1645. Exploring Clinical and Antiviral Efficacy of Baloxavir Marboxil in a Phase 3, Randomized, Double-Blind, Placebo- and Active-Controlled Study of Otherwise Healthy Adults/Adolescents in Seasonal Influenza: Impact on Regional Participants, Treatment Time and Influenza Type B Virus Infection (CAPSTONE-1 Study)

BACKGROUND: Baloxavir marboxil (BXM), a selective cap-dependent endonuclease inhibitor, has demonstrated efficacy + safety for influenza in otherwise healthy patients. We present subgroup analyses for (i) United States vs. Japan (J), (ii) time (t) of treatment (early: ≥0 to ≤24 hours, vs. late: >24 to ≤48 hours), and (iii) influenza type B infections from the global Ph 3 trial (16/17 season). METHODS: A multicenter, randomized, double-blind, placebo (PLC)- and oseltamivir (OV)-controlled study recruited patients in Japan (n = 846) and United States (n = 590). Inclusion criteria: age 12–64 years, fever + flu symptoms, and ≤48 hours from symptom onset. Patients (20–64 years) randomized (2:2:1) to a single oral dose of BXM, PLC, or 75 mg OV BID for 5 days; patients 12–19 years were randomized (2:1) to receive BXM or PLC. BXM dose: 40/80 mg for BW </≥80 kg. Primary endpoint: time to alleviation of symptoms (TTAS) in ITTI population (pop). Viral titers measured from pre-/postdose nasal swabs. RESULTS: BXM reduced the median TTAS by 30.6 hours versus PLC (87.3 versus 117.9 hours, P = 0.1373) in the US pop and t to cessation of viral shedding: 24 versus 72 hours for PLC (P < .0001). Median TTAS in the United States versus J pop was longer, due to imbalances between groups. In both early/late treatments from symptom onset, BXM reduced TTAS versus PLC (Table 1). Regardless of the t to treatment from symptom onset, BXM reduced virus titer significantly from BL versus PLC and OV (Table 2). No significant reduction in TTAS was seen, while BXM reduced virus titer versus PLC and OV in type B virus infection. CONCLUSION: Outcomes for United States were aligned with the Ph 3 Results. Early treatment with BXM leads to a significantly faster TTAS vs. PLC. BXM caused significant viral titer reduction regardless of treatment time versus OV. BXM reduced virus titer vs. PLC and OV in type B virus infection. DISCLOSURES: K. Kawaguchi, Shionogi & Co., Ltd.: Employee, Salary. S. Portsmouth, Shionogi Inc: Employee, Salary. T. Shishido, Shionogi & Co., Ltd.: Employee, Salary.T. Uehara, Shionogi & Co., Ltd.: Employee, Salary. F. Hayden, Shionogi & Co., Ltd.: Scientific Advisor, Consulting fee (donated) and travel support for attending 6th ESWI meeting, 10–13 September 2017, Latvia, to present phase 3 OWH results.