A Comprehensive Review on Photodynamic Therapy (PDT) and Photothermal Therapy (PTT) for Cancer Treatment

Photodynamic Treatment (PDT)
Light has been used for therapy since the past 3000 years.[,] Ancient Egyptian, Indian, and Chinese cultures used light to cure numerous conditions, including psoriasis, rickets, vitiligo, and skin cancer.[] Niels Finsen invented PDT in the 19^th century. More than 100 years ago, scientists discovered that combining light and chemicals can induce cell death.[] PDT comprises two components, a PS and a light source (usually in the red spectral zone, as red light penetrates deeper into the tissue), for cancer diagnosis. The benefit of PDT is that it can be repeated multiple times without producing any immunosuppressive and myelosuppressive effects and that it can be administered even after radiotherapy, chemotherapy, or surgery.

An optimal PS agent should be a single pure compound that allows quality assurance research with low production costs and reasonable stability. An ideal PS agent should have a high absorption peak in the range of 600 to 800 nm (red to dark red) as photon absorption with a wavelength >800 nm does not have adequate energy to excite oxygen to its single state and achieve significant yield. For example, chlorines, bacteriochlorins, and phthalocyanine can provide improvement in tumor regulation. In addition, penetration of a dark-red light into the tissues with suitable wavelength agents helps in reducing toxicity and in rapid removal from healthy tissues, thereby decreasing the phototoxic side-effects.[] Since the delay between drug administration and light irradiation is usually long, the sensitizer provides ample time to disperse from the healthy tissues. It is hence proposed that the tumor response is often more reliable when light is delivered at a shorter intermediate druglight. Simultaneously, PS is already present in the arteries, which can result in significant vascular damage.[]

Some past studies have suggested that a marked inflammatory response and necrotic cell death after illumination is essential for the immune-stimulating role of PDT.[] On the other hand, it has been suggested that PSs that induce more apoptosis and that less inflammation is appropriate for applications such as for brain tumors, where swelling is undesirable. The first PS used for cancer therapy was a water-soluble porphyrin mixture called the hematoporphyrin derivative (HPD), a refined porfimer sodium form, which later came to be known as photofrin.[-] Some PSs have been used to treat cancer (Table 1).

Table 1: Photosensitizers used in PDT

Mechanism of Action
PS can deliver through various means, such as via topical and intravenous injections. However, the change in biodistribution over a period of time gets affected; another way to control the impact of PDT is the time of light exposure. The light absorption (photons), the sensitizer, is converted into a short-lived, excited single-state form from its ground state (a single state) to the long-lived electronically excited state (a triplet state). This new triple- state responds in two ways. First, it reacts directly to the substrate, such as the cell membrane or a molecule, and transfers the atom of hydrogen into radicals. When these radicals interact with oxygen, oxygenated products (type I reaction) are formed. Alternatively, the tripletstate form can directly transfer its energy to oxygen, thus converting the singlet oxygen into a highly ROS (type II reaction). While nearly all effects of PDT drugs are oxygen- dependent, photosensitization typically does not occur in the tissue"s anoxia region. Past in vivo studies have shown that induction by clamping of tissue hypoxia eliminates the porphyrin's PDT effects.[]

The types mentioned are formed through specific mechanisms I and II, which are performed simultaneously,[] which in turn depends on the type of sensitizer used and the substrate and oxygen concentration. These factors rely on the substrate sensitizer"s binding affinity. Types I and II reactions occur simultaneously. The ratio between Types I and II reactions depends on the type of sensitizer, the substrate, the oxygen concentration, and the binding affinity of the substrate sensitizer. PDT influenced only the cells near the ROS production site (such as the PS location sites), owing to the short half-life and the high reactivity of ROS.

Single oxygen has a half-life of <0.04 µs in the biological systems. Thus, the action radius of a single oxygen is <0.02 µm of photo-damage, and the cytotoxicity is multifactorial and linked to the sensitizer size, its extracellular or intracellular position, total administered dosage, the total dose of light exposure, light fluidity, availability of oxygen, and the time between the drug administration. These factors are all interrelated. The PDT¹⁴ mechanism is summarized in Figures 1 and 2.

