ZSP1273 is a novel small-molecule anti-influenza drug that targets the RNA polymerase PB2 subunit, while oseltamivir is the first-line medication that inhibits neuraminidase. ZSP1273 showed high efficacy against human influenza viruses both and , including oseltamivir-resistant strains . In future clinical applications, the combination of these two antiviral drugs with different mechanisms can reduce the potential for antiviral resistance that may arise from monotherapy. To evaluate the drug-drug interaction between ZSP1273 and oseltamivir by the pharmacokinetics and safety of co-administration in healthy subjects, a phase I, single-center, randomized, open-label, three-period crossover study was conducted. Thirty-six subjects enrolled were randomized in a 1:1:1 ratio into three crossover treatment sequences with oral administration detailed as follows: treatment A: ZSP1273 tablets 600 mg once daily (QD) for 5 days; treatment B: oseltamivir capsules 75 mg twice daily (BID) for 5 days; treatment C: ZSP1273 tablets 600 mg once daily (QD) + oseltamivir capsules 75 mg twice daily (BID) for 5 days. Plasma samples were collected from all subjects at scheduled time points after drug administration to measure the plasma concentrations of ZSP1273, oseltamivir, and its active metabolite oseltamivir carboxylate, for pharmacokinetic analysis. Compared with monotherapy, the geometric mean ratios (90% confidence intervals) of C, AUC, AUC, and AUC for ZSP1273 after co-administration were all within the ineffective boundary range of 80% to 125%, supporting that no drug-drug interaction occurs with ZSP1273. After co-administration, the AUC, AUC, and AUC of oseltamivir were all within 80% to 125%, while C decreased by 39.9%. The pharmacokinetic parameters above of oseltamivir carboxylate remained within 80%-125%, except only the lower bound of the 90% CI for C slightly below 80% (77.0%). Considering the rapid metabolism of oseltamivir into the active metabolite oseltamivir carboxylate and the minor impact of co-administration on the pharmacokinetic parameters of oseltamivir carboxylate, it is believed that no clinically significant drug-drug interaction was observed with the combination of these two drugs. During the trial, the safety and tolerability of both combination therapy and monotherapy were good, with no increased safety risks observed from the combination therapy.CLINICAL TRIALSThis study is registered with ClinicalTrials.gov as NCT05108051.