Immunological defects after suicide gene therapy of experimental graft-versus-host disease

Donor T cells are beneficial for engraftment, immune reconstitution, and antileukemic effects after allogeneic marrow transplantation, but they also cause graft-versus-host disease. Treatment with ganciclovir can control graft-versus-host disease if donor T cells are genetically engineered to express viral thymidine kinase. Clinical protocols with thymidine kinase-expressing T cells currently prescribe the curative use of ganciclovir for genetic immunosuppression only after clinical manifestations of graft-versus-host disease have appeared. The aim of this work was to compare early/preventive versus delayed/curative treatment of GVHD. Here, we found that ganciclovir administered early after experimental marrow transplantation was highly effective in preventing graft-versus-host disease caused by thymidine kinase-expressing T cells, and surviving recipient mice were able to mount a T cell-dependent B cell response. In contrast, curative ganciclovir administration later after transplantation was much less effective in treating graft-versus-host disease and surviving recipients had markedly impaired immune function. These findings should be considered in the development of future clinical trials using thymidine kinase-expressing T cells; to date, such trials have envisaged the use of GCV to treat only declared graft-versus-host disease. The use of thymidine kinase-expressing T cells for conditional elimination of activated T cells after allogeneic marrow transplantation offers a promising new approach for the control of graft-versus-host disease. The versatility of the thymidine kinase/ganciclovir system offers clinical options depending on whether thymidine kinase-expressing T cells are infused at the time of bone marrow transplantation or in a delayed manner, and depending on whether GCV is administered in an early/preventive or delayed/curative manner. The rationale underlying these options is more complex than it may appear and is likely to have a profound impact on the efficacy of such treatments. In the present work, we analyze the immunological impact when GCV is administered in an early/preventive or delayed/curative manner. Our results demonstrate that the delayed/curative strategy is clearly associated with severe immunological defects. To our knowledge, this is the first report of immunodeficiency subsequent to suicide gene therapy for GVHD.     

The equine herpes virus 4 thymidine kinase is a better suicide gene than the human herpes virus 1 thymidine kinase.

The herpes simplex virus type 1 thymidine kinase suicide gene (HSV1tk) together with ganciclovir (GCV) have been successfully used for in vivo treatment of various experimental tumors, and many clinical trials using this system have been launched. With the aim to improve this therapeutic system, we compared the potential efficacy of different herpes virus derived thymidine kinases (HSV1, varicella-zoster virus, equine herpes virus type-4 and Epstein-Barr virus) as suicide genes in association with the nucleoside analogs acyclovir, ganciclovir and bromovinyldeoxyur- idine. Using various murine and human cell lines expressing these viral tk, we show that HSV1- and EHV4tk are the more efficient suicide genes for the different nucleoside analogs tested. Moreover, EHV4tk expressing murine and human cells were three- to 12-fold more sensitive to GCV than HSV1tk expressing cells. This was correlated with the presence of five-fold higher amounts of the toxic triphosphated-GCV in EHV4- versus HSV1tk expressing cells. Altogether, these experiments underline the potential advantages of the EHV4tk as a suicide gene.     

Preservation of graft-versus-infection effects after suicide gene therapy for prevention of graft-versus-host disease

The main complications following allogeneic hematopoietic stem cell transplantation are graft-versus-host disease and poor immune reconstitution leading to severe infections. Mature donor T cells present in the transplant facilitate T cell reconstitution in adults, but also induce graft-versus-host disease, which itself impairs immune reconstitution. Thus, infusing a large number of donor T cells with a diverse repertoire should accelerate functional immune reconstitution after transplantation, only if graft-versus-host disease can be controlled. We previously demonstrated that preventive treatment with ganciclovir could control graft-versus-host disease in mice if donor T cells are made to express viral thymidine kinase as a "suicide" gene. Here we evaluated the recovery of functional antiviral immune responses in such mice. Irradiated mice received an allogeneic hematopoietic stem cell transplantation with thymidine kinase-expressing T cells and were protected from graft-versus-host disease by ganciclovir treatment, and then challenged with lymphocytic choriomeningitis virus. Grafted mice could mount efficient antilymphocytic choriomeningitis virus immune responses leading to viral elimination. Furthermore, when transplanted cells were obtained from mice previously immunized against lymphocytic choriomeningitis virus, grafted mice developed memory-type accelerated responses against the virus. We conclude that efficient graft-versus-infection effects can be mediated by naive T cells and memory donor T cells that persist after suicide gene therapy for prevention of graft-versus-host disease.     

