Comparison of mutant frequencies at the transgenic lambda LacI and cII/cI loci in control and ENU-treated Big Blue mice.

We compared the lambda cII/cI transgenic mutation assay described by Jakubczak et al. [(1996): Proc Natl Acad Sci USA 93:9073-9078] to the previously established Big Blue assay. Genomic DNA isolated from liver, spleen, and lung tissue of control or ethylnitrosourea (ENU)-treated Big Blue mice (100 mg/kg i.p., single dose) was packaged into phage (five animals, two packagings per DNA sample) which were simultaneously plated for lacI and cII/cI mutant frequency (MF) and titer. Mean MF of control animals was higher for cII/cI than lacI for all three tissues examined (spontaneous cII/cI MF divided by spontaneous lacI MF = 2.9, 3.1, and 1.7 for liver, spleen, and lung, respectively). The differences were statistically significant for liver and spleen, but not lung. The ENU-induced MF measured by subtracting control MFs from ENU-treated MFs was higher in the cII/cI assay than lacI (liver = 23.0 x 10(-5) for cII/cI vs. 15.1 x 10(-5) for lacI; spleen = 64.8 x 10(-5) for cII/cI vs. 36.1 x 10(-5) for lacI; lung = 17.1 x 10(-5) for cII/cI vs. 15.8 x 10(-5) for lacI). Fold increase over control values measured by dividing MF of ENU-treated animals by appropriate control values was higher for lacI than cII/cI (liver = 4.4-fold for lacI vs. 2.7 for cII/cI; spleen = 13.1-fold for lacI vs. 8.4 for cII/cI; and lung = 5.6-fold for lacI vs. 4.0 for cII/cI). Despite these differences, overall results were similar for the two mutational endpoints. These results suggest that the cII/cI assay may be an acceptable alternative to lacI where transgenic mutation studies are indicated.

Lack of response to multiple genotoxic agents at the hprt locus in peripheral blood T-lymphocytes of cynomolgus monkeys.

We tested the ability of a series of known genotoxic agents to cause mutations at the hprt locus in peripheral blood T-lymphocytes of cynomolgus monkeys as measured by the ability to form clones in the presence of 6-thioguanine. Ethylmethane sulfonate (EMS, 300 mg/kg i.p.), chloroethylmethane sulfonate (CI-EMS, 35 or 50 mg/kg i.p.), and the Pharmacia & Upjohn antitumor agents adozelesin (1.6, 4, 6, or 8 microg/kg i.v.) and CC-1065 (6 microg/kg i.v.) were all negative in the hprt mutation test. Results with cyclophosphamide (CP, 75 mg/kg i.v.) were equivocal. Adozelesin, CC-1065, and CI-EMS treatments increased the percentage of T-lymphocytes with chromosome aberrations, as well as inducing types of aberrations not seen in control cells. EMS and CP were not tested for chromosome aberrations. We have previously shown that treatment of monkeys with 77 mg/kg ENU substantially increased the hprt mutant frequency, with a lag time of approximately 77 days between treatment and peak MF values. The results of the present study suggest a low sensitivity of the hprt mutation assay to certain classes of genotoxic agents in cynomolgus monkeys.

Spontaneous mutation spectrum at the lambda cII locus in liver, lung, and spleen tissue of Big Blue transgenic mice.

Big Blue mice harbor a recoverable transgene in a lambda/LIZ shuttle vector. In the standard assay, in vivo mutations are measured in the bacterial lacI gene using a labor-intensive color plaque assay. Applying a simpler assay [Jakubczak et al. (1996): Proc Natl Acad Sci USA 93:9073-9078], we measured mutations in the lambda cII gene portion of the transgene. Spontaneous clear plaque mutants were analyzed from liver, lung, and spleen of five untreated mice. Of 314 mutants, 182 (58%) had independent mutations, 74 (23.5%) appeared clonal, and 58 (18.5%) showed no cII mutations. Of 182 independent cII mutations, 156 (85.7%) were base substitutions, 20 (10.9%) were frameshifts, and 6 (3.2%) were multiple substitutions and one deletion. G:C --> A:T transitions were the predominant base substitution (78% of these at CpG sites). The major mutation hotspot, a six G run and its 3' flanking T at bases 179 to 185, comprised 18.7% of the independent mutations. Other hotspots were positions 103, 196, and 212. The in vivo cII spectrum had a significantly higher proportion of G --> A and G --> T mutations and fewer frameshifts than reported in vitro. The cII and published lacI spectra are similar, though G --> A transitions and deletions were fewer in the cII gene. The cI gene was sequenced in 48 mutants with no cII mutations and most had cI mutations: 81.3% base substitutions and 18.7% frameshifts. We conclude that the cII/cI system is insensitive to deletion events, but is useful for detecting point mutations.

