Druglikeness

Druglikeness is a complex balance of various molecular properties and structure features which determine whether particular molecule is similar to the known drugs. It generally means 'molecules which contain functional groups and/or have physical properties consistent with most of known drugs'. These properties, mainly hydrophobicity, electronic distribution, hydrogen bonding characteristics, molecule size and flexibility and presence of various pharmacophoric features influence the behavior of molecule in a living organism, including bioavailability, transport properties, affinity to proteins, reactivity, toxicity, metabolic stability and many others. The presence of structural fragements typically found in drugs. Molecules with score between 2 and 7 are classified as drugs; otherwise they are classified as non-drugs. 

A more recent example of the functional approach to identify drug like molecules is the work of Muegge et al [1]. They assigned each molecule a score based on the presence of structural fragments typically found in drugs. Compounds containing some specific single pharmacophoric group can also classify as drug. One of the groups is amine. Drugs having nitro groups can be be rejected because Nitro groups, tend to begin aromatic rings, may increase a molecule's tendency to generate false positives under assay conditions [2].

1. C.Y. Wu, L.Z. Benet, 2005, Predicting drug disposition via applications of BCS : transport/ absorption/ elimination/ interplay and development of a biopharmaceutics drug disposition classification system , Pharma Res, 22, 11-23

2.  N. Watari, Y. Sugiyama, N. Kaneniwa, M. Hiura, 1988, Prediction of hepatic first-pass metabolism and plasma levels following intravenous and oral administration of barbiturates in the rabbit based on quantitative structure–pharmacokinetic relationships, J. Pharmacokinet. Biopharm. 16, 279–301