5 mm once weekly for 3 weeks) and PDT (2 mg/ kg Photofrin, followed by 200 J/cm2 illumination 48 h post-infusion). Group one received HDRrst and Group two received PDT rst. In Group one patients, local tumor control was achieved in 6 of 7 patients for: 3 months (until death), 15 months, 2+ years (until death), 2+ years (ongoing), and 5+years (ongoing, N = 2). In Group two, local control was achieved in only one patient, for 84 days. Authors conclude that combined HDR/PDT treatment for endobronchial tumors is well tolerated and can achieve prolonged local control with acceptable morbidity when PDT follows HDR and when the spacing between treatments is 1 month or less [106].

Commentary

As a palliative treatment, PDT can be used as a single method or in a combined therapy. The excellent work published by McCaughan and Williams is very convincing in the sense that patients with endobronchial obstruction caused by advanced cancer still have a median survival of 7 months and then aggressive palliation seems to be indicated. The correct treatment is something that physicians evaluate considering patient characteristics, availability of the different methods, technique costs, and nally, his or her own personal experience.

Nd-YAG laser and PDT are effective in improving airway obstruction caused by intraluminal tumors. Resection with Nd-YAG laser seems to be the best method in centrally located tumors, which are easily reached with the rigid bronchoscope, coagulated, and then resected. PDT, on the other hand, its application through a fexible bronchoscope can treat more peripheral lesions and does not require mechanical removal after irradiation. However, a cleaning bronchoscopy after PDT to remove debris is needed.

PDT cannot be used in patients with tracheal lesions extended to both main bronchi and extensive carina involvement or in patients with pneumonectomy. The infammatory reaction following PDT treatment generates edema that can be severe, worsening obstruction and risking patient’s life.

When the tumor has in ltrated the tracheobronchial wall or vascular structures, PDT application may cause perforation and/or fatal bleeding. Another disadvantage is that PDT does not relieve the obstruction immediately, and patients who present acute obstructive symptoms are not candidates for this treatment and should be treated with a fast re-opening method.

Photoresection with Nd-YAG laser and PDT are both ineffective when there is submucosal in ltration or extrinsic compression. In this case, patients can bene t from radiation therapy, and if necessary, placement of airway prosthesis. Since most patients have a combination of intraluminal, submucosal, and peri-bronchial tumor involvement, it seems reasonable to use a combination of PDT and external radiation [106]. PDT can also be used as palliation for debulking an obstructed or a stenotic bronchus or to reduce tumor extension in order to perform a less aggressive surgery [55, 107].

PDT is recognized worldwide as a palliative option for advanced non-small cell lung cancer. In fact, many studies report a poor quality of life with a shorted life in patients with metastatic non-small lung cancer [108, 109].

PDT can be a palliative option for patients with locally advanced or metastatic non-small cell lung cancer, decreasing dyspnea, and airway obstruction improving respiratory function and quality of life [110].

Complementary Endoscopic Methods for PDT Applications

In the evaluation of early-stage lung cancer, EBUS can determine the real depth of “in situ” diseases, because in many cases, the mucosa appears macroscopically intact or has only minimal changes. With EBUS submucosa invasion or peri-bronchial extension can be detected more accurately. The absence of invasion con rmed by the EBUS suggests localized tumor and can be treated endoscopically with curative intent [111].

Other authors also suggest that the absence of tracheobronchial wall invasion assessed by EBUS de nes injury as “early disease” and there-

fore should be considered an indication for the successful use of PDT [112].

Kurimoto et al. demonstrated the usefulness of EBUS in evaluating the depth of bronchial tumor and its accuracy. EBUS was in accordance with histopathological ndings in 95.8% of the cases. Five layers of the bronchial wall in ultrasound images are de ned. From the third to thefth layer, it corresponds to the cartilage. Phototherapeutic treatment response is complete in lesions whose third (sonographically de ned) layer is intact [113].

Takahashi et al. performed EBUS to evaluate the degree of carcinoma invasion into the bronchial wall in 22 lesions suspected of central early lung cancer before treatment. Fourteen lesions were diagnosed to be intracartilaginous lesions, and ten of them were treated by PDT. A complete remission was obtained in nine patients [114].

In fact, the endoscopic view is limited to the surface of the airway. Ultrasound can evaluate structures in depth. Processes located on the wall or outside the lumen can only be suspected by indirect signs such as discoloration, edema of the wall, changes in the vasculature, elevation of the mucosa, and distortion of the bronchial wall. Many abnormalities involving the peribronchial structures have no visible signs. Advanced imaging techniques such as computerized axial tomography or magnetic resonance imaging can be useful, but they are limited in detecting carcinomatous spread in peribronchial areas.

