Critical PDT theory: Viability, cytotoxicity, proliferation and clonogenicity-Is there a difference?

Comparison of MTT versus clonogenic effects of photodynamic therapy in MCF-7c3 breast cancer cells in culture. Left: MTT data; right clonogenic data. A collection of photosensitizers was used to produce these results.

Sites of sub-cellular photodamage: Apoptotic and autophagic responses

Log dose-response curves relating to the effect of mitochondrial photodamage on clonogenic survival using benzoporphyrin derivative. With wild-type cells, autophagy produces a "shoulder" on the curve which is absent when an ATG5 knockdown is examined. Loss of ATG5 prevents the process of autophagy, which is seen to be cytoprotective.

Critical PDT Theory VII: Preclinical Translation

The translation of photodynamic effects into clinical practice is a complex process that involves the pharmacokinetics of photosensitizing agents, light dosimetry and oxygenation levels. But even the 'translation' of basic photobiology into meaningful preclinical information can be challenging. Some thoughts on directions for progress in clinical trials are suggested.

Adventures in Photodynamic Therapy: Location, Location, Location

In the context of photodynamic therapy, reports periodically turn up in journals where reviewers are apparently unacquainted with the essentials. Bizarre procedures and results can thereby appear. This appears to be a byproduct of the publishing industry, especially for some of the "pay to play" options.

Critical PDT Theory VI: Detection of Reactive Oxygen Species: Trials and Errors

Fluorescence intensity of DCFH-DA in hepatoma 1c1c7 cells after 10 min incubations.

Critical PDT Theory V: What it Takes

First evidence for an apoptotic response to photodamage provided by Oleinick's group in 1991.

Critical PDT Theory VII: The Saga of Ruthenium

Recent studies involving photosensitizing agents containing ruthenium (Ru) are interpreted as an indication that this approach might be useful for treatment of bladder cancer. The absorbance spectrum of such agents tends to be limited to wavelengths < 600 nm. While this can spare underlying tissues from photodamage, this will limit applications to instances where only a thin layer of malignant cells is present. Among the more potentially interesting results is a protocol that uses only Ru nanoparticles. Other issues in Ru-based photodynamic therapy are discussed including the limited absorbance spectrum, questions relating to methodology and a general lack of details concerning localization and death pathways.

Photochemical Targeting of Mitochondria to Overcome Chemoresistance in Ovarian Cancer †

Ovarian cancer is the most lethal gynecologic malignancy with a stubborn mortality rate of ~65%. The persistent failure of multiline chemotherapy, and significant tumor heterogeneity, has made it challenging to improve outcomes. A target of increasing interest is the mitochondrion because of its essential role in critical cellular functions, and the significance of metabolic adaptation in chemoresistance. This review describes mitochondrial processes, including metabolic reprogramming, mitochondrial transfer and mitochondrial dynamics in ovarian cancer progression and chemoresistance. The effect of malignant ascites, or excess peritoneal fluid, on mitochondrial function is discussed. The role of photodynamic therapy (PDT) in overcoming mitochondria-mediated resistance is presented. PDT, a photochemistry-based modality, involves the light-based activation of a photosensitizer leading to the production of short-lived reactive molecular species and spatiotemporally confined photodamage to nearby organelles and biological targets. The consequential effects range from subcytotoxic priming of target cells for increased sensitivity to subsequent treatments, such as chemotherapy, to direct cell killing. This review discusses how PDT-based approaches can address key limitations of current treatments. Specifically, an overview of the mechanisms by which PDT alters mitochondrial function, and a summary of preclinical advancements and clinical PDT experience in ovarian cancer are provided.

Critical PDT theory II: Current concepts and indications

While photodynamic therapy (PDT) is effective for the eradication of select neoplasia and certain other pathologic conditions, it has yet to achieve wide acceptance in clinical medicine. A variety of factors contribute to this situation including relations with the pharmaceutical industry that have often been problematic. Some current studies relating to photodynamic effects are 'phenomenological', i.e., they describe phenomena that only reiterate what is already known. The net result has been a tendency of granting agencies to become disillusioned with support for PDT research. This report is intended to provide some thoughts on current research efforts that improve clinical relevance and those that do not.

Paul K Strudler: The Oracle of RAD

Paul Strudler who facilitated the support of photodynamic therapy by an NIH study section.

