Phototruncation cell tracking with near-infrared photoimmunotherapy using heptamethine cyanine dye to visualise migratory dynamics of immune cells

BACKGROUND

Noninvasive in vivo cell tracking is valuable in understanding the mechanisms that enhance anti-cancer immunity. We have recently developed a new method called phototruncation-assisted cell tracking (PACT), that uses photoconvertible cell tracking technology to detect in vivo cell migration. This method has the advantages of not requiring genetic engineering of cells and employing tissue-penetrant near-infrared light.

METHODS

We applied PACT to monitor the migration of immune cells between a tumour and its tumour-draining lymph node (TDLN) after near-infrared photoimmunotherapy (NIR-PIT).

FINDINGS

PACT showed a significant increase in the migration of dendritic cells (DCs) and macrophages from the tumour to the TDLN immediately after NIR-PIT. This migration by NIR-PIT was abrogated by inhibiting the sphingosine-1-phosphate pathway or Gαi signaling. These results were corroborated by intranodal immune cell profiles at two days post-treatment; NIR-PIT significantly induced DC maturation and increased and activated the CD8+ T cell population in the TDLN. Furthermore, PACT revealed that NIR-PIT significantly enhanced the migration of CD8+ T cells from the TDLN to the tumour four days post-treatment, which was consistent with the immunohistochemical assessment of tumour-infiltrating lymphocytes and tumour regression.

INTERPRETATION

Immune cells dramatically migrated between the tumour and TDLN following NIR-PIT, indicating its potential as an immune-stimulating therapy. Also, PACT is potentially applicable to a wide range of immunological research.

FUNDING

This work was supported by the Intramural Research Program of the National Institutes of Health, National Cancer Institute, Centre for Cancer Research (grant number: ZIA BC011513 and ZIA BC011506).

Formation of Trideuteromethylated Artifacts of Pyrrole-Containing Natural Products

Pyrrole-containing natural products form a large group of structurally diverse compounds that occur in both terrestrial and marine organisms. In the present study the formation of trideuteromethylated artifacts of pyrrole-containing natural products was investigated, focusing on the discorhabdins. Three deuterated discorhabdins, 1, 3, and 5, were identified to be isolation procedure artifacts caused by the presence of DMSO-d6 during NMR sample preparation and handling. Three additional semisynthetic derivatives, 7-9, were made during the investigation of the mechanism of formation, which was shown to be driven by trideuteromethyl radicals in the presence of water, methanol, TFA, and traces of iron in the deuterated solvent. Generation of trideuteromethylated artifacts was also confirmed for other classes of pyrrole-containing metabolites, namely, makaluvamines, tambjamines, and dibromotryptamines, which had also been dissolved in DMSO-d6 during the structure elucidation process. Semisynthetic discorhabdins were assessed for antiproliferative activity against a panel of human tumor cell lines, and 14-trideuteromethyldiscorhabdin L (3) averaged low micromolar potency.

Autonomous System for Tumor Resection (ASTR) - Dual-Arm Robotic Midline Partial Glossectomy

Head and neck cancers are the seventh most common cancers worldwide, with squamous cell carcinoma being the most prevalent histologic subtype. Surgical resection is a primary treatment modality for many patients with head and neck squamous cell carcinoma, and accurately identifying tumor boundaries and ensuring sufficient resection margins are critical for optimizing oncologic outcomes. This study presents an innovative autonomous system for tumor resection (ASTR) and conducts a feasibility study by performing supervised autonomous midline partial glossectomy for pseudotumor with millimeter accuracy. The proposed ASTR system consists of a dual-camera vision system, an electrosurgical instrument, a newly developed vacuum grasping instrument, two 6-DOF manipulators, and a novel autonomous control system. The letter introduces an ontology-based research framework for creating and implementing a complex autonomous surgical workflow, using the glossectomy as a case study. Porcine tongue tissues are used in this study, and marked using color inks and near-infrared fluorescent (NIRF) markers to indicate the pseudotumor. ASTR actively monitors the NIRF markers and gathers spatial and color data from the samples, enabling planning and execution of robot trajectories in accordance with the proposed glossectomy workflow. The system successfully performs six consecutive supervised autonomous pseudotumor resections on porcine specimens. The average surface and depth resection errors measure 0.73±0.60 mm and 1.89±0.54 mm, respectively, with no positive tumor margins detected in any of the six resections. The resection accuracy is demonstrated to be on par with manual pseudotumor glossectomy performed by an experienced otolaryngologist.

Engineering CD276/B7-H3-targeted antibody-drug conjugates with enhanced cancer-eradicating capability

CD276/B7-H3 represents a promising target for cancer therapy based on widespread overexpression in both cancer cells and tumor-associated stroma. In previous preclinical studies, CD276 antibody-drug conjugates (ADCs) exploiting a talirine-type pyrrolobenzodiazepine (PBD) payload showed potent activity against various solid tumors but with a narrow therapeutic index and dosing regimen higher than that tolerated in clinical trials using other antibody-talirine conjugates. Here, we describe the development of a modified talirine PBD-based fully human CD276 ADC, called m276-SL-PBD, that is cross-species (human/mouse) reactive and can eradicate large 500-1,000-mm3 triple-negative breast cancer xenografts at doses 10- to 40-fold lower than the maximum tolerated dose. By combining CD276 targeting with judicious genetic and chemical ADC engineering, improved ADC purification, and payload sensitivity screening, these studies demonstrate that the therapeutic index of ADCs can be substantially increased, providing an advanced ADC development platform for potent and selective targeting of multiple solid tumor types.

Bluer Phototruncation: Retro-Diels-Alder of Heptamethine Cyanine to Trimethine Cyanine through an Allene Hydroperoxide Intermediate

The photoconversion of heptamethine to pentamethine cyanines and of pentamethine to trimethine cyanines was recently reported. Here, we report mechanistic studies and initial experimental evidence for a previously unexplored 4-carbon truncation reaction that converts the simplest heptamethine cyanine to the corresponding trimethine cyanine. We propose a DFT-supported model describing a singlet oxygen (1O2)-mediated formation of an allene hydroperoxide intermediate and subsequent 4-carbon loss through a retro-Diels-Alder process. Fluorescence and mass spectrometry measurements provide evidence of this direct conversion process. This 4-carbon truncation reaction adds to a growing body of cyanine reactivity and may provide an optical tool leading to a substantial blue-shift (Δλem) of ∼200 nm.

