Paul K Strudler: The Oracle of RAD.

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

Corrigendum to "Naturally occurring sesquiterpene lactones and their semi-synthetic derivatives modulate PGE2 levels by decreasing COX2 activity and expression" [Heliyon 5 (3) (March 2019) e01366].

[This corrects the article DOI: 10.1016/j.heliyon.2019.e01366.].

Naturally occurring sesquiterpene lactones and their semi-synthetic derivatives modulate PGE2 levels by decreasing COX2 activity and expression.

Plants of the Asteraceae family have been used in traditional medicine for centuries due to their main antimicrobial and analgesic activities. A liniment from Artemisia californica has recently been tested on patients affected by either acute pain or chronic pain conditions with great success. The aim of this study was to evaluate the anti-inflammatory activity of sesquiterpene lactones (SLs), representing the majority in the Asteraceae family. Leucodin, α-santonin and sclareolide (three SLs) were chosen to undergo chemical modifications. This pool of molecules underwent molecular modeling experiments using an in-house program, WATGEN, predicting the water network and its contribution to the overall affinity of the enzyme-ligand complex. The anti-inflammatory activity and the ability of compounds to modulate COX-2 expression have been evaluated in LPS-stimulated RAW 264.7 cells and in RIF-1 cells treated according to the Photodynamic Therapy (PDT) protocols using Photoprin (PH) as photosensitizer. Furthermore, commercially available assay kits were used to evaluate the concentration of PGE-2 and the direct inhibition of COX-2. All the tested molecules fit well in the enzyme binding pocket, but to get a substantial inhibition of the expression and activity of the enzyme as well as a reduction in the PGE2 concentration, high concentrations of the compounds are needed. The only exceptions being leucodin itself and FP6, one of the α-santonin derivatives, presenting a CF3 functional group. We believe that this class of compounds has some interesting potential in the treatment of pain and inflammation. Although, the activity seems to be due to a mechanism related to the expression of the COX enzymes rather than on a direct inhibition.

Rational drug design and synthesis of new α-Santonin derivatives as potential COX-2 inhibitors.

Sesquiterpene compounds are widely known for their numerous pharmacological activities. Herein the focus of the authors was on α-Santonin, a sesquiterpene lactone from the Artemisia genus: the aim was to determine whether α-Santonin could be considered in the treatment of inflammation and pain. To this purpose, a small series of derivatives was designed and screened in silico against the enzyme COX-2 along with the parent compound. Drug-likeness parameters were also assessed. The compounds were eventually synthesized, and few were tested to determine their efficacy in the inhibition of COX-2 activity and expression. Overall, compound A2 was the only one with a detectable inhibitory potential of COX-2 activity whilst two of its ether derivatives demonstrated improved ability in the inhibition of COX-2 expression.

Rabbit model of ocular indirect photodynamic therapy using a retinoblastoma xenograft.

PURPOSE: The goal of this project was to demonstrate the feasibility of coupling the indirect ophthalmoscope laser delivery system with the 690 nm wavelength diode laser used to perform photodynamic therapy (PDT) in the treatment of retinoblastoma. METHODS: For phase 1, a total of six pigmented rabbits were treated with the indirect laser delivery system. The laser source was provided by the Lumenis Opal 690 nm laser unit, delivered through a 810 nm Indirect ophthalmoscope headpiece and a hand-held 28-diopter indirect lens (1.0 mm spot size). Four rabbits received intravenous verteporfin at doses of 0.43 or 0.86 mg/kg, and two rabbits did not receive verteporfin (controls). A second phase of the study involved eight rabbits using a retinoblastoma xenograft to determine the effect of indirect PDT on subretinal tumors. RESULTS: For phase 1, a total of 20 laser treatments were performed in the right eyes of six rabbits. Laser power levels ranged between 40 and 150 mW/cm2 and treatment duration ranged between 1 and 3 min. In the four rabbits that received verteporfin, focal retinal scars were noted at 40 mW/cm2 and higher power levels. In the two control rabbits that did not receive verteporfin, thermal burns were confirmed at 75 mW/cm2 and higher power levels. Histopathology showed focal retino-choroidal scars at the site of PDT treatment, without evidence of generalized ocular damage. Using the retinoblastoma xenograft, the indirect PDT system was shown to cause areas of tumor necrosis on histopathology. CONCLUSIONS: The results of this pre-clinical study suggest verteporfin may be activated in the rabbit retina with the indirect delivery system and the 690 nm laser unit (i.e., Indirect PDT). Using verteporfin, treatment effects were observed at 40-50 mW/cm2 in the rabbit retina, while photocoagulation was achieved at 75 mW/cm2 and higher power levels. Fundoscopic and histopathologic examination of treated areas showed circumscribed areas of retinal damage and a lack of generalized ocular toxicity, suggesting that this modality may represent a safe and localized method for treating intraocular retinoblastoma.

