Evolution of Angiogenesis Biomarkers: What the Future Holds
By Jennifer Levin Carter MD, MPH
Since Dr. Judah Folkman’s discovery of the process of angiogenesis, there has been rapid growth in the understanding of its regulation as well as the targeting of its underlying mechanisms.1 To date, there are 13 agents with anti-angiogenic properties approved for the treatment of cancer in the US (Table 1). Despite this progress, the development of angiogenesis inhibitors has experienced multiple setbacks, and continues to be a challenging area in cancer therapeutics. This problem can be attributed to three major causes:
- Short-term efficacy or inconsistent benefit of existing therapies. This lack of efficacy has led to FDA approvals for these agents for narrow indications, thus limiting their commercial and clinical potential.
- Reliable predictive biomarkers for identifying patients who are most likely to respond are not currently available.
- Many anti-angiogenesis agents are associated with the risk of serious adverse events, including bleeding and severe hypertension.2
In order to overcome these barriers, it is critical to define the appropriate use of anti-angiogenesis inhibitors, in addition to developing new anti-angiogenic drugs with improved toxicity profiles. Finding reliable biomarkers that predict efficacy and/or resistance will be necessary if angiogenesis inhibitors are to find more widespread applicability in the treatment of cancer.
Evidence for the Potential of Biomarkers of Response to
Biomarkers have been traditionally classified into 3 categories:
- prognostic—these predict patient outcomes regardless of treatment
- predictive—these predict how a patient will respond to a specific treatment, and can be predictive of sensitivity or of resistance
- pharmacodynamic—these monitor known signaling pathways downstream of a given drug target, to assess in-tissue on target effects
Angiogenesis biomarkers include the angiogenic growth factors—mainly VEGF and its receptors, soluble growth factor receptors, key players in other angiogenic pathways, as well as circulating endothelial cells (CECs) and their progenitors. Current research is attempting to evaluate the prospective applications of each of these types of biomarkers.
VEGF, VEGF Receptors, and Related Growth Factors:
VEGF, which attracts tumor endothelial cells, has been the most studied and frequently targeted angiogenic growth factor. In some cancer types, VEGF levels were found to be up-regulated in tumors and in circulation in cancer patients undergoing anti-VEGF treatment, and were reported to correlate with improved clinical outcomes. For instance, in the AVADO Phase 3 trial of first-line bevacizumab (Avastin) in combination with docetaxel for HER2-negative metastatic breast cancer, high plasma VEGFA and VEGFR2 levels were associated with greater progression-free survival.3
In contrast, in other cancer types, VEGFR1 levels (as measured in the tumor and in circulation) correlated inversely with bevacizumab treatment outcomes. The AViTA trial in 154 pancreatic cancer patients revealed a single nucleotide polymorphism (SNP) on VEGFR1 that correlated with increased VEGFR1 expression and poor outcome with bevacizumab treatment.4
Furthermore, Niers et al., proposed that the “true” circulating VEGF levels in most cancer patients are low, and that the recorded high levels are, in fact, due to VEGF release from activated platelets.5
Other VEGF family growth factors have also shown changes in expression in cancer patients. Circulating levels of placental growth factor (PlGF) in plasma have been reported by multiple research groups to consistently increase in response to anti-VEGF therapy.6 Thus, PlGF has the potential to serve as a pharmacodynamic biomarker and may provide clues to mechanisms of therapy-resistance.
Evolution of Angiogenesis Biomarkers:
What the Future Holds (continued)
Soluble Growth Factor Receptors:
In addition to being embedded in the plasma membranes of cells, growth factor receptors are also present in circulation. The circulating levels of these soluble receptors have been proposed to serve as biomarkers for anti-angiogenic treatment. Significantly attenuated soluble VEGFR-2 (sVEGFR-2) levels in plasma have been reported in response to anti-VEGFR tyrosine kinase inhibitors in several studies.6
In a neoadjuvant study of rectal cancers treated with bevacizumab and chemoradiation, Duda et al. found that low pre-treatment levels of circulating sVEGFR-1, an endogenous blocker of VEGF, correlated both with response to treatment and with treatment toxicity. They suggest that sVEGFR1 is potentially a predictive biomarker in this disease.7
Other Molecular Biomarkers:
Apart from VEGF receptors, there are other soluble factors in circulation that may be exploited as biomarkers. The Ang-TIE pathway of angiogenesis includes cytokines that help maintain the endothelial cell lining of blood vessels.8 Angiopoeitin and its receptor, both of which are components of the Ang-TIE pathway, have been identified to be potentially useful.
