Comparative pharmacology and toxicology of pharmaceuticals in the environment: diphenhydramine protection of diazinon toxicity in Danio rerio but not Daphnia magna

Abstract

Pharmaceuticals and other contaminants of emerging concern present unique challenges to environmental risk assessment and management. Fortunately, mammalian pharmacology and toxicology safety data are more readily available for pharmaceuticals than other environmental contaminants. Identifying approaches to read-across such pharmaceutical safety information to non-target species represents a major research need to assess environmental hazards. Here, we tested a biological read-across hypothesis from emergency medicine with common aquatic invertebrate and vertebrate models. In mammals, the antihistamine diphenhydramine (DPH) confers protection from poisoning by acetylcholinesterase inhibition because DPH blocks the acetylcholine receptor. We employed standardized toxicity methods to examine individual and mixture toxicity of DPH and the acetylcholinesterase inhibitor diazinon (DZN) in Daphnia magna (an invertebrate) and Danio rerio (zebrafish, a vertebrate). Though the standardized Fish Embryo Toxicity method evaluates early life stage toxicity of zebrafish (0-3 days post fertilization, dpf), we further evaluated DPH, DZN, and their equipotent mixture during three development stages (0-3, 3-6, 7-10 dpf) in zebrafish embryos. Independent action and concentration addition mixture models and fish plasma modeling were used to assist interpretation of mixture toxicity experiments. Though our primary hypothesis was not confirmed in acute studies with Daphnia magna, DPH conferred a protective effect for acute DZN toxicity to zebrafish when DPH plasma levels were expected to be greater than mammalian therapeutic, but lower than acutely lethal, internal doses. We further observed that timing of developmental exposure influenced the magnitude of DZN and DPH toxicity to zebrafish, which suggests that future zebrafish toxicity studies with pharmaceuticals and pesticides should examine exposure during developmental stages.

Fig. 1

Daphnia magna subchronic (10 days) a survival and b fecundity responses to either diphenhydramine or diazinon. Statistical differences from controls are denoted with asterisks (*p < 0.05)

Fig. 2

Evaluation of Daphnia magna acute toxicity (48 h EC₅₀) for equipotent mixtures of diphenhydramine (DPH) and diazinon (DZN). S values denote equipotent mixtures based on individual chemical respective EC₅₀ values (1S = 0.5(EC_50DZN) + 0.5(EC_50DPH))

Fig. 3

a Interactive effects of equipotent mixtures of diphenhydramine (DPH) and diazinon (DZN) on mean (±SD) Daphnia magna fecundity (10 days). b Comparison of concentration addition (CA) and independent action (IA) models to mean Daphnia magna fecundity (10 days, percent of control) of equipotent mixtures of DPH and DZN. S values denote equipotent mixtures based on individual chemical respective LOEC values (1S = 0.5(LOEC_DPH) + 0.5(LOEC_DZN)). Statistical differences from controls are denoted with asterisks (*p < 0.05)

Fig. 4

Acute (72 h) Danio rerio mortality of a diazinon (DZN) and b diphenhydramine (DPH) during three stages of development (0–3, 3–6, and 7–10 days post fertilization, dpf)

Fig. 5

Comparison of concentration addition (CA) and independent action (IA) models compared to observed mortality (72 h LC₅₀) from equipotent mixtures of diphenhydramine (DPH) and diazinon (DZN) in Danio rerio for a 0–3 dpf; b 3–6 dpf; and c 7–10 dpf studies. S values denote equipotent mixtures based on individual chemical respective LC₅₀ values (1S = 0.5(LC_50DZN) + 0.5(LC_50DPH)). dpf = days post fertilization