Enzyme-Activatable Near-Infrared Photosensitizer with High Enrichment in Tumor Cells Based on a Multi-Effect Design

Enzyme-activatable near-infrared (NIR) fluorescent probes and photosensitizers (PSs) have emerged as promising tools for molecular imaging and photodynamic therapy (PDT). However, in living organisms selective retention or even enrichment of these reagents after enzymatic activation at or near sites of interest remains a challenging task. Herein, we integrate non-covalent and covalent retention approaches to introduce a novel "1-to-3" multi-effect strategy-one enzymatic stimulus leads to three types of effects-for the design of an enzyme-activatable NIR probe or PS. Using this strategy, we have constructed an alkaline phosphatase (ALP)-activatable NIR fluorogenic probe and a NIR PS, which proved to be selectively activated by ALP to switch on NIR fluorescence or photosensitizing ability, respectively. Additionally, these reagents showed significant enrichment (over 2000-fold) in ALP-overexpressed tumor cells compared to the culture medium, accompanied by massive depletion of intracellular thiols, the major antioxidants in cells. The investigation of this ALP-activatable NIR PS in an in vivo PDT model resulted in complete suppression of HeLa tumors and full recovery of all tested mice. Encouragingly, even a single administration of this NIR PS was sufficient to completely suppress tumors in mice, demonstrating the high potential of this strategy in biomedical applications.

Effective synthesis, development and application of a highly fluorescent cyanine dye for antibody conjugation and microscopy imaging

An asymmetric cyanine-type fluorescent dye was designed and synthesized via a versatile, multi-step process, aiming to conjugate with an Her2+ receptor specific antibody by an azide-alkyne click reaction. The aromaticity and the excitation and relaxation energetics of the fluorophore were characterized by computational methods. The synthesized dye exhibited excellent fluorescence properties for confocal microscopy, offering efficient applicability in in vitro imaging due to its merits such as a high molar absorption coefficient (36 816 M-1 cm-1), excellent brightness, optimal wavelength (627 nm), larger Stokes shift (26 nm) and appropriate photostability compared to cyanines. The conjugated cyanine-trastuzumab was constructed via an effective, metal-free, strain-promoted azide-alkyne click reaction leading to a regulated number of dyes being conjugated. This novel cyanine-labelled antibody was successfully applied for in vitro confocal imaging and flow cytometry of Her2+ tumor cells.

Ru(II)-Cyanine Complexes as Promising Photodynamic Photosensitizers for the Treatment of Hypoxic Tumours with Highly Penetrating 770 nm Near-Infrared Light

Light-activated treatments, such as photodynamic therapy (PDT), provide temporal and spatial control over a specific cytotoxic response by exploiting toxicity differences between irradiated and dark conditions. In this work, a novel strategy for developing near infrared (NIR)-activatable Ru(II) polypyridyl-based photosensitizers (PSs) was successfully developed through the incorporation of symmetric heptamethine cyanine dyes in the metal complex via a phenanthrimidazole ligand. Owing to their strong absorption in the NIR region, the PSs could be efficiently photoactivated with highly penetrating NIR light (770 nm), leading to high photocytotoxicities towards several cancer cell lines under both normoxic and hypoxic conditions. Notably, our lead PS (Ru-Cyn-1), which accumulated in the mitochondria, exhibited a good photocytotoxic activity under challenging low-oxygen concentration (2 % O2 ) upon NIR light irradiation conditions (770 nm), owing to a combination of type I and II PDT mechanisms. The fact that the PS Protoporphyrin IX (PpIX), the metabolite of the clinically approved 5-ALA PS, was found inactive under the same challenging conditions positions Ru-Cyn-1 complex as a promising PDT agent for the treatment of deep-seated hypoxic tumours.

Molecular Engineering Design of Enhanced Donor-Acceptor Therapeutic Reagent for Efficient Image-Guided Photodynamic Therapy

The greatest barrier to the further development and clinical application of tumor image-guided photodynamic therapy (PDT), is the inconsistency between the fluorescence intensity and singlet oxygen generation yield of the photosensitizer under light excitation. Herein, a novel donor-acceptor (D-A) system is designed from the point of molecular selection by wrapping a classical porphyrin molecule (5,10,15,20-tetraphenylphorphyrin, H2 TPP) as an acceptor into conjugated polymer (Poly[N,N'-bis(4-butylpheny)-N,N'-bis(phenyl)benzidine], ADS254BE) as a donor through fluorescence resonance energy transfer (FRET) mechanism, which exhibits bright red emission centered at 650 nm (quantum yield, 0.12), relatively large Stoke shift of 276 nm, enhanced singlet oxygen generation rate of 0.73, and excellent photostability. The investigations on distribution and killing effect of nanomaterials in cancer cells reveal that ADS254BE/H2 TPP NPs can accumulate in the cytoplasm for imaging while simultaneously producing a large amount of singlet oxygen to remarkably kill cancer cells, which can be used for real-time image-guided PDT. In the xenograft tumor model, real-time imaging and long-term tracing in tumor tissue with ADS254BE/H2 TPP NPs disclose that the growth of lung cancer in mice can be effectively inhibited during in situ imaging. From the standpoint of molecular engineering design, this work provides a feasible strategy for novel D-A systems to improve the development of image-guided PDT.

Engineering of a GSH activatable photosensitizer for enhanced photodynamic therapy through disrupting redox homeostasis

Although disrupted redox homeostasis has emerged as a promising approach for tumor therapy, most existing photosensitizers are not able to simultaneously improve the reactive oxygen species level and reduce the glutathione (GSH) level. Therefore, designing photosensitizers that can achieve these two aspects of this goal is still urgent and challenging. In this work, an organic activatable near-infrared (NIR) photosensitizer, CyI-S-diCF₃, is developed for GSH depletion-assisted enhanced photodynamic therapy. CyI-S-diCF₃, composed of an iodinated heptamethine cyanine skeleton linked with a recognition unit of 3,5-bis(trifluoromethyl)benzenethiol, can specifically react with GSH by nucleophilic substitution, resulting in intracellular GSH depletion and redox imbalance. Moreover, the activated photosensitizer can produce abundant singlet oxygen (¹O₂) under NIR light irradiation, further heightening the cellular oxidative stress. By this unique nature, CyI-S-diCF₃ exhibits excellent toxicity to cancer cells, followed by inducing earlier apoptosis. Thus, our study may propose a new strategy to design an activatable photosensitizer for breaking the redox homeostasis in tumor cells.

