recent risk assessments of dioxins comparing the who, cot, scf and jecfa evaluations j.c. larsen...
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Recent Risk Assessments Of Dioxins
Comparing the WHO, COT, SCF and JECFA evaluations
J.C. Larsen(using many slides prepared by A.G. Renwick)Danish Institute of Food and Veterinary Research, Division of Toxicology and Risk Assessment, Mørkhøj Bygade 19, DK-2860 Søborg, Denmark
O
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Polychlorinated dibenzodioxins
O
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ClPolychlorinated dibenzofurans
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Co-planar polychlorinated biphenyls
HAZARD IDENTIFICATION
Human studies
Epidemiology
Local accidental high exposures
Sevesoe.g. Bertazzi et al., 2001
Differences in background low level exposures
Rotterdam and Groningen cohortse.g. Huisman et al., 1995Koopman-Esseboom et al., 1996
Exposed workers
e.g.Fingerhut et al., 1991Ott and Zober, 1996Manz et al., 1991Flesch-Janys et al., 1995Becher et al., 1996Coggon et al., 1991
Environmental
HAZARD IDENTIFICATION (TCDD)
Genetic Studies in bacteria and mammalian cells were generally negative, does not bind to DNA, not genotoxic
Acute Oral LD50 is 1 - 5000g/kg (guinea pig – hamster)
Short-term Studies in mice rats, guinea pigs and monkeys; metabolic effects and changes in liver, thymus and haematology
Long-term andcarcinogenicity
Decreased weight gain, increased mortality (F), splenic and thymic atrophy, hepatic degeneration plus increase in thyroid follicular and hepatic carcinomas in rats
Reproductive Multi-generation studies – deceased fertility (rats)Single-generation studies – decreased sperm count (rats)Developmental studies – cleft palate and hydronephrosis (mice), effects on testes, prostate and sperm count (rats)
In vitro and animal studies
HAZARD CHARACTERISATION
1. Selection of effects that are of relevance to humans
2. Identification of effect(s) produced at the lowest doses
3. Dose-response assessment of effects and consideration of mode of action (threshold or non-threshold)
4. Derivation of a health-based advice related to the risk associated with the critical effect(s) (threshold or non-threshold)
For Dioxins the effects are considered to be mediated via the AhR [or other threshold mechanisms] so that the usual NOAEL + uncertainty factor approach is suitable
Recent studies with knockout mice show that the AhR is important in:-
Effects on liver, thymus(?), epididymis, testes – Lin et al., 2001Effects on prostate - Lin et al., 2002Oxidative stress and vascular inflammation – Hennig et al., 2002T-lymphocyte suppression – Kerkvliet et al., 2002Keratinocyte differentiation – Loertscher et al., 2002
Are the extensive epidemiological data adequate for risk assessment?Are exposure data reliable - both quantitatively and in relation to confounding exposures?Does hazard characterisation in human studies include all hazards identified in animal studies?
HAZARD CHARACTERISATION
Human studies
Chloracne – no clear dose-response reported
Liver – increases in liver enzymes
Cardiovascular disease – inconsistent evidence
Blood lipids – inconsistent changes
Reproductive hormones – inconsistent changes
Reproductive outcomes – change in sex ratio (Serveso)
Thyroid function – small and inconsistent changes
Neurological effects – inconsistent findings
Respiratory system – inconsistent evidence
Urinary system – no major changes reported
Immunological effects – inconsistent findings
Neurodevelopmental – some differences in ongoing Dutch studies
Cancer – largely based on occupational exposures
Based on COT, 2001
Mably et al (1992a,b,c) – reported decreased weights of epididymis and sperm numbers in rats after a single oral dose of 64ng/kg on gestation day 15 (GD15).
Animal studies
HAZARD CHARACTERISATION
Gray et al (1995, 1997a, 1997b) – reported greater effects on male offspring following single oral dosage on GD15 compared with GD8, with significant decreases in sperm count after 50ng/kg
Faqi et al (1998) – reported effects on sperm production in offspring following 25ng/kg as a subcutaneous loading dose 2 weeks prior to mating followed by weekly maintenance doses of 5ng/kg
Ohsako et al (2001) – reported decreased anogenital distance in offspring following 50ng/kg by gavage on GD15. NOAEL was 12.5 ng/kg bw.
