Abstract Background: Letrozole (LET) and trastuzumab (TRA) are established agents for the treatment of hormone-receptor (HR) positive metastatic breast cancer (MBC) patients and HER2 positive disease, respectively. The “eLEcTRA” trial was designed to compare the efficacy and safety of LET combined with TRA to LET alone in patients that are both HER2 and HR positive. Furthermore, LET alone was compared in patients with HER2 and HR positive disease vs. those with HER2 negative, HR positive tumors.Methods: In this multicenter trial, 92 pts from 32 sites in 7 countries were enrolled from 2003 to 2007. Initially, enrollment of 370 pts was planned, however, due to slow recruitment the trial was prematurely closed in 2007. Patients with HER2 and HR positive MBC were randomized to either LET alone (Arm A, n=31) or LET + TRA (Arm B, n=26) as first-line treatment. In addition, 35 patients with HER2 negative and HR positive tumors were assigned to receive LET alone as first-line treatment (Arm C). Tumor response assessments were based on RECIST. Primary endpoint was time to progression (TTP), secondary endpoints were overall response rate (ORR) and clinical benefit rate (CBR).Results: Median age in the three arms was 61 (arm A), 61,5 (arm B), and 70 years (arm C), respectively. Median TTP in the LET alone arm (A) was 3.3 months compared to 14.1 months in the LET + TRA combination arm (B) (hazard ratio=0.67; p=0.23) and 15.2 months in arm C (hazard ratio=0.71; p=0.03). CBR was 39% for arm A compared to 65% in arm B (odds ratio 3.15, CI 1.07-9.26) and 77% in arm C (odds ratio 5.34, CI 1.83-15.58), respectively. ORRs were 13% for arm A, 27% for arm B, and 11% for arm C.LVEF as cardiac safety parameter showed no change from baseline to the minimum value during the treatment for arms A and C and a slight decrease by 5% for arm B.The incidence of other cardiac adverse events was comparable in all arms (10%, 8%, and 9%). Gastrointestinal disorders or musculoskeletal and connective tissue disorders were slightly more frequent for patients receiving LET and TRA (arm B).Conclusion: The results of the eLEcTRA trial show that the combination of letrozole and trastuzumab can be safely administered and is superior to letrozole alone as first-line therapy in patients with HER2 and HR positive MBC. Our results also confirm earlier evidence that aromatase-inhibitor therapy alone is only associated with a rather short TTP in HER2 and HR positive MBC. In contrast, outcome in letrozole-treated patients with HER2 negative HR positive tumors is significantly better compared to that associated with tumors with both HR and HER2 expression.(Trial was sponsored by Novartis Pharmaceuticals) Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 4094.