CHAPTER 1: The Journey of PDT Throughout History: PDT from Pharos to Present

The journey of photodynamic therapy (PDT) throughout history and its endless development could be simply represented to the reader through what has been previously cited in the literature of natural science publishing as Abdel-Kader’s PDT Tree.¹  As is well known, a tree consists of three main parts: roots, trunk and branches. For the origins of PDT, as represented by the roots of the tree, this treatment was originally known as heliotherapy, and its use dates back to ancient civilizations such as Egypt, China, India, Greece and Rome. The development of PDT is symbolized by the tree trunk, and PDT reappeared with Arnold Rikli with the significant outcome of the first approved PDT drug in 1999 by the Food and Drug Administration (FDA).²  The PDT process consists of three key components: a photosensitizer (PS), a light source and tissue oxygen. The crown at the top of the tree that consists of outgrowing branches portrays the combination of these three components that results in fruitful oncological and non-oncological treatment applications. PDT has been recognized and used for many years, but the approach has only recently been applied widely. Throughout the representation of the PDT journey, in this chapter, we will trace the three parts of the PDT tree in detail, starting from the PDT origins in different civilizations, progressing to the stages of development and moving through to its current status (Figure 1.1).

Light has been utilized in the treatment of many physical and mental illness since antiquity, when such treatment was classically known as heliotherapy.^3,4  Ancient cultures worshiped the sun and believed it was a health-bringing deity with the power to heal many diseases. All over the world, evidence has been found of cults worshipping sun gods. Phototherapy began in ancient civilizations such as Egypt, China, Greece, India and Rome, but disappeared for many centuries, and it was not until the early 20th century that this form of therapy re-emerged and was rediscovered by western civilization through Arnold Rikli, Oscar Raab, Niels Finsen and Herman von Tappeiner.⁵ 

In ancient civilizations, sunlight was used in the treatment of various diseases such as vitiligo, psoriasis, rickets, skin cancer and psychosis. The utilization of sunlight as a therapeutic agent was introduced in ancient civilizations such as Egypt and Greece 3000 years ago. These civilizations practiced various forms of heliotherapy in which patients had their total body exposed to the sun in specially set-aside areas (Figure 1.2).⁵ 

In Ancient Egypt, where sunlight as a treatment was well known, the Ebres Papyrus, dating from 1550 BC (the oldest maintained medical documents), mentioned the Pharaoh’s utilization of phototherapy, using plants such as Ammi majus, parsnip, parsley and Saint John’s wort to make a powder that was applied on depigmented lesions. The extracts of the Ammi majus plant were used by the Ancient Egyptians to treat diseases such as vitiligo.³  The Pharaohs used to build temples without roofs and have their bodies exposed to sunlight in order to benefit from its healing rays. These temples were dedicated to the light god, Aton.⁴ 

Moreover, Indian medical literature dating to 1500 BC describes a treatment combining herbs with natural sunlight in order to treat non-pigmented skin areas.⁶  In one of India’s sacred books, Atharava-Veda (1400 BC), patients suffering from vitiligo were given certain plant extracts of the Bavachee plant, Psoralea corylifolia, and were asked to stand in sunlight for some time.³ 

In Ancient China, what has been termed heliotherapy was one of the immortalizing techniques of early Daoism, introduced by Lingyan Tzu-Ming in the first century AD during the Han dynasty.⁴  One technique, described approximately four centuries later during the Tang dynasty, was to stand in the early morning sunshine, holding in the right hand a piece of green paper on which the character for the sun was written in red, circumscribed by a red rectangle. At the end of the ritual, the insolated paper was shredded in water and consumed in an attempt to trap some of the essence of the sun in the body. Even though sunbathing and heliotherapy were early phototherapy modalities, the scientific basics were not laid out until the end of the Ming and beginning of the Qing dynasty in the 17th and 18th centuries.⁴  Buddhist literature from approximately 200 AD and 10th century Chinese documents made similar references regarding treating non-pigmented skin areas by combining herbs with sunlight.⁶ 

The Greek civilization was also one of the great civilizations that believed in the power of sunlight. The name “heliotherapy” was first used in the 2nd century BC by the Greek doctor Hippocrates, who was also called the “father” of medical science. Hippocrates taught the value of sun exposure in the restoration of health. Hippocrates introduced the benefits and healing powers of sunlight from his journeys to Egypt, where sunlight treatments were well known.^2–4  The Greeks had a famous city called Heliopolis (city of the sun) that was well known for its healing temples and light rooms containing windows covered with specially dyed cloths.⁵ 

