A theranostic agent for in vivo near-infrared imaging of β-amyloid species and inhibition of β-amyloid aggregation

Design and synthesis of hemicyanine-based NIRF probe for detecting Aβ aggregates in Alzheimer's disease

Alzheimer's disease (AD), a progressive neurodegenerative disorder, has garnered increased attention due to its substantial economic burden and the escalating global aging phenomenon. Amyloid-β deposition is a key pathogenic marker observed in the brains of Alzheimer's sufferers. Based on real-time, safe, low-cost, and commonly used, near-infrared fluorescence (NIRF) imaging technology have become an essential technique for the detection of AD in recent years. In this work, NIRF probes with hemicyanine structure were designed, synthesized and evaluated for imaging Aβ aggregates in the brain. We use the hemicyanine structure as the parent nucleus to enhance the probe's optical properties. The introduction of PEG chain is to improve the probe's brain dynamice properties, and the alkyl chain on the N atom is to enhance the fluorescence intensity of the probe after binding to the Aβ aggregates as much as possible. Among these probes, Z2, Z3, Z6, X3, X6 and T1 showed excellent optical properties and high affinity to Aβ aggregates (Kd = 24.31 ∼ 59.60 nM). In vitro brain section staining and in vivo NIRF imaging demonstrated that X6 exhibited superior discrimination between Tg mice and WT mice, and X6 has the best brain clearance rate. As a result, X6 was identified as the optimal probe. Furthermore, the docking theory calculation results aided in describing X6's binding behavior with Aβ aggregates. As a high-affinity, high-selectivity, safe and effective probe of targeting Aβ aggregates, X6 is a promising NIRF probe for in vivo detection of Aβ aggregates in the AD brain.

Molecular engineering of a theranostic molecule that detects Aβ plaques, inhibits Iowa and Dutch mutation Aβ self-aggregation and promotes lysosomal biogenesis for Alzheimer's disease

Extracellular clustering of amyloid-β (Aβ) and an impaired autophagy lysosomal pathway (ALP) are the hallmark features in the early stages of incurable Alzheimer's disease (AD). There is a pressing need to find or develop new small molecules for diagnostics and therapeutics for the early stages of AD. Herein, we report a small molecule, namely F-SLCOOH, which can bind and detect Aβ1-42, Iowa mutation Aβ, Dutch mutation Aβ fibrils and oligomers exhibiting enhanced emission with high affinity. Importantly, F-SLCOOH can readily pass through the blood-brain barrier and shows highly selective binding toward the extracellular Aβ aggregates in real-time in live animal imaging of a 5XFAD mice model. In addition, a high concentration of F-SLCOOH in both brain and plasma of wildtype mice after intraperitoneal administration was found. The ex vivo confocal imaging of hippocampal brain slices indicated excellent colocalization of F-SLCOOH with Aβ positive NU1, 4G8, 6E10 A11 antibodies and THS staining dye, affirming its excellent Aβ specificity and targetability. The molecular docking studies have provided insight into the unique and specific binding of F-SLCOOH with various Aβ species. Importantly, F-SLCOOH exhibits remarkable anti-fibrillation properties against toxic Aβ aggregate formation of Aβ1-42, Iowa mutation Aβ, and Dutch mutation Aβ. F-SLCOOH treatment also exerts high neuroprotective functions and promotes autophagy lysosomal biogenesis in neuronal AD cell models. In summary, the present results suggest that F-SLCOOH is a highly promising theranostic agent for diagnosis and therapeutics of AD.

Emerging paradigms in Alzheimer's Therapy

Alzheimer's disease is a neurodegenerative disorder that affects elderly, and its incidence is continuously increasing across the globe. Unfortunately, despite decades of research, a complete cure for Alzheimer's disease continues to elude us. The current medications are mainly symptomatic and slow the disease progression but do not result in reversal of all disease pathologies. The growing body of knowledge on the factors responsible for the onset and progression of the disease has resulted in the identification of new targets that could be targeted for treatment of Alzheimer's disease. This has opened new vistas for treatment of Alzheimer's disease that have moved away from chemotherapeutic agents modulating a single target to biologics and combinations that acted on multiple targets thereby offering better therapeutic outcomes. This review discusses the emerging directions in therapeutic interventions against Alzheimer's disease highlighting their merits that promise to change the treatment paradigm and challenges that limit their clinical translation.

Recent Advances in Fluorescent Theranostics for Alzheimer's Disease: A Comprehensive Survey on Design, Synthesis, and Properties

Alzheimer's disease (AD) is the most common form of neurodegenerative dementia that is rapidly becoming a major health problem, especially in developed countries because of their increasing life expectancy. Two main problems are often associated with the disease: (i) the absence of a widely accessible "gold-standard" for early diagnosis and (ii) lack of effective therapies with disease-modifying effects. The recent success of the monoclonal antibody lecanemab played an important role not only in clarifying a possible druggable pathway but also in spelling the revival of small molecule drug discovery. Unlike bulky biologics, small molecules are structurally less complex, generally cheaper, and compatible with at-home oral consumption, making it feasible for people to start their drug regimen early and stay on it longer. In this sense, small-molecule near-infrared fluorescent theranostics have been gaining more and more attention from the scientific community, as they have the potential to simultaneously provide diagnostic outputs and deliver therapeutic action, paving the way toward personalized medicine in AD patients. They also have the potential to shift the diagnostic "status-quo" from expensive and limited-access PET radiotracers toward inexpensive and handy imaging tools widely available for primary patient screening and preclinical animal studies. Herein, we review the most recent advances in the field of fluorescent theranostics for Alzheimer's disease, detailing their design strategies, synthetic approaches and imaging and therapeutic properties in vitro and in vivo. With this Review, we intend to provide a milestone in the acquired knowledge in the field of AD theranostics, encouraging the future development of properly designed theranostic compounds with improved chances to reach clinical applications.

