Microtubule assembly inhibition by porphyrins and related compounds

Preclinical photodynamic therapy research in Spain 4: Cytoskeleton and adhesion complexes of cultured tumor cells as targets of photosensitizers

Tumor cell death induced by photodynamic therapy (PDT) with different photosensitizers (PSs) is due to the selective damage of several membranous organelles including mitochondria, lysosomes and Golgi apparatus. Other cell structures such as the cytoskeleton (CSK) (microtubules, actin microfilaments and cytokeratin intermediate filaments) and the cell adhesion components (cadherins and integrins) are also implicated in cell death induced by PSs. CSK and adhesion components are responsible for many cellular functions such as the maintenance of morphology, motility, division and adhesion, all of them of fundamental importance for growth and dissemination of tumors. Therefore, they are considered very important targets for anticancer therapies, including PDT. In addition, similarly to the rest of the anticancer therapies, PDT often leaves a significant number of surviving tumor cells. The reorganization of CSK as well as the adhesion proteins in the PDT resistant cells affect their invasive migratory capabilities. Taking into account all these features, both CSK and adhesion proteins are crucial targets of PDT.

Porphyrins affect the self-assembly of tubulin in solution

Self-assembly of tubulin heterodimers in solution has been studied in the past to predict the effects that ligands and/or conformational changes have on the formation of tubulin filaments. Self-assembly of tubulin in solution has produced formations similar to cellular microtubules (MTs). The present study reports on the effects that two porphyrins (protoporphyrin IX, PPIX and tetrakis(4-sulfonatophenyl)porphyrin, TPPS) produce on the self-assembly of tubulin alpha,beta-heterodimers in buffer solution. The study shows that, when incubated simultaneously with MT-stabilizing ligands (i.e., paclitaxel and guanosine triphosphate, GTP), porphyrins do not affect the ability of tubulin to form MT. However, if paclitaxel and GTP are added after tubulin has been allowed to self-assemble in the presence of either porphyrin, the ability to form MT-like structures is reduced or suppressed. We suggest that this effect is due to the formation of porphyrin-mediated aggregates that cannot be broken or elongated by the addition of GTP or paclitaxel.

Irradiation of the porphyrin causes unfolding of the protein in the protoporphyrin IX/beta-lactoglobulin noncovalent complex

Porphyrins such as protoporphyrin IX (PPIX) are known to occasionally cause conformational changes in proteins for which they are specific ligands. It has also been established that irradiation of porphyrins noncovalently intercalated between bases or bound to one of the grooves can cause conformational effects on DNA. Conversely, there is no evidence reported in the literature of conformational changes caused by noncovalently bound PPIX to globular proteins for which the porphyrin is not a specific ligand. This study shows that the irradiation of the porphyrin in the PPIX/lactoglobulin noncovalent complex indeed causes a local and limited (approximately 7%) unfolding of the protein near the location of Trp19. This event causes the intrinsic fluorescence spectrum of the protein to shift to the red by 2 nm and the average decay lifetime to lengthen by approximately 0.5 ns. The unfolding of lactoglobulin occurs only at pH >7 because of the increased instability of the protein at alkaline pH. The photoinduced unfolding does not depend on the presence of O2 in solution; therefore, it is not mediated by formation of singlet oxygen and is likely the result of electron transfer between the porphyrin and amino acid residues.

Dynamics of photoinduced endosomal release of polyplexes

Endosomal escape is a well-known bottleneck for successful delivery of macromolecular drugs and genes. Photochemical disruption of endosomal membranes is an approach to overcome this bottleneck. In this study, we used the photosensitizer disulphonated meso-tetraphenylporphine with sulfonate groups on adjacent phenyl rings (TPPS(2a)) to investigate photoinduced endosomal release in living cells with high resolution fluorescence wide-field microscopy in real time. We studied the release dynamics of 10 kDa dextran and polyplexes consisting of DNA condensed with the cationic polymers linear polyethyleneimine (LPEI), poly-(L)-lysine (PLL) or poly-(D)-lysine (PDL). By means of dual-color microscopy and the use of double-labeled polyplexes DNA and polymer were imaged simultaneously. We show that the characteristics of the cationic polymer significantly influence the release behavior of the polyplexes. The release of dextran occurred within 100 ms. For LPEI/DNA particles, LPEI quickly spread throughout the cytosol similar to dextran, whereas DNA was released slowly (within 4 s) and remained immobile thereafter. In case of PLL particles, both DNA and polymer showed quick release. PDL particles remained condensed upon photosensitizer activation. In addition, we demonstrate that TPPS(2a) has biological side effects. Besides stop of microtubule dynamics in the dark, the movement of endosomes ceased after photosensitizer activation.

