A phase 1 clinical trial evaluating marizomib, pomalidomide and low-dose dexamethasone in relapsed and refractory multiple myeloma (NPI-0052-107): final study results.

Marizomib (MRZ) is an irreversible, pan-subunit proteasome inhibitor (PI) in clinical development for relapsed/refractory multiple myeloma (RRMM) and glioma. This study analysed MRZ, pomalidomide (POM) and low-dose dexamethasone (Lo-DEX) [PMD] in RRMM to evaluate safety and determine the maximum tolerated dose (MTD) and/or recommended Phase 2 dose (RP2D). Intravenous MRZ (0·3-0·5 mg/m2 ) was administered over 2 h on days 1, 4, 8, 11; POM (3-4 mg) on days 1-21; and Lo-DEX (5 or 10 mg) on days 1, 2, 4, 5, 8, 9, 11, 12, 15, 16, 22 and 23 of every 28-day cycle. Thirty-eight patients were enrolled that had received a median of 4 (range 1-10) prior lines of therapy; all patients received prior lenalidomide and bortezomib. No dose-limiting toxicities (DLTs) were observed and 0·5 mg/m2 MRZ was determined to be the RP2D. The most common treatment-related ≥Grade 3 adverse events were: neutropenia (11/38 patients: 29%), pneumonia (4/38 patients 11%), anaemia (4/38 patients; 11%) and thrombocytopenia (4/38 patients; 11%). The overall response rate and clinical benefit rate was 53% (19/36) and 64% (23/36), respectively. In conclusion, PMD was well tolerated and demonstrated promising activity in heavily pre-treated, high-risk RRMM patients.

Marizomib for central nervous system-multiple myeloma.

Marizomib, a natural marine product, is an irreversible proteasome inhibitor currently under investigation in relapsed-refractory multiple myeloma (RRMM) and malignant glioma. Central nervous system-multiple myeloma (CNS-MM) is a rare manifestation of extra-medullary disease with few therapeutic options, highlighting the unmet clinical need in these patients. Marizomib demonstrated encouraging activity in RRMM and has emerging clinical activity in glioma, making it a potential CNS-MM therapeutic intervention. Herein, we present two patients with RRMM and CNS involvement who benefited from marizomib-based therapy. These cases provide the first proof of principle for further exploring marizomib in CNS-MM patients.

Marizomib activity as a single agent in malignant gliomas: ability to cross the blood-brain barrier.

BACKGROUND: The proteasome plays a vital role in the physiology of glioblastoma (GBM), and proteasome inhibition can be used as a strategy for treating GBM. Marizomib is a second-generation, irreversible proteasome inhibitor with a more lipophilic structure that suggests the potential for penetrating the blood-brain barrier. While bortezomib and carfilzomib, the 2 proteasome inhibitors approved for treatment of multiple myeloma, have little activity against malignant gliomas in vivo, marizomib could be a novel therapeutic strategy for primary brain tumors. METHODS: The in-vitro antitumor activity of marizomib was studied in glioma cell lines U-251 and D-54. The ability of marizomib to cross the blood-brain barrier and regulate proteasome activities was evaluated in cynomolgus monkeys and rats. The antitumor effect of marizomib in vivo was tested in an orthotopic xenograft model of human GBM. RESULTS: Marizomib inhibited the proteasome activity, proliferation, and invasion of glioma cells. Meanwhile, free radical production and apoptosis induced by marizomib could be blocked by antioxidant N-acetyl cysteine. In animal studies, marizomib distributed into the brain at 30% of blood levels in rats and significantly inhibited (>30%) baseline chymotrypsin-like proteasome activity in brain tissue of monkeys. Encouragingly, the immunocompromised mice, intracranially implanted with glioma xenografts, survived significantly longer than the control animals (P < .05) when treated with marizomib. CONCLUSIONS: These preclinical studies demonstrated that marizomib can cross the blood-brain barrier and inhibit proteasome activity in rodent and nonhuman primate brain and elicit a significant antitumor effect in a rodent intracranial model of malignant glioma.

Chk2 protects against radiation-induced genomic instability.

