Measurement of the rate of uptake and subcellular localization of porphyrins in cells using fluorescence digital imaging microscopy

Synthesis, characterization, and photodynamic therapy activity of 5,10,15,20-Tetrakis(carboxyl)porphyrin

Water-soluble porphyrins are considered promising drug candidates for photodynamic therapy (PDT). This study investigated the PDT activity of a new water-soluble, anionic porphyrin (1-Zn), which possesses four negative charges. The photodynamic anticancer activity of 1-Zn was investigated by the MTT assay, with mTHPC as a positive control. The cellular distribution was determined by fluorescence microscopy. Holographic and phase contrast images were recorded after 1-Zn treatment with a HoloMonitor™ M3 instrument. The inhibition of A549 cell growth achieved by inducing apoptosis was investigated by flow cytometry and fluorescence microscopy. DNA damage was investigated by the comet assay. The expression of apoptosis-related proteins was also measured by western blot assays. 1-Zn had better phototoxicity against A549 cells than HeLa and HepG2 cancer cells. Interestingly, 1-Zn was clearly located almost entirely in the cell cytoplasmic region/organelles. The late apoptotic population was less than 1.0% at baseline in the untreated and only light-treated cells and increased to 40.5% after 1-Zn treatment and irradiation (P < 0.05). 1-Zn triggered significant ROS generation after irradiation, causing ΔΨm disruption (P < 0.01) and DNA damage. 1-Zn induced A549 cell apoptosis via the mitochondrial apoptosis pathway. In addition, 1-Zn bound in the groove of DNA via an outside binding mode by pi-pi stacking and hydrogen bonding. 1-Zn exhibits good photonuclease activity and might serve as a potential photosensitizer (PS) for lung cancer cells.

Interactions Between Spermine-Derivatized Tentacle Porphyrins and The Human Telomeric DNA G-Quadruplex

G-rich DNA sequences have the potential to fold into non-canonical G-Quadruplex (GQ) structures implicated in aging and human diseases, notably cancers. Because stabilization of GQs at telomeres and oncogene promoters may prevent cancer, there is an interest in developing small molecules that selectively target GQs. Herein, we investigate the interactions of meso-tetrakis-(4-carboxysperminephenyl)porphyrin (TCPPSpm4) and its Zn(II) derivative (ZnTCPPSpm4) with human telomeric DNA (Tel22) via UV-Vis, circular dichroism (CD), and fluorescence spectroscopies, resonance light scattering (RLS), and fluorescence resonance energy transfer (FRET) assays. UV-Vis titrations reveal binding constants of 4.7 × 10⁶ and 1.4 × 10⁷ M⁻¹ and binding stoichiometry of 2–4:1 and 10–12:1 for TCPPSpm4 and ZnTCPPSpm4, respectively. High stoichiometry is supported by the Job plot data, CD titrations, and RLS data. FRET melting indicates that TCPPSpm4 stabilizes Tel22 by 36 ± 2 °C at 7.5 eq., and that ZnTCPPSpm4 stabilizes Tel22 by 33 ± 2 °C at ~20 eq.; at least 8 eq. of ZnTCPPSpm4 are required to achieve significant stabilization of Tel22, in agreement with its high binding stoichiometry. FRET competition studies show that both porphyrins are mildly selective for human telomeric GQ vs duplex DNA. Spectroscopic studies, combined, point to end-stacking and porphyrin self-association as major binding modes. This work advances our understanding of ligand interactions with GQ DNA.

Porphyrin Cyclodextrin Conjugates Modulate Amyloid Beta Peptide Aggregation and Cytotoxicity

Although fibrillar amyloid beta peptide (Aβ) aggregates are one of the major hallmarks of Alzheimer's disease, increasing evidence suggests that soluble Aβ oligomers are the primary toxic species. Targeting the oligomeric species could represent an effective strategy to interfere with Aβ toxicity. In this work, the biological properties of 5[4-(6-O-β-cyclodextrin)-phenyl],10,15,20-tri(4-hydroxyphenyl)-porphyrin and its zinc complex were tested, as new molecules that interact with Aβ and effectively prevent its cytotoxicity. We found that these systems can cross the cell membrane to deliver Aβ intracellularly and promote its clearance. Our results provide evidence for the use of cyclodextrin-porphyrin derivatives as a promising strategy to target amyloid aggregation.

