The Fc Domain of Immunoglobulin Is Sufficient to Bridge NK Cells with Virally Infected Cells.

Clearance of pathogens or tumor cells by antibodies traditionally requires both Fab and Fc domains of IgG. Here, we show the Fc domain of IgG alone mediates recognition and clearance of herpes simplex virus (HSV1)-infected cells. The human natural killer (NK) cell surface is naturally coated with IgG bound by its Fc domain to the Fcγ receptor CD16a. NK cells utilize the Fc domain of bound IgG to recognize gE, an HSV1-encoded glycoprotein that also binds the Fc domain of IgG but at a site distinct from CD16a. The bridge formed by the Fc domain between the HSV1-infected cell and the NK cell results in NK cell activation and lysis of the HSV1-infected cell in the absence of HSV1-specific antibody in vitro and prevents fatal HSV1 infection in vivo. This mechanism also explains how bacterial IgG-binding proteins regulate NK cell function and may be broadly applicable to Fcγ-receptor-bearing cells.

In vitro screening of clinical drugs identifies sensitizers of oncolytic viral therapy in glioblastoma stem-like cells.

Oncolytic viruses (OV) have broad potential as an adjuvant for the treatment of solid tumors. The present study addresses the feasibility of clinically applicable drugs to enhance the oncolytic potential of the OV Delta24-RGD in glioblastoma. In total, 446 drugs were screened for their viral sensitizing properties in glioblastoma stem-like cells (GSCs) in vitro. Validation was done for 10 drugs to determine synergy based on the Chou Talalay assay. Mechanistic studies were undertaken to assess viability, replication efficacy, viral infection enhancement and cell death pathway induction in a selected panel of drugs. Four viral sensitizers (fluphenazine, indirubin, lofepramine and ranolazine) were demonstrated to reproducibly synergize with Delta24-RGD in multiple assays. After validation, we underscored general applicability by testing candidate drugs in a broader context of a panel of different GSCs, various solid tumor models and multiple OVs. Overall, this study identified four viral sensitizers, which synergize with Delta24-RGD and two other strains of OVs. The viral sensitizers interact with infection, replication and cell death pathways to enhance efficacy of the OV.

Comparative effectiveness of antinociceptive gene therapies in animal models of diabetic neuropathic pain.

Peripheral neuropathic pain is one of the most common and debilitating complications of diabetes. Several genes have been shown to be effective in reducing neuropathic pain in animal models of diabetes after transfer to the dorsal root ganglion using replication-defective herpes simplex virus (HSV)1-based vectors, yet there has never been a comparative analysis of their efficacy. We compared four different HSV1-based vectors engineered to produce one of two opioid receptor agonists (enkephalin or endomorphin), or one of two isoforms of glutamic acid decarboxylase (GAD65 or GAD67), alone and in combination, in the streptozotocin-induced diabetic rat and mouse models. Our results indicate that a single subcutaneous hindpaw inoculation of vectors expressing GAD65 or GAD67 reduced diabetes-induced mechanical allodynia to a degree that was greater than daily injections of gabapentin in rats. Diabetic mice that developed thermal hyperalgesia also responded to GAD65 or endomorphin gene delivery. The results suggest that either GAD65 or GAD67 vectors are the most effective in the treatment of diabetic pain. The vector combinations, GAD67+endomorphin, GAD67+enkephalin or endomorphin+enkephalin also produced a significant antinociceptive effect but the combination did not appear to be superior to single gene treatment. These findings provide further justification for the clinical development of antinociceptive gene therapies for the treatment of diabetic peripheral neuropathies.

Expression of HSV-1 receptors in EBV-associated lymphoproliferative disease determines susceptibility to oncolytic HSV.

Epstein-Barr virus (EBV)-associated B-cell lymphoproliferative disease (LPD) after hematopoietic stem cell or solid organ transplantation remains a life-threatening complication. Expression of the virus-encoded gene product, EBER, has been shown to prevent apoptosis via blockade of PKR activation. As PKR is a major cellular defense against Herpes simplex virus (HSV), and oncolytic HSV-1 (oHSV) mutants have shown promising antitumor efficacy in preclinical models, we sought to determine whether EBV-LPD cells are susceptible to infection by oHSVs. We tested three primary EBV-infected lymphocyte cell cultures from neuroblastoma (NB) patients as models of naturally acquired EBV-LPD. NB12 was the most susceptible, NB122R was intermediate and NB88R2 was essentially resistant. Despite EBER expression, PKR was activated by oHSV infection. Susceptibility to oHSV correlated with the expression of the HSV receptor, nectin-1. The resistance of NB88R2 was reversed by exogenous nectin-1 expression, whereas downregulation of nectin-1 on NB12 decreased viral entry. Xenografts derived from the EBV-LPDs exhibited only mild (NB12) or no (NB88R2) response to oHSV injection, compared with a NB cell line that showed a significant response. We conclude that EBV-LPDs are relatively resistant to oHSV virotherapy, in some cases, due to low virus receptor expression but also due to intact antiviral PKR signaling.

