Citations

Abstract Background: Gene expression profiling is an important tool in prediction, prognosis, and cancer modeling. The DASL® assay (cDNA –mediated Annealing, extension, Selection, and Ligation; Illumina; San Diego, CA) is a gene expression profiling system designed for degraded RNAs such as those derived from formalin-fixed paraffin-embedded (FFPE) tumor samples. FFPE tumor samples are invaluable resources for cancer research, being the most widely available materials for which patient outcomes are known. Purpose: The objective of this study was to evaluate the performance of Illumina's Whole Genome DASL platform (24,526; 24K) at distinguishing HER2-positive (HER2+) from HER2-negative (HER2-) FFPE breast tumors. Methods: We measured the relative gene expression of biological and technical replicates using nine HER2+ and 11 HER2- primary breast tumors from patients that were all node-positive and estrogen receptor-positive. The HER2 status of the tumors was determined using the Dako Hercept Test following the 2007 ASCO/CAP guidelines for HER2 positivity. The non-background corrected probe-expression data were exported from Illumina BeadStudio software and normalized using the software R and the function fastlo. Log2 transformed expression values were used to obtain the relative gene expression per probe. Spearman correlations were calculated between both types of replicates. Unequal-variance t-tests were conducted to test for differences in expression levels between HER2+ and HER2- tumors. An overall ERBB2 gene expression level was calculated by taking the average of the three ERBB2 probes for each tumor across all replicates. To simulate the possible outcome if only one replicate of each tumor was measured for gene expression, differential expression was tested for all possible combinations of single replicate probe expression values. Results: Spearman correlations for technical and biological replicates within tumor samples ranged from 0.9804 – 0.9946, and 0.9822 – 0.9945, respectively. The overall ERBB2 gene expression was significantly lower in HER2- tumors than in HER2+ tumors (p = 0.0000018). The top five most differentially expressed genes (p < 0.0005) included ERBB2, GRB7, C17ORF37, PERLD1, and ASF1B. For each of the three ERBB2 probes, there were 65,536 possible ways to combine the individual probe expression values. The p-values for testing differential expression in each of these 65,536 combinations of HER2+ tumors versus HER2- tumors ranged from 0.00029 – 0.0091 for ILMN_1717902, 0.0000078 – 0.00026 for ILMN_1728761, and 0.000000002 – 0.0000032 for ILMN-2352131. Conclusions: Illumina's Whole Genome Platform provided highly reproducible gene expression values among biological and technical replicates of breast tumors. This platform also demonstrated significant differential ERBB2 gene expression between HER2+ and HER- breast tumors, which correctly classified the breast tumors as HER2+ and HER2-. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 6127.

Abstract BackgroundRIBBON-1, a Phase III, multicenter, randomized, placebo-controlled trial in patients with previously untreated MBC was designed to evaluate the efficacy and safety of adding bevacizumab (B) to chemotherapy regimens including capecitabine (Cape; n=615); a taxane (T; n=307), or an anthracycline (Anth; n=315) compared with chemotherapy alone. Pre-specified analyses of progression-free survival PFS) in the Cape and pooled T/Anth cohorts, the primary endpoints of the study were increased upon addition of B. In the Cape cohort, median PFS increased from 5.7 to 8.6 mo (HR=0.69, p-value=0.0002) and in the T/Anth cohort, it increased from 8.0 to 9.2 mo (HR=0.64; p-value<0.0001).MethodsRECIST was used to determine objective response rate (ORR). Patients without a post-baseline tumor assessment were considered non-responders. The primary analysis for the ORR and the duration of response was performed using only patients with measurable disease at baseline. ORR was formally compared between the two treatment arms using the Mantel-Haenszel Chi-squared test, using the randomization stratification factors. Fisher's exact test was also performed. Clinical benefit