Investigation of the interactions between Pt(II) and Pd(II) derivatives of 5,10,15,20-tetrakis (N-methyl-4-pyridyl) porphyrin and G-quadruplex DNA

G-quadruplexes are non-canonical DNA structures formed by guanine-rich DNA sequences that are implicated in cancer and aging. Understanding how small molecule ligands interact with quadruplexes is essential both to the development of novel anticancer therapeutics and to the design of new quadruplex-selective probes needed for elucidation of quadruplex biological functions. In this work, UV-visible, fluorescence, and circular dichroism spectroscopies, fluorescence resonance energy transfer (FRET) melting assays, and resonance light scattering were used to investigate how the Pt(II) and Pd(II) derivatives of the well-studied 5,10,15,20-tetrakis(N-methyl-4-pyridyl)porphyrin (TMPyP4) interact with quadruplexes formed by the human telomeric DNA, Tel22, and by the G-rich sequences from oncogene promoters. Our results suggest that Pt- and PdTMPyP4 interact with Tel22 via efficient π-π stacking with a binding affinity of 10(6)-10(7) M(-1). Under porphyrin excess, PtTMPyP4 aggregates using Tel22 as a template; the aggregates reach maximum size at [PtTMPyP4]/[Tel22] ~8 and dissolve at [PtTMPyP4]/[Tel22] ≤ 2. FRET assays reveal that both porphyrins are excellent stabilizers of human telomeric DNA, with stabilization temperature of 30.7 ± 0.6 °C for PtTMPyP4 and 30.9 ± 0.4 °C for PdTMPyP4 at [PtTMPyP4]/[Tel22] = 2 in K(+) buffer, values significantly higher as compared to those for TMPyP4. The porphyrins display modest selectivity for quadruplex vs. duplex DNA, with selectivity ratios of 150 and 330 for Pt- and PdTMPyP4, respectively. This selectivity was confirmed by observed 'light switch' effect: fluorescence of PtTMPyP4 increases significantly in the presence of a variety of DNA secondary structures, yet the strongest effect is produced by quadruplex DNA.

Cationic Porphyrin-Anionic Surfactant Mixtures for the Promotion of Self-Organized 1:4 Ion Pairs in Water with Strong Aggregation Properties

We performed a systematic study on the spectroscopic and aggregation properties of stoichiometric mixtures (1:4) of the tetracationic meso-tetrakis(4-N-methylpyridinium)porphyrin (H2 TMPyP) and three sodium alkylsulfate surfactants (tetradecyl, hexadecyl, and octadecylsulfate) in an aqueous solution. The objective was to build a supramolecular aggregate, which would favor the internalization of tetracationic porphyrins in cells without chemical modification of the structure of the porphyrin. We show that stoichiometric H2 TMPyP/alkylsulfate (1:4) mixtures lead to the formation of large hollow spherical aggregates (60-160 nm). The TEM images show that the membrane of these aggregates are composed of smaller aggregates, which are probably rod-like micelles. These rod-like micelles have a hydrophobic core composed of the alkyl chains of the alkylsulfate surfactant, whereas the charged surface corresponds to the tetracationic porphyrins.

