Photodynamic antimicrobic therapy ( PACT ) involves the use of photosensitizers activated by exposure to seeable visible radiation in order to eliminate bugs ( this method has already been applied in photodynamic therapy of tumors ) . Photodynamic consequence of the peculiar PS is attributed to its ability to perforate into the sensitive micro-organism, to absorb the visible radiation of certain wave-length, and to bring forth reactive cytotoxic O merchandises. The mark micro-organism for photoinactivation are bacteriums, Fungis, viruses and Protozoa. Photodynamic antimicrobic therapy is proposed as a potentially topical, noninvasive attack suited for intervention of locally happening infection. The fact that bacteriums are going progressively immune to antibiotics and antiseptics has lead to an increased involvement in the field of developing new alternate obliteration methods, such as PACT. Research and development of light-sensitive substances have been traveling on with the purpose to happen effectual antimicrobic substances, which would hold a broad-spectrum authority.
Keywords: photodynamic antimicrobic therapy, photoinactivation, photosensitizers, phthalocyanines, antibacterial consequence
Photodynamic antimicrobic therapy ( PACT ) is based on the construct that a non-toxic photosensitiser localized in certain cells can be activated by low doses of seeable visible radiation of the appropriate wave-length, and generates vest O and free groups that are cytotoxic to aim cells. This phenomenon was foremost described by Oscar Raab in 1890 when he noted toxicity of acridine orange, which was dependent on light – against Paramecium caudatum. However, surveies of this phenomenon and its practical usage didn?t appear until the 2nd half of the twentieth century. At first, the possibilities of usage in the country of tumour therapy and, subsequently, besides their usage in antimicrobic therapy were studied ( 1 ) . Photodynamic anti-tumour therapy ( PDT ) is based on the fact that light-sensitive substance administered consistently is preferentially incorporated in quickly proliferating tumorous tissue, and after its irradiation the cell constructions are damaged due to the development of reactive cytotoxic merchandises and subsequent programmed cell death of tumorous cells. The confining factor is localization of function of the tumor and the possibility of targeted irradiation. The first successful usage of PDT was described in skin tumor therapy – by local application of eosin, which accumulates rapidly in the proliferating neoplastic cells ( 1-3 ) . Nowadays, porphyrin derived functions ( Photofrin, Foscan ) are used clinically ; they were approved for the usage in oncologic patients in the USA, Canada, France, the European Union and Japan. They are used for intervention of vesica tumors, lung tumors, gallet and tummy tumors, and for the tumor of cervix and caput. Although porphyrin substances are effectual in PDT, it is known that after parenteral disposal there are some unwanted side effects such as drawn-out tegument radiosensitivity ( 4 ) . In dermatology, PDT is successfully used for larger inoperable basal cell carcinomas ; substances incorporating the delta-aminolevulinic acid ( precursor of light-sensitive protoporphyrin IX ) are applied. The application of these compounds is less widespread in intervention of psoriasis and actinic keratoses. PDT is besides used in ophthalmology for age-related macular devolution therapy.
In the 1980 ‘s and 1990 ‘s, PDT began to be studied for the usage in antimicrobic intervention because of the increasing opposition of infective bugs to assorted types of antibiotics and the subsequent demand for alternate therapeutical agencies. The initial research proved that light-sensitive substances – when activated by light – are much more toxic to microbes than to human cells ( 4-7 ) .
2. Mechanism of photoinactivation
During photoinactivation of bugs or tumor cells, mark cells are damaged by the interaction of harmless seeable visible radiation with light-sensitive substance in the presence of O ( 8 ) . The light-sensitive substance ( PS ) itself has no or negligible antibacterial consequence but after its irradiation by visible radiation of appropriate moving ridge length ( during this procedure, photon is absorbed by the light-sensitive substance ) , the substance gets from the initial quiescent province to an unstable, aroused phase, in which extremely reactive cytotoxic merchandises can develop after the reaction with the environment in the presence of O. The consequence of the reaction are cytotoxic merchandises: hydroxyl extremist OH- , superoxid extremist or singlet O 1O2 ( molecular O O2 in the primary province contains 6 conductivity negatrons, 2 are unpaired after accepting certain sum of energy ; these negatrons conjugate and singlet O is generated ) . Singlet O has a really short half-time period ( nanoseconds ) and diffusion limited to the distance up to 100nm ; hence this activity is confined merely to immediate environment. Therefore, the accretion of the light-sensitive substance in the mark cell or its close milieus is required for successful photoinactivation. Lethal cell harm is caused by the devastation of nucleic acids, cell wall or cytoplasmic membrane while the cellular enzymatic and conveyance mechanisms are inactivated.
