Targeting metastatic breast cancer: problems and potential

Introduction

Breast cancer remains one of the leading causes of cancer-related mortality among women in the US¹. Since the primary tumor is usually resected upon detection, the majority of mortality is due to metastases. While 5-year relative survival for patients diagnosed with localized breast cancer is 98.5%, it plummets to a dismal 25% for patients with distant overt metastases¹. Moreover, an estimated 20–50% of breast cancer patients diagnosed at an early stage are expected to develop metastatic disease, which can occur years or even decades after surgical removal of the primary tumor². These statistics indicate that the establishment of effective therapies that target and prevent metastasis is of critical importance. Although the past decade has seen significant advances in the development of methods for detection and treatments of primary breast cancer, these treatments are generally ineffective at eliminating metastatic disease. This suggests either an inherent biological difference between primary tumors and distant metastases, a role of the microenvironment at the secondary site in inducing therapeutic resistance, or likely both.

Recent progress in breast cancer treatment has been characterized by a shift from cytotoxic drugs, which broadly target highly proliferating cells, to more targeted therapies designed to attack a specific molecule or pathway through a known mechanism of action. This approach has been possible due to enormous technological leaps in genome sequencing, molecular biology, cancer genetics, genomics, and bioinformatics that allow for the identification of individual patient mutations and clinically actionable targets from tumor biopsy samples. Sequence information and patient-derived xenograft methods have shown promise in identifying individualized therapeutic strategies that are effective for specific patients, rather than the traditional population-based clinical strategies. However, while various sequencing projects have characterized the genomic landscape of breast cancer^3,4, relatively little is known about the somatic diversity of metastatic tumors. Expression profiling has revealed multiple subtypes with different prognostic outcomes⁵ and led to identification of gene signatures that can stratify patients into groups with low- or high-risk for developing distant metastatic disease⁶. Still, targeted therapies against metastases have proven to be more challenging to develop compared to those targeting the primary tumor, since the critical driver molecules have not yet been firmly established.

In this review, we discuss the major challenges facing the development of efficient therapies against metastasis. We also present a strategy that we believe will improve the targeting of metastatic disease: blocking of further dissemination after diagnosis, eradication of disseminated tumor cells and prevention of the dormant-to-proliferative switch of those cells not eradicated, and elimination of established metastatic tumors. This strategy requires the elucidation of mechanisms that drive the establishment and maintenance of metastases.

Current anti-metastatic treatments and therapies in development

Current therapies against metastatic breast cancer address different stages of tumor progression⁷. Broad-spectrum therapies, such as cytotoxic chemotherapy, aim to kill actively dividing cancer cells. According to an extensive meta-analysis, combination chemotherapy can increase the 15-year survival rate of patients with early breast cancer⁸. However, a substantial number of these patients develop metastases or recurrent disease years after cessation of treatment, indicating the presence of dormant cells at the secondary site that are resistant to standard therapies. Therefore, the pathways that allow cancer cells to remain viable in a dormant state would provide ideal targets for eliminating minimal residual disease and preventing metastatic outgrowth after the cessation of treatment. Among them are the survival pathways involving Src and p38^9–12. Recent findings have shown that pharmacological inhibition of Src family kinase (SFK) signaling inhibited the proliferative outgrowth of dormant disseminated cells and the development of macrometastatic lesions¹². Furthermore, dormant cell proliferation required ERK1/2 activation, and the treatment of cells undergoing the dormant-to-proliferative switch with the combination of Src inhibitor (AZD0530, saracatinib, AstraZeneca) and MEK1/2 inhibitor (AZD6244, selumetinib, AstraZeneca) resulted in apoptosis of dormant cells; neither of the inhibitors alone achieved this effect. Since MEK1/2 is a well-described upstream activator of ERK1/2, these observations complement previous reports showing that an ERK^low/p38^high signaling ratio promotes tumor cell quiescence through a combination of signaling pathways promoting adaptive and basal survival and G0-G1 quiescence¹¹. These reports indicate that therapies targeting signaling involved in the dormant-to-proliferative switch might represent promising clinical interventions against metastatic disease.

Major efforts of the pharmaceutical industry are now focused on the development of novel targeted therapies that function in a molecule-specific manner⁷. Examples of targeted therapies that are presently in clinical use for breast cancer are aromatase/estrogen synthase inhibitors (anastrozole, letrozole, and exemestane)¹³ and trastuzumab (Herceptin^®), a monoclonal antibody against epidermal growth factor receptor 2 (ERBB2/HER2/neu)¹⁴. Initially investigated as a monotherapy and later in combination with chemotherapy, trastuzumab significantly improved patient response rate, time to progression, and overall survival compared to chemotherapy alone¹⁴. However, its use is limited only to HER2-positive breast cancers, the expression of which, as explained in the next section of this review, may not be constant between primary tumors and metastases, reducing the effectiveness of this strategy.

Based on their specific role in the metastatic cascade, potential molecular targets for therapeutic intervention can be categorized as involved in either metastasis initiation or progression^15,16. Due to the early dissemination of tumor cells, which is discussed in the next section, molecules important in metastasis initiation, such as regulation of cellular motility, angiogenesis, and invasion of local extracellular matrix (ECM), may not be efficient targets for preventing metastatic disease since patients at risk already have disseminated cells by the time of primary tumor diagnosis. However, if those molecules also play key roles in the later stages (progression) of metastasis, such as continued trafficking after initial escape from the primary tumor¹⁷, their targeting might be of clinical importance. In particular, molecules that promote survival at the distant sites, the dormancy-to-proliferation switch, and colonization of distant organs are very attractive targets for drug development.

