Early expectations of antitumor activity across all cancer types have been tempered by clinical disappointment in many cases and only modest efficacy in others. Nonetheless, VEGF signaling continues to present a promising target.
As a key regulator of tumor angiogenesis, the vascular endothelial growth factor (VEGF) signaling pathway has been the focus of efforts to treat cancer for more than a decade. The FDA has approved 10 VEGF-targeting agents since 2004, when bevacizumab (Avastin) became the poster child of antiangiogenic therapy. Bevacizumab now carries indications in six different tumor types.
Early expectations of antitumor activity across all cancer types have been tempered by clinical disappointment in many cases and only modest efficacy in others, hindered in part by a lack of validated biomarkers and the intricacies of the tumor vasculature that these drugs target.
Nonetheless, VEGF signaling continues to present a promising target. Clinical experience and an improved understanding of the nuances of this pathway have yielded a growing armory of VEGF-targeting drugs with novel mechanisms of action and expanded indications (Table).The origins of tumor angiogenesis can be traced to the seminal observations of physician-scientist Judah Folkman, MD, a Giant of Cancer Care award winner, and his colleagues at Harvard Medical School. Although the concept initially met with skepticism, his theory that tumors need to establish a new vasculature to grow beyond the boundaries of the existing blood vessels has set the stage for decades of research in this field.
aTrial information from NIH Clinical Trials Registry, www.ClinicalTrials.gov. Some phase III trials are ongoing but not enrolling new participants.
bBevacizumab approvals in mCRC were given for first-line, second-line, and retreatment after progression, respectively.
c Ramucirumab is approved separately for single agent and combination therapy in gastric/GEJ cancers. AML indicates acute myeloid leukemia; CRC, colorectal cancer; DTC, differentiated thyroid carcinoma; GEJ, gastroesophageal junction; GBM, glioblastoma; GIST, gastrointestinal stromal tumors; HCC, hepatocellular carcinoma; MTC, medullary thyroid cancer; NSCLC, non-small cell lung cancer; pNET, pancreatic neuroendocrine tumor; RCC, renal cell carcinoma; STS, soft tissue sarcoma.
Beyond the establishment of the primitive vascular network (vasculogenesis) in embryonic development, new blood vessels are typically shaped from preexisting ones via angiogenesis during physiological processes like wound healing. Normally, angiogenesis is tightly regulated by a delicate balance between pro- and anti-angiogenic signals.
Observing that tumors failed to grow beyond a certain size without the supporting blood vessels to provide oxygen and nutrients, Folkman proposed that cancer cells secreted some kind of angiogenic factor to stimulate vascularization to support their continued growth. Ultimately, numerous angiogenic factors have since been identified. Central among them are VEGFs, a family of secreted growth factors that includes VEGF-A, B, C, D and placental growth factor. They bind to three VEGF receptors (VEGFR-1, 2, and 3) on the surface of endothelial cells, the major cell type involved in the formation of new blood vessels. VEGF-A is the most extensively studied isoform and VEGFR-2 is the principal receptor through which its angiogenic effects are mediated.
VEGF-A stimulation triggers VEGFR-2 phosphorylation and the subsequent recruitment and activation of a number of downstream proteins that trigger a cascade of different signaling pathways involved in endothelial cell proliferation, and migration and vascular permeability. It is thought that the oxygen-poor environment of the tumor stimulates the expression of high levels of VEGF on the surface of tumor cells, as well as on normal cells in the surrounding microenvironment.
Fresh Approaches With Bevacizumab
Other factors can lead to elevated VEGF expression, including many of the genetic drivers of cancer. For example, a key genetic driver of renal cell carcinoma (RCC) is loss of the tumor suppressor gene VHL. Loss of VHL increases expression of the hypoxia inducible factor (HIF) proteins, which in turn are key regulators of VEGF expression. Ultimately, the high levels of VEGF contribute to an angiogenic switch that tips the balance toward proangiogenic signaling by activating the VEGF pathway in endothelial cells to promote their angiogenic functions, and creates characteristically tangled and leaky tumor blood vessels.Bevacizumab, a monoclonal antibody targeting VEGF-A, gained its first approval in the United States for metastatic colorectal cancer (CRC) in 2004. Several randomized clinical trials confirmed the efficacy of bevacizumab in combination with chemotherapy (a fluoropyrimidine plus irinotecan or oxaliplatin), which has since become standard of care in the frontline setting. Bevacizumab was subsequently approved in the second-line setting in combination with fluorouracil- based chemotherapy in 2006 and with fluoropyrimidine-based chemotherapy in 2013. More recently, a number of trials have been evaluating the optimal backbone therapy to be used in combination with bevacizumab in the frontline metastatic CRC setting.