The Romans continued utilizing light therapy, especially for the purposes of skin treatments. In Roman baths (therms), famous throughout history, it was also possible to sunbathe in a solarium.^2–4  Ancient Rome was the first civilization to treat acne with baths. In the times of the Roman Empire, it was thought that pores could be cleared and cleansed by mixing sulfur in mineral baths. Romans believed that this type of cleansing reduced the amount of bacteria affecting the skin and causing acne.⁷  However, with the decline of the Roman Empire and the rise of Christianity, heliotherapy disappeared.⁴ 

Later, in the 13th century, Ibn Al-Bitar stated in his book, Mofradat Al Adwiya (Terminologies of Pharmaceuticals), the treatment of vitiligo with a solution of honey and powdered Aatrillal seeds (that was later classified as Ammi majus). Administration of this tincture was both topical and oral, followed by exposure to direct sunlight for 1–2 hours.⁸ 

As previously mentioned, the stages of PDT development are illustrated by the tree trunk. It was not until the late 1800s and early 1900s that phototherapy started thriving once more. The Swiss doctor Arnold Rikli (1823–1906) is considered the pioneer of modern phototherapy. His famous quote was, “Water is good; air is better and light is best of all”.⁵  Rikli was the first in the 19th century to introduce sunbathing as a treatment for chronic diseases and functional disorders.⁹  One of his first great accomplishments was the foundation of a National Medicine Institute in 1855 in Bled, Slovenia.¹⁰  Rikli played a key role in rediscovering the positive effects of sunlight that had been forgotten for hundreds of years. He developed therapeutic guidelines and ideas that are still applicable today. For his great accomplishments and consistent work for 50 years, the name of Arnold Rikli became an international award dedicated to all of the scientific disciplines represented in photobiology that focus on the effects of optical radiation on humans.⁵ 

The PDT concept of using a dye as a PS in the photodynamic process was initiated by Oscar Raab, who was the first to examine photosensitized reactions in a scientific way in 1898. He examined the effects of light and dyes on paramecia. He noted that the toxic effect of acridine dye on paramecia was minimal on days during which there was a thunderstorm in comparison to its efficacy on normal days. From this observation, he concluded that light, in some way, activates the acridine dye to kill paramecia. He hypothesized that acridine dye converts light into a form of active chemical energy, which was a finding that formed the basis of PDT.³ 

Furthermore, phototherapy was developed into a science and recognized by the Danish physician Niels Finsen, a main pioneer of modern phototherapy who opened a new avenue for medical science, given that he was the first to use carbon arc phototherapy for lupus vulgaris, and was awarded the Nobel Prize for his contribution to the treatment of diseases in 1903.⁴  He began recording the effects of sunlight on insects and amphibians. One of his great findings was discovering how ultraviolet (UV) light from the sun or from electric light could kill bacteria. He then proved the beneficial effects of UV rays on the human body.⁴  Moreover, he published several papers in 1893 and 1894 on the beneficial uses of phototherapy. He stated in his publication the effect of red light in the treatment of smallpox, which prevented suppuration of pustules. The Finsen Light Institute was established in Copenhagen in April 1896 and still exists today.⁶  In his institute, he had a sun garden where he treated patients with lupus vulgaris through sunbathing.¹¹ 

During Finsen’s era, both X-rays and gamma rays were discovered by the German physicist Wilhelm Rontgen (1845–1923) and the French physicist Antoine-Henri Becquerel (1852–1908). With Finsen’s achievements in the field of phototherapy, the idea of radiotherapy (RT) was introduced. Since Finsen’s time, X-rays and gamma rays have been frequently used for the diagnosis and treatment of diseases.⁵ 

As an encouraging step towards apply phototherapy treatments in hospitals at that time, Princess Alexandra, wife of the future Edward VII, encouraged doctors to apply the treatment at the London Hospital, of which she was president.⁵ 

