Dynamics of photoinduced cell plasma membrane injury.

We have developed a video microscopy system designed for real-time measurement of single cell damage during photolysis under well defined physicochemical and photophysical conditions. Melanoma cells cultured in vitro were treated with the photosensitizer (PS), tin chlorin e6 (SnCe6) or immunoconjugate (SnCe6 conjugated to a anti-ICAM monoclonal antibody), and illuminated with a 10 mW He/Ne laser at a 630 nm wavelength. Cell membrane integrity was assessed using the vital dye calcein-AM. In experiments in which the laser power density and PS concentration were varied, it was determined that the time lag before cell rupture was inversely proportional to the estimated singlet oxygen flux to the cell surface. Microscopic examination of the lytic event indicated that photo-induced lysis was caused by a point rupture of the plasma membrane. The on-line nature of this microscopy system offers an opportunity to monitor the dynamics of the cell damage process and to gain insights into the mechanism governing photolytic cell injury processes.     

Determination of the antibacterial efficacy of a new porphyrin-based photosensitizer against MRSA ex vivo.

Following extensive in vitro screening of new photosensitizers the purpose of the present study was to examine penetration as well as antibacterial efficacy of a lead photosensitizer against MRSA using an ex vivo porcine skin model. Two different applications were performed: (i) preincubation of bacteria in solution with a porphyrin-based photosensitizer XF73 and subsequent application on the ex vivo porcine skin; (ii) application of pure bacteria on the explants followed by an incubation with XF73 in a water-ethanol formulation for up to 60 min under occlusion. The localisation of XF73 was restricted to the stratum corneum. Different concentrations (0-10 microM) of XF73 and different incubation times (5-60 min) were used to determine phototoxicity against methicillin-resistant and methicillin-sensitive S. aureus, which was applied on the explants. Preincubation of S. aureus with 0.1 microM XF73 in solution prior to the application of these XF73-incubated bacteria on the skin demonstrates a higher efficacy (>3 log10) after irradiation. Antibacterial photodynamic inactivation resulted in a approximately 1 log10 (0.1 microM)-3.64+/-0.035 (10 microM) log10 growth reduction independently of the antibiotic resistance pattern of used S. aureus strains. Irradiation of applied bacteria without photosensitizer incubation did not show any marked decrease (<1 log10) of bacteria cell number, indicating a significant phototoxicity of the XF73. Histological evaluations of untreated and treated skin areas upon irradiation within 24 h showed no significant degree of necrosis or apoptosis determined by TUNEL-assay indicating that the porcine skin is still vital. This study demonstrates that this XF73 porphyrin-based photosensitizer had concentration-dependent differences in killing efficacy of MRSA in comparison to skin cells using an ex vivo porcine skin model. The results described here imply that topical delivery of XF73 may be considered as a possible treatment in patients with superficial infections of the skin.     

Green fluorescent protein photobleaching: a model for protein damage by endogenous and exogenous singlet oxygen

Characterization of protein damage during photosensitization of chlorin e6-treated cells was performed using the green fluorescent protein (GFP). The GFP-chromophore damage caused by singlet oxygen was studied in COS 7 kidney cells and E. coli bacteria following light irradiation. Electron spin resonance (ESR) revealed the generation of endogenous singlet oxygen (1O2) by photoactivated GFP, an effect similar to that produced by the exogenous photosensitizer chlorin e6. A light dose-dependent photobleaching effect of GFP was pronounced at low pH or upon photosensitization with chlorin e6. However, the 1O2 quenchers beta-carotene and sodium azide minimized GFP photo-bleaching. Gel electrophoresis of photosensitized GFP followed by fluorescence multi-pixel spectral imaging revealed the binding of chlorin e6 to GFP, affecting the photobleaching efficacy. Fluorescence multi-pixel spectral imaging of GFP-transfected COS 7 cells demonstrated the presence of GFP in the cytoplasm and nucleus, while chlorin e6 was found to be concentrated in the perinuclear vesicles. Exposure of the cells to light induced GFP photobleaching in the close vicinity of chlorin e6 vesicles. We conclude that photoactivated GFP generates endogenous 1O2, inducing chromophore damage, which can be enhanced by the cooperation of exogenous chlorin e6.     