DNA sequence analysis of hprt mutants persisting in peripheral blood of cynomolgus monkeys more than two years after ENU treatment.

We have been studying in vivo mutagenesis at the hypoxanthine phosphoribosyl transferase (hprt) locus in cynomolgus monkey T-lymphocytes. This primate model allows us to study mutations and their kinetics under well-controlled conditions. Previously, we reported mutations detected at various times after intraperitoneal treatment with ethylnitrosourea (ENU, 77 mg/kg). At 832 days after that first treatment, the monkey received a second dose of 77 mg/kg ENU. Up to 1,331 days after the second treatment, the T-cell mutant frequency (44.2 x 10(-6)) was still 26-fold higher than background (1.7 x 10(-6)), suggesting that mutants persisted in the peripheral blood. Mutant clones from Days 974, 1,164, and 1,311 after the second treatment were selected in thioguanine. Hprt cDNA was prepared from a cell lysate, PCR-amplified, and sequenced. Of 45 mutants, 30 yielded PCR product and 26 were sequenced. Base substitutions were found in 21 (81%) of the 26 mutants and consisted of one G:C --> A:T and five A:T --> G:C transitions, one G:C --> C:G, eight A:T --> T:A, and six A:T --> C:G transversions. Therefore, most base substitutions occurred at A:T basepairs, characteristic of ENU-induced mutations in vivo, and were detected up to 3.6 years after the second treatment. Deletions of exons 2 and 3 occurred in two mutants and exon 7 was deleted in one mutant. There were two insertion mutants: one was a single base insertion and the other contained an insertion of 277 basepairs which was nearly identical to a simian retroviral sequence.

Culture and propagation of Hprt mutant T-lymphocytes isolated from mouse spleen.

The optimization of the mouse lymphocyte Hprt mutation assay has been impeded by the relatively poor growth potential of mouse T-cells in vitro, which leads to low cloning efficiencies (CEs) and limited expansion of Hprt mutant clones for molecular analysis of mutations occurring in control and treated mice. In this study, the addition and manipulation of concanavalin A (Con A), mouse interleukin-2 (IL-2), and a commercially available culture supplement, rat T-STIM with Con A, were used to identify growth conditions producing relatively high CEs for mouse T-cells. Supplementation of medium with 10% rat T-STIM, along with appropriate amounts of Con A for priming and exogenous IL-2 for cloning, resulted in average CEs of 15-16% in lymphocytes isolated from spleens of control mice (n = 32) or mice exposed to 1,3-butadiene (n = 27). In addition, several reagents were assessed for their potential to stimulate long-term growth of Hprt mutant clones; these T-cell stimulatory agents included Con A, phytohemagglutinin, and a calcium ionophore ionomycin combined with a tumor promoter phorbol 12-myristate 13-acetate. In a pilot study, stimulation with Con A proved to be the most effective means for propagating mouse T-cell clones under the various conditions tested. In follow-up experiments, transfer of mutant clones to 24-well plates and repeated stimulation with Con A in IL-2 and rat T-STIM supplemented medium was found to expand 76% of 536 mutant clones to about 400,000 to several million cells per clone. These data indicate that rat T-STIM-supplemented medium enhances the initial outgrowth of mouse T-cells, and that repeated mitogenic stimulation with Con A in the presence of IL-2 and rat T-STIM provides a means for propagating mouse T-cell clones for mutation analyses by a variety of methods.

Effect of plating medium and phage storage on mutant frequency and titer in the lambda cII transgenic mutation assay.

