The gene encoding cytosine deaminase (CD) has been expressed in the human colorectal carcinoma cell line WiDr. Metabolism studies confirm that tumor cells expressing CD convert the very nontoxic prodrug 5-fluorocytosine (5FCyt) to 5-fluorouracil (5FUra) and 5FUra metabolites. Tumor xenografts composed of CD-expressing cells can selectively generate tumor levels of >400 iAM 5FUra when the host mouse is dosed with nontoxic levels of 5FCyt. The selective metabolic conversion of 5FCyt to 5FUra in CD-expressing tumor cells results in the inhibition of thymidylate synthase and incorporation of 5FUra into RNA. 5FUra is also liberated into the surrounding environment when CD-expressing tumor cells are treated with 5FCyt. The liberated 5FUra is able to kill neighboring, non-CD-expressing tumor cells in vitro and in vivo.Most importantly, when only 2% of the tumor mass contains CD-expressing cells (98% non-CD-expressing cells), significant regresions in all tumors are observed when the host mouse is dosed with nontoxic levels of 5FCyt.We have previously reported our efforts to develop a therapeutic approach using gene transfer technology for the treatment of primary and metastatic tumors (1-4). This approach, virus-directed enzyme/prodrug therapy, exploits transcriptional differences between normal and neoplastic cells to selectively kill cancer cells. An artificial chimeric gene is created that is composed of tissue-specific transcriptional regulatory sequences linked to the coding domain of a nonmammalian enzyme. The nonmammalian enzyme can metabolically activate a nontoxic prodrug to a cytotoxic anabolite. Ifthe tissue-specific regulatory sequences are from a tumor-associated marker gene such as the carcinoembryonic antigen gene (5, 6), the prostate-specific antigen gene (7,8), or the a-fetoprotein gene (9, 10), then the artificial chimeric gene will result in tumor-specific expression of the nonmammalian enzyme and, consequently, in tumor-specific production of the cytotoxic metabolite.A critical issue influencing the therapeutic benefit of this approach will be the efficiency of gene transfer into a solid tumor. However, the efficacy of this gene therapy approach is dependent not only on the efficiency of gene transfer but also on the enzyme/prodrug system. Various enzyme/prodrug systems have been reported (1)(2)(3)(11)(12)(13)(14). A prodrug that is metabolically converted into a toxic anabolite that is not readily diffusible from one tumor cell into another will require very high gene transfer efficiency. However, if the toxic anabolite is readily diffusible, then the efficiency of gene transfer into the tumor mass may be quite low but still be able to achieve a significant therapeutic effect. Cells were lysed by freeze-thawing and centrifuged for 10 min at 8000 x g. The conditioned medium and the cell extract supernatant were evaporated to dryness, dissolved in deionized water (1 ml for medium; 100 !A for extract). Aliquots (20 1.l) of each sample were analyzed by HPLC on a Partisphere C18 column (4.6 mm x...

An approach involving retroviral-mediated gene therapy for the treatment of neoplastic disease is described. This therapeutic approach is called "virus-directed enzyme/prodrug therapy" (VDEPT). The VDEPT approach exploits the transcriptional differences between normal and neoplastic cells to achieve selective killing of neoplastic cells. We now describe development of the VDEPT approach for the treatment of hepatocellular carcinoma. Replication-defective, amphotrophic retroviruses were constructed containing a chimeric varicella-zoster virus thymidine kinase (VZV TK) gene that is transcriptionally regulated by either the hepatomaassociated a-fetoprotein or liver-associated albumin transcriptional regulatory sequences. Subsequent to retroviral infection, expression of VZV TK was limited to either a-fetoprotein-or albumin-positive cells, respectively. VZV TK metabolically activated the nontoxic prodrug 6-methoxypurine arabinonucleoside (araM), ultimately leading to the formation of the cytotoxic anabolite adenine arabinonucleoside triphosphate (araATP). Cells that selectively expressed VZV TK became selectively sensitive to araM due to the VZV TK-dependent anabolism of araM to araATP. Hence, these retroviraldelivered chimeric genes generated tissue-specific expression of VZV TK, tissue-specific anabolism of araM to araATP, and tissue-specific cytotoxicity due to araM exposure. By utilizing such retroviral vectors, araM was anabolized to araATP in hepatoma cells, producing a selective cytotoxic effect.

