Hyperoxygenation enhances the tumor cell killing of photofrin-mediated photodynamic therapy

Tumor hypoxia, either preexisting or as a result of oxygen depletion during photodynamic therapy (PDT) light irradiation, can significantly reduce the effectiveness of PDT-induced cell killing. To overcome tumor hypoxia and improve tumor cell killing, we propose using supplemental hyperoxygenation during Photofrin-PDT. The mechanism for the tumor cure enhancement of the hyperoxygenation-PDT combination is investigated using an in vivo-in vitro technique. A hypoxic tumor model was established by implanting mammary adenocarcinoma in the hind legs of mice. Light irradiation (200 J/cm2 at either 75 or 150 mW/cm2), under various oxygen supplemental conditions (room air, carbogen, 100% normobaric or hyperbaric oxygen), was delivered to animals that received 12.5 mg/kg Photofrin 24 h before light irradiation. Tumors were harvested at various time points after PDT and grown in vitro for colony formation analysis. Treated tumors were also analyzed histologically. The results show that when PDT is combined with hyperoxygenation, the hypoxic condition could be improved and the cell killing rate at various time points after PDT could be significantly enhanced over that without hyperoxygenation, suggesting an enhanced direct and indirect cell killing associated with high-concentration oxygen breathing. This study further confirms our earlier observation that when a PDT treatment is combined with hyperoxygenation it can be more effective in controlling hypoxic tumors.     

Interaction between photodynamic therapy and BCG immunotherapy responsible for the reduced recurrence of treated mouse tumors

Subcutaneous mouse EMT6 tumors were treated by individual or combined regimens of a single Bacillus Calmette-Guérin (BCG) vaccine administration and photodynamic therapy (PDT). Six clinically relevant photosensitizers characterized by different action mechanisms were used: Photofrin, benzoporphyrin derivative, tetra(m-hydroxyphenyl)chlorin (foscan), mono-L-aspartylchlorin e6, lutetium texaphyrin or zinc phthalocyanine. Irrespective of the type of photosensitizer used, the optimized BCG protocols improved the cure rate of PDT-treated tumors. This indicates that the interaction does not take place during the early phase of tumor ablation but at later events involved in preventing tumor recurrence. Beneficial effects on tumor cure were observed even when the BCG injection was delayed to 7 days after PDT. The accumulation of activated myeloid cells that markedly increases in tumors treated by Photofrin-based PDT was not additionally affected by BCG treatment. However, the incidence of immune memory T cells in tumor-draining lymph nodes that almost doubled at 6 days after Photofrin-PDT further increased close to three-fold with adjuvant BCG. This suggests that BCG immunotherapy amplifies the T-lymphocyte-mediated immune response against PDT-treated tumors. Since both these modalities are established for the treatment of superficial bladder carcinomas, use of their combination for this condition should be clinically tested.     

Effect of photosensitizer dose on fluence rate responses to photodynamic therapy

Photodynamic therapy (PDT) regimens that conserve tumor oxygenation are typically more efficacious, but require longer treatment times. This makes them clinically unfavorable. In this report, the inverse pairing of fluence rate and photosensitizer dose is investigated as a means of controlling oxygen depletion and benefiting therapeutic response to PDT under conditions of constant treatment time. Studies were performed for Photofrin-PDT of radiation-induced fibrosarcoma tumors over fluence rate and drug dose ranges of 25-225 mW cm(-2) and 2.5-10 mg kg(-1), respectively, for 30 min of treatment. Tumor response was similar among all inverse regimens tested, and, in general, tumor hemoglobin oxygen saturation (SO2) was well conserved during PDT, although the highest fluence rate regimen (225 mWx2.5 mg) did lead to a modest but significant reduction in SO2. Regardless, significant direct tumor cell kill (>1 log) was detected during 225 mWx2.5 mg PDT, and minimal normal tissue toxicity was found. PDT effect on tumor oxygenation was highly associated with tumor response at 225 mWx2.5 mg, as well as in all other regimens tested. These data suggest that high fluence rate PDT can be carried out under oxygen-conserving, efficacious conditions at low photosensitizer dose. Clinical confirmation and application of these results will be possible through use of minimally invasive oxygen and photosensitizer monitoring technologies, which are currently under development.     

