2-((1‑(4‑Phenoxyphenoxy)propan‑2‑yl)oxy)pyridine (MPEP) is a juvenile hormone mimetic pesticide used to control a variety of insects, including tsetse flies, cockroaches, and whiteflies, and is added to potable water in Zika virus‑endemic areas to control mosquitoes. It has been proposed that MPEP might contribute to the increased incidence of microcephaly in babies born to mothers who could be consuming MPEP in potable water. Because limited information is available about the potential hazard of MPEP to pregnant women, the National Toxicology Program (NTP) conducted prenatal developmental toxicity studies of MPEP to assess possible harm to the developing conceptus and pregnant animal. In these studies, time‑mated Sprague Dawley (Hsd:Sprague Dawley SD) rats and New Zealand White (Hra:NZW SPF) rabbits were administered MPEP in corn oil by gavage once daily from implantation on gestation day (GD) 6 (rats) or GD 7 (rabbits) to the day before expected parturition (GD 20 for rats; GD 28 for rabbits). In the prenatal developmental toxicity study in rats, fetuses were examined for evidence of MPEP fetal toxicity. A dose range‑finding study in rabbits was conducted, followed by a prenatal developmental toxicity study, to confirm the absence of a response in a second species. An assessment of maternal and fetal MPEP concentrations following exposure demonstrated maternal‑fetal transfer of MPEP in both rats on GD 18 and rabbits on GD 28.
Prenatal developmental toxicity study in rats
Dose selection was informed by summary data provided for marketing approval, and an additional dose level was added to aid in dose‑response characterization. Groups of 25 time‑mated female rats were administered 0, 62.5, 125, 250, or 500 mg MPEP/kg body weight/day (mg/kg/day) in corn oil by gavage once daily from GD 6 to GD 20.
After initiation of dosing (GD 6–9), dams administered either 250 or 500 mg/kg/day displayed similar significantly decreased (~25%) mean body weight gains relative to vehicle control animals. This finding occurred concomitantly with significantly decreased (~11%) feed consumption in the 500 mg/kg/day group, demonstrating limited maternal toxicity. Exposure to MPEP did not affect any pregnancy or litter parameters. Fetal weight in the 500 mg/kg/day group was slightly lower (<4%), with a significant trend, and was not associated with an increased incidence of ossification variants.
Fetal visceral findings included small increases in the incidences of liver discoloration (a variation), which could have been incidental but may be indirect effects of MPEP on fetal liver function or metabolism. No external, visceral, head, or skeletal malformations were attributed to MPEP exposure.
Dose range‑finding study in rabbits
Groups of eight time‑mated female rabbits were administered 0, 300, 400, or 500 mg/kg/day MPEP in corn oil by gavage once daily from GD 7 to GD 28. Information from previous work indicated a likely sharp dose‑response curve for maternal toxicity. Decreased feed consumption and decreases in mean body weight indicative of overt maternal toxicity were observed at doses of 400 and 500 mg/kg/day, resulting in those dose groups being removed from the study and fetuses not examined. Similar, but less severe, maternal findings were noted at 300 mg/kg/day. Uterine and fetal weights were slightly lower in the 300 mg/kg/day dose group relative to the vehicle control group, and these findings may have been secondary to maternal toxicity. No external or placental observations were attributed to MPEP exposure. A high dose of 250 mg/kg/day was therefore selected for the prenatal developmental toxicity study.
Prenatal developmental toxicity study in rabbits
Time‑mated rabbits (23 or 24 per dose group) were administered 0, 62.5, 125, or 250 mg/kg/day of MPEP in corn oil by gavage once daily from GD 7 to GD 28. An additional 3 or 4 does per dose group, used for biological sampling, were administered the same doses of MPEP. The 250 mg/kg/day dose was generally well tolerated by most does; however, decreases in feed consumption, mean body weight, and body weight gain were observed in this dose group, resulting in three animals being removed early from the study. These early removals, in addition to two nonpregnant does and two does that underwent parturition prior to laparotomy, collectively resulted in 16 litters in the 250 mg/kg/day group available for examination. Litter size, postimplantation loss, and fetal weight were not affected by MPEP exposure, demonstrating that although the 250 mg/kg/day dose resulted in some maternal toxicity, the does and fetuses received the highest dose possible without overt impact on maternal function that may impact fetal outcomes.
No external, visceral, or head malformations were attributed to MPEP exposure. A single incidence of hydrocephaly was noted in one fetus in the 125 mg/kg/day group, but this finding was considered incidental. Three fetuses from two litters and three fetuses from one litter in the 125 and 250 mg/kg/day dose groups, respectively, displayed increased incidences of a skeletal malformation: seventh costal cartilage not fused to sternum. This structure is recognized by the International Federation of Teratology Societies but has not specifically been reported in the historical control data of commercial contract laboratories used for animal sourcing. The absence of this cartilaginous structure could result from a delay in development independent of maternal toxicity, given that individual doe mean body weight gains and feed consumption were similar to those of does whose fetuses were not observed with this malformation.
Exposure to MPEP was confirmed in pregnant rats and rabbits and MPEP was detected in the fetuses, demonstrating that fetuses were exposed to MPEP.
Under the conditions of the rat prenatal developmental toxicity study, there was no evidence of developmental toxicity of 2‑((1‑(4‑phenoxyphenoxy)propan‑2‑yl)oxy)pyridine (MPEP) in Sprague Dawley (Hsd:Sprague Dawley SD) rats administered 62.5, 125, 250, or 500 mg/kg/day based on the absence of effects on reproductive parameters, fetal weight, or increased incidence of fetal malformations or variations. The highest dose administered was 500 mg/kg/day, which did not result in overt maternal toxicity.
Under the conditions of the rabbit prenatal developmental toxicity study, there was equivocal evidence of developmental toxicity of MPEP in New Zealand White (Hra:NZW SPF) rabbits based on the occurrence of the malformation “seventh costal cartilage not fused to sternum” in dosed groups. This finding was observed at 250 mg/kg/day, a dose that induced some maternal toxicity.
National Toxicology Program (NTP). 2022. NTP developmental and reproductive toxicity technical report on the prenatal development studies of 2-((1-(4-phenoxyphenoxy)propan-2-yl)oxy)pyridine (CASRN 95737-68-1) in Sprague Dawley (Hsd:Sprague Dawley SD) rats and New Zealand White (Hra:NZW SPF) rabbits. Research Triangle Park, NC: National Toxicology Program. DART Report 07. https://doi.org/10.22427/NTP-DART-07