New Highly Fluorescent Water Soluble Imidazolium-Perylenediimides: Synthesis and Cellular Response

The synthesis and characterization of two new water soluble 2,6-bis(imidazolylmethyl)-4-methylphenoxy-containing perylenediimides, PDI-1 and PDI-2, are described. These compounds demonstrate a high fluorescence quantum yield in water and were investigated as potential photosensitizers for generating reactive oxygen species with applications in anticancer activities. The HeLa cell line (VPH18) was used to evaluate their efficacy. Fluorescence microscopy was employed to confirm the successful internalization of PDI-1 and PDI-2, while confocal microscopy revealed the specific locations of both PDIs within the lysosomes and mitochondria. In vitro studies were conducted to evaluate the anticancer activity of PDI-1 and PDI-2. Remarkably, these photosensitizers demonstrated a significant ability to selectively eliminate cancer cells when exposed to a specific light wavelength. The water solubility, high fluorescence quantum yield, and selective cytotoxicity of these PDIs toward cancer cells highlight their potential as effective agents for targeted photodynamic therapy. In conclusion, the findings presented here provide a strong foundation for the future exploration and optimization of PDI-1 and PDI-2 as effective photosensitizers in photodynamic therapy, potentially leading to improved treatment strategies for cancer patients.

Photothermal-driven disassembly of naphthalocyanine nano-photosensitizers for photothermal and photodynamic therapy

The disassembly of nanomaterials is of particular interest for high-quality imaging and targeted therapies in the field of nanomedicine. In this study, we developed a novel strategy for fabricating self-assembled naphthalocyanine photosensitizers (SiNc@CEL) with intrinsically unique photochemical and photophysical properties. SiNc@CEL could be disassembled under the photothermal effect, and its photoactivity could be enhanced by 780 nm laser irradiation. Moreover, SiNc@CEL generates reactive oxygen species, including superoxide radicals (O₂^•-) and singlet oxygen (¹O₂), as well as good photothermal properties, facilitating the application of multifunctional phototherapy. In vitro evaluation indicated that SiNc@CEL possesses an excellent bactericidal effect under a combination of photodynamic (PDT) and photothermal therapy (PTT). The in vivo treatment of a full-layer skin defect model of Escherichia coli (E. coli) infection showed that SiNc@CEL had superior antibacterial and wound-healing abilities. These results provide the basis for a feasible strategy to enhance the phototherapeutic effect of photosensitizer (PS) systems.

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.

Cytotoxicities of Tumor-Seeking Dyes: Impact on Future Clinical Trials

Heptamethine (Cy7) dyes with meso-Cl substituents injected intravenously (iv) into mice accumulate in tumors and persist there over several days. We believe this occurs via meso-Cl displacement by the only free cysteine residues of albumin; therefore, conjugating tumor-seeking dyes with fragments can increase selective therapeutic delivery to tumors and drug residence. This strategy has elevated significance recently because the first tumor-seeking dye-drug conjugate has moved into clinical trials. Options for further clinical research include modifying the dye, and use of preformed albumin adducts instead of dyes alone. Herein we show correlations of cytotoxicities, lipophilicities, organelle localization, apoptosis, cell-cycle arrest, wound healing/migration assays, and reactivities/affinities with human serum albumin are difficult to observe. However, our studies arrived at an important conclusion: preformed dye-drug-HSA adducts are less cytotoxic, and therefore preferable for subsequent clinical work, relative to direct injection of meso-Cl-containing forms.

Probing tricarbocyanine dyes for targeted delivery of anthracyclines

Along with bright fluorescence in the near-IR range, heptamethine carbocyanine dyes possess affinity to cancer cells. Thus, these dyes could be utilized as fluorescent labels and vectors for drug delivery as covalent conjugates with cytotoxic compounds. To test the properties, structure-activity relationship, and scope of such conjugates, we synthesized drug-dye dyads of tricarbocyanine dyes with anthracycline drug daunorubicin. We used hydrophilic zwitterionic and hydrophobic positively charged benzoindoline-benzothiazole-based heptamethine dyes as terminal alkyne derivatives and N-acylated or oxime-linked daunorubicin as azido-derivatives. These two alkynes and two azides were coupled to each other by Cu-catalyzed Huisgen-Meldal-Sharpless cycloaddition (click reaction) to afford four conjugates. Molecules based on hydrophobic dyes possess submicromolar cytotoxicity to HCT116 cells. Cytotoxicity, cell penetration, intracellular distribution, apoptosis induction and the effect of antioxidants on toxicity were evaluated. The results show that the structure of the cyanine-anthracycline conjugate (hydrophilicity/hydrophobicity, charge, linker, attachment site) is important for its biological activity, thus, expansion of the chemical space of such conjugates could provide new molecular research tools for diagnostics and therapy.

Hybrid Cyanine/Methotrexate Nanoparticles for Synergistic PDT/Chemotherapy of Breast Cancer

Typically, nanomedicine was prepared using a nanocarrier to load cargo for specific purposes. In this work, a carrier-free nanosystem for imaging and photodynamic (PDT)/chemo combination therapy was developed using simple self-assembly of a dye and a chemotherapeutic agent. The resulting nanoparticles (I₂-IR783/MTX@NPs) exhibited a spherical morphology with a size of 240.6 ± 2.5 nm. I₂-IR783/MTX@NPs had substantial internalization in 4T1 murine breast cancer cells and showed a synergistic anticancer effect after NIR light irradiation. Additionally, the 3D tumor model exhibits the same phototoxicity of nanoparticles as a 2D cell culture. The PDT efficiency of the nanosystem in the physiological environment was confirmed by the detection of intracellular reactive oxygen species as well as the live/dead viability/cytotoxicity assay following NIR light exposure. In addition, optical coherence tomography (OCT) was used as an alternative tool to monitor the response after treatment. Therefore, I₂-IR783/MTX@NPs show great potential use in theranostic application for breast cancer PDT-chemotherapy.

