Highly bright and stable NIR-BRET with blue-shifted coelenterazine derivatives for deep-tissue imaging of molecular events in vivo

Regiospecific Coelenterazine Analogs for Bioassays and Molecular Imaging

Bioluminescence (BL) generated by luciferase-coelenterazine (CTZ) reactions is broadly employed as an optical readout in bioassays and in vivo molecular imaging. In this study, we demonstrate a systematic approach to elucidate the luciferase-CTZ binding chemistry with a full set of regioisomeric CTZ analogs, where all the functional groups were regiochemically modified. When the chemical structures were categorized into Groups 1-6, the even-numbered Groups (2, 4, and 6) of the CTZ analogs are found to be exceptionally bright with NanoLuc enzyme. A CTZ analogue M2 was the brightest with NanoLuc and the reason was deciphered by a computational analysis of the binding modes. We also report that (i) the regioisomeric CTZ analogs collectively create unique intensity patterns according to each marine luciferase, (ii) the quantitative structure-activity relationship analysis revealed the roles of respective functional groups of CTZ analogs, and (iii) the regioisomeric CTZ analogs also exert red shifts of the BL spectra and color variation: that is, the λmax values are near 500 nm with NanoLuc, near 530 nm with ALuc16, and near 570 nm with RLuc86SG. The advantages of the regioisomeric CTZ analogs were finally demonstrated using (i) a dual-luciferase system with M2-specific NanoLuc and native CTZ-specific ALuc16, (ii) an estrogen activatable single-chain BL probe by imaging, and (iii) BL imaging of live mice bearing tumors expressing NanoLuc and RLuc8.6SG. This study is the first systematic approach to elucidate the regiochemistry in BL imaging studies. This study provides new insights into how CTZ analogs regiochemically work in BL reporter systems and guides the specific applications to molecular imaging.

A Vector Nanoplatform for the Bioimaging of Deep-Seated Tumors

Today, in preclinical studies, optical bioimaging based on luminescence and fluorescence is indispensable in studying the development of neoplastic transformations, the proliferative activity of the tumor, its metastatic potential, as well as the therapeutic effect of antitumor agents. In order to expand the capabilities of optical imaging, sensors based on the bioluminescence resonance energy transfer (BRET) mechanism and, therefore, independent of an external light source are being developed. A targeted nanoplatform based on HER2-specific liposomes whose internal environment contains a genetically encoded BRET sensor was developed in this study to visualize deep-seated tumors characterized by overexpression of human epidermal growth factor receptor type 2 (HER2). The BRET sensor is a hybrid protein consisting of the highly catalytic luciferase NanoLuc (an energy donor) and a LSSmKate1 red fluorescent protein with a large Stokes shift (an energy acceptor). During the bioimaging of disseminated intraperitoneal tumors formed by HER2-positive SKOV3.ip1cells of serous ovarian cystadenocarcinoma, it was shown that the developed system is applicable in detecting deep-seated tumors of a certain molecular profile. The developed system can become an efficient platform for optimizing preclinical studies of novel targeted drugs.

Pseudo-Luciferase Activity of the SARS-CoV-2 Spike Protein for Cypridina Luciferin

Enzymatic reactions that involve a luminescent substrate (luciferin) and enzyme (luciferase) from luminous organisms enable a luminescence detection of target proteins and cells with high specificity, albeit that conventional assay design requires a prelabeling of target molecules with luciferase. Here, we report a luciferase-independent luminescence assay in which the target protein directly catalyzes the oxidative luminescence reaction of luciferin. The SARS-CoV-2 antigen (spike) protein catalyzes the light emission of Cypridina luciferin, whereas no such catalytic function was observed for salivary proteins. This selective luminescence reaction is due to the enzymatic recognition of the 3-(1-guanidino)propyl group in luciferin at the interfaces between the units of the spike protein, allowing a specific detection of the spike protein in human saliva without sample pretreatment. This method offers a novel platform to detect virus antigens simply and rapidly without genetic manipulation or antibodies.

Current advances in the development of bioluminescent probes toward spatiotemporal trans-scale imaging

Bioluminescence imaging has recently attracted great attention as a highly sensitive and non-invasive analytical method. However, weak signal and low chemical stability of the luciferin are conventional drawbacks of bioluminescence imaging. In this review article, we describe the recent progress on the development and applications of bioluminescent probes for overcoming the aforementioned limitations, thereby enabling spatiotemporal trans-scale imaging. The detailed molecular design for manipulation of their luminescent properties and functions enabled a variety of applications, including in vivo deep tissue imaging, long-term imaging, and chemical sensor.

