Sulfonated Aluminum Phthalocyanine Research Articles

Aluminum phthalocyanine and gold nanorod conjugates: the combination of photodynamic therapy and photothermal therapy to kill cancer cells

The sulfonated aluminum phthalocyanines (AlPcSs), popularly used photosensitizers, were linked on the surfaces of gold nanorods (AuNRs) by the electrostatic binding to form AlPcS–AuNRs conjugates, in order to improve the photo-therapy efficiency of cancer cells by combining the photodynamic therapy (PDT) of AlPcSs and the photothermal therapy (PTT) of AuNRs . The AlPcS's fluorescence is two-fold enhanced when they adhered on the surfaces of AuNRs probably due to the surface Plasmon coupling, which would facilitate the AlPcS detection. The fluorescence images show that AuNRs can carry loaded AlPcSs to penetrate into human nasopharyngeal carcinoma cells with a fast speed, achieving the effective intracellular delivery of AlPcSs . The PTT effect of cellular AuNRs alone under the white light irradiation of 50 minutes decreased the cell viability to 77%, and the PDT effect of cellular AlPcS–AuNRs with filtered red light (670–710 nm) irradiation of 50 min lowered the cell viability to 79%. However, with the same white light irradiation of 50 min, the AlPcS–AuNRs destroyed most cells leaving the cell viability to 28%, reflecting a typical synergistic effect on cell killing. These results suggest that the combination of PTT and PDT with AlPcS–AuNRs is a promising strategy for improving the phototherapy of cancers.

Gallium Phthalocyanine Photosensitizers: Carboxylation Enhances the Cellular Uptake and Improves the Photodynamic Therapy of Cancers

The octacarboxyl gallium (GaPcC) and metal-free (H2PcC) phthalocyanines were prepared using the carboxyl as the peripheral substituent. The carboxylation improves the intracellular delivery of these two PcCs into KB and QGY cancer cells as compared to that of sulfonated aluminum phthalocyanines (AlPcS), a popularly used photosensitizer (PS). Moreover, GaPcC maintains high photoproduction of singlet oxygen. With a short incubation time of 3 hours, GaPcC accumulates sufficiently in both KB and QGY cells and improves photodynamic therapy (PDT) by effectively killing these cancer cells. AlPcS and H2PcC show much lower PDT effects under the same conditions, because AlPcS have a slow cellular uptake rate resulting in a low cellular amount and the ability of H2PcC to produce 1O2 is low. Carboxylation is a promising way to prepare water-soluble metal phthalocyanines (MPcCs) and facilitates the cellular uptake of MPcCs for PDT improvement.

Enhancement of Intracellular Delivery of Anti-cancer Drugs by the Tat Peptide

The arginine-rich cationic Tat peptides have been reported to enhance the intracellular delivery of macromolecules, including DNA, RNA, and proteins. In this work an arginine cationic peptide derived from the HIV-1 Tat protein was conjugated with noncovalent bonds to sulfonated aluminum phthalocyanine (AlPcS, a photosensitizer for the light-activated photodynamic cancer therapy), doxorubicin (DOX, a chemotherapeutic agent), or quantum dots (QDs, often used as carriers for the delivery of anticancer drugs). The fluorescence of intracellular conjugates of AlPcS-Tat, DOX-Tat, and QDs-Tat was studied by means of confocal laser scanning microscopy in the human nasopharyngeal carcinoma KB cells and cervical carcinoma Hela cells in vitro. The Tat peptide with noncovalent links can enhance at least a twofold of intracellular delivery of AlPcS, DOX, and QDs via an endocytotic pathway in the two tumor cell lines. This finding may suggest that the Tat peptide-mediated intracellular delivery of anticancer drugs may have the potential for improving efficacy of cancer therapy.

Plasmonic Gold Nanorods Can Carry Sulfonated Aluminum Phthalocyanine To Improve Photodynamic Detection and Therapy of Cancers

