Biochemical and Molecular Toxicology
UNC ENVR/TOXC 442
November 8th, 2011
Toxic Effects of Pesticides
Matthew T. Martin, Ph.D.
National Center for Computational Toxicology
Office of Research & Development
U.S. Environmental Protection Agency
RTP, NC 27711
Email: martin.matt@epa.gov
http://www.epa.gov/ncct/
Pesticides
• Prevent, destroy, repel or mitigate any pest ranging from insects,
animals and weeds, to microorganisms such as fungi, molds,
bacteria and viruses
• Fungicides, Rodenticides, Herbicides, Insecticides, Antimicrobials
• Inert & Other Ingredients make up final pesticide formulation
• OPP (US) & PMRA (Canada) Regulate Pesticides
• FIFRA, FFDCA, FQPA – Covers Most of US Pesticide Legislation
• Why have a lecture on pesticide toxicology?
OPP= USEPA Office of Pesticide Programs
PMRA=Pest Management Regulation Agency
FIFRA =Federal Insecticide, Fungicide, Rodenticide Act
FFDCA=Federal Food, Drug, and Cosmetic Act
FQPA =Food Quality Protection Act
2
Pesticides
Pesticide Mass
• Toxicity data rich chemicals
• Potential for high human exposure
• Designed to be bioactive
• What makes a pesticide different from a commodity chemical?
–
–
–
–
Designed to be bioactive
Is any pesticide just a pesticide? (“biocide”)
Statutory authority to require toxicity tests
Indirect & direct application to food/crops
• What makes a pesticide different from a pharmaceutical?
– No direct human exposure
– Design intentions (destroy vs treat)… Are they really different?
– Molecular target potencies & efficacy differences
3
Pesticides (by the numbers)
• About 8800 total pesticidal
ingredients
• About 4200 active ingredients
–
–
–
–
1800 conventional pesticides
300 antimicrobial pesticides
250 biopesticides
400 food-use (direct or indirect
contact with the food supply)
– Remaining are unsupported
• About 4600 inert or other
ingredients
• ~200 Chemical class
represented
4
Pesticides in the News
August 23, 2009
Debating How Much Weed Killer Is Safe in Your Water Glass
By CHARLES DUHIGG
For decades, farmers, lawn care workers and professional green thumbs have relied on the popular weed killer atrazine to
protect their crops, golf courses and manicured lawns.
But atrazine often washes into water supplies and has become among the most common contaminants in American reservoirs
and other sources of drinking water.
Now, new research suggests that atrazine may be dangerous at lower concentrations than previously thought. Recent studies
suggest that, even at concentrations meeting current federal standards, the chemical may be associated with birth defects, low
birth weights and menstrual problems.
Laboratory experiments suggest that when animals are exposed to brief doses of atrazine before birth, they may become
more vulnerable to cancer later.
An investigation by The New York Times has found that in some towns, atrazine concentrations in drinking water have spiked,
sometimes for longer than a month. But the reports produced by local water systems for residents often fail to reflect those
higher concentrations.
5
Pesticides in the News
October 7, 2009
Regulators Plan to Study Risks of Atrazine
By CHARLES DUHIGG
The Environmental Protection Agency plans to conduct a new study about the potential
health risks of atrazine, a widely used weedkiller that recent research suggests may be more
dangerous to humans than previously thought.
Atrazine — a herbicide often used on corn fields, golf courses and even lawns — has become
one of the most common contaminants in American drinking water.
For years, the E.P.A. has decided against acting on calls to ban the chemical from
environmental activists and some scientists who argued that runoff was polluting ecosystems
and harming animals.
More recently, new studies have suggested that atrazine in drinking water is associated with
birth defects, low birth weights and reproductive problems among humans, even at
concentrations that meet current federal standards.
The E.P.A. is expected to announce on Wednesday that it will conduct a new evaluation of the
pesticide to assess any possible links between atrazine and cancer, as well as other health
problems, such as premature births. The E.P.A. may determine that new restrictions are
necessary.
