Combined photothermal and photodynamic therapy

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Photodynamic/photothermal combination therapy involves the usage of a chemical compound or nanomaterial that, when irradiated at a certain wavelength, converts light energy into reactive oxygen species (ROS) and heat. This has shown to be highly effective in the treatment of skin infections, showing increased wound healing rates and a lower impact on human cell viability than photodynamic (PD) or photothermal (PT) therapies. The compounds involved often employ additional mechanisms of action or side effect reduction mechanisms, further increasing their efficacy. [1] [2] [3]

Contents

Phototherapies are minimally invasive, with the primary toxicity issues surrounding phototoxicity and the nonspecific ROS and heat mechanisms of action affecting healthy human cells (albeit in lower amounts than the target cells). In skin wound infections, multiple phototherapeutic approaches have observed increased rates of wound closure over nontreated controls. This is typically due to an upregulation of vascular endothelial growth factor (VEGF) and hypoxia-inducible factor (HIF). Phototherapies are also active against both gram-positive and gram-negative bacteria, with photodynamic therapy having some exceptions. [4]

To apply this technique, a photosensitizer is localized to the wound or tumor site, either topically or intravenously. Once localized, the target area is exposed to a laser of a selected wavelength and intensity for a predetermined irradiation time. The wavelength, localization technique, laser intensity, and irradiation period are determined based on the individual phototherapeutic agent, as these factors can vary greatly from compound to compound. Topical applications may be through the incorporation of the phototherapeutic agent with a hydrogel that will slowly leech the compound into the wound, allowing for a more controlled production of ROS and/or heat. [5] [6]

Phototherapy Types

Photodynamic Therapy

Photosensitizers and approved treatments

A photosensitizer is a chemical compound or nanomaterial capable of capturing light energy and using this energy to generate ROS. Currently, there are 6 photosensitizers that are clinically-approved or undergoing clinical trials for the treatment of cancers and 1 approved for the treatment of eye disorders and diseases. [7] Photodynamic therapy (PDT) is also often used for acne treatment as well as various dermatological conditions such as psoriasis, atopic dermatitis, and vitiligo. [8] It is highly unlikely that bacteria would gain resistance to a photosensitizer or PDT treatment, as the photosensitizers can generate ROS within or outside of the target cell, both of which damage the membrane [9]

Mechanism of action

A photosensitizer generates ROS through one of two processes. Type I involves a redox reaction that results in the creation of superoxides (O2), hydroxyl radicals (OH•), and radical peroxides, whereas Type II generates singlet oxygen directly through an electron transfer from the photosensitizer. [5] These ROS go on to nonspecifically damage a variety of cellular components, including proteins, DNA, and lipids as they seek to remove the radical.

Limitations

Due to the necessity of oxygen for PDT, these treatments do not work as well in hypoxic environments, including in developed tumors and some deep wounds. Dental infections tend to also respond better to photothermal therapy than photodynamic therapy, though both have a strong effect. [10] [11] [12] The efficacy of PDT for antimicrobial usage is limited by the properties of the membrane of the target cell such as the electrical gradient (membrane potential) and lipid composition. Whereas high cell death is observed for Escherichia coli and Staphylococcus aureus , other bacterial species such as Klebsiella pneumoniae and Acinetobacter baumannii tend to see very low impact from PDT due to these factors. [4] This limits potential as a broadband antibiotic, but may also allow for specificity in targeting the pathogenic cells over human and skin microbiome cells.

Photothermal Therapy

Indocyanine green is an FDA-approved photothermal agent that is primarily used in imaging techniques, but also displays anticancer and antimicrobial activity through photothermal therapy (PTT) treatments. [11] Photothermal agents are active against diseased cells by accumulating in or around target cells, then converting light energy directly to heat, killing the target through heat-related damage.

