Stage: Preclinical
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 ICH, which can result in miscarriage, loss of the newborn or severe life-long neurological disability
Therapeutic Modality: Human hyperimmune globulin
Mechanism of Action: Prevention of HPA-1a alloimmunization through AMIS

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 a pregnant woman to develop antibodies that attack the platelets of her fetus. The destruction of platelets in the fetus can result in low platelet counts (thrombocytopenia) in the baby, potentially leading to intracranial hemorrhage (ICH). The consequences of ICH can be devastating, and include miscarriage of the fetus, 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 fetal and neonatal alloimmune thrombocytopenia (FNAIT) resulting from human platelet antigen-1a (HPA-1a) alloimmunization.

With RLYB211, Rallybio hopes to eradicate FNAIT resulting from HPA-1a incompatibility. RLYB211 is designed to prevent FNAIT through a mechanism known as antibody-mediated immune suppression (AMIS). This product candidate is a human hyperimmune immunoglobulin product derived from plasma donated by women who became alloimmunized to HPA-1a during a prior pregnancy. RLYB211, when administered to mothers at risk, is designed to prevent HPA-1a alloimmunization though AMIS, thereby preventing FNAIT from occurring in the fetus or newborn.

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