Targeting Survivin Enhances Chemosensitivity in Retinoblastoma Cells and Orthotopic Tumors.

Treatments for retinoblastoma (Rb) vary depending on the size and location of the intraocular lesions and include chemotherapy and radiation therapy. We examined whether agents used to treat Rb induce a pro-survival phenotype associated with increased expression of survivin, a member of the inhibitor of apoptosis family of proteins. We document that exposure to carboplatin, topotecan or radiation resulted in elevated expression of survivin in two human Rb cell lines but not in normal retinal pigmented epithelial (RPE) cells. Cellular levels of survivin were attenuated in Rb cells exposed to an imidazolium-based survivin suppressant, Sepantronium bromide (YM155). Protein expression patterns of survivin in RPE cells were not altered following treatment protocols involving exposure to YM155. Including YM155 with chemotherapy or radiation increased levels of apoptosis in Rb cells but not in RPE cells. Intraocular luciferase expressing Rb tumors were generated from the Rb cell lines and used to evaluate the effects of carboplatin and YM155 on in-vivo survivin expression and tumor growth. Carboplatin induced expression of survivin while carboplatin combined with YM155 reduced survivin expression in tumor bearing eyes. The combination protocol was also most effective in reducing the rate of tumor regrowth. These results indicate that targeted inhibition of the anti-apoptotic protein survivin provides a therapeutic advantage for Rb cells and tumors treated with chemotherapy.

Induction of prosurvival molecules during treatment: rethinking therapy options for photodynamic therapy.

Photodynamic therapy (PDT) not only causes direct cytotoxicity to malignant cells within a tumor but also appears to have both direct and indirect effects on nonmalignant components of the tumor microenvironment. A host of preclinical studies have been performed to document how PDT modulates the tumor microenvironment. This article explores the role of cellular components such as the hypoxia-inducible factor 1α, vascular endothelial growth factor, cyclooxygenase-2, matrix metalloproteinases, the antiapoptotic protein survivin, and 17-AAG (an inhibitor of heat shock proteins), with the hope that combined modality regimens targeting these processes may improve PDT tumor responsiveness.

Pro-apoptotic and anti-inflammatory properties of the green tea constituent epigallocatechin gallate increase photodynamic therapy responsiveness.

BACKGROUND: A polyphenol constituent of green tea, epigallocatechin gallate (EGCG), has anti-carcinogenic properties. A growing number of studies document EGCG-mediated induction of apoptotic pathways and inhibition of pro-survival factors when combined with chemotherapy or radiation. We evaluated the efficacy of EGCG in modulating photofrin (PH)-mediated photodynamic therapy (PDT) responses. METHODS: Mouse mammary carcinoma (BA) cells and transplanted BA tumors growing in C3H mice were treated with PH-mediated PDT. Select groups of treated cells and mice also received EGCG and then cytotoxicity, tumor response, and expression of survival molecules were evaluated in all experimental groups. RESULTS: EGCG increased apoptosis and cytotoxicity in BA cells exposed to PH-mediated PDT. The initial pro-survival phase of the unfolded protein response (UPR), characterized by increased expression of the 78 kDa glucose-regulated protein (GRP-78), was induced by PDT. The second pro-apoptotic phase of the UPR, characterized by phospho-c-Jun N-terminal kinase (p-JNK) expression, activation of caspases-3 and 7, poly ADP ribose polymerase (PARP) cleavage, and expression of C/EBP homologous protein was observed when PDT was combined with EGCG. EGCG also decreased the expression of the pro-survival proteins GRP-78 and survivin, and attenuated PDT-induced prostaglandin E2 (PGE2 ) expression in PDT-treated cells. Comparable responses also were observed when BA tumors were treated with PDT and EGCG. In addition, PDT-induced expression of metalloproteinases (MMPs) and vascular endothelial growth factor (VEGF) was down-regulated in treated tumor tissue by EGCG. CONCLUSIONS: The polyphenol EGCG improves PDT efficacy by increasing tumor apoptosis and decreasing expression of pro-survival and angiogenic molecules within the tumor microenvironment.