Critical PDT Theory IV: The Strange Case of 'KillerRed'?

A 27 kDa protein termed 'KillerRed' (KR) contains a chromophore with photodynamic properties. Upon irradiation with green light, this protein behaves much like a 'classical' photosensitizing agent. Fluorescence is produced and a photophysical reaction results in the conversion of nearby oxygen molecules to reactive oxygen species. It is periodically claimed that these effects would permit KR to mimic the anti-tumor properties of photosensitizing agents that have demonstrated clinical efficacy for cancer control. Based on the photophysical properties of KR and issues associated with its use, the likelihood appears to be dim. Moreover, there are many concerns with regard to some current reports on KR, summarized here.

Critical PDT Theory III: Events at the Molecular and Cellular Level

Photodynamic therapy (PDT) is capable of eradicating neoplastic cells that are accessible to sufficient light and oxygen. There is adequate information now available for assessing conditions where PDT might be the therapy of choice, but limited access to clinical facilities and impediments to regulatory approval of new agents have limited clinical usage. Early reports mainly involved clinical data with few thoughts towards finding death pathways. In 2022, there is a clear understanding of the determinants of successful tumor eradication. While PDT may be the optimal method for many clinical indications, support for this approach has lagged. This report provides a commentary on some elements of recent progress in PDT at the molecular and cellular levels, along with a discussion of some of the limitations in current research efforts.

Photodynamic Therapy: Critical PDT Theory

Photodynamic therapy can be useful for eradication of malignant cells at sites that are accessible to light delivery. There are few adverse effects, with many clinical reports indicating that PDT has curative potential. Patients with minimal disease, where success is more likely, are also sought by those promoting other protocols. New photosensitizing agents that initiate light-catalyzed reactions continue to be discovered. Reports describing advances in understanding fundamental aspects of photobiology are always of interest. But implications for treatment of neoplasia and other diseases are not always justified, especially when poorly-penetrating wavelengths of light are employed, often at very high light doses. Efficacy is sometimes estimated by protocols that may not accurately measure photokilling. Many reports claiming potential clinical relevance for in vitro observations are based on a limited understanding of the determinants of clinical efficacy. The future of photodynamic therapy depends on an appreciation of what can be accomplished, especially when used with other modalities, but will also depend on the goals and interests of granting agencies, pharmaceutical groups and clinical personnel. This commentary is intended to provide some thoughts on current research efforts, especially where clinical implications are suggested, hinted at or otherwise implied.

Detection of Paraptosis After Photodynamic Therapy

Photodynamic therapy (PDT) is a procedure for the selective photosensitization of neoplastic tissues. Subsequent irradiation with visible light can lead to cell death along with vascular shutdown and other responses that lead to selective eradication of malignant cells. Among the cellular responses to PDT are necrosis, apoptosis, and autophagy. These pathways have generally been associated with cell death, although autophagy can also be cytoprotective. A fourth effect that has hitherto been somewhat neglected is termed "paraptosis," a lethal response that can be identified by detecting an extensive collection of cytoplasmic vacuoles. Unlike autophagy, these vacuoles are bound by single membranes. Paraptosis has been characterized as a response to misfolded endoplasmic reticulum proteins that must be "cleared" if the affected cell is to survive. At present, there is no simple biochemical test for paraptosis. This chapter describes the procedure for detection of paraptosis using phase-contrast microscopy, along with some confirmatory approaches.

Photodynamic Therapy: Deaths in the Family

The field of photodynamic therapy has suffered some significant losses during the past decade. Among those was James Kennedy, shown at the Innsbruck IPA conference (2011).

Characteristics of an Impaired PDT Response

A concurrent human papilloma virus (HPV) infection potentiates the efficacy of ionizing radiation for treatment of head and neck cancer by promoting apoptosis. Studies in cell culture indicated an opposite effect for photodynamic therapy (PDT) when this leads to mitochondrial and ER photodamage. The explanation for this difference in PDT efficacy remains to be established. While apoptosis was impaired in HPV(-) cells, such cells can be killed via photodamage directed at the ER: this leads to a nonapoptotic death pathway termed paraptosis. No differences in photosensitizer uptake or reactive oxygen species (ROS) production were observed in HPV(+) vs. HPV(-) tumors. We now provide evidence that death pathways initiated by ER/mitochondrial photodamage leading to either paraptosis or apoptosis are impaired in an HPV(+) head and neck cell line. These results illustrate the complex determinants of PDT efficacy, a topic that has yet to be fully explored.