3. Photosensitizers and light beginnings
The procedure of photoinactivation of bug is dependent on light-sensitive substance and visible radiation. Particularly, of import is the type of PS, its concentration, optical maser type ( wave-length of the beam should be every bit near as possible to the soaking up upper limit of the PS ) and the dosage of light applied by irradiation.
Many natural every bit good as man-made photosensitive substances are known, typically dyes, normally belonging to aromatic compounds. For photoinactivation of bugs by PACT were used derivates of thiodiphenylamines – toluidine bluish O and methylene blue ; porphyrins – hematoporphyrin, delta-aminolevulinic acid ; xanthenes – rose bengal, chlorins – chlorin ( e6 ) , derivates of chlorin ( e6 ) with poly-L-lysine and polyethyleneimine ; phthalocyanines – disulphonated phthalocyanine of aluminum, cationic phthalocyanines of Zn, disulphonated phthalocyanine of Zn.
Photodynamic consequence of the peculiar PS is attributed to its ability to perforate into the sensitive micro-organism, to absorb the visible radiation of certain wave-length and to bring forth reactive cytotoxic O merchandises. In this regard, phthalocyanines seem to be a promising group of light-sensitive substances ( 9,10 ) .
Phthalocyanines are heterocyclic adducts composed of tetrapyrrole karyon connected by N Bridgess. Some of their derivates are used as dyes for pressmans ( blue coloring material ) , pigments or plastics. They are effectual light-sensitive substances. After their irradiation, a big measure of vest O is generated ( they are able to stay in the aroused province for a longer period compared to methylene or toluidine blue ) . Furthermore, they are more immune to chemical or photochemical debasement. They absorb visible radiation with the wave-length between 660-700nm, which verges on the infra-red terminal of the spectrum.
Practical usage of phthalocyanines was studied in connexion with decontamination of blood merchandises. The soaking up upper limit of phthalocyanines varies from that of red blood cells and therefore the hazard of their possible harm is low. Phthalocyanines besides showed promising consequences in photodynamic experimental therapy of malignant neoplastic diseases. Contrary to the first coevals of porphyrins, phthalocyanines were more effectual when tested on animate beings during anti-tumour therapy, and the happening of unwanted side effects was lower. Study of antimicrobic features of phthalocyanines in PACT is chiefly in the phase of in vitro experiments.
Exposure to seeable visible radiation of peculiar wave-length is necessary for the activation of light-sensitive substances. Most of the PS are activated by ruddy visible radiation in the scope of 630 – 700 nanometer. That corresponds to the incursion of the light beam to the tissue up to 0.5 – 1.5 centimeter, and, at the same clip, represents the bound for therapeutical consequence of PDT. Technically, it is possible to utilize assorted beginnings of visible radiation. However, optical masers and lamps with the possibility of higher visible radiation energy seem to be a better option. Lasers are able to bring forth monochromatic visible radiation beam and, harmonizing to the measure of the light energy, they are divided into high-power and low-power optical masers. It is known that while utilizing the high-power optical masers, deadly antimicrobic consequence as a consequence of the irradiation itself might happen. A strong antibacterial consequence while utilizing the Erbium: YAG optical maser for intervention of root canal was described ( 11 ) , but the necessity of maintaining the precise measure of applied light energy was emphasised. When the subliminal doses were used, there was no disinfectant consequence, nevertheless, higher doses of energy brought on unwanted complication in the signifier of structural harm to the root canal. For irradiation of light-sensitive substances by PACT, low-power optical masers are used ( it is of import to act upon merely the light-sensitive substance and therefore minimise the unwanted harm of the environing tissue ) . Photoinactivation of bacteriums is achieved by light doses in the scope of mW. Irradiation of bacteriums in the absence of PS has no influence on the viability of bacterial cells.