Enhancement of photodynamic therapy by 2,5-dimethyl celecoxib, a non-cyclooxygenase-2 inhibitor analog of celecoxib.

Photodynamic therapy (PDT) effectiveness can be improved by employing combined modality approaches involving pharmaceuticals targeting the tumor microenvironment and/or tumor cell death pathways. In one approach, combining PDT with celecoxib improves long-term tumoricidal activity without increasing normal tissue photosensitization. However, side effects arising from the use of coxib based cyclooxygenase-2 (COX-2) inhibitors, including cardiovascular injury, decreases the clinical applications of this class of compounds. A growing number of studies demonstrate that the tumoricidal actions of coxibs such as celecoxib involve non-COX-2 mediated mechanisms. The celecoxib analog, 2,5-dimethyl celecoxib (DMC), lacks COX-2 inhibitory activity but exhibits cytotoxic properties comparable to the COX-2 inhibitor celecoxib. We compared the effectiveness of DMC and celecoxib in modulating PDT response at both the in vitro and in vivo level using a C3H/BA murine mammary carcinoma model. Both DMC and celecoxib blocked PDT induced expression of the pro-survival protein survivin, enhanced the endoplasmic reticulum stress (ERS) response of PDT, and increased both apoptosis and cytotoxicity in BA cells exposed to combination protocols. DMC enhanced the in vivo tumoricidal responsiveness of PDT without altering PGE2 levels. Our data demonstrates that DMC improved PDT by increasing apoptosis and tumoricidal activity without modulating COX-2 catalytic activity. Our results also suggest that celecoxib mediated enhancement of PDT may involve both COX-2 dependent and independent mechanisms.

Identification of MAP kinase pathways involved in COX-2 expression following photofrin photodynamic therapy.

Photodynamic therapy (PDT) using the photosensitizer Photofrin is approved for the clinical treatment of solid tumors. PDT causes cytotoxic oxidative stress, but additionally induces prosurvival molecules such as cyclooxygenase-2 (COX-2). Combining PDT with COX-2 inhibitors increases the efficacy of in vivo treatment. Understanding mechanisms leading to prosurvival molecule induction is relevant to the design of more effective treatments. Using COX-2 promoter constructs, transcription factor-binding assays, identification of protein kinase activation, and inhibitors of transcription factor binding we were able to determine that COX-2 expression following PDT involves the p38 MAP kinase pathway.

Targeting the tumor microenvironment using photodynamic therapy combined with inhibitors of cyclooxygenase-2 or vascular endothelial growth factor.

Photodynamic therapy (PDT) involves the administration of a photosensitizer (PS) followed by localized exposure of a targeted tissue to PS adsorbing light. PDT induces cytotoxicity to exposed malignant cells and also effects non-malignant components of the tumor microenvironment. This indirect action of PDT leads to inflammatory and proangiogenic responses and modulates treatment effectiveness. Preclinical studies designed to determine how PDT modulates the tumor microenvironment use murine tumor models to investigate the expression and/or the activation of growth factors, proteinases, and inflammatory molecules following treatment. These studies demonstrate that improvements in treatment responsiveness following PDT are achieved using inhibitors targeting angiogenic and/or inflammatory pathways.

Targeting the 90 kDa heat shock protein improves photodynamic therapy.

The geldanamycin derivative, 17-allylamino-17-demethoxygeldanamycin (17-AAG), binds to the amino-terminal ATP binding pocket of the 90 kDa heat shock protein (Hsp-90) and inhibits this chaperone from stabilizing client proteins involved with the malignant phenotype. We examined the effects of a combined modality protocol involving photodynamic therapy (PDT) and 17-AAG in mouse mammary carcinoma cells and tumors. PDT increased the expression of the anti-apoptotic and pro-angiogenic proteins survivin, Akt, HIF-1alpha, MMP-2 and VEGF in tumor tissue and this expression decreased significantly when 17-AAG was included in the treatment regimen. Tumor bearing mice treated with PDT and 17-AAG had improved long-term tumoricidal responses when compared with individual treatment protocols. We conclude that Hsp-90 plays an active role in modulating tumor responsiveness following PDT and targeting Hsp-90 with 17-AAG enhances the therapeutic effectiveness of PDT.

Cyclooxygenase-2 expression induced by photofrin photodynamic therapy involves the p38 MAPK pathway.

