Pesticides

Principles of Toxicology

Pesticides

• EPA definition: “substances or mixtures of substances intended for preventing, destroying, repelling or mitigating any pest…”

A bit of history…

– Chinese - 1000BC– Europe - 1800s– CA - today !

•Sulfur

•Arsenic-containing•Strychnine•Nicotine (tobacco leaves extracts - 1690)•Pyrethrum (chrysanthemum extract)•Bordeaux mix: copper, lime (Ca(OH)2), water

Later…• 1930’s - modern era chemistry

– Alkylthiocyanate– Dithiocarbamate– Bromide compounds

• WWII - – DDT– Dinitrocresol– 2,4 D

• Since then, synthesis with goal improved specificity, reduced toxicity…

• No such a thing as “safe pesticide”

Integral part of crop and health Integral part of crop and health protectionprotectionPoisonings are anticipated…Poisonings are anticipated…

3mil acute cases annually (ww)3mil acute cases annually (ww)220,000 deaths220,000 deaths

CA - 25,000 pesticide related CA - 25,000 pesticide related illnesses, annuallyillnesses, annually

USA - 80,000 USA - 80,000

Efficacy of crop protection

Pesticide poisonings by occupational

activity

Medical successes

• DDT– Typhus in Naples, Italy– River blindness, West Africa– Malaria - Africa, Asia, Middle East

There are still many parasitic and vector-borne diseases…

Regulations…• 1906 - First Federal Food and Drugs Act• 1938 - Federal Food, Drugs and Cosmetics Act• 1958 amend. - Delaney clause: “no additive shall be

deemed safe if …found to induce cancer…”• 1947 - FIFRA: Federal Insecticide, Fungicide and

Rodenticide Act: all pest control products under one law - by USDA

• 1972 - FIFRA reorganized and passed to EPA• FIFRA Amendments - 1975, ‘78, ‘80, 84• 1996 - Food Quality protection Act (children)• Developing countries adapt or lack regulations

Nervous System

Central Peripheral

EfferentAfferent

Somatic Autonomic

Para-Sympathetic Sympathetic

ENS

Anatomic Classification

CNS Skull and Spinal cord

PNS 12 pairs of cranial nerves 31 pairs of spinal nerves

CNS PNSafferent

efferent

Sympathetic ANS

Fight or flight

Parasympathetic ANS

Rest and digest

Sympathetic: Adrenergic

Parasympathetic: Cholinergic

thoraco-lumbar

cranio-sacral

Parasympathetic nerves

4 CranialIII oculomotor

VII facialIX glossopharyngial

X vagus

SacralS2S3S4

L1L2L3

T1....

T12

Sympathetic nerves

AfferentNerves Efferent

Mixed

Spinal nerves are mixed for major length

Afferent dorsal root

Efferent ventral root

– then divide

Neurons

Structure of a typical neuron

Pre-ganglionic neuron

Post-ganglionic neuron

GanglionMulti-neuron synapse

organ

Neuroeffector junction

Somatic

AchParasympathetic -cholinergic

Ach Ach

Sympathetic - adrenergic

Ach Adr/NA

Feature Sympathetic Parasympathetic

Start point thoraco-lumbar cranio-sacralGanglion near spine on organ (terminal)Ganglion synapse Ach AchPregangl. neuron short longPostgangl. neuron long shortEffector synapse Adr AchEffector organs throughout body limited

Typical Synapse

Cholinergic Transmission

Acetate Choline

CH3C-O- + HO-CH2-CH2-N+(CH3)3

CH3C-O-CH2-CH2-N+(CH3)3

O

O

Acetylcholine

Acetylcholine Synthesis

Choline Acetylase

Coenzyme A

Acetate Choline

CH3C-O- + HO-CH2-CH2-N+(CH3)3

CH3C-O-CH2-CH2-N+(CH3)3

O

O

Acetylcholine

CholinesteraseCholinesterase

Acetylcholine Catabolism

Cholinergic Receptors

Nicotinic: NM skeletal muscleNN ganglia (post-), presynaptic

Muscarinic:M1 CNS, sympathetic (exceptions), presynapticM2 Smooth muscle, heart, presynapticM3 Exocrine glands, blood vessels

