Time-related induction of DNA repair synthesis in rat hepatocytes following in vivo treatment with cyproterone acetate

A core in vitro genotoxicity battery comprising the Ames test plus the in vitro micronucleus test is sufficient to detect rodent carcinogens and in vivo genotoxins

In vitro genotoxicity testing needs to include tests in both bacterial and mammalian cells, and be able to detect gene mutations, chromosomal damage and aneuploidy. This may be achieved by a combination of the Ames test (detects gene mutations) and the in vitro micronucleus test (MNvit), since the latter detects both chromosomal aberrations and aneuploidy. In this paper we therefore present an analysis of an existing database of rodent carcinogens and a new database of in vivo genotoxins in terms of the in vitro genotoxicity tests needed to detect their in vivo activity. Published in vitro data from at least one test system (most were from the Ames test) were available for 557 carcinogens and 405 in vivo genotoxins. Because there are fewer publications on the MNvit than for other mammalian cell tests, and because the concordance between the MNvit and the in vitro chromosomal aberration (CAvit) test is so high for clastogenic activity, positive results in the CAvit test were taken as indicative of a positive result in the MNvit where there were no, or only inadequate data for the latter. Also, because Hprt and Tk loci both detect gene-mutation activity, a positive Hprt test was taken as indicative of a mouse-lymphoma Tk assay (MLA)-positive, where there were no data for the latter. Almost all of the 962 rodent carcinogens and in vivo genotoxins were detected by an in vitro battery comprising Ames+MNvit. An additional 11 carcinogens and six in vivo genotoxins would apparently be detected by the MLA, but many of these had not been tested in the MNvit or CAvit tests. Only four chemicals emerge as potentially being more readily detected in MLA than in Ames+MNvit--benzyl acetate, toluene, morphine and thiabendazole--and none of these are convincing cases to argue for the inclusion of the MLA in addition to Ames+MNvit. Thus, there is no convincing evidence that any genotoxic rodent carcinogens or in vivo genotoxins would remain undetected in an in vitro test battery consisting of Ames+MNvit.

Follow-up testing of rodent carcinogens not positive in the standard genotoxicity testing battery: IWGT workgroup report

At the Plymouth Third International Workshop on Genotoxicity Testing in June 2002, a new expert group started a working process to provide guidance on a common strategy for genotoxicity testing beyond the current standard battery. The group identified amongst others "Follow-up testing of tumorigenic agents not positive in the standard genotoxicity test battery" as one subject for further consideration [L. Müller, D. Blakey, K.L. Dearfield, S. Galloway, P. Guzzie, M. Hayashi, P. Kasper, D. Kirkland, J.T. MacGregor, J.M. Parry, L. Schechtman, A. Smith, N. Tanaka, D. Tweats, H. Yamasaki, Strategy for genotoxicity testing and stratification of genotoxicity test results-report on initial activities of the IWGT Expert Group, Mutat. Res. 540 (2003) 177-181]. A workgroup devoted to this topic was formed and met on September 9-10, 2005, in San Francisco. This workgroup was devoted to the discussion of when it would be appropriate to conduct additional genetic toxicology studies, as well as what type of studies, if the initial standard battery of tests was negative, but tumor formation was observed in the rodent carcinogenicity assessment. The important role of the standard genetic toxicology testing to determine the mode of action (MOA) for carcinogenesis (genotoxic versus non-genotoxic) was discussed, but the limitations of the standard testing were also reviewed. The workgroup also acknowledged that the entire toxicological profile (e.g. structure-activity relationships, the nature of the tumor finding and metabolic profiles) of a compound needed to be taken into consideration before the conduct of any additional testing. As part of the meeting, case studies were discussed to understand the practical application of additional testing as well as to form a decision tree. Finally, suitable additional genetic toxicology assays to help determine the carcinogenic MOA or establish a weight of evidence (WOE) argument were discussed and formulated into a decision tree.

