SUICIDE GENE THERAPY SYSTEM FOR THE TREATMENT OF BRAIN TUMOURS

Abstract: In a suicide gene therapy system, a nucleic acid molecule, in particular a DNA molecule, encoding a glycosidase is applied with a non-toxic cyanogenic glycoside substrate, which upon degradation by the glycosidase yields hydrogen cyanide, for the treatment of brain tumours in animals or humans. The DNA preferably encodes linamarase, the substrate is preferably linamarin. It is carried by a viral vector or a plasmid. The two interacting components are administered separately.

Description: Suicide gene therapy system for the treatment of brain tumours The invention relates to the field of gene therapy for brain tumours. Glioblastoma is one of the most frequent (1 per 50,000 people per year) and deadly (mortality close to 100%) brain tumours in humans. In order to find a cure for this disease, a number of strategies have been developed in the past. One of the most promising strategies was to sensitize tumour cells to specific prodrugs. Among these strategies, one of the most successful systems to date is the herpes simplex thymidine kinase (Hsf/f)/ganciclovir (Culver et al., 1992; Ram et al., 1993; Izquierdo et al., 1995). HsTK phosphorylates nucleoside analogs (such as ganciclovir or acyclovir) that normal eukaryotic cells fail to phosphorylate. The incorporation of these analogs into the replicating DNA blocks the process, thereby killing the cell. Malignant brain tumours (glioblastoma) have been cured in the rat animal model provided the volume of the tumour does not exceed 100-150 mm 3 in size (Izquierdo et al., 1997). When the same treatment is applied to humans, complete remission of the tumour is not achieved, although there is, in some cases, a measurable size reduction probably responsible for a considerable increase in the expected survival time of these patients (Culver et al., 1994; Izquierdo et al., 1996). In a similar way, tumour cells may be sensitized to cytosine arabinoside (ara-C) or 2'-2'difluorodeoxycytidine (gemcitabine) by delivery to the neoplastic cell of the deoxycytidine kinase gene enabling the prodrugs to be phosphorylated and thus block replication. The cytosine deaminase gene, as a further example, is capable of making cells sensitive to the prodrug 5-fluorocytosine. To date, only four prodrug activating systems have been used in transgene systems for brain tumours: the above-mentioned herpes thymidine kinase for the activation of ganciclovir and the E. coli cytosine deaminase for 5-fluorocytosine, the human cytochrome P450 2B1 for cyclophosphamide, and the E. coli purine nucleoside phosphorylase for activation of 6-methyl- purine-2'-deoxyribonucleoside. It has already been suggested by Deonarain, 1994, to explore the enzyme linamarase, which converts amygdalin to cyanide, for the treatment of hepatoma, melanoma, pancreatic and breast cancer. It was the object of the present invention to provide an alternative killer- suicide gene therapy system for the treatment of brain tumours, in particular of the malignant type such as glioblastoma. To solve the problem underlying the object of the invention, an alternative suicide gene system for the preparation of a medicament for the treatment of brain tumours was developed. The present invention is directed to the use of a nucleic acid molecule encoding a glycosidase in combination with a non-toxic cyanogenic glycoside which upon degradation by the glycosidase yields hydrogen cyanide, for the preparation of a medicament for the treatment of brain tumours. Preferably, the nucleic acid molecule is a DNA molecule. Glycosidases (O-Glycosyl hydrolases) are a widespread group of enzymes which hydrolyse the glycosidic bond between two or more carbohydrates or between a carbohydrate and a non-carbohydrate moiety. The catabolism of cynogenic glycosides is initiated by cleavage of the carbohydrate moiety by a β-glucosidase, which yields the corresponding alpha-hydroxynitrile. The hydroxynitriie decomposes by producing hydrogen cyanide and an aldehyde ketone. In a preferred embodiment of the invention, the glycosidase is a glucosidase, in particular a β-glucosidase, and the substrate is a cyanogenic glucoside. In an even more preferred embodiment of the invention, the enzyme β-glucosidase is linamarase and the cyanogenic glucoside is linamarin. The linamarase hydrolyzes the innocuous substrate linamarin (2-OH-isobutyronitrile-β-D-glucopyranoside) to acetone cyanohydrin and glucose (Mkpong et al., 1990; Hughes et al., 1992) (Fig. 1 ). Acetone cyanohydrin is unstable at pH above 6 and spontaneously decomposes to acetone and HCN (Selmar et al., 1987a, b). The cyanogenic glucoside, linamarin, is not hydrolyzed by most mammalian tissues, therefore in the absence of linamarase, once absorbed, it is excreted unchanged in the urine (Frakes and Sharma, 1986; Hernandez et al., 1995). The β-glucosidase , e.g. linamarase, may be derived from different plants, e.g. cassava, flax, hevea, phaseolus, white clover, etc. (Kakes, 1985; Fie'des and Gerhardt; 1994; Frehner and Conn, 1987; Hughes and Dunn, 1982). Alternatively to linamarase and linamarin, other glycosidases and corresponding substrates may be employed. Depending on the sugar moiety and the glycosidic bond, the enzyme may be selected, inter alia, from monosaccharidases and disaccharidases (e.g. galactosidases, lactases, fucosidases). The enzymes may be derived from plants or microorganisms. To obtain the DNA encoding an enzyme whose sequence has not yet been identified, methods based on standard protocols in molecular biology (Sambrook et al., 1989) can be employed. An example for such a method was used for determining the nucleotide and amino acid sequence of the linamarase from white clover (Oxtoby et al., 1991). Apart from naturally occuring enzymes, or nucleic acid molecules, in particular DNAs, encoding them, respectively, modified enzyme molecules, or DNAs encoding them, respectively, may be used. Such molecules may be truncated or modified, as long as they hydrolize the cyanogenic carbohydrate substrate, such that the toxic agent HCN is released. Whether a modified molecule is suitable for application in the present invention, can be easily verified in preliminary enzymatic assays in which the substrate is incubated with the enzyme and the release of HCN is detected. Besides linamarin, the substrate may be selected from other naturally occuring cyanogenic glycosides, e.g. amygdalin, sambunigrin, prunasin, holocalin, zierin, lotaustralin, taxiphyllin, vicianin, dhurrin (Gmelin et al., 1973; Jensen et al., 1973; Stevens et al., 1968; Nartey, 1968). Apart from naturally occuring cyanogenic glycoside substrates, synthetic cyanogenic glycosides may be designed to be used as substrate for the enzyme. Examples for synthetic substrates may be monosaccharides based on those with the general formula NC R C H 2 R 11 R H, —O n R 2- - R Rs F Rg wherein R, and R 2 ,which may be different from one another or the same, denote branched or unbranched C r C 4 -alkyl; R 3 , R 4 , R 5 , R 6 , R 7 , R 8 ,R 9 , R 10 and R 11f which may the same or different fom one another, denote hydrogen, hydroxy C | -C 4 -alkyl or C C 4 -alkoxy. Similarly the expert may design corresponding disaccharide derivatives, for example derivatives of cellobiose, maltose, lactose, saccharose, gentobiose, trehalose or turanose. In order for a substrate to be suitable for the present invention, it must be non-toxic and amenable to decomposition by a glycosidase to yield hydrogen cyanide. The person skilled in the art may test the suitability of an enzyme/substrate combination in routine enzyme assays, in which the substrate is incubated with the enzyme under the appropriate conditions and the production of hydrogen cyanide is determined, as described, inter alia, by Selmar et al., 1987; Itoh-Nashida et al., 1987; Mao and Anderson, 1967; Stevens et al., 1968. The invention makes, for the first time, use of a nucleic acid molecule, whose product action provokes release of hydrogen cyanide, for the treatment of brain tumours, in particular malignant brain tumours. In the following, for simplicity, the term "linamarase