The MEK/ERK signaling pathway plays a crucial role in the pathogenesis of human multiple myeloma (MM). Importantly, this pathway promotes interactions of MM cells with bone marrow stromal cells (BMSCs) that secrete cytokines and growth factors for MM cell growth, survival, and resistance to chemotherapeutic drugs. We here investigate cytotoxic effects of a novel MEK1/2 inhibitor AS703026 (abstract#3861) in MM. AS703026 inhibited proliferation of a panel of drug sensitive and resistant MM cell lines in a dose-dependent manner. Specifically, LD50 of AS703026 for U266, INA6, and H929 are 5, 20, and 200 nM, respectively. AS703026 blocked MM cell growth in a mehylcellulose-based colony formation assay. It specifically inhibited pERK1/2, but neither pSTAT3 nor pAKT. Phosphorylation of AKT was upregulated following 4h treatment. Importantly, AS703026 blocked pERK1/2, but not pSTAT3, induced by MM cell adhesion to BMSCs. Selective MEK1/2 inhibition by AS703026 led to a cessation of cell proliferation accompanied by G0-G1 cell cycle arrest, as shown by increased subG0 cells, and concurrently abolished S phase cells. AS703026 further induced apoptosis in the presence or absence of BMSCs, as manifested by caspase 3 and PARP cleavages. Notably, AS703026 reduced expression of MM-related c-maf oncogene in a time-dependent manner, suggesting a MEK1/2-dependent regulation of c-maf that may contribute MM cell growth inhibition. AS703026 further decreased VEGF and IL6 secretion induced by MM cell adhesion to BMSCs, but not baseline cytokine secretion by BMSCs. It blocked dose-dependent osteoclastogenesis in vitro, as measured by number of TRAP-positive multinuclear cells following culturing PBMCs with RANKL and M-CSF. Moreover, AS703026 sensitized MM cells to both conventional (dexamethasone) and novel (perifosine, lenalidomide, rapamycin) therapies. In vivo studies demonstrate that treatment of H929-bearing mice with AS703026 (n=4 at 30 mg/kg; n=6 at 15 mg/kg), but not vehicle alone (n=6), blocked MM tumor growth in a dose-dependent manner (p<0.02). Immunoblotting showed downregulated pERK1/2 and PARP cleavage in AS703026-treated, but not control, tumors, confirming specific pERK1/2 inhibition and apoptosis in vivo. Immunohistochemistry (IHC) studies demonstrated upregulated TUNEL and caspase 3 cleavages triggered by AS703026 in H929 tumors. AS703026 further reduced CD34 staining by IHC, suggesting inhibition of angiogenesis in vivo. Significantly, AS703026 of <200 nM is cytotoxic against the majority of MM patient cells (n=14), as measured in 5-day DNA synthesis or MTT assays. Importantly, the increased viability of MM patient cells cocultured with BMSCs was inhibited within the same dose range (n=4). Our results therefore strongly support clinical protocols evaluating AS703026, alone or with other anti-MM agents, to improve patient outcome in MM.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 3695.

100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO