Novel PET/SPECT probes for imaging of tau in Alzheimer's disease

Aggregation, Transmission, and Toxicity of the Microtubule-Associated Protein Tau: A Complex Comprehension

The microtubule-associated protein tau is an intrinsically disordered protein containing a few short and transient secondary structures. Tau physiologically associates with microtubules (MTs) for its stabilization and detaches from MTs to regulate its dynamics. Under pathological conditions, tau is abnormally modified, detaches from MTs, and forms protein aggregates in neuronal and glial cells. Tau protein aggregates can be found in a number of devastating neurodegenerative diseases known as "tauopathies", such as Alzheimer's disease (AD), frontotemporal dementia (FTD), corticobasal degeneration (CBD), etc. However, it is still unclear how the tau protein is compacted into ordered protein aggregates, and the toxicity of the aggregates is still debated. Fortunately, there has been considerable progress in the study of tau in recent years, particularly in the understanding of the intercellular transmission of pathological tau species, the structure of tau aggregates, and the conformational change events in the tau polymerization process. In this review, we summarize the concepts of tau protein aggregation and discuss the views on tau protein transmission and toxicity.

Structure-Activity Relationships of Radioiodinated 6,5,6-Tricyclic Compounds for the Development of Tau Imaging Probes

Tau aggregate, which is the main component of the neurofibrillary tangle, is an attractive imaging target for diagnosing and monitoring the progression of Alzheimer's disease (AD). In this study, we designed and synthesized six radioiodinated 6,5,6-tricyclic compounds to explore novel scaffolds for tau imaging probes. On in vitro autoradiography of AD brain sections, pyridoimidazopyridine and benzimidazopyrimidine derivatives ([¹²⁵I]21 and [¹²⁵I]22, respectively) showed selective binding affinity for tau aggregates, whereas carbazole, pyrrolodipyridine, and pyridoimidazopyrimidine derivatives ([¹²⁵I]10, [¹²⁵I]12, and [¹²⁵I]23, respectively) bound β-amyloid aggregates. In a biodistribution study using normal mice, [¹²⁵I]21 and [¹²⁵I]22 showed high initial uptakes (5.73 and 5.66% ID/g, respectively, at 2 min postinjection) into and rapid washout (0.14 and 0.10% ID/g, respectively, at 60 min postinjection) from the brain. Taken together, two novel scaffolds including pyridoimidazopyridine and benzimidazopyrimidine may be applied to develop useful tau imaging probes.

Tauopathies: Mechanisms and Therapeutic Strategies

Tauopathies are morphologically, biochemically, and clinically heterogeneous neurodegenerative diseases defined by the accumulation of abnormal tau proteins in the brain. There is no effective method to prevent and reverse the tauopathies, but this gloomy picture has been changed by recent research advances. Evidences from genetic studies, experimental animal models, and molecular and cell biology have shed light on the main mechanisms of the diseases. The development of radiology and biochemistry, especially the development of PET imaging, will provide important biomarkers for the clinical diagnosis and treatment. Given the central role of tau in tauopathies, many treatments have constantly emerged, including targeting phosphorylation, targeting aggregation, increasing microtubule stabilization, tau immunization, clearance of tau, anti-inflammatory treatment, and other therapeutics. There is still a long way to go before we obtain drug therapy targeted at multifactor mechanisms.

Current progress, challenges and future prospects of diagnostic and therapeutic interventions in Alzheimer's disease

The diverse pathological mechanisms and their implications for the development of effective diagnostic and therapeutic interventions in Alzheimer's disease are presented with current progress, challenges and future prospects.

Detection and Characterization of Small Molecule Interactions with Fibrillar Protein Aggregates Using Microscale Thermophoresis

Neurodegenerative diseases such as Parkinson's and Alzheimer's disease share the pathological hallmark of fibrillar protein aggregates. The specific detection of these protein aggregates by positron emission tomography (PET) in the patient brain can yield valuable information for diagnosis and disease progression. However, the identification of novel small compounds that bind fibrillar protein aggregates has been a challenge. In this study, microscale thermophoresis (MST) was applied to assess the binding affinity of known small molecule ligands of α-synuclein fibrils, which were also tested in parallel in a thioflavin T fluorescence competition assay for further validation. In addition, a MST assay was also developed for the detection of the interaction between a variety of small molecules and tau fibrils. The results of this study demonstrate that MST is a powerful and practical methodology to quantify interactions between small molecules and protein fibrillar aggregates, which suggests that it can be applied for the identification and development of PET radioligands and potentially of therapeutic candidates for protein misfolding diseases.

Clinical Applications of Small-molecule PET Radiotracers: Current Progress and Future Outlook

Radiotracers, or radiopharmaceuticals, are bioactive molecules tagged with a radionuclide used for diagnostic imaging or radiotherapy and, when a positron-emitting radionuclide is chosen, the radiotracers are used for PET imaging. The development of novel PET radiotracers in many ways parallels the development of new pharmaceuticals, and small molecules dominate research and development pipelines in both disciplines. The 4 decades since the introduction of [¹⁸F]FDG have seen the development of many small molecule PET radiotracers. Ten have been approved by the US Food and Drug Administration as of 2016, whereas hundreds more are being evaluated clinically. These radiotracers are being used in personalized medicine and to support drug discovery programs where they are greatly improving our understanding of and ability to treat diseases across many areas of medicine including neuroscience, cardiovascular medicine, and oncology.

An updated radiosynthesis of [18F]AV1451 for tau PET imaging

BACKGROUND

[18F]AV1451 is a commonly used radiotracer for imaging tau deposits in Alzheimer's disease (AD) and related non-AD tauopathies. Existing radiosyntheses of [18F]AV1451 require complex purifications to provide doses suitable for use in clinical imaging studies. To address this issue, we have modified the synthesis of [18F]AV1451 to use only 0.5 mg precursor, optimized the Boc-deprotection step and developed a simplified method for HPLC purification of the radiotracer.

RESULTS

An optimized [18F]AV1451 synthesis using a TRACERLab FXFN module led to high radiochemical yield (202 ± 57 mCi per synthesis) and doses with excellent radiochemical purity (98 ± 1%) and good specific activity (2521 ± 623 Ci/mmol).

CONCLUSION

An updated and operationally simple synthesis of [18F]AV1451 has been developed that is fully automated and prepares radiotracer doses suitable for use in clinical tau PET studies.

Structure–activity relationships of radioiodinated diphenyl derivatives with different conjugated double bonds as ligands for α-synuclein aggregates

We investigated the structure–activity relationships of radioiodinated diphenyl (IDP) derivatives with different conjugated double bonds as ligands for α-syn aggregates.

Synthesis of a PET tau tracer [(11)C]PBB3 for imaging of Alzheimer's disease

The authentic standard PBB3 and its precursor N-desmethyl-PBB3 as well as TBS-protected N-desmethyl-PBB3 precursor for radiolabeling were synthesized from 5-bromo-2-nitropyridine, acrolein diethyl acetal, 6-methoxy-2-methylbenzothiazole, and diethylchlorophosphate with overall chemical yield 1% in six steps, 2% in five steps, and 1% in six steps, respectively. [(11)C]PBB3 was prepared from either desmethyl-PBB3 or TBS-protected desmethyl-PBB3 with [(11)C]CH3OTf through N-[(11)C]methylation and isolated by HPLC combined with SPE in 20-25% and 15-20% radiochemical yield, respectively, based on [(11)C]CO2 and decay corrected to end of bombardment (EOB). The radiochemical purity was >99%, and the specific activity at EOB was 370-1110 GBq/μmol with a total synthesis time of ~40-min from EOB.