Paraptosis after ER Photodamage Initiated by m-tetra(hydroxyphenyl) Chlorin

Two cell lines, A549 (human-derived nonsmall-cell lung cancer) and 1c1c7 (mouse hepatoma), were photosensitized with m-THPC and irradiated under LD₉₀ conditions. After 4 h, a pattern of cytoplasmic vacuoles had formed consistent with the initiation of paraptosis. After irradiation, there was no detectable loss of the mitochondrial membrane potential indicating no significant photodamage to mitochondria. We did, however, observe localization of m-THPC in the endoplasmic reticulum (ER), as indicated by fluorescence microscopy. Subsequent ER perturbation is known to result in initiation of paraptosis, another pathway to cell death. While an apoptotic response to m-THPC has been reported, the ability to target ER and induce paraptosis could explain the efficacy of this agent which could therefore eradicate cell types with an impaired apoptotic response.

Death Pathways Associated with Photodynamic Therapy

This report describes studies involving ER vs. lysosomal targeting and is designed to assess the initiation of different death pathways as a function of subcellular targeting and PDT dose. Photodamage directed at mitochondria or lysosomes initiates apoptosis, a death pathway generally considered to be irreversible. Photodamage that involves the ER can lead to another death pathway termed paraptosis. This does not involve caspase activation, can eradicate cell types with impaired apoptosis; at high levels of irradiation, apoptosis and necrosis were observed. Autophagy has a cytoprotective function unless lysosomes are targeted; loss of lysosomal integrity can interfere with the autophagic recycling processes.

Photodynamic Therapy as a Potent Radiosensitizer in Head and Neck Squamous Cell Carcinoma

Simple Summary

Despite the advances in multimodality treatment strategies, more than 30% of patients with advanced head and neck squamous cell carcinoma (HNSCC) experience recurrence of the disease that is usually derived from the residual tumor. The goal of our study is to understand the molecular basis underlying radiotherapy resistance in advanced HNSCC and to identify a mechanism-based radiosensitizer. We found that the autophagic cell survival pathway is upregulated in therapy-resistant HNSCC. Photodynamic therapy (PDT) directed at the endoplasmic reticulum (ER)/mitochondria induces programmed cell death such as paraptosis and apoptosis in an autophagic adaptor p62-dependent manner, promoting radiotoxicity.

Abstract

Despite recent advances in therapeutic modalities such as radiochemotherapy, the long-term prognosis for patients with advanced head and neck squamous cell carcinoma (HNSCC), especially nonviral HNSCC, remains very poor, while survival of patients with human papillomavirus (HPV)-associated HNSCC is greatly improved after radiotherapy. The goal of this study is to develop a mechanism-based treatment protocol for high-risk patients with HPV-negative HNSCC. To achieve our goal, we have investigated molecular mechanisms underlying differential radiation sensitivity between HPV-positive and -negative HNSCC cells. Here, we found that autophagy is associated with radioresistance in HPV-negative HNSCC, whereas apoptosis is associated with radiation sensitive HPV-positive HNSCC. Interestingly, we found that photodynamic therapy (PDT) directed at the endoplasmic reticulum (ER)/mitochondria initially induces paraptosis followed by apoptosis. This led to a substantial increase in radiation responsiveness in HPV-negative HNSCC, while the same PDT treatment had a minimal effect on HPV-positive cells. Here, we provide evidence that the autophagic adaptor p62 mediates signal relay for the induction of apoptosis, promoting ionizing radiation (XRT)-induced cell death in HPV-negative HNSCC. This work proposes that ER/mitochondria-targeted PDT can serve as a radiosensitizer in intrinsically radioresistant HNSCC that exhibits an increased autophagic flux.