Photodynamic effect of novel chlorin e6 derivatives on a single nerve cell

Chlorin e(6) and its derivatives are promising sensitizers for photodynamic therapy (PDT). In order to compare the photodynamic effects of 8 novel derivatives of chlorin e(6) and to explore some mechanisms of their effects at the cellular level, we studied PDT-induced changes in bioelectric activity of crayfish mechanoreceptor neuron that was used as a sensitive experimental model. Neurons were insensitive to red laser irradiation (632.8 nm; 0.3 W/cm(2)) or to photosensitizers alone, but changed firing rate and died under the photodynamic effect of nanomolar concentrations of sensitizers. The dynamics of neuron responses depended on photosensitizer type and concentration. The dependence of neuron lifetime on photosensitizer concentration allowed comparing efficiencies of different photosensitizers. Radachlorin was the most potent photosensitizer comparable with mTHPC. High photodynamic efficiency of some chlorin e(6) derivatives was related to weak dependence of neuron lifetime on sensitizer concentration, indicating to the initiation of 2-3 secondary processes such as free radical membrane damage by one absorbed photon. Photodynamic efficiency of sensitizers depended on amphiphilicity influencing their intracellular localization.     

Fast and Effective Photodynamic Inactivation of Multiresistant Bacteria by Cationic Riboflavin Derivatives

Photodynamic inactivation of bacteria (PIB) proves to be an additional method to kill pathogenic bacteria. PIB requires photosensitizer molecules that effectively generate reactive oxygen species like singlet oxygen when exposed to visible light. To allow a broad application in medicine, photosensitizers should be safe when applied in humans. Substances like vitamin B2, which are most likely safe, are known to produce singlet oxygen upon irradiation. In the present study, we added positive charges to flavin derivatives to enable attachment of these molecules to the negatively charged surface of bacteria. Two of the synthesized flavin derivatives showed a high quantum yield of singlet oxygen of approximately 75%. Multidrug resistant bacteria like MRSA (Methicillin resistant Staphylococcus aureus), EHEC (enterohemorrhagic Escherichia coli), Pseudomonas aeruginosa, and Acinetobacter baumannii were incubated with these flavin derivatives in vitro and were subsequently irradiated with visible light for seconds only. Singlet oxygen production in bacteria was proved by detecting its luminescence at 1270 nm. After irradiation, the number of viable bacteria decreased up to 6 log₁₀ steps depending on the concentration of the flavin derivatives and the light dosimetry. The bactericidal effect of PIB was independent of the bacterial type and the corresponding antibiotic resistance pattern. In contrast, the photosensitizer concentration and light parameters used for bacteria killing did not affect cell viability of human keratinocytes (therapeutic window). Multiresistant bacteria can be safely and effectively killed by a combination of modified vitamin B2 molecules, oxygen and visible light, whereas normal skin cells survive. Further work will include these new photosensitizers for topical application to decolonize bacteria from skin and mucosa.     

Compromising the plasma membrane as a secondary target in photodynamic therapy-induced necrosis

Photodynamic therapy (PDT) is a non-invasive treatment widely applied to different cancers. The goal of PDT is the photo-induced destruction of cancer cells by the activation of different cell death mechanisms, including apoptosis and/or necrosis. Recent efforts focusing on understanding the mechanisms of cell death activated by PDT find that it depends on the type of photosensitizer (PS), targeted organelles, and nature of the light used. It is generally accepted that very short incubation times are required to direct the PS to the plasma membrane (PM), while longer periods result in the accumulation of the PS in internal compartments such as the endoplasmic reticulum or mitochondria. Glycosylation of the PS targets cancer via saccharide receptors on the cell surface, and is generally assumed that these compounds rapidly internalize and accumulate, e.g. in the endoplasmic reticulum. Herein we demonstrate that a minor fraction of a glycosylated chlorin compound residing at the PM of cancer cells can activate necrosis upon illumination by compromising the PM independently of the length of the incubation period. The results presented here show that the PM can also be targeted by glycosylated PS designed to accumulate in internal organelles. PS activation to induce necrosis by compromising the plasma membrane has the benefits of fast cell death and shorter irradiation times. The findings described here expand our understanding of the cellular damage induced by phototherapies, presenting the possibility of activating another cell death mechanism based on the incubation time and type of light used.     

