The efficacy and safety of SYN023 (Zamerovimab and Mazorelvimab injection), the recombinant humanized anti-rabies virus monoclonal antibody mixture, combined with rabies vaccine in a WHO category III rabies post-exposure population: A randomized, double-blind, positive control, phase III clinical trial.

The SYN023-006 study was conducted in WHO Category III rabies-exposed patient populations to assess the safety and efficacy of post-exposure prophylaxis that included either the monoclonal antibody mixture SYN023 or human rabies immunoglobulin (HRIG). This phase 3, double-blind, randomized, controlled trial was conducted at multiple clinical disease control sites in China. Patients were randomized 3:1 (stratified by study site) to wound infiltration with 0.3 mg/kg SYN023 or 20 IU/kg HRIG. All patients received thorough wound washing and 5 doses intramuscular Vero cell rabies vaccine. The composite primary study objective was to demonstrate: 1) superiority of Day 8 geometric mean (GM) concentration of serum rabies virus neutralizing antibodies (RVNA) with SYN023 versus HRIG (protocol-defined superiority margin: GM RVNA ratio 95 % confidence interval [CI] lower limit >1.2) and 2) no rabies in SYN023 recipients. Efficacy was evaluated in the Per-Protocol population. Safety endpoints included solicited and unsolicited adverse events (AEs) analyzed in all patients receiving any study treatment. ClinicalTrials.govNCT04644484. From 23 September 2020 to 26 June 2021, 537 male and 463 female patients were randomized (n = 750 SYN023, n = 250 HRIG). Day 8 GM RVNA was 4.339 IU/mL (standard error [SE]: 1.035) with SYN023 and 0.232 IU/mL (SE: 1.060) with HRIG. The SYN023:HRIG GM RVNA ratio was 18.695 (95 % CI: 16.440, 21.260) indicating superior RVNA with SYN023. No suspected rabies cases or deaths occurred. AEs were generally similar between treatment groups except greater local solicited AEs frequencies with HRIG (SYN023: 165/750 patients [22.0 %]; HRIG: 97/250 [38.8 %]). This study indicates that SYN023, a monoclonal antibody product, may be used as part of rabies post-exposure prophylaxis and provides superior protection sooner after exposure than HRIG.