Citations

Abstract Background: Stage III triple negative(TN) inflammatory breast cancer(IBC) is associated with a poor prognosis evidenced by a 15 month(mo) median disease free survival(DFS) and overall survival(OS) of 34 mo. The substantial incidence of developing distant metastasis may be due to the prevalence of cancer cells with stem cell-like features (e.g. CD44+/CD24-) in TNIBC. The transcriptional pathway JAK2/STAT3 is associated with the survival of CD44+/CD24- cells, and preclinical data demonstrates overexpression of activated STAT3(pSTAT3) in > 95% of TNIBC. Preclinical studies have shown that ruxolitinib (Incyte®Corporation), an approved JAK1/JAK2 inhibitor, suppresses pSTAT3 in IBC patient derived xenograft models, and when combined with paclitaxel, results in a synergistic reduction in tumor weight. Given the lack of a known therapeutic target in TNIBC, this preoperative proof of principle study exploits the survival mechanism of CD44+/CD24- stem cells prevalent in this disease, by combining ruxolitinib(Rux) with paclitaxel(T) followed by doxorubicin/cyclophosphamide(AC). Methods: Up to 64 pts with newly diagnosed stage III TNIBC (cT4d, any N, M0) are eligible if they have adequate organ function and are willing to undergo 2 research biopsies(rbx) of the affected breast. Following baseline rbx, pts are randomized to a 7 day(d) run-in phase of Rux vs Rux(15 mg bid) + T(80mg/m2/wkx1). A 2nd rbx is obtained after the run-in phase. Pts randomized to RuxT continue to receive a total of Tx12 wks+Rux. Pts randomized to Rux alone, are re-randomized to receive Tx12 wks+Rux vs Tx12wks alone. Following T, all pts receive AC (A-60mg/m2,C-600mg/m2) every 14d x 4. Pts proceed to modified radical mastectomy (MRM) followed by chest wall/regional lymph nodes radiation therapy. Correlatives: To assess the effect of JAK inhibition with Rux on pSTAT3 and STAT3 related gene expression, molecular and genomic markers (e.g. RNA-seq, ChIPseq, FISH) will be determined in each rbx and residual tumor at MRM. The relative frequency and topology of CD44+/CD24- cell population and pSTAT3 expression by IHC will also be assessed in these tumor specimens. IL-6 and CRP plasma concentrations will be measured at baseline, prior to T and AC and prior to MRM. Statistics: The primary endpoint is change in markers of JAK/STAT inhibition. If the proportion of rbx exhibit a biologic response to Rux alone (i.e. change from pSTAT3 expression to pSTAT3 negative) is <10%, then Rux alone is minimally effective on JAK inhibition vs alternative hypothesis that Rux inhibits JAK if the proportion of biologic response is ≥33%. If ≥5/25 rbx treated with Rux alone have a biologic response then the hypothesis that biologic response is ≤10% is rejected with an error rate of 0.098 (target 0.10). If ≤4/25 rbx have a biologic response then the hypothesis that biologic response is ≥33% is rejected with an error rate of 0.05 (target 0.10). Biologic response of rbx with Rux alone will also be compared with the proportion of biologic response to RuxT (33% vs. 66% based upon presumed synergy with RuxT). Secondary endpoints are clinical: pathologic complete response in breast/lymph nodes, Residual Cancer Burden, DFS and OS. Clinical Trial Information: NCT02876302. Citation Format: Overmoyer B, Regan M, Polyak K, Brock J, Van Poznak C, King T, Haddad T, Stearns V, Hwang S, Winer E. TBCRC 039: Phase II study of combination ruxolitinib (INCB018424) with preoperative chemotherapy for triple negative inflammatory breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr OT3-05-01.

