Definition of a PesticideDefinition of a Pesticide

Any substance intended for preventing, destroying, repelling, or mitigating Any substance intended for preventing, destroying, repelling, or mitigating any pestany pest

InsecticidesInsecticides Herbicides Herbicides FungicidesFungicides

Pesticides are Not NewPesticides are Not New

• FumigantsFumigants Sulfur (1000 BC)Sulfur (1000 BC) Bordeaux mixture (CuSOBordeaux mixture (CuSO44, lime (Ca(OH), lime (Ca(OH)22, H, H22O)- 1882)O)- 1882)

• InsecticidesInsecticides Arsenic (16Arsenic (16thth Century) Century) Tobacco leaves (Tobacco leaves (Nicotiana tabacumNicotiana tabacum - 1690) - 1690) Rotenone (Rotenone (Derris elipticaDerris eliptica – 1800s) – 1800s) Pyrethrum (Pyrethrum (Chrysanthemum cinerriafolumChrysanthemum cinerriafolum- 1800s)- 1800s) Paris Green (copper arsenite 1800s)Paris Green (copper arsenite 1800s)

• RodenticidesRodenticides Nux vomica (Nux vomica (Strychnos nux-vomicaStrychnos nux-vomica) )

There is No Such Thing as a There is No Such Thing as a Completely SAFE PesticideCompletely SAFE Pesticide

• But Pesticides can be USED safely.But Pesticides can be USED safely.• All Pesticides have some type of All Pesticides have some type of

biological activitybiological activity• The trick is to balance efficacy and The trick is to balance efficacy and

safety. safety.

Pesticide RegulationsPesticide Regulations

Wiley or Sherman Act (1906)Wiley or Sherman Act (1906) Federal Food, Drug, and Cosmetic Act (1938)Federal Food, Drug, and Cosmetic Act (1938)

• Pesticide tolerances set in 1954 and 1958Pesticide tolerances set in 1954 and 1958• 1958: Delaney Act1958: Delaney Act

No additive is deemed safe if found to cause cancerNo additive is deemed safe if found to cause cancer• Pesticides were one type of additivePesticides were one type of additive

Federal Insecticide, Fungicide, and Rodenticide ActFederal Insecticide, Fungicide, and Rodenticide Act• 1947 (initially administered by USDA)1947 (initially administered by USDA)• 1972 (switch to EPA)1972 (switch to EPA)

Defines registration and labeling requirementsDefines registration and labeling requirements Can cancel registrationCan cancel registration Set tolerancesSet tolerances

• FDA monitors residue levelsFDA monitors residue levels Food Quality Protection ActFood Quality Protection Act

• 19961996• Special considerations for childrenSpecial considerations for children

Higher susceptibilityHigher susceptibility Consider child consumption patternsConsider child consumption patterns

Development of a New Pesticide is Development of a New Pesticide is a Lengthy and Costly Processa Lengthy and Costly Process

FIFRAFIFRA• Product and residue chemistryProduct and residue chemistry• Environmental fateEnvironmental fate• ToxicologyToxicology

Acute toxicityAcute toxicity• Oral, dermal, inhalation, irritation, ocular, delayed neurotox, dermal sensitizationOral, dermal, inhalation, irritation, ocular, delayed neurotox, dermal sensitization

SubchronicSubchronic• Rat, mouse, dog (dermal, inhalation, neurotox)Rat, mouse, dog (dermal, inhalation, neurotox)

ChronicChronic• Rodent, dog, carcinogenicityRodent, dog, carcinogenicity

ReproductiveReproductive• Fertility, teratology, in vitro mutagenicityFertility, teratology, in vitro mutagenicity

• BiotransformationBiotransformation• Occupational exposureOccupational exposure• Spray driftSpray drift• Environmental impact on non-target speciesEnvironmental impact on non-target species• EfficacyEfficacy

Costs: $30 to 80M (full development and testing of a new

pesticide may take 10 years)

The Majority of Insecticides are The Majority of Insecticides are NeurotoxicNeurotoxic

Toxicological Syndromes Toxicological Syndromes Associated with InsecticidesAssociated with Insecticides

Acute syndromesAcute syndromes• Cholinesterase inhibitorsCholinesterase inhibitors• ArsenicArsenic• Pyrethrins and pyrethroidsPyrethrins and pyrethroids