Von Tappeiner, one of the pioneers of photobiology, took over Raab’s research regarding fluorescent materials as therapeutic agents in dermatology.⁴  In 1905, von Tappeiner, in coordination with a dermatologist named Jesionek, published clinical data using PSs in the treatment of skin cancer, lupus of the skin and condylomata of female genitalia using different dyes such as eosin, fluorescein, sodium dichloroanthracene disulfonate and Grubler’s Magdalene red. Moreover, in 1905, von Tappeiner and Jesionek investigated the effects of the PS eosin on facial basal cell carcinoma after long-term exposure either to sunlight or arc-lamp light, causing tumor resolution and a 12-month relapse-free period in two-thirds of the patients. With more studies on PDT, von Tappeiner and Jodlbauer reported in 1904 that the presence of oxygen was a must for photosensitization. Therefore, he was the first to come up with the term “photodynamic therapy” (Figure 1.3).¹² 

The discovery and development of hematoporphyrin (HP) is considered the most important event in the progress of PDT. It was produced in an impure form by Scherer in 1841.⁶  In 1871, Hoppe-Seyler gave it its name. In the period of 1908–1913, the photodynamic properties of HP were studied on paramecia, erythrocytes, mice, guinea pigs and humans after exposure to sunlight.⁶  Friedrich Meyer Betz was the first to study the phototoxic effects of HP in 1912. He self-administered 200 mg intravenously then exposed himself to sunlight and suffered from edema and hyperpigmentation for more than 2 months.⁶  In 1924, Policard observed red fluorescence in experimental rat sarcomas after exposure to UV radiation and hypothesized that the produced fluorescence was associated with tumor accumulation of endogenous HP, and this was considered to be an important phase in the discovery of HP.⁶  In 1942, Auler and Banzer were the first to study the accumulation of injected porphyrins in tumors; they injected tumor-bearing rats with HP, which accumulated in primary and metastatic tumors as well as lymph nodes.⁶  In 1948, Figge and his co-workers introduced the use of porphyrins as a cancer treatment when they discovered their high affinity not only to malignant cells, but also to rapidly growing tissues, including embryonic and regenerating cells.¹³  The following phase was the discovery of the HP derivatives (HpD). This was a giant step forward in the evolution of PDT and was achieved by Schwartz (1955), who was the first to observe that HP itself was impure and consisted of a mixture of porphyrins and other impurities. Schwartz managed to separate them by treatment with a mixture of concentrated sulfuric and acetic acid.⁶  He found that HP itself had poor tumor-localizing properties and was a weak phototoxic agent compared to other components in the mixture “which were later called, HpD”.⁶  The modern era of PDT was initiated with the studies of Schwartz and Lipson in 1960. Lipson further demonstrated the property of tumor localization, and in the early 1960s, he focused on the potential use of HpD in tumor detection and diagnosis.⁶  In 1966, Lipson et al. reported the first use of HpD to treat cancer in a patient with a large, ulcerating, recurrent breast carcinoma. The patient was treated with several HpD injections with light exposure of xenon arc lamp.¹⁴  Perria et al. were the first to use PDT in the treatment of human gliomas using HP derivatives.¹⁵  Over the years, many studies have been implemented to compare the efficacy of several HP derivatives in PDT. One of these studies by Karagianis et al. in 1996 concluded that porphyrins achieve selective tumor killing and sparing of normal brain tissue.¹⁶  Szurko et al. (2003) found that the photodynamic actions of different types of porphyrins were able to inhibit the growth of melanomas at non-toxic concentrations and cell death was caused by necrosis.¹⁷  Photofrin® is one of the most widely used HP derivatives. A study by Muller and Wilson in 1995 showed that Photofrin-PDT prolonged the survival of patients suffering from malignant gliomas.¹⁸  Protoporphyrin IX (PpIX) and 5-aminolevulinic acid (5-ALA) are endogenous PSs and intermediates in heme synthesis.¹⁹ 

Second-generation porphyrins and porphyrin derivatives as well as third-generation PSs have arisen with the aim of alleviating the problems encountered with first-generation porphyrins and improving the efficacy of PDT. In 1998, Schmidt et al. investigated the use of the benzoporphyrin derivative (BPD) and light-emitting diode light sources. Results suggested that BPD is a possible new-generation PS that could be used for the treatment of different malignant brain disorders.²⁰  Verteporfin is a second-generation BPD that is indicated for the treatment of patients with predominantly classic sub-foveal choroidal neovascularization due to age-related macular degeneration, pathologic myopia or presumed ocular histoplasmosis.²¹ 

