Retrovirus—mediated herpes simplex virus thymidine kinase gene therapy approach for hepatocellular carcinoma

INTRODUCTI0NHepatocellularcarcinoma(HCC)is0ne0fthem0stc0mm0nhumanmalignancies,causinganestimatedl,250,OOOdeatht0llperyearworldwide[1].Thep0orprognosisencounteredintreatment0fsuchcarcinomaismainlycausedbylatediagn0sisandinsufficiency0feffectivestrategies,especiallyforadvanced-stagedpatients.However,recentknowledge0fpathogenesisofHCCatm0lecularlevelprovidesanalternativeappr0achwhenc0nsideringgenetherapyastreatmelltf0rHCC.Am0ngthevari0usgenetherapystrategiesincancer)itwaJsrep0rtedthatth…     

健脾化瘀中药提高胞嘧啶脱氨酶/单纯疱疹病毒胸苷激酶基因治疗肝细胞癌的实验研究

目的:观察健脾化瘀中药提高胞嘧啶脱氨酶/单纯疱疹病毒胸苷激酶基因治疗肝细胞癌的作用。方法:脂质体lipofectamine将含有双自杀基因的腺病毒载体pAd-CD/TK导人293细胞,收集病毒上清转染人肝癌细胞BEL7402,MTT法测定BEL7402细胞存活率。裸鼠人肝癌模型转染CD/TK双自杀基因后,给予5-FC500mg/kg,GCV 100mg/kg腹腔注射,同时予健脾化瘀中药960复方灌胃。观察肿瘤生长情况。结果:给予前体药物5-FC和GCV后,CD/TK转染细胞被杀死。并表现出较强的旁观者效应。转染细胞比例达到10%即表现出较强的杀伤作用(P<0.01)。健脾化瘀中药960复方具有提高旁观者效应作用,1.67ml/kg和2.5ml/kg960复方含药血清组细胞存活率显著低于对照组(P<0.01)。转染基因组应用5-FC和GCV治疗后,裸鼠肝癌的生长明显受到抑制(P<0.05),抑瘤率39.42%,单用中药组抑瘤率18.04%,中药与CD/TK 5-FC/GCV联合运用组,较单纯CD/5-FC/HSV-tk/GCV对裸鼠肿瘤模型的生长抑制作用更加明显(P<0.05),抑瘤率55.10%。结伦:腺病毒介导CD/TK自杀基因可有效地杀死人肝癌BEL7402细胞,健脾化瘀中药960复方具有显著提高CD/TK双自杀基因对人肝癌细胞的抑杀作用。     

带HSV—tk基因的重组腺病毒基因治疗小鼠B16黑色素瘤的实验研究

我父成功开展了携带单纯疹疹Ⅰ型病毒胸腺嘧啶激酶基因（Ｈｅｒｐｅｓｓｉｍｐｌｅｘｖｉｒｕｓｔｈｙ－ｍｎｉｄｉｎｅｋｉｎａｓｅ,ＨＳＶ－ｔｋ）的复制缺陷型重组腺病毒Ａｄ（ＨＳＶ－ｔｋ）结合使用ＧＣＶ治疗Ｃ５７ＢＬ／６小鼠Ｂ１６黑色素瘤的离体及动物试验。     

Co-expression of bfl-1 enhances host response in the herpes simplex virus-thymidine kinase/ganciclovir gene therapy system

Anticancer suicide gene therapy using herpes simplex virus-thymidine kinase (HSV-tk) and ganciclovir (GCV) features the unique advantage of being able to elicit brisk host immune response against tumors and the host response reportedly can be potentiated with the co-expression of other appropriate immune- or apoptosis-related genes. We introduced a novel antiapoptotic gene, bfl-1, to test its applicability in the HSV-tk/GCV system. CT-26 murine colon cancer cells transfected with HSV-tk, alone or in combination with bcl-xL or bfl-1, were either grown in vitro or injected into syngeneic mice, followed by GCV administration. The co-expression of bfl-1 was associated with the upregulation of CD95 and CD40 ligand (CD40L) in vitro and with pronounced intratumoral T-lymphocyte infiltration in vivo. These results add to the previous findings that antiapoptotic genes can be used as an adjunctive component in the HSV-tk/GCV system to enhance host immune response against tumors.     