A closer look at amyloid ligands, and what they tell us about protein aggregates

The accumulation of amyloid fibrils is characteristic of neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease. Detecting these fibrils with fluorescent or radiolabelled ligands is one strategy for diagnosing and better understanding these diseases. A vast number of amyloid-binding ligands have been reported in the literature as a result. To obtain a better understanding of how amyloid ligands bind, we have compiled a database of 3457 experimental dissociation constants for 2076 unique amyloid-binding ligands. These ligands target Aβ, tau, or αSyn fibrils, as well as relevant biological samples including AD brain homogenates. From this database significant variation in the reported dissociation constants of ligands was found, possibly due to differences in the morphology of the fibrils being studied. Ligands were also found to bind to Aβ(1-40) and Aβ(1-42) fibrils with similar affinities, whereas a greater difference was found for binding to Aβ and tau or αSyn fibrils. Next, the binding of ligands to fibrils was shown to be largely limited by the hydrophobic effect. Some Aβ ligands do not fit into this hydrophobicity-limited model, suggesting that polar interactions can play an important role when binding to this target. Finally several binding site models were outlined for amyloid fibrils that describe what ligands target what binding sites. These models provide a foundation for interpreting and designing site-specific binding assays.

An activatable small-molecule fluorogenic probe for detection and quantification of beta-amyloid aggregates

Extracellular accumulation of β amyloid (Aβ) peptides in the brain is thought to be a pathological hallmark and initial event before the symptom starts of Alzheimer's patients. Herein, we developed two series of benzo[d]thiazole-based small-molecule compounds (BM1-BM4, BPM1-BPM4) with a donor-acceptor (D-A) or donor-π-acceptor (D-π-A) architecture, respectively, based on structure-activity relationship. Among them, the optimized BPM1 not only displayed the highest binding affinity to Aβ aggregates over other proteins or Aβ monomers, but was readily activated its fluorescence with 10-fold fluorescence enhancement, allowing for specifically and sensitively detecting Aβ aggregates. BPM1 also exhibits several other advantages including low molecular weight, low cytotoxicity and excellent biological stability. Besides, cell staining results confirmed that SK-N-BE(2) cells can be fluorescently lighted up as well as cell permeability and damage when treated with BPM1-bound Aβ1-42 aggregates.

Recent progress of small-molecule-based theranostic agents in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of neurodegenerative dementia. As a multifactorial disease, AD involves several etiopathogenic mechanisms, in which multiple pathological factors are interconnected with each other. This complicated and unclear pathogenesis makes AD lack effective diagnosis and treatment. Theranostics, exerting the synergistic effect of diagnostic and therapeutic functions, would provide a promising strategy for exploring AD pathogenesis and developing drugs for combating AD. With the efforts in small drug-like molecules for both diagnosis and treatment of AD, small-molecule-based theranostic agents have attracted significant attention owing to their facile synthesis, high biocompatibility and reproducibility, and easy clearance from the body through the excretion systems. In this review, the small-molecule-based theranostic agents reported in the literature for anti-AD are classified into four groups according to their diagnostic modalities. Their design rationales, chemical structures, and working mechanisms for theranostics are summarized. Finally, the opportunities for small-molecule-based theranostic agents in AD are also proposed.

Imaging of Carbonic Anhydrase Level in Epilepsy with an Environment-Sensitive Fluorescent Probe

Carbonic anhydrases (CAs) participate in various physiological and pathological activities by catalyzing the interconversion between carbon dioxide and bicarbonate ions. Under normal circumstances, they guarantee that the relevant biological reactions in our body occur within an appropriate time scale. Abnormal expression or activity alteration of CAs is closely related to the pathogenesis of diverse diseases. This work reports an inhibitor-directed fluorescent probe FMRs-CA for the detection of CAs. Excellent selectivity, favorable biocompatibility, and desirable blood-brain barrier (BBB) penetration endow the probe with the ability to image the fluctuation of CAs in cells and mice. We achieved in situ visualization of the increased CAs in hypoxic cells with this probe. Additionally, probe FMRs-CA was mainly enriched within the liver and gradually metabolized by the liver. With the help of FMRs-CA, the increase of CAs in epileptic mouse brains was revealed first from the perspective of imaging, providing the mechanism connection between abnormal CA expressions and epilepsy.

Recent Advances in the Development of Near-Infrared Fluorescent Probes for the in Vivo Brain Imaging of Amyloid-β Species in Alzheimer's Disease

The deposition of β-amyloid (Aβ) plaques in the parenchymal and cortical regions of the brain of Alzheimer's disease (AD) patients is considered the foremost pathological hallmark of the disease. The early diagnosis of AD is paramount in order to effective management and treatment of the disease. Developing near-infrared fluorescence (NIRF) probes targeting Aβ species is a potential and attractive approach suitable for the early and timely diagnosis of AD. The advantages of the NIRF probes over other tools include real-time detection, higher sensitivity, resolution, comparatively inexpensive experimental setup, and noninvasive nature. Currently, enormous progress is being observed in the development of NIRF probes for the in vivo imaging of Aβ species. Several strategies, i.e., the classical push-pull approach, "turn-on" effect, aggregation-induced emission (AIE), and resonance energy transfer (RET), have been exploited for development. We have outlined and discussed the recently emerged NIRF probes with different design strategies targeting Aβ species for ex vivo and in vivo imaging. We believe that understanding the recent development enables the prospect of the rational design of probes and will pave the way for developing future novel probes for early diagnosis of AD.

Specific Fluorescent Probes for Imaging DNA in Cell-Free Solution and in Mitochondria in Living Cells

New styryl dyes consisting of N-methylpyridine or N-methylquinoline scaffolds were synthesized, and their binding affinities for DNA in cell-free solution were studied. The replacement of heterocyclic residue from the pyridine to quinoline group as well as variation in the phenyl part strongly influenced their binding modes, binding affinities, and spectroscopic responses. Biological experiments showed the low toxicity of the obtained dyes and their applicability as selective dyes for mitochondria in living cells.

