Transformation of rabbit lymphocytes by an Epstein-Barr virus-related herpesvirus from Macaca arctoides.

Herpesvirus Macaca arctoides (HVMA), an Epstein-Barr virus (EBV)-related herpesvirus of macaque origin, induces malignant lymphomas in rabbits. To get more insights into the oncogenesis of the EBV/HVMA infection the aim of the present study was to prove the in vitro transforming ability of HVMA for rabbit lymphocytes as well as human umbilical cord blood lymphocytes. As a result, B-cell transformation could be demonstrated after infection with HVMA in all mononuclear cell samples of 20 rabbits. The transformation was evaluated microscopically and confirmed by the expression of EBV-related nuclear antigens. The transforming activity led to the establishment of permanent rabbit lymphoblastoid cell lines cultured up to more than 90 passages. The cell lines contained EBV-like HVMA-DNA. Interestingly, the transformed rabbit lymphocytes showed chromosomal abnormalities with a subtetraploid karyotype. The low extent of lytic cycle-dependent expression of virus capsid antigen in the established cell lines increased after treatment with the inducing agents iododeoxyuridine and mitomycin C. In contrast, no transformation could be induced after exposure of human umbilical cord blood lymphocytes to HVMA. The permanent rabbit lymphoblastoid cell lines provide a model for further studies on the role of EBV/HVMA in oncogenesis of lymphomas. In addition, it might be suitable for testing potential antiviral compounds in vitro.

Cytogenetic detection of a trans-species bystander effect: induction of sister chromatid exchanges in murine 3T3 cells by ganciclovir metabolized in HSV thymidine kinase gene-transfected Chinese hamster ovary cells.

Due to the very limited transduction capacity of hitherto available vectors, the success of gene therapy of tumours by means of suicide genes has so far essentially depended on the transfer of toxic drug metabolites from transduced (metabolizing) cells to adjacent non-transduced cells via gap junctions (bystander effect). Most experimental systems for the detection of a bystander effect yield net data of cell losses and cannot differentiate between killed transduced versus killed bystander cells. Here we report on metabolic cooperation in vitro between CHO cells stably transfected with the thymidine kinase gene of herpes simplex virus type-1 (HSVtk) (metabolizing cells) and Swiss albino 3T3 cells (bystander cells). Both cell lines are readily distinguishable by single cell and colony morphology and by their chromosomal constitution. While 3T3 cells cultured alone were refractory to the antiviral drug ganciclovir (GCV), co-culture with CHO-HSVtk(+) cells led to a dramatic reduction in plating efficiency as well as to a 4-fold increase in sister chromatid exchange rates induced by very low GCV concentrations in the 3T3 bystander cells. The modulator of gap junctional cooperation, all-trans retinoic acid, caused a strong augmentation of the bystander effect, while 18alpha-glycyrrhetinic acid, an inhibitor of gap junctional communication, drastically diminished the toxicity of GCV in the bystander cells. Whereas CHO-HSVtk(+) cells showed a distinct immunoreactivity for connexin43 in the cell membranes, 3T3 cells were almost negative. The co-culture system described here allows unequivocal distinction between metabolizing and bystander cells. In this way, mechanistic aspects of the transfer of genotoxic/cytotoxic metabolites to cells, which per se are unable to form them, become accessible to investigation.

Establishment and cidofovir sensitivity of a cell line from a heart transplant recipient with multiple cutaneous tumors.

A new cell line, designated as Tuwei00, is described. It originated from an Epstein-Barr virus-positive skin tumor biopsy of a heart transplant recipient, whose numerous cutaneous neoplasms were treated with the antiviral drug cidofovir what caused at least transient remissions. The cell line was established in vitro and maintained for more than 70 passages. Cells of early passages were characterized by a slower growth, the inability to form colonies and a higher sensitivity to cidofovir. After overcoming a crisis, the cells grew faster, to a higher density and were able to form adherent colonies from single cells as well as colonies in soft agar. Chromosome analysis showed diploidy/hyperdiploidy at the earlier and hypodiploidy at the later passages. Sensitivity to cidofovir was distinctly higher in early passages of Tuwei00 cells than in later passages and was characterized by distinct decline of cell survival after long term cidofovir exposure. Established normal human keratinocytes, HaCaT cells, which were checked for comparison, showed a low cidofovir sensitivity similar to late passage Tuwei00 cells.