PTT has a low level of selectivity beyond the accumulation stage, in which it tends to preferentially accumulate within diseased and bacterial cells. This increases broadband antibiotic activity and decreases the likelihood of resistance development, but also raises the impact on human cells. Human cells experience irreversible damage in the range of 46-60 °C, which is below temperatures reached by some photothermal agents during photothermal therapy. [2] [13] Human cell viability may be maintained through low temperature PTT (≤ 45 °C), which is typically only possible in combination with an additional antibiotic or photodynamic activity. [14]

Combination Therapy - Antibacterial

Photodynamic/photothermal combination therapy combines the mechanisms of ROS production and heat generation into one treatment for a heightened effect on the target bacterial cells. In many cases, this can be done with a single compound or nanomaterial (phototherapeutic agent) and wavelength.

Advantages over monotherapy

Increased antibiotic efficacy

Due to the presence of both ROS and excess heat, target cells are less able to resist each effect. Increased heat corresponds to heightened cell membrane permeability, [15] allowing the generation of ROS within the target cell. This also removes/reduces the selectivity observed for PDT, as it is able to enter the cell unhindered.

Lower side effects

Both photosensitizers and photothermal agents have some degree of selectivity for target cells over healthy human cells, but in utilizing both of these mechanisms this selectivity is bolstered. Increased antibiotic efficacy indicates a lower likelihood of requiring follow-up treatments, so the damage is minimal. In addition, some of these combination phototherapeutic agents have antioxidant/reactive oxygen scavenging properties, reducing the amount of collateral damage sustained by the surrounding human cells. [3] [6]

Incorporation of tertiary mechanisms

Many phototherapeutic agents that display both PD and PT activity come with added effects, such as antibiotic metal ions, [2] [13] [14] [16] [17] physical antibiotic mechanisms, [1] or peroxidase-like activity. [18] These added effects further increase antibiotic activity, often demonstrating broadband activity with 99% cell death or above regardless of strain or drug resistance.

Related Research Articles

<span class="mw-page-title-main">Antibiotic</span> Antimicrobial substance active against bacteria

An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of such infections. They may either kill or inhibit the growth of bacteria. A limited number of antibiotics also possess antiprotozoal activity. Antibiotics are not effective against viruses such as the ones which cause the common cold or influenza; drugs which inhibit growth of viruses are termed antiviral drugs or antivirals rather than antibiotics. They are also not effective against fungi; drugs which inhibit growth of fungi are called antifungal drugs.

<span class="mw-page-title-main">Photodynamic therapy</span> Form of phototherapy

Photodynamic therapy (PDT) is a form of phototherapy involving light and a photosensitizing chemical substance used in conjunction with molecular oxygen to elicit cell death (phototoxicity).

<span class="mw-page-title-main">Aminolevulinic acid</span> Endogenous non-proteinogenic amino acid

δ-Aminolevulinic acid, an endogenous non-proteinogenic amino acid, is the first compound in the porphyrin synthesis pathway, the pathway that leads to heme in mammals, as well as chlorophyll in plants.

<span class="mw-page-title-main">Light therapy</span> Therapy involving intentional exposure to sunlight

Light therapy, also called phototherapy or bright light therapy is the exposure to direct sunlight or artificial light at controlled wavelengths in order to treat a variety of medical disorders, including seasonal affective disorder (SAD), circadian rhythm sleep-wake disorders, cancers, and skin wound infections. Treating skin conditions such as neurodermatitis, psoriasis, acne vulgaris, and eczema with ultraviolet light is called ultraviolet light therapy.