Chronic toxicosisChronic toxicosis• Delayed neuropathy (OPIDN)Delayed neuropathy (OPIDN)

ResiduesResidues• Organochlorine insecticidesOrganochlorine insecticides

DDTDDT Wild-life problemWild-life problem

Anticholinesterase Anticholinesterase InsecticidesInsecticides

Organophosphorus InsecticidesCarbamate Insecticides

Acetylcholinesterase Acetylcholinesterase InhibitorsInhibitors

Organophosphates (OP)Organophosphates (OP)• Phosphoric acid derivativesPhosphoric acid derivatives

30 in use30 in use First synthesized in 1937 (Bayer)First synthesized in 1937 (Bayer)

• Tetraethyl pyrophosphate (TEPP)Tetraethyl pyrophosphate (TEPP)• Chemical warfare agentsChemical warfare agents

Tabun (GA)Tabun (GA) Sarin (GB) – Japanese subway attacksSarin (GB) – Japanese subway attacks Soman (GD)Soman (GD) VXVX CMPF (GF)CMPF (GF)

• Parathion (Parathion (O,OO,O-diethyl -diethyl OO pp-nitrophenyl phosphate)-nitrophenyl phosphate) Replaced DDT in 1950sReplaced DDT in 1950s

• Number of acute poisoning cases resultedNumber of acute poisoning cases resulted

• ChlorpyrifosChlorpyrifos

Parathion

Acetylcholinesterase Acetylcholinesterase InhibitorsInhibitors

CarbamatesCarbamates• Carbamic acid derivativesCarbamic acid derivatives

~25 in use~25 in use Originally developed as fungicides (1930s)Originally developed as fungicides (1930s)

• CarbarylCarbaryl Physostigmine (alkaloid from Calabar Physostigmine (alkaloid from Calabar

bean)bean)

Interference With Neurotransmitter Interference With Neurotransmitter

(Acetylcholine) Function(Acetylcholine) Function

AcetylcholineAcetylcholine• Synthesized in neuron cell bodySynthesized in neuron cell body• Release triggered by an action potentialRelease triggered by an action potential

Sudden influx CaSudden influx Ca2+2+ ---> ACh release ---> ACh release

• Broken down by acetylcholinesteraseBroken down by acetylcholinesterase• Primary neurotransmitter in PNSPrimary neurotransmitter in PNS

Smooth and skeletal muscleSmooth and skeletal muscle

• CNS (distributed throughout)CNS (distributed throughout)• In the developing brain, every neuron In the developing brain, every neuron

expresses cholinesterase activity even if it expresses cholinesterase activity even if it isn’t cholinergic in adulthood. isn’t cholinergic in adulthood.

Mode of ActionMode of Action

CholinergicCholinergicneuronneuron

Effector organs: Effector organs: Smooth muscleSmooth muscleSkeletal muscle Skeletal muscle

CNSCNS

AcetylcholineAcetylcholine

Mode of ActionMode of Action

Acetylcholinesterase inhibitorsAcetylcholinesterase inhibitors• CarbamatesCarbamates

Carbamylation of enzyme occursCarbamylation of enzyme occurs ReversibleReversible

• OrganophosphatesOrganophosphates Enzyme phosphorylation occursEnzyme phosphorylation occurs

• ““Aging”Aging” IrreversibleIrreversible

• Aging occurs over a 12-24 hr time periodAging occurs over a 12-24 hr time period

Mode of ActionMode of Action

AcetylcholineAcetylcholine

Acetycholinesterase Acetycholinesterase inhibitorsinhibitors

MuscarinicMuscarinic NicotinicNicotinic

Synaptic cleftSynaptic cleft

CholineCholine

AcetateAcetate++

AcetylcholinesteraseAcetylcholinesterase

Carbamate

Organophosphate

C

Op

Treatment:Atropine2-PAM

Clinical SignsClinical Signs Clinical signs related to excessive stimulation of Clinical signs related to excessive stimulation of

nicotinic and muscarinic receptorsnicotinic and muscarinic receptors• Muscle tremorsMuscle tremors• CNS effectsCNS effects• Respiratory paralysis (this is what causes death)Respiratory paralysis (this is what causes death)• SSalivation, alivation, LLacrimation, acrimation, UUrination, and rination, and DDefecationefecation• CNS signsCNS signs• BronchospasmBronchospasm• Bronchial secretions Bronchial secretions • MiosisMiosis• BradycardiaBradycardia