Hexadecyltrimethylammonium bromide-coated gold nanorods (AuNRs) with positive charges were effectively bound to negatively charged sulfonated aluminum phthalocyanine (AlPcS), a photosensitizer for photodynamic detection and therapy, due to the electrostatic force, with a loading content of 10(4) AlPcS molecules per rod. A 5-fold increase in the AlPcS fluorescence of the AlPcS-AuNRs complex was seen. The excitation fluorescence spectrum of the AlPcS-AuNRs with a typical 520 nm band fits well with the resonance band of AuNR surface plasmons, suggesting that such increased AlPcS fluorescence is produced from the strong surface plasmons of AuNRs. The intracellular distribution of AlPcS-AuNRs was studied in the QGY liver cancer cells by respectively imaging the AlPcS fluorescence and AuNRs reflectance with a confocal microscope. Furthermore, the AlPcS-AuNRs-loaded cells were photodynamically damaged after being exposed to red light in a light-dose-dependent manner. In contrast, no phototoxicity of the cells was seen after incubation with the same amount of free AlPcS, indicating that the AlPcS-AuNRs can enhance the AlPcS-mediated photodynamic effect. In addition, the loaded AlPcS can be photothermally released from AuNRs in the cells by the irradiation with an 800 nm femtosecond laser, demonstrating the potential for controlled drug release.

Generation of Singlet Oxygen via the Composites of Water-Soluble Thiol-Capped CdTe Quantum DotsSulfonated Aluminum Phthalocyanines

Singlet oxygen (1O2), one of the reactive oxygen species, plays an important role in many biomedical applications. The various compounds including the phthalocyanines, quantum dots (QDs) and QD complex, which may have potential to produce 1O2, thus received more and more attentions in recent years. By means of the direct detection of near-infrared 1270 nm, we found that the water-soluble thiol-capped CdTe QDs can photoproduce 1O2 in deuterated water with a low quantum yield (QY) of 1%. When sulfonated aluminum phthalocyanines (AlSPc's) were connected to these QDs, forming water-soluble QD-Pc composites, the 1O2 QY of the composites increased to 15% under the excitation of 532 nm, while little 1O2 production can be found for AlSPc alone at the same excitation because of the poor absorption of AlSPc in this region. The results of indirect measurements of 1O2, obtained from the photodegradation of the 1O2 chemical trap anthracene-9,10-diyl-bis-methylmalonate (ADMA), confirmed 1O2 yields in both QD and QD-Pc composite solutions. The QD-Pc composites have the advantage of extending the excitation region to 400-600 nm with remarkably enhanced extinction coefficients as compared with that of AlSPc. Therefore QD-Pc composites can fully utilize visible region light excitation to effectively produce 1O2, which may facilitate the applications of QD-Pc composites in broad areas.

Targeting gastrin-releasing peptide receptors of prostate cancer cells for photodynamic therapy with a phthalocyanine–bombesin conjugate

Sulfonated aluminum phthalocyanines (AlPcS) are potent photosensitizers for the photodynamic therapy (PDT) of cancer. In this study we evaluate the possibility to improve the efficacy of AlPcS-PDT for prostate cancer by targeting tetrasulfonated aluminum phthalocyanines (AlPcS(4)) to the gastrin-releasing peptide receptor (GRPR) through coupling to bombesin. A mono-carbohexyl derivative of AlPcS(4) is attached to 8-Aoc-bombesin(7-14)NH(2) via an amide bridge to yield a bombesin-AlPcS(4) conjugate linked by a C-14 spacer chain. The conjugate is characterized by mass spectroscopy and shown to bind to the GRPR with a relative binding affinity (RBA) of 2.3, taking bombesin (RBA=100) as unity. The in vitro photodynamic efficacy of the conjugate against PC-3 human prostate cancer cells is improved by a factor 2.5 over the non-conjugated mono-carbohexyl derivative of AlPcS(4).

Light-induced effects in sulfonated aluminum phthalocyanines — potential photosensitizers in the photodynamic therapy: Spectroscopic and kinetic study

Sodium salts of sulfonated hydroxyaluminum phthalocyanines [Al(OH)Pc(SO 3Na) i ] containing various numbers of sulfonate groups ( i = 1–4), were studied by spectroscopic techniques. The lifetimes of the singlet states were found to range between 5 and 8 ns, experimentally determined fluorescence quantum yields ranging between 0.45 and 0.74. Both parameters depend on the number of sulfonate groups, the singlet lifetime depending additionally on concentration. The triplet lifetime was indirectly estimated for aqueous solutions of Al(OH)Pc(SO 3Na) 3 to amount to ca. 0.6 μs. The decay time of singlet oxygen in the same system was also found to be close to 0.6 μs. The molecules were found to dimerize in acidic and neutral solutions, the second order rate constants of the dimerization being of the order of 10 mol −1 dm 3 s −1, and the equilibrium constants of the order of 10 4. The experiments were supplemented with quantum chemical calculations allowing us to assign electronic transitions in monomers and dimers.

