Porphyrin sensitizers in tumour phototherapy. Novel sensitizers of the chlorin and bacteriochlorin class with amphiphilic properties

Fluorescence-based method for sensitive and rapid estimation of chlorin e6 in stealth liposomes for photodynamic therapy against cancer

Chlorin e6 is a chlorine-based porphyrin containing photosensitizer mainly used for the therapy in cancers like neck and head, early-stage lung cancer, and topical skin cancers. The present study provides a comprehensive account of a highly sensitive, precise, and validated method for the quantification of chlorin e6 in its liposomal formulation. This method is based on the systemic study of the fluorescence action of chlorin e6 in acetonitrile solvent. This experiment follows the analytical method validation parameters as per the International Conference on Harmonization (ICH). Chlorin e6 molecule exhibits strong fluorescence at a wavelength of emission 665 nm, upon excitation at a wavelength of excitation 400 nm in acetonitrile. The linearity of the fluorescence concentration plot was observed over a concentration range of 50 to 1000 ng/mL. The developed and validated method was successfully applied for the estimation of encapsulation efficiency in in-house developed stealth liposomes. Also, stock solution stability and photodegradation study of chlorin e6 were further conducted.

Redox behaviour of the β-dihydroporphycene cobalt complex: study on the effect of hydrogenation of the ligand

The dihydrogenated porphycene cobalt(ii) complex was synthesized and electrochemical experiments were carried out. The one-electron reduction of the complex proceeded at the central metal to afford the Co(i) species; in contrast, for the non-hydrogenated porphycene cobalt(ii) complex, the one-electron reduction gave the ligand reduced radical anion species. The reactivity of the one-electron reduced species with alkyl halides showed clear differences between the complexes. Hydrogenation of the β-position of the porphycene makes it possible to generate a central cobalt reduced species possessing a higher reactivity than the ligand reduced radical anion species.

5,20-BIS(4-Sulphophenyl)-10,15-Bis(2-Methoxy-4-Sulphophenyl)-21-Thiaporphyrin as a new Potent Sensitizer in Photodynamic Therapy

The main purpose of the study was to investigate the effectiveness of a new photosensitizer for photodynamic therapy. 5,20-bis(4-sulphophenyl)-10,15-bis(2-methoxy-4-sulphophenyl)-21-thiaporphyrin (21-thiaporphyrin) was compared to chlorin e6 and tetra(m-hydroxyphenyl)porphyrin (m-THPP) for its ability to sensitize tumors and skin to light. Chlorin e6 and m-THPP induced a strong tumor and skin photosensitization. In contrast, the same doses of 21-thiaporphyrin produced no skin sensitization and gave approximately 10 mm tumor necrosis after light exposure, in comparison to the 5-6 mm necrosis induced by chlorin e6 or m-THPP under identical conditions. 21-Thiaporphyrin, tested as a potential photosensitizer, induced no skin sensitization even at doses as high as 7.5 mg/kg body weight. 21-Thiaporphyrin presents a high potency in tumor sensitizing, i.e. a feature required for an efficient photosensitizer in photodynamic therapy applications.

Synthesis and evaluation of cationic bacteriochlorin amphiphiles with effective in vitro photodynamic activity against cancer cells at low nanomolar concentration

Bacteriochlorins are attractive candidates as photosensitizers for photodynamic therapy (PDT) due to their intense absorption in the near-infrared (NIR) region of the spectrum where light transmission through tissue is maximal. Many naturally occurring bacteriochlorins are inherently unstable due to adventitious atmospheric oxidation. A de novo synthesis affords bacteriochlorins that contain a geminal dimethyl group in each reduced pyrrole ring to increase stability against oxidation. Here, three new synthetic bacteriochlorins, each bearing a single side-chain containing one or two positive charges, were investigated for their in vitro PDT activity against HeLa human cancer cells. All bacteriochlorins were active at low nanomolar concentration when activated with NIR light; those bearing a single positive charge exhibited faster uptake and higher activity. The bacteriochlorins were localized in mitochondria, lysosomes and endoplasmic reticulum as shown by organelle specific fluorescent probes. Cell death was via apoptosis as shown by cell morphology and nuclear condensation. Taken together, the results show the importance of appropriate peripheral groups about a photosensitizer for effective PDT applications.