Complement activation cascade and its regulation: relevance for the response of solid tumors to photodynamic therapy

The complement system has emerged as a prominent participant in host response elicited following treatment of solid tumors by photodynamic therapy (PDT). Activity of the complement system is tightly controlled by a series of endogenous regulatory proteins. The expression of decay-accelerating factor (DAF), complement-receptor-1-related protein y (Crry), and protectin, which are the three major mouse membrane-bound complement regulatory proteins (mCRPs), was examined following treatment of murine squamous cell carcinomas SCCVII by PDT mediated by the photosensitizer Photofrin. A marked decrease was detected in the expression of all three mCRPs on cancer cells from tumors following PDT, indicating that these cells were made more vulnerable to complement attack. In order to amplify this effect, following PDT mice were injected with antibodies neutralizing either Crry, protectin, or DAF. With anti-Crry and anti-protectin this resulted in increased tumor cure rate compared to PDT alone, while the contrary was observed with PDT plus anti-DAF combination (presumably owing to additional role of DAF in T cell signaling). Further examination including other complement regulatory proteins showed that combining antitumor PDT with antibody-mediated neutralization of factor H (soluble negative complement regulator) or injection of properdin (positive complement regulator) increased the cure rates of PDT-treated tumors. The use of various agents promoting complement activity appears promising for employment as adjuvants to PDT.     

Hypoxia Induced by Upconversion‐Based Photodynamic Therapy: Towards Highly Effective Synergistic Bioreductive Therapy in Tumors

Local hypoxia in tumors is an undesirable consequence of photodynamic therapy (PDT), which will lead to greatly reduced effectiveness of this therapy. Bioreductive pro‐drugs that can be activated at low‐oxygen conditions will be highly cytotoxic under hypoxia in tumors. Based on this principle, double silica‐shelled upconversion nanoparticles (UCNPs) nanostructure capable of co‐delivering photosensitizer (PS) molecules and a bioreductive pro‐drug (tirapazamine, TPZ) were designed (TPZ‐UC/PS), with which a synergetic tumor therapeutic effect has been achieved first by UC‐based (UC‐) PDT under normal oxygen environment, immediately followed by the induced cytotoxicity of activated TPZ when oxygen is depleted by UC‐PDT. Treatment with TPZ‐UC/PS plus NIR laser resulted in a remarkably suppressed tumor growth as compared to UC‐PDT alone, implying that the delivered TPZ has a profound effect on treatment outcomes for the much‐enhanced cytotoxicity of TPZ under PDT‐induced hypoxia.     

Self-oxygenated co-assembled biomimetic nanoplatform for enhanced photodynamic therapy in hypoxic tumor

Photodynamic therapy (PDT) has shown great application potential in cancer treatment and the important manifestation of PDT in the inhibition of tumors is the activation of immunogenic cell death (ICD) effects. However, the strategy is limited in the innate hypoxic tumor microenvironment. There are two key elements for the realization of enhanced PDT: specific cellular uptake and release of the photosensitizer in the tumor, and a sufficient amount of oxygen to ensure photodynamic efficiency. Herein, self-oxygenated biomimetic nanoparticles (CS@M NPs) co-assembled by photosensitizer prodrug (Ce6-S-S-LA) and squalene (SQ) were engineered. In the treatment of triple negative breast cancer (TNBC), the oxygen carried by SQ can be converted to reactive oxygen species (ROS). Meanwhile, glutathione (GSH) consumption during transformation from Ce6-S-S-LA to chlorin e6 (Ce6) avoided the depletion of ROS. The co-assembled (CS NPs) were encapsulated by homologous tumor cell membrane to improve the tumor targeting. The results showed that the ICD effect of CS@M NPs was confirmed by the significant release of calreticulin (CRT) and high mobility group protein B1 (HMGB1), and it significantly activated the immune system by inhibiting the hypoxia inducible factor-1alpha (HIF-1α)-CD39-CD73-adenosine a2a receptor (A2AR) pathway, which not only promoted the maturation of dendritic cells (DC) and the presentation of tumor specific antigens, but also induced effective immune infiltration of tumors. Overall, the integrated nanoplatform implements the concept of multiple advantages of tumor targeting, reactive drug release, and synergistic photodynamic therapy-immunotherapy, which can achieve nearly 90% tumor suppression rate in orthotopic TNBC models.     

