A Phase I Study of Alpelisib in Combination with Trastuzumab and LJM716 in Patients with PIK3CA-Mutated HER2-Positive Metastatic Breast Cancer

Purpose: Activating mutations in PIK3CA promote resistance to HER2-targeted therapy in breast cancer; however, inhibition of PI3K alone leads to escape via feedback upregulation of HER3. Combined inhibition of HER2, HER3, and PI3K overcomes this mechanism preclinically. Patients and Methods: This phase I study investigated the MTD of alpelisib given in combination with trastuzumab and LJM716 (a HER3-targeted antibody) in patients with PIK3CA-mutant HER2-positive (HER2(+)) metastatic breast cancer (MBC) using the continual reassessment method. Secondary analyses included efficacy and exploratory correlative studies. Results: Ten patients were treated initially with daily alpelisib (arm A). Grade >= 3 adverse events seen in >= 2 patients included diarrhea (n = 6), hypokalemia (n = 3), abnormal liver enzymes (n = 3), hyperglycemia (n = 2), mucositis (n = 2), and elevated lipase (n = 2). The MTD of alpelisib in arm A was 250 mg daily. This prompted the opening of arm B in which 11 patients received intermittently dosed alpelisib. Grade >= 3 adverse events seen in >= 2 patients included diarrhea (n = 5), hypokalemia (n = 3), and hypomagnesemia (n = 2). The MTD of alpelisib in arm B was 350 mg given 4 days on, 3 days off. Among 17 patients assessed, 1 had a partial response, 14 had stable disease, and 2 had disease progression at best response. Five patients had stable disease for >30 weeks. mRNA profiling of pre- and on-treatment tissue demonstrated PIK3CA target engagement by alpelisib via induction of downstream signaling and feedback pathways. Conclusions: Combination treatment with alpelisib, trastuzumab, and LJM716 was limited by gastrointestinal toxicity. Further efforts are warranted to target the PI3K pathway in HER2(+) MBC.

Automated summary

The study found that combination treatment with alpelisib, trastuzumab, and LJM716 in PIK3CA-mutant HER2-positive metastatic breast cancer showed some efficacy, but was limited by gastrointestinal toxicity. Further efforts are needed to target the PI3K pathway in HER2(+) MBC.