<span class="mw-page-title-main">Photosensitizer</span> Type of molecule reacting to light

Photosensitizers are light absorbers that alter the course of a photochemical reaction. They usually are catalysts. They can function by many mechanisms, sometimes they donate an electron to the substrate, sometimes they abstract a hydrogen atom from the substrate. At the end of this process, the photosensitizer returns to its ground state, where it remains chemically intact, poised to absorb more light. One branch of chemistry which frequently utilizes photosensitizers is polymer chemistry, using photosensitizers in reactions such as photopolymerization, photocrosslinking, and photodegradation. Photosensitizers are also used to generate prolonged excited electronic states in organic molecules with uses in photocatalysis, photon upconversion and photodynamic therapy. Generally, photosensitizers absorb electromagnetic radiation consisting of infrared radiation, visible light radiation, and ultraviolet radiation and transfer absorbed energy into neighboring molecules. This absorption of light is made possible by photosensitizers' large de-localized π-systems, which lowers the energy of HOMO and LUMO orbitals to promote photoexcitation. While many photosensitizers are organic or organometallic compounds, there are also examples of using semiconductor quantum dots as photosensitizers.

Photomedicine is an interdisciplinary branch of medicine that involves the study and application of light with respect to health and disease. Photomedicine may be related to the practice of various fields of medicine including dermatology, surgery, interventional radiology, optical diagnostics, cardiology, circadian rhythm sleep disorders and oncology.

<span class="mw-page-title-main">Porfimer sodium</span> Pharmaceutical drug

Porfimer sodium, sold as Photofrin, is a photosensitizer used in photodynamic therapy and radiation therapy and for palliative treatment of obstructing endobronchial non-small cell lung carcinoma and obstructing esophageal cancer.

<span class="mw-page-title-main">Artificial enzyme</span>

An artificial enzyme is a synthetic organic molecule or ion that recreates one or more functions of an enzyme. It seeks to deliver catalysis at rates and selectivity observed in naturally occurring enzymes.

Photothermal therapy (PTT) refers to efforts to use electromagnetic radiation for the treatment of various medical conditions, including cancer. This approach is an extension of photodynamic therapy, in which a photosensitizer is excited with specific band light. This activation brings the sensitizer to an excited state where it then releases vibrational energy (heat), which is what kills the targeted cells.

<span class="mw-page-title-main">Photooxygenation</span> Light-induced oxidation reaction

A photooxygenation is a light-induced oxidation reaction in which molecular oxygen is incorporated into the product(s). Initial research interest in photooxygenation reactions arose from Oscar Raab's observations in 1900 that the combination of light, oxygen and photosensitizers is highly toxic to cells. Early studies of photooxygenation focused on oxidative damage to DNA and amino acids, but recent research has led to the application of photooxygenation in organic synthesis and photodynamic therapy.

Photoimmunotherapy (PIT) is an oncological treatment that combines photodynamic therapy of tumor with immunotherapy treatment. Combining photodynamic therapy with immunotherapy enhances the immunostimulating response and has synergistic effects for metastatic cancer treatment.

<span class="mw-page-title-main">Sonodynamic therapy</span>

Sonodynamic therapy (SDT) is a noninvasive treatment, often used for tumor irradiation, that utilizes a sonosensitizer and the deep penetration of ultrasound to treat lesions of varying depths by reducing target cell number and preventing future tumor growth. Many existing cancer treatment strategies cause systemic toxicity or cannot penetrate tissue deep enough to reach the entire tumor; however, emerging ultrasound stimulated therapies could offer an alternative to these treatments with their increased efficiency, greater penetration depth, and reduced side effects. Sonodynamic therapy could be used to treat cancers and other diseases, such as atherosclerosis, and diminish the risk associated with other treatment strategies since it induces cytotoxic effects only when externally stimulated by ultrasound and only at the cancerous region, as opposed to the systemic administration of chemotherapy drugs.

<span class="mw-page-title-main">Gold nanoparticles in chemotherapy</span> Drug delivery technique using gold nanoparticles as vectors

Gold nanoparticles in chemotherapy and radiotherapy is the use of colloidal gold in therapeutic treatments, often for cancer or arthritis. Gold nanoparticle technology shows promise in the advancement of cancer treatments. Some of the properties that gold nanoparticles possess, such as small size, non-toxicity and non-immunogenicity make these molecules useful candidates for targeted drug delivery systems. With tumor-targeting delivery vectors becoming smaller, the ability to by-pass the natural barriers and obstacles of the body becomes more probable. To increase specificity and likelihood of drug delivery, tumor specific ligands may be grafted onto the particles along with the chemotherapeutic drug molecules, to allow these molecules to circulate throughout the tumor without being redistributed into the body.