Photodynamic therapy: autophagy and mitophagy, apoptosis and paraptosis

Macroautophagy/autophagy can play a cytoprotective role after photodynamic damage to malignant cells, depending on the site of subcellular damage initiated by reactive oxygen species. There is evidence for such protection when mitochondria are among the targets. Targeting lysosomes has been reported to be more effective for photokilling, perhaps because autophagy offers no cytoprotection. Photodynamic damage to both lysosomes and mitochondria can, however, markedly enhance the overall level of photokilling. Two mechanisms have been proposed to account for this result. Lysosomal photodamage leads to the release of calcium ions, resulting in the activation of the protease CAPN (calpain). CAPN then cleaves ATG5 to a fragment (tATG5) capable of interacting with mitochondria to enhance pro-apoptotic signals. It has also been proposed that targeting lysosomes for photodynamic damage can impair mitophagy, a process that could mitigate the pro-apoptotic effects of mitochondrial targeting. The level of lysosomal photodamage required for suppression of mitophagy is unclear. The "tATG5 route" involves the catalytic action of CAPN, activated by a degree of lysosomal photodamage barely detectible by a viability assay. ER photodamage can also initiate paraptosis, a death pathway functional even in cell types with impaired apoptosis and apparently unaffected by autophagy. Abbreviations: ALLN: N-acetyl-Leu-Leu-norleucinal (cell-permeable inhibitor of calpain); ATG: autophagy related; BPD: benzoporphyrin derivative (Visudyne); ER: endoplasmic reticulum; EtNBS: 5-ethylamino-9-diethyl-aminobenzo[a]phenothiazinium chloride; MTT: a tetrazolium dye; NPe6: mono N-aspartyl chlorin e6; PDT: photodynamic therapy; ROS: reactive oxygen species.

Hypericin Accumulation as a Determinant of PDT Efficacy

Efficacy of photodynamic therapy (PDT) with hypericin was compared using two cell lines in monolayer culture. Photokilling was substantially impaired in the A549 (human-derived lung tumor) when compared with the 1c1c7 murine hepatoma. The efficacy of PDT with regard to photokilling was correlated with the steady-state intracellular hypericin concentration. There was otherwise no difference in subcellular targets or the pathway to cell death: paraptosis. In a prior study involving cell lines derived from patients with head and neck cancer, differences in response to PDT had shown no correlation with photosensitizer uptake.

Exploring Modes of Photokilling by Hypericin

Effects of photodynamic therapy (PDT) using the anthraquinone hypericin were explored with OVCAR-5 cells in vitro. Irradiation resulted in ER > lysosomal photodamage. Paraptosis was identified as a primary death pathway resulting from ER perturbation. This is characterized by an extensive pattern of cytoplasmic vacuole formation. As the PDT dose increased, apoptotic death was also detected. The cytoprotective effect of autophagy, observed when certain other subcellular sites are PDT targets, appears to be absent. These results, together with prior evidence that paraptosis can be lethal to cells with an impaired apoptotic pathway, suggest a role for agents with this targeting profile in photodynamic therapy. A limitation to be overcome for hypericin is a suboptimal absorbance profile.

Effects of HPV Status on Responsiveness to Ionizing Radiation vs Photodynamic Therapy in Head and Neck Cancer Cell lines

Efficacy of ionizing radiation (I/R) was compared with phototoxic effects of photodynamic therapy (PDT) in vitro using two cell lines derived from patients with head and neck squamous cell carcinoma (HNSCC). A cell line derived from a donor with a human papilloma virus (HPV) infection was more responsive to I/R but significantly less responsive to PDT than a cell line derived from an HPV-free patient. Cell death after I/R in the HPV(+) cell line was associated with increased DEVDase activity, a hallmark of apoptosis. The HPV(-) line was considerably less responsive to I/R, with DEVDase activity greatly reduced, suggesting an impaired apoptotic program. In contrast, the HPV(-) cells were readily killed by PDT when the ER was among the targets for photodamage. While DEVDase activity was enhanced, the death pathway appears to involve paraptosis until the degree of photodamage reached the LD99 range. These data suggest that PDT-induced paraptosis can be a death pathway for cells with an impaired apoptotic program.