We examined several experimental parameters of the lambda cI/cII transgenic mutation assay. In the assay, clear plaque lambda phage mutants are identified in a positive selection scheme following rescue of the lambda/LIZ shuttle vector from frozen tissues of Big Blue" transgenic mice. Mutant frequency and titer of phage from various tissues of control and ENU-treated animals was essentially the same on LB or TB1 plating medium, and storage of isolated DNA at 4 degrees C for up to 4 months did not affect either mutant frequency or titer. Storage of packaged phage for 28 days at 4 degrees C did not affect titer. The mean mutant frequency of packaged phage stored 28 days at 4 degrees C was consistently higher than phage plated the same day as packaging (day 0), though the difference was statistically significant in only two of the four samples tested. Reconstruction experiments in which numerically defined titers of known cII mutants were plated on both G1217 and G1225 E. coli strains and incubated at 37 degrees C or 24 degrees C showed highest titers on G1217 at 37 degrees C. The fraction of the G1217, 37 degrees C titer seen in the other strains and conditions varied widely with the cII mutation.

In vivo mutagenesis in the cynomolgus monkey: time course of HPRT mutant frequency at long time points following ethylnitrosourea exposure.

We have monitored mutant frequency at the HPRT locus in peripheral blood lymphocytes of cynomolgus monkeys using a clonal assay in which mutants are selected by resistance to 6-thioguanine. Among untreated animals, the mean spontaneous mutant frequency was 2.9 +/- 2.9 x 10(-6) (standard deviation, based on 131 determinations in 33 animals), in good agreement with HPRT mutant frequencies in other species. In four animals treated with a single intraperitoneal dose of 77 mg/kg ethylnitrosourea, mutant frequency increased with time, peaking 70 to 100 days after treatment. Mutant frequency in two of the four animals was monitored at intervals for 6 years, and a second identical treatment was given about 830 days after the first. Mutant frequency again peaked in these two animals 70 days after the second dose and decreased following peak values, declining to a plateau that was higher than the predose mutant frequency in both animals. This pattern was repeated following the second ethylnitrosourea treatment. Fractionating the dose of ethylnitrosourea into five equal daily injections had no effect on mutant frequency in two animals when compared to a single dose.

Glucose and amino acid turnover in untreated gestational diabetes.

OBJECTIVE: Although gestational diabetes affects as many as 3% of all pregnant women, specific aspects of glucose and protein metabolism in this population have not been clearly delineated. We tested the hypothesis that gestational diabetes mellitus (GDM) results in increased glucose production and proteolysis during fasting. RESEARCH DESIGN AND METHODS: Using tracer isotope infusions, the rate of appearance (Ra) of glucose, leucine, phenylalanine and tyrosine, phenylalanine hydroxylation, leucine oxidation, and urea nitrogen excretion were determined after an overnight fast in 10 GDM subjects, within 2 weeks of diagnosis and before initiation of treatment, and in a matched control group of nine healthy nondiabetic pregnant women. RESULTS: Fasting glucose Ra was similar in GDM patients and control subjects (GDM, 12.8 +/- 1.1 vs. control subjects, 12.8 +/- 0.9 mumol . kg-1 . min-1). Leucine and phenylalanine Ra (reflecting proteolysis) also were not different between GDM patients and control subjects (GDM leucine Ra, 128 +/- 14 vs. control subjects, 124 +/- 5; phenylalanine Ra GDM, 35 +/- 4 vs. control subjects, 40 +/- 2 mumol . kg-1 . h-1). Furthermore, leucine oxidation and phenylalanine hydroxylation were not increased in GDM subjects, urea nitrogen excretion was actually lower in GDM patients. However, fasting insulin concentrations were significantly elevated in GDM subjects (GDM, 165 +/- 35 vs. control subjects, 30 +/- 5 pmol/l; P < 0.01). CONCLUSIONS: Hepatic glucose release and whole-body proteolysis in GDM patients were remarkably similar to matched pregnant control subjects. This was achieved with insulin concentrations three- to fivefold higher than normal, suggesting significant insulin resistance for both glucose and protein metabolism in GDM.

End points for biomonitoring: assay sensitivity/selectivity.