Intracellular calcium dynamics during photolysis.

The objective of this investigation was to gain a deeper understanding of the intracellular events that precede photolysis of cells. A model system, consisting of malignant melanoma cells pretreated with the calcium sensitive fluorescent dye, Fluo-3, was used to examine the intracellular calcium dynamics in single-cell photolysis experiments. Exposure of the cells to 632 nm laser light in the presence of photosensitizer, tin chlorin e6, resulted in a rise in intracellular calcium. The increase in intracellular calcium was blocked using a variety of calcium channel blocking agents, including verapamil, nifedipine, and nickel. Treatment with the channel blockers was also effective in either decreasing or eliminating cell death despite the presence of lethal doses of photosensitizer and irradiation. These results show that intracellular calcium rises prior to plasma membrane lysis, and that this early rise in intracellular calcium is necessary for membrane rupture.     

Comparative study of the phototoxicity of long-wavelength photosensitizers targeted by the MornigaG lectin

Morniga G is a plant lectin selective for high density of tumor-associated carbohydrate T and Tn antigens on the surface of cells. The interaction of the protein with Tn induces its cell penetration. This property was used for targeting photosensitizers (consisting of the porphyrins TrMPyP and TPPS, the Al(III)-phthalocyanin AlPcS(4), and the chlorin e6) against leukemic Jurkat T cells after covalent coupling to the protein. The control of MornigaG/photosensitizer loading allowed the comparison of the toxicity of the different photosensitizer conjugates. Conjugate including a single AlPcS(4) per protein appeared promising, since it is poorly toxic when irradiated under white light, while it shows a strong phototoxicity (LD(50) = 4 nM) when irradiated in the therapeutic window, it preferentially kills cancerous lymphocytes, and the sugar binding specificity of the lectin part of the molecule remains unaltered.     

In vitro T cell-mediated killing of Pseudomonas aeruginosa. IV. Nonresponsiveness in polysaccharide-immunized BALB/c mice is attributable to vinblastine-sensitive suppressor T cells

We reported previously that BALB/c mice immunized with a polysaccharide (PS) antigen isolated from immunotype 1 Pseudomonas aeruginosa and vinblastine sulfate develop T cell-mediated protective immunity, despite their failure to produce specific antibody. In vitro, Lyt-1-,2+, I-J+ T cells from vinblastine- and PS-immunized mice kill P. aeruginosa by secretion of a bactericidal lymphokine. BALB/c mice immunized with PS alone generate neither protective antibodies nor a protective T cell response. The current studies indicate that T cells from mice immunized with PS alone significantly suppress the bactericidal activity of T cells from mice immunized with vinblastine and PS. The suppressor T cells are of the same Lyt-1-,2+, I-J+ phenotype as the bactericidal T cells. Suppression is mediated by a soluble product of these suppressor T cells which both inhibits T cell proliferation and interferes with the production or release of the bactericidal lymphokine. Cyclophosphamide, used in other systems to remove suppressor T cells, fails to enhance bacterial killing and does not inhibit suppressor cell activity. These studies indicate that immunization with PS elicits responses in two functionally distinct subgroups of Lyt-1-,2+, I-J+ T cells, and that these cells are distinguishable by their sensitivity to vinblastine sulfate.     

