**6. Anti-angiogenic treatments**

The pathological angiogenesis of glioblastomas is a hallmark of the disease process, with multiple mechanisms hypothesized, including the transdifferentiation of tumor cells into endothelial cells, vascular mimicry, and vessel co-opting [27]. Tumor angiogenesis has been shown to be associated with the recruitment of hematopoietic and circulating precursor cells [28].

The VEGF (vascular endothelial growth factor) pathway is highly expressed in glioma angiogenesis with overexpression of VEGF-A. There have been a multitude of factors identified to propagate and inhibit the VEGF pathway, including hypoxia inducible angio‐ genic factors, and endogenous factors like placenta growth factor. The anti-VEGF/VEGR compounds inhibit the proliferation of endothelial cells and neoangeogenesis, with a corre‐ sponding decrease in the permeability of the blood–brain barrier. Within 48 hours of anti-VEGF-A therapy with bevacizumab (Avastin®), there is decreased contrast enhancement, which may be misleading and hence to be read as a pseudoresponse. In contrast the T2/FLAIR progression is seen on serial radiological imaging, which has been postulated to be a nonan‐ giogenic invasive growth pattern and the likelihood of T2 progress predicting subsequent T1 and in turn tumor progression [14, 29]. Hence, our above discussion on RANO criteria will be called upon here to be borne in mind while analysing the imaging characteristics of patients on antiangiogenic therapy.

Bevacizumab (Avastin®) is the antibody to VEGF-A which has been utilized in Phase I, II and III trials to investigate its role in both newly diagnosed and recurrent glioblastoma. Of note, the AVAglio (Avastin® in Glioblastoma) study was undertaken in a for newly diagnosed glioblastoma patients in a randomized manner (bevacizumab versus placebo) with doubleblinding. Postsurgical resection, the patients were commenced on the Stupp protocol (concurrent radiotherapy 2 Gy 5 days a week and temozolomide 75 mg/kg) in combination with intravenous bevacizumab 10 mg/kg (or placebo) every fortnight. After a 28-day treat‐ ment break, the patients were commenced on a maintenance dose of temozolomide (150– 200 mg/kg) and fortnightly intravenous bevacizumab (10 mg/kg) or placebo for 6 weeks. This was followed by bevacizumab (10 mg/kg) every three weeks as monotherapy. The patients were assessed clinically at predetermined, regular time points. The results of the AVAglio study echoed those of the Phase III Radiation Therapy Oncology Group (RTOG-0825) with

both studies showing a trend toward increase in progression free survival but no significant difference in overall survival [30, 31]. It is important to note, as with other previous studies, the adverse effects of the bevacizumab group were noted to be higher than in the placebo group and noted to include hypertension, proteinuria, and (arterial) thromboembolism. The question arises regarding the failure of progression-free survival to overall survival, and it is postulat‐ ed there are possible escape mechanisms in the anti-VEGF pathway and treatment which results in an aggressive, recurrent tumor [29]. The crossover seen in the AVAglio trial may have had considerable impact on the true survival data, and in comparison the BELOB [singleagent bevacizumab or lomustine versus a combination of bevacizumab plus lomustine in patients with recurrent glioblastoma] Phase II trial had virtual exclusion of patient cross-over to the bevacizumab arm, and also surprisingly there were fewer of the above-described adverse effects of bevacizumab [8]. Additionally, while the predictive value of MGMT promoter methylation and treatment with temozolomide is well known [32], the prognostic signifi‐ cance in association with anti-angiogenic or other chemotherapeutic agents is less well understood. Thus, in increasingly more of the recent trials, the MGMT status is included and required to allow the study of temozolomide-free arms [29]. It has also been noted in prelimi‐ nary clinical trial data that angiopoietin-1/-2 may potentially destabilize vessel and when used in association with VEGF-A, angiogenic synergy is exhibited and further clinical trials being undertaken with this hypothesis in mind [27].

Moreover, genetic expression data of glioblastoma subgroups has been recently retrospec‐ tively explored using the AVAglio trial data. To recap, the Cancer Genome Atlas subdivides the heterogeneous entity of glioblastoma into the following subtypes: proneural, classical, and mesenchymal (with the previously known neural subtype possibly being an artefact) [33]. In this most recent study, the addition of bevacizumab is shown to be associated with in‐ creased overall survival in the proneural subtype GBM, with naïve, nonmutated IDH [34]. This report came as a welcome surprise to the Neuro-Oncology community, as patients with the proneural subtype lacking IDH mutations have historically a poorer survival compared with proneural subtype with IDH mutations.

We also note that on preclinical GBM models, it has been shown that bevacizumab induces hypoxia in treated tumors, which is accompanied by increased glycolytic activity and tumor invasiveness [35]. This is an area for further research to exploit in view of anaerobic glycolyt‐ ic dependency of glioblastomas and is discussed below in subsection 8 in further detail.