Effect of Fluoride Anions on Gramicidin Photoinactivation Sensitized by Sulfonated Aluminum Phthalocyanines¶

Interaction of potent photodynamic agents, sulfonated aluminum phthalocyanines (AlPcSn where n is a number of sulfonic groups), with biological membranes was studied here using model systems: sensitized photoinactivation of gramicidin channels in planar lipid bilayers and adsorption on lipid monolayers. Fluoride anions known to form complexes with aluminum were found to inhibit both the adsorption of aluminum phthalocyanines on lipid monolayers, as measured with a Langmuir trough by surface pressure and surface potential changes, and photodynamic efficacy of the dyes, as studied by gramicidin channel photoinactivation. The similar effects were caused by the alkalinization of the medium. Fluoride anions appeared to be much more effective in the case of AlPcS4 as compared to AlPcS3. The suppression of the photodynamic potency of aluminum phthalocyanines was attributed to desorption of the dyes from lipid bilayers induced by fluoride or hydroxyl ions. With AlPcS4 an enhancement of the dye aggregation leading to a decrease in the sensitizing activity was probably involved in the fluoride effect as revealed by absorption and fluorescence spectral measurements. Capillary electrophoresis was employed to understand the mechanism of the dye desorption. The results of these experiments indicated that the reduction in the membrane affinity was associated with an increase in the negative charge of the dye molecules due to the binding of fluoride or hydroxyl ions.

Involvement of adenylate cyclase and tyrosine kinase signaling pathways in response of crayfish stretch receptor neuron and satellite glia cell to photodynamic treatment

Neuroglial interactions are most profound during development or damage of nerve tissue. We studied the responses of crayfish stretch receptor neurons (SRN) and satellite glial cells to photosensitization with sulfonated aluminum phthalocyanine Photosens. Although Photosens was localized mainly in the glial envelope, neurons were very sensitive to photodynamic treatment. Photosensitization gradually inhibited and then abolished neuron activity. Neuronal and glial nuclei shrank. Some neurons and glial cells lost the integrity of the plasma membrane and died through necrosis after the treatment. The nuclei of other glial cells but not neurons become fragmented, indicating apoptosis. The number of glial nuclei around neuron soma increased, probably indicating proliferation for enhanced neuron protection. Adenylate cyclase (AC) inhibition by MDL-12330A, or tyrosine kinase (TK) inhibition by genistein, shortened neuron lifetime, whereas AC activation by forskolin or protein tyrosine phosphatases (PTP) inhibition by sodium orthovanadate prolonged neuronal activity. Therefore, cAMP and phosphotyrosines produced by AC and TK, respectively, protected SRN against photoinactivation. AC inhibition reduced photodamage of the plasma membrane and subsequent necrosis in neuronal and glial cells. AC activation prevented apoptosis in photosensitized glial cells and stimulated glial proliferation. TK inhibition protected neurons but not glia against photoinduced membrane permeabilization and subsequent necrosis whereas PTP inhibition more strongly protected glial cells. Therefore, both signaling pathways involving cAMP and phosphotyrosines might contribute to the maintenance of neuronal activity and the integrity of the neuronal and glial plasma membranes. Adenylate cyclase but not phosphotyrosine signaling pathways modulated glial apoptosis and proliferation under photooxidative stress.

Interactions of a Sulfonated Aluminum Phthalocyanine and Cytochrome c in Micellar Systems: Binding and Electron-Transfer Kinetics

The interaction of the tetrasulfonated aluminum phthalocyanine with cytochrome c was studied in micellar systems, after previous work that was conducted in aqueous solutions [Laia et al., J. Phys. Chem. B, 2004, 108, 7506−7514]. It is found that, in ionic micellar aggregates, the interaction is disrupted whereas, in neutral micelles and reversed micelles, it is maintained. The binding constant decreases, but it still reaches high values in neutral micellar systems. The electrostatic nature of the complex formation was assessed in sucrose/water mixtures and was observed to be strongly dependent on the dielectric constant of the mixture. The electron-transfer process within the complex is affected by the microviscosity, leading to faster decays in more-viscous systems, which is attributed to small dynamic conformational changes of the complex. In Triton X-100 micelles, the result is similar to that obtained in aqueous solution; however, in Brij 35 micelles, it shows features that are unique, reflecting an apolar environment. High electron-transfer rate constants are obtained in cyclohexane/n-hexanol/Triton X-100/water microemulsions, and these rate constants cannot be atributed solely to a more-viscous environment. It is suggested that, in such systems, other effects, such as confinement and water structure, have a major role.

