To determine infant exposure to 6-thioguanine and 6-methylmercaptopurine nucleotides (6-TGN and 6-MMPN, respectively) during maternal use of azathioprine in breastfeeding.
Mother-infant pairs provided blood for determination of 6-TGN and 6-MMPN concentrations, and TPMT genotype.
Four women taking azathioprine 1.2–2.1 mg kg−1 day−1 and their infants were studied. All had the wild-type TPMT genotype. Maternal 6-TGN and 6-MMPN concentrations ranged from 234 to 291 and 284 to 1178 pmol per 8 × 108 red blood cells, respectively, and were consistent with those associated with improved therapeutic outcomes. Neither 6-TGN nor 6-MMPN was detected in any of the infants, despite a sensitive assay.
The data suggest that azathioprine may be ‘safe’ during breastfeeding in patients with the wild-type TPMT genotype (∼90% of caucasian patients) taking ‘normal’ doses.