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Abstract

Photodynamic therapy (PDT) is a relatively new modality for anticancer treatment and although the interest has increased greatly in the recent years, it is still far from clinical routine. As PDT consists of administering a nontoxic photosensitizing chemical and subsequently illuminating the tumor with visible light, the treatment is not subject to dose-limiting toxicity, which is the case for established anticancer treatments like radiation therapy or chemotherapy. This makes PDT an attractive adjuvant therapy in a combined modality treatment regimen, as PDT provides an antitumor immune response through its ability to elicit the release of damage-associated molecular patterns and tumor antigens, thus providing an increased antitumor efficacy, potentially without increasing the risk of treatment-related toxicity. There is great interest in the elicited immune response after PDT and the potential of combining PDT with other forms of treatment to provide potent antitumor vaccines. This review summarizes recent studies investigating PDT as part of combined modality treatment, hopefully providing an accessible overview of the current knowledge that may act as a basis for new ideas or systematic evaluations of already promising results.

Keywords

photodynamic therapy cancer immunogenicity combined-modality treatment

Photodynamic Therapy

In recent years, there has been increasing interest in the potential of using photodynamic therapy (PDT) to treat various types of cancers, either on its own or in combination with other anticancer treatments. The principle of PDT includes the administration of a photosensitizing drug and subsequently illuminating the target area with visible light corresponding to the absorbance wavelength of the photosensitizing drug, triggering a series of biological effects.^1,2 Thus, there are 2 main components that govern the applicability of PDT in cancer therapy, the localization of the photosensitizing drug in tumor tissue and delivering the appropriate dose of light (near-infrared for most photosensitizing drugs) to that tissue. The characteristics of an ideal photosensitizing drug have been extensively described elsewhere,^3
–5 but a few key features include high-specific tumor uptake, negligible dark toxicity (biological effects without light application), and a fairly long absorption wavelength as longer wavelengths allow deeper tissue penetration. Some of the most investigated photosensitizing drugs include hypericin,^6
–8 Photofrin,^4,9 and 5-aminolevulinic acid.^4,10 Alongside a wide variety of photosensitizing drugs, there is a vast availability of light sources for PDT such as various types of lasers, broad wavelength band lamps, and light-emitting diodes.^9,11 Figure 1 illustrates the photophysical and photochemical mechanisms that lead to the biological effects of PDT.

Figure 1.

A ground state photosensitizer (PS) molecule in singlet state is excited by light boosting an electron into a higher energy orbital. The excited state reverts to the ground state via fluorescence or internal conversion (˜nanoseconds) or the spin of the excited electron inverts resulting in an excited triplet state PS at lower energy, which is a relatively long-lived state (˜microseconds) since the excitation energy is lost by phosphorescence in a spin-forbidden transfer directly back to the ground singlet state. The excited triplet state can then either transfer its energy to triplet molecular oxygen resulting in biologically reactive excited singlet state oxygen or transfer an electron to superoxide anions resulting in various reactive oxygen species (ROS) capable of causing substantial biological damage. The resulting biological effects of photodynamic therapy include cell death through different pathways and in many cases also a potent immune response involving both the innate and the adaptive immune system.

Immunogenic Cell Death and PDT

Considerable research efforts have been undertaken to clarify the biological mechanisms responsible for the anticancer effectiveness of PDT. These mechanisms can generally be divided into direct antitumor effects and indirect effects related to an induced immunological response. The cell death mechanisms of direct PDT-induced cytotoxicity include apoptosis (programmed cell death), necrosis (unregulated cellular breakdown), and macroautophagy (degradation of cellular components by lysosomes).^2,7,12 Direct effects can also include damage to tumor vasculature, which may however initiate subsequent tumor angiogenesis.^7,12,13 Which cell death mechanism is responsible for the antitumor effects depends on the light dose and photosensitizer (PS) uptake. The amount of apoptotic versus necrotic cell death depends on the resulting reactive oxygen species (ROS) concentration within the cells after PDT.⁷

Photodynamic therapy has been shown to induce immunogenic cell death (ICD) that can trigger a considerable immune response further enhancing the antitumor effect.¹⁴ A detailed review on ICD was recently published by Kroemer et al¹⁵ where the authors provide the following operational definitions of ICD:

Malignant cells are considered to undergo ICD in vitro if upon subsequent transplantation they provide the host with an immune response that protects against challenges with tumor cells of the same strain, that is, acting as a cancer vaccine. In vivo, cell death is defined as immunogenic if it triggers a response of the innate and adaptive immune system leading to antitumor effects that are a result of mechanisms dependent on the immune system.

The innate immune response includes activation of the complement system with a strong inflammatory response and neutrophil infiltration, along with macrophage activation, maturation of dendritic cells (DCs), and increased natural killer cell activity.^14,16 Adaptive immunity is mediated via B- and T cells. B cells produce antibodies that can in turn eliminate tumor cells. T-helper (T_h) cells can both induce and suppress immune effects. On one hand, T_h1 cells (CD4+T-bet+ T-cells) activate and stimulate antitumor M1 macrophages as well as license CD8+ T-cells to become cytotoxic T-lymphocytes (CTLs) that kill cancer cells. While on the other hand T_h2 cells (CD4+GATA-3+ T-cells) stimulate B cells into proliferation, induce B-cell antibody class switching as well as activate and stimulate protumor M2 macrophages. Additionally, regulatory T cells (T_regs; CD4+CD25+FoxP3+ T-cells) suppress the antitumor immune response through induction of anergy in T_h1 cells and CTLs.^12,14

Immunogenic cell death is triggered by the release of damage-associated molecular patterns (DAMPs), which are normally retained within the cell but act as danger signals in response to damage or stress by being translocated to the cell surface, secreted into the cytoplasm, or released extracellularly.^16,17 The release of DAMPs prompts an efficient donation of tumor antigens priming the adaptive immune system to create antibodies and seek out the corresponding cancer cells. Damage-associated molecular patterns include calreticulin, heat shock proteins (HSPs) 70 and HSP90, high-mobility group box 1 (HMGB1), adenosine triphosphate (ATP), uric acid, interlukin 1α, DNA, and RNA.¹⁶

Thus, it has been shown that a robust approach to determine whether cancer cells subjected to a certain treatment undergo ICD is to measure subsequent calreticulin exposure, ATP secretion, and HMGB1 release.¹⁵

It has been shown that 2 essential components required to trigger the intracellular mechanisms resulting in ICD are endoplasmic reticulum (ER) stress and the generation of ROS.¹⁸ This is explained in further detail in the review by Krysko et al,¹⁸ but it has been extensively shown that both ER stress and ROS are required to elicit ICD. Interestingly, the strongest immune response is provided by treatments inducing focused primary ER stress rather than ER stress induced as a secondary effect through damage to other intracellular targets. This makes PDT an important candidate for ICD-based cancer therapy as hypericin-PDT has been shown to induce massive ROS production generating direct ER stress, as hypericin localizes within the ER.^19,20 This suggests that the immune response resulting from hypericin-PDT will be more efficient than immune responses from treatments inducing secondary ER stress, as focused ROS-based ER stress has been shown to result in an increased number of emitted DAMPs as well as simplifying the DAMP trafficking pathways.¹⁸

In the following sections, we summarize the current preclinical, and in a few cases also clinical, evidence of the efficacy of PDT when used as part of combined modality anticancer treatment.

