Leukemia (2009) 23, 10–24 & 2009 Macmillan Publishers Limited All rights reserved 0887-6924/09 $32.00 www.nature.com/leu SPOTLIGHT REVIEW Bone marrow microenvironment and the identification of new targets for myeloma therapy K Podar, D Chauhan and KC Anderson Department of Medical Oncology, LeBow Institute for Myeloma Therapeutics, Dana Farber Cancer Institute, Jerome Lipper Multiple Myeloma Center, Harvard Medical School, Boston, MA, USA The development of multiple myeloma (MM) is a complex multi- Signaling cascades activated by cytokines, growth factors and/ step process involving both early and late genetic changes in or adhesion in MM cells include the Ras/Raf/MEK/MAPK- the tumor cell as well as selective supportive conditions by the k bone marrow (BM) microenvironment. Indeed, it is now well pathway, PI3K/Akt-pathway, the JAK/Stat3-pathway, the NF B- established that MM cell-induced disruption of the BM homeo- pathway and the Wnt-pathway. Promising intracellular targets stasis between the highly organized cellular and extracellular for novel therapies also include protein kinase C (PKC) and SPOTLIGHT compartments supports MM cell proliferation, survival, migra- heat-shock proteins (HSPs). Moreover, genomic profiling has tion and drug resistance through activation of various signaling now identified additional stage-specific intracellular targets, (for example, PI3K/Akt, JAK/Stat-, Raf/MEK/MAPK-, NFjB- and which are now under investigation as novel potential therapeutic Wnt-) pathways. Based on our enhanced understanding of the 2,4 functional importance of the MM BM microenvironment and its targets. inter-relation with the MM cell resulting in homing, seeding, Cell surface receptors include integrins, cadherins, selectins, proliferation and survival, new molecular targets have been syndecans, and the immunoglobulin superfamily of cell adhe- identified and derived treatment regimens in MM have already sion molecules including syndecan-1 (CD138), H-CAM (CD44), changed fundamentally during recent years. These agents VLA-4 (CD49d/CD29), ICAM-1 (CD54), N-CAM (CD56), LFA-3 include thalidomide, its immunomodulatory derivative lenali- (CD58), avb3, CD56, CD74, HM1.24, VLA-5 (CD49e/CD29), domide and the proteasome inhibitor bortezomib, which mediate 5,6 tumor cytotoxicity in the BM milieu. Ongoing studies are further VLA-6 and CD51. Most recently, a functional role in MM has delineating MM pathogenesis in the BM to enhance cytotoxicity, been shown for cell surface glycoprotein CD2 subset-1 (CS-1), a 7 avoid drug resistance and improve patient outcome. member of the immunoglobulin gene superfamily; as well as Leukemia (2009) 23, 10–24; doi:10.1038/leu.2008.259; HLA-A;8 and b2-microglobulin, which stabilizes MHC class I published online 9 October 2008 molecule and is a key prognostic marker in MM.9 Keywords: multiple myeloma; bone marrow; novel therapies Several approaches to target these surface receptors are under investigation. Initially considered to be the sole contributor of maintenance and expansion of MM cells within the BM,10,11 the liquid milieu consists of cytokines and growth factors, including IL-6, VEGF, insulin-like growth factor-1 (IGF-1), TNF-a, SDF-1a and CD40; Introduction as well as TGF-b, bFGF, MIP-1a, SCF, HGF, IL-1b, IL-3, IL-10, IL-15 and IL-21;10–12 as well as angiopoietin-1 and matrix Despite the presence of a variety of chromosomal aberrations, metalloproteinases, for example, MMP-2 and MMP-9. Several translocations and mutations in essential growth and tumor approaches to neutralize these cytokines, growth factors or their suppressor genes in multiple myeloma (MM) cells, onco- respective receptors are under investigation. genomic studies have identified few differences distinguish- In summary, novel biologically based treatment regimens aim ing monoclonal gammopathy of unknown significance from to target not only MM cells alone but also MM cell–stromal cell MM.1–3 This finding highlights the essential role of the bone interactions and the BM milieu. Here, we discuss three classes marrow (BM) microenvironment in disease maintenance and of new therapeutic targets and derived agents: first, those progression. Indeed, direct and indirect interactions with other targeting pathways of MM cells; second, agents targeting the cells and the extracellular matrix (ECM) within the liquid milieu cellular BM compartment; and third, those targeting the non- of the BM environment are key requirements for MM cellular BM compartment (http://clinicaltrials.gov/; http:// pathogenesis, MM cell growth, survival, migration and drug www.multiplemyeloma.org/; http://myeloma.org/). resistance. Cytokines and growth factors are produced and secreted by MM and other cells within the BM microenviron- ment and regulated both