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Breastfeeding mothers who were taking etanercept (Enbrel) for autoimmune arthritic conditions passed the drug into their breast milk, but at very low concentrations and their infants showed no abnormalities, a small study found.

Median etanercept concentration in milk collected from six women was 12.9 ng/mL (interquartile range [IQR] 6.0-46.9), for a of 1.9%, according to Kerri Bertrand, MPH, and colleagues at the University of California San Diego (UCSD).

"There were no serious AEs [adverse events] reported for any infant," they wrote in a appearing in the Journal of Rheumatology. "All infants had growth measures within normal range at their 6-month well-child visit."

While the sample was too small to offer a definitive conclusion about etanercept's safety for breastfed infants, Bertrand and colleagues said their findings were "reassuring." The estimated RID of 1.9% -- indicating that the infant's exposure was 1.9% of the mother's dose -- was well below the 10% level generally accepted as the threshold for safety, they noted.

The question was important to address because rheumatoid arthritis and related conditions primarily affect women and age at onset is often within the childbearing years. Withholding controller medications such as etanercept while new mothers nurse their babies increases the risk of disease flares. But the potential for these drugs to enter breast milk and thus pass to infants can't be ignored. The cites "limited data" from published studies indicating that the drug does enter breast milk "at low levels," but its effects on the breastfed infant are unknown.

For the current study, Bertrand and colleagues drew on samples from six women taking etanercept who agreed to participate in UCSD's Human Breast Milk Repository program. Four had rheumatoid arthritis and the other two had ankylosing spondylitis. All were using etanercept at 50 mg/week; five at single 50-mg doses while the other was taking it in divided doses twice a week. For one woman, the infant was her first; the others had already had at least one previous child. Median maternal age was 36, and all were white and non-Hispanic. Median infant age at the time of milk collection was 10.3 months (IQR 2.5-24.4).

Etanercept levels in breast milk increased with infant age and with mothers' age. For infants 6 months and younger, concentrations were in the range of 5-15 ng/mL, reaching more than 40 ng/mL for the infant still breastfeeding at 24 months (P=0.02). The correlation was similarly significant for maternal age. Hours since last feeding, however, did not appear to influence drug concentration in milk significantly.

Five of the children underwent neurocognitive assessment via the Ages and Stages Questionnaire 2 at one or more points from age 4 to 30 months. Three were judged normal on all domains and two were "at risk" in a single domain at 12 months. One of these latter children was later rated as normal in that domain and the other had no additional follow-up recorded.

Overall, Bertrand and colleagues concluded that "these preliminary data suggest that systemic infant exposure to [etanercept] through human milk ingestion would be minimal."

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