Photodynamic therapy (PDT), using the porphyrin photosensitizer Photofrin (PH), is approved for the clinical treatment of solid tumors. In addition to the direct cytotoxic responses of PH-PDT-mediated oxidative stress, this procedure also induces expression of angiogenic and prosurvival molecules including cyclooxygenase-2 (COX-2). In vivo treatment efficacy is improved when PH-PDT is combined with inhibitors of COX-2. In the current study we evaluated the signaling pathways involved with PH-PDT-mediated COX-2 expression in a mouse fibrosarcoma cell line. COX-2 promoter reporter constructs with mutated transcription elements documented that the nuclear factor kappa B (NFkappaB) element, cyclic-AMP response element 2 (CRE-2), CCAAT/enhancer binding protein (C/EBP) element and activator binding protein-1 (AP-1) element were responsive to PH-PDT. Transcription factor binding assays demonstrated that nuclear protein binding to NFkappaB, CRE-2, c-fos and c-jun elements were elevated following PH-PDT. Kinase phosphorylation upstream of COX-2 expression was also examined following PH-PDT. Stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) and c-Jun were phosphorylated following PH-PDT but the SAPK/JNK inhibitor SP600125 failed to attenuate COX-2 expression. In contrast, p38 mitogen-activated protein kinase (MAPK), which activates CRE-2 binding, was phosphorylated following PH-PDT and inhibitors of p38 MAPK, SB203580 and SB202190, decreased PH-PDT-induced COX-2 expression at both the mRNA and protein levels. Extracellular signal-regulated kinase (ERK1/2) phosphorylation, which also increases CRE-2 binding activity, was initially high in untreated cells, decreased immediately following PH-PDT and then rapidly increased. MEK1/2 is immediately upstream of ERK1/2 and the MEK1 inhibitor PD98059 failed to attenuate COX-2 expression while the MEK1/2 inhibitor U0126 induced a slight decrease in COX-2 expression. The NFkappaB inhibitor SN50 failed to reduce COX-2 expression. These results demonstrate that multiple protein kinase cascades can be activated by oxidative stress and that the p38 MAPK signaling pathway and CRE-2 binding are involved in COX-2 expression following PH-PDT.

Multiple components of photodynamic therapy can phosphorylate Akt.

A growing number of clinically relevant molecular and cellular responses are observed following photodynamic therapy (PDT). PDT-mediated oxidative stress and PDT-induced tissue hypoxia can elicit the transcriptional and/or translational expression of genes associated with cellular stress, inflammation, angiogenesis, immuno-modulation, apoptosis and signal transduction. One of the signaling molecules activated by oxidative stress is Akt/protein kinase B. Phosphorylation of Akt/protein kinase B activates this signaling molecule and induces a survival response in effected cells and tissue. We hypothesized that PDT using Photofrin (PH) as the photosensitizer could also induce increased levels of Akt phosphorylation. Results from our initial set of experiments demonstrated that in vitro and in vivo PDT treatments induced Akt phosphorylation. Interestingly, incubation of mouse and human breast cancer cells with the porphyrin-based photosensitizer, PH, increased the expression of Akt phosphorylation in the absence of light. Exposure of the corresponding mouse and human-derived breast cancer tumors growing in mice to 630 nm light in the absence of PH administration also induced Akt phosphorylation. These results demonstrate that individual components of the PDT process, photosensitizer alone and light alone, as well as the complete PDT procedure can activate the Akt signaling pathway.

Survivin, a member of the inhibitor of apoptosis family, is induced by photodynamic therapy and is a target for improving treatment response.

We observed that photodynamic therapy (PDT) induces the expression and phosphorylation of the inhibitor of apoptosis (IAP) protein survivin in murine and human cancer cells and tumors. Survivin inhibits caspase-9, blocks apoptosis, and is associated with resistance to chemotherapy and radiation. Survivin is a client protein for the 90-kDa heat shock protein (Hsp-90), and the binding of survivin to Hsp-90 assists in the maturation, proper folding, assembly, and transport of this IAP protein. A derivative of the antibiotic geldanamycin, 17-allylamino-17-demethoxygeldanamycin (17-AAG), interferes with proper binding of client proteins, such as survivin, to Hsp-90 and leads to misfolding of client proteins, ubiquination, and proteasome degradation. We hypothesized that PDT efficacy may be reduced by treatment-mediated expression and phosphorylation of survivin, and therefore, targeting the survivin pathway could increase PDT responsiveness. To address this hypothesis, we examined cellular and molecular responses following exposure to PDT, 17-AAG, and the combination of PDT plus 17-AAG in human BT-474 breast cancer cells using Photofrin and NPe6 as photosensitizers. Cells treated with the combination of PDT and 17-AAG exhibited decreased expression of the Hsp-90 client proteins phosphorylated survivin, phosphorylated Akt, and Bcl-2. The decreased expression of these client proteins was accompanied by higher apoptotic indexes and increased cytotoxicity. To confirm a specific role for survivin in modulating PDT, we used a human melanoma cell line, YUSAC2/T34A-C4, stably transfected with an inducible dominant-negative survivin gene under the control of a tetracycline-regulated (tet-off) promoter. PDT treatment of melanoma cells expressing the dominant-negative survivin resulted in increased cleavage of the caspase substrate poly(ADP-ribose) polymerase, apoptosis, and cytotoxicity when compared with results following PDT of the same melanoma cell line expressing wild-type survivin. These results show for the first time that targeting survivin and possibly other Hsp-90 client proteins improves in vitro PDT responsiveness and suggest that manipulation of the antiapoptotic pathway maintained by survivin may enhance PDT-mediated cancer therapy.

