Singlet Oxygen Generation by Metallotexaphyrins

Lutetium Texaphyrin-Celecoxib Conjugate as a Potential Immuno-Photodynamic Therapy Agent

Immuno-photodynamic therapy (IPDT) has emerged as a new modality for cancer treatment. Novel photosensitizers can help achieve the promise inherent in IPDT, namely, the complete eradication of a tumor without recurrence. We report here a small molecule photosensitizer conjugate, LuCXB. This IPDT agent integrates a celecoxib (cyclooxygenase-2 inhibitor) moiety with a near-infrared absorbing lutetium texaphyrin photocatalytic core. In aqueous environments, the two components of LuCXB are self-associated through inferred donor-acceptor interactions. A consequence of this intramolecular association is that upon photoirradiation with 730 nm light, LuCXB produces superoxide radicals (O2-•) via a type I photodynamic pathway; this provides a first line of defense against the tumor while promoting IPDT. For in vivo therapeutic applications, we prepared a CD133-targeting, aptamer-functionalized exosome-based nanophotosensitizer (Ex-apt@LuCXB) designed to target cancer stem cells. Ex-apt@LuCXB was found to display good photosensitivity, acceptable biocompatibility, and robust tumor targetability. Under conditions of photoirradiation, Ex-apt@LuCXB acts to amplify IPDT while exerting a significant antitumor effect in both liver and breast cancer mouse models. The observed therapeutic effects are attributed to a synergistic mechanism that combines antiangiogenesis and photoinduced cancer immunotherapy.

Fractional dependence of the free energy of activation on the driving force of charge transfer in the quenching of the excited states of substituted phenanthroline homoleptic ruthenium(ii) complexes in aqueous medium

The photophysical characteristics of some homoleptic ruthenium(ii) phenanthroline derivatives are investigated in aqueous medium. The lifetimes of the excited ³MLCT state of the studied complexes were found to be very sensitive to the type of the substituents on the phenanthroline ligand and were found to increase from about 0.96 μs in case of the parent [Ru(Phen)₃]²⁺ complex to 2.97 μs in case of [Ru(DPPhen)₃]²⁺. The transient absorption spectra of the current set of complexes were studied also in aqueous medium. Quenching of the excited ³MLCT states of the studied complexes by molecular oxygen were studied and quenching rate constants were found to be in the range 1.02-4.83 × 10⁹ M^-1 s^-1. Values of singlet oxygen quantum yields were found to be in the range 0.01 to 0.25, and the corresponding efficiencies of singlet oxygen thereby produced, f ^T _Δ, were in the range 0.03-0.52. The mechanism by which the excited ³MLCT state is quenched by oxygen is discussed in light of the spin statistical factor rate constants and the competition between charge transfer and non-charge transfer quenching pathways. The partial charge transfer parameters, p _CT, were obtained and found to be about 0.88 for all complexes except for complexes with f ^T _Δ values lower than 0.25. The correlation of the activation free energies ΔG ^≠ of the exciplexes formation with the driving force for charge transfer, ΔG _CET, gives a charge transfer character of the exciplexes of about 35.0%.

Lanthanide-tetrapyrrole complexes: synthesis, redox chemistry, photophysical properties, and photonic applications

Tetrapyrrole derivatives such as porphyrins, phthalocyanines, naphthalocyanines, and porpholactones, are highly stable macrocyclic compounds that play important roles in many phenomena linked to the development of life. Their complexes with lanthanides are known for more than 60 years and present breath-taking properties such as a range of easily accessible redox states leading to photo- and electro-chromism, paramagnetism, large non-linear optical parameters, and remarkable light emission in the visible and near-infrared (NIR) ranges. They are at the centre of many applications with an increasing focus on their ability to generate singlet oxygen for photodynamic therapy coupled with bioimaging and biosensing properties. This review first describes the synthetic paths leading to lanthanide-tetrapyrrole complexes together with their structures. The initial synthetic protocols were plagued by low yields and long reaction times; they have now been replaced with much more efficient and faster routes, thanks to the stunning advances in synthetic organic chemistry, so that quite complex multinuclear edifices are presently routinely obtained. Aspects such as redox properties, sensitization of NIR-emitting lanthanide ions, and non-linear optical properties are then presented. The spectacular improvements in the quantum yield and brightness of Yb^III-containing tetrapyrrole complexes achieved in the past five years are representative of the vitality of the field and open welcome opportunities for the bio-applications described in the last section. Perspectives for the field are vast and exciting as new derivatizations of the macrocycles may lead to sensitization of other Ln^III NIR-emitting ions with luminescence in the NIR-II and NIR-III biological windows, while conjugation with peptides and aptamers opens the way for lanthanide-tetrapyrrole theranostics.