The 5' sequences from the human carcinoembryonic antigen gene (CEA) were analyzed using luciferase reporter gene assays. This analysis identified important cis-acting sequences needed for selective expression in CEA-positive cells. Over 50 CEA/luciferase reporter clones were constructed and analyzed in two CEA-positive and two CEA-negative cell lines. The CEA sequences analyzed extended from the translational start to 14.5 kb 5' of the CEA gene. A 408-bp region from the CEA 3' untranslated region was also examined for its effect on reporter gene activity. The CEA promoter was located between bases -90 and +69 of the transcriptional start site. Sequences between -41 and -18 were essential for expression from the CEA promoter. Multimerization of sequences between -89 and -40 resulted in copy number-related increases in both expression level and selectivity for CEA-positive cells. Two upstream regions of CEA, -13.6 to -10.7 kb or -6.1 to -4.0 kb, when linked to the multimerized promoter led to high-level, selective expression in CEA-positive cell lines. Several CEA/luciferase constructs demonstrated 80- to 120-fold higher expression in CEA-positive cell lines compared to expression in CEA-negative Hep3B cells. The expression from these constructs was quite strong in CEA-positive cells, being two- to four-fold higher than an SV40 enhancer/promoter construct. The most promising CEA transcriptional regulatory sequences were used to regulate the expression of cytosine deaminase (CD) in stable cell lines. The expression of CD was assessed directly by an enzymatic assay and indirectly by determining the in vitro IC50 to 5-fluorocytosine (5FC). The chimeric gene pCEA/CD-145 displayed the desired expression spectrum--high-level expression in the CEA-positive cells and low-level expression in CEA-negative cells. CD expression from this chimera correlated well with the expression of the endogenous CEA gene. Treatment of mice bearing NCI H508 pCEA/CD-145 tumor xenografts with 5FC lead to significant antitumor effects in vivo. The CEA/CD chimeric gene should be useful for tumor-specific suicide gene therapy of CEA-positive tumors.

In mammalian cells, ras genes code for 21-kDa GTP-binding proteins. Increased expression and mutations in specific amino acids have been closely linked to alterations of normal cell morphology, growth, and differentiation and, in particular, to neoplastic transformation. The signal transduction induced by these p21' proteins is largely unknown; however, the signaling pathways of several growth factors have been reported to involve phosphatidylinositol (Ptdlns) 3-kinase. In the present study of a Ha-ras-transformed epithelial cell line, we demonstrated increased Ptdlns 3-kinase activity in anti-phosphotyrosine and anti-receptor (insulin and hybrid insulin-like growth factor I) immunoprecipitates of cells that had been stimulated with insulin or insulin-like growth factor I. The PtdIns 3-kinase activity was also immunoprecipitated in these experiments by the anti-Ras monoclonal antibody Y13-259. The specificity ofthis association with p21t was ascertained by the neutralizing effect of the antigen peptide and the absence of PtdIns 3-kinase activity in Y13-259 immunoprecipitates from cells in which the ras gene was turned off.These data indicate that Ptdlns 3-kinase activity is an important step in the cascade of reactions in p21m signal transduction, suggesting that the alterations of the cytoskeleton and growth in ras-transformed cells could be mediated by Ptdlns 3-kinase activity.

Escalation therapy with mitoxantrone (MX) in highly active multiple sclerosis is limited by partially dose-dependent side-effects. Predictors of therapeutic response may result in individualized risk stratification and MX dosing. ATP-binding cassette-transporters ABCB1 and ABCG2 represent multi-drug resistance mechanisms involved in active cellular MX efflux. Here, we investigated the role of ABC-gene single nucleotide polymorphisms (SNPs) for clinical MX response, corroborated by experimental in vitro and in vivo data. Frequencies of ABCB1 2677G>T, 3435C>T and five ABCG2-SNPs were analysed in 832 multiple sclerosis patients (Germany, Spain) and 264 healthy donors. Using a flow-cytometry-based in vitro assay, MX efflux in leukocytes from individuals with variant alleles in both ABC-genes (designated genotype ABCB1/ABCG2-L(ow), 22.2% of patients) was 37.7% lower than from individuals homozygous for common alleles (ABCB1/ABCG2-H(igh), P < 0.05, 14.8% of patients), resulting in genotype-dependent MX accumulation and cell death. Addition of glucocorticosteroids (GCs) inhibited MX efflux in vitro. ABC-transporters were highly expressed in leukocyte subsets, glial and neuronal cells as well as myocardium, i.e. cells/tissues potentially affected by MX therapy. In vivo significance was further corroborated in experimental autoimmune encephalomyelitis in Abcg2(-/-) animals. Using a MX dose titrated to be ineffective in wild-type animals, disease course and histopathology in Abcg2(-/-) mice were strongly ameliorated. Retrospective clinical analysis in MX monotherapy patients (n = 155) used expanded disability status scale, relapse rate and multiple sclerosis functional composite as major outcome parameters. The clinical response rate [overall 121 of 155 patients (78.1%)] increased significantly with genotypes associated with decreasing ABCB1/ABCG2-function [ABCB1/ABCG2-H 15/24 (62.5%) responders, ABCB1/ABCG2-I(ntermediate) 78/98 (79.6%), ABCB1/ABCG2-L 28/33 (84.8%), exact Cochran-Armitage test P = 0.039]. The odds ratio for response was 1.9 (95% CI 1.0-3.5) with each increase in ABCB1/ABCG2 score (from ABCB1/ABCG2-H to -I-, and -I to -L). In 36 patients with severe cardiac or haematological side effects no statistically relevant difference in genotype frequency was observed. However, one patient with biopsy proven cardiomyopathy only after 24 mg/m2 MX exhibited a rare genotype with variant, partly homozygous alleles in 3 ABC-transporter genes. In conclusion, SNPs in ABC-transporter genes may serve as pharmacogenetic markers associated with clinical response to MX therapy in multiple sclerosis. Combined MX/GC-treatment warrants further investigation.