Abstract
A 35-year-old pregnant woman visited our outpatient clinical questioning the safety of once daily 50 mg mercaptopurine (MP) during pregnancy and lactation, which was successfully treating her Crohn's disease. We measured MP and its metabolites in plasma and breast milk and found after 4 hours of intake of MP, no MP or its metabolites in breast milk. We concluded that 4 hours after intake of MP, no exposure of the suckling infant to MP and its metabolites was found while being breastfed.
Introduction
A 35-year-old pregnant woman visited our outpatient clinic questioning the safety of once daily 50 mg mercaptopurine (MP) during pregnancy and lactation, which was successfully treating her Crohn's disease as she was in remission at that time. First, we reassured her that MP can be safely used in pregnancy, based on the review published by Hutson et al., describing no signs of elevated risks of congenital malformations. 1 A prospective study of Kanis et al. also found no adverse pregnancy outcomes after maternal use of thiopurines during pregnancy and follow-up till 1 year after birth. 2 Although MP does not appear to be a major human teratogen, the drug has been associated with newborn toxicity when used near delivery, especially concerns about immunosuppression in the child. Therefore, complete blood count and liver function tests should be performed in the neonate after delivery. 3
Because less information can be found about the safe use of MP during breastfeeding, safety information can be found of the prodrug of MP, azathioprine. Azathioprine is rapidly and almost completely (88%) converted to MP, which is further metabolized through different steps in 6-thioguanine (6TG) (the active metabolite) and 6-methylmercaptopurine (6MMP) (predictive for side effects such as leucopenia, thrombocytopenia, anemia, pancreatitis, and hepatotoxicity). 4 Plasma 6TG concentrations are undetectable after 6 hours as it is rapidly transported into the cell. 4
Breastfeeding is recommended by the World Health Organization (WHO) during the first 6 months of life, because of the health benefits. 5 Our patient was very motivated to breastfeed her baby after birth, but the patient was doubting about breastfeeding her baby after birth, because of conflicting information about safety in literature (Internet, handbooks). Some sources advise not to breastfeed because of severe toxicity in the newborn, 3 whereas others advise to withhold breastfeeding during 4 hours after intake of MP. 6 We discussed the findings from literature with our patient and the possible safety issues. Especially those related to time of intake of MP related to times of breastfeeding. We decided to analyze thiopurine both intra- and extracellular plasma concentrations 1 month before delivery both 2 and 4 hours after intake. MP is rapidly transported into the cell, 4 and, therefore, possibly not available for transport into breast milk. When MP or metabolites are undetectable in plasma (extracellular), no MP or metabolites could reach the mammary gland and so breastfeeding is free from MP and metabolites. To further test this hypothesis, thiopurine concentrations were also analyzed postpartum in the patients' breast milk 4 hours after intake of MP. For our patient, when the thiopurine concentrations would be below the limit of quantification in both matrices, she would breastfeed her child.
Materials and Methods
Analysis
We used the method earlier described by Dervieux and Boulieu to measure 6TG and 6MMP plasma concentrations. 7 Some slight modifications for 6TG and 6MMP were done. The method is in-house validated according to EMA and FDA guidelines, with inter- and intra-assay variations <10% and using 5-bromouracil as internal standard. The lower limit of quantification was 100 pmol/8 × 10*8 red blood cells (RBCs) for 6TG and 200 pmol/8 × 10*8 RBCs 6MMP. Reference values for 6MMP and 6TG are >5,700 pmol/10*8 RBCs and 300–600 pmol/10*8 RBCs, respectively. 6 Reference values for MP are not given in literature.