DNA polymerase beta mediates protection of mammalian cells against ganciclovir-induced cytotoxicity and DNA breakage.

The efficacy of suicide herpes simplex virus-1 thymidine kinase (HSVtk)/ganciclovir (GCV) gene therapy is often limited by intrinsic resistance of tumor cells. Here we show that repair of GCV incorporated in DNA is a factor involved in GCV resistance. A protective role of DNA repair in GCV-induced cell killing is supported by the following findings: (a) GCV-exposed Chinese hamster ovary-HSVtk cells exhibited both reduced repair of GCV and cloning efficiency in the presence of a specific polymerase beta (beta-pol) inhibitor, prunasin; (b) DNA beta-pol-deficient mouse fibroblasts were more sensitive to the cytotoxic, apoptosis-inducing, and genotoxic (DNA breakage and chromosomal aberration-inducing) effects of GCV as compared with wild-type and beta-pol-complemented cell lines; (c) methoxyamine, an inhibitor of beta-pol-dependent short-patch base excision repair, sensitized wild-type and complemented beta-pol cells to GCV, whereas it had no effect on the sensitivity of beta-pol-null cells to GCV. Because methoxyamine-mediated sensitization of beta-pol wild-type and beta-pol-complemented cells to GCV did not reach the level of null cells, we suggest that both beta-pol-dependent short- and long-patch base excision repair are involved in protection of cells to GCV. Some implications for HSVtk/GCV gene therapy are being discussed.

Genetic risks of antiviral nucleoside analogues--a survey.

The available informations on the genotoxic effects in experimental systems of the antiherpesvirus nucleosides aciclovir, penciclovir, ganciclovir, brivudine and cidofovir as well as of the antiretrovirals zidovudine (AZT), lamivudine, zalcitabine (ddC), didanosine and stavudine are reviewed. Furthermore, data on carcinogenic activity of these drugs in laboratory rodents are compiled. Most nucleoside analogue antivirals induce chromosomal aberrations but are inactive in gene mutation assays. Carcinogenicity findings in mice and rats are variable but clearly positive for AZT and ddC. The possible mechanisms by which these agents may cause damage in the genetic information are still largely hypothetical, and experimental findings do not permit relevant extrapolations to the situation in man. There is no conclusive evidence that any of the drugs caused tumours in humans. The use of nucleoside analogues in antiviral therapy remains a pragmatic option that seems justified by risk/benefit assessment.

Virucidal and chlamydicidal activities of eye drops with povidone-iodine liposome complex.

Povidone-iodine (PVP-I) is a broad-spectrum microbicide with in vitro activity against bacteria, viruses, fungi and protozoans. A 5% solution of PVP-I proved to be highly effective in ophthalmic surgery for the prophylaxis of endophthalmitis. For the antiseptic treatment of eye infections a novel application form of PVP-I has been developed by using a PVP-I liposome complex which demonstrated an excellent antimicrobial efficacy. In this study it could be shown that the novel liposomal formulations containing 2.5 or 5% PVP-I were as active as the aqueous solution against herpes simplex virus type 1, adenovirus type 8, coxsackievirus A9 and Chlamydia trachomatis in cell culture referring to equal PVP-I concentrations. Long-term cytotoxicity experiments demonstrated a moderate cytotoxicity for both formulations with a better tolerability of the liposomal PVP-I formulation compared with the aqueous solution. There is no evidence for a genotoxic activity of these agents.

Comparison of the genotoxic and apoptosis-inducing properties of ganciclovir and penciclovir in Chinese hamster ovary cells transfected with the thymidine kinase gene of herpes simplex virus-1: implications for gene therapeutic approaches.

