Reappraising antiangiogenic therapy for breast cancer

SUMMARY

Phase III trials of antiangiogenic drugs for metastatic breast cancer have either had only limited success, e.g. the monoclonal anti-VEGF antibody bevacizumab when used with various conventional chemotherapy regimens, or have failed altogether, e.g. the small molecule oral tyrosine kinase inhibitor (TKI) sunitinib. No phase III trial has yet demonstrated an overall survival benefit and the progression free survival (PFS) benefits, when attained with bevacizumab are short, with perhaps one exception. Together, these results call for a reappraisal of using antiangiogenic drugs for breast cancer and possible strategies to improve their efficacy. Among the reasons to help explain the limited benefits observed thus far include the possibility that angiogenesis may not be a major driver of breast cancer growth, compared to some other types of cancer; that acquired resistance may develop rapidly to VEGF-pathway targeting antiangiogenic drugs, in part due to angiogenic growth factor redundancy; that optimal chemotherapy regimens have not been used in conjunction with an antiangiogenic drug; and that antiangiogenic drugs may secondarily aggravate biologic aggressiveness of the tumors, thereby reducing their overall efficacy after inducing an initial benefit. Several possible strategies are discussed for improving the efficacy of antiangiogenic drugs, including combination with different chemotherapy regimens, e.g. long term and less toxic metronomic chemotherapy protocols; validation of predictive biomarkers to individualize patient therapy; development of improved preclinical therapy models, e.g. involving advanced metastatic breast cancer, and combination with other types of anti-cancer agents especially biologics such as trastuzumab for Her2-positive breast cancer. Reasons for the current concern regarding use of antiangiogenic drug treatments for early stage cancers, including breast cancer, are also discussed.

Introduction

A seminal development in oncology therapeutics occurred in 2004 when the FDA approved the first antiangiogenic drug, namely, bevacizumab (Avastin®), the monoclonal antibody to VEGF on the basis of a phase III trial result, for the treatment of metastatic colorectal cancer.¹ Based mostly on phase III trial results, bevacizumab was subsequently given FDA accelerated approval for Her-2 negative metastatic breast cancer in 2008,² and full approval for non small cell lung cancer,³ renal cell cancer (RCC)^4,5 and second line treatment of glioblastoma.⁶ These successes were paralleled by approvals of oral small molecule antiangiogenic receptor tyrosine kinase inhibitors (TKIs) which target multiple RTKs, including VEGF receptors such as VEGFR-2, the major signaling receptor for VEGF-mediated angiogenesis. The marketed approvals include sunitinib (Sutent®), sorafenib (Nexavar®), and pazopanib (Votrient®) for RCC and sorafenib for hepatocellular carcinoma (HCC), all given as monotherapies.^7–9 In contrast, bevacizumab is administered intravenously, mostly as part of cocktail with various approved chemotherapy regimens since, on its own, it seems to have little or no clinical (survival) benefit,¹ though it can be used successfully sometimes as a follow-up maintenance therapy,¹⁰ and with interferon alpha for kidney cancer.⁴

Together, these approvals constitute a major advance in oncology drug development and medical oncology practice. However, over the last few years there have been many setbacks in the clinical application of antiangiogenic therapeutics. Thus, it is becoming increasingly evident that the clinical benefits using these drugs with respect to progression free survival (PFS) or overall survival (OS) are mostly very small, usually in the range of a few months; moreover, benefits in PFS are frequently not followed by OS benefits.^10–12 There have also been a number of failed randomized phase III trials of antiangiogenic TKIs alone or with chemotherapy^12,13 and four of these trials involved sunitinib as a first line treatment with chemotherapy (either docetaxel, paclitaxel, or capecitabine) and also as a second line monotherapy treatment against capecitabine or when combined with capecitabine for metastatic breast cancer.^14–17 In addition, there have been a number of failures in other indications of bevacizumab plus chemotherapy in the metastatic setting.^12,18 Furthermore, phase III adjuvant therapy trial results in early stage colorectal cancer (called CO8 and AVANT) have been extremely disappointing^19,20; indeed the latter trial showed a worse survival outcome in patients who received bevacizumab plus chemotherapy compared to the chemotherapy,^20,21 a result consistent with some preclinical adjuvant antiangiogenic drug therapy findings published in 2009.^22,23 Together these results have raised concerns about adjuvant antiangiogenic drug therapy in early stage breast cancer.

