Establishment of parameters for optimal transduction efficiency and antitumor effects with purified high-titer HSV-TK retroviral vector in established solid tumors

Tumor-specific suicide gene therapy for hepatocellular carcinoma by transcriptionally targeted retroviral replicating vectors

Replicating virus vectors are attractive tools for anticancer gene therapy, but the potential for adverse events due to uncontrolled spread of the vectors has been a major concern. To design a tumor-specific retroviral replicating vector (RRV), we replaced the U3 region of the RRV ACE-GFP with a regulatory sequence consisting of the hepatitis B virus enhancer II (EII) and human α-fetoprotein (AFP) core promoter to produce ACE-GFP-EIIAFP, a hepatocellular carcinoma (HCC)-targeting RRV. Similar to ACE-GFP, ACE-GFP-EIIAFP exhibited robust green fluorescent protein (GFP) expression in HCC cells and, most importantly, it exhibited HCC-specific replication and did not replicate in non-HCC tumor cells or normal liver cells. We sequenced the promoter region of ACE-GFP-EIIAFP collected from serial infection cycles to examine the genomic stability of the vector during its replicative spread, and found that the vector could retain the hybrid promoter in the genome for at least six infection cycles. In vitro studies revealed that ACE-CD-EIIAFP and ACE-PNP-EIIAFP, which express the yeast cytosine deaminase and Escherichia coli purine nucleoside phosphorylase, respectively, exert a highly potent cytotoxic effect on HCC cells in the presence of their respective prodrugs. In vivo, ACE-CD-EIIAFP-mediated suicide gene therapy efficiently suppressed HCC tumor growth and no detectable RRV signal was observed in extratumoral tissues. These results suggest that the tumor-specific, suicide-gene-encoding RRV may fulfill the promise of retroviral gene therapy for cancer.

Improved retroviral suicide gene transfer in colon cancer cell lines after cell synchronization with methotrexate

Background

Cancer gene therapy by retroviral vectors is mainly limited by the level of transduction. Retroviral gene transfer requires target cell division. Cell synchronization, obtained by drugs inducing a reversible inhibition of DNA synthesis, could therefore be proposed to precondition target cells to retroviral gene transfer. We tested whether drug-mediated cell synchronization could enhance the transfer efficiency of a retroviral-mediated gene encoding herpes simplex virus thymidine kinase (HSV-tk) in two colon cancer cell lines, DHDK12 and HT29.

Methods

Synchronization was induced by methotrexate (MTX), aracytin (ara-C) or aphidicolin. Gene transfer efficiency was assessed by the level of HSV-TK expression. Transduced cells were driven by ganciclovir (GCV) towards apoptosis that was assessed using annexin V labeling by quantitative flow cytometry.

Results

DHDK12 and HT29 cells were synchronized in S phase with MTX but not ara-C or aphidicolin. In synchronized DHDK12 and HT29 cells, the HSV-TK transduction rates were 2 and 1.5-fold higher than those obtained in control cells, respectively. Furthermore, the rate of apoptosis was increased two-fold in MTX-treated DHDK12 cells after treatment with GCV.

Conclusions

Our findings indicate that MTX-mediated synchronization of target cells allowed a significant improvement of retroviral HSV-tk gene transfer, resulting in an increased cell apoptosis in response to GCV. Pharmacological control of cell cycle may thus be a useful strategy to optimize the efficiency of retroviral-mediated cancer gene therapy.

