Phenylpyrazoleinsecticide poisoning

Michael Eddleston

Clinical Pharmacology Unit, University of Edinburgh

South Asian Clinical Toxicology Research Collaboration,

Scottish Poisons Information BureauRoyal Infirmary of Edinburgh

Funded by the

Insecticides

Cholinergic system• organophosphorus pesticides• carbamates• pyrethroids• neonicotinoids

GABAergic system • organochlorines• N-phenylpyrazoles• avermectins

Miscellaneous: eg chitin synthesis inhibitors

GABAA receptor

Organochlorine insecticides

• Widely used since the 1950s. More recently their use has fallen worldwide after recognition of their environmental persistence. Still cases in India, Iran & Turkey

• Two main groups – cyclodienes (eg. endosulfan) and cycloalkanes (eg lindane).

• 1980s found to bind to and block the GABAA chloride channel in the CNS, resulting in hyperexcitability.

• Severe poisoning causes status epilepticus that is often resistant to therapy. CFR >25%

• Treatment = early benzodiazepines, rapidly followed by phenobarbital, and then general anaesthesia with thiopental

N phenylpyrazole insecticides

EPA CPH 03 Toxline INTOX

• acetoprole (CAS: 209861-58-5) x x 0 x

ethiprole (CAS: 181587-01-9) x √ 2 x

fipronil (CAS: 120068-37-3) 1994 √ 961 x

pyraclofos (CAS: 77458-01-6) (Jap) √ 17,078 OP

pyrafluprole (CAS: 315208-17-4) x x 0 x

pyriprole (CAS: 394730-71-3) x x 0 x

vaniliprole (CAS: 145767-97-1) x x 0 x

• Recent development - fipronil first used in agriculture in the mid-1990s to kill pests of corn, rice and cotton

• Also against animal ticks (fipronil, pyriprole) and ants (fipronil).

fipronil

pyraclofos vaniliprole

Structure

phenyl group + pyrazole carbonitrile

Acute toxicity and fat solubility

rat oral LD50 (mg/kg) logK[ow]

acetoprole NK NK

ethiprole 7080 2.0

fipronil 92 4.0

pyraclofos 237 3.8

pyrafluprole NK NK

pyriprole NK NK

vaniliprole NK NK

chlorpyrifos 135 5.1

dimethoate 150 0.8

endosulfan 80 3.1

Clinical experience of phenylpyrazole poisoning - 1

• 3 papers reporting fipronil poisoning.

No reports for any of the other pesticides.

• Fung 2003 - 77yr old woman unintentionally ate ~0.1mg of fipronil in an ant bait. No clinical features occurred.

• Chodorowski 2004 - 50yr old man occupationally exposed to a 20% formulation during preparation and 0.005% during spraying. Only clinical features: headache, nausea, vertigo, and weakness.

• Mohamed 2004 - 6 men and 1 woman who had intentionally ingested a 5% agricultural formulation of fipronil. Two developed seizures (1x 1 seizure, 1x 2 seizures). The most ill patient was drowsy on admission, sweating profusely, and vomiting. N&V was the commonest feature.

Clinical experience of phenylpyrazole poisoning - 2

• Mohamed 2004 - fipronil was detected in blood for all 6 patients in whom a blood sample was obtained in this prospective study.

• This paper also reported a retrospective case of a 23 yr old male who was reported to have ingested 100 ml of 5% fipronil. He developed status epilepticus and died with bronchopneumonia after 17 days. Ingestion of fipronil was not proven in this case, but the bottle label was present in the notes.

• CURRENT CLINICAL EXPERIENCE

Nausea and vomiting, profuse sweating, self-terminating seizures.

(SACTRC case series 23 patients, CFR: 0 [95%CI 0.0-14.8%])

Retrospective case suggests possibility of severe status epilepticus

Fipronil pharmacokinetics 1

0 10 20 30 40 50 60 700

50

100

1000

2000

3000

4000

#2

#3

#4

#5

#6

#7

3744 ug/L

1600 ug/L

*

*

Hours post-ingestion

Pla

sm

a f

ipro

nil

co

nc

en

tra

tio

n (

g/L

)

Metabolism of fipronil

Fipronil pharmacokinetics - 2

0 10 20 30 400

10

20

30

40

50

60

70

80

90 fipronil sulfone

fipronil

Hours post-ingestion

Pla

sm

a c

on

cen

trat

ion

(

g/L

)

Total

Patient 7

Q - why are phenylpyrazoles relatively less toxic?

• Examined the specificity of binding of fipronil to human and insect GABA receptors via expression of subunits in Sf9 cells

• Endosulfan, lindane and fipronil compete for the same [3H]EBOB binding site on the GABAA with affinities of 0.3–7 nM.

• Resistance to cyclodienes in drosophila (alanine302serine mutation) also confers low sensitivity to fipronil

• Specific combinations of , and subunits confer differential insecticide binding sensitivity

John E Casida, University of Berkeley

Insecticides bind to all 5 subunits from within the pore to prevent

flow of chloride ions

Chen 2006 PNAS

Binding of insecticides to human GABA receptors

Ratra 2001 TAP 172:233

Human 3 monomeric GABAA receptor

Insecticide Hum b3 Native insect Selectivity

GABA R IC50 (nM)alpha-endosulfan 0.34 6.4 0.05lindane 0.66 7.1 0.09fipronil 1.8 4 0.45

Ratra 2001 ToxL 122:215

Has sequence homology toinsect GABA receptor

Binding of insecticides to 3receptor correlates well withbinding to insect receptor

Phenylpyrazole insecticide selectivity

Potency

Insecticide Mammal Insect Selectivity

GABA R IC50 (nM)alpha-endosulfan 28 10 2.8lindane 833 11 76fipronil 4300 2.3 1870

LD50 (mg/kg)alpha-endosulfan 10 5.5 1.8lindane 40 5.5 7.3fipronil 32 0.25 128

Ratra 2001 TAP 172:233

Ikeda et al 2004 - patch clamped dorsal root ganglion cells

Showed that fipronil reduced both frequency of Cl- channels opening and length of time open

Fipronil blocks insectglutamate-activated chloride channels

• Glutamate gated chloride channels exist at central synapses in insects, but not mammals

Narahisha 2007 HET

• Potency compares favourably with GABA chloride channel inhibition: IC50 10.0 nM vs 2.3 nM

Phenylpyrazole insecticides - conclusions

• Recently introduced into agricultural practice

• Only one commonly used - fipronil

• Inhibit the the GABAA chloride channel in the CNS at the same site as organochlorines. May also block glutamate chloride channels. Result in neuronal hyperexcitability.

• Relatively less toxic to mammals due to selective binding to insect vs mammalian GABA receptors.

• Low toxicity in poisoning due to high potency of fipronil. Allows use of small quantities and low concentrations.

• Treatment = benzodiazepines. No evidence currently that other Rx is required. Low case fatality.