Potentiation of chlorin e6 photodynamic activity in vitro with peptide-based intracellular vehicles

Photodynamic therapy (PDT) is a targeted treatment modality where photosensitizers accumulate into cells and are selectively activated by light leading to the production of toxic species and cell death. Focusing the action of photosensitizers to a unique intracellular target may enhance their cytotoxicity. In this study, we demonstrate that the routing of the porphyrin-based photosensitizer chlorin e(6), to the nucleus of cells can significantly alter its toxicity profile. The cellular localization of chlorin e(6) was achieved by coupling the chromophore during solid-phase synthesis to a nucleus-directed linear peptide (Ce6-peptide) or a branched peptide (Ce6-loligomer) composed of eight identical arms displaying the sequence of the Ce6-peptide. These constructs incorporated signals guiding their cytoplasmic uptake and nuclear localization. Ce6-peptide and Ce6-loligomer displayed an enhanced photodynamic activity compared to unconjugated chlorin e(6), lowering the observed CD(50) values for CHO and RIF-1 cells by 1 or more orders of magnitude. The intracellular accumulation of Ce6-peptide and Ce6-loligomer was assessed by electron and confocal microscopy as well as by flow cytometry. Constructs were internalized by cells within an hour and by 6 h, the release of active oxygen species could be observed within the nucleus of cells pretreated with Ce6-loligomer. These results highlight the utility of designing peptides as vehicles for regulating the intracellular distribution of photosensitizers such as chlorin e(6) in order to maximize their efficacy in PDT.     

Photodynamic therapy exploiting the anti-tumor activity of mannose-conjugated chlorin e6 reduced M2-like tumor-associated macrophages

M2-like tumor-associated macrophages (M2-TAMs) in cancer tissues are intimately involved in cancer immunosuppression in addition to growth, invasion, angiogenesis, and metastasis. Hence, considerable attention has been focused on cancer immunotherapies targeting M2-TAMs. However, systemic therapies inhibit TAMs as well as other macrophages important for normal immune responses throughout the body. To stimulate tumor immunity with fewer side effects, we targeted M2-TAMs using photodynamic therapy (PDT), which damages cells via a nontoxic photosensitizer with harmless laser irradiation. We synthesized a light-sensitive compound, mannose-conjugated chlorin e6 (M-chlorin e6), which targets mannose receptors highly expressed on M2-TAMs. M-chlorin e6 accumulated more in tumor tissue than normal skin tissue of syngeneic model mice and was more rapidly excreted than the second-generation photosensitizer talaporfin sodium. Furthermore, M-chlorin e6 PDT significantly reduced the volume and weight of tumor tissue. Flow cytometric analysis revealed that M-chlorin e6 PDT decreased the proportion of M2-TAMs and increased that of anti-tumor macrophages, M1-like TAMs. M-chlorin e6 PDT also directly damaged and killed cancer cells in vitro. Our data indicate that M-chlorin e6 is a promising new therapeutic agent for cancer PDT.     

Synthesis and biological evaluation of new chlorin derivatives as potential photosensitizers for photodynamic therapy

Three new water-soluble chlorin derivatives 3, 5 and 8 for potential use as photosensitizers in photodynamic therapy (PDT) for cancer were synthesized from photoprotoporphyrin IX dimethyl ester (1). The in vivo biodistribution and clearance of chlorin derivatives 3, 5 and 8 were investigated in tumor-bearing mice. Iminodiacetic acid derivative 8 showed the greatest tumor-selective accumulation among the new chlorin derivatives with maximum accumulation in tumor tissue at 3 h after intravenous injection and rapid clearance from normal tissues within 24 h after injection. The in vivo therapeutic efficacy of PDT using 8 was evaluated by measuring tumor growth rates in tumor-bearing mice with 660 nm light-emitting diode irradiation at 3 h after injection of 8. Tumor growth was significantly inhibited by PDT using 8. These results indicate that iminodiacetic acid derivative 8 is useful as a new photosensitizer to overcome the disadvantages of photosensitizers that are currently in clinical use.     

