Application of the framework to carcinogenicity testing

We have applied the assessment framework presented in Chap-ter I to analyze various options as potential alternatives to the cancer bioassay (DECD TG 451; DECD, 2009), which is con-ducted as a 2-year bioassay in rats and mice and is currently the only accepted test for carcinogenicity.

Testing with the cancer bioassay in two species can involve 600-800 animals, the histopathological examination of more than 40 tissues per animal, and costs approximately 0 I mil-lion per chemical and species (Vanparys et al., 20 II). This bio-assay is obviously time-consuming and expensive, and uses large numbers of animals. In addition, the assayCil predictiv-ity for humans has been challenged (Knight et aI., 2006a,b,c;

Shanks et aI., 2009). Thus, while protection against potential carcinogenic effects of environmental chemicals is a key de-sire of the public, this assay is not suitable for broad use, nor is it broadly used.

5.2.1 Abolition of useless tests

The concept that genotoxicity is the first and foremost mecha-nism of chemical carcinogenicity is rarely challenged. How-ever, there are little or no epidemiological data that support the hypothetical existence of widespread chemical carcinogen-esis. Not only has average age increased continuously over the last 150 years (Kirkwood, 2008), during which period about 100,000 chemicals were introduced into our environment, but age-adjusted cancer rates did not increase over this time period (Jemal et aI., 2009). Furthermore, exposure to mutagens did not correlate with oncomutations in people (Thilly, 2003).

·31!.1M4I.H!~B'

APOPTOSIS CELL TOXICITY

Fig. 5.2: Chemical carcinogenesis stages and the occurrences involved in each one

(modified from Oliveira et aI., 2007)

It is important to note that carcinogenicity testing was devel-oped as a result of historical cases of adverse effects, and the test models currently in place were developed with the existing knowledge at that time. However, the fact that there has been much scientific progress relevant to this field since then, com-bined with the degree of public concern about potential chemi-cal carcinogenicity, has led us to focus this paper on carcino-genicity testing.

Standardization of protocols

The cancer bioassay is astonishingly young, given the impor-tance of the health effect in question: the standardized protocol was suggested by the US National Cancer Institute in 1976 and adopted by DECD in 1981. The ICH (International Council on Harmonisation of Technical Requirements for the Registration of Pharmaceuticals for Human Use) only adopted the test for use in pharmaceuticals in 1997.

1 http://guidance.echa,europa,eu/docs/guidance_documenVinformation_requirements_r7a_en.pdf (last accessed 08.09.2011)

ACTIVATION INACTIVATION

GENOTOXIC MECHANISMS DNA adducts

Chromosome breakage.

fusion. deletion. mis-segregation.

non-NON-GENOTOXIC MECHANISMS Inflammation

disjunction

Hypermutability Genetic instability Loss of proliferation control

Resistance to apoptosis

EXCRETION (kidneys. liver. lungs)

Fig. 5.3: Metabolic activation of chemical compounds and genotoxic and non-genotoxic effects of carcinogens (modified from Oliveira et aI., 2007)

Although it is in many respects a well-standardized protocol, it has been criticized as having poorly defined endpoints and a high level of uncontrolled variation. Suggestions for aspects of the test that could be optimized include proper randomiza-tion, blinding, better necroscopy work, and adequate statistics (Freedman and Zeisel, 1988). However, 20 years after its adop-tion by OECD, the most recent test guidelines (OECD, 2009) still do not make randomization and blinding mandatory, and the guideline statistics do not control for multiple testing, de-spite the fact that about 60 endpoints are assessed in the assay.

Furthermore, the data analysis is ill-defined: "When applicable, numerical results should be evaluated by an appropriate and generally acceptable statistical method.D

Reducing the duration of the assay to 18 months has also been suggested (Davies et aI., 2000), although others contradicted the applicability of this option (Haseman et aI., 2001).