Acetylcholine Nicotinic Receptor

Nicotinic Receptor structure

Ion Channel

Na+

Na+

Muscarinic receptor

G protein

Outside

Inside

Agonist

Muscarinic receptor

G- proteins

Excitatory action Inhibitory action

Phosholipase C Adenylic cyclase

K+ channels

Signal transduction cascade involving Adenylic cyclase

Muscarinic receptor stimulation

Gi

-

Protein Kinase A

-

-

-

Signal transduction cascade involving Phospholipase C

Muscarinic receptor stimulation

Gq

Cholinergic agonists

Adrenergic Transmission

Sympathetic

Nerve

Terminal

1

Tyrosine

mitochondria contains

MAO, oxidizes amines

2

DA, Nepi

stored with ATP

in granules

3

4 exocytosis

Receptors

5

re-uptake

transport

Adr (NA) synthesis

Adrenergic Receptors

Alpha most effector cells

presynaptic, lipocytes, platelets, some smooth muscle

Betaeffector cells (*heart), brain, lipocytes, presynaptic

smooth muscle and myocardium

lipocytes

Adrenergic receptors: 2 and

receptor receptor+ -

Adenylylcyclase

receptor+

Phospholipase C

Adrenergic receptors: 1

Pesticides

•Organochlorines•ChE inhibitors

•Organophosphates•Carbamates

•Phenoxyherbicides•Pyrethroids•Bromine-based•Phenol- derivatives•Dipyridyl derivatives

Organochlorine insecticides

Organochlorine insecticides• DDT

– first commercially produced insecticide (1940’s)

– banned in the US in the 1970’s but is still manufactured and exported (1 ton/day)banned in the US in the 1970’s but is still manufactured and exported (1 ton/day)

• Cyclodienes– Most toxic (CNS) and persistent pesticides known

• HCH and Cl-benzene– Mixtures of isomers

– Medicinal use (lice shampoo) (lindane)

•t1/2 = 7-30y

•Bioaccumulates•Persistent•Lipophilic

•Non-selective•Endocrine disrupter•Reproductive toxins•Neurotoxins (Lindane)

Observed effects

• DDT– Enzyme induction– Competes with estradiol for receptor

• Cyclodienes– Reproductive toxicity (reduced fertility, loss of

pups, teratogenic)– CNS toxicity

• HCH and Cl-benzene– CNS toxicity– Increased hepatocellular tumors (mice)

Mechanisms of action• DDT

– Peripheral sensory neurons– prolonged negative afterpotential in neurons– K+ transport, inactivate Na+ channel closure, inhibit

Na+ /K+ and Ca2+ /Mg2+ ATPases, inhibit calmodulin-transport of Ca2+ (fig. 22-4)

• Cyclodienes– CNS localized– GABAA receptor/channel antagonists, inhibit Cl--uptake and

Na+ /K+ and Ca2+ /Mg2+ ATPases

• HCH and Cl-benzene– Suggested similar to cyclodienes but unknown

Cholinesterase inhibitors

• Organophosphates (OP) and Carbamates– Strong Acute neurotoxicity - AChE inhibition

(cholinergic effects)

– Nervous system toxins - nerve gas (sarin)

WWII chemical warfareWWII chemical warfare1988 Iraq- against Kurds1988 Iraq- against Kurds1994 Japan 1994 Japan 1995 Tokyo subway1995 Tokyo subway

1st OP: TEPP 1st OP: TEPP (tetraethylpyrophosphate), (tetraethylpyrophosphate), followed by parathionfollowed by parathion1st carbamic: 19301st carbamic: 1930

Cholinesterase

CH3C-O- + HO-CH2-CH2-N(CH3)3

CH3 C - O - CH2 - CH2 - N(CH3)3

O

O

+

+

Esteratic Anionic

O

P X

R1R1

R2R2

= aliphaticR1-2R1-2

ParathionMalathionSomanEcothiophate

Organophosphates

= e- withdrawingX

X

Strong Covalent Bond, Inactivates Enzyme

(stable >100h)

Organophosphates

P

R1

R2

Esteratic Anionic

O

Aging of complex

Organophosphates are Organophosphates are slower to release from slower to release from AchE - “also aging effect”AchE - “also aging effect”

Carbamates are faster: Carbamates are faster: reversiblereversible

Phase I metabolic Phase I metabolic activation activation Multiple metabolic Multiple metabolic reactionsreactions

