Background: Neratinib (HKI-272) is an orally administered irreversible pan-ErbB receptor tyrosine kinase inhibitor. In an ongoing phase II study, the preliminary objective response rate was 26% in patients with ErbB2+ advanced breast cancer with prior trastuzumab therapy. This study assessed the safety and preliminary efficacy of the combination of neratinib plus trastuzumab. Methods: Patients with advanced ErbB2+ breast cancer that progressed following trastuzumab therapy were enrolled. The primary endpoint was 16-week progression free survival rate (PFS). In part 1 (dose escalation), patients received neratinib 160 mg or 240 mg daily plus trastuzumab 4 mg/kg IV loading dose then 2 mg/kg weekly. In part 2, patients received weekly trastuzumab with neratinib 240 mg daily. Timed blood samples were collected for PK analyses. PK analysis is ongoing. Results: 45 patients (part 1 n = 8; part 2 n = 37) were enrolled (mean age 52 yr); 9 are active. In part 1, cohorts 1 and 2 were fully enrolled with 4 patients each. No dose limiting toxicities were observed. Most common AEs, any grade, were diarrhea (91%), nausea (51%), anorexia (40%), vomiting (38%), and asthenia (27%). Grade 3/4 AEs were diarrhea (13%), nausea (4%), vomiting (4%). Two patients receiving neratinib 240 mg reported AEs leading to withdrawal. No AEs of congestive heart failure and no significant drops of left ventricular ejection fraction were reported. Among 33 patients evaluable for efficacy, objective response rate was 27% (95% CI, 13% - 46%); 16-week PFS rate (for part 2) 47% (95% CI, 29% - 63%); median PFS was 19 weeks (95% CI 15 - 32 weeks). Conclusions: Neratinib plus trastuzumab was well tolerated with no significant or unexpected toxicities, and demonstrated clinical activity.