Suicide genes for cancer therapy

Bifidobacterium infantis-mediated HSV-TK/GCV suicide gene therapy induces both extrinsic and intrinsic apoptosis in a rat model of bladder cancer

Bladder cancer is the most common cancer in the urological tract. Here, we investigated the molecular pathways involved in the apoptosis of rodent bladder cancer, which was treated with Bifidobacterium infantis (BI)-mediated herpes simplex virus thymidine kinase (HSV-TK)/ganciclovir (GCV) suicide gene therapy. We engineered the BI-TK system, which consists of BI with the recombinant plasmid PGEX-1λT carrying HSV-TK (that is, TK) gene. Tumor-bearing rats were randomly divided into three groups and tail vein injected with normal saline (group A), BI/PGEX-1λT (group B) or BI-TK (group C), followed by intraperitoneal injection of GCV. The BI-TK/GCV system (group C) was shown to inhibit tumor growth and caused the apoptosis of tumor cells as assessed by in situ terminal transferase dUTP nick-end labeling assay. While varied extents of apoptosis were detected in tumor cells from all groups, tumor cells treated with BI-TK/GCV system (group C) exhibited the highest level of apoptosis (P < 0.05), consistent with our previous studies. Furthermore, we found that the expression levels of Fas, FasL, Cyt-C and caspase-9 in tumor tissues derived from group C were significantly higher than the other two groups (P < 0.001). Therefore, our results have demonstrated that the BI-TK/GCV therapy system exhibits a sustainable antitumor growth activity in the rodent model of bladder cancer. Mechanistically, both extrinsic and intrinsic apoptosis pathways are involved in the BI-TK/GCV antitumor functionality.

Chemovirotherapy of malignant melanoma with a targeted and armed oncolytic measles virus

Effective treatment modalities for advanced melanoma are desperately needed. An innovative approach is virotherapy, in which viruses are engineered to infect cancer cells, resulting in tumor cell lysis and an amplification effect by viral replication and spread. Ideally, tumor selectivity of these oncolytic viruses is already determined during viral cell binding and entry, which has not been reported for melanoma. We engineered an oncolytic measles virus entering melanoma cells through the high molecular weight melanoma-associated antigen (HMWMAA) and proved highly specific infection and spread in melanoma cells. We further enhanced this oncolytic virus by inserting the FCU1 gene encoding the yeast-derived prodrug convertases cytosine deaminase and uracil phosphoribosyltransferase. Combination treatment with armed and retargeted MV-FCU1-αHMWMAA and the prodrug 5-fluorocytosine (5-FC) led to effective prodrug conversion to 5-fluorouracil, extensive cytotoxicity to melanoma cells, and excessive bystander killing of noninfected cells. Importantly, HMWMAA-retargeted MV showed antitumor activity in a human xenograft mouse model, which was further increased by the FCU1/5-FC prodrug activation system. Finally, we demonstrated susceptibility of melanoma skin metastasis biopsies to HMWMAA-retargeted MV. The highly selective, entry-targeted and armed oncolytic virus MV-FCU1-αHMWMAA may become a potent building block of future melanoma therapies.

Targeted expression of herpes simplex virus thymidine kinase in adenovirus-infected cells reduces virus titers upon treatment with ganciclovir in vitro

BACKGROUND

Adenoviruses are a frequent cause of life-threatening infections in immunocompromised patients. Available therapeutics still cannot completely prevent fatal outcomes. By contrast, herpes viruses are well treatable with prodrugs such as ganciclovir (GCV), which are selectively activated in virus-infected cells by virus-encoded thymidine kinases. This effective group of prodrugs is not applicable to adenoviruses and other DNA viruses because they lack those kinases.

METHODS

To render adenoviruses amenable to GCV treatment, we generated an adenoviral vector-based delivery system for targeted expression of herpes simplex virus thymidine kinase (HSV-TK) in wild-type adenovirus 5 (wt Ad5)-infected cells. HSV-TK expression was largely restricted to wt virus-infected cells by transcription of the gene from the Ad5 E4 promoter. Its activity is dependent on the adenoviral E1A gene product which is not produced by the vector but is only provided in cells infected with wt adenovirus. The anti-adenoviral effect of HSV-TK expression and concomitant treatment with GCV was assessed in vitro in four different cell lines or primary cells.

RESULTS

E4 promoter-mediated HSV-TK background expression was sufficiently low to prevent cytotoxicity in the presence of low-levels GCV in cells not infected with wt Ad5. However, expression was several-fold increased in wt Ad5-infected cells and treatment with low levels of GCV efficiently inhibited wt Ad5 DNA replication. Genome copy numbers and output of infectious particles were reduced by up to > 99.99% and cell viability was greatly increased.

CONCLUSIONS

We extended the concept of enzyme/prodrug therapy to adenovirus infections by selectively sensitizing adenovirus-infected cells to treatment with GCV.

Evaluation of a gene-directed enzyme-product therapy (GDEPT) in human pancreatic tumor cells and their use as in vivo models for pancreatic cancer

Background

Gene-directed enzyme prodrug therapy (GDEPT) is a two-step treatment protocol for solid tumors that involves the transfer of a gene encoding a prodrug-activating enzyme followed by administration of the inactive prodrug that is subsequently activated by the enzyme to its tumor toxic form. However, the establishment of such novel treatment regimes to combat pancreatic cancer requires defined and robust animal model systems.

Methods

Here, we comprehensively compared six human pancreatic cancer cell lines (PaCa-44, PANC-1, MIA PaCa-2, Hs-766T, Capan-2, and BxPc-3) in subcutaneous and orthotopical mouse models as well as in their susceptibility to different GDEPTs.

Results

Tumor uptake was 83% to 100% in the subcutaneous model and 60% to 100% in the orthotopical mouse model, except for Hs-766T cells, which did not grow orthotopically. Pathohistological analyses of the orthotopical models revealed an infiltrative growth of almost all tumors into the pancreas; however, the different cell lines gave rise to tumors with different morphological characteristics. All of the resultant tumors were positive for MUC-1 staining indicating their origin from glandular or ductal epithelium, but revealed scattered pan-cytokeratin staining. Transfer of the cytochrome P450 and cytosine deaminase suicide gene, respectively, into the pancreatic cancer cell lines using retroviral vector technology revealed high level infectibility of these cell lines and allowed the analysis of the sensitivity of these cells to the chemotherapeutic drugs ifosfamide and 5-fluorocytosine, respectively.

Conclusion

These data qualify the cell lines as part of valuable in vitro and in vivo models for the use in defined preclinical studies for pancreas tumor therapy.