Liver metastasis of cancer facilitated by chemokine receptor CCR6

When injected subcutaneously, mouse plasmacytoma (MOPC315) grew rapidly in situ, and metastatic cells became detectable first in the lymph nodes (LNs) and bone marrow, and later in the liver and lungs. We studied MOPC315 cell migration by tracking metastatic cells labelled with green fluorescent protein (GFP). We measured the levels of their chemokine receptor mRNA (by semiquantitative and real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), because chemokines can regulate organ predilection of metastasis. Freshly sorted metastatic cells and tumour cell lines derived from the liver of BALB/c mice overexpressed functional CCR6 and CCR7 molecules compared with primary tumour. Preincubation with the CCR6 ligand (CCL20) induced liver-sorted tumour cells to preferentially colonize the liver, demonstrating an association between liver metastasis and CCR6 expression in the mouse. Because the liver is a common site for metastasis, second only to draining LNs, we wished to ascertain whether this finding could be generalized, i.e. whether other cancers can use the similar mechanism of metastasis to the liver, and whether it holds true for humans. We found that CCR6 is overexpressed in small liver metastases of colon, thyroid and ovarian carcinomas compared with normal liver. Because human liver constitutively expresses CCL20, it could attract and select CCR6+ cancer cells. We suggest that chemotaxis via CCR6 might be a common mechanism by which malignant cancers metastasize to the liver. As metastasis in patients with cancer poses the biggest peril for survival, inhibition of CCR6 signalling, either during or after medical or surgical treatment, might be useful in preventing liver metastasis.

Highly efficient and tumor-restricted gene transfer to malignant gliomas by replication-competent retroviral vectors

The first large randomized phase III trial in gene therapy demonstrated no improvement in the survival of patients injected with packaging cells that produced conventional replication-defective retroviral vectors carrying the herpes simplex virus thymidine kinase gene, a disappointing result that was attributed to extremely poor levels of transduction efficiency. To circumvent this problem, we have developed a modified replication-competent retrovirus (RCR) that is capable of transducing human glioma cell lines A-172, U-87, T-98G, U-373, and U-138 and rat glioma cell lines C6 and 9L, over multiple infection cycles in vitro, resulting in a tremendous enhancement in transduction efficiency over conventional replication-defective retroviral vectors at the same dose. Whereas the transduction efficiency of conventional retroviral vectors injected into preestablished subcutaneous U-87 tumors at a dose of 1.0 x 10(5) transducing units (TU) was only 0.2% at 6 weeks postinjection, the same dose of RCR vector resulted in up to 97.2% transduction. When RCR vectors at a dose of 1.0 x 10(4) TU were injected into preestablished intracranial U-87 tumors, transduction efficiency at 2 and 3 weeks was 74 and 98.1%, respectively. Notably, however, intracranial injection of RCR vectors did not result in detectable infection of normal brain cells. Furthermore, using a sensitive polymerase chain reaction assay, no detectable RCR signal could be observed in any extracerebral tissues, including lung, liver, kidney, upper gastrointestinal tract (esophagus and stomach), lower gastrointestinal tract (colon and small intestine), skin, spleen, and bone marrow. Treatment of U-87 intracranial gliomas with RCR vectors carrying the yeast cytosine deaminase suicide gene followed by 5-fluorocytosine prodrug administration resulted in 100% survival over a 60-day follow-up period, compared with 0% survival of control groups receiving vector alone or prodrug alone. Our results demonstrate that RCR vectors can achieve therapeutically significant levels of transduction in malignant human gliomas, and that RCR vector spread after intratumoral injection is restricted to the tumor itself.

Tissue-specific transcriptional targeting of a replication-competent retroviral vector