Paraptosis and Photodynamic Therapy: A Progress Report

Photodamage to the endoplasmic reticulum (ER) can initiate a death pathway termed paraptosis. The "canonical" model of paraptosis, initiated by certain drugs and other stimuli, requires a brief interval of protein synthesis, involves the action of MAP kinases and can be followed by biochemical markers. The latter include changes in expression of AIP-1/Alix and IGF-1R proteins and translocation of HMGB-1 from nucleus to plasma membrane. There is also a report indicating that an enhanced level of autophagy can impair death by paraptosis. The pathway to paraptosis follows the canonical pathway when ER photodamage is minor (<LD₅₀ ). When the extent of ER photodamage approaches LD₉₀ levels, there are deviations from the "canonical" pathway: interfering with protein synthesis does not prevent paraptosis nor does a brief chilling of cells after irradiation, MAP kinases are not involved, and stimulation of autophagy was not cytoprotective. We had previously speculated that ER protein cross-linking might potentiate paraptosis (Photochem. Photobiol. 95, 2019, 1239) but this appears to be incorrect. At higher PDT doses, substantial cross-linking of a typical ER protein (BiP, binding immunoglobin protein, an HSP chaperone) was detected and paraptosis was impaired. This may relate to decreased mobility of cross-linked proteins. Other pathways to cell death were then observed.

Thomas J. Dougherty: An Appreciation

Without the efforts of Tom Dougherty, it is unlikely that photodynamic therapy would have been more than a minor biomedical curiosity. His contributions to the field not only involved demonstrating the clinical relevance of the procedure but organizing that eventually led to FDA approval. This report summarizes his contributions to the field and illustrates the scope of discoveries that became feasible after his work brought PDT to the attention of others and to assorted funding agencies.

A Combination of Visudyne and a Lipid-anchored Liposomal Formulation of Benzoporphyrin Derivative Enhances Photodynamic Therapy Efficacy in a 3D Model for Ovarian Cancer

A major objective in developing new treatment approaches for lethal tumors is to reduce toxicity to normal tissues while maintaining therapeutic efficacy. Photodynamic therapy (PDT) provides a mechanistically distinct approach to treat tumors without the systemic toxicity of chemotherapy drugs. PDT involves the light-based activation of a small molecule, a photosensitizer (PS), to generate reactive molecular species (RMS) that are toxic to target tissue. Depending on the PS localization, various cellular and subcellular components can be targeted, causing selective photodamage. It has been shown that targeted lysosomal photodamage followed by, or simultaneous with, mitochondrial photodamage using two different PS results in a considerable enhancement in PDT efficacy. Here, two liposomal formulations of benzoporphyrin derivative (BPD): (1) Visudyne (clinically approved) and (2) an in-house formulation entrapping a lipid conjugate of BPD are used in combination with direct PS localization to mitochondria, endoplasmic reticulum and lysosomes, enabling simultaneous photodamage to all three organelles using a single wavelength of light. Building on findings by our group, and others, this study demonstrates, for the first time in a 3D model for ovarian cancer, that BPD-mediated photodestruction of lysosomes and mitochondria/ER significantly enhances PDT efficacy at lower light doses than treatment with either PS formulation alone.

3D Culture Models of Malignant Mesothelioma Reveal a Powerful Interplay Between Photodynamic Therapy and Kinase Suppression Offering Hope to Reduce Tumor Recurrence

In this issue, Cramer et al. introduce 3D culture models of metastatic mesothelioma to investigate basic cancer biology and new combination therapies for combating this complex and lethal disease. The results suggest that erlotinib-enhanced photodynamic therapy could further improve the efficacy of intraoperative light-activation to mop up residual tumor deposits in the clinic following surgical removal of macroscopic mesothelioma metastases.

Nanolipid Formulations of Benzoporphyrin Derivative: Exploring the Dependence of Nanoconstruct Photophysics and Photochemistry on Their Therapeutic Index in Ovarian Cancer Cells

With the rapidly emerging designs and applications of light-activated, photodynamic therapy (PDT)-based nanoconstructs, photonanomedicines (PNMs), an unmet need exists to establish whether conventional methods of photochemical and photophysical characterization of photosensitizers are relevant for evaluating new PNMs in order to intelligently guide their design. As a model system, we build on the clinical formulation of benzoporphyrin derivative (BPD), Visudyne^® , by developing a panel of nanolipid formulations entrapping new lipidated chemical variants of BPD with differing chemical, photochemical and photophysical properties. These are 16:0 and 20:0 lysophosphocholine-BPD (16:0/20:0 BPD-PC), DSPE-PEG-BPD and BPD-cholesterol. We show that Visudyne^® was the most phototoxic formulation to OVCAR-5 cells, and the least effective was liposomal DSPE-PEG-BPD. However, these differences did not match their optical, photophysical and photochemical properties, as the static BPD quenching was highest in Visudyne, which also exhibited the lowest generation of singlet oxygen. Furthermore, we establish that OVCAR-5 cell phototoxicity also does not correlate with rates of photosensitizer photobleaching and fluorescence quantum yields in any nanolipid formulations. These findings warrant critical future studies into subcellular targets and molecular mechanisms of phototoxicity of photodynamic nanoconstructs, as more reliable prognostic surrogates for predicting efficacy to appropriately and intelligently guide their design.