Oncolytic virus therapy of multiple tumors in the brain requires suppression of innate and elicited antiviral responses.

The occurrence of multiple tumors in an organ heralds a rapidly fatal course. Although intravascular administration may deliver oncolytic viruses/vectors to each of these tumors, its efficiency is impeded by an antiviral activity present in complement-depleted plasma of rodents and humans. Here, this activity was shown to interact with complement in a calcium-dependent fashion, and antibody neutralization studies indicated preimmune IgM has a contributing role. Short-term exposure to cyclophosphamide (CPA) partially suppressed this activity in rodents and humans. At longer time points, cyclophosphamide also abrogated neutralizing antibody responses. Cyclophosphamide treatment of rats with large single or multiple intracerebral tumors substantially increased viral survival and propagation, leading to neoplastic regression.

The nuclear DICER-circular RNA complex drives the deregulation of the glioblastoma cell microRNAome.

The assortment of cellular microRNAs ("microRNAome") is a vital readout of cellular homeostasis, but the mechanisms that regulate the microRNAome are poorly understood. The microRNAome of glioblastoma is substantially down-regulated in comparison to the normal brain. Here, we find malfunction of the posttranscriptional maturation of the glioblastoma microRNAome and link it to aberrant nuclear localization of DICER, the major enzymatic complex responsible for microRNA maturation. Analysis of DICER's nuclear interactome reveals the presence of an RNA binding protein, RBM3, and of a circular RNA, circ2082, within the complex. Targeting of this complex by knockdown of circ2082 results in the restoration of cytosolic localization of DICER and widespread derepression of the microRNAome, leading to transcriptome-wide rearrangements that mitigate the tumorigenicity of glioblastoma cells in vitro and in vivo with correlation to favorable outcomes in patients with glioblastoma. These findings uncover the mechanistic foundation of microRNAome deregulation in malignant cells.

An oncolytic viral mutant that delivers the CYP2B1 transgene and augments cyclophosphamide chemotherapy.

Herpes simplex viruses type 1 (HSV-1) with an inactivated viral ribonucleotide reductase (Hsrr, ICP6) were designed to target tumor cells with upregulated mammalian ribonucleotide reductase (mRR), an enzyme whose expression is regulated by the p16/pRB tumor suppressor pathway. A recombinant HSV-1 was generated by knock-out of Hsrr and insertion of the rat CYP2B1 transgene responsible for the bioactivation of the prodrugs, cyclophosphamide and ifosfamide. The mutant virus replicated selectively in rat and human tumor cells that express mRR. Addition of cyclophosphamide potentiated oncolytic effects against cultured tumor cells and subcutaneous tumor xenografts established in athymic mice.

An infectious transfer and expression system for genomic DNA loci in human and mouse cells.

The recent completion of the human genome sequence allows genomics research to focus on understanding gene complexity, expression, and regulation. However, the routine-use genomic DNA expression systems required to investigate these phenomena are not well developed. Bacterial artificial chromosomes (BACs) and P1-based artificial chromosomes (PACs) have proved excellent tools for the human genome sequencing projects. We describe a system to rapidly and efficiently deliver and express BAC and PAC library clones in human and mouse cells by converting them into infectious amplicon vectors. We show packaging and intact delivery of genomic inserts of >100 kilobases with efficiencies of up to 100%. To demonstrate that genomic loci transferred in this way are functional, the complete human hypoxanthine phosphoribosyltransferase (HPRT) locus contained within a 115-kilobase BAC insert was shown to be expressed when delivered by infection into both a human HPRT-deficient fibroblast cell line and a mouse primary hepatocyte culture derived from Hprt-/- mice. Efficient gene delivery to primary cells is especially important, as these cells cannot be expanded using antibiotic selection. This work is the first demonstration of infectious delivery and expression of genomic DNA sequences of >100 kilobases, a technique that may prove useful for analyzing gene expression from the human genome.