Development of Iodinated Indocyanine Green Analogs as a Strategy for Targeted Therapy of Liver Cancer

Liver cancer is one of the leading causes of cancer-related deaths, with a significant increase in incidence worldwide. Novel therapies are needed to address this unmet clinical need. Indocyanine green (ICG) is a broadly used fluorescence-guided surgery (FGS) agent for liver tumor resection and has significant potential for conversion to a targeted therapy. Here, we report the design, synthesis, and investigation of a series of iodinated ICG analogs (I-ICG), which can be used to develop ICG-based targeted radiopharmaceutical therapy. We applied a CRISPR-based screen to identify the solute carrier transporter, OATP1B3, as a likely mechanism for ICG uptake. Our lead I-ICG compound specifically localizes to tumors in mice bearing liver cancer xenografts. This study introduces the chemistry needed to incorporate iodine onto the ICG scaffold and defines the impact of these modifications on key properties, including targeting liver cancer in vitro and in vivo.

Superior Photoprotection of Cyanine Dyes with Thio-imidazole Amino Acids

Preventing fluorophore photobleaching and unwanted blinking is crucial for single-molecule fluorescence (SMF) studies. Reductants achieve photoprotection via quenching excited triplet states, yet either require counteragents or, for popular alkyl-thiols, are limited to cyanine dye Cy3 protection. Here, we provide mechanistic and imaging results showing that the naturally occurring amino acid ergothioneine and its analogue dramatically enhance photostability for Cy3, Cy5, and their conformationally restrained congeners, providing a biocompatible universal solution for demanding fluorescence imaging.

Rejuvenating old fluorophores with new chemistry

The field of organic chemistry began with 19th century scientists identifying and then expanding upon synthetic dye molecules for textiles. In the 20th century, dye chemistry continued with the aim of developing photographic sensitizers and laser dyes. Now, in the 21st century, the rapid evolution of biological imaging techniques provides a new driving force for dye chemistry. Of the extant collection of synthetic fluorescent dyes for biological imaging, two classes reign supreme: rhodamines and cyanines. Here, we provide an overview of recent examples where modern chemistry is used to build these old-but-venerable classes of optically responsive molecules. These new synthetic methods access new fluorophores, which then enable sophisticated imaging experiments leading to new biological insights.

Doubly Strapped Zwitterionic NIR-I and NIR-II Heptamethine Cyanine Dyes for Bioconjugation and Fluorescence Imaging

Heptamethine cyanine dyes enable deep tissue fluorescence imaging in the near infrared (NIR) window. Small molecule conjugates of the benchmark dye ZW800-1 have been tested in humans. However, long-term imaging protocols using ZW800-1 conjugates are limited by their instability, primarily because the chemically labile C4'-O-aryl linker is susceptible to cleavage by biological nucleophiles. Here, we report a modular synthetic method that produces novel doubly strapped zwitterionic heptamethine cyanine dyes, including a structural analogue of ZW800-1, with greatly enhanced dye stability. NIR-I and NIR-II versions of these doubly strapped dyes can be conjugated to proteins, including monoclonal antibodies, without causing undesired fluorophore degradation or dye stacking on the protein surface. The fluorescent antibody conjugates show excellent tumor-targeting specificity in a xenograft mouse tumor model. The enhanced stability provided by doubly strapped molecular design will enable new classes of in vivo NIR fluorescence imaging experiments with possible translation to humans.

Assessment of a novel biliary-specific near-infrared fluorescent dye (BL-760) for intraoperative detection of bile ducts and biliary leaks during hepatectomy in a preclinical swine model

OBJECTIVES

Postoperative bile leakage is a common complication of hepatobiliary surgery and frequently requires procedural intervention. Bile-label 760 (BL-760), a novel near-infrared dye, has emerged as a promising tool for identifying biliary structures and leakage, owing to its rapid excretion and strong bile specificity. This study aimed to assess the intraoperative detection of biliary leakage using intravenously administered BL-760 compared with intravenous (IV) and intraductal (ID) indocyanine green (ICG).

MATERIALS AND METHODS

Laparotomy and segmental hepatectomy with vascular control were performed on two 25-30 kg pigs. ID ICG, IV ICG, and IV BL-760 were administered separately, followed by an examination of the liver parenchyma, cut liver edge, and extrahepatic bile ducts for areas of leakage. The duration of intra- and extrahepatic fluorescence detection was assessed, and the target-to-background (TBR) of the bile ducts to the liver parenchyma was quantitatively measured.

RESULTS

In Animal 1, after intraoperative BL-760 injection, three areas of leaking bile were identified within 5 min on the cut liver edge with a TBR of 2.5-3.8 that was not apparent to the naked eye. In contrast, after IV ICG administration, the background parenchymal signal and bleeding obscured the areas of bile leakage. A second dose of BL-760 demonstrated the utility of repeated injections, confirming two of the three previously visualized areas of bile leakage and revealing one previously unseen leak. In Animal 2, neither ID ICG nor IV BL-760 injections showed obvious areas of bile leakage. However, fluorescence signals were observed within the superficial intrahepatic bile ducts after both injections.

CONCLUSIONS

BL-760 enables the rapid intraoperative visualization of small biliary structures and leaks, with the benefits of fast excretion, repeatable intravenous administration, and high-fluorescence TBR in the liver parenchyma. Potential applications include the identification of bile flow in the portal plate, biliary leak or duct injury, and postoperative monitoring of drain output. A thorough assessment of the intraoperative biliary anatomy could limit the need for postoperative drain placement, a possible contributor to severe complications and postoperative bile leak.

Red-Shifted Coumarin Luciferins for Improved Bioluminescence Imaging

Multicomponent bioluminescence imaging in vivo requires an expanded collection of tissue-penetrant probes. Toward this end, we generated a new class of near-infrared (NIR) emitting coumarin luciferin analogues (CouLuc-3s). The scaffolds were easily accessed from commercially available dyes. Complementary mutant luciferases for the CouLuc-3 analogues were also identified. The brightest probes enabled sensitive imaging in vivo. The CouLuc-3 scaffolds are also orthogonal to popular bioluminescent reporters and can be used for multicomponent imaging applications. Collectively, this work showcases a new set of bioluminescent tools that can be readily implemented for multiplexed imaging in a variety of biological settings.