Preclinical photodynamic therapy research in Spain 3: Localization of photosensitizers and mechanisms of cell deathin vitro

Photodynamic therapy (PDT) is a subject of increasing biomedical research and represents a very promising therapeutic modality for palliative or even curative treatment of some superficial or endoscopically accessible tumors. In addition to the first photosensitizers (PSs) applied (hematoporphyrin-based drugs), second generation PSs with improved photophysical and photobiological properties are now studied using cell cultures, experimental tumors and clinical trials. On the other hand, there is a growing interest in the analysis of cell death mechanisms by apoptosis, which is especially relevant in oncology, because many anticancer drugs work, at least in part, by triggering apoptosis in neoplastic cells both in vitro and in vivo. The evaluation of cell death mechanisms is an important parameter to determine the efficacy and the potential toxicity of a treatment, allowing better adjustment of protocol. Using cell cultures, our research team has studied the mechanisms of cell damage and death implicated in the photodynamic processes, as well as the relationship between the cellular localization of the PS and the organelle damage during photosensitization. The results obtained in our laboratory provide a deeper understanding on the action mechanisms that lead to cell inactivation by PDT, and also allow selection of PSs with higher potential for clinical application than those currently in use.

Proteomic approaches in understanding action mechanisms of metal-based anticancer drugs

Medicinal inorganic chemistry has been stimulating largely by the success of the anticancer drug, cisplatin. Various metal complexes are currently used as therapeutic agents (e.g., Pt, Au, and Ru) in the treatment of malignant diseases, including several types of cancers. Understanding the mechanism of action of these metal-based drugs is for the design of more effective drugs. Proteomic approaches combined with other biochemical methods can provide comprehensive understanding of responses that are involved in metal-based anticancer drugs-induced cell death, including insights into cytotoxic effects of metal-based anticancer drugs, correlation of protein alterations to drug targets, and prediction of drug resistance and toxicity. This information, when coupled with clinical data, can provide rational basses for the future design and modification of present used metal-based anticancer drugs.

Chain-dependent photocytotoxicity of tricationic porphyrin conjugates and related mechanisms of cell death in proliferating human skin keratinocytes

Photodynamic therapy (PDT) is a poor treatment option for nodular basal cell carcinomas and squamous cell carcinomas. As a result, the search for new photosensitizers with better effectiveness is of current interest. The photocytotoxicity of conjugates (P-R) of a water-soluble tri-cationic porphyrin (P-H) having similar efficiency of production of singlet oxygen, the PDT cytotoxin, has been assessed in vitro. Links between uptake, intracellular localization, photooxidative stress, photocytotoxicity and ability to induce programmed cell death are established. Conjugates bearing methyl (P-Me), Di-O-isopropylidene-(-d-galactopyranosyl (P-OGal) or N,N'-dicyclohexylureidooxycarbonyl (P-DDC) chains are efficiently taken-up by proliferating NCTC 2544 keratinocytes. The relative order of photocytotoxicity is P-OGal >P-DDC=P-Me≫P-H. The photocytotoxic potential of P-Me, P-OGal and P-DDC equals that of endogenous protoporphyrin IX induced by δ-aminolevulinic acid or its esters, the pro-drugs currently employed for PDT of skin lesions. Microfluorometry shows that P-Me, P-OGal, and P-DDC localize in endocytotic or pinocytotic vesicles but not in mitochondria or nucleus. Absence of annexin V binding, caspase activation or chromatin condensation suggests that cell photosensitization by P-R does not induce apoptosis. On the other hand, P-OGal photocytotoxicity correlates with appearance of multiple vesicles that have hallmarks of autophagy compartments, being decorated with the marker LC3 in cells transfected with an expression vector encoding GFP-LC3. p38 and JNK phosphorylation and inhibition of ERK1/2 phosphorylation suggest close relationship between mortality of NCTC 2544 keratinocytes and MAPK pathway impairment. Given their potentially easy formulation, water-soluble P-R are promising powerful photosensitizers for PDT of skin lesions.

A new protocol in photodynamic therapy: enhanced tumour cell death by combining two different photosensitizers

The combined application of two photosensitisers (PSs), zinc(II) phthalocyanine (ZnPc) and the cationic porphyrin meso-tetrakis(4-N-methylpyridyl)porphine (T4MPyP), on HeLa cells produced an enhanced lethal effect relative to treatments with single PSs. Thus, the proper combination of PSs may constitute a new strategy to improve the efficacy of clinical photodynamic therapy.

