Efficient synthesis and photodynamic activity of porphyrin-saccharide conjugates: targeting and incapacitating cancer cells

Synthesis and photophysical properties of thioglycosylated chlorins, isobacteriochlorins, and bacteriochlorins for bioimaging and diagnostics

The facile synthesis and photophysical properties of three nonhydrolyzable thioglycosylated porphyrinoids are reported. Starting from meso-perfluorophenylporphyrin, the nonhydrolyzable thioglycosylated porphyrin (PGlc₄), chlorin (CGlc₄), isobacteriochlorin (IGlc₄), and bacteriochlorin (BGlc₄) can be made in 2-3 steps. The ability to append a wide range of targeting agents onto the perfluorophenyl moieties, the chemical stability, and the ability to fine-tune the photophysical properties of the chromophores make this a suitable platform for development of biochemical tags, diagnostics, or as photodynamic therapeutic agents. Compared to the porphyrin in phosphate buffered saline, CGlc₄ has a markedly greater absorbance of red light near 650 nm and a 6-fold increase in fluorescence quantum yield, whereas IGlc₄ has broad Q-bands and a 12-fold increase in fluorescence quantum yield. BGlc₄ has a similar fluorescence quantum yield to PGlc₄ (<10%), but the lowest-energy absorption/emission peaks of BGlc₄ are considerably red-shifted to near 730 nm with a nearly 50-fold greater absorbance, which may allow this conjugate to be an effective PDT agent. The uptake of CGlc₄, IGlc₄, and BGlc₄ derivatives into cells such as human breast cancer cells MDA-MB-231 and K:Molv NIH 3T3 mouse fibroblast cells can be observed at nanomolar concentrations. Photobleaching under these conditions is minimal.

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.

Evaluation of the specific interactions between glycodendrimeric porphyrins, free or incorporated into liposomes, and concanavalin A by fluorescence spectroscopy, surface pressure, and QCM-D measurements

In photodynamic therapy, the specificity of a photosensitizer and its penetration into tumor cells are crucial. We have analyzed the ability of newly synthesized meso-(tetraphenyl)porphyrins to be recognized by a model of mannose-specific proteins overexpressed at the surface of retinoblastoma cells. The specific interaction of porphyrin with Con A was studied by surface pressure measurements, fluorescence spectroscopy, dynamic light scattering, and QCM-D. The extent of porphyrins binding to Con A was highly dependent upon their chemical structure. Glycodendrimeric porphyrins showed the higher binding constant to Con A. The length of the spacer separating the sugar from the tetrapyrrolic ring appeared to be crucial in controlling the interaction of the compounds with the lectin in solution or immobilized onto a solid substrate. The methodology used proved to be efficient for the selection of potentially active compounds. The glycodendrimeric porphyrins, especially the derivative having the longer spacer, interacted more significantly with the lectin than the compound devoid of any sugar.

A New Crosslinkable Oxygen Sensor Covalently Bonded into Poly(2-hydroxyethyl methacrylate)-CO-Polyacrylamide Thin Film for Dissolved Oxygen Sensing

A new oxygen sensor, compound 2, was synthesized through a chemical modification of a popularly used oxygen sensor of platinum(II)-5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorophenyl)-porphyrin (PtTFPP). The new sensor compound 2 possesses four crosslinkable methacrylate functional moieties, enabling it to be polymerized and crosslinked with other monomers for polymer sensing film (also called membrane) preparation. Using this characteristic, compound 2 was covalently bonded to hydrophilic poly(2-hydroxyethyl methacrylate)-co-polyacrylamide (referred to as PHEMA to simplify) and hydrophobic polystyrene (PS) films. To better understand the advantages and disadvantages of chemical crosslinking approaches and the influence of polymer matrices on sensing performance, PtTFPP was physically incorporated into the same PHEMA and PS matrices to compare. Response to dissolved oxygen (DO), leaching of the sensor molecules from their matrices, photostability of the sensors, and response time to DO changes were studied. It was concluded that the chemical crosslinking of the sensor compound 2 in polymer matrices: (i) alleviated the leaching problem of sensor molecules which usually occurred in the physically doped sensing systems and (ii) significantly improved sensors' photostability. The PHEMA matrix was demonstrated to be more suitable for oxygen sensing than PS, because for the same sensor molecule, the oxygen sensitivity in PHEMA film was higher than that in PS and response time to DO change in the PHEMA film was faster than that in PS. It was the first time oxygen sensing films were successfully prepared using biocompatible hydrophilic PHEMA as a matrix, which does not allow leaching of the sensor molecules from the polymer matrix, has a faster response to DO changes than that of PS, and does not present cytotoxicity to human lung adenocarcinoma epithelial cells (A549). It is expected that the new sensor compound 2 and its similar compounds with chemically crosslinking characteristics can be widely applied to generate many interesting oxygen sensing materials for studying biological phenomena.

