Activatable imaging probes with amplified fluorescent signals

Activatable fluorescent probes for real-time imaging-guided tumor therapy

Surgery and drug therapy are the two principal options for cancer treatment. However, their clinical benefits are hindered by the difficulty of accurate location of the tumors and timely monitoring of the treatment efficacy of drugs, respectively. Rapid development of imaging techniques provides promising tools to address these challenges. Compared with conventional imaging techniques such as magnetic resonance imaging and computed tomography etc., fluorescence imaging exhibits high spatial resolution, real-time imaging capability, and relatively low costs devices. The advancements in fluorescent probes further accelerate the implementation of fluorescence imaging in tumor diagnosis and treatment monitoring. In particular, the emergence of site-specifically activatable fluorescent probes fits the demands of tumor delineation and real-time feedback of the treatment efficacy. A variety of small molecule probes or nanoparticle-based probes have been developed and explored for the above-mentioned applications. This review will discuss recent advances in fluorescent probes with a special focus on activatable nanoprobes and highlight the potential implementation of activatable nanoprobes in fluorescence imaging-guided surgery as well as imaging-guided drug therapy.

Peptide-based diagnostic and therapeutic agents: Where we are and where we are heading?

Peptides are increasingly present in all branches of medicine as innovative drugs, imaging agents, theragnostic, and constituent moieties of other sophisticated drugs such as peptide-drug conjugates. Due to new developments in chemical synthesis strategies, computational biology, recombinant technology, and chemical biology, peptide drug development has made a great progress in the last decade. Numerous natural peptides and peptide mimics have been obtained and studied, covering multiple therapeutic areas. Even though peptides have been investigated across the wide therapeutic spectrum, oncology, metabolism, and endocrinology are the most frequent medical indications of them. This review summarizes the current use of and the emerging new opportunities of peptides for diagnosis and treatment of various diseases.

Polymeric nanocarriers: A promising tool for early diagnosis and efficient treatment of colorectal cancer

BACKGROUND

Colorectal cancer (CRC) is the third most prevalent type of cancer for incidence and second for mortality worldwide. Late diagnosis and inconvenient and expensive current diagnostic tools largely contribute to the progress of the disease. The use of chemotherapy in the management of CRC significantly reduces tumor growth, metastasis, and morbidity rates. However, poor solubility, low cellular uptake, nonspecific distribution, multiple drug resistance and unwanted adverse effects are still among the major drawbacks of chemotherapy that limit its clinical significance in the treatment of CRC. Owing to their remarkable advantages over conventional therapies, the use of nanotechnology-based delivery systems especially polymeric nanocarriers (PNCs) has revolutionized many fields including disease diagnosis and drug delivery.

AIM OF REVIEW

In this review, we shed the light on the current status of using PNCs in the diagnosis and treatment of CRC with a special focus on targeting strategies, surface modifications and safety concerns for different types of PNCs in colonic cancer delivery.

KEY SCIENTIFIC CONCEPTS OF REVIEW

The review explores the current progress on the use of PNCs in the diagnosis and treatment of CRC with a special focus on the role of PNCs in improvement of cellular uptake, drug targeting and co-delivery of chemotherapeutic agents. Possible toxicity and biocompatibility issues related to the use of PNCs and imitations and future recommendation for the use of those smart carriers in the diagnosis and treatment of CRC are also discussed.

Hemicyanine-Based Near-Infrared Activatable Probes for Imaging and Diagnosis of Diseases

Molecular activatable probes with near-infrared (NIR) fluorescence play a critical role in in vivo imaging of biomarkers for drug screening and disease diagnosis. With structural diversity and high fluorescence quantum yields, hemicyanine dyes have emerged as a versatile scaffold for the construction of activatable optical probes. This Review presents a survey of hemicyanine-based NIR activatable probes (HNAPs) for in vivo imaging and early diagnosis of diseases. The molecular design principles of HNAPs towards activatable optical signaling against various biomarkers are discussed with a focus on their broad applications in the detection of diseases including inflammation, acute organ failure, skin diseases, intestinal diseases, and cancer. This progress not only proves the unique value of HNAPs in preclinical research but also highlights their high translational potential in clinical diagnosis.

