The mouse spot test. Evaluation of its performance in identifying chemical mutagens and carcinogens

The Pivotal Role of Preclinical Animal Models in Anti-Cancer Drug Discovery and Personalized Cancer Therapy Strategies

The establishment and utilization of preclinical animal models constitute a pivotal aspect across all facets of cancer research, indispensably contributing to the comprehension of disease initiation and progression mechanisms, as well as facilitating the development of innovative anti-cancer therapeutic approaches. These models have emerged as crucial bridges between basic and clinical research, offering multifaceted support to clinical investigations. This study initially focuses on the importance and benefits of establishing preclinical animal models, discussing the different types of preclinical animal models and recent advancements in cancer research. It then delves into cancer treatment, studying the characteristics of different stages of tumor development and the development of anti-cancer drugs. By integrating tumor hallmarks and preclinical research, we elaborate on the path of anti-cancer drug development and provide guidance on personalized cancer therapy strategies, including synthetic lethality approaches and novel drugs widely adopted in the field. Ultimately, we summarize a strategic framework for selecting preclinical safety experiments, tailored to experimental modalities and preclinical animal species, and present an outlook on the prospects and challenges associated with preclinical animal models. These models undoubtedly offer new avenues for cancer research, encompassing drug development and personalized anti-cancer protocols. Nevertheless, the road ahead continues to be lengthy and fraught with obstacles. Hence, we encourage researchers to persist in harnessing advanced technologies to refine preclinical animal models, thereby empowering these emerging paradigms to positively impact cancer patient outcomes.

Final Report on the Safety Assessment of Trilaurin, Triarachidin, Tribehenin, Tricaprin, Tricaprylin, Trierucin, Triheptanoin, Triheptylundecanoin, Triisononanoin, Triisopalmitin, Triisostearin, Trilinolein, Trimyristin, Trioctanoin, Triolein, Tripalmitin, Tripalmitolein, Triricinolein, Tristearin, Triundecanoin, Glyceryl Triacetyl Hydroxystearate, Glyceryl Triacetyl Ricinoleate, and Glyceryl Stearate Diacetate

Triesters of glycerin and aliphatic acids, known generically as glyceryl triesters and specifically as Trilaurin, etc., are used in cosmetic products as occlusive skin-conditioning agents and/or non-aqueous viscosity-increasing agents. Hundreds of glyceryl triesters are used in a wide variety of cosmetic products at concentrations ranging from a few tenths of a percent to 46%. Glyceryl triesters are also known as triglycerides; ingested triglycerides are metabolized to monoglycerides, free fatty acids, and glycerol, all of which are absorbed in the intestinal mucosa and undergo further metabolism. Dermal absorption of Triolein in mice was nil; the oil remained at the application site. Only slight absorption was seen in guinea pig skin. Tricaprylin and other glyceryl triesters have been shown to increase the skin penetration of drugs. Little or no acute, subchronic, or chronic oral toxicity was seen in animal studies unless levels approached a significant percentage of caloric intake. Subcutaneous injections of Tricaprylin in rats over a period of 5 weeks caused a granulomatous reaction characterized by oil deposits surrounded by macrophages. Dermal application was not associated with significant irritation in rabbit skin. Ocular exposures were, at most, mildly irritating to rabbit eyes. No evidence of sensitization or photosensitization was seen in a guinea pig maximization test. Most of the genotoxicity test systems were negative. Tricaprylin, Trioctanoin, and Triolein have historically been used as vehicles in carcinogenicity testing of other chemicals. In one study, subcutaneous injection of Tricaprylin in newborn mice produced more tumors in lymphoid tissue than were seen in untreated animals, whereas neither subcutaneous or intraperitoneal injection in 4-to 6-week-old female mice produced any tumors in another study. Trioctanoin injected subcutaneously in hamsters produced no tumors. Trioctanoin injected intraperitoneally in pregnant rats was associated with an increase in mammary tumors in the offspring compared to that seen in offspring of untreated animals, but similar studies in pregnant hamsters and rabbits showed no tumors in the offspring. One study of Triolein injected subcutaneously in rats showed no tumors at the injection site. As part of an effort to evaluate vehicles used in carcinogenicity studies, the National Toxicology Program conducted a 2-year carcinogenicity study in rats given Tricaprylin by gavage. This treatment was associated with a statistically significant dose-related increase in pancreatic acinar cell hyperplasia and adenoma, but there were no acinar carcinomas, the incidence of mononuclear leukemia was less, and nephropathy findings were reduced, all compared to corn oil controls. Overall, the study concluded that Tricaprylin did not offer significant advantages over corn oil as vehicles in carcinogenicity studies. Trilaurin was found to inhibit the formation of neoplasms initiated by dimethylbenzanthracene (DMBA) and promoted by croton oil. Tricaprylin was not teratogenic in mice or rats, but some reproductive effects were seen in rabbits. A low level of fetal eye abnormalities and a small percentage of abnormal sperm were reported in mice injected with Trioctanoin as a vehicle control. Clinical tests of Trilaurin at 36.3% in a commercial product applied to the skin produced no irritation reactions. Trilaurin, Tristearin, and Tribehenin at 40%, 1.68%, and 0.38%, respectively, in commercial products were also negative in repeated-insult patch tests. Tristearin at 0.32% in a commercial product induced transient, mild to moderate, ocular irritation after instillation into the eyes of human subjects. Based on the enhancement of penetration of other chemicals by skin treatment with glyceryl triesters, it is recommended that care be exercised in using them in cosmetic products. On the basis of the available data, the following 23 glyceryl triesters are considered safe as used in cosmetics: Trilaurin, Triarachidin, Tribehenin, Tricaprin, Tricaprylin, Trierucin, Triheptanoin, Triheptylundecanoin, Triisononanoin, Triisopalmitin, Triisostearin, Trilinolein, Trimyristin, Trioctanoin, Triolein, Tripalmitin, Tripalmitolein, Triricinolein, Tristearin, Triundecanoin, Glyceryl Triacetyl Hydroxystearate, Glyceryl Triacetyl Ricinoleate, and Glyceryl Stearate Diacetate. Some of these are not currently in use, but would be considered safe if used at concentrations similar to those glyceryl triesters that are in use as cosmetic ingredients.

