Harnessing cyanine photooxidation: from slowing photobleaching to near-IR uncaging

Ultrabright Dibenzofluoran-Based Polymer Dots with NIR-IIa Emission Maxima and Unusual Large Stokes Shifts for 3D Rotational Stereo Imaging

Emerging organic molecules with emissions in the second near-infrared (NIR-II) region are garnering significant attention. Unfortunately, achieving accountable organic emission intensity over the NIR-IIa (1300 nm) region faces challenges due to the intrinsic energy gap law. Up to the current stage, all reported organic NIR-IIa emitters belong to polymethine-based dyes with small Stokes shifts (<50 nm) and low quantum yield (QY; ≤0.015%). However, such polymethines have proved to cause self-absorption with constrained emission brightness, limiting advanced development in deep-tissue imaging. Here a new NIR-IIa scaffold based on rigid and highly conjugated dibenzofluoran core terminated by amino-containing moieties that reveal emission peaks of 1230-1305 nm is designed. The QY is at least 10 times higher than all synthesized or reported NIR-IIa polymethines with extraordinarily large Stokes shifts of 370-446 nm. DBF-BJ is further prepared as a polymer dot to demonstrate its in vivo 3D stereo imaging of mouse vasculature with a 1400 nm long-pass filter.

Near-infrared duocarmycin photorelease from a Treg-targeted antibody-drug conjugate improves efficacy of PD-1 blockade in syngeneic murine tumor models

Regulatory T cells (Tregs) play a crucial role in mediating immunosuppression in the tumor microenvironment. Furthermore, Tregs contribute to the lack of efficacy and hyperprogressive disease upon Programmed cell death protein 1 (PD-1) blockade immunotherapy. Thus, Tregs are considered a promising therapeutic target, especially when combined with PD-1 blockade. However, systemic depletion of Tregs causes severe autoimmune adverse events, which poses a serious challenge to Treg-directed therapy. Here, we developed a novel treatment to locally and predominantly damage Tregs by near-infrared duocarmycin photorelease (NIR-DPR). In this technology, we prepared anti-CD25 F(ab')2 conjugates, which site-specifically uncage duocarmycin in CD25-expressing cells upon exposure to NIR light. In vitro, CD25-targeted NIR-DPR significantly increased apoptosis of CD25-expressing HT2-A5E cells. When tumors were irradiated with NIR light in vivo, intratumoral CD25+ Treg populations decreased and Ki-67 and Interleukin-10 expression was suppressed, indicating impaired functioning of intratumoral CD25+ Tregs. CD25-targeted NIR-DPR suppressed tumor growth and improved survival in syngeneic murine tumor models. Of note, CD25-targeted NIR-DPR synergistically enhanced the efficacy of PD-1 blockade, especially in tumors with higher CD8+/Treg PD-1 ratios. Furthermore, the combination therapy induced significant anti-cancer immunity including maturation of dendritic cells, extensive intratumoral infiltration of cytotoxic CD8+ T cells, and increased differentiation into CD8+ memory T cells. Altogether, CD25-targeted NIR-DPR locally and predominantly targets Tregs in the tumor microenvironment and synergistically improves the efficacy of PD-1 blockade, suggesting that this combination therapy can be a rational anti-cancer combination immunotherapy.

Light in a Heartbeat: Bond Scission by a Single Photon above 800 nm

Photocages enable scientists to take full control over the activity of molecules using light as a biocompatible stimulus. Their emerging applications in photoactivated therapies call for efficient uncaging in the near-infrared (NIR) window, which represents a fundamental challenge. Here, we report synthetically accessible cyanine photocages that liberate alcohol, phenol, amine, and thiol payloads upon irradiation with NIR light up to 820 nm in aqueous media. The photocages display a unique chameleon-like behavior and operate via two distinct uncaging mechanisms: photooxidation and heterolytic bond cleavage. The latter process constitutes the first example of a direct bond scission by a single photon ever observed in cyanine dyes or at wavelengths exceeding 800 nm. Modulation of the beating rates of human cardiomyocytes that we achieved by light-actuated release of adrenergic agonist etilefrine at submicromolar concentrations and low NIR light doses (∼12 J cm-2) highlights the potential of these photocages in biology and medicine.

Ratiometric G-quadruplex/hemin DNAzymes with low-dosage associative substrates

BACKGROUND

G-quadruplex (G4)/hemin DNAzymes with conversion of substrates into colorimetric readouts are well recognized as convenient biocatalysis tools in sensor development. However, the previously developed colorimetric G4/hemin DNAzymes are diffusive substrate-based DNAzymes (DSBDs). The current colorimetric DSBDs have several drawbacks including high dosage (∼mM) of diffusive substrates (DSs), colorimetric product toxicity, and single colorimetric readout without tolerance to fluctuation of experimental factors and background. In addition, the usage of high-dosage DSs can smear the G4 foldings and their discard is more harmful to environment. Therefore, exploring alternative DNAzymes with potential to overcome these drawbacks of DSBDs is urgently needed.

RESULTS

We herein developed associative substrate-based DNAzymes (ASBDs). Cyanine dyes were selected as associative substrates (ASs) due to their binding competency with G4/hemin DNAzymes. With respect to DSBDs, ASBDs needed only low dosage (∼10 μM) of ASs to be able to cause a rapid and visible substrate conversion. In addition, since cyanine dyes are NIR dyes with high extinction coefficients and their conversion products have absorption bands at shorter wavelength. Therefore, a colorimetric ratio response can be developed to follow activities of G4/hemin DNAzymes with competency to tolerate fluctuation of experimental factors and background. In particular, herein developed ASBDs can endure somewhat concentration fluctuation of H2O2. ASBDs are able to cowork with other enzymes (for example, glucose oxidase) to realize cascade sensing.

SIGNIFICANCE

The developed ASBDs can operate at low dosage of substrates with a colorimetric ratio response and can overcome the drawbacks met in DSBDs. We expect that, by designing ASs with fruitful color panel in the future, our work will inspire more interesting in developing environment-benign and low-carbon G4/hemin DNAzymes and desired colorful high-performance sensors.

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.

Bioorthogonal chemistry for prodrug activation in vivo

Prodrugs have emerged as a major strategy for addressing clinical challenges by improving drug pharmacokinetics, reducing toxicity, and enhancing treatment efficacy. The emergence of new bioorthogonal chemistry has greatly facilitated the development of prodrug strategies, enabling their activation through chemical and physical stimuli. This "on-demand" activation using bioorthogonal chemistry has revolutionized the research and development of prodrugs. Consequently, prodrug activation has garnered significant attention and emerged as an exciting field of translational research. This review summarizes the latest advancements in prodrug activation by utilizing bioorthogonal chemistry and mainly focuses on the activation of small-molecule prodrugs and antibody-drug conjugates. In addition, this review also discusses the opportunities and challenges of translating these advancements into clinical practice.

Triplet-Induced Singlet Oxygen Photobleaches Near-Infrared Dye-Sensitized Upconversion Nanosystems

The rapid photobleaching of near-infrared (NIR) dye-sensitized upconversion nanosystems is one of the crucial problems that has blocked their technological applications. Uncovering the photophysical and photochemical pathways of NIR dyes would help to elucidate the photobleaching mechanism and thereby improve the photostability of the system. Here we investigate the triplet dynamics of NIR dyes and their interaction with triplet oxygen in the typically investigated IR806-sensitized upconversion nanoparticle (UCNP) nanosystem. Low-temperature fluorescence at 77 K provides direct proof of the generation of singlet oxygen (1O2) under 808 nm laser irradiation. Mass spectrometry indicates that all three double bonds in the structure of IR806 can be broken in the photochemical process. Coupling IR806 to the surface of UCNPs can accelerate its triplet dynamics, thus producing more 1O2 to photocleave IR806. Importantly, we find that the addition of β-carotene can scavenge the generated 1O2, thereby providing a simple method to effectively inhibit photobleaching.

Method To Diversify Cyanine Chromophore Functionality Enables Improved Biomolecule Tracking and Intracellular Imaging

Heptamethine indocyanines are invaluable probes for near-infrared (NIR) imaging. Despite broad use, there are only a few synthetic methods to assemble these molecules, and each has significant limitations. Here, we report the use of pyridinium benzoxazole (PyBox) salts as heptamethine indocyanine precursors. This method is high yielding, simple to implement, and provides access to previously unknown chromophore functionality. We applied this method to create molecules to address two outstanding objectives in NIR fluorescence imaging. First, we used an iterative approach to develop molecules for protein-targeted tumor imaging. When compared to common NIR fluorophores, the optimized probe increases the tumor specificity of monoclonal antibody (mAb) and nanobody conjugates. Second, we developed cyclizing heptamethine indocyanines with the goal of improving cellular uptake and fluorogenic properties. By modifying both the electrophilic and nucleophilic components, we demonstrate that the solvent sensitivity of the ring-open/ring-closed equilibrium can be modified over a wide range. We then show that a chloroalkane derivative of a compound with tuned cyclization properties undergoes particularly efficient no-wash live cell imaging using organelle-targeted HaloTag self-labeling proteins. Overall, the chemistry reported here broadens the scope of accessible chromophore functionality, and, in turn, enables the discovery of NIR probes with promising properties for advanced imaging applications.

