Prophylactic Administration of HPA-1a–Specific Antibody RLYB212 Safely Prevents Fetal/Neonatal Alloimmune Thrombocytopenia in Pregnant Mice

Fetal/Neonatal Alloimmune Thrombocytopenia (FNAIT) is a life-threatening bleeding disorder caused by maternal alloantibodies targeting fetal platelet alloantigens. These alloantibodies, developed during pregnancy, lead to fetal and neonatal thrombocytopenia and, in severe cases, spontaneous or trauma-induced intracranial hemorrhage. Unlike Hemolytic Disease of the Fetus and Newborn (HDFN), which occurs in pregnancies subsequent to parturition-induced alloimmunization, an estimated 25% to 50% of FNAIT cases occur without warning during the gestation of the first pregnancy. Though long proposed, there are currently no approved therapies for the prevention of FNAIT. We have previously demonstrated that the HPA-1a-specific monoclonal antibody RLYB212 induces rapid and complete elimination of HPA-1a+ platelets from circulation and prevents alloimmunization in non-pregnant mice, making it a potential prophylactic candidate to prevent FNAIT. This study aims to evaluate the efficacy and safety of RLYB212 in preventing alloimmunization and FNAIT in pregnant mice. Wild-type (WT) female BALB/c mice were bred with HPA-1a-homozygous male C57Bl/6 mice. On days 8 and 15 after mating, pregnant females carrying HPA-1a positive fetuses were administered RLYB212 prophylactically (via IV injection) at two different doses (1.01 or 5.05 µg/kg body weight), followed by transfusion of 1.2x10⁷ HPA-1a+ platelets (equivalent to fetal maternal hemorrhage (FMH) of ~30 ml in humans). Control mice received HPA-1a+ platelets but no RLYB212. Maternal and neonatal HPA-1a-specific alloantibody titers and neonatal platelet counts were assessed. Administration of RLYB212 at either 1.01 or 5.05 µg/kg prevented alloimmunization induced by transfusion of HPA-1a+ platelets in pregnant mice. RLYB212-treated, but not untreated, pregnant mice gave birth to pups with normal platelet counts, indicating prevention of FNAIT. In addition, the safety of administering RLYB212 during pregnancy was evaluated by monitoring pregnant mice administered RLYB212 (1.01 or 5.05 µg/kg body weight) alone without challenge with HPA-1a+ platelet transfusion. RLYB212 alone at either 1.01 or 5.05 µg/kg did not cause thrombocytopenia in HPA-1a+ pups born to treated pregnant mice. Taken together, these data establish that prophylactic administration of the HPA-1a-specific antibody RLYB212 to pregnant mice is both effective and safe in preventing maternal alloimmunization and FNAIT. These preclinical findings further support the potential of RLYB212 as a prophylactic therapy, paving the way for human studies aimed at preventing FNAIT in pregnant women.