Are There Any Clinically Relevant Subgroups of Triple-Negative Breast Cancer in 2018?

Transcription

1 Clinical Review Are There Any Clinically Relevant Subgroups of Triple-Negative Breast Cancer in 2018? Jack J. Chan, Tira J.Y. Tan, and Rebecca A. Dent National Cancer Centre Singapore and Duke-National University of Singapore Medical School, Singapore ASSOCIATED CONTENT See accompanying commentary on page 290 Abstract The working immunohistochemical definition of triple-negative breast cancer (TNBC) is admittedly reductionist and has only limited usefulness for informing oncologists about therapeutic decisions beyond chemotherapy. Early molecular taxonomies of TNBC based heavily on gene expression profiling, which is not readily available in the clinic today, do not necessarily encompass other molecular targets already incorporated into rationally designed clinical trials. We state that it is possible to delineate five subgroups of TNBC relevant to present-day clinical practice and cover the evidence that lends credence to emerging biomarker-directed treatment strategies for each subgroup. DOI: INTRODUCTION Triple-negative breast cancers (TNBCs) lack expression of estrogen, progesterone, and human epidermal growth factor receptor 2 (HER2)/c-erbB2 receptors but otherwise differ in histomolecular phenotypes. They have greater and earlier risk of relapse 1,2 and shortersurvival 2 thannon-tnbcs.although patients with TNBC have greater sensitivity to chemotherapy, those who have early TNBC have unfavorable prognosis compared with their hormone receptor positive and HER2-positivecounterparts. 2 The term triple-negative paradox epitomizes such observations and represents an unmet clinical need for precision medicine in TNBC. Sadly, a dearth of druggable oncogenic drivers andapprovednoncytotoxictreatmentsstill plague clinical management of TNBC. TNBCs are heterogeneous in their molecular drivers and immune traits. Various groups have sought to categorize TNBCs, often with overlap (Fig 1). One such taxonomy by Lehmann et al 3,4 uses gene expression profiling that encompasses four intrinsic molecular subtypes in a revised version with distinct gene ontologies and therapeutic sensitivities. Perou 5 led a collaboration to simplify the classification: TNBCs are stratified as either luminal/ androgen receptor (AR; 20% to 30%) or basal (70% to 80%). The luminal/ androgen receptor type is subdivided into luminal A plus B or HER2-enriched, and the basal type is subdivided into claudin-low/mesenchymal or basal-like. These models enhance our understanding of the molecular underpinning of TNBCs and engender clinical research. However, they leverage sophisticated genomic expression measurement techniques not readily accessible for most oncology practices, and they do not include tractable molecular targets among those already elucidated within TNBCs. 6,7 In this review, we propose five clinically relevant subgroups of TNBC: defective DNA repair, inflamed phenotype, androgen receptor positive, PI3K/AKT/PTEN altered, and unique antigen expressing. Here,wesummarizetheevidencethat supports refining therapy selection by Copyright 2018 by American Society of Clinical Oncology Volume 14 / Issue 5 / May 2018 n jop.ascopubs.org 281

2 Chan, Tan, and Dent TNBC 20% to 30% 70% to 80% Luminal/androgen receptor Basal Luminal A + B HER2-enriched Claudin-low/ mesenchymal Basal-like Basal-like 1, basal-like 2, mesenchymal Fig 1. Overlaps in the stratification schemas of triple-negative breast cancer (TNBC) from Lehmann et al 4 and the collaboration led by Perou. 5 Lehmann s revised intrinsic molecular subtypes are in italics. HER2, human epidermal growth factor receptor 2. using these subsets and suggest an algorithm to test for these subsets (Fig 2). DEFECTIVE DNA REPAIR SUBGROUP This subgroup is marked by defective DNA repair, focusing on homologous recombination repair of single-strand breaks, a process that requires functional BRCA1 and BRCA2 genes among others. Most breast cancers that occur in BRCA1 mutation carriers are triple-negative and/or basal-like. 8 Tumors resulting from a deleterious germline mutation in BRCA1/2 or one of the homologous recombination repair pathway genes have homologous recombination deficiency (HRD) and are characterized by genomic instability, increased frequency of copy number gains and losses, and heightened sensitivity to damage by DNA crosslinking. 9 A subset of TNBCs has been postulated to be phenotypically and molecularly similar to hereditary BRCA1-associated breast cancers, a concept known as BRCAness. 10 Acquired HRD can TNBC Germline BRCA1/2 and HR pathway gene mutation testing (Somatic BRCA mutation testing) (HRD score, HRD scar biomarkers) Histologic examination for tumor-infiltrating lymphocytes (?) (Immune signature by gene expression microarray) IHC for androgen receptor (?) (Androgen-related gene signature by genomic diagnostic assay) (Sequencing for PIK3CA/AKT1/ PTEN alterations) IHC for targetable cancer epithelial antigens Tested negative Defective DNA repair: Inflamed phenotype: Androgen receptor positive: PI3K/AKT/PTEN altered: Unique antigenexpressing: Unclassified TNBCs: platinums and PARP inhibitors immunotherapy androgen blockade AKT inhibitors antibody-drug conjugates chemotherapy and clinical trials Fig 2. Proposed algorithm to define five clinically relevant subgroups of triple-negative breast cancer (TNBC) that are matched to their therapeutic sensitivities. Diagnostic tests with less well-established roles are shown in parentheses. TNBCs that tested negative are unclassified and can be treated with cytotoxic chemotherapy or in clinical trials. HR, homologous recombination; HRD, homologous recombination deficiency; IHC, immunohistochemistry; PARP, poly (ADP-ribose) polymerase. 282 Volume 14 / Issue 5 / May 2018 n Journal of Oncology Practice Copyright 2018 by American Society of Clinical Oncology