Texaphyrins: a new approach to drug development

The texaphyrins are prototypical metal-coordinating expanded porphyrins. They represent a burgeoning class of pharmacological agents that show promise for an array of medical applications. Currently, two different water-soluble lanthanide texaphyrins, namely motexafin gadolinium ( Gd-Tex , 1) and motexafin lutetium ( Lu-Tex , 2), are involved in multi-center clinical trials for a variety of indications. The first of these agents, XCYTRIN® (motexafin gadolinium) Injection, is being evaluated as a potential X-ray radiation enhancer in a randomized Phase III clinical trial in patients with brain metastases. The second, in various formulations, is being evaluated as a photosensitizer for use in: (i) the photodynamic treatment of recurrent breast cancer (LUTRIN® Injection; now in Phase IIb clinical trials); (ii) photoangioplastic reduction of atherosclerosis involving peripheral and coronary arteries (ANTRIN® Injection; now in Phase II and Phase I clinical trials, respectively); and (iii) light-based age-related macular degeneration (OPTRIN™ Injection; currently under Phase II clinical evaluation), a vision-threatening disease of the retina. In this article, these developments, along with fundamental aspects of the underlying chemistry are reviewed.

Pharmaceutical development and medical applications of porphyrin-type macrocycles

The current state of pharmaceutical development of porphyrin-type macrocycles in medicine is highlighted. Currently, several porphyrinoid-based drugs are under various stages of development as phototherapeutic agents, X-ray radiation enhancers and boron neutron capture agents. These compounds represent a burgeoning class of pharmacological agents that are potentially useful in an array of treatment areas.

Mechanisms of porphyrinoid localization in tumors

The photosensitizing and pharmacokinetic properties of porphyrin-type compounds have been investigated for nearly a century. In the last decade, two porphyrin derivatives were approved in the U.S.A. and in several other countries for the photodynamic treatment of various lesions. An overview of the different mechanisms for preferential porphyrinoid localization in malignant tumors is presented herein. Several uptake pathways are possible for each photosensitizer, which are determined by its structure, mode of delivery and tumor type. Comparisons of the different mechanisms and correlations with the structure of the sensitizer are presented. Current delivery systems for porphyrin sensitizers are described, as well as recent strategies for enhancing their tumor-specificity, including conjugation to a carrier system that selectively targets a tumor-associated receptor or antigen.

Increasing damage to tumor blood vessels during motexafin lutetium-PDT through use of low fluence rate

Photodynamic therapy (PDT) with low light fluence rate has rarely been studied in protocols that use short drug-light intervals and thus deliver illumination while plasma concentrations of photosensitizer are high, creating a prominent vascular response. In this study, the effects of light fluence rate on PDT response were investigated using motexafin lutetium (10 mg/kg) in combination with 730 nm light and a 180-min drug-light interval. At 180 min, the plasma level of photosensitizer was 5.7 ng/microl compared to 3.1 ng/mg in RIF tumor, and PDT-mediated vascular effects were confirmed by a spasmodic decrease in blood flow during illumination. Light delivery at 25 mW/cm(2) significantly improved long-term tumor responses over that at 75 mW/cm(2). This effect could not be attributed to oxygen conservation at low fluence rate, because 25 mW/cm(2) PDT provided little benefit to tumor hemoglobin oxygen saturation. However, 25 mW/cm(2) PDT did prolong the duration of ischemic insult during illumination and was correspondingly associated with greater decreases in perfusion immediately after PDT, followed by smaller increases in total hemoglobin concentration in the hours after PDT. Increases in blood volume suggest blood pooling from suboptimal vascular damage; thus the smaller increases after 25 mW/cm(2) PDT provide evidence of more widespread vascular damage, which was accompanied by greater decreases in clonogenic survival. Further study of low fluence rate as a means to improve responses to PDT under conditions designed to predominantly damage vasculature is warranted.

