Among them, dilazep, an inhibitor of nucleoside transporters, has been clinically used for the treatment of cardiac dysfunction via postoral administration. Some homopiperazine derivatives, such as K-7174 and K-11706, were shown in pre-clinical studies to inhibit cell adhesion and to rescue BKM120 944396-07-0 anemia of chronic disorders via the activation of erythropoietin production in vitro and in vivo. In PD 0332991 827022-32-2 addition, K-7174 was reported to exert anti-inflammatory action via induction of the UPR. These observations prompted us to consider that HPDs could be orally active PIs; however, this possibility has not been tested so far. In this study, we demonstrated that HPDs, including K-7174, have the ability to inhibit proteasome activity via different mechanisms of action from bortezomib and other conventional PIs. In the present study, we show that HPDs constitute a novel class of PIs with a unique mode of proteasome binding. Although many kinds of small molecular PIs with various chemical structures have been developed, this is the first demonstration of the proteasome-inhibitory activity of HPDs. In addition, most of the previous PIs mainly acted on one or two catalytic subunits and their mechanisms of action are not fully understood. In contrast, we have demonstrated that HPDs act on all three catalytic subunits of the proteasome by direct binding to the active pockets of the ?1, ?2 and ?5 subunits with a similar binding 
mode and kinetics. These results indicate the unique features of homopiperazine-derived PIs in chemical structures and effects on the proteasome. Moreover, we have identified the critical chemical structure of homopiperazine-derived PIs; therefore, these observations may contribute to the development of novel PIs with higher activity and specificity. The high concentrations to trigger cytotoxicity might be the obstacle for clinical application of K-7174. Crystal structure analyses revealed that K-7174 interacts with ? subunits largely via hydrophobic interaction, whereas bortezomib binds to the ?5 subunit via a hydrogen-bond network, explaining why higher concentrations are required for HPDs compared with bortezomib. Therefore, the development of novel HPDs with higher activity and specificity is essential for clinical translation. Our finding on the chemical structure of homopiperazine-derived PIs may be of great help in this regard. Despite the great success of bortezomib in the treatment of refractory malignancies such as MM and mantle cell lymphoma, we still intend to develop orally bioavailable PIs with distinct mechanisms of action from bortezomib. Several novel PIs, such as carfilzomib, NPI-0052, CEP-18770, MLN9708, and ONX-0912, are now undergoing clinical trials and show considerable benefits for refractory/relapsed cases as well as untreated MM patients. Among them, carfilzomib and its derivative ONX-0912 are peptide derivatives and have greater selectivity for the ?5 subunit than bortezomib. Although NPI-0052 is a non-peptide PI targeting all three proteasome subunits, its effect was strong for chymotrypsin-like, moderate for trypsinlike, and weak for caspase-like activities. In addition, NPI-0052 is intravenously administered in clinical studies, although it is expected to have oral bioactivity. MLN9708 is orally available and its efficacy has been demonstrated in phase I clinical trials with oral administration.