Abstract Background: DNA methylation is a promising prognostic and predictive biomarker of response to treatment in advanced breast cancer (Fackler MJ. Can Res 2014). We evaluated whether baseline and change in tissue and serum methylation would predict pathological complete response (pCR) in patients with HER2-negative early breast cancer treated with PST. Methods: TBCRC008 was a multicenter placebo-controlled trial that investigated pCR (no invasive cancer in breast/axilla) following 12 weeks of preoperative carboplatin and albumin-bound paclitaxel with or without the histone deacetylase (HDAC) inhibitor vorinostat in 62 patients with HER2-negative breast cancer. We performed an exploratory planned biomarker study correlating baseline and change (D15) in tumor tissue and serum methylation with pCR. Methylation was measured using cumulative methylation index (CMI) in quantitative multiplexed methylation-specific polymerase chain reaction (QM-MSP) assays; cMethDNA assay for serum (Fackler MJ. Can Res 2014) and QM-MSP for tissue (Fackler MJ. Can Res 2004). The analysis population included all patients with available methylation and pCR data. Association between CMI level (log transformed baseline, D15, change) and pCR was evaluated using univariate as well as multivariable logistic regression models controlling for treatment arm and estrogen receptor (ER) status. Additional subgroup analyses assessed association of CMI level with pCR stratified by ER status and treatment arms, respectively. Spearman's correlation coefficient was performed to evaluate the correlation between tissue and serum CMI. Results: pCR data were available for 61 of 62 patients. One patient with unknown response was treated as a non-responder. The pCR rate was similar in both arms (vorinostat 25.8%, placebo 29%). Both tissue and serum CMI data were available in 55 (baseline), 50 (D15) and 46 (change) patients. Elevated serum methylation based on a predefined threshold (6.9 units) was identified in 23/59 (39%) patients. In univariate analysis, one log unit increase in tissue CMI levels at D15 was associated with a 40% lower chance of obtaining a pCR (odds ratio, OR=0.60, 95% CI 0.37-0.97; p=0.037). The multivariable analysis did not show a statistically significant association of tissue or serum CMI with pCR, although a similar trend in magnitude towards association with pCR was observed for D15 and change in tumor tissue CMI (OR 0.67 and 0.57, p=0.129 and 0.147 respectively). Subgroup analyses suggested a significant association between tissue CMI levels at D15 and pCR in the group treated with vorinostat [tissue OR: 0.44 (0.20, 0.93), p=0.03; serum OR 0.37 (0.13, 1.07), p=0.07] but not in the placebo group. No significant correlation was observed between tissue and serum CMI. Conclusion: This is the first study to evaluate the predictive role of tissue and serum methylation (CMI) in patients with early breast cancer treated with PST ± an HDAC inhibitor. Methylation was detected more frequently in tissue than serum. Tissue CMI levels at D15 may predict poor response to this regimen. Further evaluation of the predictive role of methylation in early breast cancer is warranted. Citation Format: Connolly RM, Fackler MJ, Zhang Z, Zhou XC, Goetz MP, Boughey JC, Walsh B, Carpenter J, Storniolo AM, Watkins S, Barielson E, Sukumar S, Stearns V. Tumor and serum DNA methylation in women receiving preoperative chemotherapy (PST) with or without vorinostat in primary operable HER2-negative breast cancer in TBCRC008 [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P1-09-15.

LBA1 Background: Adjuvant endocrine therapy with AI vs T improves outcomes in postmenopausal HR+ BC. TEXT and SOFT were designed to test whether adjuvant AI improves outcomes in premenopausal women with HR+ BC treated with OFS (AI question) and to determine the value of OFS in women who remain premenopausal and are suitable for adjuvant T (OFS question). Methods: TEXT and SOFT, randomized phase 3 trials, enrolled 5,738 premenopausal women with HR+ early BC from Nov03 to Apr11 (2672 TEXT; 3066 SOFT). TEXT randomized women within 12wk of surgery to 5y E+OFS vs T+OFS; chemotherapy (CT) was optional and concurrent with OFS. SOFT randomized women to 5y E+OFS vs T+OFS vs T alone, either within 12wk of surgery if no CT planned, or within 8mo of completing (neo)adjuvant CT. OFS was by choice of 5y triptorelin, oophorectomy, or ovarian irradiation. The primary endpoint is disease-free survival (DFS: randomization until invasive local, regional, distant recurrence, or contralateral breast; 2nd malignancy; death). Due to low event rates, protocol amendments in 2011 changed the analysis plans to answer the AI question (E+OFS vs T+OFS) by joint analysis of TEXT and SOFT. By Q3’2013 with >5y median follow-up, 436 DFS events were projected, providing 84% power for HR=0.75 with E+OFS vs T+OFS (stratified logrank 2-sided α=0.05). Results: At 5.7y median follow-up, 514 (11%) DFS events were reported in the ITT population comparing E+OFS (n=2346) vs T+OFS (n=2344). Patients assigned E+OFS had significantly reduced DFS hazard (HR=0.72; 95% CI, 0.60-0.86; P=0.0002) vs T+OFS; 5y DFS was 91.1% vs 87.3%. Reductions were similar for secondary endpoints of BC-free interval (HR=0.66 (0.55-0.80) 5y BCFI 92.8% vs 88.8%) and distant recurrence-free interval (HR=0.78 (0.62-0.97)), though not overall survival (HR=1.14 (0.86-1.51)) at this early follow-up (194 (4%) deaths). Grade 3-4 targeted AEs were reported in 31% E+OFS vs 29% T+OFS patients. Conclusion: In premenopausal women with HR+ BC, adjuvant treatment with E+OFS significantly reduced the risk of recurrence compared to T+OFS. Clinical trial information: NCT00066703 / NCT00066690 .