Genetic strategies for brain tumor therapy.

Gene therapy is a potentially useful approach in the treatment of human brain tumors, which are notoriously refractory to conventional approaches. Most human clinical trials to date have been unsuccessful in terms of improving patient outcome. Recent improvements in viral vectors, the development of stem cell technology, and increased understanding of the mechanism of action of therapeutic transgenes provide hope that the next generation of gene therapeutics may show increased efficacy in treatment of this devastating disease.

Synergistic anticancer effects of ganciclovir/thymidine kinase and 5-fluorocytosine/cytosine deaminase gene therapies.

BACKGROUND: A bacterial enzyme, Escherichia coli cytosine deaminase, which converts the prodrug 5-fluorocytosine into the toxic drug 5-fluorouracil, and a viral enzyme, herpes simplex virus thymidine kinase, which converts ganciclovir from an inactive prodrug to a cytotoxic agent by phosphorylation, are being actively investigated for use in gene therapy for cancer. The purpose of this study was to determine whether combining these prodrug-activating gene therapies might result in enhanced anticancer effects. METHODS: Rat 9L gliosarcoma cells were transfected with plasmids containing the E. coli cytosine deaminase gene (9L/CD cells), with plasmids containing the herpes simplex virus thymidine kinase gene (9L/TK cells), or with both expression plasmids (9L/CD-TK cells). The drug sensitivities of the cell lines were evaluated; in addition, the sensitivities of 9L and 9L/CD-TK cells mixed in varied proportions were measured. The effects of prodrug treatment on 9L/CD-TK tumor growth (i.e., size and volume) in nude mice were monitored. The isobologram method of Loewe and the multiple drug-effect analysis method of Chou-Talalay were used to measure the interaction between the two prodrug-activating gene therapies. To elucidate the mechanism of interaction, the phosphorylation of ganciclovir in 9L/CD-TK cells after varying prodrug treatments was studied. RESULTS AND CONCLUSIONS: The presence of transfected cytosine deaminase and thymidine kinase genes in 9L gliosarcoma cells reduced cell survival, both in vitro and in vivo, following treatment with the relevant prodrugs; the effects of the two components appeared to be synergistic and related mechanistically to the enhancement of ganciclovir phosphorylation by thymidine kinase following 5-fluorouracil treatment.

Comparative analyses of transgene delivery and expression in tumors inoculated with a replication-conditional or -defective viral vector.

Viral vectors for cancer can be classified into those that do not replicate (replication-defective vectors) and those that selectively replicate in neoplastic cells (replication-conditional or oncolytic vectors). Both of these can deliver anticancer cDNAs for therapeutic purposes. Opposite hypotheses can be made regarding the advantages of each vector type with regard to anatomic transgene expression. For the former vector, because cDNA delivery occurs in neoplastic cells that have the ability to migrate into the tumor mass, relatively extensive anatomic and temporal expression of anticancer functions may occur. For the latter vector, active viral replication may permit anatomically and temporally extensive delivery of the foreign cDNA into the tumor mass. Herein, we performed a simple comparative analysis to test which of these hypotheses is valid. Direct inoculation of s.c. tumors with a replication-conditional or a replication-defective viral vector, each of which expressed lacZ cDNA, was performed. Tumors were excised and analyzed for anatomic delivery of beta-galactosidase and for neoplastic viral titers. We find that lacZ cDNA expression is observed in approximately 40% of the tumor area 3, 7, and 14 days after injection with the replication-conditional vector, whereas approximately 10% of the tumor area expresses the transgene 3 days after injection with the replication-defective vector, with a rapid decline in expression thereafter. Titers of the replication-conditional virus remain stable within injected tumors for the 14 days of the assay (approximately 1:1,000 of the initial injection dose), whereas titers of the replication-defective vector decrease rapidly after injection (to a value of 1:100,000 of the initial injection dose). Taken in conjunction, these studies show that transgene delivery and expression in tumors last longer and are found throughout an anatomically more extensive area after injection with replication-conditional gene therapy vectors than after injection with replication-defective gene therapy vectors.

Treatment of glioma by engineered interleukin 4-secreting cells.