Photosensitizing action of isomeric zincN-methylpyridylporphyrins in human carcinoma cells

The success of photodynamic therapy (PDT), as a minimally invasive approach, in treating both neoplastic and non-neoplastic diseases has stimulated the search for new compounds with potential application in PDT. We have previously reported that Zn(II) N-alkylpyridylporphyrins (ZnTM-2(3,4)-PyP(4+) and ZnTE-2-PyP(4+)) can act as photosensitizers and kill antibiotic-resistant bacteria. This study investigated the photosensitizing effects of the isomers of ZnTMPyP(4+) (ZnTM-2(3,4)-PyP(4+)) and respective ligands on a human colon adenocarcinoma cell line. At 10 microM and 30 min of illumination the isomeric porphyrins completely inhibited cell growth, and at 20 microM killed approximately 50% of the cancer cells. All these effects were entirely light-dependent. The isomers of the ZnTMPyP(4+) and the respective ligands show high photosensitizing efficiency and no toxicity in the dark. Their efficacy as photosensitizers is comparable to that of hematoporphyrin derivative (HpD).

Spatial and temporal dynamics of in vitro photodynamic cell killing: extracellular hydrogen peroxide mediates neighbouring cell death

Photodynamic killing of a cell population is generally considered to result from direct effects that occur in each cell. In some scenarios this may be an over-simplification and the potential for cell-cell signaling processes to contribute to the response of a population to photodynamic stress is addressed in this paper. Photodynamic killing of EMT6 cells in culture was studied in time and space using computerized time-lapse microscopy. The rate of cell killing was dependent on the fluence with both rapid and slower processes evident, the proportion of the former increasing with fluence. The spatial distribution of cell death was non-random and for the slow cell killing process was found to occur preferentially in the vicinity of dead or dying cells, suggesting a local signaling process. An inhibitory effect of extracellular catalase indicated the involvement of hydrogen peroxide in the spread of cell death and NADPH oxidase was determined as the principal source of hydrogen peroxide. This cell signaling pathway was observed for membrane-bound and mitochondrial photosensitizers but not for a nuclear photosensitizer. These secondary cell signalling pathways extend the oxidative damage to cells in space and time.

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.

Cellular pharmacological properties of gold(III) porphyrin 1a, a potential anticancer drug lead

The development of gold(III) complexes as potential anticancer drugs with higher cytotoxicity and fewer side effects than existing metal anticancer drugs has been actively pursued in recent years. In this study, we explored the cellular pharmacological properties of gold(III) porphyrin 1a, an anticancer drug lead we previously described. The cytotoxicity study of gold(III) porphyrin 1a by naphthol blue black (NBB) staining assay demonstrated that the higher cytotoxicity of gold(III) porphyrin 1a was not related to its photosensitizing activity. Serum dependent test revealed that serum proteins exhibited lesser effects on the activity of gold(III) porphyrin 1a. In addition, in vivo and in vitro binding assays showed that gold(III) porphyrin 1a acted on DNA noncovalently, which was differently from cisplatin. Flow cytometric study indicated that gold(III) porphyrin 1a inhibited cell growth partly through abrogating cell cycle at G(0)-G(1), and induced apoptosis in SUNE1 cells. The enhanced expression of p53, a cell cycle-controlling and apoptosis-related protein, further demonstrated that the cell cycle arrest and apoptosis induced by gold porphyrin 1a were p53 dependent. Our results highlighted the potential of gold(III) porphyrin 1a as an anticancer drug.