Emerging implications of nonmammalian cytosine deaminases on cancer therapeutics

Nonmammalian cytosine deaminases (CDs) have been investigated for last 30 years in the context of cancer therapy. The therapeutic effect of CD is based on its ability to catalyze the conversion of nontoxic prodrug 5-fluorocytosine (5FC) into the anticancer drug 5-fluorouracil (5FU) by deamination of the number 4 carbon of 5FC. This deamination property of CD has been explored to develop innovative therapeutic approach for treatment of cancer. A general overview is needed for the identification of efficient cytosine deaminases for potential use in cancer therapy. In this review, we have discussed about nonmammalian CDs for a variety of prodrug gene/enzyme therapy applications with several recent examples. Finally, we have provided a prospective on the future aspects of CDs and their applications in cancer therapy.

A caspase 8-based suicide switch induces apoptosis in nanobody-directed chimeric receptor expressing T cells

In accordance with the two-step hypothesis of T cell activation and the observation that stimulation through the T cell receptor (TCR) alone may lead to anergy, we focused on the introduction of co-stimulatory signaling to this type of receptors to achieve optimal activation. Enhanced mRNA and cell surface receptor expression via the co-stimulatory gene fragment (OX40) was confirmed by RT-PCR and flow cytometry. Inclusion of the OX40 co-stimulatory signaling region in series with the TCR led to enhanced antigen-induced IL-2 production after stimulation by MUC1-expressing cancer cell lines as compared to the chimeric receptor without OX40. Moreover, with the aim of maintaining high efficiency, while providing a means of controlling any possible unwanted proliferation in vivo, a regulation system was used. This controls the dimerization of a membrane-bound caspase 8 protein. Toward that goal, pFKC8 and CAR constructs were co-transfected into Jurkat cells, and the level of apoptosis was measured. 24 h after addition of the dimerizer, a 91% decrease in transfected cells was observed.

Antitumor potential of SLPI promoter controlled recombinant caspase-3 expression in laryngeal carcinoma

The purpose of this study is to develop a specific and efficient targeted gene therapy candidate approach for laryngeal carcinomas. Several promoters of human squamous cell carcinoma antigen 2(SCCA2), secretory leukocyte protease inhibitor (SLPI) and Survivin genes were cloned from human genomic DNA and evaluated for tumor-specific transcription potential in human laryngeal carcinoma Hep-2 cells by dual luciferase assays. One SLPI promoter fragment (677 bp) showed the highest efficiency and specificity, and was used to control the expression of a recombinant active caspases-3 (revCasp3), which could trigger apoptosis without activation of its upstream cascade elements once expressed in a cell, in an adenoviral vector (Ad-SLPI-revCasp3), and its antitumor efficacy was assessed. In vitro infection with Ad-SLPI-revCasp3 showed revCasp3 could be specifically expressed in Hep-2 cells, resulting in efficient activation of endogenous Caspase-3 and subsequent apoptosis of Hep-2 cells. In Hep-2 nude mice xenograft model, intratumoral administration of Ad-SLPI-revCasp3 significantly inhibited tumor growth without obvious loss of body weight and obvious hepatic toxicity. In summary, our study showed the specific and efficient apoptosis-inducing potential of Ad-SLPI-revCasp3, and this makes it a new candidate approach of targeted gene therapy for laryngeal squamous cell carcinoma, which needs further systematic investigation.

5-Fluorouracil-loaded poly(ε-caprolactone) nanoparticles combined with phage E gene therapy as a new strategy against colon cancer

This work aimed to develop a new therapeutic approach to increase the efficacy of 5-fluorouracil (5-FU) in the treatment of advanced or recurrent colon cancer. 5-FU-loaded biodegradable poly(ε-caprolactone) nanoparticles (PCL NPs) were combined with the cytotoxic suicide gene E (combined therapy). The SW480 human cancer cell line was used to assay the combined therapeutic strategy. This cell line was established from a primary adenocarcinoma of the colon and is characterized by an intrinsically high resistance to apoptosis that correlates with its resistance to 5-FU. 5-FU was absorbed into the matrix of the PCL NPs during synthesis using the interfacial polymer disposition method. The antitumor activity of gene E from the phage ϕX174 was tested by generating a stable clone (SW480/12/E). In addition, the localization of E protein and its activity in mitochondria were analyzed. We found that the incorporation of 5-FU into PCL NPs (which show no cytotoxicity alone), significantly improved the drug's anticancer activity, reducing the proliferation rate of colon cancer cells by up to 40-fold when compared with the nonincorporated drug alone. Furthermore, E gene expression sensitized colon cancer cells to the cytotoxic action of the 5-FU-based nanomedicine. Our findings demonstrate that despite the inherent resistance of SW480 to apoptosis, E gene activity is mediated by an apoptotic phenomenon that includes modulation of caspase-9 and caspase-3 expression and intense mitochondrial damage. Finally, a strongly synergistic antiproliferative effect was observed in colon cancer cells when E gene expression was combined with the activity of the 5-FU-loaded PCL NPs, thereby indicating the potential therapeutic value of the combined therapy.

Baculoviruses as gene therapy vectors for human prostate cancer

Prostate cancer is the most commonly diagnosed cancer in ageing men in the western world. While the primary cancers can be treated with androgen ablation, radiotherapy and surgery, recurrent castration resistant cancers have an extremely poor prognosis, hence promoting research that could lead to a better treatment. Targeted therapeutic gene therapy may provide an attractive option for these patients. By exploiting the natural ability of viruses to target and transfer their genes into cancer cells, either naturally or after genetic manipulation, new generations of biological control can be developed. In this review we present the advantages and practicalities of using baculovirus as a vector for prostate cancer gene therapy and provide evidence for the potential of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) as a safer alternative vehicle for targeting cancer cells. Strategies to target baculovirus binding specifically to prostate cell surfaces are also presented. The large insertion capacity of baculoviruses also permits restricted, prostate-specific gene expression of therapeutic genes by cloning extended human transcriptional control sequences into the baculovirus genome.

