Phenylpyrazole insecticide poisoning Michael Eddleston Clinical Pharmacology Unit, University of Edi

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Phenylpyrazoleinsecticide poisoning Michael
EddlestonClinical Pharmacology Unit,
University of EdinburghSouth Asian Clinical
Toxicology Research Collaboration,Scottish
Poisons Information BureauRoyal Infirmary of
EdinburghFunded by the
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Insecticides

• Cholinergic system
• organophosphorus pesticides
• carbamates
• pyrethroids
• neonicotinoids
• GABAergic system
• organochlorines
• N-phenylpyrazoles
• avermectins
• Miscellaneous eg chitin synthesis inhibitors

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GABAA receptor
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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 gt25
• Treatment early benzodiazepines, rapidly
  followed by phenobarbital, and then general
  anaesthesia with thiopental

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N phenylpyrazole insecticides

• EPA CPH 03 Toxline INTOX
• acetoprole (CAS 209861-58-5) x x 0
  x
• ethiprole (CAS 181587-01-9) x v 2
  x
• fipronil (CAS 120068-37-3) 1994 v
  961 x
• pyraclofos (CAS 77458-01-6) (Jap) v
  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).

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Structurephenyl group pyrazole carbonitrile
fipronil
vaniliprole
pyraclofos
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Acute toxicity and fat solubility

• rat oral LD50 (mg/kg) logKow
• 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

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Clinical experience of phenylpyrazole poisoning -
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• 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. NV was the commonest
  feature.

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Clinical experience of phenylpyrazole poisoning -
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• 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 95CI
  0.0-14.8)
• Retrospective case suggests possibility of
  severe status epilepticus

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Fipronil pharmacokinetics 1
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Metabolism of fipronil
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Fipronil pharmacokinetics - 2
Patient 7
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Q - why are phenylpyrazoles relatively less toxic?
John E Casida, University of Berkeley

• 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 3HEBOB binding site on the GABAA with
  affinities of 0.37 nM.
• Resistance to cyclodienes in drosophila
  (alanine302serine mutation) also confers low
  sensitivity to fipronil
• Specific combinations of a, b and g subunits
  confer differential insecticide binding
  sensitivity

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Insecticides bind to all 5 subunits from within
the pore to prevent flow of chloride ions
Chen 2006 PNAS
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Binding of insecticides to human GABA receptors
Ratra 2001 TAP 172233
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Human b3 monomeric GABAA receptor
Has sequence homology to insect GABA
receptor Binding of insecticides to b3 receptor
correlates well with binding to insect receptor
Ratra 2001 ToxL 122215
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Phenylpyrazole insecticide selectivity
Ratra 2001 TAP 172233
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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
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Fipronil blocks insectglutamate-activated
chloride channels

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

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

Narahisha 2007 HET
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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.