The results of the MAVERICC trial, presented at the 2016 Gastrointestinal Cancer Symposium, suggested that there was no difference in progression-free survival (PFS) among patients treated with bevacizumab in combination with modified leucovorin/5-fluorouracil/ oxaliplatin (mFOLFOX6) or leucovorin/5-fluorouracil/ irinotecan (FOLFIRI). The exception was patients with left-sided tumors who did better with the FOLFIRI/bevacizumab combination.
In 2015, updated results of the phase III TRIBE trial demonstrated that the use of FOLFOXIRI (fluorouracil, leucovorin, oxaliplatin, and irinotecan) with bevacizumab improved survival compared with FOLFIRI plus bevacizumab. Interim results from the phase II STEAM trial, presented at the 2016 Gastrointestinal Cancer Symposium, appeared to confirm these findings when bevacizumab was given concurrently with chemotherapy.
Bevacizumab also has been approved for a number of other tumor types, most recently in combination with chemotherapy for the treatment of ovarian cancer and cervical cancer, representing the first drug approvals for these tumor types in close to a decade.
Approval in ovarian cancer was based on the phase III AURELIA trial, which found that bevacizumab significantly improved PFS when added to paclitaxel, pegylated liposomal doxorubicin, or topotecan, compared with chemotherapy alone, although no statistically significant improvement in overall survival (OS) was seen. In patients with persistent, recurrent, or metastatic cervical cancer, bevacizumab improved OS by almost 4 months when combined with paclitaxel plus either cisplatin or topotecan, in the GOG 240 study.
Bevacizumab is also a first-line treatment option for patients with non—small cell lung cancer (NSCLC) and the results of trials examining its use in the second- and third-line are eagerly awaited. The combination of bevacizumab with erlotinib has also shown promise, improving median PFS from 9.7 months to 16 months, in patients with EGFR mutation-positive NSCLC compared with erlotinib alone. The phase II BELIEF trial evaluated the same combination in patients with the T790M gatekeeper mutation and observed a similar PFS rate. Phase III trials examining the potential of this combination as first-line therapy are ongoing.
More Agents Developed
Bevacizumab has not always proved effective and has a particularly checkered record in breast cancer. The FDA granted an accelerated approval in combination with paclitaxel in metastatic HER2-negative disease in 2008, but revoked the indication several years later when the benefits could not be confirmed in subsequent trials. It continues to be evaluated in this setting, and a number of recent trials have suggested a benefit to bevacizumab in HER2-negative metastatic disease, although its role remains unclear. Bevacizumab also was evaluated in gastric cancer, but the phase III AVAGAST and AVATAR trials failed to show improvements in OS.Despite approvals in numerous different tumor types, bevacizumab did not provide the singleagent success that was anticipated in the majority of cases. In the years since bevacizumab’s approval, a number of other VEGF-targeting agents with different mechanisms of action have emerged that have had greater success as monotherapy. These include a growing list of small-molecule tyrosine kinase inhibitors (TKIs) which target the three VEGFRs and additional kinases, including those with roles in angiogenesis such as platelet- derived growth factor receptor (PDGFR) and fibroblast growth factor receptor (FGFR) or other important cancer hallmarks such as Raf and c-KIT.
First-generation multitargeted TKIs include sorafenib (Nexavar), sunitinib (Sutent) and pazopanib (Votrient). A variety of next-generation TKIs subsequently have been developed, including regorafenib (Stivarga), vandetanib (Caprelsa), and axitinib (Inlyta). These drugs have enjoyed particular success in the treatment of metastatic RCC, but also have notched marketing approvals in CRC, hepatocellular carcinoma (HCC), gastric cancer, soft tissue sarcoma, and rarer tumors such as pancreatic neuroendocrine tumors and thyroid cancer. These agents continue to be evaluated in numerous phase III clinical trials across a variety of tumor types.
Cabozantinib (Cometriq) targets multiple kinases, predominantly VEGFR-2 and MET, which makes it of particular interest since MET has been implicated in resistance to anti-VEGF therapy. It has shown promise in the treatment of HCC and RCC, and phase III trials are ongoing.
The results of a trial comparing cabozantinib with everolimus in patients with metastatic RCC after progression on VEGF-targeted therapy were recently published. The phase III METEOR study demonstrated a significant PFS benefit to cabozantinib treatment (7.4 months vs 3.8 months), with a trend toward improved OS. The FDA has awarded breakthrough therapy designation to cabozantinib in RCC on the basis of this trial.
Nintedanib and cediranib also are being evaluated in several ongoing phase III trials. The AGO-OVAR 12 trial of nintedanib in combination with chemotherapy in advanced ovarian cancer demonstrated a modest improvement in PFS for the combination (17.2 months vs 16.6 months), but was associated with a higher rate of gastrointestinal adverse events.