For example, in a Phase 3 trial of gemcitabine (Gemzar) with bevacizumab or placebo in pancreatic cancer patients, low levels of Ang-2 and the chemokine, SDF1, as well as high levels of VEGF-D correlated with a lack of benefit from bevacizumab treatment.9 These data support the potential application of Ang2, SDF1, and VEGF-D as predictive biomarkers for bevacizumab treatment in pancreatic cancer. Similar data with these and other potential biomarkers have been demonstrated in other cancer types, including renal cancer.10
CECs and CEPCs:
Circulating endothelial cells (CECs) released from the tumor vasculature and circulating endothelial progenitor cells (CEPCs) from the bone marrow have also been investigated as angiogenic biomarkers in preclinical as well as clinical studies.11 Changes in the number of CECs and CEPCs in the peripheral blood of patients undergoing treatment have been evaluated. However, the results have been inconsistent.12
Challenges and Missing Links:
The above-mentioned studies are only a few examples of the existing data suggesting that biomarkers may help to better guide the use of anti-angiogenic drugs and thereby result in greater clinical efficacy. However, these data are collectively inconsistent and thus, insufficient to permit the routine use of any of these biomarkers clinically, particularly for predictive applications.
Another major challenge hindering clinical progress is that the currently approved tyrosine kinase inhibitors (TKIs) that have anti-angiogenic properties target multiple kinases, which are affected in a distinct manner. For instance, sunitinib (Sutent) is capable of inhibiting VEGFR-1, -2, -3, PDGFRA, and PDGFRB. Thus, it is difficult to correlate its effects with the inhibition of a particular target kinase.2 To further complicate this issue, TKIs and other anti-angiogenic agents are generally used in combination with chemotherapy, making it challenging to attribute an effect to a particular agent-biomarker pair.
Additionally, variability in testing may have an impact on data interpretation. These include factors related to2, 12:
- tissue sampling, which can lead to variable results due to tumor heterogeneity,
- the type of clinical specimen, e.g. serum, plasma, or tissue,
- the availability of concurrent biomarker test results and patient outcome data, and
- the selection of the appropriate endpoints to determine the efficacy of the biomarker as predictive.
The existing data around angiogenic molecules in different cancer types, coupled with the discovery of novel biomarkers and the strategic design of future validation trials, may help address some of these issues.12
Leveraging Vascular Normalization and Immune Modulation:
More than a decade ago, the concept of tumor vasculature normalization of angiogenesis emerged13. It was proposed that certain anti-angiogenic agents could temporarily normalize the leaky structure of tumor vessels, thus improving tumor blood flow, oxygenation, and drug delivery of chemotherapeutic agents to the tumor cells. The transient period was referred to as the “normalization window”, in which combined chemotherapy or radiation therapy demonstrated better response in the presence of angiogenesis inhibitors.14
For instance, treatment with vandetanib (Caprelsa) (a VEGFR TKI that promotes vascular normalization) in combination with the chemotherapeutic agent docetaxel significantly improved PFS in advanced non-small cell lung cancer patients compared with docetaxel alone (median PFS was 4 months versus 3.2 months).15 Another study showed that radiation treatment during vascular normalization upon sunitinib treatment leads to a synergistic delay in tumor growth in mice.16
Mechanistically, vascular normalization transiently reduces the vessel diameter and permeability, and thins the abnormally thickened basement membrane. Therefore, it was postulated that the extent of normalization could be predictive of response to anti-angiogenic therapy.14
This hypothesis was strengthened by pre-clinical data showing that VEGFR2 blockade led to increased pericyte recruitment to tumor blood vessels in an orthotopic brain tumor model via upregulation of Ang1. This also led to thinning of the abnormally thickened vascular basement membrane via matrix metalloproteinase (MMP) activation.14 Further clinical research showed that evaluation of vascular normalization indicators one-day post-anti-VEGF therapy with cediranib was predictive of response in patients with recurrent glioblastoma.17
