AS703026 is a novel, selective, non-competitive, orally bioavailable MEK1/2 inhibitor with experimental IC(50) values of 5-11nM. In use on multiple myeloma cells (MM), AS703026 inhibited growth and survival of MM cells and cytokine-induced osteoclast differentiation more potently (~10X) than AZD6244. Inhibition of proliferation induced by AS703026 was mediated by G0-G1 cell cycle arrest and was accompanied by reduction of MAF oncogene expression. AS703026 further induced apoptosis via caspase 3 and Poly ADP ribose polymerase (PARP) cleavage in MM cells, both in the presence or absence of bone marrow stromal cells (BMSCs). Importantly, AS703026 sensitized MM cells to a broad spectrum of conventional (dexamethasone, melphalan), novel or emerging (lenalidomide, perifosine, bortezomib, rapamycin) anti-MM therapies. Significant tumour growth reduction in AS703026- vs. vehicle-treated mice bearing H929 MM xenograft tumours correlated with downregulated pERK1/2, induced PARP cleavage, and decreased microvessels in vivo. Moreover, AS703026 (<200nm) was="" cytotoxic="" against="" the="" majority="" of="" tumour="" cells="" tested="" from="" patients="" with="" relapsed="" and="" refractory="" mm="" (84%),="" regardless="" of="" mutational="" status="" of="" ras="" and="" braf="" genes.="" importantly,="" bmsc-induced="" viability="" of="" mm="" patient="" cells="" was="" similarly="" blocked="" within="" the="" same="" dose="">