Effect of Solubilizing Group on the Antibacterial Activity of Heptamethine Cyanine Photosensitizers

Antibiotic resistance of pathogenic bacteria dictates the development of novel treatment modalities such as antimicrobial photodynamic therapy (APDT) utilizing organic dyes termed photosensitizers that exhibit a high cytotoxicity upon light irradiation. Most of the clinically approved photosensitizers are porphyrins that are poorly excitable in the therapeutic near-IR spectral range. In contrast, cyanine dyes function well in the near-IR region, but their phototoxicity, in general, is very low. The introduction of iodine atoms in the cyanine molecules was recently demonstrated to greatly increase their phototoxicity. Herein, we synthesized a series of the new iodinated heptamethine cyanine dyes (ICy7) containing various solubilizing moieties, i.e., negatively charged carboxylic (ICy7COOH) and sulfonic (ICy7SO₃H) groups, positively charged triphenylphosphonium (ICy7PPh₃), triethylammonium (ICy7NEt₃) and amino (ICy7NH₂) groups, and neutral amide (ICy7CONHPr) group. The effect of these substituents on the photodynamic eradication of Gram-positive (S. aureus) and Gram-negative (E. coli and P. aeruginosa) pathogens was studied. Cyanine dyes containing the amide and triphenylphosphonium groups were found to be the most efficient for eradication of the investigated bacteria. These dyes are effective at low concentrations of 0.05 µM (33 J/cm²) for S. aureus, 50 µM (200 J/cm²) for E. coli, and 5 µM (100 J/cm²) for P. aeruginosa and considered, therefore, promising photosensitizers for APDT applications. The innovation of the new photosensitizers consisted of a combination of the heavy-atom effect that increases singlet oxygen generation with the solubilizing group's effect improving cell uptake, and with effective near-IR excitation. Such a combination helped to noticeably increase the APDT efficacy and should pave the way for the development of more advanced photosensitizers for clinical use.

Near-Infrared Dyes: Towards Broad-Spectrum Antivirals

Broad antiviral activity in vitro is known for many organic photosensitizers generating reactive oxygen species under irradiation with visible light. Low tissue penetration of visible light prevents further development of antiviral therapeutics based on these compounds. One possible solution to this problem is the development of photosensitizers with near-infrared absorption (NIR dyes). These compounds found diverse applications in the photodynamic therapy of tumors and bacterial infections, but they are scarcely mentioned as antivirals. In this account, we aimed to evaluate the therapeutic prospects of various NIR-absorbing and singlet oxygen-generating chromophores for the development of broad-spectrum photosensitizing antivirals.

Nanotherapeutic Intervention in Photodynamic Therapy for Cancer

The clinical need for photodynamic therapy (PDT) has been growing for several decades. Notably, PDT is often used in oncology to treat a variety of tumors since it is a low-risk therapy with excellent selectivity, does not conflict with other therapies, and may be repeated as necessary. The mechanism of action of PDT is the photoactivation of a particular photosensitizer (PS) in a tumor microenvironment in the presence of oxygen. During PDT, cancer cells produce singlet oxygen (¹O₂) and reactive oxygen species (ROS) upon activation of PSs by irradiation, which efficiently kills the tumor. However, PDT's effectiveness in curing a deep-seated malignancy is constrained by three key reasons: a tumor's inadequate PS accumulation in tumor tissues, a hypoxic core with low oxygen content in solid tumors, and limited depth of light penetration. PDTs are therefore restricted to the management of thin and superficial cancers. With the development of nanotechnology, PDT's ability to penetrate deep tumor tissues and exert desired therapeutic effects has become a reality. However, further advancement in this field of research is necessary to address the challenges with PDT and ameliorate the therapeutic outcome. This review presents an overview of PSs, the mechanism of loading of PSs, nanomedicine-based solutions for enhancing PDT, and their biological applications including chemodynamic therapy, chemo-photodynamic therapy, PDT-electroporation, photodynamic-photothermal (PDT-PTT) therapy, and PDT-immunotherapy. Furthermore, the review discusses the mechanism of ROS generation in PDT advantages and challenges of PSs in PDT.

Necroptosis activation is associated with greater methylene blue-photodynamic therapy-induced cytotoxicity in human pancreatic ductal adenocarcinoma cells

Pancreatic ductal adenocarcinomas (PDAC) are the fourth leading cause of death due to neoplasms. In view of the urgent need of effective treatments for PDAC, photodynamic therapy (PDT) appears as a promising alternative. However, its efficacy against PDAC and the mechanisms involved in cell death induction remain unclear. In this study, we set out to evaluate PDT's cytotoxicity using methylene blue (MB) as a photosensitizer (PS) (MB-PDT) and to evaluate the contribution of necroptosis in its effect in human PDAC cells. Our results demonstrated that MB-PDT induced significant death of different human PDAC models presenting two different susceptibility profiles. This effect was independent of MB uptake or its subcellular localization. We found that the ability of triggering necroptosis was determinant to increase the treatment efficiency. Analysis of single cell RNA-seq data from normal and neoplastic human pancreatic tissues showed that specific necroptosis proteins RIPK1, RIPK3 and MLKL presented significant higher expression levels in cells displaying a transformed phenotype providing further support to the use of approaches that activate necroptosis, like MB-PDT, as useful adjunct to surgery of PDAC to tackle the problem of microscopic residual disease as well as to minimize the chance of local and metastatic recurrence.

Norsquaraine endowed with anticancer and antibacterial activities

BACKGROUND

Photodynamic therapy (PDT) is a method for the treatment of cancer. Furthermore, PDT can also be used for the eradication of bacteria. The photo-sensitizing drug, a.k.a photosensitizer, is critical for the success of PDT. Although norsquaraines are analogs of squaraine dyes, they are overlooked as photosensitizers.

METHODS

In this work, synthesis, characterization, bioimaging and in vitro PDT applications of a new norsquaraine dye 1 were described. We also prepared nanoparticles from norsquaraine 1 and Pluronic F127 to obtain 1@F127.

