In situ delivery of suicide genes for cancer treatment

Novel selective human mitochondrial kinase inhibitors: Design, synthesis and enzymatic activity

Selective and effective TK2 inhibitors can be obtained by introduction of bulky lipophilic chains (acyl or alkyl entities) at the 2' position of araT and BVaraU, nucleoside analogues naturally endowed with a low TK2 affinity. These derivatives showed a competitive inhibitory activity against TK2 in micromolar range. BVaraU nucleoside analogues, modified on the 2'-O-acyl chain with a terminal N-Boc amino-group, conserved or increased the inhibitory activity against TK2 (7l and 7m IC(50): 6.4 and 3.8 microM, respectively). The substitution of an ester for a carboxamide moiety at the 2' position of araT afforded a consistent reduction of the inhibitory activity (25, IC(50): 480 microM). On the contrary, modifications at 2'-OH position of araC and araG, have provided inactive derivatives against TK2 and dGK, respectively. The biological activity of a representative compound, 2'-O-decanoyl-BVaraU, was also investigated in normal human fibroblasts and was found to impair mitochondrial function due to TK2 inhibition.

Increased anti-tumor effect by a combination of HSV thymidine kinase suicide gene therapy and interferon-gamma/GM-CSF cytokine gene therapy in CT26 tumor model

The potential therapeutic benefit of introducing IFN-gamma and GM-CSF genes in combination with the HSVtk suicide gene into subcutaneously implanted CT26 tumor cells was compared with that from each treatment alone. Cells, unmodified or retrovirally transduced with HSVtk or IFN-gamma/GM-CSF genes, were inoculated subcutaneously into syngeneic BALB/c mice in various combinations. HSVtk gene, with intraperitoneal ganciclovir treatment, reduced tumor volume by 81% at locally inoculated tumor sites (p < 0.01) and by 25% at distantly inoculated tumor sites (p = 0.052). IFN-gamma/GM-CSF genes showed a 56% tumor volume reduction at local tumor sites (p < 0.01) and 15% volume reduction at remote tumor sites, although this was not statistically significant. The combination of HSVtk (with GCV) and IFN-gamma/GM-CSF genes showed an 81% volume reduction at local tumor sites (p < 0.01) and a 43% volume reduction at remote tumor sites (p < 0.01). Thus, the combination of HSVtk and IFN-gamma/GM-CSF gene therapy produced greater therapeutic efficacy than either treatment alone.

The role of new agents in the treatment of non-small cell lung cancer

Lung cancer is one of the most frequent causes of cancer deaths worldwide. Non-small cell lung cancer (NSCLC) accounts for approximately 80% of cases and no curative treatment is available for the advanced stages of disease (stages III and IV), which comprise the majority of cases. Current treatment regimens with standard chemotherapy offer only a limited survival benefit, and, therefore, the development of new therapeutic strategies is needed. Novel chemotherapeutic drugs such as the epothilones, MEN 10755 and S-1 are being studied in patients with advanced stages of disease. Furthermore, a large number of therapies targeted against critical biological abnormalities in NSCLC are being investigated in clinical trials. The latter approach includes inhibition of growth factors, interference with abnormal signal transduction, inhibition of angiogenesis and gene replacement therapy. Promising results have thus far been obtained with some of these therapies. This review describes the role of new therapeutic agents in the treatment of NSCLC.

Clinical impact of novel treatment strategies

Following two decades of research on the biology of cancer and in particular of lung cancer, we have now a large number of molecular targets that can be utilized to create specific medicines against these cancers. Non-small cell lung cancer represents numerically the most important solid tumor in Western world, and is poorly affected by current therapies, where surgery represents almost the only curative therapy for about 25% of patients who are resectable at diagnosis. An abundant number of targeted therapies are being investigated in NSCLC. Among them are the metalloproteinase inhibitors, several tyrosine kinase inhibitors and several attempts of gene replacement have also been made. Promising results have so far been obtained with some of these approaches, and the outcome of large randomized studies is awaited. Small cell lung cancer (SCLC) represents about 20% of lung carcinomas, and several of the novel approaches that are being attempted for NSCLC, are also being investigated for SCLC. All these novel therapies open a new era of anticancer treatment that will likely complement the currently available therapies in the near future.

