Dimethyl Phosphite (DMP) is rapidly absorbed via the oral and dermal routes. The main metabolic pathway in rodents is demethylation to monomethyl hydrogen phosphite (MMP) and further oxidation to CO2. DMP was mainly eliminated via urine and expired air. Over the studied dose range between 10 and 200 mg/kg bw and 5 x 200 mg/kg bw, respectively, only little evidence of bioaccumulation or saturation of absorption and elimination was observed. The only difference in studied toxicokinetics between rats and mice was the more rapid metabolism and elimination in mice.
An inhalation LC50 value is not available, but an exposure of 7100 mg/m³ (concentration estimated based on air flow and net loss of material) over 6 hours was not lethal for rats, mice and guinea pigs. Clinical signs were observed in mice only, and included occasionally laboured respiration after approximately 2 hours of exposure and ptosis after 5 hours. The acute dermal LD50 was 681 mg/kg bw (rabbits). Signs of intoxication were depression, ptosis, labored respiration, ataxia and placidity. The acute oral LD50 values were: 3283 mg/kg bw for male rats, 3040 mg/kg bw for female rats, 2815 mg/kg bw for male mice, and between 2150 and 3160 mg/kg bw for female mice. Clinical signs were inactivity, weakness, prostration and shallow breathing at doses near to or exceeding the LD50 values. White opaque eyes were seen in male mice.
Dimethyl Phosphite is irritating to the skin and eyes of rabbits. After prolonged or repeated exposures moderate to severe irritation of skin and mucosa was observed in rats. No sensitisation studies are available.
Proposed metabolic pathways of DMP in rats and mice (Nomeir and Matthews, 1997).