Genotoxic potential of cyproterone acetate: a possible role of reactive oxygen species

The genotoxicity of cyproterone acetate was studied in human lymphocytes using chromosomal aberrations (CAs), mitotic index (MI) and sister chromatid exchanges (SCEs) as a parameter. Cyproterone acetate was found to be genotoxic at 20 and 30 microM. To study the possible mechanism of the genotoxicity of cyproterone acetate, superoxide dismutase (SOD) and catalase (CAT) were used separately and in combination along with the 30 microM of cyproterone acetate at different doses. SOD treatment increases CAs, SCEs and decreases MI as compared to treatment with 30 microM CPA alone, at both of the tested doses. CAT treatment decreases the frequencies of CAs and SCEs and increases MI in both, as compared to treatment with 30 microM CPA alone, separately and in combination with SOD, suggesting a possible role of reactive oxygen species for the genotoxic damage.

Genotoxicity of hormonal steroids

Hormonal steroids have a widespread use in medicine and their side effects are continuously debated. The possible genotoxic activity of steroids has been the subject of many investigations. The natural estrogens estradiol, estrone and estriol are generally negative in the ICH core battery of tests, but several positive results have been obtained when using additional endpoints of genotoxicity. The genotoxic activity of the 4-hydroxy metabolites of estradiol and estrone is well established. The synthetic steroidal estrogens have a comparable profile of negative and positive test results. Cyproterone acetate and some of its analogues have a special position within the group of progestins. Their genotoxic potential has been established. Other progestins are generally negative in the routine tests. Anti-glucocorticoids, anti-progestins, corticosteroids, androgens, anabolics and anti-androgens appear to be devoid of genotoxic activities. The genotoxic potential of estradiol, estrone and cyproterone acetate with its analogues may play no role under normal physiological and therapeutic conditions. The metabolic conditions that are needed for the formation of DNA-reactive metabolites and oxygen radicals may not be present in humans. Epidemiological cancer data seem to support this view. The importance of thresholds in the dose-effect-relationship of genotoxicity data and their use in risk assessment is discussed.

Are some progestins genotoxic liver carcinogens?

Progestins (progestogens) are classified by the International Agency for Research on Cancer (IARC) as possibly carcinogenic to humans. In the last decade evidence has shown that a synthetic drug of this family, cyproterone acetate, is activated to a reactive species by the liver, and forms DNA adducts and elicits DNA repair in hepatocytes from both rats and humans. The response is similar in humans of both genders but markedly higher in female than in male rats. The promutagenic character of DNA lesions is indicated by the increase in liver of female rats of the frequency of micronucleated cells, of mutations, and of enzyme-altered preneoplastic foci. Two other synthetic progestins, chlormadinone acetate and megestrol acetate, and an aldosterone antagonist, potassium canrenoate, share with cyproterone acetate the 17-hydroxy-3-oxopregna-4,6-diene structure. While less extensively studied, results so far obtained indicate that they are capable of inducing genotoxic effects qualitatively similar to those of cyproterone acetate. The majority of progestins have not been systematically tested for genotoxicity and the generally negative responses obtained with the standard battery of genotoxicity tests might be the consequence of the use of inappropriate target cells and/or metabolic activation systems. Cyproterone acetate, is activated by the hepatocytes to reactive species of such short half-life that they react only with the DNA of the cell in which are formed. Therefore, it cannot be excluded a priori that other progestins will not display genotoxic effects when tested adequately. This hypothesis is supported by the knowledge that estrogen-progestin combinations used as oral contraceptives are classified by the IARC as carcinogenic to humans due to the increased risk of hepatocellular carcinoma. This risk should probably be ascribed to the progestin component, since estrogens are carcinogenic to humans due to the increased risk of endometrial and possibly of breast cancer but not liver cancer.

ICH-harmonised guidances on genotoxicity testing of pharmaceuticals: evolution, reasoning and impact

