The document discusses the threshold of toxicological concern (TTC). It describes how the TTC was developed based on regulatory standards for food additives in the US. The TTC approach is proposed for use in various regulatory contexts to screen for substances that pose negligible risk based on their exposure levels. The benefits of the TTC include being fast, simple, and conservative. Downsides include uncertainty in the approach and its reliance on animal data and assumptions about exposure levels. Future developments may integrate TTC with improved exposure assessments and toxicity screening methods.

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2. Regulatory Relevance of the
Threshold of Toxicological Concern
(TTC)
R B Cope BVSc BSc (Hon 1) PhD cGLPCP
DABT ERT
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3. Content & Focus
• What is the TTC and how was it developed?
• What is the regulatory relevance of the TTC?
• Future directions of the TTC approach.
• Benefits and risks of the TTC approach?
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4. The glass metaphor: A toxicologist’s perspective
Economics Department: The glass is twice as big as it needs to be.
Somebody needs to talk to manufacturing. We could save a fortune by
only producing the bottom half of the glass.
Manufacturing Department: Response to the Economics Department - the
glass is clearly full: half with water and half with air. What is your
problem?
Accounts Department: Who needs economists when you have
accountants?
Marketing Department: If the glass was in a forest with nobody watching,
what would it be?
Toxicology Department: The glass has acceptable safety properties
allowing for an uncertainty factor of 100, erring on the side of the
precautionary principle, assuming: that animal to human extrapolation is
valid, that there was adequate experimental power, the study Klimisch
score was 1, life is good today, the mice felt fine right up until they died,
marketing has not complained about the toxicology department lately, the
manufacturing department ignores us anyway, the economists don’t see
the point of the toxicology department, marketing does not understand
toxicology, the accountants don’t like the CRO we are using, the regulators
are saying nice things about our submission, the product stewards like us
at the moment, my paper has just been accepted for publication, its Friday
and I have a great weekend coming up, its payday, …………………
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5. Slide acknowledgement: Watze de Wolf, DuPont 5

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8. Where does the TTC come from?
• US FD&C Act amendments 1958
– Required premarket approval or review of food additives
prior to their inclusion in food:
• Precisely defined what is a “food additive;”
• Established the standards of review and safety for food-additive
approvals;
• "food additive" to include components of food-contact articles
(food packaging and food processing equipment) that migrate, or
may reasonably be expected to migrate, into food, making them
subject to premarket approval [Section 201(s) of the Act];
• For premarket approval for use of food additives, the Act required
that the safety of the proposed use be demonstrated by data
submitted to the FDA in a food-additive petition;
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9. Where does the TTC come from?
• Amendments resulted in numerous petitions for
exemptions from the food additive requirements
of the act (regulation must not be overly
burdensome to industry);
• For next 37 years or so exemptions were granted
on an “ad hoc” basis;
• During this time, industry also approached the
FDA to propose that the Agency handle the issue
of minimal migration of these substances into
food through a so-called "Threshold of
Regulation" (TOR) policy.
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10. Where does the TTC come from?
• TOR
– 1967: Industry TOR proposal of 100 ppb dietary
concentration for food-packaging
materials, excluding pesticides and heavy
metals, based on the results of two-year chronic
oral feeding studies of 220 compounds;
– 1969: FDA proposed a TOR of 50 ppb for levels of
migrants into food or food-simulating solvents;
– 1977: Society of the Plastics Industry proposed a
TOR of 50 ppb for migrants into food.
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11. Where does the TTC come from?
• TOR
– 1995: FDA establishes a TOR on the basis of
• Reasonable certainty of no harm;
• Did not violate the Delany Clause of the FD&C Act;
• Provided acceptable protection even if the exempted
substance was later shown to be carcinogenic
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12. Where does the TTC come from?
• US FDA TOR
– Substance is not a carcinogen nor bears chemical resemblance to known
carcinogens;
– Substance does not contain carcinogenic impurities OR if it does, does not
contain a carcinogenic impurity with a lowest TD50 ≤ 6.25mg/kg BW/d;
– Dietary concentration ≤ 0.5 ppb (≤1.5 μg/person/d based on a diet of 1,500 g
of solid food and 1,500 g of liquid food/person/d);
– OR
– The substance is currently regulated for direct addition into food, and the
dietary exposure to the substance resulting from the proposed use is at or
below 1 percent of the acceptable daily intake as determined by safety data in
the Food and Drug Administration's files or from other appropriate sources;
– The substance has no technical effect in or on the food into which it migrates;
– The substance use has no significant adverse impact on the environment.
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13. Where does the TTC come from?
• US FDA TOR
– FDA has right of refusal irrespective of the data;
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14. Main Example: TTC For Food
An Example of TTCs in Complex Mixtures
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15. Main Example: TTC For Food
1. How do you prioritize?
2. Can you safely store this food item?
3. Do you really have to chemically identify each new peak?
4. Do you really have to chemically isolate all the unknowns, purify them and develop a
toxicology package to ensure human safety?
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5. Could you realistically perform all the studies required?

