Photodynamic therapy

Water-filtered infrared-A radiation: a novel technique for localized hyperthermia in combination with bacteriochlorophyll-based photodynamic therapy

A novel application of an infrared-A (IR-A) radiation source equipped with a water-filter in the radiation path is described, which allows for tumour treatment with a simultaneous combination of localized hyperthermia (HT) and bacteriochlorophyll-serine (Bchl-ser) based photodynamic therapy (PDT). Using this system, the IR-A radiation was used to heat tumours to 43 degrees C for 60 min, while at the same time activating the Bchl-ser which was injected i.v. at a dose of 20 mg/kg, 10 min following commencement of HT. The growth of tumours undergoing this combined therapy was compared to that of tumours undergoing HT alone or sham-treated controls. Within the 90 day observation period, 100% of tumours in sham-treated animals, 80% in HT-treated animals and only 17% in HT + Bchlser-treated animals reached the end point target volume of 3.5 ml. Thus, the tumour growth inhibition effect of HT can be substantially enhanced by combination with Bchl-ser-PDT. This novel technique has proved to be well-tolerated, easy to apply and should be suitable for treatment of superficial malignancies, especially where hypoxic tumour areas are present.

Photodynamic parameters in the chick chorioallantoic membrane (CAM) bioassay for topically applied photosensitizers

The relative efficacy of Photofrin-based photodynamic therapy (PDT) has been compared with that of the second-generation photosensitizers 5-aminolevulinic acid (ALA), sulfonated chloro-aluminum phthalocyanine (AlPcSn), benzoporphyrin derivative monoacid ring A (BPD-MA), and lutetium texaphyrin (Lutex). PDT-induced vascular damage in the chick chorioallantoic membrane (CAM) is measured following topical application of the photosensitizers. In order to make meaningful comparisons, care is taken to keep treatment variables the same. These include light dose (5 and 10 J/cm2), power density (33 and 100 mW/cm2), and drug uptake time (30 and 90 min). The drug dose ranges from 0.1 microgram/cm2 for BPD to 5000 micrograms/cm2 for ALA. Results are also analyzed statistically according to CAM vessel type (arterioles versus venules), vessel diameter, and vessel development (embryonic age). For each photosensitizer, the order of importance for the various PDT parameters is found to be unique. The differences between the sensitizers are most likely due to variation in biophysical and biochemical characteristics, biodistribution, and uptake kinetics.

A photosensitising adenovirus for photodynamic therapy

We have developed a new approach to photodynamic therapy based on adenoviral transduction of the rate-limiting enzyme in heme synthesis. Conventional phototherapy uses porphyrin-based chemical photosensitisers, including delta-aminolaevulinic acid (ALA) which is converted to protoporphyrin IX (PpIX) by the enzymes of the heme biosynthetic pathway. The lack of a specific mechanism for targeting chemical photosensitisers and PpIX to tumour cells means that therapeutic irradiation can damage normal tissue and exposure to sunlight following treatment can cause severe burns. The rate limiting enzyme in PpIX synthesis is ALA-synthase (ALA-S). We have developed a new yeast vector system for manipulation of the adeno- virus genome and used it to construct a virus expressing a mutant form of ALA-S lacking the iron response elements which regulate ALA-S translation and the heme regulatory motifs which regulate import of ALA-S into mitochondria. The virus induces a large increase in PpIX expression and confers photosensitivity on cultured cells. Unlike conventional photodynamic therapy, a viral approach makes it possible to restrict photosensitivity by biological rather than purely physical or chemical means. As with HSV thymidine kinase, ALA-S expression is a general mechanism for sensitisation to a therapeutic agent which can easily be adapted to whatever means of gene delivery is most effective.