3 Triple-Negative Breast Cancer Subgroups occur in sporadic breast cancers through epigenetic mechanisms such as methylation of BRCA1/2 or somatic mutations as part of the BRCAness spectrum. 10,11 DNA-based companion diagnostic assays have been designed on the basis of whole genome tumor loss-of-heterozygosity score, 12 telomeric allelic imbalance score, 13 and large-scale state transitions score. 14 Hypothesizing that BRCAness in sporadic TNBC predicts for sensitivity to DNA-damaging agents, Isakoff et al 15 reported that a BRCA-like genomic instability signature, including HRD score, heterozygosity score, and large-scale state transitions, in 32 (48%) of 66 patients with sporadic TNBCs could discriminate responders from nonresponders. More recently, the use of mutational signatures derived from whole genome sequencing (HRDetect) identified BRCA1/2-deficient tumors with 98.7% sensitivity. 16 Two drug classes adopted in the DNA repair defect targeted approach are platinums and poly (ADP-ribose) polymerase (PARP) inhibitors. Role of Platinums Enthusiasm for using platinums against BRCA-related TNBCs was founded on their mechanism of inflicting DNA damage via adducts and DNA crosslinking. A proof-of-principle study reported a striking pathologic complete response (pcr) of 61% to neoadjuvant cisplatin in 107 Polish patients with stage I to III germline BRCA1-associated breast cancers, most of which were triple negative. 17 The phase II GepartoSixto trial evaluated an investigational neoadjuvant chemotherapy backbone (once-per-week paclitaxel with nonpegylated liposomal doxorubicin for 18 weeks) in patients with either TNBC or HER2-positive breast cancer. Bevacizumab was administered to patients with TNBC, with or without once-perweek carboplatin at area under the curve of 1.5 to In the TNBC subgroup, carboplatin improved pcr rate and diseasefree survival. 18a A hypothesis-generating secondary analysis of the TNBC subpopulation suggested that patients with BRCA1/2 wild-type disease benefited from the addition of carboplatin but not those with mutations. 19 Conversely, CALBG is another randomized phase II trial in TNBC which showed that pcr rate, although not event-free survival or overall survival, was increased by incorporating once-every-3-week carboplatin to a standard alkylator-containing chemotherapy backbone. 19a The most recent BrighTNess phase III study confirmed that the inclusion of carboplatin to standard neoadjuvant chemotherapy for TNBC enhanced pcr rate, but no incremental benefit was gained from adding veliparib, a PARP inhibitor, plus carboplatin. 19b Data concerning the predictive value of HRD scar biomarkers to neoadjuvant platinum sensitivity are discordant. In the singlearm phase II PrECOG 0105 study of carboplatin, gemcitabine, and iniparib, high HRD score and HRD identified TNBC tumors, including BRCA1/2 nonmutated tumors, more likely to respond to platinum-containing therapy. 20 In contrast, HRD tumors responded better than non-hrd tumors in the GepartoSixto trial, regardless of carboplatin exposure (test of interaction P 5.77). 21 Shifting to the metastatic setting, the phase II TBCRC009 trial that evaluated first- or second-line platinum monotherapy demonstrated that individuals with BRCA1/2 mutations were more likely to respond than wild-type individuals (55% v 20%; P 5.02) but without durability of response or improvement in progression-free survival (PFS) or overall survival. Platinumresponsive tumors exhibited higher values for large-scale state transitions and heterozygosity score assays. 15 In the randomized phase III TNT trial (N 5 376), carboplatin was compared with docetaxel as first-line treatment. Correlative analyses according to BRCA1/2 mutation status showed that patients with mutations fared better with carboplatin than with docetaxel treatment, with greater overall response rate (ORR; 68% v 33%; P 5.03; test of interaction P 5.01) and PFS (6.8 v 3.1 months; P 5.03; test of interaction P 5.03). 