Eunha Kim

Emission Wavelength Prediction of a Full-Color-Tunable Fluorescent Core Skeleton, 9-Aryl-1,2-dihydropyrrolo[3,4-b]indolizin-3-one

In this paper we report on a novel fluorescent core skeleton, 9-aryl-1,2-dihydropyrrolo[3,4-b]indolizin-3-one, which we named Seoul-Fluor, having tunable and predictable photophysical properties. Using a concise and practical one-pot synthetic procedure, a 68-member library of new fluorescent compounds was synthesized with diverse substituents. In Seoul-Fluor, the electronic characteristics of the substituents, as well as their positional changes, have a close correlation with their photophysical properties. The systematic perturbation of electronic densities on the specific positions of Seoul-Fluor, guided with the Hammett constant, allows emission wavelength tunability covering the full color range. On the basis of these observations and a computational analysis, we extracted a simple first-order correlation of photophysical properties with the theoretical calculation and accurately predicted the emission wavelength of Seoul-Fluors through the rational design. In this study, we clearly demonstrate that Seoul-Fluor can provide a powerful gateway for the generation of desired fluorescent probes without the need for a tiresome synthesis and trial-and-error process.

Combinatorial Discovery of Full-Color-Tunable Emissive Fluorescent Probes Using a Single Core Skeleton, 1,2-Dihydropyrrolo[3,4-β]indolizin-3-one

We developed a novel fluorescent core skeleton, 1,2-dihydropyrrolo[3,4-beta]indolizin-3-one, by complexity-generating one-pot reactions through 1,3-dipolar cyclization followed by oxidative aromatization. This fluorescent core skeleton can accommodate various wavelengths of emission maxima by changing the electronic properties of substituents, which was postulated by computational studies. The full-color-tunable emission maxima were achieved with a single core skeleton by changing the substituents using the combinatorial approach. These novel fluorophores have excellent photophysical and photochemical properties: moderate to excellent quantum yields, resistance to the photobleaching, pH-independent fluorescence, large Stokes shifts, druglike lipophilicity for membrane permeability, etc. Further, we successfully demonstrated the bioapplication of fluorophores B1 and B5 in the immunofluorescence for visualizing cellular compartments of HeLa cells.

Chemistry as a Prism: A Review of Light‐Emitting Materials Having Tunable Emission Wavelengths

Photoluminescent materials have been extensively applied in various fields of science because of their numerous advantages, such as excellent sensitivity, good specificity, a large linear range of analysis, ease of handling, and so on. Many strategies have been used to understand and manipulate the photophysical properties of photoluminescent materials. This Focus Review describes recent progress focused on tuning the photophysical properties, especially the emission wavelengths of pi-conjugated oligomers, photoluminescent organometallic complexes, and fluorescent organic dyes by chemical modification.

Discovery, understanding, and bioapplication of organic fluorophore: a case study with an indolizine-based novel fluorophore, Seoul-Fluor.

Owing to its high sensitivity and great applicability, the fluorescence phenomenon has been considered as an inevitable research tool in the modern scientific fields of chemistry, biology, materials science, biomedical science, and their interfaces. Many strategies have been pursued to understand and manipulate the photophysical properties of fluorescent materials, but the scientific community has been focused on the repeated application of existing organic fluorophores or the identification of unique fluorescence properties in a trial-and-error basis without systematic studies. Moreover, recent studies are emphasizing the necessity of deeper understanding about the structure-photophysical property relationship of organic fluorophores for the development of better fluorescent probes. Herein, we provide an overview of a novel fluorescent molecular framework, Seoul-Fluor, which can be rationally engineered to furnish a wide variety of fluorophores in terms of the photophysical properties. Seoul-Fluor is built on an indolizine-based fluorescent platform with three different positions to introduce various substituents: R(1) and R(2) substituents for electronic perturbation; R(3) substituent as a functional handle for bioconjugation. Over the past decade, we have demonstrated that the Seoul-Fluor system has (i) tunable and predictable emission wavelength covering a full visible-color range; (ii) controllable quantum yield via photoinduced electron transfer phenomenon; and (iii) environment-sensitive fluorogenic properties that can be modified through intramolecular charge transfer processes. We convincingly demonstrated the prediction of photophysical properties, that is, emission wavelength and quantum yield, through the construction of a systematic set of analogues and the subsequent analysis of their photophysical properties without the highly sophisticated theoretical support. Guided by quantifiable parameters such as the Hammett substituent constants or energy levels of the molecular orbitals, this unique organic fluorophore can serve as a versatile molecular platform for the development of novel fluorescent switchable biosensors and fluorogenic bioprobes. In this Account, we will discuss the discovery and recent progress made on Seoul-Fluor, the rational design of Seoul-Fluor-based bioprobes, and their practical applications to specific biological processes that are facilitated by systematic studies of the structure-photophysical property relationships.

Rational Perturbation of the Fluorescence Quantum Yield in Emission‐Tunable and Predictable Fluorophores (Seoul‐Fluors) by a Facile Synthetic Method Involving C—H Activation†

Abstract Fluorescence imaging enables the uniquely sensitive observation of functional‐ and molecular‐recognition events in living cells. However, only a limited range of biological processes have been subjected to imaging because of the lack of a design strategy and difficulties in the synthesis of biosensors. Herein, we report a facile synthesis of emission‐tunable and predictable Seoul‐Fluors, 9‐aryl‐1,2‐dihydrolopyrrolo[3,4‐b]indolizin‐3‐ones, with various R1 and R2 substituents by coinage‐metal‐catalyzed intramolecular 1,3‐dipolar cycloaddition and subsequent palladium‐mediated C—H activation. We also showed that the quantum yields of Seoul‐Fluors are controlled by the electronic nature of the substituents, which influences the extent of photoinduced electron transfer. On the basis of this understanding, we demonstrated our design strategy by the development of a Seoul‐Fluor‐based chemosensor 20 for reactive oxygen species that was not accessible by a previous synthetic route.