Photodynamic Therapy in Combination With Immunostimulants

Since PDT has the potential of triggering a strong antitumor immune response, there has been great interest in potentiating this effect by stimulating various components of the immune system.²¹ This can be done by combining PDT with the administration of various immunostimulants to, for example, increase neutrophil infiltration, enhance tumor antigen presentation, upregulate T-cell activation, and suppress expression of T_regs. Immunostimulants can be administered intratumorally, intravenously, or even topically depending on the type of malignancy.²¹

Table 1 summarizes recent studies exploring different immunostimulatory agents and their effect on immune response when combined with PDT for various malignancies.

Table 1.

Studies Combining Photodynamic Therapy With Immunostimulants.

Therapy Combination	In Vivo Tumor Model	Photosensitizer	Light Source and Dose	Main Results	Reference
Glycated chitosan, which stimulates the immune system by producing TNF-α and IL-6 or T-cell upregulation and was administered immediately after PDT at 0.5% or 1.5% concentration	EMT6 mammary sarcoma and line 1 lung adenocarcinoma subcutaneously implanted into BALB/c mice	5 mg/kg Photofrin at 24 h before PDT for EMT6 tumors and 0.1 mg/kg mTHPC 24 h before PDT for line 1 lung tumors	Photofrin-PDT: a 150-W lamp with 630 ± 10 nm filter delivered 60 J/cm² at 100 mW/cm² mTHPC-PDT: a 652-nm laser delivered 30 J/cm² at 110 mW/cm²	Significantly more tumor-free mice at >30 days when glycated chitosan was added compared to Photofrin-PDT alone or mTHPC-PDT alone	Chen et al²²
Complement-activating agents zymosan and heat-aggregated g globulin (HAGG) administered intratumorally directly after PDT	Lewis lung carcinoma cells subcutaneously implanted in C57BL/6 mice	10 mg/kg Photofrin 24 h before PDT	A 150-W lamp with 630 ± 10 nm filter delivered 150 J/cm² at 110 mW/cm²	Significant increase in tumor-free mice and C3 complement protein levels with Photofrin-PDT + 0.5 mg/mouse zymosan or 1.25 mg/mouse HAGG	Korbelik et al²³
Phase II clinical trial of topical imiquimod, which stimulates the innate immune system through toll-like receptors TLR7 and TLR8, for grade 2/3 vulval intraepithelial neoplasia, applied in the weeks prior to PDT		Methylaminolevulinate photosensitizer cream topically applied 3 h before PDT	Aktlite 128 lamp delivering 630 nm light to a dose of 50 J/cm²	(1) At 1 year 65% of patients symptom free (2) More T-cells found in biopsies of responders (3) More T_reg-cells found in nonresponders	Winters et al²⁴
The neutrophil- and macrophage-activating streptococcal preparation OK-432 was intratumorally injected either 3 h before or immediately after PDT	NR-S1 squamous cell carcinoma transplanted into the dorsum of C3H/HeNCrj mice	Hematoporphyrin oligomers administered intraperitoneally with 20 mg/kg at 48 h before PDT	10 Hz pulsed laser light at 630 nm was used to deliver a dose of 360 J/cm² at 15 mJ/cm² per pulse	(1) Significant increase in survival and decrease in tumor volumes if OK-432 given before PDT (2) No effect if given after PDT	Uehara et al²⁵
Dendritic cell and macrophage activating Bacilus Calmette-Guerin (BCG) administered subtumorally either 24 before or immediately after PDT	EMT6 mammary sarcoma cells were intramuscularly transplanted to BALB/c mice	Six different photosensitizers; Photofrin, BPD, zinc phthalocyanine, mTHPC (Foscan), lutetium texaphyrin, and NPe6	Corresponding wavelengths of 630, 690, 671, 652, 732, and 664 nm were delivered at 60-90 mW/cm² using a tunable Xenon light bulb source to different light doses	(1) Percentage tumor-free mice increased with BCG after PDT, for all tested photosensitizers (2) Substantial increase in immune memory T-cells in draining lymph nodes	Korbelik et al²⁶
Administration of mycobacterium cell wall extract (MCWE), which stimulates humoral- and cell-mediated immunity was performed subtumorally at different time points in relation to PDT	EMT6 mammary sarcoma cells were intramuscularly transplanted to BALB/cJ mice	Four different photosensitizers: Photofrin, BPD, zinc phthalocyanine, and mTHPC (Foscan)	Corresponding wavelengths of 630, 690, 671, and 652 nm were delivered at 100-130 mW/cm² using a tunable Xenon light bulb source to different light doses	(1) MCWE immediately after PDT increased percentage of tumor-free mice (2) Also increased neutrophil infiltration MCWE before PDT had no effect	Korbelik and Cecic²⁷
PDT was combined with vitamin D3-binding protein-derived macrophage-activating factor (DBPMAF) at varying time points administered either intraperitoneally, intravenously, or peritumorally	Squamous cell carcinoma (SCCVII) cells were subcutaneously implanted to C3H/HeN	Photofrin was administered intravenously with 10 mg/kg at 24 h before PDT	A Xenon light bulb source was used to deliver 150 J/cm² of 630 ± 10 nm light at 130 mW/cm²	(1) Optimal treatment: PDT + DBPMAF after 0, 4, 8, and 12 days (2) 100% tumor-free mice compared to 25% with PDT alone (3) Result of the recruitment of activated macrophages to the tumor	Korbelik et al²⁸
PDT was combined with low-dose (50 mg/kg) cyclophosphamide (CY) 2 days before PDT to enhance antitumor immunity through CY-driven depletion of T_regs	Wild-type CT26 colon carcinoma cells were subcutaneously transplanted into BALB/c mice. If surviving is >90 days, mice were rechallenged with CT26 cells	Liposomal benzoporphyrin derivative (BPD) mono acid ring A at 0.3 mg/mL was intraperitoneally injected 15 min before PDT	A diode laser was used to deliver 690 nm light at 100 mW/cm² to a dose of 120 J/cm²	(1) Low-dose CY prior to PDT substantially decreased tumor volumes and increased survival (2) Only if CY was administered at the time of rechallenge, the tumor was rejected	Reginato et al²⁹
Granulocyte colony-stimulating factor (G-CSF), which causes enhanced neutrophil production was intratumorally administered prior to PDT twice daily for 5 days	Colon carcinoma C26 cells and Lewis lung carcinoma cells were transplanted into the footpad of BALB/c and (C57BL/6xDBA/2)F₁ mice, respectively	Photofrin was administered intraperitoneally 24 h before PDT at 15 mg/kg	A He–Ne ion laser was used to deliver 630 nm light at 80 mW/cm² to a dose of 150 J/cm²	(1) G-CSF + PDT showed increased number of neutrophils (2) Significantly increased survival and decreased tumor volumes	Golab et al³⁰
Immature dendritic cells (DCs) obtained from mouse bone marrow were administered intratumorally 1 h after PDT	Colon carcinoma C26 cells were injected into the footpad of BALB/c mice	Photofrin was administered intraperitoneally 24 h before PDT at 10 mg/kg	A He–Ne laser was used to deliver 630 nm light at 80 mW/cm² to a dose of 90 J/cm²	(1) PDT + immature DCs showed DC maturation and increased IL-12 (2) Significant inhibition of contralateral untreated C26 tumors	Jalili et al³¹
Immature dendritic cells (DCs) obtained from mouse bone marrow were injected intratumorally on 4 separate days following PDT	BALB/c mice were injected with C26 colon carcinoma cells, and C57BL/6 mice were subcutaneously inoculated with B16 melanoma cells	ATX-S10 Na(II) photosensitizer was administered 5 h before PDT at 5 mg/kg	A diode laser was used to deliver 670 nm light to a dose of 150 J/cm²	(1) PDT + immature DCs resulted in increased survival for C26 and B16 tumors (2) Adoptive transfer of splenocytes from treated mice inhibited C26 tumor growth in naive mice	Saji et al³²

Abbreviations: IL interleukin; mTHPC, meta-tetra(hydroxyphenyl)chlorine; PDT, photodynamic therapy; TNF, tumor necrosis factor; T_regs, regulatory T cells.

