UVA Activation Research Articles

Genotoxicity and the stability of N-nitrosomorpholine activity following UVA irradiation

This study investigated N-nitrosomorpholine (NMOR) genotoxicity following UVA irradiation without metabolic activation. Following UVA irradiation, the photo treated NMOR (irradiated NMOR) was directly mutagenic, without UVA or metabolic activation, in the Ames test. The activity was relatively stable, and approximately 79% of the activity remained after 10 days of storage at 37 °C, 4 °C, or −20 °C. Micronuclei (MN) formation was observed in HaCaT cells after treatment with irradiated NMOR without metabolic activation. The action spectrum of MN formation in response to NMOR irradiation followed the NMOR absorption curve. In vivo, MN formation was observed in the peripheral blood reticulocytes of mice injected with irradiated NMOR under the inhibition of cytochrome P450-mediated metabolism of NMOR. Volatile NMOR may attach to environmental materials and be irradiated with environmental UVA light. Photoactivated NMOR-attached air pollutants could float in the air and fall onto the human body, leading to genotoxicity induced by the irradiated NMOR.

UV-A activation of peroxymonosulfate for the removal of micropollutants from secondary treated wastewater

The occurrence of micropollutants (MPs) in the aquatic environment poses a threat to the environment and to the human health. The application of sulfate radical-based advanced oxidation processes (SR-AOPs) to eliminate these contaminants has attracted attention in recent years. In this work, the simultaneous degradation of 20 multi-class MPs (classified into 5 main categories, namely antibiotics, beta-blockers, other pharmaceuticals, pesticides, and herbicides) was evaluated for the first time in secondary treated wastewater, by activating peroxymonosulfate (PMS) with UV-A radiation, without any pH adjustment or iron addition. The optimal PMS concentration to remove the spiked target MPs (100 μg L−1) from wastewater was 0.1 mM, leading to an average degradation of 80% after 60 min, with most of the elimination occurring during the first 5 min. Synergies between radiation and the oxidant were demonstrated and quantified, with an average extent of synergy of 69.1%. The optimized treatment was then tested using non-spiked wastewater, in which 12 out of the 20 target contaminants were detected. Among these, 7 were degraded at some extent, varying from 10.7% (acetamiprid) to 94.4% (ofloxacin), the lower removals being attributed to the quite inferior ratio of MPs to natural organic matter. Phytotoxicity tests carried out with the wastewater before and after photo-activated PMS oxidation revealed a decrease in the toxicity and that the plants were able to grow in the presence of the treated water. Therefore, despite the low degradation rates obtained for some MPs, the treatment effectively reduces the toxicity of the matrix, making the water safer for reuse.

CaproGlu: Multifunctional tissue adhesive platform

Driven by the clinical need for a strong tissue adhesive with elastomeric material properties, a departure from legacy crosslinking chemistries was sought as a multipurpose platform for tissue mending. A fresh approach to bonding wet substrates has yielded a synthetic biomaterial that overcomes the drawbacks of free-radical and nature-inspired bioadhesives. A food-grade liquid polycaprolactone grafted with carbene precursors yields CaproGlu. The first-of-its-kind low-viscosity prepolymer is VOC-free and requires no photoinitiators. Grafted diazirine end-groups form carbene diradicals upon low energy UVA (365 nm) activation that immediately crosslink tissue surfaces; no pre-heating or animal-derived components are required. The hydrophobic polymeric environment enables metastable functional groups not possible in formulations requiring solvents or water. Activated diazirine within CaproGlu is uniquely capable of crosslinking all amino acids, even on wet tissue substrates. CaproGlu undergoes rapid liquid-to-biorubber transition within seconds of UVA exposure–features not found in any other bioadhesive. The exceptional shelf stability of CaproGlu allows gamma sterilization with no change in material properties. CaproGlu wet adhesiveness is challenged against current unmet clinical needs: anastomosis of spliced blood vessels, anesthetic muscle patches, and human platelet-mediating coatings. The versatility of CaproGlu enables both organic and inorganic composites for future bioadhesive platforms.