The various bevacizumab-based phase III metastatic breast cancer trials deserve further comment. The E2100 trial which led to the accelerated approval of bevacizumab for Her2-negative metastatic breast cancer involved an open label weekly paclitaxel regimen plus bevacizumab given every 2 weeks.² This trial led to a progression free survival benefit of 5.5 months and a hazard ratio (HR) of 0.48. No OS benefit, as discussed above, was obtained.² One of the required subsequent confirmatory phase III trials undertaken to gain full approval evaluated bevacizumab with docetaxel given once every 3 weeks (the AVADO trial) which resulted in only a PFS benefit of only 0.9 months (HR 0.62).^24,25 The other trial, the Ribbon-1,²⁶ involved either an anthracycline or a taxane, and resulted in a PFS benefit of 1.2 months (HR 0.64) or capecitabine, and gave a PFS benefit of 2.9 months (HR 0.69).²⁶ The lesser PFS benefits accompanied by the lack of an OS benefit in these follow-up trials was a major factor in the FDA decision to revoke the label of bevacizumab for breast cancer²⁷ – though other factors are almost certainly involved, which include the toxicities associated with bevacizumab treatment¹⁸ and, though not supposedly a factor in the decision, the high cost of the drug.^24,28 Simply put, the risks (and costs) associated with using the drug were considered to outweigh its benefits.^24,27 One question which inevitably arises from these clinical trial results is why a repeat, or at least an extension of the weekly paclitaxel-based E2100 bevacizumab trial was not undertaken. For example, such a trial could have involved a placebo in the control arm, and/or extended maintenance bevacizumab monotherapy after the initial course of bevacizumab + paclitaxel treatment was completed. Maintenance bevacizumab therapy has been found to improve survival (PFS only, thus far) in ovarian cancer phase III trials,¹⁰ though only incrementally, and at excessively high financial cost.^28,29

Taken together, the bevacizumab based phase III trials along with the TKI (e.g. sunitinib-based) phase III trials in metastatic breast cancer have recently provoked much discussion and debate^24,27 about the future prospects and present wisdom or value of antiangiogenic based therapies, not only for the treatment of metastatic breast cancer, but also for early stage breast cancer when using antiangiogenic drugs for either neoadjuvant and adjuvant treatment.^12,23 Below I discuss a reappraisal of the rationale and prospects of antiangiogenic therapeutics for breast cancer treatment, primarily from a basic research perspective.

Looking back: Reexamining the original rationale for antiangiogenic drugs in breast cancer treatment