Synthesis and evaluation of chalcogenopyrylium dyes as potential sensitizers for the photodynamic therapy of cancer

A series of thiopyrylium (2), selenopyrylium (3), and telluropyrylium dyes (4) was prepared via the addition of Grignard reagents to either 2, 6-di(4-dimethylamino)phenylchalcogenopyran-4-ones (5a) or 2-[4-(dimethylamino)phenyl]-6-phenylchalcogenopyran-4-ones (5b) followed by elimination and ion exchange to give the chloride salts. The absorption spectra and quantum yields for singlet oxygen generation of these dyes suggested that the dyes would have utility as sensitizers for PDT. Selenopyrylium dyes 3a and 3d with quantum yields for singlet oxygen generation of 0.040 and 0.045, respectively, were phototoxic to Colo-26 cells in culture. The toxicity of the dyes 2-4 was evaluated in clonogenic assays of human carcinoma cell lines. Importantly, the presence of a sulfur, selenium, or tellurium heteroatom in the molecules had no predictable impact on the toxicity of any particular dye set. Substituents at the 2-, 4-, and 6-positions of the dye had a much greater impact on cytotoxicity. The IC(50) values determined in the clonogenic assays did not correlate with chemical properties in the dye molecules such as reduction potential or lipophilicity. Initial in vivo toxicity studies showed no toxicity for these dyes at dosages between 7.2 and 38 micromol/kg in BALB/c mice.

2,4,6-triarylchalcogenopyrylium dyes related in structure to the antitumor agent AA1 as in vitro sensitizers for the photodynamic therapy of cancer

Cationic chalcogenopyrylium dyes 2-4 were synthesized in six steps from 4-(dimethylamino)phenylethyne (7), have absorption maxima in methanol of 594, 631, and 672 nm, respectively, and generate singlet oxygen with quantum yields [Phi((1)O(2))] of 0.020, 0.064, and 0.037, respectively. Dyes 2-4 are hydrolytically more stable than other chalcogenopyrylium dyes evaluated previously as sensitizers for photodynamic therapy. At 10 microM final concentration, all dyes 2-4 inhibited cytochrome c oxidase during irradiation of tumor mitochondrial suspensions treated with 10 microM dye. The degree of enzyme inhibition was abated in a reduced oxygen environment and in the presence of imidazole, a singlet oxygen trap. Superoxide dismutase, at a final concentration of 30 U, did not alter the photosensitized inhibition of mitochondrial cytochrome c oxidase by dyes 2-4. These data suggest that singlet oxygen may play a major role in the photosensitized inhibition of mitochondrial cytochrome c oxidase. Irradiation of R3230AC rat mammary adenocarcinoma cells in the presence of dyes 2-4 caused a significant loss in cell viability with thiopyrylium dye 2 displaying the greatest phototoxicity. Initial acute toxicity studies in vivo demonstrate that, at 10 mg/kg, none of the three dyes displayed overt toxicity.

Combination photoimmunotherapy and cisplatin: effects on human ovarian cancer ex vivo

BACKGROUND

Patients with ovarian cancer that is clinically resistant to cisplatin-based chemotherapy have little hope of a cure of their disease. Photoimmunotherapy, which involves the antibody-targeted delivery of a nontoxic photosensitizer that is activated to a cytotoxic state with visible light, may offer a new treatment option. Photoimmunotherapy may be applied intraperitoneally to target disseminated tumor. We tested the hypothesis that this treatment in combination with cisplatin potentiates cytotoxicity in ovarian cancer cell lines and primary cultures of human tumors.

METHODS

Five human cancer cell lines (ovarian and breast) and 19 primary cultures were studied. The primary cultures were from solid and ascites tumor samples obtained from 14 patients with ovarian cancer who were undergoing primary surgery. The photosensitizer chlorin e(6) was conjugated to the F(ab')(2) fragment of the murine monoclonal antibody OC-125, which is directed against the antigen CA 125. Cytotoxicity was measured by the microculture tetrazolium assay. Treatments consisted of cisplatin alone, photoimmunotherapy alone, and photoimmunotherapy followed by cisplatin. The fractional product method was used to assess synergy in treatment effects. Ex vivo cultured human cells exhibiting 80% or greater survival at cisplatin concentrations of 10 microM for 24 hours were defined as cisplatin resistant for this study.

