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- Material and methods
The immunosuppressive drug azathioprine (AZA) is used to treat patients with severe inflammatory bowel disease (IBD), organ transplants and several autoimmune diseases.1, 2 Although these diseases often affect women of child-bearing age,1, 3 data on birth outcome among women exposed to AZA during pregnancy remain limited.
Azathioprine and its metabolite mercatopurine (MP), inhibit purine ribonucleotide and hence DNA synthesis.4, 5 Recent research has shown that the placenta forms a relative barrier to AZA and its pharmacologically active end-metabolites: the metabolite 6-thioguaninenucleotide (6-TGN) crosses the placenta, whereas 6-methylmercaptopurine does not.6 However, the role of specific AZA metabolites in inducing adverse effects during pregnancy is unknown.6 As AZA and mercaptopurine do not appear to differ substantially in efficacy and toxicity, the two drugs are usually studied together.7, 8
Azathioprine teratogenicity has been demonstrated in animals, with congenital abnormalities (CAs), such as cleft palate, limb abnormalities and ocular anomalies.5, 9 Human data on birth outcome after in utero exposure to AZA or MP are limited mostly to case reports or case series of women with organ transplants10–16 or women with IBD.17, 18 Only five controlled studies have reported birth outcome among AZA- or MP-exposed women, with conflicting findings.3, 7, 8, 19, 20 Only one small study8 estimated the relative risks of specific birth outcomes.
In our nationwide cohort study, we examined the risk of preterm birth, low birth weight at term and CAs among offspring of Danish women who received prescriptions for AZA or MP during their pregnancies.
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- Material and methods
We found an increased risk of preterm birth among women who filled prescriptions for AZA or MP during pregnancy, compared with women who were not prescribed any kind of reimbursable medication during the period extending from 3 months before conception to the end of pregnancy. This finding, however, applied primarily to induced preterm deliveries, and is likely to be a result of usual obstetric practice for these patients. Our data also indicated an increased risk of low birth weight at term and CAs for newborns of AZA- or MP-exposed women. However, these associations may be confounded by the underlying disease or concurrent use of other drugs. When we used the third comparison cohort in an attempt to take the underlying disease into consideration, the association became weaker for induced preterm birth and disappeared for spontaneous preterm birth, for low birth weight at term, and for CAs, implying that the adverse birth outcome were caused by the underlying disease and not by use of AZA or MP.
The main strengths of our study are its population-based design, made possible by Denmark's uniformly organized national healthcare system, and the minimization of selection bias through use of exposure data from a complete nationwide prescription database. Another strength is that measurement of exposure was based on prescriptions rather than self-reported use (which may lead to recall bias or under ascertainment28). Study weaknesses included a lack of information on patient compliance, as filling a prescription was used as a proxy for actual use of a drug. However, because the drugs are used for long-term treatment of severe chronic diseases, the likelihood of compliance is high. Still, if some women who filled prescriptions for AZA or MP stopped using the drug when they discovered they were pregnant, misclassification of exposure could ensue. It is also possible that some women in the third comparison cohort filled a prescription for AZA or MP earlier than 3 months before pregnancy, but actually used it during pregnancy. Another limitation is that we had no information on the prescribed daily dose of AZA or MP or on length of treatment. Furthermore, data on drugs administered during hospitalization, including AZA or MP, are not registered in the prescription database and therefore could not be included in the study. Any potential misclassification of exposure resulting from these limitations would tend to underestimate our risk estimates.
The quality of the outcome variables in the MBR (and thus in the FTDB) are high.29 However, its data on gestational age are subject to some misclassification, since the gestational age recorded in the Birth Registry is a week longer in some cases than that recorded in the medical records.29 This misclassification was more likely to affect women in the comparison cohorts, as women with severe chronic diseases who were treated with AZA or MP during pregnancy probably underwent more prenatal ultrasound examinations and therefore had more precise data on gestational age. Thus, differential misclassification of gestational age may have led to an overestimated risk of preterm birth for AZA- or MP-exposed women, compared with women who did not fill prescriptions during pregnancy.
