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Osmium (Os) based photosensitizers (PSs) are a unique class of nontetrapyrrolic metal-containing PSs that absorb red light. We recently reported a highly potent Os(II) PS, rac-[Os(phen)

We explore the photophysical properties of a family of Ru(II) complexes, Ru-ip-nT, designed as photosensitizers for photodynamic therapy (PDT). The complexes incorporate a 1H-imidazo[4,5-f][1,10]-phenanthroline (ip) ligand appended to one or more thiophene rings. One of the complexes studied herein, Ru-ip-3T (known as TLD1433), is currently in Phase II human clinical trials for treating bladder cancer by photodynamic therapy (PDT). The potent photocytotoxicity of Ru-ip-3T is attributed to a long-lived intraligand charge-transfer triplet state. The accessibility of this state changes upon varying the length (n) of the oligothiophene substituent. In this paper we highlight the impact of n on the ultrafast photoinduced dynamics in Ru-ip-nT, leading to the formation of the function-determining long-lived state. Femtosecond time-resolved transient absorption combined with resonance Raman data was used to map the excited-state relaxation processes from the Franck-Condon point of absorption to the formation of the lowest-energy triplet excited state, which is a triplet metal-to-ligand charge transfer ((3)MLCT) excited state for Ru-ip-0T-1T and an oligothienyl-localized triplet intraligand charge transfer ((3)ILCT) excited state for Ru-ip-2T-4T. We establish the structure-activity relationships with regard to changes in the excited state dynamics as a function of thiophene chain length, which alters the photophysics of the complexes and presumably impacts the photocytotoxicity of these photosensitizers.

Sub/supercritical fluid chromatography (SFC) is a green separation technique that has been used to separate a wide variety of compounds and is proven to be immensely useful for chiral separations. However, SFC is currently not thought to be applicable for ionic compounds due to their low solubility in CO

Ru(II) complexes that undergo photosubstitution reactions from triplet metal-centered ((3)MC) excited states are of interest in photochemotherapy (PCT) for their potential to produce cytotoxic effects in hypoxia. Dual-action systems that incorporate this stoichiometric mode to complement the oxygen-dependent photosensitization pathways that define photodynamic therapy (PDT) are poised to maintain antitumor activity regardless of oxygenation status. Herein, we examine the way in which these two pathways influence photocytotoxicity in normoxia and in hypoxia using the [Ru(dmp)(2)(IP-nT)](2+) series (where dmp=2,9-dimethyl-1,10-phenanthroline and IP-nT=imidazo[4,5-f][1,10]phenanthroline tethered to n=0–4 thiophene rings) to switch the dominant excited state from the metal-based (3)MC state in the case of Ru-phen–Ru-1T to ligand-based (3)ILCT state for Ru-3T and Ru-4T. Ru-phen–Ru-1T, having dominant (3)MC states and the largest photosubstitution quantum yields, were inactive in both normoxia and hypoxia. Ru-3T and Ru-4T, with dominant (3)IL/(3)ILCT states and long triplet lifetimes (τ(TA)=20–25 μs), had the poorest photosubstitution quantum yields yet were extremely active. In the best instances, Ru-4T exhibited attomolar phototoxicity toward SKMEL28 cells in normoxia and picomolar in hypoxia, with PI values in normoxia of 10(5)–10(12) and 10(3)–10(6) in hypoxia. While maximizing excited state deactivation through photodissociative (3)MC states did not result in bonafide dual-action PDT/PCT agents, the study has produced the most potent photosensitizer we know of to date. The extraordinary photosensitizing capacity of Ru-3T and Ru-4T may stem from a combination of very efficient (1)O(2) production and possibly complementary Type I pathways via (3)ILCT excited states.

