By Robert S. Kerbel, PhD
- Tumor angiogenesis factors are switched on and secreted by tumor cells, as well as various normal cells, and stimulate the formation in and around tumors of new blood vessel capillaries. Foremost among them is the VEGF family and their cognate signaling receptors, especially VEGFR-2.
- There are four types of approved VEGF pathway–targeting drugs in oncology: monoclonal neutralizing antibodies to the circulating VEGF ligand, monoclonal VEGFR-2 blocking antibodies, an ever-expanding list of oral small-molecule TKIs that primarily act intracellularly to block the catalytic signaling function of VEGFR-2, and an antibody-like decoy trap agent that binds strongly to VEGF and placental growth factor.
- These drugs have had a number of notable successes based on randomized phase III clinical trials, leading to multiple marketed approvals; however, these successes also have to be viewed in the context of a number of limitations and failures.
- A number of strategies to improve VEGF pathway–targeting drugs are currently being investigated.
We now know that there are numerous TAFs, but foremost among them is the family of vascular endothelial growth factors (VEGFs) and their cognate signaling receptors, especially VEGF receptor 2 (VEGFR-2). More than anything else, it has been a series of pioneering discoveries to elucidate the components and functions of this pathway that resulted in the theory that eventually led to successful clinical practice using VEGF pathway inhibitors as antiangiogenic drugs.2
The major components of the VEGF family are VEGF-A, VEGF-B, VEGF-C, VEGF-D, and placental growth factor, as well as three receptor tyrosine kinases, VEGFR-1, VEGFR-2, and VEGFR-3 (Fig. 1). VEGF-A, usually referred to simply as VEGF, comes in a variety of isoforms, two of which circulate in blood—VEGF121 and VEGF165—and bind to endothelial cell VEGFRs. Binding to VEGFR-2 (but not VEGFR-1) sets in motion a number of intracellular signaling pathways that lead to multiple functions necessary for sprouting neoangiogenesis, including cell division, migration, vascular permeability, and promotion of cell survival.2,3 For a more detailed and updated account of the role of other VEGF receptors, such as VEGFR-1 and VEGFR-3, and various co-receptors, such as neuropilin-1 and -2, see the review by Ferrara and Adamis.2
Working together, these genetic and epigenetic factors help explain why expression and activation of the VEGF pathway are common features of most types of cancer and, hence, targets for cancer therapy. As discussed in this article, the rationale for targeting VEGF may extend beyond its dedicated proangiogenic function, in part because of the growing realization that it can cause significant immunosuppressive effects within the tumor microenvironment by multiple mechanisms.5-9
With respect to oncology, there are four types of approved VEGF pathway–targeting drugs:10 monoclonal neutralizing antibodies to the circulating VEGF ligand (i.e., bevacizumab2), monoclonal VEGFR-2 blocking antibodies (i.e., ramucirumab11,12), and an ever-expanding list of oral small-molecule tyrosine-kinase inhibitors (TKIs) including sunitinib, sorafenib, pazopanib, axitinib, regorafenib, nintedanib, cabozantinib, and vatalanib,13 that primarily act intracellularly to block the catalytic signaling function of VEGFR-2. However, these TKIs can target multiple other receptor kinases, such as PDGF receptors, c-Kit, CSF-R1, FLT3, RET, RAF (in the case of sorafenib), bFGF receptors (regorafenib and nintedanib), and TIE2 (regorafenib). A fourth type of drug, aflibercept, although used less often, is an antibody-like decoy trap agent that binds strongly to VEGF and placental growth factor.2,14
Together, these drugs have had a number of notable successes that are based on randomized phase III clinical trial results in patients with advanced metastatic disease, leading to multiple marketed approvals.2,14 An updated and detailed list of these trials and approvals in the United States and other countries can be found in the recent review by Ferrara and Adamis2 and in another by Jayson et al.14 The approvals include bevacizumab for first-line treatment of colorectal, ovarian, cervical, and non–small cell lung cancers (NSCLC); second-line treatment of colorectal cancer (bevacizumab, ramucirumab, and aflibercept); second-line treatment of NSCLC (nintedanib and ramucirumab); first- or second-line treatment of renal cell carcinoma (sunitinib, sorafenib, pazopanib, axitinib, and bevacizumab with interferon); first-line treatment of hepatocellular carcinoma (sorafenib); second-line treatment of gastric cancer (ramucirumab); and