Investigation of the substituent effect of indocyanine green derivatives for lymph imaging

Fluorescence lymph imaging with indocyanine green (ICG) is widely utilized as diagnostic tool for lymphatic diseases. While this technique offers numerous advantages, the kinetics of ICG at the injection site can pose challenges for a detailed diagnosis. In this study, we synthesized various ICG derivatives possessing cationic, anionic, or uncharged substituents and examined their photochemical properties, binding affinity to human serum albumin, as well as their correlation to pharmacokinetics in mice. The introduction of different substituents not only affected certain physiochemical properties, but also impacted the pharmacokinetics within the lymph nodes. Immunofluorescence imaging suggested that the extent of uptake of the ICG derivatives by phagocytic cells may affect the retention of the contrast ratios in the lymph nodes. These findings can provide new insights in the pharmacokinetics in lymphatic tissues, which could be useful for the development of novel fluorescent agents for lymph imaging.

Lighting up Self-Quenching Nanoaggregates with Protein Corona for Simultaneous Intraoperative Imaging and Photothermal Theranostics of Metastatic Cancer

Near-infrared (NIR) photothermal transduction agents (PTAs) with large rigid π-extended and planar structures are prone to aggregate in a physiological environment where their emission is often quenched due to the strong intermolecular dipole-dipole or π-π interactions. This aggregation-caused quenching effect greatly impedes their applications in image-guided photothermal theranostics. Herein, we made an interesting finding that engineering a bioinspired protein corona (PC), once thermodynamically stabilized in preferred orientations on PTA nanoaggregates, can produce brilliant NIR fluorescence with a high quantum yield (∼6.2%) without compromising their photothermal properties. Both experimental data and computational modeling suggest that the mechanism of fluorescence enhancement is due to the high-affinity binding of nano-sized PTA to albumin, which regulates the molecular conformation and aggregation state of PTA. High spatial and temporal resolution imaging of albumin PC-coated PTA aggregates enables image-guided photothermal therapy for cancer cells in sentinel lymph nodes to remarkably inhibit pulmonary metastasis. Such a treatment combined with the surgical removal of the primary tumor can prolong animal survival, which is a promising candidate for clinical applications in the treatment of advanced metastatic cancers.

A genetic engineering strategy for editing near-infrared-II fluorophores

The second near-infrared (NIR-II) window is a fundamental modality for deep-tissue in vivo imaging. However, it is challenging to synthesize NIR-II probes with high quantum yields (QYs), good biocompatibility, satisfactory pharmacokinetics, and tunable biological properties. Conventional long-wavelength probes, such as inorganic probes (which often contain heavy metal atoms in their scaffolds) and organic dyes (which contain large π-conjugated groups), exhibit poor biosafety, low QYs, and/or uncontrollable pharmacokinetic properties. Herein, we present a bioengineering strategy that can replace the conventional chemical synthesis methods for generating NIR-II contrast agents. We use a genetic engineering technique to obtain a series of albumin fragments and recombinant proteins containing one or multiple domains that form covalent bonds with chloro-containing cyanine dyes. These albumin variants protect the inserted dyes and remarkably enhance their brightness. The albumin variants can also be genetically edited to develop size-tunable complexes with precisely tailored pharmacokinetics. The proteins can also be conjugated to biofunctional molecules without impacting the complexed dyes. This combination of albumin mutants and clinically-used cyanine dyes can help widen the clinical application prospects of NIR-II fluorophores.

Human Serum Albumin Dimerization Enhances the S2 Emission of Bound Cyanine IR806

Cyanine molecules are important phototheranostic compounds given their high fluorescence yield in the near-infrared region of the spectrum. We report on the frequency and time-resolved spectroscopy of the S₂ state of IR806, which demonstrates enhanced emission upon binding to the hydrophobic pocket of human serum albumin (HSA). From excitation-emission matrix spectra and electronic structure calculations, we identify the emission as one associated with a state having the polymethine chain twisted out of plane by 103°. In addition, we find that this configuration is significantly stabilized as the concentration of HSA increases. Spectroscopic changes associated with the S₁ and S₂ states of IR806 as a function of HSA concentration, as well as anisotropy measurements, confirm the formation of HSA dimers at concentrations greater than 10 μM. These findings imply that the longer-lived S₂ state configuration can lead to more efficient phototherapy agents, and cyanine S₂ spectroscopy may be a useful tool to determine the oligomerization state of HSA.

