Treating Triple-Negative Breast Cancer: Where Are We?

e8 Treating Triple-Negative Breast Cancer: Where Are We? Aki Morikawa, MD, PhD, and Andrew D. Seidman, MD Abstract Unlike estrogen receptor (ER) positive and HER2-positive breast cancer, triple-negative breast cancer (TNBC) lacks a repertoire of targeted therapies. Hence, chemotherapy is the only available systemic option in current clinical practice. In general, survival of patients with TNBC is worse than that for those with ER-positive and HER2-positive breast cancer, especially for advanced-stage disease. Thus, a great unmet need exists. Conventional chemotherapeutic agents such as platinumsalts have been examined for specific benefit in TNBC; however, no one agent has yet been shown to offer a differential incremental benefit in metastatic TNBC. The hope is that ongoing research in targetable molecular pathways will lead to new therapeutic options for TNBC. Because these patients are likely to be increasingly subcategorized using potentially targetable molecular alterations, investigators will need to be mindful of the challenges in designing and conducting clinical trials in smaller subpopulations. The successful incorporation of targeted therapies in routine clinical practice for TNBC, which mirrors the success achieved by anti-her2 and endocrine therapies, is awaited. (J Natl Compr Canc Netw 2015;13:(e8 e18) Contemporary systemic therapy is commonly informed by the classification of breast cancer based solely on hormonal receptor and HER2 status. Triple-negative is a category of exclusion defined by immunohistochemical and/or fluorescence in situ hybridization based criteria, which has undergone modification over time (ASCO/ CAP guidelines). 1,2 Histologically, triple-negative breast cancer (TNBC) is a heterogeneous group of breast cancers that includes common infiltrating ductal carcinoma From Breast Medicine Service, Memorial Sloan Kettering Cancer Center, New York, New York. Submitted April 20, 2014; accepted for publication August 6, 2014. Dr. Morikawa has received research support from the Susan G. Komen Foundation and the ASCO Conquer Cancer Foundation Bonadonna Fellowship. Dr. Seidman has served as a consultant for Genentech/Roche and Pfizer and has received payment for lectures, including service on speaker bureaus from Genentech, Genomic Health, Eisai, and Celgene, and research support from the Susan G. Komen Foundation. Correspondence: Andrew D. Seidman, MD, Evelyn H. Lauder Breast Center, Memorial Sloan Kettering Cancer Center, 300 East 66th Street, New York, NY 10065. E-mail: seidmana@mskcc.org and rare types, such as adenoid cystic, metaplastic, apocrine, and neuroendocrine carcinoma. 3 Nevertheless, the clinical utility of this categorization has been robust as a prognostic and predictive biomarker. Patients with TNBC are more likely to have a poor prognosis than those with estrogen receptor (ER)/ progesterone receptor (PR) positive and/or HER2-positive breast cancer. 4 This is likely partially attributable to a dearth of novel therapeutic options for TNBC, in contrast to the recent expansion of endocrine and anti- HER2 therapies (eg, ado-trastuzumab-emtansine, pertuzumab, everolimus with exemestane). 5 7 Therefore, great interest has been shown in exploring potential biomarkers (prognostic and predictive) and developing new therapeutic options. This article reviews current treatment options ( where are we now? ) and ongoing therapeutic research ( where are we going? ) for TNBC. Heterogeneity of TNBC Because the utilitarian definition of TNBC is simply based on the exclusion of ER/PR/HER2 expression, it is not unexpected that this clinical subtype remains biologically heterogeneous. Studies examining tumor characteristics have found correlations between TNBC and certain molecular genomic features, such as basal cytokeratin (eg, CK5/6) and epidermal growth factor receptor (EGFR) expression, checkpoint kinase (Chk1), p53 alterations, and BRCA1 germline mutations. 8 13 TNBC has also been shown to have overlapping features with the basal-like subtype (intrinsic breast cancer classification), 14,15 including a high frequency of poorly differentiated tumors, expression of cytokeratins and EGFR, and p53 mutations. Although the basal-like subtypes are most often triple-negative, they are not equivalent groups. 16,17 In a gene expression study of 579 TNBCs, 73% of TNBCs showed basal-like characteristics. 18

