summary points of sensitivity and specificity values. We also investigated relevant confounders of CSF biomarkers' diagnostic performance such as age, disease duration, and global cognitive impairment.

RESULTS

The p-tau/Aβ42 ratio showed the best diagnostic performance. No statistically significant effects of the confounders were observed. Nonetheless, the p-tau/Aβ42 ratio may be especially indicated for younger patients. P-tau may be preferable for less cognitively impaired patients (high MMSE scores) and the t-tau/Aβ42 ratio for more cognitively impaired patients (low MMSE scores).

CONCLUSION

The p-tau/Aβ42 ratio has potential for being implemented in the clinical routine for the differential diagnosis between AD and FTLD. It is of utmost importance that future studies report information on confounders such as age, disease duration, and cognitive impairment, which should also stimulate understanding of the role of these factors in disease mechanisms and pathophysiology.

Tau Immunotherapies for Alzheimer's Disease and Related Tauopathies: Progress and Potential Pitfalls

Tau immunotherapies have now advanced from proof-of-concept studies to Phase II clinical trials. This review briefly outlines developments in the field and discusses how these therapies may work, which involves multiple variables that are connected in complex ways. These various factors are likely to define therapeutic success in humans and have not been thoroughly investigated, at least based on published reports.

CSF neurofilament light concentration is increased in presymptomatic CHMP2B mutation carriers

Objective

A rare cause of familial frontotemporal dementia (FTD) is a mutation in the CHMP2B gene on chromosome 3 (FTD-3), described in a Danish family. Here we examine whether CSF biomarkers change in the preclinical phase of the disease.

Methods

In this cross-sectional explorative study, we analyzed CSF samples from 16 mutation carriers and 14 noncarriers from the Danish FTD-3 family. CSF biomarkers included total tau (t-tau) and neurofilament light chain (NfL) as a marker for neurodegeneration, phosphorylated tau (p-tau) as a marker for tau pathology, β-amyloid (Aβ) 38, 40, and 42 (Aβ₃₈, Aβ₄₀, and Aβ₄₂) to monitor Aβ metabolism, and YKL-40 as a marker of neuroinflammation. Aβ isoform concentrations were measured using a multiplexed immunoassay; t-tau, p-tau, NfL, and YKL-40 concentrations were measured using sandwich ELISAs.

Results

CSF NfL concentration was significantly increased in mutation carriers vs noncarriers. Further, CSF NfL concentration was significantly higher in symptomatic mutation carriers compared to presymptomatic carriers, and also significantly higher in presymptomatic carriers compared to noncarriers. No differences in t-tau and p-tau and YKL-40 concentrations between controls and mutation carriers were observed. CSF concentrations of the Aβ peptides Aβ₃₈ and Aβ₄₀ but not Aβ₄₂ were significantly lower in mutation carriers compared to noncarriers.

Conclusions

Increased NfL levels in presymptomatic individuals and in symptomatic patients with FTD-3 indicate a continuous process of neurodegeneration from the presymptomatic to symptomatic state. Although not specific for FTD-3 pathology, our data suggest that CSF NfL could serve as a valuable biomarker to detect onset of neurodegeneration in FTD-3 mutation carriers.

Cerebrospinal Fluid Biomarkers in Familial Forms of Alzheimer's Disease and Frontotemporal Dementia

As dementia is a fast-growing health care problem, it is becoming an increasingly urgent need to provide an early diagnosis in order to offer patients the best medical treatment and care. Validated biomarkers which reflect the pathology and disease progression are essential for diagnosis and are important when developing new therapies. Today, the core protein biomarkers amyloid-β42, total tau and phosphorylated tau in the cerebrospinal fluid (CSF) are used to diagnose Alzheimer's disease (AD), because these biomarkers have shown to reflect the underlying amyloid and tau pathology. However, the biomarkers have proved insufficient predictors of dementias with a different pathology, e.g. frontotemporal dementia (FTD); furthermore, the biomarkers are not useful for early AD diagnosis. Familial dementias with a known disease-causing mutation can be extremely valuable to study; yet the biomarker profiles in patients with familial dementias are not clear. This review summarizes CSF biomarker findings from studies on symptomatic and presymptomatic individuals carrying a mutation in one of the genes known to cause early-onset familial AD or FTD. In conclusion, the biomarker profile of inherited AD is quite similar between carriers of different mutations as well as similar to the profile found in sporadic AD, whereas familial FTD does not seem to have a clear biomarker profile. Hence, new biomarkers are needed for FTD.

Antibody-derived in vivo imaging of tau pathology

Antibodies or their derivatives as imaging probes for pathological tau protein have great potential, but have not been well studied. In particular, smaller, single-chain-variable antibody fragments (scFv's) are attractive for detecting tau lesions in live subjects. Here, we generated libraries of scFv's and identified numerous phospho-tau-selective scFv's. Peripheral injection of one of these scFv's consistently resulted in a strong in vivo brain signal in transgenic tauopathy mice, but not in wild-type or amyloid-β plaque mice. The parent tau antibody provided similar results, albeit with a weaker signal intensity. The imaging signal correlated very well with colocalization of the probe with intraneuronal tau aggregates. Both were associated with markers of endosomes, autophagosomes, and lysosomes, suggesting their interaction in these degradation pathways. Such specific antibody-derived imaging probes have great potential as diagnostic markers for Alzheimer's disease and related tauopathies.

Biomarkers in dementia: clinical utility and new directions

Imaging, cerebrospinal fluid (CSF) and blood-based biomarkers have the potential to improve the accuracy by which specific causes of dementia can be diagnosed in vivo, provide insights into the underlying pathophysiology, and may be used as inclusion criteria and outcome measures for clinical trials. While a number of imaging and CSF biomarkers are currently used for each of these purposes, this is an evolving field, with numerous potential biomarkers in varying stages of research and development. We review the currently available biomarkers for the three most common forms of neurodegenerative dementia, and give an overview of research techniques that may in due course make their way into the clinic.