Compound 18, which contained a benzyl substituent and lipophilic side chain, displayed strong inhibitory activity against the ChT-L site of the proteasome (Ki′: 0.12 nM) and cytotoxicity towards human RPMI8226 multiple myeloma cells (IC50: 2.2 nM).66 Further optimization of 18 has focused on improving cytotoxicity.73 This evidence concerns the gene SLC5A7 and plasma cell myeloma.