Waterborne pharmaceutical uptake and toxicity is modified by pH and dissolved organic carbon in zebrafish

Human and veterinary pharmaceuticals have been observed in natural aquatic environments around the world, and many have been shown to impact fish health. Presently, we examined the influence of pH, dissolved organic carbon (DOC) and Na⁺ or Ca²⁺ on the bioavailability and toxicity of waterborne pharmaceuticals in larval zebrafish. Drugs included sertraline (selective serotonin reuptake inhibitor; SSRI), fluoxetine (SSRI), diclofenac (nonsteroidal anti-inflammatory drug) and ethinyl estradiol (estrogen; EE₂). The 96 h-LC₅₀s for sertraline, fluoxetine and diclofenac were influenced by pH over an environmentally relevant range (pH 5.8-8.2). Toxicity was related to the predicted concentration of non-ionized compounds, which more readily cross cell membranes than ionized compounds. For example, sertraline was 4.1-fold more toxic (as measured by 96 h-LC₅₀s) at pH 8.2 compared to pH 5.8, while the predicted amount of non-ionized sertraline was also greater at pH 8.2 (based on previously reported pKa values). Experiments with radiolabelled drugs demonstrated that sertraline uptake was also 5.4-fold higher at pH 8.2 compared to pH 5.8. Terrigenous and autochthonous DOC samples (as low as 1 mg/L) protected against sertraline uptake and toxicity, although they were more effective at lower (environmentally relevant) drug concentrations. In contrast, the uptake of EE₂, which was principally non-ionized in all water chemistries tested, was not altered by pH or DOC. There was no change in sertraline toxicity with the addition of 12 mM Na⁺ or 3 mM Ca²⁺. In conclusion, the influence of pH and DOC on drug uptake and toxicity in fish appears to be predictable based on the physicochemical properties of the drug (e.g. pKa, polar surface area). The influence of water chemistry on drug bioavailability in fish is likely relevant to all aquatic life.