Single-chain multicolor-reporter templates for subcellular localization of molecular events in mammalian cells

Single-chain multicolor-reporter imaging templates were developed for the subcellular localization of molecular events in mammalian cells. The templates were constructed by tandem linkage of fluorescent protein variants - fused with luciferases and the subcellular localization signal peptides. The templates simultaneously reported steroid hormonal activities at different optical spectra in the subcellular compartments. The templates contribute to the expansion of a toolbox of optical probes for subcellular localization of molecular events in intact cells.

The Molecular Basis of Organic Chemiluminescence

Bioluminescence (BL) and chemiluminescence (CL) are interesting and intriguing phenomena that involve the emission of visible light as a consequence of chemical reactions. The mechanistic basis of BL and CL has been investigated in detail since the 1960s, when the synthesis of several models of cyclic peroxides enabled mechanistic studies on the CL transformations, which led to the formulation of general chemiexcitation mechanisms operating in BL and CL. This review describes these general chemiexcitation mechanisms-the unimolecular decomposition of cyclic peroxides and peroxide decomposition catalyzed by electron/charge transfer from an external (intermolecular) or an internal (intramolecular) electron donor-and discusses recent insights from experimental and theoretical investigation. Additionally, some recent representative examples of chemiluminescence assays are given.

Bright Molecular Strain Probe Templates for Reporting Protein-Protein Interactions

Imaging protein-protein interactions (PPIs) is a hot topic in molecular medicine in the postgenomic sequencing era. In the present study, we report bright and highly sensitive single-chain molecular strain probe templates which embed full-length Renilla luciferase 8.6-535SG (RLuc86SG) or Artificial luciferase 49 (ALuc49) as reporters. These reporters were deployed between FKBP-rapamycin binding domain (FRB) and FK506-binding protein (FKBP) as a PPI model. This unique molecular design was conceptualized to exploit molecular strains of the sandwiched reporters appended by rapamycin-triggered intramolecular PPIs. The ligand-sensing properties of the templates were maximized by interface truncations and substrate modulation. The highest fold intensities, 9.4 and 16.6, of the templates were accomplished with RLuc86SG and ALuc49, respectively. The spectra of the templates, according to substrates, revealed that the colors are tunable to blue, green, and yellow. The putative substrate-binding chemistry and the working mechanisms of the probes were computationally modeled in the presence or absence of rapamycin. Considering that the molecular strain probe templates are applicable to other PPI models, the present approach would broaden the scope of the bioassay toolbox, which harnesses the privilege of luciferase reporters and the unique concept of the molecular strain probes into bioassays and molecular imaging.

Bioluminescent imaging systems boosting near-infrared signals in mammalian cells

Bioluminescence (BL) is broadly used as an optical readout in bioassays and molecular imaging. In this study, the near-infrared (NIR) BL imaging systems were developed. The system was harnessed by prototype copepod luciferases, artificial luciferase 30 (ALuc30) and its miniaturized version picALuc, and were characterized with 17 kinds of coelenterazine (CTZ) analogues carrying bulky functional groups or cyanine 5 (Cy5). They were analyzed of BL spectral peaks and enzymatic kinetics, and explained with computational modeling. The results showed that (1) the picALuc-based system surprisingly boosts the BL intensities predominantly in the red and NIR region with its specific CTZ analogues; (2) both ALuc30- and picALuc-based systems develop unique through-bond energy transfer (TBET)-driven spectral bands in the NIR region with a Cy5-conjugated CTZ analogue (Cy5-CTZ); and (3) according to the computational modeling, the miniaturized version, picALuc, has a large binding pocket, which can accommodate CTZ analogues containing bulky functional groups and thus allowing NIR BL. This study is an important addition to the BL imaging toolbox with respect to the development of orthogonal NIR reporter systems applicable to physiological samples, together with the understanding of the BL-emitting chemistry of marine luciferases.