The molecules identified and currently being investigated comprise both the tumor cell autologous factors, such as those involved in signal transduction, adhesion, motility, growth and survival, and the microenvironmental factors, including resident stromal cells, components of the immune system, chemokines, and promoters of angiogenesis. A growing catalog of potential molecular targets identified for inhibition of metastatic progression has been extensively reviewed^7,18,19, including a recent overview of drug candidates in current pharmaceutical pipelines that target the mechanisms of tumor cell migration²⁰. Effective anti-metastatic therapies need to address metastasis-specific characteristics that allow for the active targeting of metastatic cells. In the next section, we will discuss in more detail the challenges facing the treatment of metastasis from the perspective of drug development, specifically heterogeneity within and between metastases, as well as between metastases and primary tumors, constraints in detection and analysis, and the timing of intervention.

Challenges in the treatment of metastatic disease

Take-home lessons for optimal design of future anti-metastatic therapies

Based on our current knowledge of metastasis biology, we envision a three-pronged strategy for clinical intervention for progressive disease: prevention, stasis, and destruction. The prevention arm would be primarily applied as a neo-adjuvant therapy intended to reduce the potential metastatic capacity of tumor cells naturally shed from the primary tumor between diagnosis and surgical removal. As discussed above, this might require combination therapy for maximal efficiency since it has been demonstrated that tumor cells use multiple motility mechanisms (amoeboid, epithelial-to-mesenchymal transition, collective migration³⁸) to initiate dissemination. This arm might also be used to help suppress potential metastatic cells displaced during surgical resection itself. Since breast cancers undergo early dissemination, the prevention strategy would not necessarily be effective against cells that disseminated to distant sites prior to diagnosis. For these cells, cytostatic or cytotoxic therapies need to be developed, specific to the biology of disseminated cells, to either hold the disseminated cells in a sub-clinical state for the remaining natural lifetime of the patient, or to eliminate the metastatic “seed” before it can establish clinically relevant lesions. Finally, for those patients unfortunate enough to develop metastases, therapeutics specifically developed and targeted to metastatic biology need to be developed to reduce morbidity and mortality associated with these secondary tumors.

For these strategies to work, appropriate targets that drive the establishment and maintenance of metastasis have to be identified. Regrettably, despite significant advances in our understanding of the cellular biology of metastatic disease, our knowledge of metastasis “driver” genes remains limited. This is due partially to the difficulty of studying the process, since much of the important biology occurs in a small fraction of disseminated single cells or micrometastases. At present, it is not possible to easily identify those cells that will evolve into macroscopic lesions for in depth analysis, or exactly when that critical event occurs. In addition, metastatic samples are difficult to obtain from human patients since they are not usually resected, but instead are subject to systemic therapy. Thus, although the common mutations that drive primary breast cancer are now known thanks to large sequencing projects, much less is known about the mutational spectrum in metastatic disease. Finally, there may be many genes that are associated with metastatic progression. Analysis of gene expression-based prognostic signatures suggests that there may be hundreds or even thousands of genes whose expression is correlated with the disease outcome³⁹. Many of these genes are most likely expressed in the tumor microenvironment and therefore investigations solely in tumor cells have the potential to miss important molecules targetable for clinical intervention.

Continued investigations based on the biology of primary tumors for the development of novel therapeutics are clearly important, and undoubtedly will provide additional benefit to patients. However, this strategy is potentially fraught with dangers, since therapies that can shrink primary tumors have been shown to have little effect, or even potentially adverse effects, on metastatic disease^40,41. Furthermore, work from many laboratories has identified metastasis-associated genes that are not frequently mutated during cancer progression but are epigenetically silenced or exert their metastatic influence through alterations at the transcriptional level. Our own laboratory uses a meiotic genetics approach to identify somatically inherited polymorphisms that affect patients’ susceptibility to metastasis. This approach has generated a growing list of metastasis susceptibility genes with tumor-autonomous or stromal effects, including some with the potential to be actionable clinical targets^42–48. The results of these studies suggest that a more comprehensive examination of metastasis biology, incorporating both genomic and transcriptional landscape mapping, may provide an entirely different set of genes that might be more advantageously targeted to specifically reduce metastatic burden, without necessarily impacting the primary tumor.

Now that The Cancer Genome Atlas (TCGA) and other genomics projects have provided detailed information regarding the etiology of primary tumors, it is time to initiate similar projects for metastatic disease. Since metastatic tumors represent highly selected cells from a subset of the primary tumor localized in completely different microenvironments, these lesions should be recognized as related (but distinct) tumors with different biology. Thus, treatment strategies that are developed based on specific features of metastasis biology, used in parallel with those designed against the primary tumor, will provide a more effective method for combatting the final lethal stages of neoplastic disease than those strategies developed solely against the primary tumor. A Metastasis Genome Atlas Project would, however, have additional challenges compared to the TCGA. The major challenge would be access to the metastatic tumor samples since surgical resection of metastases is usually not performed due to increased risk without perceived patient benefit. In addition, metastatic tissue collected postmortem may be compromised or selected by therapies that potentially induce genomic changes, complicating analysis and interpretation of the results. Furthermore, it will be important to screen multiple metastases from different secondary sites within individual patients to help identify tissue-specific differences that might expose therapeutic vulnerabilities or resistance. Regardless of the challenges facing the Metastasis Genome Atlas Project concept, improved therapeutic strategies to reduce breast cancer morbidity and mortality would significantly benefit from a better understanding of the biology underlying the primary cause of patient distress, disseminated metastatic disease.