Photodynamic therapy: combined modality approaches targeting the tumor microenvironment.

BACKGROUND AND OBJECTIVES: Photodynamic therapy causes direct cytotoxicity to malignant cells within a tumor. Photodynamic therapy (PDT) can also have both direct and indirect effects upon various non-malignant components of the tumor microenvironment. This action can lead to PDT-mediated angiogenesis and inflammation, which are emerging as important determinants of PDT responsiveness. STUDY DESIGN/MATERIALS AND METHODS: Preclinical studies have been performed to document how PDT modulates the tumor microenvironment. The expression, function, and treatment relevance of angiogenic growth factors, proteinases, and inflammatory molecules have been monitored following PDT using mouse tumor models. RESULTS: Photofrin-mediated PDT was shown to be a strong activator of VEGF, MMPs, and COX-2 derived prostaglandins within the tumor microenvironment. Inhibitors that target these angiogenic and pro-survival molecules can enhance the effectiveness of PDT. CONCLUSIONS: Improvements in PDT tumor responsiveness may be achieved by employing combined modality regimens targeting malignant cells as well as treatment-induced angiogenesis and/or inflammation.

Avastin enhances photodynamic therapy treatment of Kaposi's sarcoma in a mouse tumor model.

The goal of the current study was to determine if the antiangiogenic drug Avastin would improve the effectiveness of Photodynamic Therapy (PDT) in a xenograft model of Kaposi's sarcoma (KS). Human KS-Imm tumors transplanted in nude mice were treated with Photofrin-mediated PDT. Expression parameters of proangiogenic molecules were documented and the tumoricidal effectiveness of PDT combined with the VEGF inhibitor Avastin was determined. PDT induced increased expression of HIF-1alpha, VEGF, PGE2, TNF-alpha, and IL-1beta within treated KS tumor tissue. Significant overexpression of KS cell derived human VEGF and to a lesser extent overexpression of host cell derived mouse VEGF were detected within treated tumors. Combining PDT with Avastin resulted in a significant increase in the long-term responsiveness of treated KS tumors when compared to individual treatments. These results demonstrate for the first time that Avastin can improve PDT treatment effectiveness and suggest that VEGF inhibitors may ameliorate the clinical efficacy of PDT.

Celecoxib and NS-398 enhance photodynamic therapy by increasing in vitro apoptosis and decreasing in vivo inflammatory and angiogenic factors.

Photodynamic therapy (PDT) elicits both apoptotic and necrotic responses within treated tumors and produces microvascular injury leading to inflammation and hypoxia. PDT also induces expression of angiogenic and survival molecules including vascular endothelial growth factor, cyclooxygenase-2 (COX-2), and matrix metalloproteinases. Adjunctive administration of inhibitors to these molecules improves PDT responsiveness. In the current study, we examined how the combination of PDT and COX-2 inhibitors improve treatment responsiveness. Photofrin-mediated PDT combined with either celecoxib or NS-398 increased cytotoxicity and apoptosis in mouse BA mammary carcinoma cells. Immunoblot analysis of protein extracts from PDT-treated cells also showed poly(ADP-ribose) polymerase cleavage and Bcl-2 degradation, which were further enhanced following combined therapy. Tumor-bearing mice treated with PDT and either celecoxib or NS-398 exhibited significant improvement in long-term tumor-free survival when compared with PDT or COX-2 inhibitor treatments alone. The combined procedures did not increase in vivo tumor-associated apoptosis. Administration of celecoxib or NS-398 attenuated tissue levels of prostaglandin E2 and vascular endothelial growth factor induced by PDT in treated tumors and also decreased the expression of proinflammatory mediators interleukin-1beta and tumor necrosis factor-alpha. Increased tumor levels of the antiinflammatory cytokine, interleukin 10, were also observed following combined treatment. This study documents for the first time that adjunctive use of celecoxib enhances PDT-mediated tumoricidal action in an in vivo tumor model. Our results also show that administration of COX-2 inhibitors enhance in vitro photosensitization by increasing apoptosis and improve in vivo PDT responsiveness by decreasing expression of angiogenic and inflammatory molecules.