In vivo MR tracking of therapeutic microglia to a human glioma model

A knowledge of the spatial localization of cell vehicles used in gene therapy against glioma is necessary before launching therapy. For this purpose, MRI cell tracking is performed by labeling the cell vehicles with contrast agents. In this context, the goal of this study was to follow noninvasively the chemoattraction of therapeutic microglial cells to a human glioma model before triggering therapy. Silica nanoparticles grafted with gadolinium were used to label microglia. These vehicles, expressing constitutively the thymidine kinase suicide gene fused to the green fluorescent protein gene, were injected intravenously into human glioma-bearing nude mice. MRI was performed at 4.7 T to track noninvasively microglial accumulation in the tumor. This was followed by microscopy on brain slices to assess the presence in the glioma of the contrast agents, microglia and fusion gene through the detection of silica nanoparticles grafted with tetramethyl rhodamine iso-thiocyanate, 3,3'-dioctadecyloxacarbocyanine perchlorate and green fluorescent protein fluorescence, respectively. Finally, gancyclovir was administered systemically to mice. Human microglia were detectable in living mice, with strong negative contrast on T(2) *-weighted MR images, at the periphery of the glioma only 24 h after systemic injection. The location of the dark dots was identical in MR microscopy images of the extracted brains at 9.4 T. Fluorescence microscopy confirmed the presence of the contrast agents, exogenous microglia and suicide gene in the intracranial tumor. In addition, gancyclovir treatment allowed an increase in mice survival time. This study validates the MR tracking of microglia to a glioma after systemic injection and their use in a therapeutic strategy against glioma.

Cancer suicide gene therapy with TK.007: superior killing efficiency and bystander effect

Suicide gene therapy is a promising concept in oncology. We have recently introduced a novel suicide gene, TK.007, which was shown to excel established herpes simplex virus thymidine kinase (HSVtk) variants when used for donor-lymphocyte modification in adoptive immunotherapy models. Here, the potential of TK.007 in killing cancer cells was studied. Initially, we transduced tumour cell lines derived from different neoplasias (glioblastoma, melanoma, lung cancer, colon cancer) with lentiviral LeGO vectors encoding TK.007 or the splice-corrected (sc)HSVtk together with an eGFP/Neo-marker. Based on direct in vitro comparison, we found that TK.007 facilitates more efficient tumour cell killing at significantly lower ganciclovir doses in all tumour cell lines tested. Also, using different readout systems, we found a significantly stronger bystander effect of TK.007 as compared to scHSVtk. Importantly, in vitro data were confirmed in vivo using a subcutaneous G62 glioblastoma model in NOD/SCID mice. In mice transplanted with scHSVtk-positive tumours, treatment with low (10 mg/kg) or standard (50 mg/kg) ganciclovir doses resulted only in short-term growth inhibition or transient tumour remission, respectively. In striking contrast, in the TK.007 group, all animals achieved continuous complete remission after both standard and low-dose ganciclovir. Finally, a substantial bystander effect for TK.007 was also confirmed with the G62 model in vivo, where significantly prolonged survival for mice bearing tumours containing only 10% or 50% TK.007-expressing cells was observed. In summary, our data indicate strongly improved anti-tumour activity of TK.007 as compared to conventional HSVtk. We therefore suppose that TK.007 is an excellent candidate for cancer suicide gene therapy.

Synergistic antitumoral effect of combination E gene therapy and Doxorubicin in MCF-7 breast cancer cells

The low effectiveness of conventional therapies to achieve the long-term survival of metastatic breast cancer patients calls for the development of novel options. Genes encoding cytotoxic proteins have been proposed as a new strategy to enhance the antiproliferative activity of drugs. Combined therapy using these genes and classical antitumoral drugs are under intensive study. The E gene from ϕX174 encodes a membrane protein with a toxic domain that leads to a decrease in the tumour cell growth rate. With the aim of improving the anti-tumour effect on breast cancer cells of the currently used chemotherapeutic drugs (Paclitaxel, Docetaxel and Doxorubicin), we investigated the association of E suicide gene with these drugs. The effect of the combined therapy (gene therapy and cytotoxic) was determined by treating transfected MCF-7 cells and multicellular tumour spheroids (MTS) with drugs gradient concentrations. Our results showed that E gene has a direct oncolytic effect inducing a significant decrease in the proliferation rate of the MCF-7 cells. The E gene antitumoral activity was mediated by the induction of apoptosis (mitochondrial pathway). In addition, a significant enhancement of proliferation inhibition was observed when E gene transfection was associated with cytotoxic drugs in comparison to single treatments. The use of the combined therapy E gene-Doxorubicin obtained the greatest effect on the MCF-7 growth arrest. This therapeutic association also induced a significant enhancement of the MTS volume growth inhibition. Anti-tumour activity of the chemotherapeutic drugs classically used in the treatment of breast cancer was enhanced by E gene. Our in vitro results indicate that experimental therapeutic strategy based in the combined therapy E gene and cytotoxic drugs may be of potential therapeutic value as a new strategy for patients with advanced breast cancer.

Antiangiogenesis enhances intratumoral drug retention

The tumor vasculature delivers nutrients, oxygen, and therapeutic agents to tumor cells. Unfortunately, the delivery of anticancer drugs through tumor blood vessels is often inefficient and can constitute an important barrier for cancer treatment. This barrier can sometimes be circumvented by antiangiogenesis-induced normalization of tumor vasculature. However, such normalizing effects are transient; moreover, they are not always achieved, as shown here, when 9L gliosarcoma xenografts were treated over a range of doses with the VEGF receptor-selective tyrosine kinase inhibitors axitinib and AG-028262. The suppression of tumor blood perfusion by antiangiogenesis agents can be turned to therapeutic advantage, however, through their effects on tumor drug retention. In 9L tumors expressing the cyclophosphamide-activating enzyme P450 2B11, neoadjuvant axitinib treatment combined with intratumoral cyclophosphamide administration significantly increased tumor retention of cyclophosphamide and its active metabolite, 4-hydroxycyclophosphamide. Similar increases were achieved using other angiogenesis inhibitors, indicating that increased drug retention is a general response to antiangiogenesis. This approach can be extended to include systemic delivery of an anticancer prodrug that is activated intratumorally, where antiangiogenesis-enhanced retention of the therapeutic metabolite counterbalances the decrease in drug uptake from systemic circulation, as exemplified for cyclophosphamide. Importantly, the increase in intratumoral drug retention induced by neoadjuvant antiangiogenic drug treatment is shown to increase tumor cell killing and substantially enhance therapeutic activity in vivo. Thus, antiangiogenic agents can be used to increase tumor drug exposure and improve therapeutic activity following intratumoral drug administration, or following systemic drug administration in the case of a therapeutic agent that is activated intratumorally.