For MP analysis in breast milk we used a registered preparation (purinethol® 50 mg tablets; Aspen, The Netherlands), dissolved in water for injection and to construct a calibration line. We used an LC-MSMS system (Xevo TQD® analyzer, Acquity column manager using a phenyl 1.7 μm column, all from Waters, Ettenleur, The Netherlands). Breast milk samples were prepared in the same way as whole blood samples. Components were eluted with a mixture of ammonium acetate 0.0154% and 0.1% formic acid in water for injection (A1) and acetonitrile (A2) were used, respectively, using gradient elution starting 97% A1 during 1.8 minutes; 5% A2 during 0.5 minutes returning to 97% A1 after 2.3 minutes until 5 minutes. The lower limit of quantification in breast milk was 25 pmol/L.
Sample collection
Blood samples were withdrawn in EDTA tubes 2 and 4 hours after intake of MP and immediately prepared for further analysis. A milk sample was collected by pumping off breast milk during day 1 after delivery, 4 hours after intake of MP. Two milliliters breast milk was used for analyses of MP and its metabolites, the remaining was given to the neonate.
Results
After 41 weeks of pregnancy a healthy girl, 2.8 kg, was born by caesarian section. No congenital malformations were seen, and blood count and liver function tests parameters were within the normal range. Intra- and extracellular concentrations 2 and 4 hours after intake of MP 1 month before delivery are presented in Table 1. The free (extracellular) concentrations of 6TG and 6MMP were below the limit of quantification. In breast milk, 4 hours after MP intake, concentrations of MP and its metabolites were undetectable.
Intra- and Extracellular Concentrations of 6TG and 6MMP After Intake of 50 mg MP in a Pregnant Woman (8 Months Pregnant) and Concentrations in Breast Milk Samples
Samples were taken 1 month before delivery.
Below the limit of detection of 25 pmol/L.
6TG, 6-thioguanine; 6MMP, 6-methylmercaptopurine; MP, mercaptopurine; NA, not applicable; RBCs, red blood cells.
Discussion
This clinical case report described a 35-year-old pregnant woman using 50 mg of MP, where we measured intra- and extracellular maternal 6TG and 6MMP concentrations and also concentrations in breast milk after delivery. We found 6TG and 6MMP intracellular concentrations 1 month before delivery within the normal range 2 and 4 hours after intake of MP. Extracellular concentrations of both metabolites 4 hours after intake were undetectable. Just after delivery we found no concentrations of MP, 6TG, and 6MMP in breast milk; therefore, breastfeeding is safe 4 hours after intake of MP. Our patient felt safe to breastfeed her child. This is in concordance with the advice given on LactMed®, which stated that avoiding breastfeeding for 4 hours after MP intake should markedly decrease the dose received by the infant through breast milk. 4 The strength of this case report is that we measured actual concentrations both intra- and extracellular, which provided evidence for this statement.
Previously published work of Christensen et al. 8 also found 4 hours after intake most MP was already excreted in the breast milk and a maximum dosage the suckling infant receives is <0.008 mg/kg body weight/24 hours. 8
A limitation of this case report is the single-point breast milk sampling, because transportation of MP or its metabolites over the mammary gland may change over time during the lactation period. However, it is well known that large molecules may pass the mammary gland during the first days of breastfeeding because of unripening of the mammary gland. 9 In this case we found no concentrations of MP or its metabolites during the first day of breastfeeding. It is assumed that during the lactation period the amount of MP transported over the mammary gland may be less in time. Another point is that we did not monitor immunosuppression and blood parameters of the neonate during the first weeks after delivery, which may occur because of in utero exposure to MP. However, we advised to check for blue spots on the neonate, and in such case the mother should call her general practitioner for further advice.
Another limitation is that concentrations of MP in the mother may change after delivery. As far as we know, no literature about this topic is available. We did not check for these possible changes.
We used purinethol commercially available tablets as standard. Because the FDA requires content of active ingredient to be 95–105% of the stated potency, our results may vary in that range. This seems not to be of importance, seeing the clinical question related to the concentrations found.
Conclusion
In this case report no concentrations of MP, 6TG, or 6MMP were found in breast milk 4 hours after intake of MP, so after 4 hours the suckling infant will not be exposed to MP or its metabolites.
Footnotes
Disclosure Statement
No competing financial interests exist.