gene" is used for the nucleic acid molecule encoding a glycosidase. The invention may be applied for the treatment for tumours like glioblastoma, meduloepitelioma, meduloblastoma, neuroblastoma, germinoma, embryonic carcinoma, malignant astrocytoma, astroblastoma, ependimoma, oligodendroglioma, plexal carcinoma, neuroepitelioma, pineoblastoma, ependimoblastoma, primitive neuroectodermic tumour, malignant meningioma, condrosarcoma, meningeal sarcomatosa, malignant melanoma, malignant schwanoma. The medicament of the present invention represents a combined preparation of two ingredients, i.e. the linamarase gene and the substrate linamarin, which are administered separately. The two components interact directly and form a functional unity. In the present invention, the linamarase gene may be used in combination with any vector system that is suitable for delivering genes into brain tumour cells. In a preferred embodiment, the nucleic acid molecule encoding linamarase is a DNA or an RNA molecule carried by a viral vector. In a particularly preferred embodiment, the vector is a retroviral vector. Retroviruses, which have RNA as their nucleic acid and use the enzyme reverse transcriptase to copy their genome to be integrated into the DNA of the host cell's chromosomes, only effectively infect proliferating cells (although they could enter quiescent ones) because they require cell division for integration and gene expression. This property makes retrovirus the most convenient choice vectors for the treatment of rapidly dividing cells in malignancies arising within a postmitotic tissue, in an anatomical compartment (the brain) that is partially separated from the rest of the body by the hematoencephalic barrier. When compared to other types of viral vectors, retroviral vectors are more selective for transgene delivery to brain tumour cells, and the transgene is inherited by all progeny cells of the infected cells. The vector must fulfil the general requirement to be defective and therefore unable to give infective progeny. It has two long terminal repeats (LTR) acting as potent promoters and a selection gene, e.g. the puromycin resistance gene (puromycin N-acetyl transferase), besides the therapeutically active linamarase gene. A great variety of retroviral vectors have been designed for gene therapy applications and are available to the person skilled in the art. Examples for suitable vectors that may be used in the present invention and methods for constructing such vectors have been reviewed by Vile and Russel, 1995, Braas et al., 1996. Most retroviral vectors that have been designed for gene therapy are based on Moloney- Murine Leukemia Virus (Mo-MLV). A survey of retroviral vectors and the requirements upon them that need to be fulfilled in order to make them suitable for use in brain tumour gene therapy is given in the article Kramm et al., 1995, the disclosure of which, including the relevant cited references, is herewith expressly incorporated by reference herein. The retroviral vector may be applied as such, i.e. as a constituent of the pharmaceutical composition, together with an agent such as protamine or polybrene to facilitate viral infection, in a suitable physiological buffer, usually a saline buffer. The viral titer should not be lower than 10 6 colony forming units per ml. The preferred route of administration is intratumoral injection of the virus and infusion for the substrate. In order to counteract the limitations of the use of retroviral vectors perse, i.e. direct application of the virus by intratumoral injection, which are due to the relatively low titers and instability of infectious particles, the retroviral vectors are preferably applied by intratumoral grafting of packaging cells (producer cells) that have been infected with the vector and allowed to grow. The packaging cells are injected as a suspension in saline buffer and can be of human or murine origin, preferably human, e.g. cells of the human fibrosarcoma cell line FLYA13 (Cosset et al., 1995). An example for a mouse cell line is ΨCRIP (Danos and Mulligan, 1988). Human cells are preferred because they are not rejected as easily as murine producer cells. The cells should give titers not inferior to 10 6 colony forming units per ml, the cell number injected is approximately 10 9 or above. In another embodiment, the viral vector is an adenoviral vector. Due to the natural tropism of adenovirus, its accessibility to easy manipulation and the high viral titers that can be obtained, adenoviral vectors have found increased interest for cancer gene therapy as an alternative to retroviral vectors, the use of which is limited due to their requirement for active cell division and by the viral titers that can be obtained. A great variety of adenoviral vectors is available for use in the present invention, a review on recent vector developments that take into account the structural, immunological and targeting problems posed by gene therapy applications, is given by Kovesdi et al., 1997. Strategies for cancer therapy using adenoviral vectors were also evaluated by Descamps et al., 1996; specifically, adenoviral vectors have been shown to be useful for applying the HSV-tk ganciclovir suicide system in the treatment of glioma (Chen et al., 1994; Bonnekoh et al., 1995). Adenovirus-mediated gene transfer to the brain has been further discussed by Kramm et al., 1995, and Yung, 1994; and methodologically assessed by Peltekian et al., 1997. The vectors disclosed in the cited references can be used for inserting the linamarase gene according to the present invention. Since adenoviruses can be produced with higher titer numbers, they can be applied as such, i.e. there is no requirement for implanting producer cells. According to a further embodiment of the invention, adenoviral/retroviral vector chimeras may be used that combine the advantageous features of both adenoviral and retroviral vectors and have been recently shown to provide a novel strategy to achieve high-efficiency stable transfection in vivo (Bilbao et al., 1997). In another embodiment of the invention, the viral vector is a Herpes simplex virus (HSV) vector. Methods for constructing HSV vectors that may be used for inserting the linamarase gene and delivering the gene to the tumour cells, have been described by Latchman, 1994; the use of HSV vectors for brain tumours has been disclosed by Kramm et al., 1995; and Yung, 1994. Regarding the other components of the pharmaceutical composition that contains a viral vector carrying the linamarase gene, they are essentially the same as for the retroviral vector compositions. The pharmaceutical composition may contain a polycationic agent that enhances viral binding to the host cells in vivo, such as polybrene (hexadimethrine bromide) or protamine, which may be present in a concentration of approximately 4-8 μg/ml. When applying such agents, the risk of possible side effects, e.g. hemorrhage in case of protamine, has to be weighed against the benefit of an increase of gene transfer efficacy achieved by the addition of that agent. Apart from viral vector systems, non-viral gene delivery systems may be used in the present invention to deliver a plasmid that carries the linamarase gene to the brain tumour cells. In this case, the linamarase gene is carried by plasmid DNA. By using non-viral systems, some of the disadvantages of viral vectors, e.g. design constraints and safety issues, can be avoided. A non-viral system that may be used in the present invention is based on receptor-mediated endocytosis. Such a system is disclosed in WO 93/07283 which is based on complexes of plasmid DNA and polycations that are conjugated with cell surface binding ligands and is equipped with an endosomolytic function, such as an inactivated adenovirus or a membrane active peptide. For the present invention, the cell surface binding ligand that serves as targeting function may be selected from brain cell binding ligands, e.g. glioma-specific antibodies. Another group of non-viral gene transfer systems that can be used in the present invention is based on lipophilic gene delivery vehicles. Examples for such