22 In contrast, HRD score, 22 BRCA1 methylation, and BRCA1 silencing 23 did not select for sensitivity to carboplatin over docetaxel (test of interaction P 5.91,.35, and.07, respectively). Mounting evidence contributes to the belief that platinums are highly active in germline BRCA-mutant TNBC. In the neoadjuvant setting, preoperative platinum-based chemotherapy is likely a reasonable choice for patients with locally advanced disease who are young and fit. However the issue of whether germline BRCA mutation status predicts for better response from neoadjuvant platinum therapy remains controversial. Data are lacking on adjuvant platinum; the phase III NRG-B003 trial (NCT ), stratified for BRCA mutation status, is in progress. In the metastatic TNBC setting, platinum monotherapy is included in our armamentarium, with data most convincing for first-line platinum for patients with germline BRCA mutation. Role of PARP Inhibitors Defining BRCAness led to development of PARP inhibition as a therapeutic strategy exploiting synthetic lethality. 24 PARP enzymes are required for the efficient repair of DNA singlestrand breaks through base excision repair. When BRCA1/2- dependent homologous recombination repair is disabled, Copyright 2018 by American Society of Clinical Oncology Volume 14 / Issue 5 / May 2018 n jop.ascopubs.org 283

4 Chan, Tan, and Dent PARP inhibition leads to the formation of double-stranded breaks that cannot be accurately repaired, chromosomal instability, cell cycle arrest, and apoptotic cell death. 25 In a proof-of-concept phase II trial, two sequential cohorts with BRCA-mutated advanced breast cancer were treated with olaparib at different doses, yielding ORRs of 25% to 54%. 26 Results of two phase III registration trials that evaluated PARP inhibitors versus chemotherapy of physician s choice in pretreated patients with BRCA-mutant, HER2- negative metastatic breast cancer were reported in Both OlympiAD (olaparib) and EMBRACA (talazoparib) reached their primary end point of PFS. 27,28 Notably, for the prespecified TNBC subgroup in both trials, the PARP inhibitor was favored over the chemotherapy comparator arm (that excluded a platinum) in terms of PFS prolongation (hazard ratio [HR], 0.43;95% CI, 0.29 to 0.63 andhr,0.60; 95% CI,0.41 to 0.87, respectively). The US Food and Drug Administration approved olaparib for this indication in January Regulatory approval for talazoparib is likely to follow. These expand our armamentarium for treating germline BRCA-mutant metastatic breast cancer and potentially offer alternative options to spare or delay the use of cytotoxic drugs. A third trial (BRAVO, NCT ) is underway that is examining niraparib in a similar patient population. Ongoing efforts focus on molecular diagnostics beyond BRCA testing to predict benefit from PARP inhibition and extend the reach of PARP inhibitors to a broader population through combination strategies. Talazoparib Beyond BRCA (TBB) trial (NCT ) is a phase II study of talazoparib in BRCA1/2-negative patients with advanced TNBC and HRD as assessed by the HRD assay, or advanced HER2-negative breast cancer with germline or somatic mutation in homologous recombination pathway genes. Premised on platinum responsiveness as being enriched for defective DNA repair in tumors, DORA (NCT ) is a planned randomized phase II trial of maintenance olaparib versus olaparib plus durvalumab after response to first- or second-line platinum therapy. This trial will be hypothesis-generating on two fronts: whether there is a subset of TNBC to which PARP inhibition is efficacious in lieu of germline BRCA testing and whether PARP inhibition is synergistic with immune checkpoint inhibition. PARP inhibitors are also being investigated in neoadjuvant and adjuvant settings. OlympiA (NCT ) is an ongoing randomized placebo-controlled trial examining 12 months of olaparib as adjuvant treatment of BRCA-mutant TNBC. INFLAMED PHENOTYPE SUBGROUP Several lines of evidence support investigation of immunotherapy in TNBC. We know from microarray signatures that immune activation is observed within TNBC. 4 Extensive analyses have also characterized the immune infiltrate in breast cancers at a histologic level not restricted to the medullary histotype. 29 Early TNBCs are enriched with high levels of tumor-infiltrating lymphocytes; increasing levels of tumorinfiltrating lymphocytes portend better prognosis. 30,31 Stromal tumor-infiltrating lymphocyte levels may also act as a surrogate for pre-existing antitumor immunity and may identify metastatic TNBCs more likely to respond to the anti-programmed cell death-1 (PD-1) monoclonal antibody pembrolizumab. 32 Checkpoint inhibitors relieve the immunosuppressive tumor microenvironment and promote antitumor immune responses. In phase I trials, pembrolizumab as well as antiprogrammed death-ligand 1 (PD-L1) antibodies atezolizumab and avelumab were well-tolerated and showed efficacy signals in TNBCs PD-L1 expression within stroma and/or tumors had been applied in these trials as either a selection or stratification criterion with variable impacts. The phase II KEYNOTE-086 assesses pembrolizumab in advanced TNBC through three cohorts: cohort A, previously treated irrespective of PD-L1 expression; cohort B, treatmentnaïve and PD-L1 positive; and cohort C, pretreated PD-L1 positive. Preliminary results for cohorts A and B were reported in Activity may be greater in patients with less heavily pretreated disease: the ORR was 23% in cohort B versus 5% in cohort A (on multivariate analysis; odds ratio, 4.19; 95% CI, 1.41 to 13.00). 