A new study reports that hematologic abnormalities may occur in one-third of infants exposed to natalizumab during the third trimester but treatment was generally safe (Triplett et al. Mult Scler Relat Disord 2020;40:101961).

The retrospective chart review examined data for 15 infants born to 13 mothers with MS who were treated with natalizumab in the third trimester. Median age at conception was 34 years; median duration of MS was 4.5 years. The median birth weight was 2778 grams (range 2100-3790 grams). Two infants had anemia and three infants had thrombocytopenia. Hematologic abnormalities did not result in morbidity or mortality. No other laboratory or congenital abnormalities were identified. The authors suggested that reduced natalizumab dosing in the third trimester, administration of intravenous immunoglobulins or umbilical cord sampling prior to delivery might reduce the hematological effects of natalizumab on neonates.

Similar results were reported in a previous case series of 12 women with 13 pregnancies (Haghikia et al. JAMA Neurol 2014;71:891-895). In that study, 10 of 13 infants had mild-to-moderate hematologic changes, including anemia and thrombocytopenia. Clinical monitoring by a pediatrician was recommended to evaluate potential complications of anemia and thrombocytopenia.

Animal studies have reported that natalizumab (up to 30 mg/kg IV every other day) appears to cross the placenta primarily during the third trimester but does not alter fetal development (Tysabri Product Monograph, August 2019). Hematologic changes in animals included a decrease in lymphocytes (CD3+ T cells, CD20+ B cells), alterations in lymphocyte subsets, anemia, a reduced platelet count and decreases in hematopoiesis. Hematologic effects in neonates appeared to resolve with clearance of natalizumab.