Background: Belimumab is a recombinant human immunoglobulin G1 lambda monoclonal antibody that binds soluble B lymphocyte stimulator protein with high affinity and inhibits its biological activity. Belimumab is not recommended for breastfeeding women due to insufficient data about its excretion into breast milk. In this study, we measured belimumab concentrations in the breast milk of one nursing mother diagnosed with mixed connective tissue disease (MCTD) and evaluated the health of her breastfed infant. Materials and Methods: Maternal serum and breast milk belimumab concentrations were collected three times (2 weeks after the first dose, the day after the second dose, and 7 weeks after the second dose) after ethical approval and informed consent. An enzyme-linked immunosorbent assay was used to detect belimumab in serum and breast milk samples. Case Report: A 39-year-old para 4 female was diagnosed with MCTD. The serum concentrations at three times were 29.45, 76.82, and 33.95 mcg/mL. The concentrations in breast milk were 0.12, 0.17, and 0.12 mcg/mL. The milk-to-serum concentration ratios at each sampling point were 0.0041, 0.0022, and 0.0035, respectively. Her infant experienced no health problems. Routine vaccinations were administered without any adverse effects such as infection or immunoreaction. Discussion and Conclusions: Breast milk levels of belimumab ranged from 1/200 to 1/500 of those in serum, and no harmful effect occurred in her infant. This is the first study reporting belimumab concentrations in human breast milk. Further studies are needed to elucidate the impact of exposure on breastfeeding infants.
Keywords: belimumab; breastfeeding; mixed connective tissue disease.