Estimation of population exposure and biological impact of potential hazards are central reasons for performing biomonitoring. The sensitivity of the biomonitoring methods and the linkage of the measured phenomenon to human disease are also important, but often overlooked, considerations. We are conducting experiments to evaluate the sensitivity of hprt mutation measurement in the nonhuman primate, the cynomolgus monkey. Our findings demonstrate in the monkey that hypoxanthine guanine phosphoribosyltransferase (hprt) mutations produced in vivo can be detected using technique originally worked out using human cells; cynomolgus monkeys were chosen to avoid many of the complications encountered in studying humans. Sequencing of mutants from the monkey using reverse transcriptase polymerase chain reaction methods has led us to conclude that there is similarity of the spectra observed between the spontaneous mutations detected in the two species. However, more recent data suggest that due to low sensitivity, the method is probably not appropriate for routine biomonitoring of randomly selected populations. For example, the inability of the hprt mutation assay to detect some very potent mutagens in the monkey and the effects of the time-dependent pattern of mutant occurrence serve to urge caution in interpretation of elevation or lack of elevation in mutant frequency. Mechanisms for splitting and archiving samples of human tissues/blood from populations at risk may prove valuable as methods improve.

Spontaneous and ethylnitrosourea-induced mutation fixation and molecular spectra at the lacI transgene in the Big Blue rat-2 embryo cell line.

Big Blue Rat-2 cells were evaluated for mutagenesis and mutational spectra (spontaneous and ethylnitrosourea [ENU]-induced). Survival, mutant frequency, population doubling time, and kinetics of mutant increase (to 120 hr) were determined. Exposures were 100, 200, 400, 600, and 1,000 micrograms ENU/ml. The spontaneous mutant frequency was similar to that previously reported in vivo, i.e., 5 X 10(5). Dose-related increases in mutant frequency were observed following ENU treatment. Kinetics (time course) of mutant frequency increase, population doubling, and mutational spectra were investigated following treatment at 1,000 micrograms ENU/ml. Among 39 spontaneous mutants, 26 independent mutations were found as follows: nine (34.6%) G:C-->A:T transitions (five at CpG sites), six (23%) G:C-->T:A transversions, three (11.5%) G:C-->C:G transversions (two at CpG sites), two (7.7%) frameshifts, five (19%) deletions or insertions, and one (3.8%) complex (deletion+insertion) mutation. Among 46 ENU-induced mutants, 37 independent mutations (all base substitutions) were found as follows: 15 (40.5%) G:C-->A:T transitions (four at CpG sites), five (13.5%) A:T-->G:C transitions, four (10.8%) G:C-->T:A transversions, 11 (30%) A:T-->T:A transversions, and two (5.4%) A:T-->C:G transversions. Nearly 50% of the base substitutions in the ENU-treated cells were at A:T base pairs, in contrast to the spontaneous mutants where none was found. Both the spontaneous and the ENU-induced mutational spectra were similar to that reported in vivo and for other cells. An important aspect of the experiment is that all mutations sequenced following ENU treatment (1,000 micrograms/ml) occurred under conditions which our experiments show corresponded to very little mitotic activity.

Southern blot analysis of T-cell receptor gene rearrangements in cynomolgus monkeys, and identification of a progenitor cell HPRT mutation.

Increases in peripheral blood T-lymphocyte HPRT mutant frequency may reflect either a number of independent HPRT gene mutational events or clonal proliferation of a single HPRT mutant. Sequence analysis of HPRT mutations in conjunction with T-cell receptor (TCR) gene rearrangement pattern analysis can distinguish these possibilities. Our laboratory previously characterized a nonhuman primate model for in vivo mutation studies using the clonal HPRT mutation assay. In the present study we report the use of probes for human TCR beta and gamma genes to characterize TCR rearrangements in cynomolgus monkeys. Together, these methods were used to examine a monkey which exhibited a mean spontaneous HPRT mutant frequency (MF) of 16.4 x 10(-6), compared to the normal mean MF of 3.03 x 10(-6). The elevated MF resulted from the occurrence of a single HPRT mutation in a lymphocyte progenitor cell or stem cell, since T-cell clones isolated from the monkey exhibited a G to T transversion at base pair 539 in the HPRT coding region, and had unique rearrangements of TCR gamma along with an apparent germline TCR beta configuration. In a preliminary in vivo mutation study, the animal was treated with the investigational potent mutagen and antitumor agent adozelesin (U-73975). No increase in HPRT mutant frequency was observed. The HPRT mutant clones isolated after treatment showed rearrangement of both TCR gamma and beta genes. Possible explanations for these findings are discussed.

