NAMs are being adopted by chemical regulators worldwide to reduce the need for live animal testing. OECD TG 249 is an in vitro test adopted in 2021 that uses a gill-cell line from rainbow trout. It is a potential alternative to standard in vivo acute fish toxicity tests accepted by chemical regulators around the world.
TG 249 is faster and cheaper than current whole animal tests, uses minimal chemical volumes and provides good reproducibility. However, two validation studies have shown notable discrepancies in the results obtained for some groups of chemicals compared to in vivo data. Regulators can determine the validity of the data generated by TG 249 using other lines of evidence, as recommended by the guideline itself.
Additional lines of evidence available to chemical assessors are scarce. Apart from QSARs, which are not applicable to all chemical groups, the only other readily available evidence of toxicity for a new chemical are Daphnia endpoints, which are a requirement of the registration process. This project aimed at determining whether a correlation exists between Daphnia and fish acute toxicity, that could be used as a line of evidence for supporting in vitro data from TG 249.
A dataset comprising 521 chemicals with different modes of action (MoA) was analysed for 13 Daphnia species and 80 fish species. A good correlation among the 290 datapoints common to both taxa was obtained by linear regression on the log scale (r2 = 0.72). On average, Daphnia are 25 times more sensitive than fish, and departures from this difference were used to identify outlier chemicals. A stepwise protocol that uses QSAR, Daphnia ecotoxicity and a chemical’s MoA has been proposed as a screening tool to help chemicals assessors determine the suitability of TG 249 data as a substitute for in vivo acute fish toxicity data.