An underlying assumption regarding the rationale for using antiangiogenic therapies is that all types of cancer are angiogenesis-dependent and therefore inhibiting angiogenesis would be highly effective as a generic cancer treatment. However, in retrospect, this rationale may be faulty for several reasons. It now seems clear that there are certain types of cancer, e.g. clear cell renal cell cancer that are not only highly angiogenic, but that the associated tumor angiogenesis is driven predominantly by VEGF.³⁰ Thus, in retrospect, it is perhaps not surprising that four different VEGF-pathway targeting antiangiogenic drugs have been approved for this type of malignancy, as noted above. But can we assume that there is a similar degree of importance for angiogenesis and a similar associated high degree of VEGF dependency in breast cancer? Perhaps not. To cite one example, Furstenberger and colleagues recently undertook an analysis of the expression of a dozen different proangiogenic growth factors in 41 samples of primary breast cancer tissue specimens.³¹ However, unlike most previous studies of this nature this was accompanied by a similar analysis of the expression of the aforementioned growth factors in normal adjacent tissues. Surprisingly, it was found that 11 of the 12 factors analyzed showed greater levels at the gene expression level in the adjacent normal tissue – the exact opposite pattern of what one might have expected. The only factor that was upregulated in tumor tissue was VEGF – but only by a very small amount.³¹ In contrast, two inhibitors of angiogenesis (thrombospondin-1 and thrombospondin-2) showed greater levels of expression in tumor tissue compared to adjacent normal tissue.³¹ The authors concluded that primary breast cancers are likely not sites of active angiogenesis. While no metastases were analyzed, it is clearly possible that the same pattern of expression might also hold true for such lesions. If so, this could constitute a serious challenge to the rationale of using antiangiogenic drugs that target the function of various growth factors such as VEGF as an effective means of treating breast cancer. In this regard consideration should be given to other types of vascular targeting strategies such as “vascular disrupting agents” (VDAs)³² or peptide-drug conjugates such as NGR-TNFa designed to target the established but abnormal tumor vasculature^33,34 as an alternative – and hopefully more effective – treatment strategy.

Looking at the present, I: Rapidly acquired drug resistance

Currently approved antiangiogenic drugs work exclusively (e.g. bevacizumab) or likely, mainly (e.g. the TKIs such as sunitinib) by targeting VEGF dependent angiogenesis. Even if VEGF is a meaningful driver of breast cancer angiogenesis, there is the distinct possibility that numerous other proangiogenic growth factors are present and/or can be upregulated, to drive angiogenesis, when the VEGF pathway is inhibited.³⁵ In fact such a possibility was implicated by a seminal study published in 1997 by Relf et al.³⁶ The authors analyzed a large number of primary breast cancer specimens from various types of disease progression for 6 different proangiogenic growth factors, which included bFGF and VEGF.³⁶ The authors found that the more advanced stage of disease the breast cancer specimen was derived from, the greater the level of proangiogenic growth factor redundancy detected.³⁶ They speculated that the anti-tumor effect of targeting a single growth factor would likely be very transient. By implication, this could also result in the selection and overgrowth of other tumor subpopulations capable of driving and sustaining angiogenesis through the production of alternate growth factors such as bFGF.³⁵ Indeed, there are now a number of preclinical studies which have indicated that exactly this type of selection process can operate as a mechanism of acquired drug resistance.^37,38 In one study, treatment of spontaneously arising islet cell pancreatic mouse carcinomas arising in a genetically engineered mouse model using an antibody to VEGF receptor-2 resulted in a transient tumor response lasting around 3–4 weeks, which was then followed by resumption (relapse) of tumor growth.³⁷ The relapse was found to be due to the upregulation of the angiogenesis stimulator, bFGF.³⁷ Similarly, treatment of human renal cell carcinoma xenografts with sunitinib resulted in the eventual emergence of drug resistant variants as a result of activation/upregulation of interleukin-8 mediated angiogenesis.³⁸ In addition, activation of alternate proangiogenic pathways can occur epigenetically by changes in the tumor microenvironment, e.g. by virtue of the antiangiogenic drug treatment causing elevated levels of tumor hypoxia as a result of reducing tumor microvascularity as well as tumor vessel blood flow and blood perfusion.³⁹ The increased tumor hypoxia can result in elevated expression of the transcription factor known as hypoxia-inducible factor-1alpha (HIF-1α) which can then induce the expression of alternate proangiogenic growth factor encoding genes as an adaptive/evasive mechanism of survival.⁴⁰ As a result, there is considerable interest in trying to devise strategies in which an antiangiogenic drug is combined with another drug or treatment capable of targeting HIF-1α expression.⁴¹ By way for example, metronomic-like chemotherapy regimens using topotecan may represent one such strategy.⁴²

