Correlation between photodynamic efficacy of differing porphyrins and membrane partitioning behavior

Mechanisms of resistance to photodynamic therapy

Photodynamic therapy (PDT) involves the administration of a photosensitizer (PS) followed by illumination with visible light, leading to generation of reactive oxygen species. The mechanisms of resistance to PDT ascribed to the PS may be shared with the general mechanisms of drug resistance, and are related to altered drug uptake and efflux rates or altered intracellular trafficking. As a second step, an increased inactivation of oxygen reactive species is also associated to PDT resistance via antioxidant detoxifying enzymes and activation of heat shock proteins. Induction of stress response genes also occurs after PDT, resulting in modulation of proliferation, cell detachment and inducing survival pathways among other multiple extracellular signalling events. In addition, an increased repair of induced damage to proteins, membranes and occasionally to DNA may happen. PDT-induced tissue hypoxia as a result of vascular damage and photochemical oxygen consumption may also contribute to the appearance of resistant cells. The structure of the PS is believed to be a key point in the development of resistance, being probably related to its particular subcellular localization. Although most of the features have already been described for chemoresistance, in many cases, no cross-resistance between PDT and chemotherapy has been reported. These findings are in line with the enhancement of PDT efficacy by combination with chemotherapy. The study of cross resistance in cells with developed resistance against a particular PS challenged against other PS is also highly complex and comprises different mechanisms. In this review we will classify the different features observed in PDT resistance, leading to a comparison with the mechanisms most commonly found in chemo resistant cells.

The role of photosensitizer molecular charge and structure on the efficacy of photodynamic therapy against Leishmania parasites

Photodynamic therapy (PDT) is emerging as a potential therapeutic modality in the clinical management of cutaneous leishmaniasis (CL). In order to establish a rationale for effective PDT of CL, we investigated the impact of the molecular charge and structure of photosensitizers on the parasitic phototoxic response. Two photosensitizers from the benzophenoxazine family that bear an overall cationic charge and two anionic porphyrinoid molecules were evaluated. The photodynamic activity of the photosensitizers decreases in the following order: EtNBSe > EtNBS > BpD > PpIX. The studies suggest that compared to hydrophobic anionic photosensitizers, the hydrophilic cationic benzophenoxazine analogs provide high effectiveness of PDT possibly due to (1) their strong attraction to the negatively charged parasitic membrane, (2) their hydrophilicity, (3) their high singlet oxygen quantum yield, and (4) their efficacy in targeting intracellular organelles.

Quinacrine enhances vesicular stomatitis virus inactivation and diminishes hemolysis of dimethylmethylene blue-phototreated red cells

Several photodynamic methods for virus inactivation in red blood cell (RBC) suspensions have resulted in unwanted hemolysis during extended 1-6 degrees C storage. To explore the possibility that hemolysis may be mediated by a membrane-bound dye, a molecule similar in structure to yet different in light absorption properties from the photosensitizer was used as an inhibitor for RBC membrane binding in virus photoinactivation and photohemolysis studies. The addition of 500 pM quinacrine to oxygenated RBC before treatment with 3.6 microM dimethylmethylene blue (DMMB) and 219 mJ/cm2 red light resulted in an increased extracellular concentration of the sensitizer, increased extracelluar viral inactivation kinetics, and decreased hemolysis during 1-6 degrees C storage without alteration of quinacrine absorption properties. These results collectively suggest that despite its recognized affinity for viral nucleic acid, DMMB also binds to RBC membranes and that the bound dye is, in part, responsible for photoinduced hemolysis.

Localization of lipoprotein-delivered benzoporphyrin derivative in the rabbit eye

PURPOSE

Photodynamic therapy (PDT) using the photosensitizer Benzoporphyrin derivative monoacid (BPD-MA or verteporfin) is currently under investigation for the treatment of choroidal neovascularization. We investigated the localization of this photosensitizer using fluorescence microscopy and quantified its presence in ocular tissues after porphyrin extraction using fluorescence spectroscopy.

METHODS

Albino rabbits were administered 2mg/kg BPD-MA pre-complexed with low density lipoprotein (LDL) intravenously, or given no treatment. The eyes were enucleated at intervals between 5 minutes and 24 hours after dye injection and were studied with light and fluorescence microscopy, or dissected for porphyrin extraction.

RESULTS

At 5 minutes after dye injection, there was bright fluorescence from the choroid and retinal pigment epithelium (RPE) with trace retinal outer segment fluorescence. After 20 minutes, there was increased photoreceptor outer segment and RPE fluorescence but decreased choroidal fluorescence. By 2 hours no fluorescence remained in either the choroid or the photoreceptors and there was diminished fluorescence of the RPE. Trace RPE fluorescence was still visible at 24 hours. Fluorescence localization of liposomal BPD (2mg/kg) at the earliest (5 minutes) time point was indistinguishable from that of the BPD-LDL complex. Using spectrofluorimetry, the highest BPD-MA levels from the eye were measured in the retina/RPE/uvea complex with lower levels measured from the sclera and other tissues.

CONCLUSIONS

BPD-MA with LDL rapidly accumulates in the choroid, RPE, and photoreceptors after intravenous injection. Future studies of PDT with BPD-MA for the treatment of fundus disorders may need to address the relationship between dye localization and photodynamically-mediated injury.

