Albumin fusion as a design strategy to keep antibody drugs on the mother's side of the placenta

A woman with rheumatoid arthritis, a patient managing certain cancers, or someone living with a chronic inflammatory disease often depends on monoclonal antibody drugs — engineered protein medicines, often given by infusion, that are an increasingly used class of biologic treatments. For patients of reproductive age, pregnancy has meant an awkward choice: keep the disease under control and accept fetal exposure to those medicines, or stop treatment and risk a flare.

A study in Science Immunology by researchers at the University of Oslo and Oslo University Hospital points to a third path. The team, led by Jan Terje Andersen, reports that the placenta's protein-sorting machinery — the receptor FcRn, which ferries protective maternal IgG antibodies across to the fetus — leaves most albumin on the mother's side. By fusing antibody drugs to albumin, the researchers say, they could exploit that sorting and keep the medicine on the maternal side in mice and in ex vivo human placental tissue.

The finding turns a familiar trick into a deliberate pregnancy-safety design lever. Albumin is a long-lived blood protein, already used as a half-life-extending fusion partner in some approved biologics to keep drugs circulating longer in the body. The change is the design intent: a fusion format chosen specifically to exploit how the placenta already sorts proteins, so the same receptor that carries protective IgG to the fetus can be steered to leave a medicine on the mother's side.

A receptor that picks sides

The placenta's job is to feed a growing fetus while keeping most of the mother's bloodstream separate from it. FcRn is the molecular bouncer at that border, the GEN report on the paper explains. In the placenta, it binds IgG, the most common antibody class, and shuttles it across to the fetal circulation, a process that normally gives newborns passive immunity before their own immune systems mature. The receptor does similar recycling work elsewhere in the body, returning IgG and albumin to the bloodstream to extend their half-lives.

The new finding is that the placental form of FcRn treats those two proteins differently. IgG crosses; albumin largely does not. That difference is what makes albumin a useful "seatbelt" for an antibody drug, the authors argue.

What the experiments showed

In mouse models and in ex vivo human placental tissue, the Oslo team fused IgG antibodies to albumin and measured how much of the resulting construct reached the fetal side. The fusion protein stayed mostly on the maternal side, the team reports, and in mice did not produce the adverse effects seen with unfused antibody. News-Medical's summary of the work and the Bioengineer coverage describe the same result.

The conclusion, as the Oslo University Hospital publication record for Andersen states it, is that albumin is an attractive fusion partner for biologics intended to minimize fetal exposure during pregnancy.

What's missing

The work is preclinical. The mouse and ex vivo tissue experiments do not establish safety in human pregnancies, and no drug built on this design has been tested in pregnant patients, let alone approved. The findings tell drug developers how to design the next generation of antibody medicines for use in pregnancy, not what to prescribe next week.

Albumin fusion is itself a familiar approach. Drug developers have used it for years to extend how long biologic drugs circulate in the bloodstream. The paper changes the design question instead: for an antibody drug aimed at a disease common in women of reproductive age, can the molecule be built so the placenta's own machinery leaves it on the maternal side? For drug developers, asking that at the design stage would mean choosing a different molecular format from the start, not adding a pregnancy-safety patch after a candidate is already in the clinic.