Introduction
Multiple myeloma (MM) is an indolent B-cell disease that develops in the bone marrow and is associated with osteolytic lesions in the advanced stages[
1]. Despite progress in prolonging myeloma patient survival, current therapies are not curative; thus, it is imperative that new treatments be developed for this debilitating disease[
2,
3].
Survival and proliferation of myeloma cells are dependent on the presence of a permissive microenvironment, which includes bone marrow stroma and soluble cytokines[
4‐
9] such as IL-6 and HGF[
8,
10]. HGF is the ligand for MET receptor tyrosine kinase. When HGF binds to and activates MET, MET is autophosphorylated on Tyr1230, Tyr1234 and Tyr1235 located in the activation loop[
11‐
14]. In addition, MET has a multisubstrate docking site that is activated at Tyr1349 and Tyr1356. The phosphorylation of this region results in the induction of MET signaling through the activation of several downstream target pathways, including the mitogen-activated protein kinase (MAPK) and AKT signaling pathways[
11]. HGF/MET-induced MAPK signaling has been shown to be essential for proliferation, migration and invasion[
7,
11,
15,
16] while the induction of AKT signaling promotes tumor cell survival[
17].
HGF/MET signaling is increasingly recognized as an important contributor to the pathogenesis of myeloma. Expression of both HGF and MET has been demonstrated in most myeloma cell lines and primary patient samples[
18,
19]. Studies correlating HGF levels with MM clinical parameters such as diagnosis[
20‐
23] disease stage, aggressiveness[
22,
24,
25], prognosis[
22,
23,
26], and response[
26‐
29]. Besides its effects on the malignant myeloma cells, HGF is involved in the pathogenesis of myeloma-related bone disease. HGF levels are increased in patients with extensive bone lesions, and correlates with expression of osteoclast stimulating cytokines[
24]. IL-11 secretion from osteoblasts is induced by HGF[
30], and HGF inhibits bone morphogenetic protein-induced osteoblastogenesis[
31].
Taken together, these clinical findings strongly support our hypothesis that targeting the HGF/MET signaling pathway is a rational approach to myeloma therapy. In line with this postulate, our laboratory studies demonstrated that genetically knocking down MET in myeloma cell lines using short hairpin RNA and ribozyme approaches resulted in growth inhibition and demise of the myeloma cells[
32,
33]. Consistent with these observations, a decline in MET transcript and protein levels induced by treatment with any of the transcription inhibitors flavopiridol, cordycepin, or 8-chloro-adenosine, promoted myeloma cell death[
32‐
34]. Collectively, these data demonstrate MET’s pivotal role in myeloma cell biology and underscore the importance of MET targeting as a therapeutic strategy in MM[
35].
While these genetic and pharmacologic strategies suggest utility of MET/HGF inhibition as therapeutic targets, these interventions are not pragmatic for clinical use. Amuvatinib (previously known as MP470, Astex Pharmaceuticals, Inc.) is a synthetic carbothioamide that inhibits MET, cKIT and platelet derived growth factor receptor (PDGFR). This small-molecule inhibitor competes with ATP for binding at the catalytic site. In solid cancers, amuvatinib has been shown to be effective in inhibiting MET at low micromolar concentrations (IC
50 ~5 μM)[
36]. Amuvatinib is a well-tolerated, orally bioavailable drug currently in phase II clinical trials[
37,
38]. Availability of a clinical candidate, its inhibitory potential for MET kinase, and the role of MET in myeloma cell survival provided compelling rationales for testing the effects of amuvatinib on myeloma cells.
In the present study, we compared mRNA levels of MET and HGF in normal and primary myeloma plasma cells. We investigated amuvatinib’s actions and cytotoxic effects in primary plasma cells obtained from patients with myeloma. To elucidate in more detail the mechanism of action of amuvatinib in myeloma cells, we evaluated its effect on MET activity and downstream signaling in the myeloma cell line U266, which over-expresses HGF. Our data demonstrate that MET receptor tyrosine kinase may be targeted in myeloma and support the investigation of small-molecule inhibitors such as amuvatinib as possible therapeutic agents against this disease.
Discussion
MET is a receptor tyrosine kinase that is activated by the ligand HGF and has been shown to be constitutively expressed, mutated, or over-expressed in many different cancer cell types. It serves as an important factor for cell survival, migration, and motility[
7,
11,
15]. Corollary to that, inhibition of MET kinase activity causes reduction of the downstream signaling that is necessary for these cells to maintain their oncogenic properties[
46]. Previous studies in our laboratory showed that while MET receptor tyrosine kinase acts as a survival factor for myeloma cells[
32,
33], it is neither mutated nor, for the most part, over-expressed in MM. However, its ligand HGF is increased in plasma or serum obtained from myeloma patients and higher HGF level has been associated with poor prognosis[
18,
20,
22,
26]. Furthermore, HGF not only promotes growth, migration, and survival of myeloma cells, it also potentiates IL-6 effects[
46].
While levels of plasma HGF have been associated with myeloma, levels of
HGF and
MET mRNA in patient plasma cells have not been well evaluated nor correlated with disease status. Our analyses of mRNA array data[
40,
41] demonstrated autocrine expression of HGF in CD138+ plasma cells from MM patients. This was consistent with previous report in 7 myeloma patient samples[
18]. Our results further elucidated that the level of the
HGF expression was directly associated with disease progression.
Together, these findings provide a rationale for targeting the HGF/MET signaling axis in myeloma. Targeting HGF directly may prove difficult, since therapeutic targeting of HGF would need to be effective at elevated levels to successfully compete and inhibit the high serum HGF concentrations in myeloma patients. Therefore, using a small-molecule MET suppressor such as amuvatinib may be a viable option to target the HGF/MET pathway. Additionally, several MET inhibitors are available for clinical testing[
11].
