Tumor associated stromal cells play a critical role on the outcome of the oncolytic efficacy of conditionally replicative adenoviruses. PLoS One. 2009;4:e5119. [PMID: 19337591 PMCID: PMC2663040 DOI: 10.1371/journal.pone.0005119]

The implications of oncolytic viruses targeting fibroblasts in enhancing the antitumoural immune response

Oncolytic viruses (OVs) are an emerging immunotherapy platform that selectively target tumour cells, inducing immunogenic cell death. This reverses the 'immune-desert' phenotype of tumours, enhancing antitumour immunity. However, oncolytic virotherapy has shown limited efficacy in solid tumours due to the presence of protumoural, immunosuppressive cancer-associated fibroblasts (CAFs). Recent studies have explored OVs that specifically target CAFs to enhance antitumoural immune responses, with promising results. Nevertheless, detailed interrogation of the experimental design of these studies casts doubt on their potential for successful clinical translation. Most studies targeted CAFs non-specifically, failing to acknowledge CAF heterogeneity, with antitumoural CAFs also present. Thus, use of transcriptomics is advisable to provide more focused targeting, limiting potential off-target toxicity. Furthermore, experiments to date have largely been conducted in murine models that do not faithfully recapitulate tumour microenvironments, potentially biasing the efficacy observed. Future work should make use of humanised patient-derived xenograft murine models for animal studies, after which primary human tumour biopsies should be utilised to more closely represent the patient population for maximal translation relevance. Additionally, approaches to enhance the antitumoural immune responses of this therapy should be prioritised, with the ultimate aim of achieving complete remission, which has not yet been observed pre-clinically.

Tutorial: design, production and testing of oncolytic viruses for cancer immunotherapy

Oncolytic viruses (OVs) represent a novel class of cancer immunotherapy agents that preferentially infect and kill cancer cells and promote protective antitumor immunity. Furthermore, OVs can be used in combination with established or upcoming immunotherapeutic agents, especially immune checkpoint inhibitors, to efficiently target a wide range of malignancies. The development of OV-based therapy involves three major steps before clinical evaluation: design, production and preclinical testing. OVs can be designed as natural or engineered strains and subsequently selected for their ability to kill a broad spectrum of cancer cells rather than normal, healthy cells. OV selection is further influenced by multiple factors, such as the availability of a specific viral platform, cancer cell permissivity, the need for genetic engineering to render the virus non-pathogenic and/or more effective and logistical considerations around the use of OVs within the laboratory or clinical setting. Selected OVs are then produced and tested for their anticancer potential by using syngeneic, xenograft or humanized preclinical models wherein immunocompromised and immunocompetent setups are used to elucidate their direct oncolytic ability as well as indirect immunotherapeutic potential in vivo. Finally, OVs demonstrating the desired anticancer potential progress toward translation in patients with cancer. This tutorial provides guidelines for the design, production and preclinical testing of OVs, emphasizing considerations specific to OV technology that determine their clinical utility as cancer immunotherapy agents.

In Vitro and In Vivo Efficacy of a Stroma-Targeted, Tumor Microenvironment Responsive Oncolytic Adenovirus in Different Preclinical Models of Cancer

More than one million women are diagnosed annually worldwide with a gynecological cancer. Most gynecological cancers are diagnosed at a late stage, either because a lack of symptoms, such as in ovarian cancer or limited accessibility to primary prevention in low-resource countries, such as in cervical cancer. Here, we extend the studies of AR2011, a stroma-targeted and tumor microenvironment responsive oncolytic adenovirus (OAdV), whose replication is driven by a triple hybrid promoter. We show that AR2011 was able to replicate and lyse in vitro fresh explants obtained from human ovarian cancer, uterine cancer, and cervical cancer. AR2011 was also able to strongly inhibit the in vitro growth of ovarian malignant cells obtained from human ascites fluid. The virus could synergize in vitro with cisplatin even on ascites-derived cells obtained from patients heavily pretreated with neoadjuvant chemotherapy. AR2011(h404), a dual transcriptionally targeted derived virus armed with hCD40L and h41BBL under the regulation of the hTERT promoter, showed a strong efficacy in vivo both on subcutaneous and intraperitoneally established human ovarian cancer in nude mice. Preliminary studies in an immunocompetent murine tumor model showed that AR2011(m404) expressing the murine cytokines was able to induce an abscopal effect. The present studies suggest that AR2011(h404) is a likely candidate as a novel medicine for intraperitoneal disseminated ovarian cancer.

