**7. Potential mechanisms of synergy between bevacizumab and chemotherapy**

Early phase clinical trials have demonstrated that bevacizumab, in combination with systemic cytotoxic chemotherapy, can potentiate treatment efficacy when given concomitantly [71]. In fact, in most clinical settings, with the exception of ovarian cancer where bevacizumab has been observed to have single agent activity [72], bevacizumab has only shown significant activity when it is combined with cytotoxic chemotherapy and the same is true for aflibercept [21].

It has been well-established that the tumour vasculature is dysfunctional and leaky, resulting in enhanced interstitial fluid pressure and thus preventing effective delivery of chemotherapy [73]. Evidence from preclinical studies showed that bevacizumab can 'normalise' the chaotic tumour vasculature, achieving reduced vessel tortuosity, reduced leakiness and reduced interstitial fluid pressure. Based on these studies, it was proposed that bevacizumab works in combination with chemotherapy to improve chemotherapy delivery [71, 73], which is now a widely accepted notion amongst many clinicians.

However, this concept is also highly controversial, with some work even refuting the normalisation hypothesis. For example, one group demonstrated that bevacizumab persistently reduced both tumour perfusion and chemotherapy delivery when NSCLC patients were treated with bevacizumab-containing chemotherapy [74]. Therefore, other potential explanations for synergy between bevacizumab and chemotherapy must be considered. Current alternative theories based mostly on preclinical data include: (1) direct synergy between the anti-angiogenic effects of bevacizumab and potential anti-angiogenic effects of chemotherapy [75], (2) targeting of VEGF signalling directly in cancer cells by bevacizumab [21], (3) chemotherapy may inhibit resistance to bevacizumab, because chemotherapy suppresses the tumour recruitment of myeloid cells that have been implicated in resistance to bevacizumab [76], (4) bevacizumab may prevent tumour rebound that may occur during breaks in chemotherapy [76].

It should be noted that vessel normalisation facilitated by anti-angiogenic agents may provide therapeutic benefit through other mechanisms, which are independent of chemotherapy delivery. For example, in glioblastoma patients, vessel normalisation induced by single agent VEGF-targeted therapy may prolong survival due to other effects, such as oedema control or improved tumour oxygenation [77].

There are two other curious observations that have yet to be properly explained. Firstly, the synergistic effect of adding bevacizumab to chemotherapy does not occur in all tumour types. For example, the addition of bevacizumab does not lead to improvements in outcome in advanced breast cancer [78]. Secondly, VEGFR TKIs show single agent activity without the need for co-administration of chemotherapy [21].

Recent insight into these two curious observations has been reported. A study examining both clinical and mouse tumour tissue specimens demonstrated that tumour types utilising a vasculature surrounded by a well-developed stroma (e.g. mCRC, NSCLC) respond better to bevacizumab when it is added to chemotherapy as opposed to tumour types that utilise a vasculature without a well-developed intervening stromal component (e.g. mRCC, PNET) which respond better to VEGF TKIs alone [79]. This suggests that tumour cell interactions with different stromal components may influence response to different anti-angiogenic agents and how they synergise with concomitant drugs. However, there is still much work to be done in order to understand the mechanisms involved.
