Table 2. SAR studies on the bottom ring (ring A) of dichloro derivatives 14 and 15.

Compound	Ring A	K d a (μM)	LE b	PDB
14		12	0.43	5OTR
15		6.5	0.41	5OTZ

^aMeasured by ITC.

^bLE = ligand efficiency*. *Ligand efficiency is defined as the ratio of the Gibbs free energy of binding of a ligand divided by the number of heavy (non-hydrogen) atoms in the molecule (LE = ΔG/[number of heavy atoms]).20