Effect of combined cytostatic cyclosporin A and cytolytic suicide gene therapy on the prevention of experimental graft-versus-host disease

The immunosuppressive drug cyclosporin A (CsA) represents the standard preventive treatment of graft-versus-host disease (GVHD), the main complication of allogeneic hematopoietic stem cell transplantation (HSCT). However, its efficacy is only partial and many patients develop lethal GVHD despite CsA. A strategy of genetic immunosuppression based on conditional elimination of donor T cells expressing the Herpes simplex type 1 thymidine kinase (TK) suicide gene was recently developed. In this system, ganciclovir (GCV) selectively kills dividing but not quiescent TK T cells. Since CsA is known to have a cytostatic effect on T cells, it could negatively interfere with the division-dependent TK gene therapy. We thus tested whether administration of CsA would antagonize elimination of alloreactive donor TK T cells mediated by GCV in a murine model of GVHD. In vivo experiments revealed that, contrary to GCV, CsA only transiently controlled alloactivation-induced T cell proliferation, and likewise could not prevent lethal GVHD. When T cells resumed proliferation under CsA, they were however still sensitive to GCV. Survival, as well as immune reconstitution, was excellent in mice treated with GCV alone or in combination with CsA. These observations should help to design improved suicide gene therapy trials in the field of allogeneic HSCT.     

GCV modulates the antitumoural efficacy of a replicative adenovirus expressing the Tat8-TK as a late gene in a pancreatic tumour model

Replication-competent adenoviruses carrying the herpes simplex thymidine kinase (TK) gene have shown contradictory evidence with regard to their antitumoural efficacy in combination with ganciclovir (GCV) treatment. We generated a replication-competent adenovirus carrying Tat8-TK, a modified form of the TK gene, under the control of the adenoviral major late promoter (AdRGDTat8-TK-L). Pancreatic cancer cell lines with different sensitivity to the TK/GCV system were infected with AdRGDTat8-TK-L, both in the presence and absence of GCV, and tested for treatment efficacy. We observed that, although the presence of GCV reduced viral replication in all infected cell lines, in three out of four GCV significantly enhanced the efficacy of the virotherapy. Interestingly, the cytotoxicity of the AdRGD-Tat8-TK-L/GCV was found more potent than that of a first generation AdTK/GCV system. In tumour xenografts from BxPC-3 and NP-18 pancreatic cells, both AdRGDTat8-TK-L and AdRGDTat8-TK-L/GCV treatment showed antitumoural activity. In BxPC-3 tumours scheduling of virus and prodrug was a key factor to determine the outcome of the therapy. Importantly, the addition of GCV enhanced the antitumoural effect of AdRGDTat8-TK-L only when applied in two rounds of virus+GCV. Interestingly, in spite of interfering with viral replication in vitro, GCV treatment of NP-18 tumours did not compromise the antitumoural efficacy of the AdRGDTat8-TK-L adenovirus. Thus, our results show that the combination therapy of a replicative adenovirus and the Tat8-TK/GCV suicide system can prove beneficial, when the appropriate regimen of virus and GCV is applied.     