Female Rhesus monkeys given 0.15 ng/kg bw/day in the diet for 3.5 years Schantz & Bowman (1989) - reported subtle, non-persistent neurobehavioural changes (object learning) in offspring (16.2 and 36.3 months).Rier et al (1993) – reported increased incidence of endometriosis after 10 years
Hazard Characterisation
Rodents require higher doses (100-200-fold) to reach the same equivalent body burdens as seen in humans at background exposures.
Body burden estimations are considered the most appropriate dosimetric parameter for interspecies comparison.
What intake in humans would give a maternal body burden of TCDD of for example 30ng/kg?
TCDD would show slow accumulation on repeated daily intake, because the half-life of TCDD in humans is very long (7.5 years)
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0 10 20 30 40 50 60 70
Time in years
Bo
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(n
g/k
g)
4-5 half-lives
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0 10 20 30 40 50 60 70
Time in years
Bo
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load
(n
g/k
g)
Body burden = daily intake x bioavailability x half-life0.693
Daily intake = body burden x 0.693bioavailability x half-life
0
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0 10 20 30 40 50 60 70
Time in years
Bo
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(n
g/k
g)
Daily intake = body burden x 0.693bioavailability x half-life
Daily intake = 30 ng/kg x 0.693 0.5 x 7.5 years
Daily intake = 15 pg/kg/day
ANIMAL BODY BURDENS OF TCDD AND ASSOCIATED HUMAN ESTIMATED DAILY INTAKES (EDI) (WHO 1998)
STUDY RESPONSE
(LOAELs)
MATERNAL BODY BURDEN
(ng/kg)
ASSOCIATED
HUMAN EDI
(pg/kg bw/day)
Gray et al., 1997a RATS: decreased sperm count in offspring
28 14
Gehrs et al., 1997b; Gehrs & Smailowicz 1998
Immune suppression in offspring
50 25
Gray et al., 1997b Increased genital malformations in offspring
73 37
Schantz & Bowman, 1989
MONKEYS: Neurobehavioural (object learning) effects in offspring
42 21
Rier et al., 1993 Endometriosis 42 21
Estimation of a TDI for dioxins (WHO
1998)
The LOAELs for the most sensitive adverse responses in experimental animals could be transformed into a range of estimated long-term human daily intakes of 14-37 pg/kg bw/ day.
Composite uncertainty factor of 10:
TDI 1 - 4 TEQ pg/kg bw (rounded figures)
Limitations of the design of the rat studies used by WHO
The bioavailability of 2,3,7,8-TCDD to the foetus is likely to be higher following a single oral bolus dose on days 15 - 16 of gestation (the sensitive window) than after low-level chronic dietary exposure at steady state.