270 Background: Neuropathy is a common and potentially enduring and disabling complication of adjuvant taxane therapy. Recent studies have identified candidate host single nucleotide polymorphisms (SNPs) associated with taxane-induced neuropathy (Schneider et al. ASCO 2011, abstr. 1000). We therefore sought to determine whether neuropathy was associated with breast cancer recurrence. Methods: This study included 4,950 eligible women with axillary lymph node positive or high-risk node-negative breast cancer who received up to 4 cycles of AC (doxorubicin 60 mg/m2 and cyclophosphamide 600 mg/m2) every 3 weeks, followed by either: (1) paclitaxel 175 mg/m2 every 3 weeks x 4 (P3), (2) paclitaxel 80 mg/m2 weekly x 12 (P1), (3) docetaxel 100 mg/m2 every 3 weeks x 4 (D3), or (4) docetaxel 35 mg/m2 weekly x 12 (D1). Chemotherapy doses were based on actual body weight. Cox proportional hazards model were used to determine the relationship between neuropathy and disease free survival (DFS) and overall survival (OS) treating neuropathy status as a time dependent covariate and using a landmark analysis. Results: Of 4,702 patients who received at least 1 taxane dose, grade 2-4 neuropathy developed in 20%, 27%, 16%, and 16% in the P3, P1, D3, and D1 arms, respectively. In a model including age, tumor size, nodal status, treatment arm, neuropathy, and the neuropathy- treatment interaction, there was no relationship between neuropathy and DFS and OS in the entire population, for any of the individual treatment arms, or for any breast cancer subtypes, whether analyzed as a time-dependent covariate or using a landmark analysis. Baseline covariates associated with an increase rate of neuropathy included black race (25% vs. 19% grade 2-4, p=0.02) and obesity (21% vs. 19%, p=0.04), but not age. Conclusions: There was no association between taxane-induced neuropathy and DFS or OS in patients treated with contemporary AC-taxane therapy, including weekly paclitaxel. These findings show that taxane-induced neuropathy is not associated with outcome, thus suggesting that validation of SNPs predictive of neuropathy may be useful in identifying patients at higher risk for neuropathy but not taxane benefit and thereby improve therapeutic individualization.

Abstract Genetic risks in breast cancer remain only partly understood. Here, we report the results of a genome-wide association study of germline DNA from 4,658 women, including 252 women experiencing a breast cancer recurrence, who were entered on the MA.27 adjuvant trial comparing the aromatase inhibitors (AI) anastrozole and exemestane. Single-nucleotide polymorphisms (SNP) of top significance were identified in the gene encoding MIR2052HG, a long noncoding RNA of unknown function. Heterozygous or homozygous individuals for variant alleles exhibited a ∼40% or ∼63% decrease, respectively, in the hazard of breast cancer recurrence relative to homozygous wild-type individuals. Functional genomic studies in lymphoblastoid cell lines and ERα-positive breast cancer cell lines showed that expression from MIR2052HG and the ESR1 gene encoding estrogen receptor-α (ERα) was induced by estrogen and AI in a SNP-dependent manner. Variant SNP genotypes exhibited increased ERα binding to estrogen response elements, relative to wild-type genotypes, a pattern that was reversed by AI treatment. Further, variant SNPs were associated with lower expression of MIR2052HG and ERα. RNAi-mediated silencing of MIR2052HG in breast cancer cell lines decreased ERα expression, cell proliferation, and anchorage-independent colony formation. Mechanistic investigations revealed that MIR2052HG sustained ERα levels both by promoting AKT/FOXO3-mediated ESR1 transcription and by limiting ubiquitin-mediated, proteasome-dependent degradation of ERα. Taken together, our results define MIR2052HS as a functionally polymorphic gene that affects risks of breast cancer recurrence in women treated with AI. More broadly, our results offer a pharmacogenomic basis to understand differences in the response of breast cancer patients to AI therapy. Cancer Res; 76(23); 7012–23. ©2016 AACR.