e9 Treating Triple-Negative Breast Cancer The Cancer Genome Atlas (TCGA) project reporting of breast cancer molecular analysis also showed significant overlap between TNBC and basal-like features and similarity to high-grade serous ovarian cancer. 19 The TCGA basal-like subanalysis and the primary TNBC analysis identified several involved pathways that included PTEN, PI3K/PTEN, cell cycle, and growth factors, and highlighted the heterogeneity of TNBC. 19,20 Recently a large metaanalysis of 587 TNBC gene expression profiles from 21 data sets described 7 TNBC subtypes with 6 potentially targetable pathways. 21 Those subtypes consist of 2 basal-like types (BL1 and BL2), immunomodulatory (IM), mesenchymal (M), mesenchymal stem-like (MSL), luminal androgen receptor (LAR), and unstable (UNS). The BL1 subtype was characterized by cell cycle/cell division and DNA damage response pathways (and association with the BRCA deficiency). The BL2 subtype was typified by growth factor signaling such as via the EGFR pathway. The IM subtype was named for a cluster that contained immune-related processes. The M and MSL types were both associated with motility and epithelial to mesenchymal transition (EMT), but the MSL had more stem cell like features. The LAR subtype contained the genes involved in androgen and steroid pathways. In addition, various cell lines were classified into these TNBC subtypes and evaluated for responses to therapy designed to match to the appropriate representative subtype. 21 A Web-based subtyping tool was also created that allowed normalized gene expression data to be analyzed for TNBC subtypes. 22 Treating TNBC: Where Are We Now? Currently, cytotoxic chemotherapy is the standard therapeutic approach for TNBC. TNBC is relatively chemosensitive, as evidenced by relatively higher pathologic complete response (pcr) rates compared with other subtypes, such as ER-positive/HER2-negative. 23 This may be explained partially by the fact that TNBC is often associated with a high grade and proliferative rate. However, despite higher response rates, they have an inferior outcome with respect to time to recurrence and overall survival (OS) after chemotherapy. 4 Preclinical studies and subgroup analyses within clinical trials have suggested that certain chemotherapeutic agents have differential antitumor activity in TNBC compared with non- TNBC. Thus, certain classes of chemotherapeutic agents have been preferentially evaluated in patients with TNBC. Microtubule-Targeting Agents Based on the TNBC subtypes, the basal-like types are postulated to respond well to taxanes because of their expression of proliferation genes. 21 Efficacy of paclitaxel specifically for TNBC (compared with non-tnbc) has been observed in subtype analyses within clinical trials. 24,25 However, these associations between TNBC s have not been consistent; a meta-analysis has shown benefit for taxane in the adjuvant setting independent of ER status. 26 Ixabepilone is an epothilone B analogue that stabilizes microtubules and has activity in patients with TNBC as a monotherapy. 27 In a phase III study of ixabepilone and capecitabine versus capecitabine, improvements in response rate and median progression-free survival (PFS) were noted for the combination regimen in the TNBC subset compared with the ER-positive subset (response rate, 27% vs 9%, and median PFS, 4.1 vs 2.1 months, respectively). 28 However, no significant benefit was noted in terms of median OS for patients with TNBC (hazard ratio [HR], 0.83; P<.05). 29 Eribulin is a halichondrin B analogue that perturbs microtubule dynamics. It demonstrated a survival benefit in heavily pretreated metastatic breast cancer (MBC) compared with the treatment of physician s choice in the EMBRACE trial. 30 The phase III 301 trial compared eribulin with capecitabine in 1102 pretreated patients with MBC, and reported a significant improvement in median OS associated with eribulin among the subset of patients with TNBC in an exploratory analysis (14.4 vs 9.4 mo; HR, 0.7; P=.01). 31 However, in the published version of this study, the authors did not report any significant difference in benefit among the subgroups. 32 Given that patients with metastatic TNBC often require multiple lines of chemotherapy, eribulin and capecitabine are both likely to remain useful. Eribulin is also being evaluated in combination with carboplatin (neoadjuvant) and everolimus (metastatic) for the treatment of for TNBC. 33,34 Platinum Salts TNBCs have been associated with BRCA mutation status. BRCA-mutated tumors have defects in their