S-Series Coelenterazine-Driven Combinatorial Bioluminescence Imaging Systems for Mammalian Cells

A unique combinatorial bioluminescence (BL) imaging system was developed for determining molecular events in mammalian cells with various colors and BL intensity patterns. This imaging system consists of one or multiple reporter luciferases and a series of novel coelenterazine (CTZ) analogues named "S-series". For this study, ten kinds of novel S-series CTZ analogues were synthesized and characterized concerning the BL intensities, spectra, colors, and specificity of various marine luciferases. The characterization revealed that the S-series CTZ analogues luminesce with blue-to-orange-colored BL spectra with marine luciferases, where the most red-shifted BL spectrum peaked at 583 nm. The colors completed a visible light color palette with those of our precedent C-series CTZ analogues. The synthesized substrates S1, S5, S6, and S7 were found to have a unique specificity with marine luciferases, such as R86SG, NanoLuc (shortly, NLuc), and ALuc16. They collectively showed unique BL intensity patterns to identify the marine luciferases together with colors. The marine luciferases, R86SG, NLuc, and ALuc16, were multiplexed into multi-reporter systems, the signals of which were quantitatively unmixed with the specific substrates. When the utility was applied to a single-chain molecular strain probe, the imaging system simultaneously reported three different optical indexes for a ligand, i.e., unique BL intensity and color patterns for identifying the reporters, together with the ligand-specific fold intensities in mammalian cells. This study directs a new combinatorial BL imaging system to specific image molecular events in mammalian cells with multiple optical indexes.

Near-Infrared Imaging of Steroid Hormone Activities Using Bright BRET Templates

Bioluminescence (BL) is an excellent optical readout for bioassays and molecular imaging. Herein, we accomplished new near infrared bioluminescence resonance energy transfer (NIR-BRET) templates for monitoring molecular events in cells with higher sensitivity. We first identified the best resonance energy donor for the NIR-BRET templates through the characterization of many coelenterazine (CTZ)-marine luciferase combinations. As a result, we found that NLuc-DBlueC and ALuc47-nCTZ combinations showed luminescence in the blue emission wavelength with excellent BL intensity and stability, for example, the NLuc-DBlueC and ALuc47-nCTZ combinations were 17-fold and 22-fold brighter than their second highest combinations, respectively, and were stably bright in living mammalian cells for at least 10 min. To harness the excellent BL properties to the NIR-BRET systems, NLuc and ALuc47 were genetically fused to fluorescent proteins (FPs), allowing large "blue-to-red" shifts, such as LSSmChe, LSSmKate2, and LSSmNep (where LSS means Large Stokes Shift). The excellent LSSmNep-NLuc combination showed approximately 170 nm large resonance energy shift from blue to red. The established templates were further utilized in the development of new NIR-BRET systems for imaging steroid hormone activities by sandwiching the ligand-binding domain of a nuclear receptor (NR-LBD) between the luciferase and the FP of the template. The NIR-BRET systems showed a specific luminescence signal upon exposure to steroid hormones, such as androgen, estrogen, and cortisol. The present NIR-BRET templates are important additions for utilizing their advantageous imaging of various molecular events with high efficiency and brightness in physiological samples.

Multiplex quadruple bioluminescent assay system

Bioluminescence (BL) is unique cold body radiation of light, generated by luciferin-luciferase reactions and commonly used in various bioassays and molecular imaging. However, most of the peak emissions of BL populate the blue-yellow region and have broad spectral bandwidths and thus superimpose each other, causing optical cross-leakages in multiplex assays. This study synthesized a new series of coelenterazine (CTZ) analogues, named K-series, that selectively illuminates marine luciferases with unique, blue-shifted spectral properties. The optical property and specificity of the K-series CTZ analogues were characterized by marine luciferases, with K2 and K5 found to specifically luminesce with ALuc- and RLuc-series marine luciferases, respectively. The results confirmed that the luciferase specificity and color variation of the CTZ analogues minimize the cross-leakages of BL signals and enable high-throughput screening of specific ligands in the mixture. The specificity and color variation of the substrates were further tailored to marine luciferases (or single-chain bioluminescent probes) to create a multiplex quadruple assay system with four integrated, single-chain bioluminescent probes, with each probe designed to selectively luminesce only with its specific ligand (first authentication) and a specific CTZ analogue (second authentication). This unique multiplex quadruple bioluminescent assay system is an efficient optical platform for specific and high-throughput imaging of multiple optical markers in bioassays without optical cross-leakages.