The ability of interleukin-4 (IL-4) to mediate an antitumor response to human gliomas was studied in vivo in nude mice. To allow the effect of IL-4 to be exerted over a relatively short distance and at an optimal concentration, a transfected tumor cell line expressing a high level of IL-4 was used in mixed tumor transplantation assays. There was a significant inhibition of growth of the U87 human glioma line when the IL-4-secreting cell line, LT-1, was implanted s.c. with the glioma in 5 nude mice when compared to contralateral control tumors consisting of the U87 glioma and IL-4-negative control cells. In addition, there was a prolongation of survival when U87 along with IL-4-secreting cells were implanted intracerebrally in 12 nude mice compared to 12 control nude mice implanted with U87 and IL-4-negative control cells and 11 control animals receiving U87 alone. Histological analysis 4 days after i.c. inoculation revealed the presence of a dramatic eosinophil infiltrate and tumor necrosis. The absence of viable glioma cells as well as resolution of inflammation 19 days after treatment suggests the potential for complete tumor regression without ongoing inflammatory sequelae resulting from cytokine treatment.

Multimodal cancer treatment mediated by a replicating oncolytic virus that delivers the oxazaphosphorine/rat cytochrome P450 2B1 and ganciclovir/herpes simplex virus thymidine kinase gene therapies.

Multimodal therapy is generally more effective than single-agent treatment for cancer. rRp450 is an engineered herpes simplex viral mutant that replicates in and kills tumor cells in a relatively selective fashion. It also expresses, in infected cells, the cyclophosphamide (CPA)-sensitive rat cytochrome P450 2B1 (CYP2B1) and the ganciclovir (GCV)-sensitive herpes simplex virus thymidine kinase (HSV-TK) transgenes. We show that cultured rat 9L and human U87deltaEGFR glioma cells, infected and lysed by rRp450, also exhibit supra-additive sensitivity to both CPA and GCV, as determined by Chou-Talalay synergy analysis. DNA cross-linking, assayed by ethidium bromide fluorescence, was significantly inhibited in the presence of GCV, suggesting that interactions between the CPA/CYP2B1 and GCV/HSV-TK gene therapies occurred at the level of DNA repair. In vivo, regression of 9L s.c. tumor volumes in athymic mice was achieved only by the multimodal treatment allowed by rRp450 viral oncolysis combined with CPA/CYP2B1 and GCV/HSV-TK gene therapies, whereas all other treatment combinations produced only tumor growth retardation.

Long-term survival of rats harboring brain neoplasms treated with ganciclovir and a herpes simplex virus vector that retains an intact thymidine kinase gene.

Survival of rats harboring cerebral 9L gliosarcomas can be significantly extended by an intratumoral inoculation with a herpes simplex virus vector, designated as hrR3. This vector, which bears the lacZ reporter gene, is defective in the gene encoding ribonucleotide reductase, allowing for replication in dividing tumor cells but not in postmitotic neural cells. It also possesses an intact viral thymidine kinase (TK) gene, which confers chemosensitivity to ganciclovir. In this study, the ability of ganciclovir to potentiate the antitumor effect of hrR3 was evaluated. In culture, there was a 23% decrease in the growth of 9L cells treated with hrR3 plus ganciclovir compared to hrR3 alone (P < 0.01). The combination of hrR3 plus ganciclovir led to the long-term survival of 48% of rats harboring intracerebral 9L gliosarcomas compared to 20% survival in the hrR3 group (P < 0.05). Ganciclovir treatment had no effect on the growth of tumor cells in vitro or in vivo when a herpes simplex virus vector with a defective TK gene was used. Immunocytochemistry confirmed selective expression of the TK gene in cells within the tumor. These findings indicate that the TK gene can potentiate the antitumor effect of the hrR3 herpes simplex virus vector and provide the basis for placing additional therapeutic genes in the genome of hrR3.

Oncolysis of diffuse hepatocellular carcinoma by intravascular administration of a replication-competent, genetically engineered herpesvirus.