CHOP activation by photodynamic therapy increases treatment induced photosensitization.

BACKGROUND AND OBJECTIVES: C/EBP homologous protein (CHOP) is an endoplasmic reticulum (ER) stress inducible transcription factor involved in the development of apoptosis, growth arrest, and differentiation. CHOP deficient (chop - / - ) mouse embryonic fibroblasts (MEFs) exposed to ER stresses such as tunicamycin exhibit decreased apoptosis and reduced toxicity when compared to chop + / + control cells. Overexpression of the 70 kDa heat shock stress protein (HSP-70) can inhibit apoptotic pathways. The biological significance of photodynamic therapy (PDT) protocols that induce cellular damage resulting in differential CHOP and stress protein expression patterns was examined. STUDY DESIGN/MATERIALS AND METHODS: Wild type mouse radiation induced fibrosarcoma (RIF) cells as well as MEFs with chop + / + and chop - / - genotypes were used with either a mitochondrial and ER localizing porphyrin (PH) photosensitizer or a lysosomal localizing chlorin (NPe6) photosensitizer. PDT induced cytotoxicity, apoptosis, and stress protein expression patterns were determined as a function of cell type and photosensitizer. RESULTS: PH mediated PDT induced expression of CHOP and 78 kDa glucose regulated protein (GRP-78), but not HSP-70 while NPe6 mediated PDT induced protein expression of HSP-70 but did not activate CHOP or GRP-78 expression. Enhanced apoptosis and toxicity were observed in chop + / + cells following exposure to tunicamycin or PH mediated PDT when compared to identical treatments in chop - / - cells. NPe6 mediated PDT induced minimally detectable apoptosis in both chop + / + and chop - / - cells and only a modest increase in survival for chop - / - cells. CONCLUSIONS: These results demonstrate that PDT activation of CHOP, GRP-78, and HSP-70 varied as a function of photosensitizer subcellular localization and that a single oxidative stress response was not observed following PDT. We also show that CHOP expression increased apoptosis following PH mediated PDT and that increased CHOP expression is associated with enhanced PDT photosensitization.

Spontaneous and controllable activation of suicide gene expression driven by the stress-inducible grp78 promoter resulting in eradication of sizable human tumors.

GRP78 is a stress-inducible chaperone protein with antiapoptotic properties that is overexpressed in transformed cells and cells under glucose starvation, acidosis, and hypoxic conditions that persist in poorly vascularized tumors. Previously we demonstrated that the Grp78 promoter is able to eradicate tumors using murine cells in immunocompetent models by driving expression of the HSV-tk suicide gene. Here, through the use of positron emission tomography (PET) imaging, we provide direct evidence of spontaneous in vivo activation of the HSV-tk suicide gene driven by the Grp78 promoter in growing tumors and its activation by photodynamic therapy (PDT) in a controlled manner. In this report, we evaluated whether this promoter can be applied to human cancer therapy. We observed that the Grp78 promoter, in the context of a retroviral vector, was highly activated by stress and PDT in three different types of human breast carcinomas independent of estrogen receptor and p53. Complete regression of sizable human tumors was observed after prodrug ganciclovir treatment of the xenografts in immunodeficient mice. In addition, the Grp78 promoter-driven suicide gene is strongly expressed in a variety of human tumors, including human osteosarcoma. In contrast, the activity of the murine leukemia virus (MuLV) long-terminal repeat (LTR) promoter varied greatly in different human breast carcinoma cell lines, and in some cases, stress resulted in partial suppression of the LTR promoter activity. In transgenic mouse models, the Grp78 promoter-driven transgene is largely quiescent in major adult organs but highly active in cancer cells and cancer-associated macrophages, which can diffuse to tumor necrotic sites devoid of vascular supply and facilitate cell-based therapy. Thus, transcriptional control through the use of the Grp78 promoter offers multiple novel approaches for human cancer gene therapy.