In 1991, Bachor et al. studied the difference between free chlorin e6 (Ce6) and microsphere-bound Ce6 on human bladder carcinoma. It was found that microsphere-bound Ce6 was more efficient due higher tumor uptake and longer residence in tumor cells.²² 

Abdel-Kader and his co-workers have recently focused their research on the newly discovered PS consisting of chlorophyll derivatives (CpDs) for both oncological and non-oncological uses.²³  In 2012, Gomaa et al. examined the efficacy of PDT using CpDs on a breast cancer cell line. Results proved that CpDs are better candidates than chemotherapeutics for breast cancer because of their higher efficacy at tumor cell killing, as well as their safety in normal cells.²⁴ 

Photochlor (2-[1-hexyloxyethyl]-2-devinyl pyropheophorbide; HPPH) is a lipophilic, second-generation, chlorin-based PS. In 2001, Lobel et al. used HPPH or Photochlor in an in vivo study for treating rat malignant gliomas using PDT. They concluded that HPPH became localized in tumor cells more than in normal brain cells and could be used as an adjuvant therapy in treating gliomas.²⁵  In 2013, Sherifa et al. performed another study on the PS Fospeg®, which is a liposomal formulation of the PS Foscan® (commercial name of meta-tetrahydroxyphenylchlorin [m-THPC]). The results indicate that Fospeg-mediated PDT is a promising strategy for the treatment of hepatocellular carcinoma and needs to be further explored in vivo.²⁶  Recently, the use of dyes as PSs has received great attention from researchers.⁶ 

In conclusion, one branch of the PDT tree portrays the rapid development of PSs, and a reasonable number of PSs have already been approved for clinical applications, as shown in Table 1.1.

Despite the fact that PDT has proved to be an effective and promising treatment modality, it still has some drawbacks, among which is the insufficient dosimetry of light sources. Therefore, much consideration has been given to development of new irradiation sources that offer the advantages of prompt delivery of light doses to the target site, improved penetration depth and offering simultaneous diagnosis and treatment at the same time. The first light sources used in PDT were non-coherent light sources (e.g. conventional arc lamps). These are safe, easy to use and inexpensive. Light-emitting diodes and xenon lamp sources are now commonly used for dermatological applications. However, today, lasers are more commonly used. These produce high-energy monochromatic light of a specific wavelength with a narrow bandwidth for a specific PS. The laser light can be focused, passed down an optical fiber and directly delivered to the target site through a specially designed illuminator tip (e.g. a microlens or a cylindrical or spherical diffuser). Argon dye, potassium-titanyl-phosphate dye, metal vapor lasers and, most recently, diode lasers have been used for clinical PDT applications. The ideal type of light for PDT should have the following characteristics: appropriate absorption by the PS to yield high singlet oxygen quantum yield; high skin penetration; and appropriate fluence and duration. Tissue penetration of the visible light varies from one wavelength to another; it increases towards the near-infrared (NIR) region, where light activation in the blue range (400 nm) allows for tissue penetration of perhaps 1 mm. In contrast, red light may penetrate tissues of 0.5 to 1 cm, allowing for illumination of more deeply seated lesions.^1,28–31 

Despite the well-established efficacy of PDT-mediated cell death and tumor destruction in various forms of cancers, there are still major obstacles to the development of an effective PS possessing all of the properties of an ideal PS, as well as obstacles due to the insufficiency of light dosimetry.³² 

As a result of the deficiencies of many PSs, and in an attempt to enhance the efficacy of PDT and overcome its drawbacks, combination therapy has been explored by many researchers. PDTs in combination with chemotherapy, RT, immunotherapy and anti-angiogenesis therapy and hypothermia have been developed.³³ 

Combination therapy in its simplest definition means the use of different modalities that act via different mechanisms in order to produce additive value and, in many cases, a synergistic effect. For example, a combined therapy might work through acting on different cell signaling pathways, enhancing tumor killing efficiency and at the same time blocking cellular resistance capabilities. An inevitable effect of this is the opportunity to reduce the dose of any/all modalities in the therapeutic combination, making it possible to reduce noxious side effects.³⁴ 

Alkylating agents constitute a group of compounds that are considered among the first lines of therapy in different cancers. These compounds add an alkyl group to important biological molecules in the target tissue, hence causing disruption of the cellular machinery of division and metabolism, an effect that can be used in combating different types of tumor cells.³⁵ 

There are different available alkylating agents that are commonly used to treat different cancers; for example, cisplatin and its derivatives (oxaliplatin and carboplatin) are commonly used drugs to treat sarcomas, lymphomas and ovarian cancers.³⁶  These alkylating agents tend to interact with cellular DNA, forming DNA adducts that force the cell to commit suicide, known biologically as apoptosis.³⁷  Despite their efficacy at the clinical level, these agents do not offer selectivity towards cancer cells; therefore, normal cells are affected in an adverse way. As such, combinations of these agents with PDT have been attempted.