32 ORR in cohort A was independent of PD-L1 expression. Median PFS was numerically similar in both cohorts: 2.0 and 2.1 months, respectively. 36,37 We await the results of KEYNOTE-119 (NCT ), a randomized phase III study comparing pembrolizumab monotherapy with investigator s choice of chemotherapy for second- or thirdline treatment of metastatic TNBC. Chemotherapy has been postulated to enhance cancer immunogenicity and thus be synergistic with immunotherapy. 38 In a phase IB trial of atezolizumab co-administered with nab-paclitaxel in metastatic TNBC, confirmed ORR in all patients was 42%. Importantly, the regimen had a tolerable safety profile, and responses were seen regardless of PD-L1 expression level. 39 The combination of pembrolizumab with eribulin was also active in patients with metastatic TNBC irrespective of PD-L1 status in the ENHANCE 1 phase IB/II trial: ORR was 26%, and median PFS was 4.2 months. 40 Such 284 Volume 14 / Issue 5 / May 2018 n Journal of Oncology Practice Copyright 2018 by American Society of Clinical Oncology

5 Triple-Negative Breast Cancer Subgroups promising efficacy signals coupled with manageable safety have fostered placebo-controlled phase III trials of first-line checkpoint inhibitor plus chemotherapy combinations in advanced or metastatic TNBCs such as KEYNOTE-355 (NCT ) and the IMpassion series (IMpassion130, NCT ; IMpassion131, NCT ; IMpassion132, NCT ). PD-L1 blockade plus chemotherapy is also actively studied in the operable TNBC setting. I-SPY2 is an adaptively randomized phase II trial that aims to match experimental regimens with responding subtypes. Regimens graduate from phase II if they have a high Bayesian predictive probability of success in a subsequent phase III neoadjuvant trial within the biomarker signature in which they performed well. Notably, pembrolizumab plus standard neoadjuvant chemotherapy graduated from the I-SPY2 platform, with the Bayesian-estimated pcr rate in TNBC three times that of chemotherapy alone (60% v 20%). 41 The ongoing KEYNOTE-522 (NCT ) trial randomly assigns patients with locally advanced TNBCs to receive neoadjuvant chemotherapy (pembrolizumab followed by adjuvant pembrolizumab) or neoadjuvant chemotherapy (placebo followed by adjuvant placebo). The field of immunotherapy for TNBCs is nascent. Only a small proportion of patients with TNBCs benefit from PD-L1 checkpoint inhibition, with ORR in unselected cohorts on the order of 5% to 25% and seemingly better when used as first-line therapyratherthanlater-linetherapy Furthermore, innate resistance of pretreated TNBC to immunotherapy may potentially be overcome by combination treatment with chemotherapy. 39,40 More research is needed to define a sensitive subset of TNBCs and develop biomarker-driven strategies to render the tumor and its microenvironment more amenable to immunotherapy-based strategies. AR-POSITIVE SUBGROUP The luminal AR subtype of TNBCs expresses AR messenger RNA in addition to downstream AR targets and coactivators. 3,42 There is significant variability in the reported frequency (7% to 75%) and prognostic significance of AR expression in TNBCs. 43 Preclinical work and early trials suggest that targeting AR has a clinical benefit in appropriately selected patients with TNBC. The first of such trials, TBCRC011, assessed bicalutamide, an AR inhibitor, in 51 hormone receptor negative patients who screened positive for AR (. 10% by immunohistochemistry [IHC]). A 24-week clinical benefit rate (CBR) of 19% was observed; median PFS was 12 weeks. 44 This trial was followed by other single-arm phase II trials. The UCBG 12-1 trial of abiraterone acetate (a CYP17 inhibitor) in a cohort of heavily pretreated AR-positive ($ 10% by IHC) TNBCs demonstrated a 6-month CBR of 20% and PFS of 2.8 months. 45 In the MDV trial of enzalutamide (a potent AR inhibitor), the 24-week CBR was 28%, and the median PFS was 3.3 months in 78 evaluable patients. 46 In MDV , AR positivity was defined as. 0% by IHC. Exploratory analysis demonstrated that an androgen-related gene signature obtained from the genomic diagnostic assay PREDICT AR was associated with greater clinical benefit. 47 The phase III ENDEAR trial (NCT ) of paclitaxel plus enzalutamide versus placebo or enzalutamide monotherapy followed by paclitaxel for diagnostic signature-positive TNBC has been withdrawn. New agents in development for AR-positive metastatic breast cancer(either TNBC or hormone receptor positive) are orteronel (TAK-700), a CYP17 inhibitor (NCT ), and seviteronel (VT-464), a dual CYP17 inhibitor-cum-ar antagonist (CLARITY-01, NCT ). PI3K/AKT/PTEN-ALTERED SUBGROUP Comprehensive pan-omic analyses of TNBCs found high rates of PI3K/AKT/mTOR pathway aberrations. 6,7 Activation of the PI3K pathway is mainly mediated at the protein level, less reliant on PI3KCA mutations (7%) but more commonly through the loss of negative regulators PTEN (mutation or loss, 35%) and/or INPP4B (loss, 30%). 6 AKT inhibitors seem more promising than mtor and PI3K inhibitors in TNBC. In I-SPY2, the AKT inhibitor MK-2206 combined with standard neoadjuvant chemotherapy showed an estimatedpcrrateof40% versus22% forchemotherapyalonefor the hormone receptor negative, HER2-negative signature. 