16. TTC
• Examples of other reasons for “new peaks”:
– Change in food contact material or new contact
materials;
– New food processes;
– Changes in storage conditions;
– Fermentation/bacterial action;
– Cooking or other heat/freeze processes;
– Changes in agrichemical use.
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18. Cohorts of Concern and TTC Excluded Classes
• Potent human-relevant mutagenic carcinogens
– Mutagens: aflatoxins, N-nitroso group, azoxy group, PAHs, etc. Included
based on linear low dose extrapolation to cancer risk of 1 x 10-6 (high
cancer slope factor);
– aH-receptor agonists: PHDDs, PHDBFs, co-planar PHBP. High potency
human-relevant non-mutagenic carcinogens;
– Steroids and other endocrine-active substances;
• Protein/peptide allergens and toxins (notably the Clostridial &
Staphylococcal toxins);
• High molecular weight polymers ( mw > 1000 Da). Note: remnant
oligomers and monomers in polymers are included in the TTC method;
• Bioaccumulative non-essential metals, notably Cd, Hg, Pb, As, Tl,
– Ag (argyria);
• Allergenic metals e.g. Ni, Cr, Ag
• Radioactive materials (Fukushima) 18

19. Cramer Classes
• Class I substances are simple chemical structures with
efficient modes of metabolism suggesting a low order of
oral toxicity;
– Problem: many toxicants are poisonous because they are
rapidly and efficiently metabolized to an ultimate toxicant (i.e.
they are really pro-toxicants);
– Efficient metabolism (excretion) does not guarantee low
toxicity;
• Class III substances are those that permit no strong initial
presumption of safety, or may even suggest significant
toxicity or have reactive functional groups;
• Class II are assigned to intermediate substances
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20. Cramer Classes
• The Cramer decision tree is set up such that it
detects few harmless compounds as Class I (low
concern) substances, and virtually all other
compounds as Class III (high concern, i.e. no
assumptions of safety possible);
• Cramer decision tree errs on the side of the
precautionary principle (conservative).
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21. WHO/IPCS TTC Exposure Thresholds
TTC Tier (μg/d) Dose Equivalent (mg/kg BW/d) Basis
0.15 2.5 x 10-6 Structural alerts for genetox
18 3 x 10-4 Organophosphate ACh inhibitors
90 0.0015 Cramer Class III 5th percentile NOAEL
540 0.009 Cramer Class II 5th percentile NOAEL
1800 0.03 Cramer Class I 5th percentile NOAEL
Assumptions:
• Continuous life-time consumption
• Default human body weight of 60 kg
• UF of 100 applied to the NOAEL
• Based on adults, but subsequent studies have demonstrated that TTC
thresholds are protective for repro/developmental effects
Trivial Nitpicking:
• NOAELs across different studies and study designs do NOT denote the same
level of risk because of different statistical power
• In general, OECD studies have a sensitivity of about 10% (5% for repro)
• BMDLo is a better measure
• However, TTC thresholds are very conservative! 21