Synergistic antitumor effect of a penicillin derivative combined with thapsigargin in melanoma cells

PURPOSE

To investigate the effect of TAP7f, a penicillin derivative previously characterized as a potent antitumor agent that promotes ER stress and apoptosis, in combination with thapsigargin, an ER stress inducer, on melanoma cells.

METHODS

The synergistic antiproliferative effect of TAP7f in combination with thapsigargin was studied in vitro in murine B16-F0 melanoma cells, and in human A375 and SB2 melanoma cells. In vivo assays were performed with C57BL/6J mice challenged with B16-F0 cells. Immunofluorescence and Western blot assays were carried out to characterize the induction of ER stress and apoptosis. Necrotic tumor areas and the potential toxicity of the combined therapy were examined by histological analysis of tissue sections after hematoxylin-eosin staining.

RESULTS

In vitro, the combination of TAP7f with thapsigargin synergistically inhibited the proliferation of murine B16-F0, and human A375 and SB2 melanoma cells. When non-inhibitory doses of each drug were simultaneously administered to C57BL/6J mice challenged with B16-F0 cells, a 50% reduction in tumor volumes was obtained in the combined group. An apoptotic response characterized by higher expression levels of Baxenhanced PARP-1 cleavage and the presence of active caspase 3 was observed in tumors from the combined treatment. In addition, higher expression levels of GADD153/CHOP and ATF4 were found in tumors of mice treated with both drugs with respect to each drug used alone, indicating the induction of an ER stress response. No signs of tissue toxicity were observed in histological sections of different organs extracted from mice receiving the combination.

CONCLUSION

The synergistic and effective antitumor action of TAP7f in combination with thapsigargin could be considered as a potential therapeutic strategy for melanoma treatment.

Oncolytic reovirus-mediated killing of mouse cancer-associated fibroblasts

Oncolytic viruses, which mediate tumor cell-specific infection, resulting in efficient tumor cell killing, have attracted much attention as a novel class of anti-cancer biopharmaceutical agents. Cancer-associated fibroblasts (CAFs) are an important component of the tumor microenvironment that strongly supports the growth, survival, and metastasis of tumor cells, suggesting that CAFs would have influence to the antitumor effects of oncolytic viruses; however, it remains to be fully evaluated whether oncolytic viruses affect the viabilities and properties of CAFs following treatment. Oncolytic reovirus, which is a non-enveloped virus that contains 10-segmented double-stranded RNA genome, shows efficient tumor cell lysis without apparent cytotoxicity to normal cells and has been tested worldwide in clinical trials against various types of tumors. In this study, we demonstrated that reovirus exhibited cytotoxicity against mouse primary CAFs isolated from subcutaneous tumors, but not against tail-tip fibroblasts. Infection with reovirus resulted in activation of caspase 3 and up-regulation of apoptosis-related gene expression, indicating that reovirus induced apoptosis of mouse primary CAFs. Intratumoral administration of reovirus induced apoptosis of mouse CAFs in the tumor. Taken together, these results indicate that reovirus has the potential to mediate antitumor effects by killing not only cancer cells but also CAFs.

Simultaneous Tumor and Stroma Targeting by Oncolytic Viruses

Current cancer therapeutics often insufficiently eradicate malignant cells due to the surrounding dense tumor stroma. This multi-componential tissue consists of mainly cancer-associated fibroblasts, the (compact) extracellular matrix, tumor vasculature, and tumor-associated macrophages, which all exert crucial roles in maintaining a pro-tumoral niche. Their continuous complex interactions with tumor cells promote tumor progression and metastasis, emphasizing the challenges in tumor therapy development. Over the last decade, advances in oncolytic virotherapy have shown that oncolytic viruses (OVs) are a promising multi-faceted therapeutic platform for simultaneous tumor and stroma targeting. In addition to promoting tumor cell oncolysis and systemic anti-tumor immunity, accumulating data suggest that OVs can also directly target stromal components, facilitating OV replication and spread, as well as promoting anti-tumor activity. This review provides a comprehensive overview of the interactions between native and genetically modified OVs and the different targetable tumor stromal components, and outlines strategies to improve stroma targeting by OVs.

