**9. Anti-angiogenesis**

Angiogenesis depends on the balance between angiogenic and anti-angiogenic regulators [124]. Among the former, VEGF has been demonstrated to play an essential role in stimulating angiogenesis by promoting the migration, proliferation, and tube formation of endothelial cells. VEGF upregulation has been shown in MNs, suggesting its role as a pro-angiogenic factor responsible for edema formation in these tumors [125–127].

Neoangiogenesis in MNs is regulated by the balance between concentrations of both VEGF and semaphorin 3A (SEMA3A) in the tumor's microenvironment rather than by VEGF alone [125]. Accordingly, neo-angiogenesis would be blocked or stimulated depending on the prevalence of VEGF or SEMA3A with a high ratio between VEGF and SEMA3A as a negative predictor of recurrences [125]. Additionally, VEGF expression in MNs seems to be enhanced by hypoxia-inducible factor 1-alpha [128] and EGF [129], and reduced by dexamethasone.

Caveolin-1 (cav-1), which is a 20-KDa protein mainly expressed by fibroblasts, endothelial cells, myocytes, and adipocytes, seems to be involved in the oncogenesis and progression of several neoplasms, including MNs [130]. Similar to what has been reported in several solid tumors, a significant correlation has been shown between tumor-cell-derived cav-1 and microvascular density (MVD) in MNs [131], suggesting that this protein behaves as a pro-angiogenic factor. Consistent with this hypothesis, cav-1 has been shown to regulate endothelial cell growth and differentiation and to stimulate capillary tubule formation in vitro [132]. Moreover, VEGF-mediated pathological angiogenesis is strikingly reduced in cav-1 knock-out mice [133]. On the other hand, the association between cav-1 expression and MVD may also be related to factors regulating both the MNs neo-angiogenesis and cav-1 expression. Indeed, cav-1 may function as a pro-tumorigenic factor that can stimulate cell proliferation, following its tyrosine-14 phosphorylation by Src kinase [134].

Endothelin-1 (ET-1) has been demonstrated to play a role in the mechanism of meningioma tumorigenesis via the ETA receptor [135]. ET-1 expression/upregulation may contribute to meningioma growth by inducing the formation of new blood vessels. Indeed, a significant correlation has been shown between the expression of ET-1 and that of VEGF or MVD in MNs, in agreement with its proangiogenic action in these tumors.

Following these biological considerations, several angiogenesis inhibitors, such as bevacizumab, sunitinib, and vatalanib, have been evaluated in phase II trials with promising results [136]. The efficacy and safety of bevacizumab were evaluated in grades II and III MNs, finding a PFS6 of 43.8%. In addition, a review of 22 additional case reports for a total of 92 patients revealed a PFS of 16.8 months with 6 months PFS of 73% in those exposed to bevacizumab [137]. A phase II trial designed for all grades recurrent MNs that included 15 patients (15, 22, and 13 grade I, II, and III, respectively) showed stability of the disease in 100% of benign tumors and 82–85% among those with high-grade injuries. In addition, the PFS6, the median PFS, and OS, were 87%, 22.5 months, and 35.6 months for patients with grade I tumors, while this distribution was 77%, 15.3 months, and not reached for

grade II, and 46%, 3.7 months, and 12.4 months for grade III, respectively [138]. There is an ongoing phase II trial evaluating bevacizumab in recurrent and progressive MNs (NCT01125046).

Kaley et al. reported a prospective, multicenter single-arm phase 2 trial that investigated the efficacy of sunitinib, a tyrosine kinase inhibitor that inhibits VEGF and PDGF receptors, which are over-expressed in MNs [139]. Thirty-six patients with grade II and III recurrent or progressive MNs were enrolled. They were heavily pre-treated (median five recurrences) and received sunitinib at 50 mg per day for days 1–28 of a 42-day cycle. The PFS6 was 42%, the median PFS was 5.2 months (95% CI 2.8–8.3), and the median overall survival was 24.6 months (16.5–38.4). Adverse events included four (8%) intratumoral hemorrhages, of which one was fatal, one (2%) grade 4 thrombotic microangiopathy, and one (2%) grade 3 gastrointestinal perforation. MRI perfusion in the exploratory group indicated that sunitinib is an active agent, and expression of VEGFR2 predicted PFS with a median of 1.4 months in VEGFR2-negative patients versus 6.4 months in VEGFR2- positive patients (*p* = 0.005) [139]. More recently, Cardona et al. reported a PFS of 9.1 months (95% CI 6.8–16.8) in a cohort of patients with high-grade MNs treated with sunitinib [140].

### **10. Hormonal therapy**

Evidence suggests that meningioma growth could be hormone dependent because of the female predominance specially after puberty and reproductive years. Additionally, that 30% of the meningiomas are estrogen receptor positive and 70% are progesterone receptor positive [76]. It is also known, that high grade meningiomas express more estrogen receptors whereas benign meningiomas express more progesterone receptors [141]. It is also important to add, that approximately 90% of meningiomas express somatostatin receptors [142]. Therefore, hormonal therapies have been utilized in high grade meningioma treatment.