Ruthenium complexes bearing protic diimine ligands are cytotoxic to certain cancer cells upon irradiation with blue light. Previously reported complexes of the type [(N,N)(2)Ru(6,6’-dhbp)]Cl(2) with 6,6’-dhbp = 6,6’-dihydroxybipyridine and N,N = 2,2’-bipyridine (bipy) (1(A)), 1,10-phenanthroline (phen) (2(A)), and 2,3-dihydro-[1,4]dioxino[2,3-f][1,10]phenanthroline (dop) (3(A)) show EC(50) values as low as 4 μM (for 3(A)) vs. breast cancer cells upon blue light irradiation (Inorg. Chem. 2017, 56, 7519). Herein, subscript A denotes the acidic form of the complex bearing OH groups, and B denotes the basic form bearing O(−) groups. This photocytotoxicity was originally attributed to photodissociation, but recent results suggest that singlet oxygen formation is a more plausible cause of photocytotoxicity. In particular, bulky methoxy substituents enhance photodissociation but these complexes are non-toxic (Dalton Trans. 2018, 47, 15685). Cellular studies are presented herein that show the formation of reactive oxygen species (ROS) and apoptosis indicators upon treatment of cells with complex 3(A) and blue light. Singlet oxygen sensor green (SOSG) shows the formation of (1)O(2) in cell culture for cells treated with 3(A) and blue light. At physiological pH, complexes 1(A)-3(A) are deprotonated to form 1(B)-3(B) in situ. Quantum yields for (1)O(2) (ɸ(Δ) ) are 0.87 and 0.48 for 2(B) and 3(B) respectively, and these are an order of magnitude higher than the quantum yields for 2(A) and 3(A). The values for ɸ(Δ) show an increase with 6,6’-dhbp derived substituents as follows: OMe < OH < O(−). TD-DFT studies show that the presence of a low lying triplet metal-centered ((3)MC) state favors photodissociation and disfavors (1)O(2) formation for 2(A) and 3(A) (OH groups). However, upon deprotonation (O(−) groups), the (3)MLCT state is accessible and can readily lead to (1)O(2) formation, but the dissociative (3)MC state is energetically inaccessible. The changes to the energy of the (3)MLCT state upon deprotonation have been confirmed by steady state luminescence experiments on 1(A)-3(A) and their basic analogs, 1(B)-3(B). This energy landscape favors (1)O(2) formation for 2(B) and 3(B) and leads to enhanced toxicity for these complexes under physiological conditions. The ability to convert readily from OH to O(−) groups allowed us to investigate an electronic change that is not accompanied by steric changes in this fundamental study.

TLD1433 is the first Ru(II) complex to be tested as a photodynamic therapy agent in a clinical trial. In this contribution we study TLD1433 in the context of structurally-related Ru(II)-imidozo[4,5-f][1,10]phenanthroline (ip) complexes appended with thiophene rings to decipher the unique photophysical properties which are associated with increasing oligothiophene chain length. Substitution of the ip ligand with ter- or quaterthiophene changes the nature of the long-lived triplet state from metal-to-ligand charge-transfer to (3)ππ* character. The addition of the third thiophene thus presents a critical juncture which not only determines the photophysics of the complex but most importantly its capacity for (1)O(2) generation and hence the potential of the complex to be used as a photocytotoxic agent. ENTRY FOR THE TABLE OF CONTENTS: A low-lying triplet intraligand state ((3)IL) determines the properties of the long-lived excited states in a series of Ru(II) complexes. The (3)IL state can be accessed by increasing the length of an oligothiophene chain. The (3)IL state is extremely efficient at generating (1)O(2) and thus enhances the potency of the complexes as PDT agents. [Image: see text]