Multifunctional theranostic carbazole-based cyanine for real-time imaging of amyloid-β and therapeutic treatment of multiple pathologies in Alzheimer's disease

Alzheimer's disease (AD) is a progressive and irreversible neurodegenerative disorder characterized by the synaptic and neuronal loss, which results in cognitive impairment in particular learning and memory. Currently, AD is incurable and no single confirmative test can clinically be used to diagnose AD. In light of the complex and multifactorial nature of AD etiology, the development of multifunctional/multi-target drugs that act on multiple pathological pathways and mechanisms shows great therapeutic potential for intervention of this devastating disease. We report herein a multifunctional theranostic cyanine, SLCOOH, which serves not only as a highly sensitive fluorescent probe for real-time imaging of amyloid-β (Aβ) contents in different age groups of transgenic (Tg) AD mice but also as an effective therapeutic agent for early AD intervention via multiple pathological targets in the AD mouse model. Remarkably, treatment with SLCOOH gives rise to multiple therapeutic benefits, including the amelioration of cognitive decline, a reduction in Aβ levels, a decrease in hyperphosphorylated tau proteins and tau depositions, and the alleviation of synaptic loss and dysfunctions in young triple Tg AD mice. Our results have demonstrated that in addition to superior Aβ imaging capability, SLCOOH exhibits versatile and effective multiple modes of drug action, signifying outstanding therapeutic potential to treat early onset AD. Our work also paves the way for the development of effective Aβ-targeted theranostic agents for AD.

Small-molecule theranostics in Alzheimer's disease

Alzheimer's Disease (AD) remains one of the most challenging health-related issues for our society. It is becoming increasingly prevalent, especially in developed countries, due to the rising life expectancy and, moreover, represents a considerable economic burden worldwide. All efforts at the discovery of new diagnostic and therapeutic tools in the last decades have invariably met with failure, making AD an incurable illness and underscoring the need for new approaches. In recent years, theranostic agents have emerged as an interesting strategy. They are molecules able to simultaneously provide diagnostic information and deliver therapeutic activity, allowing for the assessment of the molecule activity, the organism response and the pharmacokinetics. This makes these compounds promising for streamlining research on AD drugs and for their application in personalized medicine. We review here the field of small-molecule theranostic agents as promising tools for the development of novel diagnostic and therapeutic resources against AD, highlighting the positive and significant impact that theranostics can be expected to have in the near future in clinical practice.

Real-Time Detection and Visualization of Amyloid-β Aggregates Induced by Hydrogen Peroxide in Cell and Mouse Models of Alzheimer's Disease

Oxidative stress, caused by an imbalance between the production and the accumulation of reactive oxygen species (ROS), is a prominent cause of the neurotoxicity induced by aggregated amyloid-β (Aβ) in Alzheimer's disease (AD). Tools that can directly detect and monitor the presence and amount of Aβ-induced ROS are still lacking. We report herein the first Aβ-targeted ratiometric H₂O₂-responsive fluorescent probe for real-time detection and monitoring of the Aβ-induced H₂O₂ level in cell and AD mouse models. The H₂O₂-responsive probe is constructed based on a methylamino-substituted quinolinium-based cyanine as the fluorescence moiety and a phenylboronate ester as the sensing reaction site. This sensing probe exhibits a large emission wavelength shift of ∼87 nm upon reacting with H₂O₂, a high binding selectivity for Aβ, and a faster response toward H₂O₂ in the presence of Aβ, concomitant with an enhanced fluorescence intensity, hence greatly boosting the sensitivity of in-situ H₂O₂ detection. This biocompatible and nontoxic probe is capable of ratiometrically detecting and imaging endogenous H₂O₂ induced by Aβ in a neuronal cell model. Remarkably, this Aβ-targeted H₂O₂-responsive probe is also able to detect, monitor, and differentiate different Aβ-induced H₂O₂ levels in real time in different age groups of transgenic AD mice in which the cerebral H₂O₂ level increases age dependently concomitant with the plaque contents. Therefore, this smart probe can act as a powerful tool to diagnose high-risk subjects and diseased brains of AD and to further study the role of ROS in AD pathology.

The regulatory mechanism and therapeutic potential of transcription factor EB in neurodegenerative diseases

The autophagy-lysosomal pathway (ALP) is involved in the degradation of protein aggregates and damaged organelles. Transcription factor EB (TFEB), a major regulator of ALP, has emerged as a leading factor in addressing neurodegenerative disease pathology, including Alzheimer's disease (AD), Parkinson's disease (PD), PolyQ diseases, and Amyotrophic lateral sclerosis (ALS). In this review, we delineate the regulation of TFEB expression and its functions in ALP. Dysfunctions of TFEB and its role in the pathogenesis of several neurodegenerative diseases are reviewed. We summarize the protective effects and molecular mechanisms of some TFEB-targeted agonists in neurodegenerative diseases. We also offer our perspective on analyzing the pros and cons of these agonists in the treatment of neurodegenerative diseases from the perspective of drug development. More studies on the regulatory mechanisms of TFEB in other biological processes will aid our understanding of the application of TFEB-targeted therapy in neurodegeneration.

Near-Infrared Fluorescent Probes as Imaging and Theranostic Modalities for Amyloid-Beta and Tau Aggregates in Alzheimer's Disease

A person suspected of having Alzheimer's disease (AD) is clinically diagnosed for the presence of principal biomarkers, especially misfolded amyloid-beta (Aβ) and tau proteins in the brain regions. Existing radiotracer diagnostic tools, such as PET imaging, are expensive and have limited availability for primary patient screening and pre-clinical animal studies. To change the status quo, small-molecular near-infrared (NIR) probes have been rapidly developed, which may serve as an inexpensive, handy imaging tool to comprehend the dynamics of pathogenic progression in AD and assess therapeutic efficacy in vivo. This Perspective summarizes the biochemistry of Aβ and tau proteins and then focuses on structurally diverse NIR probes with coverages of their spectroscopic properties, binding affinity toward Aβ and tau species, and theranostic effectiveness. With the summarized information and perspective discussions, we hope that this paper may serve as a guiding tool for designing novel in vivo imaging fluoroprobes with theranostic capabilities in the future.

