Objective Belimumab is a human IgG1-lambda monoclonal antibody that inhibits the B-cell survival factor BLyS. It is used in patients with active systemic lupus erythematosus (SLE). Since data on the safety of belimumab during breastfeeding is scarce, we investigated belimumab concentrations measured in breast milk of a woman with SLE initiating treatment with subcutaneous belimumab due to a lupus flare three months after delivery.

Methods Our patient stopped giving breastfeeding when she started on subcutaneously administered belimumab. During the phasing out of breastfeeding she pumped breast milk to prevent engorgement. Milk samples were collected two and four weeks after the initiation of belimumab. Belimumab concentrations were measured by using an in-house developed enzyme-linked immunoabsorbent assay (ELISA).

Results Belimumab concentrations measured in two breastmilk samples were 0.264 ug/mL at day 14 and 0.885 ug/mL at day 28 (table 1).

Conclusion This case report shows that subcutaneously administered belimumab was transferred to breastmilk. The higher concentration at day 28 is probably caused by the low volume and long stasis of the breastmilk during phasing out. The concentrations measured in breast milk were much lower than those measured in serum of adult patients (around 70 ug/mL).¹ The actual serum level of belimumab in the infant is supposed to be low, because of proteolysis in the infant’s stomach and low intestinal absorption. However data on infant serum levels are not available and urgently needed in order to be able to weight the possible risk of transfer of belimumab to the infant against the positive effects of breastfeeding for mother and infant.

Reference

1. Struemper H, M Thapar, D Roth. Population pharmacokinetic and pharmacodynamic analysis of belimumab administered subcutaneously in healthy volunteers and patients with systemic lupus erythematosus. Clin Pharmacokinet, 2018;57(6):717–728.

Funding None