Combined multiphoton microscopy and somatostatin receptor type 2 imaging of pancreatic neuroendocrine tumors

Pancreatic neuroendocrine tumors (PNETs) are a rare but increasingly more prevalent cancer with heterogeneous clinical and pathological presentation. Surgery is the preferred treatment for all hormone-expressing PNETs and any PNET greater than 2 cm, but difficulties arise when tumors are multifocal, metastatic, or small in size due to lack of effective surgical localization. Existing techniques such as intraoperative ultrasound provide poor contrast and resolution, resulting in low sensitivity for such tumors. Somatostatin receptor type 2 (SSTR2) is commonly overexpressed in PNETs and presents an avenue for targeted tumor localization. SSTR2 is often used for pre-operative imaging and therapeutic treatment, with recent studies demonstrating that somatostatin receptor imaging (SRI) can be applied in radioguided surgery to aid in removal of metastatic lymph nodes and achieving negative surgical margins. However not all PNETs express SSTR2, indicating labeled SRI could benefit from using a supplemental label-free technique such as multiphoton microscopy (MPM), which has proven useful in improving the accuracy of diagnosing more common exocrine pancreatic cancers. Our work tests the suitability of combined SRI and MPM for localizing PNETs by imaging and comparing samples of PNETs and normal pancreatic tissue. Specimens were labeled with a novel SSTR2-targeted contrast agent and imaged using fluorescence microscopy, and subsequently imaged using MPM to collect four autofluorescent channels and second harmonic generation. Our results show that a combination of both SRI and MPM provides enhanced contrast and sensitivity for localizing diseased tissue, suggesting that this approach could be a valuable clinical tool for surgical localization and treatment of PNETs.

Photochemical Dimerization of Plakinidine B Leads to Potent Inhibition of the E3 Ubiquitin-Protein Ligase CBL-B

A new dimeric alkaloid plakoramine A [(±)-1] was identified from a marine sponge Plakortis sp. Chiral-phase HPLC separation of (±)-1 led to the purified enantiomers (+)-1 and (-)-1 which both potently inhibited CBL-B E3 ubiquitin ligase activities. The absolute configurations of the enantiomers were determined by quantum chemical calculations. Scrutinization of the purification conditions revealed a previously undescribed, nonenzymatic route to form (±)-1 via photochemical conversion of its naturally occurring monomeric counterpart, plakinidine B (2).

Modified norcyanines enable ratiometric pH imaging beyond 1000 nm

Activatable fluorophores with emission beyond 1000 nm have the potential to enable high contrast imaging in complex in vivo settings. However, there are few scaffolds that can be applied to this challenge. Here we detail the synthesis and evaluation of benzo[c,d]indole-substituted norcyanines that enable pH responsive fluorescence imaging in the long wavelength (>1150 nm) range. A key component of our molecular design is the installation of a hydrophilic substituted quaternary amine in the central dihydropyridine ring system. A compound with a C4'-phenyl substituent, but not the C4'-protio homologue, exhibits absorbance maxima of 740 nm and 1130 nm in basic and acidic media, respectively, with evidence of J-aggregate-like properties. These two distinct absorbances enabled ratiometric imaging of probe internalization in a tumor model. Overall, these studies provide a new class of activatable long-wavelength responsive fluorophores with promising photophysical properties.

High-Contrast Detection of Somatostatin Receptor Subtype-2 for Fluorescence-Guided Surgery

Dye design can influence the ability of fluorescently labeled imaging agents to generate tumor contrast and has become an area of significant interest in the field of fluorescence-guided surgery (FGS). Here, we show that the charge-balanced near-infrared fluorescent (NIRF) dye FNIR-Tag enhances the imaging properties of a fluorescently labeled somatostatin analogue. In vitro studies showed that the optimized fluorescent conjugate MMC(FNIR-Tag)-TOC bound primarily via somatostatin receptor subtype-2 (SSTR2), whereas its negatively charged counterpart with IRDye 800CW had higher off-target binding. NIRF imaging in cell line- and patient-derived xenograft models revealed markedly higher tumor contrast with MMC(FNIR-Tag)-TOC, which was attributed to increased tumor specificity. Ex vivo staining of surgical biospecimens from primary and metastatic tumors, as well as involved lymph nodes, demonstrated binding to human tumors. Finally, in an orthotopic tumor model, a simulated clinical workflow highlighted our unique ability to use standard preoperative nuclear imaging for selecting patients likely to benefit from SSTR2-targeted FGS. Our findings demonstrate the translational potential of MMC(FNIR-Tag)-TOC for intraoperative imaging and suggest broad utility for using FNIR-Tag in fluorescent probe development.

Structure-Activity Relationships of Antibody-Drug Conjugates: A Systematic Review of Chemistry on the Trastuzumab Scaffold

Antibody-drug conjugates (ADCs) are a rapidly growing class of cancer therapeutics that seek to overcome the low therapeutic index of conventional cytotoxic agents. However, realizing this goal has been a significant challenge. ADCs comprise several independently modifiable components, including the antibody, payload, linker, and bioconjugation method. Many approaches have been developed to improve the physical properties, potency, and selectivity of ADCs. The anti-HER-2 antibody trastuzumab, first approved in 1998, has emerged as an exceptional targeting agent for ADCs, as well as a broadly used platform for testing new technologies. The extensive work in this area enables the comparison of various linker strategies, payloads, drug-to-antibody ratios (DAR), and mode of attachment. In this review, these conjugates, ranging from the first clinically approved trastuzumab ADC, ado-trastuzumab emtansine (Kadcyla), to the latest variants are described with the goal of providing a broad overview, as well as enabling the comparison of existing and emerging conjugate technologies.

Cyanine Phototruncation Enables Spatiotemporal Cell Labeling

Photoconvertible tracking strategies assess the dynamic migration of cell populations. Here we develop phototruncation-assisted cell tracking (PACT) and apply it to evaluate the migration of immune cells into tumor-draining lymphatics. This method is enabled by a recently discovered cyanine photoconversion reaction that leads to the two-carbon truncation and consequent blue-shift of these commonly used probes. By examining substituent effects on the heptamethine cyanine chromophore, we find that introduction of a single methoxy group increases the yield of the phototruncation reaction in neutral buffer by almost 8-fold. When converted to a membrane-bound cell-tracking variant, this probe can be applied in a series of in vitro and in vivo experiments. These include quantitative, time-dependent measurements of the migration of immune cells from tumors to tumor-draining lymph nodes. Unlike previously reported cellular photoconversion approaches, this method does not require genetic engineering and uses near-infrared (NIR) wavelengths. Overall, PACT provides a straightforward approach to label cell populations with spatiotemporal control.