Glucose 6-phosphate dehydrogenase inhibition sensitizes melanoma cells to metformin treatment

Most cancer cells exacerbate the pentose phosphate pathway (PPP) to enhance biosynthetic precursors and antioxidant defenses. Metformin, which is used as a first-line oral drug for the treatment of type 2 diabetes, has been proposed to inhibit the malignant progression of different types of cancers. However, metformin has shown poor efficacy as single agent in several clinical trials. Thus, the aim of the present work was to investigate whether the pharmacological inhibition of G6PDH, the first and rate-limiting enzyme of the PPP, by 6-amino nicotinamide (6-AN) potentiates the antitumoral activity of metformin on different human melanoma cell lines. Our results showed that 6-AN has sensitizing properties to metformin cytotoxicity. The combination of metformin and 6-AN decreased glucose consumption and lactate production, altered the mitochondrial potential and redox balance, and thereby blocked melanoma cell progression, directing cells to apoptosis and necrosis. To our knowledge, this is the first study describing the effect of this combination. Future preclinical studies should be performed to reveal the biological relevance of this finding.

Synthetic Tumor-Specific Promoters for Transcriptional Regulation of Viral Replication

Here we describe a collection of methods that have been adapted to isolate and modify tumor-specific promoters (TSPs ) to drive viral replication for cancer therapy and other uses. We will describe as examples the secreted protein acidic and rich in cysteine (SPARC ) and the protease-activated receptor-1 (PAR-1) promoter. We outline strategies to select appropriate TSPs using bioinformatics resources and the methods utilized in their subsequent cloning, assessment of transcriptional activity, and their use in conditionally replicative oncolytic adenoviruses .

Prospects for combined use of oncolytic viruses and CAR T-cells

With the approval of talimogene laherparepvec (T-VEC) for inoperable locally advanced or metastatic malignant melanoma in the USA and Europe, oncolytic virotherapy is now emerging as a viable therapeutic option for cancer patients. In parallel, following the favourable results of several clinical trials, adoptive cell transfer using chimeric antigen receptor (CAR)-redirected T-cells is anticipated to enter routine clinical practice for the management of chemotherapy-refractory B-cell malignancies. However, CAR T-cell therapy for patients with advanced solid tumours has proved far less successful. This Review draws upon recent advances in the design of novel oncolytic viruses and CAR T-cells and provides a comprehensive overview of the synergistic potential of combination oncolytic virotherapy with CAR T-cell adoptive cell transfer for the management of solid tumours, drawing particular attention to the methods by which recombinant oncolytic viruses may augment CAR T-cell trafficking into the tumour microenvironment, mitigate or reverse local immunosuppression and enhance CAR T-cell effector function and persistence.

Application of Synthetic Tumor-Specific Promoters Responsive to the Tumor Microenvironment

Activity of endogenous promoters can be altered by including additional responsive elements (REs). These elements can be responsive to features of the tumor environment or alternatively to signaling pathways specifically activated in cancer cells. These REs incorporated into tumor-specific promoters can improve cancer targeting, the replicative capacity, and lytic activity of conditionally replicative adenovirus. Here we outline an approach to incorporate hypoxia and inflammation REs into a specific fragment of the SPARC promoter and the steps to clone a nucleosome positioning sequence (NPS ) identified in the osteocalcin promoter that contains a Wnt RE upstream of a heterologous synthetic promoter.

Oncolytic Adenovirus-Loaded Menstrual Blood Stem Cells Overcome the Blockade of Viral Activity Exerted by Ovarian Cancer Ascites

Patients with ovarian cancer present peritoneal ascites at recurrence as a marker of disseminated disease and dismal prognosis. Oncolytic immunotherapy is an emerging approach for the treatment of disseminated cancer. In the present work, we constructed a novel oncolytic adenovirus, AR2011, to target malignant ovarian tumors. AR2011 exhibited a clear lytic effect in vitro in human ovarian cancer cell lines and malignant cells obtained from ascitic fluids (AFs) of patients with ovarian cancer. AR2011 activity was neutralized by antibodies present in 31 samples of patient-derived AFs. However, this blockade was overridden by preloading menstrual blood stem cells (MenSCs) with AR2011 (MenSC-AR), since AFs exerted no in vitro inhibitory effect on viral lytic activity under these conditions. Moreover, soluble factors present in AFs act as MenSC chemoattractants. MenSC-AR treatment of nude mice carrying established peritoneal carcinomatosis following administration of human ovarian cancer cells was able to inhibit tumor growth at levels similar to those observed with AR2011 alone. This study demonstrates that MenSCs can be used to override the blockade that AFs exert on viral oncolytic effects.