13,15-N-cycloimide derivatives of chlorin p6 with isonicotinyl substituent are photosensitizers targeted to lysosomes.

Four monocationic cycloimide derivatives of chlorin p(6) (CICD) were studied as photosensitizers and compared to a structurally similar neutral derivative. Cationic CICD are highly photostable (quantum yield of photobleaching is about 1 x 10(-5), generate singlet oxygen under irradiation (quantum yields are 0.3-0.45), can be involved in a photo-induced substrate-dependent generation of superoxide radicals, but do not produce OH . 17,18-delta-lacton 13(2)-(N-methylisonicotinylamido)-13,15-cycloimide mesochlorin p(6) () and 13(2)-(N-methylisonicotinylamido)-13,15-cycloimide mesochlorin p(6) methyl ester () possess high cancer cell killing photodynamic activity, but they provide no photoinduced bactericidal effect. Substitution of an ethyl group with a hydroxyethyl or acetyl group at position 3 of the macrocycle results in a decrease in extinction and intracellular accumulation that finally leads to the reduced photocytotoxicity. Cationic CICD are targeted to lysosomes, and their intracellular penetration occurs most probably via caveolae-dependent endocytosis. Photodynamic treatment with cationic CICD results in the cell death via necrosis at both sub-phototoxic (40-70% of dead cells) and phototoxic (90-100% of dead cells) regimes of cell treatment. Irradiation induces lysosome damage, leakage of CICD from lysosomes and development of protease activity in cytoplasm, whereas mitochondria are not affected with irradiation. A positive charge of cationic CICD modified drastically an internalization pathway, sites of intracellular localization and mechanisms of photoinduced cytotoxicity as compared to previously studied neutral and anionic CICD. Our experiments with different CICD show that varying charge and structure of substituents it is possible to modulate many cellular properties of CICD in order to find the best molecular template of the advanced near-IR photosensitizer for photodynamic therapy.     

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

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

Photodynamic therapy: a new antimicrobial approach to infectious disease?

Photodynamic therapy (PDT) employs a non-toxic dye, termed a photosensitizer (PS), and low intensity visible light which, in the presence of oxygen, combine to produce cytotoxic species. PDT has the advantage of dual selectivity, in that the PS can be targeted to its destination cell or tissue and, in addition, the illumination can be spatially directed to the lesion. PDT has previously been used to kill pathogenic microorganisms in vitro, but its use to treat infections in animal models or patients has not, as yet, been much developed. It is known that Gram-(-) bacteria are resistant to PDT with many commonly used PS that will readily lead to phototoxicity in Gram-(+) species, and that PS bearing a cationic charge or the use of agents that increase the permeability of the outer membrane will increase the efficacy of killing Gram-(-) organisms. All the available evidence suggests that multi-antibiotic resistant strains are as easily killed by PDT as naive strains, and that bacteria will not readily develop resistance to PDT. Treatment of localized infections with PDT requires selectivity of the PS for microbes over host cells, delivery of the PS into the infected area and the ability to effectively illuminate the lesion. Recently, there have been reports of PDT used to treat infections in selected animal models and some clinical trials: mainly for viral lesions, but also for acne, gastric infection by Helicobacter pylori and brain abcesses. Possible future clinical applications include infections in wounds and burns, rapidly spreading and intractable soft-tissue infections and abscesses, infections in body cavities such as the mouth, ear, nasal sinus, bladder and stomach, and surface infections of the cornea and skin.     