Dual-Modality Detection of Early-Stage Drug-Induced Acute Kidney Injury by an Activatable Probe

Early detection of drug-induced acute kidney injury (AKI) is crucial for effective treatment and prevention of further injury. It remains challenging, however, because of the lack of activatable indicators with multimodality imaging capability that could increase the accuracy of diagnosis by mutual verification. Herein, we report an activatable probe, FDOCl-22, that enabled dual-modality detection of the early-stage drug-induced AKI. FDOCl-22 was completely soluble in water and highly sensitive to hypochlorous acid (HOCl). Dramatic increases of both near-infrared (NIR) emission and absorption were observed after reaction with HOCl. A correlation between HOCl concentration and drug-induced AKI was established using FDOCl-22 as a tool. As a consequence, the HOCl-activated probe was able to detect the early-stage drug-induced AKI by dual-modality imaging, irrespective of the drug stimulation time or dosage, by combining NIR fluorescence and photoacoustic imaging.

Activatable Molecular Probes for Second Near-Infrared Fluorescence, Chemiluminescence, and Photoacoustic Imaging

Optical imaging plays a crucial role in biomedicine. However, due to strong light scattering and autofluorescence in biological tissue between 650-900 nm, conventional optical imaging often has a poor signal-to-background ratio and shallow penetration depth, which limits its ability in deep-tissue in vivo imaging. Second near-infrared fluorescence, chemiluminescence, and photoacoustic imaging modalities mitigate these issues by their respective advantages of minimized light scattering, eliminated external excitation, and ultrasound detection. To enable disease detection, activatable molecular probes (AMPs) with the ability to change their second near-infrared fluorescence, chemiluminescence, or photoacoustic signals in response to a biomarker have been developed. This Minireview summarizes the molecular design strategies, sensing mechanisms, and imaging applications of AMPs. The potential challenges and perspectives of AMPs in deep-tissue imaging are also discussed.

Ultra pH-sensitive polymeric nanovesicles co-deliver doxorubicin and navitoclax for synergetic therapy of endometrial carcinoma

An ultra pH-sensitive polymeric nanovesicle was constructed to co-deliver doxorubicin and navitoclax for highly efficient synergetic treatment of endometrial carcinoma.

Near infra-red polymeric nanoparticle based optical imaging in Cancer diagnosis

Cancer disease is a foremost health concern and top basis of death in comparison with many diseases including cardiovascular disorders. During initial diagnosis (usually late diagnosis), a majority of cancer patients suffer from metastatic and advanced cancer stages which resulted in limited therapeutic modalities based interventions and effectiveness. Though considerable advancement has been made in combating the disease, continuous and intense efforts are ongoing for early diagnosis and development of therapies. Generally applied treatment options for cancer are surgery, chemotherapy and radiotherapy, which are restricted by failure to early diagnose, insufficient on-targeted drug delivery, systemic toxicity, and lack of real-time monitoring of therapeutic responses in cancer. Noninvasive imaging or minimally invasive imaging methodology is valuable in clinical diagnostic settings. Specifically, noninvasive optical imaging integrated with polymeric nanomaterial have been extensively investigated in the field of cancer diagnostics and therapy. Currently, optical imaging methods go together with polymer-based fluorescent nanoparticles in accomplishing the molecular level detection of tumor boundaries. NIR probe tagged polymeric nanoparticles have potential to provide an advantage in the early cancer detection, therapeutic monitoring and image guided surgery procedures. This article review the recent progress in state-of-the-art NIRF polymeric nanoparticles used for optical imaging particularly on cancer diagnosis.

Hyaluronic Acid-Based Activatable Nanomaterials for Stimuli-Responsive Imaging and Therapeutics: Beyond CD44-Mediated Drug Delivery

There is a rapidly increasing interest in developing stimuli-responsive nanomaterials for treating a variety of diseases. By enabling the activation of function locally at the sites of interest, it is possible to increase therapeutic efficacy significantly while simultaneously reducing adverse side effects. While there are many sophisticated nanomaterials available, they are often highly complex and not easily transferrable to industrial scales and clinical settings. However, nanomaterials based on hyaluronic acid offer a compelling strategy for reducing their complexity while retaining several desirable benefits such as active targeting and stimuli-responsive degradation. Herein, the basic properties of hyaluronic acid, its binding partners, and natural routes for degradation by hyaluronidases-hyaluronic-acid-degrading enzymes-and oxidative stresses are discussed. Recent advances in designing hyaluronic acid-based, actively targeted, hyaluronidase- or reactive-oxygen-species-responsive nanomaterials for both diagnostic imaging and therapeutic delivery, which go beyond merely the classical targeting of CD44, are summarized.