Interferon-β gene transfer inhibits melanoma cells adhesion and migration

We evaluated the effects of expression of interferon-β (IFNβ) after lipofection on melanoma cells adhesion and migration. Three canine mucosal (Ak, Br and Ol) and one human dermal (SB2) melanomas were assayed. By means of the wound healing assay, we found a significant inhibitory effect of canine IFNβ gene expression on cells migration in Br and Ol monolayers. This effect could be reproduced on unlipofected Ol cells with conditioned culture media obtained from canine IFNβ gene-lipofected Ol cells, and with recombinant human IFNβ on unlipofected SB2 cells. Furthermore, IFNβ gene expression of the four tested tumor cells significantly inhibited their adhesion to extracellular matrix (ECM) proteins and their spreading from multicellular spheroids onto gelatin coating. The addition of catalase reverted the increase of reactive oxygen species (ROS) in Ol cells and the inhibition of cell migration in monolayers (Ol) and spheroids (Ol an SB2) produced by canine and human IFNβ expression, suggesting the involvement of ROS as mediators of IFNβ action on the cells interactions with ECM. Together with its known immune, antiangiogenic and cytotoxic effects, the present data strongly support more studies exploring the clinical potential of IFNβ for cancer therapy.

Oncolytic viruses: a new class of immunotherapy drugs

Oncolytic viruses represent a new class of therapeutic agents that promote anti-tumour responses through a dual mechanism of action that is dependent on selective tumour cell killing and the induction of systemic anti-tumour immunity. The molecular and cellular mechanisms of action are not fully elucidated but are likely to depend on viral replication within transformed cells, induction of primary cell death, interaction with tumour cell antiviral elements and initiation of innate and adaptive anti-tumour immunity. A variety of native and genetically modified viruses have been developed as oncolytic agents, and the approval of the first oncolytic virus by the US Food and Drug Administration (FDA) is anticipated in the near future. This Review provides a comprehensive overview of the basic biology supporting oncolytic viruses as cancer therapeutic agents, describes oncolytic viruses in advanced clinical trials and discusses the unique challenges in the development of oncolytic viruses as a new class of drugs for the treatment of cancer.

Docetaxel-carboxymethylcellulose nanoparticles target cells via a SPARC and albumin dependent mechanism

Cellax, a polymer-docetaxel (DTX) conjugate that self-assembled into 120 nm particles, displayed significant enhancements in safety and efficacy over native DTX across a number of primary and metastatic tumor models. Despite these exciting preclinical data, the underlying mechanism of delivery of Cellax remains elusive. Herein, we demonstrated that serum albumin efficiently adsorbed onto the Cellax particles with a 4-fold increased avidity compared to native DTX, and the uptake of Cellax by cells was primarily driven by an albumin and SPARC (secreted protein acidic and rich in cysteine, an albumin binder) dependent internalization mechanism. In the SPARC-positive cells, a >2-fold increase in cellular internalization of Cellax was observed in the presence of albumin. In the SPARC-negative cells, no difference in Cellax internalization was observed in the presence or absence of albumin. Evaluation of the internalization mechanism using endocytotic inhibitors revealed that Cellax was internalized predominantly via a clathrin-mediated endocytotic mechanism. Upon internalization, it was demonstrated that Cellax was entrapped within the endo-lysosomal and autophagosomal compartments. Analysis of the tumor SPARC level with tumor growth inhibition of Cellax in a panel of tumor models revealed a positive and linear correlation (R(2) > 0.9). Thus, this albumin and SPARC-dependent pathway for Cellax delivery to tumors was confirmed both in vitro and in vivo.