Vacata- and divacataporphyrin: new photosensitizers for application in photodynamic therapy-an in vitro study

BACKGROUND AND OBJECTIVE

The photodynamic therapy is a well-known method of treatment of both malignant tumors and non-tumor lesions in human patients. In the present study, we aimed at evaluating the in vitro efficacy of the new photosensitizing agents, vacataporphyrin (VP), and divacataporphyrin (DVP).

MATERIALS AND METHODS

The effectiveness of VP and DVP was compared to well-known photosensitizers, that is, hematoporphyrin derivative (HPD) and chlorin e6 (Ce6) in identical in vitro conditions. The experiment was performed on a well-established breast cancer cell line, MCF-7 and compared to HCV 29T11-2-D1 cell line. Cells were incubated in standard conditions and they were exposed to different concentrations of VP, DVP, HPD, and Ce6, that is, 180, 90, 45, 22.5, and 10 µg/ml. After incubation with photosensitizers, the cells were washed, medium was exchanged and the cells were subjected to irradiation at the proper wavelengths, light intensity (100 mW/sq cm), and total light doses 4.5 and 9 J/sq cm.

RESULTS

Our results showed that the VP and DVP are potent photosensitizers and the photocytotoxic effect after the incubation with DVP was much better than that of VP. The cytotoxic effects of VP and DVP were less intensive than these of HPD and Ce6. VP and DVP also accumulated well in the tumor cells. Our results also indicated that the VP and DVP effectiveness on MCF-7 cells was photosensitizer dose and light dose dependent.

CONCLUSION

The overall properties revealed by both new porphyrins and particularly a possibility for excitation at a higher wavelength and thus a deeper tissue penetration, make them promising candidates for further in vivo experiments.

Evaluation of tetraphenyl-2,3-dihydroxychlorins as potential photosensitizers

A series of β,β-dihydroxychlorins derived from meso-tetraphenylporphyrins (TPPs) have been synthesized. Their in vitro cytotoxicity has been measured and compared to BPDMA (verteporfin). Under the assay conditions BPDMA had an LD 50 (lethal dose to kill 50% of cells) value of 0.007 μM (5 ng/mL). The LD 50 values for the TPP derivatives varied from 1.7 × 10-2 to 9.9 μM depending upon the substituents and their position on the phenyl groups. One example of the dihydroxychlorin prepared from unsubstituted 5,15-diphenylporphyrin was examined and this exhibited an LD 50 of 2.4 × 10-3 μ M !

Synthesis of novel asymmetric zinc (II) phthalocyanines bearing octadecyloxyl and glucosyl groups

A novel asymmetric zinc (II) phthalocyanine substituted by one lipophilic octadecyloxyl group and three hydrophilic glucosyl groups was synthesized. Using Q-TOF MS, the major byproducts formed during the deprotection processes were identified. An improved procedure was worked out to convert these byproducts to the desired product in quantitative yield.

Pharmacokinetics of a tri-glucoconjugated 5,10,15-(meta)-trihydroxyphenyl-20-phenyl porphyrin photosensitizer for PDT. A single dose study in the rat

