Stage: Phase 1/2 Clinical Study
Indication:Fetal and Neonatal Alloimmune Thrombocytopenia (FNAIT)
Disease Mechanism: Destruction of platelets in the fetus by maternal alloantibodies targeting the HPA-1a platelet antigen
Disease Impact: Severe thrombocytopenia leading to intracranial hemorrhage (ICH), which can result in miscarriage, stillbirth, loss of the newborn or severe life-long neurological disability
Therapeutic Modality: Plasma-derived polyclonal anti-HPA-1a antibody
Mechanism of Action: Administration of RLYB211 is designed to drive rapid elimination of HPA-1a antigen, thereby preventing alloimmunization

Fetal and Neonatal Alloimmune Thrombocytopenia

FNAIT is potentially life-threatening rare disease that can cause uncontrolled bleeding in fetuses and newborns. FNAIT is a disorder that can occur during pregnancy and is caused by an incompatibility between mother and fetus of a specific human platelet antigen (HPA), most commonly HPA-1.

This incompatibility can cause an expectant mother to develop antibodies that attack the platelets of her fetus. The destruction of platelets in the fetus can result in severe thrombocytopenia, potentially leading to ICH. The consequences of ICH can be devastating, and include miscarriage, stillbirth, loss of the newborn, and severe lifelong neurological disability in those babies who survive.

There is currently no approved therapy for the prevention or treatment of FNAIT.


RLYB211 is an investigational drug in development for the prevention of FNAIT resulting from HPA-1a alloimmunization.

RLYB211 is an intravenously administered polyclonal HPA-1a antibody purified from plasma of naturally HPA-1a immunized women. The administration of RLYB211 to expectant at-risk mothers is designed to rapidly eliminate antigenic fetal platelets from the mother’s circulation and prevent maternal alloimmunization. The prevention of maternal alloimmunization eliminates the risk of FNAIT in the fetus.

Rallybio acquired RLYB211 from Prophylix AS. RLYB211 received an Orphan Drug Designation (ODD) from both the U.S. Food and Drug Administration (FDA) and the European Medicines Agency (EMA), and a Rare Pediatric Disease (RPD) designation from the FDA.