In addition, the assay has not been standardized for animal strains, with the only definition being that "young healthy adult animals of commonly used laboratory strains should be em-ployed.CIThis is contrary to evidence that strain standardization

appears to be a most critical factor. Even when the same strain is used, there appear to be problems with standardization that hamper the use of historical control groups (Haseman et aI., 1997). In this study, the most commonly used strains showed strong weight gain and changes in some tumor incidences that resulted in reduced survival over just one decade, which was attributed to the intentional or inadvertent selection of breeding stocks with faster growth and easier reproduction. Other fac-tors that have been suggested that could possibly influence the bioassay protocol over time include caging protocols, diet, en-vironmental factors, genetic drift, study duration, and survival differences.

An analysis of 1,872 individual species/gender group tests in the US National Toxicology Program (NTP) showed that 243 of these tests resulted in Clquivocal evidenceDor were judged as Dnadequate studiesD(Seidle, 2006), suggesting the protocol as it stands is not robust. The two-species paradigm also has been challenged (Alden et aI., 1996) by studies showing that rats are more sensitive, and regulatory action is rarely taken on the basis of bioassay results in mice (Van Oosterhout et aI., 1997; van

Ravenzwaay, 20 I 0). It is estimated that $ 1-2 million and up to I ,000 mice over a 3-year period would be saved by eliminating the mouse section of each chemical test (Alden et aI., 1996).

Reproducibility

Gottmann et al. (2001) compared 121 replicate rodent carcino-genicity assays from the two sections (National Cancer Insti-tute/National Toxicology Program and literature) of the Carci-nogenic Potency Database (CPDB) to estimate the reliability of these experiments. They found a concordance of 57% between the overall rodent carcinogenicity classifications from both sources; this result did not substantially improve when species, sex, strain, and target organ information was considered. They concluded: CI'hese results indicate that rodent carcinogenicity assays are much less reproducible than previously expected, an effect that should be considered in the development of structure-activity relationship models and the risk assessment process.O Ironically, cell transformation assays (CTA, discussed in more detail below) appear to reproduce the cancer bioassay better than it reproduces itself. Thus, it appears likely that the existing bioassay would fail any validation investigation that a replace-ment test would be subjected to.

Potency correlation between species

This is not a classical validation criterion, but it is part of the Bradford-Hill criteria to support associations. The apparent cor-relation between potency of carcinogens in mice and rats has been shown to be largely an artifact (Bernstein et aI., 1985).

Interspecies and organ site correlation

Concordances of 57% were reported between mouse and rat bio-assays. Better correlations that were previously reported (71 % rat to mouse, 76% mouse to rat) were driven by the abundance of strong mutagens studied, which are typically positive in all sexes, many species', and several organs (Gray et aI., 1995).

An analysis of bioassays in rats, mice, and hamsters, as well as comparisons with humans for known carcinogens, has shown that the likelihood of a chemical that induces tumors in one spe-cies in a certain organ also inducing tumors in another spespe-cies in the same organ is less than 50% (Gold et aI., 1991,1998).

Sex specificity

A critical appraisal of the role of sex hormones (endocrine sta-tus) on species susceptibilities in chemical carcinogenesis (Toth, 2002) concluded: CI'here are compelling indications, particu-larly in the fields of physiology and metabolism, to conclude the limited usefulness of the various animal species in sex hormone research. The findings allow only restricted inferences for the human species.O

Scientific relevance

The first critical issue is that of high-dose to low-dose extrapo-lation. The use of maximum tolerated doses appears to be the source of many artifacts. Jay Goodman, Michigan State Uni-versity, is cited (Schmidt, 2002) as saying: Df we're dealing with a situation in which the likely human exposure is in the same ballpark, then these (dosing regimens) may be applicable.

But doses that are hundreds to thousands of times higher than normal exposures (such as those often given during animal test-ing) might be carcinogenic simply because they overwhelm de-toxification pathways. In these cases, we see tumors along with gross histopahologic evidence of tissue damage.O However, dose regimens are defended by others (Bucher, 2000), and many substances test positive for carcinogenicity only at maximum tolerated doses, including some accepted human carcinogens.