二氢卟吩e6在艾氏荷瘤小鼠体内的分布

目的研究二氢卟吩e6(Ce6)在移植瘤小鼠体内吸收、分布及代谢的动态变化,以期为声动力疗法处理不同部位肿瘤的时间点提供科学依据。方法艾氏移植瘤小鼠尾静脉注射Ce6后,利用荧光分光光度法和小动物活体成像技术测定Ce6在小鼠不同组织的富集分布变化规律。结果小鼠尾静脉给药后,Ce6迅速分布于全身各组织,在2 h内,各组织药物浓度均达到峰值,其中以肝含量最高。随后各组织中药物浓度均开始下降,以肝中清除速度最快。肿瘤组织中的Ce6含量在注射后不断上升,2 h时达到最高,随后开始下降,2～10 h代谢比较缓慢,24 h时浓度降至最低,但仍高于其他组织。结论 Ce6在艾氏移植瘤中具有肿瘤组织选择性好、潴留时间长并可迅速从体内排出等优点,有着很好的临床应用前景,同时提出了不同组织类型不同部位的肿瘤应根据各自适当的时间点进行处理。     

Self-assembled chlorin e6 conjugated chondroitin sulfate nanodrug for photodynamic therapy

Acetylated-chondroitin sulfate/chlorin e6 conjugates (Ac-CS/Ce6 1, 2, 3) were synthesized via the formation of an ester linkage between CS and Ce6 and evaluated as nanoscale drugs for photodynamic therapy. Ac-CS/Ce6 2 and 3 with higher Ce6 contents of 11.7 and 17.6%, respectively, had average diameters of <150 nm and were very stable in phosphate-buffered saline (PBS) for 1 month. The critical self-quenching concentration (CQC) of Ac-CS/Ce6 decreased as the conjugated-amount of Ce6 increased. All samples displayed autophotoquenching properties in aqueous solution, whereas their fluorescence intensity strongly correlated with the amount of Ce6 in the organic solvent dimethyl sulfoxide (DMSO). Compared with free Ce6, Ac-CS/Ce6 nanodrug photoactivity was maintained in terms of fluorescence properties and singlet oxygen ((1)O(2)) generation. In a HeLa cell culture system, we observed rapid cellular uptake of the Ac-CS/Ce6 nanodrug without any other ligands using confocal imaging and fluorescence-activated cell sorting (FACS) analysis. Upon light irradiation following cellular uptake, phototoxicity was detected via 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay. The self-quenching effect and fluorescence recovery of Ac-CS/Ce6 were also determined both in vitro and in vivo. Taken together, our results indicate that Ac-CS/Ce6 has potential as an effective photodynamic therapy (PDT) prodrug for clinical application.     

Vascular targeting to the SST2 receptor improves the therapeutic response to near-IR two-photon activated PDT for deep-tissue cancer treatment

Background

Broader clinical acceptance of photodynamic therapy is currently hindered by (a) poor depth efficacy, and (b) predisposition towards establishment of an angiogenic environment during the treatment. Improved depth efficacy is being sought by exploiting the NIR tissue transparency window and by photo-activation using two-photon absorption (2PA). Here, we use two-photon activation of PDT sensitizers, untargeted and targeted to SST2 receptors or EGF receptors, to achieve deep tissue treatment.

Methods

Human tumor lines, positive or negative for SST2r expression were used, as well as murine 3LL cells and bovine aortic endothelial cells. Expression of SST2 receptors on cancer cells and tumor vasculature was evaluated in vitro and frozen xenograft sections. PDT effects on tumor blood flow were followed using in vivo scanning after intravenous injection of FITC conjugated dextran 150 K. Dependence of the PDT efficacy on the laser pulse duration was evaluated. Effectiveness of targeting to vascular SST2 receptors was compared to that of EGF receptors, or no targeting.