Looking at the present, II: The chemotherapy backbone/partner matters

There was an initial hope or speculation that an antiangiogenic drug such as bevacizumab could be combined with virtually any chemotherapy regimen so as to enhance the efficacy of the chemotherapy.⁴³ However, it is becoming increasingly clear that the nature of the chemotherapy drug (or chemotherapy drug combinations) as well as the schedules and doses used, can have a significant impact on whether the presence of a drug such as bevacizumab makes a difference, or not, when combined with the chemotherapy.⁴³ By way of example, the E2100 trial involved a weekly paclitaxel regimen whereas the AVADO involved once every 3 week docetaxel. One consequence of this is that the duration of treatment in the E2100 trial could be maintained for a longer period of time compared to the treatment given in the AVADO trial. Longer duration treatments using either chemotherapy or bevacizumab^10,44 appear to increase overall efficacy. As such, this also highlights what might be an additional rationale for using less toxic and hence longer term metronomic chemotherapy regimens in combination with an antiangiogenic agent, especially a drug such as bevacizumab, as discussed below. In any event it seems increasingly clear that much more needs to be learned about optimizing chemotherapy regimens for use in conjunction with an antiangiogenic drug for the treatment of cancer in general, including breast cancer. This could also apply to antiangiogenic TKIs given the inability of such drugs to improve outcomes when combined with conventional chemotherapy regimens; for example would they provide such benefits, and to a significant degree, when combined with metronomic chemotherapy, as some preclinical studies have shown^45,46?

Looking at the present, III: Changes in biologic aggressiveness induced by antiangiogenic therapy

A current topic of considerable interest and debate concerns the possibility that antiangiogenic drug treatments, after possibly causing an initial clinical benefit/tumor response, may alter the natural history of the disease in such a way as to make it more aggressive, such that the initial benefit is either compromised, lost altogether, or even that the final overall survival outcome is worsened.^12,22,47 For example, returning to the observation that an antiangiogenic therapy can increase tumor hypoxia and thus HIF-1α expression, such effects could also result in the upregulation of a number of HIF-1α regulated genes involved in promoting growth, invasion, and metastasis. One such potential ‘invasion’-inducing gene encodes the proto-oncogenic tyrosine kinase known as c-met.⁴⁸ The growth factor that binds c-met is hepatocyte growth factor (HGF) which has been implicated as a multifaceted mitogen, proangiogenic growth factor, and invasion/motility (scatter) factor.⁴⁸ While at the present time there is no compelling clinical evidence to indicate antiangiogenic drug treatments aggravate or accelerate metastatic disease in patients who had metastatic disease when treatment was initiated^19,25 (though it should be noted that these metastatic cancer trials involved treatment of patients with chemotherapy plus an antiangiogenic drug) preliminary evidence from the AVANT adjuvant colorectal trial indicates that this may be a concern, at least for early stage disease treatment settings.^13,49 Furthermore, it is theoretically possible that the failure to achieve OS benefit in the breast cancer (and other) trials where PFS benefits were attained could be explained, at least in part, by a change in biology of the treated cancers such that their growth rate is slightly accelerated, i.e., a subtle ‘rebound’ is induced. This could translate over time to a shortened period between relapse and death compared to those patients not receiving the antiangiogenic drug, thereby resulting in the absence of an OS benefit.¹²

Looking ahead: How can antiangiogenic-based therapy for breast cancer be improved?