HSV vector cytotoxicity is inversely correlated with effective TK/GCV suicide gene therapy of rat gliosarcoma

Herpes simplex virus (HSV)-mediated delivery of the HSV thymidine kinase (tk) gene to tumor cells in combination with ganciclovir (GCV) administration may provide an effective suicide gene therapy for destruction of malignant glioblastomas. However, because HSV is a highly cytotoxic agent, gene expression from the virus is short-lived which may limit the effectiveness of HSVtk/GCV therapy. Using different replication-defective HSVtk gene vectors, we compared HSV vector backgrounds for their cytotoxic activity on infection of 9L gliosarcoma cells in culture and brain tumors in rats and evaluated the impact of vector toxicity on the effectiveness of tk/GCV-mediated suicide gene therapy. As reported previously for other cell lines, a vector deleted for both copies of the immediate-early (IE) gene ICP4 (SOZ.1) was highly toxic for 9L cells in culture while a vector deleted in addition for the ICP22 and ICP27 IE genes (T.1) reduced or arrested 9L cell proliferation with more limited cell killing. Nevertheless, both vectors supported widespread killing of uninfected cells in the presence of GCV following low multiplicity infections, indicating that vector cytotoxicity did not preempt the production of vector-encoded TK enzyme necessary for the killing of uninfected cells by the HSV-tk/GCV bystander effect. Although an SOZ.1-related vector (SHZ.2) caused tumor cell necrosis in vivo, injection of SHZ.2 at multiple coordinates thoughout the tumor followed by GCV administration failed to prolong markedly the survival of tumor-bearing rats. In contrast, a single injection of T.1 produced a life-extending response to GCV. These results indicate that vector cytotoxicity can limit the efficacy of HSV-tk/GCV treatment in vivo, which may be due to premature termination of tk gene expression with attendant abortion of the bystander effect.     

Synergy of adoptive T-cell therapy and intratumoral suicide gene therapy is mediated by host NK cells

In situ tumor cell killing by the herpes simplex virus thymidine kinase (HSVtk) gene can effectively prime antitumor T-cell responses, at least in part through local induction of a pro-inflammatory environment. Therefore, we reasoned that tumor-associated HSVtk expression would significantly enhance the efficacy of adoptive T-cell transfer (ACT) of (tumor) antigen-specific T cells into tumor-bearing hosts. When B16ovaHSVtk tumors were treated with ganciclovir (GCV), along with suboptimal numbers of activated OT-1T cells, complete tumor regressions were observed where GCV, or ACT, alone was completely ineffective. To our surprise, analysis of regressing tumors showed no increases in intratumoral OT-1T cell trafficking. However, the intratumoral percentages of both OT-1 and endogenous natural killer (NK) cells were substantially increased over controls. Depletion of endogenous NK cells abrogated the efficacy of the combination therapy and reduced the percentages of interferon-gamma(IFNgamma)-secreting OT-1T cells in mice that received combined therapy to levels similar to those of control mice. These data suggest that even relatively low levels of gene transfer of suicide genes into tumors may have therapeutic value as an adjuvant for other T-cell therapies, by providing immunological signals that support T-cell activation and expansion in vivo.     

Genetic prodrug activation therapy (GPAT) in two rat prostate models generates an immune bystander effect and can be monitored by magnetic resonance techniques