A NIR-II fluorescent probe for articular cartilage degeneration imaging and osteoarthritis detection

Articular cartilage (AC) is a complex water-bearing tissue consisting of chondrocytes, proteoglycans, and collagen. AC degeneration, which occurs in the early stage and throughout the entire course of osteoarthritis (OA), is one of the main pathological changes of OA. However, current clinical approaches are unable to detect AC degradation during the early stage of OA. Herein, a novel NIR-II probe, CH1055-WL, was developed with an organic fluorophore (CH1055) and type II collagen-binding peptide (WYRGRL) for AC targeting and degeneration imaging. In vitro and in vivo imaging studies demonstrated that CH1055-WL specifically bound to AC and permitted sensitive detection of age-related or surgically induced AC degeneration in living mice. In vitro imaging of cartilage samples from pig knee joint and in vivo imaging of live mice with the probe administered via local injection in joint cavities demonstrated that CH1055-WL specifically and efficiently bound to AC. Further evaluation of CH1055-WL revealed sensitive detection of age-related AC degeneration and surgically induced AC degeneration in living mice. Our results indicated that the cartilage-targeting probe CH1055-WL allowed visual monitoring of AC degeneration in living subjects, thus displaying promise for early OA detection.

MMP-2-responsive fluorescent nanoprobes for enhanced selectivity of tumor cell uptake and imaging

It is difficult to develop highly selective substrate-based fluorescent nanoprobes for specific matrix metalloproteinases (MMPs) due to overlapping substrate specificities among the family of MMP enzymes. To resolve this issue, we have developed novel fluorescent nanoprobes that are highly selective for soluble MMP-2. Herein, MMP-2-responsive nanoprobes were prepared by immobilizing fluorescent fusion proteins on nickel ferrite nanoparticles via the His-tag nickel chelation mechanism. The fusion protein consisted of a fluorescent mCherry protein with a cell penetrating peptide (CPP) moiety. An MMP-2 cleavage site was also introduced within the fusion protein, which was directly linked to the nickel ferrite nanoparticles. The selectivity of nanoprobes was modulated by hiding the cleavage site of MMP-2 substrates deeply inside the system, which could result in strong steric hindrance between the nanoprobes and MMPs, especially for membrane-tethered MMPs such as MMP-14. A cell-based assay demonstrated that the nanoprobes could only be activated by tumor cells secreting soluble MMP-2, but not membrane-tethered MMP-14. To further evaluate the contribution of the steric hindrance effect on the nanoprobes, a truncated recombinant MMP-14 was employed to confer their cleavage activity as compared to native membrane-tethered MMP-14. Furthermore, a designed probe with a diminished steric hindrance effect was proved to be activated by membrane-tethered type MMP-14. The results indicated that the design of fluorescent nanoprobes employing the steric hindrance effect can greatly enhance the selectivity of MMP-responsive nanoprobes realizing the specific detection of soluble MMP-2 in a tumor microenvironment. We believe that highly selective MMP-2-responsive fluorescent nanoprobes have broad impacts on biomedical applications including molecular imaging and labeling for tumor detection.

Design and Synthesis of Quenched Activity-based Probes for Diacylglycerol Lipase and α,β-Hydrolase Domain Containing Protein 6

Diacylglycerol lipases (DAGL) are responsible for the biosynthesis of the endocannabinoid 2-arachidonoylglycerol. The fluorescent activity-based probes DH379 and HT-01 have been previously shown to label DAGLs and to cross-react with the serine hydrolase ABHD6. Here, we report the synthesis and characterization of two new quenched activity-based probes 1 and 2, the design of which was based on the structures of DH379 and HT-01, respectively. Probe 1 contains a BODIPY-FL and a 2,4-dinitroaniline moiety as a fluorophore-quencher pair, whereas probe 2 employs a Cy5-fluorophore and a cAB40-quencher. The fluorescence of both probes was quenched with relative quantum yields of 0.34 and 0.0081, respectively. The probes showed target inhibition as characterized in activity-based protein profiling assays using human cell- and mouse brain lysates, but were unfortunately not active in living cells, presumably due to limited cell permeability.