RESULTS

When all cell types (cisplatin sensitive and cisplatin resistant) were considered together, combination treatment yielded cytotoxicity that was, on average, 6.9 times (95% confidence interval = 1.86-11.94) greater than that of cisplatin alone (two-sided P =.023). Cisplatin-resistant cells showed a synergistic effect of the two treatments (two-sided P =.044), while cisplatin-sensitive cells showed an additive effect.

CONCLUSION

These ex vivo data suggest that platinum resistance in human ovarian cancer cells may be reversible by pretreatment with OC-125-targeted photoimmunotherapy. Further studies are required to confirm the efficacy of this approach in vivo.

Foscan (mTHPC) photosensitized macrophage activation: enhancement of phagocytosis, nitric oxide release and tumour necrosis factor-alpha-mediated cytolytic activity

Photodynamic activation of macrophage-like cells contributes to an effective outcome of photodynamic therapy (PDT) treatment. The possibility for an enhancement of macrophage activity by photosensitization with meta-tetra(hydroxyphenyl)chlorin (mTHPC) (1 microg ml(-1)) was studied in U937, monocyte cell line differentiated into macrophages (U937) cells). Phagocytic activity of U937phi cells was evaluated by flow-cytometry monitoring of ingestion of fluorescein-labelled Escherichia coli particles. Increase in irradiation fluence up to 3.45 mJ cm(-2) (corresponding irradiation time 15 s) resulted in significant increase in fluorescence signal (145%), while at higher light fluences the signal lowered down to the untreated control values. A light energy-dependent production of tumour necrosis factor-alpha (TNF-alpha) by photosensitized macrophages was further demonstrated using the L929 assay. The maximum TNF-alpha mediated cytolysis was observed at 28 mJ cm(-2) and was 1.7-fold greater than that in control. In addition, we demonstrated a fluence-dependent increase in nitric oxide (NO) production by mTHPC-photosensitized macrophages. NO release increased gradually and reached a plateau after irradiation fluence of 6.9 mJ cm(-2). Cytotoxicity measurements indicated that the observed manifestations of mTHPC-photosensitized macrophage activation took place under the sublethal light doses. The relevance of the present findings to clinical mTHPC-PDT is discussed.

Biopharmaceutics of boronated radiosensitizers: liposomal formulation of MnBOPP (manganese chelate of 2,4-(alpha, beta-dihydroxyethyl) deuterioporphyrin IX) and comparative toxicity in mice

Binary treatment modalities such as photodynamic therapy (PDT) and neutron capture therapy (NCT) combine low-toxicity electromagnetic irradiation with an appropriate radiation sensitizer to enhance selectivity for tumor targets. The porphyrin derivative tetrakiscarborane carboxylate ester of 2,4-(alpha, beta-dihydroxyethyl) deuterioporphyrin IX (BOPP) shows tumor-selective uptake and is active in both treatment modalities. BOPP also chelates paramagnetic ions such as Mn(2+), and therefore its tissue accumulation and selectivity can be detected noninvasively by using magnetic resonance imaging. However, local and systemic toxicity appears elevated for the Mn(2+) chelate (MnBOPP), but is poorly characterized. Here we have developed a liposomal formulation of MnBOPP and compared its toxicity with that of MnBOPP administered to mice in saline. The optimal liposome composition and maximal capacity to accommodate MnBOPP were investigated by differential scanning calorimetry and by encapsulation efficiency. MnBOPP was encapsulated quantitatively at up to 12 mol % (drug:lipid) in liposomes of varying composition, and remained incorporated during extended dialysis. Phase separation of drug- and lipid-rich domains was observed above 12% drug. MnBOPP in buffered saline was lethal to animals at 90 micromol/kg, and caused severe necrosis at the injection site at dose levels of 60 micromol/kg or greater. In contrast, MnBOPP formulated in liposomes was well tolerated at the highest tested dose of 135 micromol/kg, with the elimination of local toxicity.

Synthesis and estrogen receptor binding affinity of a porphyrin-estradiol conjugate for targeted photodynamic therapy of cancer

A tetraphenylporphyrin-C11-beta-estradiol conjugate has been synthesized. Competitive binding assay of the conjugate with estrogen receptor (ER)-ligand-binding domain showed that the conjugate binds specifically to the protein with high affinity. Potential use of this conjugate to selectively deliver cytotoxic porphyrins to ER-positive cells in various carcinomas is discussed.

