Lipinski’s ‘rule of 5’ is used to describe common characteristics observed in many small molecule drugs. It is important to note that there are 4 rules, and that there are many exceptions to these rules (including all biological drugs).
The four rules are: a molecular weight >500Da, >5 hydrogen bond donors, >10 hydrogen bond acceptors, and a log(P)>5 (partition coefficient). Where these criteria are satisfied, it is likely that the small molecule drug being studied is not going to be effective at meeting the ADME descriptions.
Aspirin (salicylic acid) is a small molecule drug that satisfies the criteria set out by Lipinski’s rules. It is small (<500Da), does not have many hydrogen bond donors or acceptors, and has a low partition coefficient. Paracetamol (acetaminophen) is similar, being described by Lipinski’s rules as a good small molecule drug.
These rules, or variations of them, may be useful in identifying potential lead candidates for further research.