The inability of replication-defective viral vectors to efficiently transduce tumor cells in vivo has prevented the successful application of such vectors in gene therapy of cancer. To address the need for more efficient gene delivery systems, we have developed replication-competent retroviral (RCR) vectors based on murine leukemia virus (MLV). We have previously shown that such vectors are capable of transducing solid tumors in vivo with very high efficiency. While the natural requirement of MLV infection for cell division imparts a certain degree of specificity for tumor cells, additional means for confining RCR vector replication to tumor cells are desirable. Here, we investigated the parameters critical for successful tissue-specific transcriptional control of RCR vector replication by replacing various lengths of the MLV enhancer/promoter with sequences derived either from the highly prostate-specific probasin (PB) promoter or from a more potent synthetic variant of the PB promoter. We assessed the transcriptional specificity of the resulting hybrid long terminal repeats (LTRs) and the cell type specificity and efficiency of replication of vectors containing these LTRs. Incorporation of PB promoter sequences effectively restricted transcription from the LTR to prostate-derived cells and imparted prostate-specific RCR vector replication but required the stronger synthetic promoter and retention of native MLV sequences in the vicinity of the TATA box for optimal replicative efficiency and specificity. Our results have thus identified promoter strength and positioning within the LTR as important determinants for achieving both high transduction efficiency and strict cell type specificity in transcriptionally targeted RCR vectors.

Transcription-targeted gene therapy for androgen-independent prostate cancer

The Escherichia coli enzyme (purine nucleoside phosphorylase, PNP) gene is delivered directly into PC3 tumors by one injection of replication-deficient human type-5 adenovirus (Ad5). Expressed PNP converts the systemically administered prodrug, 6MPDR, to a toxic purine, 6MP, causing cell death. We sought to increase the specificity of recombinant Ad vectors by controlling PNP expression with the promoter region from the androgen-dependent, prostate-specific rat probasin (Pb) gene. To increase its activity, the promoter was combined with the SV40 enhancer (SVPb). Cell lines were transfected with plasmids containing both a reporter gene, under SVPb control, and a reference gene cassette to allow normalization of expression levels. Plasmids expressed approximately 20-fold more reporter in prostate cancer than in other cells, but surprisingly, the SVPb element was both androgen-independent and retained substantial prostate specificity. Killing by Ad5-SVPb-PNP vector of cell lines cultured with 6MPDR for 6 days was 5- to 10-fold greater in prostate cancer than in liver or lung cells. In vivo, a single intratumoral injection of Ad5-SVPb-PNP (4 x 10(8) pfu), followed by 6MPDR administration twice daily for 6 days, significantly suppressed the growth of human prostate tumors in nude mice and increased their survival compared to control animals. Thus, the androgen-independent, prostate-targeting Ad5 vector reduces human prostate cancer growth significantly in vitro and in vivo. This first example of an androgen-independent vector points the way toward treatment of emerging androgen-independent prostate cancer in conjunction with hormone ablation therapy at a time when the tumor burden is low.

Toxicity assessment of intratumoral injection of the herpes simplex type I thymidine kinase gene delivered by retrovirus in patients with refractory cancer

Introduction of the herpes simplex type I thymidine kinase (HSV-TK) gene into tumor tissue, followed by ganciclovir, initiates a phosphorylation cascade that induces formation of a toxic ganciclovir triphosphate. Animal trials suggest that this ganciclovir triphosphate has antitumor activity. Here we report application of the HSV-TK transfection approach using a retroviral construct. Sixteen patients (median age 61.5 years) with refractory carcinoma (13 melanoma, 1 breast cancer, 1 nonsmall-cell lung cancer, and 1 osteogenic sarcoma) received intratumoral injection of HSV-TK retroviral vector at escalating doses (0.2x10(7) cfu per injection x 5 daily doses) and we evaluated them for toxicity and activity. We observed grade III pain associated with cellulitis in one patient following injection. Analysis of blood samples drawn between 3 and 28 weeks from 14 patients for replication-competent retrovirus by PCR analysis of the amphotrophic envelope revealed no replication-competent retrovirus. We injected 21 lesions. We identified no tumor responses of the injected lesions. Of 13 patients with advanced melanoma, 6 survived over one year. Thus, injection of retroviral delivered HSV-TK in patients with refractory cancer was well-tolerated.