Studies have been carried out on several different tumor models including lung cancer, colon cancer, squamous cell carcinomas, melanoma, and breast cancer. Many of the different strategies showed promising in vivo results with increased survival and reduction in tumor volumes. There were also some clear immunotherapeutic effects of these combination strategies showing effective rejection of tumor rechallenge or significant antitumoral effects of untreated contralateral tumors. Many of the studies used Photofrin as the PS agent, although some studies showed that the increase in therapeutic effect from combination therapy was independent of the choice of PS.^26,27 As different studies addressed different mechanisms for enhancing tumor immunogenicity, there may also be potential for combining some of the strategies to further improve treatment results. This should of course be done with caution and considerable thought has to be given to how immunostimulants are administered, as systemic effects could result in severe autoimmune reactions.

Photodynamic Therapy in Combination With Established Anticancer Therapy

The combination of PDT and established anticancer therapies such as ionizing radiation and/or chemotherapy provides an exciting platform for potential new treatment options, especially since PDT does not have the inherent dose-limiting toxicity of either radiation therapy or chemotherapy.

The combination with ionizing radiation also includes the potential of certain PS agents to act as radiosensitizers.³³ Since ionizing radiation causes cell death mainly via direct DNA damage, there are potential synergistic effects to be explored as PDT can cause DNA degradation leading to increased cell death as the DNA may lose its capacity to repair normally sublethal single-strand breaks.³⁴

When it comes to combining chemotherapy with PDT, there are many possible ways of achieving a combined or synergistic effect, depending on the cell death mechanisms of the chemotherapeutic drug in question. Not only are the cell death mechanisms important but also the intracellular target of the chemotherapy agent, as this will likely affect how the combination with PDT affects the tumor cells. As PDT also affects the cell membrane permeability, adding it as an adjuvant to chemotherapy may increase the deliverability of cytotoxic drugs. Some chemotherapy drugs can act both as a cytotoxic agent and as a PS, providing the option to illuminate the tumor tissue after chemotherapy administration in order to elicit a potentially synergistic treatment effect.³⁵ The interest in this combination therapy does not only lie in enhancing the antitumor effects but also in the ability to reduce the risk of severe side effects by reducing the required chemotherapy dose.³⁶

Tables 2 and 3 summarize the experiments and results of several recent studies combining PDT with either chemotherapy or ionizing radiation. Most of the studies combining ionizing radiation with PDT showed an additive antitumor effect of the 2 treatments, when either a cobalt-60 (⁶⁰Co) or cesium-137 (¹³⁷Cs) source was used. There was, however, a potent increase in cytotoxicity shown in 1 study of human MCF-7 breast cancer cells receiving PDT and treatment with 100 kV x-rays.³⁵ Interestingly, the studies investigating the order in which PDT and ionizing radiation were administered found that the effect of combination therapy was independent on which treatment preceded the other.^33,37,38

Table 2.

Studies Combining Photodynamic Therapy With Ionizing Radiation.

Therapy Combination	In Vitro Tumor Model	Photosensitizer	Light Source and Dose	Main Results	Reference
A ⁶⁰Co source was used to deliver different doses of ionizing radiation in combination with PDT at different time sequence	Ehrlich ascites carcinoma cells were inoculated intraperitoneally to BALB/c mice, tumor cells were then excluded from the intraperitoneum, treated with PDT and radiation, and then inoculated into healthy BALB/c mice	HPde photosensitizer was injected intraperitoneally 3 h before exclusion of intraperitoneal cells at concentrations of 10-60 mg/kg	A tungsten lamp was used to deliver light between 370 and 680 nm at 30 mW/cm² for 90-180 s	(1) HPde acts both as a photosensitizer and a radiosensitizer (2) Additive effect on tumor growth, independent on treatment order	Luksiene et al³³
PDT was combined with ionizing radiation at 1.2 Gy/min with a ¹³⁷Cs source delivered either 24 h before or 24 h after PDT	Squamous cell carcinoma cell lines V134, V175, and SCC-61 were cultured and used in the exponential growth phase	Cells were incubated for 4 h prior to treatment with 1 mmol/L/L 5-aminolevulinic acid (5-ALA)	A tungsten-halogen lamp with a 400-750 nm filter was used to deliver light at 50 mW/cm² for 3 min	(1) Tumor cells accumulated in the G2/M phase after ionizing radiation (2) Additive effect on survival with combination therapy, independent of order	Allman et al³⁷
PDT was combined with ⁶⁰Co ionizing radiation at 1.4-1.9 Gy/min to a dose of 2 or 7 Gy either 15-30 min before PDT or 15-30 min after PDT	Nonsmall cell lung cancer lines A549, SKMES1, NCIH460, and NCIH23 were used along with normal human fibroblasts	Cells were incubated for 24 h before treatment with 2.5 mg/mL Photofrin	An LED light source was used to deliver 20 mW/cm² light for 100 or 400 s depending on cell line	(1) Less than additive effect on cell survival, irrespective of treatment order (2) Significantly less than additive cytotoxic effect on normal fibroblasts	Sharma et al³⁸
Clinical case study of 4 patients with Bowen disease where fractionated electron beam therapy was applied to 12 Gy, 30 min after PDT		A topical solution of 5-aminolevulinic acid (5-ALA) was applied at 20% concentration 4-6 h before treatment	Laser light at 630 nm was delivered in 4 fractions 2-3 days apart to a total dose of 200 J/cm²	All lesions disappeared following combination therapy and no recurrences after 14-month average follow-up	Nakano et al³⁹
PDT and chemotherapy were combined with concurrent 100 kV x-ray treatment at 1.1 Gy/min to a dose of 4 Gy	Human MCF-7 breast cancer cells were used	1 μmol/L mitoxantrone was used as a photo- and radiosensitizer, and cells were incubated 90 min before treatment	A Lumacare LC122-A fiber optic probe delivered 10 J/cm² at fluence rate 75 mW/cm²	PDT, chemotherapy, and 100 kV x-rays had a potent cytotoxic effect on MCF-7 cells, exceeding that of an additive effect	Sazgarnia et al³⁵

Abbreviations: ⁶⁰Co, cobalt-60; ¹³⁷Cs, cesium-137; HPde, hematoporphyrin dimethyl ether; LED, light emitting diode; MCF-7, Michigan Cancer Foundation 7; PDT, photodynamic therapy; SCC-61, squamous cell carcinoma 61.

Table 3.

Studies Combining Photodynamic Therapy With Chemotherapy.