Tanshinone IIA Increases the Bystander Effect of Herpes Simplex Virus Thymidine Kinase/Ganciclovir Gene Therapy via Enhanced Gap Junctional Intercellular Communication

The bystander effect is an intriguing phenomenon by which adjacent cells become sensitized to drug treatment during gene therapy with herpes simplex virus thymidine kinase/ganciclovir (HSV-tk/GCV). This effect is reported to be mediated by gap junctional intercellular communication (GJIC), and therefore, we postulated that upregulation of genes that facilitate GJIC may enhance the HSV-tk/GCV bystander effect. Previous findings have shown Tanshinone IIA (Tan IIA), a chemical substance derived from a Chinese medicine herb, promotes the upregulation of the connexins Cx26 and Cx43 in B16 cells. Because gap junctions are formed by connexins, we hypothesized that Tan IIA might increase GJIC. Our results show that Tan IIA increased GJIC in B16 melanoma cells, leading to more efficient GCV-induced bystander killing in cells stably expressing HSV-tk. Additionally, in vivo experiments demonstrated that tumors in mice with 10% HSV-tk positive B16 cells and 90% wild-type B16 cells became smaller following treatment with the combination of GCV and Tan IIA as compared to GCV or Tan IIA alone. These data demonstrate that Tan IIA can augment the bystander effect of HSV-tk/GCV system through increased gap junction coupling, which adds strength to the promising strategy that develops connexins inducer to potentiate the effects of suicide gene therapy.     

Transfection of oral cancer cells mediated by transferrin-associated lipoplexes: mechanisms of cell death induced by herpes simplex virus thymidine kinase/ganciclovir therapy

The Herpes Simplex Virus thymidine kinase (HSV-tk) suicide gene/ganciclovir (GCV) approach has been used for the treatment of a variety of cancers. The purpose of the present study was to evaluate the cytotoxic effect of ganciclovir in oral squamous cancer cells, previously transfected with HSV-tk gene delivered by transferrin-associated complexes (Tf-lipoplexes), as well as to investigate the mechanisms involved in the bystander effect and in the process of cell death. The delivery of HSV-tk gene to the oral cancer cells, HSC-3 and SCC-7, mediated by Tf-lipoplexes followed by ganciclovir treatment resulted in essentially 100% cytotoxicity, the observed toxic effect being dependent both on GCV dose and incubation time. Cell death was shown to occur mainly by an apoptotic process. Different experimental approaches demonstrated that the observed cytotoxicity was mainly due to diffusion of the toxic agent into neighbouring, non-transfected cells, via gap junctions. Preliminary in vivo studies in a murine model for oral squamous cell carcinoma have shown a significant inhibition of tumor growth upon injection of Tf-lipoplexes carrying HSV-tk followed by intraperitonal injection of GCV, as compared to controls.     

Transfection of oral cancer cells mediated by transferrin-associated lipoplexes: Mechanisms of cell death induced by herpes simplex virus thymidine kinase/ganciclovir therapy

The Herpes Simplex Virus thymidine kinase (HSV-tk) suicide gene/ganciclovir (GCV) approach has been used for the treatment of a variety of cancers. The purpose of the present study was to evaluate the cytotoxic effect of ganciclovir in oral squamous cancer cells, previously transfected with HSV-tk gene delivered by transferrin-associated complexes (Tf-lipoplexes), as well as to investigate the mechanisms involved in the bystander effect and in the process of cell death. The delivery of HSV-tk gene to the oral cancer cells, HSC-3 and SCC-7, mediated by Tf-lipoplexes followed by ganciclovir treatment resulted in essentially 100% cytotoxicity, the observed toxic effect being dependent both on GCV dose and incubation time. Cell death was shown to occur mainly by an apoptotic process. Different experimental approaches demonstrated that the observed cytotoxicity was mainly due to diffusion of the toxic agent into neighbouring, non-transfected cells, via gap junctions. Preliminary in vivo studies in a murine model for oral squamous cell carcinoma have shown a significant inhibition of tumor growth upon injection of Tf-lipoplexes carrying HSV-tk followed by intraperitonal injection of GCV, as compared to controls.     