Cell-specific viral gene therapy of a Hurthle cell tumor

We evaluated the effectiveness of a replication-defective adenovirus-transducing thymidine kinase (TK) gene under the control of the rat Tg (rTg) promoter (AdrTgtk) in therapy of a human Hurthle cancer (XTC-1 cell) in vitro and in vivo. The ganciclovir (GCV) sensitivity of infected XTC-1 cells was assessed in vitro by H(3)-thymidine incorporation assay and Trypan-blue exclusion, and by an in vivo tumor development assay. Proliferation was strongly inhibited by adding GCV into the culture medium of infected cells, but not uninfected cells, proving cell infection and expression of TK in the XTC-1 cells. AdrTgtk, and also viruses that have the noncell-specific cytomegalovirus (CMV) promoter-directing expression of TK (AdCMVtk), or luciferase (AdCMVLuc), were used to transduce XTC-1 cells to evaluate killing effects. After infection with AdCMVtk or AdrTgtk, followed by GCV treatment, 70% of infected cells were killed in the presence of GCV, compared with less than 20% of cells infected by AdCMVLuc and treated with GCV. In vivo toxicity was studied in BALB/c mice. When adenovirus is given iv, liver is the major organ infected. No significant changes of the serum transaminase levels and no histological abnormalities were found in animals treated with AdrTgtk/GCV given iv, compared with control animals. High levels of serum transaminases, lymphocyte infiltration, some Kupffer's cell prominence, and extensive single-cell hepatocyte death were found in AdCMVtk/GCV-treated animals, indicating severe liver damage induced, as expected, by the noncell-specific CMV promoter. XTL-1 cells (2 x 10(6)) were injected sc into BALB/c-severe combined immunodeficient mice (BALB/c-SCID), and the mice developed tumors after 3 wk. After intratumoral injection of AdrTgtk and treatment with GCV, tumors stabilized in 15 of 17 mice within 3 wk, 9 tumors remained stabilized after 5 wk of treatment, and 2 disappeared during observation. In AdCMVLuc/GCV-treated control mice, almost all tumors grew continuously. The average tumor size in AdrTgtk-treated mice was significantly smaller than that of control animals after 2 wk of treatment. Our data confirm the effectiveness and specificity of an adenovirus using rTg promoter to express TK, and support its future application to thyroid cancer gene therapy in humans.

Novel approaches to the treatment of non-small cell lung cancer

Prognosis of non-small cell lung carcinomas (NSCLC) remains poor, especially in advanced disease. The introduction of new cytotoxic agents in the past decade did only attain minor improvements in survival. It is rather clear that chemotherapy may have reached a plateau, and that it will be difficult to obtain better results in advanced NSCLC by chemotherapy alone. Novel treatment modalities are urgently needed in advanced NSCLC. Backed-up by advances in the understanding of tumor cell biology, a new generation of anticancer agents specifically directed at targets such as tyrosine kinases, farnesyl transferase, angiogenesis factors, matrixmetalloproteinases and oncogenes has been developed in recent years. In this review, we give a brief summary of the state-of-the-art treatment of NSCLC, highlighting its limitations. Novel systemic approaches are then discussed in detail with focus on their mechanistic rationale, stage of clinical development and possible drawbacks. Finally, perspectives of future applications and impact on the treatment of NSCLC are also discussed.

Advances in gene therapy for malignant melanoma

BACKGROUND

The recent developments in the field of gene transfer have advanced the use of gene therapy as a novel strategy against a variety of human malignancies. Due to its unique set of characteristics, melanoma represents a suitable target for the clinical translation of the different gene transfer approaches recently developed. The goal of gene therapy targeted to melanoma cells is to introduce "suicide" genes, to transfer tumor suppressor genes, to inactivate aberrant oncogene expression, or to introduce genes encoding immunologically relevant molecules. Gene therapy targeted to the host's immune cells has been developed as an additional strategy to redirect immune responses against melanoma.

METHODS

The authors reviewed the published gene transfer studies in experimental models, as well as the results of gene therapy clinical trials for patients with melanoma.

RESULTS

Clinical trials have shown the feasibility and safety of gene therapy against malignant melanoma. Although no major successes have been reported, the positive results observed in some patients support the potential for gene therapy in the management of this disease.

CONCLUSIONS

Gene therapy of melanoma using current gene transfer approaches is feasible and safe. Better vector technology as well as increased understanding of the "bystander effect" triggered by gene transfer approaches would provide the tools to validate gene therapy as an effective modality of treatment for malignant melanoma.