Vascular normalization may also play a critical role in the behavior of the immune environment of tumors. In 2012, Huang et al. demonstrated in a pre-clinical animal model that subclinical doses of an anti-VEGFR2 antibody cause vascular normalization and are capable of reprogramming the immune component of the tumor microenvironment to relieve their immunosuppressive effects. Consequently, the antibody potentiated the anti-tumor action of cancer vaccine therapy.18
Evolution of Angiogenesis Biomarkers:
What the Future Holds (continued)
Furthermore, the results from a Phase 2 trial presented at ASCO 2014 in patients with ovarian cancer, who had not received prior anti-angiogenic therapy in the recurrent setting or prior PARP inhibitors, demonstrated a median PFS of 17.7 months in those treated with the combination of cediranib and the PARP inhibitor, olaparib, in comparison to 9 months with olaparib alone.19 Blood samples from a subset of 13 patients in this trial were subjected to biomarker analyses at baseline and day 3 of treatment. CECs, CEPCs, and plasma cytokine (IL-6, IL-8, VEGF and soluble VEGFR2) levels were measured. The olaparib-cediranib combination treatment caused a greater decrease in IL-8 and a median 3.5 fold increase in CECs when compared to treatment with olaparib alone; the increase in CECs was significantly correlated with PFS>6 months in 6 patients on the combination arm.
These highly promising results make cediranib an interesting candidate for ovarian cancer treatment, and suggest that the identification of biomarkers to identify appropriate patients for treatment and to follow their responses may improve patient care. It has also been proposed that such strategies in other types of cancer, including non-small cell lung cancer and colorectal cancer, may offer predictive power to identify patients that are most likely to respond to treatment.20, 21
What the Future Holds:
For decades, the role of angiogenesis in cancer has intrigued researchers and clinicians. There have been some significant breakthroughs with drugs like bevacizumab and some of the TKIs that have anti-angiogenic properties, but a means by which to predict which patients will respond remains elusive. Researchers have discovered novel mechanisms of manipulating the process of angiogenesis via the discovery of connections between angiogenesis inhibition and immune modulation; these mechanisms can be further exploited to discover effective combination therapies.
In addition, new biomarkers have been identified that may predict efficacy of therapy. However, there are likely multiple biomarkers and complex pathways involved in angiogenesis, and it will require many more years of research to elucidate these complex mechanisms. Moreover, it will be essential to better understand the mechanism of action and toxicities of anti-angiogenesis agents and identify sub-populations of patients across cancer types that will respond to therapy with tolerable toxicity.
In recent years, new angiogenesis biomarkers have been discovered, and are now the focus of a growing number of research efforts. There has been much progress both in biomarker discovery and therapeutic development. Additional study of the interface between angiogenesis and the immune microenvironment represents a particularly promising direction. Together, these novel therapeutic approaches and the advanced understanding of the complexity of anti-angiogenic biomarkers will likely transform the way we target angiogenesis, and help deliver more personalized approaches to the use of these drugs in cancer treatment.
About the Contributor
Chief Medical Officer and Founder, N-of-One, Jennifer Carter, MD has been an early driver in shaping and delivering personalized medicine for oncologists at the point of care. Today, N-of-One is the leading provider of molecular interpretation and therapeutic strategies for oncology.
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- Miles DW, de Haas SL, Dirix LY, Romieu G, Chan A, Pivot X, et al. Biomarker results from the AVADO phase 3 trial of first-line bevacizumab plus docetaxel for HER2-negative metastatic breast cancer. Br J Cancer. 2013 Mar 19;108(5):1052-60.
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