e10 Morikawa and Seidman homologous recombination repair mechanism and are thought to be particularly vulnerable to DNAdamaging agents. 35 In addition to the association of TNBC with germline and somatic BRCA pathway defects, 19 some TNBCs, even in the absence of known BRCA mutations, have demonstrated homologous DNA-repair deficiency and potential vulnerability to DNA-damaging agents, such as platinum salts, similar to that observed in BRCA-mutated tumors. 36 In this context, cisplatin and carboplatin have been evaluated in unselected TNBC. Most recently, the TBCRC009 phase II study reported a response rate of 25.6% to platinum monotherapy (cisplatin or carboplatin) as first- or secondline therapy of metastatic TNBC. 37 Of the 87 patients enrolled, 11 were known BRCA mutation carriers. The response rate was higher in BRCA mutants (54.5%) than wild-type BRCA (19.7%), but median PFS (3.3 months for BRCA mutants and 2.8 months for wild-type BRCA) and median OS (13.7 months for BRCA mutants and 10.9 months for wild-type BRCA) were not statistically significantly different (P>.05). In the metastatic setting, the combination of gemcitabine and carboplatin was evaluated in a prospective trial showing modest antitumor activity. 38 40 Selected examples of platinum regimens in locally advanced/metastatic TNBC along with nonplatinum regimens are shown in Table 1. Notably, gemcitabine/carboplatin was the comparator arm in phase II and III trials in metastatic TNBC evaluating the addition of the investigational drug iniparib. 41,42 In the gemcitabine/carboplatin control arm, the objective response rate was 30% and median PFS and OS were 4.1 and 11.1 months, respectively. Notably, no direct comparison of gemcitabine and platinum versus other chemotherapy combinations has ever been performed in a prospective randomized trial for metastatic TNBC. Therefore, whether this combination has any particular or unique advantage over other nonplatinum standard chemotherapy regimens in TNBC remains unclear. Notably, results of the phase III TNT trial comparing single-agent docetazel and carboplatin in patients with TNBC were recently reported. In unselected patients with TNBC, the efficacy of carboplatin monotherapy was similar to that of docetaxel, which is regarded as one of the most active agents against MBC in general. In the subset of patients with a BRCA mutation, carboplatin showed differential benefit compared with docetaxel (response rate, 68.0% vs 33.3% for patients with BRCA mutations and 28.1% vs 36.6% for those with wild-type BRCA status, respectively; PFS, 6.8 vs 3.1 months, respectively, in patients with BRCA mutations). An assay for homologous repair deficiency used in this study was not predictive of a differential carboplatin benefit. 43 Currently, a randomized phase III trial is evaluating nab-paclitaxel in combination with gemcitabine or carboplatin versus gemcitabine and carboplatin alone in metastatic TNBC (tnacity trial; ClinicalTrials.gov identifier: NCT01881230). The neoadjuvant use of platinum salts has been evaluated in BRCA-mutated and unselected TNBC. A recent meta-analysis of 28 neoadjuvant studies showed a pooled pcr rate of 45% (95% CI, 40% 49%) for platinum-containing regimens. 44 The inclusion of a platinum seemed to be associated with a higher proportion of pcrs compared with non platinum-containing regimens (response rate, 1.45; 95% CI, 1.25 1.68; P<.001). 44 The GeparSixto study evaluated the addition of carboplatin to paclitaxel and pegylated liposomal doxorubicin (plus bevacizumab in TNBC) in the neoadjuvant setting. 45 The addition of carboplatin was associated with a significant increase in pcr for TNBC (37.9% vs 58.7%; P<.05), although a high rate of discontinuation (39%) was noted for the regimen that included carboplatin. 45 In the CALGB 40603 trial in which carboplatin and/or bevacizumab was added to anthracycline/ paclitaxel chemotherapy in 454 patients with TNBC, the breast pcr rate improved with the addition of carboplatin (from 46% to 60%; P=.018), with more marrow suppression noted in the carboplatin group: grade 3/4 neutropenia (56% vs 20%) and thrombocytopenia (22% vs 4%). 