Results

Tumor vasculature stained for SST2 receptors even in tumors from SST2 receptor negative cell lines, and SST2r targeted PDT led to tumor vascular shutdown. Stretching the pulse from ~ 120 fs to ~ 3 ps led to loss of the PDT efficacy especially at greater depth. PDT targeted to SST2 receptors was much more effective than untargeted PDT or PDT targeted to EGF receptors.

General significance

The use of octreotate to target SST2 receptors expressed on tumor vessels is an excellent approach to PDT with few recurrences and some long term cures.     

Extracorporeal Photo-Immunotherapy for Circulating Tumor Cells

It is well established that metastasis through the circulatory system is primarily caused by circulating tumor cells (CTCs). In this preliminary effort, we report an approach to eliminate circulating tumor cells from the blood stream by flowing the blood though an extracorporeal tube and applying photodynamic therapy (PDT). Chlorin e6 (Ce6), a photosensitizer, was conjugated to CD44 antibody in order to target PC-3, a prostate cancer cell line. PC-3 cells were successfully stained by the Ce6-CD44 antibody conjugate. PDT was performed on whole blood spiked with stained PC-3 cells. As the blood circulated through a thin transparent medical tube, it was exposed to light of 660 nm wavelength generated by an LED array. An exposure of two minutes was sufficient to achieve selective cancer cell necrosis. In comparison, to PDT of cells growing inside a tissue culture, the PDT on thin tube exhibited significantly enhanced efficiency in cell killing, by minimizing light attenuation by blood. It suggests a new extracorporeal methodology of PDT for treating CTCs as well as other hematological pathogens.     

Targeted photodynamic therapy of established soft-tissue infections in mice.

The worldwide rise in antibiotic resistance necessitates the development of novel antimicrobial strategies. Although many workers have used photodynamic therapy (PDT) to kill bacteria in vitro, the use of this approach has seldom been reported in vivo in animal models of infection. We have previously described the first use of PDT to treat excisional wound infections by Gram-(-) bacteria in living mice. However, these infected wound models involved a short timespan between infection (30 min) and treatment by PDT. We now report on the use of PDT to treat an established soft-tissue infection in mice. We used Staphylococcus aureus stably transformed with a Photorhabdus luminescenslux operon (luxABCDE) that was genetically modified to be functional in Gram-(+) bacteria. These engineered bacteria emitted bioluminescence, allowing the progress of the infection to be monitored in both space and time with a low light imaging charge-coupled device (CCD) camera. One million cells were injected into one or both thigh muscles of mice that had previously been rendered neutropenic by cyclophosphamide administration. Twenty-four hours later, the bacteria had multiplied more than one hundredfold; poly-L-lysine chlorin e6 conjugate or free chlorin e6 was injected into one area of infected muscle and imaged with the CCD camera. Thirty minutes later, red light from a diode laser was delivered as a surface spot or by interstitial fiber into the infection. There was a light dose dependent loss of bioluminescence (to <5% of that seen in control infections) not seen in untreated infections or those treated with light alone, but in some cases, the infection recurred. Treatment with conjugate alone led to a lesser reduction in bioluminescence. Infections treated with free chlorin e6 responded less well and the infection subsequently increased over the succeeding days, probably due to PDT-mediated tissue damage. PDT-treated infected legs healed better than legs with untreated infections. This data shows that PDT may have applications in drug-resistant soft-tissue infections.     

