The Ecstasy and Agony of Assay Interference Compounds
There is an editorial in ACS Central Science DOI that I would encourage everyone involved in hit identification to read.
A couple of quotes will give you an idea of the content
Alarmingly, up to 80–100% of initial hits from screening can be artefacts if appropriate control experiments are not employed.
it is important to realize that no PAINS-containing drug has ever been developed starting from a protein-reactive PAINS target-based screening hit
They also emphasise the critical need for experimental validation for any screening hit.
Such validation experiments include classic dose response curves, lack of incubation effects, imperviousness to mild reductants, and specificity versus counter-screening targets. If a molecule is flagged as a potential PAINS or aggregator using published patterns but is well-behaved by these criteria, it may be a true, well-behaved ligand. Ultimately, genuine SAR combined with careful mechanistic study provides the most convincing evidence for a specific interaction. Covalent and spectroscopic interference molecules act via specific physical mechanisms, for which controls are known. Colloidal aggregation, fortunately, is readily identified by rapid mechanistic tests and by counter-screening.
In addition you need to consider compound identify and purity, reproducing the activity with an authentic sample is essential.
Whilst time-consuming this validation work will save a fortune in the future.