Enhanced apoptotic response to photodynamic therapy after bcl-2 transfection

Apoptosis is a cellular death process involving the sequential activation of a series of caspases, endonucleases, and other enzymes. The initiation of apoptosis can be inhibited by overexpression of bcl-2 and certain other members of a related family of proteins. We examined the effects of bcl-2 overexpression on the apoptotic response to photodynamic therapy (PDT), using aluminum phthalocyanine as the photosensitizing agent. In this study, we compared the immortalized human breast epithelial cell line MCF10A with a subline (MCF10A/bcl-2) transfected with the human bcl-2 gene. The latter was approximately 2-fold more sensitive to the phototoxic effects of PDT. At a 50 mJ/cm2 light dose, photodamage to MCF-10A/bcl-2 resulted in a greater loss of the mitochondrial membrane potential (delta(psi)m), enhanced release of mitochondrial cytochrome c, a more rapid and greater activation of caspase-3, and a greater apoptotic response. Western blot analysis revealed that the transfected cell line showed overexpression of both bcl-2 and bax, and that PDT caused selective destruction of bcl-2, leaving bax unaffected. The greater apoptotic response by the transfected line is, therefore, attributed to the higher bax:bcl-2 ratio after photodamage.

Photodynamic therapy: a review

Photodynamic therapy (PDT) of malignant tumours is a new technique for treating cancers. After intravenous injection, a photosensitiser is selectively retained by the tumour cells so after time there is more sensitiser in the tumour than in the normal adjacent tissue. The photosensitiser must be able to absorb the wavelength of light being delivered to it, and the amount of light getting to the photosensitiser depends on the characteristics of the tissue it passes through. When exposed to light with the proper wavelength, the sensitiser produces an activated oxygen species, singlet oxygen, that oxidises critical elements of neoplastic cells. Because there is less sensitiser in the adjacent normal tissue, less reaction occurs to it. Since this is an entirely different process, the use of chemotherapy, ionising radiation or surgery does not preclude the use of PDT. Also, unlike ionising irradiation, repeated injections and treatments can be made indefinitely. Different molecules and atoms absorb different wavelengths of energy. Since the light energy must be absorbed to start the photochemical reaction, the absorption spectrum of the photosensitiser determines the wavelength used to initiate the reaction. However, this can be qualified by the tissue the light has to travel through to get to the photosensitiser. The photosensitiser porfimer sodium has a peak absorption in the area of 405 nm (blue-violet) and a much lower absorption peak at 630 nm (red). However, because the longer red wavelength penetrates tissue deeper than 405 nm, we use the red wavelength, usually delivered from a laser system. This permits coupling of the red light beam to quartz fibres which can then be used with modifications to treat external surface tumours, inserted interstitially directly into large tumours, passed though any endoscope to treat intraluminal tumours, or inserted behind the retina to treat tumours of the retina. Twenty years after the pioneering work of Dr. Thomas Doherty, the US Food and Drug Administration (FDA) has approved the use of porfimer sodium for photodynamic therapy of endobronchial and oesophageal tumours. Research continues towards approval for management of skin cancers and metastatic cutaneous and subcutaneous breast cancers. The realisation that one of the mechanisms of photodynamic therapy is thrombosis of vessels led to the development of verteporfin to treat macular degeneration. Multiple other areas are being investigated as well as new photosensitisers. Photodynamic therapy is an entirely new treatment modality and its development can be likened to that of the discovery of antibiotics. This is just the beginning, and its possible uses are only limited by the imagination.