Measuring the lifetime of singlet oxygen in a single cell: addressing the issue of cell viability

Singlet molecular oxygen, O(2)(a(1)Delta(g)), has been detected from single neurons and HeLa cells in time-resolved optical experiments by its 1270 nm phosphorescence (a(1)Delta(g)--> X(3)Sigma(-)(g)) upon irradiation of a photosensitizer incorporated into the cell. The cells were maintained in a buffered medium and their viability was assessed by live/dead assays. To facilitate the detection of singlet oxygen, intracellular H(2)O was replaced with D(2)O by an osmotic de- and rehydration process. The effect of this insult on the cells was likewise assessed. The data indicate that, in the complicated transition from a "live" to "dead" cell, the majority of our cells have the metabolic activity and morphology characteristic of a live cell. Quenching experiments demonstrate that the singlet oxygen lifetime in our cells is principally determined by interactions with intracellular water and not by interactions with other cell constituents. The data indicate that in a viable, metabolically-functioning, and H(2)O-containing cell, the lifetime of singlet oxygen is approximately 3 micros. This is consistent with our previous reports, and confirms that the singlet oxygen lifetime in a cell is much longer than hitherto believed. This implies that, in a cell, singlet oxygen is best characterized as a selective rather than reactive intermediate. This is important when considering roles played by singlet oxygen as a signaling agent and as a component in events that result in cell death. The data reported herein also demonstrate that spatially-resolved optical probes can be used to monitor selected events in the light-induced, singlet-oxygen-mediated death of a single cell.

Proteomic characterization of the cytotoxic mechanism of gold (III) porphyrin 1a, a potential anticancer drug

There has been increasing interest in the potential applications of gold (III) complexes as anticancer drugs with higher cytotoxicity and fewer side effects than existing metal anticancer drugs. Our previous findings demonstrated that gold (III) porphyrin 1a preferentially induced apoptosis in a cancer cell line (SUNE1). In this study, we identified differentially expressed proteins related to the drug's cytotoxic action by comparing the protein alterations induced by gold (III) porphyrin 1a and cisplatin treatments. Several clusters of altered proteins were identified, including cellular structure and stress-related chaperone proteins, proteins involved in reactive oxygen species and enzyme proteins, translation factors, proteins that mediate cell proliferation or differentiation, and proteins participating in the internal degradation systems. Our results indicated that multiple factors leading to apoptosis were involved in drug cytotoxicity in SUNE1 cells. The balance between pro-apoptotic and anti-apoptotic signals determined the final fate of cancer cells.

5,10,15,20-Tetrakis(N-Methyl-4-Pyridyl)-21H,23H-Porphine (TMPyP) as a Sensitizer for Singlet Oxygen Imaging in Cells: Characterizing the Irradiation-dependent Behavior of TMPyP in a Single Cell†

Singlet molecular oxygen, a1Delta(g), can be detected from a single cell by its weak 1270 nm phosphorescence (a1Delta(g)-->X3Sigma(g)-) upon irradiation of the photosensitizer 5,10,15,20-tetrakis(N-methyl-4-pyridyl)-21H,23H-porphine (TMPyP) incorporated into the cell. The behavior of this sensitizer in a cell, and hence the behavior of the associated singlet oxygen phosphorescence signal, depends on the conditions under which the sample is exposed to light. Upon irradiation of a neuron freshly incubated with TMPyP, the intensity of TMPyP fluorescence initially increases and there is a concomitant increase in the singlet oxygen phosphorescence intensity from the cell. These results appear to reflect a photoinduced release of TMPyP bound to DNA in the nucleus of the cell, where TMPyP tends to localize, and the subsequent relocalization of TMPyP to a different microenvironment in the cell. Upon prolonged irradiation of the cell, TMPyP photobleaches and there is a corresponding decrease in the singlet oxygen phosphorescence intensity from the cell. The data reported herein provide insight into key factors that can influence photosensitized singlet oxygen experiments performed on biological samples.

Direct optical detection of singlet oxygen from a single cell

Singlet oxygen has been detected in single nerve cells by its weak 1270 nm phosphorescence (a1deltag --> X3sigmag-) upon irradiation of a photosensitizer incorporated in the cell. Thus, one can now consider the application of direct optical imaging techniques to mechanistic studies of singlet oxygen at the single-cell level.