Alzheimer's disease (AD) is a chronic neurodegenerative disorder characterized by progressive and irreversible damage to the brain. One of the hallmarks of the disease is the presence of both soluble and insoluble aggregates of the amyloid beta (Aβ) peptide in the brain, and these aggregates are considered central to disease progression. Thus, the development of small molecules capable of modulating Aβ peptide aggregation may provide critical insight into the pathophysiology of AD. In this work we investigate how photoactivation of three distorted Ru(II) polypyridyl complexes (Ru1–3) alters the aggregation profile of the Aβ peptide. Photoactivation of Ru1–3 results in the loss of a 6,6′-dimethyl-2,2′-bipyridyl (6,6′-dmb) ligand, affording cis-exchangeable coordination sites for binding to the Aβ peptide. Both Ru1 and Ru2 contain an extended planar imidazo[4,5-f][1,10]phenanthroline ligand, as compared to a 2,2′-bipyridine ligand for Ru3, and we show that the presence of the phenanthroline ligand promotes covalent binding to Aβ peptide His residues, and in addition, leads to a pronounced effect on peptide aggregation immediately after photoactivation. Interestingly, all three complexes resulted in a similar aggregate size distribution at 24 h, forming insoluble amorphous aggregates as compared to significant fibril formation for peptide alone. Photoactivation of Ru1–3 in the presence of pre-formed Aβ1–42 fibrils results in a change to amorphous aggregate morphology, with Ru1 and Ru2 forming large amorphous aggregates immediately after activation. Our results show that photoactivation of Ru1–3 in the presence of either monomeric or fibrillar Aβ1–42 results in the formation of large amorphous aggregates as a common endpoint, with Ru complexes incorporating the extended phenanthroline ligand accelerating this process and thereby limiting the formation of oligomeric species in the initial stages of the aggregation process that are reported to show considerable toxicity.

Intracellular excited-state properties of a photodynamic active Osmium-complex are investigated. The key to the excited-state relaxation is the interplay of 3MLCT and 3ILCT states on the oligothiophene-substituted ligand. We successfully tested the model derived from solution with the complex internalized in cells to bridge the gap between cuvette and in vivo measurements for the first time. This contribution describes the excited-state properties of an Osmium-complex when taken up into human cells. The complex 1 [Os(bpy)2(IP-4T)](PF6)2 with bpy=2,2′-bipyridine and IP-4T=2-{5′-[3′,4′-diethyl-(2,2′-bithien-5-yl)]-3,4-diethyl-2,2′-bithiophene}imidazo[4,5-f][1,10]phenanthroline) can be discussed as a candidate for photodynamic therapy in the biological red/NIR window. The complex is taken up by MCF7 cells and localizes rather homogeneously within in the cytoplasm. To detail the sub-ns photophysics of 1, comparative transient absorption measurements were carried out in different solvents to derive a model of the photoinduced processes. Key to rationalize the excited-state relaxation is a long-lived 3ILCT state associated with the oligothiophene chain. This model was then tested with the complex internalized into MCF7 cells, since the intracellular environment has long been suspected to take big influence on the excited state properties. In our study of 1 in cells, we were able to show that, though the overall model remained the same, the excited-state dynamics are affected strongly by the intracellular environment. Our study represents the first in depth correlation towards ex-vivo and in vivo ultrafast spectroscopy for a possible photodrug., publishedVersion

TLD1433 is the first ruthenium (Ru)-based photodynamic therapy (PDT) agent to advance to clinical trials and is currently in a Phase II study for treating non-muscle bladder cancer with PDT. Herein, we present a photophysical study of TLD1433 and its derivative TLD1633 using complex, biologically relevant solvents to elucidate the excited state properties that are key for biological activity. The complexes incorporate an imidazo [4,5-f][1,10]phenanthroline (IP) ligand appended to α-ter- or quaterthiophene, respectively, where TLD1433 = [Ru(4,4′-dmb)(2)(IP-3T)]Cl(2) and TLD1633 = [Ru(4,4′-dmb)(2)(IP-4T)]Cl(2) (4,4′-dmb=4,4′-dimethyl-2,2′-bipyridine; 3T=α-terthiophene; 4T= α-quaterthiophene). Time-resolved transient absorption experiments demonstrate that the excited state dynamics of the complexes change upon interaction with biological macromolecules (e.g., DNA). In this case, the accessibility of the lowest energy T(1) state is increased at the expense of a higher-lying triplet intraligand charge transfer ((3)ILCT) state. We attribute this behavior to the increased rigidity of the ligand framework upon binding to DNA, which prolongs the lifetime of the T(1) state. This lowest lying state is primarily responsible for O(2) sensitization and hence photoinduced cytotoxicity. Therefore, in order to gain a realistic picture of the excited-state kinetics that underlie the photoinduced function of the complexes, it is necessary to interrogate their photophysical dynamics in the presence of biological targets once they are known.