Photodynamic therapy (PDT) involves a non invasive treatment of small and superficial cancers using a photosensitive drug and light to kill tumoral cells. 5,10,15-meso-tri-(meta-O-beta-D-glucosyloxyphenyl)-20-phenylporphyrin [m-TPP(glu)3] is a new photosensitizer (PS) with more enhanced photocytotoxicity relative to 5,10,15,20-meso-tetra-(meta-hydroxyphenyl) chlorin [m-THPC] (Foscan). It was injected intravenously once to healthy rats at three different doses (0.25, 0.5 and 1 mg kg(-1)) and compared to m-THPC (0.3 mg kg(-1)). Pharmacokinetic parameters for both photosensitizers were derived from plasma concentration-time data using a non-compartmental analysis and a two-compartment pharmacokinetic model. m-TPP(glu)3 is more rapidly eliminated throughout the organism than m-THPC. Its mean plasma clearance is 19 mL h(-1) kg(-1) (6 mL h(-1) kg(-1) for m-THPC), and its mean residence time is 5h (20 h for m-THPC). The area under curve (AUC) and initial mean serum concentration (C0) were found to be proportional to the dose. As for Foscan, no metabolite of m-TPP(glu)3 was detected in plasma. The biodistribution study demonstrates that the most significant amount of m-TPP(glu)3 was concentrated in organs such as lung, liver and spleen which are rich in reticulo-endothelial cells. Maximum concentrations were reached in organs 14 h after IV administration. At 48 h, the photosensitizer was essentially eliminated from all organs. Because of its shorter elimination time, m-TPP(glu)3 is more attractive than m-THPC as a PDT agent since secondary side effects of shorter duration could be expected.

Enhanced detection of seven glucoconjugated and hydroxylated porphyrins and chlorins by nonaqueous capillary electrophoresis combined with stacking

The nonaqueous capillary electrophoresis mode which includes a preconcentration step based on a transient pseudo-isotachophoresis to the simultaneous separation of seven glucoconjugated and hydroxylated porphyrins and chlorins, exhibiting very close structures, is reported. A high methanol content, of the buffer solution, was necessary in order to prevent self-assembly of the compounds and to enhance their solubility during separation. With the addition of 66% (v/v) methanol and 1% (w/v) NaCl in the aqueous sample solution, large volumes could be injected (44% capillary volume) without a loss in resolution. Sensitivity of detection was therefore improved by a 100-fold factor with regard to the method employing normal injection (2% capillary volume). Optimum electrophoretic conditions, in terms of sensitivity and performance, were obtained by using 20 mM phosphoric acid buffer, pH 2.2 and 50% methanol. The method was validated and applied to qualitative analysis of glucoconjugates in serum samples.

A study of the stability of tri(glucosyloxyphenyl)chlorin, a sensitizer for photodynamic therapy, in human colon tumoural cells: a liquid chromatography and MALDI-TOF mass spectrometry analysis

Asymmetrical glycoconjugated tetrapyrrolic macrocycles are under study as efficient sensitizers for photodynamic therapy (PDT). In this context, tri(meta-O-beta-glucopyranosyloxyphenyl)chlorin [TPC(m-O-Glu)(3)] 2a/3a was found to be four times more photoactive in vitro than Foscan. In a further study of this interesting glycoconjugate, its metabolism by cellular glycosidases in HT29 cells has to be explored. Cellular extracts of HT29 cells incubated with TPC(m-O-Glu)(3) (24h, 6microM) were analyzed by MALDI-TOF mass spectrometry and high performance liquid chromatography (HPLC). In MALDI-TOF mass spectra, the presence of compounds distinct from TPC(m-O-Glu)(3) (m/z 1151) were observed at m/z 989, 827 and 665 corresponding to the loss of one, two or three glucose units (162u) and were be ascribed to TPC(m-OH)(m-O-Glu)(2) 2/3b,b',b", TPC(m-OH)(2)(m-O-Glu) 2/3c,c',c" and TPC(m-OH)(3) isomers 2d/3d, respectively. The porphyrins resulting from chlorin oxidation TPP(m-O-Glu)(3) 4a, TPP(m-OH)(m-O-Glu)(2) 4b,b", TPP(m-OH)(2)(m-O-Glu) 4c,c" and TPP(m-OH)(3) 4d were also observed. The HPLC profile (lambda(anal)=420 nm) showed eight peaks consistent with mass spectra. The kinetics of deglucosylation was studied from HPLC profiles between 1 and 48h incubation. The concentration of triglucoconjugated and diglucoconjugated molecules was maximum around 3 and 8h incubation, respectively, whereas, totally deglucosylated species appeared only after incubation for more than 10h. The fully deglycosylated porphyrin TPP(m-OH)(3) is the final metabolite, being observed at a concentration 15 times higher than that of the remaining TPC(m-O-Glu)(3) 2a/3a. Compared to the photobiological activity of the parent molecule [TPC(m-O-Glu)(3)], a three times higher TPP(m-OH)(3) concentration was necessary to observe a similar in vitro photoactivity.