6
Pesticides in the Scientific Literature
Silent Spring
Published in 1962
7
Pesticides in the Regulatory Process
PESTICIDE REGULATORY SUBMISSIONS BY YEAR
6000
Major Amendment
To FIFRA
5000
4000
3000
2000
1000
0
2005
2000
1995
1990
1985
1980
1975
1970
1965
1960
1955
1950
1945
8
Resource for High Quality Pesticide
Chemical Use/Class Annotation
http://www.alanwood.net/pesticides/
9
Pesticide Regulation
• Federal Insecticide, Fungicide, and Rodenticide Act
(FIFRA, 1947) administered by USDA
– Major amendments in 1972 and 1988
• Federal Food, Drug, and Cosmetic Act (FFDCA, 1954)
established pesticide tolerances on food
– Delaney Clause, forbade the use of carcinogens as food
additives
• Food Quality Protection Act (FQPA, 1996) reauthorized
FFIFRA provisions
–
–
–
–
–
Tolerances reassessed as part of re-registrations
single, health-based standard
aggregate risk from all routes of non-occupational exposure
evaluating endocrine effects
extra tenfold uncertainty factor for children/in utero
10
Vulnerability of Children
Greater exposure
• On a caloric consumption:body-weight ratio Children are
2.5x adults. Diet less varied (fruit and milk)
•  Hand to mouth activity
• Skin surface area per body weight is double that of an
adult
•  Rate of respiration
Greater physiological susceptibility
•
•
•
•
Period of rapid development of nerve cells
Loss of organ function can be permanently imprinted
 Absorption and  elimination of pesticides
Metabolizing enzymes not fully developed
11
Pesticide Testing- US EPA
Harmonized Test Guidelines
810 - Product Performance Test Guidelines
830 - Product Properties Test Guidelines
835 - Fate, Transport and Transformation Test Guidelines
840 - Spray Drift Test Guidelines
850 - Ecological Effects Test Guidelines
860 - Residue Chemistry Test Guidelines
870 - Health Effects Test Guidelines
875 - Occupational and Residential Exposure Test Guidelines
880 - Biochemicals Test Guidelines
885 - Microbial Pesticide Test Guidelines
890 - Endocrine Distruptor Screening Program Test Guidelines
http://www.epa.gov/ocspp/pubs/frs/home/guidelin.htm
12
http://www.epa.gov/opp00001/reregistration/status.htm
13
What is the acute and chronic Point-ofDeparture for pesticide toxicity?
Reference Dose (RfD) = NOAEL x UF
14
ToxRefDB website: http://actor.epa.gov/toxrefdb/
15
Chronic Rat & Mouse Endpoints
16
Reproductive Toxicity Profiling
Systemic Toxicity
&
Delayed Sexual
Maturation
Decreased
Reproductive
Performance
17
http://toxsci.oxfordjournals.org/cgi/reprint/kfp080
Pesticide Carcinogenicity
• Roughly 50% of all conventional pesticides cause tumors
in rodents
• Generally pesticides are non-genotoxic carcinogens
(screened out in development process)
• Human relevance?
– Site and tissue specificity (liver tumor in rodent vs lymphoma inc
in humans)
– Mechanistic relevance (peroxisome proliferators)
– High dose vs real world exposure potential
21
37
248 Chemicals
122 w/ No Liver Pathology
126 w/ Liver Pathology
No Pathology
Proliferative Lesions
Pre-neoplastic Lesions
Neoplastic Lesions
68
18
Chemicals Evaluated for Carcinogenic
Potential by US EPA
http://www.epa.gov/pesticides/carlist/
19
Is a pesticide an endocrine
disruptor?
•
•
•
•
Estrogenic
(anti) Androgenic
Thyrotoxic
Other
Panzica et al 2005
20
Endocrine Disruptor Screening Program
•The Food Quality Protection Act (FQPA) of 1996 and subsequent amendments to the
Federal Food, Drug, and Cosmetic Act (FFDCA) and Safe Drinking Water Act (SDWA)
required EPA to “develop a screening program, using appropriate validated test
systems and other scientifically relevant information, to determine whether certain
substances may have an effect in humans that is similar to an effect produced by a
naturally occurring estrogen, or other such endocrine effect as the Administrator may
designate.”