delivery vehicles are liposomes which have been suggested for various gene therapy applications (Feigner et al., 1994; San et al., 1993). The DNA plasmid carrying the linamarase gene can be encapsulated into liposomes according to known methods (Hug and Sleight, 1991 ). Other examples are cationic lipids such as lipofectin (Feigner et al., 1987). To increase the uptake of non-viral delivery vehicles and to enhance linamarase gene expression, several routes of administration that take into account the specific requirements posed by the necessity to open the blood-tumour barrier in the brain have been suggested for the application of drugs to brain tumours may be applied in the present invention as an accompanying measure. Such routes have been reviewed by Kramm et al., 1995; an example is the application of bradykinin or bradykinin analogues. A further example to improve uptake of the gene by brain tumor cells is the conjugation of targeting molecules to the delivery vehicle, e.g. coupling liposomes with a glioma-specific antibody, as suggested by Yoshida and Mizzuno, 1994. Targeting molecules may also be monoclonal antibodies to proteins that are more abundant in gliomas, such as the fibrillary acidic protein (Abaza et al., 1997). An alternative route for applying the linamarase plasmid is the transfer of naked DNA. In principal, this transfer method would, for several economical and technical reasons, be an ideal system, furthermore it is advantageous due to the low antigenicity of naked DNA. The disadvantage of the low efficiency of this gene transfer route has lately been overcome by a method that combines plasmid injection followed by in vivo electroporation of the tumour tissue. My means of electroporation, cell membranes are permealized, thus markedly enhancing drug and/or gene transfer (Nishi et al., 1996). 10 As for the viral vectors, the preferred route of administration for the above- mentioned non-viral vectors is intratumoral injection. Administration of the prodrug, e.g. linamarin, is preferably local, although it may also be peritoneal or intravenous. The preference for local administration is based on the estimation and experimental confirmation in other systems (tk ganciclovir or tk/5-2-bromovinyl-2'-deoxyuridine) that the amount of prodrug needed to eradicate 1 mm 3 of tumour is one thousand times lower if the prodrug is administered locally than intraperitoneally, which substantially decreases the cost of the treatment and reduces possible adverse effects on other tissues. Local administration may be achieved by infusion with a cannula from the tumour cavitiy to the outside, connected for several hours a day with a continuos infusion pump, that will deliver approximately 0.2 ml/hour. The treatment lasts for about a week. The total dose is approximately 50 mg of linamarin per day. This is equivalent to 5 mg of cyanide per day, which is far from the toxic doses of 50-60 mg. The prodrug is preferably administered in a physiological salt solution, e.g. PBS (phosphate buffered saline). In general, 50 mg of the prodrug is the minimum dose per day that is administered for treatment of a tumour. This dosis, which is far below the toxic concentration in terms of the released cyanide, may be increased in order to eradicate larger tumours. Depending on the specific requirements of the treatment, i.e. the individual patient, the size and localization of the tumour, the number of tumours, etc., the mode of application of the substrate may be continuous, which is the preferred mode of application, or sequential in several doses per day, or which may, in some cases, be one single dose. Consequently, the substrate concentration may vary. In case of continuous administration, the minimum substrate concentration is 10 mg/ml. If the substrate is 11 administered in one single or several doses, the concentration may be increased accordingly to approximately 30mg/ml. The substrate is administered after a time sufficient for the linamarase gene to be expressed in the tumour cells. If, according to the preferred embodiment of the invention, the linamarase gene is administered by means of a retroviral vector, this period of time is determined by the rate of infection of the tumour cells by the virus. Depending on the accessibility of the tumour cells, this time is usually several days to approximately a week. In a preferred embodiment, the invention comprises a kit, wherein a first container contains the preparation with the nucleic acid molecule encoding the glycosidase, preferably the linamarase, and a second container contains the preparation with the substrate, preferably linamarin. Both preparations are in pharmaceutically acceptable formulations, e.g. physiological salt solutions. In a preferred embodiment, the first container contains retroviral vectors carrying the linamarase gene, with a titer of >_10 6 colony forming units. In a preferred embodiment, the first container contains producer cells, infected with the retroviral vectors, which are provided in deep frozen form. The number of cells contained in the preparation may be the number of cells ready for application, i.e. 10 9 cells or more, alternatively, a stock preparation of cells is provided to be grown in the appropriate medium, which may be provided in a further container, to obtain the number of cells required for the specific application. In another preferred embodiment, the first container contains a preparation of adenoviral vectors. In another preferred embodiment, the first container contains a preparation of plasmid DNA, optionally together with a non-viral gene delivery system, e.g. liposomes, into which the plasmid DNA has been incorporated. In the case of a gene delivery system based on receptor-mediated endocytosis, the components facilitating the entry of the DNA into the cells and its 12 expression, e.g. a polycation conjugated with a cellular ligand, may be present in an additional container. The second container contains the substrate, preferably linamarin. In an preferred embodiment, the linamarin is present in solid form in the total dosis to be applied, and the buffer is provided separately. In this embodiment, the substrate solution may be prepared prior to administration in a suitable concentration according to the administration route. It has been shown in the experiments of the present invention that neoplastic retroviral cells that express the linamarase gene have, upon introduction into tumour cells, the potential of causing a large bystander effect among neighbouring cells. The data obtained suggest a substantial killing effect that would counteract the poor estimates (1-10%) for in vivo retroviral infection of solid tumours. The experiments conducted in the present invention demonstrate that transduction of mouse or rat cells by retroviral vectors expressing the cassava linamarase gene, cas 5 (Hughes et al., 1992), drastically increases sensitivity of these cells to the cytotoxic effects of linamarin in vitro and in vivo. Neighbouring cells can also be affected by the cyanide released, thereby amplifying the killing effect. In the experiments of the present invention a therapeutic retrovirus was engineered by inserting the linamarase gene (cas 5) into the Moloney murine leukaemia based vector pBabepwro (Morgenstem and Land, 1990). The resulting recombinant retroviral plasmid carries the cas 5 gene expressed under the control of the 5' LTR (long terminal repeat) promoter and the selection gene pac (puromycin N-acetyl transferase) under the SV40 promoter. Amphotropic retroviral packaging cells ΨCRIP were grown in increasing amounts of linamarin to determine the sensitivity of these cells to the compound. Values up to 2 mg/ml of linamarin did not substantially decrease cell survival values. No cyanide was detected in any type of cell line tested (C6, ΨCRIP and 3T3) even after treatment with very high concentrations of 13 linamarin, showing the stability of the substrate and the absence of linamarase activity in these mammalian cells. The ecotropic murine packaging cell line ΨCRE was transfected by the retroviral plasmid vector pLlinSp and supernatants from ΨCRE lawns, were