Speciation of new tri- and tetra-glucoconjugated tetrapyrrolic macrocycles (porphyrins and chlorins): an electronic molecular spectroscopy study

In this work, we study the physicochemical properties of some newly developed glycoconjugated photosensitizers that can be used in photodynamic therapy (PDT) of cancers: meso-tri- and tetra-(meta-O-beta-D-glucosyloxyphenyl)porphyrins and meso-, tri-, and tetra-(meta-O-beta-D-glucosyloxyphenyl)chlorins. Their properties are compared to the non-glycosylated hydroxylated parent compounds meso-tetra-(meta-hydroxyphenyl) porphyrin and meso-tetra-(meta-hydroxyphenyl)chlorin. It was found that at the ground state, all porphyrins present, independent of the substitution, have the same mean ionization constant (pKa = 2.7), corresponding to two indistiguishable steps of protonation of tetrapyrrolic nitrogens. On the other hand, in the case of chlorins, one proton process can be observed and the corresponding nitrogen exhibits a slightly superior basicity (pKa = 3.0) with respect to porphyrins. Hydroxylated compounds present a second transition at high pH corresponding to the ionization of phenol groups (pKa = 10.5). Consequently, all photosensitizers are not charged at physiological pH (approximately 7.4), and so the ionization process does not influence their activity in biological media. Ionization induces very important variations in photosensitizer absorption and emission spectra. For example, absorption in the red region (band V), one of the most important characteristics of a good photosensitizer, is only important for diprotonated porphyrins and neutral chlorins. As far as fluorescence emission is concerned, neutral chlorins are almost six times more fluorescent than the corresponding neutral porphyrins (phi(chlorin)/phi(porphyrin) approximately = 6). It should be emphasized that the spectra modifications induced by pH variations can find interesting applications in the optimization of visible and fluorescence detection in high-performance liquid chromatography (HPLC) as well as in the development of direct, rapid fluorimetric analytical methods.

Synthesis, cellular internalization and photodynamic activity of glucoconjugated derivatives of tri and tetra(meta-hydroxyphenyl)chlorins

Glucoconjugated tri and tetra(meta-hydroxyphenyl)chlorins have been synthesized in order to explore how glucoconjugation of the macrocycle affects the photoactivity of the molecule. Internalization processes, photosensitizing efficacy of TPC(m-O-GluOH)(3) and TPC(m-O-GluOH)(4), in HT29 human adenocarcinoma cells have been compared to those of tetra(meta-hydroxyphenyl) chlorin (m-THPC, Foscan). The tetra glucoconjugated chlorin, TPC(m-O-GluOH)(4), was found to be poorly internalized and weakly photoactive. In contrast, the asymmetric and more amphiphilic compound TPC(m-O-GluOH)(3), exhibited superior phototoxicity compared to m-THPC. Drug concentration, temperature and sodium azide effects indicated that TPC(m-O-GluOH)(3) internalization partly proceeds via an active receptor-mediated endocytosis mechanism. Cellular uptake appeared as a saturable process and remained 30% lower than for mTHPC. However, a maximum phototoxicity in HT29 cells (survival fraction of 2+/-0.6%) were observed for concentration as low as 2 microM. A 4-fold higher concentration of m-THPC was necessary to observe the same level of photoactivity. This higher phototoxicity has been correlated to a greater mitochondrial affinity. On the basis of these results, work is in progress to further evaluate the potential of glycosylated chlorins in photodynamic therapy (PDT).

