Effects of metformin on wild fathead minnows (Pimephales promelas) using in-lake mesocosms in a boreal lake ecosystem

Due to its widespread use for the treatment of Type-2 diabetes, metformin is routinely detected in surface waters globally. Laboratory studies have shown that environmentally relevant concentrations of metformin can adversely affect the health of adult fish, with effects observed more frequently in males. However, the potential risk to wild fish populations has yet to be fully elucidated and remains a topic of debate. To explore whether environmentally relevant metformin exposure poses a risk to wild fish populations, the present study exposed wild fathead minnows (Pimephales promelas) to 5 or 50 μg/L metformin via 2 m diameter in-lake mesocosms deployed in a natural boreal lake in Northern Ontario at the International Institute for Sustainable Development - Experimental Lakes Area (IISD-ELA). Environmental monitoring was performed at regular intervals for 8-weeks, with fish length, weight (body, liver and gonad), condition factor, gonadosomatic index, liver-somatic index, body composition (water and biomolecules) and hematocrit levels evaluated at test termination. Metabolic endpoints were also evaluated using liver, brain and muscle tissue, and gonads were evaluated histologically. Results indicate that current environmental exposure scenarios may be sufficient to adversely impact the health of wild fish populations. Adult male fish exposed to metformin had significantly reduced whole body weight and condition factor and several male fish from the high-dose metformin had oocytes in their testes. Metformin-exposed fish had altered moisture and lipid (decrease) content in their tissues. Further, brain (increase) and liver (decrease) glycogen were altered in fish exposed to high-dose metformin. To our knowledge, this study constitutes the first effort to understand metformin's effects on a wild small-bodied fish population under environmentally relevant field exposure conditions.

Environmental Concentrations of the Type 2 Diabetes Medication Metformin and its Transformation Product Guanylurea in Surface Water and Sediment in Ontario and Quebec, Canada

Metformin, used to treat Type 2 Diabetes, is the active ingredient of one of the most prescribed drugs in the world, with over 120 million yearly prescriptions globally. In wastewater treatment plants (WWTPs), metformin can undergo microbial transformation to form the transformation product guanylurea, which could have toxicological relevance in the environment. Surface water samples from 2018-2020 and sediment samples from 2020 were collected from 6 mixed-use watersheds in Quebec and Ontario, Canada and analyzed to determine the metformin and guanylurea concentrations at each site. Metformin and guanylurea were present above their limits of quantification in 51.0 and 50.7% of all water samples, and in 64% and 21% of all sediment samples, respectively. In surface water, guanylurea was often present at higher concentrations than metformin, while the inverse was true in sediment, with metformin frequently detected at higher concentrations than guanylurea. Additionally, at all sites influenced solely by agriculture, concentrations of metformin and guanylurea were lower than 1 µg/L in surface water, suggesting that agriculture is not a significant source of these compounds in the investigated watersheds. These data suggest that WWTPs and potentially septic system leaks are the most likely sources of the compounds in the environment. Guanylurea was detected at many of these sites above environmental concentrations of concern, where critical processes in fish may be affected. Due to the scarcity of available ecotoxicological data and the prominence of guanylurea across all sample sites, there is a need to perform more toxicological investigations of this transformation product and revisit regulations. This report will help provide toxicologists with environmentally relevant concentration ranges in Canada.

Developmental and Full-Life Cycle Exposures to Guanylurea and Guanylurea-Metformin Mixtures Results in Adverse Effects on Japanese Medaka (Oryzias latipes)

Metformin is currently thought to be the highest drug by weight released into the aquatic environment, as a direct result of its widespread use in the treatment of a number of human health disorders. The removal of metformin from wastewaters is directly related to the formation of guanylurea (metformin's only known persistent degradation product), which is generally present at higher concentrations in surface waters than the parent compound. With metformin use rising steadily, it is important to characterize the effects of guanylurea on nontarget aquatic organisms. We recently demonstrated the effects of developmental exposure to environmentally relevant concentrations of metformin on the growth of early life stage (ELS) medaka as well as effects on the body weight of adult male fish following full-life cycle exposures. In the present study, we describe similar effects of guanylurea exposure on these endpoints and life stages. Guanylurea led to effects on growth in a 28-d ELS assessment that were similar to those of metformin; however, these effects occurred at concentrations in the ng/L range compared with the μg/L range for metformin. A possible sex-dependent association with body weight changes was also observed in adults following a 165-d full-life cycle exposure to guanylurea alone or in a mixture with metformin. To our knowledge, the present is the first study to report the toxicity of guanylurea to nontarget aquatic organisms. Environ Toxicol Chem 2019;00:1-6. © 2019 SETAC.

Effects of environmentally relevant metformin exposure on Japanese medaka (Oryzias latipes)

Metformin is one of the most prevalent pharmaceuticals in both surface and waste waters, yet little is known about the bioavailability and/or effects of developmental exposure on early life stage (ELS) fish. Here, we demonstrate that embryo-larval stages of medaka are capable of taking up metformin from the aquatic environment, provided exposure occurs prior to chorion hardening (∼6-hpf). Once transferred to clean water, ELS medaka are able to completely depurate metformin in <24-hours. Furthermore, ELS medaka exposed to a range of relevant concentrations of waterborne metformin (from 6 hpf through 28-days post hatch) had significantly reduced growth metrics, altered metabolomes, and changes in the expression of genes associated with cell growth. The range of concentrations investigated were 1.0, 3.2, 10, 32, and 100 μg·L^-1. To examine effects of chronic, low level metformin exposure across the full medaka life-cycle, we exposed newly fertilized embryos to 3.2 μg L^-1 waterborne metformin for 165-days. The weight and length of adult fish were examined, as were effects on the production of some steroid hormones, specifically a significant increase (control females: 0.161 ± 0.023 pg/mg; metformin treated females: 3.42 ± 0.543) in the production of 11-ketotestosterone was observed in adult female medaka. Collectively, these results suggest that current environmental exposure scenarios may be sufficient to cause effects on developing fish.