A uniquely stable replication-competent retrovirus vector achieves efficient gene delivery in vitro and in solid tumors

A major obstacle in cancer gene therapy is the limited efficiency of in vivo gene transfer by replication-defective retrovirus vectors in current use. One strategy for circumventing this difficulty would be to use vectors capable of replication within tumor tissues. We have developed a replication-competent retrovirus (RCR) vector derived from murine leukemia virus (MuLV). This vector utilizes a unique design strategy in which an internal ribosome entry site-transgene cassette is positioned between the env gene and the 3' long terminal repeat (LTR). The ability of this vector to replicate and transmit a transgene was examined in culture and in a solid tumor model in vivo. The RCR vector exhibited replication kinetics similar to those of wildtype MuLV and mediated efficient delivery of the transgene throughout an entire population of cells in culture after an initial inoculation with 1 plaque-forming unit (PFU) of vector per 2000 cells. After injection of 6 x 10(3) PFU of vector into established subcutaneous tumors, highly efficient spread of the transgene was observed over a period of 7 weeks, in some cases resulting in spread of the transgene throughout the entire tumor. MuLV-based RCR vectors show significant advantages over standard replication-defective vectors in efficiency of gene delivery both in culture and in vivo. This represents the first example of the use of an RCR vector in an adult mammalian host, and their first application to transduction of solid tumors.

Factors affecting long-term expression of a secreted transgene product after intravenous administration of a retroviral vector

We have studied parameters affecting in vivo expression of human growth hormone (hGH) in mice after intravenous administration of a retroviral vector encoding the protein as a model system for clotting factor VIII gene therapy. Such treatment results in a brief burst of high-level expression followed by lower level sustained expression of the hGH in the circulation. The major targets for transduction in the mouse are liver and spleen. Such direct transduction (i.e., without surgical or chemical induction of cell division) requires vector at high titer (>/=10(8) cfu/ml) and is dose dependent. Transduction efficiency decreases with increasing age of the recipient. Nevertheless, long-term expression in adults is observed after administration of vector as a split dose on 2 consecutive days. We also show that anti-vector immune responses may enhance long-term expression and that both anti-vector and anti-transgene immunity can be modulated. This work provides a framework for the rational development of means to enhance the efficiency of retroviral vectors for use in clinical gene replacement therapy.

Expression of herpes simplex virus-thymidine kinase gene controlled by a promoter region of the midkine gene confers selective cytotoxicity to ganciclovir in human carcinoma cells

A selective expression of suicide gene(s) in tumor cells should produce a preferential cytotoxic effect on tumors. Promoter region(s) of a gene that is expressed in tumors but not in normal tissues can be useful for tumor-specific transcription of a suicide gene. Midkine (MK), a growth/differentiation factor, is expressed predominantly in various types of human tumors, whereas its expression in adult normal tissues is highly restricted. In our study, we showed that a 2.3-kb fragment of genomic DNA in the 5' upstream region of the MK gene could activate transcription of a fused reporter gene in MK-positive cells but not in MK-negative cells. Efficiency of the cis-acting sequence to permit expression of an exogenous gene in tumor cells was comparable with that of the SV40 promoter. Regulated expression of the herpes simplex virus-thymidine kinase (HSV-TK) gene under the control of the MK promoter conferred increased sensitivity to ganciclovir (GCV) on MK-positive tumor cells. Administration of GCV into nude mice that were implanted with MK-positive tumor cells that expressed the HSV-TK gene under the control of the MK promoter could suppress the subsequent tumor growth. Expression of therapeutic genes restricted to tumors can be achieved by the use of the putative cis-acting MK promoter.