Photodynamic therapy of age-related macular degeneration: History and principles

We briefly review the history and principles of photodynamic therapy (PDT), especially as it is applied to choroidal neovascularization (CNV) in age-related macular degeneration (AMD). After a brief general history of PDT, we discuss the relationship between the physicochemical structure and photodynamic activity of the second-generation photosensitizers, such as those in current clinical use. We then discuss the basic photophysics of photosensitizer molecules, and describe the initial chemical reactions induced by activated sensitizers. We outline a novel method for screening photosensitizers to be used in treating CNV, as well as the complex biomolecular pathways modulated by PDT-induced oxidative stress and the vascular effects of PDT in solid tumors. The paper closes with a discussion of how all this information might be used to improve the selectivity and efficacy of clinically useful photosensitizers.

Characterization of the photoproducts of protoporphyrin IX bound to human serum albumin and immunoglobulin G

Clinically useful photosensitisers (PSs) are likely bound to subcellular and molecular targets during phototherapy. Binding to a macromolecule has the potential to change the photophysical and photochemical characteristics of the PSs that are crucial for their phototoxicity and cell-killing activity. We investigated the effects of binding of a specific PS (protoporphyrin IX or PPIX) to two proteins, human serum albumin (HSA) and a commercially available immunoglobulin (IgG). These two proteins provide two different environments for PPIX. The albumin binds PPIX in hydrophobic binding sites located in subdomain IIA and IIIA, conversely IgG leaves PPIX exposed to the solvent. We show that photophysical parameters such as emission maxima and fluorescence lifetime depend on the binding site. Our results indicate that the different binding site yields very different rates of formation of photoproducts (more than three times higher for PPIX bound to HSA than to IgG) and that different mechanisms of formation may be occurring. Our characterization shows the relevance of protein binding for the photochemistry and ultimately the phototoxicity of PSs.

Neurotoxicity of tetraphenylporphinesulfonate (TPPS4) and a hematoporphyrin derivative (Photosan) in organotypic cultures of chick embryonic dorsal root ganglia

The neurotoxic effect of tetraphenylporphinesulfonate (TPPS4) and a hematoporphyrin derivative (HPD, Photosan) has been studied in organotypic cultures of chick dorsal root ganglia maintained in a semi-solid culture medium. The changes in two characteristics of neurite outgrowth, the mean radial length of neurites growing out from the ganglia and the area of neurite outgrowths, are used as parameters to evaluate the toxic effect. The porphyrins are tested over the concentration range 10-160 micrograms ml-1. TPPS4 is slightly more toxic than the HPD Photosan. The median inhibitory concentration (IC50) for TPPS4 is 45-50 micrograms ml-1 and for the HPD Photosan 50-60 micrograms ml-1, respectively. Nevertheless, the toxicity of the two drugs is relatively low compared to that of commonly used anticancer drugs, such as cisplatin or taxol.