Enhancing VSV oncolytic activity with an improved cytosine deaminase suicide gene strategy

Oncolytic viruses (OVs) are promising therapeutic agents for cancer treatment, with recent studies emphasizing the combined use of chemotherapeutic compounds and prodrug suicide gene strategies to improve OV efficacy. In the present study, the synergistic activity of recombinant vesicular stomatitis virus (VSV)-MΔ51 virus expressing the cytosine deaminase/uracil phosphoribosyltransferase (CD::UPRT) suicide gene and 5-fluorocytosine (5FC) prodrug was investigated in triggering tumor cell oncolysis. In a panel of VSV-sensitive and -resistant cells-prostate PC3, breast MCF7 and TSA, B-lymphoma Karpas and melanoma B16-F10-the combination treatment increased killing of non-infected bystander cells in vitro via the release of 5FC toxic derivatives. In addition, we showed a synergistic effect on cancer cell killing with VSV-MΔ51 and the active form of the drug 5-fluorouracil. Furthermore, by monitoring VSV replication at the tumor site and maximizing 5FC bioavailability, we optimized the treatment regimen and improved survival of animals bearing TSA mammary adenocarcinoma. Altogether, this study emphasizes the potency of the VSV-CD::UPRT and 5FC combination, and demonstrates the necessity of optimizing each step of a multicomponent therapy to design efficient treatment.

Selectivity and efficiency of late transgene expression by transcriptionally targeted oncolytic adenoviruses are dependent on the transgene insertion strategy

Key challenges facing cancer therapy are the development of tumor-specific drugs and potent multimodal regimens. Oncolytic adenoviruses possess the potential to realize both aims by restricting virus replication to tumors and inserting therapeutic genes into the virus genome, respectively. A major effort in this regard is to express transgenes in a tumor-specific manner without affecting virus replication. Using both luciferase as a sensitive reporter and genetic prodrug activation, we show that promoter control of E1A facilitates highly selective expression of transgenes inserted into the late transcription unit. This, however, required multistep optimization of late transgene expression. Transgene insertion via internal ribosome entry site (IRES), splice acceptor (SA), or viral 2A sequences resulted in replication-dependent expression. Unexpectedly, analyses in appropriate substrates and with matching control viruses revealed that IRES and SA, but not 2A, facilitated indirect transgene targeting via tyrosinase promoter control of E1A. Transgene expression via SA was more selective (up to 1,500-fold) but less effective than via IRES. Notably, we also revealed transgene-dependent interference with splicing. Hence, the prodrug convertase FCU1 (a cytosine deaminase-uracil phosphoribosyltransferase fusion protein) was expressed only after optimizing the sequence surrounding the SA site and mutating a cryptic splice site within the transgene. The resulting tyrosinase promoter-regulated and FCU1-encoding adenovirus combined effective oncolysis with targeted prodrug activation therapy of melanoma. Thus, prodrug activation showed potent bystander killing and increased cytotoxicity of the virus up to 10-fold. We conclude that armed oncolytic viruses can be improved substantially by comparing and optimizing strategies for targeted transgene expression, thereby implementing selective and multimodal cancer therapies.

TK.007: A novel, codon-optimized HSVtk(A168H) mutant for suicide gene therapy

Conditional elimination of infused gene-modified alloreactive T cells, using suicide gene activation, has been shown to be an efficient strategy to abrogate severe graft-versus-host disease (GvHD) in the context of adoptive immunotherapy. To overcome shortcomings of the most widely used suicide gene, wild-type (splice-corrected) herpes simplex virus thymidine kinase (scHSVtk), we generated two new variants: the codon-optimized coHSVtk and, by introducing an additional mutation (A168H), the novel TK.007. We transduced human hematopoietic cell lines and primary T cells with retroviral "sort-suicide vectors" encoding combinations of selection markers (tCD34 and OuaSelect) with one of three HSVtk variants. In vitro we observed higher expression levels and sustained long-term expression of TK.007, indicating lower nonspecific toxicity. Also, we noted significantly improved kinetics of ganciclovir (GCV)-mediated killing for TK.007-transduced cells. In an experimental (murine) allogeneic transplantation model, TK.007-transduced T cells mediated severe GvHD, which was readily abrogated by application of GCV (10 mg/kg). Last, we established a modified allotransplantation model that allowed quantitative comparison of the in vivo activities of TK.007 versus scHSVtk. We found that TK.007 mediates both significantly faster and higher absolute killing at low GCV concentrations (10 and 25 mg/kg). In summary, we demonstrate that the novel TK.007 suicide gene combines better killing performance with reduced nonspecific toxicity (as compared with the frequently used splice-corrected wild-type scHSVtk gene), thus representing a promising alternative for suicide gene therapy.

The emerging role of viruses in the treatment of solid tumours

There is increasing optimism for the use of non-pathogenic viruses in the treatment of many cancers. Initial interest in oncolytic virotherapy was based on the observation of an occasional clinical resolution of a lymphoma after a systemic viral infection. In many cancers, by comparison with normal tissues, the competency of the cellular anti-viral mechanism is impaired, thus creating an exploitable difference between the tumour and normal cells, as an unimpeded viral proliferation in cancer cells is eventually cytocidal. In addition to their oncolytic capability, these particular viruses may be engineered to facilitate gene delivery to tumour cells to produce therapeutic effects such as cytokine secretion and anti -tumour immune responses prior to the eventual cytolysis. There is now promising clinical experience with these viral strategies, particularly as part of multimodal studies, and already several clinical trials are in progress. The limitations of standard cancer chemotherapies, including their lack of specificity with consequent collateral toxicity and the development of cross-resistance, do not appear to apply to viral-based therapies. Furthermore, virotherapy frequently restores chemoradiosensitivity to resistant tumours and has also demonstrated efficacy against cancers that historically have a dismal prognosis. While there is cause for optimism, through continued improvements in the efficiency and safety of systemic delivery, through the emergence of alternative viral agents and through favourable clinical experiences, clinical trials as part of multimodal protocols will be necessary to define clinical utility. Significant progress has been made and this is now a major research area with an increasing annual bibliography.

Suicide gene therapy for graft-versus-host disease

In allogeneic hematopoietic stem cell transplantation, donor-derived T cells are key players for early immune reconstitution and efficient engraftment, as well as the graft-versus-leukemia and graft-versus-infection effects. However, a severe and quite common life-threatening complication is the development of graft-versus-host disease, during which the alloreactive donor T cells attack the host. Controlling graft-versus-host disease while preserving the benefits of graft-versus-leukemia still constitutes a challenge. A promising approach for the control of graft-versus-host disease is suicide gene therapy, which involves the ex vivo genetic modification of donor T cells with a suicide gene that allows for the selective elimination of the cells in vivo if graft-versus-host disease occurs. This article presents an overview of such approaches with special reference to lessons learned from previous clinical experiences, as well as a discussion of critical factors in suicide gene therapy.