With regard to possible confounders, we were able to consider the influence of maternal age and parity, but lacked data on maternal smoking and alcohol use, as well as sufficient data to study the influence of co-medication. If AZA- or MP-exposed women were more prone to smoke during pregnancy than were comparison women, the effect of AZA or MP on preterm birth could be overestimated, as smoking during pregnancy has been associated with an increased risk of preterm birth (OR = 1.4, 95% CI: 1.1–1.9).30 In fact, women with Crohn's disease who smoke are more likely to have severe disease31 and therefore may be more likely to be treated with AZA. With respect to CAs, tobacco use or moderate alcohol consumption during pregnancy has not been identified as independent risk factors for any of the CAs occurring among newborns of AZA- or MP-exposed women in our study.32, 33 However, concomitant use of other drugs during pregnancy could have confounded our effect estimates for CAs, as some of the drugs used by AZA-exposed women may have teratogenic effects themselves (i.e. prednisone34). Furthermore, aminosalicylates (i.e. mesalazine and sulfasalazine) may interact with AZA and lead to higher concentrations of the pharmacologically active metabolite, 6-TGN.35
Information on CAs was obtained from the HDR, whose data are of generally high quality with an 85% correct coding rate.26 However, we were not able to procure data on miscarriages or induced abortions. Selection bias could have occurred if women exposed to AZA or MP had more miscarriages or induced abortions related to fetal abnormalities than did comparison mothers. This would lead to underestimation of the risk of CAs. It is also important to consider that teratogens increase the rate of selected CAs but not all CAs.36 Cohort studies can detect only large increases in the risk of specific CAs, and are limited in their ability to provide an assurance of safety. The CAs observed among exposed newborns in the present study affected different organ systems, and none of the affected children had limb malformations, ocular anomalies or cleft palate, as found in animal studies.5, 9
Our finding of an overall increased risk of preterm birth following AZA or MP exposure in pregnancy corroborates two cohort studies of renal transplant recipients.19, 20 Sgro et al. reported an increased prevalence of preterm birth, stillbirth and low birth weight among 44 pregnancies in women who had undergone renal transplantation, and who had been treated with prednisone, together with AZA, ciclosporin, or both during pregnancy.19 Similarly, Bar et al. observed significantly more preterm births and children born with intrauterine growth restriction (IUGR), in a study of 38 renal transplant recipients who were treated with combinations of prednisone, AZA, ciclosporin and tacrolimus during pregnancy. In this study, the comparison cohort was composed of newborns of women with primary renal disease who were not treated with immunosuppressive drugs.20 Both studies did not find an increased prevalence of CAs among children of women exposed to immunosuppressive drugs during pregnancy.19, 20 However, neither study estimated relative risks, distinguished between spontaneous and induced preterm births, controlled for confounding, or specified whether children with adverse birth outcomes were born to women exposed to AZA, ciclosporin, or tacrolimus.
Based on a record review of 155 male and female patients who had conceived after developing IBD, Francella et al.7 concluded that use of MP before conception or during pregnancy appears to be safe. These data were compared with IBD patients who completed their pregnancies before taking MP. The authors reported an overall RR of 0.85 (95% CI: 0.47–1.55) for a successful pregnancy outcome for exposed parents, controlled for maternal age and parent gender. Similarly, another recent cohort study encompassing 101 pregnancies among women with IBD treated with AZA or MP found no overall association with poor pregnancy outcomes, after adjustment for maternal age.3 However, the relative risks of specific adverse birth outcomes were not estimated in either study.
To date, only one small cohort study has estimated the relative risks of specific adverse birth outcomes among children of AZA- or MP-exposed women.8 This study, conducted by some co-authors of this work, compared birth outcomes among 11 women with IBD or other diseases with those for 19 418 controls who did not receive a prescribed medication during pregnancy. Two CAs were documented among nine women exposed in early pregnancy (OR = 6.7, 95% CI: 1.4–32.4), and three preterm children with low birth weight were delivered by 10 women exposed during the entire pregnancy, corresponding to odds ratios of 6.6 (95% CI: 1.7–25.9) for preterm birth, and 3.8 (95% CI: 0.4–33.3) for low birth weight. However, no distinction was made between spontaneous and induced preterm births. The study included women who filled prescriptions in a Danish county between 1991 and 2000. Thus, some of their pregnancies were also included in the present study. Our nationwide study corroborates this regional study with respect to the overall increased risk of preterm birth.
A key problem complicating the interpretation of data in our study is confounding by indication. Because drug use is closely correlated with underlying disease and disease activity, it is difficult to distinguish between the influence of the disease itself on the risk of adverse birth outcome and possible adverse drug effects.
In conclusion, we found an overall increased risk of preterm birth and an indication of an increased risk of low birth weight at term and CAs among newborns of AZA- or MP-exposed women, compared with newborns of women who did not use any prescription drugs during pregnancy. These results are difficult to interpret, because we were unable to study the influence of disease activity and co-medication. However, when the comparison was limited to newborns of women with the same types of underlying disease, only the association for induced preterm birth remained elevated, implying that the adverse birth outcome were caused by the underlying disease and not by the use of AZA/MP.