Rallybio Announces Proof-of-Concept Results and Development Updates for RLYB212, a Novel Monoclonal anti-HPA-1a Antibody to Prevent Fetal and Neonatal Alloimmune Thrombocytopenia

Rallybio Corporation (Nasdaq: RLYB), a clinical-stage biotechnology company committed to identifying and accelerating the development of life-transforming therapies for patients with severe and rare diseases, today reported data from the Phase 1b proof-of-concept study of RLYB212, a novel monoclonal anti-HPA-1a antibody in development for the prevention of fetal and neonatal alloimmune thrombocytopenia (FNAIT). The data, delivered in an oral presentation by Christof Geisen, M.D., at the 31^st Congress of the International Society on Thrombosis and Haemostasis (ISTH), showed that subcutaneous (SC) RLYB212 administration produced a dose-dependent, rapid and complete elimination of transfused HPA-1a positive platelets in HPA-1a negative subjects, with both doses meeting the prespecified proof-of-concept criteria of ≥90% reduction in mean platelet elimination half-life. Mean platelet elimination half-life was 5.8 hours (0.09mg) and 1.5 hours (0.29mg) for RLYB212 compared to 71.7 hours for placebo.

Christof Geisen, M.D., Institute of Transfusion Medicine and Immunohaematology, German Red Cross Blood Transfusion Service, and the lead author for the RLYB212 abstract stated, “These proof-of-concept results provide further understanding as to how FNAIT may have the potential to be prevented through the rapid and complete elimination of fetal platelet antigens prior to maternal alloimmunization. These data are a critical step forward to address a significant unmet need with no currently approved treatments or therapies to prevent maternal alloimmunization as well as the occurrence of FNAIT in babies.”

Phase 1b Proof-of-Concept (POC) Study of RLYB212 in FNAIT

The Phase 1b single-blind, placebo-controlled proof-of-concept study was designed to establish the ability of SC RLYB212 to rapidly eliminate HPA-1a positive platelets transfused to HPA-1a negative healthy subjects. The study included 11 males aged 18 to 65 years, randomized to RLYB212 0.09mg (n=4), RLYB212 0.29mg (n=5), or placebo (n=2).

Summary of Proof-of-Concept Results

• RLYB212 demonstrated a dose-dependent, rapid and complete elimination of transfused HPA-1a positive platelets in HPA-1a negative subjects.
• Mean platelet elimination half-life was 5.8 hours (0.09mg) and 1.5 hours (0.29mg) for RLYB212 compared to 71.7 hours for placebo, meeting the study’s prespecified criteria for proof-of-concept in both dose groups (≥ 90% reduction in mean platelet elimination half-life).
• The study’s broad range of pharmacodynamic and pharmacokinetic (PK) data will allow substantive modeling of the RLYB212 concentration-effect relationship and inform the target dose regimen for the planned future studies.
• Platelet elimination profiles after SC administration of RLYB212 were consistent with those of Rhesus factor D (RhD)-positive erythrocytes after intramuscular administration of anti-RhD agents, which are well-established to safely and effectively prevent RhD alloimmunization during pregnancy.
• Consistent with previously reported data, RLYB212 was observed to be well-tolerated with no reports of serious or severe adverse events.

“The data reported today at ISTH continue to support our belief in the potential use of subcutaneous RLYB212 as a prophylactic therapeutic for the prevention of HPA-1a alloimmunization and FNAIT. We are pleased to see rapid and complete platelet elimination in our target concentration range, with both dose groups meeting the proof-of-concept criteria of at least 90% reduction in mean platelet elimination half-life,” commented Róisín Armstrong, Ph.D., Rallybio’s RLYB212 Program Lead. “With no currently approved therapies for the prevention of FNAIT, there remains a substantial unmet need for a treatment that can effectively eliminate fetal platelet antigen and, as a result, significantly decrease the risk of severe bleeding in the fetus and neonate, including intracranial hemorrhage and its devastating consequences.”