We studied the genotoxic and apoptosis-inducing properties of ganciclovir (GCV) and penciclovir (PCV) using Chinese hamster ovary cells stably transfected with the thymidine kinase (tk) gene of herpes simplex virus-1 (HSV-1). Cells expressing HSVtk were 300 and 100 times more sensitive than their isogenic HSVtk- counterparts to the cytotoxic effects of GCV and PCV, respectively. Using radiolabeled drugs, GCV was found to be incorporated into the genomic DNA much more effectively than PCV. GCV was highly potent in inducing chromosomal aberrations compared with PCV, which provoked less sister chromatid exchanges and chromosomal changes using equimolar or equitoxic doses. For both agents, apoptosis was shown to be the major route of cell killing. Time course experiments revealed that neither genotoxicity nor apoptosis were induced within the cell cycle exposed to the drug; they are late events provoked in the following cell cycle(s). This indicates that the incorporation/exposure step of GCV or PCV into DNA is not decisive for triggering genotoxicity and apoptosis, but that events occurring subsequently, presumably during replication of a DNA containing the nucleotide analogs, are of major importance. Because PCV, unlike GCV, induced highly effectively apoptosis without exerting much genotoxicity, the use of PCV as a relatively safe alternative drug for suicide gene therapy of malignant diseases is recommended.

Cytogenetic genotoxicity of anti-herpes purine nucleoside analogues in CHO cells expressing the thymidine kinase gene of herpes simplex virus type 1: comparison of ganciclovir, penciclovir and aciclovir.

The three anti-herpes nucleoside analogues ganciclovir, penciclovir and aciclovir were investigated as to their recombinogenic [sister chromatid exchange (SCE) inducing] and clastogenic activity in CHO cells expressing the thymidine kinase gene of HSV-1, which is a precondition of therapeutic activity of these drugs. The compounds were applied for the duration of one cell cycle and cytogenetic end-points were measured between 0 and 42 h after exposure. Although the nucleoside analogues are quite similar with respect to chemical structure, they differ basically in their genotoxic potency, aberration types induced as well as the time course of chromosomal damage. Aciclovir induced SCEs and chromosomal aberrations immediately after exposure but only in a concentration range much higher than that reached in blood plasma during anti-herpes therapy. The direct genotoxic activity is explained by the obligate chain terminating property of aciclovir upon incorporation into genomic DNA. On the other hand, genotoxic damage caused by ganciclovir and penciclovir is of the delayed type requiring at least one post-exposure cell cycle for its expression. Unlike aciclovir, ganciclovir is an extremely potent inducer of SCEs and chromosome breaks and translocations at concentrations far below those impairing the proliferative activity and triggering apoptosis of the target cells (as shown by our previous investigation). Penciclovir is essentially devoid of genotoxic activity. It induces SCEs only at cytotoxic/apoptotic concentrations, is only weakly clastogenic and induces premature chromosome condensation which appears to result from uncoupling of karyokinesis and cytokinesis. The genotoxic activity of ganciclovir is explained as due to repair processes triggered in the second post-exposure replication cycle at the sites of nucleoside analogue incorporation into genomic DNA. The findings have considerable implications with respect to the use of ganciclovir or other antiviral drugs in suicide gene therapy of malignant diseases.

Cytogenetic genotoxicity of antiherpes virostatics in Chinese hamster V79-E cells. I. Purine nucleoside analogues.

The antiherpes virostatics acyclovir (ACV), valaciclovir (VACV), penciclovir (PCV), famciclovir (FCV) and ganciclovir (GCV), which belong to the group of purine acyclic nucleoside analogues, were tested for clastogenic and sister chromatid exchange (SCE)-inducing activity in Chinese hamster V79-E cells upon chronic application with and without a recovery period. ACV induced borderline effects in both cytogenetic assays, a dose-dependent reduction of the mitotic index and an increasing cell cycle delay. With VACV and PCV only a decrease of the mitotic index and an increase of cell cycle delay were observed. FCV was negative with respect to the four parameters studied, presumably due to the incapacity of the target cells of metabolizing FCV to PCV. GCV was a very potent genotoxin in both assays. It induced a statistically significant SCE response even in the range of the cytomegalovirus IC50 of < 10 microM. By variation of the experimental protocol it was shown that SCEs are induced in the second cell cycle following exposure to GCV but not in the first one. It is assumed that the drugs under study are metabolized to their respective triphosphates and then inhibit DNA replication as detected by decreasing mitotic index and increasing cell cycle delay. In the case of GCV it is suggested that GCV-TP is incorporated into the target cell DNA and that chromosomal aberrations and SCEs are secondary lesions due to repair processes at the substituted template.