Other researchers have attempted to chemically conjugate the alkylating agent to the PS in a single molecule, producing a HP-based platinum compound that possesses the cytostatic activity of cisplatin and the cytotoxic capability of the PS at the same time against bladder cancer.³⁸  This produced superior antiproliferative and selective effects over cisplatin and HP alone, as well as a non-chemical combination of the two drugs.

Doxorubicin in combination with 5-ALA/PDT in mice bearing transplantable mammary adenocarcinomas and disulfonated aluminum phthalocyanine/PDT in mice bearing murine leukemia and lymphoma have advantages over the single-modality therapies.³⁹  Finally, the anticancer efficacy of doxorubicin combined with methylene blue/PDT has been taken to a whole new level; hybrid nanoparticles were prepared for simultaneous and selective tumor delivery of the PS and doxorubicin. This nanotechnology-based therapeutic combination offered enhanced tumor eradication and improved animal survival.⁴⁰ 

Mitomycin C is an anti-tumor antibiotic that inhibits DNA synthesis through electrophilic attack of cellular nucleophiles—basically DNA—causing DNA alkylation and subsequent cytotoxicity. In addition, it has the ability to alkylate rRNA, preventing protein expression and glutathione causing impaired anti-oxidative potential in the target cells.⁴¹ 

Methotrexate is a known inhibitor of DNA synthesis that works by acting as a structural analog of folic acid. This inhibits the dihydrofolate reductase enzyme that is required for the synthesis of thymidylate and purine nucleotides during DNA synthesis. Its cytostatic effect leads to inhibition of tumor progression.⁴² 

Methotrexate also has a stimulatory effect on the coproporphyrinogen oxidase enzyme, which is a major enzyme in the synthesis of the endogenous PS, PpIX, making it an excellent candidate for combination with PDT. Therefore, it has been combined with 5-ALA, which is also a prodrug of PpIX, and their combination has led to excessive production of the endogenous PS, producing synergistic tumor destruction in human prostate carcinoma and leading to reductions of the toxic methotrexate dose.⁴³ 

Radiation therapy or RT is a treatment modality that utilizes ionizing radiation in order to damage the DNA of cancerous cells, disrupting their reproduction capability.⁴⁴  Despite the fact that undifferentiated malignant cells are more prone to killing by ionizing radiation because they reproduce more rapidly and have impaired DNA repair machinery, normal cells are still not spared in RT because of its lack of selectivity, causing noxious side effects.⁴⁵ 

In an attempt to reduce the side effects that follow RT application in Bowen’s disease and to increase PDT efficacy and reduce the recurrence rate, 5-ALA/PDT has been combined with low-dose RT in a small-group clinical trial and has shown improved efficacy in the single-modality therapies and reduced side effects.⁴⁶ 

Any cancer treatment modality should not only destroy the tumor at its primary site, but also activate the immune response for complete recession of any residual cells both at the site of the primary tumor and secondary ones. This makes it less likely for the tumor to recur.^47,48 

PDT is one of those ideal treatment modalities, as it induces local inflammation at the site of application and causes stimulation of the host’s immune response. Activation of the immune response at PDT-treated tissues was determined by observation of the infiltration of lymphocytes, leukocytes and macrophages into PDT-treated tissue. The inflammatory process is mediated by factors such as vasoactive substances, components of the complement and clotting cascades, acute-phase proteins, peroxidases, reactive oxygen species (ROS), leukocyte chemo-attractants, inflammatory cytokines, growth factors and other immune regulators.^31,49 

Hence, the combination of immunotherapy with PDT was suggested to produce synergistic tumor eradication responses. Intratumoral injection of various immunostimulants as adjuncts to PDT has been given much consideration in order to avoid the severity of the powerful activation of the immune system if these immunostimulants are given systemically, which can be toxic or fatal.⁵⁰ 