Current Status of Short-Term Tests for Evaluation of Genotoxicity, Mutagenicity, and Carcinogenicity of Environmental Chemicals and NCEs

The advent of the industrial revolution has seen a significant increase in the number of new chemical entities (NCEs) released in the environment. It becomes imperative to check the toxic potential of NCEs to nontarget species before they are released for commercial purposes because some of these may exert genotoxicity, mutagenicity, or carcinogenicity. Exposure to such compounds produces chemical changes in DNA, which are generally repaired by the DNA repair enzymes. However, DNA damage and its fixation may occur in the form of gene mutations, chromosomal damage, and numerical chromosomal changes and recombination. This may affect the incidence of heritable mutations in man and may be transferred to the progeny or lead to the development of cancer. Hence, adequate tests on NCEs have to be undertaken for the risk assessment and hazard prediction. Compounds that are positive in tests that detect such damages have the potential to be human mutagens/carcinogens. Only long-term animal bioassays, involving lifetime studies on animals, were used earlier to classify substances as mutagens/carcinogens. These tests were cumbersome and time consuming and required a lot of facilities and personnel. Short-term tests, therefore, were brought into practice. A "battery" of three to four of these short-term tests has been proposed now by a number of regulatory authorities for the classification of compounds as mutagenic or carcinogenic. This review deals with the current status of these short-term tests.

Mutagenicity testing with transgenic mice. Part II: Comparison with the mouse spot test

The mouse spot test, an in vivo mutation assay, has been used to assess a number of chemicals. It is at present the only in vivo mammalian test system capable of detecting somatic gene mutations according to OECD guidelines (OECD guideline 484). It is however rather insensitive, animal consuming and expensive type of test. More recently several assays using transgenic animals have been developed. From data in the literature, the present study compares the results of in vivo testing of over twenty chemicals using the mouse spot test and compares them with results from the two transgenic mouse models with the best data base available, the lacI model (commercially available as the Big Blue(R) mouse), and the lacZ model (commercially available as the Mutatrade mark Mouse). There was agreement in the results from the majority of substances. No differences were found in the predictability of the transgenic animal assays and the mouse spot test for carcinogenicity. However, from the limited data available, it seems that the transgenic mouse assay has several advantages over the mouse spot test and may be a suitable test system replacing the mouse spot test for detection of gene but not chromosome mutations in vivo.