Therapy Combination	In Vitro Tumor Model	In Vivo Tumor Model	Photosensitizer	Light Source and Dose	Main Results	Reference
The potential enhancement in cytotoxicity when administering cisplatin (6.25 μg/mL in vitro and 2 mg/kg in vivo) along with the photosensitizer prior to PDT was evaluated	Human head and neck squamous cell carcinoma (AMC-HN3) cells were used	AMC-HN3 cells were also subcutaneously implanted in BALB/c/nu/nu mice	In vitro: cells were incubated with 5-aminolevulnic acid (5-ALA) at 25 or 50 μg/mL fro 24 h In vivo: 375 mg/kg 5-ALA was administered 6 h before PDT	A 632-nm laser was used to illuminate cells to 6.0 J/cm² and in vivo tumors to 132 J/cm²	(1) Combined 5-ALA PDT and cisplatin increased cytotoxicity (2) More efficient against tumor recurrence	Ahn et al⁴⁰
HeLa cells subjected to combination treatment with 24 h incubation with low-dose cisplatin followed by PDT, evaluated through cell viability and cell death mechanisms	HeLa cells were used as a human cervical cancer model		After incubation with cisplatin, cells were incubated with 5-aminolevulininc acid (5-ALA) for 4 h prior to PDT	A 635-nm laser was used to deliver a light dose of 5 J/cm²	(1) Combination treatment with cisplatin doses >1 mg/L synergistically enhanced cytotoxicity (2) Increased apoptosis rate, related to upregulation of p53 and changes in p21, Bcl-2, and Bax expression	Wei et al⁴¹
The effect of combined PDT and doxorubicin of varying concentration (4-16 μmol/L) on a multidrug-resistant cell line was evaluated in vitro	The multidrug-resistant human uterine sarcoma cell line MES-SA/Dx5 was used		Cells were incubated with pheophorbide a (Pa) photosensitizer 2 h before PDT	A quartz-halogen lamp was used to illuminate the cells with 610 nm light at 70 mW/cm² to a dose of 84 J/cm²	(1) Synergistic effect on cytotoxicity from combined doxorubicin + PDT mediated by intracellular ROS generation (2) Synergistic effect seen only in the multidrug-resistant line	Cheung et al³⁶
Gefitinib, which can inhibit ABCG2 protein-mediated efflux of porphyrin out from malignant cells, was evaluated at different concentrations in vitro in combination with PDT	Human glioma cell lines U87MG, U118MG, A172, and T98G were used		Following gefitinib incubation, cells were incubated with 1 mmol/L 5-aminolevulinic acid (5-ALA) for 6 h prior to PDT	A 405-nm laser was used to treat cells at 10 mW/cm² to a light dose of either 0.6, 1.8, or 3.0 J/cm²	(1) Gefitinib + PDT resulted in dose-dependent reduction of the surviving glioma fraction (2) Effect due to decreased ABCG2 expression and subsequent increase in intracellular porphyrin levels	Sun et al⁴²
The apoptosis-inducing protein apoptin was tested in combination with PDT via PVP3 plasmid administration	Nasopharyngeal carcinoma CNE-2 cells were used	The CNE-2 cells were also transplanted subcutaneously to BALB/c nude mice after PVP3 incubation	Cells were incubated with 5-aminolevulinic acid (5-ALA) at 1 mmol/L/L for 6 h before PDT, and the mice were administered 5-ALA at 100 mg/kg 3 h prior to PDT	Laser light at 630 nm was used to deliver 1, 3, or 6.25 J/cm² to CNE-2 cells and 100 J/cm² at 100 mW/cm² to the mice	Apoptin gene therapy + PDT resulted in significantly stronger antitumor effects in vitro and in vivo compared to monotherapies	Fang et al⁴³
5-FU, gemcitabine, oxaliplatin and cis-diamminedichloroplatinum chemotherapy in combination with PDT in vitro and gemcitabine and oxaliplatin in combined with PDT in vivo	Human biliary cancer (NOZ) cells were cultured	The NOZ cells were also transplanted subcutaneously to BALB/cANcrj mice	Cells were incubated for 24 h with 20 μg/mL talaporfin sodium (TPS) photosensitizer. The mice were injected with 5 mg/kg TPS at 2 h before PDT	A semiconductor laser was used to illuminate cells with 664 ± 2 nm light to 12 J/cm² and mice to a dose of 60 J/cm²	(1) Significant increase in tumor necrotic area and apoptosis-positive cells (2) Synergistic cytotoxicity increase from oxaliplatin and gemcitabine + PDT	Nonaka et al⁴⁴
PDT was combined with 5 mg/kg Adriamycin to investigate increased antitumor effects through potential enhanced apoptosis and inhibited tumor angiogenesis		4T1 mammary carcinoma cells were subcutaneously transplanted to the right flank of BALB/c mice	The photosensitizer benzoporphyrin derivate monoacid ring (BPD-MA) was intravenously injected 24 h before PDT at 1 mg/kg	Laser light at 690 nm was used to deliver 120 J/cm² to the tumors	(1) Adriamycin + PDT resulted in significantly reduced tumor volumes (2) Also considerable increase in survival compared to separate Adriamycin or PDT	Tong et al⁴⁵
The effect of combining doxorubicin or vincristine with PDT in the treatment of sensitive or resistant murine leukemia cells was tested	Vincristine-resistant LBR-V160 cells, doxorubicin-resistant LBR-D160 cells, and sensitive LBR-murine leukemic cells were tested		Cells were incubated for 4 h prior to PDT with 1 mmol/L 5-aminolevulinic acid (5-ALA)	Fluorescent lamps at 400-700 nm were used to illuminate cells for 0, 5, 10, or 20 min	(1) Chemotherapy-resistant LBR-D160 and LBR-V160 cell lines were sensitive to 5-ALA PDT (2) No increase in treatment efficacy from combination therapy	Diez et al⁴⁶
The polytherapy combination of Navelbine or cisplatin chemotherapy followed by PDT, and by adoptive immunotherapy with splenic lymphocytes from PDT-treated mice was tested		Hybrid DBA/2x BALB/c mice were intraperitoneally injected with L1210 murine leukemia cells	mTHPC (Foscan) was administered postchemotherapy and 24 h prior to PDT at 0.3 mg/kg	A continuous wave dye laser was used to deliver 670 nm light at 100 mW/cm² to a dose of 100 J/cm²	(1) Chemotherapy, PDT and adoptive immunotherapy successfully treated this aggressive metastatic tumor (2) PDT or chemotherapy alone showed no survival advantage over control	Canti et al⁴⁷

Abbreviations: ABCG2, adenosine triphosphate-binding cassette subfamily G member 2; mTHPC, meta-tetra(hydroxyphenyl)chlorine; PDT, photodynamic therapy; PVP, polyvinylpyrrolidone; ROS, reactive oxygen species.

As for the combination with chemotherapy, many alternatives have been explored and one quite interesting finding was the highly increased treatment efficacy of drug-resistant murine leukemia and human uterine sarcoma cell lines, when PDT was added.^36,46 In these cases, PDT might prove a promising addition or even alternative to chemotherapy. It was also shown that PDT in combination with cisplatin resulted in a considerably increased in vitro and in vivo antitumor effect, which may prove very useful considering the often dose-limiting toxicity of cisplatin.^40,41 One study also examined the combination of PDT, chemotherapy, and adoptive immunotherapy with splenic lymphocytes, adding a further dimension to the combination approach.⁴⁷ This triple combination proved effective in treating an aggressive murine leukemia model in vivo, where either PDT alone or chemotherapy alone was ineffective. Although very promising, great care should be taken when attempting to translate this into a clinical alternative as the addition of each new treatment regimen potentially increases the risk of severe, and possibly systemic, treatment-related toxicity.

Photodynamic Therapy in Combination With Experimental Anticancer Therapy

There are several anticancer treatment strategies that are currently not well established in the clinic, used on an experimental basis, or are still being tested in the laboratory. Two such strategies are hyperthermia and focused ultrasound (sometimes referred to as sonodynamic therapy).