Chitosan is another compound that is known for its immunostimulatory activity. It has been suggested that it produces different cytokines, such as tumor necrosis factor alpha and interleukin-6. Another mechanism of immunostimulation was observed in animal models, in which chitosan was found to boost the immune response via upregulation of T cells.⁵⁰  In an attempt to combine this immunoregulator and PDT, a water-soluble form of chitosan—named glycated chitosan—was combined with Photofrin®/PDT and m-THPC/PDT for the treatment of EMT6 mammary sarcoma and Line 1 lung adenocarcinoma, respectively.⁵¹  The PDT application in conjunction with the presence of the immunostimulator caused prominent release of cytokines and initiation of a strong inflammatory cascade that improved tumor eradication capacities.

PDT may induce direct vascular damage and subsequently more extensive injury due to internalization of PSs by endothelial cells and subsequent release of ROS upon irradiation, leading to cytotoxicity of endothelial cells and a widening of the inter-endothelial cell junctions. This causes increases of vascular permeability and releases of clotting and vasoconstricting factors, platelet adherence to damaged cell walls and thrombus formation. Ultimately, this results in tissue hypoxia as a means of tumor destruction.^49,52,53 

An alternative mechanism of the PDT effect on angiogenesis has been suggested, as it was observed that PDT could induce the expression of angiogenic factors such as vascular endothelial growth factor (VEGF), COX-2 and matrix metalloproteinase, a mechanism that can cause reperfusion of obstructed blood vessels and formation of new ones.^54–56  Therefore, the combination of PDT with angiogenesis inhibitors that act via blocking the previously mentioned mediators was suggested as a promising treatment approach. For example, Avastin® (bevacizumab), a fully humanized monoclonal IgG1 antibody that specifically neutralizes all isoforms of VEGF,⁵⁷  has been applied simultaneously with Photofrin/PDT and hypericin/PDT in Kaposi’s sarcoma and murine bladder tumor and bladder tumor xenografts, respectively.^58–60 

The well-known mechanism of PDT action on tumorous and non-tumorous cells can be simplified by the induction of oxidative stress in the respective cells, causing these cells to undergo organized or disorganized death, named apoptosis and necrosis, respectively.⁶¹  Therefore, it can be simply concluded that any approach that acts to combat this oxidative stress can reduce PDT efficacy, such as the use of free radical scavengers or antioxidants. However, it has been previously reported that employment of antioxidants during PDT could produce synergistic tumor eradication effects.³⁹  This has been explained by the observation that some antioxidants can produce pro-oxidant effects at very low concentrations, as in the case of the combinatory approach of ascorbate with 5-ALA/PDT in rat dimethylaminostillbine (DS) sarcoma cancer cells.⁶²  Other researchers point to an alternative mechanism of interaction between the antioxidant, ascorbate, and the singlet oxygen produced in the human promyelocytic leukemia cells (HL60) after treatment with BPD/PDT, which can produce more toxic species such as hydrogen peroxide, which activates myeloperoxidase, thereby producing more toxic oxidants.⁶³ 

Cell growth is mediated through different receptors and downstream cell signaling cascades. Therefore, approaches that target such receptors and their downstream molecules could eventually inhibit cell growth and stop tumor progression. As such, receptor inhibition strategies could be combined with PDT in order to produce synergistic tumor elimination results. Epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase that has a major role in cell cycle progression and proliferation, and it also protects cells against apoptosis. Therefore, EGFR can act to decrease PDT efficacy; hence, its inhibition during PDT application using cetuximab (Erbitux®), a monoclonal antibody that binds competitively to EGFR, was hypothesized to produce better results than PDT alone. This was investigated in epithelial ovarian cancer and non-small-cell lung cancer using BPD as a PS. It was found that PDT induced better cytotoxicity via the apoptotic mechanism, hence giving more hope for the prevention of metastasis and recurrence of tumors.⁶⁴ 

Photothermal therapy (PTT) is the therapeutic modality in which energetic photons are converted into heat, which then causes target cell destruction through a process known clinically as hyperthermia. The sources of irradiation include NIR, visible light and others.⁶⁵ 

