Ask the experts: past, present and future of the rule of five.

Coined in 1997, by Christopher Lipinki et al., the rule of five (Ro5) comprises a set of parameters that determine drug-likeness for oral delivery. The parameters are as follows: no more than five hydrogen bond donors (nitrogen or oxygen atoms with one or more hydrogen atoms); no more than ten hydrogen bond acceptors (nitrogen or oxygen atoms); a molecular mass less than 500 Da; and an octanol-water partition coefficient log P no greater than 5. Future Medicinal Chemistry invited a selection of leading researchers to express their views on Lipinski's Ro5, which has influenced drug design for over a decade. Their enlightening responses provide an insight into the current and future role of Ro5, and other rules of thumb, in the evolving world of medicinal chemistry.