Chemical probes for biological systems.

According to the latest definition in use by the NIH Molecular Libraries Screening Centers Network, a compound to be nominated as a chemical probe should have, on the one hand, an affinity below 100 nM for the primary target and, on the other hand, at least tenfold selectivity against related targets. Taking drugs as the ultimate product of an affinity and selectivity optimization process, it is found that only 14.4% of them would actually qualify as chemical probes under those criteria. Therefore, if chemical probes are expected to give rise to new medicines, strict adherence to the current probe definition might result in many compounds of potential therapeutic interest being overlooked.