Time-resolved fluorescence in photobiology

Perspectives of photodynamic therapy in biotechnology

Photodynamic therapy (PDT) is a current and innovative technique that can be applied in different areas, such as medical, biotechnological, veterinary, among others, both for the treatment of different pathologies, as well as for diagnosis. It is based on the action of light to activate photosensitizers that will perform their activity on target tissues, presenting high sensitivity and less adverse effects. Therefore, knowing that biotechnology aims to use processes to develop products aimed at improving the quality of life of human and the environment, and optimizing therapeutic actions, researchers have been used PDT as a tool of choice. This review aims to identify the impacts and perspectives and challenges of PDT in different areas of biotechnology, such as health and agriculture and oncology. Our search demonstrated that PDT has an important impact around oncology, minimizing the adverse effects and resistance to chemotherapeutic to the current treatments available for cancer. Veterinary medicine is another area with continuous interest in this therapy, since studies have shown promising results for the treatment of different animal pathologies such as Bovine mastitis, Malassezia, cutaneous hemangiosarcoma, among others. In agriculture, PDT has been used, for example, to remove traces of antibiotics of milk. The challenges, in general, of PDT in the field of biotechnology are mainly the development of effective and non-toxic or less toxic photosensitizers for humans, animals and plants. We believe that there is a current and future potential for PDT in different fields of biotechnology due to the existing demand.

Photoinactivation of multidrug resistant bacteria by monomeric methylene blue conjugated gold nanoparticles

Multidrug resistant (MDR) bacterial infections have become a severe threat to the community health due to a progressive rise in antibiotic resistance. Nanoparticle-based photodynamic therapy (PDT) is increasingly been adopted as a potential antimicrobial option, yet the cytotoxicity associated with PDT is quite unspecific. Herein, we show Concanavalin-A (ConA) directed dextran capped gold nanoparticles (GNP_DEX-ConA) enhanced the efficacy and selectivity of methylene blue (MB) induced killing of multidrug resistant clinical isolates. Here, we show that our complex MB@GNP_DEX-ConA is effective against range of MDR clinical isolates, including Escherichia coli, Klebsiella pneumoniae and Enterobacter cloacae. In our treatment modality negligible dark toxicity suggests photochemically driven process with 97% killing of MDR bacteria. GNP_DEX-ConA with monomeric form of MB departs maximum fluorescence decay time (τf: 1.7ns in HSA) and singlet oxygen (ΔΦ; 0.84) for improved activity in albumin rich infection sites. Further, the complex show least toxicity when tested against HEK293 mammalian cells. The principle component analysis (PCA) and confocal microscopy illustrates cytosolic ¹O₂ mediated type-II PDT as mechanism of action. Hence, MB@GNP_DEX-ConA mediated PDT is potential therapeutic approach against MDR infections and can be tailored to fight other infectious diseases.

Non-monotonic changes in clonogenic cell survival induced by disulphonated aluminum phthalocyanine photodynamic treatment in a human glioma cell line

BACKGROUND

Photodynamic therapy (PDT) involves excitation of sensitizer molecules by visible light in the presence of molecular oxygen, thereby generating reactive oxygen species (ROS) through electron/energy transfer processes. The ROS, thus produced can cause damage to both the structure and the function of the cellular constituents resulting in cell death. Our preliminary investigations of dose-response relationships in a human glioma cell line (BMG-1) showed that disulphonated aluminum phthalocyanine (AlPcS2) photodynamically induced loss of cell survival in a concentration dependent manner up to 1 microM, further increases in AlPcS2concentration (>1 microM) were, however, observed to decrease the photodynamic toxicity. Considering the fact that for most photosensitizers only monotonic dose-response (survival) relationships have been reported, this result was unexpected. The present studies were, therefore, undertaken to further investigate the concentration dependent photodynamic effects of AlPcS2.

METHODS

Concentration-dependent cellular uptake, sub-cellular localization, proliferation and photodynamic effects of AlPcS2 were investigated in BMG-1 cells by absorbance and fluorescence measurements, image analysis, cell counting and colony forming assays, flow cytometry and micronuclei formation respectively.

RESULTS

The cellular uptake as a function of extra-cellular AlPcS2 concentrations was observed to be biphasic. AlPcS2 was distributed throughout the cytoplasm with intense fluorescence in the perinuclear regions at a concentration of 1 microM, while a weak diffuse fluorescence was observed at higher concentrations. A concentration-dependent decrease in cell proliferation with accumulation of cells in G2+M phase was observed after PDT. The response of clonogenic survival after AlPcS2-PDT was non-monotonic with respect to AlPcS2 concentration.

CONCLUSIONS

Based on the results we conclude that concentration-dependent changes in physico-chemical properties of sensitizer such as aggregation may influence intracellular transport and localization of photosensitizer. Consequent modifications in the photodynamic induction of lesions and their repair leading to different modes of cell death may contribute to the observed non-linear effects.

