lipophilic ligand efficiency
Lipophilic ligand efficiency, or LLE, asks an honest question: is your molecule binding because it genuinely fits the target, or just because it is greasy? Smearing oil on a key might make it slide into more locks, but that is not real fit. LLE separates skillful binding from cheap, fat-driven stickiness.
It is calculated very simply as potency expressed on a logarithmic scale (pIC50 or pKi) minus logP. A high LLE means you are getting a lot of binding for relatively little lipophilicity, which is a sign of high-quality, specific interactions. A low LLE warns that much of the apparent potency is coming from grease, the kind of binding that tends to drag along poor solubility, promiscuity, and toxicity.
Medicinal chemists track LLE across a series to make sure they are improving the molecule the right way. A compound that gets more potent only because it got more lipophilic shows no LLE gain and is often a trap; the prized analogs are those that raise potency while holding or lowering lipophilicity, lifting LLE in the process.
Two analogs share an IC50 of 100 nM, but one has logP 2 and the other logP 5; the lower-logP compound has the higher LLE and is usually the better starting point.
Same potency, very different quality of binding.
A commonly cited target is LLE above about 5–7 for an attractive lead, though the right value depends on the target class.