Tetrapyrroles: A chemical class of potent photosensitizers for the photodynamic treatment of tumours

Theoretical study of C6F5-corrole molecules functionalized with aromatic groups for Photodynamic Therapy

The singlet oxygen generation by electronically excited molecules in photodynamic therapy (PDT) requires light absorption within a specific wavelength window, and a subsequent intersystem crossing transition to a triplet excited state that is, at least, 0.98 eV higher in energy than the singlet ground state. Tetrapyrrolic macrocycles, such as porphyrin and corrole, have been widely used in oxygen singlet generation for PDT. Suitable functionalization can potentialize these macrocycles as photosensitizers. In this contribution, we use Density Functional Theory (DFT) calculations to determine the structural, electronic and spectroscopic properties of corrole macrocycles bound to different polycyclic aromatic groups in the gas phase, dichloromethane, and water. We also calculate the spin-orbit coupling (SOC) matrix elements of the intersystem crossing channels involving the first excited singlet states and excited triplet states. The results for optical absorption show that the threshold wavelength for optical absorption increases with the polarity of the environment and the number of aromatic rings of the ligands, whereas the oscillator strengths increase with the polarity of the environment but decrease with the number of aromatic rings. It is verified that the triplet excited states involved in the intersystem crossing transitions satisfy the energy requirement for the oxygen singlet generation. The magnitude of spin-orbit coupling (SOC) matrix elements associated with the intersystem crossing are also seen to be dependent on the environment involving the corrole molecules, and on the number of aromatic rings of the ligands connected to the corrole. Further, the binding of the functionalized corrole molecules with biomolecules as the calf thymus DNA and human serum albumin is studied and characterized through molecular docking. These results show that the corrole macrocycles, suitably functionalized with polycyclic aromatic groups, fulfill several criteria to be considered as good PDT photosensitizers.

The sound of drug delivery: Optoacoustic imaging in pharmacology

Optoacoustic (photoacoustic) imaging offers unique opportunities for visualizing biological function in vivo by achieving high-resolution images of optical contrast much deeper than any other optical technique. The method detects ultrasound waves that are generated inside tissue by thermo-elastic expansion, i.e., the conversion of light absorption by tissue structures to ultrasound when the tissue is illuminated by the light of varying intensity. Listening instead of looking to light offers the major advantage of image formation with a resolution that obeys ultrasonic diffraction and not photon diffusion laws. While the technique has been widely used to explore contrast from endogenous photo-absorbing molecules, such as hemoglobin or melanin, the use of exogenous agents can extend applications to a larger range of biological and possible clinical applications, such as image-guided surgery, disease monitoring, and the evaluation of drug delivery, biodistribution, and kinetics. This review summarizes recent developments in optoacoustic agents, and highlights new functions visualized and potent pharmacology applications enabled with the use of external contrast agents.

Anthelmintic activity and non-cytotoxicity of phaeophorbide-a isolated from the leaf of Spondias mombin L

ETHNOPHARMACOLOGICAL RELEVANCE

Helminthosis (worm infection) is a disease of grazing livestock, with significant economic implications. Increasing resistance to existing synthetic anthelmintics used to control helminthosis and the unwanted presence of residues of the anthelmintics reported in meat and dairy products present a serious global health challenge. These challenges have necessitated the development of novel anthelmintics that could combat drug resistance and exhibit better safety profiles. Spondias mombin L. (Anacardiaceae) is a plant that has been used traditionally as a worm expeller.

AIM OF STUDY

The aim of the work reported herein was to isolate and characterise anthelmintic compound(s) from S. mombin leaf, establishing their bioactivity and safety profile.

MATERIALS AND METHODS

Adult Haemonchus placei motility assay was used to assess anthelmintic bioactivity. Bioassay-guided chromatographic fractionation of acetone extract of S. mombin leaf was carried out on a silica gel stationary phase. The structure of the compound was elucidated using spectroscopy (¹H and ¹³C NMR) and Liquid Chromatography-Mass Spectrometry (LC-ESI-MS). Screening to exclude potential cytotoxicity against mammalian cells (H460, Caco-2, MC3T3-E1) was done using alamar blue (AB) and CellTitreGlo (CTG) viability reagents.

RESULTS

The acetone extract yielded an active fraction 8 (Ethyl acetate: methanol 90:10; anthelmintic LC₅₀: 3.97 mg/mL), which yielded an active sub-fraction (Ethyl acetate: Methanol 95:5; anthelmintic LC₅₀: 53.8 μg/mL), from which active compound 1 was isolated and identified as phaeophorbide-a (LC₅₀: 23.0 μg/mL or 38.8 μM). The compound was not toxic below 200 μM but weakly cytotoxic at 200 μM.

CONCLUSIONS

Phaeophorbide-a (1) isolated from S. mombin leaf extract and reported in the plant for the first time in this species demonstrated anthelmintic activity. No significant toxicity to mammalian cells was observed. It therefore represents a novel anthelmintic pharmacophore as a potential lead for the development of novel anthelmintics.