Fluorescence imaging of Foscan and Foslip in the plasma membrane and in whole cells

A fluorescence microscope equipped with a condenser for total internal reflection (TIR) illumination was combined with a pulsed laser diode and a time-gated image intensifying camera for fluorescence lifetime measurements of single cells. In particular, fluorescence patterns, decay kinetics, and lifetime images of the lipophilic photosensitizers Foscan and Foslip were studied in whole cells as well as in close vicinity to their plasma membranes. Fluorescence lifetimes of both photosensitizers in cultivated HeLa cells decreased from about 8 ns at an incubation time of 3 h to about 5 ns at an incubation time of 24 h. This seems to result from an increase in aggregation (or self-quenching) of the photosensitizers during incubation. Selective measurements within or in close proximity to the plasma membrane indicate that Foscan and Foslip are taken up by the cells in a similar way, but may be located in different cellular sites after an incubation time of 24 h. A combination of TIR and fluorescence lifetime imaging microscopy (FLIM), described for the first time, appears to be promising for understanding some key mechanisms of photodynamic therapy (PDT).

Photodynamic therapy with pulsed light sources: a theoretical analysis

The effectiveness of photodynamic therapy using pulsed sources was evaluated using a mathematical model describing the time-dependent excitation and de-excitation of the photosensitizer molecule. Using the various numerical data available in the literature on haematoporphyrin we calculated that the effectiveness of pulsed excitation in PDT is identical to that of CW excitation for peak fluence rates below 4 x 10(8) W m-2. Above this threshold the effectiveness drops significantly. In practice this effect will occur with sources with high pulse energy and low repetition frequency. The commonly used dye lasers pumped by either a Cu vapour laser or a frequency doubled Nd:YAG laser have a PDT effectiveness identical to that of a CW source of the same wavelength and the same average fluence rate.

Fluorescence lifetime imaging microscopy: homodyne technique using high-speed gated image intensifier

In the previous sections we demonstrated imaging of intracellular Ca2+ using our approach to FLIM. What other analytes can be imaged using FLIM? We have now characterized the lifetime of a good number of ion indicators. Based on these studies we know that Cl- can be imaged using FLIM with probes such as SPQ or MQAE, pH can be imaged using resorufin and probes of the SNAFL and SNARF (Molecular Probes) series, and Mg2+ can be imaged using Magnesium Green, Mag-quin-2, or Mag-quin-1 (Molecular Probes). At present, the probe for K+, as PBFI, are just adequate as a lifetime probe, but it seems likely that newer probes for Na+ (Sodium Green) and K+ will be practical for effective imaging. Of course, imaging of oxygen is possible using a wide variety of fluorophores. It should be noted that a wide variety of substances and/or phenomena are known to alter decay times, acting as quenchers. These include the phenomena of resonance energy transfer, collisional quenching, temperature effects, and viscosity effects. Also, the FLIM method is not limited to microscopic objects but can be possibly used in remote imaging of any object. Hence, FLIM will allow the imaging of the chemical and physical properties of objects based on the effects of the local environment on the decay kinetics of fluorophores. The instrumentation for FLIM is presently complex and requires a moderately complex laser source, a gain-modulated image intensifier, and a slow-scan CCD camera. However, one can readily imagine the instrumentation becoming rather compact, and even all solid-state, owing to advances in laser and CCD technologies and, more importantly, advances in probe chemistry. To be specific, the dye laser shown in Fig. 1 may be replaced by a simpler UV laser, such as the 354 nm HeCd laser which has become available (Fig. 11). Intensity modulation of a continuous wave sources can be accomplished with acoustooptic modulators. The scientific slow-scan CCD cameras are presently rather expensive, but they are used in the present instrumentation because of their linearity and high dynamic range. However, the increasing use of CCD detectors suggest that even the scientific-grade CCD cameras will soon become less costly. Additionally, the frame rates of these detectors continue to increase in response to the need for faster imaging. Furthermore, the performance of the video CCD cameras is increasing, as seen by the introduction of 10-bit video analog-to-digital (A/D) converters.(ABSTRACT TRUNCATED AT 400 WORDS)

Time-resolved in-vivo fluorescence of photosensitizing porphyrins

Various components of photosensitizing porphyrins (e.g. monomers, aggregates, ionic species) have been recently localized in single cells by time-resolved fluorescence microscopy. Novel time-resolving techniques, based on picosecond laser diodes, a frequency-doubled Nd:YAG laser and time-gated microscopic equipment, were used for in-vivo measurements of the chick chorioallantoic membrane (CAM) exhibiting a pronounced vasculature. Changes of the fluorescence decay kinetics after light exposure were correlated with the formation of a photoproduct (Photosan, aminolaevulinic acid) or changes of the intracellular binding sites (tetraphenyl-porphyrins). Fluorescent components with different decay times were shown to be distributed differently within the tissue.