Spectroscopic and biological studies of a novel synthetic chlorin derivative with prospects for use in PDT

Photosensitizers with desirable combinations of chemical, photophysical and biological properties are essential for improving the efficacy of photodynamic therapy (PDT) against various cancers. Chlorins seem to be promising candidates for photodynamic therapy (PDT) owing to their photophysical properties. This paper reports spectroscopic and biological properties of a novel synthetic chlorin derivative. Cytotoxicity, phototoxicity as well as subcellular localization of the novel derivative was studied using Lewis lung carcinoma cultured cells (LLC). In the examined concentration range no significant cytotoxic effects were found but high phototoxicity was observed. Confocal laser scanning microscopy demonstrated that the compound, upon entering cells, was localized in the perinuclear cytoplasm of LLC cells. Using fluorescent microscopy we investigated the impact of PDT based on the novel compound upon cytoskeleton and DNA structure of LLC cells. Our results indicate that liposomes are effective in transferring the chlorin photosensitizer into the studied cells, leading to their high photosensitization, whereas the non-carrier delivery mode (i.e., DMSO) is rather useless for such purposes.     

生物相容透皮给药微针治疗浅表肿瘤

复杂的肿瘤微环境导致抗肿瘤药物在肿瘤组织内递送效率低下，严重阻碍了药物对浅表肿瘤的治疗效果。生物相容透皮给药微针凭借较高的机械强度，刺穿皮肤角质层，将微针内的药物递送至浅表肿瘤组织内，提高生物利用度，改善静脉注射、口服给药的肝肾毒性等问题。本文介绍了生物相容透皮给药微针的设计及其在癌症化疗、光动力治疗、光热治疗、免疫治疗、基因治疗等领域的研究进展，对浅表肿瘤的微创、局部递药和精准、高效治疗具有重要指导意义。     

Cellular uptake and subcellular distribution of chlorin e6 as functions of pH and interactions with membranes and lipoproteins

The uptake and more importantly the subcellular distribution of photosensitizers are major determinants of their efficacy. In this paper, the cellular internalization of chlorin e6 (Ce6), a photosensitizer bearing three carboxylic chains, is considered with emphasize on pH effects. Small unilamellar vesicles are used as models to investigate the dynamics of interactions of Ce6 with membranes. The entrance and exit steps from the outer lipid hemileaflet are very fast (~ms). A slow transfer of Ce6 through the membrane was observed only for thin bilayers made of dimyristoleoyl-phosphatidylcholine. Ce6 did not permeate through bilayers consisting of longer phospholipids more representative of biological membranes. These results along with previous data on the interactions of Ce6 with low-density lipoproteins (LDL) are correlated with cellular studies. After 15 min incubation of HS68 human fibroblasts with Ce6, fluorescence microscopy revealed labeling of the plasma membrane and cytosolic vesicles different from lysosomes. When vectorized by LDL, Ce6 was mainly localized in lysosomes but absent from the plasma membrane. Internalization of LDL bound photosensitizer via ApoB/E receptor mediated pathway was demonstrated by overexpression experiments. A pH decrease from 7.4 to 6.9 did not affect the intracellular distribution of Ce6, but significantly increased its overall cellular uptake.     

Novel Drug Delivery System for Age-related Macular Degeneration Using Nanotechnology

Age-related macular degeneration (AMD) is a leading cause of legal blindness in developed countries. Even with the recent advent of several treatment options such as photodynamic therapy (PDT) and anti-vascular endothelial growth factor (VEGF) therapy for the treatment of exudative AMD, characterized by choroidal neovascularization (CNV), their efficacy is still limited. Thus, in this review article, we investigated novel drug delivery system for AMD using nanotechnology. Polyion complex (PIC) micelle has a size range of several tens of nanometers formed through electrostatic interaction, and accumulates in solid tumors through enhanced permeability and retention (EPR) effect. The distribution of the PIC micelle encapsulating fluorescein isothiocyanate-labeled poly-l-lysine (FITC-P(Lys)) in experimental CNV in rats was investigated. PIC micelle accumulates in the CNV lesions and is retained in the lesion for as long as 168 h after intravenous administration. PIC micelles can be used for achieving effective drug delivery system to CNV. Although PDT is a main treatment option for CNV, most patients require repeated treatments. For effective PDT against AMD, the selective delivery of photosensitizer to the CNV lesions and an effective photochemical reaction at the CNV site are necessary. The characteristic dendritic structure of the photosensitizer prevents aggregation of its core sensitizer, thereby inducing a highly effective photochemical reaction. A supramolecular nanomedical device, i.e., a novel dendritic photosensitizer encapsulated by a polymeric micelle formulation was employed for an effective PDT for AMD. With its highly selective accumulation on CNV lesions, this treatment resulted in a remarkably efficacious CNV occlusion with minimal unfavorable phototoxicity. Our results will provide a basis for an effective approach to PDT for AMD.     