by autocrine as well as paracrine loops Targeting pathways within MM cells and cell–cell adhesion. The complexity of signaling events is further enhanced by co-stimulation triggered by cell–cell Ras/MEK/MAPK-pathway in MM contact and cytokine/growth factors. Several cytokines and growth factors within the MM BM microenvironment including IL-6, IGF-1 and VEGF, as well as Correspondence: Dr K Podar, Department of Medical Oncology, direct cell–cell contact through integrins trigger MM cell LeBow Institute for Myeloma Therapeutics, Dana-Farber Cancer growth, survival and drug resistance by inducing the Ras/MEK/ Institute, Jerome Lipper Multiple Myeloma Center, Harvard Medical MAPK-pathway (Figure 1). Moreover, MM patients with mutant School, 44 Binney Street, Boston, MA 02115, USA. E-mail: [email protected] Ras have a shortened survival and are less likely to respond to Received 10 June 2008; revised 4 August 2008; accepted 14 August chemotherapy. The initial transfer of farnesyl groups from 2008; published online 9 October 2008 farnesyl diphosphate to the CAAX motif of the oncoprotein MM and the BM microenvironment K Podar et al 11 Cytokines and Growth factors ECM integrins SKF Wnt Cav-1 Ras frizzled PKC DSH Rock DSH PI3K JAK/ Rho Raf NFκB STAT APC GSK3-β 2+ [Ca ]i proliferation MAPK drug resistance β-catenin PKC Bcl9 survival β -catenin proliferation survival Pygo TCF integrins migration proliferation selectins cadherins autocrine and paracrine PGs (e.g. CD138) cytokine and growth factor Igs proliferation secretion migration MM cell/ stromal cell and ECM interaction triggering MM cell proliferation, survival, drug- resistance (CAMDR) Figure 1 Signaling cascades in MM. Ras is, at least in part, responsible for related downstream signal targets Akt is perifosine/KRX-0401. Specifically, perifosine: transduction, and targeted inhibition of Ras/Raf/MEK/ERK inhibits Akt phosphorylation; triggers JNK activation followed signaling using farnesyltransferase inhibitors (FTI) abrogates by caspase-8/9 and poly-ribose polymerase cleavage; triggers tumor cell growth.6,12 However, the transfer of farnesyl groups, the formation of the death-inducing signaling complex as well as and therefore the effect of FTIs, on Ras may not be specific as FTI the recruitment of TRAIL-R1/DR4, TRAIL-R2/DR5 and Bid in Tipifarnib (R115777) also induces MM cell apoptosis through lipid rafts; induces apoptosis through cytochrome c release as Ras-independent pathways.13 A clinical phase II trial demon- well as downregulation of survivin; and increases ERK strated that the FTI Tipifarnib was well tolerated, induced phosphorylation.19–21 Importantly, MEK inhibitors for example, stabilization of disease, and inhibited farnesylation and tumor AZD-6244 (see previous section) or exogenous TRAIL synergisti- survival pathways in patients with advanced MM.14 Interest- cally enhance perifosine-induced cytotoxicity in MM cells.17,22 ingly, the combination of bortezomib (Bort) with Tipifarnib Clinical studies using perifosine in combination with Bort and/or induces synergistic MM cell apoptosis through downregulation with dexamethasone (Dex) or Len with Dex are now ongoing. of HDAC6 and inhibition of aggresome formation.15 An ongoing Other PI3K inhibitors include SF1126 and BEZ235.23,24 clinical trial is testing the efficacy of Tipifarnib together with Activated Akt phosphorylates forkhead transcription factor, Bort.16 glycogen synthase 3b and mammalian target of rapamycin. SPOTLIGHT Raf-1 is a key regulator of cellular proliferation and survival Mammalian target of rapamycin in turn mediates phosphoryla- within the MAPK pathway, and BAY 43-9006/sorafenib/Nexavar tion of P70S6 and 4E-BP1 and subsequent upregulation of 6,12 (Bayer Pharmaceuticals, West Haven, CT, USA) is the first oral D-cyclins and c-Myc. Several preclinical studies demonstrate multikinase inhibitor (PDGFR, VEGFR-1,-2,-3 and c-Kit) that promising results using mammalian target of rapamycin targets Raf and affects tumor signaling and tumor vasculature. inhibitors. A phase II clinical trial using temsirolimus alone is Several clinical studies using sorafenib, alone or in combination now completed, and a clinical phase I trial using deforolimus is with novel or conventional therapies, are now ongoing in solid ongoing. Combination clinical trials using temsirolimus with and hematologic malignancies including MM. Bort or Len; sorafenib with everolimus; and sorafenib with Bort Finally, our own recent data demonstrate significant inhibi- are also ongoing (Table 1). tory activity of the MEK1/2 inhibitor AZD6244 (ARRY-142886) against MM cell growth and survival as well as osteoclasto- genesis,17,18 and a clinical trial with AZD6244 is planned (Table 1). Stat signaling pathways in MM Dysregulated cytokine,