A Bright Monomeric Near-Infrared Fluorescent Protein with an Excitation Peak at 633 nm for Labeling Cellular Protein and Reporting Protein-Protein Interaction

Bright monomeric near-infrared fluorescent proteins (NIR-FPs) are useful as markers for labeling proteins and cells and as sensors for reporting molecular activities in living cells and organisms. However, current monomeric NIR-FPs are dim under excitation with common 633/635/640 nm lasers, limiting their broad use in cellular/subcellular level imaging. Here, we report a bright monomeric NIR-FP with maximum excitation at 633 nm, named mIFP663, engineered from Xanthomonas campestris pv Campestris phytochrome (XccBphP). mIFP663 has high molecular brightness with a large extinction coefficient (86,600 M^-1 cm^-1) and a decent quantum yield (19.4%), and high cellular brightness that is 3-6 times greater than those of spectrally similar NIR-FPs in HEK293T cells in the presence of exogenous BV. Moreover, we demonstrate that mIFP663 is able to label critical cellular and viral proteins without perturbing subcellular localization and virus replication, respectively. Finally, with mIFP663, we engineer improved bimolecular fluorescence complementation (BiFC) and new bioluminescent resonance energy transfer (BRET) systems to detect protein-protein interactions in living cells.

BRET Sensors for Imaging Membrane Integrity of Microfluidically Generated Extracellular Vesicles

Extracellular vesicles (EVs) derived from various cell lines have been extensively used as natural nanodelivery vehicles for drug, protein, and nucleic acid deliveries in therapeutic applications for cancer. Recently, we developed a microfluidic-based reconstruction strategy as a novel method to generate microRNA-loaded membrane vesicles for cancer therapy in vivo. We used EVs and cell membranes isolated from different source of cells for this reconstruction process. The microfluidic system produced reconstructed vesicles of uniform sizes with high microRNA loading efficiency independent of input membrane sources (EVs or cell membranes). To address the functional integrity of the membrane structure and of proteins in the reconstructed EVs, we introduce a membrane-insertable bioluminescence resonance energy transfer (BRET) sensor system. This sensor, with its membrane-insertable palmitoylation signal peptide sequence derived from a growth-associated protein 43 (GAP43), helps in trafficking the fusion protein to the cell membrane upon its expression in cells and allows for imaging reconstructed membrane vesicles using optical imaging. In this chapter, we detail the stepwise methods used for the engineering of cells using this sensor, isolation of EVs from the engineered cells, preparation of reconstructed EVs by microfluidic processing, and BRET imaging of reconstructed EVs for membrane integrity evaluation.

Visible Light Bioluminescence Imaging Platform for Animal Cell Imaging

The present protocol introduces a visible light bioluminescence imaging (BLI) platform together with 12 novel coelenterazine (CTZ) analogues and luciferase sets. We exemplify to create diverse hues of bioluminescence (BL) ranging from blue to far red with the combination of marine luciferases and the three groups of CTZ analogues. We also show how to characterize the new CTZ analogues in detail such as the kinetic parameters, dose dependency, and luciferase specificity. The 2-series CTZ analogues interestingly have specificity to artificial luciferases and are completely dark with Renilla luciferase derivatives in contrast. The 3d is highly specific to only NanoLuc. This BL imaging system covering the visible region provides a useful multicolor imaging portfolio that efficiently images molecular events in mammalian cells.

How to Select Firefly Luciferin Analogues for In Vivo Imaging

Bioluminescence reactions are widely applied in optical in vivo imaging in the life science and medical fields. Such reactions produce light upon the oxidation of a luciferin (substrate) catalyzed by a luciferase (enzyme), and this bioluminescence enables the quantification of tumor cells and gene expression in animal models. Many researchers have developed single-color or multicolor bioluminescence systems based on artificial luciferin analogues and/or luciferase mutants, for application in vivo bioluminescence imaging (BLI). In the current review, we focus on the characteristics of firefly BLI technology and discuss the development of luciferin analogues for high-resolution in vivo BLI. In addition, we discuss the novel luciferin analogues TokeOni and seMpai, which show potential as high-sensitivity in vivo BLI reagents.

Color-tunable bioluminescence imaging portfolio for cell imaging

The present study describes a color-tunable imaging portfolio together with twelve novel coelenterazine (CTZ) analogues. The three groups of CTZ analogues create diverse hues of bioluminescence (BL) ranging from blue to far red with marine luciferases. We found that the hue completes the whole color palette in the visible region and shows red-shifted BL with a marine luciferase: for example, Renilla luciferase 8 (RLuc8) and Artificial Luciferase 16 (ALuc16) show 187 nm- and 105 nm-redshifted spectra, respectively, by simply replacing the substrate CTZ with 1d. The optical properties of the new CTZ analogues were investigated such as the kinetic parameters, dose dependency, and luciferase specificity. The 2-series CTZ analogues interestingly have specificity to ALucs and are completely dark with RLuc derivatives, and 3d is highly specific to only NanoLuc. We further determined the theoretical background of the red-shifted BL maximum wavelengths (λ_BL) values according to the extended π conjugation of the CTZ backbone using Density Functional Theory (DFT) calculations. This color-tunable BL imaging system provides a useful multicolor imaging portfolio that efficiently images molecular events in mammalian cells.