The key to the use of these data for risk assessment came with the studies of Hurst et al (2000a,b) who reported the tissue distribution of 3H in maternal and fetal tissue on GD16 after administration of [3H]-TCDD as a single dose on GD15 and following subchronic exposure (5 days per week for 13 weeks)
Animal studies
HAZARD CHARACTERISATION
Maternal Fetal
Dose on GD 15Dose on GD 15
Maternal Fetal
Daily doseDaily dose
Animal studies
HAZARD CHARACTERISATION
Single dose on GD15
Doseng/kg
Maternalng/kg
Fetalng/kg
Mat/Fet ratio
1000 585 55.7 10.5
50 30 5.3 5.7
200 97 13.2 7.4
800 523 39.1 13.4
Subchronic exposure
Daily doseng/kg
Maternalng/kg
Fetalng/kg
Mat/Fet ratio
0.71 20 1.4 14.3
7.1 120 7.5 16.0
21.3 300 15.2 19.7
The key to the use of these data for risk assessment came with the studies of Hurst et al (2000a,b) who reported the tissue distribution of 3H in maternal and fetal tissue on GD16 after administration of [3H]-TCDD as a single dose on GD15 and following subchronic exposure (5 days per week for 13 weeks)
Correction Factor Applied
SCF – fitted power functions to the data for the GD15 and subchronic data
and derived a correction factor of 2.5 at maternal body burdens of
<30ng/kg and 2.6 from 30-100ng/kg. The SCF established corresponding values of fetal, acute maternal and subchronic steady state maternal body burdens for 2,3,7,8-TCDD
COT - used ratio of 2.5 based on the 2 lowest doses
Single dose on GD15
Doseng/kg
Maternalng/kg
Fetalng/kg
Mat/Fet ratio
50 30 5.3 5.7
Subchronic exposure
Daily doseng/kg
Maternalng/kg
Fetalng/kg
Mat/Fet ratio
0.71 20 1.4 14.3
Ratio
2.5
JECFA – fitted power functions to the data for the GD15 and subchronic
data and derived a correction factor of 2.6; also fitted a linear model and estimated possible TDI values using each correction method
Selection of a NOAEL or LOAEL on which to base the TDI
Faqi et al (1998) appear to have detected the most sensitive endpoints for which the LOAEL is 25ng/kg loading dose plus 5ng/kg every 7 days
Table 33. Studies providing the body burdens at which no effect and the lowest observed effect were seen for the most sensitive adverse effects of TCDD on developmental end-points in rats (From: JECFA, 2002)
Reference; rat strain End-point Dosing regimen No-effect body burden (ng/kg bw)
Lowest effective body burden (ng/kg bw)
Ohsako et al. (2001); Holtzman
Ventral prostate weight; decreased anogenital distance in male offspring
Single oral bolus dose by gavage on day 15 of gestation
13 51
Faqi et al. (1998); Wistar Decreased sperm production and altered sexual behaviour in male offspring
Loading dose; maintenance dose by subcutaneous injection
25
Gray et al. (1997a,b); Long-Evans
Accelerated eye opening and decreased sperm count in offspring
Single oral bolus dose by gavage on day 15 of gestation
28
Mably et al. (1992c); Holtzman
Decreased sperm count in offspring
Single oral bolus dose by gavage on day 15 of gestation
28
Gehrs et al (1997); Gehrs & Smialowicz (1998);
Immune suppression in offspring Single oral bolus dose by gavage on day 14 of gestation
50
0
5
10
15
20
25
30
35
-14 -7 0 7 14 21
Time in days
Mat
ern
al b
od
y b
urd
en (
ng
/kg
)
Loading dose
Dose 2
Dose 3
Dose 4
Final dose
Estimation of the maternal subchronic steady state body burden (BB) associated with the Faqi et al. (1998) LOAEL
At GD 16 the maternal BB consisted of a (“pseudo”) steady state BB of 20 ng/kg bw and an acute BB of 5 ng/kg bw from the injected loading dose. Using the tabulated corresponding values of fetal, acute maternal and subchronic steady state maternal BBs the SCF estimated a fetal BB of 1.8 + 1.2 = 3.0 ng/kg bw. The 3.0 ng/kg bw fetal BB would correspond to a maternal subchronic steady state BB of 39 ng/kg bw.