first-line treatment of soft tissue sarcomas (pazopanib), as well as some other less common tumors, such as pancreatic neuroendocrine tumors (sunitinib) and thyroid cancer.2,14 Bevacizumab is also approved in some countries (although no longer in North America) for metastatic breast cancer and for glioblastoma.2,14 Bevacizumab has also recently been shown to be active in mesothelioma,15 although it is not yet approved for this indication. An important aspect of the marketed approvals is that antibodies, such as bevacizumab or ramucirumab, are administered concurrently with conventional chemotherapy regimens, followed in one case (i.e., in ovarian cancer) as maintenance.2,14 Also, ramucirumab has been approved as second-line monotherapy in gastric and gastroesophageal junction adenocarcinoma.2,14 In contrast, the TKIs thus far are only approved as monotherapies, with the exception of nintedanib, which is approved in Europe for second-line use with docetaxel in NSCLC.2,14
Although these successes have changed clinical practice for the treatment of many types of cancer, they also have to be viewed in the context of a number of limitations and failures. These include the following: (1) Modest benefits were gained in positive phase III trials of metastatic therapy in progression-free survival or overall survival (OS), especially the latter. Indeed, in many instances, progression-free survival benefits were not followed by an OS benefit.2,14 (2) VEGF pathway–targeting drugs failed in phase III trials of certain cancers, e.g., prostate and pancreatic cancer.2,14 (3) Many phase III adjuvant trials involving bevacizumab, sunitinib, or sorafenib for treatment of early-stage breast, colorectal, hepatocellular, or renal cell cancer failed to meet their primary endpoints of benefit in disease-free survival (DFS).16-23 (4) Numerous antiangiogenic TKI-plus-chemotherapy combinations failed in phase III trials of various common indications, including breast cancer, NSCLC, and colorectal cancer.2,14 Some comments about these clinical limitations and failures follow.
First, in the outcomes of the phase III trial with metastatic therapy, clinical trial design factors, such as crossover and/or other therapies available to patients with long post-progression survival, can reduce or eliminate the chances of obtaining an OS benefit.24 Second, no biomarker predictive of possible clinical benefit for any VEGF pathway–targeting antiangiogenic drug, including elevated VEGF in tumors or in the circulation, has yet been validated.3 As a result, in contrast to most other targeted therapies, none of the numerous phase III trials previously undertaken of VEGF pathway antiangiogenic therapy regimens involved any prior selection or enrichment of patients considered to have an increased probability of benefiting from the therapy.2,14 Efforts continue in this critical area of antiangiogenic therapy research, and there are some hints that certain biomarkers—such as restricted gene signatures associated with angiogenesis or hypoxia25 or patients whose tumors have particular genetic mutations that are associated with significant tumor response rates (regressions) to a VEGF pathway monotherapy—might yet have promise.26 However, these require validation in prospective trials that involve greater patient numbers.
Regarding the aforementioned negative antiangiogenic drug–based adjuvant clinical trial results, a recurring finding is that patients in the experimental arm who receive the antiangiogenic drug for 12 to 18 months have a benefit in delayed tumor progression during therapy and, subsequently, for an extended period when therapy is stopped (e.g., for up to 1 year). However, this benefit fades over time and is eventually lost altogether, so the prespecified endpoint of a benefit in DFS at a later time point (e.g., 3 years) is not met.14-21 Why this happens is unknown; it suggests the possibility that longer-term therapy could provide a benefit in DFS, but this clearly poses serious ethical and financial challenges, given the adverse effects of such drugs and their high costs. Furthermore, a large fraction of patients enrolled in such adjuvant trials do not need any therapy, because they are already cured by surgery. Finally, with respect to the numerous unsuccessful phase III trials testing an antiangiogenic TKI with concurrent standard chemotherapy, a major problem has been the toxicity of such combinations, which require drug dose reductions or therapy breaks that likely result in reduced efficacy;2,14 additional biological factors may be involved as well, highlighting possible differences between antibodies and TKIs.