Adenoviral delivery of pan-caspase inhibitor p35 enhances bystander killing by P450 gene-directed enzyme prodrug therapy using cyclophosphamide+

Background

Cytochrome P450-based suicide gene therapy for cancer using prodrugs such as cyclophosphamide (CPA) increases anti-tumor activity, both directly and via a bystander killing mechanism. Bystander cell killing is essential for the clinical success of this treatment strategy, given the difficulty of achieving 100% efficient gene delivery in vivo using current technologies. Previous studies have shown that the pan-caspase inhibitor p35 significantly increases CPA-induced bystander killing by tumor cells that stably express P450 enzyme CYP2B6 (Schwartz et al, (2002) Cancer Res. 62: 6928-37).

Methods

To further develop this approach, we constructed and characterized a replication-defective adenovirus, Adeno-2B6/p35, which expresses p35 in combination with CYP2B6 and its electron transfer partner, P450 reductase.

Results

The expression of p35 in Adeno-2B6/p35-infected tumor cells inhibited caspase activation, delaying the death of the CYP2B6 "factory" cells that produce active CPA metabolites, and increased bystander tumor cell killing compared to that achieved in the absence of p35. Tumor cells infected with Adeno-2B6/p35 were readily killed by cisplatin and doxorubicin, indicating that p35 expression is not associated with acquisition of general drug resistance. Finally, p35 did not inhibit viral release when the replication-competent adenovirus ONYX-017 was used as a helper virus to facilitate co-replication and spread of Adeno-2B6/p35 and further increase CPA-induced bystander cell killing.

Conclusions

The introduction of p35 into gene therapeutic regimens constitutes an effective approach to increase bystander killing by cytochrome P450 gene therapy. This strategy may also be used to enhance other bystander cytotoxic therapies, including those involving the production of tumor cell toxic protein products.

Oncolytic herpes simplex virus vectors and chemotherapy: are combinatorial strategies more effective for cancer?

Despite aggressive treatments, including chemotherapy and radiotherapy, cancers often recur owing to resistance to conventional therapies. Oncolytic viruses such as oncolytic herpes simplex virus (oHSV) represent an exciting biological approach to cancer therapy. A range of viral mutations has been engineered into HSV to engender oncolytic activity. While oHSV as a single agent has been tested in a number of cancer clinical trials, preclinical studies have demonstrated enhanced efficacy when it is combined with cytotoxic anticancer drugs. Among the strategies that will be discussed in this article are combinations with standard-of-care chemotherapeutics, expression of prodrug-activating enzymes to enhance chemotherapy and small-molecule inhibitors. The combination of oHSV and chemotherapy can achieve much more efficient cancer cell killing than either single agent alone, often through synergistic interactions. This can be clinically important not just for improving efficacy but also for permitting lower and less toxic chemotherapeutic doses. The viral mutations in an oHSV vector often determine the favorability of its interactions with chemotherapy, just as different cancer cells, due to genetic alterations, vary in their response to chemotherapy. As chemotherapeutics are often the standard of care, combining them with an investigational new drug, such as oHSV, is clinically easier than combining multiple novel agents. As has become clear for most cancer therapies, multimodal treatments are usually more effective. In this article, we will discuss the recent progress of these combinatorial strategies between virotherapy and chemotherapy and future directions.

Human and viral nucleoside/nucleotide kinases involved in antiviral drug activation: structural and catalytic properties

Antiviral nucleoside and nucleotide analogs, essential for the treatment of viral infections in the absence of efficient vaccines, are prodrug forms of the active compounds that target the viral DNA polymerase or reverse transcriptase. The activation process requires several successive phosphorylation steps catalyzed by different kinases, which are present in the host cell or encoded by some of the viruses. These activation reactions often are rate-limiting steps and are thus open to improvement. We review here the structural and enzymatic properties of the enzymes that carry out the activation of analogs used in therapy against human immunodeficiency virus and against DNA viruses such as hepatitis B, herpes and poxviruses. Four major classes of drugs are considered: thymidine analogs, non-natural L-nucleosides, acyclic nucleoside analogs and acyclic nucleoside phosphonate analogs. Their efficiency as drugs depends both on the low specificity of the viral polymerase that allows their incorporation into DNA, but also on the ability of human/viral kinases to provide the activated triphosphate active forms at a high concentration at the right place. Two distinct modes of action are considered, depending on the origin of the kinase (human or viral). If the human kinases are house-keeping enzymes that belong to the metabolic salvage pathway, herpes and poxviruses encode for related enzymes. The structures, substrate specificities and catalytic properties of each of these kinases are discussed in relation to drug activation.

Pancreatic cancer: gene therapy approaches and gene delivery systems

IMPORTANCE OF THE FIELD

Due to the absence of early diagnosis, the highly invasive and metastatic features and the lack of effective therapeutic modalities, the prognosis of patients with pancreatic cancer is poor. Gene therapy is currently regarded as a potential and promising therapeutic modality for pancreatic cancer.

AREAS COVERED IN THIS REVIEW

This article summarizes an update of gene therapy approaches and reviews the latest progress in gene delivery systems that have been tested on pancreatic cancer.

WHAT THE READER WILL GAIN

The treatment effectiveness of gene combination therapy is better than that of the regulation of single-gene or single gene therapy approaches. Naked DNA is limited because of degradation by intracellular and extracellular nucleases. Virus vectors show high transfection efficiency but are limited due to immunogenicity, inflammatory response and potential carcinogenicity. Non-viral vectors, such as cationic polymers or inorganic nanoparticles, show an important feature that they can be easily modified, and the progress of materials science will provide more and better non-viral vectors, accordingly improving the efficiency and safety of gene therapy, which will make them the most promising vectors for pancreatic cancer.

Locoregional intravascular viral therapy of cancer: precision guidance for Paris's arrow?

Viral therapy of cancer includes strategies such as viral transduction of tumour cells with 'suicide genes', using viral infection to trigger immune-mediated tumour cell death and using oncolytic viruses for their direct anti-tumour action. However, problems still remain in terms of adequate viral delivery to tumours. A role is also emerging for single-organ isolation and perfusion. Having begun with the advent of isolated limb perfusion for extremity malignancy, experimental systems have been developed for the perfusion of other organs, particularly the liver, kidneys and lungs. These are beginning to be adopted into clinical treatment pathways. The combination of these two modalities is potentially significant. Locoregional perfusion increases the exposure of tumour cells to viral agents. In addition, the avoidance of systemic elimination through the immune and reticulo-endothelial systems should provide a mechanism for increased transduction/infection of target cells. The translation of laboratory research to clinical practice would occur within the context of perfusion programmes, which are already established in the clinic. Many of these programmes include the use of vasoactive cytokines such as tumour necrosis factor-alpha, which may have an effect on viral uptake. Evidence of activation of specific anti-tumour immunological responses by intratumoural and other existing methods of viral administration raises the intriguing possibility of a locoregional therapy, with the ability to affect distant sites of disease. In this review, we examined the state of the literature in this area and summarized current findings before indicating likely areas of continuing interest.