48 Ipatasertib is an oral ATP-competitive AKT inhibitor. Preclinical studies provided the rationale for its combination with paclitaxel. Sensitivity to ipatasertib was associated with high levels of phosphorylated AKT, PTEN protein loss, and mutations in PTEN or PIK3CA. 49 LOTUS is a randomized, double-blind phase II study that investigated the efficacy of ipatasertib versus placebo plus paclitaxel in treatment-naïve advanced TNBC (N 5 124). LOTUS met one of its co-primary end points, wherein PFS in the intention-to-treat population was modestly but significantly longer with ipatasertib versus placebo (6.2 v 4.9 months; HR, 0.60; P 5.037). Of note, in a prespecified subgroup of patients with PIK3CA/AKT1/PTENaltered tumors defined through next-generation sequencing; Copyright 2018 by American Society of Clinical Oncology Volume 14 / Issue 5 / May 2018 n jop.ascopubs.org 285

6 Chan, Tan, and Dent n 5 42), median PFS was 9.0 months with ipatasertib, longer than 4.9 months with placebo (HR, 0.44; P 5.04). 50 Further research regarding AKT inhibitors for TNBC includes another randomized placebo-controlled phase II trial (NCT ) of first-line paclitaxel with or without AZD5363 and the completed randomized phase II FAIRLANE trial (NCT ) assessing the addition of ipatasertib to paclitaxel in the neoadjuvant setting. A phase III trial of ipatasertib in PIK3CA/AKT1/PTEN-altered locally advanced or metastatic TNBC or hormone receptor positive, HER2- negativebreast cancerisunderway(ipatunity130,nct ). UNIQUE ANTIGEN-EXPRESSING SUBGROUP The characterization of antigens that can be expressed by TNBCwithin the cytoplasmicmembrane and serve as potential therapeutic targets has galvanized interest in antibody-drug conjugates (ADCs). ADCs amalgamate the targeted nature of monoclonal antibody with a cytotoxic payload for an improved therapeutic index. Candidate surface antigens and ADCs along with the status of their clinical development are presented in Table 1. Encouraging results have been reported for ADCs in advanced TNBC. The following paragraphs describe two surface marker-adc pairings. Trop-2 is expressed in more than 80% of TNBCs and is associated with poor prognosis and aggressive disease. 53 Sacituzumab govitecan (IMMU-132) consists of humanized immunoglobulin G antibody against Trop-2 linked to SN-38, an active metabolite of irinotecan. In a first-in-human phase I/II trial, sacituzumab govitecan had an acceptable safety profile. Promising durable responses (ORR, 30%; median duration of response, 8.9 months) and PFS of 6.0 months were seen in a heavily pretreated population of patients with Table 1. Targetable Cancer Epithelial Antigens in TNBC, Candidate Antibody-Drug Conjugates, and the Status of Drug Development Status of Drug Development Surface Antigen Antibody-Drug Conjugate Status Trial Acronym Trial No. Trop-2 Sacituzumab govitecan (IMMU-132) Phase I/II trial reported 51 NCT Phase III trial recruiting; FDA breakthrough therapy and fast-track designation ASCENT NCT Glycoprotein nonmetastatic B (gpnmb) Glembatumumab vedotin (CDX-011) Phase I/II trial reported 51a NCT Phase II trial reported 52 EMERGE NCT Phase IIb trial active, not recruiting METRIC NCT LIV-1 Ladiratuzumab vedotin (SGN-LIV1A) Interim results of phase I trial reported NCT Phase Ib/2 trial in combination with pembrolizumab planned NCT Mesothelin Anetumab ravtansine (BAY ) Phase I trial (MTD) reported NCT Phase Ib multi-indication trial including TNBC recruiting NCT Carbonic anhydrase 6 (CA6) SAR Phase I trial (MTD) reported NCT Phase II trial recruiting NCT Protein tyrosine kinase 7 (PTK7) PF Interim results of phase I trial reported NCT Phase I trial in combination with gedatolisib planned NCT Abbreviations: FDA, US Food and Drug Administration; MTD, maximum tolerated dose; TNBC, triple-negative breast cancer. 286 Volume 14 / Issue 5 / May 2018 n Journal of Oncology Practice Copyright 2018 by American Society of Clinical Oncology

7 Triple-Negative Breast Cancer Subgroups TNBC. 51 The US Food and Drug Administration has accorded sacituzumab govitecan breakthrough therapy and fast-track designation. A confirmatory phase III randomized trial versus physician s choice of treatment in refractory/relapsed metastatic TNBC is recruiting (ASCENT, NCT ). Glycoprotein nonmetastatic B (gpnmb) is highly expressed in TNBC compared with normal tissue and is also an adverse prognosticmarker. 