Oprea distinguished the lead-like chemical space from the drug-like space. The following lead-likeness constraints are applied to ilib diverse's Oprea lead-likeness filter:
Oprea, Tudor I.; Davis, Andrew M.; Teague, Simon J.; Leeson, Paul D.
Is There a Difference between Leads and Drugs? A Historical Perspective.
Journal of Chemical Information and Computer Sciences (2001), 41(5), 1308-1315.