Organophosphate InsecticidesOrganophosphate Insecticides

ToxicologyToxicology• Delayed effects (OPIDN)Delayed effects (OPIDN)

Ginger Jake paralysisGinger Jake paralysis• Tri-o-tolyl phosphate (TOTP)Tri-o-tolyl phosphate (TOTP)

Leptophos, mipafox, chlorpyrifos, DFPLeptophos, mipafox, chlorpyrifos, DFP All organophosphates are required to be All organophosphates are required to be

tested for their ability to produce OPIDN tested for their ability to produce OPIDN before they are marketed (Hen test)before they are marketed (Hen test)

OPIDNOPIDN

OOrganophosphate rganophosphate IInduced nduced DDelayed elayed NNeuropathyeuropathy• Clinical signsClinical signs

Ataxia and paralysisAtaxia and paralysis Develop 10 to 14 days after exposureDevelop 10 to 14 days after exposure

• NeuropathologyNeuropathology Wallerian-type degenerationWallerian-type degeneration

• Mode of ActionMode of Action Inhibition of neurotoxic esterase (NTE) is Inhibition of neurotoxic esterase (NTE) is

generally predictivegenerally predictive

As Animals Mature, They Become Less Sensitive to

Chlorpyrifos ToxicityMales

Control CPF 20 mg/kgBra

in C

ho

lin

este

rase A

cti

vit

y (

X ±

S.E

.M.)

0

2

4

6

8

10

12

14

16

18

20

11%

31%

61%

PND17PND27Adult

(Moser et al, Toxicol. Sci, 1998)

Generic OP PathwayGeneric OP Pathway

Parent Pesticide Oxon

Hydroylzed by A-Esterases

Bind to CaEs

InhibitAChE

Hepatic Activation

Acetylcholinesterase in the Young Brain is NOT more Sensitive to Organophosphorus Pesticide

Inhibition

Pesticide Concentration

1 nM 10 nM 100 nM 1 µM 10 µM

Per

cent

Con

trol

AC

hE A

ctiv

ity (

X ±

S.E

.M.)

0

20

40

60

80

100 PND4Adult

Chlorpyrifos-oxonMalaoxon Aldicarb

Mortensen et al., Toxicol. 1998

Developmental Profiles of Carboxylesterase and A-Esterase in

Rats

Chlorpyrifos-oxonase

Age (days)

0 10 20 30 40 50 60 70 80 900

20

40

60

80

100

120

140

Carboxylesterases

Age (days)

0 10 20 30 40 50 60 70 80 90

Per

cent

Adu

lt A

ctiv

ity (

X ±

S.E

.M.)

0

20

40

60

80

100

120

140LiverPlasma

(Mortensen et al., J. Biochem. Toxicol. 1996; Moser et al, Toxicol. Sci, 1998)

Generic OP PathwayAdult vs Young Rat

Parent Pesticide Oxon

Hydroylzed by A-Esterases

Bind to CaEs

InhibitAChE

Hepatic Activation and probably Deactivation

0

20

40

60

80

100

120

140

0 5 10 15 20 25

106 RR 107 RR

110 RR 100 QR

0

20

40

60

80

100

120

140

0 5 10 15 20 25

105 QQ 102 QQ

108 QQ 109 QQ

113 QQ

0

20

40

60

80

100

120

140

0 5 10 15 20 25

Ary

lest

era

se (

U/m

L)

Ary

lest

era

se (

U/m

L)

Ary

lest

ea

se (

U/m

L)

Age (months)

Age (months)Age (months)

C

Developmental Profiles of A-Esterase in Humans

Cole et al, 2003

Pyrethrin and Pyrethrin and Pyrethroid Pyrethroid

InsecticidesInsecticides

Pyrethrin and Pyrethroid Pyrethrin and Pyrethroid InsecticidesInsecticides

PyrethrinsPyrethrins• Naturally derived Naturally derived

insecticideinsecticide ChrysathemumChrysathemum Natural pyrethrins Natural pyrethrins

include pyrethrin I, include pyrethrin I, pyrethrin II, jasmolin pyrethrin II, jasmolin I, jasmolin II, cinerin I I, jasmolin II, cinerin I and cinerin II.3 and cinerin II.3