4. Applications of PACT
4. a. Target micro-organisms
The mark micro-organism for photoinactivation are chiefly bacteriums. The possibility of bacterial photoinactivation by PACT has been by experimentation proved many times – assorted chemical photosensitive substances have been used to different bacterial strains. ( Table 1. ) Significant difference in the effectivity of PACT was noted with regard to photoinactivation of Gram-positive and Gram-negative bacteriums. Gram-positive bacteriums were instead sensitive to photoinactivation ( 12-14 ) . The antibacterial consequence was achieved by photoinactivation mediated by light-sensitive substances with different chemical construction ( positively and negatively charged substances were effectual every bit good as impersonal 1s ) . Gram-negative bacteriums were normally immune to the action of negatively charged or impersonal photosensitizers ( 15-17 ) . This difference is being explained by different construction of the bacterial cell wall. There is a permeable outer peptidoglycan bed in Gram-positive bacteriums, which enables the incursion of light-sensitive substance to the cytoplasmic membrane, the mark of PACT. Thickening of the cell wall of MRSA ( methicilin-resistant Staphylococcus aureus ) or VRE ( vankomycin-resistant enterococci ) diminishes the incursion of antibiotics, antiseptics and germicides, nevertheless, the lessening in effectivity of PACT has non been proved so far ( 18 ) . The wall of Gram-negative bacteriums is more complex. There is an outer membrane composed of phospholipid double-layer with lipopolysaccharides above the dilutant peptidoglycan bed. The wall of Gram-negative bacteriums is usually merely small permeable for big and hydrophobic molecules. Negatively charged PS are non able to perforate the lipopolysaccharide but they can be partly effectual in higher concentration when more singlet O is generated in close propinquity to the bacterial cells. The sensitiveness of Gram-negative cells to photoinactivation is increased by utilizing PS with polycationic molecular construction – natural or unnaturally produced by adhering of positively charged functional groups ( they can advance a tight electrostatic interaction with the negatively charged sites on the outer surface of the bacterial cells ) . Cationic PS are able to demobilize wider scope of bacteriums species than impersonal or anionic PS. In micromolar concentration, they can take down the sum of bacteriums by 4-5 logs after incubation up to 5-10 proceedingss and irradiation of about 50mW/cm2 ( 19 ) . It is besides effectual against bacteriums immune to antibiotics and bacteriums strains turning in biofilm ( 20-22 ) . The sensitiveness of Gram-negative bacteriums to photoinactivation can besides be increased by adding substances such as EDTA ( ethylenediaminetetraacetic acid ) or polymyxin B, which increase the permeableness of the outer wall of Gram-negative bacteriums by let go ofing up to 50 % of lipopolysaccharide ( 23-25 ) . Nevertheless, some bacteriums ( e.g. Burholderia cepacia, Proteus Mirabilis ) are already of course immune to cationic constructions, which may diminish the consequence of inactivation mediated by positively charged PS. Furthermore, a survey published in 1988 describes the isolation of Salmonella enterica serovar Typhimurium, demoing a low grade of opposition to cationic constructions as lipopolysaccharide of this strain had by and large lower negative charge when compared to sensitive strains ( 26 ) . While analyzing PACT and different bacterial strains, it was found out that there is a heterogenous group of bacteriums, which is inactivated merely by irradiation by visible radiation of certain wave-length, even without the presence of light-sensitive substance. Thorough analysis of these bacterial strains proved that their radiosensitivity is given by the accretion of of course generated porphyrins inside the bacterial organic structure ; chiefly it concerned precursors generated at synthesis of hematoporphyrins. This group contains chiefly black-pigmented anaerobes Porphyromonas sp. and Prevotela sp. , in which porphyrins are accumulated, and therefore they are light-sensitive depending on the outer environment and its conditions. The ability to roll up porphyrins is for good proved in Propionibacterium acnes and Helicobacter pylori ( 27 ) . The cognition of bacteriums of course bring forthing photoactive porphyrins is still merely disconnected and the account of the causes of this phenomenon is still unknown.
The available beginnings show that during PACT wild every bit good as multi-resistant bacterial strains are killed with equal effectivity. This has been repeatedly proved in the MRSA and Pseudomonas aeruginosa ( 28-30 ) . The fact is that the opposition to photochemical devastation of bugs mediated by vest O and other reactive cytotoxic merchandises is extremely unlikely ( it is a natural mechanism of endocellular devastation ) and merely the theoretical possibility of its happening has been discussed. However, it was proved in vitro that bacteriums are able to chuck out the molecules of porphyrins derived functions by the outflow mechanism and, theoretically, to diminish their sensitiveness by take downing the permeableness of cell wall for PS ( 31 ) . The same mechanism is the footing of bacteriums opposition to antiseptics and antibiotics.
Other possible campaigners for PACT are infective Fungis, particularly Candida albicans ( 32,33 ) . Fungi are more immune to PACT due to larger cells and the presence of atomic membrane, which is another obstruction to incursion of PS to the mark construction. During the experiment, it was necessary to utilize higher concentrations and longer exposure of the PS, and higher doses of light than during the photoinactivation of bacteriums. Aspergillus fumigatus has besides been tested ( 34 ) .
Photoinactivation of viruses by PACT has been tested in some states to extinguish viruses from blood merchandises in particular fluorescent boxes. The new indicants for PACT include many types of viral tegument infections that are caused by the human villoma virus ( different sorts of wart ) and herpes simplex virus ( herpes simplex ) . In recent old ages, the possibility to photoinactivate parasitic Protozoa has been studied. Photoinactivation of promastigotes and amastigotes of Leishmania amazonensis was described in laboratory conditions ( 35 ) . Photoinactivation of Plasmodium falciparum and Acanthamoeba palestinensis was investigated, excessively ( 36, 37 ) .