In relation to PDT, hyperthermia can cause damage to the tumor vasculature that would affect angiogenesis and, just like PDT, damage the cell membranes, which could allow for a synergistic effect between the 2 treatments.⁴⁸

As the use of focused ultrasound in the management of cancer is fairly new, the combination between PDT and ultrasound has only started to be explored in the last few years. The main hypothesis suggesting that this combination may yield a considerable antitumor effect is based on the fact that there are PS agents that can be triggered by both light energy and ultrasound energy, which should potentiate the response.⁴⁹

Tables 4 and 5 summarize the results of studies performed to evaluate the potential antitumor efficacy of PDT in combination with either hyperthermia or focused ultrasound. Photodynamic therapy in combination with hyperthermia has been shown to yield a strong direct cytotoxic response but also to prompt a considerable antitumor immune response.^13,50 Some studies have shown that the timing between therapies is of critical importance and that hyperthermia applied after PDT is more effective than the reversed order.^13,48 There also appears to be a considerable dose–response effect dependent both on the light dose and on the length and temperature of hyperthermia,^53,54 suggesting that there is a need for systematically evaluating this dose–response relationship, in vitro and in vivo, to obtain the optimal treatment parameters for different cancer types.

Table 4.

Studies Combining Photodynamic Therapy With Hyperthermia.

Therapy Combination	In Vitro Tumor Model	In Vivo Tumor Model	Photosensitizer	Light Source and Dose	Main Results	Reference
Hyperthermia consisting of a 43°C water bath was applied immediately after PDT for 1 h		RIF-1 tumor cells were subcutaneously implanted into the hind leg of C3H/Km mice	Hypericin was administered intravenously 6 h before PDT illumination at 5 mg/kg	Laser light at 595 nm was used to deliver a dose of 120 J/cm² at a fluence rate of 100 mW/cm²	(i) Combination therapy lead to increased damage to tumor vasculature (ii) Most pronounced effect of interaction was the increased direct cytotoxicity	Chen et al¹³
Tumor vaccines were generated through a combination of PDT and hyperthermia, which was applied at 41°C for 1 h during cell incubation	Colon carcinoma C26 cells were treated with PDT and hyperthermia and then subjected to freeze/thawing to create the tumor vaccine. Balb/c mice were subcutaneously injected with C26 cells to form tumor models		Cells were incubated in medium containing 10 or 30 μg/mL hematoporphyrin monomethyl (HMME) for 12 h prior to PDT	Laser light at 630 nm was used to treat the cells for 20 min to a dose of 5 J/cm²	(1) PDT + hyperthermia resulted in significantly smaller tumor volumes (2) Also higher expression of CD8+ T cells, IFN-γ and HSP70, indicating immune response	He et al⁵⁰
Infrared light hyperthermia (940-1600 nm) at 310 mW/cm² to a dose of 437.5 J/cm² was combined with PDT to enhance treatment efficacy deep within the tumor		BALB/cAjcl mice were subcutaneously implanted with human squamous cell carcinoma cells	5-Aminolevulinic acid (5-ALA) was administered subcutaneously 3 h before PDT at 250 mg/kg	A linear polarized near-infrared irradiator was used to deliver 50 J/cm² of 580-740 nm light at 35 mW/cm²	(1) PDT + hyperthermia led to significant tumor growth inhibition (2) PDT or hyperthermia alone had no effect on these tumors with progression in the deep layer	Yanase et al⁵¹
Magnetoliposomes loaded with photosensitizer-based complex were used to evaluate the combined treatment of PDT and magnetohyperthermia using an AC magnetic field at 1 MHz	Pigmented melanoma B16-F10 cells were incubated with the magnetoliposomes		Liposomes were loaded with a zinc phthalocyanine (ZnPc)-based complex, and cells were incubated at a concentration of 0.25 μg/mL for 3 h before PDT	Laser light was used to deliver 670 nm light at 84 mW/cm² to doses between 0.5 and 2 J/cm²	(1) Applied separately PDT was more effective than magnetohyperthermia (2) Combined treatment reduced the B16-F10 cell viability to about half that of PDT alone	Bolfarini et al⁵²
Hyperthermia was applied simultaneously with PDT using a hot plate heating cells to 43°C ± 0.5°C during treatment to evaluate potential increase in treatment efficacy	Human osteosarcoma cell lines HOSM-1 and HOSM-2 were used		Cells were incubated for 6 h before PDT with medium containing 0.2 mmol/L 5-aminolevulinic acid hexylester (5-hALA)	A near infrared halogen lamp was used to deliver 580-740 nm light at 42 mW/cm² to doses ranging from 10 to 80 J/cm²	(1) PDT alone was effective against HOSM-2 cells (2) Reduction in cell survival of HOSM-1 cells was only seen with combination therapy, from 40 J/cm2, 16 min heat and upward	Yanase et al⁵³
A temperature-controlled incubator was used to deliver hyperthermia at different temperatures concurrently with PDT	Human grade IV glioblastoma spheroids (ACBT) and rat BT₄C rat glioma cells		Cells were incubated in either 100 or 500 μg/mL 5-aminolevulinic acid (5-ALA) for 4 h before PDT	An optical fiber was used to deliver 635 nm light at 25 mW/cm² to doses ranging from 12 to 100 J/cm²	(1) Substantial cytotoxicity at 49°C and light doses >25 J/cm² (2) Between 40°C and 46°C there was a highly synergistic effect of PDT + hyperthermia on surviving fraction	Hirschberg et al⁵⁴

Abbreviations: AC, alternating current; HSP70, heat-shock protein 70; IFN-γ, interferon γ.

Table 5.

Studies Combining Photodynamic Therapy With Focused Ultrasound.

Therapy Combination	In Vitro Tumor Model	In Vivo Tumor Model	Photosensitizer	Light Source and Dose	Main Results	Reference
Continuous-wave ultrasound at 1 MHz and 0.36 W/cm² for 1 min in combination with PDT to increase cell membrane permeability and enhance intracellular photosensitizer uptake	Murine 4T1 mammary carcinoma cells were used		Cells were incubated with chlorin e6 photosensitizer at 1 μg/mL for 4 h prior to PDT	A 650-nm laser was used to illuminate cells to 1.2 J/cm² at 10.4 mW/cm²	(1) Significant enhancement in antitumor efficacy with combined PDT and ultrasound (2) Suggested mechanism: increased ROS generation via loss of cell membrane potential and caspase-dependent apoptosis	Li et al⁴⁹
Continuous-wave ultrasound at 1 MHz and 0.36 W/cm² for 1 min was used in combination with PDT at different time points	Human breast cancer MDA-MB-231 cells were used		Cells were incubated with chlorin e6 photosensitizer at 1 μg/mL for 4 h prior to PDT	Laser light at 650 nm was used to illuminate tumor cells to a dose of 1.2 J/cm² at 1.3 mW/cm²	(1) Cell survival significantly reduced with combination therapy (2) Slightly more effective when ultrasound preceded PDT, likely due to enhanced cell membrane permeability	Wang et al⁵⁵
Combination therapy was evaluated with ultrasound at 1 MHz and 0.5 W/cm² for 90 s delivered prior to PDT	Rat C6 glioma cells were used		Tumor cells were incubated for 4 h prior to PDT with 10 μg/mL hematoporphyrin (HMME)	Laser light at 630 nm was used to illuminate cells to doses of 20, 40, 80, 120, 160, 200, and 240 J/cm² at a fluence rate of 100 mW/cm²	(1) Ultrasound + PDT effectively killed C6 glioma cells synergistically (2) 80 J/cm² was resulted in the highest apoptotic rate	Li et al⁵⁶
Combination therapy consisted of ultrasound delivered prior to PDT at 1 MHz and either 0.4, 0.7, or 1.0 W/cm² for 10 min		White random-bred rats were subcutaneously implanted with rat C6 glioma cells	Chlorin e6 conjugated with polyvinyl pyrolidone (Photolon) was injected intravenously 2.5 h prior to PDT at 2.5 mg/kg	Laser light at 661 nm was used to deliver a dose of either 50 or 100 J/cm² at 170 mW/cm²	Tumor necrosis area significantly increased in rat glioma models with ultrasound + Photolon-PDT	Tserkovsky et al⁵⁷

Abbreviations: PDT, photodynamic therapy; ROS, reactive oxygen species.