Induction of gene mutations by 5-(2-chloroethyl)-2'-deoxyuridine (CEDU), an antiviral pyrimidine nucleoside analogue

5-(2-chloroethyl)-2'-deoxyuridine (CEDU) had been developed for the treatment of herpes simplex infections. In the Salmonella reverse mutation test, the compound was found to be mutagenic in strains TA1535 and TA102 at very high concentrations (> or =2500 micro g/plate), both with and without S9-mix. The mutagenic potential of CEDU was further investigated in vivo and in vitro. It did not induce DNA repair in rat hepatocyte primary cultures, and was negative in the micronucleus test in V79 cells and in the comet assay in human leukocytes. In vivo, CEDU was negative in the bone marrow micronucleus test in CD1 mice. The mouse spot test provided a clearly positive result. Treatment of mice on day 9 of pregnancy with 2000 mg/kg resulted in 5.9% of the F1 animals having genetically relevant spots, whereas the corresponding vehicle control group had a spot rate of 1.9%. Since these data clearly identified CEDU as an inducer of gene mutations in vivo, this potential was further investigated in lacZ transgenic Muta Mouse. Six female animals were treated daily on five consecutive days with 2000 mg/kg/day and sacrificed, after a treatment-free sampling time, 14 days later. The data showed a clear increase in the mutant frequency in the bone marrow, the lung and in the spleen. CEDU is an exception in the group of nucleoside analogues, because it was found to be a strong gene mutagen and, in contrast to the other compounds of this group investigated so far, had no considerable clastogenic effects.

Mouse models for induced genetic instability at endogenous loci

Exposure to environmental factors and genetic predisposition of an individual may lead individually or in combination to various genetic diseases including cancer. These diseases may be a consequence of genetic instability resulting in large-scale genomic rearrangements, such as DNA deletions, duplications, and translocations. This review focuses on mouse assays detecting genetic instability at endogenous loci. The frequency of DNA deletions by homologous recombination at the pink-eyed unstable (p(un)) locus is elevated in mice with mutations in ATM, Trp53, Gadd45, and WRN genes and after exposure to carcinogens. Other quantitative in vivo assays detecting loss of heterozygosity events, such as the mammalian spot assay, Dlb-1 mouse and Aprt mouse assays, are also reviewed. These in vivo test systems may predict hazardous effects of an environmental agent and/or genetic predisposition to cancer.

Benzo(a)pyrene and X-rays induce reversions of the pink-eyed unstable mutation in the retinal pigment epithelium of mice

The pink-eyed unstable (p(un)) mutation is the result of a 70kb tandem duplication within the murine p gene. Homologous deletion/recombination of the locus to wild-type occurs spontaneously in embryos and results in pigmented spots in the fur and eye that persist for life. Such deletion events are also inducible by a variety of DNA damaging agents, as we have observed previously with the fur spot assay. Here, we describe the use of the retinal pigment epithelium (RPE) of the eye to detect reversion events induced with two differently acting agents. Benzo(a)pyrene (B(a)P) induces a high frequency, and X-ray exposure a more modest increase, of p(un) reversion in both the fur and the eye. The eye-spot assay requires fewer mice for significant results than the fur spot assay. Previous work had elucidated the cell proliferation pattern in the RPE and a position effect variegation phenotype in the pattern of p(un) reversions, which we have confirmed. Acute exposure to B(a)P or X-rays resulted in an increased frequency of reversion events. The majority of the spontaneous reversions lie toward the periphery of the RPE whereas induced events are found more centrally, closer to the optic nerve head. The induced distribution corresponds to the major sites of cell proliferation in the RPE at the time of exposure, and further advocates the proposal that dividing cells are at highest risk to develop deletions.

Involvement of p53 in X-ray induced intrachromosomal recombination in mice

The tumor suppressor gene Trp53 (also known as p53) is the most frequently mutated gene in human cancers. p53 is induced in response to DNA damage and effects a G(1) cell cycle arrest. It is believed that p53 plays a key role in maintaining genomic integrity following exposure to DNA-damaging agents. We determined the frequency of spontaneous and DNA damage-induced homologous intrachromosomal recombination in p53-deficient mouse embryos. Homologous intrachromosomal recombination events resulting in deletions at the pink eyed unstable (p(un)) locus result in reversion to the p gene. Reversions occurring in embryonic premelanocytes give rise to black spots on the gray fur of the offspring. Pregnant C57BL/6J p(un)/p(un) p53(+/-) mice were exposed to X-rays (1 Gy) or administered benzo¿apyrene (B¿aP; 30 or 150 mg/kg i.p.) 10 days after conception. Frequencies of spontaneous p(un) reversions in p53(-/-) and p53(+/-) animals were not significantly different compared with their wild-type littermates. X-ray treatment increased the recombination frequency in wild-type and p53(+/-), but surprisingly not in p53(-/-) offspring. In contrast, B¿aP treatment caused a dose-dependent increase in p(un) reversion frequencies in all three genotypes. Western blot analysis of embryos indicated that p53 protein levels increased approximately 3-fold following X-ray treatment, while B¿aP had no effect on p53 expression. These results are in agreement with the proposal that p53 is involved in the DNA damage response following X-ray exposure and suggest that X-ray-induced double-strand breaks are processed differently in p53(-/-) animals.