54 Glembatumumab vedotin (CDX- 011) is composed of a fully human immunoglobulin G2 monoclonal antibody with high affinity for the extracellular domain of gpnmb conjugated to the microtubule inhibitor monomethyl auristatin E. Activity was noted in metastatic gpnmb-overexpressing ($ 25% by IHC) TNBCs in the phase II EMERGE study that compared glembatumumab vedotin with the investigator s choice of chemotherapy. Post hoc analysis showed that the ORR was 18% versus 0% in patients with TNBC and 40% versus 0% in gpnmb-overexpressing TNBC. 52 The METRIC trial (NCT ), a randomized phase IIb study that evaluated glembatumumab vedotin versus capecitabine in gpnmb-overexpressing metastatic TNBC, is underway. ORRs in the previously mentioned phase II studies compare favorably against historical values for late-line cytotoxic monotherapy in the refractory setting. It is conceivable that more compounds will be efficacious against TNBCs as new cancer epithelial antigens are discovered. Immunostaining to screen for targetable tumor antigens may become de rigeur in the coming era of personalized medicine for TNBC. DISCUSSION In this review, we discussed five clinically applicable subpopulations of TNBC. Our schema is motivated primarily by practicability for the everyday clinic. It is not intended to be a substitute for holistic, methodologically rigorous approaches to classify this heterogeneous disease. Apart from platinums and PARP inhibitors against the defective DNA repair subgroup, novel agents matched to the other four subgroups are still investigational, and we would not use them at this time to treat TNBC outside of clinical trials. Combinatorial strategies involving cytotoxics, targeted drugs, and/or immunotherapy are now available. Molecular diagnostics are being evaluated as therapeutic decision-making tools in TNBC. Examples include using high-throughput genome analysis to direct maintenance therapy after cytotoxic induction in the metastatic setting (SAFIR02_Breast, NCT ) and using gene expression signature with imaging to select chemotherapy nonresponders in the neoadjuvant setting for enrollment in targeted therapy trials (ARTEMIS, NCT ). Prospective validation of circulating tumor DNA may potentially revolutionize treatment planning, disease monitoring, and prognostic evaluation of breast cancer. The treatment landscape will transform substantially in the coming years. Our five TNBC subsets in 2018 may be refined or become obsolete in the face of evolving evidence. As emerging agents fulfill their potential, we look forward to more robust discovery and validation of predictive biomarkers to help better select, individualize, and sequence treatments for TNBC. Authors Disclosures of Potential Conflicts of Interest Disclosures provided by the authors are available with this article at jop.ascopubs.org. Author Contributions Conception and design: All authors Manuscript writing: All authors Final approval of manuscript: All authors Accountable for all aspects of the work: All authors Corresponding author: Rebecca A. Dent, MD, National Cancer Centre Singapore, Division of Medical Oncology, 11 Hospital Dr, Singapore ; rebecca.dent@duke-nus.edu.sg. References 1. Dent R, Trudeau M, Pritchard KI, et al: Triple-negative breast cancer: Clinical features and patterns of recurrence. Clin Cancer Res 13: , Liedtke C, Mazouni C, Hess KR, et al: Response to neoadjuvant therapy and longterm survival in patients with triple-negative breast cancer. J Clin Oncol 26: , Lehmann BD, Bauer JA, Chen X, et al: Identification of human triple-negative breast cancer subtypes and preclinical models for selection of targeted therapies. J Clin Invest 121: , Lehmann BD, Jovanović B, Chen X, et al: Refinement of triple-negative breast cancer molecular subtypes: Implications for neoadjuvant chemotherapy selection. PLoS One 11:e , Perou CM: Triple negative breast cancer: Biology and heterogeneity. Presented at the 2016 San Antonio Breast Cancer Symposium, San Antonio, TX, December 6-10, Cancer Genome Atlas Network: Comprehensive molecular portraits of human breast tumours. Nature 490:61-70, Shah SP, Roth A, Goya R, et al: The clonal and mutational evolution spectrum of primary triple-negative breast cancers. Nature 486: , Sørlie T, Tibshirani R, Parker J, et al: Repeated observation of breast tumor subtypes in independent gene expression data sets. Proc Natl Acad Sci USA 100: , Roy R, Chun J, Powell SN: BRCA1 and BRCA2: Different roles in a common pathway of genome protection. Nat Rev Cancer 12:68-78, Turner N, Tutt A, Ashworth A: Hallmarks of BRCAness in sporadic cancers. Nat Rev Cancer 4: , Stefansson OA, Jonasson JG, Johannsson OT, et al: Genomic profiling of breast tumours in relation to BRCA abnormalities and phenotypes. Breast Cancer Res 11:R47, Abkevich V, Timms KM, Hennessy BT, et al: Patterns of genomic loss of heterozygosity predict homologous recombination repair defects in epithelial ovarian cancer. Br J Cancer 107: , Birkbak NJ, Wang ZC, Kim JY, et al: Telomeric allelic imbalance indicates defective DNA repair and sensitivity to DNA-damaging agents. Cancer Discov 2: , 2012 Copyright 2018 by American Society of Clinical Oncology Volume 14 / Issue 5 / May 2018 n jop.ascopubs.org 287