•The first phase of EDSP assays are designated Tier 1 tests with a purpose of
identifying chemicals that exhibit potential to interact with endocrine pathways or
mechanisms (i.e. the estrogen, androgen, and/or thyroid hormone systems) and
ultimately determine which chemicals should undergo more definitive in vivo testing
(i.e., Tier 2).
First test orders have been issued:
http://www.epa.gov/endo/pubs/regaspects/testorders.htm
22
http://www.epa.gov/endo/pubs/regaspects/testorders.htm
Reproductive
and
Endocrine
Organ
Toxicity
Endpoints
from
ToxRefDB
24
Interpretation of In Vitro Assay Results is
Challenging
ERa_TRANS
ERE_CIS
Concentration (µM)
25
Importance of Biotransformation
Methoxychlor (human ERa)
Methoxychlor (bovine ERa)
HPTE (human ERa)
Parent/Metabolite
ERa radioligand
binding assay
% of Inhibition
120
100
HPTE (bovine ERa)
80
Bottom
Top
LogIC50
HillSlope
IC50
60
40
20
HPTE (hER a )
2.448
= 100.0
-1.349
-0.9197
0.04476
HPTE (bER a )
1.725
= 100.0
-1.697
-0.9877
0.02009
0
-20
-3
-2
-1
0
1
2
Conc (log M)
Parent/Metabolite
100
Methoxychlor
HPTE
ERa cellular
(HEK293)
transactivation
assay
% of Control
80
60
Bottom
Top
LogEC50
HillSlope
EC50
40
20
Methoxychlor
0.3017
41.86
0.5714
5.466
3.727
HPTE
-3.912
35.70
-0.7131
2.258
0.1936
0
-3
-20
-2
-1
0
1
2
Conc (log M)
26
Pesticidal MOA vs. Toxicological MOA
*Methoxychlor was intended to be a
replacement for DDT (“Silent Spring”)
http://www.irac-online.org/wpcontent/uploads/2009/09/MoA_Classification.pdf
27
28
Focus On a Mode of Action …
NR activators stimulate intracellular processes that lead to hyperplasia
Chronic stimulation increases the risk of neoplasms
Environmental
Chemicals
Chemicals
Pesticides
Conazoles
Pyrethroids
Toxics
DE-71
PCBs
Phthalates
PFOA/PFOS
Molecular
response
Molecular Response (Early)
NR-sig
CAR
PXR
PPARa
Cellular
response
Tissue
response
Cell fate
Adverse
Outcome
Proliferation
Hyperplasia
Gene-reg. Transcription
cis-reg.
trans-reg.
Xen. Met.
Phase I
Phase II
Death
Apoptosis
Necrosis
Tumor
Cancer
Phase III
29
30
HISTORY OF DDT
1,1,1-trichloro-2,2-bis-(p-chlorophenyl) ethane
DDT was discovered to be an insecticide in 1939 by
Paul Muller. He was a scientist working for Geigy, a
Swiss firm that was focused on the chemical
development of agricultural insecticides. Products with
DDT entered the Swiss market in 1941. Seven years
later, in 1948, Muller received the Nobel Prize for
medicine and physiology in recognition for the lives
DDT saved.
• WWII – DDT was used by the allies to suppress a
typhus epidemic in Naples
• 1943-1944 DDT was applied directly to the head
of humans to control lice
• Success with DDT hastened the development of
aldrin, dieldrin, endrin, chlordane, benzene
hexachloride etc.