Rigid and Photostable Shortwave Infrared Dye Absorbing/Emitting beyond 1200 nm for High-Contrast Multiplexed Imaging

The shortwave infrared (SWIR) spectral region beyond 1200 nm offers optimal tissue penetration depth and has broad potential in diagnosis, therapy, and surgery. Here, we devised a novel class of fluorochromic scaffold, i.e., a tetra-benzannulated xanthenoid (EC7). EC7 absorbs/emits maximally at 1204/1290 nm in CH₂Cl₂ and exhibits an unparalleled molar absorptivity of 3.91 × 10⁵ cm^-1 M^-1 and high transparency to light at 400-900 nm. It also exhibited high resistance toward both photobleaching and symmetry breaking due to its unique structural rigidity. It is feasible for in vivo bioimaging and particularly suitable to couple with the shorter-wavelength analogues for high-contrast multiplexing. High-contrast dual-channel intraoperative imaging of the hepatobiliary system and three-channel in vivo imaging of the intestine, the stomach, and the vasculature were showcased. EC7 is a benchmark fluorochrome for facile biomedical exploitation of the SWIR region beyond 1200 nm.

Spin-Vibronic Control of Intersystem Crossing in Iodine-Substituted Heptamethine Cyanines

Spin-orbit coupling between electronic states of different multiplicity can be strongly coupled to molecular vibrations, and this interaction is becoming recognized as an important mechanism for controlling the course of photochemical reactions. Here, we show that the involvement of spin-vibronic coupling is essential for understanding the photophysics and photochemistry of heptamethine cyanines (Cy7), bearing iodine as a heavy atom in the C3' position of the chain and/or a 3H-indolium core, as potential triplet sensitizers and singlet oxygen producers in methanol and aqueous solutions. The sensitization efficiency was found to be an order of magnitude higher for the chain-substituted than the 3H-indolium core-substituted derivatives. Our ab initio calculations demonstrate that while all optimal structures of Cy7 are characterized by negligible spin-orbit coupling (tenths of cm^-1) with no dependence on the position of the substituent, molecular vibrations lead to its significant increase (tens of cm^-1 for the chain-substituted cyanines), which allowed us to interpret the observed position dependence.

Antibody Drug Conjugates of Near-Infrared Photoimmunotherapy (NIR-PIT) in Breast Cancers

Worldwide, the incidence rate of breast cancer is the highest in women. Approximately 2.3 million people were newly diagnosed and 0.685 million were dead of breast cancer in 2020, which continues to grow. Triple-negative breast cancer (TNBC) is the most aggressive breast cancer subtype with a higher risk of recurrence and metastasis, but disappointly, there are no effective and specific therapies clinically, especially for patients presenting with metastatic diseases. Therefore, it is urgent to develop a new type of cancer therapy for survival improvisation and adverse effects alleviation of breast cancers. Near-infrared photoimmunotherapy (NIR-PIT) is a newly developed, photochemistry-based cancer therapy. It was drive by an antibody-photoabsorber conjugate (APC) which is triggered by near-infrared light. The key part of APC is a cancer-targeting monoclonal antibody (mAb) that can bind to receptors or antigens on the surface of tumor cells. Because of this targeted conjugate accumulation, subsequent deployment of focal NIR-light results in functional damage on the targeted cell membranes without harming the immediately adjacent receptor-negative cells and evokes a kind of photochemical, speedy, and highly specific immunogenic cell death (ICD) of cancer cells with corresponding antigens. Subsequently, immature dendritic cells adjacent to dying cancer cells will become mature, further inducing a host-oriented anti-cancer immune response, complicatedly and comprehensively. Currently, NIR-PIT has progressed into phase 3 clinical trial for recurrent head and neck cancer. And preclinical studies have illustrated strong therapeutic efficacy of NIR-PIT targeting various molecular receptors overexpressed in breast cancer cells, including EGFR, HER2, CD44c, CD206, ICAM-1 and FAP-α. Thereby, NIR-PIT is in early trials, but appears to be a promising breast cancer therapy and moving into the future. Here, we present the specific advantages and discuss the most recent preclinical studies against several transmembrane proteins of NIR-PIT in breast cancers.

