(E)-5-(2-bromovinyl)-2'-deoxyuridine potentiates ganciclovir-mediated cytotoxicity on herpes simplex virus-thymidine kinase--expressing cells

Novel delivery strategies for glioblastoma

Brain tumors--particularly glioblastoma multiforme--pose an important public health problem in the United States. Despite surgical and medical advances, the prognosis for patients with malignant gliomas remains grim: current therapy is insufficient with nearly universal recurrence. A major reason for this failure is the difficulty of delivering therapeutic agents to the brain: better delivery approaches are needed to improve treatment. In this article, we summarize recent progress in drug delivery to the brain, with an emphasis on convection-enhanced delivery of nanocarriers. We examine the potential of new delivery methods to permit novel drug- and gene-based therapies that target brain cancer stem cells and discuss the use of nanomaterials for imaging of tumors and drug delivery.

Introduction to the background, principles, and state of the art in suicide gene therapy

Gene therapy is defined as a technology that aims to modify the genetic component of cells to gain therapeutic benefits. Suicide gene therapy (or gene-directed enzyme prodrug therapy [GDEPT]) is a two-step treatment for cancer (especially, solid tumors). In the first step, a gene for a foreign enzyme is delivered to the tumor by a vector. Following the expression of the foreign enzyme, a prodrug is administered during the second step, which is selectively activated in the tumor. This article discusses the principles and the theorectical background of GDEPT. A special emphasis is put on enzyme/prodrug systems developed for GDEPT, the design of prodrugs and the kinetic of their activation, the types and the mechanisms of bystander effect and its immunological implications. The possible strategies to improve GDEPT are also discussed.

Use of suicide genes for cancer gene therapy: study of the different approaches

Cancer is a disease of high incidence for which conventional treatments are not necessarily effective. There is a need for the development of new alternative strategies. Among them, suicide gene therapy has been developed. In this approach, a gene encoding for a protein toxic under particular conditions is delivered to the target cells, resulting in their death. Although this approach has been in development for a long time, new combinations with other gene therapy areas, such as selective replicative viruses, tumour targeting, or conventional treatments such as chemo- or radiotherapy, are currently being tested. This review will summarise some of these approaches.

Functional expression of thymidine kinase in human leukaemic and colorectal cells, delivered as EGFP fusion protein by herpesvirus saimiri-based vector

Herpesvirus saimiri (HVS) has the capacity to incorporate large amounts of heterologous DNA and can infect many different human cell types. To develop its potential as a gene therapy vector, we cloned herpes simplex virus thymidine kinase (TK) gene into the HVS genome in the form of an enhanced green fluorescent protein (EGFP) fusion protein, using a cosmid-based approach. At multiplicity of infection = 100 over 90% of human leukemic K562 and Jurkat cells were transduced with HVS/EGFP-TK. Conditions of no selective pressure expression were maintained at > 92% per cell division. Expression of the EGFP-TK fusion protein rendered transfected leukaemic cells sensitive to cytotoxic treatment with the prodrugs ganciclovir (GCV) and (E)-5-(2-bromovinyl)-2'deoxyuridine (BVDU) at concentrations as low as 10 ng/ml. The viral vector was also screened against a panel of colorectal and pancreatic carcinoma cell lines. All cell lines were transduced but showed a range of sensitivity to infection. Three of the most easily transduced cell lines: Mia PaCa, HCT116 and SW948 transduced with HVS/EGFP-TK were effectively ablated by subsequent treatment with GCV or BVDU. Our results show that in its current form HVS/EGFP-TK could be utilized as an antitumour agent, or it could be developed further by inclusion of a therapeutic gene, with TK presence ensuring a mechanism of controlled removal of modified cells when no longer necessary. These results suggest that HVS/EGFP-TK has a great potential for a number of gene therapy applications.

Apoptosis induced by (E)-5-(2-bromovinyl)-2'-deoxyuridine in varicella zoster virus thymidine kinase-expressing cells is driven by activation of c-Jun/activator protein-1 and Fas ligand/caspase-8