Diazoalkane decay kinetics from UVA-active protein labelling molecules: Trifluoromethyl phenyl diazirines

Trifluoromethyl phenyl diazirine (TPD) molecules are relatively stable carbene precursors, that readily form carbon covalent bonds with proteins. The stability of the diazoalkane intermediates is unknown, as are the factors which control carbene/diazoalkane ratios. This leads to incomplete carbene insertion onto desired compounds. Herein, stability and decay kinetics of diazoalkanes are evaluated from TPDs with various electron drawing groups, including 3-Phenyl-3-(trifluoromethyl)-3H-diazirine (TPD-H), p-benzyl alcohol (TPD-CH2OH), p-4-benzoic acid (TPD-COOH), and p-benzyl bromide (TPD-CH2Br). The spectroscopic analysis before and after UVA activation is performed both in dilute chloroform and neat by 19F NMR and ATR-FTIR, respectively. The increase of diazoalkane concentration after UVA exposure was in the order of: TPD-H > TPD-CH2Br > TPD-CH2OH > TPD-COOH. Indirect carbene/diazoalkane ratios ranged from 6: 1 to 3:1. Diazoalkane was relatively stable over the evaluation period of 30 min (post-UVA activation) in all compounds except TPD-CH2Br, which exhibits an 11 min half-life.

Naphthyl quinoxaline thymidine conjugate is a potent anticancer agent post UVA activation and elicits marked inhibition of tumor growth through vaccination

Anticancer anthracyclines are cytotoxic drugs that can induce antitumor immune response as a secondary effect through immunogenic cell death (ICD) mechanism. However, the immunogenic potency is quite limited, possibly due to that these chemotherapeutic agents are not specifically developed as ICD inducers. Thus, new drug entities through studies focusing on enhanced ICD induction would significantly promote antitumor immune response in the vaccination application. We report here a naphthyl quinoxaline thymidine conjugate as a new class of cytotoxic compounds that effectively induced in vivo antitumor activity through the vaccination application. Synthesized naphthyl quinoxaline conjugates were weak fluorescent thymidine analog yet exhibited a pronounced anticancer activity in the low nanomolar range post UVA activation. The potent activity of naphthyl conjugate was able to induce the marked detection of ICD markers including ATP and HMGB1 extracellular and calreticulin intracellularly at 2 h post UVA activation. Most importantly, mice vaccinated with cells treated with naphthyl conjugate plus UVA exhibited complete tumor growth inhibition in the tumor challenge study, and the induced immunogenic inhibition was much more effective than that of mitoxantrone anthracycline drug. All these results demonstrate the high potential of naphthyl quinoxaline conjugate for the cancer cell vaccine against tumor.

CRGD target liposome delivery system promoted immunogenic cell death through enhanced anticancer potency of a thymidine conjugate under UVA activation as a cancer vaccine

Conventional chemotherapeutic and photodynamic therapy have recently been shown to also elicit immune response against cancer through the immunogenic cell death mechanism, which can be potentially translated into effective cancer vaccines. However, there are few studies on the potential of nanodelivery system to promote the immunogenic cell death as a cancer vaccine. We reported here that cRGD target liposome delivery system was capable to promote the immunogenic cell death through enhanced potency of a thymidine conjugate post UVA activation as a cancer vaccine. Liposomes and cRGD target liposomes were found to significantly increase the cellular accumulation of the thymidine conjugate and subsequently translated into enhanced cytotoxic potency after UVA activation. More importantly, cRGD target liposomes of the thymidine conjugate markedly promoted the early detection of immunogenic cell death markers including ATP, HMGB1 and calreticulin. Subsequent in vivo vaccination−challenge study confirmed effective tumor growth inhibition by the cRGD liposomal thymidine conjugate and UVA treated cancer cells as the cancer vaccine. In addition, liposomes and cRGD target liposomes alone did not shown any induction of the immunogenic cell death markers, revealing the adjuvant nature of the nanodelivery system.

Sn-doped and porogen-modified TiO2 photocatalyst for solar light elimination of sulfure diethyle as a model for chemical warfare agent

In the context of the increase in chemical threat due to warfare agents, the development of efficient methods for destruction of Chemical Warfare Agents are of first importance both for civilian and military purposes. Here, we demonstrate that optimized Sn-TiO2 doped and PEG-modified photocatalysts allow increased and high performances under UVA and solar-light irradiations leading to total elimination of highly contaminated environments containing Diethylsulfide (DES) used as a model molecule mimicking Yperite (Mustard Gas). It has been shown that Sn doping induces significant modifications on the structural, morphological, surface, electronic and optical properties of TiO2. For example, the addition of 1% Sn increases significantly the surface area from 30 to 80 m2/g and decreases the particle size, while Sn-doping results in a reduction of the anatase band-gap from 3.2 to 2.95 eV. Total DES elimination could be reached for 90 and 120 min under continuous contaminant flux under UVA and solar light activation, respectively accompanied with limited deactivation phenomenon. Correlations between the resulting physico-chemical properties of the doped and PEG-modified materials and the photocatalytic activities were carried out. The results open up extremely promising way for the decontamination of highly contaminated environments containing real warfare agents under UVA but also under solar light illumination.