Due to estrogen receptors low expression, treatment with tamoxifen (estrogen receptor antagonist) has not shown effective results. Additionally, there is not any reports of androgen receptor antagonists in meningiomas [143]. In 1993 Goodwin et al. in a retrospective case series of 21 patients with meningioma treated with tamoxifen, they reported response in only 1 patient and disease progression in 10 patients [144]. Additionally, in a case study from Markwalder et al. a small group of patients with inoperable meningiomas that received tamoxifen were studied and only two patients show radiographical partial response [145].

Currently, due to the lack of evidence of anti-estrogenic agents' effect on meningioma no recommendation is available. Mifepristone is a progesterone receptor inhibitor. In a study published in 1991 by Wolfsberger et al., they used mifepristone as treatment of unresectable meningioma patients, they reported that five patients showed reduction of tumor size on neuroimaging and visual field improvement; in addition, three patients experienced headache relief and improvement in extraocular muscle function. No toxicities were reported [141]. Other study by Lamberts et al. reported stable disease in three patients, tumor size reduction in other three patients and no toxicities were reported [146]. These studies were limited because of the small sample size and tumor stage wasn't described in any of them. Therefore, more studies are needed to conclude the effect of mifeprisotne in high grade meningiomas. Other trial by Ji et al. reported a median PFS of 10 months and a median OS of 31 months in the mifepristone arm of patients with recurrent meningioma [147]. Additionally, in 2006 Grunberg et al. reported a reduction of less than 10% of the tumor area without clinical improvement in eight patients with unresectable meningioma who received mifepristone [148].

Megestrol acetate is an oral progesterone agonist that was used in a small trial. However no response was observed in high grade meningiomas [76]. So far there is no evidence that supports the use of progesterone receptor inhibitors in high grade meningiomas.

Somatostatin is important in regulation and proliferation of normal cells and tumor cells. It is known that meningiomas report the highest frequency of somatostatin receptor expression in brain tumors, especially the sst2A subtype. It is also reported that somatostatin inhibits meningioma growth in vitro in most studies, but increases meningioma proliferation in some [76].

Chamberlan et al. reported that 31% of patients demonstrated a partial radiographic response and 44% achieved PFS at 6 months with minimal toxicity in patients treated with octreotide (a somatostatin agonist). Furthermore, one-third of patients showed stable disease after treatment [149]. Therefore, somatostatin analogs are recommended for systemic treatment of unresectable or radiorefractory relapsed meningiomas [150]. The phase II CEVOREM trial explored the efficacy of the combination of everolimus (an mTOR inhibitor) and octreotide in high grade meningiomas treatment. The trial reported that the 6-month progression-free survival rate was 55% and the 6-month overall survival was 90% and 12-month survival rate was 90%. Additionally, a decrease of more than 50% was observed in the growth rate at 3 months in 78% of the tumors. That happens because, octreotide suppressed AKT activation during everolimus treatment and synergistically reduced expression of downstream proteins [121]. The previous results suggest that the combination of everolimus and octreotide could be a very good option to treat high grade meningiomas, however more studies are needed. In other phase II trial by Johnson et al. only 2 of 12 high grade cases experience long progression-free intervals, but at the end all patients experienced disease progression with median time of 17 weeks; a median survival 2.7 years was reported and octreotide was well-tolerated [151]. Additionally, an in- vitro study by Graillon et al. reported a significant anti-proliferative effects octreotide, but no apoptotic response [152].

Parasoreotide (SOM230C) is an intramuscularly long-acting somatostatin analogue. In the phase II trial by Norden et al., they reported that pasireotide has limited activity in recurrent meningiomas, a PFS-6 of 17% and median PFS of 15 weeks were reported. Furthermore, expression of somatostatin receptor was predictive of favorable response. However the findings in this trial require further investigation [153]. These findings are promising, nevertheless, larger randomized studies should be conducted to make a solid conclusion.

Growth hormone is secreted by the pituitary gland, and it induces production of insulin-like growth factor-I (IGF-I-), these hormones influence normal growth and metabolism [73]. There is existent evidence that reports abuntant growth hormone receptors expression in meningioma cells. There is also reported that inhibition of these receptors represents a decreased meningioma cell proliferation [154]. McCutcheon et al. reported that administration of pegvisomant reduces meningioma growth and in some cases causes tumor regression. Pegvisomant blocks growth hormone receptors and induces downregulation of the GH/IGF-I axis [155]. In other study, Puduvalli et al. reported that fenretinide, a synthetic retinoid, induced apoptosis in meningioma primary cells tested, it also increases levels of the death receptor DR5 and causes mitochondrial membrane depolarization. They also reported eradication of IGF-I proliferation in the meningioma cells [156].

Finally, insulin-like growth factor-II acts like IGF-I. In multiple studies have reported that the invasiveness of meningiomas is correlated to levels of IGF-II expression [157]. However, several studies are needed to establish IGF-II blockade could be an option to treat patients with meningiomas. These results provide

preliminary evidence, but further studies are needed to explore these options as treatment against meningioma.