Interferon-β gene transfer induces a strong cytotoxic bystander effect on melanoma cells

A local gene therapy scheme for the delivery of type I interferons could be an alternative for the treatment of melanoma. We evaluated the cytotoxic effects of interferon-β (IFNβ) gene lipofection on tumor cell lines derived from three human cutaneous and four canine mucosal melanomas. The cytotoxicity of human IFNβ gene lipofection resulted higher or equivalent to that of the corresponding addition of the recombinant protein (rhIFNβ) to human cells. IFNβ gene lipofection was not cytotoxic for only one canine melanoma cell line. When cultured as monolayers, three human and three canine IFNβ-lipofected melanoma cell lines displayed a remarkable bystander effect. As spheroids, the same six cell lines were sensitive to IFNβ gene transfer, two displaying a significant multicell resistance phenotype. The effects of conditioned IFNβ-lipofected canine melanoma cell culture media suggested the release of at least one soluble thermolabile cytotoxic factor that could not be detected in human melanoma cells. By using a secretion signal-free truncated human IFNβ, we showed that its intracellular expression was enough to induce cytotoxicity in two human melanoma cell lines. The lower cytoplasmatic levels of reactive oxygen species detected after intracellular IFNβ expression could be related to the resistance displayed by one human melanoma cell line. As IFNβ gene transfer was effective against most of the assayed melanomas in a way not limited by relatively low lipofection efficiencies, the clinical potential of this approach is strongly supported.

A Novel CDC25B Promoter-Based Oncolytic Adenovirus Inhibited Growth of Orthotopic Human Pancreatic Tumors in Different Preclinical Models

PURPOSE

We decided to construct a novel oncolytic adenovirus whose replication was driven by the CDC25B promoter for its use in preclinical models of pancreatic cancer.

EXPERIMENTAL DESIGN

We placed the essential E1A gene under control of the CDC25B promoter. Based on preliminary data, we pseudotyped the adenovirus with a chimeric fiber of serotypes 5/3. We investigated the in vitro lytic effect and the in vivo therapeutic efficacy in combination with gemcitabine on human pancreatic tumor xenografts orthotopically growing in nude mice and in tumors growing in Syrian hamsters. We also assessed biochemical markers of hepatic toxicity and CA19.9 levels.

RESULTS

AV25CDC exhibited a strong in vitro lytic effect on pancreatic cancer cells. In vivo administration of AV25CDC combined with gemcitabine in mice harboring subcutaneously growing SW1990 pancreatic tumors almost abrogated tumor growth. Nude mice harboring 15-day-old orthotopic tumors, treated intratumorally or systemically with AV25CDC combined with gemcitabine, exhibited 70% to 80% reduction in tumor size compared with control mice that lasted for at least 60 days. Chemovirotherapy treatment induced a return to normal levels of biochemical parameters of hepatic toxicity; these mice exhibited more than 90% reduction in CA19.9 serum levels compared with control. Chemovirotherapy efficacy was confirmed in mice harboring Mia PaCa-2 tumors and in Syrian hamster harboring HaP-T1 tumors. We observed that viral treatment disrupted tumor architecture and induced an increase in MMP-9 activity that might facilitate gemcitabine penetrability.

CONCLUSION

These data demonstrate that AV25CDC is an effective oncolytic agent candidate for pancreatic cancer chemovirotherapy combination.

SIBLINGs and SPARC families: Their emerging roles in pancreatic cancer

Pancreatic cancer has a considerably poor prognosis with a 5-year survival probability of less than 5% when all stages are combined. Pancreatic cancer is characterized by its dense stroma, which is involved in the critical interplay with the tumor cells throughout tumor progression and furthermore, creates a barrier restricting efficient penetration of therapeutics. Alterations in a large number of genes are reflected by a limited number of signaling pathways, which are potential targets. Understanding more about the molecular basis of this devastating cancer type regarding tumor microenvironment, distinct subpopulations of cells, epithelial-to-mesenchymal transition and inflammation will lead to the development of various targeted therapies for controlling tumor growth and metastasis. In this complex scenario of pancreatic cancer, especially members of the “small integrin binding ligand N-linked glycoproteins” (SIBLINGs) and “secreted protein acidic and rich in cysteine” (SPARC) families have emerged due to their prominent roles in properties including proliferation, differentiation, apoptosis, adhesion, migration, angiogenesis, wound repair and regulation of extracellular matrix remodeling. SIBLINGs consist of five members, which include osteopontin (OPN), bone sialoprotein, dentin matrix protein 1, dentin sialophosphoprotein and matrix extracellular phosphoglycoprotein. The SPARC family of modular extracellular proteins is comprised of SPARC/osteonectin (ON) and SPARC-like 1 (hevin); secreted modular calcium binding proteins; testicans and follistatin-like protein. In this review, we especially focus on OPN and ON, elaborating on their special and growing importance in pancreatic cancer diagnosis and prognosis.

