Phenylpyrazole insecticide poisoning Michael Eddleston Clinical Pharmacology Unit, University of Edinburgh South Asian Clinical Toxicology Research Collaboration, Scottish Poisons Information Bureau Royal 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

GABA A 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 GABA A 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 EPACPH 03 Toxline INTOX acetoprole (CAS: ) x x 0 x ethiprole (CAS: ) x √ 2 x fipronil (CAS: ) 1994 √ 961 x pyraclofos (CAS: ) ( Jap) √ 17,078 OP pyrafluprole (CAS: ) x x 0 x pyriprole (CAS: ) x x 0 x vaniliprole (CAS: ) 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 fipronil pyraclofos pyrafluprole NK NK pyriprole NK NK vaniliprole NK NK chlorpyrifos dimethoate endosulfan 803.1

Clinical experience of phenylpyrazole poisoning papers reporting fipronil poisoning. No reports for any of the other pesticides. Fung yr old woman unintentionally ate ~0.1mg of fipronil in an ant bait. No clinical features occurred. Chodorowski yr old man occupationally exposed to a 20% formulation during preparation and 0.005% during spraying. Only clinical features: headache, nausea, vertigo, and weakness. Mohamed 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 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 %]) Retrospective case suggests possibility of severe status epilepticus

Fipronil pharmacokinetics 1

Metabolism of fipronil

Fipronil pharmacokinetics - 2 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 GABA A 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 Ratra 2001 ToxL 122:215 Has sequence homology to insect GABA receptor Binding of insecticides to  3 receptor correlates well with binding to insect receptor

Phenylpyrazole insecticide selectivity Ratra 2001 TAP 172:233

Ikeda et al patch clamped dorsal root ganglion cells Showed that fipronil reduced both frequency of Cl - channels opening and length of time open

Fipronil blocks insect glutamate-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: IC nM vs 2.3 nM

Phenylpyrazole insecticides - conclusions Recently introduced into agricultural practice Only one commonly used - fipronil Inhibit the the GABA A 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.