Amuvatinib is an orally available drug that is currently in clinical trials for the treatment of solid tumors[
37,
38,
47]. This compound was designed, developed, and selected via a computation-driven
in silico process whereby drug scaffolds were screened, docked, and fitted against a homologous model of KIT. After additional screening in biochemical and cell-based assays, amuvatinib was selected as a tyrosine kinase inhibitor with activity against wild-type and mutant KIT, MET, RET, FLT3 and PDGFRα[
48,
49]. Later, amuvatinib inhibition of MET activity was found to lead to reduction of RAD51 expression and to radiosensitization of tumor cells[
50].
Since amuvatinib is a small-molecule inhibitor that suppresses MET activity, we tested this agent as a proof-of-concept to therapeutically target MET in myeloma. Our study demonstrated that amuvatinib was effective in inhibiting growth and DNA synthesis at low micromolar concentrations in cell lines grown under normal conditions (10% FBS). Moreover, amuvatinib treatment resulted in cell death in U266 myeloma cell line dependent on MET/HGF signaling, as measured by annexin V/PI staining and PARP cleavage. This cytotoxic effect remained even when these MET-addicted cells were grown on bone marrow stromal cells. In contrast, the drug did not induce apoptosis in another myeloma cell line (RPMI-8226/S) that is not dependent on the MET/HGF signaling axis due to lower levels of HGF (75% less) and MET (95% less).
Because amuvatinib also impairs KIT and PDGFR signaling, we tested impact of imatinib (an established KIT and PDGFR inhibitor) in myeloma cells. Imatinib induced no significant amount of cell death in U266 cells demonstrating that amuvatinib’s effect was due to MET inhibition. This statement was in line with the data regarding decreased phosphorylation of MET after amuvatinib treatment. Because >95% of the compound is bound and sequestered by serum proteins (Unpublished data), the dose required to achieve maximum inhibition of MET phosphorylation in serum starved conditions was lower than the dose to induce apoptosis in full serum conditions. Likewise, under serum starved conditions, the maximum induction of apoptosis was seen at the same dose which achieved maximum inhibition of MET phosphorylation. As expected, in imatinib treated cells, there was no reduction of p-MET (data not shown) as well as no significant reduction in survival. These correlation data suggest that amuvatinib mediated growth inhibition and cell death is due to its action on MET and not its action on KIT or PDGFR.
In conjunction with a decrease in MET phosphorylation, there was a decline in HGF-dependent ERK1/2 and AKT phosphorylation as well as the phosphorylation of the AKT targets GSK3β and caspase-9 (data not shown). Diminution of phosphorylated MET and associated decreases in ERK1/2 and AKT phosphorylation has been shown to be important in growth, migration and cell survival pathways for other cancer cell types[
11].
Amuvatinib proved to be effective in inducing cell death not only in a MET dependent myeloma cell line but also in primary CD138+ malignant plasma cells obtained from patients with myeloma. In contrast, amuvatinib did not cause cell death in normal CD138– cells obtained from the same individuals (Figure
2). These data provide evidence of the selectivity of amuvatinib, suggesting that it may be used specifically for myeloma treatment without impairing other normal hematological cells in the bone marrow. In line with this selective cytotoxic effect on CD138 plasma cells, MET phosphorylation was reduced by amuvatinib treatment in primary plasma cells but not CD138– cells.
The effects of amuvatinib described here provide proof-of-concept that MET is important for the survival of myeloma cells and that reduction of its kinase activity may prove to be an effective targeted therapy. The 25 μM dose of amuvatinib needed to robustly induce apoptosis in cell lines and plasma cells under full serum conditions may not be achievable
in vivo. Pharmacokinetic studies of amuvatinib during a phase I trial indicated that plasma levels reached between 1 and 2 μM[
51]. Hence, newer generation and more potent MET tyrosine kinase inhibitors are needed[
11]. ARQ 197 (tivantinib) is a small-molecule, non-ATP-competitive inhibitor which is highly specific for MET[
52,
53]. This drug is well tolerated in clinical trials and has shown efficacy in solid tumors[
54‐
58]. Pharmacodynamic studies from a phase I trial indicated that at an oral dosing of 360 mg, twice daily, ARQ 197 reached steady-state plasma concentrations of 6–7 μM[
55]. This correlated with decreases in total MET and phospho-FAK (Tyr861) and increases in TUNEL-positive cells in patients’ tumors.
Our results with amuvatinib provided the impetus to pursue testing of ARQ 197 in myeloma cells. Our preclinical studies indicated that treatment with ARQ 197 for 48 hours was cytotoxic to myeloma cell lines (≥ 60% increase in annexin V/PI-positive cells) at clinically achievable doses[
59]. Moreover, these studies provided the foundation for a Cancer Therapy Evaluation Program, National Cancer Institute sponsored phase 2 clinical trial of ARQ 197 in myeloma patients, which is currently underway at MD Anderson Cancer Center[
60].
Competing interest
SR and PT are employed by Astex Pharmaceuticals, Inc., Dublin, CA. For the remaining authors, none was declared.
Authors’ contributions
CJP, SZ, KB, and CMS designed and performed experiments; CJP, SZ, JZ, VB, and CMS analyzed data; SS performed experiments; PT and SR provided amuvatinib; MW provide sorted bone marrow samples; CJP and CMS wrote the manuscript; CMS and VG directed study; VG provided laboratory resources; All authors critically read and approved manuscript.