With this brief background in mind, a number of strategies are being explored to improve the efficacy of VEGF pathway–targeting drugs. A few of the more important initiatives are discussed here. In addition to the ongoing efforts in the discovery and validation of predictive biomarkers, some of the more promising approaches include the following: (1) elucidating mechanisms of intrinsic and acquired resistance; (2) assessing combination treatments in which the VEGF pathway plus an additional important proangiogenic pathway are concurrently or sequentially targeted; (3) combining VEGF pathway–targeting drugs with immune checkpoint therapies; (4) elucidating the mechanisms by which VEGF pathway inhibition with an antiangiogenic drug increases the efficacy of chemotherapy; and (5) targeting endothelial cell–derived “angiocrines.” For all of these initiatives, preclinical studies with mouse tumor therapy models that have an improved chance of predicting future clinical outcomes would clearly be helpful.27
Resistance to Antiangiogenic Drugs Targeting the VEGF/VEGFR-2 Pathway
A broad and diverse number of mechanisms have been proposed to explain instances of either intrinsic or acquired resistance to antiangiogenic therapies that work primarily or exclusively by inhibition of the VEGF pathway.28 A number of reviews on this topic have addressed this issue in depth.28-31 Here, only a small number of mechanisms will be discussed, to illustrate their diversity.
The most cited theory involves the activation of compensatory (bypass) pathways of angiogenesis. This is similar in principle to numerous mechanisms that have been proposed to explain acquired resistance to other types of targeted drugs. The basic idea is that, by inhibiting the proangiogenic function of VEGF, evolutionary-like pressures are created that force induction or upregulation of an alternative and otherwise silent proangiogenic pathway.25
One obvious way that this could occur is by sustained and effective inhibition of VEGF function, which presumably would lead to significant suppression of tumor vascularity, blood flow, and perfusion, which, in turn, would lead to elevated tumor hypoxia.4,25 This physiologic change would serve to trigger one or more alternate compensatory proangiogenic pathways (e.g., upregulation of hypoxia-inducible factor 1a [HIF-1a]). This transcription factor is known to regulate many different genes encoding proangiogenic proteins, including basic fibroblast growth factor (bFGF), interleukin 8 (IL-8) or other inflammatory cytokines, and angiopoietin 2 (ANG-2).4 This theory of acquired (adaptive/evasive) resistance derives mostly from preclinical studies and suggests that one strategy to prolong the efficacy of VEGF pathway inhibition would be to either concurrently or sequentially target an implicated bypass pathway that is or can be activated.25,28 Thus, treatments with drugs that target bFGF, ANG-2, or IL-8 have been shown to be successful in prolonging the antitumor effects of agents that block the VEGF pathway.28,32 Although preclinical studies support this strategy,28,32 evidence in the clinic, so far, is lacking validity.33
An alternative strategy would be to concurrently or sequentially administer drugs that have, among their properties, an ability to suppress HIF-1a. There are many such agents; topoisomerase II inhibitors, such as topotecan,34 or an investigational camptothecin nanoparticle drug conjugate known as CRLX10136 are among the more interesting. Because of this apparent HIF-1a–targeting property, such agents (provided that they are not overly toxic) may be ideal chemotherapy partners to pair with a VEGF pathway–targeting drug.35,36
A second mechanism that may help explain many instances of either intrinsic or acquired resistance to VEGF pathway–targeting antiangiogenic drugs stems from the nature and heterogeneity of the tumor vasculature. There is evidence that there are numerous blood vessel subtypes within tumors, some that are VEGF dependent but others that are not.37 Hence, tumors that are heavily populated with VEGF-independent angiogenic blood vessels will be minimally responsive or nonresponsive to a VEGF pathway–targeting therapy.38 Moreover, therapy with such drugs might lead to a gradual switch from tumors being populated mainly by VEGF-dependent to VEGF-independent vessel subtypes.
A major question that arises from such findings is whether the heterogeneity of blood vessels in tumors, and particularly co-opted/hijacked tumor blood vessels, can be targeted by an alternative antivascular therapy. For example, co-opted vessels in an organ such as the lungs can be abnormal in terms of such functions as permeability/leakiness and, thus, can express various markers not found in their truly normal vasculature counterpart, potentially rendering them vulnerable to a particular therapeutic intervention. Vascular disrupting agents may have such an effect,44 and drugs that inhibit tumor invasion/migration, a process that may precede a switch to vessel co-option,43 could also represent potential strategies to delay the switch to vessel co-option.