Photodynamic Properties of Hypocrellin A, Complexes with Rare Earth Trivalent Ions: Role of the Excited State Energies of the Metal Ions

Fifteen complexes of hypocrellin A (HA) with rare earth trivalent ions (except Pm3+) along with the complex of HA with Sc3+ were prepared, and their photodynamic activities, including absorption in the phototherapeutic window (600-900 nm); water-solubility; triplet lifetime; generation of reactive oxygen species (ROS), such as singlet oxygen (1O2), superoxide anion radical (O2-*), and hydroxyl radical (OH*); generation of semiquinone anion radical; and affinity to DNA, as well as photosensitized damage on calf thymus DNA (CT DNA), were compared in detail using the UV-visible spectrum, fluorescence spectrum, spin-trapping EPR technique, and laser photolysis technique. All complexes exhibit a red-shifted absorption spectrum, an increased absorbance above 600 nm, improved water solubility, and an enhanced affinity to CT DNA over the parent HA. For ions that possess low-energy excited states, including Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, and Yb3+, the corresponding complexes show undetectable or nearly undetectable fluorescence, a triplet excited-state lifetime, generation of ROS, and photodamage in CT DNA. In contrast, for ions that do not possess low-energy excited states, including Sc3+, Y3+, La3+, Gd3+, and Lu3+, the corresponding complexes exhibit higher photodamage abilities with CT DNA with respect to HA, benefitting from both their comparable or even higher 1O2 quantum yields and an electrostatic affinity that is higher for DNA than HA.

Mechanism of quenching by oxygen of the excited states of ruthenium(ii) complexes in aqueous media. Solvent isotope effect and photosensitized generation of singlet oxygen, O2(1Δg), by [Ru(diimine)(CN)4]2−complex ions

In this study we report on the photophysical properties of some [RuL(CN)4](2-) complex ions where L = 2,2'-bipyridine (bpy), 5,5'-dimethyl-2,2'-bipyridine (dmb), 1,10-phenanthroline (phen), 1-ethyl-2-(2-pyridyl)benzimidazole (pbe), 2,2':6',2'''-terpyridine (tpy) and [RuL3](2+) where L = bpy or phen. Measurements were carried out in H2O and D2O. The effect of the deuterium isotope effect on the lifetime of these complexes is discussed. It has also been found that the presence of cyano groups has a pronounced effect on the lifetime of the excited metal-to-ligand charge transfer ((3)MLCT) of these complexes. Quenching of the (3)MLCT states by oxygen is reported in H2O and D2O. The rate constants, k(q), for quenching of the (3)MLCT states of these ruthenium complex ions by molecular oxygen are in the range (2.55 to 7.01) x 10(9) M(-1) s(-1) in H2O and (3.38 to 5.69) x 10(9) M(-1) s(-1) in D2O. The efficiency of singlet oxygen, O2((1)Delta(g)), production as a result of the (3)MLCT quenching by oxygen, f(Delta)(T), is reported in D2O and found to be in the range 0.29-0.52. The rate constants, k(q)(Delta), for quenching of singlet oxygen by ground state sensitizers in D2O is also reported and found to be in the range (0.15 to 3.46) x 10(7) M(-1) s(-1). The rate constants and the efficiency of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition of a non-charge transfer (nCT) and a CT deactivation channel. nCT deactivation occurs from a fully established spin-statistical equilibrium of (1)(T1(3)Sigma) and (3)(T1(3)Sigma) encounter complexes by internal conversion (IC) to lower excited complexes that dissociate to yield O2((1)Delta(g)), and O2((3)Sigmag-). The balance between CT and nCT deactivation channels which is described by the relative contribution p(CT) of CT induced deactivation is discussed. The kinetic model proposed for the quenching of pi-pi* triplet states by oxygen can also be applied to the quenching of (3)MLCT states by oxygen.

Photosensitized generation of singlet oxygen from rhenium(i) and iridium(iii) complexes

Photophysical properties in dilute acetonitrile solution are reported for a number of iridium(III) and rhenium(I) complexes. The nature of the lowest excited state of the complexes under investigation is either metal-to-ligand charge transfer ((3)MLCT) or a ligand centred ((3)LC) state. Rate constants, k(q), for quenching of the lowest excited states by molecular oxygen are in the range 1.5 x 10(8) to 1.4 x 10(10) M(-1) s(-1). Efficiency of singlet oxygen production, f(Delta)(T), following oxygen quenching of the lowest excited states of these complexes, are in the range of 0.27-1.00. The rate constants and the efficiency of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition between a non-charge transfer (nCT) and a CT deactivation channel. The balance between CT and nCT deactivation channels, which is described by the relative contribution p(CT) of CT induced deactivation, is discussed. The kinetic model is found to be successfully applied in the case of quenching of the excited triplet states of coordination compounds by oxygen in acetonitrile, as was proposed for the quenching of pi-pi* triplet states by oxygen.

