Residue-based interpretation of toxicity and bioconcentration QSARs from aquatic bioassays: Neutral narcotic organics

Abstract

The critical body residue (CBR), estimated from aquatic toxicity QSARs and bioconcentration-log K_ow relationships, appears to be relatively constant, at about 4 mmol L⁻¹ of fish, for the acute toxicity of a variety of hydrophobic narcotic organic chemicals examined by the U.S. Environmental Protection Agency (Duluth, MN) in tests with the fathead minnow. However, for hydrophilic chemicals (log K_ow < 1.5) the bulk of the toxicant is in the water phase rather than the organic/lipid phase of the organism, so the whole-body residues in these cases should be similar to the LC50 water concentration. Over the log K_ow range of – 1.5 to 6, acutely toxic whole-body residues for narcotics can be approximated by the QSAR-derived equation: CBR (mM) = 2.5 mM + 50/K_ow. Estimates obtained by this method are in reasonable agreement with the limited literature data available for acutely toxic whole-body residues of hydrophobic narcotic organic chemicals. Elimination half-lives estimated from nonlinear curve fitting to time-toxicity information were relatively constant for the Duluth bioassay data at approximately 3 h. Despite the relatively high variability of this type of kinetics data, the literature information for small aquatic organisms, from both toxicity-and bioconcentration-based tests, was in a similar range. It appears that QSARs created with raw aquatic bioassay data occur primarily as a result of the influence of chemical-physical properties on the partitioning process. Log K_ow appears to have little to do with the inherent potency of the neutral, narcotic organic chemicals examined.