Near-infrared-II ratiometric fluorescence probes for non-invasive detection and precise navigation surgery of metastatic sentinel lymph nodes

Sentinel lymph node (SLN) biopsy is the key diagnostic procedure to determine tumor metastasis and treatment plan. Current SLN biopsy has considerable drawbacks in that SLNs (both malignant and normal) must be removed by navigation surgery, followed by a time-consuming pathological examination. The selective, non-invasive, and real-time diagnosis of metastatic status in SLNs is becoming essential. Methods: Here, we design two lanthanide-doped nanoparticles as a pair of NIR-II ratiometric fluorescence probes, one of which is conjugated with tumor-targeting moiety, while the other is conjugated with PEG as an internal reference. The NIR-II ratiometric fluorescence signal (I_{1060 nm}/I_{1525 nm}) from two well-separated channels were used to identify the tumor-draining SLNs. The precise navigation surgery of metastatic SLNs was performed and we further evaluated their surgery outcomes. Results: The NIR-II ratiometric fluorescence facilitates an ideal fluorescence-guided surgery with only resection of tumor-positive SLNs, thereby avoiding unnecessary removal of the normal SLNs. In addition, our system has a time-saving operation procedure and can be performed under the operation light without altering the appearance of surgical settings. Conclusion: The present study enables non-invasive and real-time detection metastatic status in SLNs with high sensitivity and selectivity. Our investigations will provide a new direction for SLN biopsy and substantially improve cancer surgery outcomes.

Repurposing Cyanine Photoinstability To Develop Near-Infrared Light-Activatable Nanogels for In Vivo Cargo Delivery

The favorable properties of cyanines (e.g., near-infrared (NIR) absorbance and emission) have made this class of dyes popular for a wide variety of biomedical applications. However, many cyanines are prone to rapid photobleaching when irradiated with light. In this study, we have exploited this undesirable trait to develop NIR-nanogels for NIR light-mediated cargo delivery. NIR-nanogels feature a photolabile cyanine cross-linker (Cy780-Acryl) that can cleave via dioxetane chemistry when irradiated. This photochemical process results in the formation of two carbonyl fragments and concomitant NIR-nanogel degradation to facilitate cargo release. In contrast to studies where cyanines are utilized as photocages, our approach does not require direct chemical attachment to the cargo, thus expanding our ability to deliver molecules that cannot be covalently modified. We showcase this feature by encapsulating a palette of small-molecule chemotherapeutics that feature a structurally diverse chemical architecture. To demonstrate site-selective release in vivo, we generated a murine model of breast cancer. Relative to nonlight irradiated and drug-free controls, treatment with NIR-nanogels loaded with paclitaxel (a potent cytotoxic agent) and NIR light resulted in significant attenuation of tumor growth. Moreover, we show via histological staining of the vital organs that minimal off-target effects are observed.

Assessment of Photoreleasable Linkers and Light-Capturing Antennas on a Photoresponsive Cobalamin Scaffold

Cobalamin has shown promise as a light-sensitive drug delivery platform owing to its ease of modification and the high quantum yields for drug photorelease. However, studies to date on the general photochemistry of alkyl cobalamins have primarily focused on methyl and adenosyl-substituted derivatives, the natural cofactors present in various enzymatic species. We describe the synthesis and photolytic behavior of cobalamin conjugates comprised of different combinations of fluorophores and β-axial ligands. In general, cobalamin conjugates containing β-axial alkyl substituents undergo efficient photolysis under aqueous conditions, with quantum yields up to >40%. However, substituents that are large and hydrophobic, or unable to readily support the presumed radical intermediate, suffer less efficient photolysis (<15%) than smaller, water-soluble, analogs. By contrast, quantum yields improve by 2-fold in DMF for cobalamins containing large hydrophobic β-axial substituents. This suggests that drug release from carriers comprised of membranous compartments, such as liposomes, may be significantly more efficient than the corresponding photorelease in an aqueous environment. Finally, we explored the impact of fluorophores on the photolysis of alkyl cobalamins under tissue-mimetic conditions. Cobalamins substituted with efficient photon-capturing fluorophores display up to 4-fold enhancements in photolysis relative to unsubstituted derivatives. In summary, we have shown that the photosensitivity of alkyl cobalamin conjugates can be tuned by altering the Co-appended alkyl moiety, modulating the polarity of the environment (solvent), and installing photon-capturing fluorophores onto the cobalamin framework.

