Population pharmacokinetic modelling of amoxicillin in human breast milk-A contribution from the ConcePTION project.

Aims Amoxicillin, a widely used β-lactam antibiotic, requires improved pharmacokinetic characterization during breastfeeding. This study used a population pharmacokinetic (PopPK) approach to model amoxicillin concentrations in breast milk, identify variability sources and estimate infant exposure, applying worst-case scenarios.Methods Breastfeeding mothers receiving amoxicillin for at least 2 days were enrolled. At steady state, three (nearly) paired blood and milk samples were collected between two doses: 15-30 min, 1-2 h and 3-4 h post-dose. Samples were analysed using LC-MS/MS. PopPK modelling was performed with Monolix. Milk-to-plasma (M/P) concentrations ratios and relative infant dose (RID) were evaluated. RID was calculated relative to both adult and paediatric dosing regimens. Monte Carlo simulations estimated infant exposure at high maternal doses (6 g/day).Results Twenty-five mother-infant pairs were included. All plasma and milk samples (n = 75/matrix) contained quantifiable amoxicillin. A two-compartment model described the data, with drug transfer from plasma to milk and elimination from milk. Simulated M/P concentrations ratios were 4.1%-5.4%, while observed values ranged from 4.5% to 9.6%. RID based on adult or paediatric dosages remained <0.4%, regardless of measured or simulated concentrations. No adverse effects were reported in breastfed infants.Conclusions These findings support the low transfer of amoxicillin into breast milk and align with the absence of adverse effects in breastfed infants, reinforcing its safety during lactation.