Prolonged, NK cell-mediated antitumor effects of suicide gene therapy combined with monocyte chemoattractant protein-1 against hepatocellular carcinoma

Tumor recurrence rates remain high after curative treatments for hepatocellular carcinoma (HCC). Immunomodulatory agents, including chemokines, are believed to enhance the antitumor effects of tumor cell apoptosis induced by suicide gene therapy. We therefore evaluated the immunomodulatory effects of a bicistronic recombinant adenovirus vector (rAd) expressing both HSV thymidine kinase and MCP-1 on HCC cells. Using an athymic nude mouse model (BALB/c-nu/nu), primary s.c. tumors (HuH7; human HCC cells) were completely eradicated by rAd followed by treatment with ganciclovir. The same animals were subsequently rechallenged with HCC cells, tumor development was monitored, and the recruitment or activation of NK cells was analyzed immunohistochemically or by measuring IFN-gamma mRNA expression. Tumor growth was markedly suppressed as compared with that in mice treated with a rAd expressing the HSV thymidine kinase gene alone (p < 0.001). Suppression of tumor growth was associated with the elevation of serum IL-12 and IL-18. During suppression, NK cells were recruited exclusively, and Th1 cytokine gene expression was enhanced in tumor tissues. The antitumor activity, however, was abolished either when the NK cells were inactivated with anti-asialo GM1 Ab or when anti-IL-12 and anti-IL-18 Abs were administered. These results indicate that suicide gene therapy, together with delivery of MCP-1, eradicates HCC cells and exerts prolonged NK cell-mediated antitumor effects in a model of HCC, suggesting a plausible strategy to prevent tumor recurrence.     

High volume hydrodynamic injection of plasmid DNA via the hepatic artery results in a high level of gene expression in rat hepatocellular carcinoma induced by diethylnitrosamine

BACKGROUND: Hydrodynamic injection of naked plasmid DNA (pDNA) via the tail vein is a safe and effective method of gene transfer to the liver. However, successful gene transfer has yet to be shown for hepatocellular carcinoma (HCC); therefore, we investigated the feasibility and efficacy of hydrodynamic injection via the tail vein and hepatic artery in a diethylnitrosamine (DEN)-induced HCC model in rats. METHODS: HCC was induced in Sprague-Dawley rats by 100 ppm DEN in drinking water. pCMV-SPORT-beta-galactosidase (beta-gal, 400 microg) was injected (i) via the tail vein in a volume of 0.1 ml/g in 30 s or (ii) via the hepatic artery in a volume of 5 or 10 ml at 1 ml/s, either with or without temporary occlusion of the inferior vena cava (IVC) and portal vein (PV). The liver was harvested 24 h after administration, and beta-gal expression was evaluated with X-gal staining and measurement of enzymatic activity in tissue homogenates. RESULTS: Hydrodynamic injection via the tail vein achieved transgene expression only in non-cancerous tissue (tumor: 0.16 +/- 0.04%, non-tumor: 5.07 +/- 1.66%). Hydrodynamic injection via the hepatic artery was tolerated, but failed to produce efficient transgene expression in tumor and non-tumor cells. On the other hand, concomitant use of temporary IVC/PV occlusion with hydrodynamic injection via the hepatic artery dramatically increased transgene expression in cancer cells, but tumor-selective gene transfer was not achieved with this procedure (tumor: 7.38 +/- 3.66%, non-tumor: 7.77 +/- 1.06%). CONCLUSIONS: High-volume hydrodynamic injection of a pDNA solution via the hepatic artery with IVC/PV occlusion achieved a high level of gene expression in a HCC rat model. This gene transfer technique may have potential in clinical gene therapy for HCC.     