Current trends in pyrrole and porphyrin-derived nanoscale materials for biomedical applications

This article is written to provide an up-to-date review of pyrrole-based biomedical materials. Porphyrins and other tetrapyrrolic molecules possess unique magnetic, optical and other photophysical properties that make them useful for bioimaging and therapy. This review touches briefly on some of the synthetic strategies to obtain porphyrin- and tetrapyrrole-based nanoparticles, as well as the variety of applications in which crosslinked, self-assembled, porphyrin-coated and other nanoparticles are utilized. We explore examples of these nanoparticles' applications in photothermal therapy, drug delivery, photodynamic therapy, stimuli response, fluorescence imaging, photoacoustic imaging, magnetic resonance imaging, computed tomography and positron emission tomography. We anticipate that this review will provide a comprehensive summary of pyrrole-derived nanoparticles and provide a guideline for their further development.

Synthesis of PSMA-targeted 131- and 152-substituted chlorin e6 derivatives and their biological properties

Prostate cancer is an extremely common cancer among older men. Conventional chemotherapy has proven to be not effective enough in battling it due to its high systemic toxicity and low selectivity. An alternative method of cancer treatment known as photodynamic therapy (PDT) has been shown to be effective. It is not without its faults either: one of the issues it’s been known to have is the insufficient selectivity of photosensitizer accumulation in tumor tissues. Recent studies, however, seem to indicate that introducing a PSMA-targeted moiety into photosensitizer might prove to be a solution to this problem. The present paper is concerned with synthesis of PSMA-targeted 131- and 152-substituted chlorin e6 conjugates and their biological characteristics. Our data suggests that the developed conjugates show potential as targeted agents for photodynamic therapy.

Photophysical properties of pheophorbide a in solution and in model membrane systems

Pheophorbide a (Pheo) is a well-known hydrophobic photosensitizer used for photodynamic treatment of various diseases. The influence of the surroundings on the electronic properties of photosensitizers mainly accumulating in membrane structures is of relevance for their photoactivity. In this paper the current knowledge about the electronic properties of Pheo in different microheterogeneous environments is summarized and new findings about its incorparation in different model membranes are discussed.

Synthesis of novel pyropheophorbide a-fullerene conjugates

Two pyropheophorbide a units were attached via a malonate addition reaction to the [60]fullerene core to give a trichromophoric system. Also, in a different approach two pyropheophorbide a moieties were coupled to a hexasubstituted [60]fullerene diol. The mono-adduct was very light and oxygen sensitive whereas the corresponding hexaadduct showed a much higher stability.

Toxic interaction of thionine to deoxyribonucleic acids: elucidation of the sequence specificity of binding with polynucleotides

The sequence specificity of the intercalative DNA damage of the phenothiazine dye thionine has been investigated by absorbance, fluorescence, circular dichroism and viscosity studies using four synthetic polynucleotides, poly(dA-dT)·poly(dA-dT), poly(dA)·poly(dT), poly(dG-dC)·poly(dG-dC) and poly(dG)·poly(dC). Strong hypochromic-bathochromic effects in absorbance and quenching in fluorescence were observed that showed strong binding of thionine to these polynucleotides. Scatchard plots revealed non-cooperative binding and analysis by McGhee-von Hippel equation provided the affinity values in the order of 10(5)M(-1). The binding clearly revealed the high preference of thionine to the alternating GC sequences followed by the homo GC sequences. The AT polynucleotides had lower binding affinities but the alternating AT sequences had higher affinity compared to the homo stretches. The results of ferrocyanide quenching studies in fluorescence and viscosity experiments conclusively proved the intercalation of thionine while circular dichroic studies provided evidence for the structural perturbations associated with the sequence specific intercalative binding. The sequence specificity of the intercalative damage of thionine to deoxyribonucleic acid is advanced from this study.

Spectroscopic studies on the thermodynamic and thermal denaturation of the ct-DNA binding of methylene blue

The ct-DNA binding properties of methylene blue (MB) including binding constant, thermodynamic parameter and thermal denaturation (T(m)) have been systematically studied by spectrophotometric method. The binding of MB to ct-DNA is quite strong as indicated by remarkable hypochromicity, red shift and equilibrium binding constant (K(b)). Van't Hoff plot of 1/T versus lnK(b) suggests that the MB dye binds exothermically to ct-DNA which is characterized by large negative enthalpy and entropy changes. According to polyelectrolyte theory, the charge release (Z) when ct-DNA interacts with MB is +1.09 which corresponds very well to the one positive charge carried by the MB dye. The K(b) at a low concentration of salt is dominated by electrostatic interaction (90%) while that at a high concentration of salt is mostly controlled by non-electrostatic process (85%). However, the stabilization of the DNA binding event in both cases is governed by non-electrostatic process. A moderate stabilization of double helix ct-DNA occurs when the MB dye binds to ct-DNA as indicated by the increase in T(m) of ct-DNA of about 5.5 degrees C in the presence of MB. This suggests that MB dye possibly binds to ct-DNA via electrostatic and intercalation modes.

