Assessing the atmospheric fate of pesticides used to control mosquito populations in Houston, TX

During the summer months, urban areas are literal hot spots of mosquito-borne disease transmission and air pollution. Public health authorities release aerosolized pesticides directly into the atmosphere to help control adult mosquito populations and thereby reduce the threat of diseases, such as Zika Virus. The primary adulticides (i.e. pesticides used to control adult mosquito populations) in Houston, TX are permethrin and malathion. These adulticides are typically sprayed at night using ultra-low volume sprayers. Particulate matter (PM) samples including total suspended and fine PM (PM < 2.5 μm in aerodynamic diameter) were collected at four ground-based sites across Houston in 2013 and include daytime, nighttime, and 24 h samples. Malathion is initially sprayed as coarse aerosol (5-25 μm), but is measured in fine aerosol (<2.5 μm) and coarse aerosol in the urban atmosphere. Particle size is relevant both for deposition velocities and for human exposure. Atmospheric permethrin concentrations measured in nighttime samples peak at 60 ng m^-3, while malathion nighttime concentrations peak near 40 ng m^-3. Malaoxon, an oxidation product of malathion, was also frequently detected at concentrations >10 ng m^-3, indicating significant nighttime oxidation. Based on the loss of malathion and the increase in malaoxon, the atmospheric half-life of malathion in Houston was estimated at <12 h, which was significantly shorter than previous half-life estimates (∼days). Importantly, malaoxon is estimated to be 22-33 times more toxic to humans than malathion. Both the aerosol size and the half-life are critical for mosquito control, human exposure, and risk assessment of these routine pesticides.

Differential lymphatic versus portal vein uptake of the synthetic pyrethroids deltamethrin and cis-permethrin in rats

The objective of this study was to obtain data on pathways of absorption of the synthetic pyrethroids deltamethrin (DLM) and cis-permethrin (CPM) following oral administration to rats. Adult male Sprague-Dawley rats with cannulated mesenteric lymph ducts and hepatic portal veins were given single doses of either 5 mg/kg DLM or 60 mg/kg CPM via the duodenum and lymph and portal blood samples collected for up to 300 min. The pyrethroid dosing vehicles (5 mL/kg body weight) were either corn oil or glycerol formal. Levels of DLM and CPM in lymph and portal blood samples were determined by high-performance liquid chromatography-mass spectrometry-mass spectrometry. Over the time period studied, levels of both DLM and CPM following administration in either corn oil or glycerol formal were greater in lymph than in portal blood. Lymphatic uptake of both DLM and CPM was enhanced following dosing in glycerol formal than in corn oil. The results of this study suggest that after oral administration to rats, these two pyrethroids are predominantly absorbed via the lymphatic system rather than via portal blood. The data obtained in this study thus support a recently developed physiologically-based pharmacokinetic (PBPK) model to evaluate age-related differences in pyrethroid pharmacokinetics in the rat, where it was assumed that absorption of pyrethroids was predominantly via lymphatic uptake.

In vitro metabolism of cis- and trans-permethrin by rat liver microsomes, and its effect on estrogenic and anti-androgenic activities

Permethrin is a widely applied broad-spectrum pyrethroid insecticide that consists of a mixture of cis- and trans-isomers. We examined the changes of estrogenic and anti-androgenic activities resulting from metabolism of the isomers. Both cis- and trans-permethrin were hydrolyzed to 3-phenoxybenzyl alcohol (PBAlc) by rat liver microsomes, but the extent of hydrolysis of trans-permethrin was much greater than that of the cis-isomer. In the presence of NADPH, PBAlc was further transformed to 4'-hydroxylated PBAlc (4'-OH PBAlc), 3-phenoxybenzaldehyde (PBAld) and 3-phenoxybenzoic acid (PBAcid). cis-Permethrin, but not trans-permethrin, also afforded its 4'-hydroxylated derivative (4'-OH cis-permethrin). trans-Permethrin was an anti-androgen, but also showed weak estrogenic activity, while cis-permethrin was a weak estrogen and a weak anti-androgen. After incubation with rat liver microsomes in the presence of NADPH, cis-permethrin but not trans-permethrin was metabolically activated for estrogenic activity. On the other hand, estrogenic activity of trans-permethrin was not changed, but its anti-androgenic activity was enhanced after incubation. 4'-OH PBAlc and PBAlc showed estrogenic activity, while PBAld and PBAlc showed anti-androgenic activity. PBAcid showed neither activity. 4'-OH cis-permethrin showed both estrogenic and anti-androgenic activities. Overall, our results indicate that permethrin is metabolically activated for estrogenic and anti-androgen activities, and the microsomal transformation of permethrin to 4'-OH cis-permethrin, 4'-OH PBAlc and PBAlc contributes to the both metabolic activations.