Lipinski's rule of five

Lipinski's rule of five also known as the Pfizer's rule of five or simply the Rule of five (RO5) is a rule of thumb to evaluate druglikeness or determine if a chemical compound with a certain pharmacological or biological activity has properties that would make it a likely orally active drug in humans. The rule was formulated by Christopher A. Lipinski in 1997, based on the observation that most medication drugs are relatively small and lipophilic molecules.

The rule describes molecular properties important for a drug's pharmacokinetics in the human body, including their absorption, distribution, metabolism, and excretion ("ADME"). However, the rule does not predict if a compound is pharmacologically active.

The rule is important to keep in mind during drug discovery when a pharmacologically active lead structure is optimized step-wise to increase the activity and selectivity of the compound as well as to insure drug-like physicochemical properties are maintained as described by Lipinski's rule. Candidate drugs that conform to the RO5 tend to have lower attrition rates during clinical trials and hence have an increased chance of reaching the market.

Components of the rule
Lipinski's rule states that, in general, an orally active drug has no more than one violation of the following criteria:


 * No more than 5 hydrogen bond donors (nitrogen or oxygen atoms with one or more hydrogen atoms)
 * Not more than 10 hydrogen bond acceptors (nitrogen or oxygen atoms)
 * A molecular mass less than 500 daltons
 * An octanol-water partition coefficient log P not greater than 5

Note that all numbers are multiples of five, which is the origin of the rule's name. As with many other rules of thumb, (such as Baldwin's rules for ring closure), there are many exceptions to Lipinski's Rule.

Variants
In an attempt to improve the predictions of druglikeness, the rules have spawned many extensions, for example the following:


 * Partition coefficient log P in −0.4 to +5.6 range
 * Molar refractivity from 40 to 130
 * Molecular weight from 180 to 500
 * Number of atoms from 20 to 70 (includes H-bond donors [e.g.;OH's and NH's] and H-bond acceptors [e.g.; N's and O's])
 * Polar surface area no greater than 140 Ǻ2

Also the 500 molecular weight cutoff has been questioned. Polar surface area and the number of rotatable bonds has been found to better discriminate between compounds that are orally active and those that are not for a large data set of compounds in the rat. In particular, compounds which meet only the two criteria of: are predicted to have good oral bioavailability.
 * 10 or fewer rotatable bonds and
 * polar surface area equal to or less than 140 Å2

Lead-like
During drug discovery, lipophilicity and molecular weight are often increased in order to improve the affinity and selectivity of the drug candidate. Hence it is often difficult to maintain drug-likeness (i.e., RO5 compliance) during hit and lead optimization. Hence it has been proposed that members of screening libraries from which hits are discovered should be biased toward lower molecular weight and lipophility so that medicinal chemists will have an easier time in delivering optimized drug development candidates that are also drug-like. Hence the rule of five has been extended to the rule of three (RO3) for defining lead-like compounds.

A rule of three compliant compound is defined as one that has:
 * octanol-water partition coefficient log P not greater than 3
 * molecular mass less than 300 daltons
 * not more than 3 hydrogen bond donors
 * not more than 3 hydrogen bond acceptors
 * not more than 3 rotatable bonds