Characterization of novel safe lentiviral vectors derived from simian immunodeficiency virus (SIVmac251) that efficiently transduce mature human dendritic cells

We describe the generation and the characterization of new lentiviral vectors derived from SIVmac251, a simian immunodeficiency virus (SIV). A methodical approach was used to engineer both efficient and safe packaging constructs allowing the production of SIV viral core proteins. SIV-vectors encoding GFP (green fluorescent protein) were generated as VSV-G-pseudotyped particles upon transient expression of the vector construct and helper functions in 293 cells. The SIV vectors were able to transduce efficiently various target cell types at low multiplicity of infection, including monocyte-differentiated human dendritic cells (DCs) which retained their capacity to differentiate into mature DCs after gene transfer. Transduction of the DCs by the SIV vectors was prevented when infections were performed in the presence of AZT, a reverse-transcriptase inhibitor. After gene transfer, expression of the GFP in the target cells remained constant after several weeks, indicating that the vectors had been stably integrated into the genome of the host cells. Preparations of SIV vectors were systematically checked for the absence of replication-competent and recombinant retroviruses but remained negative, suggesting the innocuousness of these novel gene delivery vectors. Side-to-side comparisons with vectors derived from HIV-1 (human immunodeficiency virus) indicated that the SIV vectors were equally potent in transducing proliferating target cells. Finally, we have determined the infectivity of SIV vectors pseudotyped with surface glycoproteins of several membrane-enveloped viruses.

Intra-articular IL-4 gene therapy in arthritis: anti-inflammatory effect and enhanced th2activity

Gene therapy has been explored as a potential method for treating chronic inflammatory diseases such as rheumatoid arthritis. To determine the efficacy of intra-articular IL-4 gene therapy in an animal model of arthritis using a retroviral vector, a retrovirus encoding rat IL-4 (DA-IL-4) was engineered, purified and concentrated to high titer (>/=109 CFU/ml). Infectivity and expression levels were demonstrated in vitro using cultured fibroblast-like synoviocytes. Efficacy was evaluated in the rat adjuvant arthritis model. DA-IL-4 or DA-beta-gal retrovirus was injected into the intra-articular joint space of the right ankle on day 12 after immunization. Three days after joint injection, the injected paw contained increased levels of IL-4 compared with control or with the contralateral uninjected paw, demonstrating successful transgene expression. Surprisingly, 8 days after treatment IL-4 levels continued to increase in the injected and contralateral paw compared with DA-beta-gal-treated animals. Serum IL-4 levels were also elevated in DA-IL-4-treated rats. RT-PCR studies demonstrated that the transgene was expressed in the injected ankle but not in the contralateral joint. IL-4 gene therapy resulted in a significant reduction in paw swelling and decreased radiographic evidence of bone destruction. This is the first demonstration of successful intra-articular retroviral gene treatment using a therapeutic gene. In addition to its anti-inflammatory effect, this study supports the potential application of intra-articular gene therapy as a method for enhancing systemic Th2 function.

The resistance of retroviral vectors produced from human cells to serum inactivation in vivo and in vitro is primate species dependent

The ability to deliver genes as therapeutics requires an understanding of the vector pharmacokinetics similar to that required for conventional drugs. A first question is the half-life of the vector in the bloodstream. Retroviral vectors produced in certain human cell lines differ from vectors produced in nonhuman cell lines in being substantially resistant to inactivation in vitro by human serum complement (F. L. Cosset, Y. Takeuchi, J. L. Battini, R. A. Weiss, and M. K. Collins, J. Virol. 69:7430-7436, 1995). Thus, use of human packaging cell lines (PCL) may produce vectors with longer half-lives, resulting in more-efficacious in vivo gene therapy. However, survival of human PCL-produced vectors in vivo following systemic administration has not been explored. In this investigation, the half-lives of retroviral vectors packaged by either canine D17 or human HT1080 PCL were measured in the bloodstreams of macaques and chimpanzees. Human PCL-produced vectors exhibited significantly higher concentrations of circulating biologically active vector at the earliest time points measured (>1, 000-fold in chimpanzees), as well as substantially extended half-lives, compared to canine PCL-produced vectors. In addition, the circulation half-life of human PCL-produced vector was longer in chimpanzees than in macaques. This was consistent with in vitro findings which demonstrated that primate serum inactivation of vector produced from human PCL increased with increasing phylogenetic distance from humans. These results establish that in vivo retroviral vector half-life correlates with in vitro resistance to complement. Furthermore, these findings should influence the choice of animal models used to evaluate retroviral-vector-based therapies.

