Azathioprine metabolite levels and outcomes during pregnancies with rheumatic disease

Objective: Despite widespread use of azathioprine (AZA) during pregnancy, no studies evaluated the impact of pregnancy on AZA metabolites 6-thioguanine nucleotide (6-TGN) and 6-methylmercaptopurine nucleotide (6-MMPN) disposition in rheumatic diseases. This study characterises changes in AZA metabolite concentrations throughout pregnancy in women with rheumatic disease and explores relationships between metabolite concentrations, maternal disease activity, and neonatal outcomes.

Methods: Patients with rheumatic disease from a single centre prescribed AZA prior to pregnancy and ≥1 blood sample during pregnancy (5/2016 to 4/2022) were included. Commercial laboratories quantified AZA metabolite concentrations. The upper safety limit for 6-MMPN was >5700 pmol/8×10⁸ RBC. The therapeutic target for 6-TGN was ≥159 pmol/8×10⁸ RBC. Repeated correlation measures were used to evaluate the relationship between metabolite concentrations and pregnancy duration, and the relationship between 6-TGN concentration and SLE Physician Global Assessment (PGA). The relationship between pregnancy average 6-TGN and neonatal gestational age at birth was analysed using linear regression.

Results: Thirty-seven pregnancies in 35 women with 108 serum samples were included. There was no significant difference in dose-adjusted 6-TGN concentrations across pregnancy and peripartum, whereas 6-MMPN concentrations appeared higher during pregnancy. No elevated transaminases or cholestasis were observed concurrently with 6-MMPN above 5700 pmol/8×10⁸ RBC. Metabolite concentrations were related to total AZA dosage, weight-based dosage and TPMT phenotype. In pregnant women with SLE achieving average 6-TGN in the therapeutic range, we observed a non-significant reduction in PGA and increase in neonatal gestational age at birth.

Conclusions: In this exploratory study, we did not observe systematic changes in 6-TGN concentrations throughout pregnancy and peripartum, whereas 6-MMPN concentrations were higher during pregnancy. Monitoring AZA metabolite concentrations in pregnancy is a potential tool to identify medication non-adherence as well as patients with high 6-MMPN in whom dosage adjustment or close laboratory monitoring may optimise safety.