Quadruplex-interactive agents as telomerase inhibitors: synthesis of porphyrins and structure-activity relationship for the inhibition of telomerase

The cationic porphyrin 5,10,15,20-tetra-(N-methyl-4-pyridyl)porphyrin (TMPyP4) binds to quadruplex DNA and is thereby an inhibitor of human telomerase (Wheelhouse et al. J. Am. Chem. Soc. 1998, 120, 3261-3262). Herein the synthesis and telomerase-inhibiting activity of a wide range of analogues of TMPyP4 are reported, from which rules for a structure-activity relationship (SAR) have been discerned: (1) stacking interactions are critical for telomerase inhibition, (2) positively charged substituents are important but may be interchanged and combined with hydrogen-bonding groups, and (3) substitution is tolerated only on the meso positions of the porphyrin ring, and the bulk of the substituents should be matched to the width of the grooves in which they putatively lie. This SAR is consistent with a model presented for the complexation of TMPyP4 with human telomeric quadruplex DNA.

Development of a digital fluorescence sensing technique to monitor the response of macrophages to external hypoxia

Oxygen plays a very important role in living cells. The intracellular level of oxygen is under tight control, as even a small deviation from normal oxygen level affects major cellular metabolic processes and is likely to result in cellular damage or cell death. This paper describes the use of the oxygen sensitive fluorescent dye tris (1,10-phenanthroline) ruthenium chloride [Ru(phen)(3)] as an intracellular oxygen probe. Ru(phen)(3) exhibits high photostability, a relatively high excitation coefficient at 450 nm (18 000 M(-1) cm(-1)), high emission quantum yield ( approximately 0.5), and a large Stoke shift (peak emission at 604 nm). It is effectively quenched by molecular oxygen due to its long excited state lifetime of around 1 micros. The luminescence of Ru(phen)(3) decreases with increasing oxygen concentrations and the oxygen levels are determined using the Stern-Volmer equation. In our studies, J774 Murine Macrophages are loaded with Ru(phen)(3), which passively permeates into the cells. Fluorescence spectroscopy and digital fluorescence imaging microscopy are used to observe the cells and monitor their response to changing oxygen levels. The luminescence intensity of the cells decreases when exposed to hypoxia and recovers once normal oxygen conditions are restored. The analytical properties of the probe and its application in monitoring the cellular response to hypoxia are described.

Synthesis and photodynamic activity of a cationic zinc monoazaporphyrin bearing a nitrogen atom at the peripheral position

A new cationic monoazaporphyrin, zinc 2-aza-8,12,13,17-tetraethyl-2,3,7,18-tetramethylporphyrinium iodide 3 was synthesized. Photodynamic activity of 3 in degradation of 2',3'-isopropylideneguanosine 4 was compared with 2-aza-8,12,13,17-tetraethyl-3,7,18-trimethylporphyrin 1, zinc 2-aza-8,12,13,17-tetraethyl-3,7,18-trimethylporphyrin 2, and hematoporphyrin 5. The quarternary ammonium 3 showed a remarkable increase of photodynamic activity compared with 5, although no appreciable difference in the activity was observed between 1 and 5.

Assays using digital fluorescence: 1985-1998

Luminescence continues to provide comprehensive literature surveys which will be published in most issues. These are a continuation of the literature surveys begun in 1986 in the Journal of Bioluminescence and Chemiluminescence which, up until 1998, encompassed more than 6000 references cited by year or specialized topic. With this newly named journal these searches are expanding to reflect the journal's wider scope. In future we will cover all fundamental and applied aspects of biological and chemical luminescence and include not only bioluminescence and chemiluminescence but also fluorescence, time resolved fluorescence, electrochemiluminescence, phosphorescence, sonoluminescence, lyoluminescence and triboluminescence. The compilers would be pleased to receive any comments from the readership. Contact by e-mail: L.J. Kricka: larry_kricka@path1a.med.upenn.edu or P.E. Stanley: Stanley@LUMIWEB.COM Copyright 1999 John Wiley & Sons, Ltd.

