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¹ University of Maryland Greenebaum Cancer Center, ² University of Maryland Department of Pharmaceutical Sciences, and ³ University of Maryland School of Pharmacy, Baltimore, Maryland and ⁴ Myeloma Institute for Research and Therapy, University of Arkansas, Little Rock, Arkansas

Requests for reprints: Naoko Takebe, University of Maryland Greenebaum Cancer Center, 655 West Baltimore Street, BRB 7-029, Baltimore, MD 21201. Phone: 410-328-6870; Fax: 410-328-1975. E-mail: ntakebe{at}som.umaryland.edu

Multiple myeloma is an incurable disease for the majority of patients, therefore requiring new biological targeted therapies. In primary myeloma cells, IMP dehydrogenase (IMPDH) was shown to be consistently overexpressed. We therefore tested the IMPDH inhibitor mycophenolate mofetil (MMF) currently available as a clinical therapeutic agent for its antimyeloma activity in vitro. MMF depleted intracellular guanosine 5'-triphosphate (GTP) levels in myeloma cells. We showed apoptosis induction in myeloma cell lines and primary myeloma cells between 1 and 5 µmol/L MMF. MMF was also cytotoxic at this concentration in dexamethasone-resistant and Mcl-1-overexpressed myeloma cell lines shown by the tetrazolium salt XTT assay along with cell survival measured by a modified flow cytometric assay. Apoptosis was not inhibited by the presence of an antioxidant, suggesting that MMF-induced apoptosis is less likely to be associated with reactive oxygen species. However, apoptosis was abrogated by exogenously added guanosine, which activates an alternative pathway for GTP formation, implicating that this effect is directly mediated by IMPDH inhibition. MMF-induced G₁-S phase cell cycle arrest and its apoptosis induction mechanism were associated with a caspase-dependent pathway as shown by alteration of mitochondrial membrane potential and cytochrome c release followed by activation of the caspases. MMF-induced apoptosis was also inhibited by a pan-caspase inhibitor Z-VAD-fmk. MMF-treated myeloma cells showed an up-regulation of Bak, which most likely together with Bax resulted in the release of cytochrome c. In summary, MMF attenuates G₁-S phase cell cycle progression and activates the pathway of mitochondrial dysfunction, leading to cytochrome c release followed by activation of caspases. [Mol Cancer Ther 2006;5(2):457–66]

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Received 8/26/05; revised 11/10/05; accepted 11/29/05.