Fig 1: Mechanism involved in PDT.

Fig 2: Type I and type II reaction in PDT.

Effect of PDT on Tumor
PDT mediates tumor destruction in 3 main ways. First, PDT-generated ROS directly destroy tumor cells and damage the tumor-associated vasculature, leading to tumor infarction. PDT eventually activates the immune response to tumor cells. The three forms can also impact each other. The long-term tumor control involves a combination of all these components.[]

The German scientist Friedrich Meyer-Betz became the first one to treat humans with porphyrins in 1913. He tested his skin for the symptoms of applying 200 mg hematoporphyrin. In the skin areas exposed to light, swelling and pain were recorded.[] In 1975, Thomas Dougherty and colleagues reported that the administration of the derivative hematoporphyrin and red light in mice led to total breast tumor destruction. In the same year, J. F. Kelly and colleagues reported that hematoporphyrin light activation could abolish bladder cancer in mice.[,]

Limitations of PDT
PDT only treats the area where the light source is accessible, hence this treatment is mainly suitable for the lining organs, where the light source can reach, considering that light cannot travel very far through the body tissues. PDT cannot be used to identify large cancers and cancers that spread to the majority of locations. The types of PS used in PDT remain in the body for longer, which makes the patient more sensitive to light for a short while. Hence, caution should be undertaken after the PS has been inserted into the body.[-]

Phototherapy Therapy (PTT)
Since the 18^th century, thermal treatments for cancer cells were known. Hyperthermia is the elevation of temperature above the physiological levels, typically to values of 40?45°C. The main goal of hyperthermia is to create an environment that facilitates eradication of tumors and spares the normal tissues involved in cancer treatment. Hyperthermia achieves this by instigating direct cytotoxic effects and physiologic effects. Clinically, hyperthermia can function synergistically with both radiation and chemotherapy. Cancer cells get subjected to permanent damage during hyperthermia as a result of degradation of the cell membrane and protein denaturation. However, this therapy often affects the normal tissues. Incorporating laser radiation treatment into cancer therapy can facilitate applicability of photothermal treatment for more selective cancer treatment.

As a consequence, laser-induced hyperthermia seems helpful for the treatment of retinal or choroidal tumors. The significant downside of this treatment is the need for a high-power laser to destroy the tumor cells. Meanwhile, a PTT with a photothermal agent has been proposed selectively for heating. A biocompatible photothermal agent with a high absorption coefficient, an NIR light source, and an NIR region is the primary requirement for PTT. The temperature increase in the PTT depends on the absorption of the NIR wavelength and the light-excitation coefficient. PTT alone or in combination may kill cancer cells in either the primary tumor or in tumor at the early metastasis stage. However, with the following methods, PTT offers an essential advantage in reducing metastasis in several cancer types:

1. Direct removal of cancer cells with PTT: NIR laser can penetrate soft tissues up to 2 cm. PTT on NIR-laser irradiation causes ablation of the primary tumors or lymph node metastasis;[-] it may also damage or destroy primary tumor cancer cells. It also kills the cells that trigger tumors and stem cells responsible for causing cancer, thereby preventing their metastasis to another organ.[-]

2. Imaging Guide: The image guidance offers details for an improved therapeutic regimen with PTT for the safety and efficacy of photothermal ablation. [] Multimodal imaging (such as X-rays, computed tomography [CT], photoacoustic imaging, and magnetic resonance imaging) may be applied to assess the position of metastasis of the lymph node and to refine the therapeutic regimens throughout PTT. The current technique is limited to mapping lymph node metastasis, which is unusual in metastasis imaging of the brain, liver, and lung deep tissues. X-ray CT and photoacoustic imaging include image-guided PTT metastasis of cancer.[-] Using nanorods of bismuth sulfide (B2S3), Zhao et al. recently developed a theranostic framework for multimodal imaging-guided cancer metastasis. Such nanorods can be used to track their real-time distribution in the tumor sites for CT contrast agents for angiography and organic imaging. For instance, Bi2S3 nanorods can ablate the primary tumor with a Near infrared (NIR laser and thereby prevent further lung metastases. Li et al. developed copper-labeled copper sulfide nanoparticles and used it in combination with PTT for metastatic breast cancer radiotherapy. The use of PTT in radiotherapy prevents tumor development and increases the survival rate of the treated patients.[]