46 Thus the available evidence suggests that platinum can improve the pcr rate. Patients with TNBC who experience a pcr have significantly better survival than those who do not. 23 However, an association between platinum use and relapse-free survival and OS still needs to be elucidated (ie, does the incremental improvement in pcr translate to fewer downstream events). The NeoALTTO and ALTTO trials recently reported that the addition of lapatinib, which improves pcr from 29.5% to 51.0% in the neoadjuvant setting, did not translate into a significant benefit in disease-free survival or OS when used as adjuvant therapy. 47,48 At the time of this writing, whether

e11 Treating Triple-Negative Breast Cancer Table 1 Selected Chemotherapy Regimens With TNBC-Specific Outcomes in the Metastatic Setting Drugs Docetaxel (doc) vs carboplatin (carb) carboplatin arm (gemcitabine and carboplatin +/ iniparib) cisplatin cisplatin Patients With TNBC Study Population TNBC RR TNBC TTP/PFS a TNBC OS a Reference Total Study Design Patients Phase III 376 376 No previous taxane or platinum in metastatic setting Phase III, gemcitabine and carboplatin only arm 31% (carb) vs 36% (doc) 3.1 (carb) vs 4.5 mo (doc) 12.4 (carb) vs 12.3 mo (doc) Tutt et al, 43 2014 519 519 Prior 0 2 lines 30% 4.1 mo 11.1 mo O Shaughnessy et al, 42 2011 Phase II 144 144 Relapsed after 1 adjuvant/nac regimen or firstline metastatic regimen with anthracycline Phase II 38 38 Second-line, anthracycline and taxane pretreated Retrospective 33 33 Prior anthracycline review Retrospective review Phase II 31 31 Prior anthracycline 33% 5.1 mo 73% at 1 y Kohail et al, 89 2012 42% 5.4 mo 13.9 mo Wang et al, 90 2012 cisplatin 27% 5 mo 14 mo Ozkan et al, 91 2012 36 17 Prior taxane 5.3 mo Koshy et al, 92 cisplatin 2010 32% 5.5 mo 11 mo Maisano et carboplatin al, 38 2011 Phase II 47 15 First-line 20% Yardley et al, 39 carboplatin 2008 Phase II 150 30 Prior taxane or no 30% Loesch et al, 40 carboplatin prior taxane 2008 Phase II 56 14 First-line 35.7% 6 mo Aogi et al, 93 paclitaxel 2011 Cisplatin (cis) arm Randomized 115 115 No more than 1 10% (cis) 1.5 mo (cis) Baselga et al, 70 (cisplatin +/ cetuximab) phase II prior 2013 Cisplatin or carboplatin Phase II 87 87 First- or secondline 25.6% 2.9 mo 11 mo Isakoff et al, 37 2014 Cisplatin (cis) and Randomized 154 72 Prior 0 1 line 30% 5.1 mo 12.8 mo O Shaughnessy irinotecan (iri) arm phase II (cis/iri) (cis/iri) (cis/iri) et al, 42 2014 (cisplatin, irinotecan +/ cetuximab) Docetaxel/cisplatin vs docetaxel/capecitabine Docetaxel Capecitabine Ixabepilone Randomized phase II Retrospective review Retrospective review Retrospective subanalysis 53 53 First-line 63% vs 15.4%; P=.002 10.9 vs 4.8 mo HR, 0.29; P<.001 32.8 vs 21.5 mo HR, 0.41; P=.027 Fan et al, 94 2013 162 28 First-line 0% at 2 y 37.8% at 2 y Gonçalves et al, 95 2009 75 15 Prior anthracycline 0% 3.6 mo Gilabert et al, 96 2011 11 42 Prior anthracycline, 6% 55% 1.6 4.6 mo Perez et al, 27 taxane, 2010 capecitabine 6.3% Aogi et al, 97 2013 Ixabepilone Phase II 52 16 Prior anthracycline Ixabepilone + capecitabine vs capecitabine Oral vinorelbine and capecitabine Metronomic oral cyclophosphamide and capecitabine Oral cyclophosphamide and capecitabine Eribulin vs capecitabine Phase III subanalysis 752 71 Prior anthracycline 27% (iri + cis) vs 9% (cis) 4.1 vs 2.1 mo HR, 0.68; P<.05 Pivot et al, 28 2009 Phase II 54 9 First-line 22.2% 4 mo Tubiana- Mathieu et al, 98 2009 Phase II 51 10 No more than 1 prior line and no prior 5-FU, capecitabine, or cyclophosphamide Phase II 45 12 No more than 1 prior line Phase III subanalysis 1102 284 Prior anthracycline 44.4% 10.7 mo Yoshimoto et al, 99 2012 41.7% 220.5 d Tanaka et al, 100 2010 14.4 vs 9.4 mo HR, 0.7; P<.05 Kaufman et al, 31 2012 Eribulin Phase II 56 12 First-line 16.7% 3.4 mo McIntyre et al, 101 2014 Anthracycline and cyclophosphamide Retrospective review 110 25 First-line 56.5% 8.1 mo 25.4 mo Yi et al, 102 2010 Paclitaxel arm (from paclitaxel +/ bevacizumab) Phase III, paclitaxel-only arm 722 233 First-line 4.6 mo Miller et al, 103 2007 Abbreviations: HR, hazard ratio; NAC, neoadjuvant chemotherapy; OS, overall survival; PFS, progression-free survival; RR, response rate; TNBC, triple-negative breast cancer; TTP, time to progression. a Median, unless otherwise noted.