Synthetic radiation-inducible promoters mediated HSV-TK/GCV gene therapy in the treatment of oral squamous cell carcinoma

OBJECTIVE

To investigate the therapeutic effect of herpes simplex virus thymidine kinase (HSV-TK) gene mediated by synthetic radiation-inducible promoters in the treatment of oral squamous cell carcinoma (OSCC) in vitro and in vivo.

METHODS

The plasmids pcDNA3.1(+)E6-HSV-TK were constructed, in which the HSV-TK genes were mediated by synthetic radiation-inducible promoters. The recombined plasmids were transfected into the Tca8113 cells and golden hamster buccal carcinoma, respectively. Low-dose radiotherapy was used to upregulate the HSV-TK genes expression. HSV-TK mRNA was assayed by RT-PCR. Apoptosis and proliferating cell nuclear antigen were detected respectively by in situ end-labeling and immunohistochemical method.

RESULTS

Compared with control group, the comparative survival rate of Tca8113 cells in HSV-TK/GCV/IR group was markedly decreased and the golden hamster buccal carcinoma in HSV-TK/GCV/IR group was obviously suppressed. Up-regulation of HSV-TK gene expression was found in the Tca8113 cells and in the golden hamster buccal carcinoma resulting from exposure to low-dose irradiation. The apoptosis indexes in Tca8113 cells or golden hamster buccal carcinoma with irradiation were markedly higher than those without irradiation. At the same time, the proliferation indexes in Tca8113 cells or golden hamster buccal carcinoma with irradiation were markedly lower than those without irradiation.

CONCLUSION

The results indicate that the synthetic radiation-inducible promoters can serve as a molecular switch to adjust the expression of HSV-TK gene in the treatment of OSCC, and low-dose induction radiation can significantly improve therapeutic efficiency.

Plant thymidine kinase 1: a novel efficient suicide gene for malignant glioma therapy

The prognosis for malignant gliomas remains poor, and new treatments are urgently needed. Targeted suicide gene therapy exploits the enzymatic conversion of a prodrug, such as a nucleoside analog, into a cytotoxic compound. Although this therapeutic strategy has been considered a promising regimen for central nervous system (CNS) tumors, several obstacles have been encountered such as inefficient gene transfer to the tumor cells, limited prodrug penetration into the CNS, and inefficient enzymatic activity of the suicide gene. We report here the cloning and successful application of a novel thymidine kinase 1 (TK1) from the tomato plant, with favorable characteristics in vitro and in vivo. This enzyme (toTK1) is highly specific for the nucleoside analog prodrug zidovudine (azidothymidine, AZT), which is known to penetrate the blood-brain barrier. An important feature of toTK1 is that it efficiently phosphorylates its substrate AZT not only to AZT monophosphate, but also to AZT diphosphate, with excellent kinetics. The efficiency of the toTK1/AZT system was confirmed when toTK1-transduced human glioblastoma (GBM) cells displayed a 500-fold increased sensitivity to AZT compared with wild-type cells. In addition, when neural progenitor cells were used as delivery vectors for toTK1 in intracranial GBM xenografts in nude rats, substantial attenuation of tumor growth was achieved in animals exposed to AZT, and survival of the animals was significantly improved compared with controls. The novel toTK1/AZT suicide gene therapy system in combination with stem cell-mediated gene delivery promises new treatment of malignant gliomas.

Molecular and traditional chemotherapy: a united front against prostate cancer

Castrate resistant prostate cancer (CRPC) is essentially incurable. Recently though, chemotherapy demonstrated a survival benefit ( approximately 2months) in the treatment of CRPC. While this was a landmark finding, suboptimal efficacy and systemic toxicities at the therapeutic doses warranted further development. Smart combination therapies, acting through multiple mechanisms to target the heterogeneous cell populations of PC and with potential for reduction in individual dosing, need to be developed. In that, targeted molecular chemotherapy has generated significant interest with the potential for localized treatment to generate systemic efficacy. This can be further enhanced through the use of oncolytic conditionally replicative adenoviruses (CRAds) to deliver molecular chemotherapy. The prospects of chemotherapy and molecular-chemotherapy as single and as components of combination therapies are discussed.

The current state of head and neck cancer gene therapy

The incidence of head and neck cancer continues to increase worldwide, with tobacco exposure and human papillomavirus type 16 infections being the major etiological factors. Current therapeutic options are ineffective in approximately half of the individuals afflicted with this malignancy. Developments in the identification of molecules that sustain head and neck squamous cell carcinoma (HNSCC) growth and survival have made molecular targeting by gene therapy approaches a feasible therapeutic strategy. Although gene therapy was originally designed to correct single gene defects, it has now evolved to encompass all forms of therapeutic interventions involving engineered cells and nucleic acids that modify the overall pattern of gene expression within target tissues. Several preclinical studies and clinical trials have tested the efficacy of targeting specific molecules in patients with HNSCC, using genetic therapy approaches. This review discusses promising preclinical and clinical approaches and new directions for HNSCC gene therapy.

HDAC inhibitor valproic acid enhances tumor cell kill in adenovirus-HSVtk mediated suicide gene therapy in HNSCC xenograft mouse model

Safety, efficacy and enhanced transgene expression are the primary concerns while using any vector for gene therapy. One of the widely used vectors in clinical trials is adenovirus which provides a safe way to deliver the therapeutic gene. However, adenovirus has poor transduction efficiency in vivo since most tumor cells express low coxsackie and adenovirus receptors. Similarly transgene expression remains low, possibly because of the chromatization of adenoviral genome upon infection in eukaryotic cells, an effect mediated by histone deacetylases (HDACs). Using a recombinant adenovirus (Ad-HSVtk) carrying the herpes simplex thymidine kinase (HSVtk) and GFP genes we demonstrate that HDAC inhibitor valproic acid can bring about an increase in CAR expression on host cells and thereby enhanced Ad-HSVtk infectivity. It also resulted in an increase in transgene (HSVtk and GFP) expression. This, in turn, resulted in increased cell kill of HNSCC cells, following ganciclovir treatment in vitro as well as in vivo in a xenograft nude mouse model.