On the photostability and luminescence of dye-sensitized upconverting nanoparticles using modified IR820 dyes

Dye sensitization is a promising route to enhance luminescence from lanthanide-doped upconverting nanoparticles (LnUCNPs) by improving the photon harvesting capability of LnUCNPs through the use of dye molecules, characterized by higher absorption coefficients. The literature does not fully address the poor photostability of NIR dyes, hindering solution-based applications. The improvements achieved by dye-sensitized LnUCNPs are usually obtained by comparison with non-dye sensitized LnUCNPs. This comparison results in exciting the LnUCNPs at different wavelengths with respect to the dye-sensitized LnUCNPs or at the same wavelengths, where, however, the non-dye sensitized LnUCNPs do not absorb. Both these comparisons are hardly conclusive for a quantification of the improvements achieved by dye-sensitization. Both shortcomings were addressed by studying the photodegradation via thorough spectroscopic evaluations of a 4-nitrothiophenol-modified and unmodified IR820-LnUCNP system. The modified IR820 dye system exhibits a 200% enhancement in the emission of NaGdF₄:Er³⁺,Yb³⁺/NaGdF₄:Yb³⁺ nanoparticles relative to the unmodified IR820-sensitized LnUCNPs and emits for over twice the duration, demonstrating a substantial improvement over previous dye-LnUCNP systems. Upconversion dynamics between the dyes and Er³⁺ establish the importance of back-transfer dynamics in modulating the dye-LnUCNP luminescence. Quantum yield measurements further illustrate the mechanism of sensitization and increased efficiency of this new nanosystem.

Shining Light on Cyclopentadienone-Norbornadiene Diels-Alder Adducts to Enable Photoinduced Click Chemistry with Cyclopentadiene

A new Diels-Alder (DA)-based photopatterning platform is presented, which exploits the irreversible, light-induced decarbonylation and subsequent cleavage of cyclopentadienone-norbornadiene (CPD-NBD) adducts. A series of CPD-NBD adducts have been prepared and systematically studied toward the use in a polymeric material photopatterning platform. By incorporating an optimized CPD-NBD adduct into polymer networks, it is demonstrated that cyclopentadiene may be unveiled upon 365 nm irradiation and subsequently clicked to a variety of maleimides with spatial control under mild reaction conditions and with fast kinetics. Unlike currently available photoinduced Diels-Alder reactions that rely on trapping transient, photocaged dienes, this platform introduces a persistent, yet highly reactive diene after irradiation, enabling the use of photosensitive species such as cyanine dyes to be patterned. To highlight the potential use of this platform in a variety of material applications, we demonstrate two proof-of-concepts: patterned conjugation of multiple dyes into a polyacrylate network and preprogrammed ligation of streptavidin into poly(ethylene glycol) hydrogels.

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.

Harnessing Hypoxia-Dependent Cyanine Photocages for In Vivo Precision Drug Release

Photocaging holds promise for the precise manipulation of biological events in space and time. However, current near-infrared (NIR) photocages are oxygen-dependent for their photolysis and lack of timely feedback regulation, which has proven to be the major bottleneck for targeted therapy. Herein, we present a hypoxia-dependent photo-activation mechanism of dialkylamine-substituted cyanine (Cy-NH) accompanied by emissive fragments generation, which was validated with retrosynthesis and spectral analysis. For the first time, we have realized the orthogonal manipulation of this hypoxia-dependent photocaging and dual-modal optical signals in living cells and tumor-bearing mice, making a breakthrough in the direct spatiotemporal control and in vivo feedback regulation. This unique photoactivation mechanism overcomes the limitation of hypoxia, which allows site-specific remote control for targeted therapy, and expands the photo-trigger toolbox for on-demand drug release, especially in a physiological context with dual-mode optical imaging under hypoxia.

Interrogating biological systems using visible-light-powered catalysis

Light-powered catalysis has found broad utility as a chemical transformation strategy, with widespread impact on energy, environment, drug discovery and human health. A noteworthy application impacting human health is light-induced sensitization of cofactors for photodynamic therapy in cancer treatment. The clinical adoption of this photosensitization approach has inspired the search for other photochemical methods, such as photoredox catalysis, to influence biological discovery. Over the past decade, light-mediated catalysis has enabled the discovery of valuable synthetic transformations, propelling it to become a highly utilized chemical synthesis strategy. The reaction components required to achieve a photoredox reaction are identical to photosensitization (catalyst, light source and substrate), making it ideally suited for probing biological environments. In this Review, we discuss the therapeutic application of photosensitization and advancements made in developing next-generation catalysts. We then highlight emerging uses of photoredox catalytic methods for protein bioconjugation and probing complex cellular environments in living cells.