RESULTS

Norsquaraine 1 boosted the generation of reactive oxygen species over a wide range of pH (pH 8.0, 7.0, 6.0, and 2.2.). Furthermore, 1 was internalized by epidermoid laryngeal carcinoma Hep-2 (Hep-2) cells and used for fluorescence imaging. Remarkably, norsquaraine 1 destroyed most of the cancer cells (ca. 77% to 89%) after illumination with red light. Most strikingly, 1 successfully inhibited the growth of Methicillin-resistant Staphylococcus aureus (MRSA) upon illumination. Last but not least, photodynamic sterilization of tomato juice, an acidic beverage, was feasible using 1 as a photo sterilizer. Nano formulation of 1 with Pluronic F127 provided 1@F127 nanoparticles. It is lucid that 1@F127 nanoparticles permeate into Hep-2 cells and boost the generation of ROS upon illumination.

CONCLUSION

Norsquaraine 1 shows superior features as a photosensitizer pertinent to PDT in a wide range of pH. This norsquaraine is endowed with anticancer and antibacterial activities. Which should be further evaluated.

Recent Developments in Heteroatom/Metal-Doped Carbon Dot-Based Image-Guided Photodynamic Therapy for Cancer

Carbon nanodots (CNDs) are advanced nanomaterials with a size of 2–10 nm and are considered zero-dimensional carbonaceous materials. CNDs have received great attention in the area of cancer theranostics. The majority of review articles have shown the improvement of CNDs for use in cancer therapy and bioimaging applications. However, there is a minimal number of consolidated studies on the currently developed doped CNDs that are used in various ways in cancer therapies. Hence, in this review, we discuss the current developments in different types of heteroatom elements/metal ion-doped CNDs along with their preparations, physicochemical and biological properties, multimodal-imaging, and emerging applications in image-guided photodynamic therapies for cancer.

Switching the NIR upconversion of nanoparticles for the orthogonal activation of photoacoustic imaging and phototherapy

Phototheranostics based on upconversion nanoparticles (UCNPs) offer the integration of imaging diagnostics and phototherapeutics. However, the programmable control of the photoactivation of imaging and therapy with minimum side effects is challenging due to the lack of ideal switchable UCNPs agents. Here we demonstrate a facile strategy to switch the near infrared emission at 800 nm from rationally designed UCNPs by modulating the irradiation laser into pulse output. We further synthesize a theranostic nanoagent by combining with a photosensitizer and a photoabsorbing agent assembled on the UCNPs. The orthogonal activation of in vivo photoacoustic imaging and photodynamic therapy can be achieved by altering the excitation modes from pulse to continuous-wave output upon a single 980 nm laser. No obvious harmful effects during photoexcitation was identified, suggesting their use for long-term imaging-guidance and phototherapy. This work provides an approach to the orthogonal activation of imaging diagnostics and photodynamic therapeutics.

Polymethine dyes-loaded solid lipid nanoparticles (SLN) as promising photosensitizers for biomedical applications

Polymethine dyes (PMD) have proved to be excellent candidates in the biomedical field for potential applications in both diagnostic and therapeutic. However, PMD application in biomedicine is hindered by their poor solubility and stability in physiological conditions. Therefore, the incorporation of these dyes in nanosystems could be important to prevent the formation of dye aggregates in aqueous environment and to protect their photophysical characteristics. In the present work, two PMD based on the benzoindolenine ring (bromine benzo-cyanine-C4 and bromine benzo-squaraine-C4) were incorporated into Solid Lipid Nanoparticles (SLN) to solubilize and stabilize them in aqueous solutions. Obtained SLN showed a high incorporation efficiency for both PMD (≈90%) and not only preserved their spectroscopic properties in the NIR region even under physiological conditions but also improved them. Viability assays showed good biocompatibility of both empty and loaded nanocarriers while the cellular uptake and intracellular localization showed the effective internalization in MCF-7 cells, with a partial mitochondrial localization for CY-SLN. Moreover, in vitro phototoxicity assay showed that cyanine loaded-SLN (CY-SLN) is more photoactive than the free dye.

Photochemical α-Aminonitrile Synthesis Using Zn-Phthalocyanines as Near-Infrared Photocatalysts

While photochemical transformations with sunlight almost exclusively utilize the UV-vis part of the solar spectrum, the majority of the photons emitted by the sun have frequencies in the near-infrared region. Phthalocyanines show high structural similarity to the naturally occurring light-harvesting porphyrins, chlorins, and mainly bacteriochlorins and are also known for being efficient and affordable near-infrared light absorbers as well as triplet sensitizers for the production of singlet oxygen. Although having been neglected for a long time in synthetic organic chemistry due to their low solubility and high tendency toward aggregation, their unique photophysical properties and chemical robustness make phthalocyanines attractive photocatalysts for the application in near-infrared-light-driven synthesis strategies. Herein, we report a cheap, simple, and efficient photocatalytic protocol, which is easily scalable under continuous-flow conditions. Various phthalocyanines were studied as near-infrared photosensitizers in oxidative cyanations of tertiary amines to generate α-aminonitriles, a synthetically versatile compound class.

Thiocarbonyl photosensitizer, a feasible way to eliminate the photosensitizer residues in photodynamic therapy

Photodynamic therapy (PDT) has been successfully applied in clinical treatment several years. However, after finished treatment process the residual photosensitizer will spread throughout body, which forces patients stay in the dark room to avoid exposure in sunlight several weeks. Therefore, develop degradable photosensitizer could effectively eliminate this inconvenience. In the past, researchers have developed degradable photosensitizers based on supramolecular structure. In this study, we achieved the same effect in small molecule level. Three thiocarbonyl photosensitizers (PS) have high photogenerated ¹O₂ quantum yield and can be photodegraded by laser irradiation within 15 min. And due to its high phototoxicity and low toxicity, thiocarbonyl PS still maintains its high phototoxicity. Especially, mitochondrial targeting PS 1a has better properties than many BODIPY or cyanine heavy-atom-free photosensitizers. It only needs 1 μM to reduce HeLa cell activity to 30%. Finally the thiocarbonyl PS provided a convenient way to solve the PS residue problem without sacrificing PDT efficiency.