The murine Chk2 kinase is activated after exposure to ionizing radiation and is necessary for p53-dependent apoptosis, but the role Chk2 plays in determining genomic stability is poorly understood. By analyzing the sensitivity of Chk2-deficient murine and human cells to a range of DNA-damaging agents, we show that Chk2 deficiency results in resistance to agents that generate double-strand breaks but not to other forms of damage. Surprisingly, the absence of Chk2 results in increased sensitivity to UV-radiation-induced DNA damage. Defective apoptosis after radiation-induced DNA damage may result in genomic instability; therefore, the consequences of Chk2 deficiency on genomic instability were assayed using an in vitro screen. Gene amplification was not detected in untreated Chk2(-/-) cells, but the rate of gene amplification after irradiation was elevated and was similar to that found in p53 compromised cells. A synergistic increase in genomic instability was seen after disruption of both Chk2 and p53 function, indicating that the two proteins have non-redundant roles in regulating genome stability after irradiation. The data demonstrate that Chk2 functions to maintain genome integrity after radiation-induced damage and has important implications for the use of Chk2 inhibitors as adjuvant cancer therapy.

c-Jun-deficient cells undergo premature senescence as a result of spontaneous DNA damage accumulation.

Mouse embryo fibroblasts deficient for the c-Jun proto-oncogene (c-Jun-/- MEF) undergo p53-dependent premature senescence in conventional culture. This phenotype becomes evident only after several cell divisions, suggesting that senescence may result from exposure to unknown environmental factors. Here, we show that c-Jun-/- MEF can proliferate successfully in low oxygen (3% O2), indicating that premature senescence under conventional culture conditions is a consequence of hyperoxic stress. c-Jun-/- MEF exhibit higher basal levels of DNA damage compared to normal fibroblasts in high but not low oxygen, implying that senescence results from chronic accumulation of spontaneous DNA damage. This accumulation may be attributable, at least in part, to inefficient repair, since DNA damage induced by gamma ionizing radiation and H2O2 persists for longer in c-Jun-/- MEF than in wild-type MEF. Unexpectedly, p53 expression, phosphorylation, and transcriptional activity are largely unaffected by oxygen exposure, indicating that the accumulation of spontaneous DNA damage does not result in chronic activation of p53 as judged by conventional criteria. Finally, we find that c-Jun associates with nuclear foci containing gammaH2AX and ATM following irradiation, suggesting a potential role for c-Jun in DNA repair processes per se.

Cell transformation by v-Jun deactivates ERK MAP kinase signalling.

Previous studies have shown that v-Jun accelerates G1 progression and enables cells to sustain S phase entry in the absence of serum growth factors. Since growth factor-dependent ERK MAP kinase signalling plays an important role in regulating the G1/S transition, we investigated whether aberrant ERK regulation might contribute to cell cycle deregulation by v-Jun. Contrary to expectation, we find that cells transformed by v-Jun exhibit a profound reduction in the basal level of active, dual-phosphorylated ERK. In addition, ERK becomes refractory to stimulation by a subset of agonists including serum, LPA, and EGF, but remains partially responsive to the phorbol ester, TPA. Biochemical analysis indicates that these defects are attributable to a combination of inefficient signal propagation between Ras and Raf within the ERK pathway and increased tonic deactivation by MAP kinase phosphatases. Taken together, these results demonstrate that cell transformation by v-Jun induces alterations in cell physiology which antagonize ERK signalling at multiple levels. The potential significance of this phenotype for oncogenesis by v-Jun is discussed.

Molecular mechanism and biological functions of c-Jun N-terminal kinase signalling via the c-Jun transcription factor.

The regulation of c-Jun transcriptional activity by Jun N-terminal kinase (JNK) has become a paradigm for understanding how mitogen-activated protein (MAP) kinase signalling pathways elicit specific changes in gene transcription through selective phosphorylation of nuclear transcription factors. Selective phosphorylation of c-Jun by JNK is determined by a specific docking motif in c-Jun, the delta region, which enables JNK to associate physically with c-Jun. Analogous MAP kinase docking motifs have subsequently been found in several other transcription factors, indicating that this is a general mechanism for ensuring specificity of signal transduction. Genetic and biochemical studies in mice, flies and cultured cells have provided evidence that signals relayed by JNK through c-Jun regulate a range of cellular processes including cell proliferation, tumourigenesis, apoptosis and embryonic development. Despite these advances, in most cases, the genes or programs of gene expression downstream of JNK and c-Jun, which control these processes, have not been defined. Here, we review the current understanding of the molecular basis and biological consequences of JNK signalling via c-Jun and highlight some of the mechanistic issues, which remain to be resolved.