The molecular mode of cell killing by the antiviral drug (E)-5-(2-bromovinyl-2'-deoxyuridine (BVDU) was studied in Chinese hamster ovary (CHO) cells stably transfected with the thymidine kinase gene (tk) of varicella zoster virus (CHO-VZVtk). The colony-forming ability of the cells was reduced to <1% at a concentration of approximately 1 microM BVDU, whereas for nontransfected cells or cells transfected with tk gene of herpes simplex virus type 1 (CHO-HSVtk), a 1000-fold higher dose was required to achieve the same response. BVDU inhibited thymidylate synthase in CHO-VZVtk but not in CHO-HSVtk and control cells. On the other hand, the drug was incorporated into DNA of VZVtk- and HSVtk-expressing cells to nearly equal amounts. Because coexposure of CHO-VZVtk cells to exogenous thymidine protected them from BVDU-induced cell killing, the cells obviously die because of thymidine depletion. At highly cytotoxic BVDU doses (50 microM) and longer exposure times (24-48 h), VZVtk cells were blocked to some extent in S and G2/M phase and underwent apoptosis (48-72 h). Not only apoptosis but also necrosis was induced. The findings also show that the drug causes the induction of c-Jun and the activation of activator protein-1 resulting in increased level of Fas ligand (FasL) and caspase-8/-3 activation. Bid and poly(ADP-ribose) polymerase were cleaved by caspases. Expression of Bax increased, whereas Bcl-2/Bcl-x(L) remained unchanged. Transfection of dominant-negative Fas-associated death domain and inhibition of caspase-8 by N-benzyloxycarbonyl-IETD-fluoromethyl ketone strongly abrogated BVDU-induced apoptosis, indicating Fas/FasL to be crucially involved. Thus, BVDU-triggered apoptosis differs significantly from that induced by ganciclovir, which induces in the same cellular background the mitochondrial damage pathway.

Prodrugs for Gene-Directed Enzyme-Prodrug Therapy (Suicide Gene Therapy)

This review focuses on the prodrugs used in suicide gene therapy. These prodrugs need to satisfy a number of criteria. They must be efficient and selective substrates for the activating enzyme, and be metabolized to potent cytotoxins preferably able to kill cells at all stages of the cell cycle. Both prodrugs and their activated species should have good distributive properties, so that the resulting bystander effects can maximize the effectiveness of the therapy, since gene transduction efficiencies are generally low. A total of 42 prodrugs explored for use in suicide gene therapy with 12 different enzymes are discussed, particularly in terms of their physiocochemical properties. An important parameter in determining bystander effects generated by passive diffusion is the lipophilicity of the activated form, a property conveniently compared by diffusion coefficients (log P for nonionizable compounds and log D(7) for compounds containing an ionizable centre). Many of the early antimetabolite-based prodrugs provide very polar activated forms that have limited abilities to diffuse across cell membranes, and rely on gap junctions between cells for their bystander effects. Several later studies have shown that more lipophilic, neutral compounds have superior diffusion-based bystander effects. Prodrugs of DNA alkylating agents, that are less cell cycle-specific than antimetabolites and more effective against noncycling tumor cells, appear in general to be more active prodrugs, requiring less prolonged dosing schedules to be effective. It is expected that continued studies to optimize the bystander effects and other properties of prodrugs and the activated species they generate will contribute to improvements in the effectiveness of suicide gene therapy.

Gene therapy of gliomas

Malignant glioma formation is associated with characteristic genetic alterations, although epigenetic mechanisms may contribute in tumorigenesis. Until recently, our knowledge has mainly been based on chromosomal and molecular studies performed in the last two decades. This has increased tremendously with the advent of new technologies, in particular expression arrays for simultaneous analysis of thousands of genes. Consequently, gene therapy of gliomas may aim at molecular interference with 'gain of function' genes (oncogenes) or replacement of 'loss of function' genes (tumor suppressor genes). Such approaches require transgene expression in whole tumor cell populations (if not other mechanisms come into play) which cannot be achieved with current vector systems. Hence other strategies have been pursued which may be independent of genes actually involved in tumorigenesis. Microbial genes (e.g. herpes simplex virus thymidine kinase) may be transferred into the tumors allowing for prodrug activation (e.g. ganciclovir). Furthermore, cytokines or other immunomodulatory genes may be used for vaccination purposes which frequently involves ex vivo transfection of autologous tumor cells with such genes. These approaches proved promising in preclinical studies performed in cell culture and different inbred rodent models. A considerable number of clinical trials have been initiated based on these approaches. Although most therapeutic strategies proved safe, clinical responses fell short of expectations raised by preclinical results. This, to a large extent, has to be attributed to a lag in the development of efficient vector systems. Although much effort has been put into this area of research, neuro-oncologists are still in await of a vector system allowing for selective and efficient tumor cell transduction. This has led to increased interest in distinct but related strategies, e.g. oncolytic viruses or direct intra-tumoral delivery of anti-sense oligonucleotides.