Bortezomib therapy has proven successful for the treatment of relapsed and/or refractory multiple myeloma (MM); however, prolonged treatment is associated with toxicity and development of drug resistance. Here, we show that the novel proteasome inhibitor NPI-0052 induces apoptosis in MM cells resistant to conventional and Bortezomib therapies. NPI-0052 is distinct from Bortezomib in its chemical structure, effects on proteasome activities, mechanisms of action, and toxicity profile against normal cells. Moreover, NPI-0052 is orally bioactive. In animal tumor model studies, NPI-0052 is well tolerated and prolongs survival, with significantly reduced tumor recurrence. Combining NPI-0052 and Bortezomib induces synergistic anti-MM activity. Our study therefore provides the rationale for clinical protocols evaluating NPI-0052, alone and together with Bortezomib, to improve patient outcome in MM.

Salinosporamide A (1, NPI-0052) is a potent proteasome inhibitor in development for treating cancer. In this study, a series of analogues was assayed for cytotoxicity, proteasome inhibition, and inhibition of NF-kappaB activation. Marked reductions in potency in cell-based assays accompanied replacement of the chloroethyl group with unhalogenated substituents. Halogen exchange and cyclohexene ring epoxidation were well tolerated, while some stereochemical modifications significantly attenuated activity. These findings provide insights into structure-activity relationships within this novel series.

The proteasome has emerged as an important clinically relevant target for the treatment of hematologic malignancies. Since the Food and Drug Administration approved the first-in-class proteasome inhibitor bortezomib (Velcade®) for the treatment of relapsed/refractory multiple myeloma (MM) and mantle cell lymphoma, it has become clear that new inhibitors are needed that have a better therapeutic ratio, can overcome inherent and acquired bortezomib resistance and exhibit broader anti-cancer activities. Marizomib (NPI-0052; salinosporamide A) is a structurally and pharmacologically unique β-lactone-γ-lactam proteasome inhibitor that may fulfill these unmet needs. The potent and sustained inhibition of all three proteolytic activities of the proteasome by marizomib has inspired extensive preclinical evaluation in a variety of hematologic and solid tumor models, where it is efficacious as a single agent and in combination with biologics, che-motherapeutics and targeted therapeutic agents. Specifically, marizomib has been evaluated in models for multiple myeloma, mantle cell lymphoma, Waldenstrom’s macroglobulinemia, chronic and acute lymphocytic leukemia, as well as glioma, colorectal and pancreatic cancer models, and has exhibited synergistic activities in tumor models in combination with bortezomib, the immunomodulatory agent lenalidomide (Revlimid®), and various histone deacetylase inhibitors. These and other studies provided the framework for ongoing clinical trials in patients with MM, lymphomas, leukemias and solid tumors, including those who have failed bortezomib treatment, as well as in patients with diagnoses where other proteasome inhibitors have not demonstrated significant efficacy. This review captures the remarkable translational studies and contributions from many collaborators that have advanced marizomib from seabed to bench to bedside.

The diketopiperazine NPI-2358 is a synthetic analog of NPI-2350, a natural product isolated from Aspergillus sp., which depolymerizes microtubules in A549 human lung carcinoma cells. Although structurally different from the colchicine-binding site agents reported to date, NPI-2358 binds to the colchicine-binding site of tubulin. NPI-2358 has potent in-vitro anti-tumor activity against various human tumor cell lines and maintains activity against tumor cell lines with various multidrug-resistant (MDR) profiles. In addition, when evaluated in proliferating human umbilical vein endothelial cells (HUVECs), concentrations as low as 10 nmol/l NPI-2358 induced tubulin depolymerization within 30 min. Furthermore, NPI-2358 dose dependently increases HUVEC monolayer permeability--an in-vitro model of tumor vascular collapse. NPI-2358 was compared with three tubulin-depolymerizing agents with vascular-disrupting activity: colchicine, vincristine and combretastatin A-4 (CA4). Results showed that the activity of NPI-2358 in HUVECs was more potent than either colchicine or vincristine; the profile of CA4 approached that of NPI-2358. Altogether, our data show that NPI-2358 is a potent anti-tumor agent which is active in MDR tumor cell lines, and is able to rapidly induce tubulin depolymerization and monolayer permeability in HUVECs. These data warrant further evaluation of NPI-2358 as a vascular-disrupting agent in vivo. Currently, NPI-2358 is in preclinical development for the treatment of cancer.