Sorafenib, a dual Raf kinase/vascular endothelial growth factor receptor inhibitor has significant anti-myeloma activity and synergizes with common anti-myeloma drugs

Multiple myeloma is characterized by increased bone marrow neovascularization driven in part by vascular endothelial growth factor (VEGF). In addition, the Ras/Raf/MEK/ERK pathway is critical for the proliferation of myeloma cells and is often upregulated. Sorafenib (Nexavar) is a novel multi-kinase inhibitor that acts predominantly through inhibition of Raf-kinase and VEGF receptor 2, offering the potential for targeting two important aspects of disease biology. In in vitro studies, sorafenib-induced cytotoxicity in MM cell lines as well as freshly isolated patient myeloma cells. It retained its activity against MM cells in co-culture with stromal cells or with interleukin-6, VEGF or IGF; conditions mimicking tumor microenvironment. Examination of cellular signaling pathways showed downregulation of Mcl1 as well as decreased phosphorylation of the STAT3 and MEK/ERK, as potential mechanisms of its anti-tumor effect. Sorafenib induces reciprocal upregulation of Akt phosphorylation; and simultaneous inhibition of downstream mTOR with rapamycin leads to synergistic effects. Sorafenib also synergizes with drugs such as proteasome inhibitors and steroids. In a human in vitro angiogenesis assay, sorafenib showed potent anti-angiogenic activity. Sorafenib, through multiple mechanisms exerts potent anti-myeloma activity and these results favor further clinical evaluation and development of novel sorafenib combinations.

ABT-737, an inhibitor of Bcl-2 family proteins, is a potent inducer of apoptosis in multiple myeloma cells

Disruption of pathways leading to programmed cell death plays a major role in most malignancies, including multiple myeloma (MM). ABT-737 is a BH3 mimetic small-molecule inhibitor that binds with high affinity to Bcl-2 and Bcl-xL, preventing the sequestration of proapoptotic molecules and shifting the cell survival/apoptosis balance toward apoptosis induction. In this study, we show that ABT-737 is cytotoxic to MM cell lines, including those resistant to conventional therapies, and primary tumor cells. Flow cytometric analysis of intracellular levels of Bcl-2 family proteins demonstrates a clear inversion of the Bax/Bcl-2 ratio leading to induction of apoptosis. Activation of the mitochondrial apoptosis pathway was indicated by mitochondrial membrane depolarization and caspase cleavage. Additionally, several signaling pathways known to be important for MM cell survival are disrupted following treatment with ABT-737. The impact of ABT-737 on survival could not be overcome by the addition of interleukin-6, vascular endothelial growth factor or insulin-like growth factor, suggesting that ABT-737 may be effective in preventing the growth and survival signals provided by the microenvironment. These data indicate that therapies targeting apoptotic pathways may be effective in MM treatment and warrant clinical evaluation of ABT-737 and similar drugs alone or in combination with other agents in the setting of MM.

Thymoglobulin targets multiple plasma cell antigens and has in vitro and in vivo activity in multiple myeloma

Multiple myeloma is characterized by the proliferation of clonal plasma cells that have a heterogeneous expression of various cell surface markers, precluding successful use of monoclonal antibodies for therapeutic targeting of the tumor cell. Thymoglobulin (rabbit-derived polyclonal anti-thymocyte globulin), by virtue of its method of preparation, contains antibodies against several B-cell and plasma cell antigens and offers an attractive option for immunotherapy of myeloma. Here, we demonstrate potent anti-myeloma activity of the rabbit anti-thymocyte globulin preparation Thymoglobulin in vitro and in vivo in an animal model of myeloma. Thymoglobulin was able to induce dose- and time-dependent apoptosis of several myeloma cell lines, including those resistant to conventional anti-myeloma agents. Importantly, the anti-myeloma activity was preserved even when myeloma cells were grown with different cytokines demonstrating the ability to overcome microenvironment-mediated resistance. Thymoglobulin induced apoptosis of freshly isolated primary myeloma cells from patients. Using a competitive flow cytometric analysis, we were able to identify the potential antigen targets for Thymoglobulin preparation. Finally, in a plasmacytoma mouse model of myeloma, Thymoglobulin delayed the tumor growth in a dose-dependent manner providing convincing evidence for continued evaluation of this agent in the clinic in patients with myeloma, either alone or in combination with other agents.

Correlation of bone marrow angiogenesis and response to thalidomide dexamethasone in multiple myeloma

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Background: Thalidomide (Thal) in combination with dexamethasone (Dex) is an effective therapy for myeloma (MM). Though initially evaluated in MM because of its anti-angiogenic properties, it is not clear how much this contribute to its anti-MM effects. We examined whether the degree of angiogenesis influences response to Thal + Dex, as well as effect of therapy on marrow angiogenesis. We also examined whether expression of angiogenic cytokines by MM cells affect response to therapy. Methods: A total of 207 pts with newly diagnosed MM were randomized on the ECOG phase III trial (E1A00) to either Thal-Dex or Dex alone. Bone marrow microvessel density (MVD) was determined on 88 pts at diagnosis and at 4 mos after therapy in 25 pts. Expression of fibroblast growth factor (FGF) and vascular endothelial growth factor (VEGF) by MM cells was determined by RTPCR on 60 pts at diagnosis and on 22 pts at 4 mos. In addition, the percentage of marrow PCs in apoptosis was determined at diagnosis and 4 mos in 43 pts. A decrease in the serum M protein by 50% or in its absence, a decrease in urine M protein by 50% was considered a response. Results: The median BM MVD at diagnosis was 33 (3–118), and was similar in the pts responding to therapy (36.4) and in the non responders (45.4). We also separately looked at those receiving the combination vs. Dex alone, and did not find any correlation. The expression of FGF and VEGF in the PCs did not predict response to therapy with either the combination or Dex. Among the 17 responding pts, MVD decreased by a mean of 12 compared to an increase in the mean MVD of 3 among the non responders (P = NS). There was no significant correlation between the level of FGF or VEGF expression and the marrow MVD. There was no correlation between the percentage of myeloma cells in apoptosis at baseline and response to therapy. Conclusion: While the anti-angiogenic activity of Thal may play a role in its anti-myeloma effect, baseline MVD or tumor cell expression of angiogenic cytokines do not predict resistance or response to Thal-Dex or Dex alone. The findings here confirm previous observations that marrow angiogenesis is not related to the degree of secretion of angiogenic cytokines by individual cells, but rather the cumulative effect of the tumor mass.

No significant financial relationships to disclose.