8 Chan, Tan, and Dent 14. Popova T, Manié E, Rieunier G, et al: Ploidy and large-scale genomic instability consistently identify basal-like breast carcinomas with BRCA1/2 inactivation. Cancer Res 72: , Isakoff SJ, Mayer EL, He L, et al: TBCRC009: A multicenter phase II clinical trial of platinum monotherapy with biomarker assessment in metastatic triple-negative breast cancer. J Clin Oncol 33: , Davies H, Glodzik D, Morganella S, et al: HRDetect is a predictor of BRCA1 and BRCA2 deficiency based on mutational signatures. Nat Med 23: , Byrski T, Huzarski T, Dent R, et al: Pathologic complete response to neoadjuvant cisplatin in BRCA1-positive breast cancer patients. Breast Cancer Res Treat 147: , von Minckwitz G, Schneeweiss A, Loibl S, et al: Neoadjuvant carboplatin in patients with triple-negative and HER2-positive early breast cancer (GeparSixto; GBG 66): A randomised phase 2 trial. Lancet Oncol 15: , a. Untch M, Schneeweiss A, Salat C, et al: Long-term survival analysis of the randomized phase II trial investigating the addition of carboplatin to neoadjuvant therapy for triple-negative (TNBC) and HER2-positive early breast cancer (GeparSixto). Ann Oncol 28:49, 2017 (suppl 5; abstr 163PD) 19. Hahnen E, Lederer B, Hauke J, et al: Germline mutation status, pathological complete response, and disease-free survival in triple-negative breast cancer: Secondary analysis of the GeparSixto randomized clinical trial. JAMA Oncol 3: , a. Sikov WM, Berry DA, Perou CM, et al: Event-free and overall survival following neoadjuvant weekly paclitaxel and dose-dense AC 6 carboplatin and/or bevacizumab in triple-negative breast cancer: Outcomes from CALGB (Alliance). Cancer Res 76, 2017 (suppl 4; abstr S2-05) 19b. Loibl S, O Shaughnessy J, Untch M, et al: Addition of the PARP inhibitor veliparib plus carboplatin or carboplatin alone to standard neoadjuvant chemotherapy in triple-negative breast cancer (BrighTNess): A randomised, phase 3 trial. Lancet Oncol 19: , Telli ML, Timms KM, Reid J, et al: Homologous recombination deficiency (HRD) score predicts response to platinum-containing neoadjuvant chemotherapy in patients with triple-negative breast cancer. Clin Cancer Res 22: , von Minckwitz G, Timms K, Untch M, et al: Prediction of pathological complete response (pcr) by homologous recombination deficiency (HRD) after carboplatincontaining neoadjuvant chemotherapy in patients with TNBC: Results from GeparSixto. J Clin Oncol 33, 2015 (suppl; abstr 1004) 22. Tutt A, Ellis P, Kilburn L, et al: TNT: A randomized phase III trial of carboplatin compared with docetaxel for patients with metastatic or recurrent locally advanced triple negative or BRCA1/2 breast cancer (CRUK/07/012). Cancer Res 75, 2015 (suppl 9; abstr S3-01) 23. Tutt A, Cheang MCU, Kilburn L, et al: BRCA1 methylation status, silencing and treatment effect in the TNT trial: A randomized phase III trial of carboplatin compared with docetaxel for patients with metastatic or recurrent locally advanced triple negative or BRCA1/2 breast cancer (CRUK/07/012). Cancer Res 77, 2017 (suppl 4; abstr S6-01) 24. Kaelin WG, Jr: The concept of synthetic lethality in the context of anticancer therapy. Nat Rev Cancer 5: , Fong PC, Boss DS, Yap TA, et al: Inhibition of poly(adp-ribose) polymerase in tumors from BRCA mutation carriers. N Engl J Med 361: , Tutt A, Robson M, Garber JE, et al: Oral poly(adp-ribose) polymerase inhibitor olaparib in patients with BRCA1 or BRCA2 mutations and advanced breast cancer: A proof-of-concept trial. Lancet 376: , Robson M, Im SA, Senkus E, et al: Olaparib for metastatic breast cancer in patients with a germline BRCA mutation. N Engl J Med 377: , Litton J, Rugo HS, Ettl J, et al: EMBRACA: A phase 3 trial comparing talazoparib, an oral PARP inhibitor, to physician s choice of therapy in patients with advanced breast cancer and a germline BRCA mutation. Cancer Res 78, 2018 (suppl 4; abstr GS6-07) 29. Rakha EA, Aleskandarany M, El-Sayed ME, et al: The prognostic significance of inflammation and medullary histological type in invasive carcinoma of the breast. Eur J Cancer 45: , Loi S, Sirtaine N, Piette F, et al: Prognostic and predictive value of tumorinfiltrating lymphocytes in a phase III randomized adjuvant breast cancer trial in node-positive breast cancer comparing the addition of docetaxel to doxorubicin with doxorubicin-based chemotherapy: BIG J Clin Oncol 31: , Adams S, Gray RJ, Demaria S, et al: Prognostic value of tumor-infiltrating lymphocytes in triple-negative breast cancers from two phase III randomized adjuvant breast cancer trials: ECOG 2197 and ECOG J Clin Oncol 32: , Loi S, Adams S, Schmid P, etal: Relationship between tumorinfiltrating lymphocyte (TIL) levels and response to pembrolizumab (pembro) in metastatic triple-negative breast cancer (mtnbc): Results from KEYNOTE-086. Ann Oncol 28:608, 2017 (suppl 5; abstr LBA13) 33. Nanda R, Chow LQ, Dees EC, et al: Pembrolizumab in patients with advanced triple-negative breast cancer: Phase Ib KEYNOTE-012 study. J Clin Oncol 34: , Schmid P, Cruz C, Braiteh FS, et al: Atezolizumab in metastatic TNBC (mtnbc): Long-term clinical outcomes and biomarker analyses. Cancer Res 77, 2017 (suppl 13; abstr 2986) 35. Dirix LY, Takacs I, Nikolinakos P, et al: Avelumab (MSB C), an anti-pd-l1 antibody, in patients with locally advanced or metastatic breast cancer: A phase Ib JAVELIN solid tumor trial. Cancer Res 76, 2016 (suppl 4; abstr S1-04) 36. Adams S, Schmid P, Rugo HS, et al: Phase 2 study of pembrolizumab (pembro) monotherapy for previously treated metastatic triple-negative breast cancer (mtnbc): KEYNOTE-086 cohort A. J Clin Oncol 35, 2017 (suppl; abstr 1008) 37. Adams S, Loi S, Toppmeyer D, et al: Phase 2 study of pembrolizumab as first-line therapy for PD-L1 positive