Adenoviral-mediated gene therapy for thyroid carcinoma using thymidine kinase controlled by thyroglobulin promoter demonstrates high specificity and low toxicity

A replication defective adenovirus transducing thymidine kinase (TK) gene under the control of the rat thyroglobulin (rTg) promoter (AdrTgtk) was developed to evaluate its cell-specific killing activity in gene therapy. We also developed adenoviruses containing the TK gene driven by the cytomegalovirus (CMV) promoter (AdCMVtk), and luciferase (Luc) gene driven by the rTg or CMV promoter (AdrTgLuc or AdCMVLuc). Luc activity in FRTL-5, HepG2, COS1, rMTC, hMTC, Hela, GH3, T98G, and CA77 cells was measured after infection with AdrTgLuc or AdCMVLuc. FRTL-5 cells produce thyroglobulin (Tg), whereas all other cells are non-Tg-producing cell lines. Transduction by AdCMVLuc caused high Luc activity in all cell lines. However, infection with AdrTgLuc induced Luc activity only in FRTL-5 cells. AdCMVtk or AdrTgtk was used to transduce various cell lines to evaluate the different killing effect. After infection with AdCMVtk vector followed by ganciclovir (GCV) treatment, cell growth was strongly suppressed in all cell lines compared both to noninfected cells and to cells infected by AdCMVLuc in the presence of GCV. When FRTL-5 cells were infected with AdrTgtk followed by GCV treatment, more than 90% were killed, but only a minimal effect was observed in other cell lines, indicating that the Tg promoter transduced TK expression only in Tg-producing cells. When adenovirus is given intravenously, liver and spleen are the major organs infected. A high Luc activity was found in liver and spleen of AdCMVLuc treated animals. No Luc activity was found in liver and spleen of AdrTgLuc-treated animals, indicating that rTg does not transduce Luc expression in non-Tg-producing tissues in vivo. No significant changes of the serum transaminase levels and histologic abnormalities were found in animals treated with AdrTgtk/GCV compared with control animals. High levels of serum transaminases, lymphocyte infiltration, some Kupffer's cell prominence, and extensive single cell hypatocyte death were found in AdCMVtk/GCV-treated animals, indicating severe liver damage induced, as expected, by a noncell-specific promoter. These results indicate that transfer of TK gene driven by the rTg promoter has thyroid cell-specific killing ability in the presence of GCV, little in vivo toxicity, and should be useful in the future for treating thyroid Tg-producing cancers.

Selective abolishment of pyrimidine nucleoside kinase activity of herpes simplex virus type 1 thymidine kinase by mutation of alanine-167 to tyrosine

Herpes simplex virus type 1 (HSV-1) encodes a thymidine kinase (TK) that markedly differs from mammalian nucleoside kinases in terms of substrate specificity. It recognizes both pyrimidine 2'-deoxynucleosides and a variety of purine nucleoside analogs. Based on a computer modeling study and in an attempt to modify this specificity, an HSV-1 TK mutant enzyme containing an alanine-to-tyrosine mutation at amino acid position 167 was constructed. Compared with wild-type HSV-1 TK, the purified mutant HSV-1 TK(A167Y) enzyme was heavily compromised in phosphorylating pyrimidine nucleosides such as (E)-5-(2-bromovinyl)-2'-deoxyuridine and the natural substrate dThd, whereas its ability to phosphorylate the purine nucleoside analogs ganciclovir (GCV) and lobucavir was only reduced approximately 2-fold. Moreover, a markedly decreased competition of natural pyrimidine nucleosides (i.e., thymidine) with purine nucleoside analogs for phosphorylation by HSV-1 TK(A167Y) was observed. Human osteosarcoma cells transduced with the wild-type HSV-1 TK gene were extremely sensitive to the cytostatic effects of antiherpetic pyrimidine [i.e., (E)-5-(2-bromovinyl)-2'-deoxyuridine] and purine (i.e., GCV) nucleoside analogs. Transduction with the HSV-1 TK(A167Y) gene sensitized the osteosarcoma cells to a variety of purine nucleoside analogs, whereas there was no measurable cytostatic activity of pyrimidine nucleoside analogs. The unique properties of the A167Y mutant HSV-1 TK may give this enzyme a therapeutic advantage in an in vivo setting due to the markedly reduced dThd competition with GCV for phosphorylation by the HSV-1 TK.

