SLC5A7 and plasma cell myeloma: Substitution of the formyl group in TP-110 with boronates resulted in enhanced inhibitory potency towards the ChT-L site.38 Optimization of this scaffold led to boronate 4, which bore an N-terminal 3,6-dichloro-2-pyridyl substituent and exhibited comparable inhibitory potency towards the chymotrypsin-like activity of the proteasome (IC50: 0.053 μM) to that of bortezomib.39 Analog 4 also displayed good cytotoxic activity towards the RPMI8226 multiple myeloma cells, and was chosen for further in vitro and solid tumor studies.40