Design and creativity in synthesis of multivalent neoglycoconjugates

From the authors' opinion, this chapter constitutes a modest extension of the seminal and inspiring contribution of Stowell and Lee on neoglycoconjugates published in this series [C. P. Stowell and Y. C. Lee, Adv. Carbohydr. Chem. Biochem., 37 (1980) 225-281]. The outstanding progresses achieved since then in the field of the "glycoside cluster effect" has witnessed considerable creativity in the design and synthetic strategies toward a vast array of novel carbohydrate structures and reflects the dynamic activity in the field even since the recent chapter by the Nicotra group in this series [F. Nicotra, L. Cipolla, F. Peri, B. La Ferla, and C. Radaelli, Adv. Carbohydr. Chem. Biochem., 61 (2007) 353-398]. Beyond the more classical neoglycoproteins and glycopolymers (not covered in this work) a wide range of unprecedented and often artistically beautiful multivalent and monodisperse nanostructures, termed glycodendrimers for the first time in 1993, has been created. This chapter briefly surveys the concept of multivalency involved in carbohydrate-protein interactions. The topic is also discussed in regard to recent steps undertaken in glycobiology toward identification of lead candidates using microarrays and modern analytical tools. A systematic description of glycocluster and glycodendrimer synthesis follows, starting from the simplest architectures and ending in the most complex ones. Presentation of multivalent glycostructures of intermediate size and comprising, calix[n]arene, porphyrin, cyclodextrin, peptide, and carbohydrate scaffolds, has also been intercalated to better appreciate the growing synthetic complexity involved. A subsection describing novel all-carbon-based glycoconjugates such as fullerenes and carbon nanotubes is inserted, followed by a promising strategy involving dendrons self-assembling around metal chelates. The chapter then ends with those glycodendrimers that have been prepared using commercially available dendrimers possessing varied functionalities, or systematically synthesized using either divergent or convergent strategies.

Conjugation of 2-(1'-hexyloxyethyl)-2-devinylpyropheophorbide-a (HPPH) to carbohydrates changes its subcellular distribution and enhances photodynamic activity in vivo

The carbohydrate moieties on conjugating with 3-(1'-hexyloxyethyl)-3-devinyl pyropeophorbide-a (HPPH) altered the uptake and intracellular localization from mitochondria to lysosomes. In vitro, HPPH-Gal 9 PDT showed increased PDT efficacy over HPPH-PDT as detectable by the oxidative cross-linking of nonphosphorylated STAT3 and cell killing in ABCG2-expressing RIF cells but not in ABCG2-negative Colon26 cells. This increased efficacy in RIF cells could at least partially be attributed to increased cellular accumulation of 9, suggesting a role of the ABCG2 transporter for which HPPH is a substrate. While such differences in the accumulation in HPPH derivatives by tumor tissue in vivo were not detectable, 9 still showed an elevated light dose-dependent activity compared to HPPH in mice bearing RIF as well as Colon26 tumors. Further optimization of the carbohydrate conjugates at variable treatment parameters in vivo is currently underway.

Synthesis of novel diselenide-linked porphyrin dimers under phase-transfer catalysis condition and their interactions with DNA

Novel diselenide-linked porphyrin dimers were synthesized under phase-transfer catalysis conditions. The targeted compounds were characterized by (1)H-NMR, high-resolution mass spectrometry, UV/VIS and fluorescence spectroscopies, redox-potential measurements, and elemental analysis. The interaction of the title compounds with DNA was studied using UV/VIS, fluorescence, and circular dichroism (CD) spectroscopies. The relative rates of singlet-oxygen production from the diselenide-linked porphyrin dimers upon photoirradiation were also measured.