Nanoplatforms for Targeted Stimuli-Responsive Drug Delivery: A Review of Platform Materials and Stimuli-Responsive Release and Targeting Mechanisms

To achieve the promise of stimuli-responsive drug delivery systems for the treatment of cancer, they should (1) avoid premature clearance; (2) accumulate in tumors and undergo endocytosis by cancer cells; and (3) exhibit appropriate stimuli-responsive release of the payload. It is challenging to address all of these requirements simultaneously. However, the numerous proof-of-concept studies addressing one or more of these requirements reported every year have dramatically expanded the toolbox available for the design of drug delivery systems. This review highlights recent advances in the targeting and stimuli-responsiveness of drug delivery systems. It begins with a discussion of nanocarrier types and an overview of the factors influencing nanocarrier biodistribution. On-demand release strategies and their application to each type of nanocarrier are reviewed, including both endogenous and exogenous stimuli. Recent developments in stimuli-responsive targeting strategies are also discussed. The remaining challenges and prospective solutions in the field are discussed throughout the review, which is intended to assist researchers in overcoming interdisciplinary knowledge barriers and increase the speed of development. This review presents a nanocarrier-based drug delivery systems toolbox that enables the application of techniques across platforms and inspires researchers with interdisciplinary information to boost the development of multifunctional therapeutic nanoplatforms for cancer therapy.

Taking phototherapeutics from concept to clinical launch

More than four decades have passed since the first example of a light-activated (caged) compound was described. In the intervening years, a large number of light-responsive derivatives have been reported, several of which have found utility under a variety of in vitro conditions using cells and tissues. Light-triggered bioactivity furnishes spatial and temporal control, and offers the possibility of precision dosing and orthogonal communication with different biomolecules. These inherent attributes of light have been advocated as advantageous for the delivery and/or activation of drugs at diseased sites for a variety of indications. However, the tissue penetrance of light is profoundly wavelength-dependent. Only recently have phototherapeutics that are photoresponsive in the optical window of tissue (600-900 nm) been described. This Review highlights these recent discoveries, along with their limitations and clinical opportunities. In addition, we describe preliminary in vivo studies of prospective phototherapeutics, with an emphasis on the path that remains to be navigated in order to translate light-activated drugs into clinically useful therapeutics. Finally, the unique attributes of phototherapeutics is highlighted by discussing several potential disease applications.

Cyanine Nanocage Activated by Near-IR Light for the Targeted Delivery of Cyclosporine A to Traumatic Brain Injury Sites

More than 2.8 million annually in the United States are afflicted with some form of traumatic brain injury (TBI), where 75% of victims have a mild form of TBI (MTBI). TBI risk is higher for individuals engaging in physical activities or involved in accidents. Although MTBI may not be initially life-threatening, a large number of these victims can develop cognitive and physical dysfunctions. These late clinical sequelae have been attributed to the development of secondary injuries that can occur minutes to days after the initial impact. To minimize brain damage from TBI, it is critical to diagnose and treat patients within the first or "golden" hour after TBI. Although it would be very helpful to quickly determine the TBI locations in the brain and direct the treatment selectively to the affected sites, this remains a challenge. Herein, we disclose our novel strategy to target cyclosporine A (CsA) into TBI sites, without the need to locate the exact location of the TBI lesion. Our approach is based on TBI treatment with a cyanine dye nanocage attached to CsA, a known therapeutic agent for TBI that is associated with unacceptable toxicities. In its caged form, CsA remains inactive, while after near-IR light photoactivation, the resulting fragmentation of the cyanine nanocage leads to the selective release of CsA at the TBI sites.

A Red Light-Triggered Drug Release System Based on One-Photon Upconversion-Like Photolysis

Photoresponsive drug release systems can enhance drug accumulation at the sites where light is applied. Nowadays, the photocleavable groups used in the systems usually require ultraviolet or blue light irradiation, which limits tissue penetration depth and is harmful to normal cells and living bodies. A one-photon upconversion-like photolysis strategy, which can cleave green light-activatable prodrugs with red light at the presence of a red light-excitable photosensitizer in organic solvents, is developed. However, both the prodrug and photosensitizer are hydrophobic and their energy transfer process is sensitive to oxygen molecules. Here, a simple strategy to address these problems by loading the two components in biocompatible and biodegradable polymeric micelles, is presented. The developed low-irradiance red light-triggered drug release system has a size around 40 nm and exhibits good stability in aqueous solutions. The micellar encapsulation protects the photolysis reaction from oxygen quenching in normoxia aqueous solutions. The therapeutic effect of the system enhanced by the redlight irradiation is demonstrated through in vitro and in vivo studies, indicating promising potential in cancer therapy. The study provides the first example and also an important reference for applying one-photon upconversion-like photolysis in biomedical applications.

