Rallybio Stock Plunges After Program For Pediatric Bleeding Disorder Disappoints

Rallybio Corporation RLYB on Tuesday discontinued the RLYB212 program to prevent fetal and neonatal alloimmune thrombocytopenia (FNAIT).

FNAIT is a condition where maternal antibodies, formed against a fetal platelet antigen inherited from the father, cross the placenta and destroy fetal platelets. This leads to thrombocytopenia (low platelet count) and a risk of bleeding.

The company said the decision was based on pharmacokinetic (PK) data from the Phase 2 trial. The data demonstrate that the RLYB212 dose regimen is unable to achieve predicted target concentrations and the minimum target concentration required for efficacy.

The single-arm Phase 2 dose confirmation trial was designed to assess the PK and safety of RLYB212 in pregnant women at higher risk for HPA-1a alloimmunization and FNAIT.

• Secondary objectives included assessing pregnancy and neonatal/infant outcomes and the occurrence of emergent HPA-1a alloimmunization.
  • Second trimester PK results from the sentinel participant demonstrated an inability of RLYB212 to achieve predicted target concentrations of 6 ng/mL to 10 ng/mL, as well as the minimum target concentration required for efficacy of 3 ng/mL, with values near or below the assay’s lower limit of quantitation.
  • Dose adjustment is not deemed feasible given that PK levels are meaningfully outside the predicted range and the absence of empiric data to inform an adjustment.
  • It is hypothesized that HPA-1a antigen expression on the placenta may be impacting plasma concentrations of RLYB212.
• No further enrollment in the trial is planned, and all participant screening has been stopped.
• The company will continue safety follow-up of the sentinel participant as specified in the clinical trial protocol.

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Rallybio remains focused on advancing RLYB116, a once-weekly low-volume C5 inhibitor for complement-driven diseases, and its emerging preclinical programs.

Rallybio remains on track to initiate dosing in the RLYB116 confirmatory clinical pharmacokinetic/ pharmacodynamic (PK/PD) study in the second quarter of 2025, with data readouts from Cohorts 1 and 2 expected in the third and fourth quarters of 2025, respectively.

The study is expected to demonstrate complete and sustained complement inhibition with improved tolerability of RLYB116.

• RLYB116 is a novel antibody mimetic fusion protein designed to inhibit C5 and to provide a once-weekly, small volume, subcutaneously injected therapy to meet patient demand for a convenient, self-administered at-home solution.
• RLYB116 has the potential across several complement mediated diseases, including paroxysmal nocturnal hemoglobinuria, antiphospholipid syndrome, and generalized myasthenia gravis, representing a commercial opportunity of more than $6 billion.

Preclinical Programs

• REV102, an ENPP1 inhibitor for hypophosphatasia under development through a joint venture with Recursion Pharmaceuticals, entered investigational new drug application-enabling studies in the first quarter of 2025 to support initiating a Phase 1 study in 2026.
  • Data evaluating REV102 in a preclinical model of later-onset HPP is expected in the second half of 2025.
• Rallybio’s portfolio also includes RLYB332, a long-acting, monoclonal anti-matriptase-2 antibody for iron overload diseases.

Price Action: RLYB stock is down 40.6% at $0.25 at the last check Tuesday.

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