These results also might be interpreted as species differences that are hidden by high-dose artifacts at the expense of many false-positives.

Predictivity of point of reference (human cancer)

An analysis by Pritchard et al. (2003) suggested 69% predictivi-ty of human carcinogenicipredictivi-ty for the two-species cancer bioassay, which ironically dropped to 65% when it was combined with in vitro genotoxicity test findings (Pritchard et aI., 2003). This con-trasts with an analysis by Knight et al. (2006a,b), who showed that in 58% of cases considered by the EPA, they deemed results from a positive cancer bioassay as insufficient for assigning hu-man carcinogenicity, even though the EPA was far more likely to assign this classification than the IARC. A previous comparison of known human carcinogens, a's classified by the IARC mainly based on epidemiology, with corresponding animal data found an unconvincing correlation (Freedman and Zeisel, 1988): CI'he research reports of the cancer community (even taken at face value) do not sustain the conventional argument for the validity of the qualitative extrapolation ... We remain sympathetic to the idea that animal data have some predictive value for carcino-genicity in humans ... But the evidence for such propositions is surprisingly weak.OIt is also w.orth noting that the most typical sites of tumor formation in humans do not correspond to those in rodents (Anisimov et aI., 2005), as shown in Table 5.1:

Tab. 5.1: Most common spontaneous cancers in humans and rodents

(adapted from Anisimov et aI., 2005)

d'~ncer

I:

^{.,~i,~~ iU'" . Rats}

Endometrial carcinomas 0 +

Leukaemia I lymphoma + 0

In the absence of human data, it might be considered reasona-ble to use data from tests in nonhuman primates for comparative purposes. Cancer bioassays in nonhuman primates were carried out on 37 compounds within 34 years (Takayama et aI., 2008);

the results were D .. Inconclusive in many cases,Dbut carcino-genicity was shown unequivocally for four of them.

Taken together, the cancer bioassay is D .. Often not relevant to human carcinogenesis risk assessment.D(Ward, 2007).

Specificity

About 50% of all chemicals tested in the cancer bioassay test positive (see Tab. 5.2), and 53% of 301 chemicals tested by the NTP were positive, with 40% of these positives classified as non-genotoxic (Ashby and Tennant, 1991). It is sometimes claimed that this high positive rate is due to the testing of suspi-cious substances, especially in early years of identification of mutagens. Of substances tested in the NTP simply because of exposure considerations, 80% were found not to be carcino-genic (Fung et aI., 1995). In contrast, Johnson identified 60%

of 128 high production volume chemicals to be rodent carcino-gens (Johnson, 2003). A similarly high proportion, around 50%

positives, can be found in various databases for pharmaceuticals (MacDonald,2004).

Pharmaceuticals are rapidly discontinued when they are found to be possibly genotoxic, but also many non-genotoxic ones test positive in the cancer bioassay (Silva Lima and Van der Laan, 2000). r::I'he database compiled from the 'Physician's Desk Reference' (PDR), including registered pharmaceuticals only, also provides a good illustration of rodent tumor findings being irrelevant to humansD(Davies and Monro, 1995; Silva Lima and Van der Laan, 2000). Over two decades, 101 out of 241 substances entered the market despite positive cancer bio-assays, presumably primarily as a result of the positive bioassay

Tab. 5.2: Proportion of chemicals evaluated as carcinogenic (modified from Ames and Gold, 2000; Gold et aI., 2005)