Graphene oxide (GO) nanovehicles possess unique NIR absorption properties. In addition, they can be functionalized or loaded with a PS in order to act simultaneously as a photodynamic and photothermal agent and so offer optimum therapeutic outcomes. These nanovehicles were used to deliver PSs such as Ce6 and methylene blue. In both cases, better antitumor effects were achieved because of the increased accumulation of the PS in the tumor cells when presented in the nano-GO as compared to the accumulation of the free PS, and also because of the additive photothermal effects provided when the tumor was subsequently exposed to NIR irradiation at 808 nm, inducing PTT by nano-GO after being irradiated at 650 nm for PDT by the PS.⁶⁶ 

Nanovesicles, namely liposomes, transferosomes, niosomes and others, are tiny vesicles, similar in composition to the cell membrane. They can be loaded with drugs and used to deliver such drugs for the treatment of cancer and other diseases.⁶⁷  They are highly biocompatible and biodegradable nanocarriers composed of a unilamellar or multilamellar lipid bilayer membrane with an aqueous inner core.⁶⁸ 

In our group, conventional liposomes and transferosomes were loaded with chlorophyllin derivatives (nano-CHL) as PSs and used in the photodynamic treatment of pigmented melanoma. Melanoma is a type of skin cancer that occurs in melanocytes, the melanin-producing cells. Melanin is known to confer resistance to different therapeutic modalities, even PDT. Melanin pigment interferes with the radiation-absorption capacity of the PS and acts to neutralize the produced ROS responsible for its cytotoxicity; hence, it decreases PDT’s overall efficacy. In this regard, depigmentation strategies have been employed prior to nano-CHL-mediated PDT. In vitro studies have shown favorable results that encourage the future implementation of these technologies in in vivo and clinical studies.⁶⁹ 

Malignant brain tumors (gliomas) are the most commonly occurring primary central nervous system tumors. The most aggressive form is glioblastoma multiforme (GBM). Nano-CHL has been employed for in vitro PDT of GBM as compared to free PS. Results have shown enhanced PDT efficacy with nano-CHL and that cytotoxicity occurs mainly via apoptosis and, to a lesser extent, necrosis, as was revealed by fluorescence microscopy.⁷⁰ 

Inorganic nanoparticles (e.g. silica nanoparticles) are of great interest as drug carriers. In our group, mesoporous silica nanoparticles were employed as vehicles for the PS, Fotolon®, possessing pores whose sizes range from 2 to 50 nm. This conjugation increased the cellular accumulation of the PS by a minimum of 1.5 times as compared to free Fotolon. This increased the efficacy of PDT against breast cancer cell lines and caused reductions of the irradiation dose.⁷¹ 

PDT is not only a therapeutic modality for oncological diseases, but also has proven to be very efficient in non-oncological diseases. Clinical trials of PDT on microbial infections demonstrated the inactivation of a variety of microbial pathogens, including Gram-positive and Gram-negative bacteria, yeasts and fungi, mycoplasmas and parasitic protozoa.^72,73  This technique is characterized by a high degree of specificity for the target microorganisms, in which the chemical structure of the PS is suitably engineered to induce the selective association of the photodynamic agent with the structural elements that are typical of microbial cells.^74,75 

The use of photochemical processes as a tool to control the populations of several types of noxious insects and parasites has been repeatedly examined in both laboratory experiments and field studies.^76,77  Most recently, a successful field implementation of using the photodynamic modality to control vector-borne diseases, such as malaria, filaria and Dengue fever, has been reported.^78,79  Field trials were performed in infested epidemic swamps in Uganda, Ethiopia and Sudan using CpDs as PSs, which were approved by the FDA as food additives. These were added to infested swamps in order to be taken up by mosquito larvae. The accumulated CpDs inside the larvae body induce oxidative stress upon sunlight exposure, which interrupts the life cycle of the parasite and results in organism death. The formulated PS that was used achieved target selectivity such that all other biological beneficiary organisms (which were present in the same treated swamps) were not affected.

The results revealed that this innovative modality against vector-borne diseases and agro-pests combines both effectiveness and efficiency with the highest levels of human safety and environmental friendliness. These efforts will pave the way to future large-scale, country-wide projects.

I would like to express my gratitude to my co-workers. Special thanks goes to the pleasant and smart Aya Sebak, Mai Rady, Engy Fadel and Shaimaa Abdel-Hamid.