rate (CBR) was defined as the proportion of patients with a complete or partial response or with stable disease at Week 24. Exploratory analyses of CBR and time to response are provided here. Summary statistics of time to response at Week 9 will be presented as will the final analysis of the secondary endpoint of overall survival.ResultsAnalyses of ORR and CBR are provided in the following table. CapeTaxaneAnth PL (n=206)B (n=409)PL (n=104)B (n=203)PL (n=103)B (n=212)Patients with measurable disease, n (%)161 (78.2)325 (61.1)85 (81.7)161 (79.3)92 (89.3)184 (86.8)ORR, %23.635.435.350.340.252.2CR, n (%)1 (0.6)7 (2.2)3 (3.5)4 (2.5)2 (2.2)3 (1.6)PR, n (%)37 (23.0)108 (33.2)27 (31.8)77 (47.8)35 (38.0)93 (50.5)Difference between arms, %11.815.012.0p-value, unstratified0.00940.030.073Duration of ORMedian, mo7.29.28.68.46.08.1HR (95% CI)0.61 (0.39–0.96)0.75 (0.45–1.27)0.53 (0.34–0.83)Log-rank p-value0.03260.2840.0047Clinical benefit rate*, %47.164.365.471.969.980.7p-value, unstratified<0.00010.240.04CR+PR at Week 9, %20.229.438.354.544.451.1B=bevacizumab, cape=capecitabine, CI=confidence interval, HR=hazard ratio, OR=objective response, PL=placebo, T/Anth=taxane/anthracycline.*For randomized patients.ConclusionsIn patients with HER2-negative MBC, the addition of bevacizumab to capecitabine, taxane or anthracycline chemotherapy induces more responses and those responses occur more rapidly. For capecitabine and anthracycline the addition of bevacizumab leads to objective responses that last longer. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 6084.

Abstract Background: Three prior Phase III trials (E2100, AVADO, and RIBBON-1) established the clinical benefit of adding bevacizumab (BV) to various chemotherapies (chemos) as first-line treatment for metastatic breast cancer (MBC). A previous Phase III study (AVF2119g) in patients with predominantly heavily pre-treated MBC, in which BV was added to capecitabine (Cape), resulted in a significant increase in objective response rate (ORR), but did not meet the primary endpoint for progression-free survival (PFS). The current study, RIBBON-2, was designed to evaluate the efficacy and safety of the addition of BV to chemotherapies used as second-line treatment for MBC.Methods: Patients were randomized in a 2:1 ratio to chemo+BV or chemo+placebo (PL). Key eligibility criteria included one prior cytotoxic treatment for MBC, ECOG performance status of 0 to 1, and HER2-negative or unknown status. Prior to randomization, investigators chose one of the following chemo agents: taxane (T; paclitaxel 90 mg/m2/wk for 3 of the 4 weeks; paclitaxel 175 mg/m2, nab-paclitaxel 260 mg/m2, docetaxel 75–100 mg/m2, all given q3wk), gemcitabine (G; 1250 mg/m2 on Days 1 and 8 q3wk), Cape (2000 mg/m2 Days 1–14 q3wk), or vinorelbine (V; 30 mg/m2/wk). BV or PL was administered at 10 mg/kg q2wk or 15 mg/kg q3wk, depending on the chemo regimen. The primary endpoint of the study was investigator-assessed PFS pooled across the chemo cohorts. Key secondary endpoints included overall survival (OS), PFS within individual chemo cohorts, ORR, and safety.Results: 684 patients (T, 304; G, 160; Cape, 144; and V, 76) at 211 sites in 19 countries were randomized between February 2006 and June 2008. Overall, the two study arms were balanced for patient characteristics at baseline. The study met its primary endpoint of PFS pooled across chemo cohorts and also demonstrated a 10% increase in ORR when BV was added to chemo. At the interim analysis for OS, the median durations were 18 mo for chemo+BV and 16.4 mo for chemo+PL (see table).Across all chemo cohorts the incidence of BV-related AEs was consistent with data from previous studies. Hypertension was the only BV-related AE consistently increased in the chemo+BV arm across all chemo cohorts.Conclusions: The addition of BV to chemotherapies used for second-line treatment of MBC led to a significant improvement in PFS. The AE profile of BV in the overall study population and across the chemotherapy cohorts was consistent with that previously observed. Additional analyses, including PFS for the individual chemo cohorts, will be presented. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 42.

