Fipronil-Related Heterocyclic Compounds: Structure–Activity Relationships for Interaction with γ-Aminobutyric Acid- and Voltage-Gated Ion Channels and Insecticidal Action

Abstract

To investigate the role of the heterocyclic moieties of nitrogen-containing phenyl heterocyclic compounds (PHCs) in interaction with γ-aminobutyric acid (GABA)-gated chloride channels, diverse classes of PHCs were examined for their ability to inhibit the specific binding of [3H]4′-ethynyl-4-n-propylbicycloorthobenzoate (EBOB), a noncompetitive GABA antagonist, to housefly head and rat brain membranes. PHCs that inhibited [3H]EBOB binding include pyrazoles; 1H-1,2,3-triazoles; a 1H-1,2,4-triazole; 1,3,4-oxadiazol-2(3H)-ones; a 1,3,4-oxadiazole-2(3H)-thione; 1,2,4-oxadiazoles; a 1,2,4-thiadiazole; thiazoles; a 4(3H)-pyrimidinone; and a 2,4(1H,3H)-pyrimidinedione. An analogue (1) of the pyrazole insecticide fipronil, bearing an SCF3 group in place of the S(O)CF3, was found to be the most potent inhibitor with IC50s of 7.55 and 177 nM in housefly head and rat brain membranes, respectively. 3-(2,6-Dichloro-4-trifluoromethyl phenyl)-5-t-butyl-1,3,4-oxadiazol-2(3H)-one exhibited the highest selectivity for housefly GABA receptors versus rat receptors (IC50 rat/IC50 fly >204). PHCs that exhibited a comparable selectivity include a pyrazole and a 1H-1,2,3-triazole. Interaction of 16 selected PHCs with rat brain GABA-gated channels was also examined using [3S]t-butylbicyclophosphorothionate (TBPS), a radioligand for the mammalian noncompetitive antagonist site. A plot of pIC50s of 12 PHCs revealed a close correlation (r = 0.93) between their potency in inhibiting [3H]EBOB and [35S]TBPS binding. Scatchard analyses of the inhibition of [35S]TBPS binding by 1 suggested a competitive-type inhibition. A plot of the potency of 14 PHCs in inhibiting [3H]EBOB binding to housefly head membranes against their piperonyl butoxide-synergized insecticidal effect on German cockroaches yielded a close correlation (r = 0.89), with the exception of five triazoles and a 2,4(1H,3H)-pyrimidinedione. Although several selected PHCs also inhibited the specific binding of [3H]batrachotoxinin A 20-α-benzoate, a tritiated analogue of the sodium channel activator batrachotoxinin, to synaptosomes and membranes prepared from mouse brains, housefly heads, and American cockroach nerve cords, the concentrations required were higher than those of standard compounds producing significant effects on this system. The results demonstrate that a variety of five- and six- membered, nitrogen-containing heterocyclic compounds, bearing a 2,6-dichloro-4-trifluoromethyphenyl or 2,4,6- trichlorophenyl group, interact with ionotropic GABA receptors. Several PHCs display higher affinities for housefly GABA receptors and high selectivity as compared to rat GABA receptors. PHCs' insecticidal activity is mediated by their interaction with GABA-gated chloride channels.