collected to infect ΨCRIP producing individual clones. Some clones showed a marked toxicity to the presence of linamarin, with estimated concentrations that resulted in 50% cytotoxicity (IC 50 ), of 7-60 μg/ml (Fig. 3a). Toxicity is due to cyanide production as measured by the Lambert method (Lambert et al 1975). In a similar way, rat glioblastoma C6 cells were infected with ΨCRE///7. The C6/t7 clones gave in general IC 50 50 times higher than ΨCRIP individual clones. Cells of the packaging cell line FlyA13 (human fibrosarcoma) have an intermedian IC 50 value of 195 μg/ml at pH 7.4 that drops to 77 μg/ml at pH 6.6 (Fig.3c). Other cell lines whose IC 50 were estimated showed values closer to C6 than to ΨCRIP (Table 1 ). The extraordinarily low IC 50 value estimated for ΨCRIP could be related to a very high linamarin permeability of these particular cells. Upon incubation with linamarin, cell extracts from C6//t7 produced even higher amounts of cyanide than ΨCRIP///7 suggesting a very efficient expression of the gene in these cells. Isolation of mRNA and Northen hybridization confirmed the colorimetric results. Furthermore, in the experiments of the present invention, the toxicity of sodium cyanide upon the cells was evaluated. It was found that glioblastoma tumor cell lines (C6, L9 or U373 MG) as well as one glioblastoma explant from a human patient, are 10 to 500 times more resistant to the toxic effect of exogenous sodium cyanide than ΨCRIP. This finding could be related to the observation that malignant tumor cells (C6) have a low oxidative respiration and a decrease in the number of mitochondria making them quite resistant to metabolic poisons such as KCN (Lichtor and Dohrmann 1986; Maduh et al. 1991 ), a property that can 14 be partly reversed in vivo by the low pH conditions maintained within the tumor. Neoplastic cells forming a tumor are exposed to a substantial and consistent decrease in extracellular pH from 7.4 to 6.6 in the case of glioblastoma, or lower for other type of tumors ( Hu et al. 1988; Gerweck and Seetharaman 1996) resulting in higher sensitivity (50% or greater) to the inhibitory effects of sodium cyanate. Nutrient deprivation due to poor vascularization within solid tumors is probably the major cause of this acidity, limited oxygen diffusion causes induction of anaerobic glycolysis and lactic acid acumulation. In vitro, cultures are normally maintained closer to pH 7.4 than 6.6; however lowering the culture pH to 6.6 reduced the IC 50 of the cell lines tested. The lower interstitial pH in tumors than normal tissues would result in a greater and specific sensitivity to the toxic effect of cyanide on tumor cells. This is very convenient because it will selectively increase the specificity of the therapeutic action towards inner tumor cells rather than normal surrounding cells. The type of euthanasic effect expected in the present system is rather different from the prototype bystander effect dependent on connexin expression and cell communication via gap junctions (Wygoda et al., 1997; Vrionis et al., 1997). Toxicity to the neighbours of a linamarase containing cell is due to cyanide release, and cyanide action does not require cell-to- cell contact or gap junctions. In order to determine the bystander effect, experiments were performed in which linamarase producer cells were mixed in a fifty/fifty initial cell ratio with non producer cells. The results of these experiments indicate that the bystander effect in this system is very efficient, being proportional to the amount of cyanide released by the producer cells and the lethal dosage for cyanide in neighbouring cells. Based on the encouraging in vitro data, the in vivo sensitivity of the system was evaluated using an intracerebral tumour model. Two types of 15 experiments were performed: in the first type, C6 cells already transfected with pLlinSp and selected for puromycin resistance, were inoculated into the animal brain and allowed to form a tumour. The glioblastoma were subsequently treated with linamarine. In the second type of experiments, brain tumours were induced by ordinary C6 cells, and ΨCRIP ///7-retroviral producing cells were inoculated into the growing tumour, 3 or 5 days later, tumours were expose to linamarin. C6//π transfected cells were inoculated into the white matter of the right cerebral hemisphere of Wistar rats. A few weeks later an intracraneal tumour was clearly visible by magnetic resonance imaging (around 70 mm 3 volume size). After linamarin delivery (a total of 2.4 mg) throughout an 8 day period the magnetic resonance image taken three weeks after the treatment showed a total absence of the tumoral mass. One of the animals having a similar size tumour, was treated with only 1 mg of linamarin. The animal finished the treatment but finally died as a consequence of the tumour, indicating a lower limit dosage for the prodrug. No apparent toxicity was observed by local administration of up to 9 mg of linamarin into normal rat brains in the same conditions as in the experimental animals. In the second type of experiments, C6 cells were inoculated into the right hemisphere of Wistar rats and allowed to develop into tumours. Three of the largest size tumour animals, with approximate volume sizes of were selected for further inoculation with ΨCRIP ///7-retroviral producing and local linamarine treatment. A total eradication of the tumour was achieved for the largest and the smallest glioblastomas. Control animals with tumours about the same volume sizes, treated only with linamarin (6 mg) in the same conditions as described earlier, died shortly afterwards. 16 The cured animals were well for several months after the treatment, with no apparent neurological damage, and no recurrence. With the present invention, it was possible for the first time to eradicate a rat glioblastoma of the dimensions described. Toxicity to normal brain tissue was estimated by direct inoculation of ΨCRIP///7 producer cells, followed by linamarin treatment. The obtained magnetic resonance images of the treated animals serve as a proof that the treatment itself does not produce appreciable damage to the brain of the animals. Transduction rate of tumour cells by retroviral vectors in vivo is usually in the range of 1-10% of the tumour cell population. An efficient bystander (good Samaritan or euthanasic) effect has been hypothesized to explain the total remission of tumours in animal models by the Hstklganciclovir system. Transfer of phosphorylated ganciclovir between cells via gap junctions appears as one of the best documented theories to explain the mechanism (Wygoda et al., 1997; Vrionis et al., 1997). Some tumour cells, like rat glioblastoma C6 cells for example, have lost the capacity to synthesize some of the connexin proteins and therefore the benefit from this important effect will be severely limited. The linamarase/linamarin enzyme/prodrug combination does not require cell-to-cell contact for its bystander effect, since evidence of cyanide in the cell culture medium indicates that it can cross cell membranes. Furthermore, the bystander effect expected in vivo should be greater than in vitro. The volatile character of cyanide makes it rather difficult to estimate the killing effect for surrounding cells in vitro because the compound dissipates into the medium and air faster than to neighbouring cells. On the other hand, in vivo, within a tightly packaged tumour, the cells nearest to the producers will be the first and main target of the cyanide released. Cyanide released within a tumour would also be more effective against neighbouring cells by the low extracellular pH increased responsive effect (Hu et al., 1988; Gerweck and Seetharaman, 1996). Cells outside the tumour having 17 extracellular pH values close to 7.4 would be rather resistant to the cyanide bystander effect. In the experiments of the present invention considerations concerning possible toxic effects have been taken into account. One of the aspects that needed to be considered is related to the possibility of linamarin breakdown in other parts of the body by alternative enzymes or situations; and the other