Abstract Background: Treatment of aromatase refractory metastatic breast cancer (MBC) is difficult and challenging. Estrogen receptor (ER) resistance causes enhanced expression of the vascular endothelial growth factor (VEGF).Several studies have shown that the ER interacts with the VEGF pathway and is an important mechanism of resistance. Therefore we embarked on a phase II study of fulvestrant, a complete ER suppressor and bevacizumab, a well studied VEGF monoclonal antibody in aromatase refractory MBC patients. Methods: A single stage phase II study with an interim analysis of fulvestrant and bevacizumab was conducted with these objectives: 6 month progression-free survival rate (PFS), tumor response, toxicity, and overall survival. Regimen: fulvestrant 250 mg day1 and 15 (cycle 1) then day 1 (cycle 2 and beyond) and bevacizumab 10mg/kg days 1 and 15 of each 4 weeks is a cycle. Results: At the time of interim analysis, 11/20 evaluable patients achieved 3-month progression-free survival status while remaining on treatment for at least 3 months, not meeting the protocol specified efficacy requirements and thus halting accrual. 36 patients were enrolled from September 2007-December 2008; 33 patients were evaluable. Number of prior metastatic chemotherapy regimens: 0 in 26 patients and 1 in 7 patients. 22 (67%) patients received prior hormonal therapy in the metastatic setting. 18 (55%) had measurable disease. A median of 6 cycles (range 1-19) were administered. 12/33 evaluable patients (95% CI:20-55%) achieved 6-month progression-free survival status while remaining on treatment for a least 6 months. Among 18 patients with measurable disease, 2 (11%) patients CI:1.4-35%) had a confirmed tumor reponse (both PR). Additionally, 2 patients had stable disease for greater than 6 months, for a clinical benefit rate of 22%. Median follow up was 8.5 months (range 1.7-17.5 months). Median progression-free survival was 6.2 months (95% CI:5.4-10.1 months). The 6 -month overall survival rate was 84.8% (95% CI 73.5-98%). The median dose level administered was 250 mg for fulvestrant and 10 mg/kg for bevacizumab for cycles 1-19. The most common grade 3/4 adverse events (AEs) were hypertension 2 (6%), headache pain 2 (6%), and confusion 2 (6%). There was 1 grade 5 central nervous system hemorrhage. 13 (39%) patients experienced a grade 3 non-hematologic AE and 4 (12%) experience a grade 4+ non-hematologic AE. Conclusion: Fulvestrant/bevacizumab is safe and tolerable. Although this regimen did not meet its statistical endpoint, 22% of evaluable patients with aromatase refractory diesease achieved clinical benefit with minimal toxicity. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 4096.