Estimation of the daily intake in humans that would give the same maternal body burden
Daily intake = 39 ng/kg x 0.693
0.5 x 7.5 years
Daily intake = 20 pg/kg bw
Uncertainty factors
Database deficiencies Usually 3 or 10
No chronic bioassay
No developmental/repro’ toxicity data
No NOAEL only a LOAEL in critical study
Usually 100
KINETICS DYNAMICSKINETICS DYNAMICS
SPECIESDIFFERENCES
HUMANVARIABILITY
10 10
Uncertainty factors
Inter-species extrapolation
Inter-individual variability
Chemical specific data can be used to replace a default uncertainty factor (UF) by an adjustment factor (AF) - based on IPCS 1999, 2001
100 - FOLD UNCERTAINTY FACTOR
INTER-SPECIES
DIFFERENCES
10 - FOLD
INTER-INDIVIDUAL
DIFFERENCES
10 - FOLD
TOXICO-DYNAMIC
ADUF
10 0.4
2.5
TOXICO-KINETIC
AKUF
10 0.6
4.0
TOXICO-DYNAMIC
HDUF
10 0.5
3.2
TOXICO-KINETIC
HKUF
10 0.5
3.2
TOXICO-DYNAMIC
ADUF
10 0.4
2.5
TOXICO-KINETIC
AKUF
10 0.6
4.0
TOXICO-DYNAMIC
HDUF
10 0.5
3.2
TOXICO-KINETIC
HKUF
10 0.5
3.2
Taken into account by calculation of maternal body burden
TOXICO-DYNAMIC
ADUF
10 0.4
2.5
TOXICO-DYNAMIC
HDUF
10 0.5
3.2
TOXICO-KINETIC
HKUF
10 0.5
3.2
Taken into account by calculation of maternal body burden
TOXICO-KINETIC
AKAF
TCDD1.0
Rats are more sensitive than humans and as sensitive as the most sensitive human
Taken into account by calculation of maternal body burden
TOXICO-DYNAMIC
ADAF
TCDD1.0
TOXICO-KINETIC
AKAF
TCDD1.0
TOXICO-DYNAMIC
HDAF
TCDD1.0
TOXICO-KINETIC
HKUF
10 0.5
3.2
Humans are generally less sensitive than rats, but the most sensitive human might be as sensitive as rats
No data available
9.6
Uncertainty factor for TCDD
Database deficiencies 3
Inter-individual variability 3.2
Inter-species extrapolation 1
20 pg/kg/day gives a maternal body burden of 39 ng/kg
Dividing by the 9.6-fold uncertainty factor gives a daily intake of 2.1 pg/kgThis was rounded to give a tolerable intake of
2pg/kg/day
Table 34. Summary of four calculations of provisional tolerable monthly intake intakes (PTMIs) for PCDDs, PCDFs and coplanar PCBs (From JECFA, 2002)
Linear model Power model
Ohsako et al. (2001)
Faqi et al. (1998)
Ohsako et al. (2001)
Faqi et al. (1998)
Administered dose (ng/kg bw)a 12.5 12.5
Maternal body burden (ng/kg bw)b 7.6 25b 7.6 25b
Equivalent maternal body burden with repeated dosing (ng/kg bw)
13c 25c 19d 39d
Body burden from feed (ng/kg bw) 3 3 3 3
Total body burden (ng/kg bw) 16 28 22 42
EHMI (pg/kg bw per month) 237 423 330 630
Safety factor 3.2 9.6 3.2 9.6
PTMI (pg/kg bw per month) 74 44 103 66
EHMI, equivalent human monthly intake
a Bolus dose (NOEL)b Target maternal body burden after repeated dosing (LOEL)c Linear relationship between fetal and maternal body burden assumed from data in Table 30d Non-linear relationship between fetal and maternal body burden assumed from data in Table 30e For humans, 7.6 year half-life and 50% uptake from food assumed (Eq. 1)
WHO (1998) – gave a range of 1- 4 pg/kg/day – because intake data are usually expressed on a daily basis
SCF (2001) – gave a value of 14pg/kg/week – to reflect the long half-life of TCDD
Guidance values expressed as 2,3,7,8-TCDD equivalents
JECFA (2002) – gave a value of 70pg/kg/month – to reflect the long half-life of TCDD
Note – one day = 0.04% of the half-life – one week = 0.25% of the half-life – one month = 1.11% of the half-life
Therefore – the main rationale of longer time intervals is to emphasise that short-term peaks of exposure will not greatly affect body burden - rather than indicate a duration of exposure that could affect body burden
Conclusions
Recent evaluations by the COT, SCF and JECFA have been based on changes in the male rat reproductive system following in utero exposure
Conversion of the animal data into a tolerable intake for humans has allowed for
the fetal to maternal ratio after bolus dose the long half-life and potential for accumulation the relative sensitivity of rats and humans potential human variability in kinetics and dynamics
The COT, SCF and JECFA differ in the time-base of the guidance value but not in the data or approach used
The intake by a significant proportion of the population exceeds the guidance value