Exploiting Enzyme Alterations in Cancer for Drug Activation, Drug Delivery, and Nanotherapy

Alterations of enzyme activity or enzyme levels in cancer tissue have been documented over the years. Taking advantage of these observations, various strategies for drug activation and drug delivery have been developed. One example is a mechanism called "prodrugs" that generates active drugs by enzyme cleavage. Another example is enzyme-induced anticancer drug release mechanisms. This system is constructed by incorporating enzyme-sensitive linkages to materials such as hydrogels and nanodelivery vehicles, including liposomes, polymer micelles, and mesoporous silica nanoparticles. Various release mechanisms for anticancer drugs and siRNA have been developed. In addition, enzyme cleavage is utilized in nanodelivery vehicles that contain nanomachines. One example is nanovalve that can be opened by enzyme cleavage. Another example is enzyme-induced release of nanoparticles from multistage vehicles. Finally, colon-specific drug delivery by azoreductase cleavable mechanism is discussed.

Recent Advances of Activatable Molecular Probes Based on Semiconducting Polymer Nanoparticles in Sensing and Imaging

Molecular probes that change their signals in response to the target of interest have a critical role in fundamental biology and medicine. Semiconducting polymer nanoparticles (SPNs) have recently emerged as a new generation of purely organic photonic nanoagents with desirable properties for biological applications. In particular, tunable optical properties of SPNs allow them to be developed into photoluminescence, chemiluminescence, and photoacoustic probes, wherein SPNs usually serve as the energy donor and internal reference for luminescence and photoacoustic probes, respectively. Moreover, facile surface modification and intraparticle engineering provide the versatility to make them responsive to various biologically and pathologically important substances and indexes including small-molecule mediators, proteins, pH and temperature. This article focuses on recent advances in the development of SPN-based activatable molecular probes for sensing and imaging. The designs and applications of these probes are discussed in details, and the present challenges to further advance them into life science are also analyzed.

Strategies for using nanoprobes to perceive and treat cancer activity: a review

Nanomedicine has seen a significant increase in research on stimuli-responsive activatable nanoprobes for tumor-specific delivery and diagnosis. The tumor microenvironment has particular characteristics that can be exploited to implement therapeutic strategies based on disparities between normal tissues and tumor tissues, including differences in pH, oxygenation, enzymatic expression, gene activation/inactivation, and vasculature. The nanocarriers of activatable nanoparticles maintain their structure while circulating in the body and, upon reaching the tumor site, are altered by unique tumoral stimuli, leading to the release of a drug or other agent. This review demonstrates the latest achievements in the use of internal stimuli-responsive, activatable nanoparticles with respect to unique design strategies and applications.

Cancer-Microenvironment-Sensitive Activatable Quantum Dot Probe in the Second Near-Infrared Window

Recent technological advances have expanded fluorescence (FL) imaging into the second near-infrared region (NIR-II; wavelength = 1000-1700 nm), providing high spatial resolution through deep tissues. However, bright and compact fluorophores are rare in this region, and sophisticated control over NIR-II probes has not been fully achieved yet. Herein, we report an enzyme-activatable NIR-II probe that exhibits FL upon matrix metalloprotease activity in tumor microenvironment. Bright and stable PbS/CdS/ZnS core/shell/shell quantum dots (QDs) were synthesized as a model NIR-II fluorophore, and activatable modulators were attached to exploit photoexcited electron transfer (PET) quenching. The quasi type-II QD band alignment allowed rapid and effective FL modulations with the compact surface ligand modulator that contains methylene blue PET quencher. The modulator was optimized to afford full enzyme accessibility and high activation signal surge upon the enzyme activity. Using a colon cancer mouse model, the probe demonstrated selective FL activation at tumor sites with 3-fold signal enhancement in 10 min. Optical phantom experiments confirmed the advantages of the NIR-II probe over conventional dyes in the first near-infrared region.

