Introduction
1. The general advice of the COM when considering the risk assessment of chemicals which are mutagenic in-vivo has been that it is prudent to assume a linear, non threshold dose response. Thus it is
assumed that any exposure to an in-vivo mutagen is associated with some damage to DNA and consequently
an increased risk of mutation leading to an increased risk of adverse health effects albeit that this may be small.
In such instances the Committee has recommended that exposures be reduced to a low as is reasonably practicable.
The COC has adopted a prudent approach to the assessment of chemical carcinogens which assumes that genotoxic carcinogens
have the potential to damage DNA at any level of exposure and that such damage may lead to tumour development.
Thus for genotoxic carcinogens it is assumed that there is no discernible threshold and that any level of exposure
carries a risk. (1,2)
2. The COM agreed to review its general approach to the risk assessment of in-vivo
mutagens following the publication of the COM guidance on a strategy for testing chemicals for mutagenicity.(3)
The Committee has previously considered specific chemicals, on a case-by-case basis, with regard to deviations
from its general approach to in-vivo
mutagens. This statement summarises
the conclusions reached at the February meeting 2001.
Evidence for existence
of in-vivo thresholds for mutagenic effects
3. The Committee recalled that there were two mechanisms for which sufficient evidence is available for the COM
to conclude that a threshold for mutagenicity exists namely (i) aneugenicity induction by tubulin inhibitors (specifically
methyl benzimadazole carbamates (MBCs), benomyl, carbendazim and thiophanate-methyl) and, (ii) the rapid detoxication
of hydroquinone and phenol via the oral route. The Committee has undertaken detailed reviews of the mutagenicity
data on these chemicals and full statements have been published on the COM Website (www.doh.gov.uk/com.htm) and in the Committee's Annual Reports.(1) A brief overview is given below with the objective
of providing background information on the approach used to provide the critical data used by the COM in its evaluation.
Methyl benzimadazole
carbamates (MBC) induced aneugenicity.
4. Benomyl, carbendazim and thiophanate-methyl belong to the methyl benzimadazole carbamate (MBCs) class of chemicals.
The MBC class of chemicals are widely used in approved pesticide products as fungicides and also in veterinary
medicines in particular as antihelmintics in both food producing and companion animals. These chemicals act by
interfering with microtubule formation during mitosis. The COM has provided advice to the U.K regulatory Authorities
namely the Pesticides Safety Directorate (PSD) and the Veterinary Medicines Directorate (VMD) of the Ministry of
Agriculture, Fisheries and Food on the most appropriate approach for the risk assessment of MBCs. (4-6)
5. In 1993 the COM agreed that it was reasonable to assume that aneuploidy
inducing chemicals (particularly those that function by interfering
with the spindle apparatus of cell division) have a threshold of action.(4)
The safety evaluation of aneuploidy inducing chemicals (aneugens) acting
by inhibition of microtubule formation is based on the identification
of a threshold dose below which aneuploidy does not occur. The Committee
provided advice on methodologies for identifying thresholds in 1993,
namely appropriate in-vitro experiments in human lymphocytes using the
detection and quantification of non-disjunction chromosome less and
centromere positive nuclei using FISH (Fluorescent in-situ hybridisation)
analysis of selected chromosomes for centromeric DNA. The Committee
considered that it was not possible to determine thresholds for aneugenicity
using the currently available in-vivo assays. This advice was used by
PSD and VMD when requesting data from approval/licence holders of products
containing MBCs. In 1996, the Committee considered the results of experiments
undertaken with benomyl and carbendazim and concluded that the studies
had been satisfactorily conducted and the data indicated No Observed
Effect Levels (NOELs) could be estimated for these two chemicals. (7-10)
It was noted that that it would be difficult to define precise thresholds
for activity from these data and the mathematical models that had been
used for their analysis. Appropriate studies which provided evidence
for a threshold effect have also been undertaken with thiophanate-methyl.(11)
Hydroquinone and
phenol
6. In 1994, the COM agreed that both hydroquinone and phenol should be regarded as somatic cell in-vivo mutagens.(12-19) The Committee agreed that for exposure to these two compounds by the
oral route there was potential for a threshold of activity as there was good evidence from appropriate toxicokinetic
studies that two protective mechanisms (namely rapid conjugation and detoxification via the glutathione pathway)
would substantially reduce systemic exposure to any active metabolites formed. However, Members agreed that there
were insufficient data on inhalation and dermal exposure and it was not possible to assume that a threshold existed
for activity when exposure was via the respiratory tract or the skin. The Committee noted the information from
one published paper that when radiolabelled phenol was given intratracheally, initially all the radiolabel in the
plasma was present as phenol. (20) These data suggested that there was little conjugation of phenol on the "first-pass"
from airways to the circulation. The Committee recommended that appropriate toxicokinetic studies were needed.