A multi-target theranostic nano-composite against Alzheimer's disease fabricated by conjugating carbon dots and triple-functionalized human serum albumin

The complex pathogenesis of Alzheimer's disease (AD) involves the aggregation and accumulation of amyloid β-protein (Aβ) as well as elevated levels of reactive oxygen species (ROS), which requires the development of comprehensive diagnostic and therapeutic interventions. In this work, a multifunctional theranostic nano-composite (HSA-BFP@CDs) is constructed by conjugating triple-functionalized human serum albumin (HSA-BFP) as a theranostic agent targeting Aβ and carbon dots (CDs) as an ROS scavenger. HSA-BFP@CDs exhibits a fluorescence "off-on" effect at 700 nm upon interaction with Aβ aggregates, showing the capability for detection of Aβ plaques and potential for early diagnosis of AD. Besides, HSA-BFP@CDs effectively inhibits the aggregation of Aβ, increasing the viability of Aβ-treated cells from 74% to over 95% at 100 µg/mL. Moreover, multiple ROS, including hydroxyl radicals, superoxide radicals, hydrogen peroxide, and Aβ-Cu²⁺-induced-ROS, can be scavenged by HSA-BFP@CDs, thus resulting in the mitigation of cellular oxidative damages. Experiments with the AD model of Caenorhabditis elegans further demonstrate the multifunctionality of HSA-BFP@CDs in imaging amyloid plaques, reducing Aβ deposition, and relieving oxidative stress in vivo, showing the prospect for Aβ- and ROS-targeted AD diagnosis and treatment. This work provided new insight into the design of protein-carbon dots conjugate and the development of multi-target therapy of AD. STATEMENT OF SIGNIFICANCE: Alzheimer's disease (AD) is the most common form of dementia, which currently affects over 55 million people worldwide. Due to the complex pathogenesis of AD involving amyloid β-protein (Aβ) aggregation as well as elevated levels of reactive oxygen species (ROS), it is highly desired to develop comprehensive diagnostic and therapeutic interventions. In this paper, we fabricated a multifunctional theranostic nano-composite (HSA-BFP@CDs) via the conjugation of triple-functionalized human serum albumin (HSA-BFP) and carbon dots (CDs). The multifunctionality of HSA-BFP@CDs for efficient detection of Aβ aggregates and inhibition of Aβ aggregation as well as scavenging of ROS was demonstrated, demonstrating the potential of the protein-carbon dots conjugate for the multi-target therapy of AD.

Rotor-Tuning Boron Dipyrromethenes for Dual-Functional Imaging of Aβ Oligomers and Viscosity

Alzheimer's disease (AD), known as a common incurable and elderly neurodegenerative disease, has been widely explored for accurate detection of its biomarker (Aβ oligomers) for early diagnosis. Although great efforts have been made, it is still of great importance to develop fluorescence probes for Aβ oligomers with good selectivity and low background. Herein, starting from BODIPY493/503 (a commercial dye for neutral lipid droplets), which exhibited a small Stokes shift and no response toward Aβ peptides, two fluorescence probes 5MB-SZ and B-SZ with a benzothiazole rotor at the 2-position of the BODIPY core and a methyl or benzyl group at the meso position have been designed and synthesized, which exhibited excellent optical properties/stability and could successfully image β-amyloid fibrils and viscosity. Upon exposure to Aβ oligomers, the fluorescence intensity of 5MB-SZ was enhanced by 43.64-fold with the corresponding fluorescence quantum yields changing from 0.85% to 27.43%. Meanwhile, probe 5MB-SZ showed a highly sensitive viscosity response in both solutions and living cells. In vitro and in vivo experiments confirmed that probe 5MB-SZ exhibited an excellent capacity for imaging β-amyloid fibrils. Therefore, 5MB-SZ, as a rotor-tuning BODIPY analogue, could possibly serve as a highly potential and powerful fluorescence probe for early diagnosis of AD.

Theranostic F-SLOH mitigates Alzheimer's disease pathology involving TFEB and ameliorates cognitive functions in Alzheimer's disease models

Accumulation of amyloid-β (Aβ) oligomers and phosphorylated Tau aggregates are crucial pathological events or factors that cause progressive neuronal loss, and cognitive impairments in Alzheimer's disease (AD). Current medications for AD have failed to halt, much less reverse this neurodegenerative disorder; therefore, there is an urgent need for the development of effective and safe drugs for AD therapy. In the present study, the in vivo therapeutic efficacy of an Aβ-oligomer-targeted fluorescent probe, F-SLOH, was extensively investigated in 5XFAD and 3XTg-AD mouse models. We have shown that F-SLOH exhibits an efficient inhibitory activity against Aβ aggregation in vivo, and acts as an effective theranostic agent for the treatment of multiple neuropathological changes in AD mouse models. F-SLOH has been found to significantly reduce not only the levels of Aβ oligomers, Tau aggregates and plaques but also the levels of amyloid precursor protein (APP) and its metabolites via autophagy lysosomal degradation pathway (ALP) in the brains of 5XFAD and 3XTg-AD mice. It also reduces astrocyte activation and microgliosis ultimately alleviating neuro-inflammation. Furthermore, F-SLOH mitigates hyperphosphorylated Tau aggregates, synaptic deficits and ameliorates synaptic memory function, and cognitive impairment in AD mouse models. The mechanistic studies have shown that F-SLOH promotes the clearance of C-terminal fragment 15 (CTF15) of APP and Paired helical filaments of Tau (PHF1) in stable cell models via the activation of transcription factor EB (TFEB). Moreover, F-SLOH promotes ALP and lysosomal biogenesis for the clearance of soluble, insoluble Aβ, and phospho Tau. Our results unambiguously reveal effective etiological capabilities of theranostic F-SLOH to target and intervene multiple neuropathological changes in AD mouse models. Therefore, F-SLOH demonstrates tremendous therapeutic potential for treating AD in its early stage.

Steroid-Quinoline Hybrids for Disruption and Reversion of Protein Aggregation Processes

Reversing protein aggregation within cells may be an important tool to fight protein-misfolding disorders such as Alzheimer's, Parkinson's, and cardiovascular diseases. Here we report the design and synthesis of a family of steroid-quinoline hybrid compounds based on the framework combination approach. This set of hybrid compounds effectively inhibited Aβ1-42 self-aggregation in vitro by delaying the exponential growth phase and/or reducing the quantity of fibrils in the steady state. Their disaggregation efficacy was further demonstrated against preaggregated Aβ1-42 peptides in cellular assays upon their endocytosis by neuroblastoma cells, as they reverted both the number and the average area of fibrils back to basal levels. The antiaggregation effect of these hybrids was further tested and demonstrated in a cellular model of general protein aggregation expressing a protein aggregation fluorescent sensor. Together, our results show that the new cholesterol-quinoline hybrids possess wide and marked disaggregation capacities and are therefore promising templates for the development of new drugs to deal with conformational disorders.