The design of near-infrared (NIR)-active photosensitizers (PSs) for light-based cancer treatments such as photodynamic therapy (PDT) has been a challenge. While several NIR-Ru(II) scaffolds have been reported, this approach has not been proven in cells. This is the first report of NIR-Ru(II) PSs that are phototoxic to cancer cells, including highly pigmented B16F10 melanoma cells. The PS family incorporated a bis(1,8-naphthyridine)-based ligand (tpbn), a bidentate thiophene-based ligand (nT; n=0–4), and a monodentate 4-picoline ligand (4-pic). All compounds absorbed light >800 nm with maxima near 730 nm. Transient absorption (TA) measurements indicated that n≥4 thiophene rings (4T) positioned the PDT-active triplet intraligand charge transfer ((3)ILCT) excited state in energetic proximity to the lowest lying triplet metal-to-ligand charge transfer ((3)MLCT). 4T had low micromolar phototoxicity with PI(vis) and PI(733nm) values as large as 90 and 12, respectively. Spectroscopic studies suggested that the long-lived (T(TA)=3–6 μs) (3)ILCT was accessible from the (3)MLCT state, but energetically uphill in the overall photophysics. The study highlights that phototoxic effects can be achieved with NIR-absorbing Ru(II) PSs as long as the reactive (3)ILCT states are energetically accessible from the low-energy (3)MLCT states. It also demonstrates that tissue-penetrating NIR light can be used to activate the PSs in highly pigmented cells where melanin attenuates shorter wavelengths of light.

Mounting evidence over the past 20 years suggests that photodynamic therapy (PDT), an anticancer modality known mostly as a local treatment, has the capacity to invoke a systemic antitumor immune response, leading to protection against tumor recurrence. For aggressive cancers such as melanoma, where chemotherapy and radiotherapy are ineffective, immunomodulating PDT as an adjuvant to surgery is of interest. Towards the development of specialized photosensitizers (PSs) for treating pigmented melanomas, nine new near-infrared (NIR) absorbing PSs based on a Ru(ii) tris-heteroleptic scaffold [Ru(NNN)(NN)(L)]Cln, were explored. Compounds 2, 6, and 9 exhibited high potency toward melanoma cells, with visible EC50 values as low as 0.292–0.602 μM and PIs as high as 156–360. Single-micromolar phototoxicity was obtained with NIR-light (733 nm) with PIs up to 71. The common feature of these lead NIR PSs was an accessible low-energy triplet intraligand (3IL) excited state for high singlet oxygen (1O2) quantum yields (69–93%), which was only possible when the photosensitizing 3IL states were lower in energy than the lowest triplet metal-to-ligand charge transfer (3MLCT) excited states that typically govern Ru(ii) polypyridyl photophysics. PDT treatment with 2 elicited a pro-inflammatory response alongside immunogenic cell death in mouse B16F10 melanoma cells and proved safe for in vivo administration (maximum tolerated dose = 50 mg kg−1). Female and male mice vaccinated with B16F10 cells that were PDT-treated with 2 and challenged with live B16F10 cells exhibited 80 and 55% protection from tumor growth, respectively, leading to significantly improved survival and excellent hazard ratios of ≤0.2., Ru(ii) photosensitizers (PSs) destroy aggressive melanoma cells, triggering an immune response that leads to protection against tumor challenge and mouse survival.