Cyanine Nanocages Activated by Near-Infrared Light for the Targeted Treatment of Traumatic Brain Injury

Traumatic brain injury (TBI) is a common and prevalent condition that affects large numbers of people across a range of ages. Individuals engaging in physical activities and victims of accidents are at a higher risk for TBI. There is a lack of available treatment specifically for TBI. Given the difficulty to determine its precise location in the brain, TBI remains difficult to fully diagnose or treat. Herein, we disclose a novel strategy for directing therapeutic agents to TBI sites, without the need to determine the precise location of the TBI activity in the brain. This novel approach is based on the use of a cyanine dye nanocage carrying Gabapentin, a known TBI therapeutic agent. Upon exposure of the cyanine nanocage to near-infrared light, the local release of Gabapentin is triggered, selectively at the TBI-affected site.

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.

Photobleaching of organic fluorophores: quantitative characterization, mechanisms, protection

Photochemical stability is one of the most important parameters that determine the usefulness of organic dyes in different applications. This Review addresses key factors that determine the dye photostability. It is shown that photodegradation can follow different oxygen-dependent and oxygen-independent mechanisms and may involve both ¹S₁-³T₁ and higher-energy ¹S_n-³T_n excited states. Their involvement and contribution depends on dye structure, medium conditions, irradiation power. Fluorescein, rhodamine, BODIPY and cyanine dyes, as well as conjugated polymers are discussed as selected examples illustrating photobleaching mechanisms. The strategies for modulating and improving the photostability are overviewed. They include the improvement of fluorophore design, particularly by attaching protective and anti-fading groups, creating proper medium conditions in liquid, solid and nanoscale environments. The special conditions for biological labeling, sensing and imaging are outlined.

Mutual impact of clinically translatable near-infrared dyes on photoacoustic image contrast and in vitro photodynamic therapy efficacy

Photodynamic therapy (PDT), a spatially localized phototoxic therapy that involves irradiation of a photosensitizer (PS) with specific wavelengths of light, has shown exceptional promise in impacting cancer treatment outcomes, particularly oral cancer. To reduce PDT outcome variability, attempts toward image-guided personalized PDT are being pursued by monitoring PS uptake either via fluorescence or photoacoustic imaging (PAI), a nonionizing modality dependent on optical absorption properties of the tissue. PAI-guided PDT requires a near-infrared contrast agent for deep tissue imaging with minimal photobleaching effect. We evaluate the impact of PDT agent, benzoporphyrin derivative (BPD), on PAI agent indocyanine green (ICG) and vice versa, given that they have different optical absorption properties and singlet oxygen quantum yields for PDT. Specifically, we demonstrate in two oral squamous cell carcinoma lines (FaDu and SCC4) that ICG has minimal effect on BPD PDT efficacy when irradiated with either a continuous or pulsed laser. Furthermore, the impact of BPD on ICG photodegradation was monitored with PAI in tissue-mimicking phantoms. These studies inform us that the combination of BPD and ICG can be utilized for PAI-guided PDT. However, researchers need to consider the photodegradation effects of ICG in the presence of BPD when designing their drug delivery strategies for PAI-guided PDT.

Construction and bioimaging application of novel indole heptamethine cyanines containing functionalized tetrahydropyridine rings

IR780 as a commercially available dye with near-infrared emission has been extensively applied in fluorescent probes and bioimaging.

Long-wavelength photoremovable protecting groups: On the way to in vivo application

Photoremovable protective groups (PPGs) and related "caged" compounds have been recognized as a powerful tool in an arsenal of life science methods. The present review is focused on recent advances in design of "caged" compounds which function in red or near-infrared region. The naive comparison of photon energy with that of organic bond leads to the illusion that long-wavelength activation is possible only for weak chemical bonds like N-N. However, there are different means to overcome this threshold and shift the uncaging functionality into red or near-infrared regions for general organic bonds. We overview these strategies, including the novel photochemical and photophysical mechanisms used in newly developed PPGs, singlet-oxygen-mediated photolysis, and two-photon absorption. Recent advances in science places the infrared-sensitive PPGs to the same usability level as traditional ones, facilitating in vivo application of caged compounds.