Cellular uptake and subcellular distribution of chlorin e6 as functions of pH and interactions with membranes and lipoproteins

The uptake and more importantly the subcellular distribution of photosensitizers are major determinants of their efficacy. In this paper, the cellular internalization of chlorin e6 (Ce6), a photosensitizer bearing three carboxylic chains, is considered with emphasize on pH effects. Small unilamellar vesicles are used as models to investigate the dynamics of interactions of Ce6 with membranes. The entrance and exit steps from the outer lipid hemileaflet are very fast (∼ ms). A slow transfer of Ce6 through the membrane was observed only for thin bilayers made of dimyristoleoyl-phosphatidylcholine. Ce6 did not permeate through bilayers consisting of longer phospholipids more representative of biological membranes. These results along with previous data on the interactions of Ce6 with low-density lipoproteins (LDL) are correlated with cellular studies. After 15 min incubation of HS68 human fibroblasts with Ce6, fluorescence microscopy revealed labeling of the plasma membrane and cytosolic vesicles different from lysosomes. When vectorized by LDL, Ce6 was mainly localized in lysosomes but absent from the plasma membrane. Internalization of LDL bound photosensitizer via ApoB/E receptor mediated pathway was demonstrated by overexpression experiments. A pH decrease from 7.4 to 6.9 did not affect the intracellular distribution of Ce6, but significantly increased its overall cellular uptake.     

Pharmacokinetic and biodistribution study following systemic administration of Fospeg® – a Pegylated liposomal mTHPC formulation in a murine model

Fospeg® is a newly developed photosensitizer formulation based on meso‐tetra(hydroxyphenyl)chlorin (mTHPC), with hydrophilic liposomes to carry the hydrophobic photosensitizer to the target tissue. In this study the pharmacokinetics and biodistribution of Fospeg® were investigated by high performance liquid chromatography at various times (0.5–18 hours) following systemic i.v. administration. As a model an experimental HT29 colon tumor in NMRI nu/nu mice was employed. Our study indicates a higher plasma peak concentration, a longer circulation time and a better tumor‐to‐skin ratio than those of Foslip®, another liposomal mTHPC formulation. Data from ex vivo tissue fluorescence and reflectance imaging exhibit good correlation with chemical extraction. Our results have shown that optical imaging provides the potential for fluorophore quantification in biological tissues. (© 2013 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

The pH-dependent distribution of the photosensitizer chlorin e6 among plasma proteins and membranes: a physico-chemical approach

Decrease in interstitial pH of the tumor stroma and over-expression of low density lipoprotein (LDL) receptors by several types of neoplastic cells have been suggested to be important determinants of selective retention of photosensitizers by proliferative tissues. The interactions of chlorin e6 (Ce6), a photosensitizer bearing three carboxylic groups, with plasma proteins and DOPC unilamellar vesicles are investigated by fluorescence spectroscopy. The binding constant to liposomes, with reference to the DOPC concentration, is 6 x 10(3) M(-1) at pH 7.4. Binding of Ce6 to LDL involves about ten high affinity sites close to the apoprotein and some solubilization in the lipid compartment. The overall association constant is 5.7 x 10(7) M(-1) at pH 7.4. Human serum albumin (HSA) is the major carrier (association constant 1.8 x 10(8) M(-1) at pH 7.4). Whereas the affinity of Ce6 for LDL and liposomes increases at lower pH, it decreases for albumin. Between pH 7.4 and 6.5, the relative affinities of Ce6 for LDL versus HSA, and for membranes versus HSA, are multiplied by 4.6 and 3.5, respectively. These effects are likely driven by the ionization equilibria of the photosensitizer carboxylic chains. Then, the cellular uptake of chlorin e6 may be facilitated by its pH-mediated redistribution within the tumor stroma.     