Proportion % Chem. tested In rats and mice 379/648 58%

• natural 861165 55%

• synthetic 293/493 59%

.Chem. tested 11'1 r~ts or mice 751 11456 52%

• Naturalpe~iicldes 41175 52%

• Commercia II pesticides 79/198 55%

• Chemicals In roasted coffee 23/32 72%

• Mold toxins 15/25 60%

Drugs (PDR) 117/241 49%

Drugs (FDA) 125/282 44%

testing being considered irrelevant compared to the medical benefit of the compounds (Davies and Monro, 1995). It is not known how many chemicals were rejected over the same period (Davies and Monro, 1995). An early analysis of 20 putative hu-man non-carcinogens found 19 false-positives, suggesting only 5% specificity (Ennever et aI., 1987). The inappropriateness of rodent carcinogenicity assays as currently performed has been examined by Roe (1987), who notes that: IThere can be no sense in testing chemicals for carcinogenicity in rats main-tained under conditions such that 50-100% of them (the control animals) develop pituitary and mammary tumors, etc. There is no identifiable population of humans for which such rats could constitute a model.DThe implications of these observations for risk assessment have been noted by Bridges (Bridges, 1988).

However, others see even this as an underestimate (Sobe1s, 1987): D .. Carcinogenicity is expressed to a different extent in different species of rodents, so that bioassay results in only two rodent species are likely to underestimate the proportion of chemicals with carcinogenic potential.D

Sensitivity

Assessing the sensitivity of the cancer bioassay is made diffi-cult by the fact that most human carcinogens were designated as such, to a large extent, by animal tests (with the discussed prob-lematic interspecies correlation), and those typically missed are not identified by other means. There are strong claims that all known human carcinogens are detected with the cancer bio-assay (Huff, 1999; Rail, 2000), but this could be considered a self-fulfilling prophecy, as most of these classifications are based on animal experiments. However, not all known human carcinogens can be modeled in animals (Silbergeld, 2004). For example, there is

D no animal model of cigarette smoke-induced lung cancer, D no rodent leukemia induced by benzene, and

D no genetic point mutations in animals induced by arsenic.

This situation does not necessarily represent a contradiction, as these agents are positive for carcinogenicity in other organs or by other modes of action. However, achieving the right clas-sification but for the wrong reason is a questionable outcome.

Furthermore, the current testing situation leads to an enormous number of false-positives; Rail suggests that only one in ten compounds is truly carcinogenic (Rail, 2000).

Despite all of these false-positives, cases of human carcinogens that are not detectable in animals remain, e.g., the anticonvulsant diphenyl-hydantoin (phenytoin) is classified as carcinogenic to humans but showed no carcinogenic effect in experimental mice and rats (Anisimov et aI., 2005). Ennever and Lave (2003) also have discussed the chemical combination of aspirin!phenacetin!

caffeine, which is classified as a human carcinogen but tested neg-ative in both rodent species (En never and Lave, 2003). Johnson (2001) presents a list of the known human carcinogens that have been tested in the NTP rat bioassay prior to 2000 (Johnson, 200 I):

r::I'he list contains 10 different chemicals, counting the variolls forms o.fasbestos as one, the three nickel compounds as one, and the 10 bellzidine-like compounds as one ... (Of) the 13 individual chemicals tested in four sex-species groups, two chemicals were positive in/our groups, one was positive in three groups, six were

positive in two groups, olle was positive in aile group, and three were positive in no (0) groups. Only two human carcinogens (thio-tepa and bellzene) are bonafide trails-species carcinogens. Thus, jar NTPRB-tested chemicals, it is not evident that human carcinogens necessarily demonstrate clear trans-species carcino-gellic effects.Drhese examples clearly contradict claims of 100%

identification of known human carcinogens. It is also worth not-ing that an early assessment of the bioassay suggested only 46%

sensitivity based on 19 human carcinogens (Salsburg, 1983). The fact that rats and mice predict each other only about 57%

does not fit with an assumption that 100% of human carcino-gens are detected, as it is fair to assume that humans are not better predicted by either species than they predict each other.