Current Multistage Drug Delivery Systems Based on the Tumor Microenvironment

The development of traditional tumor-targeted drug delivery systems based on EPR effect and receptor-mediated endocytosis is very challenging probably because of the biological complexity of tumors as well as the limitations in the design of the functional nano-sized delivery systems. Recently, multistage drug delivery systems (Ms-DDS) triggered by various specific tumor microenvironment stimuli have emerged for tumor therapy and imaging. In response to the differences in the physiological blood circulation, tumor microenvironment, and intracellular environment, Ms-DDS can change their physicochemical properties (such as size, hydrophobicity, or zeta potential) to achieve deeper tumor penetration, enhanced cellular uptake, timely drug release, as well as effective endosomal escape. Based on these mechanisms, Ms-DDS could deliver maximum quantity of drugs to the therapeutic targets including tumor tissues, cells, and subcellular organelles and eventually exhibit the highest therapeutic efficacy. In this review, we expatiate on various responsive modes triggered by the tumor microenvironment stimuli, introduce recent advances in multistage nanoparticle systems, especially the multi-stimuli responsive delivery systems, and discuss their functions, effects, and prospects.

Rotaxane probes for protease detection by 129Xe hyperCEST NMR

We report on the design and implementation of a cucurbit[6]uril rotaxane probe for the detection of matrix metalloproteases by Xe hyperCEST NMR.

Highly selective and rapidly activatable fluorogenic Thrombin sensors and application in human lung tissue

A fast and selective fluorogenic probe for Thrombin is reported and applied in ex vivo fibrotic human lung tissue.

Synthesis and application of water-soluble, photoswitchable cyanine dyes for bioorthogonal labeling of cell-surface carbohydrates

The synthesis of cyanine dyes addressing absorption wavelengths at 550 and 648 nm is reported. Alkyne functionalized dyes were used for bioorthogonal click reactions by labeling of metabolically incorporated sugar-azides on the surface of living neuroblastoma cells, which were applied to direct stochastic optical reconstruction microscopy (dSTORM) for the visualization of cell-surface glycans in the nm-range.

Optical imaging probes in oncology

Cancer is a complex disease, characterized by alteration of different physiological molecular processes and cellular features. Keeping this in mind, the possibility of early identification and detection of specific tumor biomarkers by non-invasive approaches could improve early diagnosis and patient management.Different molecular imaging procedures provide powerful tools for detection and non-invasive characterization of oncological lesions. Clinical studies are mainly based on the use of computed tomography, nuclear-based imaging techniques and magnetic resonance imaging. Preclinical imaging in small animal models entails the use of dedicated instruments, and beyond the already cited imaging techniques, it includes also optical imaging studies. Optical imaging strategies are based on the use of luminescent or fluorescent reporter genes or injectable fluorescent or luminescent probes that provide the possibility to study tumor features even by means of fluorescence and luminescence imaging. Currently, most of these probes are used only in animal models, but the possibility of applying some of them also in the clinics is under evaluation.The importance of tumor imaging, the ease of use of optical imaging instruments, the commercial availability of a wide range of probes as well as the continuous description of newly developed probes, demonstrate the significance of these applications. The aim of this review is providing a complete description of the possible optical imaging procedures available for the non-invasive assessment of tumor features in oncological murine models. In particular, the characteristics of both commercially available and newly developed probes will be outlined and discussed.

Targeted Near-Infrared Fluorescent Turn-on Nanoprobe for Activatable Imaging and Effective Phototherapy of Cancer Cells

A novel and green multifunctional nanoplatform as a nanocarrier for drug delivery, cell imaging, and phototherapy has been engineered. The nanoplatform is composed of stabilized carbon spheres (CSs) as cores, a coated polydopamine (PDA) shell, targeted folic acid (FA), and the loaded anticancer drug indocyanine green (ICG), obtaining CSs@PDA-FA@ICG nanocomposites (NCs). The biocompatible PDA shell provided a high fluorescence quenching efficiency and a surface rich in functional groups for anchoring FA for targeting cancer cells. Aromatic ICG could be effectively loaded into the CSs@PDA-FA system via hydrophobic interactions and π-π stacking with a loading efficiency of 58.9%. Notably, the activated NIR fluorescence in an intracellular environment made CSs@PDA-FA@ICG a sensitive "OFF" to "ON" nanoprobe that can be used for NIR imaging. Moreover, compared to ICG alone, the CSs@PDA-FA@ICG NCs could induce efficient photoconversion for simultaneous synergetic photodynamic therapy (PDT) and photothermal therapy (PTT) under a single NIR laser irradiation. The results demonstrated that CSs@PDA-FA@ICG NCs as a targeted and activated nanoplatform provide new opportunities to facilitate the accurate diagnosis of cancer and enhanced treatment efficacy. This work stimulates more interest in the design of the facile surface functionalization strategy to construct other multifunctional nanocomposites, such as nanotubes and nanorods.