Phase I trial of interferon gamma retroviral vector administered intratumorally with multiple courses in patients with metastatic melanoma

The purpose of this study was to determine the safety and antitumor activity of IFN-gamma retroviral vector in patients with advanced melanoma. Seventeen patients (9 single courses, 8 multiple courses) received a total of 363 intratumor injections of IFN-gamma retroviral vector (1 x 10(7) PFU/ml administered at 0.3, 0.5, and 1.0 ml per cohort). No grade III/IV adverse events were attributed to study medication. Replication-competent retrovirus was not detected in any of the 17 patients by polymerase chain reaction studies. Eight patients showed elevated anti-tumor antibody responses in comparison with baseline by ELISA. One of nine patients treated with a single course had an optimal response of stable disease, compared with eight of eight multiple-injected patients. Median survival of single-injected patients was 150 days, and patients who received multiple injections have still not achieved median survival duration, with four of eight still living (p = 0.0462, Wilcoxon; p = 0.0273, log rank). We conclude that intratumor injection of IFN-gamma is safe and well tolerated. Evidence of antitumor activity is suggested in patients with advanced malignancy that received multiple injections.

Adenovirus-mediated tissue-targeted expression of the HSVtk gene for the treatment of breast cancer

In an effort to develop a genetic therapy for the treatment of breast cancer, we constructed adenoviral vectors containing either the beta-galactosidase (beta-gal) reporter gene or the herpes simplex thymidine kinase (HSVtk) suicide gene driven by breast tissue-specific promoters. We utilized upstream regulatory sequences from either the human alpha-lactalbumin (hALA) gene, or the ovine beta-lactoglobulin (oBLG) gene in these vector constructs to target expression of heterologous genes transcriptionally to breast cancer cells both in vitro and in vivo. Data derived from breast tissue-specific reporter vectors in vitro demonstrate that expression from the hALA and oBLG promoters are indeed specific for breast cells (T47D, MCF-7, ZR75-1) when compared with non-breast cells (U2OS, HeLa). Moreover, these vectors displayed tumor cell specificity when compared with the normal MCF-10A breast cell line. These vectors also displayed breast tissue specificity when injected systemically (i.v.) into lactating Balb/c mice, which suggests that these promoters maintain their tissue-specific expression pattern within the context of the adenoviral genome in vivo. Tumors, derived from T47D human breast cancer cells, were established in nude mice and injected with either the tissue-specific reporter or suicide vectors. Results from tumors injected (i.t.) with reporter adenoviruses demonstrate that these promoters are active in T47D cells when grown as established tumors and we observed a marked regression of tumors injected with suicide vectors and treated systemically with gancyclovir (150 mg/kg/day) when compared with control animals. Moreover, mouse survival was prolonged after 35 days in mice undergoing therapy with the suicide vectors in conjunction with gancyclovir when compared with the control animals. These data suggest that the transcriptionally targeted hALA or oBLG driven expression of the HSVtk gene may be a feasible therapy for the treatment of human breast cancer.

Complete cure of established murine hepatocellular carcinoma is achievable by repeated injections of retroviruses carrying the herpes simplex virus thymidine kinase gene