Hydroxyphthalocyanines as potential photodynamic agents for cancer therapy

A series of benzyl-substituted phthalonitriles, substituted at the 3-, 4-, and 4,5-positions, underwent varied condensations with phthalonitrile to give a series of protected (monohydroxy- and polyhydroxyphthalocyaninato)zinc(II) derivatives which were readily cleaved to give several hydroxyphthalocyanines (ZnPc) (phthalocyanine phenol analogues). Their efficacy as sensitizers for the photodynamic therapy (PDT) of cancer was evaluated on the EMT-6 mammary tumor cell line. In vitro, the 2-hydroxy ZnPc (32) was the most active, followed by the 2,3- and 2,9-dihydroxy ZnPc (39 and 45), with the 2,9,16-trihydroxy ZnPc (33) exhibiting the least activity. In vivo, the monohydroxy derivative 32 and the 2,3-dihydroxy derivative 39 were both efficient in inducing tumor necrosis at 1 micromol kg-1, but complete tumor regression was poor, even at 2 micromol/kg. In contrast, the 2,9-dihydroxy isomer 45, at 2 micromol kg-1, induced tumor necrosis in all animals treated, with 75% complete regression. These results underline the importance of the position of the substituents on the Pc macrocycle to optimize tumor response and confirm the PDT potential of the unsymmetrical Pcs bearing functional groups on adjacent benzene rings.

Vascular effects of photodynamic therapy

Vascular damage and blood flow stasis are consequences of photodynamic therapy (PDT) of solid tumors using many photosensitizers. Microvascular stasis and resulting hypoxia are effective means to produce cytotoxicity and tumor regression. The observation of blood flow stasis after photodynamic therapy results from a combination of damage to sensitive sites within the microvasculature and the resulting physiological responses to this damage. A generalized hypothesis for the mechanisms leading to vessel stasis begins with perturbation and damage to endothelial cells during light treatment of photosensitized tissues. Endothelial cell damage leads to the establishment of thrombogenic sites within the vessel lumen and this initiates a physiological cascade of responses including platelet aggregation, the release of vasoactive molecules, leukocyte adhesion, increases in vascular permeability, and vessel constriction. These effects from damage combine to produce blood flow stasis.

Photodynamic effects of the cationic porphyrin, mesotetra(4N-methylpyridyl)porphine, on microtubules of HeLa cells

The treatment of HeLa human carcinoma cells with mesotetra(4N-methylpyridyl)porphine (T4MPyP) and blue light led to damage of the microtubules (MTs). The morphologies of interphase MTs and the mitotic spindle apparatus were analysed by immunofluorescence staining of alpha-tubulin. The extent of MT damage depended on the light dose and the time after photodynamic treatment. After a period of 1 h after irradiation with doses of 0.3 or 1.5 J cm-2 (sublethal conditions, corresponding to survival rates of 90% and 60% respectively), the normal MT network arrangement of interphase cells and the mitotic spindle apparatus of many cells were clearly affected. However, these effects were found to be transient, and several hours after irradiation most MTs resumed control morphology. Higher irradiation doses (4.5 J cm-2, lethal conditions, less than 10% cell survival) resulted in the irreversible alteration of interphase and mitotic MTs. The change in MT organization appeared to be the reason for the observed increase in the mitotic index (MI) after sublethal doses. The largest increase in MI was detected 6 h after treatment (twofold increase over untreated cells) for both sublethal light doses. Most of the cells in mitosis corresponded to metaphase, the number of ana-telophase cells being greatly reduced for the first hours after irradiation with a dose of 1.5 J cm-2. The results, which resemble those observed with inhibitors of MT assembly, suggest that MTs might represent an important target for T4MPyP action.

Stereochemical factors in the transport and binding of photosensitizers in biological systems and in photodynamic therapy

The uptake and biological activity of porphyrins and phthalocyanines in tumours were correlated with the geometrical features of the photosensitizer molecules. The data suggest that a critical distance of approximately 1.2 nm between oxygen atoms (originating in SO3-, COO- or OH substituents) characterizes a biologically active photosensitizer for photodynamic therapy. We propose that tubulin, which is available in large amounts during mitosis, is the main receptor molecule which binds these photosensitizers. Basic amino acid residues or tightly bound cations in tubulin or homologous proteins may act as binding sites on the receptor molecule.

The unpolymerized form of tubulin is the target for microtubule inhibition by photoactivated tetra(4-sulfonatophenyl)porphine

Several porphyrins, including tetra(4-sulfonatophenyl)porphine, sensitize cells to photoinactivation. The treatment leads to an accumulation of cells in mitosis, directly or indirectly due to a perturbation of the mitotic spindle. The present work relates to the target for this mode of action. Cells incubated with tetra(4-sulfonatophenyl)porphine were exposed to light and the microtubules were quantified 30 min after light exposure. The amount of microtubules decreased with increasing fluences. The reduction in the amount of microtubules after light exposure was enhanced by prior treatment with nocodazole (1 microgram/ml for 20 min) or low temperature (1 degree C for 60 min). When nocodazole was combined with the photochemical treatment the extent of the inhibition of microtubule formation was dose-dependent only for the lowest fluences applied. Additional light exposure did not further reduce the amount of microtubules 30 min after light exposure. The results presented indicate that the unpolymerized fraction of tubulin is the target for photochemical inhibition of microtubule formation.