Adeno-associated viral-mediated gene transfer to hepatoma: thymidine kinase/interleukin 2 is more effective in tumor killing in non-ganciclovir (GCV)-treated than in GCV-treated animals

Interleukin 2 (IL-2) enhancement of herpes simplex virus-thymidine kinase (HSV-TK)/ganciclovir (GCV)-induced tumor killing was studied by cloning the human interleukin 2 gene into an HSV-TK-bearing adeno-associated viral (AAV) vector (TK/IL-2). The mouse hepatocellular carcinoma cell line Hepa 1-6 was used as a model in this study. We found that TK/IL-2-transduced Hepa 1-6 cells were more susceptible to ganciclovir treatment than tumor cells transduced with only TK in both nude mice and immunocompetent C57L/J mice. TK/IL-2-transduced tumors also showed shrinkage without GCV treatment. The tumor-killing effect of AAV-mediated TK/IL-2 gene transfer was further studied by inoculating animals with TK/IL-2- or TK-transduced tumor cells mixed with unmodified cells with or without GCV treatment. Although tumor growth in each group was inhibited, the best result was obtained from the TK/IL-2-transduced group without GCV treatment. In this group, 10% of the transduced tumor cells could eradicate the whole tumor in 50% of the animals tested as well as provide long-term protection against tumor cell rechallenge. When this group was treated with GCV, the antitumor effect of TK/IL-2 was reduced. We attribute this to the early ablation of transgene-bearing tumor cells by GCV treatment, which thus reduces the duration of IL-2 expression. We conclude that (i) TK/IL-2 plus GCV treatment generates a stronger tumor-killing effect than HSV-TK plus GCV and (ii) tumor killing of TK/IL-2 is more effective in non-GCV-treated animals than in GCV-treated animals.

Replicating herpes simplex virus vectors for cancer gene therapy

Attenuated viral vectors based on herpes simplex virus (HSV) are capable of killing cancer cells directly while sparing normal tissue in animal models of disease. This selective ability is likely due to the evolutionary constraints on the virus to establish lifelong infection in its host without causing destruction of normal tissues. However, extensive experimental animal data show that cancer cells are able to sustain a productive viral infection, which ultimately leads to cell death and tumour regression. Moreover, preliminary results generated in two Phase I clinical studies of modified replicating HSV for the treatment of brain tumours (e.g., glioblastoma multiforme) have been encouraging and suggest that the safety data generated in animals are predictive of human safety. Although much progress has been made in developing oncolytic HSV vectors for clinical use, there is still a long way to go to determine which combinations of virus, surgery, radiation and chemotherapy will provide improved therapy for the control and eradication of a variety of human cancers.

Gene therapy of established medullary thyroid carcinoma with herpes simplex viral thymidine kinase in a rat tumor model: relationship of bystander effect and antitumor efficacy

Bystander effect (BSE) refers to killing of cells adjacent to a cell engineered to express a killing gene segment. BSE is considered an important aspect of suicide gene therapy with thymidine kinase. We evaluated the BSE of adenovirus expressing herpes simplex thymidine kinase (AdCMVtk) in rat medullary thyroid carcinomas (rMTC) and three rat thyroid epithelial cancer cell lines using an in vitro BSE assay. In the assay, different proportions of infected and uninfected cells are mixed. Only the proportion of directly infected cells was inhibited in the proliferation assay using rMTC cells. This indicates that there is little BSE in this cell line. One rat thyroid epithelial cancer cell line (RTC-R2) has a high BSE, with BSE index (BSEi) of 7. In the proliferation assay a greater proportion of cells was inhibited than those directly infected. BSE was also evaluated during in vivo tumor growth by subcutaneous injection of mixtures of AdCMVtk infected and uninfected cells. Ganciclovir (GCV) treatment of tumors developing from a 1:1 mixture of infected to uninfected rMTC cells failed to inhibit their growth. In contrast, GCV treatment of a 2:8 mixture of infected to uninfected RTC-R2 cells completely inhibit tumor development, indicating a high BSE. BSE is related to in vivo antitumor efficacy when replication-defective adenovirus AdCMVtk is directly injected into rMTC tumors. After treatment with 100 mg/kg per day of GCV, a growth-retardation effect was observed in small tumors (<100 mm3), but there was little antitumor activity in large tumors (>100 mm3). Our results indicate that there is a good correlation between this in vitro BSE assay and in vivo treatment efficacy. Not all kinds of tumors are suitable for thymidine kinase (TK)/GCV gene therapy because some lack BSE. Methods to improve BSE and/or transduction efficiency are needed in order to obtain an effective therapeutic result. It will be appropriate to test the BSE in human tumor cells before performing clinical trials with current adenoviral vectors expressing TK.