A study of the binding of merocyanine 540 to myeloid leukemia M1 cells using an intensified charge-coupled device for fluorescence imaging microscopy

The binding of merocyanine 540 (MC540) to murine myeloid leukemia (M1) cells and normal erythrocytes was measured by fluorescence digital imaging microscopy using an intensified charge-coupled device. It was found that, on average, about three times more MC540 were bound to a unit membrane area of M1 cells than erythrocytes, a result consistent with previous studies. However, it was shown for the first time that MC540 binding varied significantly from one M1 cell to the next, and about 15% of the sensitized M1 cells were as MC540-negative as normal erythrocytes. Using the leukemic inhibitory factor as a differentiation inducer, M1 cells were induced to differentiate into mature macrophage-like cells in vitro. Such treatment lowered the average MC540 binding by about one-third but did not affect the cell-to-cell variation significantly.

Structure and biodistribution relationships of photodynamic sensitizers

Photodynamic therapy (PDT) has, during the last quarter century, developed into a fully fledged biomedical field with its own association, the International Photodynamic Association (IPA) and regular conferences devoted solely to this topic. Recent approval of the first PDT sensitizer, Photofrin (porfimer sodium), by health boards in Canada, Japan, the Netherlands and United States for use against certain types of solid tumors represents, perhaps, the single most significant-indicator of the progress of PDT from a laboratory research concept to clinical reality. The approval of Photofrin will undoubtedly encourage the accelerated development of second-generation photosensitizers, which have recently been the subject of intense study. Many of these second-generation drugs show significant differences, when compared to Photofrin, in terms of treatment times postinjection, light doses and drug doses required for optimal results. These differences can ultimately be attributed to variations in either the quantum efficiency of the photosensitizer in situ, which is in turn affected by aggregation state, localized concentration of endogenous quenchers and primary photophysics of the dye, or the intratumoral and intracellular localization of the photosensitizer at the time of activation with light. The purpose of this review is to bring together data relating to the biodistribution and pharmacokinetics of second-generation sensitizers and attempt to correlate this with structural and electronic features of these molecules. As this requires a clear knowledge of photosensitizer structure, only chemically well-characterized compounds are included, e.g. Photofrin and crude sulfonated phthalocyanines have been excluded as they are known to be complex mixtures. Nonporphyrin-based photosensitizers, e.g. rose bengal and the hypericins, have also been omitted to allow meaningful comparisons to be made between different compounds. As the intracellular distribution of photosensitizers to organelles and other subcellular structures can have a large effect on PDT efficacy, a section will be devoted to this topic.

Subcellular damage kinetics within co-cultivated WI38 and VA13-transformed WI38 human fibroblasts following 5-aminolevulinic acid-induced protoporphyrin IX formation

The generation of the photosensitizer protoporphyrin IX (PpIX) in cells can be induced by externally applied 5-aminolevulinic acid (ALA), with that bypassing the feedback control mechanism. The aim of the present study was to investigate the onset of destructive changes in living cocultivated WI38 and VA13-transformed WI38 human fibroblasts following ALA incubation, PpIX production and subsequent irradiation by white halogen light with a dose of 2.2 kJ/m2. Specific fluorescence markers such as 3,3'-dihexyloxacarbocyanine iodide for endoplasmic reticulum (ER) staining and dihydrorhodamine for intact mitochondria mapping combined with a low light imaging system are a versatile and sensitive tool to examine the photoinduced destruction of organelles in living cells, while artifacts are minimized. Mitochondria as primary targets of PpIX undergo a condensation under irradiation and are finally destroyed. Photodynamic treatment induces further a significant decomposition of ER, although PpIX localization could not be determined. Initial destabilization and vesiculation of ER is followed by a porous network with large cisternae (indicating the breakdown of cell integrity and cell/nucleus membrane damage). Normal cocultivated lung fibroblasts showed a delay in destruction compared to the transformed WI38-VA13 cells. The observed decomposition pattern resembles the morphological pattern of apoptosis.