Treatment of hormone refractory prostate cancer requires new treatment strategies. Genetic prodrug activation therapy (GPAT) may provide a new therapeutic avenue. In this study the antitumour efficacy of the gene encoding herpes simplex virus thymidine kinase (HSVtk) activating the prodrug ganciclovir (GCV) was compared in two models of ectopic (subcutaneous) rat prostate cancer. Both models, which differ in their characteristics, were previously shown to be weakly immunogenic but susceptible to immunotherapy. Tumour cell lines were stably transfected with HSVtk and were rendered highly sensitive to GCV. Little or no bystander killing effect was observed by tk-transfected cells on wild-type cells in vitro. However, a significant in vivo bystander effect was observed suggesting an immune-mediated response. Indeed, such an immune response was capable of slowing the growth of distant wild-type tumours and increased overall animal survival. A T helper 1 immune response was generated as a result of GCV activation and cell kill, demonstrated by the secretion of IFNgamma by cultured splenocytes in response to tumour cells. BrDU staining of tk-transfected cells treated with GCV in vitro suggested apoptotic cell death, but Annexin V staining was less marked for one of the cell lines. Serial in vivo monitoring by non-invasive magnetic resonance spectroscopy (MRS) of the tk-transfected MATLyLu tumours demonstrated a decreased ATP/Pi ratio (a measure of cell energy status) during growth and an increase in the ATP/Pi ratio during regression initiated by treatment with GCV. Further, significant differences were found in the phosphomonester (PME) to total phosphate (SigmaP) ratios in treated compared with untreated tumours, a result rarely seen in animal models, but commonly observed in patients. This study showed that a Th1-biased immune response generated by killing prostate tumour cells with tk/GCV can kill distant as well as local wild-type tumour cells. These findings suggest that GPAT may have a potential application in patients with both confined and metastatic prostate cancer and MRS may provide a method of monitoring response to treatment.     

A guanylate kinase/HSV-1 thymidine kinase fusion protein enhances prodrug-mediated cell killing

Herpes simplex virus thymidine kinase (HSVTK) with the guanosine analog ganciclovir (GCV) is currently the most widely used suicide gene/prodrug system for gene therapy of cancer. Despite the broad application of the HSVTK/GCV approach, phosphorylation of GCV to its active state is inefficient such that high, myelosuppressive doses of GCV are needed to observe an antitumor effect. One strategy used to overcome the poor substrate specificity of HSVTK towards GCV (Km = 45 microM) has been to create novel forms of TK with altered substrate preferences. Such mutant TKs have shown benefit and are currently in clinical use. We describe here a second strategy to increase the amount of intracellular triphosphorylated GCV by involving the second enzyme in the GCV activation pathway, guanylate kinase (GMK). As a means to overcome the bottleneck of prodrug activation from the monophosphate to the diphosphate, we sought to combine both the critical HSVTK and GMK activities together. In this report we describe the construction of a fusion or chimeric protein of HSVTK and guanylate kinase, show data that demonstrate it confers a approximately 175-fold decrease in IC50 compared to HSVTK alone in response to ganciclovir treatment in stably transfected C6 glioma cells and finally, we present biochemical evidence of a kinetic basis for this improved cell killing.     

Enhanced pancreatic tumor regression by a combination of adenovirus and retrovirus-mediated delivery of the herpes simplex virus thymidine kinase gene.

We have evaluated the effectiveness of combining the different characteristics of retrovirus and adenovirus to apply the herpes simplex virus thymidine kinase gene (HSVtk) and ganciclovir (GCV) treatment for gene therapy of pancreatic cancer. Transduction of NP-18 human pancreatic cells in culture by either the adenoviral vector (ADV/tk) or the retroviral vector (Rv/tk) followed by GCV treatment resulted in a GCV dose-dependent cytotoxic effect. A bystander effect was determined, both in NP-18 cultures and in xenogeneic cell mixtures of NP-18 and PA317 cells. Studies in vivo indicated that the effectiveness of tumor regression after HSVtk gene transfer and GCV treatment was dependent first on the tumor size at the time of viral injection and secondly, in large tumors, on the type of virus administered. The administration of the viral combination (ADV/tk + vector producer cells VPC-Rv/tk) was the best approach tested and resulted in a dramatic reduction in tumor mass after 4 days of GCV treatment which was maintained for the treatment period. Remarkably, two animals presented a complete eradication of the tumor. Thus, the HSVtk/GCV system when administered using a viral combination (ADV/tk + VPC-Rv/tk), may be a promising suicide gene therapy for pancreatic carcinomas.     