metastatic triple-negative breast cancer (mtnbc): Preliminary data from KEYNOTE-086 cohort B. J Clin Oncol 35, 2017 (suppl; abstr 1088) 38. Zitvogel L, Apetoh L, Ghiringhelli F, et al: Immunological aspects of cancer chemotherapy. Nat Rev Immunol 8:59-73, Adams S, Diamond JR, Hamilton EP, et al: Phase Ib trial of atezolizumab in combination with nab-paclitaxel in patients with metastatic triple-negative breast cancer (mtnbc). J Clin Oncol 34, 2016 (suppl; abstr 1009) 40. Tolaney SM, Kalinsky K, Kaklamani V, et al: Phase 1b/2 study to evaluate eribulin mesylate in combination with pembrolizumab in patients with metastatic triple-negative breast cancer (ENHANCE 1). Cancer Res 78, 2018 (suppl 4; abstr PD6-13) 41. Nanda R, Liu MC, Yau C, et al: Pembrolizumab plus standard neoadjuvant therapy for high-risk breast cancer (BC): Results from I-SPY 2. J Clin Oncol 35, 2017 (suppl; abstr 506) 42. Doane AS, Danso M, Lal P, et al: An estrogen receptor-negative breast cancer subset characterized by a hormonally regulated transcriptional program and response to androgen. Oncogene 25: , Rampurwala M, Wisinski KB, O Regan R: Role of the androgen receptor in triplenegative breast cancer. Clin Adv Hematol Oncol 14: , Gucalp A, Tolaney S, Isakoff SJ, et al: Phase II trial of bicalutamide in patients with androgen receptor-positive, estrogen receptor-negative metastatic breast cancer. Clin Cancer Res 19: , Bonnefoi H, Grellety T, Tredan O, et al: A phase II trial of abiraterone acetate plus prednisone in patients with triple-negative androgen receptor positive locally advanced or metastatic breast cancer (UCBG 12-1). Ann Oncol 27: , Traina TA, Miller K, Yardley DA, et al: Enzalutamide for the treatment of androgen receptor-expressing triple-negative breast cancer. J Clin Oncol 36: , Traina TA, Miller K, Yardley DA, et al: Results from a phase 2 study of enzalutamide (ENZA), an androgen receptor (AR) inhibitor, in advanced AR1 triple-negative breast cancer (TNBC). J Clin Oncol 33, 2015 (suppl; abstr 1003) 48. Tripathy D, Chien AJ, Hylton N, et al: Adaptively randomized trial of neoadjuvant chemotherapy with or without the Akt inhibitor MK-2206: Graduation results from the I-SPY 2 Trial. J Clin Oncol 33, 2015 (suppl; abstr 524) 49. Lin J, Sampath D, Nannini MA, et al: Targeting activated Akt with GDC-0068, a novel selective Akt inhibitor that is efficacious in multiple tumor models. Clin Cancer Res 19: , Volume 14 / Issue 5 / May 2018 n Journal of Oncology Practice Copyright 2018 by American Society of Clinical Oncology

9 Triple-Negative Breast Cancer Subgroups 50. Kim SB, Dent R, Im SA, et al: Ipatasertib plus paclitaxel versus placebo plus paclitaxel as first-line therapy for metastatic triple-negative breast cancer (LOTUS): A multicentre, randomised, double-blind, placebo-controlled, phase 2 trial. Lancet Oncol 18: , Bardia A, Mayer IA, Diamond JR, etal: Efficacy and safety of anti-trop-2 antibody drug conjugate sacituzumab govitecan (IMMU-132) in heavily pretreated patients with metastatic triple-negative breast cancer. J Clin Oncol 35: , a. Bendell J, Saleh M, Rose AA, et al: Phase I/II study of the antibody-drug conjugate glembatumumab vedotin in patients with locally advanced or metastatic breast cancer. J Clin Oncol 32: , Yardley DA, Weaver R, Melisko ME, et al: EMERGE: A randomized phase II study of the antibody-drug conjugate glembatumumab vedotin in advanced glycoprotein NMB-expressing breast cancer. J Clin Oncol 33: , Goldenberg DM, Cardillo TM, Govindan SV, et al: Trop-2 is a novel target for solid cancer therapy with sacituzumab govitecan (IMMU-132), an antibody-drug conjugate (ADC). Oncotarget 6: , Rose AA, Grosset AA, Dong Z, et al: Glycoprotein nonmetastatic B is an independent prognostic indicator of recurrence and a novel therapeutic target in breast cancer. Clin Cancer Res 16: , 2010 Copyright 2018 by American Society of Clinical Oncology Volume 14 / Issue 5 / May 2018 n jop.ascopubs.org 289

10 Chan, Tan, and Dent AUTHORS DISCLOSURES OF POTENTIAL CONFLICTS OF INTEREST Are There Any Clinically Relevant Subgroups of Triple-Negative Breast Cancer in 2018? The following represents disclosure information provided by authors of this manuscript. All relationships are considered compensated. Relationships are self-held unless noted. I 5 Immediate Family Member, Inst 5 My Institution. Relationships may not relate to the subject matter of this manuscript. For more information about ASCO s conflict of interest policy, please refer to or ascopubs.org/jop/site/ifc/journal-policies.html. Jack J. Chan Consulting or Advisory Role: Eisai Research Funding: OncoQuest (I) Travel, Accommodations, Expenses: Synthon, Novartis Rebecca A. Dent Honoraria: Genentech, AstraZeneca, Pfizer Consulting or Advisory Role: Roche, Pfizer, Merck, Eisai, AstraZeneca Travel, Accommodations, Expenses: Roche, Pfizer Tira J.Y. Tan Consulting or Advisory Role: Novartis, Pfizer Research Funding: Bayer HealthCare Pharmaceuticals, Novartis Travel, Accommodations, Expenses: Merck Sharp & Dohme, AstraZeneca, Eisai, Pfizer Volume 14 / Issue 5 / May 2018 n Journal of Oncology Practice Copyright 2018 by American Society of Clinical Oncology