Core-Modified Coelenterazine Luciferin Analogues: Synthesis and Chemiluminescence Properties

In this work on the design and studies of luciferins related to the blue-hued coelenterazine, the synthesis of heterocyclic analogues susceptible to produce a photon, possibly at a different wavelength, is undertaken. Here, the synthesis of O-acetylated derivatives of imidazo[1,2-b]pyridazin-3(5 H)-one, imidazo[2,1-f][1,2,4]triazin-7(1 H)-one, imidazo[1,2-a]pyridin-3-ol, imidazo[1,2-a]quinoxalin-1(5 H)-one, benzo[f]imidazo[1,2-a]quinoxalin-3(11 H)-one, imidazo[1',2':1,6]pyrazino[2,3-c]quinolin-3(11 H)-one, and 5,11-dihydro-3 H-chromeno[4,3-e]imidazo[1,2-a]pyrazin-3-one is described thanks to extensive use of the Buchwald-Hartwig N-arylation reaction. The acidic hydrolysis of these derivatives then gave solutions of the corresponding luciferin analogues, which were studied. Not too unexpectedly, even if these were "dressed" with substituents found in actual substrates of the nanoKAZ/NanoLuc luciferase, no bioluminescence was observed with these compounds. However, in a phosphate buffer, all produced a light signal, by chemiluminescence, with extensive variations in their respective intensity and this could be increased by adding a quaternary ammonium salt in the buffer. This aspect was actually instrumental to determine the emission spectra of many of these luciferin analogues.

Highly Bright NIR-BRET System for Imaging Molecular Events in Live Cells

The present protocol demonstrates a novel mammalian cell imaging platform exerting a bioluminescence resonance energy transfer (BRET) system. This platform achieves a ~300 nm blue-to-near infrared shift of the emission (NIR-BRET) with the development of a unique coelenterazine (CTZ) derivative named BBlue2.3 and a fusion reporter protein probe named iRFP-RLuc8.6-535SG. The best NIR-BRET shift was achieved by tuning the blue emission peak of BBlue2.3 to a Soret band of the iRFP. In mammalian cells, BBlue2.3 emits light that is ~50-fold brighter than DeepBlueC in cell imaging when combined with RLuc8.6-535SG. This NIR-BRET platform is sufficiently brighter to be used for imaging live mammalian cells at single-cell level, and also for imaging metastases in deep tissues in live mice without generating considerable autoluminescence. This unique optical platform provides the brightest NIR-BLI template that can be used for imaging a diverse group of cellular events in living subjects.

Coelenterazine Analogue with Human Serum Albumin-Specific Bioluminescence

A synthetic luciferin comprising an imidazopyrazinone core, named HuLumino1, was designed to generate specific bioluminescence with human serum albumin (HSA) in real serum samples. HuLumino1 was developed by attaching a methoxy-terminated alkyl chain to C-6 of coelenterazine and by eliminating a benzyl group at C-8. HSA levels were quantified within 5% error margins of an enzyme-linked immunosorbent assay without the need for any sample pretreatments because of the high specificity of HuLumino1.

Long-term single cell bioluminescence imaging with C-3 position protected coelenterazine analogues

Bioluminescence is a powerful imaging modality for monitoring biological phenomena both in vitro and in vivo. Bioluminescence imagin (BLI) is becoming a seamless imaging technology covering the range from cells to organs of small animals. Long-term imaging at the single cell level would lead to a true understanding of the dynamics of life phenomena. This work presents a long-term single cell bioluminescence imaging technology accomplished with C-3 position protected furimazines (FMZs), a CTZ analogues, which generate intense blue emission when paired with a highly stable engineered luciferase, Nanoluc. Four types of FMZs protected at the C-3 position have been synthesized. The type and steric bulkiness of the protection group strongly contributed to storage stability and the kinetics of the bioluminescence reactions of the analogues in human living cells. In particular, two developed FMZ analogues resulted in significantly longer bioluminescence emission with higher S/N ratio than FMZ at single cell level. Long-term bioluminescence single cell imaging technology with the developed FMZ analogues will lead to seamless imaging in the range from cells to organs of small animals.

