First Participant Dosed in Phase 1/2 RLYB211 Trial for FNAIT

In a recent press release, biopharmaceutical company Rallybio announced that the first participant was dosed in a Phase 1/2 clinical trial. Currently, only healthy male participants are enrolled in the trial. The Phase 1/2 trial will evaluate RLYB211 as a potential treatment for fetal and neonatal alloimmune thrombocytopenia (FNAIT).


Developed by Rallybio, RLYB211 is an investigational plasma-derived hyperimmune globulin. Typically, in FNAIT, mothers and fetuses do not have the same human platelet antigen (HPA), which is HPA-1. Thus, the mothers go through an alloimmunization process. In an article published in Revista Brasileira de Hematologia e Hemoterapia, researchers explain:

Alloimmunization is defined as an immune response to foreign antigens after exposure to genetically different cells or tissues.

In this case, the mother’s immune system marks the fetus’ HPA-1 as foreign and moves to attack it. However, this can be dangerous, as it destroys fetal platelets. RLYB211 is created from plasma sourced from women alloimmunized to HPA-1a (the fetal HPA). By inhibiting the immune system from attacking, RLYB211 improves fetal outcomes. Currently, there are no approved treatments for FNAIT. However, RLYB211 seems to be on its way – with both Orphan Drug and Rare Pediatric Disease designations.

The Phase 1/2 clinical trial will evaluate the appropriate dose, safety, efficacy, and pharmacokinetics of RLYB211. Three cohorts are included.

Fetal and Neonatal Alloimmune Thrombocytopenia (FNAIT)

Typically, thrombocytopenia is known as not having enough blood platelets. Normally, platelets prevent bleeding by forming clots. However, thrombocytopenia means people have extremely low levels of platelets, causing easy and excessive bleeding, bloody urine and stool, fatigue, jaundice, and even an enlarged spleen. Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a blood disorder that can cause severe thrombocytopenia in newborns. According to the Genetic and Rare Diseases Information Center:

FNAIT results in the destruction of platelets in the fetus or infant due to a mismatch between the mother’s platelets and those of the baby. Certain molecules (antigens) on the surface of the baby’s platelets are recognized as foreign by the mother’s immune system, [which] then creates antibodies that attack and destroy the baby’s platelets.

The organization continues:

Though FNAIT can occur whenever the mother’s blood mixes with that of the baby, it is usually triggered when the mother is exposed to the baby’s blood during delivery.

Symptoms include:

  • Thrombocytopenia
  • Purpura or petechiae, rashes caused by blood vessels under the skin
  • Cephalohematoma, or blood pooling between the skull and scalp
  • Easy bruising and bleeding
  • Bloody urine

However, in severe cases, FNAIT-related bleeding episodes can be fatal.

Jessica Lynn

Jessica Lynn

Jessica Lynn has an educational background in writing and marketing. She firmly believes in the power of writing in amplifying voices, and looks forward to doing so for the rare disease community.

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