The Use of Heptamethine Cyanine Dyes as Drug-Conjugate Systems in the Treatment of Primary and Metastatic Brain Tumors

Effective cancer therapeutics for brain tumors must be able to cross the blood-brain barrier (BBB) to reach the tumor in adequate quantities and overcome the resistance conferred by the local tumor microenvironment. Clinically approved chemotherapeutic agents have been investigated for brain neoplasms, but despite their effectiveness in peripheral cancers, failed to show therapeutic success in brain tumors. This is largely due to their poor bioavailability and specificity towards brain tumors. A targeted delivery system might improve the efficacy of the candidate compounds by increasing the retention time in the tumor tissue, and minimizing the numerous side effects associated with the non-specific distribution of the chemotherapy agent. Heptamethine cyanine dyes (HMCDs) are a class of near-infrared fluorescence (NIRF) compounds that have recently emerged as promising agents for drug delivery. Initially explored for their use in imaging and monitoring neoplasms, their tumor-targeting properties have recently been investigated for their use as drug carrier systems. This review will explore the recent developments in the tumour-targeting properties of a specific group of NIRF cyanine dyes and the preclinical evidence for their potential as drug-delivery systems in the treatment of primary and metastatic brain tumors.

Hows and Whys of Tumor-Seeking Dyes

Active targeting uses molecular fragments that bind receptors overexpressed on cell surfaces to deliver cargoes, and this selective delivery to diseased over healthy tissue is valuable in diagnostic imaging and therapy. For instance, targeted near-infrared (near-IR) dyes can mark tissue to be excised in surgery, and radiologists can use active targeting to concentrate agents for positron emission tomography (PET) in tumor tissue to monitor tumor metastases. Selective delivery to diseased tissue is also valuable in some treatments wherein therapeutic indexes (toxic/effective doses) are key determinants of efficacy. However, active targeting will only work for cells expressing the pivotal cell surface receptor that is targeted. That is a problem because tumors, even ones derived from the same organ, are not homogeneous, patient-to-patient variability is common, and heterogeneity can occur even in the same patient, so monotherapy with one actively targeted agent is unlikely to be uniformly effective. A particular category of fluorescent heptamethine cyanine-7 (Cy-7) dyes, here called tumor seeking dyes, offer a way to circumvent this problem because they selectively accumulate in any solid tumor. Furthermore, they persist in tumor tissue for several days, sometimes longer than 72 h. Consequently, tumor seeking dyes are near-IR fluorescent targeting agents that, unlike mAbs (monoclonal antibodies), accumulate in any solid lesion, thus overcoming tumor heterogeneity, and persist there for long periods, circumventing the rapid clearance problems that bedevil low molecular mass drugs. Small molecule imaging agents and drugs attached to tumor-seeking dyes have high therapeutic indices and long residence times in cancer cells and tumor tissue. All this sounds too good to be true. We believe most of this is true, but the controversy is associated with how and why these characteristics arise. Prior to our studies, the prevailing hypothesis, often repeated, was that tumor seeking dyes are uptaken by organic anion transporting polypeptides (OATPs) overexpressed on cancer cells. This Account summarizes evidence indicating tumor seeking Cy-7 dyes have exceptional accumulation and persistence properties because they covalently bind to albumin in vivo. That adduct formation provides a convenient way to form albumin-bound pharmaceuticals labeled with near-IR fluorophores which can be tracked in vivo. This understanding may facilitate more rapid developments of generally applicable actively targeted reagents.

On the Mechanisms of Uptake of Tumor-Seeking Cyanine Dyes

Molecular entities that localize in tumor tissue are clinically important for targeted delivery of diagnostic, imaging, and therapeutic reagents. Often these targeting entities are designed for specific receptors (e.g., EGFR or integrin receptors). However, there is a subset of cyanine-7 dyes that apparently localize in every type of solid tumor tissue (at least, no exceptions have been reported so far), and they persist there for several days. Consequently, these dyes can be used for near-IR optical imaging of tumors in animal studies, they can be conjugated with cytotoxic species to give experimental theranostics, and there is potential for expanding their use into the development of clinically useful derivatives. Data presented in the literature and in this work indicate that the half-lives of these compounds in serum at 37 °C is on the order of minutes to a few hours, so what accounts for the persistent fluorescence of these dyes in tumor tissue over periods of several days? Literature, solely based on tissue culture experiments featuring a particular receptor blocker, indicates that uptake of these dyes is mediated by the organic anion transporter proteins (OATPs). Data presented in this paper agrees with that conclusion for short-term uptake, but significantly expands understanding of the likely reasons for long-term uptake and persistent tumor localization in vivo.