Membrane transport enhancement of chlorin e6‐polyvinylpyrrolidone and its photodynamic efficacy on the chick chorioallantoic model

We have determined the influences of polyvinylpyrrolidone (PVP) on the topical delivery of chlorin e6 (Ce6) in malignant bladder cells. The chick chorioallantoic membrane (CAM) was used to model the tumor spheroids that resemble small residual bladder tumors prior to vascularization. Macroscopic fluorescence imaging showed that Ce6‐PVP‐induced fluorescence had a higher sensitivity and specificity for delineating tumor from the adjacent normal CAM compared to Ce6 alone. Nonlinear regression analyses have shown that Ce6‐PVP has a longer half‐life in the tumor compared to Ce6. The uptake ratio of Ce6‐PVP was found to have a 2‐fold increase across the CAM when compared to that of Ce6, indicating that PVP was able to facilitate diffusion of Ce6 across the membrane. Confocal laser scanning microscopy further confirmed that Ce6‐PVP has better penetration in the CAM as well as in the tumor cells compared to Ce6. The present work contributes to our understanding of the Ce6‐PVP drug–polymer system by demonstrating for the first time that the presence of PVP facilitates the transport of Ce6 across the chorioallantoic membrane. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Light Fractionation Significantly Increases the Efficacy of Photodynamic Therapy Using BF-200 ALA in Normal Mouse Skin

Background

Light fractionation significantly increases the efficacy of 5-aminolevulinic acid (ALA) based photodynamic therapy (PDT) using the nano-emulsion based gel formulation BF-200. PDT using BF-200 ALA has recently been clinically approved and is under investigation in several phase III trials for the treatment of actinic keratosis. This study is the first to compare BF-200 ALA with ALA in preclinical models.

Results

In hairless mouse skin there is no difference in the temporal and spatial distribution of protoporphyrin IX determined by superficial imaging and fluorescence microscopy in frozen sections. In the skin-fold chamber model, BF-200 ALA leads to more PpIX fluorescence at depth in the skin compared to ALA suggesting an enhanced penetration of BF-200 ALA. Light fractionated PDT after BF-200 ALA application results in significantly more visual skin damage following PDT compared to a single illumination. Both ALA formulations show the same visual skin damage, rate of photobleaching and change in vascular volume immediately after PDT. Fluorescence immunohistochemical imaging shows loss of VE-cadherin in the vasculature at day 1 post PDT which is greater after BF-200 ALA compared to ALA and more profound after light fractionation compared to a single illumination.

Discussion

The present study illustrates the clinical potential of light fractionated PDT using BF-200 ALA for enhancing PDT efficacy in (pre-) malignant skin conditions such as basal cell carcinoma and vulval intraepithelial neoplasia and its application in other lesion such as cervical intraepithelial neoplasia and oral squamous cell carcinoma where current approaches have limited efficacy.     

Construction of photodynamic‐effect immunofluorescence probes by a complex of quantum dots,immunoglobulin G and chlorin e6 and their application in HepG2 cell killing