Although very few studies have been conducted so far, the experience of PDT combined with focused ultrasound suggests that this combination can provide a strong and synergistic antitumor response. Interestingly, some studies found that the response was most pronounced when ultrasound preceded PDT, and it has been suggested that this is caused by increased ROS generation due to enhanced PS uptake as a result of ultrasound-induced increase in cell membrane permeability through sonoporation.^49,55 This also suggests that a combination of focused ultrasound, PDT, and chemotherapy may be promising as both PS and chemotherapy drug uptake can be enhanced.

Combination Therapy and Tumor Hypoxia

Tumor hypoxia presents a major challenge in cancer therapy, and hypoxic tumors are often much harder to treat than well-oxygenated tumors, something that is a common problem in, for example, radiation therapy of head and neck tumors. In PDT, hypoxia is clearly a substantial problem as the lack of oxygen in the tumor cells leads to significantly less ROS generation. To achieve an efficient tumor response under hypoxic conditions, combination therapy would have to be optimized to promote some form of tumor reoxygenation. It is often the center of the tumor that is the most hypoxic, and in this case, one option could be to apply fractionated radiation therapy to effectively treat the oxygenated outer parts of the tumor. This would eventually lead to some reoxygenation of previously hypoxic tumor cells that are still viable, and at this point radiation therapy could be combined with PDT to effectively treat the previously hypoxic parts of the tumor. Another option would be to combine PDT with hyperthermia, which has been shown to target tumor vasculature, which can subsequently initiate tumor angiogenesis, leading to reoxygenation of tumor tissue.^12,13 Such an approach should, however, be explored with caution since increased tumor angiogenesis can lead to an increase in tumor growth. All in all, the effects of a hypoxic tumor environment will impact the effectiveness of combination therapy involving PDT and should be considered when evaluating different treatment options.

Summary and Conclusions

A considerable amount of preclinical evidence has been gathered regarding the combination of PDT and other forms of anticancer treatment, and this seems to be increasing rapidly. In this review, we have shown that the potential of using PDT as part of combination therapy is being investigated from many different directions, with promising results in terms of direct cytotoxicity as well as prompting a strong antitumor immune response.

A promising option that has been explored is the addition of low-dose cyclophosphamide (CY) prior to PDT in order to deplete the number of T_regs.²⁹ This combination has led to a strong antitumor response, and more importantly tumor rechallenges were only resisted in mice receiving a second CY administration at the time of rechallenge. This shows the importance of T_reg depletion and the potential of combining this not only with PDT but possibly with other anticancer treatments as well. Another promising option is anticancer vaccination using immature DCs that have been treated with PDT.^31,32 This DC priming elicited substantial immune responses, and there was even a clear growth inhibition of C26 tumors in naive mice that received splenocytes from mice treated with PDT and immature DC injections.

Adding a further dimension to this approach, Canti et al tested the combination of PDT and chemotherapy, plus adoptive immunotherapy with splenic lymphocytes from PDT-treated mice.⁴⁷ This resulted in successful treatment of an aggressive metastatic murine leukemia model, a quite promising result, especially considering the many potential extensions or alterations in this therapy combination, including also other treatment modalities. Yet a further interesting result was obtained when combining PDT with ⁶⁰Co ionizing radiation for various nonsmall cell lung cancer (NSCLC) lines.³⁸ Using this combination of therapeutic agents, a less synergistic effect on normal lung fibroblasts compared to the NSCLC cells was found, suggesting a potential increase in the therapeutic window using this approach.

Yanase et al used hyperthermia in combination with PDT in order to more effectively treat deep-seated tumors.⁵¹ Their results showed promising antitumor effects from combination therapy, where neither PDT nor hyperthermia alone was an effective treatment. The inefficiency of treating deep-seated tumors is an inherent difficulty in PDT as most of the PS absorbance wavelengths do not penetrate far into tissue. Thus combination with hyperthermia seems to be one promising alternative, although combinations with ionizing radiation or chemotherapy could also provide a way to circumvent this problem.

The use of focused ultrasound to increase cell membrane permeability seems to be an effective method to increase intratumoral ROS generation and substantially enhance the antitumor efficacy of PDT.^49,55 This approach holds great potential as neither PDT nor ultrasound is inherently dose-limiting treatments, so applying this combination also as an adjuvant to chemo- or radiation therapy may be a very interesting option.

Future Perspectives

With the availability of many different photosensitizing drugs and the large number of possible treatment combinations, there is a need for systematically evaluating the optimal dose–response combinations and making sure the risks of increasing treatment-related toxicities are low. This need for systematic evaluation is further highlighted by the heterogeneity in PDT parameters among the studies included in this review. Taking the in vivo experiments as an example, the drug concentrations ranged from 0.1 to 250 mg/kg and light doses from 50 to 360 J/cm², with large variation even between studies using the same PS. The ideal PDT setting in a combination therapy approach will therefore have to be assessed by systematically investigating different photosensitizing agents, concentrations, and light doses to find the optimal parameters for a certain indication. This will require a great deal of work, and the results will likely not be directly transferable between different tumor types.

The ideal combination therapy involving PDT will be one that can reach deep-seated tumors, show synergistic cytotoxic efficacy, promote a strong antitumor immune response, enhance tumor-specific drug uptake, and reoxygenate hypoxic tumor tissue. Based on this review, it is not straightforward to conclude which of the combination approaches comes closest to the ideal situation, but it will likely require the combination of several different treatment modalities.

Despite the fact that many of the suggested strategies may never reach clinical applicability, and the ideal combination therapy has yet to be found, PDT as part of combination anticancer treatment shows great potential and promise based on the current preclinical evidence.

Footnotes

Abbreviations

Authors’ Note

Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIH.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work has been supported in part by R01 EB009040 from the United States National Institutes of Health (NIH).

References

Castano

Demidova

Hamblin

. Mechanisms in photodynamic therapy: part one—photosensitizers, photochemistry and cellular localization. Photodiagnosis Photodyn Ther. 2004;1(4):279–293. doi:http://dx.doi.org/10.1016/S1572-1000(05)00007-4.

Mroz

Yaroslavsky

Kharkwal

Hamblin

. Cell death pathways in photodynamic therapy of cancer. Cancers(Basel). 2011;3 (2):2516–2539. doi:10.3390/cancers3022516.

Allison

Downie

Cuenca

Childs

CJH

Sibata

. Photosensitizers in clinical PDT. Photodiagnosis Photodyn Ther. 2004;1(1):27–42. doi:http://dx.doi.org/10.1016/S1572-1000(04)00007-9.