31
•
•
•
CURRENT STATUS:
No US registration, most uses cancelled in 1972, all uses by 1989
No US production, import, or export
DDE (metabolite of DDT) is regulated as a hazardous air pollutant
(Clear Air Act)
Priority toxic pollutant (Clean Water Act)
DDT
•
• DDT can take more than 15 years to break down
• Found in animals far from where they were it is used
• Bio-accumulates in fish and marine mammals. Found concentrations in
these animals are many thousands of times higher than levels in water
• DDT can be absorbed by some plants and by animals and humans who
eat those plants
• DDT is fat-soluble and is stored in adipose tissues of humans and
animals
HUMAN EXPOSURE FROM:
• Eating contaminated fish and shellfish
• Eating imported food exposed to DDT
• Infant exposed through breast milk
• Eating products from crops grown in contaminated soil
Insecticide advantages of DDT
•
•
•
•
Low volatility
Chemical stability
Lipid solubility
Slow rate of biotransformation and degradation
Disadvantages of DDT
•
•
•
•
Persistence in the environment
Bioconcentration
Biomagnification in food chain
Profound effects on wild life (“Silent Spring”)
Health Effects of DDT
•
•
•
Paresthesia of tongue, lips, and
face
Irritability, dizziness, vertigo,
tremor, and convulsions
Hypersusceptibility to external
stimuli (light, touch, and sound)
•
•
•
•
•
Hypertrophy of hepatocytes
Hepatic tumors
No epidemiological evidence linking DDT
to carcinogenicity in humans
Low rate of absorption through the skin
Human health effects minor
33
34
Klaassen, CD. CASARETT AND DOULL's Toxicology: The Basic Science of Poisons. McGraw-Hill 2001
Sites of DDT poisoning
35
Klaassen, CD. CASARETT AND DOULL's Toxicology: The Basic Science of Poisons. McGraw-Hill 2001
Neurotoxicity: Inhibition of choline
esterase or action potential
• Organochlorine
Insecticides
• Organophosphate
Insecticides
• Carbamates
• Pyrethroid
insecticides
• Botanical
Insecticides
36
Klaassen, CD. CASARETT AND DOULL's Toxicology: The Basic Science of Poisons. McGraw-Hill 2001
• Most chemical
insecticides act by
poisoning the nervous
system of the target
organisms
• CNS of insects are
highly developed and
similar to that of the
mammal
• Chemicals that act on
the insect nervous
system may have
similar effects on
higher forms of life
Stenersen, J. Chemical pesticides: Mode of Action and Toxicology. CRC Press 2004
General Modes of Action
Pesticides acting on the
axon (impulse
transmission):
•
Interference with transport of,
Na+, K+, Ca2+, or Cl- ions
Pesticides acting on
synaptic transmission:
•
Inhibition of specific enzyme
activities:
GABA-ergic (inhibitory)
synapses
Cholinergic synapses
•
Contribution to the release or
persistence of chemical
transmitters at nerve endings
38
Stenersen J, Chemical Pesticides Mode of Action and Toxicology, CRC Press 2004
Pyrethroid Insecticides
•
•
Newest class of insecticides
New analogs will be (hopefully):
– More stable in light and air
– Better persistence
– Low mammalian toxicity
Soderlund et al. (2002)
39
Importance of
Structure-Activity-Toxicity
Relationships
Soderlund et al. (2002)
Pyrethroid Use
•
•
•
•
Household sprays
Flea preparations for pets
Plant sprays for home
Plant sprays for greenhouses
Pyrethroid Poisoning
• Similar to DDT
• Not highly toxic in animals
• Toxic ingredients
– Chrysanthemic acid
– Pyrethric acid
41
Figure 1. Nine neonicotinoid
insecticides and four nicotinoids.
The neonicotinoids are nitromethylenes
(C==CHNO2), nitroguanidines
(C==NNO2), and cyanoamidines
(C==NCN). Compounds with 6-chloro3-pyridinylmethyl, 2-chloro-5thiazolylmethyl, and 3-tetrahydrofuranmethyl moieties are referred to as
chloropyridinyls (or chloronicotinyls),
chlorothiazolyls (or thianicotinyls), and
tefuryl, respectively. The nicotinoids
are naturally occurring [(−)-nicotine
and (−)-epibatidine] and synthetics
(ABT-594 and desnitroimidacloprid).
Tomizawa & Casida (2004)
Tomizawa & Casida (2004)