3. Combination of PTT in chemotherapy: Chemotherapy is commonly used to treat cancer with metastasis.[] Recently, scientists observed the synergistic effects in cancer metastasis, with the application of combined PTT and chemotherapy. Gold nanostructure and doxorubicin (DOX) have been used in chemotherapy, mainly for treatment of cancer . DOX used in chemotherapy as an anticancer drug and gold nanoclusters or nanorods as the photothermal agent in PTT, provide a synergistic combination therapy. Recently, gold nanorods primed for combination therapy in metastatic breast cancer were wrapped with DOX-loaded DNA after NIR radiation, this combination therapy with gold nanoparticles and doxorubicin nanoparticles significantly inhibited the growth of breast tumor and lung metastasis.[] Moreover, DOX-powered mesoporous magnetic gold nanoclusters for the combination of PTT in breast cancer metastasis chemotherapy were prepared by Qian et al. Nanoclusters can be effectively applied to target tumor sites in breast cancer model 4T1 by using an extramagnetic field. The use of combination therapy effectively prevents pulmonary and mediastinal metastases, which contributes to animal survival.[]

4. Gold nanostructure as photothermal agents in cancer treatment: Gold nanoparticles have been used owing to their simplicity in preparation, bioconjugation, nontoxicity, and inert nature.[] Gold nanoparticles are useful as imaging agents, heat-absorbents, and therapeutic agents. Gold hyperthermic- based nanoparticles have shown promising outcomes in animal research, and the study on their applicability in early clinical trials are underway. Gold nanoparticles possess unique optical properties that are useful in photothermal and ultrasensitive detection. As light falls on gold nanoparticles at a particular wavelength, the conduction band on the surface of the gold nanoparticles oscillates with one another, producing a phenomenon known as the surface plasmon resonance. This phenomenon heats the light, which is emitted by gold nanoparticle. The wavelength of light at which particles disperse and light energy is absorbed depends on the shape, size, and composition of nanoparticles. Changing the size and shape of gold nanoparticles can also alter the peak, which is tunable in the NIR region, which penetrates the tissues more effectively than other light regions.[] Gold nanostructure diagram is depicted in Figure 3.

Fig 3: Gold nanosphere, nanoshell, nanorod, nanocage.

5. Advantages of gold nanoparticles in the treatment of cancer: It can be administered in specific areas so that the chances of nonspecific distribution is reduced. It can penetrate deep into the biological tissues. By creating gold nanoparticles, it enables the delivery of drugs through passive transportation (i.e., it improves permeation and retention effect) and is safe to excrete via the urinary system.[]

Gold Nanospheres
It is possible to synthesize gold nanosphere of size 2-100 nm (in diameter) via reduction of HAuCl4 using specific-reducing agents. Citrate is commonly used as a reductive agent. The size of the nanosphere can be modified by adding citrate/gold in 1:1 ratio. Several methods have been investigated for the synthesis of gold nanoparticles using various reducing agents.[] Pitsilides et al. used light-absorbing microparticles and nanoparticles for the treatment of cancer cells, including iron oxide microparticles and gold nanospheres. In the presence of gold nanospheres, the radiation of lymphocytes with laser (20 ns) increases the plasma membrane permeability, which results in cell death.[]

Gold Nanorods
Gold nanorods are synthesized using a template method based on the electrochemical gold deposition in nanoporous polycarbonate or alumina template membrane pores.[] The nanorod length is managed by depositing gold in the membrane pores. The downside of this approach is that it generates only a small amount of nanorods. Narrow absorption band and a higher two-photon luminescence than nanospheres and nanoshells are the characteristics of gold nanorods. Hence, two-photon luminescence method provides a reliable, label-free approach for three-dimensional (3D) cancer diagnostics.[]