aspect concerns to the putative damage of healthy brain tissue surrounding the tumour. Regarding the first issue, there exists one β-glucosidase in mammals which has broad specificity and an obscure physiological function which enables the organism to hydrolize toxic plant glycosides found in the diet. In general, the richest source of this cytosolic β-glucosidase in most vertebrates appears to be the liver and the small intestine except in the mouse and in the rat, where kidney is the richest source of the enzyme (Glew et al., 1993; Lima Gόrniak et al., 1993; Newmark et al., 1981 ). In experiments of the present invention it was confirmed that cyanogenic glycosides, such as linamarin, are not hydrolyzed by cultured mammalian cells, rat brain tumour or normal tissue and tumour or normal human brain tissue. In addition, it has to be considered that gut microbial flora would be able to metabolize, to a certain extent, some cyanogenic glycosides such as amygdalin or linamarin, in the intestinal lumen by bacterial β-glucosidases, resulting in the release and absorption of hydrogen cyanide. Female hamsters given an oral dose of 108 mg of linamarin/kg body weight show signs of cyanide poisoning causing death in 18% of the animals (Frakes and Sharma, 1986; Hernandez et al., 1995). The toxicity is much lower in animals receiving similar doses intravenously and is practically absent when administered intraperitoneally (Glew et al., 1993; Lima Gόrniak et al., 1993; Newmark et al., 1981). Local administration of linamarin is as safe or probably safer than intraperitoneal delivery to avoid gastrointestinal interference and it is clear from the in vitro and in vivo tests of the present 18 invention that the are well below dangerous concentrations of the prodrug. One alternative way for the oral administration of cyanogenic glucosides is to pre-treat the animals (or patients) with especially extended spectrum antibiotics to reduce gastrointestinal flora and be less susceptible to the compound toxicity (Newton et al., 1981 ; Padmaja, 1995; Halstrom and Moller, 1945). In many tropical communities a potential health hazard is recognized by the consumption of leaves and roots of the cassava plant (Manihot esculenta Crantz). When mechanical damage of tissues of this plant occurs, the cyanogenic glucoside (linamarin) is exposed to the action of the endogenous plant linamarase and α-hydroxynitrile lyase and hydrocyanic acid is released. Cooking/boiling, drying, soaking and fermentation are the usual processing techniques to inactivate linamarase and breakdown linamarin before consumption, the occasional poisoning reported in humans is generally due to inadequately processed leaves or roots from cassava. Cyanide doses of 50 to 100 mg are reported to be lethal to adults (Newton et al., 1981 ; Padmaja, 1995; Halstrom and Moller, 1945). This would be equivalent to 3 mg /kg of sodium or potassium cyanide and to 500 to 1000 mg of linamarin. The dose of linamarin potentially needed to eradicate a human tumour of about 4 cm diameter, is below dangerous concentrations of the prodrug and would be administered over a week time period. The second concern is related to the damage that cyanide release could cause in healthy parts of the brain. Cyanide can rapidly enter the plasmatic membrane and slowly accumulate in mitochondria and membrane elements of the neuronal cell, in a proportional way to the concentration available. An advantage for applying the present invention is the local extracellular lower pH created within tumours and the increased susceptibility of such condition to cyanide. Physiological extracellular pH values in surrounding normal cells will make them rather resistant to the cyanide bystander effect. In one of the in vivo experiments that have been performed, a large majority (if not all) of tumoral cell had incorporated the linamarase gene and 19 therefore could process linamarin to glucose, acetone and cyanide (C6 transfected with pLlinSp). Nevertheless magnetic resonance images show a fibrous scar comparable to the one obtained previously with the Hstklganciclovir system (Culver et al., 1992; Ram et al., 1993; Izquierdo et al., 1995; Izquierdo et al., 1997) or to the C6 infected in vivo by retroviral- lin, indicating the absence of a devastating effect in the brain. All the surviving animals were healthy and well several months after the treatment, with no apparent signs of neurological damage and no recurrence of the tumour. Glioblastoma is a particularly nasty tumour because of the infiltrating characteristic of its cellular constituents, recurrence tumours are generally not farther than 2 cm from the primary site; it maybe difficult to eliminate infiltrated neoplastic cells without damaging some surrounding brain cells. In the past it has not been possible to cure rat glioblastomas larger than 150 mm 3 volume size using the Hstklganciclovir system (Izquierdo et al., 1997), implying that the new linamarase/linamarine approach is more efficient. When the Hstklganciclovir system was applied to a small number of patients by others and the inventors (Culver et al., 1994; Izquierdo et al., 1996; Roth and Christiano, 1997), a genuine tumour size reduction in about 30% of the treated patients was generally acknowledged, but no total disappearance of the residual tumour has been achieved despite several consecutive treatments. Survival time was increased in many cases but recurrence finally killed the patient. The method of the invention has yielded results in animal experiments that can be considered highly encouraging. Brief description of the figures: Fig. 1. Enzymatic action of linamarase. Schematic representation of linamarin breakdown by linamarase into glucose and acetone cyanohydrin and further spontaneous decomposition of this product releasing free cyanide and acetone Fig. 2. Recombinant retroviral plasmid pLlinSp. 20 Fig. 3. Sensitivity to linamarin as a percentage of cell survival. A) ΨCRIP sensitivity to linamarin compared to one clone of retroviral Im ' infected ΨCRIP B) C6 sensitivity to linamarin compared to one clone of retroviral Im ' transfected C6 C) FLYA13 sensitivity to linamarin compared with one clone retroviral Im ' infected FLYA13 Fig. 4 Gadolinium enhanced magnetic resonance views in sagittal (a^b,) and axial (a 2 , b 2 ) projections of a rat brain tumor, before (a 1 ? a 2 ) and after linamarin treatment (b 1 t b 2 ) Fig. 5 Gadolinium enhanced magnetic resonance views in axial (a- X>7) sagittal (a 2 , b 2 ) and coronal (a 3 , b 3 ) projections of a rat brain tumor, before (a^ a 2 a 3 ) and after mouse retroviral producer cells ΨCRIP//t? inoculation and linamarin treatment (b 1 τ b 2„ b 3 ). Fig. 6 Axial T 2 ( a.,, b.,) and sagittal T, (a 2 , b 2 ) magnetic resonance views of two control animals Fig. 7 Cyanide presence in tissue homogenates. (a) ΨCRIP/t ' n (b) C6 (c) C6 Im ' (d) Normal rat brain (e) Normal human brain (f) Human glioblastoma. In the experiments of the invention, the following materials and methods were used a) Construction of a recombinant plasmid encoding linamarase 21 The plasmid pCas 5 (Hughes et al., 1992) was opened with the restriction enzyme Sacll and filled with Klenow, then cut with Sal I to generate the fragment blunt-Sal I. The fragment was inserted in pBabe Puro (Morgenstern and Land, 1990), opened via Sna B1/Sal I restriction enzymes (Sna B1 gives blunt end, so the insert was oriented by the Sal enzyme). The recombinant (Fig. 2) is called pLlinSp and can be selected by its resistance to puromycin (1-2,5 μg/ml puromycin) under the promoter of SV40; linamarase is under the LTR promoter. The prokaryotic moiety allowed for growing the recombinant plasmid in bacteria prior to transduction of mammalian cells. b) Cell lines and culture Cells: C6 (ATCC CLL 107) and L9 (rat glioblastoma; Weizsaecker et al., 1981 ), MGU 373 (U-373 MG; ATCC HTB-17; human glioblastoma), NIH-3T3 (ATCC CRL6331 , ΨCRE (ecotropic packaging line; Danos and Mulligan, 1988) ,ΨCRIP (amphotropic packaging line; Danos and Mulligan, 1988), Ag9181 (human fibroblast), and FLYA13 (Cosset et al., 1995) were grown in Dulbecco modified Eagles medium (DMEM). Some were supplemented with 10% heat inactivated fetal calf serum: C6, L9, MGU 373, Hs683, Ag9181 ) and the rest in calf serum. Cells were cultured at 37°C, 6% CO2 and 97% relative humidity. The pH of DMEM was artificially lowered to pH 6.6 in some experiments by decreasing the NaHCO3 concentration from 3.7 g/L to 0.7 g/L. c) Transfections and infections The retroviral plasmid pLlinSp was introduced to ecotropic ΨCRE packaging cells using the AVET technique for DNA delivery (Cotten, et al., 1992; Wagner, et al., 1992; von Rϋden, et al., 1995), and clones were selected by puromycin (48h later). The same plasmid was introduced to ecotropic ΨCRE packaging cells. Transfections were essentially performed as described previously (von Rϋden, et al., 1995). 