Single-cell pharmacokinetic imaging reveals a therapeutic strategy to overcome drug resistance to the microtubule inhibitor eribulin

Single-cell pharmacokinetic analysis of a fluorescent eribulin derivative in vivo revealed drug resistance mediated by MDR1-driven efflux, which was overcome by a nanoencapsulated MDR1 inhibitor. Single-Cell Imaging of Cancer Drug Resistance Resistance to drugs is common in cancer and is often caused by the multidrug resistance protein 1 (MDR1). To get to the bottom of this mechanism of drug resistance—and hopefully develop better therapeutics from this knowledge—Laughney et al. created a fluorescent cancer drug and probed tissue distribution and kinetics at both the single-cell and population levels. The authors developed a mouse model of drug-resistant human cancer, with tumors that were heterogeneously resistant (various levels of MDR1 expression). By tracking the fluorescent drug eribulin in vivo, they saw that MDR1-expressing cells accumulated less drug than their wild-type counterparts—and this was a function of distance from blood vessels. MDR1 inhibitors did not appear to increase drug uptake in MDR1-overexpressing cells in vivo, so the authors redesigned the MDR inhibitor as a nanoparticle. In mice, inhibitor loaded in nanoparticles demonstrated bioactivity in the tumor. The combination of single-cell pharmacokinetics, intravital imaging, and drug reengineering suggests a new platform for understanding and overcoming resistance in human cancer. Eribulin mesylate was developed as a potent microtubule-targeting cytotoxic agent to treat taxane-resistant cancers, but recent clinical trials have shown that it eventually fails in many patient subpopulations for unclear reasons. To investigate its resistance mechanisms, we developed a fluorescent analog of eribulin with pharmacokinetic (PK) properties and cytotoxic activity across a human cell line panel that are sufficiently similar to the parent drug to study its cellular PK and tissue distribution. Using intravital imaging and automated tracking of cellular dynamics, we found that resistance to eribulin and the fluorescent analog depended directly on the multidrug resistance protein 1 (MDR1). Intravital imaging allowed for real-time analysis of in vivo PK in tumors that were engineered to be spatially heterogeneous for taxane resistance, whereby an MDR1-mApple fusion protein distinguished resistant cells fluorescently. In vivo, MDR1-mediated drug efflux and the three-dimensional tumor vascular architecture were discovered to be critical determinants of drug accumulation in tumor cells. We furthermore show that standard intravenous administration of a third-generation MDR1 inhibitor, HM30181, failed to rescue drug accumulation; however, the same MDR1 inhibitor encapsulated within a nanoparticle delivery system reversed the multidrug-resistant phenotype and potentiated the eribulin effect in vitro and in vivo in mice. Our work demonstrates that in vivo assessment of cellular PK of an anticancer drug is a powerful strategy for elucidating mechanisms of drug resistance in heterogeneous tumors and evaluating strategies to overcome this resistance.

Red Si–rhodamine drug conjugates enable imaging in GFP cells

Here we evaluated a series of Si-derivatized rhodamine (SiR) dyes for their ability to visualize a model drug in live cells. We show that a charge neutral SiR derivative (but not others) can indeed be used to follow the intracellular location of the model therapeutic drug in GFP cells.

Quantitating drug-target engagement in single cells in vitro and in vivo

Quantitation of drug target engagement in single cells has proven to be difficult, often leaving unanswered questions in the drug development process. We found that intracellular target engagement of unlabeled new therapeutics can be quantitated using polarized microscopy combined with competitive binding of matched fluorescent companion imaging probes. We quantitated the dynamics of target engagement of covalent BTK inhibitors, as well as reversible PARP inhibitors, in populations of single cells using a single companion imaging probe for each target. We then determined average in vivo tumor concentrations and found marked population heterogeneity following systemic delivery, revealing single cells with low target occupancy at high average target engagement in vivo.

Concise and diversity-oriented synthesis of novel scaffolds embedded with privileged benzopyran motif

A branching DOS strategy for an unbiased natural product-like library with embedded privileged benzopyran motif was established to provide complexity and diversity of resulting heterocycles with desired drug-likeness. The importance of skeletal diversity conducted on a privileged substructure was demonstrated through the biological evaluation of a small molecule library representing 22 unique core skeletons via in vitro cytotoxicity assay.

Single cell imaging of Bruton's Tyrosine Kinase using an irreversible inhibitor

A number of Brutons tyrosine kinase (BTK) inhibitors are currently in development, yet it has been difficult to visualize BTK expression and pharmacological inhibition in vivo in real time. We synthesized a fluorescent, irreversible BTK binder based on the drug Ibrutinib and characterized its behavior in cells and in vivo. We show a 200 nM affinity of the imaging agent, high selectivity, and irreversible binding to its target following initial washout, resulting in surprisingly high target-to-background ratios. In vivo, the imaging agent rapidly distributed to BTK expressing tumor cells, but also to BTK-positive tumor-associated host cells.