These figures of 57% concordance between species, 10% real human carcinogens, and 53% positives in the rat, combine to give a sensitivity of 100% with a specificity of 47%. Lave et al. previously arrived at an estimate of 70% sensitivity as well as specificity, assuming 10% real human carcinogens (Lave et aI., 1988).

If the same calculation is performed with the assumption that 20% of all chemicals are carcinogenic in humans, this results in 75% sensitivity with 53% specificity. Inten:stingly, when we use the suggested 28% positives in rat, if non-suspicious chemicals are tested the result is 0% sensitivity and 65% specificity. Thus, whatever assumption is used, the assay does not perform well by any standard.

In a telling modeling exercise, Gaylor (2005) showed that in-creasing the number of animals per group from 50 to 200 would result in statistically significant (p<O.OI) dose-responses for 92% of substances tested (Gaylor, 2005). This shows how the inherent variability of the test produces false-positives and re-duces specificity using the current data analysis process.

Applicability domain

An applicability domain, i.e., the part of the chemical universe where the cancer bioassay is applicable to make sufficiently cor-rect predictions, has not been established for the rodent cancer bioassay. Occasional reference is made to a better prediction of (strong) genotoxic substances, but these substances are exactly the ones filtered out by the in vitro genotoxicity testing battery and are unlikely to be tested in the bioassay.

Peljormance standards

Performance standards have been introduced for test meth-ods as a guide to demonstrating that a given variant of a test is equivalent to the originally validated test. No such perform-ance standards exist for the bioassay, although they would be very helpful for evaluation of alternative test methods. Bucher reported a discordance rate of 13/38 for the transgenic approach (Bucher, 1998), or a level of agreement of 68%, which barely differs from the 65% shown by the Salmonella mutation test.

In response, Johnson showed the fingerprint pattern of organ sites affected (Johnson, 1999), concluding D .. It seems unlikely that transgenic models co?tld ever replicate or jaithfully emu -late the carcinogenic response observed in natural whole

ani-mals.DMore extensive evaluations were conducted by an ILSI!

HESI committee (Cohen et aI., 200 I). An article on these

ef-forts (Schmidt, 2002) resulted in a discussion of whether this is [lJashing the cancer bioassayD(Johnson and Huff, 2002). While possibilities for improving the animal test are outside the scope of this paper, the discussion shows the difficulties of using the cancer bioassay as point of comparison.

These comments, taken together, indicate that the cancer bio-assay Dalthough it has never been formally assessed Dappears to have severe limitations. Furthermore, the assay would not stand up to the assessment criteria that any potential replace-ment test would have to fulfill. However, these limitations are not fully understood by many who use the assay for validation of alternative methods or regulatory purposes.

It appears timely to address these limitations before em-barking on the expensive process of developing and validat-ing replacement strategies that would only then be measured against this wrongly-considered [8old standardDtest. It might be debated whether this represents a case for formal invalidation (Balls et aI., 2006; Balls and Combes, 2005), but an approach based on the principles of evidence-based toxicology (Hartung, 2010c) seems to be more appropriate in this scenario than for-mal validation. A forfor-mal assessment of the assay would allow widespread dissemination Dand encourage acceptance Dof the evidence for the assay)] limitations.

In line with these suggestions, the REACH guidance by ECHA is already quite cautious in its recommendations for use of the cancer bioassay (ECHA, 2008): D\ carcinogenicity study may, on occasion, be justified.

If

there are clear suspicions that the substance may be carcinogenic, and available injormation (from both testing and non-testing data) are not conclusive in this, both in terms oj hazard and potency, then the need jor a carcinogenicity study should be explored. In particular, such a study may be required jor substances with a widespread, dis -persive use or jor substances producing frequent or long-term human exposures. However, it should be considered only as a last resort.D

5.2.2 Reduction to key events

This approach aims to replace in vivo testing with stand-alone

This approach aims to replace in vivo testing with stand-alone

Im Dokument A roadmap for the development of alternative (non-animal) methods for systemic toxicity testing (Seite 40-52)