Stabilization and fluorescence light-up of G-quadruplex nucleic acids using indolyl-quinolinium based probes

G-Quadruplexes (G4s) are four-stranded motifs formed by G-rich nucleic acid sequences. These structures harbor significant biological importance as they are involved in telomere maintenance, transcription, and translation. Owing to their dynamic and polymorphic nature, G4 structures relevant for therapeutic applications need to be stabilized by small-molecule ligands. Some of these ligands turn on fluorescence upon binding to G4 structures, which provides a powerful detection platform for G4 structures. Herein, we report the synthesis of fluorescent ligands based on the indolyl-quinolinium moiety to specifically stabilize G4 structures and sense DNA. CD titration and melting experiments have shown that the lead ligand induces the formation of parallel G4 with preferential stabilization of the c-MYC and c-KIT1 promoter G4s over the telomeric, h-RAS1 G4, and duplex DNA. Fluorimetric titration data revealed fluorescence enhancement when these ligands interact with G4 DNA structures. The fluorescence lifetime experiment of the ligand with different DNAs revealed three excited state lifetimes (ns), which indicates more than one binding site. MD studies showed that the ligand exhibits 3 : 1 stoichiometry of binding with c-MYC G4 DNA and revealed the unique structural features, which impart selectivity toward parallel topology. The ligand was found to have low cytotoxicity and exhibited preferential staining of DNA over RNA. Collectively, the results presented here offer avenues to harness indolyl-quinolinium scaffolds for sensing and selective stabilization of G4 structures.

Strategic Design of Amyloid-β Species Fluorescent Probes for Alzheimer's Disease

Alzheimer's disease (AD) is a high mortality and high disability rates neurodegenerative disease characterized by irreversible progression and poses a significant social and economic burden throughout the world. However, currently approved AD therapeutic agents only alleviate symptoms and there is still a lack of practical therapeutic regimens to stop or slow the progression of this disease. Thus, there is urgently needed novel diagnosis tools and drugs for early diagnosis and treatment of AD. Among several AD pathological hallmarks, amyloid-β (Aβ) peptide deposition is considered a critical initiating factor in AD. In recent years, with the advantages of excellent sensitivity and high resolution, near-infrared fluorescence (NIRF) imaging has attracted the attention of many researchers to develop Aβ plaque probes. This review mainly focused on different NIRF probe design strategies for imaging Aβ species to pave the way for the future design of novel NIRF probes for early diagnosis AD.

Distinguishing normal and inflammatory models by viscosity changes with sensitively mitochondrial-trackable fluorescent probe

Biological microenvironment plays a momentous role in the regulation of various vital activities, and its abnormal changes are often closely related to some diseases. Viscosity, as an indispensable part of microenvironment parameters, has always been one of the research hotspots of investigators. Herein, we constructed a new red-emitting fluorescent probe (HVM) to identify the abnormal situation of mitochondria through viscosity changes in the biological microenvironment. Interestingly, HVM has excellent optical properties such as large stokes shift (160 nm), viscosity sensitivity (195-fold), high photostability, and biochemical properties with low cytotoxicity and excellent biocompatibility. For these reasons, the novel probe could successfully be used to identify the normal and inflammatory models via viscosity changes in biological experiments. Therefore, we provided a convenient synthetic route to obtain viscosity sensor HVM with excellent application properties.

Coassembled Chitosan-Hyaluronic Acid Nanoparticles as a Theranostic Agent Targeting Alzheimer's β-Amyloid

β-Amyloid (Aβ) fibrillogenesis is closely associated with the pathogenesis of Alzheimer's disease (AD), so detection and inhibition of Aβ aggregation are of significance for the theranostics of AD. In this work, the coassembled nanoparticles of chitosan and hyaluronic acid cross-linked with glutaraldehyde (CHG NPs) were found to work as a theranostic agent for imaging/probing and inhibition of Aβ fibrillization both in vitro and in vivo. The biomass-based CHG NPs of high stability exhibited a wide range of excitation/emission wavelengths and showed binding affinity toward Aβ aggregates, especially for soluble Aβ oligomers. CHG NPs displayed weak emission in the monodispersed state, while they remarkably emitted increased red fluorescence upon interacting with Aβ oligomers and fibrils, showing high sensitivity with a detection limit of 0.1 nM. By comparing the different fluorescence responses of CHG NPs and Thioflavin T to Aβ aggregation, the Aβ oligomerization rate during nucleation can be determined. Moreover, the fluorescence recognition behavior of CHG NPs was selective. CHG NPs specifically bind to negatively charged amyloid aggregates but not to positively charged amyloids and negatively charged soluble proteins. Such enhancement in fluorescence emission is attributed to the clustering-triggered emission effect of CHG NPs after interaction with Aβ aggregates via various electronic conjugations and hydrogen bonding, electrostatic, and hydrophobic interactions. Besides fluorescent imaging/probing, CHG NPs over 360 μg/mL could almost completely inhibit the formation of Aβ fibrils, exhibiting the capability of regulating Aβ aggregation. In-vivo assays with Caenorhabditis elegans CL2006 demonstrated the potency of CHG NPs as an effective theranostic nanoagent for imaging Aβ plaques and inhibiting Aβ deposition. The findings proved the potential of CHG NPs for development as a potent agent for the diagnosis and treatment of AD.