Protective effect of O6-methylguanine-DNA methyltransferase (MGMT) on the cytotoxic and recombinogenic activity of different antineoplastic drugs.

The DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) removes alkyl groups from the O6 position of guanine in DNA and thus may protect cells against genotoxic effects of agents inducing this lesion. To analyze quantitatively the level of protection mediated by MGMT against antineoplastic drugs, we determined the cytotoxic and recombinogenic (sister-chromatid exchange inducing) effects of various chemotherapeutic agents in a pair of isogenic Chinese hamster cell lines deficient and proficient for MGMT, generated upon transfection with human MGMT cDNA. Furthermore, we compared the responses of the human cell lines HeLa MR (MGMT deficient) and HeLa S3 (MGMT proficient) to the various agents. It is shown that: (1) MGMT proficient cells are resistant in cell killing to the methylating drug streptozotocin and all the chloroethylating nitrosoureas tested. There was a marked agent specificity in protection. The level of resistance provoked by MGMT increased in the order BCNU < CCNU < ACNU < HeCNU < streptozotocin. (2) MGMT did not protect cells against killing induced by chlorambucil, cisplatin, melphalan, activated cyclophosphamide (malosfamide) and activated ifosfamide (4-hydroperoxy-ifosfamide). (3) MGMT caused protection against the recombinogenic effect of all nitrosoureas tested. The lowest level of protection was again observed for BCNU, followed by CCNU, ACNU < HeCNU < streptozotocin. (4) MGMT proficient cells did not exhibit resistance in SCE induction towards cyclophosphamide (activated by microsomes), 4-hydroperoxy-ifosfamide, mafosfamide, chlorambucil and melphalan. Some protection was afforded, however, against cisplatin (and transplatin). This effect was abolished by pretreatment of cells with O6-benzylguanine, which depletes MGMT, indicating that some lesion(s) induced by cisplatin giving rise to SCEs can be repaired by MGMT. Taken together, these results indicate that streptozotocin, HeCNU and ACNU are more selective than CCNU and BCNU in killing MGMT deficient cells, and that in the cases of cyclophosphamide, ifosfamide, chlorambucil, cisplatin and melphalan MGMT is not involved in mediating cytotoxic drug resistance.

Structure/activity investigations in eight arylalkyltriazenes comparison of chemical stability, mode of decomposition, and SCE induction in Chinese hamster V79-E cells.

A series of seven 1-aryl-3.3-dialkyltriazenes, including 1-phenyl-3.3-dimethyltriazene (DMPT), 1-phenyl-3.3-di-(trideuteromethyl)-triazene (DMPT-ds), 1-p-methylphenyl-3.3-dimethyltriazene (DMpMPT), 1-p-nitrophenyl-3.3-dimethyltriazene (DMpNPT), 1-phenyl-3.3-diethyltriazene (DEPT), 1-phenyl-3.3-di-n-propyltriazene (DnPrPT) and 1-phenyl-3.3-diisopropyltriazene (DiPrPT) and 1.3-diphenyl-3-methyltriazene (DPMT), was synthesized and characterized by UV/VIS, IR and 1H-NMR spectroscopy. Chemical half-life was determined in phosphate buffer at 37 degrees using UV/VIS spectroscopy. With the exception of DMpNPT, which was stable, the triazenes underwent pH-dependent hydrolytic decomposition (acid catalysis). By means of UV/VIS spectra, TLC and HPLC, phenol, aniline and secondary azocoupling products were identified after complete hydrolytic cleavage of the parent compounds. Pathways of spontaneous hydrolysis are proposed and discussed. Genotoxic activity of the triazenes was assayed by measurement of sister chromatid exchanges (SCE) in V79-E cells without and with rat liver S9 mix as an exogenous metabolizing system. In the direct SCE assay (without S9 mix), all triazenes except DMpNPT exerted a toxic action (cell cycle delay) in a narrow concentration range between no effect and overt cytotoxicity. This non-specific toxicity depended on the pH of the incubation system and was inversely proportional to chemical half-life. The toxicity of these agents is most likely due to the arenediazonium cation which is a relatively stable intermediate. In a sublethal concentration range most triazeness induced significant increases of SCE rates. These are interpreted as an indirect consequence of cytotoxicity. Upon metabolic activation, the compounds were genotoxic in a dose-dependent fashion. Their SCE-inducing capacity depended on the nature of the alkylating species generated, i.e., the alkyldiazonium cation, and on chemical stability. Surprisingly, no deuterium isotope effect was observed in DMPT-d6. The order of genotoxic activity among the aryldialkyltriazenes was DMpNPT much greater than DMPT = DMPT-ds greater than DMpMPT much greater than DEPT greater than DnPrPT greater than or equal to DiPrPT. DPMT was a marginal SCE inducer but very toxic upon metabolic activation. As monooxygenation of DPMT, like spontaneous hydrolysis, should generate a phenyldiazonium cation, the results suggest that arylation of DNA causes a very low SCE induction, if any.