A fluorinated ruthenium porphyrin as a potential photodynamic therapy agent: synthesis, characterization, DNA binding, and melanoma cell studies

When the new porphyrin 5,10-(4-pyridyl)-15,20-(pentafluorophenyl)porphyrin is reacted with 2 equiv of Ru(bipy)(2)Cl(2) (where bipy = 2,2'-bipyridine) formation of the target ruthenated porphyrin is achieved with 40% yield. Strong electronic transitions are observed in the visible region of the spectrum associated with the porphyrin Soret and four Q-bands. A shoulder at slightly higher energy than the Soret band is attributed to the Ru(dpi) to bipy(pi*) metal to ligand charge transfer (MLCT) band. The bipyridyl pi to pi* transition occurs at 295 nm. Cyclic voltammetry experiments reveal two single-electron redox couples in the cathodic region at E(1/2) = -0.80 and -1.18 V vs Ag/AgCl associated with the porphyrin. Two overlapping redox couples at E(1/2) = 0.83 V vs Ag/AgCl due to the Ru(III/II) centers is also observed. DNA titrations using calf thymus (CT) DNA and the ruthenium porphyrin give a K(b) = 7.6 x 10(5) M(-1) indicating a strong interaction between complex and DNA. When aqueous solutions of supercoiled DNA and ruthenium porphyrin are irradiated with visible light (energy lower than 400 nm), complete nicking of the DNA is observed. Cell studies show that the ruthenated porphyrin is more toxic to melanoma skin cells than to normal fibroblast cells. When irradiated with a 60 W tungsten lamp, the ruthenium porphyrin preferentially leads to apoptosis of the melanoma cells over the normal skin cells.

Effects of the number and position of the substituents on the in vitro photodynamic activities of glucosylated zinc(II) phthalocyanines

A series of mono-beta-, di-alpha- and di-beta-substituted phthalonitriles which contain one or two tetraethylene-glycol-linked 1,2:5,6-di-O-isopropylidene-alpha-D-glucofuranose unit(s) were prepared by typical substitution reactions. These precursors underwent self-cyclisation or mixed-cyclisation with an excess of unsubstituted phthalonitrile in the presence of Zn(OAc)(2) x 2 H(2)O and DBU to give the corresponding zinc(II) phthalocyanines with 1, 2 or 4 glucosylated substituent(s). For the di-alpha- and tetra-beta-glucosylated analogues, removal of the isopropylidene groups was also performed by the treatment with trifluoroacetic acid and water to give the corresponding water-soluble deprotected glucosylated derivatives. All of these glucoconjugated phthalocyanines were fully characterised with various spectroscopic methods and studied for their photophysical properties and in vitro photodynamic activities against HT29 human colon adenocarcinoma and HepG2 human hepatocarcinoma cells. The tetra-beta-glucosylated phthalocyanines ZnPc(beta-PGlu)(4) (4) and ZnPc(beta-Glu)(4) (5) were found to be essentially non-cytotoxic. By contrast, the mono- and di-glucosylated analogues ZnPc(beta-PGlu) (7), ZnPc(alpha-PGlu)(2) (11), ZnPc(alpha-Glu)(2) (12) and ZnPc(beta-PGlu)(2) (20) exhibited substantial photocytotoxicity. The isopropylidene-protected di-alpha-substituted derivative 11 was particularly potent, having IC(50) values as low as 0.03 microM. The different photodynamic activities of these compounds can be attributed to their different extent of cellular uptake and aggregation tendency in the biological media, which greatly affect their singlet oxygen generation efficiency.

Photosensitiser delivery for photodynamic therapy. Part 2: systemic carrier platforms

BACKGROUND

The treatment of solid tumours and angiogenic ocular diseases by photodynamic therapy (PDT) requires the injection of a photosensitiser (PS) to destroy target cells through a combination of visible light irradiation and molecular oxygen. There is currently great interest in the development of efficient and specific carrier delivery platforms for systemic PDT.

OBJECTIVE

This article aims to review recent developments in systemic carrier delivery platforms for PDT, with an emphasis on target specificity.

METHODS

Recent publications, spanning the last five years, concerning delivery carrier platforms for systemic PDT were reviewed, including PS conjugates, dendrimers, micelles, liposomes and nanoparticles.

RESULTS/CONCLUSION

PS conjugates and supramolecular delivery platforms can improve PDT selectivity by exploiting cellular and physiological specificities of the targeted tissue. Overexpression of receptors in cancer and angiogenic endothelial cells allows their targeting by affinity-based moieties for the selective uptake of PS conjugates and encapsulating delivery carriers, while the abnormal tumour neovascularisation induces a specific accumulation of heavy weighted PS carriers by enhanced permeability and retention (EPR) effect. In addition, polymeric prodrug delivery platforms triggered by the acidic nature of the tumour environment or the expression of proteases can be designed. Promising results obtained with recent systemic carrier platforms will, in due course, be translated into the clinic for highly efficient and selective PDT protocols.