The International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH) has convened an expert working group which consisted of the authors of this paper and their respective committees, consulting groups and task forces. Two ICH guidances regarding genotoxicity testing have been issued: S2A, 'Guidance on Specific Aspects of Regulatory Genotoxicity Tests' and S2B, 'Genotoxicity: A Standard Battery for Genotoxicity Testing of Pharmaceuticals.' Together, these guidance documents now form the regulatory backbone for genotoxicity testing and assessment of pharmaceuticals in the European Union, Japan, and the USA. These guidances do not constitute a revolutionary new approach to genotoxicity testing and assessment, instead they are an evolution from preexisting regional guidelines, guidances and technical approaches. Both guidances describe a number of specific criteria as well as a general test philosophy in genotoxicity testing. Although these guidances were previously released within the participating regions in their respective regulatory communiqués, to ensure their wider distribution and better understanding, the texts of the guidances are reproduced here in their entirety (see Appendix A) and the background for the recommendations are described. The establishment of a standard battery for genotoxicity testing of pharmaceuticals was one of the most important issues of the harmonisation effort. This battery currently consists of: (i) a test for gene mutation in bacteria, (ii) an in vitro test with cytogenetic evaluation of chromosomal damage with mammalian cells or an in vitro mouse lymphoma tk assay, (iii) an in vivo test for chromosomal damage using rodent hematopoietic cells. A major change in testing philosophy is the acceptance of the interchangeability of testing for chromosomal aberrations in mammalian cells and the mouse lymphoma tk assay. This agreement was reached on the basis of the extensive review of databases and newly generated experimental data which are in part described in this publication. The authors are fully aware of the fact that some of the recommendations given in these ICH guidances are transient in nature and that the dynamic qualities and ongoing evolution of genetic toxicology makes necessary a continuous maintenance process that would serve to update the guidance as necessary.

Formation of DNA adducts by cyproterone acetate and some structural analogues in primary cultures of human hepatocytes

Recently, we have shown that the therapeutically-used progestin and antiandrogen cyproterone acetate (CPA) causes the formation of high levels of DNA adducts in rat hepatocytes and rat liver [J. Topinka, U. Andrae, L.R. Schwarz, T. Wolff, Cyproterone acetate generates DNA adducts in rat liver and in primary rat hepatocyte cultures, Carcinogenesis 14 (1993) 423-427: J. Topinka, B. Binkova, L.R. Schwarz, T. Wolff, Cyproterone acetate is an integral part of hepatic DNA adducts induced by this steroidal drug, Carcinogenesis 17 (1996) 167-169; S. Werner, J. Topinka, T. Wolff, L.R. Schwarz, Accumulation and persistence of DNA adducts of the synthetic steroid cyproterone acetate in rat liver, Carcinogenesis 16 (1995) 2369-2372; J. Topinka, B. Binkova, H.K. Zhu, U. Andrae, I. Neumann, L.R. Schwarz, S. Werner, T. Wolff, DNA damaging activity of the cyproterone acetate analogues chlormadinone acetate and megestrol acetate in rat liver, Carcinogenesis 16 (1995) 1483-1487]. Its structural analogues, chlormadinone acetate (CMA) and megestrol acetate (MGA) were much less active in this respect [J. Topinka, B. Binkova, H.K. Zhu, U. Andrae, I. Neumann, L.R. Schwarz, S. Werner, T. Wolff, DNA damaging activity of the cyproterone acetate analogues chlormadinone acetate and megestrol acetate in rat liver, Carcinogenesis 16 (1995) 1483-1487]. In the present study we addressed the question whether these compounds and two further analogues, medroxyprogesterone acetate (MPA) and progesterone, induce the formation of DNA adducts in primary cultures of human hepatocytes. Incubation of CPA with human hepatocytes from two male and four female donors induced the formation of significant levels of CPA-DNA adducts detectable by 32P-postlabeling. The by far most prevalent DNA adduct is most likely identical with adduct A formed in CPA-treated rats. DNA binding was found even at 0.03 microM CPA, the lowest concentration used. The maximum effect occurred at about 10 microM in 5 of the 6 cell preparations. At this concentration 480 and 2690 adducts x 10(-9) nucleotides were detected in hepatocytes of the two male donors and 1072, 816, 613 and 659 adducts x 10(-9) nucleotides in the hepatocytes of the four female donors after an exposure of 6 h with CPA. Extending the incubation time to 20 h resulted in a roughly three-fold higher binding. CMA and MGA were assayed in two of the liver cell preparations from the female donors. At a concentration of 20 microM and an incubation time of 6 h, DNA adduct levels for CMA were 21 and 43% and for MGA 31 and 65% of the levels observed with 20 microM CPA. In contrast, DNA binding of MPA amounted to less than 1% of that observed with CPA and DNA binding of the natural occurring progestin progesterone was below the level of detection. The results point to a genotoxic risk associated with the therapeutic use of CPA and possibly of CMA and MGA. Furthermore, the findings suggest that the significant genotoxicity observed with CPA, MGA and CMA is associated with the presence of the double bond in position 6-7 of the steroid, which is absent in MPA and progesterone.