In this study, tri‐functional immunofluorescent probes (Ce6–IgG–QDs) based on covalent combinations of quantum dots (QDs), immunoglobulin G (IgG) and chlorin e6 (Ce6) were developed and their photodynamic ability to induce the death of cancer cells was demonstrated. Strategically, one type of second‐generation photosensitizer, Ce6, was first coupled with anti‐IgG antibody using the EDC/NHS cross‐linking method to construct the photosensitive immunoconjugate Ce6–IgG. Then, a complex of Ce6–IgG–QDs immunofluorescent probes was obtained in succession by covalently coupling Ce6–IgG to water soluble CdTe QDs. The as‐manufactured Ce6–IgG–QDs maintained the bio‐activities of both the antigen–antibody‐based tumour targeting effects of IgG and the photodynamic‐related anticancer activities of Ce6. By way of polyclonal antibody interaction with rabbit anti‐human epidermal growth factor receptor (anti‐EGFR antibody, N‐terminus), Ce6–IgG–QDs were labelled indirectly onto the surface of human hepatocarcinoma (HepG2) cells in cell recognition and killing experiments. The results indicated that the Ce6–IgG–QDs probes have excellent tumour cell selectivity and higher photosensitivity in photodynamic therapy (PDT) compared with Ce6 alone, due to their antibody‐based specific recognition and location of HepG2 cells and the photodynamic effects of Ce6 killed cells based on efficient fluorescence resonance energy transfer between QDs and Ce6. Copyright © 2015 John Wiley & Sons, Ltd.     

Somatostatin Analogues for Receptor Targeted Photodynamic Therapy

Photodynamic therapy (PDT) is an established treatment modality, used mainly for anticancer therapy that relies on the interaction of photosensitizer, light and oxygen. For the treatment of pathologies in certain anatomical sites, improved targeting of the photosensitizer is necessary to prevent damage to healthy tissue. We report on a novel dual approach of targeted PDT (vascular and cellular targeting) utilizing the expression of neuropeptide somatostatin receptor (sst₂) on tumor and neovascular-endothelial cells. We synthesized two conjugates containing the somatostatin analogue [Tyr3]-octreotate and Chlorin e6 (Ce6): Ce6-K₃-[Tyr3]-octreotate (1) and Ce6-[Tyr3]-octreotate-K₃-[Tyr3]-octreotate (2). Investigation of the uptake and photodynamic activity of conjugates in-vitro in human erythroleukemic K562 cells showed that conjugation of [Tyr3]-octreotate with Ce6 in conjugate 1 enhances uptake (by a factor 2) in cells over-expressing sst₂ compared to wild-type cells. Co-treatment with excess free Octreotide abrogated the phototoxicity of conjugate 1 indicative of a specific sst₂-mediated effect. In contrast conjugate 2 showed no receptor-mediated effect due to its high hydrophobicity. When compared with un-conjugated Ce6, the PDT activity of conjugate 1 was lower. However, it showed higher photostability which may compensate for its lower phototoxicity. Intra-vital fluorescence pharmacokinetic studies of conjugate 1 in rat skin-fold observation chambers transplanted with sst₂ ⁺ AR42J acinar pancreas tumors showed significantly different uptake profiles compared to free Ce6. Co-treatment with free Octreotide significantly reduced conjugate uptake in tumor tissue (by a factor 4) as well as in the chamber neo-vasculature. These results show that conjugate 1 might have potential as an in-vivo sst₂ targeting photosensitizer conjugate.     

Direct tumor damage mechanisms of photodynamic therapy

Photodynamic therapy (PDT) is a clinically approved and rapidly developing cancer treatment regimen. It is a minimally invasive two-stage procedure that requires administration of a photosensitizing agent followed by illumination of the tumor with visible light usually generated by laser sources. A third component of PDT is molecular oxygen which is required for the most effective antitumor effects. In the presence of the latter, light of an appropriate wavelength excites the photosensitizer thereby producing cytotoxic intermediates that damage cellular structures. PDT has been approved in many countries for the treatment of lung, esophageal, bladder, skin and head and neck cancers. The antitumor effects of this treatment result from the combination of direct tumor cell photodamage, destruction of tumor vasculature and activation of an immune response. The mechanisms of the direct photodamage of tumor cells, the signaling pathways that lead to apoptosis or survival of sublethaly damaged cells, and potential novel strategies of improving the antitumor efficacy of PDT are discussed.