Abstract
Anti-angiogenic therapy has increased the progression-free survival of many cancer patients but has had little effect on overall survival, even in colon cancer (average 6–8 weeks) due to resistance. The current licensed targeted therapies all inhibit VEGF signalling (Table1). Many mechanisms of resistance to anti-VEGF therapy have been identified that enable cancers to bypass the angiogenic blockade. In addition, over the last decade, there has been increasing evidence for the role that the hypoxic and metabolic responses play in tumour adaptation to anti-angiogenic therapy. The hypoxic tumour response, through the transcription factor hypoxia-inducible factors (HIFs), induces major gene expression, metabolic and phenotypic changes, including increased invasion and metastasis. Pre-clinical studies combining anti-angiogenics with inhibitors of tumour hypoxic and metabolic adaptation have shown great promise, and combination clinical trials have been instigated. Understanding individual patient response and the response timing, given the opposing effects of vascular normalisation versus reduced perfusion seen with anti-angiogenics, provides a further hurdle in the paradigm of personalised therapeutic intervention. Additional approaches for targeting the hypoxic tumour microenvironment are being investigated in pre-clinical and clinical studies that have potential for producing synthetic lethality in combination with anti-angiogenic therapy as a future therapeutic strategy.
Highlights
The original version of the ‘Hallmarks of Cancer’ highlighted the role of neo-angiogenesis in tumour progression (Hanahan & Weinberg, 2000)
Causes of adaptive resistance may include the following: tumour expression and metabolic reprogramming by hypoxia-inducible factors (HIFs) after induction of hypoxia, hypoxia-induced increased invasion or vessel co-option, up-regulation of alternative pro-angiogenic factors by the tumour or by the stromal cells such as tumourassociated macrophages (TAMs) recruited by hypoxia, and increased recruitment of protective pericyte coverage may occur (Bergers & Hanahan, 2008; Welti et al, 2013; Gilkes et al, 2014)
Further to the resistance mechanisms that explain the lack of increase in overall patient survival, a number of clinical factors could reduce the quantifiable effects of anti-angiogenics, including the lack of patient enrichment with predictive biomarkers, patient a 2015 The Authors crossover in clinical trials and subsequent therapies post progression
Summary
The original version of the ‘Hallmarks of Cancer’ highlighted the role of neo-angiogenesis in tumour progression (Hanahan & Weinberg, 2000). Most anti-angiogenic therapies target the key pro-angiogenic factors or their endothelial cell-expressed receptors including VEGF, VEGFR1, 2 and 3, ANG 1 and 2, TIE2, FGF2, FGFR, c-Met and HGF (Bridges & Harris, 2011).
Accepted Version
Published Version
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