Peri-implantitis cleaning instrumentation influences the integrity of photoactive nanocoatings

ObjectiveTo determine in vitro the loss of integrity caused on photocatalytic anatase coated implant surfaces by clinical instrumentation through changes in surface topography and loss of functionality. MethodsAnatase-coated titanium discs were treated with diamond burs, polishers, plastic and metal hand instruments, air scaler and air flow devices. The pressure exerted through instrumentation was measured online. Surface topography was evaluated through scanning electron microscopy and contact profilometry, surface function through hydrophilization capacity upon UV-A activation. ResultsTreatment with diamond burs and instruments with metal tips resulted in an increase of roughness. Use of silicone polishers led to smoothening, which was more pronounced on the anatase surface. Plastic instruments, the air abrasive system and rubber cups left the surfaces intact. Functionality was partially lost on surfaces subjected to hand instruments and completely lost upon diamond burs and silicone polishers. SignificanceThe integrity of functional nanocoatings depends on the applied instrumentation. Air flow device, rubber cup with polishing paste and plastic tipped instruments prevent damage on these nanosurfaces and may be preferably used when decontaminating anatase and other nanocoatings in a clinical setting.

UVA photoactivation of DNA containing halogenated thiopyrimidines induces cytotoxic DNA lesions

Photochemotherapy, the combination of a photosensitiser and ultraviolet (UV) or visible light, is an effective treatment for skin conditions including cancer. The high mutagenicity and non-selectivity of photochemotherapy regimes warrants the development of alternative approaches. We demonstrate that the thiopyrimidine nucleosides 5-bromo-4-thiodeoxyuridine (SBrdU) and 5-iodo-4-thiodeoxyuridine (SIdU) are incorporated into the DNA of cultured human and mouse cells where they synergistically sensitise killing by low doses of UVA radiation. The DNA halothiopyrimidine/UVA combinations induce DNA interstrand crosslinks, DNA-protein crosslinks, DNA strand breaks, nucleobase damage and lesions that resemble UV-induced pyrimidine(6-4)pyrimidone photoproducts. These are potentially lethal DNA lesions and cells defective in their repair are hypersensitive to killing by SBrdU/UVA and SIdU/UVA. DNA SIdU and SBrdU generate lethal DNA photodamage by partially distinct mechanisms that reflect the different photolabilities of their C–I and C–Br bonds. Although singlet oxygen is involved in photolesion formation, DNA SBrdU and SIdU photoactivation does not detectably increase DNA 8-oxoguanine levels. The absence of significant collateral damage to normal guanine suggests that UVA activation of DNA SIdU or SBrdU might offer a strategy to target hyperproliferative skin conditions that avoids the extensive formation of a known mutagenic DNA lesion.

UVA activation of N-dialkylnitrosamines releasing nitric oxide, producing strand breaks as well as oxidative damages in DNA, and inducing mutations in the Ames test

We investigated the photo-mutagenicity and photo-genotoxicity of N-dialkylnitrosamines and its mechanisms of UVA activation. With simultaneous irradiation of UVA, photo-mutagenicity of seven N-dialkylnitrosamines was observed in Ames bacteria ( Salmonella typhimurium TA1535) in the absence of metabolic activation. Mutagenicity of pre-irradiated N-dialkylnitrosamines was also observed with S. typhimurium hisG46, TA100, TA102 and YG7108 in the absence of metabolic activation. UVA-mediated mutation with N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) decreased by adding either the NO or OH radical scavenger. When superhelical DNA was irradiated with N-dialkylnitrosamines, nicked circular DNA appeared. Ten N-dialkylnitrosamines examined produced strand breaks in the treated DNA in the presence of UVA. The level of single-strand breaks in ϕX174 DNA mediated by N-nitrosomorpholine (NMOR) and UVA decreased by adding either a radical scavenger or superoxide dismutase. When calf thymus DNA was treated with N-dialkylnitrosamines (NDMA, NDEA, NMOR, N-nitrosopyrrolidine (NPYR) and N-nitrosopiperidine (NPIP)) and UVA, the ratio of 8-oxodG/dG in the DNA increased. Action spectra were obtained to determine if nitrosamine acts as a sensitizer of UVA. Both mutation frequency and NO formation were highest at the absorption maximum of nitrosamines, approximately 340 nm. The plots of NO formation and mutation frequency align with the absorption curve of NPYR, NMOR and NDMA. A significant linear correlation between the optical density of N-dialkynitrosamines at 340 nm and NO formation in each irradiated solution was revealed by ANOVA. We would like to propose the hypothesis that the N-nitroso moiety of N-dialkylnitrosamines absorbs UVA photons, UVA-photolysis of N-dialkylnitrosamines brings release of nitric oxide, and subsequent production of alkyl radical cations and active oxygen species follow as secondary events, which cause DNA strand breaks, oxidative and alkylative DNA damages and mutation.