HSV-tk/GCV自杀基因疗法及其影响树突状细胞功能的实验研究

目的 研究单纯疱疹病毒－胸苷激酶／更昔洛韦（ＨＳＶ－ｔｋ／ＧＣＶ）自杀基因系统的特性,探讨转ＨＳＶ－ｔｋ基因后,肿瘤细胞被更昔洛韦（Ｇａｎｃｉｃｌｏｖｉｒ,ＧＣＶ）杀伤时的凋亡现象及对树突状细胞功能的影响。方法 以逆转录病毒法将ＨＳＶ－ｔｋ基因转人乳腺癌细胞株ＭＣＦ－７中,以Ｓｏｕｔｈｅｒｎ ｂｌｏｔ法对肿瘤细胞基因组ＤＮＡ中ｔｋ基因的整合进行鉴定,以流式细胞仪及电镜观察ＧＣＶ杀伤ＭＣＦ－７／ｔｋ细胞时的凋亡现象,由脐血ＣＤ３４＾＋细胞诱生树突状细胞,以＾３Ｈ－ＴｄＲ掺入法检测树突状细胞刺激同种异体Ｔ淋巴细胞增殖的能力。结果 以逆转录病毒法成功地将ＨＳＶ－ｔｋ基因转入ＭＣＦ－７细胞中,转染ｔｋ基因的ＭＣＦ－７细胞能被ＧＣＶ有效杀伤并存在凋亡现象,凋亡率可达３１．３％;由脐血ＣＤ３４＾＋细胞在ＧＭ－ＣＳＦ、ＴＮＦ     

Induction of Epstein-Barr Virus Kinases To Sensitize Tumor Cells to Nucleoside Analogues

The presence of Epstein-Barr virus (EBV) in the tumor cells of some EBV-associated malignancies may facilitate selective killing of these tumor cells. We show that treatment of an EBV(+) Burkitt's lymphoma cell line with 5-azacytidine led to a dose-dependent induction of EBV lytic antigen expression, including expression of the viral thymidine kinase (TK) and phosphotransferase (PT). Azacytidine treatment for 24 h modestly sensitized the cell line to all nucleosides tested. To better characterize EBV TK with regard to various nucleoside analogues, we expressed EBV TK in stable cell clones. Two EBV TK-expressing clones were moderately sensitive to high doses of acyclovir and penciclovir (PCV) (62.5 to 500 microM) and to lower doses of ganciclovir (GCV) and bromovinyldeoxyuridine (BVdU) (10 to 100 microM) compared to a control clone and were shown to phosphorylate GCV. Similar experiments in a transient overexpression system showed more killing of cells transfected with the EBV TK expression vector than of cells transfected with the control mutant vector (50 microM GCV for 4 days). A putative PT was also studied in the transient transfection system and appeared similar to the TK in phosphorylating GCV and conferring sensitivity to GCV, but not in BVdU- or PCV-mediated cell killing. Induction of EBV kinases in combination with agents such as GCV merits further evaluation as an alternative strategy to gene therapy for selective killing of EBV-infected cells.     

Graft-versus-leukemia Effect of HLA-haploidentical Central-memory T-cells Expanded With Leukemic APCs and Modified With a Suicide Gene

Allogeneic hematopoietic stem cell transplantation (HSCT) from a human leukocyte antigen (HLA)-haploidentical family donor (haplo-HSCT) is a readily available and potentially curative option for high-risk leukemia. In haplo-HSCT, alloreactivity plays a major role in the graft-versus-leukemia (GVL) effect, which, however, is frequently followed by relapse due to emerging leukemic cell variants that have lost the unshared HLA haplotype as a mechanism of immune escape. We report that stimulation of HLA-haploidentical donor T lymphocytes with leukemic antigen-presenting cells (L-APCs) expands a population of leukemia-reactive T cells, which, besides alloreactivity to unshared HLAs, contain leukemia-associated specificities restricted by shared HLAs. According to a preferential central-memory (T_CM) phenotype and to high interleukin (IL)-7Rα expression, these T cells persist in vivo and sustain a major GVL effect in a clinically relevant xenograft model. Moreover, we demonstrate that modifying L-APC–expanded T cells to express the herpes simplex virus thymidine kinase (HSV-tk) suicide gene enables their elimination with the prodrug ganciclovir (GCV), therefore providing a safety switch in case of graft-versus-host disease (GVHD). These results warrant the clinical investigation of L-APC–expanded T cells modified with a suicide gene in the setting of haplo-HSCT.     