Internalization of aggregated photosensitizers by tumor cells: subcellular time-resolved fluorescence spectroscopy on derivatives of pyropheophorbide-a ethers and chlorin e6 under femtosecond one- and two-photon excitations

Amphiphilic sensitizers self-associate in aqueous environments and form aggregated species that exhibit no or only negligible photodynamic activity. However, amphiphilic photosensitizers number among the most potent agents of photodynamic therapy. The processes by which these sensitizers are internalized into tumor cells have yet to be fully elucidated and thus remain the subject of debate. In this study the uptake of photosensitizer aggregates into tumor cells was examined directly using subcellular time-resolved fluorescence spectroscopy with a high temporal resolution (20-30 ps) and high sensitivity (time-correlated single-photon counting). The investigations were performed on selected sensitizers that exhibit short fluorescence decay times (< 50 ps) in aggregated form. Derivatives of pyropheophorbide-a ether and chlorin e6 with varying lipophilicity were used for the study. The characteristic fluorescence decay times and spectroscopic features of the sensitizer aggregates measured in aqueous solution also could be observed in A431 human endothelial carcinoma cells administered with these photosensitizers. This shows that tumor cells can internalize sensitizers in aggregated form. Uptake of aggregates and their monomerization inside cells were demonstrated directly for the first time by means of fluorescence lifetime imaging with a high temporal resolution. Internalization of the aggregates seems to be endocytosis mediated. The degree of their monomerization in tumor cells is strongly influenced by the lipophilicity of the compounds.

Biodistribution and bioactivity of tetra-pegylated meta-tetra(hydroxyphenyl)chlorin compared to native meta-tetra(hydroxyphenyl)chlorin in a rat liver tumor model

It has been proposed that the construction of a photosensitizer-polymer conjugate would lead to an increased selective retention of the drug in tumor tissue resulting in an enhancement of selective tumor destruction by light in photodynamic therapy. In this study the kinetics of a tetra-pegylated derivative of meta-tetra(hydroxyphenyl)chlorin (mTHPC-PEG) were compared with those of native meta-tetra(hydroxyphenyl)chlorin (mTHPC) in a rat liver tumor model. In addition, the time course of bioactivity of both drugs was studied in normal liver tissue. Pegylation of mTHPC resulted in a two-fold increase in the plasma half-life time, a five-fold decrease in liver uptake and an increase in the tumor selectivity at early time intervals after drug administration. However, although mTHPC concentrations in liver decrease rapidly with time, mTHPC-PEG liver concentrations increased as a function of time. This led to a loss of tumor selectivity at all but the earliest time points, whereas with mTHPC tumor selectivity increased with time. For both drugs the time course of bioactivity in the liver parallels drug concentration levels with extensive necrosis after irradiation of mTHPC-PEG-sensitized liver tissue up to drug-light intervals of 120 h. It is concluded that on balance mTHPC-PEG does not appear to show any benefits over native mTHPC for the treatment of liver tumors, as normal liver tissue accumulates the compound. However, pegylation is a potentially promising strategy with an increase in tumor selectivity and reduced liver uptake if accumulation in the liver can be prevented.

