Tab. 1:
Worrisome analyses as to the relevance of traditional systemic toxicity studies
| Repeated-Dose Toxicity (RDT) | Developmental and reproductive toxicity (DART) | Cancer bioassay |
|---|---|---|
| Interspecies concordance of mice with rats (37 chemicals): 57%–89% (average 75%) in short-term and 65%–89% (average 80%) in long-term studies (Wang and Gray, 2015) | No relevant contribution to regulatory decision-making by second generation testing (Janer et al., 2007; Martin et al., 2009a) | While 53% of all chemicals test positive, age-adjusted cancer rates did not increase over the last century (Jemal et al., 2009) |
| Mouse-to-rat organ prediction (37 chemicals) in long-term studies with an average of 55%, in short-term studies with an average of 45%. For rat-to-mouse, the averages were 27% and 49%, respectively (Wang and Gray, 2015) | 254 chemicals in ToxRefDB tested in both multi-generation and 2-year chronic studies, and 207 chemicals tested in both multigeneration and 90-day subchronic studies (Martin et al., 2009b); with an assessment factor of 10, the hazard of reproductive toxicity might be covered for 99.8 % of substances | Exposure to mutagens does not correlate with oncomutations in people (Thilly, 2003) |
| Species concordance (310 chemicals) for non-neoplastic pathology between mouse and rat was 68% (Wang and Gray, 2015) | No experience for industrial chemicals: < 25 two-generation-studies and < 100 one-generation studies in EU and US in 30 years (Bremer et al., 2007) | Protocol has poorly defined endpoints and a high level of uncontrolled variation; could be optimized to include proper randomization, blinding, better necroscopy work, and adequate statistics (Freedman and Zeisel, 1988). |
| Inter-species differences mouse vs. rat (95th percentile) of 8.3 for RDT (Bokkers and Slob, 2007) | Large number of individual skeletal variations (sometimes > 80%) even in control animals (Daston and Seed, 2007) | Most recent test guidelines (OECD, 2009) still do not make randomization and blinding mandatory, and statistics do not control for multiple testing, although about 60 endpoints are assessed (Basketter et al., 2012). |
| Low correlation between 28-day and 90-day NOAEL for 773 chemicals (Luechtefeld et al., 2016b, Fig.4) | Of those agents thought not to be teratogenic in man, only 28% are negative in all species tested (Brown and Fabro, 1983) | Not standardized for animal strains (“young healthy adult animals of commonly used laboratory strains should be employed”) ( Basketter et al., 2012) |
| A limited set of only six targets consisting of liver, kidney, clinical chemistry, body weight, clinical symptoms and hematology within a study gives a probability of 86% to detect the LOEL (Batke et al., 2013) | Of 1223 definite, probable and possible animal teratogens, fewer than 2.3% were linked to human birth defects (Bailey et al., 2005) | Problems with standardization of strains that hamper the use of historical control groups (Haseman et al., 1997): the most commonly used strains showed large weight gain and changes in some tumor incidences that resulted in reduced survival over just one decade (attributed to intentional or inadvertent selection of breeding stocks with faster growth and easier reproduction) |
| Not robust with about 25% equivocal studies (Bailey et al., 2005) | Analysis of 1,872 individual species/gender group tests in the US National Toxicology Program (NTP) showed that 243 of these tests resulted in “equivocal evidence” or were judged as “inadequate studies” ( Seidle, 2006) | |
| 74 industrial chemicals tested in New Chemicals Database: 34 showed effects on offspring, but only 2 chemicals were classified as developmental toxicants (Bremer and Hartung, 2004) | Questionable two-species paradigm as rats are more sensitive, and regulatory action is rarely taken on the basis of results in mice (Van Oosterhout et al., 1997; van Ravenzwaay, 2010) | |
| 55% of positives in screening studies not in multi-generation studies (Bremer and Hartung, 2004) | Concordance of 57% comparing 121 replicate rodent carcinogenicity assays ( Gottmann et al., 2001) | |
| Group size limits statistical power (Hotchkiss, 2008) | The apparent correlation between potency of carcinogens in mice and rats is largely an artifact (Bernstein et al., 1985). | |
| 61% inter-species correlation (Hurtt, 2003; Bailey et al., 2005) | Concordance of 57% between mouse and rat bioassays (Gray et al., 1995). | |
| Given 2.5% true reproductive toxicants and 60% inter-species correlation, testing with two species will find 84% of the toxic but label 64% of the negatives falsely (Hartung, 2009b) | Less than 50% probability for known carcinogens that induce tumors in one species in a certain organ to also induce tumors in another species the same organ comparing rats, mice, and hamsters, as well as humans (Gold et al., 1991, 1998). | |
| Of 38 human teratogens, the following percentages tested positive in other species: mouse 85%, rat 80%, rabbit 60%, hamster 45%, monkey 30%, two or more species 80%, any one species 97% (Brown and Fabro, 1983) | Doses are hundreds to thousands of times higher than normal exposures and might be carcinogenic simply because they overwhelm detoxification pathways (Schmidt, 2002) | |
| Of 165 human non-teratogens, the following percentages tested negative in other species: mouse 35%, rat 50%, rabbit 70%, hamster 35%, monkey 80%, two or more species 50%, all species 28% (Brown and Fabro, 1983) | 69% predictivity of human carcinogenicity for the two-species cancer bioassay (Pritchard et al., 2003) | |
| Reproductive toxicity within 10-fold of maternal repeated-dose toxicity for 99.8% of 461 chemicals (Martin et al., 2009b) | In 58% of cases considered by the EPA, the positive cancer bioassay was insufficient for assigning human carcinogenicity ( Knight et al., 2006a,b ) | |
| Cancer bioassays in nonhuman primates on 37 compounds were “… inconclusive in many cases” but carcinogenicity was shown unequivocally for four of them ( Takayama et al., 2008) | ||
| About 50% of all chemicals tested positive in the cancer bioassay test (Basketter et al., 2012), and 53% of 301 chemicals tested by the NTP were positive, with 40% of these positives classified as non-genotoxic (Ashby and Tennant, 1991) | ||
| An early analysis of 20 putative human non-carcinogens found 19 false-positives, suggesting only 5% specificity (Ennever et al., 1987). | ||
| Only one in ten positive compounds is truly carcinogenic (Rall, 2000) | ||
| Not all human carcinogens are found: Diphenyl-hydantoin (phenytoin) (Anisimov et al., 2005); the combination of aspirin/ phenacetin/ caffeine (Ennever and Lave, 2003); asbestos, nickel, benzidine-like compounds (Johnson, 2001); no cigarette smoke-induced lung cancer, no rodent leukemia induced by benzene, and no genetic point mutations induced by arsenic (Silbergeld, 2004). | ||
| Estimate 70% sensitivity as well as specificity, assuming 10% real human carcinogens (Lave et al., 1988) | ||
| Of 167 chemicals that caused neoplastic lesions in rat or mouse chronic/cancer studies, 35% caused neoplastic lesions in both rat and mouse (Martin et al., 2009a) | ||
| Increasing the number of animals per group from 50 to 200 would result in statistically significant (p < 0.01) dose-responses for 92% of substances tested (Gaylor, 2005) |