e12 Morikawa and Seidman platinum agents deserve routine inclusion in regimens for TNBC of any stage remains uncertain. Based on a mechanistic rationale and efficacy in nonrandomized trials, platinum is perhaps a reasonable choice for a subset of patients with TNBC who are more likely to have a DNA-repair deficiency. 44 Several studies have reported biomarkers that may be helpful in selecting an appropriate population. 49 For example, a homologous recombination deficiency assay is currently being evaluated as a biomarker in sporadic TNBC, with a primary end point of correlation with pcr (ClinicalTrials.gov identifier: NCT01982448). Treating TNBC: Where Are We Going? As knowledge of TNBC molecular features expands, several potential therapeutic targets have emerged, which include the hallmarks of cancer : DNA repair and damage checkpoint mechanism, proliferation and growth, angiogenesis, and invasion/metastases. Whether disease can successfully be controlled through targeting a single pathway in a subpopulation of TNBC (as in the case of some HER2-driven tumors) is unclear. It is likely that a rational combination of systemic agents will need to be found, which may include conventional chemotherapy (eg, platinum drug and poly(adp-ribose) polymerase [PARP] inhibitor). The following are selected examples of biologic and immunologic agents currently under investigation. PARP Inhibition Molecular genomic analyses have identified potentially targetable pathways and areas of drug development for TNBC (Table 2). In addition to the platinum drugs, PARP inhibitors have been evaluated in TNBC to target their BRCA-ness. PARP1 is a DNA repair enzyme involved in single-strand break (SSB). When used in a setting of BRCA-related repair deficiency, it is thought to facilitate synthetic lethality, wherein SSB repair deficiency leads to double-strand breaks that cannot be repaired by homologous recombination. 50 Preclinical and clinical studies have shown a particular sensitivity to PARP inhibition in BRCA-mutated tumors. 50 Initially, the investigational drug iniparib was thought to be a PARP inhibitor and was initially evaluated in a randomized phase II study with gemcitabine and carboplatin. 41 Despite the promising initial data, 41 a larger phase III trial did not show significant benefit for iniparib when added to chemotherapy. 42 Additional studies showed that the drug demonstrated poor PARP inhibition. However, other PARP inhibitors are being evaluated in patients with BRCA-mutated and sporadic TNBC. Olaparib as a monotherapy did not show any confirmed objective response in a phase II study for TNBC, 51 although modest efficacy was noted in patients with a BRCA mutation. 52 It is currently being evaluated in combination with chemotherapy or other biologic agents, such as cediranib (anti vascular endothelial growth factor [VEGF]). 53,54 Veliparib in combination with carboplatin (V+C) was evaluated in the I-SPY-2 neoadjuvant trial. There was a 92% predictive probability (Bayesian) that the addition of V+C would be statistically superior to the standard anthracycline/taxane-based therapy alone based on the observed pcr rates: 52% (28% 67%) for V+C and 26% (9% 43%) in the control arm. 55 Finally, perturbation of the PI3K/ AKT and cell cycle pathways (eg, CDK1 inhibition) have been shown to sensitize cells to PARP inhibition even in BRCA-proficient tumors in preclinical models, and clinical studies of combination therapy with PARP and PI3K inhibitors are underway (ClinicalTrials.gov identifier: NCT01623349). 56 Other efforts to target DNA damage repair and cell cycle checkpoint mechanisms include use of histone deactylase (HDAC), heat-shock protein 90 (Hsp90), and Chk1 inhibition. Although a phase II study of Chk1 inhibition in combination with irinotecan demonstrated limited activity in TNBC (response rate, 4%), preclinical studies have suggested that these strategies may be particularly relevant in TNBC associated with p53 mutation and BRCA mutation and await further clinical evaluation. 