Topical use of liposomal copper palmitate formulation blocks porphyrin-induced photosensitivity in rats

Photodynamic therapy (PDT) is a new treatment modality that uses porphyrin derivatives and visible light, especially for the treatment of cancer. However, PDT with certain photosensitisers can cause prolonged skin photosensitization. This is particularly true for Photofrin II (Photofrin)-mediated PDT where patients are required to avoid direct exposure to sunlight for a period of 4-6 weeks. This is the only long-term adverse reaction to the drug. Recent studies have shown that topical copper treatment avoids this type of inflammatory reaction. In this study, we have tested the efficiency of the liposomal formulation of copper palmitate on porphyrin-photosensitized rats. Initially, adult male Sprague-Dawley rats were rendered photosensitive either by administration of Photofrin or aminolevulinic acid (ALA), a precursor of protoporphyrin IX (PpIX). Prior to this, their dorsal skin was shaved and treated topically with a cream consisting of either empty or copper palmitate-encapsulated liposomal formulation. After being kept in a dimmed light environment, the rats were exposed to visible light, and inflammatory responses were inspected. Histological studies revealed that no inflammatory cells were present at the skin sites treated with liposomal cream containing copper palmitate in the Photofrin-sensitized group while no reduction in the number of inflammatory cells was observed at the skin samples treated with the empty liposomes. In conclusion, the data demonstrate the significant protective effect of topically-applied liposome-encapsulated copper palmitate against both Photofrin and ALA-induced PpIX photosensitivity.

Biodistribution Assessment of a Lutetium(III) Texaphyrin Analogue in Tumor-bearing Mice Using NIR Fourier-transform Raman Spectroscopy¶

The use of near-infrared (NIR)-excited Fourier-transform (FT) Raman spectroscopy as a technique for evaluating the extent of photosensitizer localization in tumor (human pancreatic adenocarcinomas)-bearing mice has been tested using lutetium(III) texaphyrin analogue Lu-T2B2Tex. The complex was injected subcutaneously in the form of three injections given during the course of 3 days. The kinetics of biodistribution were then followed over a time scale of 1-6 days. The NIR-FT-Raman spectra of tissue samples obtained from the xenographic tumor, muscle, heart, brain, liver, spleen, kidney and blood were recorded and used to identify the presence of Lu-T2B2Tex in these tissues. Five Raman sensitizer markers were used to estimate the relative content of Lu-T2B2Tex in tumor at various postinjection times. UV-Visible (Vis) absorption spectroscopic detection of this sensitizer in tissue extracts was applied as a conventional method. Both spectroscopic methods were in good agreement with each other and confirm that Lu-T2B2Tex localizes well in tumor tissue. Maximal drug content was observed 3 days after the final injection. This time delay seems to be optimal for tumor irradiation in photodynamic therapy.

Quantitation of motexafin lutetium in human plasma by liquid chromatography-tandem mass spectrometry and inductively coupled plasma-atomic emission spectroscopy

Liquid chromatography-tandem mass spectrometry (LC-MS/MS) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) methods were developed and validated for the evaluation of motexafin lutetium (MLu, lutetium texaphyrin, PCI-0123) pharmacokinetics in human plasma. The LC-MS/MS method was specific for MLu, whereas the ICP-AES method measured total elemental lutetium. Both methods were fast, simple, precise, and accurate. For the LC-MS/MS method, a closely related analogue (PCI-0353) was used as the internal standard (IS). MLu and the IS were extracted from plasma by protein precipitation and injected into an LC-MS/MS system configured with a C18 column and an electrospray interface. The lower limit of quantitation was 0.05 microg MLu mL(-1), with a signal-to-noise ratio of 15:1. The response was linear from 0.05 to 5.0 microg MLu mL(-1). For the ICP-AES method, indium was used as the IS. The sample was digested with nitric acid, diluted, filtered, and then injected into the ICP-AES system. Two standard curve ranges were validated to meet the expected range of sample concentrations: 0.5 to 50, and 0.1 to 10 microg Lu mL(-1). The LC-MS/MS and ICP-AES methods were validated to establish accuracy, precision, analyte stability, and assay robustness. Interday precision and accuracy of quality control samples were < or =6.3% coefficient of variation (CV) and within 2.2% relative error (RE) for the LC-MS/MS method, and < or =8.7% CV and within 4.9% RE for the ICP-AES method. Plasma samples from a subset of patients in a clinical study were analyzed using both methods. For a representative patient, over 90% of the elemental lutetium in plasma could be ascribed to intact MLu at early time points. This percentage decreased to 59% at 48 hours after dosing, suggesting that some degradation and/or metabolism of the drug may have occurred.

Photodynamic therapy for choroidal neovascularization: a review

PURPOSE

To review the biophysical basis and current state of therapy for photodynamic closure of subfoveal choroidal neovascularization in the eye.

METHODS

A review of the literature is included, which encompasses the chemical structure, biophysical mechanism of action, range of available agents, status of clinical trials, clinical indications, results of treatments, complications, and future directions.