Oxidative and alkylating damage in DNA.

Modification of cellular DNA upon exposure to reactive oxygen and nitrogen species is the likely initial event involved in the induction of the mutagenic and lethal effects of various oxidative stress agents. Evidence has been accumulated for the significant implication of singlet oxygen (1O(2)), generated as the result of UVA activation of endogenous photosensitizers as porphyrins and flavins. 7,8-Dihydro-8-oxo-2'-deoxyguanosine (8-oxodGuo) has been shown to be the exclusive product of the reaction of 1O(2) with the guanine moiety of cellular DNA, in contrast to the hydroxyl radical, which reacts almost indifferently with all the nucleobases and the sugar moiety of DNA. Furthermore 8-oxodGuo is also produced by other oxidants and can be used as an ubiquitous biomarker of DNA oxidation but can not be a specific marker of any particular species. The role of DNA etheno adducts in mutagenic and carcinogenic processes triggered by known occupational and environmental carcinogens has also been studied. Much interest in etheno adducts resulted from the detection of increased levels of 1,N(6)-etheno-2'-deoxyadenosine and 3,N(4)-etheno-2'-deoxycytidine in DNA from human, rat and mouse tissues under pathophysiological conditions associated with oxidative stress. A method involving on-line HPLC with electrospray tandem mass spectrometry detection has been developed for the analysis of 1,N(2)-etheno-2'-deoxyguanosine (1,N(2)-epsilondGuo) in DNA. This methodology permits direct quantification of 20 fmol (7.4 adducts/10(8) dGuo) of the etheno adduct from approximately 350 microg of crude DNA hydrolysates. This method provides the first evidence of the occurrence of 1,N(2)-epsilondGuo as a basal endogenous lesion and may be utilized to better assess the biological consequences of etheno DNA damage under normal and pathological conditions. This work addresses the importance of isotope labeling associated with mass spectrometry technique for biomolecule damage studies.

P38 mitogen-activated protein kinase activation by ultraviolet A radiation in human dermal fibroblasts.

UVA radiation penetrates deeply into the skin reaching both the epidermis and the dermis. We thus investigated the effects of naturally occurring doses of UVA radiation on mitogen-activated protein kinase (MAPK) activities in human dermal fibroblasts. We demonstrated that UVA selectively activates p38 MAPK with no effect on extracellular-regulated kinases (ERK1-ERK2) or JNK-SAPK (cJun NH2-terminal kinase-stress-activated protein kinase) activities. We then investigated the signaling pathway used by UVA to activate p38 MAPK. L-Histidine and sodium azide had an inhibitory effect on UVA activation of p38 MAPK, pointing to a role of singlet oxygen in transduction of the UVA effect. Afterward, using prolonged cell treatments with growth factors to desensitize their signaling pathways or suramin to block growth factor receptors, we demonstrated that UVA signaling pathways shared elements with growth factor signaling pathways. In addition, using emetine (a translation inhibitor altering ribosome functioning) we detected the involvement of ribotoxic stress in p38 MAPK activation by UVA. Our observations suggest that p38 activation by UVA in dermal fibroblasts involves singlet oxygen-dependent activation of ligand-receptor signaling pathways or ribotoxic stress mechanism (or both). Despite the activation of these two distinct signaling mechanisms, the selective activation of p38 MAPK suggests a critical role of this kinase in the effects of UVA radiation.

Synthesis and biological evaluation of a new furo[2,3-h]quinolin-2(1H)-one.

