Understanding Tysabri and Its Role in MS Treatment
Tysabri, with the active ingredient natalizumab, is a highly effective monoclonal antibody used to treat relapsing forms of multiple sclerosis (MS) and Crohn's disease [1.2.2, 1.5.7]. It works by preventing inflammatory immune cells from crossing the blood-brain barrier and entering the central nervous system, thereby reducing the inflammation that drives MS disease activity [1.2.4]. Given that MS often affects women during their reproductive years, the decision to continue, stop, or restart a disease-modifying therapy (DMT) like Tysabri after childbirth is a significant concern [1.2.4]. Stopping treatment can pose a risk of disease reactivation or relapse, which occurs in approximately 40% of women who discontinue the drug [1.4.9].
Natalizumab Transfer into Breast Milk
Research has consistently shown that natalizumab is excreted into breast milk, but the amounts are generally low [1.2.1]. Natalizumab is a large molecule, which limits its ability to pass into breast milk in significant quantities [1.6.2]. Studies measuring the concentration of the drug in breast milk have found variable but low levels, with peak concentrations often occurring within the first one to two weeks after an infusion [1.2.1, 1.4.9].
The key metric for assessing infant exposure is the Relative Infant Dose (RID), which compares the dose an infant receives through milk to the mother's dose. A RID below 10% is generally considered safe [1.4.9]. Studies on natalizumab have estimated the RID to be well below this threshold, with some calculating it as low as 0.04% of the maternal dose and others noting a maximum of 5.3% [1.3.6, 1.3.7]. Importantly, limited studies that have tested for natalizumab in the blood of breastfed infants have found it to be undetectable, suggesting poor oral absorption by the infant from the gastrointestinal tract [1.2.1, 1.4.8].
Assessing the Safety and Risks for the Infant
The central question for mothers and clinicians is whether this low-level exposure poses any risk to the nursing infant. A growing body of evidence from observational studies and registries suggests that breastfeeding while on Tysabri is safe.
Multiple studies, including a 2024 preliminary study presented to the American Academy of Neurology involving 125 mothers on natalizumab, found no differences in hospitalization rates, antibiotic use, or developmental milestones for infants exposed to the medication through breast milk compared to unexposed infants [1.2.3, 1.4.6]. Another long-term study published in Neurological Sciences confirmed these favorable neonatal outcomes, reporting normal growth, neuromotor development, and language development in children followed for up to five years [1.2.4, 1.2.5]. These children also received scheduled vaccinations without complications, and no hematological disorders or serious infections were recorded during or after the breastfeeding period [1.2.5].
While the current data is reassuring, it is important to note that some sources maintain a cautious stance due to the lack of large-scale, long-term clinical trials [1.2.2, 1.5.3]. The official Tysabri prescribing information advises patients to talk with their doctor about the best way to feed their baby, as it is known to pass into breast milk, but the potential effects on the infant are not fully known [1.5.1, 1.5.6].
Comparison of MS Breastfeeding Treatment Considerations
The decision to use Tysabri while breastfeeding involves weighing the benefits of controlling maternal MS against the potential, though seemingly low, risks to the infant. Below is a comparison table outlining these factors.
| Factor | Considerations for Using Tysabri While Breastfeeding | Considerations for Discontinuing Tysabri |
|---|---|---|
| Maternal Health | Effective control of MS disease activity; prevention of postpartum relapses [1.4.9]. | Significant risk of disease rebound or relapse (approx. 40% of patients) [1.4.9]. |
| Infant Exposure | Low transfer into breast milk; Relative Infant Dose (RID) well below 10% safety threshold [1.4.2, 1.4.9]. | No medication exposure through breast milk. |
| Infant Safety Data | Studies show no negative impact on growth, development, or infection rates in the first few years of life [1.2.3, 1.2.5]. Undetectable in infant serum [1.2.1]. | Avoids any theoretical or unknown long-term risks associated with drug exposure. |
| Expert Guidance | Most expert guidelines and recent studies consider it acceptable and safe [1.2.1, 1.4.8]. | Some official labels remain cautious due to limited long-term data [1.5.3, 1.5.6]. |
| Alternatives | Other DMTs like interferon-beta and glatiramer acetate are also considered safe but may be less effective for highly active MS [1.6.2, 1.6.5]. | Bridging with other therapies or a planned treatment hiatus, which carries its own risks [1.6.2]. |
Recommendations and Clinical Consultation
Given the high risk of postpartum relapse if Tysabri is discontinued, many experts recommend continuing treatment while breastfeeding, viewing the benefits for the mother as outweighing the minimal risks to the infant [1.4.8]. Some guidelines suggest that waiting at least two weeks postpartum to resume therapy may help minimize the transfer to the infant, as the gut of a newborn is more permeable [1.2.1].
The decision is ultimately a personal one that must be made in close consultation with a neurologist and pediatrician. This discussion should cover the mother's specific disease activity, the benefits of both breastfeeding and effective MS management, and a thorough review of the most current safety data. Continuous monitoring of the infant's health and development is also recommended.
Conclusion
For mothers with multiple sclerosis, the question 'Is Tysabri safe for breastfeeding?' is a complex one, but the current body of research is overwhelmingly positive. Evidence indicates that while natalizumab passes into breast milk, the amount is low, infant absorption is minimal, and follow-up studies have not identified any negative effects on infant health or development [1.2.3, 1.2.5]. Most expert guidelines now support the use of Tysabri during lactation, especially when considering the significant risk of MS relapse for the mother if treatment is stopped [1.2.1, 1.4.8]. As always, a personalized risk-benefit discussion with your healthcare team is essential to make the best choice for you and your baby.
Authoritative Link: For detailed drug information, consult the Drugs and Lactation Database (LactMed®) entry on Natalizumab from the National Institutes of Health. [1.2.1]
