An HPA-1a-positive platelet-depleting agent for prevention of fetal and neonatal alloimmune thrombocytopenia: a randomized, single-blind, placebo-controlled, single-center, phase 1/2 proof-of-concept study

Background: Fetal/neonatal alloimmune thrombocytopenia (FNAIT) is a rare and potentially life-threatening bleeding disorder of the fetus/newborn. Antibodies against human platelet antigen 1a (HPA-1a) are associated with the most frequent FNAIT cases. There are no approved therapies for FNAIT prevention or treatment. RLYB211 is a polyclonal HPA-1a hyperimmune IgG being developed to prevent FNAIT. Objectives: To investigate whether a single dose of anti–HPA-1a (1000 IU) could markedly accelerate the elimination of HPA-1ab platelets transfused into healthy, HPA1a–negative participants as compared with placebo. Methods: This randomized, single-blind, placebo–controlled, single-center, phase 1/2 proof-of-concept study (EudraCT: 2019-003459-12) included HPA-1a– and HLA-A2– negative healthy men. Cohort 1 received intravenous RLYB211 or placebo 1 hour after transfusion of HPA-1ab platelets. Cohort 1B received RLYB211 or placebo, followed by platelet transfusion 1 week later. Primary endpoint was the half-life of transfused platelets in circulation after administration of RLYB211 or placebo, determined by flow cytometry. Proof of concept was ≥90% reduction of half-life relative to placebo. Results: Twelve participants were allocated to cohort 1 or 1B and randomized to receive RLYB211 (n = 9) or placebo (n = 3). RLYB211 markedly accelerated the elimination of HPA-1ab platelets in all participants vs placebo. In cohort 1B, this effect was observed 7 days after RLYB211 administration. Two treatment–emergent adverse events were possibly related to treatment, both in RLYB211–treated participants. No participants developed HPA-1a antibodies at 12 or 24 weeks. Conclusion: These data support the hypothesis that anti–HPA-1a could be used as prophylaxis in women at risk of having an FNAIT–affected pregnancy