Optical coherence tomography (OCT) examination of the airways provides high-resolution images of the bronchial surface, making possible a detailed examination of intraepithelial lesions. Tissue layers between epithelium and basement membrane are clearly demonstrated, which is helpful to evaluate the depth of invasion of bronchial tumors [115, 116].

EBUS evaluation helps to determine the extent and depth of tumor invasion and to select the optimal treatment modality. In the near future, OCT will be widely applied and may prove to be a better complementary method for PDT treatment.

Clinical Applications of New

Sensitizers

The investigation of the new sensitizers of the second and third generations, presents new perspectives of treatment obtaining similar to better results than those obtained with sensitizers of therst generation.

Clinical experience is available for non surgical patients, advanced non-small cell carcinoma, applying NPe6 talapor n sodium, Laserphyrin®, combined with chemotherapy [117]. At 1 month after PDT, symptoms and QOL were improved in all patients, and there was an objective response to treatment, as indicated by a substantial increase in the openings of the bronchial lumen and the prevention of obstructive pneumonia, with minimum adverse events. Even when the number of patients was small, authors were enthusiastic that, if the results are supported by additional studies, they would recommend that procedures be adopted as standard therapy.

Radachlorine, another second-generation photosensitizer, was applied in ten patients with advanced non-small lung cancer with central airway obstruction (Stage IIIA or higher). All patients received 1 mg/kg of Radachlorine®, 4 h before light irradiation. Twenty percent of patients showed successful results, 70% showed partially successful results and 10% showed an unsuccessful result. The 1 year survival rate after PDT was 70% and was signi cantly improved than that obtained by patients with non-small cell lung carcinoma treated with conventional therapy. Authors conclude that Radachlorine®-based PDT is safe and effective treatment for relieving central airway obstruction in advanced NSCLC[118].

Neoadjuvant therapy is often given in an attempt to shrink tumors and improve the chance of successful surgery. In one study 42 patients were randomized to either neoadjuvant chemotherapy and endobronchial PDT or chemotherapy alone followed by surgical resection. Chlorine E6 and laser light at 662 nm were used to perform PDT before each of the three courses of chemotherapy.

After neoadjuvant treatment partial response was obtained in 19 pts (90%) in the PDT arm and

16 pts (76%) in the No-PDT arm, these patients underwent thoracotomy. In the group of PDT, 14 pneumonectomies and 5 lobectomies were performed. In the non-PDT group, patients underwent ten pneumonectomies and three lobectomies, with three patients showing unresectable tumors. Completeness of resection was signi cantly higher in the PDT group.

The authors concluded that neoadjuvant PDT along with chemotherapy made possible to convert to surgical candidates and to improve resection completeness in stage III central NSCLC patients, and it is effective and safe. The addition of Radachlorine®-PDT to preoperative CT signi cantly increased the number of patients eligible for radical resection compared to neoadjuvant CT alone [119].

An alternative approach in which light activation enables spatiotemporal speci city and control of the intracellular drug release, called Photo-Chemical Internalization (PCI), is a modi-ed form of PDT with a light-controlled drug-­ delivery [120]. This novel approach combined with CSPG4-targeting immunotox in 225.28-saporin was applied in three cases of TNBC (triple negative breast cancer cells) and two mutated malignant melanoma cells. Results showed that the combination of the drug delivery technology PCI and CSPG4-targeting immunotoxins is an ef cient, speci c, and light-­controlled strategy for the elimination of aggressive cells of TNBC and malignant melanoma origin [121].

New Perspectives

The choice of optimal combinations of photosensitizers, light sources, and treatment parameters are crucial for effective PDT The ef cacy of therapy depends upon type of photosensitizer, oxygen concentration within target tumor cells, dose of light applied, and concentration of photosensitizer with cancer cells [122].

In this sense, research on the transport of photosensitizers by means of nanoparticles, improving permeability and retention, opens up a promising horizon in improving the ef cacy of PDT in the treatment of tumors [123–126].

Other PDT Applications

Microbacterial Resistance  In the last decade, microbial resistance to antibiotics has increased in a large number of diseases [127]. The treatment of Infectious diseases using PDT is a relatively recent application that does not discriminate between strains that are and are not resistant to antibiotics [128]. Antimicrobial Photodynamic Therapy (aPDT) has become an important component in the treatment of human infections [129].

Multiple and extreme (a lack of susceptibility to four or more drugs) antibiotic resistance has necessitated the use of alternative treatment methods for microbial infection including electroporation; antimicrobial peptides; photothermal therapy; or photodynamic therapy (PDT). Photoactivated porphyrins display a potent cytotoxic activity toward a variety of Gram-positive bacteria, mycoplasma, and yeasts, but not Gram-­ negative microbial cells, as show Malik et al. in an study where they observed the changes produced by PDT in the structures of gram-positive and gram-negative microorganisms, analyzing the differences between them and their sensitivity to PDT[130].

