In the phase II HER2CLIMB trial, a 34% reduction in the risk of death was observed with the addition of tucatinib to the combination of trastuzumab and capecitabine in patients with heavily pretreated unresectable locally advanced or metastatic HER2-positive breast cancer compared to the combination alone, according to results from the phase II HER2CLIMB trial presented at the 2019 San Antonio Breast Cancer Symposium.
Rashmi K. Murthy, MD
In the phase II HER2CLIMB trial, a 34% reduction in the risk of death was observed with the addition of tucatinib to the combination of trastuzumab (Herceptin) and capecitabine (Xeloda) in patients with heavily pretreated unresectable locally advanced or metastatic HER2-positive breast cancer compared to the combination alone, according to results from the phase II HER2CLIMB trial presented at the 2019 San Antonio Breast Cancer Symposium.1,2
The median overall survival (OS) was 21.9 months (95% CI,18.3-31.0) with the tucatinib triplet compared with 17.4 months (95% CI, 13.6-19.9) with trastuzumab and capecitabine alone (HR, 0.66; 95% CI, 0.50-0.88; P = .0048). The 1- and 2-year OS rates were 76% versus 62% and 45% versus 27% in the tucatinib and control arms, respectively. The OS benefit was upheld across all prespecified subgroups.
The addition of the small molecule TKI tucatinib also led to a 46% reduction in the risk of disease progression or death compared with trastuzumab and capecitabine alone, with a median progression-free survival (PFS) of 7.8 months (95% CI, 7.5-9.6) versus 5.6 months (95% CI, 4.2-7.1), respectively (HR, 0.54; 95% CI, 0.42-0.71; P <.00001). The 6-month and 1-year PFS rates were 63% versus 46% and 33% versus 12%, respectively. The PFS benefit was upheld across all clinically significant prespecified subgroups.
Notably, the tucatinib triplet reduced the risk of disease progression or death by 52% (HR, 0.48; 95% CI, 0.34-0.69; P <.00001) in patients with brain metastases at baseline. The median PFS in this subpopulation with high unmet medical need was 7.6 months with tucatinib versus 5.4 months in the control arm. The 1-year PFS rates were 25% versus 0%, respectively. Subgroup analysis also indicated an OS benefit with tucatinib in this subgroup (HR, 0.58, 95% CI, 0.40-0.85).
Seattle Genetics, the manufacturer of tucatinib, plans to submit a new drug application to the FDA for tucatinib by the first quarter of 2020.
“The overall survival benefit was seen in patients who had received trastuzumab, pertuzumab, and T-DM1, and included patients who had brain metastases,” said lead study author Rashmi K. Murthy, MD, assistant professor, Department of Breast Medical Oncology, Division of Cancer Medicine, at The University of Texas MD Anderson Cancer Center.
“These study results represent at important advance toward improving outcomes for patients with HER2-positive metastatic breast cancer,” added Murthy. “Tucatinib in combination with trastuzumab and capecitabine has the potential to become a new standard of care in this population.”
The international, double-blind, placebo-controlled HER2CLIMB trial (NCT02614794) included 612 patients with unresectable locally advanced or metastatic HER2-positive breast cancer who were previously treated with trastuzumab, pertuzumab (Perjeta), and ado-trastuzumab emtansine (T-DM1; Kadcyla). Patients were randomized 2:1 to receive trastuzumab and capecitabine combined with either tucatinib (n = 410) or placebo (n = 202).
Tucatinib was administered orally at 300 mg twice daily during each 21-day cycle. In both arms, patients received capecitabine at 1000 mg/m2 orally twice daily on days 1 to 14 of each 21-day cycle, and trastuzumab at 8 mg/kg intravenously on day 1 of cycle 1, followed by 6 mg/kg thereafter on day 1 of each 21-day cycle.
The primary endpoint was PFS, with secondary endpoints including OS, PFS in patients with brain metastases, and confirmed objective response rate (ORR). The primary PFS endpoint was measured in the first 480 patients enrolled. Through an early amendment to the study protocol, the target population was increased from 480 to 600 patients to provide adequate power to the secondary endpoint of PFS in patients with brain metastases.
Key baseline characteristics were well balanced between the 2 arms. The median patient age was around 55, 99% of patients were female, and there was about a 50/50 split in each treatment arm between patients with an ECOG performance score of 0 or 1. About two-thirds of patients in each arm were ER- and/or PR-receptor positive. Patients in both arms had received a median of 4 prior therapies overall and a median of 3 prior therapies in the metastatic setting.
In the tucatinib arm, 48% (n = 198) of patients had brain metastases at baseline and 46% (n = 93) of patients in the control arm had them. “HER2CLIMB is the first randomized trial completed in patients with HER2-positive metastatic breast cancer that included patients with untreated or previously treated, progressing brain metastases,” said Murthy.
The data cutoff date was September 4, 2019, and the median follow-up was 14 months. The confirmed objective ORR was 41% in the tucatinib arm compared with 23% in the control arm (P = .00008). The ORR in the tucatinib group comprised a 1% complete response (CR) rate and 40% partial response (PR) rate. The stable disease (SD) rate 46% and the progressive disease (PD) rate was 8%. In the control group, the CR rate was 1%, the PR rate was 22%, the SD rate was 59%, and the PD rate was 14%.
The safety population comprised patients who received at least 1 dose of study treatment (n = 601). The median duration of treatment exposure was 5.8 months in the tucatinib arm and 4.4 months in the placebo arm. At the data cutoff, 29% of patients remained on treatment in the tucatinib group compared with 14% in the placebo group. In both arms, disease progression was the main reason for treatment discontinuation at 49% versus 66%, respectively.
Grade ≥3 adverse events (AEs) occurred in 55% of the tucatinib arm compared with 49% of the control arm. There were 6 AE-related deaths in the tucatinib group and 5 in the control group.
Diarrhea was the most frequently occurring all-grade AE in both arms at 81% (13% grade ≥3) versus 53% (9% grade ≥3) in the tucatinib versus control arms, respectively; however, antidiarrheal prophylaxis was not required.
All-grade palmar-plantar erythrodysesthesia syndrome was also common in both arms at 63% (13% grade ≥3) versus 53% (9% grade ≥3) with the triplet versus trastuzumab and capecitabine alone, respectively. “Palmar-plantar erythrodysesthesia is a clinically well-known side effect of capecitabine. Longer duration of exposure in the tucatinib arm contributed to the observed difference in rates,” explained Murthy.
Regarding liver transaminase elevations, Murthy noted that AST and ALT increases were, “mostly low-grade, transient, and reversible.”
Looking ahead, Murthy said next steps with this research include further drilling down on the data for the population with brain metastases for other key details, such as intracranial response or progression rates. Another potential research avenue, said Murthy, would be to evaluate whether continuation of tucatinib has benefit beyond progression.