间充质干细胞靶向转运胸苷激酶基因联合更昔洛韦杀伤鼻咽癌细胞

背景：研究证实,间充质干细胞能通过基因修饰成为肿瘤治疗的靶向载体。目的：观察转染胸苷激酶基因的胸苷激酶-间充质干细胞联合更昔洛韦对鼻咽癌细胞的靶向杀伤作用。方法：应用LipofectamineTM2000将表达胸苷激酶基因的重组质粒pGL3-EGFP-胸苷激酶转染至SD大鼠间充质干细胞,观察其增殖能力。应用Transwell小室观察胸苷激酶-间充质干细胞的归巢特性;将胸苷激酶-间充质干细胞植入裸鼠,观察其致瘤性;用胸苷激酶-间充质干细胞/更昔洛韦干预人鼻咽癌细胞5-8F,观察其对细胞的杀伤作用及旁观者效应。结果与结论：荧光显微镜观察及RT-PCR检测结果提示实验成功将胸苷激酶基因转染至间充质干细胞,CCK-8检测结果显示胸苷激酶-间充质干细胞与间充质干细胞增殖能力差异无显著性意义（P〉0.05）。Transwell小室迁移实验显示胸苷激酶-间充质干细胞具有归巢特性,裸鼠移植瘤实验显示胸苷激酶-间充质干细胞无致瘤性。CCK-8检测检测发现胸苷激酶-间充质干细胞/更昔洛韦具有旁观者效应,可明显降低5-8F的生存率（P〈0.01）。提示胸苷激酶-间充质干细胞/更昔洛韦对鼻咽癌细胞具有靶向迁移及杀伤作用,间充质干细胞可作为治疗鼻咽癌的理想靶向转运载体。     

Thymidine Kinase Suicide Gene-mediated Ganciclovir Ablation of Autologous Gene-modified Rhesus Hematopoiesis

Despite the genotoxic complications encountered in clinical gene therapy trials for primary immunodeficiency diseases targeting hematopoietic cells with integrating vectors; this strategy holds promise for the cure of several monogenic blood, metabolic and neurodegenerative diseases. In this study, we asked whether the inclusion of a suicide gene in a standard retrovirus vector would allow elimination of vector-containing stem and progenitor cells and their progeny in vivo following transplantation, using our rhesus macaque transplantation model. Following stable engraftment with autologous CD34⁺ cells transduced with a retrovirus vector encoding a highly sensitive modified Herpes simplex virus thymidine kinase SR39, the administration of the antiviral prodrug ganciclovir (GCV) was effective in completely eliminating vector-containing cells in all hematopoietic lineages in vivo. The sustained absence of vector-containing cells over time, without additional GCV administration, suggests that the ablation of TkSR39 GCV-sensitive cells occurred in the most primitive hematopoietic long-term repopulating stem or progenitor cell compartment. These results are a proof-of-concept that the inclusion of a suicide gene in integrating vectors, in addition to a therapeutic gene, can provide a mechanism for later elimination of vector-containing cells, thereby increasing the safety of gene transfer.     