Abstract Background: Aromatase inhibitor (AI) therapy effectively increases disease-free survival in postmenopausal women (PMW) with ER+ and/or PR+ breast cancer (BCa). However, the use of AIs results in nearly complete ablation of estrogen production which can lead to accelerated bone loss and increased fracture risk. The Z-FAST study evaluated the efficacy and safety of zoledronic acid (ZOL) in preventing AI associated bone loss in PMW with early breast cancer (EBC) who received adjuvant letrozole (LET).Material and Methods: 602 PMW with stage I-IIIa ER+ and/or PR+ BCa starting LET (2.5 mg qd x 5 yrs) were randomized (1:1) to upfront ZOL (4 mg IV q 6 mos) vs delayed ZOL. The delayed arm (D) received ZOL when either the post-baseline T-score decreased to <-2 or a clinical fracture occurred. All patients (pts) were treated with calcium and vitamin D. The primary endpoint, the percent change in lumbar spine (LS) bone mineral density (BMD) at 12 mos, was previously reported (JCO; 25:829, 2007). The 5 year (5y) final study results are reported here.Discussion: Baseline characteristics were similar between groups. 180 pts in upfront ZOL arm (U) and 175 pts in D completed full 5y study. Of pts with BMD data available, U (n=140) showed a mean increase of 6.2% in LS BMD while D (n=132) showed a mean decrease of 2.4%, resulting in an absolute difference of 8.6% (p<0.001). U (n=141) showed a mean increase of 2.6% in total hip (TH) BMD while D (n=132) showed a mean decrease of 4.1%, resulting in an absolute difference of 6.7% (p<0.001). When BMD data in D was excluded after pts started ZOL (censored analysis), the absolute difference in LS and TH BMD between the two arms was 11.3% and 8.7%, respectively. Among pts with baseline LS T-score between -1 and -2, 27.9% (19) U pts [8.6% (7) D pts] returned to normal T-score (T-score >-1), and no U pts as compared to 4.9%(4) D pts became severely osteopenic (T <-2). 17.7% (53) D pts met criteria that required initiation of ZOL. Although the study was not designed to detect a significant difference in the fracture rate between treatment arms, fractures occurred in 10.7% (29) U pts and 12.4% (33) D pts. Administration of ZOL q 6 mos for up to 5y was safe and well tolerated. No serious renal adverse events suspected related to ZOL and no confirmed osteonecrosis of the jaw cases (ONJ) were reported. Disease recurrence including death due to disease progression was reported in 7.0%, 95% C.I. (3.7%-10.3%) from K-M (16) pts in U, and 8.8%, 95% C.I. (5.2%-12.5%) (21) pts in D.Conclusion: The 5y follow-up of the Z-FAST trial show that the overall difference in the percentage change in BMD between U and D, at both LS and TH, progressively increased from baseline through 5y. These data demonstrate that ZOL 4mg IV q 6 mos is effective in preventing bone loss associated with adjuvant AI therapy in PMW with EBC. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 4083.

Abstract Background: Despite the impressive results of the trastuzumab adjuvant therapy trials, 15% of patients (pts) with HER2 overexpressing or amplified breast cancer developed tumor relapse at 4 years. Lapatinib is a reversible tyrosine kinase inhibitor against ErbB1 and ErbB2. The current study was developed to assess the cardiac safety and feasibility of adding lapatinib to paclitaxel and trastuzumab following doxorubicin and cyclophosphamide as part of adjuvant therapy.Methods: A single-arm phase II study of doxorubicin (A, 60 mg/m2 day 1) and cyclophosphamide (C, 600 mg/m2 day 1) [q2w or q3w for 4 cycles]; followed by PTL: paclitaxel (P, 80 mg/m2 weekly), trastuzumab (T, 4 mg/kg loading dose then 2 mg/kg weekly), and lapatinib (L, 1000 mg PO daily which was later amended to 750 mg) [total of 12 weeks of treatment]; followed by T (6 mg/kg day 1, every 3 weeks) and L (1000 mg PO daily) [total of 40 weeks of treatment] was conducted. The primary endpoint was severe symptomatic congestive