Oncolytic Adenoviruses in Cancer Treatment

The therapeutic use of viruses against cancer has been revived during the last two decades. Oncolytic viruses replicate and spread inside tumors, amplifying their cytotoxicity and simultaneously reversing the tumor immune suppression. Among different viruses, recombinant adenoviruses designed to replicate selectively in tumor cells have been clinically tested by intratumoral or systemic administration. Limited efficacy has been associated to poor tumor targeting, intratumoral spread, and virocentric immune responses. A deeper understanding of these three barriers will be required to design more effective oncolytic adenoviruses that, alone or combined with chemotherapy or immunotherapy, may become tools for oncologists.

Therapeutic improvement of a stroma-targeted CRAd by incorporating motives responsive to the melanoma microenvironment

We have previously designed a conditionally replicative oncolytic adenovirus (CRAd) named Ad-F512 that can target both the stromal and the malignant melanoma cell compartments. The replication capacity of this CRAd is driven by a 0.5-Kb SPARC promoter fragment (named F512). To improve CRAd's efficacy, we cloned into F512 motives responsive to hypoxia (hypoxia-responsive element (HRE)) and inflammation (nuclear factor kappa B) to obtain a chimeric promoter named κBF512HRE. Using luciferase as a reporter gene, we observed 10-15-fold increased activity under hypoxia and 10-80-fold induction upon tumor necrosis factor-α addition. We next constructed a CRAd (Ad-κBF512HRE) where E1A activity was under κBF512HRE regulation. Treatment of nude mice harboring established tumors made of a mix of SB2 melanoma cells and WI-38 fibroblasts with Ad-κBF512HRE led to the complete elimination of tumors in 100% of mice (8/8). Moreover, Ad-5/3-κBF512HRE, a viral variant pseudotyped with a chimeric 5/3 fiber, exerted a strong lytic effect on CAR-negative melanoma cells and was highly effective in vivo on established tumors made of melanoma cells and WI-38 fibroblasts, leading to the complete elimination of 4/5 tumors. These results indicate that this improved stroma-targeted oncolytic adenovirus can override the resistance of melanoma tumors and might become of significant importance for melanoma therapeutics.

Chapter four--Design of improved oncolytic adenoviruses

During the last decade adenovirus has lost its appeal in gene therapy due to a high immunogenicity that leads to a transient gene expression. However, adenovirus has gained attention as replication-competent vector to treat cancer. Designed for virotherapy, adenovirus has been successfully modified to replicate selectively in tumor cells. After the initial clinical trials with tumor-selective adenoviruses, it has become clear that further improvements on tumor targeting, intratumoral dissemination, and modulation of antiviral and antitumor immune responses are needed to effectively treat cancer. The non-viral delivery of infectious DNA encoding an oncolytic adenovirus armed with extracellular matrix-degrading genes and with genes that regulate the immune system to favor antitumor instead of antiviral immunity are key in the design oncolytic adenovirus.

A tumor-stroma targeted oncolytic adenovirus replicated in human ovary cancer samples and inhibited growth of disseminated solid tumors in mice

Targeting the tumor stroma in addition to the malignant cell compartment is of paramount importance to achieve complete tumor regression. In this work, we modified a previously designed tumor stroma-targeted conditionally replicative adenovirus (CRAd) based on the SPARC promoter by introducing a mutated E1A unable to bind pRB and pseudotyped with a chimeric Ad5/3 fiber (Ad F512v1), and assessed its replication/lytic capacity in ovary cancer in vitro and in vivo. AdF512v1 was able to replicate in fresh samples obtained from patients: (i) with primary human ovary cancer; (ii) that underwent neoadjuvant treatment; (iii) with metastatic disease. In addition, we show that four intraperitoneal (i.p.) injections of 5 × 10(10) v.p. eliminated 50% of xenografted human ovary tumors disseminated in nude mice. Moreover, AdF512v1 replication in tumor models was enhanced 15-40-fold when the tumor contained a mix of malignant and SPARC-expressing stromal cells (fibroblasts and endothelial cells). Contrary to the wild-type virus, AdF512v1 was unable to replicate in normal human ovary samples while the wild-type virus can replicate. This study provides evidence on the lytic capacity of this CRAd and highlights the importance of targeting the stromal tissue in addition to the malignant cell compartment to achieve tumor regression.