Bystander or no bystander for gene directed enzyme prodrug therapy

Gene directed enzyme prodrug therapy (GDEPT) of cancer aims to improve the selectivity of chemotherapy by gene transfer, thus enabling target cells to convert nontoxic prodrugs to cytotoxic drugs. A zone of cell kill around gene-modified cells due to transfer of toxic metabolites, known as the bystander effect, leads to tumour regression. Here we discuss the implications of either striving for a strong bystander effect to overcome poor gene transfer, or avoiding the bystander effect to reduce potential systemic effects, with the aid of three successful GDEPT systems. This review concentrates on bystander effects and drug development with regard to these enzyme prodrug combinations, namely herpes simplex virus thymidine kinase (HSV-TK) with ganciclovir (GCV), cytosine deaminase (CD) from bacteria or yeast with 5-fluorocytodine (5-FC), and bacterial nitroreductase (NfsB) with 5-(azaridin-1-yl)-2,4-dinitrobenzamide (CB1954), and their respective derivatives.

Not gene therapy, but genetic surgery-the right strategy to attack cancer

In this review, I will suggest to divide all the approaches united now under common term "gene therapy" into two broad strategies of which the first one uses the methodology of targeted therapy with all its characteristics, but with genes in the role of agents targeted at a certain molecular component(s) presumably crucial for cancer maintenance. In contrast, the techniques of the other strategy are aimed at the destruction of tumors as a whole using the features shared by all cancers, for example relatively fast mitotic cell division or active angiogenesis. While the first strategy is "true" gene therapy, the second one is more like genetic surgery when a surgeon just cuts off a tumor with his scalpel and has no interest in knowing delicate mechanisms of cancer emergence and progression. I will try to substantiate the idea that the last strategy is the only right one, and its simplicity is paradoxically adequate to the super-complexity of tumors that originates from general complexity of cell regulation, strongly disturbed in tumor cells, and especially from the complexity of tumors as evolving cell populations, affecting also their ecological niche formed by neighboring normal cells and tissues. An analysis of the most widely used for such a "surgery" suicide gene/prodrug combinations will be presented in some more details.

HSV-tk expressing mesenchymal stem cells exert bystander effect on human glioblastoma cells

Previously we have reported adipose-tissue derived human mesenchymal stem cells (AT-MSC) as cellular delivery vehicles for tumor-targeted cancer gene therapy. In this report we aimed to determine whether Herpes simplex virus - thymidine kinase (HSV-tk) expressing AT-MSC (TK-MSC) could exert cytotoxic effect on tumor cells upon treatment with prodrug ganciclovir (GCV). Direct co-cultures of human glioblastoma cells 8-MG-BA, 42-MG-BA and U-118 MG with TK-MSC/GCV resulted in substantial viability decrease in vitro. This therapeutic paradigm was most efficient against 8-MG-BA glioblastoma cells exhibiting cytotoxicity (>50%) in the presence of TK-MSC and 0.1microM GCV. Rapid apoptosis induction in three glioblastoma cell lines and TK-MSC demonstrated both bystander cytotoxic effect on tumor cells and GCV conversion-mediated suicide effect on TK-MSC. Furthermore, we were able to demonstrate formation of gap junctions between AT-MSC and human glioblastoma cells as a mechanism contributing to bystander cytotoxicity. Inability of human HeLa and MCF7 to form gap junctions with AT-MSC rendered these cell refractory to the TK-MSC/GCV mediated cytotoxicity. Gap junction intercellular communication (GJIC) capability of AT-MSC with tumor cells further supports the exploitation of mesenchymal stem cells for approaches relying on the bystander effect. Biological consequences of these capabilities remain to be further explored.

Design of anticancer prodrugs for reductive activation

Anticancer prodrugs designed to target specifically tumor cells should increase therapeutic effectiveness and decrease systemic side effects in the treatment of cancer. Over the last 20 years, significant advances have been made in the development of anticancer prodrugs through the incorporation of triggers for reductive activation. Reductively activated prodrugs have been designed to target hypoxic tumor tissues, which are known to overexpress several endogenous reductive enzymes. In addition, exogenous reductive enzymes can be delivered to tumor cells through fusion with tumor-specific antibodies or overexpressed in tumor cells through gene delivery approaches. Many anticancer prodrugs have been designed to use both the endogenous and exogenous reductive enzymes for target-specific activation and these prodrugs often contain functional groups such as quinones, nitroaromatics, N-oxides, and metal complexes. Although no new agents have been approved for clinical use, several reductively activated prodrugs are in various stages of clinical trial. This review mainly focuses on the medicinal chemistry aspects of various classes of reductively activated prodrugs including design principles, structure-activity relationships, and mechanisms of activation and release of active drug molecules.

A mammalianized synthetic nitroreductase gene for high-level expression

Background

The nitroreductase/5-(azaridin-1-yl)-2,4-dinitrobenzamide (NTR/CB1954) enzyme/prodrug system is considered as a promising candidate for anti-cancer strategies by gene-directed enzyme prodrug therapy (GDEPT) and has recently entered clinical trials. It requires the genetic modification of tumor cells to express the E. coli enzyme nitroreductase that bioactivates the prodrug CB1954 to a powerful cytotoxin. This metabolite causes apoptotic cell death by DNA interstrand crosslinking. Enhancing the enzymatic NTR activity for CB1954 should improve the therapeutical potential of this enzyme-prodrug combination in cancer gene therapy.

Methods

We performed de novo synthesis of the bacterial nitroreductase gene adapting codon usage to mammalian preferences. The synthetic gene was investigated for its expression efficacy and ability to sensitize mammalian cells to CB1954 using western blotting analysis and cytotoxicity assays.

Results

In our study, we detected cytoplasmic protein aggregates by expressing GFP-tagged NTR in COS-7 cells, suggesting an impaired translation by divergent codon usage between prokaryotes and eukaryotes. Therefore, we generated a synthetic variant of the nitroreductase gene, called ntro, adapted for high-level expression in mammalian cells. A total of 144 silent base substitutions were made within the bacterial ntr gene to change its codon usage to mammalian preferences. The codon-optimized ntro either tagged to gfp or c-myc showed higher expression levels in mammalian cell lines. Furthermore, the ntro rendered several cell lines ten times more sensitive to the prodrug CB1954 and also resulted in an improved bystander effect.

Conclusion

Our results show that codon optimization overcomes expression limitations of the bacterial ntr gene in mammalian cells, thereby improving the NTR/CB1954 system at translational level for cancer gene therapy in humans.

Genetic control of wayward pluripotent stem cells and their progeny after transplantation

The proliferative capacity of pluripotent stem cells and their progeny brings a unique aspect to therapeutics, in that once a transplant is initiated the therapist no longer has control of the therapy. In the context of the recent FDA approval of a human ESC trial and report of a neuronal-stem-cell-derived tumor in a human trial, strategies need to be developed to control wayward pluripotent stem cells. Here, we focus on one approach: direct genetic modification of the cells prior to transplantation with genes that can prevent the adverse events and/or eliminate the transplanted cells and their progeny.