One other interesting consideration regarding acquired resistance to antiangiogenic VEGF pathway–targeting drugs is that, unlike resistance to many other drugs that target specific genetic mutations and that often lead to a stable, heritable, acquired-resistant phenotype, evasive/acquired resistance to VEGF pathway–targeting drugs may not necessarily be stable.43,45-48 This may help explain certain clinical results, such as the benefit of switching from one antiangiogenic TKI to another similar one as a strategy to prolong therapeutic benefit in patients with renal cell carcinoma49 or use of bevacizumab beyond tumor progression in colorectal cancer in conjunction with a switch in the chemotherapy partner backbone.50
Combining VEGF Pathway–Targeting Drugs With Immune Checkpoint Drugs
What is the basis for this? VEGF has been shown to impact a number of processes that could potentially affect T-cell effector functions. For example, VEGF has been shown to impair the maturation of antigen-presenting dendritic cells.56,57 Other studies also have shown that anti-VEGF therapy can reduce infiltration of tumors by T regulatory cells, whereas, in contrast, VEGF can stimulate tumor infiltration by CD8+ cytotoxic T cells.6,54,58,59 The latter facilitation effect may be a consequence of affecting the expression of various endothelial cell adhesion molecules that the cytotoxic T cells require for extravasation from blood vessels into the tumor parenchyma.6 VEGF has also been shown to promote the infiltration of tumors by immunosuppressive myeloid-derived suppressor cells.6 Pre-treatment VEGF serum levels were associated with clinical response and benefit in patients with melanoma who were treated with ipilimumab.60 VEGF can also modulate expression of inhibitory checkpoints by CD8+ T cells,61 and an inverse relationship has been reported between the PD-L1 and VEGF pathway.62 Thus, putting all this together, blunting the function of VEGF should, in theory, reverse any or all of these potentially localized VEGF-mediated immunosuppressive effects and, in so doing, stimulate effector T-cell function in tumors.8,9 In effect, one might argue that VEGF is acting as a kind of quasi–immune checkpoint in the tumor microenvironment. Conversely, it could be argued that, by inducing elevated degrees of intratumoral hypoxia, anti-VEGF therapy might actually impair the efficacy of an immune checkpoint inhibitor.
Thus far, although the results are limited, the indications are that a combination of a drug such as bevacizumab with an agent such as ipilimumab or nivolumab results in increased efficacy. This has been reported both preclinically63 and clinically,52 although the clinical studies so far have involved small clinical trials that require confirmation in larger, prospective, randomized phase II and III trials (which are currently underway). Also intriguing is the reported association between PD-L1 expression with clinical outcomes to VEGF-TKI therapy—high PD-L1 is correlated with poor responsiveness.64 The property of a VEGF pathway–targeting drug to reverse VEGF-mediated immunosuppression may also help explain some limited but intriguing preclinical findings. For example, it is usually the case preclinically that administration of a drug such as an anti–VEGFR-2 antibody in mice results in tumor growth delays (which would be termed progressive disease or treatment failure in the clinic) rather than causing stable disease or tumor regression. However, there are some examples of tumor regressions observed when the treatments are performed in immunocompetent mice, and this response is associated with evidence of increased T-cell–mediated immunity.7 Perhaps PD-L1 expression by endothelial cells65 may cause antivascular effects via PD-L1 antibodies. More work needs to be done to evaluate the effects of VEGF pathway–targeting drugs in immunocompetent versus immunosuppressed mice, preferably using tumor models that involve treatment not only of established primary tumors but also of microscopic and macroscopic metastatic disease.
Elucidating the Basis for VEGF Pathway Inhibition in Augmenting Chemotherapy Efficacy
Another important strategy to improve VEGF pathway–targeting drugs is to determine how they actually work. Bevacizumab and ramucirumab are examples of molecularly targeted drugs; we know that they target VEGF and VEGFR-2, respectively. As discussed in this article, these two drugs are mostly used in conjunction with concurrent chemotherapy. But, how do they actually increase the efficacy of chemotherapy when they do so? There are a number of different theories;66-70 however, almost 2 decades after the development of VEGF and VEGFR-2 antibodies, the clinical mechanisms by which they enhance chemotherapy remain elusive. It seems clear that the nature of the chemotherapy partner—the actual drug,6 the dose, and the schedule—has an impact on outcomes,71 but we are still not sure why. Thus, this remains an important area of research and underscores the enduring importance of systemic chemotherapy when paired with molecularly targeted biologics, including VEGF pathway–targeting drugs.
Finally, a new possible and intriguing approach to improving antiangiogenic therapy is based on the concept that the endothelial cells of tumor blood vessels may promote tumor growth in a perfusion-independent manner through the secretion of a variety of paracrine growth factors called “angiocrines.”72,73 These angiocrines can create vascular niches that can promote tumor cell growth, “stemness,” and even resistance to chemotherapy. Of interest in this regard is how VEGF inhibition or use of other anti-vascular therapies may affect the tumor vessel angiocrine phenotype and, hence, tumor growth and drug sensitivity.