Allison

Sibata

. Oncologic photodynamic therapy photosensitizers: a clinical review. Photodiagnosis Photodyn Ther. 2010;7 (2):61–75. doi:10.1016/j.pdpdt.2010.02.001.

Detty

Gibson

Wagner

. Current clinical and preclinical photosensitizers for use in photodynamic therapy. J Med Chem. 2004;47 (16):3897–3915. doi:10.1021/jm040074b.

Agostinis

Vantieghem

Merlevede

de Witte

. Hypericin in cancer treatment: more light on the way. Int J Biochem Cell Biol. 2002;34 (3):221–241. doi :10.1016/S1357-2725(01)00126-1.

Kiesslich

Krammer

Plaetzer

. Cellular mechanisms and prospective applications of hypericin in photodynamic therapy. Curr Med Chem. 2006;13 (18):2189–2204. doi:10.2174/092986706777935267.

Nakajima

Kawashima

. A basic study on hypericin-PDT in vitro. Photodiagnosis Photodyn Ther. 2012;9 (3):196–203. doi:10.1016/j.pdpdt.2012.01.008.

Wilson

Patterson

. The physics, biophysics and technology of photodynamic therapy. Phys Med Biol. 2008;53 (9):R61–R109. doi:10.1088/0031-9155/53/9/R01.

10.

Ishizuka

Abe

Sano

. Novel development of 5-aminolevurinic acid (ALA) in cancer diagnoses and therapy. Int Immunopharmacol. 2011;11 (3):358–365. doi:10.1016/j.intimp.2010.11.029.

11.

Brancaleon

Moseley

. Laser and non-laser light sources for photodynamic therapy. Lasers Med Sci. 2002;17 (3):173–186. doi:10.1007/s101030200027.

12.

Garg

Nowis

Golab

Agostinis

. Photodynamic therapy: illuminating the road from cell death towards anti-tumour immunity. Apoptosis. 2010;15 (9):1050–1071. doi:10.1007/s10495-010-0479-7.

13.

Chen

Roskams

de Witte

. Enhancing the antitumoral effect of hypericin-mediated photodynamic therapy by hyperthermia. Lasers Surg Med. 2002;31 (3):158–163. doi:10.1002/lsm.10089.

14.

Mroz

Hashmi

Huang

Lange

Hamblin

. Stimulation of anti-tumor immunity by photodynamic therapy. Expert Rev Clin Immunol. 2011;7 (1):75–91. doi:10.1586/eci.10.81.

15.

Kroemer

Galluzzi

Kepp

Zitvogel

. Immunogenic cell death in cancer therapy. Annu Rev Immunol. 2013;31:51–72. doi:10.1146/annurev-immunol-032712-100008.

16.

Garg

Nowis

Golab

Vandenabeele

Krysko

Agostinis

. Immunogenic cell death, DAMPs and anticancer therapeutics: an emerging amalgamation. Biochim Biophys Acta. 2010;1805 (1):53–71. doi:10.1016/j.bbcan.2009.08.003.

17.

Garg

Dudek

Agostinis

. Cancer immunogenicity, danger signals, and DAMPs: what, when, and how? Biofactors. 2013;39 (4):355–367. doi:10.1002/biof.1125.

18.

Krysko

Garg

Kaczmarek

Krysko

Agostinis

Vandenabeele

. Immunogenic cell death and DAMPs in cancer therapy. Nat Rev Cancer. 2012;12 (12):860–875. doi:10.1038/nrc3380.

19.

Garg

Krysko

Vandenabeele

Agostinis

. The emergence of phox-ER stress induced immunogenic apoptosis. Oncoimmunology. 2012;1 (5):786–788. doi:10.4161/onci.19750.

20.

Garg

Krysko

Verfaillie

. A novel pathway combining calreticulin exposure and ATP secretion in immunogenic cancer cell death. EMBO J. 2012;31 (5):1062–1079. doi:10.1038/emboj.2011.497.

21.

St Denis

Aziz

Waheed

. Combination approaches to potentiate immune response after photodynamic therapy for cancer. Photochem Photobiol Sci. 2011;10 (5):792–801. doi:10.1039/c0pp00326c.

22.

Chen

Korbelik

Bartels

Liu

Sun

Nordquist

. Enhancement of laser cancer treatment by a chitosan-derived immunoadjuvant. Photochem Photobiol. 2005;81 (1):190–195. doi:10.1562/2004-07-20-RA-236.

23.

Korbelik

Sun

Cecic

Serrano

. Adjuvant treatment for complement activation increases the effectiveness of photodynamic therapy of solid tumors. Photochem Photobiol Sci. 2004;3 (8):812–816. doi:10.1039/b315663J.

24.

Winters

Daayana

Lear

. Clinical and immunologic results of a phase II trial of sequential imiquimod and photodynamic therapy for vulval intraepithelial neoplasia. Clin Cancer Res. 2008;14 (16):5292–5299. doi:10.1158/1078-0432.CCR-07-4760.

25.

Uehara

Sano

Wang

Sekine

Ikeda

Inokuchi

. Enhancement of the photodynamic antitumor effect by streptococcal preparation OK-432 in the mouse carcinoma. Cancer Immunol Immunother. 2000;49 (8):401–409. doi:10.1007/s002620000134.

26.

Korbelik

Sun

Posakony

. Interaction between photodynamic therapy and BCG immunotherapy responsible for the reduced recurrence of treated mouse tumors. Photochem Photobiol. 2001;73 (4):403–409. doi :10.1562/0031-8655(2001)0730403IBPTAB2.0.CO2.

27.

Korbelik

Cecic

. Enhancement of tumour response to photodynamic therapy by adjuvant mycobacterium cell-wall treatment. J Photochem Photobiol B. 1998;44 (2):151–158. doi:10.1016/S1011-1344(98)00138-9.

28.

Korbelik

Naraparaju

Yamamoto

. Macrophage-directed immunotherapy as adjuvant to photodynamic therapy of cancer. Br J Cancer. 1997;75 (2):202–207. doi :10.1038/bjc.

29.

Reginato

Mroz

Chung

Kawakubo

Wolf

Hamblin

. Photodynamic therapy plus regulatory T-cell depletion produces immunity against a mouse tumour that expresses a self-antigen. Br J Cancer. 2013;109 (8):2167–2174. doi:10.1038/bjc.2013.580.

30.

Golab

Wilczynski

Zagozdzon

. Potentiation of the anti-tumour effects of Photofrin-based photodynamic therapy by localized treatment with G-CSF. Br J Cancer. 2000;82 (8):1485–1491. doi:10.1054/bjoc.1999.1078.

31.

Jalili

Makowski

Switaj

. Effective photoimmunotherapy of murine colon carcinoma induced by the combination of photodynamic therapy and dendritic cells. Clin Cancer Res. 2004;10 (13):4498–4508. doi:10.1158/1078-0432.CCR-04-0367.

32.

Saji

Song

Furumoto

Kato

Engleman

. Systemic antitumor effect of intratumoral injection of dendritic cells in combination with local photodynamic therapy. Clin Cancer Res. 2006;12 (8):2568–2574. doi:10.1158/1078-0432.CCR-05-1986.

33.

Luksiene

Kalvelyte

Supino

. On the combination of photodynamic therapy with ionizing radiation. J Photochem Photobiol B Biol. 1999;52 (1-3):35–42. doi :10.1016/S1011-1344(99)00098-6.

34.