In 1999, further data from published papers on the kinetics of hydroquinone in rats following intratracheal instillation
and on its percutaneous absorption in in-vitro studies using rat skin and human stratum corneum were provided
to the Committee. (21,22) The new toxicokinetic study in which rats were given a single intratracheal dose of 14C-hydroquinone
showed detectable free hydroquinone in arterial blood within 5-10 seconds after dosing. (21) This new information
suggested a potential risk of site-of- contact and systemic mutagenic effects following inhalation exposure to
hydroquinone.
7. The data on MBCs and hydroquinone and phenol show that if there were specific data on a chemical to invoke such
mechanisms then the possibility of threshold could be considered. Members agreed that data on threshold-related
mechanisms would in most instances come from in-vitro studies. Any observed NOEL from such in-vitro mechanistic studies could be used to inform a risk assessment but it was unlikely that
the data could be used in a quantitative way. In those cases where a potential threshold mechanism is based on
chemical detoxication, then appropriate in-vivo
data will be required.
Possible mechanisms
for thresholded mutagenicity
8. The Committee reviewed a number of papers which reported proceedings
of an international symposium held in Salzberg in September 1998. (23)
Two broad categories of potentially threshold mechanisms were interaction
with non-DNA targets and rapid detoxication which were consistent with
the COM's experience regarding MBCs and hydroquinone and phenol. In
addition further papers presented to the symposium noted that threshold
dose responses could involve exposure to redundant or multiple cellular
targets with inactivation or modification before a toxic response is
produced. (24) Lists of potential cellular targets for threshold-related
genotoxicity were presented at the Symposium which essentially identify
protein targets such as microtubules, DNA synthetases, topoisomerases.
(25) The Committee agreed that appropriate supporting evidence would
be required in order to invoke any of these mechanisms on a chemical-by-chemical
basis.
Conclusions
9. The COM reaffirmed that for in-vivo mutagens, it is prudent to assume that there is no threshold
for mutagenicity. Where a potential threshold related mechanism can be identified, appropriate data should be generated
on a chemical-by-chemical basis. In many situations this will involve in-vitro
studies of mechanism. An appropriate strategy should be devised for each chemical under consideration and this
may, in some instances, include in-vivo studies. The regulatory approach to such chemicals can
then be based on the identification of a critical NOAEL and use of uncertainty factors.
June 2001
References
1. Annual Reports of the Committees on Toxicity, Mutagenicity and Carcinogenicity
of Chemicals in Food , Consume Products and the Environment. www.doh.gov.uk/com/com.htm
(In particular see Annual reports for 1998 and 1999).
2. Department of Health (1991). Report on health and Social Subjects. No 42. Guidelines for the Evaluation of Chemicals
for Carcinogenicity. Committee on Carcinogenicity of Chemicals in Food, Consumer Products and the Environment,
London HMSO.
3. Committee on Mutagenicity of Chemicals in Food, Consumer Products and the Environment. Guidance on a Strategy
for testing of Chemicals for Mutagenicity. December 2000.
4. 1993 Annual Report of the Committees on Toxicity, Mutagenicity, Carcinogenicity of Chemicals in Food, Consumer
Products and the Environment.
5. 1995 Annual Report of the Committees on Toxicity, Mutagenicity, Carcinogenicity of Chemicals in Food, Consumer
Products and the Environment.
6. 1996 Annual Report of the Committees on Toxicity, Mutagenicity, Carcinogenicity of Chemicals in Food, Consumer
Products and the Environment.
7. Unpublished report 1996. Carbendazin induction of aneuploidy in cultured peripheral blood lymphocytes. Final
Report.
8. Unpublished report 1996. Benomyl induction of aneuploidy in cultured peripheral blood lymphocytes. Final Report.
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