A final overall survival (OS) analysis showed that the addition of tamoxifen to abemaciclib demonstrated a median OS of more than 24 months with progression-free survival (PFS) consistent to primary study results in patients with hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer. These findings from the phase II nextMONARCH study were presented on 19 September at the ESMO Virtual Congress 2020 by Erika P. Hamilton of the Sarah Cannon Research Institute in Nashville, USA.

Abemaciclib, which is an oral, cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor, was previously reported to improve PFS when combined with endocrine therapy in patients with HR-positive, HER2-negative metastatic breast cancer in both of the phase III MONARCH 2 and MONARCH 3 studies.

These results were confirmed in the phase II nextMONARCH study, wherein the primary PFS and overall response rate (ORR) analyses showed that abemaciclib had robust single agent activity in patients with heavily pre-treated HR-positive, HER2-negative metastatic breast cancer. However, PFS and the ORR were not found to be significantly improved by the addition of tamoxifen.

In nextMONARCH, abemaciclib activity was evaluated in 234 patients with heavily pre-treated HR-positive, HER2-negative metastatic breast cancer and disease progression on or after endocrine therapy and at least 2 lines of chemotherapy.

The primary endpoint of the multicentre, randomised, open-label phase II nextMONARCH trial was PFS and OS was a preplanned secondary endpoint.

The investigators randomly assigned patients in a 1:1:1 ratio to receive oral abemaciclib at 150 mg plus tamoxifen at 20 mg, or abemaciclib at 150 mg, or to receive abemaciclib at 200 mg plus prophylactic loperamide.

The final OS analysis was performed 24 months after the last patient entered treatment.

The PFS findings remained the same in the 12-month analysis, but OS was improved with the addition of tamoxifen to single agent abemaciclib

At data cut-off on 28 June 2019, twelve patients remained on study treatment.

Median OS was most improved in patients treated with the combination of abemaciclib and tamoxifen. In the abemaciclib plus tamoxifen arm, the median OS was 24.2 months compared with 17.0 months in the abemaciclib 200 mg monotherapy cohort with prophylactic loperamide (hazard ratio [HR] 0.620, 95% confidence interval [CI] 0.397-0.969; p = 0.034).

An intermediate OS was observed in the abemaciclib 150 mg monotherapy cohort where the median OS was 20.8 months (for the comparison of abemaciclib 150 mg versus abemaciclib 200 mg: HR 0.956; 95% CI, 0.635-1.438; p = 0.832).

The PFS and ORR findings remained unchanged at the 24-month analysis.

Commonly reported treatment-emergent adverse events across all three abemaciclib arms occurring in ≥25% of patients included diarrhoea in 61.1% of patients, neutropenia in 49.6%, anaemia in 40.6%, nausea in 36.3%, leukopenia in 30.8%, fatigue in 29.9% and abdominal pain in 27.4% of patients. Grade 3 diarrhoea was infrequent across all 3 abemaciclib arms at 4.7%.

Conclusions

The investigators concluded that median OS was significantly improved by the addition of tamoxifen to abemaciclib compared with abemaciclib monotherapy patients with heavily pre-treated HR-positive, HER2-negative metastatic breast cancer. They also found that PFS findings at 24 months remained consistent with the primary results of nextMONARCH with no significant difference. The investigators commented that no new safety findings were observed across the abemaciclib treatments.

The study was funded by Eli Lilly.

This article is a news release from ESMO 2020 Press Meeting. Read the original here.