Although xenotransplantation of retrovirus-producing cells into a tumor has been shown to be effective for the treatment of cancer, injections of recombinant retroviruses are much more feasible for clinical applications. We established a clone producing retroviruses carrying the herpes simplex virus thymidine kinase (HSVtk) gene with titers of up to 4 x 10(7) colony-forming units/ml, and examined the effectiveness of in vivo gene therapy against cancer. Syngeneic mice were inoculated subcutaneously with murine hepatocellular carcinoma (HCC) cells, BNL1ME A.7R.1, and the treatment was initiated after tumors were established. When mice were given an intratumoral injection of HSVtk-carrying retroviruses or their producing cells followed by ganciclovir (GCV) treatment, significantly prolonged survival periods were observed. When mice were treated with repeated intratumoral injections of HSVtk-carrying retrovirus-producing cells, significant antitumor responses and some cures were induced by GCV treatment. Furthermore, repeated intratumoral injections of HSVtk-carrying retroviruses and GCV treatment resulted in complete regression of established HCC tumors in all animals used in the experiment. Mice that completely eradicated tumors exhibited protective immunity against wild-type HCC tumors. These results suggest that repeated injections of HSVtk-carrying retroviruses followed by GCV treatment is a potent modality for the treatment of solid tumors.

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.

Generation of a systemic antitumor response with regional intratumoral injections of interferon gamma retroviral vector

The generation of a lasting systemic immune response is a primary goal for cancer immunotherapy. Here we examine the ability of high-titer IFN-gamma retroviral vector injected into an accessible tumor to generate significant antitumor responses at a distal untreated site. CT26 or B16F10 murine tumors were inoculated subcutaneously to form solid tumors in BALB/c or C57BL/6 mice. Seven to 10 days postinoculation, high-titer IFN-gamma retroviral vector was directly injected into the subcutaneous tumor nodule, and optimal dose and course of therapy were determined. As a model for disseminated disease, mice were inoculated intravenously with CT26 cells to form pulmonary lesions, at the same time as the subcutaneous injections. Regression of subcutaneous tumor correlated with a systemic response at the distal lung metastases in the IFN-gamma-treated group (p < 0.0005). Splenocytes from mice with completely regressed tumors had a twofold increase in percent specific cytotoxicity in a standard CTL assay as compared with nonresponding mice. CD8+ T cells were shown to be essential for the regional and systemic antitumor response, as determined by in vivo cell depletion experiments. These data demonstrate that IFN-gamma retroviral vector gene therapy delivered intralesionally can generate significant inhibition of pulmonary tumor formation distal to the treatment site. The data from these preclinical studies suggest the potential clinical value of retroviral vector-mediated cytokine gene therapy for systemic cancer.

In vivo gene therapy for prostate cancer: preclinical evaluation of two different enzyme-directed prodrug therapy systems delivered by identical adenovirus vectors

Advanced prostate cancer is invariably lethal once it becomes androgen independent (AI). With the aim of developing a new treatment we have used the human androgen-independent prostate cancer cell line, PC-3, to evaluate the effectiveness of two enzyme-directed prodrug therapy (EPT) systems as a novel means for promoting tumor cell destruction in vivo. We have confined our study to the use of a PSA promoter, in a preliminary attempt to achieve prostate specificity. The two EPT systems used were the HSVTK/GCV and PNP/6MPDR systems. These were chosen for their differential dependence on DNA replication for their mechanism of action. In the present work, either the HSVTK or PNP gene, each controlled by a PSA promoter fragment, was delivered by an E1-, replication-deficient human adenovirus (Ad5) into PC-3 tumors growing subcutaneously in BALB/c nude mice. Tumors were injected with a single dose of recombinant Ad5 and mice were treated intraperitoneally with the appropriate prodrug, twice daily, for 6 days thereafter. The growth of established PC-3 tumors was significantly suppressed and host survival increased with a single course of HSVTK/GCV or PNP/6MPDR treatment. HSVTK/GCV-treated PC-3 tumor growth was 80% less than that of control treatments on day 33, while PNP/6MPDR-treated tumor growth was approximately 75% less than that of control treatments on day 52. Survival data showed that 20% of HSVTK/GCV- or PNP/6MPDR-treated animals lived >45 and >448 days, respectively, longer than control animals. These results demonstrate that both HSVTK/GCV and PNP/6MPDR therapies interrupt the growth of an aggressive human prostate cancer cell line in vivo.