Hypoxia presents a two-fold challenge in the treatment of cancer, as low oxygen conditions induce biological changes that make malignant tissues simultaneously more aggressive and less susceptible to standard chemotherapy. This paper reports the first metal-based photosensitizer that approaches the ideal properties for a phototherapy agent. The Os(phen)2-based scaffold was combined with a series of IP-nT ligands, where phen = 1,10-phenanthroline and IP-nT = imidazo[4,5-f][1,10]phenanthroline tethered to n = 0-4 thiophene rings. Os-4T (n = 4) emerged as the most promising complex in the series, with picomolar activity and a phototherapeutic index (PI) exceeding 106 in normoxia. The photosensitizer exhibited an unprecedented PI > 90 (EC50 = 0.651 μM) in hypoxia (1% O2) with visible and green light, and a PI > 70 with red light. Os-4T was also active with 733 nm near-infrared light (EC50 = 0.803 μM, PI = 77) under normoxia. Both computation and spectroscopic studies confirmed a switch in the nature of the lowest-lying triplet excited state from triplet metal-to-ligand charge transfer (3MLCT) to intraligand charge transfer (3ILCT) at n = 3, with a lower energy and longer lifetime for n = 4. All compounds in the series were relatively nontoxic in the dark but became increasingly phototoxic with additional thiophenes. These normoxic and hypoxic activities are the largest reported to date, demonstrating the utility of osmium for phototherapy applications. Moreover, Os-4T had a maximum tolerated dose (MTD) in mice that was >200 mg kg-1, which positions this photosensitizer as an excellent candidate for in vivo applications.

Tumor hypoxia renders treatments ineffective that are directly (e.g., radiotherapy and photodynamic therapy) or indirectly (e.g., chemotherapy) dependent on tumor oxygenation. This study introduces a ruthenium compound as a light-responsive anticancer agent that is water soluble, has minimal dark cytotoxicity, is active at concentrations as low as 170 pM in ~18.5% O(2) normoxia and near 10 nM in 1% O(2) hypoxia, and exhibits phototherapeutic indices as large as >500,000 in normoxia and >5,800 in 1% O(2) hypoxia using broadband visible and monochromatic blue light treatments. These are the largest values reported to date for any compound class. We highlight the response in four different cell lines to improve rigor and reproducibility in the identification of promising clinical candidates.

The advent of cisplatin as a cancer drug in the late 1960s generated considerable interest in the use of transition metal complexes as cancer therapy agents. Despite enhanced research in this area, there has yet to be any non-platinum-based transition metal complex cancer drugs approved by the Food and Drug Administration (FDA). Recently a Ru(II) metal-organic dyad (TLD1433) has provided promising results as a photodynamic therapy (PDT) agent for some types of cancer. This particularly effective PDT compound has an oligothiophene chain appended to an imidazophenanthroline ligand which chelates Ru(II). The entire complex is chiral and is synthesized as a racemate. Five such chiral Ru(II) and Os(II) PDT agents were synthesized and their enantiomers separated for the first time. The enantiomers of these compounds are not easily crystalized. However, preparative LC provided sufficient amounts of these novel PDT agents to determine their absolute configurations by vibrational circular dichroism (VCD). The synthesis, separation and absolute configuration determinations are described and discussed in detail.

