Phase II study of sequential S-1 and cyclophosphamide therapy in patients with metastatic breast cancer

S-1 and cyclophosphamide (CPA) can be given orally, and their combination may have great potential for treating metastatic breast cancer (MBC). A phase I study of sequential S-1 and CPA therapy was conducted in patients with MBC; the recommended doses that were determined for this regimen were 80 mg/m²/day for S-1 and 100 mg/m²/day for CPA. We then conducted a phase II study of this oral S-1 and CPA regimen.

This was a single-arm, open-label, single-center prospective phase II study to evaluate the efficacy of a sequential S-1 and CPA regimen for MBC. S-1 was administered orally 2×/day for 14 consecutive days, and then CPA was administered orally 2×/day for 14 consecutive days in a repeating 4-week cycle (S-1 for 2 weeks, CPA for 2 weeks). The primary endpoint was the overall response rate (ORR). Secondary endpoints included the overall survival (OS), progression-free survival (PFS), clinical benefit rate (CBR) and safety.

Thirty-six patients were enrolled in this study. The overall response was complete response in 0 (0%), partial response in 12 (33.3%), stable disease in 12 (33.3%), and progressive disease in 11 (30.1%) patients. The ORR was 33.3% (12/36). The CBR was 66.7% (24/36). The median PFS was 9.5 months (95%CI: 7.8-12.6 months). The median OS was 20.2 months (95%CI: 15.0-25.4 months) Grade 3/4 adverse events included leukopenia in seven patients (19.4%). Dose reductions because of adverse events occurred in 12 patients (33.3%). There was no treatment-related mortality.

The combination of sequential therapy with S-1 and CPA was tolerable and had efficacy with good disease control. Sequential therapy with S-1 and CPA may be a feasible new treatment option for patients with MBC; however, further study is warranted to explore the efficacy of this therapy.

JRCT, JRCTs031180296 . Registered 2 December 2019 - Retrospectively registered.