Abstract Background: Biomarkers to predict response to platinum-based therapy in early breast cancer are needed. HRD is a promising predictor of response to DNA damaging agents, such as platinums. We hypothesized that HRD (high HRD score ≥ 42 and/or tumor BRCA [tBRCA] mutation) would predict pathological complete response (pCR) in patients with HER2-negative early breast cancer treated with PST comprising a platinum, regardless of estrogen receptor (ER) status. Methods: TBCRC008 was a multicenter placebo-controlled trial that investigated pCR (no invasive cancer in breast/axilla) following 12 weeks of preoperative carboplatin and albumin-bound paclitaxel with or without vorinostat in patients with ER-positive or triple-negative breast cancer (TNBC) (Connolly RM. JNM 2015). The pCR rate was similar in both arms (vorinostat 25.8%, placebo 29%). We performed an exploratory biomarker study correlating baseline tumor biopsy HRD status with pCR. The analysis population included all patients with available HRD and pCR data. We compared the proportion of patients with pCR by HRD status using Fisher's exact test. A subset analysis compared pCR proportions by high (≥42) vs. low (<42) HRD score in those patients who did not have tBRCA mutation. A logistic regression model included HRD status, ER status, treatment arm, tumor grade and use of additional anthracycline-based non-study chemotherapy prior to definitive surgery. Results: HRD status and pCR data were available for 48/62 patients (30 ER-positive, 18 TNBC). Of these, 46% of tumors were HR deficient (n=22/48, 33% ER-positive [10/30], 67% TNBC [12/18]). We observed a significantly higher pCR rate in patients with HR deficiency vs not (50% vs 7.7%, p=0.002) in the overall population. A similar trend was observed in ER-positive (30% vs 5%, p=0.095) and TNBC (66.7% vs 16.7%, p=0.13) patients. There was no significant difference when analyzed by treatment arm (vorinostat vs placebo). In a subgroup analysis (n=40) of patients without tBRCA (25 ER-positive, 15 TNBC), a significantly higher pCR rate was observed in those with high vs low HRD score (64.3% vs 7.7%, p <0.001). After adjusting for ER status, randomized treatment, use of AC treatment, and tumor grade, patients whose tumors exhibited HR deficiency had a greater than 6 fold increase in pCR compared to those without HR deficiency (adjusted odds ratio = 6.76, 95% CI = 0.85-53.99, p=.072). Conclusion: This is the first study to evaluate the predictive role of HRD status in patients with ER-positive, HER2-negative breast cancer treated with platinum-based therapy. Our results also support prior observations that HRD status is a promising predictive biomarker of response to platinum agents in TNBC. Further evaluation of this question is warranted in both TNBC and ER-positive breast cancer. Citation Format: Connolly R, Elkin E, Timms K, Goetz M, Boughey J, Zhang Z, Walsh B, Carpenter J, Storniolo A, Watkins S, Gabrielson E, Hartman A-R, Stearns V. Homologous recombination deficiency (HRD) as a predictive biomarker of response to preoperative systemic therapy (PST) in TBCRC008 comprising a platinum in HER2-negative primary operable breast cancer. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P3-07-13.