Cell death via mitochondrial apoptotic pathway due to activation of Bax by lysosomal photodamage

Lysosomal photosensitizers have been used in photodynamic therapy. The combination of such photosensitizers and light causes lysosomal photodamage, inducing cell death. Lysosomal disruption can lead to apoptosis but its signaling pathways remain to be elucidated. In this study, N-aspartyl chlorin e6 (NPe6), an effective photosensitizer that preferentially accumulates in lysosomes, was used to study the mechanism of apoptosis caused by lysosomal photodamage. Apoptosis in living human lung adenocarcinoma cells (ASTC-a-1) after NPe6-photodynamic treatment (NPe6-PDT) was studied using real-time single-cell analysis. Our results demonstrated that NPe6-PDT induced rapid generation of reactive oxygen species (ROS). The photodynamically produced ROS caused a rapid destruction of lysosomes, leading to release of cathepsins, and the ROS scavengers vitamin C and NAC prevent the effects. Then the following spatiotemporal sequence of cellular events was observed during cell apoptosis: Bcl-2-associated X protein (Bax) activation, cytochrome c release, and caspase-9/-3 activation. Importantly, the activation of Bax proved to be a crucial event in this apoptotic machinery, because suppressing the endogenous Bax using siRNA could significantly inhibit cytochrome c release and caspase-9/-3 activation and protect the cell from death. In conclusion, this study demonstrates that PDT with lysosomal photosensitizer induces Bax activation and subsequently initiates the mitochondrial apoptotic pathway.     

Mouse dendritic cells pulsed with capsular polysaccharide induce resistance to lethal pneumococcal challenge: roles of T cells and B cells

Mice are exceedingly sensitive to intra-peritoneal (IP) challenge with some virulent pneumococci (LD50 = 1 bacterium). To investigate how peripheral contact with bacterial capsular polysaccharide (PS) antigen can induce resistance, we pulsed bone marrow dendritic cells (BMDC) of C57BL/6 mice with type 4 or type 3 PS, injected the BMDC intra-foot pad (IFP) and challenged the mice IP with supra-lethal doses of pneumococci. We examined the responses of T cells and B cells in the draining popliteal lymph node and measured the effects on the bacteria in the peritoneum and blood. We now report that: 1) The PS co-localized with MHC molecules on the BMDC surface; 2) PS-specific T and B cell proliferation and IFNγ secretion was detected in the draining popliteal lymph nodes on day 4; 3) Type-specific resistance to lethal IP challenge was manifested only after day 5; 4) Type-specific IgM and IgG antibodies were detected in the sera of only some of the mice, but B cells were essential for resistance; 5) Control mice vaccinated with a single injection of soluble PS did not develop a response in the draining popliteal lymph node and were not protected; 6) Mice injected with unpulsed BMDC also did not resist challenge: In unprotected mice, pneumococci entered the blood shortly after IP inoculation and multiplied exponentially in both blood and peritoneum killing the mice within 20 hours. Mice vaccinated with PS-pulsed BMDC trapped the bacteria in the peritoneum. The trapped bacteria proliferated exponentially IP, but died suddenly at 18-20 hours. Thus, a single injection of PS antigen associated with intact BMDC is a more effective vaccine than the soluble PS alone. This model system provides a platform for studying novel aspects of PS-targeted vaccination.     

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

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

In vitro T cell-mediated killing of Pseudomonas aeruginosa. V. Generation of bactericidal T cells in nonresponder mice

We have previously reported that BALB/c mice immunized with 10 micrograms of a Pseudomonas aeruginosa polysaccharide antigen (PS) and 100 micrograms vinblastine sulfate develop T cell-mediated protective immunity, but fail to generate an antibody response. Vinblastine functions in this system to remove a suppressor cell that normally inhibits expression of this form of immunity after PS immunization. T cells from CB.20 mice immunized with the 10 micrograms of PS and 100 micrograms vinblastine fail to kill P. aeruginosa in vitro. These mice are allotype congenic with BALB/c mice, differing at loci closely linked to the IgH-1 locus. Immunization of CB.20 mice with 10 micrograms PS and 100 micrograms vinblastine results in the appearance of T cells which suppress in vitro bactericidal activity of BALB/c T cells. In the current study we found that T cell-mediated bactericidal activity can be generated in CB.20 mice by increasing the dose of vinblastine given at the time of PS immunization. The phenotype of the CB.20 bactericidal T cell generated by high dose vinblastine is identical to that of the BALB/c bactericidal T cell, and the CB.20 bactericidal T cell can adoptively transfer protective immunity to granulocytopenic mice. After immunization of BALB/c and CB.20 mice with PS alone, approximately one log fewer CB.20 T cells than BALB/c T cells are required to suppress bacterial killing in vitro. Furthermore, the number of CB.20 T cells required to suppress in vitro bacterial killing is directly correlated with the dose of vinblastine administered at the time of immunization. Increasing the immunizing dose of PS overcomes suppressor activity and allows the generation of bactericidal T cells in BALB/c mice without a requirement for vinblastine. CB.20 mice fail to generate bactericidal T cells after immunization with high doses of PS. These results indicate that CB.20 and BALB/c mice both possess the full repertoire of T cells required to express bactericidal T cell activity and that the differences in their responses reflect only quantitative differences in suppressor cell activity.     

