Julian Adams

Phase I Trial of the Proteasome Inhibitor PS-341 in Patients With Refractory Hematologic Malignancies

PURPOSEnTo determine the maximum-tolerated dose (MTD), dose-limiting toxicity (DLT), and pharmacodynamics (PD) of the proteasome inhibitor bortezomib (previously known as PS-341) in patients with refractory hematologic malignancies.nnnPATIENTS AND METHODSnPatients received PS-341 twice weekly for 4 weeks at either 0.40, 1.04, 1.20, or 1.38 mg/m(2), followed by a 2-week rest. The PD of PS-341 was evaluated by measurement of whole blood 20S proteasome activity.nnnRESULTSnTwenty-seven patients received 293 doses of PS-341, including 24 complete cycles. DLTs at doses above the 1.04-mg/m(2) MTD attributed to PS-341 included thrombocytopenia, hyponatremia, hypokalemia, fatigue, and malaise. In three of 10 patients receiving additional therapy, serious reversible adverse events appeared during cycle 2, including one episode of postural hypotension, one systemic hypersensitivity reaction, and grade 4 transaminitis in a patient with hepatitis C and a substantial acetaminophen ingestion. PD studies revealed PS-341 induced 20S proteasome inhibition in a time-dependent manner, and this inhibition was also related to both the dose in milligrams per meter squared, and the absolute dose of PS-341. Among nine fully assessable patients with heavily pretreated plasma cell dyscrasias completing one cycle of therapy, there was one complete response and a reduction in paraprotein levels and/or marrow plasmacytosis in eight others. In addition, one patient with mantle cell lymphoma and another with follicular lymphoma had shrinkage of nodal disease.nnnCONCLUSIONnPS-341 was well tolerated at 1.04 mg/m(2) on this dose-intensive schedule, although patients need to be monitored for electrolyte abnormalities and late toxicities. Additional studies are indicated to determine whether incorporation of dose/body surface area yields a superior PD model to dosing without normalization. PS-341 showed activity against refractory multiple myeloma and possibly non-Hodgkins lymphoma in this study, and merits further investigation in these populations.

A phase 2 study of two doses of bortezomib in relapsed or refractory myeloma

In a phase 2 open‐label study of the novel proteasome inhibitor bortezomib, 54 patients with multiple myeloma who had relapsed after or were refractory to frontline therapy were randomized to receive intravenous 1·0 or 1·3u2003mg/m2 bortezomib twice weekly for 2u2003weeks, every 3u2003weeks for a maximum of eight cycles. Dexamethasone was permitted in patients with progressive or stable disease after two or four cycles respectively. Responses were determined using modified European Group for Blood and Marrow Transplantation criteria. The complete response (CR)u2003+u2003partial response (PR) rate for bortezomib alone was 30% [90% confidence interval (CI), 15·7–47·1] and 38% (90% CI, 22·6–56·4) in the 1·0u2003mg/m2 (8 of 27 patients) and 1·3u2003mg/m2 (10 of 26 patients) groups respectively. The CRu2003+u2003PR rate for patients who received bortezomib alone or in combination with dexamethasone was 37% and 50% for the 1·0 and 1·3u2003mg/m2 cohorts respectively. The most common grade 3 adverse events were thrombocytopenia (24%), neutropenia (17%), lymphopenia (11%) and peripheral neuropathy (9%). Grade 4 events were observed in 9% (five of 54 patients). Bortezomib alone or in combination with dexamethasone demonstrated therapeutic activity in patients with multiple myeloma who relapsed after frontline therapy.

Phase II Clinical Experience With the Novel Proteasome Inhibitor Bortezomib in Patients With Indolent Non-Hodgkin's Lymphoma and Mantle Cell Lymphoma

PURPOSEnTo determine the antitumor activity of the novel proteasome inhibitor bortezomib in patients with indolent and mantle-cell lymphoma (MCL).nnnPATIENTS AND METHODSnPatients with indolent and MCL were eligible. Bortezomib was given at a dose of 1.5 mg/m2 on days 1, 4, 8, and 11. Patients were required to have received no more than three prior chemotherapy regimens, with at least 1 month since the prior treatment, 3 months from prior rituximab, and 7 days from prior corticosteroids; absolute neutrophil count more than 1,500/microL (500/microL if documented bone marrow involvement); and platelet count more than 50,000/microL.nnnRESULTSnTwenty-six patients were registered, of whom 24 were assessable. Ten patients had follicular lymphoma, 11 had MCL, three had small lymphocytic lymphoma (SLL) or chronic lymphocytic leukemia (CLL), and two had marginal zone lymphoma. The overall response rate was 58%, with one complete remission (CR), one unconfirmed CR (CRu), and four partial remissions (PR) among patients with follicular non-Hodgkins lymphoma (NHL). All responses were durable, lasting from 3 to 24+ months. One patient with MCL achieved a CRu, four achieved a PR, and four had stable disease. One patient with MCL maintained his remission for 19 months. Both patients with marginal zone lymphoma achieved PR lasting 8+ and 11+ months, respectively. Patients with SLL or CLL have yet to respond. Overall, the drug was well tolerated, with only one grade 4 toxicity (hyponatremia). The most common grade 3 toxicities were lymphopenia (n = 14) and thrombocytopenia (n = 7).nnnCONCLUSIONnThese data suggest that bortezomib was well tolerated and has significant single-agent activity in patients with certain subtypes of NHL.