An oxygen self-sufficient NIR-responsive nanosystem for enhanced PDT and chemotherapy against hypoxic tumors

The efficacy of photodynamic therapy and chemotherapy is largely limited by oxygen deficiency in the hypoxic tumor microenvironment. To solve these problems, we fabricated a novel NIR-responsive nanosystem which could co-deliver oxygen and anticancer drug DOX. An oxygen self-sufficient amphiphile (F-IR780-PEG) was first synthesized and subsequently utilized to load anticancer drug DOX to form nanoparticles (F/DOX nanoparticles). Due to the high oxygen capacity of such nanoparticles, the hypoxic tumor microenvironment was greatly modulated after these nanoparticles reached the tumor region, and the results revealed that hypoxia-inducible factor α (HIF-1α) was down-regulated and the expression of P-glycoprotein (P-gp) was then reduced, which were in favor of chemotherapy. Under light irradiation at 808 nm, IR780 could efficiently produce singlet oxygen to damage cancer cells by photodynamic therapy (PDT). Simultaneously, the IR780 linkage could be cleaved by singlet oxygen generated by itself and resulted in DOX release, which further caused cell damage by chemotherapy. With the combination of PDT and chemotherapy, F/DOX nanoparticles showed remarkable therapeutic efficacy under in vitro and in vivo conditions. Furthermore, the F/DOX nanoparticles are favorable for imaging-guided tumor therapy due to the inherent fluorescence properties of IR780. We thus believe that the synergistic treatment described here leads to an ideal therapeutic approach to hypoxic tumors.

Development of photolabile protecting groups and their application to the optochemical control of cell signaling

Many biological processes are naturally regulated with spatiotemporal control. In order to perturb and investigate them, optochemical tools have been developed that convey similar spatiotemporal precision. Pivotal to optochemical probes are photolabile protecting groups, so called caging groups, and recent developments have enabled new applications to cellular processes, including cell signaling. This review focuses on the advances made in the field of caging groups and their application in cell signaling through caged molecules such as neurotransmitters, lipids, secondary messengers, and proteins.

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.

On the impact of competing intra- and intermolecular triplet-state quenching on photobleaching and photoswitching kinetics of organic fluorophores

While buffer cocktails remain the most commonly used method for photostabilization and photoswitching of fluorescent markers, intramolecular triplet-state quenchers emerge as an alternative strategy to impart fluorophores with 'self-healing' or even functional properties such as photoswitching. In this contribution, we evaluated combinations of both approaches and show that inter- and intramolecular triplet-state quenching processes compete with each other. We find that although the rate of triplet-state quenching is additive, the photostability is limited by the faster pathway. Often intramolecular processes dominate the photophysical situation for combinations of covalently-linked and solution-based photostabilizers and photoswitching agents. Furthermore we show that intramolecular photostabilizers can protect fluorophores from reversible off-switching events caused by solution-additives, which was previously misinterpreted as photobleaching. Our studies also provide practical guidance for usage of photostabilizer-dye conjugates for STORM-type super-resolution microscopy permitting the exploitation of their improved photophysics for increased spatio-temporal resolution. Finally, we provide evidence that the biochemical environment, e.g., proximity of aromatic amino-acids such as tryptophan, reduces the photostabilization efficiency of commonly used buffer cocktails. Not only have our results important implications for a deeper mechanistic understanding of self-healing dyes, but they will provide a general framework to select label positions for optimal and reproducible photostability or photoswitching kinetics in different biochemical environments.

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.

A Platinum(II) Complex of Heptamethine Cyanine for Photoenhanced Cytotoxicity and Cellular Imaging in Near-IR Light

Controlled generation of cytotoxic agents with near-IR light is a current focus of photoactivated cancer therapy, including that involving cytotoxic platinum species. A heptamethine cyanine scaffolded PtII complex, IR797-Platin exhibits unprecedented Pt-O bond scission and enhancement in DNA platination in near-IR light. This complex also displayed significant singlet oxygen quantum yield thereby qualifying as a near-IR photodynamic therapeutic agent. The complex showed 30-60 fold enhancement of cytotoxicity in near-IR light in various cancer cell lines. The cellular imaging properties were also leveraged to observe its significant co-localization in cytoplasmic organelles. This is the first demonstration of a near-IR light-initiated therapy involving the cytotoxic effects of both active cisplatin and singlet oxygen.