Placental and breast milk transfer of belimumab in three patients with systemic lupus erythematosus treated throughout pregnancy.

The safety of belimumab during pregnancy, particularly in the third trimester, remains unclear. This study aimed to assess the placental and breast milk transfer of belimumab in pregnancies complicated by systemic lupus erythematosus and to evaluate immunological effects and vaccination responses in offspring. We prospectively followed three patients with systemic lupus erythematosus who received belimumab throughout pregnancy. Belimumab concentrations were measured in maternal serum, cord blood, breast milk, and neonatal serum, along with infant development and vaccination histories. Belimumab was continued throughout pregnancy to control refractory thrombocytopenia in two cases and haemolytic anaemia in one case. All patients delivered full-term infants without obstetric complications. Overall, belimumab concentrations in cord blood and neonatal serum were comparable to those in the maternal serum, suggesting transplacental transfer. A decrease in peripheral B and transitional B cells was observed in all neonates at birth, while serum IgG levels and peripheral T cell counts were within normal ranges. Only one infant was diagnosed with a complication (left vesicoureteral reflux). Belimumab concentrations in breast milk were low, and no adverse events occurred in the vaccinated infants. Continuation of belimumab throughout pregnancy may be an option to control refractory disease activity and achieve successful outcomes in pregnancies complicated by systemic lupus erythematosus. However, careful monitoring during pregnancy and postnatal follow-up is essential to ensure safety, given that belimumab can be transferred to the placenta, detected in the neonatal peripheral blood, and affect the neonatal lymphocyte subset counts at birth.