Antisense bcl-2 retrovirus vector increases the sensitivity of a human gastric adenocarcinoma cell line to photodynamic therapy

The bcl-2 oncoprotein directly prolongs cellular survival by blocking apoptosis and its overexpression is associated with cellular resistance to killing by chemotherapeutic drugs and gamma-irradiation. Meanwhile, it has been shown that bcl-2 antisense oligonucleotide can induce apoptosis or increase toxicity of the treatment in tumors in vivo and in vitro. However, it is difficult to obtain stable transfection by this approach and there are no reports about the effect of an antisense bcl-2 on the sensitivity to oxidative stress induced by photodynamic therapy (PDT). Here we investigated the effect of an antisense bcl-2 RNA retrovirus vector transfer on the sensitivity of 2-butylamino-2-demethoxy-hypocrellin A (2-BA-2-DMHA) photosensitization in a human gastric adenocarcinoma MGC803 cell line. The results indicate that antisense bcl-2-infected MGC803 cells expressed exogenous antisense bcl-2 mRNA measured by reverse transcription polymerase chain reaction and significantly reduced bcl-2 protein determined by western blotting analysis. The decreased expression of bcl-2 protein was accompanied by increased phototoxicity and susceptibility to apoptosis induced by 2-BA-2-DMHA PDT. Our finding suggests that reduction of bcl-2 protein in gastric cancers, and possibly also in a variety of other tumors, may be a novel and rational approach to improve photosensitivity and the treatment outcome.

Effect of delivery system on the pharmacokinetic and phototherapeutic properties of bis(methyloxyethyleneoxy) silicon-phthalocyanine in tumor-bearing mice

A Si(IV)-phthalocyanine bearing two methoxyethyleneglycol axial ligands bound to the central metal ion (SiPc) has been prepared by chemical synthesis and analyzed for its phototherapeutic activity after administration in a Cremophor or liposome formulation to C57B1/6 mice bearing a subcutaneously transplanted Lewis lung carcinoma (LLC). The maximum drug accumulation in the tumor is found at 24 h after intraperitoneal injection, independent of the delivery system. However, the tumor concentration of SiPc in the Cremophor formulation is about two-fold higher, while the drug concentration in liver and skin shows similar trends with the two delivery systems. The drug accumulation and retention in the brain is much larger when using Cremophor emulsion. Photodynamic therapy (672 nm, 370 mW m-2, 360 J cm-2) at 24 h after the injection of Cremophor emulsion- or DPPC liposome-formulated SiPc causes a very efficient and similar response for the LLC (approximately 8 versus 22 mm mean tumor diameter for the control groups at 21 days after phototreatment). These very promising effects, obtained both at higher and lower tumor drug concentrations, clearly demonstrate the potential phototherapeutical activity of the newly synthesized SiPc.

BPD-MA-mediated photosensitization in vitro and in vivo: cellular adhesion and beta1 integrin expression in ovarian cancer cells

Benzoporphyrin derivative monoacid (BPD-MA) photosensitization was examined for its effects on cellular adhesion of a human ovarian cancer cell line, OVCAR 3, to extracellular matrix (ECM) components. Mild BPD-MA photosensitization (approximately 85% cell survival) of OVCAR 3 transiently decreased adhesion to collagen IV, fibronectin, laminin and vitronectin to a greater extent than could be attributed to cell death. The loss in adhesiveness was accompanied by a loss of beta1 integrin-containing focal adhesion plaques (FAPs), although beta1 subunits were still recognized by monoclonal antibody directed against human beta1 subunits. In vivo BPD-MA photosensitization decreased OVCAR 3 adhesiveness as well. Photosensitized adhesion was reduced in the presence of sodium azide and enhanced in deuterium oxide, suggesting mediation by singlet oxygen. Co-localization studies of BPD-MA and Rhodamine 123 showed that the photosensitizer was largely mitochondrial, but also exhibited extramitochondrial, intracellullar, diffuse cytosolic fluorescence. Taken together, these data show that intracellular damage mediated by BPD-PDT remote from the FAP site can affect cellular-ECM interactions and result in loss of FAP formation. This may have an impact on long-term effects of photodynamic therapy. The topic merits further investigation.

