Background: Pertuzumab (P) is a monoclonal antibody which binds to extracellular domain II of HER2 distally from trastuzumab (H), disrupting HER2 dimerization and signaling. The CLEOPATRA phase III trial showed that HP + docetaxel in HER2+ MBC prolonged progression-free survival (PFS) compared to placebo + H + docetaxel. We report preliminary results of a phase II study to evaluate the safety and efficacy of weekly paclitaxel (T) with HP (THP). Methods: Patients (pts) with HER2+ MBC with 0-1 prior treatment (Rx) are eligible. Pts receive weekly (w) paclitaxel (80mg/m²), q3w trastuzumab (loading dose 8mg/kg → 6mg/kg), and q3w pertuzumab (flat loading dose 840mg → flat dose 420mg). The primary endpoint is PFS at 6 months (mo). Secondary endpoints include response, safety (including cardiac events), and tolerability. Evaluable pts are those who have started study Rx and are assessed at 6 mo for PFS. Left ventricular ejection fraction (LVEF) is monitored by echocardiogram every 3 mo. Cardiac events are defined as symptomatic LV systolic dysfunction (LVSD), non-LVSD cardiac death, or probable cardiac death. Results: As of 5-1-12, 33 of the planned 69 pts were enrolled; 16 were evaluable at 6 mo. Of the 16 pts, G 3/4 toxicities included sepsis (1pt, 6%), cholecystitis (1pt, 6%), fatigue (1pt, 6%), skin ulceration (1pt, 6%) and cystic macular degeneration (1 pt with prior prolonged Rx with paclitaxel, 6%). G 1/2 toxicities included alopecia (16 pts, 100%), fatigue (15 pts, 94%), ALT/AST elevation (14 pts, 88%), neuropathy (14 pts, 88%), diarrhea (12 pts, 75%), rash (9 pts, 56%), nail changes (8 pts, 50%), nausea (7 pts, 44%), mucositis (7 pts, 44%), and dry skin (6 pts, 38%). Median LVEF was 63% at baseline, 60% at 3 mo and 58% at 6 mo. There were no cardiac events. At 6 mo, 12/16 pts (75%) were progression-free (2 CR, 7 PR and 3 SD); 4 pts progressed. Conclusions: Our single-center phase II study continues to accrue, with no significant diarrhea or signal of increased cardiac toxicity to date. If the estimate of activity is similar to results with docetaxel in CLEOPATRA, this will provide support for THP as an alternative option in this setting.