A new furoquinolinone derivative, namely 4-hydroxymethyl-1,6,8-trimethylfuro[2,3-h]quinolin-2(1H)-one (HOFQ), was synthesized and its biological activity studied. By UVA activation, HOFQ induced strong antiproliferative effects in Ehrlich ascite cells, which lost their ability to transmit the tumor by transplantation. HOFQ exhibited poor genotoxicity and absence of skin phototoxicity. Actually, HOFQ sensitization forms DNA-protein cross-linkages but not interstrands cross-links. Therefore, HOFQ appears to be a new promising drug for PUVA photochemotherapy and photopheresis.

13] Mitogen-activated protein kinase activation by singlet oxygen and ultraviolet A

Publisher Summary Ultraviolet A radiation induces a pattern of MAP kinase activation in human skin fibroblasts consisting of a rapid and transient induction of p38 and JNK MAPK activity but not of ERKs. UVA activation of p38 and JNK is inhibitable by the singlet oxygen quenchers azide and imidazole but not by the hydroxyl radical scavengers mannitol or DMSO, pointing to the involvement of singlet oxygen ( 1 O 2 ). Like UVA, 1 O 2 generated intracellularly on photo-excitation of rose bengal activates p38 and JNK but not ERKs. p38 and JNK activation is also elicited by chemi-excitation for the intracellular generation of 1 O 2 by the lipophilic 1,4-endoperoxide of 3,3'-(2,3-dihydroxypropyl)-l,4-naphthalene dipropionamide. In contrast, extracellular generation of 1 O 2 by irradiation of rose bengal immobilized on agarose beads or by chemi-excitation employing the hydrophilic 1,4-endoperoxide of disodium 3,3'-(1,4-naphthylidene) dipropionate is ineffective in activating p38 or JNK. These data suggest that the activation of p38 and JNK by singlet oxygen occurs only when the electronically excited molecule is generated intracellularly.

Synthesis and biological activity of (hydroxymethyl)- and (diethylaminomethyl)benzopsoralens.

Some benzopsoralens, carrying a hydroxymethyl or a diethylaminomethyl group at the 3, 5, 8, and 11 positions, were prepared, and their biological activity was compared with that of 4-(hydroxymethyl)benzopsoralen (BP). 5-(Hydroxymethyl)benzopsoralen (7b), 11-(hydroxymethyl)benzopsoralen (7c), and 11-(diethylaminomethyl)benzopsoralen (8c) induced marked antiproliferative effects in mammalian cells by simple incubation in the dark; this activity appeared to be related to their ability to inhibit topoisomerase II. Benzopsoralens appeared to be more active, especially BP and 7c, upon UVA activation. Compounds carrying a methyl group at the 4 position together with a hydroxymethyl or diethylaminomethyl at the 8 position (7d and 8d, respectively) were also effective, although to a lower extent; instead, a substituent at the 3 position canceled all activity. Benzopsoralens did not induce interstrand cross-links in DNA in vitro, as seen in the induction of cytoplasmic <<petite>> mutations and double-strand breaks in yeast. This behavior is also compatible with their low mutagenic activity in E. coli WP2 and with the absence of any phototoxicity on the skin. For these features, benzopsoralens seem to be interesting potential drugs for PUVA photochemotherapy and photopheresis. The activity shown in the dark is not sufficient for their possible use as antitumor drugs, but it does offer a new model for the study of topoisomerase inhibitors.

Mitogen-activated protein kinase (p38-, JNK-, ERK-) activation pattern induced by extracellular and intracellular singlet oxygen and UVA.

Ultraviolet A (UVA; 320-400 nm) radiation in human skin fibroblasts induces a pattern of mitogen-activated protein kinase (MAPK) activation consisting of a rapid and transient induction of p38 and c-Jun-N-terminal kinase (JNK) activity but not extracellular signal-regulated kinases (ERK). UVA activation of p38 can be inhibited by the singlet oxygen (1O2) quenchers azide and imidazole, but not by the hydroxyl radical scavengers mannitol or dimethylsulfoxide, pointing to the involvement of 1O2. The same effect has been shown for JNK. Like UVA, 1O2 generated intracellularly upon photoexcitation of Rose Bengal activates p38 and JNK but not ERK. p38 and JNK activation was also elicited by chemiexcitation for the intracellular generation of 1O2 by the lipophilic 1,4-endoperoxide of N,N'-di(2,3-dihydroxypropyl)-1, 4-naphthalene dipropionamide. In contrast, extracellular generation of 1O2, by irradiation of Rose Bengal immobilized on agarose beads or by chemiexcitation employing the hydrophilic 1,4-endoperoxide of disodium 3,3'-(1,4-naphthylidene) dipropionate, was ineffective in activating p38 or JNK. These data suggest that the activation of p38 and JNK by 1O2 occurs only when the electronically excited molecule is generated intracellularly.