Indomethacin-based near-infrared photosensitizer for targeted photodynamic cancer therapy

Near-IR fluorescent sensitizers based on heptamethine cyanine (Cy820 and Cy820-IMC) were synthesized and their abilities to target and abolish tumor cells via photodynamic therapy (PDT) were explored. Some hepthamethine cyanine dyes can be transported into cancer cells via the organic anion transporting polypeptides (OATPs). In this study, we aimed to enhance the target ability of the sensitizer by conjugation Cy820 with indomethacin, a non-steroidal anti-inflammatory drug (NSAID), to obtain Cy820-IMC that aimed to target cyclooxygenase-2 (COX-2) which overexpresses in cancer cells. The results showed that Cy820-IMC internalized the cancer cells faster than Cy820 which was verified to be related to COX-2 level and OATPs. Based on PDT experiments, Cy820-IMC has higher photocytotoxicity index than Cy820, >7.13 and 4.90, respectively, implying that Cy820-IMC showed better PDT property than Cy820. However, Cy820 exhibits slightly higher normal-to-cancer cell toxicity ratio than Cy820-IMC, 6.58 and 3.63, respectively. Overall, Cy820-IMC has superior cancer targetability and enhanced photocytoxicity. These characteristics can be further improved towards clinically approved sensitizers for PDT.

QuatCy-I2 and MHI-I2 in Photodynamic Therapy

MHI-I₂ (1) and QuatCy-I₂ (2) were compared in terms of properties important for early-stage photodynamic therapy preclinical candidates. Thus, experiments were performed to monitor dark cytotoxicities, light/dark cytotoxicity ratios, selectivity of localization in tumors over other organs, and clearance from the plasma.

Effect of morpholine and charge distribution of cyanine dyes on cell internalization and cytotoxicity

To improve the potency of Heptamethine cyanines (Hcyanines) in cancer research, we designed and synthesized two novel Hcyanines based theranostic probes, IR794-Morph and IR794-Morph-Mpip, to enhance cancer cell internalization and targeting. In acidic conditions that resemble to tumour environment, both IR794 derivatives exhibited broad NIR absorption band (704‒794 nm) and fluorescence emission (798‒828 nm) that is suitable for deep seated tumour imaging. Moreover, in vitro study revealed that IR794-Morph-Mpip exhibited better cancer targetability towards various cancer cell lines under physiological and slightly acidic conditions compared to normal cells. IR794-Morph-Mpip was fast internalized into the cancer cells within the first 5 min and mostly localized in lysosomes and mitochondria. In addition, the internalized signal was brighter when the cells were in the hypoxic environment. Furthermore, cellular uptake mechanism of both IR794 dyes, investigated via flow cytometry, revealed that endocytosis through OATPs receptors and clathrin-mediated endocytosis were the main routes. Moreover, IR794-Morph-Mpip, displayed anti-cancer activity towards all tested cancer cell types with IC₅₀ below 7 μM (at 6 h incubation), which is approximately three times lower than that of the normal cells. Therefore, increasing protonated cites in tumour environment of Hcyanines together with incorporating morpholine in the molecule can enhance structure-inherent targeting of these dyes.

Multifunctional photodynamic/photothermal nano-agents for the treatment of oral leukoplakia

Oral leukoplakia (OLK) has gained extensive attention because of the potential risk for malignant transformation. Photosensitizers (PSs) played an indispensable role in the photodynamic therapy (PDT) of OLK, but the poor light sensitivity greatly hampered its clinical application. Herein, a novel organic photosensitive ITIC-Th nanoparticles (ITIC-Th NPs) were developed for OLK photodynamic/photothermal therapy (PTT). ITIC-Th NPs present both high photothermal conversion efficiency (~ 38%) and suitable reactive oxygen species (ROS) generation ability under 660 nm laser irradiation, making them possess excellent PDT and PTT capability. In 4-nitroquinoline 1-oxide (4NQO)-induced oral precancerous animal models, ITIC-Th NPs effectively suppress the OLK's cancerization without apparent topical or systemic toxicity in vivo. This study offers a promising therapeutic strategy for PDT and PTT in OLK treatment, and this study is the first interdisciplinary research in the field of multimodal therapy for OLK.

Photoacoustic nanodroplets for oxygen enhanced photodynamic therapy of cancer

Photodynamic therapy (PDT) is a well-known cancer therapy that utilizes light to excite a photosensitizer and generate cytotoxic reactive oxygen species (ROS). The efficacy of PDT primarily depends on the photosensitizer and oxygen concentration in the tumor. Hypoxia in solid tumors promotes treatment resistance, resulting in poor PDT outcomes. Hence, there is a need to combat hypoxia while delivering sufficient photosensitizer to the tumor for ROS generation. Here we showcase our unique theranostic perfluorocarbon nanodroplets as a triple agent carrier for oxygen, photosensitizer, and indocyanine green that enables light triggered spatiotemporal delivery of oxygen to the tumors. We evaluated the characteristics of the nanodroplets and validated their ability to deliver oxygen via photoacoustic monitoring of blood oxygen saturation and subsequent PDT efficacy in a murine subcutaneous tumor model. The imaging results were validated with an oxygen sensing probe, which showed a 9.1 fold increase in oxygen content inside the tumor, following systemic administration of the nanodroplets. These results were also confirmed with immunofluorescence. In vivo studies showed that nanodroplets held higher rates of treatment efficacy than a clinically available benzoporphyrin derivative formulation. Histological analysis showed higher necrotic area within the tumor with perfluoropentane nanodroplets. Overall, the photoacoustic nanodroplets can significantly enhance image-guided PDT and has demonstrated substantial potential as a valid theranostic option for patient-specific photodynamic therapy-based treatments.