The role of photodynamic therapy in dermatology

Photodynamic therapy (PDT) involves selective photosensitization of a target tissue by means of a topically or systemically administered agent which is then activated by light to effect an oxygen dependent cytotoxic reaction. The production of reactive oxygen intermediates, including singlet oxygen, is localized to where the photosensitizer accumulates and induces apoptosis and vascular endothelial damage. First-generation photosensitizers are haematoporphyrin derivatives and are effective in treating certain nonmelanoma skin cancers. However, they induce cutaneous photosensitization for at least 4-6 weeks and have a limited role in dermatology. Many second-generation photosensitizers, such as 5-aminolaevulinic acid, are associated with less prolonged photosensitization. Although PDT remains largely experimental, it has potential applications in both benign and malignant skin disease.

Non-invasive measurement of chemotherapy drug concentrations in tissue: preliminary demonstrations of in vivo measurements

Measurements of the tissue concentrations of two chemotherapy agents have been made in vivo on an animal tumour model. The method used is based on elastic scattering spectroscopy (ESS) and utilizes a fibre-optic probe spectroscopic system. A broadband light source is used to acquire data over a broad range of wavelengths and, therefore, to facilitate the separation of absorptions from various chromophores. The results of the work include measurements of the time course of the drug concentrations as well as a comparison of the optical measurements with high performance liquid chromatography (HPLC) analysis of the drug concentrations at the time of sacrifice. It is found that the optical measurements correlate linearly with HPLC measurements, but give lower absolute values.

Wild-type p53 protein potentiates phototoxicity of 2-BA-2-DMHA in HT29 cells expressing endogenous mutant p53

To better understand the effects of p53 on the process of photodynamic therapy (PDT)-induced cell death, we introduced a wild-type p53 gene into the HT29 colorectal carcinoma cell line, which bears an endogenous mutant p53, using a lipofectin system. The influence of p53 status on the sensitivity induced by 2-butylamino-2-demethoxy-hypocrellin A (2-BA-2-DMHA) photosensitization was then examined. The results indicate that infection with wild-type p53 induces a growth arrest but does not induce cell death, and sensitizes the cells to PDT. At a concentration of 5 microM 2-BA-2-DMHA with a red light of 18 J/cm2 (lambda = 600-700 nm), the survival is reduced from 58.72% in HT29 cells to 13.49% in wild-type p53-infected HT29 cells. Apoptosis following PDT appears earlier in HT29 cells infected with wild-type p53 than in parent HT29 cells and empty vector-infected HT29 cells. These findings suggest that although wild-type p53 is, by itself, insufficient to induce apoptosis in cells with p53 mutation, it enhances the photosensitivity of 2-BA-2-DMHA by strongly potentiating the induction of apoptosis.

Contribution of myeloid and lymphoid host cells to the curative outcome of mouse sarcoma treatment by photodynamic therapy

Selective depletion or inactivation of specific myeloid populations (neutrophils, macrophages) and lymphoid populations (helper T cells, cytolytic T cells) in EMT6 sarcoma-bearing mice was used to determine the contribution of each of these host immune cell types to the curative outcome of Photofrin-based photodynamic therapy (PDT). Immunodepletion of neutrophils and cytolytic T cells initiated immediately after PDT resulted in a marked reduction in PDT-mediated tumor cures. Significant reduction in the cures of EMT6 tumors was also achieved by immunodepletion of helper T cells and inactivation of macrophages by silica treatment. The initial tumor ablation by PDT was not affected by any of the above depletion treatments. These results provide direct evidence that the contribution of neutrophils, macrophages and T lymphocytes is essential for the maintenance of long-term control of PDT-treated tumors.

Tumor-enhancement effect of a Mn3+ metalloporphyrin derivative (ATN-4T) in magnetic resonance imaging

Magnetic resonance imaging is routinely used for tumor recognition in cancer diagnosis. The tumor image-enhancing characteristics of ATN-4T (THF-Mn-Asp), a Mn3+ metalloporphyrin derivative, were evaluated in rabbits. ATN-4T (10 mM) was diluted in gelatin to final concentrations ranging from 100 to 1 microM. Increasing concentrations of ATN-4T resulted in higher signal intensities. VX2 (squamous cell carcinoma) tumor-bearing rabbits were injected with 50 micromol/kg ATN-4T intravenously and T1 -weighted images were recorded continuously. Tumor images were compared with images of surrounding muscle tissue. T1-weighted images from ATN-4T-treated rabbits showed a marked enhancement of tumor contrast from 30 to 240 min postinjection. Microscopic examination revealed that carcinoma cells were scattered throughout the high contrast area of the tumor and were not seen in the surrounding muscles. ATN-4T appears useful for enhancing the intensity of tumors imaged by magnetic resonance.