A tryptophan-containing fluorescent intramolecular complex as a designer peptidic proton sensor

Pyrene and tryptophan groups judiciously placed on a novel molecular scaffold, namely, bispidine exhibited fluorescence due to the formation of an unprecedented emissive intramolecular complex in polar solvents. Upon protonation, the emission signal from the pyrene unit enhances at the expense of the emission signal from the complex. The probe demonstrates good sensitivity, excellent selectivity, and adequate reversibility towards proton sensing. The present design based on the bispidine scaffold opens up newer opportunities for the design of novel bispidine-peptide sensors.

Peptide-substituted phthalocyanine photosensitizers: design, synthesis, photophysicochemical and photobiological studies

Activatable molecular beacons were synthesized bearing phthalocyanine, peptide sequence and fluorophore groups. The phototoxicity and cytotoxicity of the systems were studied against the cervical cancer cell line named HeLa for evaluation of their suitability for photodynamic therapy.

Recent developments in hyaluronic acid-based nanomedicine for targeted cancer treatment

INTRODUCTION

Hyaluronic acid (HA) has emerged as a promising applicant for the tumor-targeted delivery of various therapeutic agents. Because of its biocompatibility, biodegradability and receptor-binding properties, HA has been extensively investigated as the drug delivery carrier. In this review, recent advances in HA-based nanomedicines are discussed.

AREAS COVERED

This review focuses on HA-based nanomedicines for the diagnosis and treatment of cancer. In particular, recent advances in HA-drug conjugates and HA-based nanoparticles for small molecular drug delivery are discussed. The bioreducible HA conjugates for small interfering ribonucleic acid delivery have been also discussed.

EXPERT OPINION

To develop a successful HA-based nanomedicine, it has to be prepared without significant deterioration of intrinsic property of HA. The chemical modification of HA with drugs or hydrophobic moieties may reduce the binding affinity of HA to the receptors. In addition, since the HA-based nanomedicines tend to accumulate in the liver after their systemic administration, new strategies to overcome this issue have to be developed.

Small Molecule Active Site Directed Tools for Studying Human Caspases

Caspases are proteases of clan CD and were described for the first time more than two decades ago. They play critical roles in the control of regulated cell death pathways including apoptosis and inflammation. Due to their involvement in the development of various diseases like cancer, neurodegenerative diseases, or autoimmune disorders, caspases have been intensively investigated as potential drug targets, both in academic and industrial laboratories. This review presents a thorough, deep, and systematic assessment of all technologies developed over the years for the investigation of caspase activity and specificity using substrates and inhibitors, as well as activity based probes, which in recent years have attracted considerable interest due to their usefulness in the investigation of biological functions of this family of enzymes.

Aptamer-based cell imaging reagents capable of fluorescence switching

We describe an aptamer-conjugated polydiacetylene imaging probe (ACP) that shows highly specific fluorescence switching upon binding to epithelial cancer cells that overexpress the tumor biomarker protein EpCAM (epithelial cell adhesion molecule) on their surface.

MMP-2/9-Specific Activatable Lifetime Imaging Agent

Optical (molecular) imaging can benefit from a combination of the high signal-to-background ratio of activatable fluorescence imaging with the high specificity of luminescence lifetime imaging. To allow for this combination, both imaging techniques were integrated in a single imaging agent, a so-called activatable lifetime imaging agent. Important in the design of this imaging agent is the use of two luminophores that are tethered by a specific peptide with a hairpin-motive that ensured close proximity of the two while also having a specific amino acid sequence available for enzymatic cleavage by tumor-related MMP-2/9. Ir(ppy)3 and Cy5 were used because in close proximity the emission intensities of both luminophores were quenched and the influence of Cy5 shortens the Ir(ppy)3 luminescence lifetime from 98 ns to 30 ns. Upon cleavage in vitro, both effects are undone, yielding an increase in Ir(ppy)3 and Cy5 luminescence and a restoration of Ir(ppy)3 luminescence lifetime to 94 ns. As a reference for the luminescence activation, a similar imaging agent with the more common Cy3-Cy5 fluorophore pair was used. Our findings underline that the combination of enzymatic signal activation with lifetime imaging is possible and that it provides a promising method in the design of future disease specific imaging agents.