57,58 Anti-VEGF Therapy VEGF is frequently expressed in TNBC and is a potential therapeutic target. 21 Anti-VEGF therapy, particularly bevacizumab, has been studied with modest and inconsistent improvements in response rate and PFS but not for OS in the metastatic setting. 59 Recently the phase III BEATRICE trial examined the addition of bevacizumab to standard adjuvant chemotherapy in TNBC and failed to show a benefit in DFS or OS. 60 In CALGB 40603, the addition of neoadjuvant bevacizumab was associated with a significant improvement in breast-only pcr but not for

e13 Treating Triple-Negative Breast Cancer Table 2 Potentially Targetable Pathways and Selected Examples of Drugs Being Preferentially Evaluated in TNBC a Pathway Drugs Mechanism ClinicalTrials.gov Identifier DNA repair Veliparib Olaparib PARP inhibitor PARP inhibitor NCT01251874 NCT01351909 NCT01167192 TP53 mutation/checkpoint UCN-01 Chk1 inhibitor NCT00031681 P276-00 CDK inhibitor NCT01333137 Ganetespib Hsp90 inhibitor NCT01677455 Dinaciclib CDK inhibitor NCT01676753 Growth factor pathway MM-121 Anti-HER3 NCT01451632 Panitumumab Anti-EGFR NCT00894504 Anti-VEGF Cabozantinib Anti-VEGF and anti-met NCT01738438 Sorafenib Multi-TKI NCT01194869 AR pathway Enzalutamide Anti-AR NCT01889238 Notch pathway RO4929097 Gamma-secretase NCT01151449 inhibitor LDE225 Smoothened antagonist NCT02027376 Apoptosis LCL161 Smac pathway NCT01617668 PI3K/Akt pathway GDC-0941 PI3K inhibitor NCT01918306 BKM120 PI3K inhibitor NCT01629615 Immunotherapy PD-1 antibody Immune checkpoint NCT01928394 MUC1 peptide vaccine Vaccination NCT00986609 Autologous cmet-redirected T cells Adoptive T-cell therapy NCT01837602 Abbreviations: AR, androgen receptor; CDK, cyclin-dependent kinase; EGFR, epidermal growth factor receptor; PARP, poly(adpribose) polymerase; TKI, tyrosine kinase inhibitor; VEGF, vascular endothelial growth factor. a Active on ClinicalTrials.gov as of June 1, 2014. breast and axillary nodes (36% increase with P=.06 for breast and axilla). 46 On the other hand, the TNBC subset analysis of GeparQuinto showed improvement in pcr (breast and lymph nodes): 27.9% to 39.3% with addition of bevacizumab (P=.003). 61 Again, as with platinums, it is unclear (perhaps unlikely) that this incremental improvement is clinically meaningful and will translate to an improvement in DFS or OS. In addition, biomarker studies (eg, MERIDIAN trial) are being conducted to better select patients who may derive greater benefit from bevacizumab therapy. 62 Small molecule anti-vegf drugs, such as sunitinib and sorafenib, have been also evaluated and have shown unclear benefit. Sunitinib failed to show benefit as a monotherapy or in combination with chemotherapy in HER2-negative MBC, 63,64 whereas sorafenib in combination with chemotherapy has shown a modest association with PFS improvement in phase II studies, although results of the phase III trial are currently awaited. 65 68 Growth Factor Signaling Pathway EGFR is another receptor frequently expressed in TNBC and associated with the BL2 subtype. The EGFR-targeting antibody cetuximab has been evaluated in otherwise unspecified TNBC, with a suggestion of either modest benefit 69,70 or improved response rate but not PFS or OS. 71 Ongoing studies for EGFR targets include panitumumab with platinumcontaining chemotherapy (ClinicalTrials.gov identifiers: NCT00894504 and NCT01009983). Other growth factor pathways identified as potential targets include the fibroblast growth factor receptor (FGFR) pathway. Preclinical studies have shown antitumor activity with anti-fgfr treatment, but a clinical trial examining the FGFR inhibitor dovitinib in breast