Involvement of retinoblastoma (Rb) and E2F transcription factors during photodynamic therapy of human epidermoid carcinoma cells A431

Photodynamic therapy (PDT), a promising new therapeutic modality for the management of a variety of solid malignancies and many non-malignant diseases, is a bimodal therapy using a porphyrin based photosensitizing chemical and visible light. The proper understanding of the mechanism of PDT-mediated cancer cell-kill may result in improving the efficacy of this treatment modality. Earlier we have shown (Proc. Natl. Acad. Sci. USA; 95: 6977-6982, 1998) that silicon phthalocyanine (Pc4)-PDT results in an induction of the cyclin kinase inhibitor WAF1/CIP1/p21 which, by inhibiting cyclins (E and D1) and cyclin dependent kinases (cdk2 and cdk6), results in a G0/G1-phase arrest followed by apoptosis in human epidermoid carcinoma cells A431. We have also demonstrated the generation of nitric oxide during PDT-mediated apoptosis (Cancer Res.; 58: 1785-1788, 1998). Retinoblastoma (pRb) and E2F family transcription factors are important proteins, which regulate the G1-->S transition in the cell cycle. Here, we provide evidence for the involvement of pRb-E2F/DP machinery as an important contributor of PDT-mediated cell cycle arrest and apoptosis. Western blot analysis demonstrated a decrease in the hyper-phosphorylated form of pRb at 3, 6 and 12 h post-PDT with a relative increase in hypo-phosphorylated pRb. Western blot analysis also revealed that PDT-caused decrease in phosphorylation of pRb occurs at serine-780. The ELISA data demonstrated a time dependent accumulation of hypo-phosphorylated pRb by PDT. This response was accompanied with down-regulation in the protein expression of all five E2F (1-5) family transcription factors, and their heterodimeric partners DP1 and DP2. These results suggest that Pc4-PDT of A431 cells results in a down regulation of hyper-phosphorylated pRb protein with a relative increase in hypo-phosphorylated pRb that, in turn, compromises with the availability of free E2F. We suggest that these events result in a stoppage of the cell cycle progression at G1-->S transition thereby leading to a G0/G1 phase arrest and a subsequent apoptotic cell death. These data provide an evidence for the involvement of pRb-E2F/DP machinery in PDT-mediated cell cycle arrest leading to apoptosis.

Photodynamic therapy-induced apoptosis in lymphoma cells: translocation of cytochrome c causes inhibition of respiration as well as caspase activation

L5178Y-R mouse lymphoma (LY-R) cells undergo rapid apoptosis when treated with photodynamic therapy (PDT) sensitized with the silicon phthalocyanine Pc 4. In this study we show that cytochrome c is released into the cytosol within 10 min of an LD99.9 dose of PDT. Cellular respiration is inhibited by 42% at 15 min, and 60% at 30 min after PDT treatment, and caspase 3-like protease activity is elevated by 15 min post-PDT. In digitonin-permeabilized cells addition of cytochrome c to the respiration buffer reverses PDT-induced inhibition of state 3 respiration via Complex I by 40-60%, and via Complex III by 50-90%. In contrast, extramitochondrial cytochrome c does not stimulate respiration in permeabilized control cells, and catalyzes only a low rate of oxygen consumption via electron transfer to cytochrome b5 on the outer mitochondrial membrane. These results demonstrate that PDT-induced inhibition of respiration is primarily due to leakage of cytochrome c into the cytosol rather than to damage to the major enzyme complexes of the electron transport chain. Whether or not inhibition of respiration influences the time course or extent of Pc 4-PDT-induced apoptosis in LY-R cells is not clear at the present time.

