For example, the ongoing phase III MERiDIAN trial is evaluating paclitaxel with or without bevacizumab in patients with metastatic breast cancer, stratified by pretreatment plasma VEGF level (101). Future directions A wealth of evidence has been published in the

past decade collectively affirming that VEGF-axis directed therapies confer clinical benefit along the continuum of care for patients with metastatic CRC (11,50,103). Within the past year, novel approaches to targeting agiogenesis have also yielded benefit in phase III trials with regorafenib and ziv-aflibercept. While the clinical effect of anti-VEGF Inhibitors,research,lifescience,medical targeted therapies may be well established in this population, not all patients experience benefit. Furthermore, patients inevitably progress while on anti-selleck chemical angiogenic treatment, and the ultimate improvement in overall survival can be modest. There are numerous complementary Inhibitors,research,lifescience,medical angiogenic pathways, which may be deregulated or circumvent the mechanism of action for current targeted agents. Alternative mechanisms of tumor vessel formation may explain the various clinical phenotypes of initial treatment nonresponse or inducible resistance to anti-angiogenesis therapies. Rational combinations of anti-angiogenic agents are needed Inhibitors,research,lifescience,medical to overcome resistance mechanisms and exploit alternative pathways of tumor blood vessel formation. Both “vertical” (targeting multiple levels of the same pathway)

and “horizontal” strategies (covering multiple different angiogenic pathways) have been attempted in several different tumor types and reviewed recently (104). Inhibitors,research,lifescience,medical Although several of these combinations have demonstrated encouraging anti-tumor activity, the unfavorable side effect profiles have proven to be difficult to overcome. Future strategies involving non-overlapping toxicity profiles of anti-angiogenic agents and dosing adjustments based on pharmacokinetic/pharmacodynamics data should be employed to optimize tolerability and balance anti-tumor effect. Lastly, routine incorporation of predictive biomarkers is imperative to tailor patient selection and increase therapeutic efficacy of Inhibitors,research,lifescience,medical novel drug combinations. Conclusions

Mechanisms of resistance to anti-angiogenic from therapies can broadly be categorized by involvement of the VEGF-axis, stromal cell interaction, and non-VEGF pathways. These mechanisms rely on a number of distinct but interrelated paracrine signaling factors and intracellular cascades. Clinical approaches targeting multiple pathways involving VEGFC, VEGFD, Tie2-Ang2, Dll4-Notch, and TGF-β may have greater benefit than monotherapies blocking VEGFA or VEGFR2 signaling alone, for example. Numerous clinical trials are ongoing to evaluate targeted therapies with specificity for these resistance mechanisms. Incorporation of biomarkers in future clinical trials will be critical to the development of next generation anti-angiogenic regimens.