Dual-Channel Imaging of Amyloid-β Plaques and Peroxynitrite To Illuminate Their Correlations in Alzheimer's Disease Using a Unimolecular Two-Photon Fluorescent Probe

Alzheimer's disease (AD) involves multiple pathological factors that mutually cooperate and closely contact to form interaction networks for jointly promoting the AD progression. Therefore, the comonitoring of different factors is particularly valuable for elucidating their level dynamics and complex interactions. However, such significant investigations remain a major challenge due to the lack of unimolecular fluorescent probes capable of simultaneous and discriminative visualization of multiple targets. To address this concern, as proof of principle, we rationally designed a unimolecular fluorescent probe to discriminate and simultaneously profile amyloid-β (Aβ) plaques and peroxynitrite (ONOO^-), which are both the pronounced AD pathological factors. Herein, a novel ONOO^- reaction trigger was installed onto an Aβ plaque binding fluorophore to generate a dual functional fluorescent probe, displaying completely separate spectral responses to Aβ plaques and ONOO^- with high selectivity and sensitivity. With this probe, for the first time, we comonitored the distribution and variation of Aβ plaques and ONOO^- through two independent fluorescence channels, demonstrating their close apposition and tight correlation during AD course in live cell and mouse models through two-photon imaging mode. Notably, Aβ aggregates induce the neuronal ONOO^- generation, which conversely facilitates Aβ aggregation. The two critical events, ONOO^- stress and Aβ aggregation, mutually amplify each other through positive feedback mechanisms and jointly promote the AD onset and progression. Furthermore, by coimaging of the level dynamics of Aβ plaques and ONOO^-, we found that the cerebral ONOO^- is a potential biomarker, which emerges earlier than Aβ plaques in transgenic mouse models. Overall, the dual-channel responsive performance renders this probe as a powerful imaging tool to decipher Aβ plaque-ONOO^- interactions, which will facilitate AD-associated molecular pathogenesis elucidation and multitarget drug discovery.

In-vitro and in-vivo characterization of CRANAD-2 for multi-spectral optoacoustic tomography and fluorescence imaging of amyloid-beta deposits in Alzheimer mice

The abnormal deposition of fibrillar beta-amyloid (Aβ) deposits in the brain is one of the major histopathological hallmarks of Alzheimer's disease (AD). Here, we characterized curcumin-derivative CRANAD-2 for multi-spectral optoacoustic tomography and fluorescence imaging of brain Aβ deposits in the arcAβ mouse model of AD cerebral amyloidosis. CRANAD-2 showed a specific and quantitative detection of Aβ fibrils in vitro, even in complex mixtures, and it is capable of distinguishing between monomeric and fibrillar forms of Aβ. In vivo epi-fluorescence microscopy and optoacoustic tomography after intravenous CRANAD-2 administration demonstrated higher cortical retention in arcAβ compared to non-transgenic littermate mice. Immunohistochemistry showed co-localization of CRANAD-2 and Aβ deposits in arcAβ mouse brain sections, thus verifying the specificity of the probe. In conclusion, we demonstrate suitability of CRANAD-2 for optical detection of Aβ deposits in animal models of AD pathology, which facilitates mechanistic studies and the monitoring of putative treatments targeting Aβ deposits.

Strategies Targeting Soluble β-Amyloid Oligomers and their Application to Early Diagnosis of Alzheimer's Disease

BACKGROUND

Alzheimer's Disease (AD) is the most common neurodegenerative disorder, and it is still incurable. Early diagnosis and intervention are crucial for delaying the onset and progression of the disease. Mounting evidence indicates that the neurotoxic effects might be attributed to Soluble β-Amyloid Oligomers (SAβO). The SAβO are believed to be neurotoxic peptides more predominant than Aβ plaques in the early stage, and their key role in AD is self-evident. Unfortunately, identification of SAβO proves to be difficult due to their heterogeneous and transient nature. In spite of many obstacles, multiple techniques have recently been developed to target SAβO effectively. This review focuses on the recent progress in the approaches towards SAβO detection in order to shed some light on the future development of SAβO assays.

METHODS

Literatures were obtained from the following libraries: Web of Science, PubMed, EPO, SIPO, USPTO. Articles were critically reviewed based on their titles, abstracts, and contents.

RESULTS

A total of 85 papers are referenced in the review. Results are divided into three categories based on the types of detection methods: small molecule fluorescence probes, oligomer-specific antibodies and electrochemical biosensors. Finally, the improvements and challenges of these approaches applied in the early diagnosis of AD were discussed.

CONCLUSION

This review article covers three kinds of strategies that could be translated into clinic practice and lead to earlier diagnosis and therapeutic interventions of AD.

Dual-functional AIE fluorescent probes for imaging β-amyloid plaques and lipid droplets

Alzheimer's disease (AD) is a chronic neurodegenerative disease. Better imaging and early diagnosis of biomarkers of AD is extremely important for therapeutic interventions. The amyloid cascade hypothesis and its revised version identify insoluble β-amyloid deposition as a good diagnostic biomarker for AD. Moreover, lipid droplets may also act as an auxiliary biomarker related to AD pathology based on recent studies. Herein, two quinoline-based AIE probes were designed and synthesized for the imaging of Aβ plaques and lipid droplets. The probes exhibited remarkable turn-on fluorescence enhancements with the Aβ aggregates. The lipid droplets-targeting probe FB exhibited high selectivity and binding affinity towards the Aβ aggregates with a detection limit as low as 26.9 nM. Furthermore, FB was capable of readily imaging Aβ plaques and lipid droplets at the cellular level and in brain sections of transgenic AD mice. The probe FB can serve as a promising tool for developing early diagnosis and innovative therapeutics of AD.

Nanomaterials for Nanotheranostics: Tuning Their Properties According to Disease Needs

Nanotheranostics is one of the biggest scientific breakthroughs in nanomedicine. Most of the currently available diagnosis and therapies are invasive, time-consuming, and associated with severe toxic side effects. Nanotheranostics, on the other hand, has the potential to bridge this gap by harnessing the capabilities of nanotechnology and nanomaterials for combined therapeutics and diagnostics with markedly enhanced efficacy. However, nanomaterial applications in nanotheranostics are still in its infancy. This is due to the fact that each disease has a particular microenvironment with well-defined characteristics, which promotes deeper selection criteria of nanomaterials to meet the disease needs. In this review, we have outlined how nanomaterials are designed and tailored for nanotheranostics of cancer and other diseases such as neurodegenerative, autoimmune (particularly on rheumatoid arthritis), and cardiovascular diseases. The penetrability and retention of a nanomaterial in the biological system, the therapeutic strategy used, and the imaging mode selected are some of the aspects discussed for each disease. The specific properties of the nanomaterials in terms of feasibility, physicochemical challenges, progress in clinical trials, its toxicity, and their future application on translational medicine are addressed. Our review meticulously and critically examines the applications of nanotheranostics with various nanomaterials, including graphene, across several diseases, offering a broader perspective of this emerging field.