Ramakrishnan

Clay

Friedman

Antunez

Oleinick

. Post-treatment interactions of photodynamic and radiation-induced cytotoxic lesions. Photochem Photobiol. 1990;52 (3):555–559. doi :10.1111/j.1751-1097.1990.tb01799.x.

35.

Sazgarnia

Montazerabadi

Bahreyni-Toosi

Ahmadi

Aledavood

. In vitro survival of MCF-7 breast cancer cells following combined treatment with ionizing radiation and mitoxantrone-mediated photodynamic therapy. Photodiagnosis Photodyn Ther. 2013;10(1):72–78. doi:10.1016/j.pdpdt.2012.06.001.

36.

Cheung

Chan

Fung

. Antiproliferative effect of pheophorbide a-mediated photodynamic therapy and its synergistic effect with doxorubicin on multiple drug-resistant uterine sarcoma cell MES-SA/Dx5. Drug Chem Toxicol. 2013;36 (4):474–483. doi:10.3109/01480545.2013.776584.

37.

Allman

Cowburn

Mason

. Effect of photodynamic therapy in combination with ionizing radiation on human squamous cell carcinoma cell lines of the head and neck. Br J Cancer. 2000;83 (5):655–661. doi:10.1054/bjoc.2000.1328.

38.

Sharma

Farrell

Patterson

. In vitro survival of nonsmall cell lung cancer cells following combined treatment with ionizing radiation and Photofrin-mediated photodynamic therapy. Photochem Photobiol. 2009;85(1):99–106. doi:10.1111/j.1751-1097.2008.00401.x.

39.

Nakano

Watanabe

Akita

Kawamura

Tamada

Matsumoto

. Treatment efficiency of combining photodynamic therapy and ionizing radiation for Bowen’s disease. J Eur Acad Dermatol Venereol. 2011;25 (4):475–478. doi:10.1111/j.1468-3083.2010.03757.x.

40.

Ahn

Biswas

Mondal

Lee

Chung

. Cisplatin enhances the efficacy of 5-aminolevulinic acid mediated photodynamic therapy in human head and neck squamous cell carcinoma. Gen Physiol Biophys. 2014;33(1):53–62. doi:10.4149/gpb_2013046.

41.

Wei

Liu

Zhang

. Synergistic anticancer activity of 5-aminolevulinic acid photodynamic therapy in combination with low-dose cisplatin on hela cells. Asian Pac J Cancer Prev. 2013;14 (5):3023–3028. doi :10.7314/APJCP.2013.14.5.3023.

42.

Sun

Kajimoto

Inoue

Miyatake

Ishikawa

Kuroiwa

. Gefitinib enhances the efficacy of photodynamic therapy using 5-aminolevulinic acid in malignant brain tumor cells. Photodiagnosis Photodyn Ther. 2013;10 (1):42–50. doi:10.1016/j.pdpdt.2012.06.003.

43.

Fang

. Combination of apoptin with photodynamic therapy induces nasopharyngeal carcinoma cell death in vitro and in vivo. Oncol Rep. 2012;28(6):2077–2082. doi:10.3892/or.2012.2044.

44.

Nonaka

Nanashima

Nonaka

. Synergic effect of photodynamic therapy using talaporfin sodium with conventional anticancer chemotherapy for the treatment of bile duct carcinoma. J Surg Res. 2013;181 (2):234–241. doi:10.1016/j.jss.2012.06.047.

45.

Tong

Miao

Liu

Jia

Liu

. Enhanced antitumor effects of BPD-MA-mediated photodynamic therapy combined with adriamycin on breast cancer in mice. Acta Pharmacol Sin. 2012;33 (10):1319–1324. doi:10.1038/aps.2012.45.

46.

Diez

Ernst

Teijo

Batlle

Hajos

Fukuda

. Combined chemotherapy and ALA-based photodynamic therapy in leukemic murine cells. Leukemia Res. 2012;36 (9):1179–1184. doi:10.1016/j.leukres.2012.04.027.

47.

Canti

Calastretti

Bevilacqua

Reddi

Palumbo

Nicolin

. Combination of photodynamic therapy + immunotherapy + chemotherapy in murine leukiemia. Neoplasma. 2010;57(2):184–188. doi :10.4149/neo_2010_02_184.

48.

Chen

Agostinis

De Witte

. Synergistic effect of photodynamic therapy with hypericin in combination with hyperthermia on loss of clonogenicity of RIF-1 cells. Int J Oncol. 2001;18 (6):1279–1285. doi :10.3892/ijo.18.6.1279.

49.

Wang

. Efficacy of chlorin e6-mediated sono-photodynamic therapy on 4T1 cells. Cancer Biother Radiopharm. 2014;29 (1):42–52. doi:10.1089/cbr.2013.1526.

50.

. Haematoporphyrin based photodynamic therapy combined with hyperthermia provided effective therapeutic vaccine effect against colon cancer growth in mice. Int J Med Sci. 2012;9 (8):627–633. doi:10.7150/ijms.4865.

51.

Yanase

Nomura

Matsumura

Kato

Tagawa

. Hyperthermia enhances the antitumor effect of photodynamic therapy with ALA hexyl ester in a squamous cell carcinoma tumor model. Photodiagnosis Photodyn Ther. 2012;9 (4):369–375. doi:10.1016/j.pdpdt.2012.04.003.

52.

Bolfarini

Siqueira-Moura

Demets

Morais

Tedesco

. In vitro evaluation of combined hyperthermia and photodynamic effects using magnetoliposomes loaded with cucurbituril zinc phthalocyanine complex on melanoma. J Photochem Photobiol B. 2012;115:1–4. doi:10.1016/j.jphotobiol.2012.05.009.

53.

Yanase

Nomura

Matsumura

Watanabe

Tagawa

. Synergistic increase in osteosarcoma cell sensitivity to photodynamic therapy with aminolevulinic acid hexyl ester in the presence of hyperthermia. Photomed Laser Surg. 2009;27 (5):791–797. doi:10.1089/pho.2008.2329.

54.

Hirschberg

Sun

Tromberg

Yeh

Madsen

. Enhanced cytotoxic effects of 5-aminolevulinic acid-mediated photodynamic therapy by concurrent hyperthermia in glioma spheroids. J Neurooncol. 2004;70 (3):289–299. doi :10.1179/1743132810Y.0000000001.

55.

Wang

Zhang

Yang

Liu

. Ultrasound enhances the efficacy of chlorin E6-mediated photodynamic therapy in MDA-MB-231 cells. Ultrasound Med Biol. 2013;39 (9):1713–1724. doi:10.1016/j.ultrasmedbio.2013.03.017.

56.

Chen

Huang

. In vitro study of low intensity ultrasound combined with different doses of PDT: effects on C6 glioma cells. Oncol Lett. 2013;5(2):702–706. doi:10.3892/ol.2012.1060.

57.

Tserkovsky

Alexandrova

Chalau

Istomin

. Effects of combined sonodynamic and photodynamic therapies with photolon on a glioma C6 tumor model. Exp Oncol. 2012;34 (4):332–335.

Photodynamic Therapy and Its Role in Combined Modality Anticancer Treatment

Abstract

Keywords

Photodynamic Therapy

Immunogenic Cell Death and PDT

Photodynamic Therapy in Combination With Immunostimulants

Photodynamic Therapy in Combination With Established Anticancer Therapy

Photodynamic Therapy in Combination With Experimental Anticancer Therapy

Combination Therapy and Tumor Hypoxia

Summary and Conclusions

Future Perspectives

Footnotes

Abbreviations

Authors’ Note

Declaration of Conflicting Interests

Funding

References