Quantum Dot anti-CD Conjugates: Are They Potential Photosensitizers or Potentiators of Classical Photosensitizing Agents in Photodynamic Therapy of Cancer?

The present study examined the potential of quantum dot bioconjugates to sensitize cells to UV irradiation and to promote the photodynamic activity of the classical photosensitizers such as trifluoperazine (TFPZ) and sulfonated aluminum phthalocyanine (SALPC). Water-soluble CdSe nanocrystals were conjugated with anti-CD antibody with known specificity to leukemia cells. Quantum dot anti-CD conjugates were incubated with the leukemia cell line Jurkat to ensure specific interaction with the cell surface. This interaction was confirmed by fluorescent confocal microscopy. Furthermore, quantum dot anti-CD90-labeled leukemia cells were mixed with normal lymphocytes and subjected to UV irradiation in the presence or absence of a classical photosensitizer (TFPZ or SALPC). The cell fractions were separated by lectin-affinity column chromatography. The cell type was confirmed with fluorescent confocal microscopy and flow cytometry using appropriate antibodies; quantum dot anti-CD90 for leukemia cells, and PE-CD44 for normal lymphocytes. The viability of the separated cell fractions was determined using flow cytometry and the methyl tetrazolium test. The results demonstrated that quantum dot anti-CD conjugates sensitized leukemia cells to UV irradiation and promoted the effect of the classical photosensitizer SALPC. The results are discussed in the context of free radical generation during combined application of quantum dot bioconjugates and UV irradiation, as well as in the context of UV-mediated liberation of free Cd ions and their harmful effect on cell viability.

Electron-Transfer Kinetics in Sulfonated Aluminum Phthalocyanines/Cytochrome c Complexes

Sulfonated aluminum phthalocyanines interact with cytochrome c forming complexes in aqueous solutions. The binding of these dyes was probed using fluorescence techniques (steady-state spectroscopy and time-correlated single photon counting), which enable the calculation of the equilibrium binding constants. The fluorescence of the complex is relatively low (φ = 0.07) due to an electron-transfer process from the phthalocyanine singlet-excited state to the cytochrome c. The binding constant determines the amount of fluorescence and is quite high for the tetrasulfonated aluminum phthalocyanine (3.3 × 105 M-1) while those for the disulfonated and monosulfonated aluminum phthalocyanines are one order of magnitude smaller. The fluorescence decays suggest more than one location for the dye adsorbed in the protein, which is affected by electrostatic interactions, leading to nonexponential decays.

Generation of singlet oxygen with anionic aluminum phthalocyanines in water

The dependence of the quantum yield of photogeneration of singlet oxygen with aluminum sulfophthalocyanines on the degree of their sulfonation was studied. Experiments showed that Fotosens preparation containing, on the average, about three sulfo groups in the macroring in aqueous buffer solution (pH 7.4) exists in the monomeric form and sensitizes singlet oxygen with a quantum yield ϕΔ 0.42±0.06. Aluminum tetra-4-sulfophthalocyanine forms dimers, and correspondingly ϕΔ is lower: 0.22±0.03. Sulfonated aluminum phthalocyanine containing, on the average, about two sulfo groups in the macroring is aggregated to a significant extent. Addition of Triton X-100 detergent results in partial deaggregation of the dye, and ϕΔ increases from ∼0.01 to 0.15±0.02. In the series including previously unknown aluminum phthalocyanines containing residues of phosphonic acids or their esters, and also aluminum and zinc octacarboxyphthalocyanines, the influence of the dye structure on the quantum yield of generation of singlet oxygen suggests participation of axial hydroxy groups in associate formation. These associates are not manifested spectroscopically and generate singlet oxygen inefficiently. The quantum yields ϕΔ for the monomers and associates are 0.3 and 0.1, respectively.

Effect of fluoride anions on gramicidin photoinactivation sensitized by sulfonated aluminum phthalocyanines.