DNA sequence analysis of spontaneous hprt mutations arising in vivo in cynomolgus monkey T-lymphocytes.

To study the mechanisms of mutagenesis in vivo, we analyzed mutations at the hypoxanthine phosphoribosyl transferase (hprt) locus using cDNA from cynomolgus monkey T-lymphocytes. In the present study, the spectrum of spontaneous hprt mutations arising in vivo in wild-caught cynomolgus monkey peripheral T-lymphocytes is described. Cells were isolated from peripheral blood, and mutant clones were selected in 6-thioguanine, propagated, and stored frozen. cDNA was copied from hprt mRNA from a lysate of 7,000 to 20,000 cells. A 780-base-pairs (bp) region including the coding region was amplified by polymerase chain reaction and directly sequenced. We sequenced 40 spontaneous mutants from 11 monkeys. Of these 40 clones, 23 (57%) had base-pair substitutions, 11 (28%) had small (< 20 bp) deletions and/or insertions, and 6 (15%) had large (> 20 bp) deletions and/or insertions. Of the 23 base substitutions, 13 were transitions (11 G:C-->A:T, 1 A:T-->G:C, and 1 tandem TT-->CC) and 10 were transversions (3 G:C-->T:A, 3 G:C-->C:G, 2 A:T-->T:A, 2 A:T-->C:G). Bases 209 and 617 were apparent substitution hotspots, which have also been observed as hotspots in human hprt. In 2 clones with large insertions, the inserted bases were of intronic origin. One of these lost 272 bp from exons 2-3 and contained a 93-bp insertion from the middle of intron 3. Two clones with small deletions and 5 clones with large deletions or insertions (7/40 or 17.5%) could be splice mutants.

Risk and benefit evaluation in development of pharmaceutical products.

Pharmaceutical products are intended to cure disease, reduce pain and suffering, prolong life, and correct metabolic deficits in patients. However, the potential patient population is intrinsically genetically heterogenous, and this factor complicates the evaluation of data on all aspects of safety evaluation of new drugs. Often the genetic heterogeneity is related to drug metabolizing capacity, but recent evidence suggests that heterogeneity in repair capacity as well as structural integrity of the chromatin (fragile X) have been shown to be relevant. Because drugs are biologically active and may have more than one type of effect, the evaluation of a large number of parameters is necessary in arriving at a rational estimate of potential risk. In this paper, several specific examples of risk assessments and some generic genotoxicity questions that are recurrent, including the question of the relevance of in vitro chromosomal aberration induction at high dose/sampling time, are raised. Other examples of the kinds of concerns from the safety evaluation of U-48753E, U-54461, and U-68,553B are discussed. The drug U-48753E was discovered to be slightly mutagenic in the AS52 assay, and significant efforts were expended in evaluation of the metabolism-based generation of a reactive intermediate. The drug U-54,461 was shown to be capable of breaking chromosomes in vitro but extensive in vivo data as well as a variety of other studies served to reduce the level of concern substantially.(ABSTRACT TRUNCATED AT 250 WORDS)

Enumeration of 6-thioguanine-resistant T-lymphocytes in the peripheral blood of nonhuman primates (cynomolgus monkeys).

We have investigated the use of cynomolgus monkeys (Macaca fascicularis) as a model of somatic cell mutagenesis in non-human primates. Using techniques described by Albertini (Mutation Research 150:411-422, 1985) for similar studies in humans, the frequency of TG-resistant T-lymphocytes in the peripheral blood was determined in animals that were either untreated or treated with ethylnitrosourea. The frequency of TG-resistant cells in untreated males was (mean +/- SD) 6.0 +/- 5.9 per 10(6) cells and for untreated females was 2.9 +/- 2.7 per 10(6) cells. The spontaneous frequency of TG-resistant cells for all animals was 4.2 +/- 4.44 per 10(6) cells. Maximum frequency of TG-resistant cells for two animals treated with a single I.P. dose of ENU was 45.1 and 77.9 per 10(6) cells. Substantial increases in frequencies of TG-resistant cells were not seen until at least 63 days after treatment. The TG-resistant phenotype of clones isolated in the assay was stable after growth for 2 weeks in the absence of selective agent. Many of the TG-resistant clones selected were frozen for future molecular analysis.