Bioresponsive probes for molecular imaging: concepts and in vivo applications

Molecular imaging is a powerful tool to visualize and characterize biological processes at the cellular and molecular level in vivo. In most molecular imaging approaches, probes are used to bind to disease-specific biomarkers highlighting disease target sites. In recent years, a new subset of molecular imaging probes, known as bioresponsive molecular probes, has been developed. These probes generally benefit from signal enhancement at the site of interaction with its target. There are mainly two classes of bioresponsive imaging probes. The first class consists of probes that show direct activation of the imaging label (from "off" to "on" state) and have been applied in optical imaging and magnetic resonance imaging (MRI). The other class consists of probes that show specific retention of the imaging label at the site of target interaction and these probes have found application in all different imaging modalities, including photoacoustic imaging and nuclear imaging. In this review, we present a comprehensive overview of bioresponsive imaging probes in order to discuss the various molecular imaging strategies. The focus of the present article is the rationale behind the design of bioresponsive molecular imaging probes and their potential in vivo application for the detection of endogenous molecular targets in pathologies such as cancer and cardiovascular disease.

Utilising polymers to understand diseases: advanced molecular imaging agents

This review describes how the highly tuneable size, shape and chemical functionality of polymeric molecular imaging agents provides a means to intimately probe the various mechanisms behind disease formation and behaviour.

Enzyme-responsive cell-penetrating peptide conjugated mesoporous silica quantum dot nanocarriers for controlled release of nucleus-targeted drug molecules and real-time intracellular fluorescence imaging of tumor cells

Here, a set of novel and personalized nanocarriers are presented for controlled nucleus-targeted antitumor drug delivery and real-time imaging of intracellular drug molecule trafficking by integrating an enzyme activatable cell penetrating peptide (CPP) with mesoporous silica coated quantum dots nanoparticles. Upon loading of antitumor drug, doxorubicin (DOX) and further exposure to proteases in tumor cell environment, the enzymatic cleavage of peptide sequence activates oligocationic TAT residues on the QDs@mSiO2 surface and direct the DOX delivery into cellular nucleus. The systematic cell imaging and cytotoxicity studies confirm that the enzyme responsive DOX-loaded CPP-QDs@mSiO2 nanoparticles can selectively release DOX in the tumor cells with high cathepsin B enzyme expression and greatly facilitate DOX accumulation in targeted nucleus, thus exhibiting enhanced antitumor activity in these cells. As contrast, there is limited nuclear-targeted drug accumulation and lower tumor cytotoxicity observed in the cells without enzyme expression. More importantly, significant antitumor DOX accumulation and higher tumor inactivation is also found in the drug resistant tumor cells with targeted enzyme expression. Such simple and specific enzyme responsive mesoporous silica-QDs nanoconjugates provide great promise for rational design of targeted drug delivery into biological system, and may thus greatly facilitate the medical theranostics in the near future.

Alginate-coated magnetic nanoparticles for noninvasive MRI of extracellular calcium

Nanoparticles (NPs) have great potential to increase the diagnostic capacity of many imaging modalities. MRI is currently regarded as the method of choice for the imaging of deep tissues, and metal ions, such as calcium ions (Ca(2+)), are essential ingredients for life. Despite the tremendous importance of Ca(2+) for the well-being of living systems, the noninvasive determination of the changes in Ca(2+) levels in general, and extracellular Ca(2+) levels in particular, in deep tissues remains a challenge. Here, we describe the preparation and contrast mechanism of a flexible easy to prepare and selective superparamagnetic iron oxide (SPIO) NPs for the noninvasive determination of changes in extracellular Ca(2+) levels using conventional MRI. We show that SPIO NPs coated with monodisperse and purified alginate, having a specific molecular weight, provide a tool to selectively determine Ca(2+) concentrations in the range of 250 µm to 2.5 mm, even in the presence of competitive ions. The alginate-coated magnetic NPs (MNPs) aggregate in the presence of Ca(2+) , which, in turn, affects the T2 relaxation of the water protons in their vicinity. The new alginate-coated SPIO NP formulations, which have no effect on cell viability for 24 h, allow the detection of Ca(2+) levels secreted from ischemic cell cultures and the qualitative examination of the change in extracellular Ca(2+) levels in vivo. These results demonstrate that alginate-coated MNPs can be used, at least qualitatively, as a platform for the noninvasive MRI determination of extracellular Ca(2+) levels in myriad in vitro and in vivo biomedical applications.