Gene therapy for lung cancer

The advances that have been made over the past decade in the field of gene transfer as well as in the fields of immunology and the molecular biology of tumorigenesis have brought to reality the possibility of using gene transfer as an anti-cancer treatment modality. The published results of clinical trials using this approach to date have been very limited, and a considerable amount of work still needs to be done in order to make this an effective treatment modality. However the developments that have occurred in the past several years indicate that this modality will become efficacious in the foreseeable future.

Receptor-mediated and enzyme-dependent targeting of cytotoxic anticancer drugs

This review is a survey of various approaches to targeting cytotoxic anticancer drugs to tumors primarily through biomolecules expressed by cancer cells or associated vasculature and stroma. These include monoclonal antibody immunoconjugates; enzyme prodrug therapies, such as antibody-directed enzyme prodrug therapy, gene-directed enzyme prodrug therapy, and bacterial-directed enzyme prodrug therapy; and metabolism-based therapies that seek to exploit increased tumor expression of, e.g., proteases, low-density lipoprotein receptors, hormones, and adhesion molecules. Following a discussion of factors that positively and negatively affect drug delivery to solid tumors, we concentrate on a mechanistic understanding of selective drug release or generation at the tumor site.

Double suicide gene (cytosine deaminase and herpes simplex virus thymidine kinase) but not single gene transfer allows reliable elimination of tumor cells in vivo

Suicide genes such as cytosine deaminase (CD) and herpes simplex virus thymidine kinase (TK) encode products that convert nontoxic substances (prodrugs) into toxic metabolites. Suicide gene transfer is currently being used in cancer therapy or can be used as a safety modality. To analyze the reliability of suicide genes as a safety modality for a vaccination study with viable cytokine/B7 gene-modified tumor cells, the individual and combined efficacy of the two suicide genes was compared for in vitro and in vivo cell killing of a murine mammary adenocarcinoma cell line (TS/A). To adapt the system to an in vivo gene delivery situation, bulk cultures cotransfected with the CD and TK gene were used instead of selected clones. In vitro, both CD and TK conferred sensitivity to the respective prodrug but the combined cytotoxic effects of both gene products were always superior. For in vivo analysis BALB/c mice were injected subcutaneously with CD- and TK-modified TS/A cells, treated with prodrugs, and tumor size was evaluated for a period of 100 days. In the in vivo situation the combination of both enzyme/prodrug systems was again most effective. The highest single concentration of 5-FC (500 mg/kg) or GCV (100 mg/kg) was not able to fully protect the animals from developing tumors, whereas a combination of 5-FC (250 mg/kg) and GCV (50 mg/kg) resulted in complete tumor eradication. In nude mice treated in the same way, most CD/TK tumors could not be eliminated. Furthermore, BALB/c mice cured of TS/A-CD/TK tumors developed a systemic tumor immunity against challenge with parental TS/A cells. These findings indicate that reliable tumor elimination by the suicide genes depends on T cells. The cooperative effect of both suicide genes was confirmed in vitro with the human renal cell carcinoma line RCC26. We conclude that TK and CD together, but neither gene alone, act as a safety mechanism for the elimination of tumor cells in a reliable fashion and suggest that a rapid and quantitative antigen release by effective TK- and CD-mediated tumor destruction is necessary for T cell immunity to develop.

Retroviral vector targeting to melanoma cells by single-chain antibody incorporation in envelope

Two strategies for targeting recombinant retroviruses to melanoma cells were compared. One was to extend the tropism of an ecotropic envelope to human melanoma cells, the other was to enhance the tropism of an amphotropic envelope for melanoma cells. Chimeric retroviral envelopes, incorporating a single-chain antibody (ScFv) directed against high-molecular-weight melanoma-associated antigen (HMWMAA) at the amino terminus are correctly processed and incorporated into virions. ScFv-ecotropic envelope chimeras allow specific, but low-titer, targeting of HMWMAA-positive cells, when co-expressed with ecotropic envelopes. ScFv-amphotropic envelope chimeras bind specifically to HMWMAA-positive cells and allow preferential infection at high titer.

Converting cancer genes into killer genes

Over the past decade, it has become clear that tumorigenesis is driven by alterations in genes that control cell growth or cell death. Theoretically, the proteins encoded by these genes provide excellent targets for new therapeutic agents. Here, we describe a gene therapy approach to specifically kill tumor cells expressing such oncoproteins. In outline, the target oncoprotein binds to exogenously introduced gene products, resulting in transcriptional activation of a toxic gene. As an example, we show that this approach can be used to specifically kill cells overexpressing a mutant p53 gene in cell culture. The strategy may be generally applicable to neoplastic diseases in which the underlying patterns of genetic alterations or abnormal gene expression are known.