Bioluminescent Imaging Systems for Assay Developments

Bioluminescence (BL) is an excellent optical readout platform that has great potential to be utilized in various bioassays and molecular imaging. The advantages of BL-based bioassays include the long dynamic range, minimal background, high signal-to-noise ratios, biocompatibility for use in cell-based assays, no need of external light source for excitation, simplicity in the measurement system, and versatility in the assay design. The recent intensive research in BL has greatly diversified the available luciferase-luciferin systems in the bioassay toolbox. However, the wide variety does not promise their successful utilization in various bioassays as new tools. This is mainly due to complexity and confusion with the diversity, and the unavailability of defined standards. This review is intended to provide an overview of recent basic developments and applications in BL studies, and showcases the bioanalytical utilities. We hope that this review can be used as an instant reference on BL and provides useful guidance for readers in narrowing down their potential options in their own assay designs.

Advanced Bioluminescence System for In Vivo Imaging with Brighter and Red-Shifted Light Emission

In vivo bioluminescence imaging (BLI), which is based on luminescence emitted by the luciferase-luciferin reaction, has enabled continuous monitoring of various biochemical processes in living animals. Bright luminescence with a high signal-to-background ratio, ideally red or near-infrared light as the emission maximum, is necessary for in vivo animal experiments. Various attempts have been undertaken to achieve this goal, including genetic engineering of luciferase, chemical modulation of luciferin, and utilization of bioluminescence resonance energy transfer (BRET). In this review, we overview a recent advance in the development of a bioluminescence system for in vivo BLI. We also specifically examine the improvement in bioluminescence intensity by mutagenic or chemical modulation on several beetle and marine luciferase bioluminescence systems. We further describe that intramolecular BRET enhances luminescence emission, with recent attempts for the development of red-shifted bioluminescence system, showing great potency in in vivo BLI. Perspectives for future improvement of bioluminescence systems are discussed.

Bioluminescence Profiling of NanoKAZ/NanoLuc Luciferase Using a Chemical Library of Coelenterazine Analogues

We describe here an extensive structure-bioluminescence relationship study of a chemical library of analogues of coelenterazine, using nanoKAZ/NanoLuc, a mutated luciferase originated from the catalytic subunit of the deep-sea shrimp Oplophorus gracilirostris. Out of the 135 O-acetylated precursors that were prepared by using our recently reported synthesis and following their hydrolysis to give solutions of the corresponding luciferins, notable bioluminescence improvements were achieved in comparison with furimazine, which is currently amongst the best substrates of nanoKAZ/NanoLuc. For instance, the rather more lipophilic analogue 8-(2,3-difluorobenzyl)-2-((5-methylfuran-2-yl)methyl)-6-phenylimidazo[1,2-a]pyrazin-3(7H)-one provided a 1.5-fold improvement of the total light output over a 2 h period, a close to threefold increase of the initial signal intensity and a signal-to-background ratio five times greater than furimazine. The kinetic parameters for the enzymatic reaction were obtained for a selection of luciferin analogues and provided unexpected insights into the luciferase activity. Most prominently, along with a general substrate-dependent and irreversible inactivation of this enzyme, in the case of the optimized luciferin mentioned above, the consumption of 2664 molecules was found to be required for the detection of a single Relative Light Unit (RLU; a luminometer-dependent fraction of a photon).

Optical and Optoacoustic Imaging

The spatiotemporal determination of molecular events and cells is important for understanding disease processes, especially in oncology, and thus for the development of novel treatments. Equally important is the knowledge of the biodistribution, localization, and targeted accumulation of novel therapies as well as monitoring of tumor growth and therapeutic response. Optical imaging provides an ideal versatile platform for imaging of all these problems and questions.

Self-Illuminating Agents for Deep-Tissue Optical Imaging

Optical imaging plays an indispensable role in biology and medicine attributing to its noninvasiveness, high spatiotemporal resolution, and high sensitivity. However, as a conventional optical imaging modality, fluorescence imaging confronts issues of shallow imaging depth due to the need for real-time light excitation which produces tissue autofluorescence. By contrast, self-luminescence imaging eliminates the concurrent light excitation, permitting deeper imaging depth and higher signal-to-background ratio (SBR), which has attracted growing attention. Herein, this review summarizes the progress on the development of near-infrared (NIR) emitting self-luminescence agents in deep-tissue optical imaging with highlighting the design principles including molecular- and nano-engineering approaches. Finally, it discusses current challenges and guidelines to develop more effective self-illuminating agents for biomedical diagnosis and treatment.