Alzheimer's disease (AD) is a chronic neurodegenerative disorder characterized by progressive and irreversible damage to the brain. One of the hallmarks of the disease is the presence of both soluble and insoluble aggregates of the amyloid beta (Aβ) peptide in the brain, and these aggregates are considered central to disease progression. Thus, the development of small molecules capable of modulating Aβ peptide aggregation may provide critical insight into the pathophysiology of AD. In this work we investigate how photoactivation of three distorted Ru(ii) polypyridyl complexes (Ru1–3) alters the aggregation profile of the Aβ peptide. Photoactivation of Ru1–3 results in the loss of a 6,6′-dimethyl-2,2′-bipyridyl (6,6′-dmb) ligand, affording cis-exchangeable coordination sites for binding to the Aβ peptide. Both Ru1 and Ru2 contain an extended planar imidazo[4,5-f][1,10]phenanthroline ligand, as compared to a 2,2′-bipyridine ligand for Ru3, and we show that the presence of the phenanthroline ligand promotes covalent binding to Aβ peptide His residues, and in addition, leads to a pronounced effect on peptide aggregation immediately after photoactivation. Interestingly, all three complexes resulted in a similar aggregate size distribution at 24 h, forming insoluble amorphous aggregates as compared to significant fibril formation for peptide alone. Photoactivation of Ru1–3 in the presence of pre-formed Aβ1–42 fibrils results in a change to amorphous aggregate morphology, with Ru1 and Ru2 forming large amorphous aggregates immediately after activation. Our results show that photoactivation of Ru1–3 in the presence of either monomeric or fibrillar Aβ1–42 results in the formation of large amorphous aggregates as a common endpoint, with Ru complexes incorporating the extended phenanthroline ligand accelerating this process and thereby limiting the formation of oligomeric species in the initial stages of the aggregation process that are reported to show considerable toxicity., Photoactivation of a series of Ru(ii) polypyridyl complexes leads to ligand exchange and modulation of amyloid-beta peptide aggregation of relevance to Alzheimer's disease.

Direct oxidation of silyl ethers to silyl esters, using RuO4 formed in situ, is reported. The reaction was optimized to minimize formation of the corresponding carboxylic acid product whose formation pathway appears to be solvent dependent. The reaction is tolerant of halides, nitriles, nitro groups, esters, epoxides, and ketones.

In an earlier study of π-expansive ruthenium complexes for photodynamic and photochemo-therapies, it was shown that a pair of structural isomers differing only in the connection point of a naphthalene residue exhibited vastly different biological activity. These isomers are further explored in this paper through the activity of their functionalized derivatives. In normoxia, the inactive 2-NIP isomer (5) can be made as photocytotoxic as the active 1-NIP isomer (1) by functionalizing with methyl or methoxy groups, while methoxy variants of the 1-NIP isomer became inactive. In all cases, the singlet oxygen sensitization quantum yield was below 1%. Hypoxic photocytotoxicity was attenuated, with only three of the series showing any activity, notwithstanding the photodissociative ligands. The results here are consistent with the earlier findings in that seemingly minor structural modifications on the non-strained ligand can dramatically modulate the normoxic and hypoxic activity of these strained compounds and that these changes appear to exert a greater influence on photocytotoxicity than singlet oxygen sensitization or rates of photosubstitution in cell-free conditions would suggest.

We report a new class of ruthenium (Ru)-based photosensitizers that induce potent cytotoxicity in melanoma cells following activation with NIR light. In addition to the direct cytotoxic effect, this Ru-based photodynamic therapy induces immunogenic cell death in melanoma cells that can be therapeutically exploited to establish protective antitumor immunity.

Hypoxia presents a challenge to anticancer therapy, reducing the efficacy of many available treatments. Photodynamic therapy (PDT) is particularly susceptible to hypoxia given that its mechanism relies on oxygen. Herein, we introduce two new osmium-based polypyridyl photosensitizers that are active in hypoxia. The lead compounds emerged from a systematic study of two Os(II) polypyridyl families derived from 2,2′-bipyridine (bpy) or 4,4′-dimethyl-2,2′-bipyridine (dmb) as coligands combined with imidazo[4,5-f][1,10]phenanthroline ligands tethered to n=0–4 thiophenes (IP-nT). The compounds were characterized and investigated for their spectroscopic and (photo)biological activities. The two hypoxia-active Os(II) photosensitizers had n=4 thiophenes, with the bpy analog 1–4T being the most potent. In normoxia, 1–4T had low nanomolar activity (EC(50)=1–13 nM) with phototherapeutic indices (PI) ranging from 5,500 to 55,000 with red and visible light, respectively. Submicromolar potency was maintained even in hypoxia (1% O(2)), with light EC(50) and PI values of 732–812 nM and 68–76, respectively — among the largest PIs to date for hypoxic photoactivity. This high degree of activity coincided with a low-energy, long-lived (0.98–3.6 μs) mixed-character intraligand-charge-transfer ((3)ILCT)/ligand-to-ligand charge transfer ((3)LLCT) state only accessible in quaterthiophene complexes 1–4T and 2–4T. The coligand identity strongly influenced the photophysical and photobiological results in this study, whereby the bpy coligand led to longer lifetimes (3.6 μs) and more potent photocytotoxicity relative to dmb. The unactivated compounds were relatively nontoxic both in vitro and in vivo. The maximum tolerated dose (MTD) for 1–4T and 2–4T in mice was ≥ 200 mg kg(−1), an excellent starting point for future in vivo validation.