Polyethylenimine-mediated suicide gene transfer induces a therapeutic effect for hepatocellular carcinoma in vivo by using an Epstein-Barr virus-based plasmid vector

The present study aimed to establish a novel efficient nonviral strategy for suicide gene transfer in hepatocellular carcinoma (HCC) in vivo. We employed branched polyethylenimine (PEI) and combined it with Epstein-Barr virus (EBV)-based plasmid vectors. The HCC cells transfected with an EBV-based plasmid carrying the herpes simplex virus-1 thymidine kinase (HSV-1 Tk) gene (pSES.Tk) showed up to 30-fold higher susceptibilities to ganciclovir (GCV) than those transfected with a conventional plasmid vector carrying the HSV-1 Tk gene (pS.Tk). The therapeutic effect in vivo was tested by intratumoral injection of the plasmids into HuH-7 hepatomas transplanted into C.B-17 scid/scid mutant (SCID) mice and subsequent GCV administrations. Treatment with pSES.Tk, but not pS.Tk, markedly suppressed growth of hepatomas in vivo, resulting in a significantly prolonged survival period of the mice. These findings suggest that PEI-mediated gene transfer system can confer efficient expression of the suicide gene in HCC cells in vivo by using EBV-based plasmid vectors.     

HSV-tk/GCV自杀基因系统与重组TNF联合杀伤肿瘤细胞的光学成像检测

目的:应用光学成像方法检测HSV-tk/GCV自杀基因系统与重组改造的人肿瘤坏死因子(recombinanthuman tumor necrosis factor,rmhTNF)的联合使用,对人涎腺腺样囊性癌细胞(adenoid cystic carcinoma,ACC-M)的杀伤效率。方法:实验分为四组,细胞模型对照组、单纯HSV-tk/GCV自杀基因系统治疗组、rmhTNF处理组、自杀基因联合rmhTNF治疗组。将稳定表达胸苷激酶-绿色荧光蛋白(thymidine kinase-green fluorescen protenin,tk-GFP)的ACC-M细胞(ACC-M-tk-GFP)按5 000个/孔接种到96孔板中,24 h后给药治疗;分别于加药前、给药后6 h、给药后24 h进行荧光成像。检测绿色荧光蛋白(green fluorescent protein,GFP)的荧光用于准确显示胸苷激酶(tk)基因表达的肿瘤细胞,检测碘化丙啶(propidium iodide,PI)的荧光用于显示死亡的肿瘤细胞。通过细胞的透射图及其PI和GFP荧光图像,分别计算tk 细胞和tk-细胞的死亡率。结果:HSV-tk/GCV自杀基因系统与rmhTNF联合治疗组的细胞在加药后6 h就已经出现细胞死亡迹象,而单纯HSV-tk/GCV自杀基因系统治疗组的细胞在加更昔洛韦(ganciclovir,GCV)后24 h才出现细胞死亡,单纯rmhTNF治疗组在加药后24 h还没有细胞死亡发生。对于给药后24 h的tk 细胞死亡率,联合治疗组(85.88%)明显高于GCV(38.13%)和rmhTNF单独治疗组(2.97%),并且联合治疗组给药24 h后会引发部分tk-细胞死亡(9.83%)。结论:HSV-tk/GCV自杀基因系统与rmhTNF联合治疗具有显著的协同抗肿瘤效应,能够明显提高对肿瘤细胞的杀伤率;结合光学成像方法,能为更直观方便地检测各组药物分别对tk 细胞和tk-细胞的杀伤率,可为深入研究"旁观者效应"提供参考。     

小鼠ALB启动子/增强子驱动HSV-tk 对肝脏细胞的杀伤效应

利用小鼠白蛋白(ALB)启动子／增强子及单纯疱疹病毒胸苷嘧啶激酶(HSV-tk)DNA构建了载体pLLTK,以研究该载体对肝脏细胞的特异性杀伤效应。首先,为了比较载体的肝脏细胞特异转录活性,以绿色荧光蛋白(GFP)基因为报告基因构建了载体pLE(仅含小鼠ALB启动子)、pLLE(含小鼠ALB启动子和上游增强子)和pLEL(含小鼠ALB启动子和下游增强子),分别转染到人肝细胞株Hep—G2与小鼠乳腺上皮细胞株HC-11,荧光显微镜与流式细胞术分析GFP的表达。然后将载体pLLTK转染到Hep-G2研究对细胞的杀伤效应。结果发现：小鼠ALB启动子／增强子能驱动GFP肝脏特异表达;HSV-tk在Hep-G2表达使细胞具有更昔洛韦(GCV)敏感性,在GCV作用7d后,MTT分析细胞的生存率,pLLTK转染细胞表现明显的细胞死亡(53％),而阴性对照组pcDNA3．1转染细胞没有明显变化(仅2％细胞死亡)。以上结果表明所有的载体具有肝脏细胞特异性,为利用该载体产生肝脏损伤的转基因小鼠提供了细胞水平的实验依据。     