Methods for predicting carcinogenic hazards: new opportunities coming from recent developments in molecular oncology and SAR studies

Without epidemiological evidence, and prior to either short-term tests of genotoxicity or long-term tests of carcinogenicity in rodents, an initial level of information about the carcinogenic hazard of a chemical that perhaps has been designed on paper, but never synthesized, can be provided by structure-activity relationship (SAR) studies. Herein, we have reviewed the interesting strategies developed by human experts and/or computerized approaches for the identification of structural alerts that can denote the possible presence of a carcinogenic hazard in a novel molecule. At a higher level of information, immediately below epidemiological evidence, we have discussed carcinogenicity experiments performed in new types of genetically engineered small rodents. If a dominant oncogene is already mutated, or if an allele of a recessive oncogene is inactivated, we have a model animal with (n-1) stages in the process of carcinogenesis. Both genotoxic and receptor-mediated carcinogens can induce cancers in 20-40% of the time required for classical murine strains. We have described the first interesting results obtained using these new artificial animal models for carcinogenicity studies. We have also briefly discussed other types of engineered mice (lac operon transgenic mice) that are especially suitable for detecting mutagenic effects in a broad spectrum of organs and tissues and that can help to establish mechanistic correlations between mutations and cancer frequencies in specific target organs. Finally, we have reviewed two complementary methods that, while obviously also feasible in rodents, are especially suitable for biomonitoring studies. We have illustrated some of the advantages and drawbacks related to the detection of DNA adducts in target and surrogate tissues using the 32P-DNA postlabeling technique, and we have discussed the possibility of biomonitoring mutations in different human target organs using a molecular technique that combines the activity of restriction enzymes with polymerase chain reaction (RFLP/PCR). Prediction of carcinogenic hazard and biomonitoring are very wide-ranging areas of investigation. We have therefore selected five different subfields for which we felt that interesting innovations have been introduced in the last few years. We have made no attempt to systematically cover the entire area: such an endeavor would have produced a book instead of a review article.

Carcinogens induce reversion of the mouse pink-eyed unstable mutation

Deletions and other genome rearrangements are associated with carcinogenesis and inheritable diseases. The pink-eyed unstable (pun) mutation in the mouse is caused by duplication of a 70-kb internal fragment of the p gene. Spontaneous reversion events in homozygous pun/pun mice occur through deletion of a duplicated sequence. Reversion events in premelanocytes in the mouse embryo detected as black spots on the gray fur of the offspring were inducible by the carcinogen x-rays, ethyl methanesulfonate, methyl methanesulfonate, ethyl nitrosourea, benzo[a]pyrene, trichloroethylene, benzene, and sodium arsenate. The latter three carcinogens are not detectable with several in vitro or in vivo mutagenesis assays. We studied the molecular mechanism of the carcinogen-induced reversion events by cDNA analysis using reverse transcriptase-PCR method and identified the induced reversion events as deletions. DNA deletion assays may be sensitive indicators for carcinogen exposure.

Consideration of the liver of embryonic lacZ transgenic mice as an analogue of the mouse coat colour spot test: preliminary data and technical problems