Simultaneous imaging of mitochondrial viscosity and hydrogen peroxide in Alzheimer's disease by a single near-infrared fluorescent probe with a large Stokes shift

Simultaneous imaging of mitochondrial viscosity and hydrogen peroxide in Alzheimer's disease by a single near-infrared fluorescent probe with a large Stokes shift.

Recent Advancements in Pathogenesis, Diagnostics and Treatment of Alzheimer's Disease

BACKGROUND

The only conclusive way to diagnose Alzheimer's is to carry out brain autopsy of the patient's brain tissue and ascertain whether the subject had Alzheimer's or any other form of dementia. However, due to the non-feasibility of such methods, to diagnose and conclude the conditions, medical practitioners use tests that examine a patient's mental ability.

OBJECTIVE

Accurate diagnosis at an early stage is the need of the hour for initiation of therapy. The cause for most Alzheimer's cases still remains unknown except where genetic distinctions have been observed. Thus, a standard drug regimen ensues in every Alzheimer's patient, irrespective of the cause, which may not always be beneficial in halting or reversing the disease progression. To provide a better life to such patients by suppressing existing symptoms, early diagnosis, curative therapy, site-specific delivery of drugs, and application of hyphenated methods like artificial intelligence need to be brought into the main field of Alzheimer's therapeutics.

METHODS

In this review, we have compiled existing hypotheses to explain the cause of the disease, and highlighted gene therapy, immunotherapy, peptidomimetics, metal chelators, probiotics and quantum dots as advancements in the existing strategies to manage Alzheimer's.

CONCLUSION

Biomarkers, brain-imaging, and theranostics, along with artificial intelligence, are understood to be the future of the management of Alzheimer's.

An Amyloid-β Targeting Chemical Exchange Saturation Transfer Probe for In Vivo Detection of Alzheimer's Disease

A reliable and reproducible detection of Aβ deposits would be beneficial for the early diagnosis of Alzheimer's disease (AD). In the present study, the feasibility of applying chemical exchange saturation transfer (CEST) for Aβ deposit detection using angiopep-2 as a probe was evaluated, and it was demonstrated that CEST could detect angiopep-2 and Aβ-angiopep-2 aggregates in vitro. Furthermore, APP/PS1 mice injected with angiopep-2 exhibited a significantly higher in vivo CEST effect when compared with controls. The distribution of Aβ deposits detected by CEST imaging was consistent with the histological staining results. The present study is the first to report a reliable exogenous CEST probe to noninvasively evaluate Aβ deposits in APP/PS1 mice. Furthermore, these results demonstrate the potential for clinical AD diagnosis and Aβ-targeted drug therapy assessment using CEST imaging with the angiopep-2 probe.

Half-curcumin analogues as PET imaging probes for amyloid beta species

In this report, we demonstrate that half-curcuminoid could be a better scaffold for PET tracer development. F-CRANAD-101 was designed and found to show significant response to both soluble and insoluble Aβs in the fluorescent spectral tests. PET imaging results indicated that 14 month and 5 month old APP/PS1 AD mice had higher signals in the brain than age-matched wild type mice.

Versatile fluorescent probes for near-infrared imaging of amyloid-β species in Alzheimer's disease mouse model

The self-aggregation of amyloid-β peptides into soluble oligomers and then into insoluble fibril-associated amyloid plaques is a key event in the progression of Alzheimer's disease (AD). The imaging of Aβ aggregates in the brain is a powerful and practical approach for the diagnosis and progression monitoring of AD and the evaluation of the effectiveness of novel therapies for this devastating disease. Near-infrared (NIR) imaging is a sensitive and noninvasive method to detect and visualize Aβ aggregates in vivo because of its good penetration depth and low autofluorescence of biological substances. In this article, we comprehensively reviewed the recent progresses made in the development of molecular NIR fluorescent probes for Aβ detection and imaging in vivo with a particular emphasis on the design strategies, optical characteristics, Aβ-binding abilities and potential applications in AD mouse models.

The CDR1 and Other Regions of Immunoglobulin Light Chains are Hot Spots for Amyloid Aggregation

Immunoglobulin light chain-derived (AL) amyloidosis is a debilitating disease without known cure. Almost nothing is known about the structural factors driving the amyloidogenesis of the light chains. This study aimed to identify the fibrillogenic hotspots of the model protein 6aJL2 and in pursuing this goal, two complementary approaches were applied. One of them was based on several web-based computational tools optimized to predict fibrillogenic/aggregation-prone sequences based on different structural and biophysical properties of the polypeptide chain. Then, the predictions were confirmed with an ad-hoc synthetic peptide library. In the second approach, 6aJL2 protein was proteolyzed with trypsin, and the products incubated in aggregation-promoting conditions. Then, the aggregation-prone fragments were identified by combining standard proteomic methods, and the results validated with a set of synthetic peptides with the sequence of the tryptic fragments. Both strategies coincided to identify a fibrillogenic hotspot located at the CDR1 and β-strand C of the protein, which was confirmed by scanning proline mutagenesis analysis. However, only the proteolysis-based strategy revealed additional fibrillogenic hotspots in two other regions of the protein. It was shown that a fibrillogenic hotspot associated to the CDR1 is also encoded by several κ and λ germline variable domain gene segments. Some parts of this study have been included in the chapter “The Structural Determinants of the Immunoglobulin Light Chain Amyloid Aggregation”, published in Physical Biology of Proteins and Peptides, Springer 2015 (ISBN 978-3-319-21687-4).