Abstract Background: CYP2D6 is the rate limiting enzyme responsible for the metabolic activation of tamoxifen (tam) to endoxifen. Compared to CYP2D6 poor metabolizers (PM), tam-treated CYP2D6 extensive metabolizers (EM) have higher endoxifen concentrations, more vasomotor symptoms (Goetz, MP J Clin Oncol 2005), and are more likely to discontinue tam (Rae, JM 2009. Pharmacogenomics J). Additionally, higher endoxifen concentrations are associated with a stepwise increase in tam side-effects (Lorizio, W Breast Cancer Res Treat 2012). The data regarding CYP2D6 genotype and recurrence is mixed. Venlafaxine is a weak CYP2D6 inhibitor not known to alter tam pharmacokinetics (PK) and commonly recommended for tam induced hot flashes. We conducted a multicenter pharmacological study to determine whether venlafaxine altered the PK of tam and to determine the distribution of CYP2D6 genotypes in this population Methods: Women taking tam for at least 4 weeks and for whom venlafaxine was recommended for the treatment of hot flashes were eligible. Blood samples were collected prior to and 8–16 weeks following initiation of venlafaxine for steady state tam and metabolites. Genotyping was performed for alleles associated with no (PM; *3, *4, *5,*6); reduced (intermediate, IM; *10, 17 and *41); and ultra-rapid (UM; *1×2) metabolism. Power calculations demonstrated that 17 patients with paired samples were required (two-sided alpha=0.05 t-test, 90% power) to detect a 25% change in endoxifen levels after at least 8 weeks of concurrent treatment. Results: 30 women (median age 48.5) initiated venlafaxine. CYP2D6 genotypes were within Hardy Weinberg Equilibrium (HWE). CYP2D6 UM allele frequency (6.7%) was higher while CYP2D6 *4 (13.3%) was lower than expected compared to an unselected population (0.5 and 21% respectively; Sachse Am. J. Hum. Genet. 1997), resulting in the absence of CYP2D6 PM/PM. Mean (min/max) baseline endoxifen concentrations (8.73; 1.5–20.5 ng/ml) were correlated with CYP2D6 phenotype as follows: intermediate (EM/PM, PM/IM): 6.8 (1.5–11.2); extensive (EM/EM, EM/IM): 9.4 (1.5–20.5) and ultra-rapid (UM/EM: 11.0; 7.8–14) (r2 = 0.35 p = 0.05). In patients with paired samples (n = 20), venlafaxine resulted in a 23% decrease in endoxifen (−2.06 ng/ml; 95% CI −0.69 to −3.04; p = 0.004), but not tam, NDMT, or 4HT concentrations. Following initiation of venlafaxine, CYP2D6 genotype was no longer associated with endoxifen concentrations (r2 = 0.28 p = 0.23). For women with reduced CYP2D6 metabolism [EM/PM (n = 9) or PM/IM (n = 1)], venlafaxine lowered endoxifen concentrations (−2.98 ng/ml) to a level (5.41 ng/ml) reported to be associated with a higher risk of recurrence in adjuvant tam treated patients (Madlensky, L Clin Pharmacol Ther 2011). Conclusions: In this study, women with tam-induced vasomotor symptoms requiring venlafaxine were comprised predominantly of CYP2D6 EM and UM metabolizers. Venlafaxine significantly decreased endoxifen concentrations. Although the optimal concentration of endoxifen is unknown, given prior data linking low endoxifen concentrations with recurrence, venlafaxine should be used with caution in tam treated patients. (Supported by R01CA133049) Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr PD10-08.

Abstract Background: Our group and others have identified a subset of ER/PR(−) breast cancers characterized by expression of the androgen receptor (AR) and androgen-dependent growth (Doane 2006). We conducted a proof-of-concept multicenter phase II study to test the efficacy of the AR-antagonist, bicalutamide for the treatment of AR(+) ER/PR(−) MBC (NCT00468715). Results of the primary endpoint, clinical benefit rate (CBR), were presented at ASCO (Gucalp 2012). Data for the impact of bicalutamide on circulating hormone levels in women are limited. Elevations in serum testosterone (T) and estradiol (E) have been observed for men treated with bicalutamide. We hypothesized comparable patterns of change in circulating endocrine markers in response to bicalutamide for women with MBC. Methods: Patients (pts) with AR(+) (IHC ≥10%), ER/PR(−) (IHC <10%) MBC were eligible for treatment (tx) if ECOG performance status ≤2 and normal organ function regardless of menopausal status. There was no limit to prior tx except prior trastuzumab required if HER2(+). Tx consisted of bicalutamide 150mg orally daily in 28-day cycles (C). Toxicity assessed q4 weeks, response q12wks. Primary endpoint was CBR. Peripheral blood was collected for total and free T, E and sex hormone binding globulin (SHBG) at baseline, start of C2 (C2) and at end of study (EOS). Standard institutional assays were used. A Wilcoxon signed-rank test was done to compare baseline to C2 and EOS values. Results: 26 patients with AR(+) ER/PR(−) MBC were treated on study. Evaluable number (n) of pts at baseline, C2 and EOS are 26, 26 and 19 respectively. Two pts remain on study. Menopausal status: pre=2, post=24. Baseline median total and free T and estradiol were consistent with expected norms, however a wide range was observed (Table). There were no significant differences observed for median free T, total T, E or SHBG between baseline and C2 or baseline and EOS. Changes in hormone levels could not be stratified by menopausal status or response to bicalutamide given small sample size. Given the wide range of baseline values, we examined the percent change for each endocrine biomarker from baseline to C2 and EOS. As shown in the Table, there was no difference in median percent change observed across time points for each biomarker. Conclusions: No discernible patterns of change in T, E or SHBG were observed in response to bicalutamide therapy when given to women for the treatment of AR(+), ER/PR(−) MBC. These circulating hormones require further evaluation for use as a pharmacodynamic marker. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P6-05-02.