Synergistic effects of photoactivated tetra(4-sulfonatophenyl)porphine and nocodazole on microtubule assembly, accumulation of cells in mitosis and cell survival

Human carcinoma cells of the line NHIK 3025 were incubated with meso-tetra(4-sulfonatophenyl)porphine (TPPS4) for 18 h and exposed to light in the absence or presence of nocodazole. Nocodazole (1 microgram ml-1) was applied to the cells 15 min prior to light exposure and washed off the cells immediately afterwards. The presence of nocodazole during photoactivation of TPPS4-loaded cells leads to a significantly reduced ability of tubulin to repolymerize after withdrawal of nocodazole, an increased accumulation of the cells in mitosis with a larger fraction in c-metaphase and a higher yield of photoactivated cells. A higher proportion of the cells accumulating in mitosis 6-12 h after exposure to light is unable to form colonies when exposed to light in the presence of nocodazole than in its absence. The present results are consistent with a specific TPPS4-induced photodamage to the unpolymerized form of the microtubule components.

Cellular inhibition of microtubule assembly by photoactivated sulphonated meso-tetraphenylporphines

This work relates to studies on modes of phototoxicity by sulphonated mesotetraphenylporphines on cultured cells. Toxicity appears to be related to inhibition of microtubule function. Treatment of human cervix carcinoma cells of the line NHIK 3025 incubated for 18 h with meso-tetraphenylporphine sulphonates (TPPSn where n = 2a, 2o or 4) and exposed to light, inhibits multiplication for the first hours after light exposure, a significant fraction of the cells accumulating in mitosis. The maximal number of cells in mitosis after treatment (approximately 20%) is dependent on the fluence but is similar for all three photosensitizers. For the first hours after treatment the mitotic cells were always mainly in metaphase; mainly seen as c-metaphases and three-group metaphases. During this time anaphase and telophase cells were absent or greatly reduced in number. Indirect immunofluorescence staining of beta-tubulin showed that the spindle apparatus of mitotic cells was perturbed in all cases. Results are presented which indicate that photoactivation of TPPSn located on the plasma membrane destroys microtubules in interphase cells and leads to arrest of the cells in mitosis. The localization of the dye which sensitizes the photoinduced perturbation of microtubules is further discussed.

In vitro killing of human malignant mesothelioma by photodynamic therapy

Photodynamic therapy was investigated as a potential new modality for the treatment of human malignant mesothelioma (HMM) utilizing the H-MESO-1 HMM cell line and the photosensitizing agent, Photofrin-II (PF-II). Up-take of PF-II by H-MESO-1 was documented by incubating H-MESO-1 cells with PF-II and measuring the fluorescence at 625 nm following excitation at 400 nm. Cytotoxicity of photodynamic therapy was determined by incubating H-MESO-1 cells (2 X 10(5)) in microtiter plates for 24 hr with concentrations of PF-II varying from 0 to 10 micrograms/ml. The wells were exposed to gold vapor laser light (628 nm) in doses ranging from 0 to 24,000 J/m2. Twenty-four hours following treatment, [3H]thymidine (1 microCi) was added to each well. Cells were harvested 24 hr later and counted for tritium incorporation. Five replicates were performed for each combination of light and drug. Peak absorption of PF-II by H-MESO-1 was reached within 8 hr. Maximal doses of light alone caused minimal cell killing. PF-II without light was cytotoxic only at the highest concentrations. However, the combination of PF-II at concentrations at or above 2.5 micrograms/ml and light produced a significant increase in cytotoxicity. These data demonstrate that photodynamic therapy can effectively kill human malignant mesothelioma cells in vitro.