Lymphoma regression induced by ganciclovir in mice bearing a herpes thymidine kinase transgene

The dose limitations imposed on cancer chemotherapeutic agents by their lack of selectivity can, in theory, be circumvented by a strategy entailing the prophylactic insertion into hosts of drug-sensitivity genes that are acquired or expressed in some but not all cells. This strategy predicts that neoplasms arising from drug-sensitive cells might be safely treatable with tumor-eradicative drug doses because the presence of a modicum of drug-insensitive stem cells will protect vital tissues from lethal depopulation. To test this prediction, lymphomas were induced with Abelson leukemia virus in mice bearing a herpes simplex virus thymidine kinase (HSV-TK) transgene selectively expressed in lymphoid cells. Of 12 transgenic mice treated with the HSV-TK-specific substrate ganciclovir (GCV), 11 exhibited complete tumor regressions; 5 of these mice remained tumor-free over observation periods that exceeded 100 days. Among the lymphomas that recurred, most appeared to represent mutant subpopulations that were GCV-insensitive because they had lost HSV-TK, implying that independent insertion of multiple HSV-TK gene copies might provide a means of preventing recurrences. The results of this study demonstrate that chemosensitivity genes can enhance the efficacy of treatment in hosts who subsequently develop a neoplasm. While the use of a germ-line gene insertion model precludes direct human application, the results also imply the merits of exploring an alternative version of the strategy in which somatic insertion of chemosensitivity genes in mosaic fashion is used prophylactically to enhance the prospect that a subsequent tumor will respond to therapy.     

Double suicide gene therapy augments the antitumor activity of a replication-competent lytic adenovirus through enhanced cytotoxicity and radiosensitization

Replication-competent adenoviruses may provide a highly efficient means of delivering therapeutic genes to tumors. Previously, we evaluated in vitro a replication-competent adenovirus (Ad5-CD/TKrep) containing a cytosine deaminase (CD)/herpes simplex type 1 thymidine kinase (HSV-1 TK) fusion gene that allows lytic viral therapy to be combined with double suicide gene therapy. Both the CD/5-FC and HSV-1 TK/GCV enzyme/prodrug systems enhanced the tumor cell-specific cytopathic effects of the Ad5-CD/TKrep virus in vitro and sensitized cells to radiation. To extend these in vitro findings in vivo, we evaluated the antitumor activity of the Ad5-CD/TKrep virus in combination with double prodrug therapy and radiation therapy. The Ad5-CD/TKrep virus independently demonstrated significant antitumor activity against C33A cervical carcinoma xenografts. Therapeutic outcome was dramatically improved with systemic administration of double, but not single, prodrug (5-FC + GCV) therapy. When used in a neoadjuvant setting, Ad5-CD/TKrep-mediated double suicide gene therapy dramatically potentiated the effectiveness of radiation therapy. The trimodal approach of Ad5-CD/TKrep viral, double suicide gene, and radiotherapies produced significant tumor regression and ultimately 100% tumor cure. The results demonstrate the high therapeutic potential of the trimodal approach and provide a solid foundation for future clinical trials.     

Analysis of phosphorylation pathways of antiherpesvirus nucleosides by varicella-zoster virus-specific enzymes.

The inhibitory activities of acyclovir (ACV), 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil (BV-araU), ganciclovir (GCV), 9-(2-deoxy-2-hydroxymethyl-beta-D-erythro-oxetanosyl)guanine (OXT-G), and (+)-9-[(1R,2R,3S)-2,3-bis(hydroxymethyl)Cyclobutyl]guanine (cOXT-G) on the replication of wild-type and thymidine kinase (TK)-negative strains of herpes simplex virus types 1 and 2 and varicella-zoster virus (VZV) and the wild-type strain of human cytomegalovirus were tested to clarity whether the phosphorylation of these compounds is catalyzed by viral TK or other enzymes. ACV and BV-araU had little effect on the replication of TK-negative virus strains. On the other hand, GCV, OXT-G, and cOXT-G inhibited the replication of TK-negative VZV at concentrations 10 times higher than those at which they inhibited wild-type VZV, indicating that a kinase other than TK phosphorylates GCV and OXT-G in VZV-infected cells. GCV phosphorylation activity was not detected in VZV-infected cell lysates; therefore, this activity was evaluated in COS 1 cells expressing viral TK and viral protein kinase (PK). The COS 1 cells expressing VZV TK were shown to be susceptible to all compounds tested. In contrast, VZV Pk-expressing COS 1 cells were susceptible to only GCV, OXT-G, and cOXT-G. These results suggest that VZV PK phosphorylates some nucleoside analogs, for example, GCV, OXT-G, and cOXT-G. This phosphorylation pathway may be important in the anti-VZV activities of some nucleoside analogs.     

