Biological monitoring for exposure to methamidophos: A human oral dosing study

- An oral dose of methamidophos was administered to six volunteers at ADI level.
- The study has quantified methamidophos in timed urinary collections.
- Methamidophos exhibited a rapid elimination half-life of 1.1 h.
- Mean dose recovery excreted as unchanged methamidophos in urine is low - only 1.1%.
- Short half-life means estimates of exposure likely to be highly variable.

An oral dose of the organophosphate insecticide methamidophos was administered to six volunteers at the acceptable daily intake (ADI, 0.004 mg/kg).Urine was collected from the volunteers at timed intervals for 24 h post-exposure. Methamidophos itself was quantified in urine using liquid/liquid extraction and LC-MS-MS analysis (detection limit 7 nmol/L/1 μg/L).Methamidophos exhibited a rapid elimination half-life of 1.1 h, (range 0.4-1.5 h). Mean metabolite levels found in 24 h total urine collections (normalised for a 70 kg volunteer) were 9.2 nmol/L (range 1.0-19.1). One volunteer was anomalous; excluding this result the range was 6.7-19.1 nmol/L, with a mean of 10.9 nmol/L. Individual urine samples collected during the first 24 h ranged from below the detection limit (ND) to 237 nmol/L. The mean dose recovery excreted as methamidophos in urine was 1.1% (range 0.04-1.71%).Three environmental studies have been reported in the literature with levels ranging from ND to 66 nmol/L. The number of positive results in all three studies was low (<1.5% of total samples analyzed). When compared with our results (ND - 237 nmol/L), the studies suggest general population exposures are within the ADI. However, the very short half-life makes determining intermittent environmental exposures difficult.