Isotropic three-dimensional dual-color super-resolution microscopy with metal-induced energy transfer

Over the past two decades, super-resolution microscopy has seen a tremendous development in speed and resolution, but for most of its methods, there exists a remarkable gap between lateral and axial resolution, which is by a factor of 2 to 3 worse. One recently developed method to close this gap is metal-induced energy transfer (MIET) imaging, which achieves an axial resolution down to nanometers. It exploits the distance-dependent quenching of fluorescence when a fluorescent molecule is brought close to a metal surface. In the present manuscript, we combine the extreme axial resolution of MIET imaging with the extraordinary lateral resolution of single-molecule localization microscopy, in particular with direct stochastic optical reconstruction microscopy (dSTORM). This combination allows us to achieve isotropic three-dimensional super-resolution imaging of subcellular structures. Moreover, we used spectral demixing for implementing dual-color MIET-dSTORM that allows us to image and colocalize, in three dimensions, two different cellular structures simultaneously.

Cyanine Masking: A Strategy to Test Functional Group Effects on Antibody Conjugate Targeting

Conjugates of small molecules and antibodies are broadly employed diagnostic and therapeutic agents. Appending a small molecule to an antibody often significantly impacts the properties of the resulting conjugate. Here, we detail a systematic study investigating the effect of various functional groups on the properties of antibody-fluorophore conjugates. This was done through the preparation and analysis of a series of masked heptamethine cyanines (CyMasks)-bearing amides with varied functional groups. These were designed to exhibit a broad range of physical properties, and include hydrophobic (-NMe2), pegylated (NH-PEG-8 or NH-PEG-24), cationic (NH-(CH2)2NMe3+), anionic (NH-(CH2)2SO3-), and zwitterionic (N-(CH2)2NMe3+)-(CH2)3SO3-) variants. The CyMask series was appended to monoclonal antibodies (mAbs) and analyzed for the effects on tumor targeting, clearance, and non-specific organ uptake. Among the series, zwitterionic and pegylated dye conjugates had the highest tumor-to-background ratio (TBR) and a low liver-to-background ratio. By contrast, the cationic and zwitterionic probes had high tumor signal and high TBR, although the latter also exhibited an elevated liver-to-background ratio (LBR). Overall, these studies provide a strategy to test the functional group effects and suggest that zwitterionic substituents possess an optimal combination of high tumor signal, TBR, and low LBR. These results suggest an appealing strategy to mask hydrophobic payloads, with the potential to improve the properties of bioconjugates in vivo.

Ketone Incorporation Extends the Emission Properties of the Xanthene Scaffold Beyond 1000 nm

Imaging in the shortwave-infrared region (SWIR, λ = 1000-2500 nm) has the potential to enable deep tissue imaging with high resolution. Critical to the development of these methods is the identification of low molecular weight, biologically compatible fluorescent probes that emit beyond 1000 nm. Exchanging the bridging oxygen atom on the xanthene scaffold (C10' position) with electron withdrawing groups has been shown to lead to significant redshifts in absorbance and emission. Guided by quantum chemistry computational modeling studies, we investigated the installation of a ketone bridge at the C10' position. This simple modification extends the absorbance maxima to 860 nm and the emission beyond 1000 nm, albeit with reduced photon output. Overall, these studies demonstrate that broadly applied xanthene dyes can be extended into the SWIR range.

Targeted Fluorogenic Cyanine Carbamates Enable In Vivo Analysis of Antibody-Drug Conjugate Linker Chemistry

Antibody-drug conjugates (ADCs) are a rapidly emerging therapeutic platform. The chemical linker between the antibody and the drug payload plays an essential role in the efficacy and tolerability of these agents. New methods that quantitatively assess the cleavage efficiency in complex tissue settings could provide valuable insights into the ADC design process. Here we report the development of a near-infrared (NIR) optical imaging approach that measures the site and extent of linker cleavage in mouse models. This approach is enabled by a superior variant of our recently devised cyanine carbamate (CyBam) platform. We identify a novel tertiary amine-containing norcyanine, the product of CyBam cleavage, that exhibits a dramatically increased cellular signal due to an improved cellular permeability and lysosomal accumulation. The resulting cyanine lysosome-targeting carbamates (CyLBams) are ∼50× brighter in cells, and we find this strategy is essential for high-contrast in vivo targeted imaging. Finally, we compare a panel of several common ADC linkers across two antibodies and tumor models. These studies indicate that cathepsin-cleavable linkers provide dramatically higher tumor activation relative to hindered or nonhindered disulfides, an observation that is only apparent with in vivo imaging. This strategy enables quantitative comparisons of cleavable linker chemistries in complex tissue settings with implications across the drug delivery landscape.

Targetable Conformationally Restricted Cyanines Enable Photon-Count-Limited Applications*

Cyanine dyes are exceptionally useful probes for a range of fluorescence-based applications, but their photon output can be limited by trans-to-cis photoisomerization. We recently demonstrated that appending a ring system to the pentamethine cyanine ring system improves the quantum yield and extends the fluorescence lifetime. Here, we report an optimized synthesis of persulfonated variants that enable efficient labeling of nucleic acids and proteins. We demonstrate that a bifunctional sulfonated tertiary amide significantly improves the optical properties of the resulting bioconjugates. These new conformationally restricted cyanines are compared to the parent cyanine derivatives in a range of contexts. These include their use in the plasmonic hotspot of a DNA-nanoantenna, in single-molecule Förster-resonance energy transfer (FRET) applications, far-red fluorescence-lifetime imaging microscopy (FLIM), and single-molecule localization microscopy (SMLM). These efforts define contexts in which eliminating cyanine isomerization provides meaningful benefits to imaging performance.

Coumarins to Cyanines: Synthesis of Hemicyanines

Near-infrared (NIR) emitting fluorophores are powerful tools for optical imaging. However, there are only a handful of broadly employed NIR-emitting scaffolds, and the synthetic methods to prepare these molecules are often problematic. Here, we describe a novel, three-step synthesis of chromene-containing hemicyanine probes exhibiting large Stokes shifts and NIR emissions. We develop a pH-activatable probe for visualizing lysosomal trafficking of mAb conjugates. These studies provide a concise approach to hemicyanines with promising properties.

