Environmental Risks of Pharmaceutical Mixtures in Aquatic Ecosystems: Reflections on a Decade of Research.

Pharmaceuticals and personal care products (PPCPs) occur as variable mixtures in surface waters receiving discharges of human and animal wastes. A key question identified a decade ago is how to assess the effects of long-term exposures of these PPCP mixtures on non-target organisms? Herein we review the recent progress made on assessing the aquatic ecotoxicity of PPCP mixtures - with a focus on active pharmaceutical ingredients (APIs) - and the challenges and research needs that remain. New knowledge has arisen from the use of whole mixture testing combined with components-based approaches, and these studies show that mixtures often result in responses that meet the concentration addition model. However, such studies have mainly been done on individual species over shorter time periods, and longer-term, multispecies assessments remain limited. The recent use of targeted and non-targeted gene analysis has improved our understanding of the diverse pathways that are impacted, and there are promising new "read-across" methods that use mammalian data to predict toxicity in wildlife. Risk assessments remain challenging given the paucity of ecotoxicological and exposure data on PPCP mixtures. As such, the assessment of PPCP mixtures in aquatic environments should remain a priority given the potential for additive - as well as non-target - effects in non-target organisms. In addition, we need to improve our understanding of which species, lifestages and relevant endpoints are most sensitive to which type of PPCP mixture, and to expand our knowledge of environmental PPCP levels in regions of the globe that have been poorly studied to date. We recommend an increased use of New Approach Methodologies, in particular 'Omics', to advance our understanding of the molecular mechanics of mixture effects. Finally, we call for systematic research on the role of PPCP mixtures in the development of antimicrobial resistance.