Anti-Tumor Immune Response  PDT not only kills the targeted cells and damages the tumor associated vasculature but also activates an antitumor immune response through processing of antigens from destroyed tumor cells by dendritic cells, which in turn serve as antigen presenting cells to T cells. However, tumor cells adapt to varying degrees of immune cell evasion. Antitumor effects of PDT derive from three interrelated mechanisms: direct cytotoxic effects on tumor cells, damage to the tumor vasculature and induction of a robust infammatory reaction, PDT produces the tumor-cell destruction in the context of acute infammation acts as a ‘danger signal’ to the innate immune system.

The infammation produced by the PDT photoreaction produces start-up of T lymphocytes with the ability to recognize and destroy tumor cells at a

distance and develop immune memory to act later against possible tumor recurrences. The escape mechanisms that the tumor has in order to progress and evade the immune attack, can be prevented by PDT in combination with immunomodulatory strategies capable of overcoming or evading those escape mechanisms used by the progressing tumor. In this way, this process leads to development of systemic immunity and makes itself vulnerable to evoke antitumor immune response followed by tumor destruction [131, 132].

The best way to restore an immune system response against tumors is to therapeutically elicit a cancer cell death pathway that is associated with high immunogenicity and possibly capable of inhibiting or reducing the infuence of protumorigenic cytokine signaling [133].

Antitumor activity of infammatory cells and immune reactions are triggered by the sensitized tumor. These reactions contribute to more complete tumor destruction. But there are some factors that limit it, such as the uneven distribution of the PS agent inside the tumor, or oxygen availability. PS should be able to induce an immunogenic response over treated cells such as changes of surface glycoproteins receptors and consequently activate a cascade of immunologic cell response and malignant cell death.

The strong infammatory process produced by PDT in the form of local edema at the site of the photothermal reaction can lead to the development of a systemic immune response, which is a non-tumor antigen-speci c process produced by the immune system itself. Therefore, the development of a novel combination of PDT and immune checkpoint blockade therapy could be bene cial for metastatic lung cancer [134].

Three immune checkpoint agents for melanoma therapy have been approved by the FDA and other drugs will be approved to treat patients with various cancer types including kidney, lung, bladder, and prostate cancer [135].

In 2011, the antibody agent against CTLA-4 (ipilimumab) was approved and 3 years later, other two antibody agents against PD-1 (pembrolizumab and nivolumab) were approved [136, 137].

Photothermal therapy is based on the conversion of light energy (usually in the near-infrared

region) into heat energy to induce subsequent cellular necrosis or apoptosis [138]. The combination of photodynamic therapy (PDT) and photothermal therapy (PTT) was used in order to test the immune responses stimulated for the treatment of advanced cancer.

In one animal lab study, Yang et al. engineered a multitask theranostic platform Gd-Ce6@ SWNHs Gd3+ and chlorin e6 loaded single-walled carbon nanohorns(Gd-Ce6@SWNHs), to study the synergistic immunologic responses triggered by sequential PDT and PTT on the primary tumor and they have demonstrated that they are a strong immune adjuvant, and have high tumor targeting and penetration ef ciency.

Sequential photodynamic and photothermal therapy ablates the primary tumors and triggers synergistic, complementary and long-lasting host immune response, inhibiting the spontaneous pulmonary metastases and tolerating cancer rechallenge.

The authors conclude that the study has demonstrated the great potency of combined immune-­stimulating therapies of PDT + PTT using Gd-Ce6@SWNHs and provided a potential way toward tumor synergistic immunotherapy, which is promising for the elimination of tumor metastases and inhibition of recurrence in the clinic. However, the antitumor efficacy by PDT or PTT alone is less potent and unsustainable against cancer metastasis and relapse [139].

Near Infrared-Photoimmunotherapy (NIR-­ PIT) is a new, highly selective tumor treatment that employs an antibody-photon absorber conjugate (APC). When the APC attaches to its target cell and is exposed to NIR light, highly selective cell killing is observed.

Nagaya et al. show the ef cacy of NIR-PIT, using hYP218 as the antibody within the APC to target a mesothelin expressing A431/H9 cell.

They conclude that the new anti-mesothelin antibody, hYP218, is suitable as an antibody-­ drug conjugate for NIR-PIT. Furthermore, NIR-­ PIT with hYP218-IR700 is a promising candidate for the treatment of mesothelinexpressing tumors that could be readily translated to humans [140].