APOPTOSIS DURING PHOTODYNAMIC THERAPY-INDUCED ABLATION OF RIF-1 TUMORS IN C3H MICE: ELECTRON MICROSCOPIC, HISTOPATHOLOGIC AND BIOCHEMICAL EVIDENCE

Abstract Very little is known about the applicability of the metabolic and biochemical events observed in cell culture systems to in vivo tumor shrinkage following photodynamic therapy (PDT). The purpose of this study was to assess whether PDT induces apoptosis during tumor ablation in vivo . We treated radiation-induced fibrosarcoma (RIF-1) tumors grown in C3H/HeN mice with PDT employing three photosensitizers, Photofrin-II, chloroaluminum phthalocyanine tetrasulfonate, or Pc IV (a promising phthalocyanine developed in this laboratory). Each photosensitizer was injected intraperitoneally and 24 h later the tumors were irradiated with an appropriate wavelength of red light using an argon-pumped dye laser. During the course of tumor shrinkage, the tumors were removed at 1, 2, 4 and 10 h post-PDT for DNA fragmentation, histopathologic, and electron microscopic studies. Markers of apoptosis, viz . the ladder of nucleosome-size DNA fragments, increased apoptotic bodies, and condensation of chromatin material around the periphery of the nucleus, were evident in tumor tissue even 1 h post-PDT; the extent of these changes increased during the later stages of tumor ablation. No changes were observed in tumors given photosensitizer alone or irradiation alone. Our data suggest that the damage produced by in vivo PDT may activate endonucleolysis and chromatin condensation, and that apoptosis is an early event in tumor shrinkage following PDT.     

PHOTODYNAMIC THERAPY OF CHEMICALLY- AND ULTRAVIOLET B RADIATION-INDUCED MURINE SKIN PAPILLOMAS BY CHLOROALUMINUM PHTHALOCYANINE TETRASULFONATE

Photodynamic therapy (PDT) of cancer combines irradiation of tumors with visible light following selective uptake of the photosensitizer by the tumor cells. PhotofrinR-II (Pf-II) is the only photosensitizer which is in clinical use in PDT, whereas chloroaluminum phthalocyanine tetrasulfonate (AlPcTS) has also shown promise in preclinical studies. In most such studies, the effectiveness of the photosensitizers has been assessed in implanted tumor model systems rather than in model systems where tumors are allowed to grow in their own connective tissue matrix. In this study the pharmacokinetics, tumor ablation capability and cutaneous photosensitization response of AlPcTS have been assessed in mice bearing chemically- and ultraviolet B radiation (UVB)-induced benign skin papillomas. When tumor-bearing animals were injected intraperitoneally with AlPcTS (5 mg/kg body wt), maximum tumor:normal skin ratio of 2.4 was observed at 48 h, at which time the mice were irradiated within the absorption spectrum of the photosensitizer. In tumor ablation studies with SENCAR mice bearing chemically-induced skin tumors, AlPcTS resulted in greater than 80% ablation in tumor volume at 20 days post-irradiation. In cutaneous photosensitization response, AlPcTS produced only transient effects (no effect after 24 h) in SENCAR mice. Pharmacokinetics data, tumor ablation effects and cutaneous photosensitization response of AlPcTS were comparable in SKH-1 hairless mice bearing UVB-induced skin tumors. Our data indicate that AlPcTS produces significant photodynamic effects towards the ablation of murine skin tumors, and that it does not produce prolonged cutaneous photosensitivity.     

Long-Circulating Prostate-Specific Membrane Antigen-Targeted NIR Phototheranostic Agent

Targeted photodynamic therapy (PDT) combined with image-guided surgical resection is a promising strategy for precision cancer treatment. Prostate-specific membrane antigen (PSMA) is an attractive target due to its pronounced overexpression in a variety of tumors, most notably in prostate cancer. Recently, we reported a pyropheophorbide-based PSMA-targeted agent, which exhibited long plasma circulation time and effective tumor accumulation. To further advance PSMA-targeted photodynamic therapy by harvesting tissue-penetrating properties of the NIR light, we developed a bacteriochlorophyll-based PSMA-targeted photosensitizer (BPP), consisting of three building blocks: (1) a PSMA-affinity ligand, (2) a peptide linker to prolong plasma circulation time and (3) a bacteriochlorophyll photosensitizer for NIR fluorescence imaging and photodynamic therapy (Qy absorption maximum at 750 nm). BPP exhibited excellent PSMA-targeting selectivity in both subcutaneous and orthotopic mouse models. The nine D-peptide linker in BPP structure prolonged its plasma circulation time (12.65 h). Favorable pharmacokinetic properties combined with excellent targeting selectivity enabled effective BPP tumor accumulation, which led to effective PDT in a subcutaneous prostate adenocarcinoma mouse model. Overall, bright NIR fluorescence of BPP enables effective image guidance for surgical resection, while the combination of its targeting capabilities and PDT activity allows for potent and precise image-guided photodynamic treatment of PSMA-expressing tumors.     