PyrethroidsPyrethroids• Synthetic insecticidesSynthetic insecticides• Slightly more Slightly more

persistentpersistent FenvalerateFenvalerate DeltamethrinDeltamethrin

PyrethroidsPyrethroids Natural pyrethrins are light sensitive and undergo rapid Natural pyrethrins are light sensitive and undergo rapid

photodegradation photodegradation Pyrethroids that contain a cyano substituent at the Pyrethroids that contain a cyano substituent at the

alpha-carbon of the phenoxy-benzyl moiety have been alpha-carbon of the phenoxy-benzyl moiety have been classified as type II; pyrethroids which lack this alpha classified as type II; pyrethroids which lack this alpha cyano moiety as type Icyano moiety as type I• Type I (T syndrome): Type I (T syndrome): TremorsTremors, tachypnea, "running fits," , tachypnea, "running fits,"

hyperthermia, and salivation within 1-2 hours of injectionhyperthermia, and salivation within 1-2 hours of injection AllethrinAllethrin Pyrethrin IPyrethrin I ResmethrinResmethrin TetramethrinTetramethrin

• Type II (CS syndrome): Whole body tremors, Type II (CS syndrome): Whole body tremors, hypersensitivity, occasional running fits, hypersensitivity, occasional running fits, choreoathetosischoreoathetosis (sinuous writhing), hypothermia, and generalized (sinuous writhing), hypothermia, and generalized seizuresseizures

CypermethrinCypermethrin DeltamethrinDeltamethrin FenvalerateFenvalerate

Pyrethrin and Pyrethroid Pyrethrin and Pyrethroid InsecticidesInsecticides

ToxicityToxicity• Low oral toxicityLow oral toxicity

LDLD5050 quite variable quite variable• 25 to 10000 mg/kg25 to 10000 mg/kg

Rapid hydrolysis in the gastrointestinal tractRapid hydrolysis in the gastrointestinal tract Liver metabolism Liver metabolism

SynergistsSynergists• Microsomal enzyme inhibitorsMicrosomal enzyme inhibitors

Piperonyl butoxidePiperonyl butoxide

Mode of Action Mode of Action (Overstimulation of the Nervous System)(Overstimulation of the Nervous System)

Interference with voltage gated Interference with voltage gated sodium channels.sodium channels.• Type I keep channels open for Type I keep channels open for

shorter period vs. Type IIshorter period vs. Type II• Enhanced sodium ion conductanceEnhanced sodium ion conductance

Clinical SignsClinical Signs Muscle tremorsMuscle tremors Mixed CNS effectsMixed CNS effects

• CNS depressionCNS depression• CNS excitationCNS excitation

SeizuresSeizures Increased salivationIncreased salivation VomitingVomiting AtaxiaAtaxia Hypo and hyperthermiaHypo and hyperthermia ParesthesiaParesthesia

Hydroylzed by CAEs

Hydrolyzed by P450s

Generic Pyrethroid Generic Pyrethroid PathwayPathway

Pyrethroid

Keep Na Channels Open

Time After Dosing (minutes)

0 50 100 150 200 250 300 350 400

Ch

oreo

atheto

sis Sco

re

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5Weanling, 4 mg/kg DLTAdult, 30 mg/kg DLT

Choreoathetosis

Time After Dosing (minutes)

0 50 100 150 200 250 300 350 400

Salivatio

n S

core

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5Weanling, 4 mg/kg DLTAdult, 30 mg/kg DLT

Salivation

Newer Pesticides:Newer Pesticides:

““Mectins” (Nicotinic receptor agonists, Mectins” (Nicotinic receptor agonists, specific for non-mammalian receptors)specific for non-mammalian receptors)• AvermectinAvermectin• IvermectinIvermectin

Anthelmentic, insecticideAnthelmentic, insecticide Management of river blindness (Onchocerca)Management of river blindness (Onchocerca) Low dermal absorptionLow dermal absorption Minimal biotransformationMinimal biotransformation

Final ThoughtsFinal Thoughts

Most of what has been presented Most of what has been presented relates to acute toxicology.relates to acute toxicology.

What about long term effects of low What about long term effects of low level exposure to pesticides?level exposure to pesticides?

What about effects on the developing What about effects on the developing nervous system?nervous system?