Target-state uncertainty in drug discovery: when precise binding data describe the wrong system

Binding measurements can be precise and reproducible while describing a molecular state insufficiently defined for the biological or pharmacological question. We define this problem as 'target-state uncertainty'. Experimental evidence shows that changes in conformation, assembly, post-translational modification and environment can alter ligand detection, compound ranking, affinity, function and transferability. State dependence can also be deliberately exploited when the relevant state is defined as the target. We propose target-state validation across four complementary dimensions: structure, stability, dispersity and function. This framework defines what molecular system was measured, what the resulting data can support and where their interpretation should stop.