e14 Morikawa and Seidman cancer did not report any responses, although the difference in the proportion of patients experiencing stable disease and a minor reduction in tumor size suggested that patients with FGFR amplification may benefit from this strategy. 72 Recently, DUSP4 loss, a negative regulator of the Ras-ERK pathway, was reported as a potential prognostic biomarker for basal-like breast cancer after neoadjuvant chemotherapy and MEK as a potential target in this setting. 73 Phase I and pharmacodynamics studies with MEK inhibitors are ongoing in patients with TNBC (ClinicalTrials.gov identifiers: NCT01467310, NCT01155453, and NCT01363232). Cell Motility and EMT Features Characteristics of the M-like subtype include cell motility and EMT features. Steroid receptor coactivator (SRC) is thought to play a role in migration and invasion. 21 Dasatinib, an oral SRC inhibitor, has shown antitumor activity in preclinical TNBC studies. In a phase II trial of dasatinib monotherapy conducted among 44 patients with metastatic TNBC, dasatinib showed modest antitumor activity (overall response rate, 4.7% with median PFS of 8.3 weeks; 95% CI, 7.3 15.3). 74 Dasatinib is currently being studied in combination with chemotherapy (weekly paclitaxel) for MBC (ClinicalTrials.gov identifier: NCT00820170). The LAR Subtype Although its prevalence is higher in ER-positive breast cancer, androgen receptor (AR) is expressed in approximately 30% of TNBC and thought to be another potential target, especially for the LAR subtype. 21,75,76 AR-targeted drugs, such as bicalutamide and enzalutamide, approved for prostate cancer, are currently being evaluated in breast cancer, including TNBC (ClinicalTrials.gov identifier: NCT 011889238). 76 In the study evaluating bicalutamide, 150 mg/d orally in ER/PR-negative breast cancer, 14% were AR-positive with no confirmed complete response or partial response, but with 19% stable disease. 76 In the TNBC subtype analysis, it is interesting to note that LAR subtype cell lines had a PI3K mutation and were sensitive to PI3K inhibition. 21 PI3K/AKT Pathway PTEN loss is commonly noted in basal-like breast cancer and is a negative regulator of the PI3K/AKT pathway. 17 mtor inhibition with everolimus has antitumor activity in a basal-like breast cancer xenograft model. 77 In the TNBC preclinical study, cell lines with PIK3CA mutations and PTEN deficiency were highly sensitivity to dual PI3K/mTOR inhibition. 21 An intact PI3K/AKT/mTOR pathway has been shown to also contribute to DNA damage repair preclinically, and pathway inhibition sensitizes cells to PARP inhibition. 78 PI3K/AKT pathway inhibition is currently being evaluated as monotherapy (ClinicalTrials.gov identifiers: NCT01629615 and NCT01790932) or in combination with other agents (ClinicalTrials.gov identifiers: NCT01918306, NCT01623349, and NCT01939418). Immunotherapy TNBC has been associated with various characteristics in which immunotherapy would be a rational and attractive mode of therapy. Tumor-infiltrating lymphocytes (TILs) have been associated with a better prognosis among patients with TNBC. 77 An analysis of stromal TILs in TNBC from the ECOG 2197 and 1199 adjuvant therapy trials reported that for every 10% incremental increase in stromal TILs there was an associated 18% reduction in the risk for distant recurrence (P=.04) and 19% reduction in risk of death (P=.01). 80 A higher programmed death-ligand 1 (PD-L1) expression has been reported for TNBC compared with other breast cancer subtypes, and immune checkpoint therapy with a PD-L1 inhibitor is now being evaluated in breast cancer 81 (ClinicalTrials.gov identifiers: NCT01714739 and NCT01375842). Pembrolizumab, an anti PD-1 antibody, was reported to have single-agent activity in the phase I Keynote-012 trial. 