Herpes simplex virus type 1 (HSV-1) replication within tumors can mediate tumor regression (oncolysis). The genetically engineered, HSV-1 mutant rRp450 does not express viral ribonucleotide reductase and is therefore replication conditional. During the course of infection, rRp450 expresses the cytochrome P450 transgene and HSV-1 thymidine kinase gene, thereby enabling it to bioactivate the prodrugs cyclophosphamide and ganciclovir, respectively. rRp450 replication in hepatocellular carcinoma (HCC) cells is cytotoxic and liberates progeny virion that infect adjacent tumor cells. rRp450-mediated oncolysis is enhanced in the presence of cyclophosphamide, whereas it is inhibited in the presence of ganciclovir. As a consequence of defective viral ribonucleotide reductase expression, the yield of rRp450 progeny virions from infection of HCC cells is 3 to 4 log orders greater than that from infection of normal hepatocytes. This is associated with dramatic tumor reduction of diffuse HCC after a single intravascular administration of rRp450. rRp450 holds the promise of the dual therapeutic benefit of selective oncolysis and P450 transgene delivery.

Intraneoplastic polymer-based delivery of cyclophosphamide for intratumoral bioconversion by a replicating oncolytic viral vector.

rRp450 is an oncolytic herpesvirus that expresses the CYP2B1 cDNA, responsible for bioconverting cyclophosphamide (CPA) into the active metabolites 4-hydroxyCPA/aldophosphamide (AP). However, formal proof of prodrug activation is lacking. We report that activation of CPA in cells infected with rRp450 generates a time-dependent increase of diffusible 4-hydroxyCPA/AP. For in vivo applications, a CPA-impregnated polymer was implanted into human tumor xenografts inoculated with rRp450. The area under the curve for 4-hydroxyCPA/AP was 806 microg/g of tumor tissue/h when CPA was administered via intraneoplastic polymer and 3 microg/g of tumor tissue/h when CPA was administered i.p. Therefore, (a) a lytic virus expressing a "suicide" gene can activate a prodrug; and (b) within rRp450-infected tumor, more prolonged and higher concentrations of activated metabolites are generated by intraneoplastic compared with systemic administration of prodrug.

Positron emission tomography-based imaging of transgene expression mediated by replication-conditional, oncolytic herpes simplex virus type 1 mutant vectors in vivo.

To evaluate the efficiency of gene delivery in gene therapy strategies for malignant brain tumors, it is important to determine the distribution and magnitude of transgene expression in target tumor cells over time. Here, we assess the time- and vector dose-dependent kinetics of recombinant herpes simplex virus (HSV)-1 vector-mediated gene expression and vector replication in culture and in vivo by a recently developed radiotracer method for noninvasive imaging of gene expression (J. G. Tjuvajev et al., Cancer Res., 55: 6126-6132, 1995). The kinetics of viral infection of rat 9L gliosarcoma cells by the replication-conditional HSV-1 vector, hrR3, was studied by measuring the accumulation rate of 2-[14C]-fluoro-5-iodo-1-beta-D-arabinofuranosyl-uracil (FIAU), a selective substrate for viral thymidine kinase (TK). The level of viral TK activity in 9L cells was monitored by the radiotracer assay to assess various vector doses and infection times, allowing vector replication and spread. In parallel, viral yields and levels of Escherichia coli beta-galactosidase activity were assessed quantitatively. To study vector replication, spread and HSV-1-tk and lacZ gene coexpression in vivo, first- or second-generation recombinant HSV-1 vectors (hrR3 or MGH-1) were injected into s.c. growing rat 9L or human U87 deltaEGFR gliomas in nude rats at various times (8 h to 8 days) and at various vector doses [1 x 10(6) to 2 x 10(9) plaque-forming units (PFUs)] prior to imaging. For noninvasive assessment of HSV-1-tk gene expression (124I-labeled FIAU % dose/g), 0.15 mCi of 124I-labeled FIAU was injected i.v. 8 h after the last vector administration, and FIAU positron emission tomography (PET) was performed 48 h later. For the assessment of HSV-1-tk and lacZ gene coexpression, 0.2 mCi of 131I-labeled FIAU was injected i.v. 24 h after the last vector administration. Forty-eight h later, animals were killed, and tumors were dissected for quantitative autoradiographical and histochemical assessment of regional distribution of radioactivity (TK expression measured as 131I-labeled FIAU % dose/g) and coexpressed lacZ gene activity. The rates of FIAU accumulation (Ki) in hrR3-infected 9L cells in culture, which reflect the levels of HSV-1-tk gene expression, ranged between 0.12 and 3.4 ml/g/min. They increased in a vector dose- and infection time-dependent manner and correlated with the virus yield (PFUs/ml), where the PFUs:Ki ratios remained relatively constant over time. Moreover, a linear relationship was observed between lacZ gene expression and FIAU accumulation 5-40 h after infection of 9L cells with a multiplicity of infection of 1.5. At later times (> 52 h postinjection), high vector doses (multiplicity of infection, 1.5) led to a decrease of FIAU accumulation rates, viral yield, and cell pellet weights, indicating vector-mediated cell toxicity. Various levels of HSV-1-tk gene expression could be assessed by FIAU-PET after in vivo infection of s.c. tumors. The levels of FIAU accumulation were comparatively low (approximately ranging from 0.00013 to 0.003% injected dose/g) and were spatially localized; this may reflect viral-induced cytolysis of infected tumor cells and limited lateral spread of the virus. Image coregistration of tumor histology, HSV-1-tk related radioactivity (assessed by autoradiography), and lacZ gene expression (assessed by beta-galactosidase staining) demonstrated a characteristic pattern of gene expression around the injection sites. A rim of lacZ gene expression immediately adjacent to necrotic tumor areas was observed, and this zone was surrounded by a narrow band of HSV-1-tk-related radioactivity, primarily in viable-appearing tumor tissue. These results demonstrate that recombinant HSV-1 vector-mediated HSV-1-tk gene expression can be monitored noninvasively by PET, where the areas of FIAU-derived radioactivity identify the viable portion of infected tumor tissue that retains FIAU accumulation ability, and that the accumulation rate of FIAU in culture, Ki, reflects the number of HSV-1 viral particles in the infected tumor cell population [4.1 +/- 0.6 x 10(6) PFUs/Ki unit (PFUs divided by ml/min/g)]. Moreover, time-dependent and spatial relationships of HSV-1-tk and lacZ gene coexpression in culture and in vivo indicate the potential for indirect in vivo imaging of therapeutic gene expression in tumor tissue infected with any recombinant HSV-1 vector where a therapeutic gene is substituted for the lacZ gene.