Comparative mutagenicity testing of a drug candidate, U-48753E: mechanism of induction of gene mutations in mammalian cells and quantitation of potential hazard.

U-48753E is a potential human drug which was subjected to a battery of short-term assays for genetic activity. The compound was negative in the Salmonella (Ames) test, the in vitro UDS assay, the mouse bone-marrow micronucleus test and the Drosophila sex-linked recessive lethal assay. However, it was weakly positive in the CHO/HPRT assay in the presence of metabolic activation (S9). The weak positive response might easily have been labeled artifactual since there was no dose response and the dose level producing positive findings varied from experiment to experiment. In addition, the weak positive response was not confirmed in V79 cells. However, a reproducible dose-related increase in mutants was observed in the AS52/XPRT assay in the presence of S9. Metabolism of this drug proceeds through conversion of aliphatic N-methyl groups to formaldehyde. Addition of formaldehyde dehydrogenase to the S9 resulted in elimination of the mutagenicity of the compound in AS52 cells. Thus, the mutants were probably induced by formaldehyde. From the endogenous levels of formaldehyde in human blood, and the limiting potential therapeutic dose levels, the genotoxic hazard associated with U-48753E is marginal. This assessment of risk and its quantitation depend upon an understanding metabolism and exposure limits imposed by known side effects of the drug. This study can serve as a model for quantitative genetic risk assessment when mutagenicity is due to N-demethylation and formation of formaldehyde in situ.

The CHO/HPRT assay: evaluation of 19 drug candidates.

The CHO/HPRT assay is used as part of basic mutagenicity screening batteries at the Upjohn Company. The results of this and other assays provides a way of identifying compounds likely to cause mutations in mammalian systems and for which additional testing may be required. This report provides results of testing 19 drugs and drug candidates for mutational properties in the CHO/HPRT assay. The results of these studies were uniformly negative. The diversity of structures and the fact that these compounds have potent (non-mutational) biological activity suggests that separation of mutagenic properties from other beneficial properties is feasible. These compounds have also been evaluated in several other assays (Aaron et al., 1989a-d) and were negative for genotoxicity. Thus, the results of this study and other available data fails to suggest any genotoxic hazard from these materials.

The Salmonella mutagenicity test: evaluation of 29 drug candidates.

The Salmonella mutagenicity test (Ames assay) is part of the routine screening battery applied to all new drugs at The Upjohn Company. The purpose of this paper is to report results for 29 compounds. These compounds are very diverse in chemical structure and represent classes of compounds selected because of known biological activity and other reasons. None of the compounds reported here produced an increase in revertant colonies in the Salmonella strains employed (TA98, TA100, TA1535, TA1537 and TA1538) and therefore the Salmonella mutagenicity results with these materials do not suggest potential for mutagenesis or carcinogenesis.

Mutagenicity of the antitumor antibiotic CC-1065 and its analogues in mammalian (V79) cells and bacteria.