In lung cancer, the combination of nintedanib and docetaxel was evaluated in the LUME-Lung 1 study. Although no significant difference in OS was observed in the study population as a whole, patients with adenocarcinoma histology who progressed after first-line treatment experienced a 3-month improvement in median OS for the combination (10.9 months vs 7.9 months). Nintedanib is already approved in the European Union for the treatment of lung adenocarcinoma.
Cediranib development was abandoned by AstraZeneca in 2011 after it failed to meet phase III targets in lung cancer and CRC, and it also disappointed in the late stages of development in glioblastoma.
It has undergone a resurrection of late following promising results from studies in ovarian cancer. In the ICON6 trial, cediranib in combination with chemotherapy and cediranib maintenance therapy both extended PFS and OS compared with chemotherapy alone.
Focusing in on VEGFR-2
Additionally, extremely promising results from a phase II study of cediranib in combination with the poly(ADP)ribose polymerase (PARP) inhibitor olaparib in platinum-sensitive ovarian cancer were reported; the combination almost doubled median PFS (17.7 months vs 9 months) compared with olaparib monotherapy.Most promising among the new drugs targeting the VEGF pathway is ramucirumab (Cyramza), a monoclonal antibody like bevacizumab, but which targets VEGFR-2 rather than the VEGF-A ligand. In 2014 it became the first antiangiogenic therapy approved for the treatment of gastric cancer, initially as monotherapy and then subsequently in combination with paclitaxel in the second-line setting on the basis of the phase III REGARD and RAINBOW trials, respectively, which both demonstrated improved OS.
A year later, based on the results of the RAISE trial, ramucirumab went on to receive regulatory approval as second-line treatment for metastatic CRC in combination with FOLFIRI chemotherapy. In combination with docetaxel, ramucirumab also was approved as second-line therapy for metastatic NSCLC, when the combination demonstrated both improved OS and PFS in the phase III REVEL trial.
Phase III trials of ramucirumab continue to recruit participants in several tumor types. The agent has shown particular promise in late-stage clinical development in patients with HCC who have elevated alpha-fetoprotein levels at baseline; the phase III REACH-2 trial is ongoing in this patient subgroup.
Evolving Understanding of Angiogenesis
Specific targeting of VEGFR-2 is also proving to be an effective strategy for tyrosine kinase inhibition. Apatinib is a novel VEGFR-2—specific TKI that is currently being evaluated in several phase III trials in patients with HCC, gastric, and lung cancer. Like ramucirumab, apatinib has demonstrated a survival benefit for patients with advanced gastric cancer, improving median OS from 4.7 months to 6.5 months compared with placebo in patients who failed prior second-line chemotherapy.It was initially hoped that VEGF-targeting therapies would provide antitumor activity across the board in all cancer types. In reality, the variability observed with these agents has reshaped our understanding of the very biological process they target.
In normal healthy tissue, there is significant heterogeneity between the vessels supplying different organs, and it seems the same may be true of tumor vasculature. A number of different types of vessels have been identified, which may have differing sensitivity to antiangiogenic therapy. For example, it has been suggested that newly formed vessels may be more dependent on VEGF signaling than mature vessels.
Furthermore, as our understanding of the tumor vasculature has improved, it has become clear that not all tumors use angiogenesis to remodel the vasculature. A range of other mechanisms have now been reported, including vascular mimicry, where tumor cells organize themselves into vessel-like structures and may even be able to de-differentiate into endothelial cells, and vessel co-option, in which tumors make use of the existing vasculature. The level of angiogenesis may also depend on the stage of disease as well as the cancer type.
There is also the question of why many VEGF-targeted therapies have been limited in their use as single agents, which led some researchers to reevaluate the way that these drugs might work. Blood vessels derived from tumor angiogenesis are leaky, which could hinder the delivery of chemotherapy to the tumor; thus, it was initially proposed that VEGF-targeted therapy might counteract this by normalizing the vasculature. On the contrary, however, some studies have indicated that VEGF-targeted therapy reduces concomitant drug delivery, and this remains an area of controversy.
The vascular endothelial growth factor (VEGF) family members and placenta growth factor (PIGF) selectively bind to VEGF receptors (VEGFRs), setting off a complex series of cell-signaling events in several pathways to promote carcinogenic activity.
Regardless, it is clear that in general, clinical benefits with existing VEGF-targeted therapies have been modest, while side effects and costs are high, and there is a pressing need for predictive biomarkers to identify patients who are most likely to benefit from this class of drugs.
Intensive research efforts have evaluated several different possible biomarkers, including changes on enhanced magnetic resonance imaging that might indicate reduced tumor vascular permeability, plasma concentrations of VEGF or VEGFR-2, predictive protein signatures, levels of circulating endothelial cells, and certain single nucleotide polymorphisms. None are clinically validated as yet.
Jane de Lartigue, PhD, is a freelance medical writer and editor based in New Haven, Connecticut.