IR-783 Labeling of a Peptide Receptor for 'Turn-On' Fluorescence Based Sensing

In this study, we examine a means for developing near-IR fluorescent sensors through streamlined, site-specific coupling with peptide-based receptors. As the penultimate step of solid-phase synthesis of a peptide-based receptor, we show a simple means of labeling the N' terminus with the near IR fluorophore IR-783 to afford a viable fluorescent sensor after cleavage from the resin. The proof-of-concept probe utilized a biotin mimetic peptide sequence as the receptive moiety. Here we revealed a "turn-on" fluorescence enhancement upon binding of the biotin mimetic probe to its intended streptavidin target. Not all peptide-receptive moieties tested were able to generate such an enhancement upon target binding, and as such, the rationale for the observed fluorescence response properties is discussed.

Photoacoustic imaging of integrin-overexpressing tumors using a novel ICG-based contrast agent in mice

PhotoAcoustic Imaging (PAI) is a biomedical imaging modality currently under evaluation in preclinical and clinical settings. In this work, ICG is coupled to an integrin binding vector (ICG-RGD) to combine the good photoacoustic properties of ICG and the favourable α_vβ₃-binding capabilities of a small RGD cyclic peptidomimetic. ICG-RGD is characterized in terms of physicochemical properties, biodistribution and imaging performance. Tumor uptake was assessed in subcutaneous xenograft mouse models of human glioblastoma (U-87MG, high α_vβ₃ expression) and epidermoid carcinoma (A431, low α_vβ₃ expression). ICG and ICG-RGD showed high PA signal in tumors already after 15 min post-injection. At later time points the signal of ICG rapidly decreased, while ICG-RGD showed sustained uptake in U-87MG but not in A431 tumors, likely due to the integrin-mediated retention of the probe. In conclusion, ICG-RGD is a novel targeted contrast agents for PAI with superior biodistribution, tumor uptake properties and diagnostic value compared to ICG.

Cathepsin B-degradable, NIR-responsive nanoparticulate platform for target-specific cancer therapy

Stimuli-responsive anticancer formulations can promote drug release and activation within the target tumour, facilitate cellular uptake, as well as improve the therapeutic efficacy of drugs and reduce off-target effects. In the present work, indocyanine green (ICG)-containing polyglutamate (PGA) nanoparticles were developed and characterized. Digestion of nanoparticles with cathepsin B, a matrix metalloproteinase overexpressed in the microenvironment of advanced tumours, decreased particle size and increased ICG cellular uptake. Incorporation of ICG in PGA nanoparticles provided the NIR-absorbing agent with time-dependent altered optical properties in the presence of cathepsin B. Having minimal dark toxicity, the formulation exhibited significant cytotoxicity upon NIR exposure. Combined use of the formulation with saporin, a ribosome-inactivating protein, resulted in synergistically enhanced cytotoxicity attributed to the photo-induced release of saporin from endo/lysosomes. The results suggest that this therapeutic approach can offer significant therapeutic benefit in the treatment of superficial malignancies, such as head and neck tumours.

Note: Design of a full photon-timing recorder down to 1-ns resolution for fluorescence fluctuation measurements

A photon timing recorder was realized in a field programmable gate array to capture all timing data of photons on multiple channels with down to a 1-ns resolution and to transfer all data to a host computer in real-time through universal serial bus with more than 10 M events/s transfer rate. The main concept is that photon time series can be regarded as a serial communication data stream. This recorder was successfully applied for simultaneous measurements of fluorescence fluctuation and lifetime of near-infrared dyes in solution. This design is not only limited to the fluorescence fluctuation measurement but also applicable to any kind of photon counting experiments in a nanosecond time range because of the simple and easily modifiable design.