Coumarin luciferins and mutant luciferases for robust multi-component bioluminescence imaging

Multi-component bioluminescence imaging requires an expanded collection of luciferase-luciferin pairs that emit far-red or near-infrared light. Toward this end, we prepared a new class of luciferins based on a red-shifted coumarin scaffold. These probes (CouLuc-1s) were accessed in a two-step sequence via direct modification of commercial dyes. The bioluminescent properties of the CouLuc-1 analogs were also characterized, and complementary luciferase enzymes were identified using a two-pronged screening strategy. The optimized enzyme-substrate pairs displayed robust photon outputs and emitted a significant portion of near-infrared light. The CouLuc-1 scaffolds are also structurally distinct from existing probes, enabling rapid multi-component imaging. Collectively, this work provides novel bioluminescent tools along with a blueprint for crafting additional fluorophore-derived probes for multiplexed imaging.

Laparoscopic cholecystectomy in a swine model using a novel near-infrared fluorescent IV dye (BL-760)

BACKGROUND AND OBJECTIVES

Bile duct injury during laparoscopic cholecystectomy has an incidence rate of 1%-2% and commonly appears under conditions of severe inflammation, adhesion, or unexpected anatomical variations. Despite the difficulties and rising concerns of identifying bile duct during surgeries, surgeons do not have a specific modality to identify bile duct except intraoperative cholangiography. While no biliary-specific fluorescent dye exists for clinical use, our team has previously described the development of a preclinical biliary-specific dye, BL-760. Here, we present our study of laparoscopic cholecystectomy using the fluorescent dye in a swine model.

STUDY DESIGN/MATERIALS AND METHODS

With an approval from Institutional Animal Care and Use Committee, two 20-25 kg swine underwent laparoscopic abdominal surgery using a Food and Drug Administration-cleared fluorescent laparoscopic system. Images of the liver and gallbladder were taken both before and after intravenous injection of the novel fluorescent dye. The dye was dosed at 60 μg/kg and injected via the ear vein. The amount of time taken to visualize fluorescence in the biliary tract was measured. Fluorescent signal was observed after injection, and target-to-background ratio (TBR) of the biliary tract to surrounding cystic artery and liver parenchyma was measured.

RESULTS

Biliary tract visualization under fluorescent laparoscopy was achieved within 5 min after the dye injection without any adverse effects. Cystic duct and extrahepatic duct were clearly visualized and identified with TBR values of 2.19 and 2.32, respectively, whereas no fluorescent signal was detected in liver. Cystic duct and artery were successfully ligated by an endoscopic clip applier with the visual assistance of highlighted biliary tract images. Laparoscopic cholecystectomy was completed within 30 min in each case without any complications.

CONCLUSIONS

BL-760 is a novel preclinical fluorescent dye useful for intraoperative identification and visualization of biliary tract. Such fluorescent dye that is exclusively metabolized by liver and rapidly excreted into biliary tract would be beneficial for all types of hepato-biliary surgeries. With the validation of additional preclinical data, this novel dye has potential to be a valuable tool to prevent any iatrogenic biliary injuries and/or bile leaks during laparoscopic abdominal and liver surgeries.

Defining the Basis of Cyanine Phototruncation Enables a New Approach to Single-Molecule Localization Microscopy

The light-promoted conversion of extensively used cyanine dyes to blue-shifted emissive products has been observed in various contexts. However, both the underlying mechanism and the species involved in this photoconversion reaction have remained elusive. Here we report that irradiation of heptamethine cyanines provides pentamethine cyanines, which, in turn, are photoconverted to trimethine cyanines. We detail an examination of the mechanism and substrate scope of this remarkable two-carbon phototruncation reaction. Supported by computational analysis, we propose that this reaction involves a singlet oxygen-initiated multistep sequence involving a key hydroperoxycyclobutanol intermediate. Building on this mechanistic framework, we identify conditions to improve the yield of photoconversion by over an order of magnitude. We then demonstrate that cyanine phototruncation can be applied to super-resolution single-molecule localization microscopy, leading to improved spatial resolution with shorter imaging times. We anticipate these insights will help transform a common, but previously mechanistically ill-defined, chemical transformation into a valuable optical tool.

Preferential Light-Chain Labeling of Native Monoclonal Antibodies Improves the Properties of Fluorophore Conjugates

Site specific labeling methods have significant potential to enhance the properties of antibody conjugates. While studied extensively in the context of antibody-drug conjugates (ADCs), few studies have examined the impact of homogenous labeling on the properties of antibody-fluorophore conjugates (AFCs). We report the application of pentafluorophenyl (PFP) esters, which had previously been shown to be reasonably selective for K188 of the kappa light chain of human IGG antibodies, toward producing AFCs. We show that simple replacement of N-hydroxy succinimide (NHS) with PFP dramatically increases the light-chain specificity of near-infrared (NIR) AFCs. Comparing the properties of AFCs labeled using NHS and PFP-activated esters reveals that the latter exhibits reduced aggregation and improved brightness, both in vitro and in vivo. Overall, the use of PFP esters provides a remarkably simple approach to provide selectively labeled antibodies with improved properties.

Norcyanine-Carbamates Are Versatile Near-Infrared Fluorogenic Probes

Fluorogenic probes in the near-infrared (NIR) region have the potential to provide stimuli-dependent information in living organisms. Here, we describe a new class of fluorogenic probes based on the heptamethine cyanine scaffold, the most broadly used NIR chromophore. These compounds result from modification of heptamethine norcyanines with stimuli-responsive carbamate linkers. The resulting cyanine carbamates (CyBams) exhibit exceptional turn-ON ratios (∼170×) due to dual requirements for NIR emission: carbamate cleavage through 1,6-elimination and chromophore protonation. Illustrating their utility in complex in vivo settings, a γ-glutamate substituted CyBam was applied to imaging γ-glutamyl transpeptidase (GGT) activity in a metastatic model of ovarian cancer. Overall, CyBams have significant potential to extend the reach of fluorogenic strategies to intact tissue and live animal imaging applications.