CC-1065 is a very potent antitumor antibiotic which binds in the minor groove of DNA with alkylation at N-3 of adenine. Since CC-1065 caused delayed deaths in mice at therapeutic doses, analogues were prepared whose antitumor and biochemical activities have been reported. In this study, the mutagenicity for V79 cells (6-thioguanine resistance) and Salmonella (histidine auxotrophy or azaguanine resistance) of selected analogues was compared to DNA-binding activity and the structure-activity relationship was determined. CC-1065, U-62,736, U-66,866, U-66,694, U-67,786, and U-68,415 all have an A segment with an intact cyclopropyl group and different B segments. The cyclopropyl group is absent from U-66,226 and U-63,360. Elimination of the cyclopropyl ring diminished the cytotoxic and mutagenic potency of the compounds such that U-63,360 was nearly three orders of magnitude less potent than CC-1065 in V79 cells. For the compounds with an intact cyclopropyl group, the order of cytotoxic and mutagenic potency (molar basis) in V79 cells generally correlated with binding to calf thymus DNA, and increased with the length of the B segment. Thus, the order of cytotoxicity was CC-1065 greater than U-68,415 greater than U-66,694 greater than U-66,866 greater than U-62,736. U-67,786 fell outside this pattern since it was more cytotoxic and mutagenic than U-66,694, although it was of a similar size and had similar DNA-binding activity. These results show that an electrophilic carbon afforded by an intact cyclopropyl group of this type is necessary but not sufficient to account for the high cytotoxic and mutagenic potency of CC-1065 and U-68,415. The size and characteristics of the B segment also affect the potency. At an equitoxic (10 or 50% lethal dose) dose, an inverse relationship exists between cytotoxic and mutagenic potency such that at the 50% lethal dose, the least cytotoxic compound (U-62,736) was more mutagenic than the most cytotoxic compound (CC-1065). We speculate that the more cytotoxic analogues are less mutagenic (at an equitoxic dose) because they may have greater structure-directed binding to less mutable DNA sites in the minor groove.

Genotoxicity of the antitumor antibiotic CC-1065.

CC-1065, a very potent antitumor antibiotic, is active against several animal tumors, and against human tumors in the cloning assay at doses 50-1000 times lower than other agents such as adriamycin. It binds and alkylates DNA, and inhibits DNA synthesis, suggesting a potential for genotoxicity. Therefore, the genotoxic effects of CC-1065 were tested in several assay systems. CC-1065 was weakly mutagenic in the Ames Salmonella mutation assay (strain TA100) without S9 activation, but lacked mutagenic activity in TA98 with or without activation. CC-1065 was a very potent mutagen in the Salmonella forward mutation assay (induction of 8-azaguanine resistance), increasing the mutation frequency 19-fold over background at 0.1 ng/ml without activation. In mammalian (V79) cells it was a very potent mutagen without activation, increasing the mutation frequency 20-fold over background a 0.5 ng/ml. CC-1065 induced chromosome aberrations in V79 cells at very low (less than 0.1 ng/ml) doses, making this assay the most sensitive. CC-1065 increased the induction of micronuclei in rats 10- to 20-fold over the background at 200 and 400 micrograms/kg, but not at 100 micrograms/kg. CC-1065 failed to cause DNA breaks or DNA--protein cross-links as measured by the DNA damage/alkaline elution assay.

Comparative genotoxicity of adriamycin and menogarol, two anthracycline antitumor agents.

Adriamycin and menogarol are anthracyclines which cause more than 100% increase in life span of mice bearing P388 leukemia and B16 melanoma. Unlike Adriamycin, menogarol does not bind strongly to DNA, and it minimally inhibits DNA and RNA synthesis at lethal doses. Adriamycin is a clinically active drug, and menogarol is undergoing preclinical toxicology at National Cancer Institute. In view of the reported mutagenicity of Adriamycin, we have compared the genotoxicity of the two drugs. Our results show that, although Adriamycin and menogarol differ significantly in their bacterial mutagenicity (Ames assay), they have similar genotoxic activity in several mammalian systems. Adriamycin is strongly mutagenic in the Ames assay with TA98 and TA100. Menogarol is nonmutagenic to TA98 and TA100. For the mammalian cell culture systems, V79 (Chinese hamster) cells are exposed for 2 hr to drug, following which cell survival, induction of sister chromatid exchanges, chromosome damage, and production of mutants resistant to 6-thioguanine are measured. The percentage of survival obtained with the two drugs ranges between 25 and 50% at 0.15 microgram/ml and 5 to 15% at 0.3 microgram/ml. At 0.15 microgram/ml, Adriamycin and menogarol increase the percentage of cells with chromosome damage from a background level of 8.8 to 30 and 22.5%, respectively. The same drug concentration causes a small but significant increase in sister chromatid exchange rate. Both drugs are equally active (increase mutation frequency about 3- to 6-fold above background) in producing 6-thioguanine-resistant mutants. The induction of micronuclei in polychromatic erythrocytes of rats is the most sensitive assay system. Both drugs cause 10- to 15-fold increase in micronuclei at nontoxic doses.