Recent advances in the development of synthetic chemical probes for glycosidase enzymes

The emergence of synthetic glycoconjugates as chemical probes for the detection of glycosidase enzymes has resulted in the development of a range of useful chemical tools with applications in glycobiology, biotechnology, medical and industrial research. Critical to the function of these probes is the preparation of substrates containing a glycosidic linkage that when activated by a specific enzyme or group of enzymes, irreversibly releases a reporter molecule that can be detected. Starting from the earliest examples of colourimetric probes, increasingly sensitive and sophisticated substrates have been reported. In this review we present an overview of the recent advances in this field, covering an array of strategies including chromogenic and fluorogenic substrates, lanthanide complexes, gels and nanoparticles. The applications of these substrates for the detection of various glycosidases and the scope and limitations for each approach are discussed.

Multiple hybridized resonances of IR-806 chromonic molecules strongly coupled to Au nanorods

Strong coupling of plasmons and molecules generates intriguingly hybridized resonance. The IR-806 molecule is a near-infrared cyanine liquid crystal dye with multiple molecular bands and its tunable absorption spectrum varies dramatically with concentration. In this article, we investigate multiple hybridized resonances of the Au nanorods (AuNRs) strongly coupled to IR-806 molecules. Five hybridized resonance peaks are observed in the extinction spectra of the AuNR@IR-806 hybrids. Two resonance peaks at approximately 840 and 912 nm in the hybrids are reported for the first time. The dependence of the multiple hybridized peaks on the bare plasmon resonance wavelength of AuNRs and the molecular concentration is also demonstrated. The observations presented herein provide a plasmon-molecule coupling route for tuning optical responses of liquid crystal molecules.

Synthesis and characterization of near IR fluorescent albumin nanoparticles for optical detection of colon cancer

Near IR (NIR) fluorescent human serum albumin (HSA) nanoparticles hold great promise as contrast agents for tumor diagnosis. HSA nanoparticles are considered to be biocompatible, non-toxic and non-immunogenic. In addition, NIR fluorescence properties of these nanoparticles are important for in vivo tumor diagnostics, with low autofluorescence and relatively deep penetration of NIR irradiation due to low absorption of biomatrices. The present study describes the synthesis of new NIR fluorescent HSA nanoparticles, by entrapment of a NIR fluorescent dye within the HSA nanoparticles, which also significantly increases the photostability of the dye. Tumor-targeting ligands such as peanut agglutinin (PNA) and anti-carcinoembryonic antigen antibodies (anti-CEA) were covalently conjugated to the NIR fluorescent albumin nanoparticles, increasing the potential fluorescent signal in tumors with upregulated corresponding receptors. Specific colon tumor detection by the NIR fluorescent HSA nanoparticles was demonstrated in a chicken embryo model and a rat model. In future work we also plan to encapsulate cancer drugs such as doxorubicin within the NIR fluorescent HSA nanoparticles for both colon cancer imaging and therapy.

Near IR fluorescent polystyrene/albumin core/shell nanoparticles for specific targeting of colonic neoplasms

Previous studies have shown that albumin has a high affinity towards tumours, and, as a result, many drug/albumin conjugates, as well as albumin nanoparticles, have been studied as antineoplastic drug carriers. Numerous studies have also shown the high affinity of cyanine dyes for albumin. This work combines the former and the latter for the preparation of NIR fluorescent and photostable nanoparticles as diagnostic biomaterials. Tumour-specific labelling by NIR fluorescent polystyrene/albumin core/shell nanoparticles is demonstrated, without the presence of additional targeting moieties, and they possess great potential for clinical applications.

Near-infrared fluorescence probes for enzymes based on binding affinity modulation of squarylium dye scaffold

We present a novel design strategy for near-infrared (NIR) fluorescence probes utilizing dye-protein interaction as a trigger for fluorescence enhancement. The design principle involves modification of a polymethine dye with cleavable functional groups that reduce the dye's protein-binding affinity. When these functional groups are removed by specific interaction with the target enzymes, the dye's protein affinity is restored, protein binding occurs, and the dye's fluorescence is strongly enhanced. To validate this strategy, we first designed and synthesized an alkaline phosphatase (ALP) sensor by introducing phosphate into the squarylium dye scaffold; this sensor was able to detect ALP-labeled secondary antibodies in Western blotting analysis. Second, we synthesized a probe for β-galactosidase (widely used as a reporter of gene expression) by means of β-galactosyl substitution of the squarylium scaffold; this sensor was able to visualize β-galactosidase activity both in vitro and in vivo. Our strategy should be applicable to obtain NIR fluorescence probes for a wide range of target enzymes.