RESULTS

Photodynamic therapy has been shown to be effective in closing both experimental choroidal neovascularization in animal models as well as subfoveal choroidal neovascularization in humans. The therapy results in temporary closure of choroidal new vessels for a period of approximately 1 to 4 weeks. By 12 weeks, most patients have reperfusion or reproliferation of choroidal new vessels resulting in the need for retreatment to achieve continued closure and visual stabilization. Differences exist in the quantum yield, clinical efficiency, and light and sensitizer dose requirements between different classes of agents. Further clinical trials will be required to determine the optimal form of therapy, with verteporfin (Visudyne) as the only currently approved agent. Other agents, including tin etiopurpurin (Purlytin) and motexafin lutetium (Optrin), are currently undergoing phase III, and phase II trials, respectively.

CONCLUSIONS

Photodynamic therapy is a promising treatment modality shown to be effective in achieving closure and stabilization of vision loss compared with placebo control in eyes with subfoveal choroidal neovascularization.

Photosensitization of red blood cell hemolysis by lutetium texaphyrin

Lutetium (III) texaphyrin photosensitizes postirradiation or "delayed" photohemolysis (DPH) of human and bovine red blood cells at 730 nm by a Type-2 pathway mediated by singlet molecular oxygen. The DPH rate increases with increasing incubation temperature and with the second power of the incident fluence. The experimental DPH curves are in good agreement with a multi-hit kinetics model based on target theory.

Texaphyrins: new drugs with diverse clinical applications in radiation and photodynamic therapy

The texaphyrins are quintessential metal-coordinating expanded porphyrins. They constitute a new series of synthetic porphyrin analogues that show promise as drugs for use in a range of medical therapies. Currently, two different water-solubilized lanthanide(III) texaphyrin complexes, namely the gadolinium(III) and lutetium(III) derivatives 1 and 2 (Gd-Tex and Lu-Tex, respectively), are being tested clinically. The first of these, XCYTRIN, is in a pivotal Phase III clinical trial as a potential enhancer of radiation therapy for patients with metastatic cancers to the brain receiving whole brain radiation therapy. The second, in various formulations, is being tested as a photosensitizer for use in: (i) the photodynamic treatment of recurrent breast cancer (LUTRIN; Phase II clinical trials complete), (ii) photoangioplastic reduction of atherosclerosis involving peripheral arteries (ANTRIN; now in Phase II testing), and (iii) light-based treatment of age-related macular degeneration (OPTRIN; currently in Phase I clinical trials), a vision-threatening disease of the retina. Taken in concert, these two metallotexaphyrins provide a powerful new class of experimental drugs whose diverse potential utility is abetted by a combination of well-optimized physical features, favorable tissue biolocalization characteristics, and novel mechanisms of action. Interestingly, these mechanisms may alter conventional wisdom regarding mechanisms of radiation therapy and the pathophysiology of atherosclerosis.

Lutetium texaphyrin (Lu-Tex): a potential new agent for ocular fundus angiography and photodynamic therapy

PURPOSE

To investigate the suitability of lutetium texaphyrin (lu-tex) as a fluorescence imaging agent in the delineation of retinal vascular and choroidal vascular diseases. The utilization of an efficient fluorescent molecule that is also a photosensitizer represents a unique opportunity to couple diagnosis and therapy.

METHODS

Fundus fluorescence angiography comparing lu-tex (motexafin lutetium, Optrin, Pharmacyclics Inc, Sunnyvale, California) with the conventional angiographic dyes, sodium fluorescein, and indocynanine green (ICG), was performed on the eyes of normal and laser-injured New Zealand white rabbits. Plasma pharmacokinetic data and plasma protein binding were assessed in addition to light microscopy of the retina in both imaged and laser-injured eyes.

RESULTS

Normal retinal and choroidal vasculature was well delineated by lu-tex angiography. Experimentally induced choroidal and retinal vascular lesions were enhanced by lu-tex and demonstrated different staining patterns than fluorescein or ICG, particularly at the margins of the lesions. Lu-tex cleared rapidly from the plasma, with 39.7% bound to the high-density lipoprotein (HDL) fraction while 15.8% was bound to the low-density lipoprotein (LDL) fraction. No evidence of retinal toxicity after dye administration was observed by either ophthalmoscopy and fundus photography or by light microscopy.

CONCLUSION

Lu-tex angiography is a potentially valuable method for retinal vascular and choroidal vascular evaluation, and it has advantages over fluorescein and ICG angiography. The same agent could conceivably be used for both the identification of abnormal vasculature and subsequent photodynamic treatment.