DNA interaction and photocleavage properties of porphyrins containing cationic substituents at the peripheral position

A series of mono- and disubstituted cationic porphyrins (1-8) were synthesized and investigated for their ability to bind and cleave DNA in the presence of light. In these porphyrins, the cationic substituents were introduced at various peripheral positions, i.e., the non-meso positions of the porphyrin system. The modes of binding of these porphyrins to DNA were investigated by UV-vis spectroscopy, circular dichroism, and an unwinding assay. The intrinsic binding constants Kb of these porphyrins to calf thymus DNA was found to be in the range 10(4)-10(5) M-1. Two of the zinc(II) complexes of non-meso-substituted cationic porphyrins (5 and 8) were found to bind to DNA via intercalation, which is in contrast to the previously reported outside-binding mode for the Zn(II) complexes of meso-substituted cationic porphyrins. Except for monocationic porphyrin 1 and Ni(II) dicationic porphyrin 6, all the other porphyrins were found to be efficient photocleavers of DNA. The DNA photocleavage characteristics of this series of cationic porphyrins were found to depend on the structural characteristics of the poprhyrins such as (a) length of the side chain of the cationic substituents (2 vs 4), (b) the position of the side chain on the porphyrin ring (4 vs 7), and (c) the presence of the chelating metal in 3, 5, and 8 as compared to the nonmetallo porphyrins 2, 4, and 7, respectively.

Photosensitization of human skin cell lines by ATMPn (9-acetoxy-2,7,12,17-tetrakis-(beta-methoxyethyl)-porphycene) in vitro: mechanism of action

9-Acetoxy-2,7,12,17-tetrakis-(beta-methoxyethyl)-porphycene (ATMPn) is a promising new photosensitizer characterized by high absorption around 640 nm and high singlet oxygen yield. To study the mechanism of action in vitro we have investigated uptake, intracellular localization, cell survival and ultrastructural changes following photodynamic treatment in human cell lines derived from the skin (SCL1 and SCL2, squamous cell carcinoma; HaCaT keratinocytes; N1 fibroblasts). Using flow cytometry we have determined the cellular fluorescence as a marker for the uptake of ATMPn after incubation for 60 min. Co-staining with ATMPn and fluorescent dyes specific for cell organelles reveals an intracellular localization of ATMPn in lysosomes. Following irradiation using an incoherent light source (580-740 nm) and a light fluence of 24 J cm-2, phototoxicity is determined by means of the 3-4.5 dimethylthiazol-2,5 diphenyl tetrazolium bromide (MTT) assay. For all cell lines ATMPn concentrations above 15 nM yield a significant phototoxic effect. The 50% effective concentration, EC50, for SCL1 cells is 11.2 +/- 2.9 nM ATMPn. ATMPn uptake and phototoxicity are more effective for HaCaT and SCL1 as compared to SCL2 and N1 cells. Growth curves confirmed the results of the MTT assay. Because of the high lysosomal accumulation of ATMPn, already low photosensitizer concentrations without dark toxicity yield a high photodynamic effect. Immunofluorescence and electron microscopy reveal damage to tonofilaments, plasma membrane and mitochondria, indicating a mechanism unrelated to apoptosis. A dose yielding complete cell killing, as needed for oncological indications, might lead to necrosis, whereas lower sub-lethal doses result in induction of apoptosis.

Hypericin-induced photosensitization of HeLa cells leads to apoptosis or necrosis. Involvement of cytochrome c and procaspase-3 activation in the mechanism of apoptosis

Here we report that photoactivated hypericin can induce either apoptosis or necrosis in HeLa cells. Under apoptotic conditions the cleavage of poly(ADP-ribose) polymerase (PARP) into the 85-kDa product is blocked by the caspase inhibitors benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (z-VAD-fmk) and benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone (z-DEVD-fmk). Both inhibitors protect cells from apoptosis but cannot prevent hypericin-induced necrosis. Conversely, HeLa cells overexpressing the viral cytokine response modifier A (CrmA), which inhibits caspase-1 and -8, still undergo hypericin-induced apoptosis and necrosis. Evidence is provided for the release of mitochondrial cytochrome c in the cytosol and for procaspase-3 activation in the hypericin-induced cell killing.