heart failure (CHF: New York Heart Association Class III or IV with a drop in left ventricular ejection fraction [LVEF] of at least 10 percentage points to below 50% confirmed by cardiologist) with secondary endpoints of cardiac death and grade 3+ cardiac adverse events (AEs).Results: From April 2007 to October 2008, 109 evaluable pts were enrolled and initiated study treatment. Median age was 51 (range 28-72). Median follow-up is 10.2 months. No pts experienced CHF or cardiac death while on active treatment. One pt had CHF and 1 pt died due to myocardial infarction (MI) after they went off study treatment. The pt with CHF refused further treatment after completing 2 cycles of PTL and had CHF 1.8 months post PTL discontinuation with LVEF of 45% and dyspnea on exertion. The pt with MI went off study treatment due to non-cardiac AEs after completing 2 cycles of PTL and died 2.9 months post PTL discontinuation. Mean LVEF at baseline was 63.6% (N=109, SD=5.7); end of AC was 63.3% (N=104, SD=6.4); end of PTL was 59.9% (N=89, SD=8.3); and cycle 12 was 59.7% (N=66, SD=7.1). Grade 3+ non-hematologic AEs with incidence >5% include diarrhea (N=42, 39% & included both 1000mg & later amended 750mg dosing), fatigue (N=15, 14%), nausea (N=7, 6%), vomiting (N=7, 6%), acne (N=6, 6%), and hypokalemia (N=6, 6%). Pre-treatment cardiac marker data (troponin-T, troponin-I, brain natriuretic peptide, creatine kinase MB isoenzyme) will also be presented.Conclusions: Preliminary data suggest that the inclusion of L does not add to the cardiac profile of T. Data from ALTTO and long-term follow data are needed to confirm the cardiac safety of this regimen. Cardiac marker data are being analyzed in the context of changes in LVEF. We gratefully acknowledge support from the Breast Cancer Research Foundation and GlaxoSmthKline for this study. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 3086.

Abstract Background: Postmenopausal women with significant osteopenia/osteoporosis are at increased risk of fracture, a risk that is exacerbated by the use of Aromatase Inhibitors (AIs). Bisphosphonates may be used for these patients because there is no known interaction with estrogen and/or progesterone receptors (ER, PR). This study evaluated the concurrent use of zoledronic acid in patients with significant osteopenia or osteoporosis who received initial adjuvant letrozole therapy for primary BC, to determine if further bone mineral density (BMD) loss could be prevented.Methods: Postmenopausal women with Stage I-IIIa, ER and/or PR + BC, no evidence of metastatic disease, and a BMD T-score < -2.0 were treated with daily letrozole 2.5 mg/d, vitamin D 400 international units/d, calcium 500 mg twice daily, and 4 mg I.V. zoledronic acid every 6 months (for 5 years). The BMD was measured at baseline and at one year. Kruskall-Wallis p-value methodology was used as the method of statistical analysis. Since this was a single-arm study, the analysis plan was primarily descriptive. The primary endpoint was the mean change in lumbar spine (LS) BMD at 1 year.Results: 60 patients were enrolled; 46 completed 1 year of treatment. Mean patient age was 67 years, with 44% having taken prior tamoxifen. At 1 year (see figure 1), LS BMD increased 2.66% (p=0.01), femoral neck (FN) BMD increased 4.81% (p=0.01), and any measured endpoint (within the LS or FN) increased 4.55% (p=0.0052). 7% of patients experienced a fracture vs.13% with a pre-existing history of fracture before enrollment. No patients had disease recurrence during year 1. Toxicity was minimal with arthralgia as the most common complaint. There were no reports of osteonecrosis of the jaw.Conclusion: Zoledronic acid prevents additional bone loss in postmenopausal women with significant osteopenia or osteoporosis initiating letrozole. Treatment with zoledronic acid was associated with an improvement in BMD. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 2103.