Acid-base properties of chlorin e6: relation to cellular uptake

Chlorins are attractive compounds for photodynamic therapy because of their high absorption in the red spectral region. In this study, the absorbance, fluorescence excitation and fluorescence emission spectra of chlorin e6 have been recorded as functions of pH in phosphate-buffered saline (PBS) solution with and without fetal calf serum (FCS). For pure PBS solutions, variation of the pH of the solution results in a shift of both the absorption and the fluorescence spectrum as well as in a decrease of the fluorescence intensity. Spectrophotometric and fluorimetric titration curves, based on observed changes, have been plotted. There is an indication of aggregate formation at low pH values (pH < 5). The presence of 5% FCS results in a shift of the titration curve, from an inflection point at about 6.5 to one at about 7.6. Pronounced spectral changes of the fluorescence emission spectra of protein-bound chlorin e6 (change of spectral shape, decrease of peak intensity) are also observed. The partition coefficients in the 1-octanol-water system increase with decreasing pH. Thus, relatively more of the drug is incorporated in the octanol phase at low pH. Cellular uptake of chlorin e6 in the presence of serum is significantly higher at pH 6.7 as compared with that at 7.3 and 7.6. We conclude that a change in the pH value of the surrounding medium leads to a change in the lipophilicity of chlorin e6. Such a change is likely to influence its binding to the serum proteins as well as its interaction with the plasma membrane of cells and may also be related to the selective tumor uptake of the drug.

Distribution and photodynamic effects of meso-tetrahydroxyphenylchlorin (mTHPC) in the pancreas and adjacent tissues in the Syrian golden hamster

Photodynamic therapy (PDT) has the potential to destroy small tumours with safe healing of adjacent normal tissue. This study looks at the effects of PDT on the normal pancreas and adjacent tissues in hamsters using the photosensitiser meso-tetrahydroxyphenylchlorin (mTHPC). Pharmacokinetic studies used fluorescence microscopy on sections of pancreas, stomach and duodenum 1 h to 6 days after mTHPC. Highest levels of sensitiser were seen in the gastric and duodenal mucosa and in the acinar pancreas after 2-4 days. For PDT, light at 652 nm was delivered by placing a 0.2 mm diameter bare-ended fibre against the tissue. An energy of 50 J was used 2 or 4 days after 0.1 or 0.3 mg kg-1 mTHPC and animals killed 1 to 7 days later. Maximum necrosis was seen 3 days after PDT with lesions up to 4 mm in pancreas, 4.5 mm in duodenum and 2.5 mm in stomach. By fractionating the light dose, the lesion size could be increased by 30%. The main complication was free or sealed duodenal perforation (avoided by shielding the duodenum). Partial, reversible bile duct obstruction was seen occasionally. There was no macroscopic damage to the bile ducts or major blood vessels. Apart from the duodenum, all lesions healed safely. In this animal model, only the duodenum was at risk of serious, irreversible damage. Treatment is likely to be safer in the much thicker human duodenum. mTHPC is a powerful photosensitiser and suitable for further study for tumours in the region of the pancreas although care is required near the duodenum.

Experimental grounds for using chlorin e6 in the photodynamic therapy of malignant tumors

The toxicity, pharmacokinetics and antitumour effect of chlorin e6 after light irradiation were studied. The LD50 value of chlorin e6 in C3H mice is 189 +/- 9 mg kg-1 and in Wistar white rats is 113 +/- 18 mg kg-1 14 days after intraperitoneal injection. The concentration of chlorin e6 in blood, liver, kidney, spleen and tumors (sarcoma M-1 and sarcoma 45) of the rats was determined by a fluorescence method, 3, 6, 12, 18, 24, 48 and 72 h after administration at a dose of 10 mg kg-1. For this purpose, chlorin e6 was extracted from tissues by the detergent Triton X-100. The depth of necrosis in sarcoma 45, the regression rate of sarcoma M-1 and the animal cure rate were evaluated after chlorin e6 administration at doses of 1-10 mg kg-1 and subsequent irradiation with krypton laser light. Depending on the dose and the time interval between chlorin e6 injection and irradiation, the depth of necrosis in sarcoma 45 varied from 5.0 to 15.0 mm. The cure rate of the animals with sarcoma M-1 varied from 10% to 60%. The antitumor effect was directly proportional to the chlorin e6 dose and light energy exposure and inversely proportional to the time interval between photosensitizer injection and irradiation.