Photoblueing of organic dyes can cause artifacts in super-resolution microscopy

Illumination of fluorophores can induce a loss of the ability to fluoresce, known as photobleaching. Interestingly, some fluorophores photoconvert to a blue-shifted fluorescent molecule as an intermediate on the photobleaching pathway, which can complicate multicolor fluorescence imaging, especially under the intense laser irradiation used in super-resolution fluorescence imaging. Here, we discuss the mechanisms of photoblueing of fluorophores and its impact on fluorescence imaging, and show how it can be prevented.

Unifying Mechanism for Thiol-Induced Photoswitching and Photostability of Cyanine Dyes

Cyanines (Cy3, Cy5, Cy3B) are the most utilized dyes for single-molecule fluorescence and localization-based super-resolution imaging. These modalities exploit cyanines' versatile photochemical behavior with thiols. A mechanism reconciling seemingly divergent results and enabling control over cyanine photoreactivity is however missing. Utilizing single-molecule fluorescence on Cy5 and Cy5B, transient-absorption spectroscopy, and DFT modeling on a range of cyanine dyes, herein we show that photoinduced electron transfer (PeT) from a thiolate to Cy in their triplet excited state and then triplet-to-singlet intersystem crossing in the nascent geminate radical pair are crucial steps. Next, a bifurcation occurs, yielding either back electron transfer and regeneration of ground state Cy, required for photostabilization, or Cy-thiol adduct formation, necessary for super-resolution microscopy. Cy regeneration via photoinduced thiol elimination is favored by adduct absorption spectra broadening. Elimination is also shown to occur through an acid-catalyzed reaction. Overall, our work provides a roadmap for designing fluorophores, photoswitching agents, and triplet excited state quenchers for single-molecule and super-resolution imaging.

Core remodeling leads to long wavelength fluoro-coumarins

Low molecular weight, uncharged far-red and NIR dyes would be enabling for a range of imaging applications. Rational redesign of the coumarin scaffold leads to Fluoro-Coumarins (FCs), the lowest molecular weight dyes with emission maxima beyond 700, 800, and 900 nm. FCs display large Stokes shifts and high environmental sensitivity, with a 40-fold increase in emission intensity in hydrophobic solvents. Untargeted variants exhibit selective lipid droplet and nuclear staining in live cells. Furthermore, sulfo-lipid derivatization enables active targeting to the plasma membrane. Overall, these studies report a promising platform for the development of biocompatible, context-responsive imaging agents.

Fluoro-Coumarins are a novel class of far-red and near-infrared solvent sensitive dyes of exceptionally low molecular weight.

Conformationally restrained pentamethine cyanines and use in reductive single molecule localization microscopy

Pentamethine cyanines are a class of far-red fluorophores that find extensive use in single-molecule localization microscopy (SMLM), as well as a broad range of other techniques. A drawback of this scaffold is its relatively low quantum yields, which is due to excited state deactivation via trans-to-cis chromophore isomerization. Here we describe a synthetic strategy to improve the photon output of these molecules. In the key synthetic transformation, a protected dialdehyde precursor undergoes a cascade reaction that forms a tetracyclic ring system. The resulting conformationally restrained analogs exhibit improved fluorescence quantum yield and extended fluorescence lifetimes. These properties, together with their ability to efficiently recover from hydride reduction, enable a uniquely simple form of single-molecule localization microscopy (SMLM).

A near-infrared light-mediated cleavable linker strategy using the heptamethine cyanine chromophore

Optical methods offer the potential to manipulate living biological systems with exceptional spatial and temporal control. Caging bioactive molecules with photocleavable functional groups is an important strategy that could be applied to a range of problems, including the targeted delivery of otherwise toxic therapeutics. However existing approaches that require UV or blue light are difficult to apply in organismal settings due to issues of tissue penetration and light toxicity. Photocaging groups built on the heptamethine cyanine scaffold enable the targeted delivery of bioactive molecules using near-IR light (up to 780nm) in live animal settings. Here we provide a detailed procedure demonstrating the utility of the heptamethine cyanine caging group to create a light-cleavable linker between an antibody, panitumumab, and a therapeutic small molecule in the duocarmycin class of natural products. Descriptions of the design and synthesis of the small molecule component, assembly of the antibody conjugate, in vitro analysis of uncaging, in vivo imaging, and impact on tumor progression are provided.

Impact of Cyanine Conformational Restraint in the Near-Infrared Range

Appending conformationally restraining ring systems to the cyanine chromophore creates exceptionally bright fluorophores in the visible range. Here, we report the application of this strategy in the near-infrared range through the preparation of the first restrained heptamethine indocyanine. Time-resolved absorption spectroscopy and fluorescence correlation spectroscopy verify that, unlike the corresponding parent unrestrained variant, the restrained molecule is not subject to photoisomerization. Notably, however, the room-temperature emission efficiency and the fluorescence lifetime of the restrained cyanine are not extended relative to the parent cyanine, even in viscous solvents. Thus, in contrast to prior reports, the photoisomerization of heptamethine cyanines does not contribute significantly to the excited-state chemistry of these molecules. We also find that the fluorescence lifetime of the restrained heptamethine cyanine is temperature-insensitive and significantly extended at moderately elevated temperatures relative to the parent cyanine. Finally, computational studies have been used to evaluate the impact of the conformational restraint on atomic and orbital structure across the cyanine series. These studies clarify the role of photoisomerization in the heptamethine cyanine scaffold and demonstrate the dramatic effect of restraint on the temperature sensitivity of these dyes.

Glypican-3-Specific Antibody Drug Conjugates Targeting Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the second most common cause of cancer-related death in the world. Therapeutic outcomes of HCC remain unsatisfactory, and novel treatments are urgently needed. GPC3 (glypican-3) is an emerging target for HCC, given the findings that 1) GPC3 is highly expressed in more than 70% of HCC; (2) elevated GPC3 expression is linked with poor HCC prognosis; and (3) GPC3-specific therapeutics, including immunotoxin, bispecific antibody and chimeric antigen receptor T cells. have shown promising results. Here, we postulate that GPC3 is a potential target of antibody-drug conjugates (ADCs) for treating liver cancer. To determine the payload for ADCs against liver cancer, we screened three large drug libraries (> 9,000 compounds) against HCC cell lines and found that the most potent drugs are DNA-damaging agents. Duocarmycin SA and pyrrolobenzodiazepine dimer were chosen as the payloads to construct two GPC3-specific ADCs: hYP7-DC and hYP7-PC. Both ADCs showed potency at picomolar concentrations against a panel of GPC3-positive cancer cell lines, but not GPC3 negative cell lines. To improve potency, we investigated the synergetic effect of hYP7-DC with approved drugs. Gemcitabine showed a synergetic effect with hYP7-DC in vitro and in vivo. Furthermore, single treatment of hYP7-PC induced tumor regression in multiple mouse models. Conclusion: We provide an example of an ADC targeting GPC3, suggesting a strategy for liver cancer therapy.