Fluence-rate-dependent photosensitized oxidation of NADH

The photosensitizing activity of dimethoxyhaematoporphyrin, excited by a laser pulse at 532 nm (YAG-Nd3+), was investigated using reduced nicotinamide adenine dinucleotide (NADH) as a substrate. The photo-oxidative modification of NADH was monitored by measuring the absorbance at 340 nm. The use of nanosecond pulses (15 and 0.5 ns) resulted in photosensitized NADH oxidation which depended on the fluence but not on the fluence rate up to a peak fluence rate of 10(7) W cm-2. At higher fluence rates a decrease in NADH photo-oxidation was observed, as well as on irradiation with picosecond pulses (35 ps). Stern-Volmer assay of the quenching by sodium azide revealed a decrease in quenching efficiency with increasing peak fluence rate. Oxidation of NADH was not suppressed by the addition of 20 mM sodium azide at peak fluence rates above 6 x 10(9) W cm-2. This observation, as well as the significant bleaching of dye absorption, indicates excitation of the photosensitizer into higher lying excited singlet states and the involvement of processes other than photodynamic action.

Intracellular localization of meso-tetraphenylporphine tetrasulphonate probed by time-resolved and microscopic fluorescence spectroscopy

The effects of solvent pH on spectral properties and fluorescence decay kinetics were investigated in order to characterize the microenvironment of meso-tetraphenylporphine tetrasulphonate (TPPS4) taken up by cells. Steady-state absorption and fluorescence spectra of TPPS4 in buffer solutions of different pH were used to identify a ring protonated species at pH less than or equal to 4. This dictation could also be distinguished from the unprotonated form by its altered fluorescence decay time (3.5 vs. 11.4 ns). In addition, time-resolved spectroscopy gave some evidence of a monocationic species existing at pH 6-9. This was concluded from the occurrence of another component with a decay time of 5 ns. Measurements of the spectral and kinetic properties of the fluorescence emission of single epithelial cells (RR1022) incubated with TPPS4 indicated that the sensitizer was mainly localized in a microenvironment with a pH of 5, a value which occurs intracellularly only within lysosomes. Cells kept in the dark exhibited the characteristic spectra of both the dication and the neutral form. The fluorescence decay showed two components with decay times of 2.6 ns and 10.6 ns. Irradiation of the cells changed the decay times to 4.6 ns and 13.4 ns and the dication fluorescence emission peak vanished, which is in accordance with the results obtained from buffer solutions at pH greater than or equal to 6. Therefore, we deduce that the photodynamic action leads to a rupture of the lysosomes and that the sensitizer is released into the surrounding cytoplasm.

Variation in the fluorescence decay properties of haematoporphyrin derivative during its conversion to photoproducts

Haematoporphyrin derivative (HpD) photoproducts are formed in aqueous solutions during light exposure in the presence of oxygen. The evaluation of the fluorescence decay of the photoproduct-enriched HpD solution shows an increase in the short-lived components, especially about 2 ns, in comparison with HpD without photoproducts. The bleaching of the HpD fluorescence and the photoproduct formation by the fluorescence-exciting radiation has to be taken into account in the evaluation of stationary as well as time-resolved fluorescence measurements.

Time-resolved polarization measurements of porphyrin fluorescence in solution and in single cells

Porphyrin monomers, dimers and aggregates, which can be differentiated on the basis of their fluorescence lifetimes, are shown to possess different degrees of fluorescence polarization. This opens up new possibilities for microscopic imaging of these individual components in photosensitization and tumour detection. A rough estimate of the size of the porphyrin aggregates is obtained from the data of time-resolved fluorescence anisotropy.

Ocular fluorometry: principles, fluorophores, instrumentation, and clinical applications

Ocular fluorometry is rapidly evolving as a versatile technique for research and diagnosis in ophthalmology. The main reasons for this increasing success are 1) the ideal characteristics of the eye as an optical device for excitation of tissue fluorescence and for the detection of the fluorescent emission; 2) the development of novel fluorometric techniques, including differential and time-resolved fluorescence spectroscopy; and 3) the increasing use of coupling geometries with high-resolution and high spatial selectivity. Both endogenous and exogenous fluorophores are of interest to ocular fluorometry. The most significant among endogenous fluorophores are the fluorescing pigments of the lens and of the retinal pigment epithelium (RPE). The nature, topography, and fluorescence properties of such pigments depend on age and pathology and on the level of light exposure. Exogenous fluorophores of interest are both intentionally induced and unintentionally accumulated drugs (some of which are phototoxic). Laser-based fluorometric techniques play a leading role in ocular fluorometry. The peculiar properties of the laser for the excitation of fluorescence make this source a favorite candidate for ocular fluorometry both in vitro and in vivo.