Activation of the phosphoinositide 3-kinase (PI3K) pathway is an important resistance mechanism to anti-HER2 therapies. This study aimed to assess the safety and activity of alpelisib (a PI3Kα isoform-specific inhibitor) with T-DM1 in trastuzumab- and taxane-resistant HER2-positive MBC.

Patients with HER2-positive MBC that had progressed on trastuzumab-based therapy were treated with alpelisib daily and T-DM1 3.6 mg/kg every 3 weeks. The dose-limiting toxicity (DLT), maximum tolerated dose (MTD), adverse events, overall response rate (ORR), and clinical benefit rate (CBR = CR + PR + SD > 6 months) were assessed with descriptive statistics. Progression-free survival (PFS) was calculated by the Kaplan-Meier method.

Seventeen patients were enrolled with a median of 3 prior therapies for metastatic disease. The DLT was a maculopapular rash and MTD was 250 mg alpelisib daily. The most frequently occurring toxicities included fatigue, rash, gastrointestinal side effects, thrombocytopenia, anemia, elevated liver enzymes, and hyperglycemia. Fourteen patients were evaluable for response with an ORR of 43%. In patients with prior treatment and progression on T-DM1 (n = 10), the ORR was 30%. The CBR was 71% in evaluable patients and 60% in those with prior T-DM1. The median PFS was 8.1 months.

The combination of alpelisib and T-DM1 is tolerable and demonstrates activity in trastuzumab-resistant HER2-positive MBC. Furthermore, activity was observed in T-DM1-resistant disease. These data suggest that PIK3CA inhibition targets an important resistance pathway to anti-HER2 therapy, providing rationale for further study of PI3K inhibition in refractory HER2-positive MBC to validate these results.