Development of the Proteasome Inhibitor Velcade™ (Bortezomib)

The dipeptide boronic acid analogue VELCADE™ (Bortezomib; formerly known as PS-341, LDP-341 and MLM341) is a potent and selective inhibitor of the proteasome, a multicatalytic enzyme that mediates many cellular regulatory signals by degrading regulatory proteins or their inhibitors. The proteasome is, thus, a potential target for pharmacological agents. Bortezomib, the first proteasome inhibitor to reach clinical trials, has shown in vitro and in vivo activity against a variety of malignancies, including myeloma, chronic lymphocytic leukemia, prostate cancer, pancreatic cancer, and colon cancer. The drug is rapidly cleared from the vascular compartment, but a novel pharmacodynamic assay has shown that bortezomib–mediated proteasome blockade is dose-dependent and reversible. Based on phase I studies demonstrating that bortezomib has manageable toxicities in patients with advanced cancers, phase II trials have been initiated for both solid and hematological malignancies.

The proteasome: structure, function, and role in the cell

The proteasome is a multisubunit enzyme complex that plays a central role in the regulation of proteins that control cell-cycle progression and apoptosis, and has therefore become an important target for anticancer therapy. Before a protein is degraded, it is first flagged for destruction by the ubiquitin conjugation system, which ultimately results in the attachment of a polyubiquitin chain on the target protein. The proteasomes 19S regulatory cap binds the polyubiquitin chain, denatures the protein, and feeds the protein into the proteasomes proteolytic core. The proteolytic core is composed of 2 inner beta rings and 2 outer alpha rings. The 2 beta rings each contain 3 proteolytic sites named for their trypsin-like, post-glutamyl peptide hydrolase-like (PGPH) (i.e., caspase-like), or chymotrypsin-like activity. Inhibition of the proteasome results in cell-cycle arrest and apoptosis. In in vitro and in vivo animal studies, inhibition of the proteasome via bortezomib (VELCADE; formerly, PS-341, LDP-341, and MLN341) had antitumor activity against numerous tumor types either alone or in combination with conventional chemotherapeutic agents; these results provided the rationale for a broad clinical trial program. Bortezomib is currently in phase III trials for myeloma and is in early clinical development for numerous other tumor types.

Enhancement of radiosensitivity by proteasome inhibition: Implications for a role of NF-κB

PURPOSEnNF-kappaB is activated by tumor necrosis factor, certain chemotherapeutic agents, and ionizing radiation, leading to inhibition of apoptosis. NF-kappaB activation is regulated by phosphorylation of IkappaB inhibitor molecules that are subsequently targeted for degradation by the ubiquitin-proteasome pathway. PS-341 is a specific and selective inhibitor of the proteasome that inhibits NF-kappaB activation and enhances cytotoxic effects of chemotherapy in vitro and in vivo. The objective of this study was to determine if proteasome inhibition leads to enhanced radiation sensitivity.nnnMETHODS AND MATERIALSnInhibition of NF-kappaB activation in colorectal cancer cells was performed by treatment of LOVO cells with PS-341 or infection with an adenovirus encoding IkappaB super-repressor, a selective NF-kappaB inhibitor. Cells were irradiated at 0, 2, 4, 6, 8, and 10 Gy with or without inhibition of NF-kappaB. NF-kappaB activation was determined by electrophoretic mobility gel shift assay, and apoptosis was evaluated using the TUNEL assay. Growth and clonogenic survival data were obtained to assess effects of treatment on radiosensitization. In vitro results were tested in vivo using a LOVO xenograft model.nnnRESULTSnNF-kappaB activation was induced by radiation and inhibited by pretreatment with either PS-341 or IkappaBalpha super-repressor in all cell lines. Inhibition of radiation-induced NF-kappaB activation resulted in increased apoptosis and decreased cell growth and clonogenic survival. A 7-41% increase in radiosensitivity was observed for cells treated with PS-341 or IkappaBalpha. An 84% reduction in initial tumor volume was obtained in LOVO xenografts receiving radiation and PS-341.nnnCONCLUSIONSnInhibition of NF-kappaB activation increases radiation-induced apoptosis and enhances radiosensitivity in colorectal cancer cells in vitro and in vivo. Results are encouraging for the use of PS-341 as a radiosensitizing agent in the treatment of colorectal cancer.