Gap junction-mediated bystander effect in primary cultures of human malignant gliomas with recombinant expression of the HSVtk gene

The ability of herpes simplex virus type 1 thymidine kinase (HSV-tk)-expressing cells incubated with ganciclovir (GCV) to induce cytotoxicity in neighboring HSV-tk-negative (bystander) cells has been well documented. Although it has been suggested that this bystander cell killing occurs via the transfer of phosphorylated GCV, the mechanism(s) of this bystander effect and the importance of gap junctions for the effect of prodrug/suicide gene therapy in primary human glioblastoma cells remains elusive. Surgical biopsies of malignant gliomas were used to establish explant primary cultures. Proliferating tumor cells were characterized immunohistochemically and found to express glial tumor markers including nestin, vimentin, glial fibrillary acidic protein (GFAP), S-100, and gap junction protein connexin 43 (Cx43). Western blot analysis revealed the presence of phosphorylated isoforms of Cx43 and Calcein/DiI fluorescent dye transfer showed evidence of efficient gap junction communication (GJC). In order to study the effect(s) of prodrug/suicide gene therapy in these cultures, human glioblastoma cell cultures were transfected with the HSVtk gene for transient or stable expression. Ganciclovir treatment of these cultures led to >90% of cells dead within 1 week. Eradication of cells could be inhibited by the addition of alpha-glycyrrhetinic acid (AGA), a GJC inhibitor. In parallel experiments, AGA decreased the immunodetection of phosphorylated Cx43 as analyzed by Western blot and inhibited fluorescent dye transfer. In conclusion, these observations are consistent with GJC as the mediator of the bystander effect in primary cultures of human glioblastoma cells by the transfer of phosphorylated GCV from HSVtk gene transfected cells to untransfected ones.     

FTY720 suppresses liver tumor metastasis by reducing the population of circulating endothelial progenitor cells

Background

Surgical procedures such as liver resection and liver transplantation are the first-line treatments for hepatocellular carcinoma (HCC) patients. However, the high incidence of tumor recurrence and metastasis after liver surgery remains a major problem. Recent studies have shown that hepatic ischemia-reperfusion (I/R) injury and endothelial progenitor cells (EPCs) contribute to tumor growth and metastasis. We aim to investigate the mechanism of FTY720, which was originally applied as an immunomodulator, on suppression of liver tumor metastasis after liver resection and partial hepatic I/R injury.

Methodology/Principal Findings

An orthotopic liver tumor model in Buffalo rat was established using the hepatocellular carcinoma cell line McA-RH7777. Two weeks after orthotopic liver tumor implantation, the rats underwent liver resection for tumor-bearing lobe and partial hepatic I/R injury. FTY720 (2 mg/kg) was administered through the inferior caval vein before and after I/R injury. Blood samples were taken at days 0, 1, 3, 7, 14, 21 and 28 for detection of circulating EPCs (CD133+CD34+). Our results showed that intrahepatic and lung metastases were significantly inhibited together with less tumor angiogenesis by FTY720 treatment. The number of circulating EPCs was also significantly decreased by FTY720 treatment from day 7 to day 28. Hepatic gene expressions of CXCL10, VEGF, CXCR3, CXCR4 induced by hepatic I/R injury were down-regulated in the treatment group.

Conclusions/Significance

FTY720 suppressed liver tumor metastasis after liver resection marred by hepatic I/R injury in a rat liver tumor model by attenuating hepatic I/R injury and reducing circulating EPCs.     