4 μg pLlinSI in 150 μl HBS were used to transfect 200000 cells during 4 hours at 37°C. Thereafter, the medium containing the transfection complex was replaced 22 by fresh culture medium, containing 10 μmol dexamethason. 48 h later, for selection, 2 mg/ml puromycin was added to the cells. Two weeks later the colonies were isolated by using cloning rings. Viral supernatants from lawns, collected 48 h after transfection, were used to infect amphotropic ΨCRIP packaging cells (ΨCRIP Im ' ). After selection with puromycin (1 ,5 μg/ml) colonies were isolated using cloning rings. Clonal lines were assayed for vector titer, using NIH 3T3 cells. Best titers were 3-6 x10 5 c.f.uJml. Cell lines shown in table 1 were infected with ΨCRIP//77 supernatants (U373MG, Hs683, Ag9181 , Sk-N-MC and FLYA13) or ΨCRE/zn supernatants (C6 and 3T3), to obtain Im ' + cell lines. After selection with puromycin, colonies were isolated and linamarin sensitivity was estimated. d) Labeling of cells Cells were labeled essentially as described previously (Tamura et al., 1997). Two milligrams of Dil (1 ,1 '-dioctedecyl-3,3,3',3'-tetramethyl- indocarbocyanine perchlorate) were dissolved in 1 ml of ethanol, and 50 μl of the solution was added to 5ml of cells in a 6-cm culture dish. After one hour incubation at 37° C, the cells were trypsinized, washed twice with 4 PBS, and resuspended in DMEM at a concentration of 5 x 10 cells/μl. e) Linamarin sensitivity and cyanide toxicity assays in culture Cells were seeded in a flat sided tube ( provided with screw cap to prevent 5 HCN release), at a density of 10 cells/tube. Following 24 h of incubation, increasing linamarin concentrations were added. Two days later, the percentage of surviving cells was determined to evaluate drug toxicity. Cells were incubated for 2.5 h with the mitochondrial substrate MTT (3-[4,5-dimethylthiazo-2-yl]-2,5-diphenyltetrazolium bromide, 200 μg/ml) which can be transformed in formazan only by living cells. Then the medium was removed and 3 ml of DMSO (dimethylsulfoxide) were added to dissolve formazan. Samples were measured spectrophotometrically at 540 nm after 10 min. 23 Cyanide toxicity was assayed as described previously except by the addition of increasing amounts of NaCN to the cultures. f) Measurement of linamarase activity and cyanide presence Linamarase activity and cyanide presence were determined by the Lambert method (Lambert et al., 1975) with the following modifications: 1 ml of the succinimide/N-chlorosuccinimide reagent was added to the flat tubes, containing the cells with differents amounts of linamarin (48 h later), cultured in 1 ml of DMEM without phenol red. Then 1 ml of the barbituric acid/pyridine reagent was added, mixed and the absorbtion at 592 nm was read after 10 min (calibration curves were made with NaCN). g) Tissue homogenates The tissues were minced and homogenized in five volumes of a medium containing sucrose (0.25 M), ethylenediamine tetraacetate (1 mM) and Tris HCI buffer (0.01 M, pH 7.2) supplemented with the protease inhibitor phenylmethylsulfonyl fluoride (PMSF) 1 mM (all procedures were carried out to 1-4°C). The homogenate was centrifuged (600xg, 10 min).The supernatant fraction was stored at -70°C (Glew, R. et al. 1976). The protein content was measured by BCA Protein Assay. The experiment in Fig. 7 was carried out at 37°C, 4 hours of incubation with 1 mM linamarin, 0.2 mg protein, pH 7.4 , in a total volume of 1 ml. Cyanide presence was determined by the Lambert method (Lambert et al., 1975). h) Wistar rat brain inoculation of C6 lin transfected cells and linamarin administration. The procedures applied were essentially those described previously (Izquierdo et al., 1995, Izquierdo et al., 1997). Wistar male rats weighing 250-300 grams were anesthetized with a mixture of ketamine (50 mg/ml), valium (5 mg/ml) and atropin (1 mg/ml) in a 5:4:1 ratio, by vol. at a dose of 0.3 ml/100 g of body weight before placing them in a stereotaxic apparatus. C6 glioma cells or the same cells transfected with the pLlinSp plasmid and selected by puromycin resistance were injected at a concentration of 24 10^ cells/μl in complete PBS (with calcium and magnesium) supplemented with 0.1 % glucose. With the aid of the manipulating arm of the stereotaxic apparatus a total of 10 μl were introduced, over a 5 minutes interval, into the fronto-parietal lobe of the right cerebral hemisphere (4 mm to the right from Bregma and 4.5 mm deep from the skull) using a 10 μl Hamilton syringe connected to a 26 gauge needle; the needle was kept in place 3 min. before and after injection. ΨCRIP lin producer cells were inoculated at the same stereotaxic coordinates as the C6 cells except in a very large tumour in which an additional tumour site was also reached, cells were delivered at different heights along the tumour. All rats received tetracycline into the drinking water (approximately 75 mg/kg) and dexamethasone (1 mg/500 ml) for three days before and one week after surgery. The linamarin treatment was started after magnetic resonance visualization of the tumour and size estimation. The prodrug was administered by a brain infusion cannula placed at the site of tumour inoculation and fixed to the skull using dental cement; a small stainless steel screw acts as anchor to secure the cannula and the dental cement. The brain infusion cannula was then connected subcutaneously (s.c.) to an osmotic pump (alzet® model 2001 ) which delivers 1 μl/h for 7 or 8 days. The osmotic pump was filled with the appropriate amount of linamarin in PBS . j) Magnetic resonance imaging Sagittal and coronal views of the skull were made with a Cpflex small coil arround the animal. T1 sagittal and axial proyection images were obtained using spin eco, with TR 650, TE 17.0/1 and adquisition 2, slide 3 mm. Field of view 50x100. Matrix 256x512 high resolution sequence post enhancement with gadolinium (0,2 ml/Kg) with magnetization transfer MT. k) Estimation of tumour volume Volumes of tumours were estimated as follows. If [0,L] is a parametrization of the tumour interval in the sagittal section and C (x), <x < L denotes the area of the corresponding coronal section, then we have the exact formula for the volume of the tumour 25 V= j o L C(x)dx, which yields the approximation N 4 N-l In practice, however, the volume was needed to be estimated with the knowledge of only one section of each type. These tumours are ussually adapted to the rat's head geometry: they tend to be rather symmetric (elliptical) in the coronal sections, while they have long and irregular sagittal shapes. Given two "well taken" sections, coordinates were introduced: ^ S(x) <EZd o 0 x y y sagittal view coronal view A plausible model for the tumour is given by the volume generated placing sections homothetical to C 0 at each position x with a ratio S(x)/d 0 . Therefore the area ratio is [S(x)/d 0 ] 2 C. L ^f . LVf Where: C 0 = area of the given coronal section. d 0 = diameter of that coronal section in the z (sagittal) direction. S(x) = length of the sagittal section in the z direction at position x. N was to chosen taking into account the shape of the sagittal picture. 