Modulation of the UVA activation of haem oxygenase, collagenase and cyclooxygenase gene expression by epigallocatechin in human skin cells

We have investigated the modifying effects of epigallocatechin, a major polyphenolic constituent of green tea, on ultraviolet-A-activated gene expression in human fibroblasts and keratinocytes using the stress responsive enzymes: haem oxygenase-1, interstitial collagenase and cyclooxygenase-2. Although epigallocatechin strongly reduced ultraviolet-A-induced haem oxygenase-1 activation in skin-derived fibroblasts, the same compound activated collagenase and cyclooxygenase expression. In a keratinocyte cell line, ultraviolet-A-mediated haem oxygenase-1 over-expression was low and epigallocatechin failed to modulate it further. In contrast to the results with fibroblasts, ultraviolet-A activation of cyclooxygenase in keratinocytes was reduced by epigallocatechin. The results indicate that the effect of this green tea polyphenol on cellular stress responses is complex and may involve direct effects on signal transduction as well as changes that may be associated with its antioxidant activity.

O VII.3 - O VII.3 UVA activation of genes — Some clues as to mechanism

O VII.3 - O VII.3 UVA activation of genes — Some clues as to mechanism

DNA damage and cytotoxicity induced in mammalian cells by a tetramethylfuroquinolinone derivative.

1,4,6,8-Tetramethyl-2H-furo[2,3-h]quinolin-2-one [FQ] is an angelicin isoster characterized by a strong photosensitizing activity FQ shows a significant antiproliferative activity also in the dark, i.e., without UVA activation. The cytotoxic activity of FQ in the dark was detected in HeLa cells and in normal human lymphocytes; FQ showed notable antiproliferative effects, barely lower in comparison with ellipticine, used as a reference Similar results were obtained studying the FQ's capacity for forming chromosome aberrations. For both FQ and ellipticine, the chromosomal damage correlated closely with cell killing, when compared with ellipticine at the same levels of survival, FQ appeared to be much less genotoxic. Using alkaline elution we have investigated the ability of FQ to damage DNA. The formation of equivalent amounts of single-strand breaks (SSB) and DNA-protein cross-links (DPC) was observed; in addition, these lesions appeared to be located at the same sites in DNA. Experiments carried out with neutral elution demonstrated the formation of double-strand breaks (DSB). All these data are consistent with an inhibition of topoisomerase II; this hypothesis was confirmed performing an enzymatic test in vitro using topoisomerase II from Drosophila melanogaster embryos.

Novel cytotoxic DNA sequence and minor groove targeted photosensitizers: Conjugates of pyrene and netropsin analogues

The design, syntheses, photochemical and biological properties of conjugates of pyrene with pyrrole- ( 1) and imidazole-containing ( 2) analogues of netropsin are reported. The results of an ethidium displacement assay and circular dichroism (CD) titration studies show both compounds bind with a higher affinity to poly(dA-dT) than to poly(dG-dC). In addition they bind as strongly to T4 coliphage DNA as to calf thymus DNA suggesting the binding occurs in the minor groove. The quenching rate constants of the singlet excited states of agents 1 and 2 by molecular oxygen were found to be 8.5 × 10 9 M −1s −1 and 7.7 × 10 9 M −1s −1, suggesting the involvement of singlet oxygen. Both compounds showed some cytotoxicity to human chronic myeloid leukemia K562 cells in the dark. Upon irradiation the activities were significantly enhanced resulting in photoinduced dose modifications of 8 and 14 for 1 and 2, respectively under the conditions employed. Both agents were markedly more phototoxic than 1-pyrenebutyric acid 8. To address the mechanism of action of compounds 1 and 2 their photoactivated abilities to produce DNA strand breaks were measured. Both agents caused increased single strand breakage with increasing UV exposure. The concentrations (EC 50) of 1 and 2 needed to cause 50% single-strand cleavage of pBR322 DNA upon UV-A activation were found to be 40 μM and 45 μM, respectively. In contrast, no DNA strand breaks were observed in the dark with either conjugate or with 8 following irradiation. DNA strand breaks were measured in drug treated K562 cells using alkaline elution. Extensive strand breaks were observed following irradiation of the cells which were rapidly repaired.