Polymethine dyes for PDT: recent advances and perspectives to drive future applications

It has been proved that the effectiveness of photodynamic therapy (PDT) is closely related to the intrinsic features of the photosensitizer (PS). Over the recent years, several efforts have been devoted to the discovery of novel and more efficient photosensitizers showing higher efficacy and lower side effects. In this context, squaraine and cyanine dyes have been reported to potentially overcome the drawbacks related to the traditional PSs. In fact, squaraines and cyanines are characterized by sharp and intense absorption bands and narrow emission bands with high extinction coefficients typically in the red and near-infrared region, good photo and thermal stability and a strong fluorescent emission in organic solvents. In addition, biocompatibility and low toxicity make them suitable for biological applications. Despite these interesting intrinsic features, their chemical instability and self-aggregation properties in biological media still limit their use in PDT. To overcome these drawbacks, the self-assembly and incorporation into smart nanoparticle systems are forwarded promising approaches that can control their physicochemical properties, providing rational solutions for the limitation of free dye administration in the PDT application. The present review summarizes the latest advances in squaraine and cyanine dyes for PDT application, analyzing the different strategies, i.e.the self-assembly and the incorporation into nanoparticles, to further enhance their photochemical properties and therapeutic potential. The in vivo assessments are still limited, thus further delaying their effective application in PDT.

Heptamethine Cyanine–Based Application for Cancer Theranostics

Cancer is the most common life-threatening malignant disease. The future of personalized cancer treatments relies on the development of functional agents that have tumor-targeted anticancer activities and can be detected in tumors through imaging. Cyanines, especially heptamethine cyanine (Cy7), have prospective application because of their excellent tumor-targeting capacity, high quantum yield, low tissue autofluorescence, long absorption wavelength, and low background interference. In this review, the application of Cy7 and its derivatives in tumors is comprehensively explored. Cy7 is enormously acknowledged in the field of non-invasive therapy that can “detect” and “kill” tumor cells via near-infrared fluorescence (NIRF) imaging, photothermal therapy (PTT), and photodynamic therapy (PDT). Furthermore, Cy7 is more available and has excellent properties in cancer theranostics by the presence of multifunctional nanoparticles via fulfilling multimodal imaging and combination therapy simultaneously. This review provides a comprehensive scope of Cy7’s application for cancer NIRF imaging, phototherapy, nanoprobe-based combination therapy in recent years. A deeper understanding of the application of imaging and treatment underlying Cy7 in cancer may provide new strategies for drug development based on cyanine. Thus, the review will lead the way to new types with optical properties and practical transformation to clinical practice.

Engineering Nanoplatform for Combined Cancer Therapeutics via Complementary Autophagy Inhibition

Despite advances in the development of tumor treatments, mortality from cancer continues to increase. Nanotechnology is expected to provide an innovative anti-cancer therapy, to combat challenges such as multidrug resistance and tumor recurrence. Nevertheless, tumors can greatly rely on autophagy as an alternative source for metabolites, and which desensitizes cancer cells to therapeutic stress, hindering the success of any current treatment paradigm. Autophagy is a conserved process by which cells turn over their own constituents to maintain cellular homeostasis. The multistep autophagic pathway provides potentially druggable targets to inhibit pro-survival autophagy under various therapeutic stimuli. In this review, we focus on autophagy inhibition based on functional nanoplatforms, which may be a potential strategy to increase therapeutic sensitivity in combinational cancer therapies, including chemotherapy, radiotherapy, phototherapy, sonodynamic therapy, and immunotherapy.

Photo-Oxidation of Therapeutic Protein Formulations: From Radical Formation to Analytical Techniques

UV and ambient light-induced modifications and related degradation of therapeutic proteins are observed during manufacturing and storage. Therefore, to ensure product quality, protein formulations need to be analyzed with respect to photo-degradation processes and eventually protected from light exposure. This task usually demands the application and combination of various analytical methods. This review addresses analytical aspects of investigating photo-oxidation products and related mediators such as reactive oxygen species generated via UV and ambient light with well-established and novel techniques.

Synthesis of N-aryl-2,6-diphenyl-2H-pyrazolo[4,3-c]pyridin-7-amines and their photodynamic properties in the human skin melanoma cell line G361

A small series of N-aryl-2,6-diphenyl-2H-pyrazolo[4,3-c]pyridin-7-amines was synthesized from easily accessible 1-phenyl-1H-pyrazol-3-ol via 7-iodo-2,6-diphenyl-2H-pyrazolo[4,3-c]pyridine and 7-iodo-4-methyl-2,6-diphenyl-2H-pyrazolo[4,3-c]pyridine intermediates and their subsequent use in palladium catalyzed Buchwald-Hartwig cross-coupling reaction with various anilines. Majority of the compounds were not significantly cytotoxic to melanoma G361 cells in the dark up to 10 µM concentration, but their activity could be increased by irradiation with visible blue light (414 nm). The most active compound 10 possessed EC₅₀ values of 3.5, 1.6 and 0.9 µM in cells irradiated with 1, 5 and 10 J/cm², respectively. The treatment caused generation of reactive oxygen species in cells and extensive DNA damage, documented by the comet assay and by detection of phosphorylated histone H2A.X, followed by apoptotic cell death. Our results suggest that N-aryl-2,6-diphenyl-2H-pyrazolo[4,3-c]pyridin-7-amines could serve as a potential source of photosensitizing compounds with anticancer activities.

Crystal structures and Hirshfeld analysis of 4,6-di-bromo-indole-nine and its quaternized salt

4,6-Di-bromo-2,3,3-trimethyl-3H-indole, C11H11Br2N, exists as a neutral mol-ecule in the asymmetric unit. The asymmetric unit of 4,6-di-bromo-2,3,3-trimethyl-3H-indol-1-ium iodide, C12H14Br2N+·I-, contains one organic cation and one iodine anion. The positive charge is localized on the quaternized nitro-gen atom. In the crystal, mol-ecules of 4,6-di-bromo-indole-nine are linked by C-Br⋯π halogen bonds, forming zigzag chains propagating in the [001] direction. The mol-ecules of the salt form layers parallel to the (010) plane where they are linked by C-H⋯Br hydrogen bonds, C-Br⋯Br and C-Br⋯I halogen bonds. The Hirshfeld surface analysis and two dimensional fingerprint plots were used to analyse the inter-molecular contacts present in both crystals.