Low concentrations of a non-hydrolysable tetra-S-glycosylated porphyrin and low light induces apoptosis in human breast cancer cells via stress of the endoplasmic reticulum

A water-soluble tetra-S-glycosylated porphyrin (P-Glu(4)) is absorbed by MDA-MB-231 human breast cancer cells whereupon irradiation with visible light causes necrosis or apoptosis depending on the concentration of the porphyrin and the power of the light. With the same amount of light irradiation power (9.4 W m(-2)), at 10-20 microM concentrations necrosis is predominantly observed, while at <10 microM concentrations, apoptosis is the principal cause of cell death. Of the various possible pathways for the induction of apoptosis, experiments demonstrate that calcium is released from the endoplasmic reticulum, cytochrome c is liberated from the mitochondria to the cytosol, pro-caspase-3 is activated, poly-(ADP-ribose) polymerase is cleaved, and the chromatin is condensed subsequent to photodynamic treatment of these cells. Confocal microscopy indicates a substantial portion of the P-Glu(4) is located in the endoplasmic reticulum at <10 microM. These data indicate that the photodynamic treatment of MDA-MB-231 cells using low concentrations of the P-Glu(4) porphyrin and low light induces apoptosis mostly initiated from stress produced to the endoplasmic reticulum.

Photophysical properties of a photocytotoxic fluorinated chlorin conjugated to four beta-cyclodextrins

A meso-tetrakis(pentafluorophenyl)-chlorin with the reduced pyrrole ring linked to an isoxazolidine ring (FC) has been conjugated to four beta-cyclodextrins (CDFC). The CDFC exhibits excellent water solubility and is a potent photosensitizer towards proliferating NCTC 2544 human keratinocytes. The study by conventional steady state absorption and fluorescence spectroscopies and by time-resolved femto- and nanosecond laser flash spectroscopies suggests that in ethanol and pH 7 buffer the beta-cyclodextrins embed the highly hydrophobic tetrakis(pentafluorophenyl)-chlorin macrocycle and strongly interact with the chlorin rings in the singlet and triplet manifolds. In these solvents, femtosecond spectroscopy suggests that the conjugate undergoes a rapid relaxation in the upper excited singlet states induced by photochemical and/or conformation change(s) at a rate of about 5 ps(-1) to fluorescent states whose lifetime is approximately 8 ns. This interaction is destroyed upon addition of Triton X100 to buffer. Both FC and CDFC strongly fluoresce (Phi(F) approximately 0.5) in micelles. Similar behavior is observed at the triplet level. In ethanol and water, the initial transient triplet state absorbance decays within 1-3 mus yielding a longer lived triplet with spectral properties indistinguishable from that of original difference absorbance spectra. The determination of the molar absorbance in the 440-460 nm region ( approximately 35 000 M(-1) cm(-1)) leads to an estimate of approximately 0.2 for the triplet formation quantum yield of FC in toluene and of FC and CDFC in Triton X100 micelles. Quenching of the CDFC triplets by dioxygen in buffer produces (1)O(2) in a good yield consistent with the effective photocytotoxicity of the chlorin-cyclodextrins conjugate towards cultured NCTC 2544 human keratinocytes. By contrast, FC which aggregates in buffer produces little if any (1)O(2).

meso-Tetra(pentafluorophenyl)porphyrin as an efficient platform for combinatorial synthesis and the selection of new photodynamic therapeutics using a cancer cell line

The four para fluoro groups on 5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorophenyl)-porphyrin (TPPF20) are known to react with a variety of nucleophiles, but the reaction conditions for this substitution reaction depend on the nature of the nucleophiles, e.g. primary amines versus thiols. Glycosylated derivatives of this core porphyrin have been shown to be effective photodynamic agents in the induction of necrosis or apoptosis in several cancer cell lines. The present report demonstrates that TPPF20 can be used as a core platform to efficiently generate a variety of solution-phase combinatorial libraries. The focused combinatorial libraries have substituents that are chosen from a set of motifs known to bind biopolymers such as DNA, be taken up by cancer cells, or to render the compounds amphipathic. Incubation of a breast cancer cell line with these solution-phase libraries, followed by cell lyses and extraction, affords a selection assay. Matrix-assisted laser desorption ionization (MALDI) mass spectrometry of the extracts allows identification of the molecules taken up by the cells. Cell binding assays of the winning compounds synthesized directly indicate that both glycosylation and amphipathicity are key properties since neither tetraglycosylated porphyrins nor those with four polar groups are selected to the same extent. In addition, photodynamic efficacy was evaluated.