Intra-operative photodynamic therapy (IO-PDT) in combination with surgery for the treatment of non-small cell lung cancer and malignant pleural mesothelioma has shown promise in improving overall survival in patients. Here, we developed a PDT platform consisting of a ruthenium-based photosensitizer (TLD1433) activated by an optical surface applicator (OSA) for the management of residual disease. Human lung adenocarcinoma (A549) cell viability was assessed after treatment with TLD1433-mediated PDT illuminated with either 532- or 630-nm light with a micro-lens laser fiber. This TLD1433-mediated PDT induced an EC50 of 1.98 &mu, M (J/cm2) and 4807 &mu, M (J/cm2) for green and red light, respectively. Cells were then treated with 10 &micro, M TLD1433 in a 96-well plate with the OSA using two 2-cm radial diffusers, each transmitted 532 nm light at 50 mW/cm for 278 s. Monte Carlo simulations of the surface light propagation from the OSA computed light fluence (J/cm2) and irradiance (mW/cm2) distribution. In regions where 100% loss in cell viability was measured, the simulations suggest that &gt, 20 J/cm2 of 532 nm was delivered. Our studies indicate that TLD1433-mediated PDT with the OSA and light simulations have the potential to become a platform for treatment planning for IO-PDT.

The ruthenium-based photosensitizer (PS) TLD1433 has completed a phase I clinical trial for photodynamic therapy (PDT) treatment of bladder cancer. Here, we investigated a possible repurposing of this drug for treatment of conjunctival melanoma (CM). CM is a rare but often deadly ocular cancer. The efficacy of TLD1433 was tested on several cell lines from CM (CRMM1, CRMM2 and CM2005), uveal melanoma (OMM1, OMM2.5, MEL270), epidermoid carcinoma (A431) and cutaneous melanoma (A375). Using 15 min green light irradiation (21 mW/cm2, 19 J.cm&minus, 2, 520 nm), the highest phototherapeutic index (PI) was reached in CM cells, with cell death occurring via apoptosis and necrosis. The therapeutic potential of TLD1433 was hence further validated in zebrafish ectopic and newly-developed orthotopic CM models. Fluorescent CRMM1 and CRMM2 cells were injected into the circulation of zebrafish (ectopic model) or behind the eye (orthotopic model) and 24 h later, the engrafted embryos were treated with the maximally-tolerated dose of TLD1433. The drug was administrated in three ways, either by (i) incubating the fish in drug-containing water (WA), or (ii) injecting the drug intravenously into the fish (IV), or (iii) injecting the drug retro-orbitally (RO) into the fish. Optimally, four consecutive PDT treatments were performed on engrafted embryos using 60 min drug-to-light intervals and 90 min green light irradiation (21 mW/cm2, 114 J.cm&minus, 2, 520 nm). This PDT protocol was not toxic to the fish. In the ectopic tumour model, both systemic administration by IV injection and RO injection of TLD1433 significantly inhibited growth of engrafted CRMM1 and CRMM2 cells. However, in the orthotopic model, tumour growth was only attenuated by localized RO injection of TLD1433. These data unequivocally prove that the zebrafish provides a fast vertebrate cancer model that can be used to test the administration regimen, host toxicity and anti-cancer efficacy of PDT drugs against CM. Based on our results, we suggest repurposing of TLD1433 for treatment of incurable CM and further testing in alternative pre-clinical models.