26S proteasome inhibition induces apoptosis and limits growth of human pancreatic cancer

The 26S proteasome degrades proteins that regulate transcription factor activation, cell cycle progression, and apoptosis. In cancer, this may allow for uncontrolled cell division, promoting tumor growth, and spread. We examined whether selective inhibition of the 26S proteasome with PS‐341, a dipeptide boronic acid analogue, would block proliferation and induce apoptosis in human pancreatic cancer. Proteasome inhibition significantly blocked mitogen (FCS) induced proliferation of BxPC3 human pancreatic cancer cells in vitro, while arresting cell cycle progression and inducing apoptosis by 24 h. Accumulation of p21Cip1‐Waf‐1, a cyclin dependent kinase (CDK) inhibitor normally degraded by the 26S proteasome, occurred by 3 h and correlated with cell cycle arrest. When BxPC3 pancreatic cancer xenografts were established in athymic nu/nu mice, weekly administration of 1 mg/kg PS‐341 significantly inhibited tumor growth. Both cellular apoptosis and p21Cip1‐Waf‐1 protein levels were increased in PS‐341 treated xenografts. Inhibition of tumor xenograft growth was greatest (89%) when PS‐341 was combined with the tumoricidal agent CPT‐11. Combined CPT‐11/PS‐341 therapy, but not single agent therapy, yielded highly apoptotic tumors, significantly inhibited tumor cell proliferation, and blocked NF‐κB activation indicating this systemic therapy was effective at the cancer cell level. 26S proteasome inhibition may represent a new therapeutic approach against this highly resistant and lethal malignancy. J. Cell. Biochem. 82: 110–122, 2001.

Potential for proteasome inhibition in the treatment of cancer

Proteasome inhibition is a new approach to treating cancer. Proteasome inhibitors specifically induce apoptosis in cancer cells, but most proteasome inhibitors are not suitable for clinical development. Peptide boronates overcome the shortcomings of earlier generation proteasome inhibitors, and bortezomib (VELCADE; formerly PS-341) is the first peptide boronate to enter clinical trials. Preclinical studies of bortezomib have demonstrated antitumor activity in a variety of tumor types. Phase I trials provided evidence of manageable toxicities and support a twice-weekly dosing regimen now being examined in a Phase III study.

Phase I/II Trial Assessing Bortezomib and Melphalan Combination Therapy for the Treatment of Patients With Relapsed or Refractory Multiple Myeloma

PURPOSEnBortezomib has shown synergy with melphalan in preclinical models. We assessed the safety, tolerability, and response rate in a dose-escalation study of this combination for relapsed or refractory multiple myeloma patients.nnnMETHODSnBortezomib was administered from 0.7 to 1.0 mg/m(2) on days 1, 4, 8, and 11 of a 28-day cycle for up to eight cycles. Oral melphalan was administered in escalating doses from 0.025 to 0.25 mg/kg on days 1 to 4.nnnRESULTSnThirty-five patients with relapsed or refractory myeloma were enrolled, 34 of whom were assessable for response. Dose-limiting toxicity of grade 4 neutropenia in two of six patients in the highest dose cohort led to the assignment of bortezomib 1.0 mg/m2 and melphalan 0.10 mg/kg as the maximum-tolerated dose (MTD). Responses (minimal [MR], partial [PR], or complete [CR]) occurred in 23 of 34 patients (68%), including two CRs (6%), three immunofixation-positive CRs (9%), 11 PRs (32%), and seven MRs (21%). Responses were observed in five of six assessable patients (83%) at the MTD. Median progression-free survival for all patients was 8 months (range, 2 to 18 months). Grade > or = 3 toxicities were related mostly to myelosuppression. Among the 15 patients with grade 1/2 neuropathy at baseline, it resolved during treatment in one, worsened in four, and remained stable in 10 patients. Eight other patients developed grade 1/2 neuropathy during the study.nnnCONCLUSIONnBortezomib plus melphalan given on a 28-day schedule showed encouraging activity with manageable toxicity and represents a promising treatment for myeloma patients.

Proteasome inhibition in cancer: Development of PS-341

The 26S proteasome regulates protein turnover in eukaryotic cells. This is relevant in human cancer because the cell cycle, tumor growth, and survival are governed by a large repertoire of intracellular proteins that are regulated by the ubiquitin-mediated proteasome degradative pathway. In the development of new antitumor agents whose mechanisms are distinct from currently available therapies, we have discovered a potent, selective inhibitor of the proteasome: PS-341, a dipeptide boronic acid. Compared with normal cells, cancer cells--and specifically myeloma--treated with PS-341 are differentially sensitive to proteasome inhibition and apoptosis. A unique feature of PS-341 involves the inhibition of nuclear factor (NF)-kappaB activation through stabilization of the inhibitor protein IkappaB. Myeloma cells depend on NF-kappaB-mediated transcription of cytokine growth factor interleukin-6, angiogenesis through vascular endothelial growth factor, and the cell adhesion molecule VCAM-1 for adherence of the plasma cells to the stromal tissue in bone marrow. At low nanomolar concentrations, PS-341 is highly effective in abrogating the transcription of these genes, which are under the direct regulation of NF-kappaB. Moreover, PS-341 appears to synergize with dexamethasone in myeloma cell culture, which may prove to be of additional benefit clinically. The safety profile in phase I trials of PS-341 in patients with cancer appears encouraging. Because proteasome inhibition with PS-341 results in potent antitumor activity in vitro, PS-341 may offer a promising new approach to treating otherwise fatal malignancy.