A Biliary Tract-Specific Near-Infrared Fluorescent Dye for Image-Guided Hepatobiliary Surgery

Despite advances, visual inspection, palpation, and intraoperative ultrasound remain the most utilized tools during surgery today. A particularly challenging issue is the identification of the biliary system due to its complex architecture partially embedded within the liver. Fluorescence guided surgical interventions, particularly using near-infrared (NIR) wavelengths, are an emerging approach for the real-time assessment of the hepatobiliary system. However, existing fluorophores, such as the FDA-approved indocyanine green (ICG), have significant limitations for rapid and selective visualization of bile duct anatomy. Here we report a novel NIR fluorophore, BL (Bile Label)-760, which is exclusively metabolized by the liver providing high signal in the biliary system shortly after intravenous administration. This molecule was identified by first screening a small set of known heptamethine cyanines including clinically utilized agents. After finding that none of these were well-suited, we then designed and tested a small series of novel dyes within a prescribed polarity range. We validated the molecule that emerged from these efforts, BL-760, through animal studies using both rodent and swine models employing a clinically applicable imaging system. In contrast to ICG, BL-760 fluorescence revealed a high target-to-background ratio (TBR) of the cystic duct relative to liver parenchyma 5 min after intravenous injection. During hepatic resection surgery, intrahepatic ducts were clearly highlighted, and bile leakage was easily detected. In conclusion, BL-760 has highly promising properties for intraoperative navigation during hepatobiliary surgery.

Elucidation of Spirodactylone, a Polycyclic Alkaloid from the Sponge Dactylia sp., and Nonenzymatic Generation from the Co-metabolite Denigrin B

Spirodactylone (1), a hexacyclic indolizidone alkaloid possessing a novel spiro ring system, was isolated from the marine sponge Dactylia sp. The structure was elucidated by extensive spectroscopic methods including application of the LR-HSQMBC NMR pulse sequence. Oxidative cyclization of denigrin B (2), an aryl-substituted 2-oxo-pyrroline derivative that was also isolated from the sponge extract, provided material identical to spirodactylone (1). This confirmed the assigned structure and provides insight into the probable biogenesis of 1.

A Nonaggregating Heptamethine Cyanine for Building Brighter Labeled Biomolecules

Heptamethine cyanines are broadly used for a range of near-infrared imaging applications. As with many fluorophores, these molecules are prone to forming nonemissive aggregates upon biomolecule conjugation. Prior work has focused on persulfonation strategies, which only partially address these issues. Here, we report a new set of peripheral substituents, short polyethylene glycol chains on the indolenine nitrogens and a substituted alkyl ether at the C4′ position, that provide exceptionally aggregation-resistant fluorophores. These symmetrical molecules are net-neutral, can be prepared in a concise sequence, and exhibit no evidence of H-aggregation even at high labeling density when appended to monoclonal antibodies or virus-like particles. The resulting fluorophore–biomolecule conjugates exhibit exceptionally bright in vitro and in vivo signals when compared to a conventional persulfonated heptamethine cyanine. Overall, these efforts provide a new class of heptamethine cyanines with significant utility for complex labeling applications.

A Cyanine Photooxidation/β-Elimination Sequence Enables Near-infrared Uncaging of Aryl Amine Payloads

Uncaging strategies that use near-infrared wavelengths can enable the highly targeted delivery of biomolecules in complex settings. Many methods, including an approach we developed using cyanine photooxidation, are limited to phenol-containing payloads. Given the critical role of amines in diverse biological processes, we sought to use cyanine photooxidation to initiate the release of aryl amines. Heptamethine cyanines substituted with an aryl amine at the C4' position undergo only inefficient release, likely due electronic factors. We then pursued the hypothesis that the carbonyl products derived from cyanine photooxidation could undergo efficient β-elimination. After examining both symmetrical and unsymmetrical scaffolds, we identify a merocyanine substituted with indolenine and coumarin heterocycles that undergoes efficient photooxidation and aniline uncaging. In total, these studies provide a new scheme-cyanine photooxidation followed by β-elimination-through which to design photocages with efficient uncaging properties.

Harnessing Natural Product Diversity for Fluorophore Discovery: Naturally Occurring Fluorescent Hydroxyanthraquinones from the Marine Crinoid Pterometra venusta

Fluorescent small molecules are important tools in many aspects of modern biology. A two-stage evaluation process involving fluorescence screening and live-cell imaging was developed to facilitate the identification of new fluorescent probes from extracts housed within the NCI Natural Products Repository. To this end, over 2000 extracts and prefractionated samples were examined, including an extract from the marine crinoid Pterometra venusta. An optically guided evaluation involving stepwise fluorescence screening and live-cell imaging was developed to enable the isolation of fluorescent natural products. These efforts resulted in the isolation of six hydroxyanthraquinone compounds, three of which are new natural products. These purified metabolites were examined for their potential as cellular imaging probes, and they demonstrate that natural product libraries can be a good source of new fluorescent agents.

