Timing (and biology) are everything.

The initial excitement over the dramatic radiographic responses often seen with bevacizumab in patients with recurrent glioblastoma (GBM) has been tempered by the reality that not everyone responds to treatment, treatment responses are often short, and treatment has yet to be associated with clear improvements in overall survival. These sobering observations have led to many attempts to add a second agent to bevacizumab in order to improve upon its clinical activity. Although a number of studies have evaluated the addition of a second antiangiogenic agent in combination with bevacizumab, the radiographic and pathological pattern of tumor progression through bevacizumab appears to be more invasive than angiogenic.1 Thus, there have been a number of attempts to add either new agents with anti-invasion properties or cytotoxic agents. The manuscript by Rahman et al in this issue of Neuro-Oncology suggests that there is little benefit to adding lomustine, the standard drug for recurrent GBM, to patients with progressive disease on bevacizumab. Although this study was a retrospective chart review rather than a prospective trial, the data appear highly convincing.2 To date, there is a relatively extensive experience with 3 cytotoxic agents in combination with bevacizumab: irinotecan, carboplatin, and lomustine/carmustine. Irinotecan has been the most commonly used agent, with bevacizumab based largely on the historical accident that the combination had already been approved for use in advanced colon cancer when bevacizumab was first explored for GBM. Nevertheless, the 2 studies that formed the basis for FDA approval of bevacizumab in recurrent GBM failed to demonstrate any additional benefit of adding irinotecan to bevacizumab, a finding consistent with the fact that multicenter phase II trials of irinotecan failed to show much single-agent activity.3–6 Carboplatin has also been commonly used in GBM as a non-nitrosourea alkylating agent, but recent data do not suggest that carboplatin has much activity, either as a single agent or in combination with bevacizumab.7 So how about lomustine? Clearly, the nitrosoureas (lomustine, carmustine) have antitumor activity in this disease as demonstrated by a series of phase III trials in the 1970s and 1980s in the upfront setting. Despite the theoretical concern of alkylating agent cross-resistance in patients who were previously treated with temozolomide, lomustine-treated patients fared better in at least 2 recent phase III trials of new targeted agents in which lomustine was used as the control, suggesting a survival advantage in recurrent GBM as well as newly diagnosed GBM.8,9 Consistent with these observations, Taal et al recently presented data from a randomized phase II trial of recurrent GBM demonstrating that lomustine extends survival when used in combination with bevacizumab from the beginning of treatment compared with bevacizumab alone in patients with recurrent GBM.10 So what is the major difference between these positive trials of lomustine and the negative data presented by Rahman? Principally, the difference between the trials was the use of lomustine in patients who were previously treated with bevacizumab and are now refractory, compared with those not yet exposed to bevacizumab. In considering why lomustine might be less active in bevacizumab-resistant patients, one needs to consider both host factors and tumor autonomous factors. Host factors that could explain this paradox include the possibility that patients are sicker and less able to tolerate lomustine following bevacizumab failure, although the data from Rahman and others do not support this supposition. Another host factor could be related to perturbed drug delivery through a bevacizumab-altered blood-brain barrier (BBB). Although certainly possible for some BBB-impermeable drugs, lomustine's ability to readily pass through an intact BBB makes this explanation less likely. The one host factor that does appear to be modified in bevacizumab-resistant gliomas is the intratumoral accumulation of a specific subpopulation of (eg. Tie-2 expressing) monocyte/macrophages.11 Although these cells have been ascribed the properties of mediating antiangiogenesis resistance and enhanced glioma invasiveness, little is truly understood about the mechanism by which these cells got there or what they are doing to the tumor. In contrast to host factors, there are more experimental data on glioma cell autonomous changes induced by bevacizumab or anti-VEGF therapy. For example, anti-VEGF–mediated intratumoral hypoxia induces the transcription factors hypoxia-induced factor-1 and -2 (HIF-1/2). Upregulation of HIF-1/2 has been shown to drive nontumorigenic GBM cells toward a less differentiated tumorigenic or glioma stem cell (GSC) state.12 It has been demonstrated that the cancer stem cell state is generally more resistant to genotoxic insults including radiation and chemotherapy such as lomustine. In addition to hypoxia-mediated changes, bevacizumab can directly change the biology of GBM cells through hypoxia-independent VEGF inhibition. For example, it has been demonstrated that VEGF induces the recruitment of the protein tyrosine phosphatase 1b (PTP1B) to the MET/VEGFR2 complex, thereby suppressing MET signaling and tumor cell invasion. Relief of this suppression through VEGF inhibition (eg, bevacizumab) induces MET signaling and results in enhanced migration/invasion (commonly seen clinically in bevacizumab-resistant GBM) as well as epithelial-to-mesenchymal transition (EMT) characterized by changes in T- and N-cadherin and integrin b1 expression, increased expression of metalloproteinases (eg, MMP2, MMP9), and mesenchymal cellular features potentially including enhanced cytotoxic drug resistance.13,14 Thus, treatment of GBM with bevacizumab (and possibly other VEGF inhibitors) appears to change the biology of GBM, leading not only to bevacizumab resistance but also potentially to a pan-resistant phenotype. With this realization, it is not hard to understand why the sequencing of bevacizumab administration relative to other therapeutic agents could be of critical importance. Future research will be needed to understand the mechanistic basis for the VEGF inhibition-induced, EMT-like effect and to either find ways of interfering with it in order to maintain sensitivity to anti-VEGF therapies and cytotoxics or find therapeutic strategies that will selectively target the invasive/mesenchymal biology that results from bevacizumab treatment. We will also need a better understanding of the role of tumor-infiltrating monocytes/macrophages to see if this is an epiphenomenon of bevacizumab or a relevant biological axis worth targeting. Although it is somewhat discouraging that our newest, and possibly most effective, treatment for recurrent GBM results in a more refractory and invasive tumor, it is promising that for the first time we have found a way to significantly perturb the natural history of GBM, forcing this previously unstoppable train to at least redirect itself. Hopefully with continued research we will one day be able to derail it.