VSV-G pseudotyped, MuLV-based, semi-replication-competent retrovirus for cancer treatment

Low levels of gene delivery in vivo using replication-defective retroviral vectors have severely limited their application for clinical protocols. To overcome this problem, we describe here a semi-replication-competent retrovirus (s-RCR) in which the gag-pol and envelope (VSV-G, vesicular stomatitis virus G protein) genes were split into two vectors. This system offers potential advantages over both replication-defective vectors, in terms of efficiency of in vivo spread through a tumor, and all-in-one replication-competent vectors in terms of the payload of therapeutic genes that can be carried. We achieved a viral titer of s-RCR viruses approximately 70-fold higher than VSV-G pseudotyped, replication-defective vectors. In addition, s-RCR vectors induced tumor killing by the cytotoxicity of VSV-G during viral spread. Inclusion of the herpes simplex virus thymidine kinase (HSVtk30) gene into vectors significantly improved tumor killing activity followed by ganciclovir (GCV) treatment in vitro under conditions of low-level viral replication. However, at high levels of viral spread, VSV-G-mediated cytotoxicity predominated. Xenografts of human fibrosarcoma HT1080 cells, preinfected by semi-replicative green fluorescent protein vectors (semi-GFP), were completely non-tumorigenic in nude mice. Implantation of cells preinfected by semi-replicative TK30 vectors (semi-TK30) mixed with parental HT1080 cells at a ratio of 1:1 efficiently prevented tumor growth in mice treated by GCV. Direct intratumoral injection of HT1080 tumors growing in nude mice, or B16 murine melanoma in immunocompetent mice, with semi-TK30 viruses significantly prolonged survival. Injection of autologous cells (B16) producing semi-TK30 vector into B16 tumors prolonged survival only in mice treated with GCV but not with phosphate-buffered saline (PBS). In contrast, when xenogeneic cells (293T) producing semi-TK30 vectors were injected into B16 tumors, an optimal survival advantage was obtained in mice treated with PBS rather than GCV. These data indicate that complex interactions exist between direct cytotoxicity of VSV-G and HSVtk expression when placed in the context of additional immune parameters, which combine to determine the efficacy of the therapy. Taken together, our data suggest that s-RCR vectors have some potential advantages for development to deliver genes into tumors for cancer treatment but that a combination of factors will impact on the decision as to whether the s-RCR strategy is worth developing to full clinical trials.     

Enhanced ganciclovir killing and bystander effect of human tumor cells transduced with a retroviral vector carrying a herpes simplex virus thymidine kinase gene mutant

Gene transfer of the herpes simplex virus thymidine kinase (TK) gene associated with ganciclovir (GCV) treatment can lead to death of TK-expressing cells, and of neighboring TK- cells because of the bystander effect. Thus, a small proportion of TK+ cells in a tumor can lead to its complete regression after GCV treatment. However, a lack of efficacy of gene transfer into tumors associated with low GCV sensitivity and poor bystander effect of human cancer cells currently limit the clinical use of this suicide gene therapy approach. To increase the potency of suicide gene therapy, we have tested the GCV sensitivity and the bystander effect of TK mutants that have been previously described. After retroviral transduction of the TK mutants into human tumor cells of various origins, we have found a strong killing effect of GCV with cells expressing the mutants TK30 or TKF161C. The GCV sensitivity of several human tumor cell types expressing TK30 was 9- to 500-fold higher than cells containing wild-type TK. Furthermore, TK30-expressing cells were able to kill bystander cells much more efficiently than TK-expressing cells. Thus, TK30 mutant is a promising candidate for suicide gene therapy clinical trials.