Photodynamic therapy for malignant mesothelioma: preclinical studies for optimization of treatment protocols

Effective photodynamic therapy (PDT) depends on the optimization of factors such as drug dose, drug-light interval, fluence rate and total light dose (or fluence). In addition sufficient oxygen has to be present for the photochemical reaction to occur. Oxygen deficits may arise during PDT if the photochemical reaction consumes oxygen more rapidly than it can be replenished, and this could limit the efficacy of PDT. In this study we investigated the influence of the drug-light interval, illumination-fluence rate and total fluence on PDT efficacy for the photosensitizer meta-tetrahydroxyphenylchlorin (mTHPC). The effect of increasing the oxygenation status of tumors during PDT was also investigated. PDT response was assessed from tumor-growth delay and from cures for human malignant mesothelioma xenografts grown in nude mice. Tumor-bearing mice were injected intravenously with 0.15 or 0.3 mg.kg-1 mTHPC, and after intervals of 24-120 h, the subcutaneous tumors were illuminated with laser light (652 nm) at fluence rates of 20, 100 or 200 mW.cm-2. Tumor response was strongly dependent on the drug-light interval. Illumination at 24 h after photosensitization was always significantly more effective than illumination at 72 or 120 h. For a drug-light interval of 24 h the tumor response increased with total fluence, but for longer drug-light intervals even high total fluences failed to produce a significant delay in tumor regrowth. No fluence-rate dependence of PDT response was demonstrated in these studies. Nicotinamide injection and carbogen breathing significantly increased tumor oxygenation and increased the tumor response for PDT schedules with illumination at 24 h after photosensitizer injection.     

Treatment of Malignant Melanoma by High-Peak-Power 1064 nm Irradiation Followed by Photodynamic Therapy

Irradiation of B16 pigmented melanoma subcutaneously transplanted in C57 mice with a single 650 mj pulse (10 ns) of 1064 nm light from a Q-switched Nd: YAG laser caused instantaneous bleaching of the pigmented tissue. Visual and histological examination of the resulting gray-colored tumor revealed the breakdown of melanosomes with no detectable alteration of the normal and tumor-overlying skin. Histological examination of the irradiated tumor showed some degree of vascular damage; the depth of the photodamage was not affected by the successive delivery of three consecutive light pulses. The bleached tumor grew at a modestly slower rate but the high-peak-power (HPP) laser treatment did not affect the tumor concentration of a photodynamic sensitizer Si(IV)-naphthalocyanine (isoBO-SiNc) intravenously injected 24 h before Nd : YAG irradiation. Treatment of the B16 pigmented melanoma by photodynamic therapy (PDT: 1 mg/kg isoBO-SiNc, 300 mW/cm², 520 J/cm²) from a 774 nm diode laser immediately after the 1064 nm irradiation resulted in a 16 day delay of tumor regrowth, which was markedly longer than the delay (ca 6 days) obtained after PDT under identical conditions without the preirradia-tion. Thus, pretreatment of pigmented tumors with HPP 1064 nm light appears to enhance their susceptibility to conventional PDT. The tumor response was further enhanced by repeating the combined HPP/PDT treatment at an interval of 10 days (regrowth delay: 27 days), as well as by applying hyperthermia immediately after HPP/PDT (regrowth delay: ca 34 days).     

Pharmacokinetics, tissue distribution and photodynamic therapy efficacy of liposomal-delivered hypocrellin A, a potential photosensitizer for tumor therapy

