Blockade of the MEK/ERK signalling cascade by AS703026, a novel selective MEK1/2 inhibitor, induces pleiotropic anti-myeloma activity in vitro and in vivo

We investigate cytotoxicity and mechanism of action of AS703026, a novel, selective, orally bioavailable MEK1/2 inhibitor, in human multiple myeloma (MM). AS703026, more potently (9-10 fold) than AZD6244, inhibited growth and survival of MM cells and cytokine-induced osteoclast differentiation. Inhibition of proliferation induced by AS703026 was mediated by G0-G1 cell cycle arrest and was accompanied by reduction of c-maf oncogene expression. AS703026 further induced apoptosis via caspase 3 and 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 tumor growth reduction in AS703026- vs. vehicle-treated mice bearing H929 MM xenograft tumors correlated with downregulated pERK1/2, induced PARP cleavage, and decreased microvessels in vivo. Moreover, AS703026 (<200 nM) was cytotoxic against the majority of tumor 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 range. Our results therefore support clinical evaluation of AS703026, alone or in combination with other anti-MM agents, to improve patient outcome.