Background: Multiple myeloma (MM) is a plasma cell proliferative disorder that results in considerable morbidity and mortality. As it is incurable with the current therapeutic approaches, more effective therapies based on better understanding of the pathobiology of the disease are needed. In MM, malignant plasma cells are characterized by low proliferative and apoptotic rates compared to other malignancies. Studies have shown elevated expression of anti-apoptotic proteins of the Bcl-2 family in MM cells, which appear to correlate with resistance to therapy with certain drugs. Hence, accelerating the apoptotic process by targeting the Bcl-2 family of proteins appears to be an attractive strategy for the treatment of MM. AT-101 is an orally bioavailable derivative of gossypol in cancer clinical trials, and is being developed by Ascenta Therapeutics. AT-101 behaves as a small molecule inhibitor of Bcl-2 and Bcl-XL, binding to the BH3-binding pocket of these proteins and inhibiting their ability to suppress the activity of pro-apoptotic proteins, resulting in apoptosis.

Methods and Results: AT-101 was cytotoxic to several different myeloma cell lines with a median effect observed at around 5μM concentration using an MTT cell proliferation assay. Additionally, at similar doses AT-101 induced cytotoxicity in myeloma cell lines resistant to conventional agents such as Melphalan (LR50), Doxorubicin (Dox40) and Dexamethasone (MM1.R), indicating non-overlapping mechanisms. To evaluate the ability of the drug to induce cell death in the tumor microenvironment, MM cells were co-cultured with marrow stromal cells or in the presence of VEGF or IL-6, two cytokines known to be important for myeloma growth and survival. AT-101 was cytotoxic to myeloma cells under these conditions as well with a median effect at concentrations of 5–10μM. AT-101 was able to induce apoptosis in myeloma cells in a dose- and time dependent fashion, as demonstrated by flow cytometry using Annexin/PI staining as well as cell cycle studies. AT-101 also resulted in cytotoxicity of freshly isolated primary patient myeloma cells, inducing apoptosis in a dose dependent manner. We also studied the effect of AT-101 on levels of different pro- and anti-apoptotic proteins using flow cytometry on permeabilized cells. A time-dependent increase in the level of BAX was observed following treatment with AT-101 without any associated change in levels of Bcl-xL or Bcl-2. Further studies evaluating the combination of AT101 with other active myeloma agents as well as a detailed evaluation of its mechanisms in myeloma are ongoing.

Conclusion: AT-101 has significant activity in vitro in the setting of myeloma as demonstrated by its effect on myeloma cell lines and primary patient cells. More importantly, it has activity against cell lines resistant to conventional anti-myeloma agents. In addition, Phase I studies with this agent are currently ongoing in patients with solid tumors. The results from these studies form the rationale for early phase clinical trials in MM, either alone or in combination with other active therapies.

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