Focused library synthesis and medicinal chemistry on an oxadiazole-isopropylamide core proteasome inhibitor provided the lead compound that strongly inhibits CT-L activity. Structure activity relationship studies indicate the amide moiety and two phenyl rings are sensitive toward synthetic modifications. Only para-substitution in the A-ring was important to maintain potent CT-L inhibitory activity. Hydrophobic residues in the A-rings para-position and meta-pyridyl group at the B-ring significantly improved inhibition. The meta-pyridyl moiety improved cell permeability. The length of the aliphatic chain at the para position of the A-ring is critical with propyl yielding the most potent inhibitor, whereas shorter (i.e. ethyl, methyl or hydrogen) or longer (i.e. butyl, propyl and hexyl) chains demonstrating progressively less potency. Introduction of a stereogenic center next to the ether moiety (i.e. substitution of one of the hydrogens by methyl) demonstrated chiral discrimination in proteasome CT-L activity inhibition (the S-enantiomer was 35-40 fold more potent than the R-enantiomer).