Measuring transferrin receptor gene expression by NMR imaging.

The human transferrin receptor (hTfR) has been used as a model molecular target to direct therapeutic agents to tumor cells and to shuttle drugs across the blood-brain-barrier. We show in the current study that receptor expression and regulation can be visualized by NMR imaging, when the receptor is probed with a sterically protected iron containing magnetic hTfR probe. We were able to demonstrate that the novel receptor probe was an iron source that could enter the cells via the hTfR but did not play an immediate role in iron downregulation of hTfR within incubation times tested. Using genetically engineered rat 9L gliosarcoma cell lines with three different forms of the hTfR, we also demonstrated that receptor expression and regulation can be visualized by NMR imaging using the probe. This research provides proof of the principle that it is possible to image receptor gene expression and regulation and it demonstrates that it may be possible to image gene transfer in vivo.

Diffusible cytotoxic metabolites contribute to the in vitro bystander effect associated with the cyclophosphamide/cytochrome P450 2B1 cancer gene therapy paradigm.

Tumor cells become sensitive to the inert prodrug cyclophosphamide (CPA) after transfer of the gene encoding cytochrome P450 2B1. This enzyme activates CPA into 4-hydroxycyclophosphamide, which ultimately degrades into acrolein and phosphoramide mustard, the anticancer and DNA-alkylating metabolite. It is imperative that any prodrug-activating gene therapy strategy against cancer possess the capacity to affect the proliferation of tumor cells even when they do not express the transgene (bystander effect), because current methodologies cannot achieve gene transduction in all tumor cells. Prodrug-activating gene therapy schemes described to date exhibit a bystander effect that is not mediated by conditioned medium in culture and may depend on cell contact. In contrast, we find that CPA-sensitized, P450-expressing C6 glioma cells (C6-P450) transfer cytotoxicity to nonexpressing cells by releasing diffusible metabolites through the medium. A 3-h exposure to the prodrug is necessary and sufficient to achieve killing of the transfected cells, and medium conditioned by these cells can kill untransfected cells with similar potency. This bystander effect occurs in the presence of CPA even when only 10% of cells in culture express the P450 2B1 gene, and it is not reproduced by cells that have been irradiated. In an animal model of intracerebral brain tumors, expression of the P450 2B1 gene within the neoplastic cells enhanced significantly the antitumor effect of CPA, even when it was administered systemically. This study shows that CPA/P450 2B1 gene therapy represents a novel tumor-killing strategy that displays an expanded range of cytotoxic action both spatially and temporally within tumor cells and significantly potentiates the anticancer action of CPA when administered i.v.