Efficient microwave-assisted synthesis of amine-substituted tetrakis(pentafluorophenyl)porphyrin

We report an efficient and rapid means for the synthesis of tetrakis(pentafluorophenyl)porphyrin (TPPF(20)) derivatives by microwave irradiation in an environmentally acceptable solvent. The selective displacement of the para-fluorine groups in TPPF(20) by primary amines occurs in yields between 70 and 95%. This method demonstrates that TPPF(20) is an ideal platform for the rapid formation of porphyrin conjugates for therapeutic, catalytic, and other applications. [reaction: see text]

Peptide-based pharmacomodulation of a cancer-targeted optical imaging and photodynamic therapy agent

We designed and synthesized a folate receptor-targeted, water-soluble, and pharmacomodulated photodynamic therapy (PDT) agent that selectively detects and destroys the targeted cancer cells while sparing normal tissue. This was achieved by minimizing the normal organ uptake (e.g., liver and spleen) and by discriminating between tumors with different levels of folate receptor (FR) expression. This construct (Pyro-peptide-Folate, PPF) is composed of three components: (1) pyropheophorbide a (Pyro) as an imaging and therapeutic agent, (2) peptide sequence as a stable linker and modulator improving the delivery efficiency, and (3) Folate as a homing molecule targeting FR-expressing cancer cells. We observed an enhanced accumulation of PPF in KB cancer cells (FR+) compared to HT 1080 cancer cells (FR-), resulting in a more effective post-PDT killing of KB cells over HT 1080 or normal CHO cells. The accumulation of PPF in KB cells can be up to 70% inhibited by an excess of free folic acid. The effect of Folate on preferential accumulation of PPF in KB tumors (KB vs HT 1080 tumors 2.5:1) was also confirmed in vivo. In contrast to that, no significant difference between the KB and HT 1080 tumor was observed in case of the untargeted probe (Pyro-peptide, PP), eliminating the potential influence of Pyro's own nonspecific affinity to cancer cells. More importantly, we found that incorporating a short peptide sequence considerably improved the delivery efficiency of the probe--a process we attributed to a possible peptide-based pharmacomodulation--as was demonstrated by a 50-fold reduction in PPF accumulation in liver and spleen when compared to a peptide-lacking probe (Pyro-K-Folate, PKF). This approach could potentially be generalized to improve the delivery efficiency of other targeted molecular imaging and photodynamic therapy agents.

Synthesis and solvent driven self-aggregation studies of meso-"C-glycoside"-porphyrin derivatives

New types of porphyrin derivatives bearing "C-glycoside" moieties, either in 5,10,15,20- or in 5,15-meso-positions, were prepared and fully characterized. The presence of the glycosidic groups imparts to the title macrocycles, besides an amphiphilic character, a clear tendency to form chiral suprastructures upon solvent-driven self-aggregation in different aqueous-organic solvent mixtures. Supra-assembly phenomena, in terms of the size and morphology of the resulting structures, as well as their kinetics of aggregation, were studied by UV-visible, fluorescence, resonance light scattering (RLS), and CD spectroscopy, indicating that the morphology of the aggregates depends strongly on the structure of the porphyrin rings, and on the bulk conditions of aggregation.

Synthesis and structural characterisation of the first N-heterocyclic carbene ligand fused to a porphyrin

The functionalisation of two neighboring beta-pyrrolic positions of a porphyrin by a fused N-heterocyclic carbene ligand, the subsequent metallation of this external coordination site by palladium(II) and the structural characterisation of the resulting compounds are presented.

Synthesis and cellular studies of PEG-functionalized meso-tetraphenylporphyrins

The total syntheses of four PEG-functionalized porphyrins, containing one to four low molecular weight PEG chains linked via amide bonds to the para-phenyl positions of meso-tetraphenylporphyrin, are reported. The hydrophobic character of the PEG-porphyrins decreases with the number of PEG chains linked to the porphyrin ring, while their tendency for aggregation in buffered aqueous solution increases. The porphyrins containing one or two PEG chains accumulated within human HEp2 cells to a much higher extent than those having three or four PEGs at the macrocycle periphery. All PEG-porphyrins were found to be non-toxic in the dark, and only those containing one or two PEG chains were phototoxic (IC(50)=2 microM at 1J/cm(2) light dose). The preferential sites of subcellular localization of the porphyrins containing one or two PEG chains were found to be the mitochondria and endoplasmic reticulum (ER), while those containing three or four PEG chains localize preferentially in the lysosomes.