26 It is possible to obtain similar formulas changing sagittal by axial. However, taking into consideration the standard geometries of these tumours, the axial section seems to be less significative than the sagittal one. In general, it was expected to have an estimate from below using the axial-coronal approach. Example 1 Construction of recombinant retrovirus carrying the linamarase gene A therapeutic retrovirus was engineered by inserting the linamarase gene (cas 5; Hughes et al., 1992) into the Moloney murine leukemia based vector pBabept/ro (Morgenstern und Land, 1990).The resulting recombinant retroviral plasmid carries the cas 5 gene expressed under the control of the 5' LTR (long terminal repeat) promoter and the selection gene pac (puromycin N-acetyl transferase) under the SV40 promoter (Fig. 2). The estimated size of the retroviral plasmid was 6.9 kilobases (kb) and was designated pLlinSp (plasmid, LTR, linamarase, SV40 promoter, puromycin N-acetyl transferase). The recombinant pLlinSpL can be selected by its resistance to puromycin (1-2,5 μg/ml puromycin) under the promoter of SV40; linamarase is under the LTR promoter. Example 2 Assessment of linamarin sensitivity of retroviral infected cells and toxicity of sodium cyanide for the cells a) Linamarin sensitivity as a percentage of cell survival. i) ΨCRIP sensitivity to linamarin was compared to one clone of retroviral lin infected ΨCRIP, at increasing linamarin concentrations and two pH values. The initial number of cells was 10 5 in all cases. Following 24 hours incubation, increasing linamarin concentrations were added. Two days later, the percentage of surviving cells was determined to evaluate drug 27 toxicity. Clonal lines were assayed for vector titer, using NIH 3T3 cells. Best titers were 3-6 x10 5 c.f.uJml. The results of the experiment are shown in Fig. 3a. Some clones showed a marked toxicity to the presence of linamarin, with estimated concentrations that resulted in 50% cytotoxicity (IC 50 ), of 7-60 μg/ml. ii) C6 sensitivity to linamarin was compared to one clone of retroviral lin infected C6, at increasing linamarin concentrations and two pH values. The initial number of cells was 10 5 in both cases and the estimated IC 50 for this C6//π clone is 345 μg/ml at pH 7.4 and 202 μg/ml at pH 6.6. The results of the experiment are shown in Fig. 3b. iii) FLYA13 sensitivity to linamarin was compared with one clone retroviral lin infected FLYA13 at increasing linamarin concentrations and two pH values. The estimated IC 50 for this FLYA13///7 clone is 195 μg/ml at pH 7.4 and 77 μg/ml at pH 6.6. The results of the experiment are shown in Fig. 3c. The sensitivity of other cell lines, whose IC 50 was determined in the same manner, is shown in table 1. b) Cyanide toxicity Next, the toxicity of sodium cyanide upon the cells was evaluated. The chemical compound itself is an order of magnitude (ten times) more toxic than linamarin in both ΨCRIP and C6. This result was interpreted as showing that only 10% of the linamarin applied to the medium ends up as cyanide. It was also found that glioblastoma tumour cell lines (C6, L9 or U373MG) as well as one glioblastoma explant from a human patient, are 10 to 500 times more resistant to the toxic effect of exogenous sodium cyanide than ΨCRIP (and therefore 3T3, from which ΨCRIP have derived), a property that can be partly related to the pH conditions maintained in vitro and in vivo. 28 Example 3 Infectivity and cyanide bystander effect In order to determine the bystander effect in this system, experiments were performed in which linamarase producer cells were mixed in a fifty/fifty initial cell ratio with non producer cells. One of the mixtures was the packaging cell line ΨCRIP infected with the retrovirus carrying lin (ΨCRIP//n) plus normal C6 cells labeled red with Dil (1 ,1'-dioctedecyl- 3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (Tamura, et al., 1997). In the presence of 2.5 mg/ml of linamarin, both types of cells, (ΨCRIP///7 and C6 labeled) died; while 86% of C6 alone, will survive at linamarin concentrations of 10 mg/ml. Lower concentrations of linamarin (100 μg/ml) affect only partially the viability of either cell type in this mixture, despite the fact that ΨCRIP alone would be very sensitive to this concentration (Fig. 3a). These results were interpreted as showing that C6 cells, which are more resistant to cyanide than ΨCRIP, are trapping sufficient cyanide to lower the free concentration in the medium. Mixtures of retroviral carrying lin infected ΨCRIP with either ΨCRIP or 3T3 labeled in red (ΨCRIP//π /ΨCRIP red or ΨCRIP/tn /3T3 red) were devastated at the lower concentration of linamarin (100 μg/ml) due to the greater sensitivity of red labeled cells to cyanide released by producer cells. The results indicate that the bystander effect in this system is very efficient, being proportional to the amount of cyanide released by the producer cells and the lethal dosage for cyanide in neighbouring cells. Example 4 Curability of rat glioblastoma in vivo Based on the encouraging in vitro data, the in vivo sensitivity of the system was evaluated by using an intracerebral tumour model. Two types of experiments were performed: in the first type, C6 cells already transfected 29 with pLlinSp and selected for puromycin resistance, were inoculated into the animal brain and allowed to form a tumour. The glioblastoma would be subsequently treated with linamarin. In the second type of experiments, brain tumours were induced by C6 ordinary cells and CRIP //n-retroviral producing cells were inoculated into the growing tumour, 3 or 5 days later, tumours were expose to linamarin. A total of 10 C6///7 transfected cells were inoculated, in the first type of experiments, into the white matter of the right cerebral hemisphere (4 mm to the right of Bregma and 4.5 mm deep from the skull) of three Wistar rats. A few weeks later (2 to 4), an intracraneal tumour was clearly visible by 3 magnetic resonance imaging (around 70 mm volume size) (Fig. 4 ai ,a2). A day later, linamarin was administered by a direct brain-tumour infusion cannula connected by a catheter tube to a osmotic mini-pump. A total of 2.4 mg of linamarin were delivered throughout an 8 day period. Release was at a constant rate of 1 μl per hour (12.5 μg of linamarin/μl). Fig. 4 shows the gadolinium enhanced magnetic resonance views in sagittal (a^b and axial (a 2 , b 2 ) projections of a rat brain tumor, before (a,, a 2 ) and after linamarin treatment (b 1 ? b 2 ). A brain tumor (70 mm 3 volume size, a^ a 2 arrow) of glioblastoma-C6//n transfected cells, has been eradicated completely (b 1 b 2 arrow) after one week treatment with constant local release of 12,5 μg of linamarin per hour. The magnetic resonance image taken three weeks after the treatment (Fig. 4 bi ,b2), shows a total absence of the tumoural mass and in its place a residual hypointense image at the site of injection, interpreted as a fibrous scar or as a small rest of haematoma with internal haemosiderin. One of the animals having a similar size tumour, was treated with only 1 mg of linamarin. The animal finished the treatment but finally died as a consequence of the tumour, indicating a lower limit dosage for the prodrug. In the second type of experiments, 10 C6 cells were inoculated into the right hemisphere of 10 more Wistar rats and allowed to develop into 30 tumours. Three of the largest size tumour animals (volume estimation shown in materials and methods), with approximate volume sizes of 3 3 3 400 mm , 600 mm and 1000 mm (Fig.5 a-| ,a2, a3), were selected for further inoculation with ΨCRIP //t7-retroviral producing cells (7x10 cells for the two smallest and 10 cells for the largest) and linamarine local treatment. The murine retroviral producer cells (ΨCRIP//π) had a titer of 5 5x10 c.f.u. (colony forming units) and were inoculated in a single place (the same used for C6 delivery) in all but the biggest size tumour in which an additional small craniotomy was made at a distal side within the tumour, in order to increase the potential access of retrovirus to all neoplastic cells. A few days later (three for the largest tumour and 5 for the others), 6 mg of linamarin in 200 μl were administered at a constant rate of 1 μl per hour (osmotic pump delivery). A total eradication of the tumour was achieved for the largest (Fig. 5 bi ,b2, 3 ) and the smallest glioblastomas; the medium size tumour animal, on the other hand, finished also the treatment but died a month later. Examination of the dead animal brain showed an haemorrhage within the tumour remains being probably the cause of death. Fig. 5 shows the gadolinium enhanced magnetic resonance views in axial (a- ,b-,) sagittal (a 2 , b 2 ) and coronal (a 3 , b 3 ) projections of a rat brain, before (a^ a 2 a 3 ) and after mouse retroviral producer cells ΨCRIP/tn inoculation and linamarin treatment (b 1 ? b 2, b 3 ). As can be seen, a very large brain tumour (approximately 1000 mm 3 volume size, a-i a 2 a 3 arrows) of rat glioblastoma C6 cells, has been eradicated completely as shown in the three magnetic resonances (bi b 2 and b 3 arrows). These images show the appearance of the living brain after murine therapeutic retroviral producer cells inoculation into the tumour (ΨCRIP/tn) and one week treatment, three days later, with constant local release of 30 μg of linamarin per hour. Two control animals with tumours of 220 and 140 mm volume sizes, that have been treated only with the prodrug (6 mg) in the same conditions as described earlier, died shortly afterwards. 