Prevention of late lumen loss after coronary angioplasty by photodynamic therapy: role of activated neutrophils

Restenosis after coronary angioplasty arises from fibrocellular intimal hyperplasia and possibly failure of the artery to enlarge adequately. Which mechanisms underlie this process is only partly understood. No drugs have been clinically effective in reducing the incidence of restenosis. Since recently, photodynamic therapy (PDT) is being investigated as a possible treatment for intimal hyperplasia. PDT involves the systemic administration of a light-excitable photosensitizer that is taken up rather preferentially by rapidly proliferating cells. During laser irradiation light energy is transferred from the photosensitizer to oxygen generating the highly reactive singlet oxygen. This potent oxidizer can cause severe cellular damage. After PDT of a balloon-injured artery from the rat and rabbit the media remained acellular for several weeks to months, and intimal hyperplasia did not occur. The endothelial lining regenerated by two weeks, but why smooth muscle cells did not repopulated the media is not known. Neutrophils seem to play an important role in the prevention of restenosis after coronary angioplasty, since the activation status of this type of phagocyte is directly related to vessel diameter at late follow-up. Furthermore, it has been observed that neutrophils adhere to the microvascular wall upon PDT in vivo. In vitro findings suggest that the increased neutrophil adherence was not dependent on a decreased release of the anti-adhesive factors NO and prostacyclin by the PDT-treated endothelial cells. Furthermore, PDT did not stimulate the expression of P-selectin by the endothelial cells, one of the adhesion receptors for neutrophils. The endothelial cells only retract upon PDT allowing the adherence of neutrophils by their beta 2-integrin adhesion receptors to the subendothelial matrix. On the basis of these findings, we presume that the successful prevention of intimal hyperplasia by PDT partly depends on the presence of the neutrophil at the site of the lesion.

Effect of haematoporphyrin-induced photosensitization on lipid membranes

In vitro cultured mouse myeloma (Sp-2/0-Ag14) cells and phosphatidylcholine liposomes were used to study the membrane effects of photosensitization with an He-Ne laser activated haematoporphyrin (HP). Lipophilic HP molecules, intercalated between the membrane lipid molecules, caused morphological changes of cell membranes on light activation. Steady state and time-resolved fluorescence spectroscopic studies of membrane-bound HP molecules provide information about the change in membrane lipid dynamics (fluidity). Increased HP fluorescence anisotropy was found after laser irradiation in the case of cell membrane. This finding can be related either to the increased rotation correlation time of the rotating fluorophore (HP) (decreased membrane fluidity) or to the decrease in the angular range of molecular rotation, which corresponds to an increased lipid order after photosensitization. Changes in the ratio of saturated:unsaturated fatty acid content of membrane lipids or other chemical events such as cross-linking of membrane components during the photosensitization process can also account for the observed effects.

Inactivation of cell surface receptors by pheophorbide a, a green pigment isolated from Psychotria acuminata

The inhibition of cytokine and monoclonal antibody binding cell surfaces caused by an extract of Psychotria acuminata, a medicinal plant used in the traditional medicine of the people of Belize (Central Africa), was attributed to the presence of pheophorbide a and pyropheophorbide a. Since the binding of tumor necrosis factor-alpha, interleukin-8, complement factor 5a as well as epidermal growth factor to target cells was dramatically reduced, the inhibition was not receptor or cytokine specific. In addition, the respective binding of radiolabeled monoclonal antibodies CL203 and R15.7 to the cell surface antigens intracellular cell adhesion molecule-1 and lymphocyte function-associated antigen-1 beta-chain was decreased by pretreatment of cells with pheophorbide a as well. In all cases, the inhibition by pheophorbides was dependent on the simultaneous presence of light, indicating causative involvement of a photodynamic process. These observations are not unique to pheophorbides and can be extended to porphyrins as well as to other photodynamic agents. Cytotoxicity resulting from photodynamic therapy (PDT) has been documented by many studies. Our investigations suggest that the inactivation of cell surface receptors contributes not only to an antitumor effect of PDT but also to the systemic immunosuppression, a serious side effect of PDT.

In vivo uptake of liposomal benzoporphyrin derivative and photothrombosis in experimental corneal neovascularization

BACKGROUND AND OBJECTIVE

Photodynamic therapy (PDT) has been used successfully to occlude neovascularizations experimentally. We evaluated the vasoocclusive potential of benzoporphyrin derivative (BPD), a new photosensitizer currently in clinical trials. Since liposomally formulated BPD strongly binds to endogenous low density lipoproteins (LDL) after i.v. injection, LDL act as carrier to deliver BPD preferentially to proliferating endothelial cells.

STUDY DESIGN/MATERIALS AND METHODS

Corneal neovascularizations in rabbits were used as model. Time-dependent uptake and retention of liposomal BPD were measured in vivo by monitoring the laser-induced fluorescence (LIF). Photothrombosis was induced using a dye laser emitting at 692 nm.

RESULTS

A maximal BPD concentration was measured at 60-90 minutes postinjection determining the optimal time interval for treatment. Exposures as low as 10 J/cm2 allowed complete and irreversible neovascular occlusion as documented angiographically. Histology revealed selective endothelial damage, adjacent corneal stroma, or iris vessels, remained intact. Identical results were obtained using BPD directly complexed with LDL suggesting use of a LDL-mediated pathway.

CONCLUSION

We suggest BPD-PDT for a selective treatment of neovascular diseases.