Targeting cancer by transcriptional control in cancer gene therapy and viral oncolysis

Cancer-specificity is the key requirement for a drug or treatment regimen to be effective against malignant disease--and has rarely been achieved adequately to date. Therefore, targeting strategies need to be implemented for future therapies to ensure efficient activity at the site of patients' tumors or metastases without causing intolerable side-effects. Gene therapy and viral oncolysis represent treatment modalities that offer unique opportunities for tumor targeting. This is because both the transfer of genes with anti-cancer activity and viral replication-induced cell killing, respectively, facilitate the incorporation of multiple mechanisms restricting their activity to cancer. To this end, cellular mechanisms of gene regulation have been successfully exploited to direct therapeutic gene expression and viral cell lysis to cancer cells. Here, transcriptional targeting has been the role model and most widely investigated. This approach exploits cellular gene regulatory elements that mediate cell type-specific transcription to restrict the expression of therapeutic genes or essential viral genes, ideally to cancer cells. In this review, we first discuss the rationale for such promoter targeting and its limitations. We then give an overview how tissue-/tumor-specific promoters are being identified and characterized. Strategies to apply and optimize such promoters for the engineering of targeted viral gene transfer vectors and oncolytic viruses-with respect to promoter size, selectivity and activity in the context of viral genomes-are described. Finally, we discuss in more detail individual examples for transcriptionally targeted virus drugs. First highlighting oncolytic viruses targeted by prostate-specific promoters and by the telomerase promoter as representatives of tissue-targeted and pan-cancer-specific virus drugs respectively, and secondly recent developments of the last two years.

A novel technique to monitor carboxypeptidase G2 expression in suicide gene therapy using 19F magnetic resonance spectroscopy

Development and evaluation of new anticancer drugs are expedited when minimally invasive biomarkers of pharmacokinetic and pharmacodynamic behaviour are available. Gene-directed enzyme prodrug therapy (GDEPT) is a suicide gene therapy in which the anticancer drug is activated in the tumor by an exogenous enzyme previously targeted by a vector carrying the gene. GDEPT has been evaluated in various clinical trials using several enzyme/prodrug combinations. The key processes to be monitored in GDEPT are gene delivery and expression, as well as prodrug delivery and activation. {4-[bis(2-chloroethyl)amino]-3,5-difluorobenzoyl}-L-glutamic acid, a prodrug for the GDEPT enzyme carboxypeptidase-G2 (CPG2; K(m) = 1.71 microM; k(cat) = 732 s(-1)), was measured with (19)F magnetic resonance spectroscopy (MRS). The 1 ppm chemical shift separation found between the signals of prodrug and activated drug (4-[bis(2-chloroethyl)amino]-3,5-difluorobenzoic acid) is sufficient for the detection of prodrug activation in vivo. However, these compounds hydrolyze rapidly, and protein binding broadens the MR signals. A new CPG2 substrate was designed with hydroxyethyl instead of chloroethyl groups (K(m) = 3.5 microM, k(cat) = 747 s(-1)). This substrate is nontoxic and stable in solution, has a narrow MRS resonance in the presence of bovine and foetal bovine albumin, and exhibits a 1.1 ppm change in chemical shift upon cleavage by CPG2. In cells transfected to express CPG2 in the cytoplasm (MDA MB 361 breast carcinoma cells and WiDr colon cancer cells), well-resolved (19)F MRS signals were observed from clinically relevant concentrations of the new substrate and its nontoxic product. The MRS conversion half-life (470 min) agreed with that measured by HPLC (500 min). This substrate is, therefore, suitable for evaluating gene delivery and expression prior to administration of the therapeutic agent.

Abrogation of microsatellite-instable tumors using a highly selective suicide gene/prodrug combination

A substantial fraction of sporadic and inherited colorectal and endometrial cancers in humans is deficient in DNA mismatch repair (MMR). These cancers are characterized by length alterations in ubiquitous simple sequence repeats, a phenotype called microsatellite instability. Here we have exploited this phenotype by developing a novel approach for the highly selective gene therapy of MMR-deficient tumors. To achieve this selectivity, we mutated the VP22FCU1 suicide gene by inserting an out-of-frame microsatellite within its coding region. We show that in a significant fraction of microsatellite-instable (MSI) cells carrying the mutated suicide gene, full-length protein becomes expressed within a few cell doublings, presumably resulting from a reverting frameshift within the inserted microsatellite. Treatment of these cells with the innocuous prodrug 5-fluorocytosine (5-FC) induces strong cytotoxicity and we demonstrate that this owes to multiple bystander effects conferred by the suicide gene/prodrug combination. In a mouse model, MMR-deficient tumors that contained the out-of-frame VP22FCU1 gene displayed strong remission after treatment with 5-FC, without any obvious adverse systemic effects to the mouse. By virtue of its high selectivity and potency, this conditional enzyme/prodrug combination may hold promise for the treatment or prevention of MMR-deficient cancer in humans.

The piggyBac transposon is an integrating non-viral gene transfer vector that enhances the efficiency of GDEPT

Gene-directed enzyme prodrug therapy (GDEPT) is a strategy developed to selectively target cancer cells. However, the clinical benefit is limited due to its poor gene transfer efficiency. To overcome this obstacle, we took advantage of piggyBac (PB) transposon, a natural non-viral gene vector that can induce stable chromosomal integration and persistent gene expression in vertebrate cells, including human cells. To determine whether the vector can also mediate stable gene expression in ovarian cancer cells, we constructed a PB transposon system that simultaneously expresses the Herpes simplex virus thymidine kinase (HSV-tk) gene and the monomeric red fluorescent protein (mRFP1) reporter gene. The recombinant plasmid, pPB/TK, was transfected into ovarian adenocarcinoma cells SKOV3 with FuGENE HD reagent, and the efficiency was given by the percentage of mRFP1-positive cells detected by flow cytometry and confocal microscopy. The specific expression of HSV-tk in transfected cells was confirmed by RT-PCR and western blotting. The sensitivity of transfected cells to pro-drug ganciclovir (GCV) was determined by methylthiazoletetrazolium (MTT) assay. A total of 56.4 +/- 8.4% cells transfected with pPB/TK were mRFP1 positive, compared to no measurable mRFP1 expression in pORF-HSVtk-transfected cells. The expression level of HSV-tk in pPB/TK-transfected cells was 10 times higher than in pORF-HSVtk-transfected cells. The results show that pPB/TK transfection increases the sensitivity of cells to GCV in a dose-dependent manner. Our data indicate that the PB transposon system could enhance the anti-tumor efficiency of GDEPT in ovarian cancer.