31 The cured animals were well, with no apparent neurological damage, and no recurrence. Some of the treated animals showed no apparent neurological damage for a period up to six months, but developed a recurrence later. This was only seen in very large tumors (over 500 mm 3 in size) in approximately 50% of the cases. In all recurrent cases the tumoral mass had crossed the interhemispheric fissure and reached the ventricles. At this point tumoral cells could be distributed via cerebrospinal fluid to parts of the brain quite distant from the original inoculum. Indeed some of the recurrent tumors were multiple and very far from the original site. Fig. 6 shows the axial T 2 ( a^ b and sagittal T., (a 2 , b 2 ) magnetic resonance views of two control animals treated in the right hemisphere with 5x10 6 mouse lin- retroviral producer cells and then with 4 (a) and 6 mg of linamarin (b). The left hemisphere image can be considered as a control of untreated brain tissue. Toxicity to normal brain tissue was estimated by direct inoculation of ΨCRIP///? producer cells, followed by linamarin treatment (2.5, 4 or 6 mg) in the absence of tumor cells. The magnetic resonance of the treated animals did not show any large damage, but a small scar similar to the one normally present in brains after the tumor has been eradicated (Fig. 6). Two types of relaxation time (in Tesla, T) images were analyzed: T and T 2 in order to visualize lesions other than tumoral. In general, ^-weighted images primarily provide anatomic information, T 2 -weighted images are more sensitive to early pathologic changes in most tissues. No lesion related to neoplasm, edema, ischemia, infection, demyelination or trauma; was seen animals only exhibited small MRI abnormalities and no gross lesions. These results may be interpreted as a proof that the treatment itself does not produce appreciable damage to the brain of the animals. Example 5 32 Toxicity experiments β-glucosidase and linamarase activities were estimated in several tissues from rats and humans. It was expected to obtain β-glucosidase but no linamarase activity in most tissues assayed . The tissues tested from the rat were: normal brain, glioblastoma C6, liver, kidney and gut. Normal brain and cells from a glioblastoma explant were tested from humans. The tissues were minced and homogenized in five volumes of: 0.25 M sucrose, 1 mM ethylenediamine and 0.01 MTris-HCI pH 7.2, and 1 mM of phenylmethylsulfonyl fluoride as protease inhibitor; at 1 - 4°C. The homogenate was centrifuged (10 min. 600xg) and supernatants stored at -70°C. The protein content was measured by BCA protein assay (Pierce). The enzyme determinations of glucosidase and linamarase activity were carried out at 37°C. Glucosidase activity was measured as release of p-nitrophenol (yellow, absorbance at 400 nm) from p-nitrophenyl-β-D- glucoside. Linamarase activity was estimated as cyanide release measured by the Lambert method (Lambert et al., 1975). In this assay flat side tubes with screw-cap to avoid cyanide release were used. 0.1 , 0.2, 0.5 and 1 mg of tissue protein were estimated. The amounts of linamarin were 1 mM, 5 mM, 10 mM and 20 mM. Reaction was measured at two pH values pH 6 and the physiological pH 7.4. Positive control were transfected Cδlin cells and transfected YCRIP-lin cells. All human tissues gave negative results for linamarase activity. In rats only the kidney gave low but detectable levels of cyanide production. Fig. 7 shows the presence of cyanide in tissue homogenates. (a) ΨCRIP//π (b) C6 (c) C6 lin (d) Normal rat brain (e) Normal human brain (f) Human glioblastoma. 33 Table 1. Sensitivity of different cell lines to linamarin Cell line % of survival IC 50 μg/ml Linamarin cells with lin + clones 1 mg/ml Linamarin Best Mean clone value" C6 100 a 345 407 Rat glioma U-373 MG 80 145 236 Human Glioblastoma Hs 683 80 230 288 Human Glioma Sk-N-MC 80 183 215 Human Neuroblastoma FLYA13 88 a 195 382 Human Amphotropic Packaging Cells ΨCRIP 85 7 32 Murine Amphotropic Packaging Cells NIH 3T3 90 a N.D. N.D. Murine Fibroblasts Ag 9181 89 a N.D. N.D. 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Claims: 37 Claims 1. Use of a nucleic acid molecule encoding a glycosidase together with a non-toxic cyanogenic glycoside substrate which upon degradation by the glycosidase yields hydrogen cyanide, for the preparation of a medicament for separate administration of the two interacting components in the treatment of brain tumours in animals or humans. 2. The use according to claim 1 , wherein the glycosidase is a glucosidase. 3. The use according to claim 2, wherein the glucosidase is a β-glucosidase. 4. The use according to claim 3, wherein the β-glucosidase is linamarase. 5. The use according to any one of claims 1 to 4, wherein the nucleic acid molecule is a DNA molecule. 6. The use according to any one of claims 1 to 5, wherein the nucleic acid molecule is a DNA or an RNA molecule that is carried by a viral vector. 7. The use according to claim 6, wherein the vector is a retroviral vector. 8. The use according to claim 7 wherein the viral titer is >10 6 colony forming units. 38 9. The use according to claim 7, wherein the medicament contains packaging cells that have been infected with retroviral vectors. 10. The use according to claim 9, wherein the medicament contains >10 9 cells. 11. The use according to claim 6, wherein the viral vector is an adenoviral vector. 12. The use according to claim 6, wherein the viral vector is a Herpes Simplex vector. 13. The use according to claim 5, wherein the DNA molecule is carried by a plasmid. 14. The use of claim 13, wherein the DNA plasmid is contained in a liposome. 15. The use according to claim 1 , wherein the substrate is a cyanogenic monosaccharide. 16. The use according to claim 3 and claim 15, wherein the monosaccharide is glucose. 17. The use according to claim 16, wherein the substrate is linamarin. 18. Kit for the treatment of brain tumours, wherein a first container contains the preparation with the nucleic acid molecule encoding the glycosidase, and wherein a second container contains the cyanogenic substrate. 19. The kit of claim 18, wherein the glycosidase is linamarase and the substrate is linamarin. 39 20. The kit of claim 18 or 19, wherein the first container contains a preparation of retroviral vectors. 21. The kit it of claim 20, the first container contains a preparation of producer cells that have been infected with retroviral vectors. 22. The kit of claim 18 or 19, wherein the first container contains a preparation of adenoviral vectors. 23. The kit of claim 18 or 19, wherein the first container contains plasmid DNA.

Inventors: CORTES MARIA LUISA, IZQUIERDO MARTA, CORTES, MARIA, LUISA, IZQUIERDO, MARTA
Applicants: BOEHRINGER INGELHEIM INT, CORTES MARIA LUISA, IZQUIERDO MARTA, BOEHRINGER INGELHEIM INTERNATIONAL GMBH, CORTES, MARIA, LUISA, IZQUIERDO, MARTA
Application year: 1999
Main classification: C12N9
IPC codes: A61K9, A61K31, A61K35, A61K38, A61K48, A61P35, C12N7, C12N9, C12N15