Apoptosis, Paraptosis and Autophagy: Death and Survival Pathways Associated with Photodynamic Therapy

The ability of photosensitizing agents to create photodamage at specific subcellular sites has proved useful for characterizing pathway(s) to cell death and for selecting optimal targets for anti-tumor efficacy. Both apoptosis and autophagy can occur after photodamage directed at mitochondria, lysosomes or the ER, with the balance often a determinant of overall efficacy. A combination of lysosomal + mitochondrial targets is associated with enhanced efficacy. More recently, ER photodamage was found to evoke a mainly unexplored mode of photokilling that involves extensive cytoplasmic vacuole formation but does not represent autophagy. This has been termed "paraptosis" and appears to be a reaction to the appearance of misfolded ER proteins. This report is designed to summarize current knowledge relating to death pathways and update information relating to paraptosis as a PDT response.

Photodynamic therapy: Promoting in vitro efficacy of photodynamic therapy by liposomal formulations of a photosensitizing agent

OBJECTIVE

A relatively low level of lysosomal photodamage has been shown capable of promoting the efficacy of photodamage simultaneously or subsequently directed to mitochondrial/ER sites. The procedure has hitherto involved the use of two photosensitizing agents that require irradiation at two different wavelengths and different formulation techniques. This, together with different pharmacokinetic profiles of the photosensitizers, adds a layer of complexity to a protocol that we have sought to circumvent. In this study, liposomal formulations were used to direct photodamage created by benzoporphyrin derivative (BPD, Verteporfin) to lysosomes, mitochondria and the ER. This resulted in the development of an optimal targeting profile using a single agent and a single wavelength of activating irradiation.

MATERIALS/METHODS

These studies were carried out in monolayer cultures of OVCAR5 tumor cells. BPD localization was modified by lipid anchoring and formulation in liposomes, and was assessed by fluorescence microscopy. Irradiation was carried out at 690 ± 10 nm with photodamage assessed also using fluorescent probes and microscopy.

RESULTS

BPD normally localizes in a wide variety of sub-cellular loci that include both mitochondria and the ER, but lysosomes are spared from photodamage. Using a liposomal formulation containing BPD anchored to a lipid resulted in the targeting of lysosomes. A mixture of liposomes containing "free" and "anchored" BPD was shown to significantly promote photokilling. Eliminating cholesterol from the formulation of the anchored product enhanced lysosomal photodamage; prior studies had revealed that excess cholesterol can have a cytoprotective effect when lysosomes are the PDT target.

DISCUSSION

The ability of a liposomal formulation to change localization patterns permits directing photodynamic therapy toward specific sub-cellular loci, thereby promoting photokilling. Incorporating chemotherapeutic agents into such formulations could represent a logical next step in assessing the ability of directed photodamage to enhance tumor eradication. Lasers Surg. Med. 50:499-505, 2018. © 2018 Wiley Periodicals, Inc.

Cell Death Pathways Associated with Photodynamic Therapy: An Update

Photodynamic therapy (PDT) has the potential to make a significant impact on cancer treatment. PDT can sensitize malignant tissues to light, leading to a highly selective effect if an appropriate light dose can be delivered. Variations in light distribution and drug delivery, along with impaired efficacy in hypoxic regions, can reduce the overall tumor response. There is also evidence that malignant cells surviving PDT may become more aggressive than the initial tumor population. Promotion of more effective direct tumor eradication is therefore an important goal. While a list of properties for the "ideal" photosensitizing agent often includes formulation, pharmacologic and photophysical elements, we propose that subcellular targeting is also an important consideration. Perspectives relating to optimizing PDT efficacy are offered here. These relate to death pathways initiated by photodamage to particular subcellular organelles.