[The pre- and postnatal carcinogenic effect of 3,3-diethyl-1-methyl-1-nitrosourea (DEMNU) in rats following intravenous application]. / Prä- und postnatale kanzerogene Wirkung von 3,3-Diethyl-1-methyl-1-nitrosoharnstoff (DEMNH) bei Ratten nach intravenöser Applikation.

The pre- and postnatal administration of DEMNU induces a high frequency of tumors when applied via the intravenous route, and the latency periods show a dose dependence (table I). Tumors of the brain, spinal cord and cranial nerves clearly predominate. Furthermore, a large number of neoplasms of kidney, heart and soft tissue was observed (table II). As DEMNU is per se a very stable compound, it is suggested that this agent is metabolized by monooxygenases. 3-Ethyl-1-methyl-1-nitrosourea should be formed as an intermediate product via this pathway, which is relatively stable and might explain the mainly neurotropic carcinogenicity of DEMNU. Species differences in the carcinogenicity of trialkyl-nitrosoureas and the mode of metabolic activation are discussed.

Serine hydrolases activate/inactivate trisubstituted nitrosoureas in dependence on intra- and extracellular enzyme location: an SCE study in Chinese hamster V79-E cells.

Trisubstituted nitrosoureas are very stable in aqueous systems. But they are potent genotoxins in Chinese hamster V79-E cells, if no exogenous metabolizing system is added, and the mechanism of their genotoxic and carcinogenic activity has been largely unknown. This investigation shows that the sister-chromatid-exchange (SCE)-inducing capacity of 1,3-dimethyl-3-phenyl-1-nitrosourea (DMPNU) is eliminated by adding diisopropylflurophosphate (DFP) or porcine liver carboxylesterase to the incubation system. These effects are caused by two different mechanisms: (i) DFP inhibits endogenous amidases existing in V79-E cells, thus preventing the intracellular decomposition, which means an activation; and (ii) exogenous carboxylesterase cleaves DMPNU extracellularly, and the genotoxic decomposition product is obviously too short-lived to reach a critical intracellular target. A second trisubstituted nitrosourea, 3,3-diethyl-1-methyl-1-nitrosourea (DEMNU), which is mainly activated by monooxygenases, but in the absence of an exogenous metabolizing system also induces SCEs in V79-E cells, was studied in the same way. It was found that the 'direct' genotoxicity of DEMNU may be inhibited by DFP as well, but carboxylesterase decomposes this trialklynitrosourea with a much lower efficiency than DMPNU suggesting a low substrate affinity. The SCE-inducing capacity of both compounds is strongly influenced by the presence of calf serum in the culture medium. The nature of the serum factor is still unknown. Pathways for the amidase catalysis of DMPNU and for the activation of DEMNU by monooxygenases and amidases are proposed and discussed with respect to the topical or systemic carcinogenicity of these agents.

The mechanism of cytogenetic genotoxicity of exogenous glutathione in V-79 cells in vitro--implication of hydrogen peroxide and general traits of oxidative chromosome damage.

