Development of a framework to derive effect-based trigger values to interpret CALUX data for drinking water quality

It is widely acknowledged that with targeted chemical monitoring, unknown chemicals, such as new emerging chemicals or transformation products, may go unnoticed (Altenburger et al., 2015; Brand et al., 2013; van der Oost et al., 2017b). Bioassays can be used as bioanalytical tools to detect such unknown (mixtures of) chemicals, as they give an integrated response to all known and unknown chemicals that induce a response (Escher and Leusch, 2011). An increased bioassay response while acceptable concentrations of target chemicals are measured with chemical analysis may indicate that other bioactive chemicals causing this effect are present. Despite this clear added value of using bioassays in water quality screening, the interpretation of bioassay results remains a challenge. Bioassays can be very sensitive, i.e. already give a positive response at low concentrations. Therefore, a positive bioassay response does not necessarily mean an increased risk. For several bioassays used in water quality monitoring, response levels that provide a trigger to further assess potential adverse effects for humans or the environment are proposed in the scientific literature. For reporter gene assays, such thresholds, also known as effect-based trigger (EBT) values (Escher et al., 2015), are expressed in equivalents of a reference chemical in the bioassay. At responses below the EBT value, adverse effects for humans or the environment are not expected, while possible adverse effects cannot be excluded at higher responses, so a further in-depth assessment is needed.
In the practice of water quality management, EBT values need to be sufficiently conservative to be useful as indicators of hazardous bioactive micropollutants in water but also not overly conservative, preventing that assets are spent on unnecessary follow-up actions (Brand et al., 2013). EBT values of in vitro bioassays are based on the biological equivalent (BEQ) concept (Escher et al., 2015; van den Berg et al., 2006). This concept assumes that if chemicals in a mixture that elicits a response in a specific bioassay share the same mode of action, their combined effects can be described by the concentration addition concept. A BEQ concentration of an unknown mixture of chemicals can be measured in a bioassay, expressed as reference compound equivalents by comparing the response in the bioassay to the concentration-response curve of the reference chemical in that same bioassay.

Van vier beschikbare Chemical Activated LUciferase gene eXpression (CALUX) in vitro bioassays is aangetoond dat deze geschikt zijn om bruikbare en betrouwbare effect-signaalwaarden (effect-based trigger value ofwel EBT) af te leiden. In dit onderzoek zijn EBTs afgeleid die zijn afgestemd op humane gezondheidsrisico’s. CALUX-bioassays worden momenteel in Nederland toegepast als bioanalytische tool voor waterkwaliteitsbeoordeling. Overschrijding van de EBT geeft aan dat een mogelijke gezondheidsrisico bij drinkwaterconsumptie niet kan worden uitgesloten. Vervolgonderzoek naar de samenstelling van de verontreiniging is dan nodig om aan te tonen of daadwerkelijk sprake is van zo’n risico. Bioassays worden in toenemende mate ingezet als screeningmethode voor waterkwaliteit, omdat ze een integraal effect laten zien van complexe mengsels werkzame stoffen in water.