Safety and antitumour activity of ipatasertib combined with endocrine therapy and a CDK4/6 inhibitor in HR+/HER2- metastatic breast cancer (TAKTIC): a single-centre, open-label, phase 1b trial

Abstract

Background: PI3K/AKT pathway activation is implicated in CDK4/6 inhibitor resistance. The use of AKT inhibition with continued CDK4/6 blockade after CDK4/6 inhibitor resistance remains unexplored. We evaluated the safety of ipatasertib and an antioestrogen with or without palbociclib in patients with treatment refractory HR+/HER2- metastatic breast cancer.

Methods: This single-centre, open-label, phase 1b trial was conducted at the Massachusetts General Hospital (Boston, MA, USA). Eligible patients were women older than 18 years with biopsy proven HR+/HER2- locally advanced, unresectable, or metastatic breast cancer; an Eastern Cooperative Oncology Group performance status of 0-2; disease progression on at least one previous therapy for metastatic disease; and measurable disease or bone lesions. Patients received 400 mg oral ipatasertib with standard 500 mg intramuscular fulvestrant dosing (ipatasertib and fulvestrant group) or with an aromatase inhibitor (oral anastrozole 1 mg per day, exemestane 25 mg per day, or letrozole 2·5 mg per day; ipatasertib and aromatase inhibitor group) on days 1-28 of each cycle. The ipatasertib and fulvestrant plus palbociclib group included a dose-escalation phase with patients assigned sequentially to escalating doses of ipatasertib and palbociclib using a standard 3 + 3 design starting at the recommended dose of palbociclib (125 mg on days 1-21) and the lowest dose of ipatasertib (200 mg on days 1-21). The primary endpoint was safety and progression-free survival was a key secondary endpoint. Safety was analysed in all patients who received at least one dose of ipatasertib and progression-free survival was assessed in all enrolled participants. This study is registered with ClinicalTrials.gov, NCT03959891 (active, not recruiting).

Findings: Between June 5, 2019, and Feb 16, 2022, 77 patients were enrolled (19 assigned to ipatasertib and fulvestrant, 16 to ipatasertib and aromatase inhibitor, and 42 to ipatasertib and fulvestrant plus palbociclib). All patients were female (77 [100%]); 75 were White (97%) and two (3%) were Asian. The median age was 62 years (range 32-88) and 66 (86%) of 77 patients received previous CDK4/6 inhibitor (median number of previous lines was 3 [range 1-13]). The median follow-up was 12·5 months (IQR 7·6-19·7). The recommended phase 2 dose was established at 400 mg ipatasertib on days 1-21 with 100 mg palbociclib on days 8-28 and standard fulvestrant 500 mg. Median progression-free survival was 5·5 months (95% CI 3·8-7·4). Serious adverse events related to study treatment occurred in seven (17%) patients in the ipatasertib and fulvestrant plus palbociclib group and one (5%) in the ipatasertib and fulvestrant group, which were related to neutropenia, leukopenia, thrombocytopenia, and hyperglycaemia. Common grade 3-4 adverse events related to study treatment (occurring in >5% of patients) were neutropenia (30 [39%] of 77), leukopenia (15 [19%]), diarrhoea (14 [18%]), rash (seven [9%]), lymphopenia (three [4%]), and anaemia (four [5%]). Four deaths occurred during the study (one possibly treatment-related due to grade 5 hyperglycaemia in the ipatasertib and fulvestrant group and two due to infectious issues and one due to pulmonary complications in the ipatasertib and fulvestrant plus palbociclib group), deemed unrelated to study treatment.

Interpretation: The combination of fulvestrant, ipatasertib, and palbociclib showed preliminary signs of clinical activity and showed expected adverse events in heavily pretreated patients with HR+/HER2- metastatic breast cancer, warranting further evaluation in those with CDK4/6 inhibitor-refractory disease.

Funding: Genentech, Howard Hughes Medical Institute, National Foundation for Cancer Research, and Breast Cancer Research Foundation.