Interaction of potent photodynamic agents, sulfonated aluminum phthalocyanines (AlPcSn where n is a number of sulfonic groups), with biological membranes was studied here using model systems: sensitized photoinactivation of gramicidin channels in planar lipid bilayers and adsorption on lipid monolayers. Fluoride anions known to form complexes with aluminum were found to inhibit both the adsorption of aluminum phthalocyanines on lipid monolayers, as measured with a Langmuir trough by surface pressure and surface potential changes, and photodynamic efficacy of the dyes, as studied by gramicidin channel photoinactivation. The similar effects were caused by the alkalinization of the medium. Fluoride anions appeared to be much more effective in the case of AlPcS4 as compared to AlPcS3. The suppression of the photodynamic potency of aluminum phthalocyanines was attributed to desorption of the dyes from lipid bilayers induced by fluoride or hydroxyl ions. With AlPcS4 an enhancement of the dye aggregation leading to a decrease in the sensitizing activity was probably involved in the fluoride effect as revealed by absorption and fluorescence spectral measurements. Capillary electrophoresis was employed to understand the mechanism of the dye desorption. The results of these experiments indicated that the reduction in the membrane affinity was associated with an increase in the negative charge of the dye molecules due to the binding of fluoride or hydroxyl ions.

Photosensitizer Binding to Lipid Bilayers as a Precondition for the Photoinactivation of Membrane Channels

The photodynamic activity of sulfonated aluminum phthalocyanines (AlPcS n , 1 ≤ n ≤ 4) was found to correlate with their affinity for membrane lipids. Adsorbing to the surface of large unilamellar vesicles (LUVs), aluminum phthalocyanine disulfonate induced the highest changes in their electrophoretic mobility. AlPcS 2 was also most efficient in mediating photoinactivation of gramicidin channels, as revealed by measurements of the electric current across planar lipid bilayers. The increase in the degree of sulfonation of phthalocyanine progressively reduced its affinity for the lipid bilayer as well as its potency of sensitizing gramicidin channel photoinactivation. The portion of photoinactivated gramicidin channels, α, increased with rising photosensitizer concentration up to some optimum. The concentration at which α was at half-maximum amounted to 80 nM, 30 nM, 200 nM, and 2 μM for AlPcS 1, AlPcS 2, AlPcS 3, and AlPcS 4, respectively. At high concentrations α was found to decrease, which was attributed to quenching of reactive oxygen species and self-quenching of the photosensitizer triplet state by its ground state. Fluoride anions were observed to inhibit both AlPcS n (2 ≤ n ≤ 4) binding to LUVs and sensitized photoinactivation of gramicidin channels. It is concluded that photosensitizer binding to membrane lipids is a prerequisite for the photodynamic inactivation of gramicidin channels.

Binding Interactions and Conformational Changes Induced by Sulfonated Aluminum Phthalocyanines in Human Serum Albumin

Phthalocyanines (Pc), which are extensively studied as tumor localizing photosensitizers for photodynamic therapy, are transported by the blood circulatory system to target tissues. Binding interactions between human serum albumin and differently sulfonated aluminum phthalocyanines (AlPcSn;n= 1–4) were studied using optical and ESR spectroscopy. AlPcSn(n= 1–3) occupy one strong binding site and eight weaker sites. The high affinity binding site interactions differ with respect to the degree of sulfonation and isomeric composition of the Pc. Phthalocyanines without SO−3groups on adjacentiso-indole rings exhibit a high affinity binding site constant ofK∼ 3–4 × 107M−1, while Pc with two or three adjacent SO−3groups show binding for this high affinity site that is no longer independent, but cooperative (α = 2), withK∼ 2–6 × 106M−1. Binding isotherms for AlPcS4and its close analog, tempoyl spin-labeled SL-AlPcS3, do not approach saturation at high ligand concentrations. Competition analyses between AlPcSnand spin-labeled fatty acids (5- and 16-doxyl stearate isomers) reveal that all compounds participate in cooperative (allosteric) interactions with the high affinity binding site of 16-DS, while extruding 5-DS isomer from certain sites and increasing the binding affinity for the remaining. Protein conformational dynamics was studied by ESR spectroscopy using covalent (alkylation of Cys34 residue) and noncovalent spin labeling (employing SL-AlPcS3). Phthalocyanines perturb conformational dynamics parameters (τcandS) depending on the degree of sulfonation and isomeric composition corresponding to the type of sites, i.e., independent or cooperative, occupied on the HSA molecule.