Fluorescent studies on cooperative binding of cationic pheophorbide-a derivative to polyphosphate

The cooperative binding of a novel water-soluble cationic derivative of pheophorbide-a (CatPheo-a) to inorganic polyphosphate (PPS) in buffered aqueous solutions was studied by means of polarized fluorescence spectroscopy in a wide range of molar phosphate-to-dye ratios (P/D). Under low P/D values, CatPheo-a forms extended stacking associates on the PPS matrix, while under high P/D the dye binds to PPS in the dimer form. The CatPheo-a self-association is accompanied by 40-fold dye fluorescence quenching and a substantial increase in the fluorescence polarization degree. The fluorescent titration data were used for determination of cooperative binding parameters by Schwarz's method.

Shielding effects and hypoxia in photodynamic therapy

Photodynamic therapy (PDT) is a new modality to treat cancer. In clinical use, the procedure is limited by the poor penetration depth of the curative light, which ranges between 3 and 6 mm. The cause of this situation is not clear. Two hypotheses are discussed: shielding effect or hypoxia. To test both hypotheses, we designed an in vitro model. Suspensions and sediments of OAT-75 cells were incubated at 37 degrees C with pheophorbide a or 13(2)-hydroxy-bacteriopheophorbide a methyl ester, both second-generation sensitizers. Irradiation was done with laser light of suitable wavelength. In all experiments, cell death was seen on the surface of the sediment, i.e., near the border of the oxygen-containing atmosphere and opposite the site of the irradiation beam. Therefore, we should accept the hypoxia thesis.

Photophysical properties of the photosensitizer pheophorbide a studied at high photon flux densities

The photophysical properties of pheophorbide a (Pheo a) under two-step laser activation were investigated. For the first time quantum yields of higher excited state formation were calculated. It was shown that the quantum yields of the formation of these states depend strongly on the pulse duration of the excitation source. The nonlinear properties of Pheo a are quite different in dependence on the excitation wavelength but the quantum yields of the higher excited state formation in both cases (lambda exc = 337 nm and lambda exc = 667 nm) rise up increasing the photon flux density of the laser light up to 10(26) phot cm-2 s-1. A further increase of the photon flux density has no effect on the quantum yields.

Lasers in surgery

In January 1991 the winter meeting of the Surgical Research Society was held at St. Bartholomew's Hospital and the Institute of Education, London. During the meeting a symposium was held entitled 'Shedding light on lasers'. Speakers addressed the general principles of lasers, laser-tissue interactions and the applications of lasers in gastro-enterology and in vascular disease. This was followed by an open discussion of the current indications, complications and outcome, together with future possible applications for lasers in medicine.

[A simple setup for measuring scattering in testing the photodynamic effectiveness of dyes]

An experimental setup for the determination of the photodynamic activity of dyes on the basis of scattering measurements is described. With its aid, photochemically induced morphological changes and the process of cell lysis can be registered, and action spectra measured by varying the irradiation wavelength. Different photoproducts of the photosensitizer hematoporphyrin derivative (HPD) were tested in human erythrocyte suspensions. The photodynamic activity of these products was found to depend on absorption behavior, but is lower than that of HPD.

Interaction of cremophor EL with human plasma

1. Interaction of cremophor EL (CRM) with human plasma lipoproteins and nonlipoproteins has been investigated by ultracentrifugation. 2. VLDL has only a low or negligible capacity to bind CRM, i.e. there is little or no change in the optical absorption at 280 nm of VLDL when CRM is added. 3. A low density subfraction of low density lipoproteins seems to associate substantially with CRM at relatively low CRM concentrations (1-3 mg/ml), but such association is not evident for CRM concentrations in the region 12-116 mg/ml. 4. Low density lipoproteins (LDL) may act as a carrier for CRM-emulsions, yet there seems to be no concomitant change in the 280 nm optical absorption of the proteins of LDL. 5. The position in the gradient (i.e. in the centrifugation tube after centrifugation) of high density lipoproteins (HDL) is shifted towards lower density in the presence of 1-4 mg CRM/ml. For higher concentrations of CRM, a destruction of HDL can be observed: the HDL distribution is converted into a bimodal distribution of respectively lighter and heavier "HDL"-particles than the normal ones; the densities at the peaks of these distributions are approximately 1.07 g/ml (light), 1.20 g/ml (heavy) and 1.11 g/ml (normal HDL). The optical extinction coefficient is apparently the same for the proteins of normal--and modified HDL. 6. Even high CRM concentrations (less than or equal to 116 mg/ml) have no perceptible effect on the gradient positions and profile of human serum albumin (HSA) and/or other heavy proteins. 7. The possible biological significance of these findings is briefly touched upon.