Recent Strategies to Develop Innovative Photosensitizers for Enhanced Photodynamic Therapy

This review presents a robust strategy to design photosensitizers (PSs) for various species. Photodynamic therapy (PDT) is a photochemical-based treatment approach that involves the use of light combined with a light-activated chemical, referred to as a PS. Attractively, PDT is one of the alternatives to conventional cancer treatment due to its noninvasive nature, high cure rates, and low side effects. PSs play an important factor in photoinduced reactive oxygen species (ROS) generation. Although the concept of photosensitizer-based photodynamic therapy has been widely adopted for clinical trials and bioimaging, until now, to our surprise, there has been no relevant review article on rational designs of organic PSs for PDT. Furthermore, most of published review articles in PDT focused on nanomaterials and nanotechnology based on traditional PSs. Therefore, this review aimed at reporting recent strategies to develop innovative organic photosensitizers for enhanced photodynamic therapy, with each example described in detail instead of providing only a general overview, as is typically done in previous reviews of PDT, to provide intuitive, vivid, and specific insights to the readers.

Nano-photosensitizers for enhanced photodynamic therapy

Photodynamic therapy (PDT) utilizes photosensitizers (PSs) together with irradiation light of specific wavelength interacting with oxygen to generate cytotoxic reactive oxygen species (ROS), which could trigger apoptosis and/or necrosis-induced cell death in target tissues. During the past two decades, multifunctional nano-PSs employing nanotechnology and nanomedicine developed, which present not only photosensitizing properties but additionally accurate drug release abilities, efficient response to optical stimuli and hypoxia resistance. Further, nano-PSs have been developed to enhance PDT efficacy by improving the ROS yield. In addition, nano-PSs with additive or synergistic therapies are significant for both currently preclinical study and future clinical practice, given their capability of considerable higher therapeutic efficacy under safer systemic drug dosage. In this review, nano-PSs that allow precise drug delivery for efficient absorption by target cells are introduced. Nano-PSs boosting sensitivity and conversion efficiency to PDT-activating stimuli are highlighted. Nano-PSs developed to address the challenging hypoxia conditions during PDT of deep-sited tumors are summarized. Specifically, PSs capable of synergistic therapy and the emerging novel types with higher ROS yield that further enhance PDT efficacy are presented. Finally, future demands for ideal nano-PSs, emphasizing clinical translation and application are discussed.

Mono-/Bimetallic Neutral Iridium(III) Complexes Bearing Diketopyrrolopyrrole-Substituted N-Heterocyclic Carbene Ligands: Synthesis and Photophysics

The synthesis and photophysics (UV-vis absorption, emission, and transient absorption) of four neutral heteroleptic cyclometalated iridium(III) complexes (Ir-1-Ir-4) incorporating thiophene/selenophene-diketopyrrolopyrrole (DPP)-substituted N-heterocyclic carbene (NHC) ancillary ligands are reported. The effects of thiophene versus selenophene substitution on DPP and bis- versus monoiridium(III) complexation on the photophysics of these complexes were systematically investigated via spectroscopic techniques and density functional theory calculations. All complexes exhibited strong vibronically resolved absorption in the regions of 500-700 nm and fluorescence at 600-770 nm, and both are predominantly originated from the DPP-NHC ligand. Complexation induced a pronounced red shift of this low-energy absorption band and the fluorescence band with respect to their corresponding ligands due to the improved planarity and extended π-conjugation in the DPP-NHC ligand. Replacing the thiophene units by selenophenes and/or biscomplexation led to the red-shifted absorption and fluorescence spectra, accompanied by the reduced fluorescence lifetime and quantum yield and enhanced population of the triplet excited states, as reflected by the stronger triplet excited-state absorption and singlet oxygen generation.

Emerging Design Principle of Near-Infrared Upconversion Sensitizer Based on Mitochondria-Targeted Organic Dye for Enhanced Photodynamic Therapy

Upconversion luminescent (UCL) triggered photodynamic therapy (PDT) affords superior outcome for cancer treatment. However, conventional UCL materials which all work by a multiphoton absorption (MPA) process inevitably need extremely high power density far over the maximum permissible exposure (MPE) to laser. Here, a one-photon absorption molecular upconversion sensitizer Cy5.5-Br based on frequency upconversion luminescent (FUCL) is designed for PDT. The unusual super heavy atom effect (SHAE) in Cy5.5-Br strongly enhances its spin-orbit coupling (0.23 cm^-1 ), triplet quantum yield (11.1 %) and triplet state lifetime (18.8 μs) while the potential hot-band absorption of Cy5.5-Br is well maintained. Importantly, Cy5.5-Br can efficiently target the tumour site and kill cancer cells by destroying mitochondria under a biosafety MPE to 808 nm laser. The photostability and antitumor results are obviously superior to that of a Stokes process. This work provides a design criterion for FUCL dyes to realize effective PDT upon a biosafety optical density, possibly bringing more clinical benefits than conventional MPA materials.

Highly Efficient Far-Red/NIR-Absorbing Neutral Ir(III) Complex Micelles for Potent Photodynamic/Photothermal Therapy

A critical issue in photodynamic therapy (PDT) is inadequate reactive oxygen species (ROS) generation in tumors, causing inevitable survival of tumor cells that usually results in tumor recurrence and metastasis. Existing photosensitizers frequently suffer from relatively low light-to-ROS conversion efficiency with far-red/near-infrared (NIR) light excitation due to low-lying excited states that lead to rapid non-radiative decays. Here, a neutral Ir(III) complex bearing distyryl boron dipyrromethene (BODIPY-Ir) is reported to efficiently produce both ROS and hyperthermia upon far-red light activation for potentiating in vivo tumor suppression through micellization of BODIPY-Ir to form "Micelle-Ir". BODIPY-Ir absorbs strongly at 550-750 nm with a band maximum at 685 nm, and possesses a long-lived triplet excited state with sufficient non-radiative decays. Upon micellization, BODIPY-Ir forms J-type aggregates within Micelle-Ir, which boosts both singlet oxygen generation and the photothermal effect through the high molar extinction coefficient and amplification of light-to-ROS/heat conversion, causing severe cell apoptosis. Bifunctional Micelle-Ir that accumulates in tumors completely destroys orthotopic 4T1 breast tumors via synergistic PDT/photothermal therapy (PTT) damage under light irradiation, and enables remarkable suppression of metastatic nodules in the lungs, together without significant dark cytotoxicity. The present study offers an emerging approach to develop far-red/NIR photosensitizers toward potent cancer therapy.