The mechanism of cytogenetic genotoxicity (clastogenicity, induction, cell cycle delay) of 10(-3) M glutathione in V79-E cells, as described by Thust and Bach (1985), was studied in detail by using different treatment conditions. It was found that 1-cystine is the essential cofactor in the incubation system. Catalase, but not superoxide dismutase, abolished the genotoxic effect, and the iron chelator desferoxamine, as well as the hydroxyl radical scavenger mannitol, diminished the activity. It is suggested that glutathione, in combination with V79-E cells and cystine, forms a hydrogen peroxide-generating system which provokes the adverse effects. Glutathione as well as 1-cysteine and 2-mercaptopropionylglycine, which were checked for comparison, show a "paradoxic genotoxicity," i.e., at 10(-2) M the effects return almost to the level of controls. Concentration dependence and other criteria of cytogenetic genotoxicity observed with glutathione show obvious similarities to those of other oxidatively acting agents and reveal striking differences to the cytogenetic effects of "typical" genotoxins.

High local carcinogenic activity of 1,3-dimethyl-3-phenyl-1-nitrosourea and its inactivation by intravenous application in rats: comparison of in vivo findings with the in vitro direct and a combined in vivo/in vitro sister chromatid exchange assay in V79-E cells.

1,3-Dimethyl-3-phenyl-1-nitrosourea (DMPNU) is a very potent local carcinogen in rats and induces a 100% frequency of forestomach carcinomas when applied i.g. in two different dosages (10 applications of 0.3 or 0.03 mmol/kg body wt, respectively, at 14-day intervals), but it is inactive upon i.v. administration (10 applications of 0.03 mmol/kg body wt at 14-day intervals). By means of the direct sister chromatid exchange (SCE) assay in V79-E cells in the presence of rat blood, serum or plasma, respectively, as well as by a 'host-mediated' SCE assay (in which the agent was given i.v. to rats, and blood taken from the animal was checked for the recovery of genotoxic activity in cell cultures), we tried to elucidate the unexpected lack of carcinogenic activity of i.v. DMPNU. The direct SCE assay revealed a drastic reduction of DMPNU genotoxicity by rat blood, serum or plasma, respectively, which is abolished by the esterase inhibitor diisopropylfluorophosphate. In the 'host-mediated' SCE assay a genotoxic activity of DMPNU was only recoverable after a very high i.v. dose and when the blood added to the cell cultures had been taken from the rat heart within 1 min after DMPNU administration in vivo. 1-Methyl-1-nitrosourea (MNU) and 1-methyl-3-phenyl-1-nitrosourea (MPNU) were used as positive controls in these experiments and also gave a lower response than theoretically expected, but the relative loss of activity with the latter compounds was much lower than with DMPNU. It is assumed that an esterase in rat blood effectively decomposes this trisubstituted nitrosourea. Problems of the novel 'host-mediated' SCE assay are discussed.

Exogenous glutathione induces sister chromatid exchanges, clastogenicity and endoreduplication in V79-E Chinese hamster cells.

Glutathione (GSH) dissolved in Eagle's MEM and added to cultures of V79-E cells in concentrations between 2.5 X 10(-4) and 10(-3) moles/l for 1 h induces a dose-dependent cell cycle delay, sister chromatid exchanges and clastogenic damage. 7-8% of the metaphases showed endoreduplication at a recovery phase of 25 and 30 h after treatment with 10(-3) moles/l GSH. Higher concentrations were lethal. The highest tolerated dose corresponds to the intracellular GSH level in V79-E cells. In the same range of concentrations, glutathione disulfide was inactive. Endoreduplication induction by GSH is G2-phase specific and endoreduplication metaphases show a reduced occurrence of single SCEs when extrapolated to the diploid complement. The adverse effects of GSH are independent of the presence of serum in the culture fluid but completely abolished when the treatment is performed in Hank's solution instead of MEM. The mechanism of genotoxicity of exogenous GSH is discussed but, at present, no pertinent explanation can be given.

Detection of a deuterium isotope effect in di- and trisubstituted alkylphenylnitrosoureas. An SCE study in Chinese hamster V79-E cells.