Neurobehavioral, Neurobehavioral, muscular and cognitive muscular and cognitive effectseffectsDelayed Neuropathy Delayed Neuropathy (OPIDN) - ginger jake(OPIDN) - ginger jake

WHO has a battery of WHO has a battery of neuropsychological testsneuropsychological tests

OPs causing OPIDN

Phase I and II biotransformation

Antidote pralidoxime, removing OP from enzyme site

PyrethroidsNewer (1980) but were 30% of all use by 1982Newer (1980) but were 30% of all use by 1982

Extensive agricultural useExtensive agricultural useIndoor useIndoor usePet flee controlPet flee controlHousehold plantsHousehold plants

Modify NaModify Na++ channel kinetics channel kineticsAbnormal repetitive dischargesAbnormal repetitive dischargesType A shorter action than type BType A shorter action than type B

Avermectins

New generation pesticides

Herbicides• 1.9% increase/year between 1980-1985 (x2 of

insecticides) due to:– Monoculture– Mechanization of agricultural processes

• Categories by application:– Pre-planting– Pre-emergent– Post-emergent

• Low mammal toxicity• Suspected mutagens, carcinogens, teratogens• Skin irritants

Phenoxyherbicides• Introduced in 1946• 2,4Dichloro- and

2,4,5Trichloro phenoxy acetic acids

• Defoliants (Vietnam war) - Forestry

• Nerve toxicity, peripheral neuropathy

• Controversy about NHL and HL

• Contaminants may be responsible for toxicity

Dipyridyl derivatives““startling human toxicity”startling human toxicity”Banned in many countries but still in use in 130 othersBanned in many countries but still in use in 130 othersLung is the most susceptible target organLung is the most susceptible target organHighly polar- poor GI absorption (5-10%)Highly polar- poor GI absorption (5-10%)

LD50=22-262mg/kgLD50=22-262mg/kg

LD50=100-400mg/kgLD50=100-400mg/kg

ChloroacetanilidesOnly slight acute toxicity but Only slight acute toxicity but Carcinogens of category 2BCarcinogens of category 2B

Metabolic activation to Metabolic activation to mutagenic metabolite (DEBQ1) mutagenic metabolite (DEBQ1)

1985 Canada incident (well 1985 Canada incident (well water contamination)water contamination)

Phosphomonomethyl aminoacids

•Non-selective systemic Non-selective systemic herbicidesherbicides•Free acids or salts - Free acids or salts - ocular and mucus ocular and mucus membrane irritantsmembrane irritants•Class E carcinogens Class E carcinogens (EPA)(EPA)•Solvent may be the Solvent may be the toxic compound (POEA)toxic compound (POEA)

Fungicides• Lipophilic, accumulate• 90% are carcinogenic in animals

--> 75 mil pounds produced annually

• 10% acreage but 60% of total dietary carcinogenic risk

• Contaminants are dioxins and furans

• Hexachlorobenzene (banned)• Pentachlorophenol (banned)• Phthalimides• Dithiocarbamates

Fungicides

• Dithiocarbamates– Ferbam, ziram, maneb, zineb, nabam (metal-based names)– Some reported as teratogenic– Degradation to ethylene thiourea (ETU): a known mutagen,

carcinogen, teratogen and antithyroid compound.– Some neurotoxicity at high doses– May cross into CNS if bound to divalent metals

Fumigants

• Very volatile - inhalation exposure• Non-selective, highly reactive and cytotoxic

– acrylonitrile– carbon disulfide– carbon tetrachloride– ethylene dibromide (gastric carcinomas, sterility)– ethylene oxide (carcinogen, developmental tox.)– phosphine (PH3) released from aluminum phosphide (AlP) in

moist conditions (grain storage)

Rodenticides• Rodents: vectors of disease

– Zinc phosphide - PH3 (cell toxicity,

necrosis, GI, liver, kidneys)

– Fluoroacetic acid and derivatives (Fluoroacetyl-CoA --> fluorocitrate: Krebs cycle collapse)

-naphthyl thiourea (ANTU) must be metabolically activated --> resistance

– Anticoagulants (coumadin, warfarin) - antagonist of vit. K in synthesis of clotting factors; requires multiple doses; resistance