A specific subpopulation of mesenchymal stromal cell carriers overrides melanoma resistance to an oncolytic adenovirus

The homing properties of mesenchymal stromal cells (MSCs) toward tumors turn them into attractive tools for combining cell and gene therapy. The aim of this study was to select in a feasible way a human bone marrow-derived MSC subpopulation that might exhibit a selective ability to target the tumor mass. Using differential in vitro adhesive capacities during cells isolation, we selected a specific MSC subpopulation (termed MO-MSCs) that exhibited enhanced multipotent capacity and increased cell surface expression of specific integrins (integrins α2, α3, and α5), which correlated with an enhanced MO-MSCs adhesiveness toward their specific ligands. Moreover, MO-MSCs exhibited a higher migration toward conditioned media from different cancer cell lines and fresh human breast cancer samples in the presence or not of a human microendothelium monolayer. Further in vivo studies demonstrated increased tumor homing of MO-MSCs toward established 578T and MD-MBA-231 breast cancer and A375N melanoma tumor xenografts. Tumor penetration by MO-MSCs was highly dependent on metallopeptidases production as it was inhibited by the specific inhibitor 1,10 phenantroline. Finally, systemically administered MO-MSCs preloaded with an oncolytic adenovirus significantly inhibited tumor growth in mice harboring established A375N melanomas, overcoming the natural resistance of the tumor to in situ administration of the oncolytic adenovirus. In summary, this work characterizes a novel MSC subpopulation with increased tumor homing capacity that can be used to transport therapeutic compounds.

Adenovirus i-leader truncation bioselected against cancer-associated fibroblasts to overcome tumor stromal barriers

Tumor-associated stromal cells constitute a major hurdle in the antitumor efficacy with oncolytic adenoviruses. To overcome this biological barrier, an in vitro bioselection of a mutagenized AdwtRGD stock in human cancer-associated fibroblasts (CAFs) was performed. Several rounds of harvest at early cytopathic effect (CPE) followed by plaque isolation led us to identify one mutant with large plaque phenotype, enhanced release in CAFs and enhanced cytotoxicity in CAF and several tumor cell lines. Whole genome sequencing and functional mapping identified the truncation of the last 17 amino acids in C-terminal end of the i-leader protein as the mutation responsible for this phenotype. Similar mutations have been previously isolated in two independent bioselection processes in tumor cell lines. Importantly, our results establish the enhanced antitumor activity in vivo of the i-leader C-terminal truncated mutants, especially in a desmotic fibroblast-embedded lung carcinoma model in mice. These results indicate that the i-leader truncation represents a promising trait to improve virotherapy with oncolytic adenoviruses.

Recent advances in oncolytic virus design

The cytolytic properties of viruses can be used to treat cancer. Replication of certain viruses is favoured in cancer cells, whereas others can be modified to obtain tumour specificity. This approach has evolved to become a new discipline called virotherapy. In addition, these replication-competent (oncolytic) viruses can be adapted as vectors for cancer gene therapy. The "armed" viruses show a double mechanism of action: direct destruction of cancer cells as a consequence of the lytic viral cycle, in combination with the effect of the therapeutic gene incorporated in the viral genome. Current trends in the field include strategies to increase the oncolytic potency of existing viruses; the evaluation of new candidates; the search for synergistic effects between different viruses and conventional therapies; and a rational approach to take advantage of the interplay between the viruses and the host immune system. This review summarises the most relevant achievements in recent years.