The genotoxicity of 1-methyl-3-phenyl-1-nitrosourea (MPNU), 1-methyl-3-(p-chlorophenyl)-1-nitrosourea (C1-MPNU), 1-ethyl-3-phenyl-1-nitrosourea (EPNU), 1,3-dimethyl-3-phenyl-1-nitrosourea (DMPNU) and their derivatives substituted by deuterium in different positions was studied using sister chromatid exchange (SCE) induction in Chinese hamster V79-E cells. Deuterium substitution in the 1-methyl group of MPNU (MPNU-d3) and C1-MPNU (C1-MPNU-d3) diminished the SCE-inducing capacity by 20-30% and by 30-40% in DMPNU (DMPNU-d3B). There was no altered SCE activity detected when the phenyl group of MPNU (MPNU-d5) or the 3-methyl group of DMPNU (DMPNU-d3A) was deuterium labeled. No isotope effect was detected in deuterated EPNU derivatives, presumably due to the instability of these compounds. It is surmised that the easier delocalization of the positive charge in the deuterated alkyl diazonium ion causes a diminished reactivity and therefore influences the type and amount of DNA alkylation. Furthermore, the experiments with DMPNU and its derivatives revealed that, in contrast to mono- and disubstituted nitrosoureas, the biological activities of these very stable trisubstituted nitrosoureas are strongly influenced by a serum factor in the culture fluid.

Studies by means of the SCE assay in V79-E Chinese hamster cells on the mode of action of tri-substituted nitrosoureas.

The genotoxic activity of 3,3-diethyl-1-methyl-1-nitrosourea ( DEMNU ), 1,3-dimethyl-3-phenyl-1-nitrosourea ( DMPNU ) and 1-chloroethyl-3-methyl-3-phenyl-1-nitrosourea ( CEMPNU ) was studied in the SCE assay in V79-E cells in vitro. These compounds are very stable in aqueous solutions, but are directly acting genotoxins . The SCE rates increase linearly with the length of the incubation period. This direct activity is presumably due to an intracellular catalytic decomposition. Whereas the SCE-inducing effect of DMPNU and CEMPNU is not influenced by addition of S9 mix, that of DEMNU is strongly potentiated by rat and Syrian hamster S9 mix. This DEMNU activation is an NADPH-dependent enzymatic reaction and is inducible by phenobarbital. The absence of a direct mutagenic effect of DEMNU in the Ames test, as reported by other authors, is probably caused by a striking insensitivity to tri-substituted nitrosoureas of the Salmonella assay. This assumption was substantiated by long-term application of very low DMPNU doses to V79-E. Long-term simultaneous treatment with DMPNU and bromodeoxyuridine (BUdR) significantly diminished the rate of SCE induction.

Incorporation in liposomes as a method for the application of genotoxins of low water solubility in the SCE assay.

Benzo[alpha]pyrene BaP), freely dissolved or incorporated in liposomes prepared from egg yolk lecithin, was checked for SCE induction in Chinese hamster V79-E and rat liver RL-19 cells in vitro. SCE induction in V79-E was observed only when freely dissolved BaP was added together with S9 mix. RL-19 cells were per se highly sensitive to SCE induction by BaP either freely dissolved or incorporated in liposomes. It is suggested that the incorporation of genotoxins in liposomes is a practicable method for the application, in mammalian genotoxicity assays, of agents which are barely soluble or completely insoluble in water, provided no exogenous metabolizing system is required.

[Carcinogen screening in mammalian cells in vitro. The clastogenicity and the SCE test]. / Kanzerogenscreening in Säugetierzellen in vitro. Der Klastogenitäts- und der SCE-Test.

A survey is given about problems related to cheking for genotoxicity of environmental chemicals. Cytogenetic effects, induction of structural chromosome aberrations (clastogenicity) and sister chromatid exchanges (SCEs), are very sensitive indicators of genetic damage in eukaryotic cells. Problems and methods of the clastogenicity and SCE assay in mammalian cells in vitro are described. These bioassays are correlation tests and do not allow a definitive prediction of carcinogenic activity of chemicals, but they are a valuable tool for the evaluation of genetic/carcinogenic risks. Problems of extrapolation to man are discussed.