Harnessing Cyanine Reactivity for Optical Imaging and Drug Delivery

Optical approaches that visualize and manipulate biological processes have transformed modern biomedical research. An enduring challenge is to translate these powerful methods into increasingly complex physiological settings. Longer wavelengths, typically in the near-infrared (NIR) range (∼650-900 nm), can enable advances in both fundamental and clinical settings; however, suitable probe molecules are needed. The pentamethine and heptamethine cyanines, led by prototypes Cy5 and Cy7, are among the most useful compounds for fluorescence-based applications, finding broad use in a range of contexts. The defining chemical feature of these molecules, and the key chromophoric element, is an odd-numbered polymethine that links two nitrogen atoms. Not only a light-harvesting functional group, the cyanine chromophore is subject to thermal and photochemical reactions that dramatically alter many properties of these molecules. This Account describes our recent studies to define and use intrinsic cyanine chromophore reactivity. The hypothesis driving this research is that novel chemistries that manipulate the cyanine chromophore can be used to address challenging problems in the areas of imaging and drug delivery. We first review reaction discovery efforts that seek to address two limitations of long-wavelength fluorophores: undesired thiol reactivity and modest fluorescence quantum yield. Heptamethine cyanines with an O-alkyl substituent at the central C4' carbon were prepared through a novel N- to O-transposition reaction. Unlike commonly used C4'-phenol variants, this new class of fluorophores is resistant to thiol modification and exhibits improved in vivo imaging properties when used as antibody tags. We have also developed a chemical strategy to enhance the quantum yield of far-red pentamethine cyanines. Using a synthetic strategy involving a cross metathesis/tetracyclization sequence, this approach conformationally restrains the pentamethine cyanine scaffold. The resulting molecules exhibit enhanced quantum yield (Φ_F = 0.69 vs Φ_F = 0.15). Furthermore, conformational restraint improves interconversion between reduced hydrocyanine and intact cyanine forms, which enables super resolution microscopy. This Account then highlights efforts to use cyanine photochemical reactivity for NIR photocaging. Our approach involves the deliberate use of cyanine photooxidation, a reaction previously only associated with photodegradation. The uncaging reaction sequence is initiated by photooxidative chromophore cleavage (using wavelengths of up to 780 nm), which prompts a C-N bond hydrolysis/cyclization sequence resulting in phenol liberation. This approach has been applied to generate the first NIR-activated antibody-drug conjugates. Tumor uptake can be monitored in vivo using NIR fluorescence, prior to uncaging with an external irradiation source. This NIR uncaging strategy can slow tumor progression and increase survival in a MDA-MB-468- luc mouse model. Broadly, the vantage point of cyanine reactivity is providing novel probe molecules with auspicious features for use in complex imaging and drug delivery settings.

Defining the conditional basis of silicon phthalocyanine near-IR ligand exchange

Bond cleavage reactions initiated by long-wavelength light are needed to extend the scope of the caged-uncaged paradigm into complex physiological settings. Axially unsymmetrical silicon phthalocyanines (SiPcs) undergo efficient release of phenol ligands in a reaction contingent on three factors - near-IR light (690 nm), hypoxia, and a thiol reductant. These studies detail efforts to define the mechanistic basis for this unique conditionally-dependent bond cleavage reaction. Spectroscopic studies provide evidence for the formation of a key phthalocyanine radical anion intermediate formed from the triplet state in a reductant-dependent manner. Computational chemistry studies indicate that phenol ligand solvolysis proceeds through a heptacoordinate silicon transition state and that this solvolytic process is favored following SiPc radical anion formation. These results provide insight regarding the central role that radical anion intermediates formed through photoinduced electron transfer with biological reductants can play in long-wavelength uncaging reactions.

Cyanine Photocages Enable Spatial Control of Inducible Cre-Mediated Recombination

Optical control over protein expression could provide a means to interrogate a range of biological processes. One approach has employed caged ligands of the estrogen receptor (ER) in combination with broadly used ligand-dependent Cre recombinase proteins. Existing approaches use UV or blue wavelengths, which hinders their application in tissue settings. Additionally, issues of payload diffusion can impede fine spatial control over the recombination process. Here, we detail the chemical optimization of a near-infrared (NIR) light-activated variant of the ER antagonist cyclofen. These studies resulted in modification of both the caging group and payload with lipophilic n-butyl esters. The appendage of esters to the cyanine cage improved cellular uptake and retention. The installation of a 4-piperidyl ester enabled high spatial resolution of the light-initiated Cre-mediated recombination event. These studies described chemical modifications with potential general utility for improving spatial control of intracellular caging strategies. Additionally, these efforts will enable future applications to use these molecules in complex physiological settings.

A chemically stable fluorescent marker of the ureter

Surgical methods guided by exogenous fluorescent markers have the potential to define tissue types in real time. Small molecule dyes with efficient and selective renal clearance could enable visualization of the ureter during surgical procedures involving the abdomen and pelvis. These studies report the design and synthesis of a water soluble, net neutral C4'-O-alkyl heptamethine cyanine, Ureter-Label (UL)-766, with excellent properties for ureter visualization. This compound is accessed through a concise synthetic sequence involving an N- to O-transposition reaction that provides other inaccessible C4'-O-alkyl heptamethine cyanines. Unlike molecules containing a C4'-O-aryl substituent, which have also been used for ureter visualization, UL-766 is not reactive towards glutathione and the cellular proteome. In addition, rat models of abdominal surgery reveal that UL-766 undergoes efficient and nearly exclusive renal clearance in vivo. In total, this molecule represents a promising candidate for visualizing the ureter during a variety of surgical interventions.

Molecularly Targeted Cancer Combination Therapy with Near-Infrared Photoimmunotherapy and Near-Infrared Photorelease with Duocarmycin-Antibody Conjugate

Near-infrared photoimmunotherapy (NIR-PIT) is a highly selective tumor treatment that uses an antibody-photoabsorber conjugate (APC). However, the effect of NIR-PIT can be enhanced when combined with other therapies. NIR photocaging groups, based on the heptamethine cyanine scaffold, have been developed to release bioactive molecules near targets after exposure to light. Here, we investigated the combination of NIR-PIT using panitumumab-IR700 (pan-IR700) and the NIR-releasing compound, CyEt-panitumumab-duocarmycin (CyEt-Pan-Duo). Both pan-IR700 and CyEt-Pan-Duo showed specific binding to the EGFR-expressing MDAMB468 cell line in vitro In in vivo studies, additional injection of CyEt-Pan-Duo immediately after NIR light exposure resulted in high tumor accumulation and high tumor-background ratio. To evaluate the effects of combination therapy in vivo, tumor-bearing mice were separated into 4 groups: (i) control, (ii NIR-PIT, (iii) NIR-release, (iv) combination of NIR-PIT and NIR-release. Tumor growth was significantly inhibited in all treatment groups compared with the control group (P < 0.05), and significantly prolonged survival was achieved (P < 0.05 vs. control). The greatest therapeutic effect was shown with NIR-PIT and NIR-release combination therapy. In conclusion, combination therapy of NIR-PIT and NIR-release enhanced the therapeutic effects compared with either NIR-PIT or NIR-release therapy alone. Mol Cancer Ther; 17(3); 661-70. ©2017 AACR.