A dual immune signature of CD8+ T cells and MMP9 improves the survival of patients with hepatocellular carcinoma

The 5-year survival of hepatocellular carcinoma (HCC) is difficult due to the high recurrence rate and metastasis. Tumor infiltrating immune cells (TICs) and immune-related genes (IRGs) bring hope to improve survival and treatment of HCC patients. However, there are problems in predicting immune signatures and identifying novel therapeutic targets. In the study, the CIBERSORT algorithm was used to evaluate 22 immune cell infiltration patterns in gene expression omnibus (GEO) and the cancer genome atlas (TCGA) data. Eight immune cells were found to have significant infiltration differences between the tumor and normal groups. The CD8+ T cells immune signature was constructed by least absolute shrinkage and selection operator (LASSO) algorithm. The high infiltration level of CD8+ T cells could significantly improve survival of patients. The weighted gene co-expression network analysis (WGCNA) algorithm identified MMP9 was closely related to the overall survival of HCC patients. K-M survival and tROC analysis confirmed that MMP9 had an excellent prognostic prediction. Cox regression showed that a dual immune signature of CD8+ T cells and MMP9 was independent survival factor in HCC. Therefore, a dual prognostic immune signature could improve the survival of patient and may provide a new strategy for the immunotherapy of HCC.     

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.     

The anti-tumor effect of human monocyte-derived dendritic cells loaded with HSV-TK/GCV induced dying cells

Herpes simplex virus thymidine kinase (HSV-TK) gene and dendritic cells (DC) have been used as the pioneering in cancer therapy. HSV-TK gene can induce apoptosis and necrosis in tumor cells in the presence of the non-toxic prodrug ganciclovir (GCV). We investigated the anti-tumor effect of DC vaccination by introducing dying cells from HSV-TK gene treatment as an adjuvant. HepG₂-TK cell line was established by transfecting human hepatoma cell line HepG₂ (HLA-A₂ positive) with HSV-TK gene. Dying tumor cells were generated by culturing HepG₂-TK cells with GCV. After engulfed dying cells efficiently, immature DCs (imDC) derived from human monocytes were fully matured and elicited marked proliferation and cytotoxicity against HLA matched HepG₂ cells in autologous peripheral blood mononuclear cells (PBMC). It also implied that HepG₂ specific CTLs played an important role in the cytotoxicity which was primarily depended on Th1 responses. Given the feasibility of inducing dying cells by HSV-TK/GCV in vivo, our results suggest an effective method in clinical human hepatocellular carcinoma (HCC) treatment by an in vitro model of applying HSV-TK gene modified human tumor cells integrated with DC vaccination.     

The role of a HSV thymidine kinase stimulating substance, scopadulciol, in improving the efficacy of cancer gene therapy

BACKGROUND: The most extensively investigated strategy of suicide gene therapy for treatment of cancer is the transfer of the herpes simplex virus thymidine kinase (HSV-TK) gene followed by administration of antiviral prodrugs such as acyclovir (ACV) and ganciclovir (GCV). The choice of the agent that can stimulate HSV-TK enzymatic activity is one of the determinants of the usefulness of this strategy. Previously, we found that a diterpenoid, scopadulciol (SDC), produced a significant increase in the active metabolite of ACV. This suggests that SDC may play a role in the HSV-TK/prodrug administration system. METHODS: The anticancer effect of SDC was evaluated in HSV-TK-expressing (TK+) cancer cells and nude mice bearing TK+ tumors. In vitro and in vivo enzyme assays were performed using TK+ cells and tumors. The phosphorylation of ACV monophosphate (ACV-MP) was measured in TK- cell lysates. The pharmacokinetics of prodrugs was evaluated by calculating area-under-the-concentration-time-curve values. RESULTS: SDC stimulated HSV-TK activity in TK+ cells and tumors, and increased GCV-TP levels, while no effect of SDC was observed on the phosphorylation of ACV-MP to ACV-TP by cellular kinases. The SDC/prodrug combination altered the pharmacokinetics of the prodrugs. In accord with these findings, SDC enhanced significantly the cell-killing activity of prodrugs. The bystander effect was also significantly augmented by the combined treatment of ACV/GCV and SDC. CONCLUSIONS: SDC was shown to be effective in the HSV-TK/prodrug administration system and improved the efficiency of the bystander effect of ACV and GCV. The findings will be considerably valuable with respect to the use of GCV in lower doses and less toxic ACV. This novel strategy of drug combination could provide benefit to HSV-TK/prodrug gene therapy.