Hypocrellin A, from Hypocrella bambusae, is a novel photosensitizer of high singlet oxygen quantum yield for photodynamic therapy (PDT). Tissue distributions were studied in tumor-bearing mice as a function of time following administration. The tumor model was S-180 sarcoma transplanted into one hind leg of male Kunming mice; hypocrellin A (HA) was delivered to the mice by intravenous injection of 5 mg/kg of body weight as a suspension either as a unilamellar liposome or in dimethyl sulfoxide (DMSO)-solubilized saline. The HA was isolated from several tissues and organs, as well as tumors and peritumoral muscles and skin. Quantitation was performed by a high-performance liquid chromatographic technique with detection that utilizes the native fluorescence of HA. Independent of the delivery system, the dye was retained in tumors at higher concentrations than in normal tissues, except for kidney, liver, lung and spleen. The dye retention in tumors was high and was vehicle dependent. For the liposomal system, the maximal accumulation in tumor and maximal ratios of dye in tumor versus peritumoral muscle and skin occurred 12 h postinjection; for the DMSO saline system, the maximal ratio occurred earlier, 6 h postadministration. Liposomal delivery improved the selective accumulation of the dye in tumor with higher maximal levels in tumor and higher ratios of tumor-to-muscle and tumor-to-skin. Levels of dye were very low or not detectable in the brain. The PDT efficacy of HA in the liposome and DMSO saline systems was determined by evaluating the tumor volume regression percent. The PDT efficacy of HA in liposomes was highest when light treatment was performed at 12 h postinjection, consistent with the highest retention of HA in tumors. Similarly, the maximal PDT efficacy in DMSO saline was attained at 6 h postinjection, the highest HA retention point in tumor. Moreover, the peak PDT efficacy of HA in liposomes was much higher than that of HA in DMSO saline and even hematoporphyrin monomethylether.     

On the Pathomorphological Pattern of the Efficiency of Photodynamic Therapy of Murine Melanoma B16 Using a New Photosensitizer Based on Chlorin e6 Conjugate with a Prostate-Specific Membrane Antigen

Photodynamic therapy (PDT) is an effective treatment for a number of solid malignancies. In this work, the antitumor efficacy of photodynamic therapy for murine B16 melanoma with intravenous administration of a new photosensitizer (PS) based on the chlorin e₆ conjugate with a prostate-specific membrane antigen (PSMA) was studied in vivo. We have previously published the data obtained in the first part of the study: the dynamics of PS accumulation in the tumor and surrounding tissues and the antitumor efficacy of the photodynamic therapy, which was evaluated by the regression parameters and morphological characteristics of the tumors—including by the complete regression of the tumors, the absolute growth rate of the tumors among the mice with continued tumor growth, and an increase in life expectancy compared to the control. The criterion for a complete cure was the absence of signs of tumor recurrence within 90 days after therapy. The conducted studies demonstrated the high efficiency of the new photosensitizer for the photodynamic therapy of B16 melanoma. This article presents a continuation of this work, including histological studies of the zones exposed to laser irradiation on the 21st day after treatment and an assessment of the therapeutic potential of photodynamic therapy for the destruction of tumor cells. Pathological studies in the zones of photodynamic exposure revealed that the effectiveness of the PDT depended on the PS dose and the laser irradiation parameters.     

Photosynthetic Tumor Oxygenation by Photosensitizer‐Containing Cyanobacteria for Enhanced Photodynamic Therapy

Sustained tumor oxygenation is of critical importance during type‐II photodynamic therapy (PDT), which depends on the intratumoral oxygen level for the generation of reactive oxygen species. Herein, the modification of photosynthetic cyanobacteria with the photosensitizer chlorin e6 (ce6) to form ce6‐integrated photosensitive cells, termed ceCyan, is reported. Upon 660 nm laser irradiation, sustained photosynthetic O₂ evolution by the cyanobacteria and the immediate generation of reactive singlet oxygen species (¹O₂) by the integrated photosensitizer could be almost simultaneously achieved for tumor therapy using type‐II PDT both in vitro and in vivo. This work contributes a conceptual while practical paradigm for biocompatible and effective PDT using hybrid microorganisms, displaying a bright future in clinical PDT by microbiotic nanomedicine.     

Photosynthetic Tumor Oxygenation by Photosensitizer-Containing Cyanobacteria for Enhanced Photodynamic Therapy

Sustained tumor oxygenation is of critical importance during type-II photodynamic therapy (PDT), which depends on the intratumoral oxygen level for the generation of reactive oxygen species. Herein, the modification of photosynthetic cyanobacteria with the photosensitizer chlorin e6 (ce6) to form ce6-integrated photosensitive cells, termed ceCyan, is reported. Upon 660 nm laser irradiation, sustained photosynthetic O₂ evolution by the cyanobacteria and the immediate generation of reactive singlet oxygen species (¹O₂) by the integrated photosensitizer could be almost simultaneously achieved for tumor therapy using type-II PDT both in vitro and in vivo. This work contributes a conceptual while practical paradigm for biocompatible and effective PDT using hybrid microorganisms, displaying a bright future in clinical PDT by microbiotic nanomedicine.     

