Rallybio Stock Plunges After Program For Pediatric Bleeding Disorder Disappoints - Rallybio (NASDAQ:RLYB)

Rallybio Corporation (NASDAQ: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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