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23. What is the regulatory relevance of the TTC?
;
What are the proposed uses of the TTC?
• WHO JECFA 1997: Adopted a TTC approach for food flavoring additives;
• EU 2003 (DG SANCO 2003): Metabolites of active substances of plant protection
products in groundwater;
• EFSA 2004: Adopted a TTC approach for food flavoring additives;
• EMEA 2004: Genetoxic and carcinogenic impurities in pharmaceuticals;
• Flavor and Extract Manufacturers Association (FEMA) 2005: Safety evaluations of
natural flavor complexes (NFC)
• Merck 2005: Used for risk assessment of workplace exposure to pharmaceutical actives,
impurities and excipients during pharma manufacturing;
• Merck 2005: Used for pharma plant cleaning validations;
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24. What is the regulatory relevance of the TTC?
;
What are the proposed uses of the TTC?
• US FDA CDER 2008: Genetoxic and carcinogenic impurities in pharmaceuticals – “In general, an exposure
level of 1.5 μg per person per day for each impurity can be considered an acceptable qualification
threshold for supporting a marketing application. Any impurity found at a level below this threshold
generally should not need further safety qualification for genetoxicity and carcinogenicity concerns. The
threshold is an estimate of daily exposure expected to result in an upper bound lifetime risk of cancer of
less than one in a million, a risk level that is thought to pose negligible safety concerns;”
• EMEA 2008: Genetoxic constituents of herbal medicinal products / preparations;
• EFSA 2009: Dietary risk assessment of metabolites, degradation and reaction products of pesticides;
• US NSF 2010: Proposed use of TTC for unknown contaminants of water and migrants from water contact
materials; (also ILSI 1998 and several other industry groups)
• EFSA 2012: Food – “The Scientific Committee concluded that the TTC approach can be recommended as a
useful screening tool either for priority setting or for deciding whether exposure to a substance is so low
that the probability of adverse health effects is low and that no further data are necessary;
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25. What is the regulatory relevance of the TTC?
;
What are the proposed uses of the TTC?
• EFSA & COSMOS 2012: Cosmetics – EFSA is currently evaluating the use of TTC
method for cosmetics;
• 2012: Multiple proposals for use of the TTC concept in ecotoxicolgy;
• 2012: Multiple proposals for use of the TTC concept in personal care products;
• 2012: Proposal for the use of TTC concept for genetoxic impurities in
veterinary medicinal products;
• 2012: Proposal for use of TTC concept for air pollutants;
• 2012: Proposal for use of TTC concept for medical devices.
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26. Benefits of the TTC Approach.
• Triage for toxicologists: provides a structured basis for focusing
resources on things that are likely to make a bigger difference;
• Fast;
• Simple and clear;
• Considers available data;
• The structure of the Cramer decision tree generally means that there is
an assumption that a lack of data ≠ absence of effect i.e. errs on the
side of safety and conservatism.
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28. Downsides of the TTC Approach.
• We don’t know what we don’t know – assessments are only as good as today’s data!
• Open to a high level of misuse and misinterpretation: requires a careful, thinking approach. There are no
short-cuts and should not be seen as “toxicology by wrote”
• Does not mean that the substances are “safe”; means that as far as we know, they have a low priority in
terms of further investigation of the risk that they pose.
• Discourages further research on interesting and potentially useful molecules;
• Accurate “worst case” and comprehensive exposure assessment is critical to the reliability of the approach;
• Potential loss of information;
• Uses uncertainty factors which are themselves arbitrary (Could potentially use a CSAF if data was
available?);
• Still largely dependent on animal to human extrapolation: a key question – are the putative effects really
human relevant?
• Some aspects are highly conservative e.g. assumes life-long continuous exposure, does not take into
account adaption/tolerance etc.
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29. Downsides of the TTC Approach.
• Heavily dependent on data quantity and quality; particularly the databases used to support the
necessary in silico modeling;
• Heavily dependent on the confidence of the fitted NOAEL distributions;
• Relies on NOAELs;
– NOAELS across different studies and experimental designs DO NOT DENOTE THE SAME LEVEL OF RISK; only
benchmark dose procedures can do this;
– NOAELs typically only have a sensitivity of 10% of the response due to experimental design constraints;
– NOAELs are strongly experimental design-dependent where as BMD techniques are not;
• Still requires massive linear extrapolation beyond measured experimental data i.e. makes assumptions
about the shape of the low dose response curve that may not be valid. This may be particularly
important with endocrine active substances and some carcinogens;
• Ignores the possibility of hormesis-like effects;
• Cramer risk alerts are based on relatively small data bases when you consider the sheer number of
chemicals out there;
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30. Future Developments.
• Integration of the TTC and high throughput in vitro
screening (“omics”) and other bioassays;
• Use of biosensor technology;
• More representative exposure assessments (e.g.
intermittent versus lifetime);
• More accurate in silico approaches;
• Inclusion of more chemical classes/modes of action
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31. Life is a fatal process. Most of us will not die from chemical exposure.
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