Abstract Background: Microtubules are crucial for spindle formation during mitosis and for cellular proliferation. The antineoplastic effect of paclitaxel is mainly related to its ability to bind the beta subunit of tubulin, thus preventing tubulin chain depolarization and inducing apoptosis. Tubulins are expressed in human peripheral nerves and the binding of paclitaxel to tubulin may lead to neuropathy. Peripheral neuropathy is a common dose limiting toxicity of paclitaxel. We hypothesized that the occurrence of peripheral neuropathy may correlate with outcome (disease-free survival; DFS).Methods: This analysis sought to describe incidence of peripheral neuropathy following paclitaxel and its association to outcome (DFS) in patients who received paclitaxel (weekly x 12) in the adjuvant HER2+ intergroup trial N9831. Only eligible pts who initiated paclitaxel and did not have peripheral neuropathy at initiation of paclitaxel that were randomized to arms A (955 pts; chemotherapy alone) and C (889 pts; chemotherapy plus concurrent trastuzumab) of N9831 were included. Cox regression analysis stratified by ER/PR status and nodal status was used to compare DFS within arm between patients with and without peripheral neuropathy.Results: Out of 1844 eligible pts, 379 developed neuropathy (20.5%). For pts in arm A, those who developed neuropathy had better DFS than pts who did not (3 yr DFS: 86.2% vs 81.8%; HR 0.65; p=0.01), despite lower doses of paclitaxel in the pts with neuropathy. Grade of neuropathy did not appear to impact DFS. No statistical difference was noted for pts treated in the trastuzumab-containing arm (3 yr DFS: 92.8% vs 91.1% for pts with neuropathy vs not; HR 0.79; p=0.34). There were no differences in paclitaxel dose intensity between arms A and C.Conclusion: Patients with early stage HER2+ breast cancer who received adjuvant paclitaxel-containing chemotherapy in arm A and developed peripheral neuropathy had a better DFS than pts who did not develop neuropathy. This effect was possibly abrogated by the use of trastuzumab in Arm C. This side effect may represent effective bindings of paclitaxel to the target tubulin, lack of point mutations in tubulin at the paclitaxel binding site and/or lack of selective overexpression of β-III tubulin. This is a hypothesis generating study and additional analysis needs to be conducted from other large taxane-based trials.Partial support from Genentech and the Breast Cancer Research Foundation Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 2100.

Abstract Background: Adenoid cystic carcinoma of the breast is a rare type of breast cancer accounting for approximately 0.1% of all breast cancer and less than 200 cases are reported in the literature. Hence, data for treatment and outcome are limited. This retrospective review reports the Mayo Clinic's experience.Methods: We identified 30 patients between 1971 and 2009 who were diagnosed with adenoid cystic breast cancer at Mayo Clinic Rochester, Arizona or Florida. Median follow-up was 62 months with a range of 1 to 424 months. Data regarding demographics (age, gender, and race), tumor characteristics (size, grade, and margin status), biological features (Estrogen receptor (ER), Progesterone receptor (PR) status, HER2 status) and clinical course (recurrence, secondary treatment) were collected. Disease free survival was calculated using Kaplan Meier estimates.Results: The median age of all patients was 58 (range, 22 – 78 years). All were female. Median tumor size was 1.3 cm (range, 0.6 – 8.5cm). Axillary lymph node sampling (sentinel or complete dissection) was performed in 15 patients. No patient had lymph node positive disease on presentation. The majority of patients had hormone receptor negative disease. Only 3 patients (10%) had weak hormone receptor (1-10%) positivity. No tumors were HER2 positive. Only two patients had high grade disease. Initial surgical data only available in twenty one patients: fifteen (50%) were treated with lumpectomy and adjuvant radiation, and 6 (20%) were treated with mastectomy. There were no local recurrences. Only one patient was treated with adjuvant chemotherapy (with cyclophosphamide, methotrexate and fluorouracil (CMF) followed by tamoxifen) but still suffered a distant recurrence within 5 years. There were a total of five patients that recurred. The 5-year disease free survival (DFS) rate for the entire cohort was 90%, and 10 year DFS was 83%. Median time to recurrence was 47.5 months (Range, 23-77 months). Recurrence occurred almost exclusively in patients with ≥ T2 disease. The primary site of metastases was lung (80%) and bone (20%) but subsequent liver and brain metastases were also seen. Treatments at relapse included surgical resection, radiosurgery and chemotherapy.Conclusion: Adenoid cystic carcinomas of the breast are rare but have overall good prognosis. This review confirms that the majority of these patients present with node negative disease, low to intermediate histologic grade and endocrine unresponsive tumors. Primary treatment is surgery (mastectomy or lumpectomy plus radiation) and there seems to be no role of any adjuvant therapy at the present time. In general, recurrences (80%) occurred with tumors larger than 2 cm and adjuvant chemotherapy and/or targeted therapy could be explored for selected patients in the context of a clinical trial. Overall mortality after recurrence was 50% but median overall survival was variable in years (1-14 years). Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 2055.