Abstract Purpose: Patients with hormone receptor–negative breast cancer generally do not benefit from endocrine-targeted therapies. However, a subset with androgen receptor (AR) expression is predicted to respond to antiandrogen therapies. This phase II study explored bicalutamide in AR-positive, estrogen receptor (ER), and progesterone receptor (PgR)-negative metastatic breast cancer. Experimental Design: Tumors from patients with ER/PgR-negative advanced breast cancer were tested centrally for AR [immunohistochemistry (IHC) > 10% nuclear staining considered positive]. If either the primary or a metastatic site was positive, patients were eligible to receive the AR antagonist bicalutamide at a dose of 150 mg daily. Clinical benefit rate (CBR), the primary endpoint, was defined as the total number of patients who show a complete response (CR), partial response (PR), or stable disease (SD) > 6 months; secondary endpoints included progression-free survival (PFS) and toxicity. Correlative studies included measurement of circulating endocrine markers and IHC surrogates for basal-like breast cancer. Results: Of 424 patients with ER/PgR-negative breast cancer, 12% tested AR-positive. The 6-month CBR was 19% [95% confidence interval (CI), 7%–39%] for bicalutamide. The median PFS was 12 weeks (95% CI, 11–22 weeks). Bicalutamide was well-tolerated with no grade 4/5 treatment-related adverse events observed. Conclusion: AR was expressed in 12% of patients with ER/PgR-negative breast cancer screened for this trial. The CBR of 19% observed with bicalutamide shows proof of principle for the efficacy of minimally toxic androgen blockade in a select group of patients with ER/PgR-negative, AR-positive breast cancer. Clin Cancer Res; 19(19); 5505–12. ©2013 AACR.

Purpose In patients with hormone-dependent postmenopausal breast cancer, standard adjuvant therapy involves 5 years of the nonsteroidal aromatase inhibitors anastrozole and letrozole. The steroidal inhibitor exemestane is partially non–cross-resistant with nonsteroidal aromatase inhibitors and is a mild androgen and could prove superior to anastrozole regarding efficacy and toxicity, specifically with less bone loss. Patients and Methods We designed an open-label, randomized, phase III trial of 5 years of exemestane versus anastrozole with a two-sided test of superiority to detect a 2.4% improvement with exemestane in 5-year event-free survival (EFS). Secondary objectives included assessment of overall survival, distant disease–free survival, incidence of contralateral new primary breast cancer, and safety. Results In the study, 7,576 women (median age, 64.1 years) were enrolled. At median follow-up of 4.1 years, 4-year EFS was 91% for exemestane and 91.2% for anastrozole (stratified hazard ratio, 1.02; 95% CI, 0.87 to 1.18; P = .85). Overall, distant disease–free survival and disease-specific survival were also similar. In all, 31.6% of patients discontinued treatment as a result of adverse effects, concomitant disease, or study refusal. Osteoporosis/osteopenia, hypertriglyceridemia, vaginal bleeding, and hypercholesterolemia were less frequent on exemestane, whereas mild liver function abnormalities and rare episodes of atrial fibrillation were less frequent on anastrozole. Vasomotor and musculoskeletal symptoms were similar between arms. Conclusion This first comparison of steroidal and nonsteroidal classes of aromatase inhibitors showed neither to be superior in terms of breast cancer outcomes as 5-year initial adjuvant therapy for postmenopausal breast cancer by two-way test. Less toxicity on bone is compatible with one hypothesis behind MA.27 but requires confirmation. Exemestane should be considered another option as up-front adjuvant therapy for postmenopausal hormone receptor–positive breast cancer.