Tanshinone IIA---sulfonic sodium Regarding brain tumors AXL

Regarding Tanshinone IIA---sulfonic sodium tumors, AXL has been implicated in gliomagenesis and chemoresistance [39]. Previous investigations found that AXL is constitutively phosphorylated in many glioma cell lines, murine xenograft tumors and primary patient tumor samples [40]. Immunohistochemical analysis of AXL and Gas6 demonstrated that co-expression of these proteins correlates with tumor recurrence and progression [41], [42]. Furthermore, inhibition of AXL using in vitro and in vivo models resulted in a reduction of tumor growth, migration and invasion, as well as prolonged overall animal survival in xenografts containing dominant negative AXL [42]. Altogether the previous data has indicating that AXL targeted therapy may also diminish glioblastoma aggressiveness [43]. In a recent pre-clinical study performed by our group, the effectiveness of two pan-RTK inhibitors (sunitinib and cediranib) in glioblastoma cell lines was assessed and AXL was found as a common candidate target for both cediranib and sunitinib [31]. Other studies have also suggested AXL as target for sunitinib in other tumor types, such as in renal cell carcinoma [38], [44]. Moreover, de novo activation of AXL was found in imatinib-treated gastrointestinal stromal tumors (GIST) and in Her-2 positive breast cancer cells treated with lapatinib and, in both cases, AXL was associated with therapy resistance [45], [46]. Nevertheless, the role of this protein as a putative modulator of sunitinib therapy response is still in need of in vitro and in vivo validation.
Materials and methods
Results
Discussion Targeted therapy with small-molecule compounds is changing the clinical practice for several advanced cancers [48], [49]. Sunitinib malate (SU11248, Sutent by Pfizer) and cediranib (AZD2171, Recentin by AstraZeneca) are multi-target kinase drugs that inhibit PDGFR-α/ß, VEGFR1-3, KIT, RET, FLT3, CSF1R, and VEGFR1-3, KIT, PDGFRα, respectively [50], [51], [52]. These therapeutic agents have shown interesting results in pre-clinical mouse glioma models, but failed to demonstrate benefit in progression free survival and overall survival in clinical trials [26], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65]. A recent phase III clinical trial with cediranib showed that this drug failed to meet its primary end point of progression free survival prolongation, either as monotherapy or in combination with chemotherapy [26]. Hitherto, and in contrast to other tumor types [66], no predictive biomarkers for molecular targeted therapy response were yet identified in glioblastoma, hampering the design of efficient tailored therapies for these patients [30], and justifying to some extent the failure of the clinical trials until now. In a previous work of our group, we reported that cediranib could also inhibit EGFR, EphA7, AXL, MET, EphB2, and sunitinib could target EphB2, ROR1 and AXL [31], besides the abovementioned “classical” targets. Other authors have recently identified AXL as target for sunitinib [67], [68]. In the present work we aimed to validate AXL as a cediranib and sunitinib target in glioblastoma cells, using two glioblastoma cell lines with distinct AXL levels, namely U251 (low basal AXL levels) and SNB-19 (high basal AXL levels). Initially, we assessed AXL inhibition by western blot when cells were stimulated with the ligand GAS6. We showed that cediranib was able to inhibit AXL phosphorylation only at high doses in the SNB-19 cell line. Sunitinib strongly inhibited AXL in a dose-dependent manner and also inhibited the AKT pathway at high doses in the SNB-19 cell line. In accordance, modulation of AXL protein levels showed that the low levels of AXL activity could be associated with the observed response of SNB-19 cells to sunitinib therapy, but not to cediranib or imatinib. These results were not observed in U251 cell line with or without AXL expression. Several issues could explain these distinct drug responses. We have previously shown that the activation profile of RTKs in these two particular cell lines is very similar, however U251 is much more sensitive to sunitinib than SNB-19 [31]. This can in part explain the similarity in the sunitinib response after AXL modulation in U251 cells, since they already exhibit a highly sensitive response. Additionally, these findings can also suggest that sunitinib response is cell line-dependent and that AXL is not the only factor modulating the cells’ responsiveness to this drug. Hence, the main predictive factor for the response to this drug remains to be discovered, as previously pointed out [56].