Abstract Background: Endoxifen, the active metabolite of tamoxifen (Tam), is currently being developed as a drug for the treatment of estrogen receptor (ER) + breast cancer (BC). HER2 expression in ER+ BC is associated with Tam resistance, and in vivo Tam administration to mice bearing ER+/HER2+ xenografts stimulates BC growth (Shou, JNCI 2004). In humans, endoxifen is the most important Tam metabolite responsible for inhibiting estrogen induced BC growth (Wu, Cancer Research 2009). CYP2D6 metabolism affects the concentrations (conc) of endoxifen (Stearns, JNCI, 2003) and associated with worse disease free survival (DFS) in Tam treated BC (Goetz, JCO, 2005). We sought to determine the activity of Tam and its metabolites in ER+/HER2+ BC cell lines and to evaluate the role of CYP2D6 metabolism in Tam-treated patients (pts) with ER+/HER2+ BC. Additionally, we sought to determine endoxifen conc in mice administered oral Tam.Methods: MCF7 (parental and HER2-expressing) and BT474 (ER+/HER2+) cells were used to compare the activity of Tam, 4HT, and endoxifen on estrogen- stimulated growth. Oral tam PK were characterized in mice treated with standard dose of Tam (4 mg/kg; 100 μg). Clinical data were obtained via a retrospective analysis of Tam-treated pts with ER+/ HER2+ BC randomized to receive 5 years of Tam (NCCTG 89-30-52). CYP2D6 metabolism (extensive or decreased) was based on CYP2D6 genotype (*3, 4, 6, 10, 17, 41) and co-administration of a CYP2D6 inhibitor (yes/no). HER2 was determined by immunohistochemistry (IHC) or FISH (tumors 0, 1, or 2+ by IHC). The association between CYP2D6 and DFS was assessed using the log-rank test and proportional hazards modeling.Results: Compared to Tam, endoxifen potently inhibited the growth of estrogen- stimulated BT474 cells. In MCF7 cells, expression of HER2 shifted the conc of endoxifen required for 50% inhibition of growth (IC50) from 54 nM (parental) to 131 nM (HER2 expressing). Using the range of conc of Tam and its metabolites observed in humans (Tam, 300-500 nM; 4HT, 5-10 nM; and endoxifen, 10-200 nM), only endoxifen potently inhibited estrogen- stimulated growth of MCF7HER2+ cells and only at conc achievable in CYP2D6 extensive metabolizers (>50nM). In mice, conc of 4HT and endoxifen were below 15 nM following an oral dose of 4 mg/kg. In NCCTG 89-30-52, both CYP2D6 phenotype and HER2 status was determined in 201/256 randomized pts. HER2 was expressed in 23/215 (11%) but not associated with DFS overall (p=0.62). In the HER2+ subset, pts with decreased CYP2D6 metabolism (n=10) had significantly shorter DFS compared to extensive metabolizers (n=9) (HR 9.5, p=0.03; 95% CI 1.16-76.9).Conclusions: Our in vitro and clinical data provide a simple pharmacological model for understanding HER2 resistance in Tam-treated breast cancer. Mice, which lack the CYP2D6 enzyme, may not be an appropriate model for understanding tam resistance given low conc of both 4HT and endoxifen. Given that the in vitro conc of endoxifen needed to inhibit the growth of ER+/HER-2+ BC are achievable in only a subset of humans (CYP2D6 extensive metabolizers), the primary administration of endoxifen could overcome de novo Tam resistance in ER+/HER2+ BC. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 2006.