Pregnant lacZ+ transgenic mice (Muta Mouse) were treated with ENU (25 mg/kg) by oral gavage on day 10.5 of pregnancy. This dose of ENU is the optimal dose observed by other investigators for activity in the mouse coat colour spot test. Day 10.5 of pregnancy represents the stage when the embryonic liver first becomes visually discernable. By day 15.5, when the maternal and embryonic livers were analysed for lacZ- mutation frequency, the embryonic liver is the largest tissue in the embryo (approximately 25 mg). These experiments therefore represent treatment of a small pool of progenitor hepatocytes just as they are entering into an intense wave of cell division--optimum conditions for the fixation of mutations. Exposure to ENU led to an average fourfold, and a maximum tenfold increase in mutation frequency in the embryonic livers. This relatively weak response is consistent with lacZ- mutants not having a growth advantage; unlike in the mouse spot test, clonal amplification of lacZ- mutants cannot be separately scored. The level of mutation in the control embryonic livers was lower than that of the maternal control livers, but the group sizes were too small to conclude this definitively. Embryonic livers from each individual mother showed a range of mutation frequencies that were not obviously related to that of the maternal liver. On a treatment group basis, ENU was non-mutagenic to maternal livers. Half of the embryonic livers yielded DNA that failed to package, despite repeated attempts and re-isolation of the DNA from the liver. The cause of this unexpected finding is not clear. What is clear is that it was not due to the ENU treatment because eight of 22 control embryonic livers behaved similarly. These preliminary results suggest that further research is required in order to establish a practical transgenic analogue of the mouse coat colour spot test.

Induction of mutation in V79 Chinese hamster cells by tetrachlorohydroquinone, a metabolite of pentachlorophenol

Tetrachlorohydroquinone (TCHQ) and tetrachlorocatechol (TCC), two metabolites of the environmental mutagen and carcinogen pentachlorophenol, were tested without exogenous activation in V79 Chinese hamster cells for the induction of mutation at the hypoxanthine phosphoribosyl transferase (HPRT) locus to 6-thioguanine resistance (TGr) and at the Na/K-ATPase locus to ouabain resistance (OuaR). Treatment was for 24 h at 37 degrees C. TCHQ produced statistically significant increases in the frequency of TGr mutants. The lowest observed effective dose (LOED) was 20 microM, where the relative cloning efficiency was 63%. The relationship between the dose of TCHQ and the frequency of TGr mutants was approximately linear over the range of 0-60 microM with an estimated slope (+/- 95% confidence limits) of 1.1 +/- 0.3 mutants per 10(6) clonable cells per microM. At the highest tested dose of TCHQ, 60 microM, the relative cloning efficiency was reduced to 7%. In contrast to TCHQ, TCC was unable to induce TGr mutants at doses up to 120 microM. The relative cloning efficiency at this dose was 5%. Both TCHQ and TCC were unable to induce OuaR mutants. The results suggest that TCHQ is at least partly responsible for the genotoxic activity of pentachlorophenol. TCHQ can produce reactive oxygen species, which may cause large genetic damage such as deletions, resulting in mutation to TGr but not to OuaR.

Mutagenic and carcinogenic potency indices and their correlation

We have analyzed a significant number of studies existing in the literature, in which the ability of different short-term tests for predicting carcinogenicity in rodents was investigated. We have separated these studies into two groups. In the better known group of studies, qualitative predictivity was investigated (sensitivity and specificity). In the second group of studies (analyzed in greater detail), positive results were examined for the correlation between carcinogenic potency and potency of response in a given short-term test. There is substantial agreement between qualitative and quantitative predictivity; both appear to be situated between a low and moderate level. We have analyzed the interesting possibility of using the quantitative approach not only for positive data but for combined positive and negative data as well. We have stressed that short-term tests of genotoxicity should be asked to predict only initiation and irreversible alterations in the genome and not to predict a combination of these events, including promotion and modulation of differentiation. Even with regard to only initiation, genotoxicity data should be related to comparative metabolism, as well as to considerations of the significance of different end points and structure-activity relationship data. In conclusion, the information coming from short-term tests of genotoxicity is probably useful but should be used in conjunction with other types of information and only for predicting one particular class of events in the entire process of carcinogenesis.

The genotoxicity of sodium saccharin and sodium chloride in relation to their cancer-promoting properties

The literature indicates that sodium saccharin is non-reactive to DNA and inactive as a gene mutagen in vitro. At elevated dose levels it is capable of producing structural disturbances in eukaryotic chromosomes in vitro, and it shows intermittent activity as a very weak germ-cell and somatic-cell mutagen in vivo. Its possible mode of action in these respects is speculated on and related to its ability to promote bladder tumours in rats at elevated dose levels. A review of the toxicology of sodium chloride reveals a profile of genotoxic activities almost identical to that of sodium saccharin. It is suggested that the recorded genotoxic and cancer-promoting activities of these chemicals will only become apparent at elevated dose levels that define them as significant contributors to the biological medium (solvent) rather than as trace xenobiotic toxins (solutes). The possible activity of acid saccharin, or of its potassium, calcium and ammonium salts, as ionic genotoxins requires urgent evaluation.