Given the limitations thus far of antiangiogenic drug based therapies for the treatment of breast cancer, a number of strategies can be considered for improving the clinical impact of such drugs. Perhaps the most important is the identification of reliable predictive markers that can be used to identify patients who are more likely to respond to an antiangiogenic drug-based therapy and gain a benefit from doing so, as all the phase III trials in breast cancer – and all other types of cancer as well – have involved unselected patient populations. Currently the major interest in this area lies in the identification of single nucleotide genetic polymorphisms (SNPs) in angiogenesis related genes such as vegf.⁵⁰ Indeed, based on such a retrospective analysis of certain vegf gene polymorphisms in tumor tissue obtained from patients in the E2100 trial, patients were identified who experienced a major clinical benefit, even in OS, as a result of receiving the bevacizumab plus weekly paclitaxel treatment.⁵⁰ Such findings are now undergoing prospective clinical trial evaluation, and hopefully validation. Other predictive biomarkers are being investigated include hypertension, based on findings not only in the E2100 trial, but in other cancer indications suggesting elevated hypertension in patients receiving bevacizumab may be a predictive marker of future clinical benefit^51,52 though this is controversial,⁵³ and has to be validated in future prospective randomized trials.

A second strategy to improve outcomes would be to improve the predictive value and power of preclinical therapy tumor models. Historically, there has always been a major gap between the frequently encouraging results found in preclinical experimental therapeutic studies from what is subsequently observed in clinical trials, namely, far less impressive or negative outcomes.⁵⁴ As a means of addressing this disparity in outcomes, several strategies are currently being pursued which include the use of genetically engineered mouse model of cancer employing clinically relevant imaging beyond endpoints of tumor response, as well as survival⁵⁵ or developing new models of advanced visceral metastatic disease to better emulate the more challenging treatment circumstance of patients who have similar stages of advanced disease.⁵⁴ Indeed, pursuing the latter approach, we developed a number of models of advanced visceral metastatic breast cancer in mice that were then used to evaluate various investigational metronomic chemotherapy regimens^54,56 or standard vs metronomic chemotherapy regimens used in combination with a targeted biologic agent such as trastuzumab.^57,58 One such study showed a remarkable therapeutic (survival) benefit using a daily oral doublet metronomic chemotherapy drug combination.⁵⁶ The drugs used were UFT (tegafur + uracil), a 5-FU prodrug, administered daily by gavage, in conjunction with metronomic cyclophosphamide administered through the drinking water on a non-stop extended 6-month basis.⁵⁶ In contrast, this drug combination showed only a transient and modest effect in treating the same tumor cell line – a metastatic variant of MDA-MB-231 called LM2.4, when grown as an established orthotopic primary tumor transplant in separate experiments.⁵⁶ This study – along with many others showing that more potent anti-tumor effects can be achieved when combining a targeted antiangiogenic drug with an investigational metronomic chemotherapy regimen^45,46,59 – was pivotal in the decision to initiate a phase II trial of metronomic cyclophosphamide/capecitabine administered on a daily non-stop basis in combination with bevacizumab for the treatment of metastatic breast cancer patients.⁶⁰ This non-randomized phase II trial indicated an overall clinical benefit (complete response + partial response + stable disease ≥ 6 months) of 68% in 41 patients with minimal associated toxicity.⁶⁰ As a result, the treatment has now moved forward to phase III clinical trial testing where the control arm is the weekly paclitaxel/bevacizumab E2100 treatment combination (http://www.clinicaltrials.gov/ct2/show/NCT01131195?term=metronomic+capecitabine+and+breast+cancer&rank=5). With respect to what might be other potentially promising drug combinations, some obvious possibilities are protocols involving bevacizumab with another biologic targeted therapy, e.g. trastuzumab or lapatinib for the treatment of Her-2⁺ breast cancer.

Summary and conclusions

There are a number of possible reasons to help explain the limited or absent clinical benefits of the antiangiogenic drugs tested thus far for metastatic breast cancer. Several strategies are available for improving outcomes, especially when using antibodies that target the VEGF pathway, e.g. bevacizumab where PFS benefits have been reported in several phase III trials. More problematic is whether small molecule antiangiogenic TKIs can be used with any success given their repeated failures thus far. Moreover, there is growing concern about whether neoadjuvant or adjuvant antiangiogenic-drug based therapies for early stage disease will prove beneficial, given the results of two adjuvant trials using bevacizumab plus chemotherapy for early stage colon cancer²⁰ and limited preclinical studies.^22,61