Effects of Combined Lysosomal and Mitochondrial Photodamage in a Non-small-Cell Lung Cancer Cell Line: The Role of Paraptosis

We previously reported that a low level of lysosomal photodamage potentiated the phototoxic effect of subsequent mitochondrial photodamage mediated by the benzoporphyrin derivative (BPD) in murine hepatoma 1c1c7 cells. This was attributed to release of Ca²⁺ from damaged lysosomes and a calpain-mediated conversion of the autophagy-related protein ATG5 to a pro-apoptotic fragment. We now report a comparison of these results with those obtained with the human non-small-cell lung cancer A549 cell line. A549 cells contained lower levels of ATG5 and were less responsive than 1c1c7 cultures to the PDT combination. A rapid appearance of caspase 3/7 activation together with formation of condensed chromatin indicated initiation of apoptosis in both cell lines, but to a lesser extent in A549 cultures. Both cell lines became highly vacuolated within 16 h of combination PDT or an equivalent phototoxic dose from BPD alone. The vacuole periphery was labeled with a fluorescent probe for the endoplasmic reticulum (ER), and vacuole formation was prevented by presence of the protein synthesis inhibitor cycloheximide. These effects are characteristics of a caspase-independent death mode termed paraptosis previously associated with ER stress. These studies suggest that paraptosis may be a more frequent outcome of PDT than has hitherto been realized.

Subcellular Targeting as a Determinant of the Efficacy of Photodynamic Therapy

In prior studies, we have identified the ability of low-level lysosomal photodamage to potentiate the phototoxic effect of subsequent photodamage to mitochondria. The mechanism involves calpain-mediated cleavage of the autophagy-associated protein ATG5 to form a proapoptotic fragment (tATG5). In this report, we explore the permissible time lag between the two targeting procedures along with the effect of simultaneously targeting both lysosomes and mitochondria. This was found to be as effective as the sequential protocol with no gap between the irradiation steps. Inhibition of calpain reversed the enhanced efficacy of the "simultaneous" protocol. It appears that even a minor level of lysosomal photodamage can have a significant effect on the efficacy of subsequent mitochondrial photodamage. We propose that these results may explain the efficacy of Photofrin, a photosensitizing product that also targets both lysosomes and mitochondria for photodamage.

Autophagic death probed by photodynamic therapy

The high degree of selectivity for photodamage to subcellular organelles can provide a means for evaluation of autophagic death pathways. While many current reports rely on ambiguous criteria, there are glimmers of unequivocal evidence.

Subclavian Stents and Stent-Grafts: Cause for Concern?

Purpose:

To report cases of stent and stent-graft fracture in the subclavian vessels.

Methods and Results:

Three patients with self-expanding stents of 3 different types in 1 subclavian artery and 2 subclavian veins presented with recurrent symptoms 6 months to 2 years after stenting. All devices showed signs of compression with stent fracture. The covered stent in the subclavian artery was excised. Of the 2 venous patients, 1 was treated with first rib resection and the other refused further treatment.

Conclusions:

The subclavian vessels are prone to flexion during movement, and the vessels may be compressed by external structures, including the clavicle and first rib. Stents that have not been designed to withstand these forces may be damaged.

Promotion of Proapoptotic Signals by Lysosomal Photodamage: Mechanistic Aspects and Influence of Autophagy

Prior studies demonstrated that a low level (LD10-15 ) of lysosomal photodamage can sensitize cells to the apoptotic death that results from subsequent mitochondrial photodamage. We have proposed that this process occurs via a calpain-catalyzed cleavage of the autophagy-associated protein ATG5 to form a proapoptotic fragment. In this report, we provide evidence for the postulated ATG5 cleavage and show that the sequential photodynamic therapy (PDT) protocol can also partly overcome the adverse effect of hypoxia on the initiation of apoptosis. While autophagy can offer cytoprotection after mitochondrial photodamage, this does not appear to apply when lysosomes are the target. This may account for the ability of very low PDT doses directed at lysosomes to evoke ATG5 cleavage. The resulting proapoptotic effect overcomes intrinsic cytoprotection from mitochondrial photodamage along with a further stimulation of phototoxicity.