Photo-induced reactions of sulfonated aluminum phthalocyanine with peptide

The photosensitized oxidation mechanism of peptide in the presence of the CIAlPcS is studied by UV-Vis spectroscopic method. It is found that the oxidation of peptide is totally quenched in 1.1 × 10−2 molk NaN3 solution, and the same reaction cannot proceed in the dark. K3Fe(CN)6 and L-Cys cannot quench the reaction. Therefore, the reaction mechanism is mainly Foote’s type II. This reaction process can be inferred as follows: (i) active oxygen can attack acylamide bond near by carbon atom; (ii) carboxyl group departs from peptide; (iii) I-benzazole cycle is destroyed when reaction is continued; (iv) the peptide is completely damaged.

Oxygen depletion during in vitro photodynamic therapy: structure-activity relationships of sulfonated aluminum phthalocyanines

Photodynamically induced oxygen depletion has been measured in an Ehrlich ascites mouse tumor cell line using a Clark-type electrode. Cells are loaded with aluminum phthalocyanines, sulfonated to different degrees (AlPcS n , n = 0,2,3,4) and consisting of various isomeric species. Different cell lines and incubation procedures are used in order to investigate the cellular uptake mechanism. Uptake (in units of molecules/cell), post-irradiation redistribution and AlPcS n photodegradation are measured using spectroscopic techniques. For a given sensitizer, the oxygen depletion rate per cell increases sublinearly with uptake and superlinearly with cell density. In order to compare oxygen depletion rates of different compounds, we have defined the biological quantum yield (BQY) as the number of oxygen molecules that disappear per absorbed photon. The BQY is independent of uptake and cell density; therefore, it denotes the intrinsic photoactivity of a sensitizer. Sensitizers with high BQY show efficient post-irradiation intracellular redistribution. Photodegradation during irradiation is similar for all sensitizers (20–30%).

Photodynamic treatment of red blood cell concentrates for virus inactivation enhances red blood cell aggregation: protection with antioxidants.

Photodynamic treatment (PDT) using phthalocyanines and red light appears to be a promising procedure for decontamination of red blood cell (RBC) concentrates for transfusion. A possible complication of this treatment may be induced aggregation of RBC. The production of RBC aggregates was measured with a novel computerized cell flow properties analyzer (CFA). The PDT of RBC concentrates with sulfonated aluminum phthalocyanine (AIPcS4) and the silicon phthalocyanine Pc 4 under virucidal conditions markedly enhanced RBC aggregation and higher shear stress was required to disperse these aggregates. The clusters of cells were huge and abnormally shaped, unlike the rouleaux formed by untreated RBC. This aggregation was prevented when a mixture of antioxidants was included during PDT. Addition of the antioxidants after PDT reduced aggregation only partially. It is concluded that inclusion of antioxidants during PDT of RBC concentrates prior to transfusion may reduce or eliminate the hemodynamic risk that the virucidal treatment may present to the recipient.

Correlation of subcellular and intratumoral photosensitizer localization with ultrastructural features after photodynamic therapy.

Photodynamic therapy (PDT) of cancer typically involves systemic administration of tumor-localizing photosensitizers followed 48-72 h later by exposure to light of appropriate wavelengths. Knowledge about the distribution of photosensitizers in tissues is still fragmentary. In particular, little is known as to the detailed localization patterns of photosensitizers in neoplastic and normal tissues as well as the relationship between such patterns and the actual targets for the photosensitizing effect. This review focuses on ultrastructural features seen in treated cells and tumors. An attempt is made to correlate these findings with the subcellular/intratumoral localization pattern of the photosensitizers in tumor cell lines in vitro and in tumor models in vivo. Several subcellular sites are main targets of PDT with different sulfonated aluminum phthalocyanines (AIPcSn) in the human tumor cell line LOX. Nuclei are not among the primary targets. Overall, the ultrastructural changes correlate well with the data about the subcellular localization patterns for each analogue of AIPcSn in the same cell line. Similar findings are also obtained for the family of sulfonated mesotetraphenylporphines (TPPSn) in the NHIK 3025 cell line. The mechanisms involved in the killing of tumors by PDT seem to be a complex interplay between direct and indirect (via vascular damage) effects on neoplastic cells according to the intratumoral localization pattern of the applied dye. Several factors can affect the localization pattern of a drug, such as its chemical character, the mode of drug delivery, the time interval between drug administration and light exposure, and tumor type. Furthermore, whether local immune reactions (such as macrophages) and apoptosis (programmed cell death) are involved in the destruction of neoplastic cells by PDT in vivo is still an enigma. A general model for PDT-induced tumor destruction is suggested.