82 MUC1 is a tumor antigen highly expressed in early-stage basal-like breast cancer, 83 and an MUC1 peptide vaccine trial is ongoing (ClinicalTrials.gov identifier: NCT00986609). With success reported in hematologic malignancies, adoptive T-cell therapy is also being evaluated in breast cancer (ClinicalTrials.gov identifier: NCT01837602). Ideally, molecularly directed studies would be conducted among specific subgroups of TNBC with targetable pathways. Because TNBC represent approximately 20% of all breast cancer, further subsetting leads to smaller source populations for clinical trials. Examples of TNBC and subtype prevalence are shown in Figure 1. In a recent sobering report, a molecular genomic alteration screening process was used for patient accrual into targeted therapeutic trials for breast cancer. Of 423 patients, rational targeted therapy was identified and offered in only

e15 Treating Triple-Negative Breast Cancer A B Missing HR /HER2+ 1% 7% 7% 5% 4% Atlanta Pathways LACE Unclassified Basal-like 4% 11% 10% 16% LACE + Pathways Carolina TN 13% 11% 30% HER2+/ER- 8% 13% HR+/HER2+ 7% 9% 11% Luminal B 9% 21% HR+/HER2 56% 68% 73% Luminal A 53% 56% 0% 10% 20% 30% 40% 50% 60% 70% Prevalence C 80% 0% 10% 20% 30% 40% 50% 60% Prevalence UNS 19% LAR 9% MSL 6% M 24% IM 18% BL2 7% BL1 17% 13% of patients. 84 If appropriate biomarkers are used to select the study population (ie, subset of TNBC), the hope is that the strength of the therapeutic effect would be strong enough to detect significant and clinically meaningful antitumor activity, even with a modest number of patients. Therefore, the concurrent development of reliable and validated biomarkers with novel targeted therapeutics is critical. Conclusions Although the amount of molecular genomic information informing potentially targetable pathways is expanding, additional predictive biomarkers and 0% 5% 10% 15% 20% 25% 30% TCGA Primary Breast Cancer (N=163) Figure 1 Subtypes of breast cancer. (A) Prevalence of breast cancer subtypes according to estrogen receptor (ER)/progesterone receptor (PR)/HER2 status from population-based studies. The Atlanta study includes young women (aged 20 54 years) diagnosed with invasive breast cancer from 1990 through 1992 (N=476). 85 The Pathway study 104 includes women diagnosed with invasive breast cancer from 2005 through 2013 (N=2172) and The Life After Cancer Epidemiology (LACE) study 105 includes women diagnosed with early-stage breast cancer from 1996 through 2000 (N=2135), reported by Sweeney et al. 86 (B) Prevalence of breast cancer subtypes according to intrinsic breast cancer subtype from population-based studies. The Carolina Breast Cancer Study (N=1424) subtype analysis was reported by Millikan et al, 87 and the combined subtype analysis on a subset of patients from each cohort (N=1319) was reported by Sweeney et al. 86 (C) Triple-negative breast cancer (TNBC) subtyping conducted on primary breast cancers from The Cancer Genome Atlas (N=163). Data from Mayer IA, Abramson VG, Lehmann BD, Pietenpol JA. New strategies for triple-negative breast cancer deciphering the heterogeneity. Clin Cancer Res 2014;20:782 790. Abbreviations: BL1/2, basal-like type 1/2; ER, estrogen receptor; HR, hormone receptor; IM, immunomodulatory; LAR, luminal androgen receptor; M, mesenchymal; MSL, mesenchymal stem-like; TCGA, The Cancer Genome Atlas; TN, triple-negative; UNS, unstable. associated treatment options have not been incorporated into clinical practice to improve the outcomes in TNBC. Conventional cytotoxic chemotherapy is still standard treatment for TNBC. An incremental benefit of platinum therapy for TNBC is demonstrated by neoadjuvant studies, but the impact of platinums on outcomes other than pcr is uncertain, limiting their incorporation into standard clinical practice. Whether the gain from the incremental improvement in pcr rates will translate to improved relapse-free survival or OS is unclear. The discovery of many candidate targetable pathways may provide more rational treatment options and approaches for TNBC. The efficiency of clinical trial design for tar-

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