Nanotheranostics, a future remedy of neurological disorders

INTRODUCTION

Effective therapy of various neurological disorders is hindered on account of the failure of various therapeutics crossing blood-brain-barrier (BBB). Nanotheranostics has emerged as a cutting-edge unconventional theranostic nanomedicine, capable of realizing accurate diagnosis together with effective and targeted delivery of therapeutics across BBB to the unhealthy regions of the brain for potential clinical success.

AREAS COVERED

We have tried to review the current status of nanotheranostic based approaches followed to manage neurological disorders. The focus has been majorly laid on to explore various theranostic nanoparticles and their application potential towards image-guided neurotherapies. Additionally, the usefulness of exceptional diagnostic, imaging techniques including magnetic resonance imaging and fluorescence imaging are being discussed by highlighting their promising opportunities in the detection, diagnosis, and treatment of the neurological disorders.

EXPERT OPINION

Inimitable diagnostic and therapeutic potential of nanotheranostics have accomplished the aim of personalized therapies by governing the therapeutic efficacy of the system along with facilitating patient pre-selection grounded on non-invasive imaging, thereby predicting the responses of patients to nanomedicine treatments. While these accomplishments are encouraging, they are still the minority and demands for a continuous effort to improve sensitivity and precision in screening/diagnosis along with improving therapeutic efficacy in various neural disorders.

Emerging vistas in theranostic medicine

Recent years have witnessed a paradigm shift in the focus of healthcare towards development of customized therapies which cater to the unmet needs in a myriad of disease areas such as cancer, infections, cardiovascular diseases, neurodegenerative disorders and inflammatory disorders. The term 'theranostic' refers to such multifunctional systems which combine the features of diagnosis and treatment in a single platform for superior control of the disease. Theranostic systems enable detection of disease, treatment and real time monitoring of the diseased tissue. Theranostic nanocarriers endowed with multiple features of imaging, targeting, and providing on-demand delivery of therapeutic agents have been designed for enhancement of therapeutic outcomes. Fabrication of theranostics involves utilization of materials having distinct properties for imaging, targeting, and programming drug release spatially and temporally. Although the field of theranostics has been widely researched and explored so far for treatment of different types of cancer, there have been considerable efforts in the past few years to extend its scope to other areas such as infections, neurodegenerative disorders and cardiovascular diseases. This review showcases the potential applications of theranostics in disease areas other than cancer. It also highlights the cardinal issues which need to be addressed for successful clinical translation of these theranostic tools.

Theranostic applications of nanoparticles in neurodegenerative disorders

The preeminent treatments for neurodegenerative disease are often unavailable due to the poor accessibility of therapeutic drugs. Moreover, the blood–brain barrier (BBB) effectively blocks the transfer of cells, particles and large molecules, ie, drugs, across the brain. The most important challenge in the treatment of neurodegenerative diseases is the development of targeted drug delivery system. Theranostic strategies are known to combine therapeutic and diagnostic capabilities together. The aim of this review was to record the response to treatment and thereby improve drug safety. Nanotechnology offers a platform for designing and developing theranostic agents that can be used as an efficient nano-carrier system. This is achieved by the manipulation of some of the properties of nanoparticles (NPs), thereby enabling the attachment of suitable drugs onto their surface. The results provide revolutionary treatments by stimulation and thus interaction with targeted sites to promote physiological response with minimum side effects. This review is a brief discussion of the administration of drugs across the brain and the advantages of using NPs as an effective theranostic platform in the treatment of Alzheimer’s, Parkinson’s, epilepsy and Huntington’s disease.

Dual-Modal NIR-Fluorophore Conjugated Magnetic Nanoparticle for Imaging Amyloid-β Species In Vivo

Senile plaques, the extracellular deposit of amyloid-β (Aβ) peptides, are one of the neuropathological hallmarks found in Alzheimer's disease (AD) brain. The current method of brain imaging of amyloid plaques based on positron emission tomography (PET) is expensive and invasive with low spatial resolution. Thus, the development of sensitive and nonradiative amyloid-β (Aβ)-specific contrast agents is highly important and beneficial to achieve early AD detection, monitor the disease progression, and evaluate the effectiveness of potential AD drugs. Here a neuroprotective dual-modal nanoprobe developed by integrating highly Aβ-specific and turn-on fluorescence cyanine sensors with superparamagnetic iron oxide nanoparticles as an effective near-infrared imaging (NIRI)/magnetic resonance imaging (MRI) contrast agent for imaging of Aβ species in vivo is reported. This Aβ-specific probe is found not only nontoxic and noninvasive, but also highly blood brain barrier permeable. It also shows a potent neuroprotective effect against Aβ-induced toxicities. This nanoprobe is successfully applied for in vivo fluorescence imaging with high sensitivity and selectivity to Aβ species, and MRI with high spatial resolution in an APP/PS1 transgenic mice model. Its potential as a powerful in vivo dual-modal imaging tool for early detection and diagnosis of AD in humans is affirmed.

Effective Theranostic Cyanine for Imaging of Amyloid Species in Vivo and Cognitive Improvements in Mouse Model

We report herein an investigation of carbazole-based cyanine, (E)-4-(2-(9-(2-(2-methoxyethoxy)ethyl)-9H-carbazol-3-yl)-vinyl)-1-methyl-quinolin-1-iumiodide (SLM), as an effective theranostic agent for Alzheimer's disease (AD). This cyanine exhibited desirable multifunctional and biological properties, including amyloid-β (Aβ)-oligomerization inhibition, blood-brain barrier permeability, low neurotoxicity, neuroprotective effect against Aβ-induced toxicities, high selectivity and strong binding interactions with Aβ peptide/species, good biostability, as well as strong fluorescence enhancement upon binding to Aβ species for diagnosis and therapy of AD. This cyanine has been successfully applied to perform near-infrared in vivo imaging of Aβ species in transgenic AD mouse model. The triple transgenic AD mice intraperitoneally treated with SLM showed significant recovery of cognitive deficits. Furthermore, those SLM-treated mice exhibited a substantial decrease in both of oligomeric Aβ contents and tau proteins in their brain, which was attributed to the induction of autophagic flux. These findings demonstrated for the first time that SLM is an effective theranostic agent with in vivo efficacy for diagnosis and treatment of AD in mouse models.