OXYGEN LIMITATION OF DIRECT TUMOR CELL KILL DURING PHOTODYNAMIC TREATMENT OF A MURINE TUMOR MODEL

The relationship between levels of in vivo accumulated photosensitizer (Photofrin II), photodynamic cell inactivation upon in vitro or in vivo illumination, and changing tumor oxygenation was studied in the radiation-induced fibrosarcoma (RIF) mouse tumor model. In vivo porphyrin uptake by tumor cells was assessed by using 14C-labeled photosensitizer, and found to be linear with injected photosensitizer dose over a range of 10 to 100 mg/kg. Cellular photosensitivity upon exposure in vitro to 630 nm light also varied linearly with in vivo accumulated photosensitizer levels in the range of 25 to 100 mg/kg injected Photofrin II, but was reduced at 10 mg/kg. Insignificant increases in direct photodynamic cell inactivation were observed following in vivo light exposure (135 J/cm2, 630 nm) with increasing cellular porphyrin levels. These data were inconsistent with expected results based on in vitro studies. Assessment of vascular occlusion and hypoxic cell fractions following photodynamic tumor treatment showed the development of significant tumor hypoxia, particularly at 50 and 100 mg/kg of Photofrin II, following very brief light exposures (1 min, 4.5 J/cm2). The mean hyupoxic cell fractions of 25 to 30% in these tumors corresponded closely with the surviving cell fractions found after tumor treatment in vivo, indicating that these hypoxic cells had been protected from PDT damage. Inoculation of tumor cells, isolated from tumors after porphyrin exposure, into porphyrin-free hosts, followed by in vivo external light treatment, resulted in tumor control in the absence of vascular tumor bed effects at high photosensitizer doses only.(ABSTRACT TRUNCATED AT 250 WORDS)     

A Bifunctional Photosensitizer for Enhanced Fractional Photodynamic Therapy: Singlet Oxygen Generation in the Presence and Absence of Light

The photosensitized generation of singlet oxygen within tumor tissues during photodynamic therapy (PDT) is self‐limiting, as the already low oxygen concentrations within tumors is further diminished during the process. In certain applications, to minimize photoinduced hypoxia the light is introduced intermittently (fractional PDT) to allow time for the replenishment of cellular oxygen. This condition extends the time required for effective therapy. Herein, we demonstrated that a photosensitizer with an additional 2‐pyridone module for trapping singlet oxygen would be useful in fractional PDT. Thus, in the light cycle, the endoperoxide of 2‐pyridone is generated along with singlet oxygen. In the dark cycle, the endoperoxide undergoes thermal cycloreversion to produce singlet oxygen, regenerating the 2‐pyridone module. As a result, the photodynamic process can continue in the dark as well as in the light cycles. Cell‐culture studies validated this working principle in vitro.     

Semiconducting polymer dots with photosensitizer loading and peptide modification for enhanced cell penetration and photodynamic effect

We utilized semiconducting polymer do-PFDTBT, photosensitizer ZnPc and functional polymer PSMA to prepare carboxyl Pdots. The carboxyl Pdots were modified with cell penetrating peptides (R8) to prepare peptide coated-Pdots, which could enhance the cell penetration and photodynamic effect.     

DH-I-180-3-mediated photodynamic therapy: biodistribution and tumor vascular damage

An important goal of photodynamic therapy (PDT) for treatment of various cancers is to shorten PDT-performing time and simultaneously enhance PDT efficacy. Here, we investigated the nontumor tissue distribution of and the tumor vascular damage caused by a new photosensitizer, DH-I-180-3, in mice with implanted EMT6 mammary tumor cells. In addition, we performed cell-based assays to evaluate the basic antitumor effect of DH-I-180-3/PDT in EMT6 cells. After administration of PDT, the type of cell death was characterized to be apoptosis, and a change in the mitochondrial membrane potential was also observed within minutes. On the other hand, tumor growth was remarkably retarded in vivo in mice that received DH-I-180-3/PDT, compared with mice in the control group, which were exposed to light irradiation alone. Finally, tumors in some mice nearly healed. The antitumor drug reached a maximum concentration approximately 3 h after administration. However, PDT was most effective when there was substantial accumulation of DH-I-180-3 in the tumor vasculature and in healthy tissue. The histological demonstration provided further evidence of tumor vascular damage. On the basis of these findings, we suggest that PDT with the photosensitizer DH-I-180-3 induces vascular damage with blood vessel shutdown, in addition to direct killing of tumor cells, in mice.