Tumor-stroma interactions reduce the efficacy of adenoviral therapy through the HGF-MET pathway

Many preclinical studies have shown the potential of adenovirus-based cancer gene therapy. However, successful translation of these promising results into the clinic has not yet been achieved. Pancreatic ductal adenocarcinoma (PDAC) is characterized by abundant desmoplastic stroma, and tumor-stromal cell interactions play a critical role in tumor progression. Therefore, we hypothesized that tumor-stroma interactions reduce the efficacy of adenoviral therapy. We investigated the effect of fibroblasts on adenovirus-based gene therapy using SUIT-2 and PANC-1 pancreatic cancer cells cultured with or without fibroblast-conditioned culture supernatant then infected with Ad-LacZ. After 48 h, the cells were stained for β-galactosidase. The results showed that the number of β-galactosidase-positive cells was significantly reduced after culture with fibroblast-conditioned supernatant (P < 0.05). Because the hepatocyte growth factor (HGF)/MET pathway plays an important role in tumor-stroma interactions we next investigated the involvement of this pathway in tumor-stroma interactions leading to the decreased efficacy of adenoviral therapy. SUIT-2 cells were cultured with or without SU11274 (a MET inhibitor) and/or fibroblast-conditioned culture supernatant, then infected with Ad-GFP. After 48 h, GFP-positive cells were counted. The number of GFP-positive cells in cultures containing fibroblast-conditioned supernatant plus SU11274 was significantly greater than in cultures without SU11274. In conclusion, our results suggest that stromal cells in PDAC reduce the efficacy of adenoviral therapy through a mechanism involving the HGF/MET pathway. Control of such tumor-stroma interactions may lead to improvements in adenoviral gene therapy for PDAC.

Liposomes targeting tumour stromal cells

Liposomes have found clinical application in cancer therapy in the delivery of cytostatic agents. As a result of the targeted delivery of these toxic molecules to the tumour cells coupled to avoidance of toxicity-sensitive tissues, the therapeutic window is widened. Over the past years the focus of cancer therapy has shifted towards the stromal cells that are present in the tumour. It appears that clinically relevant tumours have acquired the ability to modulate the microenvironment in such a way that a chronic pro-inflammatory and pro-angiogenic state is achieved that contributes to invasion and metastasis and continued proliferation. Over the past years, liposomal formulations have been designed that target key stromal cell types that contribute to tumour growth. At the same time, many promising cell types have not been targeted yet and most of the studies employ drugs that aim at depleting stromal cells rather than modulating their activity towards an anti-tumour phenotype. In this review these target cell types will be addressed. Complementing these targeted formulations with the appropriate drugs to optimally suppress tumour-promoting signals while preserving anti-tumour action will be the challenge for the future.

Mesenchymal stem cells as therapeutic tools and gene carriers in liver fibrosis and hepatocellular carcinoma

Mesenchymal stem (stromal) cells (MSCs) are a source of circulating progenitors that are able to generate cells of all mesenchymal lineages and to cover cellular demands of injured tissues. The extent of their transdifferentiation plasticity remains controversial. Cells with MSC properties have been obtained from diverse tissues after purification and expansion in vitro. These cellular populations are heterogeneous and under certain conditions show pluripotent-like properties. MSCs present immunosuppressive and anti-inflammatory features and high migratory capacity toward inflamed or remodeling tissues. In this study we review available data regarding factors and signaling axes involved in the chemoattraction and engraftment of MSCs to an injured tissue or to a tissue undergoing active remodeling. Moreover, experimental evidence in support of uses of MSCs as vehicles of therapeutic genes is discussed. Because of its regenerative capacity and its particular immune properties, the liver is a good model to analyze the potential of MSC-based therapies. Finally, the potential application of MSCs and genetically modified MSCs in liver fibrosis and hepatocellular carcinoma (HCC) is proposed in view of available evidence.

Oncolytic Viruses for Cancer Therapy: Overcoming the Obstacles

Targeted therapy of cancer using oncolytic viruses has generated much interest over the past few years in the light of the limited efficacy and side effects of standard cancer therapeutics for advanced disease. In 2006, the world witnessed the first government-approved oncolytic virus for the treatment of head and neck cancer. It has been known for many years that viruses have the ability to replicate in and lyse cancer cells. Although encouraging results have been demonstrated in vitro and in animal models, most oncolytic viruses have failed to impress in the clinical setting. The explanation is multifactorial, determined by the complex interactions between the tumor and its microenvironment, the virus, and the host immune response. This review focuses on discussion of the obstacles that oncolytic virotherapy faces and recent advances made to overcome them, with particular reference to adenoviruses.