SNAP-tag technology mediates site specific conjugation of antibody fragments with a photosensitizer and improves target specific phototoxicity in tumor cells

Cancer cells can be killed by photosensitizing agents that induce toxic effects when exposed to nonhazardous light, but this also causes significant damage to surrounding healthy cells. The specificity of photodynamic therapy can be increased by conjugating photosensitizing agents to antibodies and antibody fragments that bind specifically to tumor cell antigens. However, standard conjugation reactions produce heterogeneous products whose targeting specificity and spectroscopic properties can be compromised. In this study, we used an antibody fragment (scFv-425) that binds to the epidermal growth factor receptor (EGFR) as a model to investigate the use of SNAP-tag fusions as an improved conjugation strategy. The scFv-425-SNAP-tag fusion protein allowed the specific conjugation of a chlorin e6 photosensitizer modified with O(6)-benzylguanine, generating a homogeneous product that was delivered specifically to EGFR(+) cancer cells and resulted in significant, tumor cell-specific cytotoxicity. The impact of our results on the development of photodynamic therapy is discussed.     

Molecular design of multifunctional antibacterial agents against methicillin resistant Staphylococcus aureus (MRSA)

Antibacterial activity of a series of alkyl gallates (3,4,5-trihydroxybenzoates) against Gram-positive bacteria, especially methicillin resistant Staphylococcus aureus (MRSA) strains was evaluated. Gram-positive bacteria are all susceptible to alkyl gallates. Dodecyl gallate was the most effective against MRSA ATCC 33591 strain with the minimum bactericidal concentration (MBC) of 25 microg/mL (74 microM). The time-kill curve study showed that dodecyl gallate was bactericidal against this MRSA strain at any growth stage. This activity was observed even in the chloramphenicol-treated cells, but the rate of decrease of cell number was slower than that in the exponentially growing cells. The bactericidal activity of medium-chain alkyl gallates was noted in combination with their ability to disrupt the native membrane-associated function nonspecifically as surface-active agents (surfactants) and to inhibit the respiratory electron transport. Subsequently, the same series of alkyl protocatechuates (3,4-dihydroxybenzoates) were studied and the results obtained are similar to those found for alkyl gallates. The length of the alkyl chain is not a major contributor but is related to the activity.     

Photodynamic effect of lanthanide derivatives of meso-tetra(N-methyl-4-pyridyl)porphine against Staphylococcus aureus

Photodynamic therapy (PDT), used for cancer treatment, is also an alternative method for eradication of drug-resistant bacteria. This method utilizes a nontoxic light-activated dye, called a photosensitizer, and visible light to produce reactive oxygen species that lead to bacterial cell death. The purpose of this study was to investigate the bactericidal effect of PDT using lanthanide derivatives of meso-tetra(N-methyl-4-pyridyl)porphine against Staphylococcus aureus strains. The new photosensitizers appeared to be photodynamically ineffective. No enhancement of antistaphylococcal activity of TMPyP was observed after the conjugation of the porphyrin with lanthanide ions. Additionally, a significant difference in the susceptibility of two bacterial strains to unmodified TMPyP was observed.