Antibiotic synergy is one of three responses possible when two or more antibiotics are used simultaneously to treat an infection. In the synergistic response, the applied antibiotics work together to produce an effect more potent than if each antibiotic were applied singly. Compare to the additive effect, where the potency of an antibiotic combination is roughly equal to the combined potencies of each antibiotic singly, and antagonistic effect, where the potency of the combination is less than the combined potencies of each antibiotic.

This is a historical timeline of the development and progress of cancer treatments, which includes time of discovery, progress, and approval of the treatments.

Hydrogels are three-dimensional networks consisting of chemically or physically cross-linked hydrophilic polymers. The insoluble hydrophilic structures absorb polar wound exudates and allow oxygen diffusion at the wound bed to accelerate healing. Hydrogel dressings can be designed to prevent bacterial infection, retain moisture, promote optimum adhesion to tissues, and satisfy the basic requirements of biocompatibility. Hydrogel dressings can also be designed to respond to changes in the microenvironment at the wound bed. Hydrogel dressings should promote an appropriate microenvironment for angiogenesis, recruitment of fibroblasts, and cellular proliferation.

Pullulan bioconjugates are systems that use pullulan as a scaffold to attach biological materials to, such as drugs. These systems can be used to enhance the delivery of drugs to specific environments or the mechanism of delivery. These systems can be used in order to deliver drugs in response to stimuli, create a more controlled and sustained release, and provide a more targeted delivery of certain drugs.

Ultrasound-triggered drug delivery using stimuli-responsive hydrogels refers to the process of using ultrasound energy for inducing drug release from hydrogels that are sensitive to acoustic stimuli. This method of approach is one of many stimuli-responsive drug delivery-based systems that has gained traction in recent years due to its demonstration of localization and specificity of disease treatment. Although recent developments in this field highlight its potential in treating certain diseases such as COVID-19, there remain many major challenges that need to be addressed and overcome before more related biomedical applications are clinically translated into standard of care.

<span class="mw-page-title-main">Ligand-targeted liposome</span> Ligand-targeted liposomes for use in medical applications

A ligand-targeted liposome (LTL) is a nanocarrier with specific ligands attached to its surface to enhance localization for targeted drug delivery. The targeting ability of LTLs enhances cellular localization and uptake of these liposomes for therapeutic or diagnostic purposes. LTLs have the potential to enhance drug delivery by decreasing peripheral systemic toxicity, increasing in vivo drug stability, enhancing cellular uptake, and increasing efficiency for chemotherapeutics and other applications. Liposomes are beneficial in therapeutic manufacturing because of low batch-to-batch variability, easy synthesis, favorable scalability, and strong biocompatibility. Ligand-targeting technology enhances liposomes by adding targeting properties for directed drug delivery.

Antimicrobial photodynamic therapy (aPDT), also referred to as photodynamic inactivation (PDI), photodisinfection (PD), or photodynamic antimicrobial chemotherapy (PACT), is a photochemical antimicrobial method that has been studied for over a century. Supported by in vitro,in vivo and clinical studies, aPDT offers a treatment option for broad-spectrum infections, particularly in the context of rising antimicrobial resistance. Its multi-target mode of action allows aPDT to be a viable therapeutic strategy against drug-resistant microorganisms. The procedure involves the application of photosensitizing compounds, also called photoantimicrobials, which, upon activation by light, generate reactive oxygen species (ROS). These ROS lead to the oxidation of cellular components of a wide array of microbes, including pathogenic bacteria, fungi, protozoa, algae, and viruses.

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