Trastuzumab emtansine (T-DM1) plus docetaxel with or without pertuzumab in patients with HER2-positive locally advanced or metastatic breast cancer: Results from a phase Ib/IIa study

Background: Trastuzumab emtansine (T-DM1) exhibited enhanced antitumor activity when combined with docetaxel or pertuzumab in preclinical studies. This phase Ib/IIa study assessed the feasibility of T-DM1 + docetaxel in patients with human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer (MBC) and T-DM1 + docetaxel ± pertuzumab in patients with HER2-positive locally advanced breast cancer (LABC). Patients and methods: Phase Ib (part 1) explored dose escalation, with T-DM1 + docetaxel administered for greater than or equal to six cycles in patients with MBC. Phase Ib (part 2) began with the maximum tolerated dose (MTD) identified in part 1. Patients with LABC were administered less than or equal to six cycles of T-DM1 + docetaxel or T-DM1 + docetaxel + pertuzumab. Phase IIa explored the MTDs identified in phase Ib. Results: Administered with T-DM1 (3.6 mg/kg), the docetaxel MTD was 60 mg/m² in MBC. In LABC, the MTD was 100 mg/ m² docetaxel in combination with T-DM1 (3.6 mg/kg), given with granulocyte colony-stimulating factor (G-CSF). Administered with T-DM1 (3.6 mg/kg) + pertuzumab (840 mg, cycle 1; 420 mg, subsequent cycles), the docetaxel MTD in LABC was 75 mg/m² with G-CSF support. Neutropenia was the most common grade 3-4 adverse event (AE; MBC, 72% and LABC, 29%). In total, 48%(12/25) of MBC patients and 47%(34/73) of LABC patients experienced AEs requiring dose modification. In MBC (median prior systemic agents = 5), the objective response rate was 80.0% (20/25; 95% confidence interval [CI] 59.3-93.2) and the median progression-free survival was 13.8 months (range, 1.6-33.5). The pathologic complete response (ypT0/is, ypN0) rate in LABC was 60.3% (44/73; 95% CI 48.1-71.5). Pharmacokinetic analyses indicated a low risk of drug-drug interaction between T-DM1 and docetaxel. Conclusions: T-DM1 combined with docetaxel ± pertuzumab appeared efficacious in MBC or LABC; however, nearly half of patients experienced AEs requiring dose reductions with these T-DM1 combinations.