Photodynamic therapy: Promotion of efficacy by a sequential protocol

Photodynamic therapy (PDT) offers a new approach to selective tumor eradication. Modifications designed to increase and optimize efficacy continue to emerge. Selective photodamage to malignant cells and their environment can bring about tumor cell destruction, shutdown of the tumor vasculature, stimulation of immunologic anti-tumor effects and potentiation of other therapeutic effects. Current development of combination protocols may provide a better rationale for integration of PDT into clinical practice. An example described here is the ability of a sequential (two-sensitizer) PDT protocol to enhance the efficacy of photokilling. The first step involves low-level lysosomal photodamage that has been shown to promote the apoptotic response to subsequent photodynamic effects directed at mitochondria. In this report, we demonstrate the ability of Photofrin, an FDA-approved photosensitizing agent, to serve as either the first or second element of the sequential protocol.

Photodynamic therapy as an effective therapeutic approach in MAME models of inflammatory breast cancer

Photodynamic therapy (PDT) is a minimally invasive, FDA-approved therapy for treatment of endobronchial and esophageal cancers that are accessible to light. Inflammatory breast cancer (IBC) is an aggressive and highly metastatic form of breast cancer that spreads to dermal lymphatics, a site that would be accessible to light. IBC patients have a relatively poor survival rate due to lack of targeted therapies. The use of PDT is underexplored for breast cancers but has been proposed for treatment of subtypes for which a targeted therapy is unavailable. We optimized and used a 3D mammary architecture and microenvironment engineering (MAME) model of IBC to examine the effects of PDT using two treatment protocols. The first protocol used benzoporphyrin derivative monoacid A (BPD) activated at doses ranging from 45 to 540 mJ/cm(2). The second PDT protocol used two photosensitizers: mono-L-aspartyl chlorin e6 (NPe6) and BPD that were sequentially activated. Photokilling by PDT was assessed by live-dead assays. Using a MAME model of IBC, we have shown a significant dose-response in photokilling by BPD-PDT. Sequential activation of NPe6 followed by BPD is more effective in photokilling of tumor cells than BPD alone. Sequential activation at light doses of 45 mJ/cm(2) for each agent resulted in >90 % cell death, a response only achieved by BPD-PDT at a dose of 360 mJ/cm(2). Our data also show that effects of PDT on a volumetric measurement of 3D MAME structures reflect efficacy of PDT treatment. Our study is the first to demonstrate the potential of PDT for treating IBC.

More Adventures in Photodynamic Therapy

Photodynamic therapy is a procedure that can provide a selective eradication of neoplastic disease if sufficient drug, light, and oxygen are available. As this description suggests, it involves the photosensitization of malignant tissues to irradiation with photons in the visible range. While not suitable for tumors at unknown loci, it can be of use for eradication of cancer at surgical margins and therapy at sites where substantial surgery might otherwise be involved. Drug development has been delayed by several factors including the reluctance of major pharmaceutical firms in the United States to invest in this technology along with some unwise approaches in the past.

Promotion of Proapoptotic Signals by Lysosomal Photodamage

We previously reported that low-level lysosomal photodamage enhanced the efficacy of subsequent mitochondrial photodamage, resulting in a substantial promotion of apoptotic cell death. We now extend our analysis of the sequential PDT protocol to include two additional lysosomal-targeting photosensitizers. These agents, because of enhanced permeability, are more potent than the agent (N-aspartyl chlorin E6, NPe6) used in the initial study. Addition of the cell-permeable cysteine protease inhibitor E-64d and calcium chelator BAPTA-AM almost completely suppressed sequential PDT-induced loss of mitochondrial membrane potential and activation of procaspases-3 and -7. These inhibitors did not, however, suppress the proapoptotic effect of a BH3 mimetic or mitochondrial photodamage. Knockdowns of ATG7 or ATG5, proteins normally associated with autophagy, suppressed photodamage induced by the sequential PDT protocol. These effects appear to be independent of the autophagic process as pharmacological inhibition of autophagy offered no such protection. Effects of ATG7 and ATG5 knockdowns may reflect the role that ATG7 plays in regulating lysosome permeability, and the likelihood that a proteolytic fragment of ATG5 amplifies mitochondrial proapoptotic processes. Our results suggest that low-dose photodamage that sequentially targets lysosomes and mitochondria may offer significant advantages over the use of single photosensitizers.