Gold Nanoshell
Gold nanoshell consists of an inner layer of silica and an outer layer of gold. Gold nanoshells are prepared by producing in situ gold nanoparticles with the coreshell cells acting as thermo-sensitive templates.[] Silica cores are prepared from ethanol by reducing tetraethyl orthosilicate. The plating of silica nanoparticles in gold is achieved in an aqueous environment using the method of seed production. This tiny nanosphere is connected to the silica core and is used as an aminefinished silane liner molecule, which allows the extra gold to be reduced to a full shell. The diameter of the gold nanoshell depends on the silica core diameter, and its thickness can be regulated by the amount of gold accumulated on the core surface.[] West and colleagues used gold nanoshells to operate on PTT in both dark-field imaging and in the treatment of HER-2-positive breast cancer cells, SKBr3. The researchers conjugated gold nanoshells with the antibody. Nanoshells with a laser NIR at 80 mW/cm2 destroyed the targeted tumor cells for 7 min while the cells without nanoshells remained unaffected. In the region of laser exposure, cell damage was restricted, indicating a high localized thermal effect.[]

Gold Nanocages
Truncated silver nanotubes and aqueous HAuCl4 are used as galvanic substitutes to produce gold nanocage. With regulated morphologies, silver nanostructures can be formed via polyol reduction, where ethylene glycol reduces AgNO3 to form silver atoms, and then nanocrystals or seeds. The combination of silver atom and seed produces nanostructure by manipulating the crystalline structures of silver seed in the presence of vinylpyrrolidone, a polymer that can selectively bind to the surface. Silver nanostructures can be used as a sacrificial template often converted by galvanic substitution reaction into gold nanostructures with hollow interiors. The wall thickness and the size of the gold nanocages can be controlled by changing the molar ratio of silver to HAuCl₄.[]

Xia and colleagues used PTT gold nanocaps to treat breast cancer. For targeting purposes, an average edge length of 65±7 nm and an overall absorption target of 800 nm of gold nanocage was combined with a monoclonal antibody (Anti-HER2). Flow cytometry was used to measure the number of gold nanocages immobilized per cell and the photothermal effect. Laser irradiation parameters (such as pulsed NIR laser), including optimum nanocage dose and laser power density and irradiation time, were calculated.[] These gold nanoparticles showed excellent success in treating cancer. Table 2 indicates the differences between the features of PDT and PTT.

Table 2: Difference between Photodynamic and Photothermal treatment

Selection of PDT and PTT for Cancer Type
As PDT is mainly based on drugs that makes cells light sensitive by producing reactive oxygen which further kills cancer cells. Recent studies showed that, PDT and PTT has diversified clinical wide spread applications for treatment of skin, head and neck cancer and also found very much useful in esophageal cancer which much useful compared to he reported treatments.

Moreover, through some studies are undertaken on PTT, the results of clinical trials are not promising compared to PDT. So, PDT treatment against cancer is considered as the best option till date.

Related Research Phototheranostic Therapy
Phototheranostics means simultaneous diagnosis and phototherapy by using light. In this therapy method, therapy phototheranostic agent is used for diagnostic imaging as well as for killing diseased cells. In this treatment phototheranostic agents upon systemic administration, targets the disease site where it shows illumination that helps to image tissue.

Irradiating light further shows activation of phototheranostic agent which can kills targeting tissues (e.g., Tumor).

Hyperthermia
Hyperthermia is used in treatment of cancer where tissues are exposed to higher temperature. When other cancer therapies combine with hyperthermia it shows synergistic effect. Like In combination of radiation, Immunotherapy, PDT, PTT with hyperthermia. Rational behind this is hyperthermia increases blood flow to the affected area which doubles the perfusion rate and improves delivery of chemotherapeutics/ Phototheranostic agent. It also increases oxygen supply to the cancer cells and thus increases chances of more damage by radiation therapy. In magnetic hyperthermia, magnetic nanoparticles show transformation of electromagnetic energy from an external high-frequency field to heat.