Enhanced Anti-Myeloma Activity by Combination of Receptor Tyrosine Kinase (RTK) Inhibition, Proteasome Inhibition, and Dexamethasone: Therapeutic Implications for t(4;14) and t(14;16) Multiple Myeloma (MM) Subgroups.

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Novel antineoplastic agents have opened perspectives to more selective treatment of multiple myeloma (MM). Targets include FGFR3 or c-maf and VEGFR1 expressed in MM subgroups carrying t(4;14)(p16.4;q32) or t(14;16)(q32;q23), respectively. The t(4;14) MM subgroup, overexpressing FGFR3, has been recently demonstrated to be sensitive to induction of apoptosis by receptor tyrosine kinase (RTK) inhibitors [Bisping, Blood 102, 661, 2003; Podar, Blood 103, 3474, 2004; Trudel, Blood 105, 2941, 2005]. The present study aimed at investigating mechanisms of enhanced anti-myeloma effects of BIBF 1000, a highly selective RTK inhibitor, blocking VEGF and FGF dependent signaling, in combination with the proteasome inhibitor bortezomib and/or dexamethasone in cytogenetically defined MM subgroups. MM cells analyzed included a panel of five t(4;14) and six t(14;16) positive myeloma cell lines as well as fourteen CD138+ sorted primary MM cells with or without t(4;14) (3/11). We found a significant, dose-dependent inhibition of proliferation and induction of apoptosis in t(4;14)+ MM lines (4/5) as well as in primary t(4;14)+ CD138+ sorted marrow derived MM cells (3/3) by incubation with the RTK inhibitor BIBF 1000. Coincubation with the proteasome inhibitor bortezomib, or dexamethasone, or their combination revealed stepwise increased apoptosis in non RAS-mutated t(4;14)+ MM (4/4). Induction of apoptosis by BIBF 1000 in t(4;14)+ cells was associated with inhibition of the phosphorylation of mitogen-activated protein kinase (MAPK p44/42). In contrast, inhibition of MAPK-phosphorylation failed and induction of apoptosis did not appear in n-RAS-mutated t(4;14)+ NCI-H929 cells. In t(14;16)+ MM cells, variable proapoptotic effects of BIBF 1000 (in 5 of 6 lines tested) were significantly enhanced by dexamethasone and/or bortezomib. Interleukin-6 partially antagonized and the pan-caspase inhibitor z-VAD almost completely reverted induced apoptosis upon exposure to these combinations. In t(14;16)+ MM.1S cells, induction of apoptosis by BIBF 1000 was associated with inhibition of the phosphatidyl-inositol-3 kinase/AKT pathway. When combining BIBF 1000, bortezomib, and/or dexamethasone, we found a markedly higher proportion of cleaved caspase-3, caspase-8 and PARP in t(4;14) and t(14;16) MM, while caspase-9 was not activated. The data provide the rationale for clinical evaluation of a combination of this class of targeted tyrosine kinase inhibitors, proteasome inhibitors and dexamethasone in MM with focus on defined cytogenetic subgroups.