An Approach to Developing Cyanines with Simultaneous Intersystem Crossing Enhancement and Excited-State Lifetime Elongation for Photodynamic Antitumor Metastasis

Heavy-atom-based photosensitizers usually exhibit shortened triplet-state lifetimes, which is not ideal for hypoxic tumor photodynamic therapy. Although several heavy-atom-free photosensitizers possess long triplet-state lifetimes, the clinical applicability is limited by their short excitation wavelengths, poor photon capture abilities, and intrinsically hydrophobic structures. Herein we developed a novel NIR heavy-atom-free photosensitizer design strategy by introducing sterically bulky and electron-rich moieties at the meso position of the pentamethine cyanine (Cy5) skeleton, which simultaneously enhanced intersystem crossing (ISC) and prolonged excited-state lifetime. We found that the ¹O₂ generation ability is directly correlated to the electron-donating ability of the meso substituent in cyanine, and the excited-state lifetime was simultaneously much elongated when the substituents were anthracene derivatives substituted at the 9-position. Our star compound, ANOMe-Cy5, exhibits intense NIR absorption, the highest ¹O₂ quantum yield (4.48-fold higher than Cy5), the longest triplet-state lifetime (9.80-fold longer than Cy5), and lossless emission intensity (nearly no change compared with Cy5). Such excellent photophysical properties coupled with its inherently cationic and hydrophilic nature enable the photosensitizer to realize photoablation of solid tumor and antitumor lung metastasis. This study highlights the design of a new generation of NIR photosensitizers for imaging-guided photodynamic cancer treatment.

Peroxidase-mimicking evodiamine/indocyanine green nanoliposomes for multimodal imaging-guided theranostics for oral squamous cell carcinoma

Here, evodiamine (EVO) and the photosensitizer indocyanine green (ICG) were integrated into a liposomal nanoplatform for noninvasive diagnostic imaging and combinatorial therapy against oral squamous cell carcinoma (OSCC). EVO, as an active component extracted from traditional Chinese medicine, not only functioned as an antitumor chemotherapeutic agent but was also capable of 68Ga-chelation, thus working as a contrast agent for positron emission tomography/computed tomography (PET/CT) imaging. Moreover, EVO could exhibit peroxidase-like catalytic activity, converting endogenous tumor H2O2 into cytotoxic reactive oxygen species (ROS), enabling Chemo catalytic therapy beyond the well-known chemotherapy effect of EVO. As proven by in vitro and in vivo experiments, guided by optical imaging and PET/CT imaging, we show that the theragnostic liposomes have a significant inhibiting effect on in situ tongue tumor through photodynamic therapy combined with chemodynamic chemotherapy.

Recent Advances in Cyanine-Based Phototherapy Agents

Phototherapies, in the form of photodynamic therapy (PDT) and photothermal therapy (PTT), are very promising treatment modalities for cancer since they provide locality and turn-on mechanism for toxicity, both of which are critical in reducing off-site toxicity. Irradiation of photosensitive agents demonstrated successful therapeutic outcomes; however, each approach has its limitations and needs to be improved for clinical success. The combination of PTT and PDT may work in a synergistic way to overcome the limitations of each method and indeed improve the treatment efficacy. The development of single photosensitive agents capable of inducing both PDT and PTT is, therefore, extremely advantageous and highly desired. Cyanine dyes are shown to have such potential, hence have been very popular in the recent years. Luminescence of cyanine dyes renders them as phototheranostic molecules, reporting the localization of the photosensitive agent prior to irradiation to induce phototoxicity, hence allowing image-guided phototherapy. In this review, we mainly focus on the cyanine dye-based phototherapy of different cancer cells, concentrating on the advancements achieved in the last ten years.

Potential of Cyanine Derived Dyes in Photodynamic Therapy

Photodynamic therapy (PDT) is a method of cancer treatment that leads to the disintegration of cancer cells and has developed significantly in recent years. The clinically used photosensitizers are primarily porphyrin, which absorbs light in the red spectrum and their absorbance maxima are relatively short. This review presents group of compounds and their derivatives that are considered to be potential photosensitizers in PDT. Cyanine dyes are compounds that typically absorb light in the visible to near-infrared-I (NIR-I) spectrum range (750-900 nm). This meta-analysis comprises the current studies on cyanine dye derivatives, such as indocyanine green (so far used solely as a diagnostic agent), heptamethine and pentamethine dyes, squaraine dyes, merocyanines and phthalocyanines. The wide array of the cyanine derivatives arises from their structural modifications (e.g., halogenation, incorporation of metal atoms or organic structures, or synthesis of lactosomes, emulsions or conjugation). All the following modifications aim to increase solubility in aqueous media, enhance phototoxicity, and decrease photobleaching. In addition, the changes introduce new features like pH-sensitivity. The cyanine dyes involved in photodynamic reactions could be incorporated into sets of PDT agents.

Balanced Intersystem Crossing in Iodinated Silicon-Fluoresceins Allows New Class of Red Shifted Theranostic Agents

Iodination of the silicon-fluorescein core revealed a new class of highly cytotoxic, red-shifted and water-soluble photosensitizer (SF-I) which is also fairly emissive to serve as a theranostic agent. Singlet oxygen generation capacity of SF-I was evaluated chemically, and up to 45% singlet oxygen quantum yield was reported in aqueous solutions. SF-I was further tested in triple negative breast (MDA MB-231) and colon (HCT-116) cancer cell lines, which are known to have limited chemotherapy options as well as very poor prognosis. SF-I induced efficient singlet oxygen generation and consequent photocytotoxicity in both cell lines upon light irradiation with a negligible dark toxicity while allowing cell imaging at the same time. SF-I marks the first ever example of a silicon xanthene-based photosensitizer and holds a lot of promise as a small-molecule-based theranostic scaffold.

Hows and Whys of Tumor-Seeking Dyes

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